Lowering the cost of access to space is now a major space policy objective. ESA is proceeding with the Future European Space Transportation Investigation Programme (FESTIP), and NASA is part-funding the X-33 and X-34 experimental spaceplanes. The X-33 is a technology demonstrator for the Venture Star reusable launch vehicle ( RLV).

Several US start-up companies have gained significant funding towards the development of RLVs, notably Kelly Space and Technology, Kistler Aerospace Corporation and Pioneer Rocketplane. Some of these US projects are due to fly next year. In 1994 Bristol Spaceplanes Limited completed a feasibility study for ESA of its Spacecab project, and the concept was endorsed by a subsequent independent review commissioned by the UK Minister for Space.

It is generally agreed that full reusability is necessary for large cost reductions. However, there is still debate about how much cost reduction is likely, how long it will take to achieve, what enabling technology is required, and what are the best types of vehicle to develop.

This paper gives provisional answers to these questions by analysing a thought experiment. We imagine that spaceplanes have matured to have operating characteristics like airliners. They are capable of several flights per day to orbit, and have a life of tens of thousands of flying hours. We then analyse the cost per launch and estimate the resulting market. We then derive a development strategy for achieving this goal, and assess how long it would take to implement.

This analysis of the thought experiment shows that developing a mature spaceplane is an achievable medium term goal, that the next step should be a small sub-orbital spaceplane, and that this could be developed at surprisingly low cost.

The logic of the paper is based on one main assumption that is difficult to prove, but seems intuitively probable. This assumption is that at some time in the future reusable launch vehicles will be almost as mature as airliners are today. Spaceplanes will be able to make several flights per day, and they will have a design life of some twenty years.

They will be certificated to airliner standards of safety, perhaps based on the JAR 25 rules to which European airliners are designed, tested, and type certificated. Additional chapters will need to be written to cover the hazards of space debris, radiation, de-pressurisation and reentry. There has been sufficient experience of manned spaceflight over the past thirty five years to be confident that there are no insoluble problems. Clearly, however, these risks will need careful management, using the enormous experience of the international airline industry.

Another assumption is that a prototype reusable launch vehicle is feasible with more-or-less existing technology. As mentioned earlier, there are now about half a dozen credible US spaceplane projects under development.

Given these assumptions, we can make conservative estimates of the probable cost and market size of a future mature space transportation service. We can then establish how best to reach this objective and how long it is likely to take.

A key parameter is the direct operating cost of a spaceplane with a payload weight fraction that can be achieved with more-or-less existing technology, but matured to have a long life and rapid turn-around.

Of the various current projects, the Lockheed Venture Star has had the most details published and is the "official" NASA candidate for the first fully reusable launch vehicle. If we assume that eventually a second or third generation Venture Star will reach airliner maturity, we can gain a feel for its cost per flight by applying airliner cost assumptions. For reasons that will become clear, it is convenient to express direct operating cost in terms of cost per seat to orbit.

Table 1 compares the costs of a mature Venture Star with a Boeing 747, and shows how the estimate was carried out. The resulting cost per seat to orbit is around $5000.

Several points should be noted about this estimate. The first is the number of people carried, 147. This was obtained by dividing the Venture Star payload of 59,000 lb by 400 lb, (assuming that each passenger weighs 200 lb, and that an accommodation module carried in the payload bay also weighs 200 lb per passenger). The present payload bay is not large enough, but we are assuming several generations ahead.

A second point is that the systems requiring the greatest improvement to reach airliner maturity are rocket motors and thermal protection systems. At present these have short lives and high maintenance costs. (A measure of the problem is that rocket motor test times are at present usually quoted in seconds.)

A third point is that the cost per seat is more than 1000 times lower than in the Space Shuttle. This is because of the combination of reusability and maturity. This cost is still an order of magnitude higher than that of a long-range airliner flight, due to lower payload fraction and higher fuel cost.

A fourth point is that the fuel cost is about 90% of the total. This is because hydrogen is some thirty times more expensive per unit weight than kerosene. (The estimate is conservative in that no reduction has been assumed in the cost of hydrogen fuel due to large scale manufacture.) There are two implications from this. Firstly, the cost estimate should be robust because the largest cost item is reliably estimated. The other assumptions could be in considerable error without greatly affecting the total. Secondly, there will be a strong incentive to minimise the use of hydrogen. This could be done by a tripropellant engine using kerosene for the early ascent, by using hypersonic air-breathing engines for the early ascent, or by a two-stage vehicle using kerosene on the lower stage, or even on both stages. Thus single stage to orbit ( SSTO) vehicles may have to wait for advanced air-breathing engines to become more economical than two-stagers.

Perhaps the most important point is that, given a prototype reusable launch vehicle, the only fundamental further developments required for drastic launch cost reductions are long life and short turn around time. Engineering history shows that these require painstaking incremental development over years of in-service evolution, rather than quantum leaps in technology.

A final point is that this estimate is conservative in that it does not assume a payload weight fraction beyond that achievable with more-or-less existing technology. It is highly probable that later generations of Venture Star, for example, would have a greater payload, as well as greater design maturity. Applying airliner cost assumptions to other spaceplane designs, such as Sänger, Skylon or Spacebus results in comparable costs. Thus the assumption of maturity overrides questions of technical detail.

Given mature launchers, the cost of building and operating space station costs will be considerably reduced. Four factors make present space stations so expensive: novelty, high political profile, low production rate, and the fact that the "call-out charge" for a plumber or electrician to fix a failure is some $500 million, the approximate cost of a Space Shuttle launch.

Given low cost access to orbit on demand by mature spaceplane, none of these factors would apply, and there is no obvious reason why a space station should cost more to build than an airliner of comparable payload volume.

Thus mature spaceplanes will lead to a "mature orbital infrastructure", as shown in fig. 1.

This concept of a complete orbital infrastructure made up of space stations, heavy lift vehicles and spaceplanes is by no means a new idea. Realistic studies were carried out by von Braun and others in the early 1950s, Ref. 2. NASA came close to achieving it twenty five years ago with Skylab, Saturn and the X-15 sub-orbital spaceplane, but did not close the gap by developing an orbital successor to the X-15.

The key development for a mature orbital infrastructure is the mature spaceplane. Each space station module has only to be launched once, but will have a life of some 20 years. Thus the cost per flight of the Heavy Lift Vehicle is far less important than that of the spaceplane, which has to fly of the order of a thousand times more frequently, if we assume a supply flight every few days.

Mature space stations depend on mature spaceplanes. A preliminary estimate of the total cost of a space holiday, Ref. 3 suggests that the spaceplane cost will be some 70% of the total. Thus, including the cost of the hotel, profits and overheads it seems probable that the fare for a few days in space will be around $10,000. The significance of $10,000 is that it is affordable by most middle income people prepared to save.

Three new commercial uses of a mature orbital infrastructure have potentially large markets - solar power generation, manufacturing and tourism. This paper will concentrate on tourism, because at this stage it has the most credible business case. Solar power involves additional major technical problems, and the market size for products manufactured in orbit is less certain than that for tourism. These markets may become larger than tourism, but the case being made in this paper does not depend on that happening.

Space tourism is defined for present purposes as fare-paying members of the general public visiting space hotels, i.e. space stations in low Earth orbit equipped for leisure, entertainment and education.

From the experiences of some 350 astronauts to date we know that trips to space are fascinating experiences. The main attractions are the views of the Earth, views of the various heavenly bodies, and playing around in microgravity. Thus it seems likely that large numbers of people will want to go as soon as it is sufficiently safe and economical. (The size of this market is discussed later.)

Space tourism is increasingly recognised as an important future market, but has never been considered seriously in space policy decisions. This may begin to change now that NASA and the Space Transportation Association have completed their joint study of space tourism, Ref. 1.

It seems highly probable that a typical future space holiday will consist of a few days or more in a space hotel i.e. a space station equipped for accommodation, leisure, education, and entertainment. The hotel will probably be assembled in low Earth orbit from modules launched by a Heavy Lift Vehicle. Tourists, crew and supplies will be ferried up and down in spaceplanes. This is one application of the "mature orbital infrastructure" mentioned earlier.

Market research carried out in Japan, (Ref. 4) suggests that around one million people per year from Japan alone would pay around $10,000 for a holiday in space. Subsequent surveys in Canada and the US indicate a comparable fraction of the population prepared to pay that amount, (Ref. 5). Assuming that this is true world-wide, the total annual demand for space tourism at a cost of $10,000 could be as high as 15 million people (the Japanese GDP is about one fifteenth of the world total.) However, given the limitations of market surveys of a product that requires a lot of imagination to appreciate, it would be prudent to assume an annual demand of just one million space tourists. Even this would require a fleet of around 50 spaceplanes of 50 seat capacity making one flight per day each.

As a quick check on this market estimate, one million people per year is equivalent to 7.5% of the industrialised population (assumed here to be one billion) making one flight per lifetime (assumed to be 75 years). This seems a conservative estimate of the likely demand for a "once in a lifetime" trip at a cost of a few months income, particularly since most people say they wish to go more than once, (Ref. 5).

The above analysis suggests that (in round but conservative numbers) a fleet of 50 spaceplanes carrying one million people to space each year at a fare of $10,000 each is a realistic goal for space policy to aim for. The technology is matured from that under development, the cost estimates seem robust, and the market assessment seems conservative.

The significance of this potential market is that it is large enough to provide both the operating experience and the commercial incentive to achieve maturity in a reasonable timescale, once it appears to be within grasp. A prototype spaceplane of a type likely to mature early would provide strong evidence of this potential. When this prototype flies it could be pointed out by the marketeers that all that is needed to achieve a thousandfold reduction in the cost of sending people to orbit is maturity.

The Venture Star was used earlier as an example to illustrate the order of magnitude costs achievable with reusability plus maturity. However, Venture Star was not designed with this as a driving requirement, and a different configuration may well be more appropriate for an early space tourism business.

Dozens of different configurations of reusable launch vehicles have been proposed over the years. The question is which of these could reach maturity first. We will start by considering the design requirements.

A useful measure of maturity is type certification for passenger carrying, i.e. meeting the space version of JAR 25 requirements mentioned earlier. It seems likely that the design capable of reaching this milestone the soonest will also mature the soonest. (Type certification is but one of several regulatory issues that will need to be addressed, (Ref. 6), but is probably the most critical for an early tourism business.)

The following discussion considers how the requirement for early civil certification affects the required fundamental design features. The arguments are summarised from those in Ref. 7.

Civil certification means higher safety factors and system redundancy than those of present or planned launchers. It would also be prudent to assume that pilots will be required, since there are no plans for pilotless airliners in the foreseeable future.

These requirements in turn mean higher weight, which for an early design favours two stages. The need for the X-33 demonstrator indicates that the engines and materials do not exist today for a reusable SSTO satellite launcher, let alone for a passenger carrying one.

Horizontal take off and landing is inherently safer than vertical take off and landing, and is therefore desirable for an early design.

For early certification the use of advanced technology should be minimised. This means that stage separation should be at supersonic speed, probably around Mach 4. Subsonic separation eases the upper stage design task compared with taking off unaided, but not enough to avoid the need for an advanced structure or advanced engine. (True SSTOs need both.) Hypersonic separation requires an advanced lower stage, and involves a difficult flyback problem. At around Mach 4 a design can be derived such that both stages can use existing technology.

Jet engines on the lower stage are desirable for ferry flights, flyback, aborted landings, and cruise to the required orbit plane. Off-the-shelf jet engines suitable for rapid development to airliner maturity are limited to about Mach 2. Additional rockets are therefore required to reach Mach 4.

Thus an early tourist carrier would be piloted, have two stages, supersonic separation, jet and rocket propulsion during the boost phase, and would take off and land horizontally.

The first spaceplane design to combine the above features was the Dassault Aerospace Transporter, shown in fig. 2. To quote from Ref. 8, "Its promises are founded.....on the basis of the very limited risks it entails .....". These words were published in 1967.

The Dassault project was one of several European Aerospace Transporter projects studied in the 1960s. There were numerous contemporary US projects. The inspiration for many of these projects was the XB-70 and the X-15, fig. 3.

Many thought at the time that an orbital successor to the X-15, hydrogen-fuelled, delta winged, and air-launched from a re-designed B-70, would be the quickest way to achieve an orbital aeroplane.

For reasons having perhaps more to do with politics than with engineering or commercial arguments this approach has never been followed.

Thus, using straightforward commercial and engineering reasoning we can say that a spaceplane designed for reaching maturity as soon as practicable would have the basic design features of the Dassault Aerospace Transporter of 1967.

The first project with the above design formula designed specifically for tourism was Spacebus, fig. 4, first described in Ref. 9 and updated in Ref. 10. Spacebus is designed to carry 50 passengers.

Figure 4. Spacebus

6.5.1 Spacecab

The business case for space tourism is not yet well enough established to raise the funding for Spacebus now. This suggests a demonstrator. Spacecab, fig. 5, is described in Ref. 7 and Ref. 10. It has the same basic design features as Spacebus, but is smaller. It is designed for launching satellites in the one tonne class, and for ferrying six crew to and from space stations.

Figure 5. Spacecab

Spacecab is essentially an updated version of the 1960s European Aerospace Transporter projects modified to use technology developed since then.

The carrier aeroplane lower stage looks not unlike Concorde, but has a simpler aerodynamic shape and structure. This is acceptable because Spacecab has only to accelerate to maximum speed, release the upper stage, and fly back. There is no long range cruise requirement, and hence no need for the last percentage point of aerodynamic or structural efficiency.

(It was primarily the range shortfall of the Concorde prototypes that necessitated the subsequent stretch to the so-called pre-production standard, and yet another stretch to the production standard. Even the first two "production" aeroplanes did not enter commercial service. It was this extended evolution that was mainly responsible for the development cost and time overrun.)

The lower stage of Spacecab has conventional jet engines for acceleration to Mach 2. Rocket engines then take over for acceleration to Mach 4 and climb to the fringe of the atmosphere. Separation takes place where the air is thin and dynamic pressure low. Air loads during separation should therefore be a manageable design problem.

The upper stage has six engines in the RL-10 or HM7 class. It has a lightweight wing/tank which is partially pressure stabilised. It can carry a satellite in the one ton class, or six space station crew, or mechanics plus spare parts to service satellites in low orbit. These missions are those in greatest need of a new launcher.

Spacecab was the subject of a feasibility study for ESA, (Ref. 7), that showed that Spacecab could use existing engines and conventional structural materials. A subsequent independent review for the UK Minister for Space "has not identified any fundamental flaws in Mr Ashford's concept"(Ref. 11).

The development cost of a Spacecab prototype should be in the region of one to two billion dollars to the point of revenue service (Ref. 10). As a check on this estimate, it can be compared with that of the X-33, at around $1.2 billion (Ref. 12). The Spacecab upper stage is not unlike an X-33 of one third the gross weight but using existing engines and proven materials. It should therefore cost less to develop, say half as much at $600 million.

A preliminary estimate for the lower stage prototype can be obtained by scaling the development cost of Concorde, which is about $12 billion at present prices. This can be multiplied by 50% because Spacecab uses existing engines whereas half the Concorde development cost went on the propulsion system. It can be multiplied by a further 10% because prototype aeroplanes cost typically 10% of the total cost to certification. Spacecab can earn revenue even as a prototype by launching satellites as soon as the flight test programme reaches orbit, which might take around 20 flights. Concorde needed some 2000 flights involving six non-deliverable aeroplanes to achieve its type certificate and to start earning money.

Multiplying these factors indicates a development cost for the lower stage prototype of around $600 million, and a total for both stages of $1.2 billion. Allowing for the preliminary nature of this estimate, it seems that the development cost of Spacecab would be recovered by substituting it for four or fewer Shuttle flights at some $500 million each.

Thus NASA, for example, could bring about a rapid reduction in launch cost and save money by sponsoring the development of a piloted two-stage launcher. A preliminary business plan on these lines is given in Ref. 3.

6.5.2 Ascender

Spacecab would be a very useful project in its own right, as well as serving as a demonstrator for Spacebus. However, there is no official requirement for a piloted two-stage launcher, and at present its development cost is probably outside the scope of an industrial private venture.

This raises the question of a demonstrator for Spacecab itself. Spacecab Demonstrator is a project described in Ref. 7. It is in effect the Spacecab upper stage scaled down and designed to take off from a runway on its own and to be able to zoom climb to space altitude on a sub-orbital trajectory.

Ascender, fig. 6, is a more recent design and is a simplified Spacecab Demonstrator designed to be able to carry four people to and from the edge of space. It takes off and climbs to about 30,000 ft using jet engines. Its rocket then takes over and Ascender pulls up into a near vertical climb, from which is can coast to 50 miles high. It is a sort of "re-invented X-15", designed for routine operations. The last flight of the X-15 took place thirty years ago, so it seems reasonable that an "everyday" spaceplane of comparable performance should now be feasible.

Figure 6. Ascender

Ascender would be used initially as a reusable sounding rocket for space research and as a spaceplane technology test-bed. After a few years in service it would be certificated for passenger carrying. It would offer about two minutes of microgravity, and would fly high enough for superb views of the Earth and for the sky to turn black with bright stars even in daytime.

Ascender could thereby start an embryonic space tourism business.

Ascender would be designed for fast turnaround at low maintenance cost. The aeroplane closest to Ascender in terms of potential operating cost is probably the Saunders-Roe SR.53 jet plus rocket fighter, which first flew in 1957, fig. 7. The two aeroplanes are of broadly comparable size and weight, and both have a jet engine and a rocket motor. Only two prototypes of the SR.53 were built, and it did not enter service. Had it done so its cost per flight would probably not have been very different from modern jet fighters, which is typically around $10,000. This provides a preliminary target for the cost per flight of Ascender when mature.

The logic of this paper so far has been to describe an attractive long term goal - a mature orbital infrastructure - and to work back to the logical next step from now - a sort of re-invented X-15. Timescales have not yet been addressed. In assessing development times it is preferable to proceed in reverse order - Ascender, Spacecab, Spacebus - , as shown in fig. 8. Each vehicle in the sequence follows naturally from the one before, technically, commercially and politically. It is not suggested that these particular designs are necessarily the ones that will get built. The detailed design process may well lead to significant changes. But they do serve to illustrate possible timescales.

The feasibility of an aeroplane like Ascender is not in doubt. The X-15 flew higher and faster more than thirty years ago. By using higher performance engines now available, Ascender avoids the need for air-launch and is smaller. Using external thermal protection, updated and simplified from that developed for the Shuttle, Ascender has a conventional aluminium airframe. Systems for life support, reaction controls, communication and navigation are all well within the state of the art.

In aerospace history, the timescale from a record-breaking experimental aeroplane to operational deployment of the new capability has usually been less than ten years. Sub-orbital aeroplanes have been neglected since the X-15, and a rapid catching up is possible.

The time to achieve three particular Ascender development milestones - first flight, certification, and maturity - will now be considered.

Ascender is a far less advanced project than the X-33, which was planned to fly three years from go-ahead. Ascender has a maximum speed of Mach 4.4, an all up weight of 8800 lb, and a propellant mass fraction of 54%. The corresponding X-33 values are Mach 15, 273,000 lb and 77%. Thus Ascender could also be flying in three years, given the required funding.

A new airliner requires typically 1000 test flights to obtain its type certificate. If we assume that Ascender requires 2000 flights and that five aeroplanes each make two research flights per week, then certification would take four years.

Thus sub-orbital space tourism could start in about seven years.

To estimate the time to reach maturity there is a useful analogy with jet engine development. The first jet aeroplane to fly was the Heinkel He 178 in 1939. The first operational jet fighters (Gloster Meteor and Messerschmitt Me 262) entered service in 1944. The first jet airliner to enter service was the de Havilland Comet, in 1952. Thus it took just thirteen years from first jet flight to first commercial jet airliner service.

The total number of flying hours to date of rocket motors (in missiles, launchers and rocket powered aeroplanes) is comparable to that of jet engines at a very early stage of development, probably around 1944. The reliability and life of jet engines have improved by three orders of magnitude since then (the time between overhauls of the Junkers Jumo 004 worked up to about eight hours), and it does not seem unreasonable to assume that rocket motors could follow broadly comparable learning curves. By mature aeroplane standards, rocket motors are at an experimental stage of development. Much the same applies to thermal protection systems. Thus it seems probable that mature Ascender operations could be approached within about 15 years.

The reason for the slow maturing of rocket motors to date has been the lack of a strong demand for long life. Their use in aeroplanes has been limited, and there has not yet been a reusable launch vehicle. The longest life high-performance rocket motor to date was the 8000 lb thrust Bristol Siddeley BS 605, used as a take-off booster by the Blackburn Buccaneer bombers of the SAAF. A life of 60 firings between overhauls was claimed.

A prototype of Spacecab could be flying within about five years, and Spacebus a few years later. The maturity gained by Ascender in terms of rocket and thermal protection system life and maintenance cost could be transferred very rapidly to Spacecab and Spacebus. Thus orbital flights could also approach maturity in about the fifteen years mentioned earlier.

Space tourism is likely to provide the largest commercial incentive to drive costs down to their mature level. Whether or not this incentive will be great enough to achieve maturity within the fifteen years that should be technically feasible requires further study.

By today's standards a mature spaceplane is no more than a thought experiment. However, it turns out to be a useful one that adds insight into the way ahead for launcher development. We have seen that sub-orbital space tourism could start in about seven years in a re-invented X-15, given the required funding, and that orbital tourism could follow a few years later. We have also seen that tourism is then likely to become the largest commercial use of space, and will thereby provide a commercial incentive to drive costs down to their mature level, which will in turn lead to a mature orbital infrastructure of benefit to all space users. The cost per person to orbit will be about 1000 times less than it is today.

The result will be a paradigm shift in astronautics, with launchers based on aeroplane technology replacing those based on ballistic missiles. The largest market will be the general public directly rather than as taxpayers. There is perhaps an analogy with the way that aeroplanes largely supplanted balloons early this century, causing a paradigm shift in aeronautics and leading to large scale passenger transportation by air.

These results may seem surprising to many, perhaps less so to those who have studied space tourism. We have become so used to the high risk and cost of human spaceflight, because of the use of launchers relying on ballistic missile technology, that the concept of a mature spaceplane takes some getting used to. When thinking about a new airliner, the maturity standard is taken for granted. When thinking about new launchers, whether reusable or not, the tendency is to assume that its maturity will be that of an expendable vehicle like a ballistic missile, or of an experimental high performance aeroplane like the X-15.

The new factor is a potential market - space tourism - large enough to require a large fleet size and to provide the operating experience and the commercial incentive for the continuous product improvement needed to approach airliner maturity.

It is interesting to note that thirty years ago the concept of a piloted two-stage spaceplane was widely considered to be the logical next step in space transportation. At that time it would have required a major development programme. Surprisingly, now that the technology exists and it would involve a development programme costing about as much as a small airliner, such a project is not even on the mainstream agenda.

The work described in this paper has been at concept level only, using first order sizing and costing methods. Nonetheless, the following conclusions are soundly based:

1. A mature orbital infrastructure is a realistic goal for space policy. The price per person to orbit would be of the order of $10,000. More than one million fare-paying passengers would visit space each year as tourists, requiring a fleet of more than 50 spaceplanes.

2. The enabling project is a mature spaceplane. It will need to be certificated to civil safety standards. To achieve this as soon as possible the spaceplane should be piloted, have two stages separating at supersonic speed, jet and rocket engines on the lower stage, and take off and land horizontally.

3. The prototype of such a spaceplane can be built with existing engines and proven materials. It would provide safe and economical transport to and from space stations, and would greatly reduce the cost of launching small satellites. It could be used for pioneering orbital tourism. Its development cost could be recovered by substituting it for four or fewer Shuttle flights.

4. Given this initial step, the only fundamental developments required are long life, improved reliability, and short turn around time. Quantum leaps in technology are not needed.

5. Once space tourism starts, it is likely soon thereafter to become the largest commercial use of space, and to provide the operating experience and commercial incentive to drive costs down towards their mature level.

6. Such a development would transform nearly all space activities, by slashing the cost of access to orbit

7. The first steps towards such airliner maturity could be gained from a sub-orbital spaceplane, a sort of re-invented X-15, capable of carrying space experiments or a few passengers. It would provide early experience of aeroplane-like space operations.

8. By analogy with jet engine development, maturity could be approached within fifteen years from now.

9. The next step should be a full business plan for the development of space tourism.

10. The sooner that space tourism gets on to the mainstream space agenda, the sooner this transformation of our space activities is likely to be achieved.

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