Carrier Aeroplane	an aeroplane used as the lower stage of a launch vehicle
ELV	expendable launch vehicle
ISS	International Space Station
LEO	low Earth orbit
MBB	Messerschmitt-Bölkow-Blohm
Operational Prototype	a prototype spaceplane fit for launching non-passenger payloads, such as satellites
RLV	reusable launch vehicles
Spaceplane	fully reusable piloted winged vehicle capable of flight to and from spaces
SS1	SpaceShipOnes
SSTO	single stage to orbits
Sub-orbital	a trajectory with sufficient speed to zoom to space height (100km) for a few minutess
TSTO	two stages to orbits
Weight	initial weight = dry weight + consumed propellants
Definitions	dry weight = inert weight + payloads
	inert weight = structure + propulsion + systems + miscellaneous

Imagine someone in 1905 tasked with writing a survey article on 'New Commercial Opportunities in Aeronautics'. He (and it would most likely have been a 'he' in those days) would probably have started with a review of the commercial uses of balloons. He would then have described the latest developments in lighter-than-air technology and how these would improve the capability of balloons to attract larger or new markets. The article would probably have discussed the recently invented aeroplane and airship. However, for the article to have had much impact, it would have had to predict nothing short of a revolution in aeronautics, with aeroplanes replacing balloons for most applications and opening up large new markets. The bulk of such an article would therefore have been about the commercial potential of aeroplanes.

We could be facing an analogous situation with regard to commercial opportunities in space. The first spaceplane designed for commercial use, SpaceShipOne (SS1), shown in Fig. 1, first flew to space (i.e., more than 100km altitude) in 2004. This was the first flight to space of a fully reusable flying machine since the last flight of the X-15 in 1968. The reasons for this 36 year hiatus are discussed later.

Figure 1. X-15 and SpaceShipOne (SS1) - the only fully reusable spacefaring vehicles to date. There was a 36-year gap between the last flight of the X-15 and the first flight to space of SS1.

SS1 could have an impact on astronautics just as far-reaching as the Wright Flyer had on aeronautics. It is reasonably obvious that airline operations are not practicable using balloons. They cannot fly into wind. In 1903 (120 years after the Montgolfier Brothers invented the balloon) the Wright Brothers showed that machines capable of flying into wind at reasonable speed were a practical proposition, and this led to an explosive growth in aeronautics.

Likewise, it is reasonably obvious that airline operations to and from orbit are not practicable using vehicles with large and complex expendable components, as used exclusively to date for spaceflight. They are far too risky and expensive. SS1 has shown that aeroplanelike space-faring vehicles with no expendable components (spaceplanes) can be developed by the private sector, at least with sub-orbital performance. If this can be extended to orbital flight, an explosive growth in astronautics could follow.

This 'if' is important, because an orbital spaceplane is a big advance on a sub-orbital one. A sub-orbital flight involves accelerating to supersonic speed on a steep climb and then coasting up to space height, which is usually defined as 100km. The time in space is no more than a few minutes. Staying up like a satellite requires about twice the height and six times the speed. Nonetheless, a suborbital spaceplane is a major step towards an orbital one.

This paper aims to assess whether or not this rapid expansion of astronautics is likely to actually happen and, if so, when. The paper starts by considering the extent to which spaceplanes can reduce the cost of access to orbit. It then considers what new markets might emerge at that cost. The results are sufficiently encouraging to postulate a 'spaceplane age', with low-cost access leading to large new commercial markets, especially space tourism, and a rapid growth of space science and exploration. The rest of the paper considers the technology required to achieve this breakthrough, how long it is likely to take, how much it will cost, and how it could be funded.

2.1 Sänger

In assessing the commercial potential of spaceplanes, a key question is the extent to which they can reduce launch costs. In order to limit the analysis to timescales of immediate interest, this paper will consider only spaceplanes using conventional materials and chemical rockets. More advanced vehicles may well offer further cost reductions but they are too far into the future to be of interest other than providing a focus for long-term research programmes.

It might seem that the logical place to begin would be a review of spaceplane project studies carried out by government agencies or prime contractors. However, it turns out that there have been remarkably few of these in recent years. Perhaps the most credible exception is the study carried out by Messerschmitt-Bölkow-Blohm ( MBB, now part of EADS) of their Sänger spaceplane(1), shown in Fig. 2. Sänger is one of the most thoroughly studied spaceplane projects and its leading design data should therefore be reliable. Moreover, it has many of the key design features needed for commercial success.

The design of Sänger dates back to the early 1960s when Junkers, a predecessor company of MBB, was involved with the European Aerospace Transporter studies (discussed later). Work on Sänger peaked in the late 1980s and early 1990s when it was the centrepiece of the German hypersonics research programme.

Sänger is a two-stage launcher, consisting of a Carrier Aeroplane and an Orbiter. The Carrier Aeroplane takes off from a runway, flies to the required orbit plane, and accelerates to Mach 6.7. The Orbiter then separates and accelerates to orbital velocity. The Carrier Aeroplane flies back to base and is prepared for the next flight. The Orbiter delivers its payload to orbit, re-enters the atmosphere, and flies back to base.

As shown in Fig. 2, there are two versions of the Orbiter, one for passengers and one for cargo. The passenger version is designed to carry 36 passengers to and from space hotels. The cargo version has a payload to low Earth orbit ( LEO) in the ten tonne class.

(c) Mach 4.4 airliner derived from the Spaceplane Carrier Aeroplane.

Figure 2. The MBB Sänger Spaceplane [EADS].

Sänger has a take-off weight comparable to that of a Boeing 747. Two major differences are that it uses liquid hydrogen fuel and has two stages. These features are to reduce the required propellant weight fraction on each stage to a practicable level. Hydrogen fuel has more energy per unit weight than kerosene, and having two stages decreases the propellant weight fraction on each stage from the very high value required for a single-stage vehicle.

Even with these palliatives, Sänger has to carry more fuel than the 747 and therefore carries about twelve times fewer people - 36 compared with around 420.

Also shown in Fig. 2 is the MBB design for a supersonic airliner based on the launcher Carrier Aeroplane with a cruising speed of Mach 4.4, which is just over twice the speed of Concorde. Other manufacturers have explored such commonality between high-speed airliners and launcher carrier aeroplanes as a means of spreading development cost, and Sänger is the most recent published example.

Both the launcher and supersonic transport versions of Sänger use advanced technology. The Sänger launcher uses advanced turboramjets on the Carrier Aeroplane to provide a stage separation speed of Mach 6.7. This gives rise to two major engineering problems that would be expensive to resolve.

First, Mach 6.7 is considerably faster than the maximum speed to date of practical air-breathing engines, which is about Mach 4. The increased speed leads to far higher temperatures and to greater difficulty in achieving adequate intake and nozzle efficiencies.

Second, stage separation has to take place in the atmosphere, leading to air and thermal loads that are both severe and hard to predict. The paper will consider later how these two difficulties can be avoided.

MBB estimated in 1987(1) that the cost per flight of the Sänger launcher was $15 million for a launch rate of twelve per year, which was about 10% of the cost per launch of the Ariane 5 large expendable launcher. This would be around $28 million today.

A flight rate of twelve per year is of course orders of magnitude less than that of an airliner fleet. This raises the question of what the cost per flight would be at higher traffic levels. A preliminary indication can be gained from the estimated cost per flight of the airliner version of Sänger, which is the same size as the Carrier Aeroplane and has much technical commonality. The estimated cost per flight of the airliner was $184,000 in 1986, (around $340,000 today), which is about 80 times less than that of the launcher. A factor of very approximately two is due to the launcher having two stages whereas the airliner has only one. Thus, the Carrier Aeroplane costs roughly 40 times more per flight than the airliner.

When estimating the cost per flight of the airliner, MBB no doubt used airliner cost-estimating relationships, which assume technical maturity and utilisation comparable with airliners in service today. When estimating the cost of the launcher version, MBB probably started with the cost breakdown of an expendable launch vehicle ( ELV) like Ariane 5 (which made six launches in 2006) and then estimated the savings due to reusability.

If a new reusable launch vehicle ( RLV) makes only twelve flights per year, there is insufficient operating experience and financial incentive to mature the technology to achieve airliner standards of life, turnaround time, and maintenance costs. The difficult technologies needed for high-speed flight, such as thermal protection systems, rocket engines, transparencies, hot structures, and seals, are almost bound to remain at the experimental stage (by airliner standards).

However, given the prospect of large new markets such as space tourism, there is then an incentive to mature these technologies. If this process led to airliner standards of maturity, then the cost per flight of the Sänger Carrier Aeroplane would become comparable with the $340,000 of the airliner. As already mentioned, it is the same size and has much design commonality.

Thus, the factor of 40 difference in cost per flight between the Sänger Carrier Aeroplane and airliner appears to be a measure of the lower level of maturity imposed on launchers by their far lower utilisation, and to spreading the fixed costs over fewer flights.

The Orbiter is smaller but more advanced and if for the sake of a preliminary estimate we assume that these factors balance out, the cost per flight of the Orbiter would also be around $340,000, to give a cost for the complete vehicle of $680,000. With 36 seats, the cost per seat to orbit would then be around $20,000, which is about 1000 times less than the potential cost today. (The Space Shuttle costs about $1 billion per flight and could be configured to carry 50 passengers, giving a cost per seat to orbit of around $20 million. By coincidence, this is close to the amount that pioneering space tourists (four to date) have had to pay for a visit to the International Space Station ( ISS) using the Soyuz for transport.)

This is clearly a very rough estimate but it does indicate the low-cost potential of a mature spaceplane. There is little doubt that a vehicle like the Sänger launcher could be developed, given sufficient priority and funding. The technology required might turn out to be more difficult than assumed by MBB, and the development costs higher. However, if the production run were long enough for design maturity to be achieved and development costs amortised, its operating cost would become comparable to the above estimate.

The fate of Concorde and proposed follow-on projects suggests that the airliner version of Sänger would probably not be commercially viable. However, this does not affect the argument being used here, which depends only on what the airliner cost per flight would be if it were built in reasonable numbers. The markets for the airliner and launcher versions are so different that the commercial viability of one hardly affects the other, except that there would be some sharing of development cost if both were developed. We will consider later the relative technical difficulties of launcher and airliner.

Generalising this result, we can state that a spaceplane designed for high traffic levels and when well down its various learning curves should have a cost per flight approximately twice that of a supersonic transport (SST) of similar size. It would carry fewer passengers than the SST and so the cost per seat would be higher. Even so, the cost per seat to orbit in a mature spaceplane would be roughly one thousand times less than the cost today.

A more detailed cost estimate(2), is summarised in Fig. 3 and shown in the Appendix. This estimate involves scaling Boeing 747 costs to derive those of the Sänger launch vehicle, assuming that the latter has maturity and utilisation comparable to the former. The cost per flight of Sänger on this basis is $556,000, which is close to the above simpler estimate and results in a cost per seat to orbit of about $15,000.

Figure 3. Cost comparison between the Sänger launcher developed to airliner maturity and a Boeing 747. The cost per seat to orbit in Sänger is about 1,000 times less than that of the Space Shuttle. Derivation shown in the Appendix.

Sänger was not optimised for a high launch rate. A vehicle so optimised, like the Bristol Spaceplanes Spacebus described later, would probably use kerosene in place of the expensive hydrogen for at least part of the ascent. This feature, together with a more efficient orbiter structure enabling more passengers to be carried, would reduce the cost per seat to orbit to about one half that estimated above for Sänger, or around $7,500(3).

The idea that the cost per flight of a spaceplane can be comparable to that of a supersonic transport, given comparable utilisation and design maturity, is not new. Several papers in the 1960s derived this result in the context of long-range sub-orbital airliners (4,7) and analysis carried out more recently(8), supports this conclusion.

The 1,000 times cost reduction is a measure of the potential benefits of using vehicles like aeroplanes in place of those like ballistic missiles.

In summary, the 'winning equation' is that:

Reusability + high traffic levels = 1,000 times reduction in the cost per seat to orbit

Space hotels (i.e., space stations equipped for passenger recreation) will be needed for orbital space tourism of more than a few hours duration. The design of these justifies a paper in its own right, but two points, taken from(2), are important in the context of the commercial opportunities for spaceplanes:

• The main causes of the high cost of present space stations are low production rate, high political profile, and high cost of access for repair and maintenance. None of these factors need apply when spaceplanes enter service and provide low-cost access. The cost per unit weight of space hotels should then become comparable to that of airliners, which is orders of magnitude less than that of space stations to date.

• Space hotels will have a life in orbit measured in decades. The cost of launching them is therefore relatively small compared with the cost of operating them, the largest component of which is the regular supply transport by spaceplane. Thus, present expendable (and therefore expensive) heavy lift vehicles, such as Ariane 5, could be used for launching their modules without greatly affecting total cost. At a later stage, technology from spaceplanes could be applied to large launchers to provide reusability and hence to reduce their cost per flight.

The combination of space stations, space hotels, reusable heavy lift vehicles for launching their modules, and spaceplanes for frequent and regular supply flights will provide a low-cost orbital infrastructure of benefit to all commercial and scientific users of space.

A preliminary estimate(2) shows that the total cost of a few days in a space hotel is about twice that of the flight there in a spaceplane. Thus, assuming the $7,500 cost per seat of Spacebus, $20,000 is a conservative round number for the cost of a space holiday when all systems are fully developed. The significance of this potential $20,000 is that middle-income people could afford it for a once-in-a-lifetime visit to space. The implications of this are discussed later.

The conclusions so far are that spaceplanes offer the potential of a thousandfold reduction in the cost of human access to orbit but that large new markets are required to provide economies of scale. This section considers the various markets for spaceplanes to see which, if any, offer such a prospect.

There will be three main markets for orbital spaceplanes: new commercial markets made possible by their low launch cost, replacing ELVs for launching satellites, and replacing ELVs for government manned space missions. Let us consider each in turn.

Spaceplanes can remove the launch cost barrier which at present prevents three commercial uses of space from developing - manufacture, solar power collection, and orbital tourism. Let us consider each in turn.

Spaceplanes will greatly reduce the cost of research in microgravity and will make affordable pilot plants of factories in orbit using zerog, vacuum, and solar energy to make products at less cost than on Earth, or that cannot be made in one-g factories. At present, the nature of such products and the demand for them are speculative, and it would not be prudent to include them in a business plan.

The energy reaching the Earth from the Sun in just three days is equal to that in all the known fossil fuel reserves. Spaceplanes will make affordable the construction of experimental solar power satellites. An operational satellite of just 250km diameter could supply most present energy needs (assuming 10% overall efficiency) without any greenhouse gas emissions during the operational phase. However, while spaceplanes could provide the required low-cost access to space, there remain the major problems of reducing the cost and weight of solar cells and of transmitting the power to Earth. It is not yet clear how well solar power generated in space will compete with that generated on the Earth's surface. As with manufacture, the demand is potentially very high but too speculative at this stage for inclusion in a business plan.

The growth of the market for space tourism is likely to follow a classic 'S-curve', typical for products that offer a new capability, like air travel, personal computers, and mobile phones. There is a pioneering phase in which the market is limited to rich and adventurous people or to those who have a strong need for the product. Then there is a rapid growth phase as the idea catches on and the costs are reduced by the economies of scale, maturing technology, and competition. Finally, there is a saturation phase in which most people who want the product have it and sales growth is driven by economic expansion and by young people entering the market.

In the early stages of such products, it is notoriously difficult to make reliable predictions about the growth of sales. In the case of space tourism, at least some statements can be made with confidence:

• The market for space tourism already exists. Four very wealthy people have each paid about $20 million for a visit to the ISS and more are planning to go. This is a far higher cost than for any other form of adventure travel. Moreover, it involves six months of training, which, for billionaires, is probably more of an obstacle than finding the money. This demand is an indication of the great fascination that space can exert.

• Sub-orbital passenger flights should start within the next few years. Virgin Galactic has firm plans and others are not far behind. Virgin has already (January 2006) taken $11 million in deposits for brief sub-orbital space experience flights in SpaceShipTwo (SS2), which will be a development of SS1.

• For the near future, a visit to a space hotel will be so expensive that it will be a once-in-a-lifetime experience for most who go. Only the very wealthy will be able to fly more than once.

These initial observations do not answer the key question of whether the demand for space tourism is likely to be large enough to fund the high cost of developing a large mature spaceplane.

There have been several market surveys to assess the demand for space tourism. Perhaps the most thorough of these is that by the Futron Corporation(9). Virgin Galactic say that this survey matches their experience to date. Among other results, this survey indicates that 30% of wealthy US citizens would pay $1 million for an orbital tourist flight. Overall, around 50% of the population would visit space if they could afford to.

The Futron study does not consider costs lower than $1 million for orbital tourism whereas, as discussed earlier, far lower costs could be achieved with mature spaceplanes. It assumes 40 years to approach market saturation at a cost per person of $5 million (orbital). Futron hardly justify this 40-year timescale - they just state that it is analogous to the development of civil aviation and other terrestrial markets. The study report does not provide enough detail to be able to estimate demand at different costs and timescales.

However, it is still possible to make useful estimates of the likely demand for early (pre-saturation) space tourism by considering the number of people alive when space tourism starts to become big business who would fly when the fare approaches the potential $20,000.

We know from the experiences of the more than 400 astronauts to date that, for most of them, a visit to space was a transforming experience. Looking at the Earth from afar, playing around in zerog, and viewing astronomical objects without atmospheric distortion were the highlights. These experiences cannot be gained on Earth.

We can therefore predict with fair certainty that the public will be highly enthusiastic about the prospect of visiting space. Thus the prediction by the Futron and other market surveys, that around 50% of the population would like to spend a few days in a space hotel if they could afford the fare, is credible.

A key point in assessing future demand is that, as mentioned earlier, the cost of a visit to orbit achievable with a mature spaceplane, around $20,000, is affordable by middle income people prepared to save.

We cannot say for sure how many people actually would visit space at this cost, but it would nonetheless be useful to have a conservative estimate for planning purposes. Unless the Futron conclusion (that 30% of wealthy US citizens would pay $1 million for an orbital tourist flight) is very optimistic, it must be conservative to assume that at least 5% of the industrialised population, alive when the cost of orbital space tourism comes down to around $20,000, would visit a space hotel for a once-in-a-lifetime experience.

If the industrialised population is 1 billion, this early demand for space tourism would then be 50 million people. This is a large number by present manned space standards but low compared with the present 1.5 billion airline passenger flights per year. It would of course take some time to fly all these 50 million people but, as an illustrative example, a fleet of 150 fifty-seat spaceplanes capable of two flights per day could carry them in ten years.

In terms of revenues, the early demand for space tourism would then be 50 million people × $20,000, which is $1 trillion. This should be a large enough market to provide the commercial incentive to develop the required mature spaceplane. This is an important point. It means that, as soon as spaceplanes are developed sufficiently for private investors to have confidence in their potential, most of the remaining funding can come from investment by the space tourism industry. This conclusion appears to be robust, in spite of the inevitable lack of precision in the estimate leading to it.

There are at present fewer than 100 ELV flights per year for launching satellites. As soon as spaceplanes become available, they will almost certainly replace ELVs of comparable payload capability by offering lower costs and higher reliability. However, this market is too small to guarantee a good return on the investment needed to develop spaceplanes, which is one reason why they have not yet been built. By contrast, the number of airliner flights per year is measured in tens of millions.

The launch services market is at present worth some $4 billion per year, and spaceplanes will eventually capture most of this. Launching satellites will provide useful early operational experience and revenue streams for spaceplane manufacturers.

Spaceplanes will lower the cost of launching satellites and make readily affordable their on-orbit servicing. It will then be possible to relax some of the design requirements for satellites, leading to further cost savings.

The present European and US annual budgets for manned spaceflight add up to about $7 billion, which is spent mainly on the Space Shuttle and the ISS. Planned future missions include sending people back to the Moon.

The low-cost orbital infrastructure (mentioned earlier in Section 2.3) will have a major impact on government space missions. Large space probes, both manned and unmanned, will be assembled in orbit from modules launched by spaceplanes or reusable heavy lift vehicles. They can be tested in orbit and repaired if necessary before being sent on their way. The result will be less expensive and more reliable space science and exploration. Because of these benefits, governments are a potential source of funding to develop prototypes of the spaceplanes and other vehicles needed for the low-cost orbital infrastructure, and this prospect is discussed later.

A large increase in traffic levels to and from orbit will inevitably lead to some pollution of the upper atmosphere. If spaceplanes use hydrogen and oxygen propellants, the main exhaust product will be water. This could be persistent at high altitudes. During the airbreathing part of the flight, there will be some emission of oxides of nitrogen.

On the other hand, spaceplanes will greatly reduce the cost of environmental science from space, which is one of the keys to understanding human impact on climate.

If space tourists react like most astronauts to date, many more people will become directly conscious of 'Spaceship Earth', and will become more environmentally aware. There is a precedent for this reaction. The famous 'Earthrise' photograph, taken from Apollo 8 in 1968, became almost overnight the icon of the environmental movement.

As mentioned earlier, solar power satellites have the potential to provide much of this planet's energy requirements without emitting greenhouse gases.

Spaceplanes will probably be the first large-scale commercial users of liquid hydrogen fuel, which will help spread hydrogen technology to aviation and maybe even to ground transport. In the longer term, and more speculatively, low-cost access to space could provide many of the resources needed for sustainable human life on Earth.

Considering the balance of these factors, it seems almost certain that the environmental benefits of spaceplanes will turn out to be far more significant than the disadvantages.

The main conclusion so far is that combining the reusability of spaceplanes with the high traffic levels of space tourism offers the eventual prospect of airline travel to orbit and a thousandfold reduction in the cost of human space transportation. Very large scientific instruments, space probes, space stations, space hotels, and experimental solar power satellites will then become affordable, and the net environmental impact is almost certain to be highly beneficial. This scenario should perhaps be called the 'spaceplane age' of astronautics, in contrast with the present 'missile age'.

Given this attractive prospect, the questions are how best to achieve it, how long it will take, how much it will cost, and how it should be funded. A key issue is the availability of the required technology, and this is discussed in the following section.

In evaluating the technology availability for spaceplanes, it is helpful to consider a sequence of progressively more advanced design concepts, starting with one that we know is already within the state of the art. We can then progress down this sequence until we arrive at a vehicle that clearly requires a major programme of technology development. This sequence is shown in Fig. 4, and consists of the following vehicles:

• Small sub-orbital spaceplane

• Large sub-orbital spaceplane

• Large sub-orbital spaceplane plus expendable upper stage

• Large sub-orbital spaceplane plus spaceplane upper stage (i.e., fully reusable orbital launcher)

• Single-stage spaceplane

The following paragraphs consider the technology availability for prototypes of each in turn. The question of maturing these prototypes towards airliner standards is considered later.

We know that small sub-orbital spaceplanes are feasible because of the X-15 and SS1. The present state of the art for a robust and practical sub-orbital spaceplane allows a propellant weight fraction (i.e., propellant weight/take-off weight) of about 60%. A robust kerosene-fuelled rocket engine will have a specific impulse (kg thrust per kg propellant consumed per second) of about 250 sec (which is equivalent to a specific fuel consumption of 14kg fuel per kg thrust per hour, which is between ten and twenty times higher than that of typical jet engines). The resulting ideal velocity (the increase in velocity that the vehicle would achieve when using all its propellant in a vacuum and in the absence of gravity) is given by the classic rocket equation as:

V_ideal = Specific Impulse × g × log_e Mass Ratio

Where the Mass Ratio is initial/final mass, assuming that the only mass loss is the consumed propellant.

Substituting the numbers for the sub-orbital spaceplane gives:

V_ideal = 250 × 9.81 × log_e (100/(100 - 60)), which equals 2,250ms^-1.

If all this kinetic energy were converted into height, the spaceplane would zoom climb to a maximum altitude of 260km. In practice, losses due to drag, gravity, and operating the rocket at low altitudes reduce the final velocity, and the height reached is lowered to about 100km. (SS1 and the X-15 were air-launched, which added about 10% to the ideal velocity and enabled them to exceed 100km height.) On a trajectory aimed at maximum speed rather than maximum height, a state of the art ground-launched spaceplane would reach a speed greater than Mach 4.

Sub-orbital spaceplanes built so far or planned for the near future are relatively small, but their technology could be applied to larger vehicles. Scaling effects are such that efficiency would improve gradually with size, as with conventional aeroplanes. The practical upper size limit is probably close to that of a large airliner.

The idea that a large Mach 4 spaceplane can be developed using existing technology may seem strange to those who have contemplated the design of transport aircraft with that speed capability. The key difference is that the spaceplane has to maintain its maximum speed for only a few minutes whereas the transport has to do so for a few hours. This relaxes the technology requirements in three ways:

• Rocket engines can be used, in spite of their high fuel consumption, and these can operate at any speed. The design of jet engines becomes more difficult as the speed increases, and present technology limits their maximum speed to about Mach 4.

• The time for heat soak is so short that thermal protection is not a major problem. SS1, for example, has a conventional structure with no more than a thin layer of thermal insulation on the nose and wing leading edges.

• A high lift to drag ratio is not required, which greatly simplifies the aerodynamic design.

The spaceplane can be compared with a drag racing car requiring only brute force to accelerate to maximum speed. By contrast, an SST is like a highly sophisticated Formula 1 racing car requiring the last percentage point of efficiency. Thus, the Sänger airliner would require significantly more advanced technology than the Sänger launcher carrier aeroplane. If the airliner were developed first, its subsequent adaptation as a carrier aeroplane would be straightforward. However, if the carrier aeroplane were developed first, it could do little more than test some of the technology for the airliner.

Large sub-orbital vehicles could be designed to carry upper stages to at least Mach 4 without needing advanced technology, and this prospect is considered in the next paragraph.

The main challenge for orbital spaceplane design is the high propellant weight fraction required to reach orbit. Satellite speed in low Earth orbit is about 7,800ms-1, and losses increase the required ideal velocity to about 9,400ms-1. A state-of-the-art hydrogen-fuelled rocket engine has a specific impulse of about 450 sec. Applying the rocket equation to a single stage to orbit vehicle ( SSTO) with these numbers gives a mass ratio of 8.4 and hence a propellant weight fraction of 88%. This leaves only 12% of the launch weight for the dry weight (i.e., inert weight plus payload, where the inert weight is everything except consumed propellant and payload and consists mainly of structure, engines, systems, and recovery equipment). In spite of this, several studies claim that a reusable rocket-powered SSTO is feasible. Even if these studies are correct, it is clear that such a vehicle would be very sensitive to weight growth and would fall short of airliner standards of robustness.

If a vehicle similar to this SSTO is launched from the large Mach 4 carrier aeroplane described in the previous section, the technology requirements are greatly relaxed. The required ideal velocity is reduced directly by the speed of the carrier aeroplane. Moreover, drag, gravity, and propulsion losses are also reduced because about 60% of these occur below Mach 4 and are therefore absorbed by the carrier aeroplane. The resulting ideal velocity is thereby reduced from 9,400 to about 7,150ms-1.

The propellant fraction is then about 80%, which increases allowable dry weight by about two-thirds (from about 12 to 20%). Moreover, air-launch helps to reduce upper stage dry weight by relaxing several more of its design requirements. For example:

• The thrust to weight ratio can be reduced to somewhat less than one compared with the 1.2 or more needed for an SSTO.

• The wing and landing gear can be sized for landing only and not for take-off.

• The rocket nozzle does not have to operate at low altitude and can be optimised for near-vacuum flight.

• Shielding on the carrier aeroplane can provide protection from high air and thermal loads during the early part of the ascent.

• The orbiter does not have to be designed for the peak bending moments due to wind shear at high dynamic pressure during ascent, as the carrier aeroplane can support these loads.

The required dry weight fraction of 20% is readily achieved with expendable hydrogen-fuelled launcher stages, such as the upper stage of Ariane 4, which have inert weight (dry weight less payload) fractions of around 12% of the propellant weight. Thus, such a stage launched from the Mach 4 carrier aeroplane could carry a payload to low orbit of about 10% of its launch weight. (The weight breakdown of the orbiter stage at launch would be roughly 80% propellant, 10% inert, and 10% payload.)

Such a combination of a Mach 4 carrier aeroplane and a single expendable upper stage is undoubtedly within the state of the art, and might be a worthwhile step towards a fully reusable spaceplane.

The large sub-orbital spaceplane plus expendable upper stage is only partially reusable. For full reusability, the upper stage needs to be able to re-enter the atmosphere and land, which requires the addition of wings, tail, thermal protection, landing gear, and flying controls. Clearly, the weight of these is an important issue. The question is whether these components can be added to an otherwise expendable upper stage while retaining a worthwhile payload.

A preliminary rule of thumb for the weight of these recovery components can be obtained from the weight breakdown of two of the very few winged vehicles to have flown back from space - the Shuttle Orbiter and the X-15.

In each case, the components needed for recovery - wings, tail, thermal protection, landing gear, and flying controls - weigh about 30% of the total landing weight. Thus, it seems reasonable to assume that recovery components added to an otherwise expendable upper stage would also weigh about 30% of the landing weight (which, for a spaceplane designed to return the payload, is very close to the dry weight).

Adding the weight of these recovery components while keeping the total and propellant weights (and hence the mass ratio and ideal velocity) constant results in a payload of about 4% of orbiter launch weight. (The weight breakdown of the reusable orbiter stage at launch is now roughly 80% propellant, 16% inert (including the recovery components), and 4% payload.) The overall payload fraction on such an early orbital spaceplane may be small, but this is more than compensated for by the reduced cost per launch compared with an expendable vehicle. These weight breakdowns are summarised in Fig. 5.

This analysis shows that the prototype of a fully reusable two-stage orbital spaceplane is within the state of the art. The design of all the components can be based on those that have already flown in aeroplanes or launchers, and the weight margins are adequate for low technical risk. The key point is that the payload of existing expendable vehicles adapted for launch from a Mach 4 carrier aeroplane would weigh considerably more than the components that would be needed for recovery.

It may be thought that recovery from orbit would require a difficult and heavy thermal protection system, but the Shuttle has demonstrated that a conventional light alloy structure can be protected by external insulation that weighs only about 7% of the landing weight. There has been significant research into thermal protection in the thirty years since the Shuttle was designed and a lighter and more practical system should now be feasible.

More detailed design work shows that, while the Mach 4 carrier aeroplane can be a practical and robust design, the spaceplane upper stage requiring the 80% propellant fraction would probably be a 'delicate' design, with a short life and high maintenance cost. The overall vehicle would nonetheless cost less to operate than today's expendable launchers and could be improved over the years towards airliner standards of robustness.

Thus, even though an orbital spaceplane is a major advance on a suborbital one, it is within the state of the art provided two stages are used and the lower (carrier aeroplane) stage is essentially an enlarged suborbital spaceplane.

As discussed later, the idea that a TSTO spaceplane can be within the state of the art is by no means new.

The spaceplanes considered so far have somewhat low payload fractions. There is, however, a technique for improving payload fraction and ease of operation that does not require advanced technology. This is to add jet engines capable of supersonic speed to the carrier aeroplane, while retaining the rocket engines. The jets are used up to their maximum speed and then switched off, greatly reducing the propellant required for this portion of the flight (because of the far lower fuel consumption of jets compared with rockets). The rockets then take over and accelerate the vehicle up to stage separation speed. The penalty is that, during the later rocketpowered phase of the flight, the jet engines add considerably to the deadweight that has to be accelerated.

If the jets can propel the carrier aeroplane to (very approximately) one third of its maximum speed, the reduction in propellant consumed up to that speed more than compensates for the deadweight of the jet engines during the rocket-powered portion of the ascent. An additional advantage is that jets are more practical than rockets for taxiing, ferry flights, flying to the required orbit plane, aborted landings, and diversion to alternative airports.

This technique is used on the Bristol Spaceplanes Limited Spacebus and Spacecab projects(2). Figure 6 shows the layout of Spacebus, which is a 50-seater optimised for carrying passengers to and from space hotels. Spacecab has a similar layout but is smaller, with a payload of six seats or a satellite in the one tonne class.

Figure 6. The Bristol Spaceplanes Spacebus fifty seat spaceplane concept.

Spacebus uses turboramjets of new design but existing technology to a speed of Mach 4 and then rocket engines to Mach 6. The rockets are also used to climb to near-space, so that air and thermal loads during separation are manageable. This technique avoids the two difficult aspects of the Sänger design - reaching hypersonic speed with airbreathing engines, and separating the Orbiter stage in the atmosphere. Thus, Spacebus is like a Sänger designed to avoid difficult technology.

In 1993/4, Bristol Spaceplanes carried out a feasibility study(10) for ESA of its Spacecab orbital spaceplane that combined what were thought to be the best features of the various 1960s European Aerospace Transporter designs. Spacecab was designed specifically to avoid advanced technology and the study showed that this was indeed possible. A subsequent independent review commissioned by the then UK Minister for Space, Ian Taylor, and carried out by BNSC broadly endorsed this conclusion(11).

A single-stage spaceplane is clearly desirable as a long-term aim. As discussed earlier, rocket powered SSTO vehicles are at best on the margins of feasibility because of the very high propellant weight fraction required. This can be reduced by using air-breathing engines for the early part of the ascent. However, air-breathing engines add a large deadweight to the subsequent rocket-powered ascent. In order to provide a net gain in payload for a single-stage orbital spaceplane, the air-breathing engines have to operate up to hypersonic speed, beyond the present state of the art. There have been several proposals for SSTO spaceplanes with advanced air-breathing engines, but all require a long programme of technology development.

Thus, the development of the large and mature spaceplane needed for low-cost access to space can be achieved sooner and at less risk with a two-stage vehicle than with a single-stager. Given that two-stage vehicles offer the prospect of a thousandfold reduction in launch cost when mature, it makes commercial sense to develop these first to build up the new markets at low technical risk. Single-stage vehicles can then be developed when the markets are ready to pay for them.

It therefore seems reasonable to conclude this section on technology availability by stating that, when considering the commercial opportunities for spaceplanes, a major programme of technology development will not be required for the prototype of a fully reusable two-stage orbital spaceplane. Design can start as soon as the funding is available.

There are two caveats to this conclusion:

• It applies only to prototypes, which will inevitably fall well short of airliner standards of life, turnaround time, and maintenance cost.

• Take-off will be noisy, which will restrict the airports that can be used. However, only a few locations will be needed for the first decade or two of space tourism, and noise is unlikely to be a significant limitation on business growth.

Passenger safety is a key issue for early space tourism flights. In the longer term, safety authorities and spaceplane manufacturers and operators should presumably aim for full type certification for carrying fare-paying passengers, as required for airliners. There are several new airworthiness issues such as stage separation, aborted launches, the thrust vector control of rocket engines, cabin pressure integrity in the vacuum of space, reaction controls, re-entry stability and control, thermal protection, and the safety of the rocket propulsion system, especially the containment of high-energy propellants and the avoidance of explosions in the combustion chamber. Clearly, a massive development effort will be needed for spaceplanes to approach airliner standards of safety.

For pioneering passenger flights, a requirement for full type certification would be prohibitively expensive and would make it very difficult for the private sector to start a space tourism industry. While spaceplane operations can realistically aim to approach aviation standards eventually, it is not practicable to expect early flights to comply fully with these.

A consensus is emerging that pioneering spaceplane operators should be allowed to carry 'intelligent' passengers, i.e., those who have had the risks explained to them and who are prepared to accept them. This still leaves unanswered the question of what risk level is acceptable. Present manned spaceflight averages about 100 flights per fatal accident. Flight-testing new airliners and business jets averages about 10,000 flights per fatal accident, as does fighter pilot combat training in peacetime. Private flying averages about 100,000 flights per fatal accident and commercial airliners more than one million flights.

There does not yet seem to be a consensus about where pioneering space tourism should fit on this safety spectrum. One hundred flights per fatal accident is clearly too risky; one million is not realistically achievable on early operations.

Rather than setting a safety target in terms of the number of flights per fatal accident, it may be preferable to set the target in terms of fatal accidents per year, to take account of the pioneering nature of early space tourism operations. For example, a tough but possibly realistic aim for a new sub-orbital spaceplane might be less than a five percent probability of a fatal accident during each year of pioneering passenger-carrying space flights. The total number of flights in the early years will be small and so the total number of fatalities would be quite low at this level of safety. This is clearly a provisional safety standard and is put forward here as the basis for discussion.

It is not practicable to aim for airliner standards of safety with spacecraft on expendable launchers, as used exclusively to date for manned spaceflight, for the following reasons:

• Expendable vehicles are based on ballistic missile technology and have a major accident rate of more than one per hundred flights. This is largely because so many components have to work right first time.

• The cost per flight is so high that only very few flights can be afforded and the design cannot progress beyond the experimental phase (by aeroplane standards). For example, the Space Shuttle has made just 115 flights in twenty-five years whereas a new airliner design makes typically 1,000 test flights in less than two years before being allowed to carry fare-paying passengers.

Spaceplanes avoid both of these problems and can in principle be made as safe as aeroplanes.

Prototypes of advanced aeroplanes require typically three and a half years between go-ahead and first flight. Given that advanced new technology is not required, the time to develop a prototype orbital spaceplane should be comparable. SS1 took this time between contract signature in April 2001 and winning the X-Prize in October 2004, and the X-15 required three months less, although of course both of these spaceplanes were sub-orbital. Allowing two years before go-ahead for feasibility studies and project definition, and six months after first flight for an incremental flight-test programme leading up to orbital velocity, results in a total time for development of six years.

It makes commercial sense that the first orbital spaceplane should be no larger than necessary for early applications, which probably implies a payload in the one tonne or six people (space station crew or passengers) class. This small orbital spaceplane would be used initially for launching small satellites, supply flights to the ISS, and transporting mechanics to maintain large satellites. It would then be used for pioneering orbital tourism.

There is a major difference between developing a new advanced aeroplane and an early spaceplane. The aeroplane will need an extensive flight test programme - typically around 1,000 flights - before it is allowed to enter commercial service. This is because the market is mature and expects high standards of reliability and safety at entry into service.

By contrast, spaceflight still uses expendable vehicles. A prototype spaceplane should be far more reliable and safer even during the flight-testing phase. Thus, prototypes of early spaceplanes should be fit for launching satellites and supplying space stations on early orbital test flights. In this way, revenue payloads on early flights can pay for much of the flight-testing required for assuring safety adequate for carrying passengers.

This is an important conclusion. It means that government agencies and commercial operators can launch satellites and supply space stations using spaceplanes that are developed only to the prototype stage and well before they have matured to airliner standards. As will be shown later, such prototypes can be developed at surprisingly low cost. Such prototypes used for pioneering operations can be called 'operational prototypes'. They could be entering service as soon as the first prototype reaches orbital velocity, i.e., in about six years.

From the analysis in previous sections, it is clear that a prototype of the first orbital spaceplane is the key to triggering progress towards the spaceplane age. For the first time, a category of launch vehicle would be in service with a cost potentially low enough to attract large new commercial markets, which is a new capability that certainly cannot be achieved with expendable vehicles. For the first time, there would be a visible demonstration of the potential of low-cost access to space.

As soon as the first spaceplane enters service, a beneficial downward cost spiral will start. The spaceplane will have a lower marginal cost per flight than competing expendable launch vehicles (the cost of propellant, crew, and maintenance compared with the cost of a new vehicle). This will encourage higher traffic levels, which will attract the investment to mature the technology and enlarge the design, which will lead in turn to even lower costs and so on until the lower cost limit of spaceplanes with conventional materials and chemical rockets is reached. As discussed earlier, most of the funding for this 1,000 times cost reduction can come from space tourism revenues.

The time taken to reach airliner maturity clearly depends on the effort devoted to this task, which will in turn depend on how rapidly the new commercial markets build up, especially space tourism. Considering the various technologies involved (aerodynamics, structures, systems, propellant tanks, thermal protection, propulsion, etc.), it seems likely that the critical path will be the development of a rocket engine with a life measured in thousands of flights compared with the present tens.

The development of the jet engine provides some insight into how long it might take to develop a long-life rocket engine. Jet engines progressed from quasi-experimental operations in 1944 (the Messerschmitt Me 262 and Gloster Meteor) to the first airliner services in 1952 (the de Havilland Comet). During this period, jet engine development was a high priority in the leading aeronautical countries. Given comparable priority, a long-life rocket engine could possibly be developed as rapidly as long-life jet engines, i.e., within about ten years of early models entering service on prototypes. This provides a preliminary indication of the minimum time required for spaceplanes to progress from prototype to approaching airliner standards of maturity.

It is relevant to note that, since the aeroplane rocket engines of the 1950s, very little effort has been allocated to developing longlife rocket engines, during which time there have been great improvements in high-temperature materials and in software for rocket design and control.

It is perhaps a remarkable fact that the cumulative flying time of rocket engines to date is comparable to that of jet engines by the mid 1940s. The only rocket-powered aeroplane to have entered service in significant numbers was the Messerschmitt Me 163 interceptor of World War II.

The conclusion from this section is that operational prototype spaceplanes could be entering service in about six years. Large and mature derivative designs could be approaching the potential $20,000 cost per seat possibly as soon as ten years later. However, it should be emphasised that achieving this latter timescale would require a massive development effort.

An important question is how much a prototype spaceplane, adequate for early operations, will cost to develop.

Fig. 7 shows the development cost plotted against dry weight for four demonstrators or prototypes of RLVs. The RLV demonstrators were (or are being) built in experimental workshops (sometimes called skunk works) set up for rapid prototype development. Experience with aeroplane development shows that the cost of a prototype built in an experimental workshop is approximately 10% of the cost to full certification.

Figure 7. Development cost trends (in 2005 dollars) for Reusable Launch Vehicle ( RLV) demonstrators built in experimental workshops.

Of these projects, the X-34, shown in Fig. 8, is perhaps the most relevant to a new orbital spaceplane. This project was funded largely by NASA as a test-bed for the technologies required for a reusable launch vehicle. It was cancelled in 2001, apparently for reasons that were mainly bureaucratic, when almost complete but before it could fly. Its development cost would have been some $250 million, and it would have been the fastest and highest fully reusable flying machine to date. It would have been air-launched from a converted Lockheed L1011 and was unpiloted. The X-34 was designed to reach about Mach 8 on a sub-orbital flight, with a take-off weight about seven times greater than that of SS1.

Figure 8. The X-34, cancelled when almost ready to fly (NASA).

The X-34 concept could be enlarged to provide a large supersonic carrier aeroplane, along the lines of those described earlier, and refined to provide the orbiter of a two-stage spaceplane. The combination of these two vehicles could provide a fully reusable orbital spaceplane. The total all-up weight would be about ten times that of the original X-34.

Development cost tends to increase somewhat less than directly proportionally to vehicle weight, and the cost of an operational prototype of this orbital spaceplane would be in the region of $2 billion (eight times that of the X-34 itself), which is comparable to that estimated for Spacecab(10), which is of similar size.

These costs are far lower than those estimated for Sänger. Spacecab and the 'Orbital X-34' are considerably smaller, lack risky advanced technology, and are suitable for development in an experimental workshop.

It may be possible to develop an orbital spaceplane prototype at even lower cost, using the sort of ingenuity that went into the development of SS1, which cost about $25 million(14). If an orbital spaceplane can be developed for ten times the cost of a sub-orbital one, the US private sector could build a prototype small orbital spaceplane for well under $1 billion, and this could be achieved in the previously mentioned six years. As will be discussed later, ten years is more likely.

Thus, the cost of a prototype small orbital spaceplane should be comparable to just one or two flights of the Space Shuttle. The major space agencies could easily afford to develop a spaceplane suitable for government missions. The case for such a development is considered later.

These development costs are far lower than those of manned spacecraft to date. This is explained by the inherent poor safety of the expendable launch vehicles used exclusively so far. Extraordinary quality control is needed to achieve even the fatal accident rate so far demonstrated of one per approximately 100 flights. This precludes development in an experimental workshop. Development cost trends are therefore more in line with those of fully certificated aeroplanes, which are about ten times higher than the cost trend line shown in Fig. 7. Manned spacecraft to date have therefore combined the high development cost of fully certificated aeroplanes with the poor safety and low flight rate imposed by the use of expendable launchers.

It may seem paradoxical, but it is precisely because spaceplanes are inherently so much safer and less expensive to fly than expendable launchers that they can cost less to develop. Operational prototypes can be built in experimental workshops and their marginal cost per flight is not a barrier to adequate testing.

Regarding the cost of developing large mature derivative spaceplanes, MBB estimated the development cost of Sänger at $17 billion(1) and, more recently, Boeing estimated the development cost of a 50-seat spaceplane to be 'at least $16 billion'(13). In neither case was there a claim of airliner maturity. The total non-recurring cost of achieving airline operations to orbit is likely to be several times these estimates.

To put these costs in context, the Space Shuttle cost about $20 billion to develop (about $50 billion today), and NASA is currently estimating the cost of returning to the Moon at more than $100 billion. The ISS will also have cost about $100 billion when it reaches the end of its useful life. Thus, the funding required for a large mature spaceplane is comparable to that of past and present major space projects. Moreover, as already discussed, revenues from space tourism should be adequate to provide most of the funding required for mature spaceplane development.

At the time of writing, there are no firm plans to fund an orbital spaceplane. This is in spite of the large potential benefits of spaceplanes, the availability of the required technology, and numerous project studies over the years. However, two parallel developments in the USA are likely to merge and lead eventually to an orbital spaceplane.

The first development is that several US companies are progressing the development of small sub-orbital spaceplanes, one or more of which should start carrying passengers within a year or two of 2010. The second development is that two start-up US companies, Rocketplane Kistler (RpK) and Space Exploration Technologies (SpaceX) are about to share $500 million in NASA funds, having won the recent NASA Commercial Orbital Transportation Services (COTS) competition(15) to start developing transports to the ISS. Initially, these will be unmanned, but crewed developments are planned.

These projects are not spaceplanes in that they are only partially reusable, but they nonetheless indicate the ability of start-up companies to develop orbital vehicles on modest budgets. SpaceX state that the cost of developing their orbital transport will be about $300 million(15).

It therefore seems probable that the US private sector will eventually merge these two lines of development and build a small orbital spaceplane. Such a project has great potential, is already under consideration(16), and sufficient funding should be available (about five US billionaires are already spending large sums on their own space projects.)

Such a development probably depends on at least one of the pioneering sub-orbital tourism operations becoming commercially successful. It seems likely that this will eventually happen. The potential market is large, and technical feasibility has been demonstrated by SS1. There are of course still many problems to sort out and these pioneering operations will probably cost more and take longer than planned. The timing is therefore uncertain, and it seems unlikely that a private-sector orbital follow-on spaceplane will enter service in less than ten years.

The alternative source of funding is government. NASA and ESA at present have no plans to develop spaceplanes. They are planning to spend large sums on the ISS and more than $100 billion sending people back to the Moon using expendable launchers exclusively, including the new Ares still at the design phase of development.

As mentioned earlier, when spaceplanes and the low-cost orbital infrastructure (mentioned earlier in Section 2.3) become available, governments will find it less expensive and more reliable to use this rather than the expendable launchers at present in use and planned for future missions. This raises the question of whether governments should support the development of spaceplanes now or wait for the private sector to develop them.

The time required for early spaceplane development is less than that of the present manned space programmes. Spaceplanes could therefore be built in time for use on at least the later part of these programmes. The cost of developing a spaceplane should be comparable to that of just one or two Shuttle flights, and the spaceplane cost per flight would be orders of magnitude less than the Shuttle (albeit with a smaller payload). Thus, the cost of developing the spaceplane would be recovered by saving just one or two Shuttle flights. It therefore seems almost certain that government agencies would recover the cost of developing the spaceplane within a year or two of its entering service on the ISS programme alone.

A preliminary indication of the possible savings is given in a recent analysis by Spaceworks Engineering Inc(18). This used illustrative cost projections to estimate that between 2010 and 2017 the US government would save about $8 billion by using commercial RLVs for ISS supply, and that the commercial companies would gain a $7 billion market. The vehicles assumed in this study were reusable but not winged and piloted like true spaceplanes, which would probably yield higher savings.

It also seems almost certain that the cost of re-visiting the Moon could be reduced greatly by evolving the system architecture to take account of the benefits of using spaceplanes and reusable heavy lift vehicles for transport to and from LEO. It would therefore be very interesting to carry out a like for like comparison of the cost of presently planned ESA and NASA manned space programmes with and without spaceplane development.

One obstacle is that most leading industrial nations have longstanding interests vested in expendable launchers and in prestige manned spaceflight, which makes it difficult for their governments to change direction and back projects that could make these interests redundant.

The major exception is the UK. Having cancelled the Blue Streak and Black Arrow launchers some thirty years ago, it has been government policy to avoid expenditure on launch vehicle development or manned spaceflight. This has been controversial, but is does mean that there are now no significant British vested interests concerned about spaceplane development.

If government agencies supported the development of a low-cost orbital infrastructure for their own use, the private sector could then commercialise it. Space tourism would then provide economies of scale and the revenues to fund a programme of maturing the vehicles and reducing their operating cost. Achieving the minimum (about ten years) timescale from prototype to mature design, mentioned earlier, depends on a 'gold rush' effect, with major players investing heavily to try to be first to market, with all the financial rewards that this could bring, once the potential of spaceplanes is more widely appreciated. If this did not happen, developing a mature spaceplane would take several years longer.

However long it takes, government programmes would benefit even further from the reduced costs made possible by space tourism. This would be a win-win situation.

Summarising this section, there is a strong case for governments to support the development of early spaceplanes to save money on their existing programmes and to speed up progress towards low-cost spaceflight. If they do not, it is likely that the US private sector will develop an orbital spaceplane a few years later.

Pressures of the Cold War led to converted ballistic missiles being used for early manned spaceflights and then to enlarged developments being used for the race to the Moon. Even so, the X-15 achieved manned flight to space height only 14 months later than the Redstone ballistic missile, which, with a Mercury capsule, achieved the first US manned sub-orbital space flight in May 1961.

The X-15 was intended to usher in an aeroplane approach to space transportation and, in the 1960s, several major aircraft companies carried out serious project studies of orbital spaceplanes, most of which had two piloted stages and used hydrogen fuel. There was a consensus in Europe and the USA that they were just about feasible with the available technology and had the potential to greatly reduce launch costs. At the time, the X-15 was making regular flights to and from space, albeit as an experimental sub-orbital aeroplane rather than an operational orbital one. Hindsight suggests that many of these designs were indeed feasible. However, others that relied on hypersonic airbreathing engines were beyond the state of the art (and still would be).

The European studies were carried out under the auspices of Eurospace and the broad technical guidance of Prof Eugen Sänger(12). The generic name 'Aerospace Transporter' was given to these projects, but they would probably now be called spaceplanes. Figure 9 shows the projects proposed by British Aircraft Corporation, Bölkow, Bristol Siddeley, Dassault, ERNO, Hawker Siddeley Aviation, and Junkers. Much of the required technology has been improved significantly since the 1960s for other projects.

In the 1970s, the early designs for the Space Shuttle were fully reusable and would have met our definition of a spaceplane. However, President Nixon then imposed a budget cut on NASA because there was the Vietnam War to pay for and the US public was losing its early enthusiasm for space. NASA could therefore no longer afford the development cost of their large fully reusable Shuttle design. They had a choice between developing a smaller but fully reusable design, like the earlier European Aerospace Transporters, and giving up on full reusability. They chose the latter, and the Shuttle as built is as expensive and risky to fly as preceding manned spacecraft launched by ballistic missiles or enlarged developments thereof.

To those who were aware of the potential of spaceplanes to reduce greatly the cost of access to space, this was a major setback that was obviously going to lead to a long delay in the introduction of an aeroplane approach to space transportation. Since then, more people have come to share this view, and a private sector spaceplane movement has evolved. This movement soon recognised that space tourism was likely to become the largest market for spaceplanes and, in 2004, the first spaceplane designed for space tourism - SpaceShipOne - reached space height, 36 years after the X-15 last achieved this feat.

Of aeroplanes that have actually flown, and with the possible exception of SpaceShipOne, the one most suitable for providing the basis of a space tourism industry is perhaps the Saunders Roe SR.53 rocket fighter that first flew in 1957. This is probably the most practical rocket-powered aeroplane yet built.

If it had entered service, the RAF would soon have had a mature rocketplane with long life and rapid turnaround. With straight-forward development, the SR.53 could have had sub-orbital performance. Indeed, when it was cancelled as a fighter in 1958, Saunders Roe did propose a space research variant(17).

A commercial development could have been built to carry passengers on space experience flights, much as now planned by Virgin Galactic using the US SpaceShipTwo. Thus, routine suborbital flights could have been achieved by the late 1960s. With economies of scale and maturing technology, the cost per seat would eventually have approached that of a long-range business jet at just a few thousand pounds. Orbital spaceplanes could have followed a few years later, probably based on one of the 1960s European Aerospace Transporter projects.

If a development along these lines had happened, spaceflight would have evolved naturally into an everyday and widely affordable business with large new commercial markets, especially space tourism. Present spaceplane initiatives from the private sector offer the prospect of catching up rapidly with what might have been.

If these conclusions seem unduly optimistic, this is perhaps due to the divergence over the years between aviation and spaceflight. Operating aeroplanes is so different from operating ballistic missiles and expendable launchers that aviation and spaceflight have developed into businesses with little overlap. An opportunity to merge aviation and spaceflight (as far as low Earth orbit) arose in the 1960s, when spaceplanes first became feasible, but was not taken. Thus, the study methods used in this paper - applying aeroplane conceptual design techniques to vehicles for space markets - have gone out of fashion. Technology has caught up with the idea of lowcost spaceplanes and the traditional space and aviation communities are being slow to react. The innovation is coming from start-up companies.

This paper has presented a broad-brush analysis of new commercial opportunities in space. While some of the argument is inevitably speculative and some of the detail liable to change, the following conclusions appear to be soundly based:

• Because of their reusability, spaceplanes have a potential cost per seat to orbit roughly 1,000 times lower than today's largely expendable vehicles, when used in large numbers and developed to airliner standards of maturity as measured by life and number of flights per day.

• Space tourism is likely to become a large enough market to provide the required funding and high traffic levels.

• The prototype of a small orbital spaceplane, needed to trigger this line of development, could be built in about six years using existing technology at a cost equivalent to one or two flights of the Space Shuttle. It would be used mainly for launching small satellites, supplying the International Space Station, and for pioneering orbital space tourism.

• The timescale required to advance from the small prototype to mature airline operations to orbit depends mainly on how rapidly the market for space tourism builds up. If there is a 'gold rush' with major players racing to invest heavily, it could possibly be achieved in as little as ten years. The pacing item is probably the development of a long-life rocket engine.

• Low-cost access to orbit will lead to rapid developments in space science and exploration, and to new commercial uses of space, especially manufacture in orbit and solar power satellites, progressing at least as far as pilot schemes. The environmental impact is likely to be highly beneficial.

• Space tourism requires an aviation approach to transport to and from orbit, which will be of wide benefit to both aviation and spaceflight. Aviation will benefit from large new markets for its products and services, and spaceflight will benefit from greatly reduced costs.

• By sponsoring spaceplane development, governments would at one stroke not only save money on presently planned programmes but would also bring forward a new 'spaceplane age' of astronautics that would be as far ahead of the present 'missile age' as the 'aeroplane age' of aeronautics was from the 'balloon age'.

1. D E Koelle and H Kuczera, October 1987, " Sänger - An advanced launcher system for Europe", IAF-87-207, presented at the 38th Congress of the IAF, Brighton, UK; The Sänger design evolved over the years. The data used here are taken from this.
2. D M Ashford, 2002, "Spaceflight Revolution", Imperial College Press.
3. D M Ashford and P Q Collins, "The prospects for european aerospace transporters", Aeronaut J, 91, (927).
4. W R Dornberger, November 1956, " The rocket propelled commercial airliner", University of Minnesota, Institute of Technology, Research Report No 135.
5. R Cornog, September 1956, " Economics of rocket-propelled aeroplanes", Aeronaut Eng Review.
6. H H Koelle, June 1964, " Assessing Re-usable Space vehicles", Astronautics and Aeronautics.
7. D M Ashford, July 1965, " Boost glide vehicles for long range transport", J R Ae Soc.
8. J P Penn and C A Lindley, 1997, "RLV design optimization for human presence in space", The Aerospace Corporation (and other papers by these authors).
9. 2002, Space Tourism Market Study, Futron Corporation.
10. February 1994, " A Preliminary Feasibility Study of the Spacecab Low-Cost Spaceplane and of the Spacecab Demonstrator", Bristol Spaceplanes Limited Report TR 6. Carried out under European Space Agency Contract No. 10411/93/F/TB. (Volume 1 reproduced as The Potential of Spaceplanes in the J Practical Applications in Space, Spring 1995.)
11. March 1995, Letter from Ian Taylor MBE MP, Parliamentary Under-Secretary of State for Trade and Technology, to the Rt Hon Sir John Cope MP.
12. H Tolle, May 1967, " Review of European Aerospace Transporter Studies", Proceedings of SAE Space Technology Conference, Palo Alto, California. (This paper describes designs by B.A.C., Bölkow, Bristol Siddeley, Dassault, ERNO, Hawker Siddeley, and Junkers.)
13. 4 February 2002, " Tourism Cost Realities", Aviation Week, p 17.
14. 21 April 2003, " Burt Rutan's Quest for Space", Aviation Week.
15. 9 October 2006, "Skin in the Game", Aviation Week, p 66.
16. 23-29 August 2005, "SpaceShipThree poised to follow if SS2 succeeds", Flight International, p 25.
17. C N Hill, 2001, " A Vertical Empire - The History of the UK Rocket and Space Programme, 1950-1971", Imperial College Press.
18. Winning in the next space market. A spaceworks engineering Inc (SEI) Case Study, January 2007.

Cost Comparison Between Sänger and Boeing 747

	747	Sänger Carrier Aeroplane	Sänger Orbiter	Total
Technical Data
Wing Span (m)	65	44	18
Length (m)	71	82	33
Wing Area (sq m)	525	880	147
Passenger Seats	420		36	36
Fuel Mass, tonnes
- Liquid Oxygen			72	72
- Liquid Hydrogen		119	11	130
- Kerosene	137
- Total	137	119	84	203
Empty Mass, tonnes	184	171	29	200
Take-off Mass, tonnes	363	290	115	405
Cost Data
Complexity Factor	1	2.5	5
Fuel Unit Cost ($/kg)
- Liquid Oxygen	0.11
- Liquid Hydrogen	2.80
- Kerosene	0.26
First Cost, $ million	130	302	102	404
Flights per day	1	2	1
Annual Costs, $ million
Amortisation	13.00	30.2	10.24
Insurance	1.95	4.53	1.54
Costs per Flight, $
Fuel	35,620	333,200	38,720	371,920
Amortisation	35,616	41,375	28,067	69,442
Insurance	5,342	6,205	4,210	10,416
Crew	18,000	18,000	18,000	18,000
Maintenance	15,600	36,245	12,293	48,538
Landing Fees, Navigation	10,000	10,000	10,000	10,000
Total	120,179	445,026	111,291	556,317
Cost per Seat, $	286			15,483

1. The main assumption is that Sänger has been developed to airliner standards of maturity, i.e., life, maintainability, and turn-around time. It is then reasonable to use airliner cost estimating relationships to assess its likely cost. The above table starts with 747 costs, provided in 2000 by an airline that wishes to remain anonymous, and uses simple scaling rules to estimate the equivalent costs of Sänger.
2. In the above table, complexity factor is a measure of relative production cost per unit mass. Thus, the Sänger Orbiter is assumed to cost five times as much per kg empty mass as the 747. This, and several other, assumptions could be significantly in error without greatly affecting the total, which is dominated by the cost of fuel.
3. The cost per kg of liquid hydrogen assumes a higher production rate than at present. Even so, it is roughly ten times more expensive than kerosene.
4. First costs have been scaled by empty weight and 'complexity factor'. The 747 is assumed to make one 12-hour flight per day, as is the Sänger Orbiter. The Sänger Carrier Aeroplane has a flight time of one to two hours and is assumed to make two flights per day.
5. Annual amortisation is 10% of first cost, and annual insurance is 1.5% of first cost.
6. Crew costs are assumed the same for all vehicles. The shorter flight time of Sänger is assumed to balance higher salaries for spacefaring pilots and cabin staff.
7. Maintenance costs are scaled in proportion to first cost.