The feasibility of reusable space vehicles has mainly been studied from the standpoint of improving rocket technology to achieve reduction in satellite launch cost. However, it is inevitable that true reusable space vehicles will significantly change the paradigm of space activities that has been assumed for past studies. According to a recent study (1), reusable space vehicles will be built in a manner similar to aircraft, and will be operated by commercial airline companies. It was estimated that about 750 000 passengers would make a three-hour spaceflight every year. Thus reusable space vehicles will create a new transportation industry which, being much larger-scale than the present commercial satellite launch business, will require a market such as space tourism involving the general public as customers. Although this number of passengers will remain only a fraction of 1 percent of the present number of air travellers, it will obviously be beyond the capability of existing space projects owned and operated by government space organizations. Predicting considerable changes in flight operations of such a space transportation system for space tourism, we consider it will be most reasonable and desirable for the new system to begin operation taking advantage of the existing infrastructure of air transportation services, such as airports and air traffic control systems.

The image of the ceremonial procedures of present day rocket operations is very wide-spread today, and planners of new space transportation systems cannot envisage these new operations, since their experience is mainly of launch operations of expendable rockets which never return. Turnaround operations of reusable vehicles impose new design requirements for vehicles. Designers of space vehicles for the space tourism era have to understand the characteristics of the new requirements, and reflect them in their design of new space transportation systems. It is also expected that a common basis for cooperation between operators of the space transportation system and the space transportation system designers will be developed through this study.

REFERENCE VEHICLE " KANKOH-MARU"

The "space tourism" under study is neither a fantasy nor just a nickname for conventional manned space flights, but an economic activity based on public demand. The space transportation system used for this purpose should be completely different from conventional rockets, including the Space Shuttle, in terms of the price of vehicles, operation cost and safety design. As in general aviation, the selection of vehicles will be one of the most important factors for this business. However, a suitable vehicle does not yet exist, though it is widely accepted that only fully reusable spaceplanes can reduce the price of spaceflight low enough for space tours by the general public. Among various concepts, there are two different types of vehicle signified by winged or wingless, and two vehicle configurations: single-stage-to-orbit ( SSTO) or two-stage-to-orbit ( TSTO). In this study, TSTO was omitted in favor of SSTO from the standpoint of operational simplicity.

Obviously winged aerospace planes are intended to be similar to aircraft in many respects, and so would be the most desirable from the standpoint of conventional airport operation. Their main difference from conventional aircraft will be more powerful and noisy engines and higher landing speed requiring longer runways. These features can be predicted by analogy to new high performance aircraft which have been introduced into general aviation. At present, however, winged vehicles are considered to be technically more difficult than a wingless ballistic type of vehicle. Because of technical uncertainty, operations of winged aerospace planes cannot be characterized in terms of take-off and landing profile, noise level and even shapes of vehicles. On the other hand, as the DC-X test vehicle demonstrated recently, operations of wingless ballistic vehicles which take-off and land vertically are much better understood than the former (2).

To make not only a qualitative but also a quantitative analysis of operations, we assume the use of wingless vehicles for the early phase of space tourism operations, and we have specifically chosen the Kankoh-maru as our reference vehicle. This vehicle is a preliminary result of the study made by the JRS transportation research committee (3). Fifty vehicles of this type in total are assumed to be operated on a global basis. Each flies 300 times per year, carrying fifty passengers per flight, so the total number of passengers is 750 000 per year. This size of transportation system is significantly smaller than the 300 million international flight passengers carried by IATA member airlines in 1993. The total number of flights is small as well. For example, if ten airports are used as bases for the fifty reference vehicles, the average frequency of departures and arrivals per airport will be five per day. The number of operational bases, and accordingly the frequency of flights at individual airports, are questions to be addressed to the operators, so we focus our study here on better understanding the characteristics of vehicle-level operations.

General characteristics of the reference vehicle " Kankoh-maru" are given in Table 1. Figure 1 shows a general view of a " Kankoh-maru" vehicle together with two typical commercial aircraft in a typical maintenance building. The reference vehicle can be operated autonomously without the support of space operation control centers and ground tracking stations. Instead navigation systems and traffic control of general aviation will be used. Similarly for ground operation, we discuss the applicability of available airport facilities for the reference vehicle operations. On the other hand, special features of space vehicles and their operations have been identified and conceptualized as necessary for propellant supply, departure and arrival facilities, passenger accommodation and air-space traffic control services.

Figure 1. A general view of the reference vehicle " Kankoh-maru"

It should be noted that any space vehicle to be used for tourism must be certified for public transportation using completely different standards from conventional rockets which were developed on the basis of weapon standards. It is also expected that this study will provide materials for discussions on these certification requirements.

Table 1 Characteristics of the reference vehicle " Kankoh-maru"

Dimensions and locations

Max. height	23.5m
Max. diameter	18.0m
Passengers cabin floor height	18.0m
Bottom surface height (clearance)	2.0m
Number of landing legs	4

Mass properties

Empty vehicle mass	50 000kg
Gross take-off mass	550 000kg
Landing mass	60 000kg

Propulsion and Power

Main engines (cryogenic temperature)
Sustainers	861 kN in vacuum x 8 engines
Boosters	725 kN sea level x 4 engines
Propellants and consumables
Liquid hydrogen	70 700 kg
Liquid oxygen	424 200 kg
Gaseous Helium
Auxiliary engines (gaseous hydrogen and oxygen)
Power units (gaseous hydrogen and oxygen)

Flight performance

Reference orbit	altitude 200 km, inclination 45o
Reference flight period	3 hours
Take-off acceleration	5m/s2
Landing deceleration	10m/s2
Landing accuracy	50m

Passenger services

Number of passengers	50
Services requiring consumables
Light meal and drinks	minimum
Lavatory and sanitary	minimum
Cabin consumables
Gases for breathing	as required
Safety provisions
Number of exit doors	2 (height: 1.9m, width 0.9m)
Number of slide raft	2

The conceptual study of the reference vehicle is lacking in the design of vehicle operations, especially for the phase of landing and post-flight maintenance, which is necessary in order to define the facilities needed for passenger accommodation and for vehicle maintenance and operations. To supplement this aspect of the vehicle design, we have developed some ideas of the turnaround operations of the reference vehicle which begin with landing and continue through the next flight, considering not only past experience but also the new economic requirements of vehicle operations. The result is summarized for normal operation in the following.

Emergency cases which have to be anticipated for passenger transportation services are considered as part of total vehicle operations as shown in Fig. 2. In cases of emergency, vehicles have to land at airports with no provision to accommodate the vehicle, or even on plain land or water. In such cases, a safe landing should be made first, and the passenger compartment should provide a short-term safe-haven for passengers and crew. An escape system for passengers to leave the vehicle will be required as a provision of each vehicle. Most such equipment onboard the vehicles is not defined yet. Some possible types of safety measures are suggested in the following.

The most substantial requirement for vehicle landing is the capability of pin-point landing which was typically demonstrated by the Apollo Lunar Excursion Modules (LEMs) which landed on the Moon. The main difference of the vertical landing vehicle under study from the LEMs is the existence of the atmosphere and wind. The stronger gravity of the Earth is another factor in the vehicle design causing greater mass. The atmosphere will be used to decelerate the vehicle and to aerodynamically control the landing path. However, near the ground surface, wind will push the vehicle to deviate from the predetermined landing point. Basically, vertical-landing space vehicles will be provided with passive aerodynamic controls for attitude and glide path control, and limited reaction control capability to enable vehicles to land softly on the ground. Therefore, an idea to control the final soft landing in horizontal as well as vertical directions is to activate the reaction control jets as appropriate in direction and in magnitude. It seems to be desirable for the pilot to watch and to maneuver the vehicle in the final phase of landing, facing the wind direction as aircraft pilots do.

After safing actions if necessary are taken by the flight crew, the vehicle will be moved by a tractor to the take-off spot where boarding bridges are provided. The passengers will find this procedure comparable to taxiing from the runway to the parking area for aircraft. This movement is necessary for the Kankoh-maru, but if the passenger compartment entrance is low enough for lift cars to access the vehicle directly, passengers will be carried by a lift car directly to the main building.

It has been assumed that no hazardous or toxic materials are used in the functional mechanism of the vehicle, such as explosives and hydrazine propellants. Liquid hydrogen and oxygen are used as propellants and for generating electric power, and these will be required to be controlled by the ground support equipment to keep the temperatures and pressures at required levels. As soon as the vehicle is fixed at the parking and departure position, the first maintenance procedure will be the establishment of closed loops to circulate cryogenic fluids in the propellant tanks of the vehicle.

Maintenance programs will be similar to those of aircraft, structured in several levels according to accumulated flight time and maintenance items. Daily inspection and minor maintenance will be made everyday similar to aircraft maintenance. For this purpose, the engine and propellant feed system of the vehicle must be improved beyond today's systems to avoid moisture and icing problems, and to make quick exchange of failed engines possible, while the propellant tank systems are kept cool. Maintenance here is defined as "work undertaken in order to restore every part of a vehicle returning from an operational flight to an acceptable level for the next flight". The word refurbishment often used for the present reusable space systems implies more intensive work than maintenance, so that it is not proper to be used here. Higher levels of maintenance and refurbishment will be performed off-site at the maintenance hangar and manufacturing factory, respectively.

Propellant refuelling will be started one hour before departure at the take-off site. The propellant feed system and the ground support system are kept connected to the propellant management system which monitors the condition of the propellants and keeps the pressures and temperatures at the engine start conditions. During this operation the departure site need not be cleared until the moment of engine start and take-off.

By the time the vehicle is ready for engine start, the passengers, leaving the main airport building, have arrived in the boarding building. When the flight crew is ready for departure, passengers will board the vehicle and take their seats. If, as described in the arrival phase, a passenger lift car can be used, the boarding facility will be simplified considerably. As soon as the thrust balance of all the engines is checked to be acceptable, the umbilical connections of the propellant management system will be disconnected, and the vehicle will accelerate vertically to depart the airport. The propellant management system should be ready for connection with a vehicle returning in either normal or contingency mode.

As a result of the analysis of the turnaround operations, there are several facilities newly required for airports to accommodate such vertical take-off and landing type of SSTO passenger vehicles. It would be premature to specify necessary facilities and their designs in detail, since the design of SSTO vehicles is preliminary and evolution of airport facilities should be considered. The concepts of facilities shown here are intended to be used as an image of facilities for further trade-off studies between the design of vehicles and provisions of airports for such services.

Though the mode of departure of the reference vehicles is the same as the take-off of current satellite launch rockets, the services required for vehicles returning from space feature the difference of the new facility. To signify this difference, the facility to service the reference vehicles will be called the Departure and Landing Facility (DLF), and the area to be used for departure is to be called the Departure and Landing Spot, as the unit of area is smaller than an airport runway and can be compared to a parking apron for an aircraft. Each spot will be occupied by a serviced vehicle from landing through departure. Although departure and arrival are very different from the point of view of vehicle design, passengers would feel comfortable with similar boarding and deplane-ing procedures as those of aircraft. This will be a design policy of the DLF. Each spot will be provided with a building to accommodate passengers and cargo, as well as services for flight and ground crew activities. Special equipment for navigation in the vicinity of the spot will be installed on the building.

An alternate idea to simplify the DLF is to eliminate the building, and to use a mobile type of passenger boarding facility recently used at airports for aircraft parking separately from the main building with boarding bridges. In the case of the reference vehicle, the boarding entrance is located so high near the top of the vehicle (18 m from the ground surface) that it will be difficult to modify existing lift-up type of mobile boarding cars for the use of the reference vehicles. If the passenger or cargo section of the vehicle can be lowered to a similar height as the floors of jumbo-jets, not only these existing airport facilities but also onboard emergency equipment can be used. A general view of the departure and landing facility is drawn for the reference vehicle as shown in Fig. 3. However, this concept will be changed by the vehicle design.

The propellants for the reference vehicle are liquid hydrogen and liquid oxygen. The demand for unprecedentedly large quantities of these liquids will be a challenge to the suppliers (4). To minimize risk, the suppliers should use the largest available production facilities. For liquid hydrogen, the largest liquefaction plant capacity is 30 tons per day which is equal to half of the required quantity for one daily flight of the reference vehicle. Therefore two units of this plant with a storage system will be used for this purpose. Production of gaseous hydrogen for liquefaction is another feature of a total system of liquid hydrogen supply. In the past, hydrogen was liquefied at the site of gaseous hydrogen production, and transported to the vehicle site. We have surveyed liquid hydrogen technology developed for fuel for cars and aircraft (5)(6)(7), and have concluded that there is no fundamental difficulty to transport, store and supply the propellants safely under the present regulations, although minor modifications are required.

Conventional satellite launchers use a relatively small proportion of the total liquid oxygen production for industrial use. However, the quantity of liquid oxygen for the vehicles under study is so large that dedicated production plants will be necessary. As a conservative approach, we assumed that one unit has a daily production capacity of 180 ton of liquid oxygen, which produces the required ratio of propellants together with a unit of the largest hydrogen liquefaction facility. Thus two units of these oxygen liquefaction facilities will be required for the reference flight model. Electricity for the liquid oxygen production system will be 15000 kW including separation of oxygen from air and liquefaction. In future, the number of units will be increased as required.

To minimize losses of liquid hydrogen, as well as to keep the temperature low, the propellant supply system will be connected to the vehicle tank system in order to keep the total system in a closed loop circulating cryogenic fluid. Figure 4 shows two modes of operation of a typical supply system with the vehicle. If liquid hydrogen is transported and loaded into the storage tank, the function of the hydrogen liquefaction facility is reliquefaction of vaporized hydrogen only, which requires relatively little electric power.

As soon as a vehicle returns and lands, a tractor moves it to the DLF where the vehicle is docked to the building with boarding gates. This can be most easily performed if the vehicle is provided with landing gear with wheels. Otherwise, more sophisticated equipment and operational procedures will be required to move the vehicle. There is still controversy concerning the vehicle design on this point.

There are various types of mobile ground support equipment (GSE) used to provide regular services for aircraft. The following is a list of typical mobile GSEs which are being considered to be used for passenger services in the present case(8).

AC 400 Hz ground power unit Aircraft towing tractor
Passenger boarding bridge
Passenger steps
Ground airconditioner
Commissary truck
Water supply truck
Cabin cleaning truck
Lavatory service truck

For cargo planes there are additional items such as container transporter, container loader and bulk cargo loader. Most of these vehicles access aircraft directly. From an economic point of view, space vehicles should take advantage of these existing services provided by airports. This point will be an important design requirement for vehicles under study.

To exchange malfunctioning parts or components as large as an engine, special handling tools and working platforms will be necessary. The rocket exhaust flow tunnel and necessary mechanical systems have to be designed to assure safety and efficiency of maintenance work, as well as the final departure operation. These equipment are not well conceptualized at the present. For the same reason as for GSE, the space tourism carrier industry should not own buildings and physical property, but should rent from airline companies. Existing aircraft maintenance buildings are large enough to carry out maintenance of the reference space vehicles.

Traditionally spaceports have been remote and isolated from populated areas, because they are designed as test ranges for weapons and projectiles under research and development, where these test objects are allowed to fail in flight control and to result in hazardous accidents. However, once qualified in terms of safety of passengers, rocket vehicles will be operated more safely and efficiently at airport type of facilities than in such inconvenient test ranges. Even high energy propellants and rocket exhaust which seem to be unique to rocket vehicle operations can be discussed on the same basis as the large amounts of aircraft fuel and noise controls at modern airports. In this respect, the rocket noise and hydrogen safety will be discussed as critical issues for selection of airports for space tourism.

The rocket noise in question is generated by rocket exhaust gases. This problem was studied theoretically and experimentally from the standpoint of structural design of aerospace vehicles as well as noise reduction of aircraft engines. According to these studies, noise power is known to be proportional to the power of a rocket nozzle flow. It is also known that a rocket exhaust channel used to deflect the exhaust gas flow for a vertical take-off rocket vehicle such as the reference vehicle affects the short-range noise and the design of the facility at the departure site. However, our present concern is in the noise level outside an airport which will be caused by the reference vehicle leaving the airport. Based on experimental studies, we tried to determine the characteristics of the rocket noise of the reference vehicle at long distance from the rocket departure site to compare them with those of the airport noise environment.

To obtain a general idea of the noise environment of the reference vehicle, a very simple model of the noise caused by rocket exhaust has been used. Firstly, a typical pattern of sound directivity given in (9) and shown in Fig. 5 was used, in which the directional characteristics of the overall sound are assumed to be axisymmetric around the flow direction of the exhaust gas, and to depend on the angle from the flow direction. In this model, the origin of the noise is assumed to be at the rocket, since far-field noise is being discussed.

Secondly the far-field overall sound pressure (OSP) at a distance R km from the sound source is assumed to decrease inversely proportional to the square of the distance R, and is proportional to the sound power (SPW). Their relation is given as follows.

10 log OSP (dB) = 10 log SPW (dB) - 20 log R + C

Applying measured data of a Saturn 1C static firing test, that is, the sound power and the maximum sound pressure at a distance of 610 m; 210.9 dB and 142.5 dB, respectively(10), we get C = -13. Similarly, the sound power and the maximum sound pressure obtained from noise measurement of a N Rocket launching at a distance of 200 m were 192 dB and 136 dB, respectively(11). In this case, C=10. To make a rough estimation of the maximum overall sound level of the reference vehicle, we interpolate the sound power level of the reference vehicle whose thrust is 820 tonf from these data of Saturn IC with a thrust of about 3200 tonf and the N Rocket and the strap-on boosters with a thrust of 145.5 ton. Then, we get the following relation in which +/-1.5 represents the difference of the values of C for the two cases, and has no physical meaning such as error or deviation.

10 log OSP (dB) = 128.5 + 1.5 (dB) - 20 log R (km)
(0 dB : 20 x10e-6N/m2, R km)

Numerical calculations of this equation were made for several horizontal distances from the take-off point while the reference vehicle climbed vertically at a constant acceleration of 5 m/s2. The results are shown in Fig. 6 as a time history of the sound pressure level at each distance. For this calculation, it was assumed that the maximum sound direction angle in Fig. 5 is 50 degrees, and that the time is elapsed time from take-off including time required for the sound to reach each distance at a speed of 300 m/sec. It should be noted that atmospheric attenuation of sound and temperature profile are not considered for this calculation, although these effects will change the sound pattern, and will significantly reduce the noise level (11).

At present, sites for rocket launching are isolated from inhabited areas by rules of safety distance determined on the basis of the potential hazards of explosion of the propellants loaded on board the rockets as shown by Fig. 7. These hazards caused by blast waves of explosion of the propellants are supposed to be evaluated by the maximum credible explosive TNT equivalent weight (12). This weight is often expressed in percent of the propellant weight as the TNT yield. At present, the yield estimates are determined based on selected worst case according to the AFR 127-100 (13). For propellants of liquid hydrogen and liquid oxygen, the factor of equivalence used is 60% which means that the reference vehicle contains 297 tons of TNT. Acceptable pressure levels from explosions at inhabited buildings, propellant supply and terminal buildings are specified to be 1.38, 41.4 and 13.8 kPa, respectively.

In this case, the distance from the launch site to inhabited buildings, propellant supply facility and airport buildings are 4000 m, 340 m and 700 m, respectively. Thus even if the current rule is applied, the distances required are within the area of an airport. However, the TNT yields of a combination of propellants vary widely depending on individual cases of accident, and usually lower than 60%. For example, the TNT yields studied for the Space Shuttle safety ranged from 5% to 50% (14), and a study for the French Guiana Ariane launch site proposed to reduce the value to 15% (15) In the case of vehicles qualified for passenger flight, this restriction will be applied. Then hydrogen safety will not be a practical problem for site selection for departure and landing facilities.

Finally, we will show some results of case studies to be compared with the case of liquid hydrogen-fuelled aircraft. As a result, it is concluded that the most important requirement of vehicle design is to assure safe departure, including abort capability and precise landing capability. In this case major airports will be able to provide the facilities needed to accommodate reusable space vehicles in the early stage of space tourism.

The first critical factor is noise level. Figure 8 shows the noise contours predicted for the first phase Kansai airport now in operation, and for the new Chubu International Airport under study, respectively. The WECPNL (Weighted Equivalent Continuous Perceived Noise Level) is generally used to evaluate the noise level at airports used for general aviation, and within the contour of level 70 is the zone within which the government subsidizes noise protection measures for hospitals and schools(6). Residential houses are subsidized inside the contour of level 75. Thus, these two levels are typical contours used for airport planning.

If the rocket noise shown before is overlaid on this figure, a circle with radius of 15 km corresponds to the noise level of 110 dB. If the WECPNL is applied for two flights of the reference vehicle per day, a noise level 110 dB is equal to a WECPNL of 85. However, this level will be significantly reduced by the sound attenuation in the atmosphere. Though more study is needed to evaluate the effect of attenuation which depends on sound frequency, we expect to correct the value by an order of 10 dB below the previous estimation. Then, it will be a reasonable target for the rocket engine designers to achieve a WECPNL of 75 within a circle with radius of 10 km. The zone of level over 90 is to be kept as a non-residential area, and the main facilities including runways and the main airport building are located in the noisiest zone with a noise-level over 95. Such high noise-level zones are too small to be shown in the contours for the airport, so a typical noise sound level contour for a B747 is included in Fig. 8 for comparison. We need more study to determine the noise level inside the airports. There is also the possibility to reduce rocket noise by improvement of rocket engines, just as the noise of jet engines has been significantly reduced from the first generation jet aircraft until the current widebody aircraft.

Besides refuelling aircraft, fuel transportation to and storage at airports are among the most important functions of efficient airports. Depending on the locations of airports, several methods of transportation are used. The New Tokyo International Airport (Narita) located inland is provided with fuel pipe-lines from the nearest harbor. The Kansai International Airport built off-shore has its own berthing facilities for fuel tankers. For some medium size airports, railway trains are used to carry fuel.

In the previous discussion, installation of a hydrogen liquefaction facility plant near the airport was examined as a technical possibility. However, from an economic point of view, it will be more probable in Japan to import liquid hydrogen from overseas, because of the high cost of electricity required for liquid hydrogen production. In this case, off-shore airports will be preferred to in-land airports. A concept of sea transportation of liquid hydrogen is shown in Fig. 9. The LH2 Barge Tank illustrated in Fig. 4 is one cargo unit of the Barge tanker shown here. This system has been originally conceptualized for a much larger scale liquid hydrogen industry(19). but it is considered that it could be used effectively for this purpose. The volume of this barge tank is 3000 kl per unit, which is enough to refuel two flights of the reference vehicle. The Euro-Quebec Hydro-Hydrogen Pilot Project proposed tanker vessels which carry five of these barge tanks per tanker.

Liquid oxygen may be liquefied at the airport site, if the production cost is acceptable. In this case, the reference flight operation requires two units of 180 ton per-day liquefaction facilities, for which 15000 kW electric power will be required. This power will be added to the present power consumption at the air port, which for example, is 50000 kW for the Kansai International Airport.

As described briefly, the questions concerning the feasibility of SSTO vehicles operating from major airports are featured by various factors which have not previously been known or well-defined. We here summarize the key issues, dividing them into several problem areas, and try to solve the questions, or at least to identify the corresponding requirements for vehicle performance.

A concept of the turnaround operation for the reference vehicle was developed to study the ground facilities for passengers. After a soft landing nearby, each vehicle will be moved to a spot provided with a boarding and deboarding facility, and will park there overnight until departure taking place from the same spot. Thus, there is no functional similarity between the SSTO facility and airport main facilities such as runways and parking aprons. Instead, a single site for departure and landing will serve exclusively for a single vehicle from landing through departure. The air-space required for the SSTO type of vehicles will be of a conical shape standing vertically with the apex fixed on the departure/landing area. The apex angle may be for example sixty degrees, and should be determined by the maximum allowable wind velocity for landing. This air space may intersect that of aircraft using the same airport, and VTOL operations will therefore require coordination with airport air traffic management.

The design of individual ground support facilities depends significantly on the design of the SSTO vehicles. From the viewpoint of passenger accommodation, the entrance and exit to the vehicle should not be located too high for existing ground support equipment (GSE) to be used. Design of the site for departure and landing will vary according to the applicability of conventional types of airport GSE, such as mobile tractors, lift cars for cargo and passengers. Provision of landing wheels would simplify significantly the post-landing operation. It has been pointed out that vehicle designers need to consider rescue and emergency capabilities in detail.

It has been found that actual rocket noise levels of the N Rocket fright 2 (N-2) measured at 1.6 km from the launch site is 10 dB lower than our predicted noise level shown in Fig. 6. The reason was explained as attenuation of the noise by the atmosphere (11). Thus, the far field noise level will be expected to be much lower than the predicted level at long distance, while atmospheric conditions will affect the sound propagation in such a case. Though there are unknown factors for the noise estimation, the noise level of conventional rocket engines will be too high to be acceptable following the current level of regulations. It will be necessary to reduce the sound level by one order of magnitude from the reference vehicle using current engines which means that the acoustic energy is 0.1% of the exhaust gas flow energy. In this case, at low flight frequency the far-field noise will be acceptable. The noise level is higher, but the duration is shorter for near-field than for far-field noise. These characteristics are more or less similar to aircraft; the main difference is the motion of the noise sources - horizontal for aircraft and vertical for SSTO.

Cryogenic propellants supply will be a key new function to be added to airports for SSTO operation. Rocket propellant operations are considered to be substantially hazardous based on past experience of weapons rocketry. Accordingly, unreasonably strict regulations are applied for the minimum distance of loaded vehicles from storage of propellants, especially for liquid hydrogen. These restrictions based on safety from explosion of the TNT equivalent of the propellants are not realistic, and are not practical for SSTO vehicles to carry passengers onboard. Nevertheless, based on the present safety distance for clearance of residential areas, the size of airports is usually large enough to keep the neighboring residential area outside the limit.

It has been found that, to reduce transportation cost, production of liquid oxygen should be made in the airport area, rather than at existing factories located in industrial areas. This will require accompanying consumption of electricity and land use in the airport system, in addition to investment in construction of the facility.

Assuming functional similarity of transportation services, we consider airport operation will become economically essential to the future space tourism industry. A vertical take-off and landing type of reusable rocket was chosen as the reference vehicle for this study because its technical characteristics are better known than a horizontal take-off and landing type.

As for air space and ground facilities for departure and landing of the vehicles, the requirements are expected to be a small fraction of a large airport. On the other hand, the noise pressure level of rocket engines especially during take-off will be considerably higher than the current allowable noise level, and the reference vehicle configuration makes it difficult to make common use of existing ground support equipment for aircraft.

These problems will impose new design requirements on rocket vehicles and engines, and trade-off studies will be made for difficult requirements. It is a finding that the required production of liquid oxygen for propellant exceeds the current industrial level, and that the economy of its production is a more crucial issue than the technology for handling liquid hydrogen safely. In conclusion, there will be no substantial difficulty for airports to be used for space tourism vehicles.

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