• Motivation
• Enabling Technology
• The Elevator System
• Launch Sites and Schedules
• Cost per Passenger
• Beyond Low Earth Orbit
• Hazards and Safety Concerns
• Radiation Concerns
• About the Author
• References, Notes, & Website Links

Many people want to go into space. Most NASA and aerospace industry insiders probably believe that "someday" tourists will be able to travel into space and visit space hotels. But the general assumptions is that to get from the current situation, in which the only commercial use of space is for unmanned communication satellites, to the space tourism era, some other industry (perhaps manufacture of exotic drugs or airplane parts, or production of electricity for use on earth, or mining for important minerals that are scarce on earth) must blaze the trail first.

However, no such industry has emerged. In fact, tourism is about the only industry in which a space based product/service doesn’t have to match the cost of the terrestrial equivalent. Of course, a higher cost will imply a smaller customer base, but the first few space hotels won’t require millions of customers, only tens of thousands. And as is the case with any high tech product or service, the cost will come down with passing time and increasing volume (thus "opening the frontier" for other industry).

If one ignores the desire for a bridge industry, and asks what sort of transportation system could be built to serve a space hotel, the space elevator is a clear choice. The unfamiliarity of the space elevator apparently results from its newness as a concept (it dates back to the 1960s whereas rockets as transportation date back to the 1900s), and its new level of feasibility (the one described here uses a graphite fiber that did not exist in the Apollo era). Science fiction authors have only occasionally explored the utility of space tethers (e.g. an episode of the T.V. show Star Trek Voyager).

It has long been recognized that only a fully Reusable Launch Vehicle ( RLV) could make space travel truly affordable. And in recent years, there has been considerable government interest in developing a launch vehicle with Single-Stage-To-Orbit capability (i.e. no booster stages are discarded during flight).

However, for round trip travel (e.g. tourism), space tethers used in conjunction with sub-orbital RLVs, provide a much better approach. Tethers (essentially long ropes connecting space craft or other bodies) allow momentum and energy transfer, so that energy removed from descending space crafts can be transferred to ascending crafts. Not surprisingly, the full Earth-to-orbit space elevator is not yet technically feasible (as envisioned by in modern times by several individuals including Arthur C. Clark, and by Tsilkovsky in antiquity, the elevator travels along a tether stretched between the surface of the earth and a counterbalance beyond geosynchronous orbit, 22,300 miles away)¹.

However, a shorter version of the space elevator, perhaps 860 miles long, is feasible with existing tether materials, and is an extremely powerful way of making low earth orbit more accessible. Space elevators (and hanging tethers in general) take advantage of the fact that higher altitude orbits have lower velocity required for orbit. One end hangs below the Center Of Gravity of the system, and therefore is traveling below orbital velocity and feels downward gravity. The other end hangs above the COG, travels faster than orbital velocity, and therefore feels the upward pull of centrifugal acceleration.

For the case of a space hotel, the space elevator (say 860 miles long in this case) links a zero gravity station orbiting 775 miles above the earth, with a space dock hanging only 160 miles from the surface, and a low gravity (low "Gee") station 1,020 miles above the earth. The low Gee station feels about 1/10 of earth normal gravity except it’s directed upwards, and serves to counterbalance the force of the hanging dock (the moon-like gravity makes it an interesting place for hotels rooms). The dock is much easier to reach by rocket than a zero-Gee station would be, since it travels at Mach 19.5, only 79% the full Mach 25 required for orbit (more precisely: 13,800 mph vs. 17,450 mph).

This 21% reduction in required launcher velocity results in approximately a 300% increase in payload capacity, for a given vehicle launch mass. This large increase is due to the fact that for a single staged vehicle burning oxygen and hydrogen (state of the art), for a Mach 25 flight, the fuel must be 88% of the launch mass, so typical design goals for this type of craft call for a payload of only 1.5% of the launch mass (fuel percentage requirement assumes an un-powered re-entry and landing). Whereas for the Mach 19.5 flight, the fuel need only be 82%², so the payload can be a much larger percentage of the gross launch mass (for comparison, with the Space Shuttle, the hydrogen/oxygen fuel is 85% of the gross, not including the solid rocket boosters). Significantly, some of the extra breathing room in the mass budget can be dedicated to optimizing for reliability and low maintenance rather than minimum mass.

For the RLV development, this could be the key to viability. NASA’s X-33 program is intended to mature and demonstrate technologies necessary for a follow-on full scale single stage to orbit vehicle, with the hope that a private company will then be willing to fund the follow-on development. However, the biggest technical uncertainty is the feasibility of the mass budget, which the X-33 will fall far short of demonstrating. The use of the space elevator allows the launch vehicle’s dry weight be be about 50% heavier (for the same gross weight and payload), thus greatly reducing investment risk.

When the launch vehicle arrives at the space dock, it latches on, and will hang from the dock (feeling about 35% of earth normal gravity), until it is ready to return to earth (presumably, the next time the dock passes over the launch site). The passengers and/or payload will move from the launch vehicle into one or more gondolas that will move along the length of the tether, up to the hotel stations. The gravity felt by the passengers will gradually decrease during the ride up (which could be many hours long), becoming negative beyond the zero-gravity point, so that at the counter balance station, one could look "upward" and see the earth.

This type of tether system would be much too massive to be practical for the monthly mission rates required for government and scientific purposes. But it’s perfectly reasonable for the daily flight rates required for tourism. In fact, the mass of tether/gondola system (i.e. space elevator) is of the same order of magnitude as the hotel itself (and two order of magnitude greater than the mass of the launch vehicle, which allows the system to remain in a relatively round orbit both before and after the launch vehicle docks).

The space elevator system consists of the dock, the gondolas, the hanging tether, the gondola track running the length of the tether, the solar electric power arrays, and the momentum (& drag compensation) engines.

The dock hangs at the lowermost end of the tether. It has parking spaces for three or four gondolas, berths for two or three launch vehicles, a pressurized & climate controlled passageway connecting the vehicles to the gondolas, and cargo loading equipment.

Each berth has a winch which lowers a magnetic docking grapple to the capture/release site 70 miles beneath the dock (this makes the effective length of the elevator system 70 miles longer for very little extra cost and weight, but still allows a launch vehicle departure a single orbit after an arrival, assuming the winches can move the vehicles at over 75 mph). The separation also allows the launch vehicle to fire rocket engines during the docking maneuver without damaging the dock or vehicles parked there.

Prior to docking, the launch vehicle will extend a steel docking plate that will latch onto the magnetic docking grapple upon contact. The grapple will have rendezvous radar, differential GPS (the Global Positioning System), and probably small rocket thrusters to stabilize its position.

The tether is the super strong structural element that holds its own enormous weight, plus that of the dock, the gondola track, and the counter balance stations. It also must withstand the intense radiation of the Van Allen belt. To make the tether as long as possible for a given mass budget (which makes the dock move as slowly as possible and therefore boosts launcher payload capacity), it’s necessary to have the strongest material. In particular the figure of merit for tether material is the tensile strength divided by the density.

Available materials are improving all the time, but rather than extrapolate what might be available in a few years, the design described here uses existing materials. The length of the tether was selected to be about a large as could be built with two flights per day for a year (to a similar elevator system).

The material with the best figure of merit is said to be Spectra-2000, which is a polyester fiber³. It’s design tensile strength is about 1.35 GPa (the ultimate strength is 3.25 GPa), and it’s density is 0.97g/cc, so it’s figure of merit is about 8 times better than that of steel (1.76 GPa and 7.85g/cc including design margin), and the feasible tether's length is squareroot(8) times longer. The problem with Spectra (and plastic fibers in general) is that it doesn’t last long in space at the altitudes of interest due to the radiation of the Van Allen belt.

Carbon fibers (such as used for graphite-epoxy tennis rackets, etc.) are radiation resistant⁴ and have a figure of merit very close to that of Spectra-2000.  Thornel T-40 from Amoco Performance Products⁵ has an ultimate tensile strength of 5.65 GPa and a density of 1.81g/cc (with a 2.4:1 design margin, the tensile strength is assumed to be 2.35 GPa).

The following graph shows the tether’s thickness profile (versus altitude), bases on construction with Thornel T-40 Carbon fibers (although Spectra-2000 could be used for the portion below a few hundred miles altitude).

The tether mass was calculated to be 34 million pounds⁶ (for the tether, dock, and solar panels) assuming: the loaded dock mass is 1.25 million pounds (including two launch vehicles and three gondolas), the gondola track and electrical cabling mass is 1.34 lbs/foot (which comes to 6 million pounds), the solar panels are 4.2 MegaWatts at 100 tons/MegaWatt, and the counterbalance station mass is 22 million pounds (to bring the effective gravity from 1/10 to a full lunar 1/6 Gee, the tether would be extended to 38 Mlbs and the counterbalance mass decreased to 8.6 Mlbs at 1190 miles). The density of the tether material was padded 20% to allow for stiffening trusses etc., and the material strength was decreased by a factor of 2.4 for safety margin.

Because of the risk of a debris impact (space dust or paint flecks from old satellites, etc.) which could sever anything that is hit, it’s desirable make the tether as several parallel cables. A possible configuration would be six cables arranged in a tube a several feet in diameter. The six cables themselves could be composed of even smaller elements configured as Hoytubes a few inches in diameter (see Tethers Unlimited site⁷). Three of the cables would support the gondola track, and the others might support electrical cables.

Rigid trusses spaced along the tether (maybe only one per mile to minimize total truss mass) would redistribute the load to remaining cables should one or two be severed (this also makes it easier to replace damaged sections of the tether). Much lighter and weaker trusses might be spaced in between the main trusses, just to keep the track from tangling. The epoxy that is used to make graphite composites isn’t radiation tolerant⁸, so it can’t be used to make the trusses, however, aluminum should be fine.

It should be noted that carbon fibers are electrically conductive. This means they must be insulated to protect them from the sparse plasma that fills space in low earth orbit. On the other hand, the tether could also be used as a neutral conductor in the electrical system, and therefore add a bit more redundancy.

It has been observed by Hans Moravec and others that a rotating tether would have a lower mass for a given dock velocity than the hanging tether of the type described here.  This concept could be useful for cargo flights, but would be inconvenient for several reasons for hotel applications.  The rotation would probably be annoying for hotel guest, particularly since the rotation rate varies somewhat as the tether spins.  A rotating tether would also dip the launch vehicle in the radiation belts every rotation (if the tether reaches an altitude above about 500 miles).  In the event that the tether fails, a rotating tether will make it more difficult to prevent the counterbalance station from re-entering the atmosphere, and more difficult for any docked launch vehicles to re-enter safely.  The rotation would complicate the use of a momentum engine, since the engine must not rotate during operation.

Since the mass of the track is the main determining factor for the tether mass, the gondola might be sized to carry a only third as many passengers as the launch vehicle; perhaps 35. Since the gondola ride will likely last several hours, the gondola will have a galley, lavatory, etc. In addition to the main door, emergency exits (including pop-out tunnels) will be provided to allow evacuation to another gondola on an adjacent track.

To minimize the required mass of the track, the gondola’s weight will be spread over a couple of miles of track, by a long "motor train". This reduces the lift requirement of the system to a couple of pounds per linear foot (with 12 lbs/sqft of radiation shielding, the gondola+motor train will likely have a mass of 50,000 lbs and the maximum Gee load is about 1/3) . The lateral pressure on the track will be similar (assuming 1000 ft or so of cable between the gondola and the motor train, the coriolis force will dominate: coriolis acceleration is 0.01 Gee at 100 mph).

Even with the track optimized for small passenger gondolas, it should still be possible to carry the full cargo load of the launch vehicles on one cargo gondola, by using an extra long motor train, and running at slower speeds.

Note that the gondolas hang beneath the motor train. As a result, they cannot go past the zero Gee point. The passengers traveling from the dock to the counter balance station must must stop at the zero Gee point and change from a earthward-hanging gondolas to one hanging away from earth.

The motor train will have several different types of cars. The regular ones may be a few dozen feet long, and form the bulk of the train. The first and last cars are extra long, and have a rigid frame. The first car must be able to push upward past a section of track that has lost power (to improve system reliability). The last car carries extra lateral load, due to the gondola cable. An independent track inspection car will travel ahead of the main train to protect again incidents resulting from damaged track.

Ballast cars (which aren’t part of the gondola’s motor train) will be stationed along the length of the elevator for energy storage. They move in a direction opposite to that of the passing gondolas (a rising car consumes electrical energy and a descending one generates electrical energy). The combined mass of the ballast cars will be large (perhaps 10 to 50x the mass of one gondola), so that only cars within one hundred or so miles of a gondola will transmit power to the gondola. The use of ballast cars reduces the electrical efficiency of the system, but eliminates the need to schedule upward passenger service at the exact same time as the downward service, and smoothes out the peaks in the electrical load on the solar electric panels (as well as allowing operation during the nighttime side of the orbit).

The tether will support three parallel monorail gondola tracks; two for simultaneous bidirectional travel, and a third track for ballast cars (which also serves as a backup and allows parking of maintenance vehicles). Track switches will be provided at perhaps a dozen or more places along the tether so that gondolas can move from one track to another to route around a damaged section or to simply change direction at the end of a run.

To avoid the need for a direct electrical connection between the gondola and track, the track could serve as the stationary portion of a linear synchronous AC motor. This is to say that the track has flat electric coils (with their axes oriented horizontally) placed in overlapping positions along its entire length. The gondolas’ motors are simply sets of permanent magnets arranged to set up a field across the coils, and therefore provide thrust when the coils are energized. The coils are energized sequentially, at a rate proportional to the gondola speed.

When coupled with a "mag-lev" system to hold the gondolas on the track, high speed operation could be possible. However, for simplicity, initial designs might use wheels and therefore limit speeds to around 100 mph. This simplifies the emergency/parking brake also (the mag-lev system would have required rocket brakes).

High voltage electrical cables running the length of the tether transmit three phase A.C. power from the solar panels to where ever it is needed (the voltage will likely exceed 50kV to minimize wire mass). Most of the solar panels will probably be located near the zero Gee point, where the sun shines 69% of the time⁹and sun-tracking is easy. But additional panels could be spread along length of the tether for diversity. The power cables will need to be transformer isolated every few dozen miles since otherwise the earth’s magnetic field would cause the moving cables to have quite a high DC voltage across their length, which would complicate insulation. It might also be desirable to pump DC power into the power cables in order to push against the earth’s magnetic field and produce additional momentum.

The momentum engine is required somewhere along the length of the tether (near the zero gravity point would be convenient) in order to offset drag and the momentum transferred to payloads as they move up the elevator, to the extent that is not offset by downward moving payloads (i.e. during construction of the hotel, or for cargo released into orbit). In this way, the elevator with engine acts as an upper stage rocket, except that more fuel efficient engines can be used.

The thrust required is relatively low. The upper atmosphere will exert about 13 lbs of drag on the the main structure (this varies a lot with design and upper atmospheric temperature)¹⁰. When the docking grapple is extended down to 90 miles, it will contribute another 30 lbs (perhaps 10 lbs average with 4 dockings per day). If cargo is carried to the zero Gee point at the rate of 100,000 lbs per day, another 84 lbs of average thrust is required¹¹. The thrust must be applied symmetrically about the orbit, so if thrust is not available during the 31.6% of the orbit that is not in direct sunlight (775 miles up¹²), then the engine must be shut down another 31.6% of the time on the sunlight side of the orbit to balance it out. The engine is then only usable for 37% of the orbit and must therefore have a thrust of 289 lbs when on to average 107 lbs overall.

For most applications rockets engines are chemically fueled (i.e. the propellant is also the energy source), such as the hydrogen-oxygen engines used in the Space Shuttle. Chemical engines have high thrust for a given engine weight, but have poor fuel economy, or "specific impulse" (specific impulse is the number of seconds that 1 lb of propellant can produce 1 lb of thrust, which is directly proportional to the exhaust velocity; the Shuttle’s main engines have a specific impulse or Isp of 455 seconds). To reduce the propellant required for a given amount of momentum, several types of engines are feasible which would use energy from the sun to the drive the propellant to higher exhaust velocities. These solar engines can’t be used for launch vehicles since they tend to be many orders of magnitude away from being able to lift their own weight. For the momentum engine on a space elevator, though, low thrust to weight ratio is tolerable, so a solar engine would be the most cost effective.

The solar engines with the best specific impulses utilize solar voltaic panels to produce electricity which is then used in various electrostatic or electromagnetic propellant acceleration devices. They can have tens to a hundreds of times better specific impulse than chemical engines, but can take megawatts of electricity to produce just a few pounds of thrust (it takes about a 100 tons of solar panels to make a megawatt of electricity).

An intermediate alternative is solar-thermal engines which have about half the propellant consumption of chemical engines (Isp = 700-1000 seconds), but would use much smaller (i.e. lighter and less expensive) solar collectors for a given thrust than electric engines. Solar thermal engines don’t involve electricity (except for the fuel pump which could potentially be electric), but simply use concentrated sunlight to heat high pressure liquid hydrogen which is then exhausted from a normal rocket nozzle. Hydrogen is the propellant of choice because for a given temperature, hydrogen molecules move the fastest, and therefore can produce the fastest moving exhaust stream.

In the 1960’s, NASA developed and ground tested a nuclear thermal engine that exhausted hydrogen. This would have the same specific impulse as a solar thermal hydrogen engine, but better thrust to weight ratio. The project was canceled due to the extremely unpleasant consequences that a crash or explosion would have had.

NASA currently has a demonstration project in the works to flight test a solar electric (electrostatic ion accelerator) engine. A solar thermal engine has been lab tested, but not flown.

The hotel and space elevator will be placed in an orbit whose period is a sub-multiple of 24 hours (i.e. an integer number of orbits per day - 13 in this case), so that it will pass over the launch sites at the same time every day. If the launcher has just a small amount of cross-range capability and the launch site is at a latitude nearly equal to the orbit inclination, then the orbit will nearly pass over the launch site in two consecutive orbits each day, so that two launches and two landings are possible from each launch site, daily. Of course, the launch sites will be better utilized if they also serve other hotels, which are also placed in 1.845 hour orbits, but pass over the launch sites at different times of the day. And the space elevator will be better utilized if it is served by several launch sites, and therefore carries more people.

It should be noted that with RLVs there is no reason to locate launch sites on a ocean coast. In fact, flying over land provides superior safety since several emergency landing sites can be provided for use during aborts. This is especially important, since the launch vehicles can be expected to occasion fail to dock with space elevator, and therefore a landing site would be required about 2,000 miles downrange from the launch site (assuming the docking altitude is 90 miles and a low-lift re-entry profile; with a high lift X-33 like re-entry profile, a lower dock altitude might be required for the 2,000 mile landing). Depending on the launchers cross range capability, different alternate landing sites might be required for the two daily launches.

During the test flights of the launch vehicle, if flights over populated areas are not desired, then it could be launched from Cape Canaveral in Florida, and land on a barge at sea, but only if it can take off and land vertically (e.g. the Delta Clipper).

After the launcher is approved for flights over land, Cape Canaveral can continue to be used for daily cargo launches, with the sea barge serving only as an emergency landing site (this might not be cost effective).

With an orbital inclination of 40 degrees or so, the orbit will only pass over Cape Canaveral once per day. The passenger launches can occur at higher latitudes, so that two overflights (and therefore eventually two launch-landing pairs) can occur each day. Passenger launches could occur initially from a launch site in Nevada (with an emergency landing site in Virginia). Later, launch sites could be added in Spain (with an emergency landing site in Turkey) and China.

Future generations of space elevators may gain an economic advantage by being placed above the equator. This would allow launches as often as once per orbit, from a South American launch site. This Mach 19.5 elevator might not allow the launch vehicle sufficient space to abort on continent though.

The space elevator will be constructed in low earth orbit, with the construction equipment located at the center of the structure. The two ends are made first, and will get farther from the center as new length is added (this allows the cargo ships to dock at the zero gravity point).

During the construction of the space elevator, if the launch vehicle does not have a reasonable payload capacity in Single Stage to Orbit mode, it could be used in a two-stage mode, and release the payloads attached to a booster engine in sub-orbital trajectories. The RLV would then land about 1,000-2,000 miles downrange, and be ferried back to the launch site atop a jumbo jet (as is currently done when the Shuttle uses its California landing site).

As an alternative to an upper stage that must be either discarded or returned to earth for reuse, a small (30x payload mass) rotating tether facility could be used boost the payloads from the resulting sub-orbital trajectory into the construction orbit (see paper by Carrol¹³).

The partially completed elevator can be used to transport the RLV back to the launch site as soon as it’s long enough (and therefore the dock is moving slowing enough) to give the RLV adequate payload capacity. When the space elevator is completed, hotel construction can begin. Also, limited passenger service can begin, with the elevator gondolas serving as makeshift space stations.

Hotel construction might start with an inflated fabric shell with an industrial sized air-lock.  The shell would then be lined with radiation shielding plates, which can be velcro-ed in place to facilitate assembly with a robotic arm.  After that, construction workers would then have a comfortable "shirt-sleeve" environment in which to work.  Note that the hotel site is in the Van Allen radiation belt, so no "extra-vehicular activity" will be done (at least not in a normal thin-walled space suit).  For the counter balance station, construction would probably occur in the zero gravity area, with special cranes used to move it to the top of the tether.

To obtain an extremely rough estimate of the costs involvolved, we can use $1 Billion for the cost of the launch vehicle ( RLV); this is in between the five of a kind space Shuttle at $1.5 billion, and sixteen of a kind Stealth bomber at $0.8 billion, and well in excess of commercial jumbo jets at under $0.1 billion. The initial deployment might be three space elevator/hotels, one launch site, and fifteen launch vehicles carrying a total of 220,000 passenger per year (several cargo launch vehicles will be required also).

Assuming each RLV launch carries 100 passengers (or 50,000 lbs of cargo, using a 500 lb/per person conversion, to allow for supplies and added structure) and flies 150 times per year, and is amortized over 2 years (the short period compensates for the maintenance cost and return on investment), the per-passenger vehicle cost is $33,333. Alternatively, this corresponds to $67 per pound of cargo.

For the Mach 19.5 dock, a LOX-H2 fueled RLV with dry weight of 12% of gross, would burn 8,800 lbs of fuel per passenger. At the 1990 price of $500/ton, this adds another $2,200 to the per passenger cost (and $4.40/lb to the cargo lift cost).

For the Hotel itself, a crude estimate of the mass can made by taking, say, half the mass of sea-faring hotel (or cruise ship), which amounts to 30,000 lbs per guest¹⁴ (or 24 million pounds for 800 guests). It should also be noted that as the materials are brought from the dock to the higher hotel site for one way trips, a rocket engine on the space elevator must make up for the momentum gained by the materials; with engines of 800 sec Isp (e.g. solar thermal engines with hydrogen propellant), the propellant for this engine adds 27% to the lift mass (or 20% for materials brought only as high as the zero-Gee point). Assuming equal mass at zero-Gee and the counterbalance, a 4 day average stay, and a 2 year amortization period, the hotel mass amortizes to 203 lbs per passenger or $13,600 of lift cost per passenger. We can add another $20/lb as a very crude estimate of material cost (for comparison, sea-faring hotels have a cost in the range of $2/lb), bringing the total amortized hotel cost to $17,700 per passenger.

For the Space Elevator, the Mach 19.5 design made from Thornel T-40 with a safety factory of 2.4 was estimated to have a mass of 34,000,000 lbs. It’s capacity is two vehicles per day to and from each launch site (higher for hotels in equatorial orbits). For 100 passengers per vehicle, and one launch sites (a through-put of 200 passenger/day which sizes the hotel to around 800 guests), 365 operating days/year, and a 2 year amortization, the amortized mass is 229 lbs per passenger (or 275 lbs allowing 20% for propellant to bring it up from the docking altitude). Using $67 and $20 for lift and material cost (the current small quantity book price of Thornel T-40 is several times this, but the discount is reasonable due to the large quantity involved), the amortized Space Elevator cost comes to $23,900 per passenger.

Summing the launch, fuel, hotel, and elevator costs, the total is $78,100 for a four day stay. It’s likely that the actual price would be subsidised somewhat by revenue from selling high-profit-margin launch services to NASA and satellite companies, and by renting hotel space to movie studios. And of course the costs will come down due to further economies of scale as more elevator, launch vehicles, and hotels are built.

Most NASA programs would surely evolve to make use of the space elevator facility. The International Space Station is likely to be replaced with a more radiation tolerant one which could be placed in the same orbit as the space elevator (perhaps it would be attached a commercial elevator, although this would be a centi-gravity lab instead of a micro-gravity lab). NASA astronauts would then book passage on a commercial flight to the space hotel, then catch a space taxi over to their space station.

The focus of NASA’s manned space program is likely to move beyond low earth orbit, to the moon, Mars, or the asteroid belt. Lunar development in particular could reduce the cost of large construction projects in earth orbit by supplying materials for lower launch costs¹⁵.

Heavy payloads could be brought up from earth in pieces, and assembled in orbit alongside the space elevator, at the zero-G altitude.

The space elevator can also serve to deploy payloads into Geosynchronous Transfer Orbit (GTO), and to deep space (escape velocity) destinations like the moon or Mars. Objects simply dropped from the counter balance station would enter a 1020*3150 mile orbit, and could use on-board rockets to obtain the extra 5200 mph needed to leave orbit.

It would be much better though to deploy the payloads using a deployable hanging tether extended from the counter balance station. From 1,020 miles up, a 600 mile long tether could release a payload in a GTO with no additional rocket thrust (tether mass is 7x that of the payload with Thornel T-40 carbon fiber and a safety factor of 2.4). Due to the facility inclination, GTO deployments can be scheduled twice per day, and lunar injection twice every 28 days. For a lunar injection, the cable would have to be 920 miles long and would have a mass 30x that of the payload.

For occasional deployments at escape velocity, a free-flying rotating tether could boost payloads from the space elevator to escape velocity, using a much less massive tether (see Tethers Unlimited site¹⁷for their lunar tether system).  However, this would be another complicated space craft with propulsion to maintain (and probably staff), and it adds an extra rendevous operation.

For better cargo capacity, the large hotel counter balancing the tether could be replaced with a smaller dock facility placed further from the zero Gee point. A counter balance station at 1,460 miles could still have 1 million pounds mass. It would feel 0.25 Gees. A cable to deploy payloads at escape velocity would then be 475 miles long, and have a mass 8.3x the payload. The GTO cable is only 160 miles long and has mass 1x the payload.

A potential application of this would be the construction of a pilot plant to demonstrate a geosynchronous orbiting solar power station that beams 100’s of megawatts of energy to earth for conversion to electricity. A cost effective multi-gigawatt production plant would probably have to be built with lunar materials, but a proof of concept plant sent from earth would be reasonable.

A 7-day lunar fly-by vacation (retracing the Apollo 8 mission) in a small ship for perhaps a dozen people would probably cost slightly more than a stay in the space hotel. Unlike the Apollo vehicles, this Lunar transfer vehicle would not aerobrake upon its return to earth. Instead it docks with the deployment tether extended from the top station of the space elevator. In this way, it conserves all momentum, and therefore the only propellant requirement of the flight is for fine course adjustment. An orbiting Lunavator (see Tethers unlimited site) could provide propellant-less lunar landings and re-launches for planetary chauvinists.

For cargo to and from the moon (or Lagrange points) that doesn’t require a short transfer time, deployments can occur daily. However, most of the cargoes will arrive at lunar distance at a time that the moon is elsewhere in its orbit, so on-board propulsion will have to be used to circularize the orbit, correct the orbital plane, and rendezvous with the moon (would could take several weeks for minimum fuel consumption).

It should be noted that a Mach 18 space elevator (which would require a material about 30% stronger than Thornel T-40 or Spectra-2000) in an orbit with an inclination matching that of the moon could provide direct deployments to and from the moon (or Lagrange points), several time per day (a low mass counter balance could be positioned so as to travel at escape velocity). This is a big win, since a mass driver or rotary sling could launch payloads to it from the surface of the moon, and therefore eliminate the need for major on-board propulsion (also, pushing against a planet is much more energy efficient than pushing against rocket exhaust). This could make it feasible to import construction materials from the moon, for use in low earth orbit. (Again, for round-trips to the lunar surface, a Lunvator would be used.)  It will probably take a bit more station keeping fuel to keep the elevator in an orbit plane matching that of the moon, due to gravity effects of the earth’s equatorial bulge ("nodal regression"), however.

The space elevator will have a redundant multi-element tether construction (perhaps using a Hoytube design), so the structure can survive many micrometeor impacts. Derelict space craft are a big concern, since one could easily sever the entire tether. Therefore traffic control will be necessary over the whole range of altitudes occupied by the space elevator: from 160 miles to 1,500 miles for the initial design, and perhaps to 2,500 miles for second generation elevators (i.e. all spacecraft and satellites in this range must yield right of way to the space elevators). Also, all satellites will have to be removed from orbit at the end of the useful service lives (i.e. end the practice of dumping old spacecraft in junk yard orbits).

With proper design, it can be insured that none of the hotel stations fall to earth if the tether is cut. For the 860 mile long tether discussed here, the lower most hotel station must be at least 670 miles above the earth (at this point, 105 miles below the zero Gee point, there is 5% of earth normal gravity). Escape pods released at or above this altitude will enter a stable orbit at least 200 miles above the surface. If a tether cut occurs below this altitude, the hotel complex will be safe (although the lower part of the space elevator, including the dock, will be lost). If a tether cut occurs above this altitude, and above a hotel station, then the tether must be deliberately severed just below the lowest hotel station, probably using explosives, to insure that no part of the hotel is pulled down with the severed tether segment.

Due to its enormous size, the space elevator is not a good neighbor for the haphazardly placed collection of satellites in low earth orbit today. The risk of collision is greater though, for other tethers and space elevators. It will probably be simplest if all permanently deployed tethers and space elevators share an identical orbital period, and maintain a fixed spacing with respect to one another. Of course, after some period of time, it will be desirable to move to the next generation of longer, lower docking-velocity space elevators. In this case existing structures would be renovated (i.e. have their main tethers replaced).

When a launch vehicle docks (or undocks), the orbit of the space elevator will shift somewhat due to the new center of gravity. But in this case, the structure is more than two orders of magnitude more massive than the vehicle, so the effect is tolerable. The perigee of the space elevator will dip by about 5 miles, and it will move away from its station in the orbit by about one degree per hour. The momentum engine can be fired to adjust for situations when launch vehicles don’t dock or depart when expected. This should allow one or two dozen elevators to share an orbit (more with heavier elevators).

Ultimately, the upper limit on the number of space elevators that can orbit the earth at the same time might be probability that one severed tether could sever others. When a hanging tether is severed in the middle, the lower half re-enters the atmosphere, and the upper half will have an elliptical orbit with a slower period. The upper severed fragment must then be deorbited or lowered into an unoccupied slot before it has a chance to collide with (and sever) another space elevator.

The most obvious objections to the whole scenario are that it’s too expensive and/or too dangerous. It’s certainly true that past space programs have been expensive and dangerous, but the same was true of the early aviation industry.

In any high technology design, new and immature products will have design defects. Fixing them takes time and engineering resources, so the thoroughness with which these defects are removed depends on market demands. A government sponsored launch system that only makes a handful of flights a year could never justify the low defect rates that a commercial airliner has (or that a commercial space vehicle would require).

Similarly, any high technology design will be expensive to produce in "onesy-twosy" quantities. Production costs and purchase price inevitable come down as production volume go up. For anything built on a government contract in particular, the supplier never has an incentive to cost optimize. A more prudent strategy is to make an attractive contract proposal, and then raise the price later by exploiting contract loop holes and fine print. Of course in the commercial world, market acceptance of a product or service is always tied to meeting cost targets, so the supplier is much more motivated to consider cost.

Unfortunately, the Van Allen radiation belt starts as soon as the atmosphere stops, in fact it’s considerably worse at the hotel altitude than it is near the dock. The hotel will need to be shielded (with perhaps 3.6 inch aluminum walls, which would be 50 lbs/sqft of shielding, and would only be 10-15% of the hotel mass). Even so, early hotels will probably have a high enough radiation level that it would be prudent to keep crew-member tours of duties down to a few months.

The elevator ride will be brief enough to allow only moderate shielding, perhaps 12 lbs/sqft , but even this amount will constitute the majority of the mass of the gondola.  This probably means the elevator gondolas will not carry a crew to limit routine crew-member exposure. The design limit will specify that even when mechanical problems strand passengers in the gondola for a day or two, they wouldn’t have a high enough dosage to suffer radiation sickness.

The radiation will be an inconvenience during hotel construction, since the construction crew will not be able use space suits to do space walk construction. But this is not such a big issue since that's an inefficient way to do construction anyway (compared to working in a pressurized assembly area).

The following graph shows radiation dosage as a function of altitude¹⁶. For reference, a medical CAT scan exposes patients to 5-10 rads, 300 rad-equivalent is considered lethal, and 25 rad-equivalent/month is the NASA limit for astronauts. The data shows that a 3g/cm^2 shield corresponding to 6.1 lbs/sqft, is not adequate (note 10^4 rads/year= 27 rads/day).  However, the radiation level clearly drops with increasing shield thickness so there is reason to believe adequate shielding is possible.

The shielding levels assumed here for early hotels is less than has been discussed for a lunar base (NASA has talked about 1000-2000 lbs per square foot), in part because the earth’s magnetic field protects the hotel from most of the energy of deadly solar flares.

Radiation will also shorten the life of the solar panels, and restrict the selection of tether materials. Spectra-2000 fiber is said to be strongest available today (for its weight), however, like most plastics, it is not radiation tolerant, and therefore will not have acceptable design life at the higher altitudes. Above 400-500 miles, the tether can be constructed of graphite fiber, which will loose strength at the rate of only a few percent per decade. The zero gravity solar panels will probably have to be replaced every 5-10 years (but panels near the dock should last much longer).

The US government must nurture this new industry, in the same it supported the communications satellite industry in the 1970’s, the aircraft industry in the 1920’s and the railroads before that.

NASA should re-align it’s goals for the manned space program to specifically support this effort. In particular, tether demonstration flight should be raised to a high priority. The X-33 program can be used to demonstrate and test engines and thermal protection systems, and maybe even provide a design that could be developed into a commercial launch vehicle.

In addition to the X-33 (VentureStar) program, NASA should also resume research into the vertical take off and land Delta Clipper launch vehicle. This configuration has a potential safety advantage of allowing landings with full or nearly full fuel tanks for an early abort capability that a glider-landing vehicle like the X-33 does not have. And if a low-fuel consumption idle mode is provide, a vertical landing vehicle could leave its engines running from launch to docking (and from un-docking to landing), to eliminate concerns about a critical air-start during atmospheric re-entry for landings and aborts (it only takes about five minutes to get from the dock to the landing site).

Last, we should adopt firm policies against leaving man made debris in orbit, ranging from old satellites to paint flecks. This particularly important given the recent trend toward deployment of communcation satellites in low earth orbit, in vast constellations involving dozen of satellites per system (e.g. Globalstar, Iridium, & Teledesic).

Nathan Wilson has dabbled in amateur "rocket science" since being inspired several years ago by the San Diego L5 society. He holds a Master of Science degree in electrical engineering from Stanford University, and works as an electrical engineer for Seagate Technology.

Comments and/or questions about this document may be addressed to Nathan2go@aol.com.

• 1. Tether background/history: Space Towers http://liftoff.msfc.nasa.gov/academy/tether/spacetowers.html Tether Past Uses http://liftoff.msfc.nasa.gov/academy/tether/past_uses.html

  See also "The Rocket/Skyhook Combination", by F. Burke Carley and Hans P. Moravec, which was published as part of a 1983 workshop proceedings.  This is distributed as part of a U.S. government micro fiche collection, NAS 1.26:171197.  It gives an overview of  tether types, including verticle, rotating, and rocking, and suggests an implementation with a Kevlar tether and the Space Shuttle as a launcher. return to text

• 2. Calculations by the Author ( SSTO spreadsheet). return to text

• 3, 7, 17. Other tether data from Tether Unlimited, inc. Tethers Unlimited Home Page http://www.tethers.com return to text3  return to text 7  return to text 17

• 4,8. Carbon Fiber radiation tolerance data came from, "The Effects of Space Radiation on a Chemically Modified Graphite-Epoxy Composite Material" by Reed, Herakovich, and Sykes. NASA Grant NAG-1-343, October 1986. Microfiche NAS 1.5:89232. They irradiated test samples with a 1.0 MeV electron beam for 1*10^10 rads and noted no degradation in the graphite fiber, but the epoxy turned to mush. return to text4  return to text8

• 5. Carbon Fiber data is from Amoco Performance Products, Inc. 4500 McGinnis Ferry Road, Alpharetta, GA 30202-9893, 800-222-2448. Amoco Chemical/Thornel http://www.amocochem.com/altsite/products/thornel/thornel.htm return to text

• 6, 11. Calculations by the Author (Sp Elev spreadsheet). return to text6  return to text11

• 9, 12. The fraction of an orbit that a satellite is in the Earth’s shadow is given by: (arcsin((radius_earth)/(radius_earth+altitude_satellite))/(180 degrees)) which is 31.6% at an altitude of 780 miles. return to text9  return to text12

• 10. Aerodynamic drag for the tether was calculated with the equation and atmospheric data from the Tethers in Space Handbook. The tether was assumed to have 72 separate cables, and the three gondola tracks were assumed to have a combined exposed width of 200 mm.

  "The Tethers in Space Handbook" is available for download from: (this also includes links to a lot of tether related sites) AIRSEDS Info http://www.airseds.com/tethersinfo.html return to text10

• 13. "Preliminary Design for a 1 Km/sec Tether Transport Facility", by Joseph A. Carroll of Tether Applications. A paper presented at the NASA OAST Third Annual Advanced Propulsion Workshop at JPL, January 1992. Carroll describes an orbital facility with a 180 mile long tether which captures payloads from the Space Shuttle in a sub-orbital flight. The tether is reeled-in to avoid the need for an elevator. A summary is given here: TU Momentum ExchangeTethers http://www.tethers.com/ M-ETethers.html return to text13

• 14. Data on Seafaring Hotels is from: Forbes magazine, 7 July 1997, "Floating Monstrosities" which describes Carnival’s new cruise ship Destiny, which was at that time, the world’s largest at 101,000 tons. It carries 3400 passengers and costs $400 million. return to text14

• 15. For a general introduction to why it’s desirable, prudent, and feasible for humans to go into space, see Gerard K. O’Neill’s book about human colonies in space, "The High Frontier". 1976 original edition. It also discusses collection of solar energy in space for use on Earth and mining of lunar resources for construction in Earth orbit. return to text15

• 16. Radiation data is from JPL Publication 83-26, "Radiation Protection for Manned Space Activities" by Thomas Jordan. This is distributed as part of a U.S. government micro fiche collection, NAS 1.26:173202. The radiation graph is subreferenced to H.R. Rugge, "Space Radiation and Its Possible Effects on Man in Space", from Space Systems and Technology Workshop II, Sept. 1982. return to text16