On February 15, 1989 Lt. Gen. Daniel O. Graham, Max Hunter, and Dr. Jerry Pournelle on behalf of this Council recommended to the National Space Council that the United States begin work on a reusable Single Stage to Orbit (SSTO) spacecraft. At that time we recommended that the new SSTO begin the revival of the "X craft" concept. Our recommendation was for a program of under one billion ($1 billion) dollars total to be completed in four years or fewer to result in a savable reusable experimental spaceship. It was our recommendation that SSTO technology would vastly reduce the expense and operational complexity of reaching space, and thus lead to the development of spacecraft that will assure the United States of routine and economical access to space.
This Council has in every one of its meetings and reports from 1980 to present concluded that routine and economic access to space will bring the nation economic, scientific, and national security benefits of great value. For a number of reasons this position is rapidly becoming non-controversial. Most professional analyses now accept the commercial, scientific, and military value of economic and routine access to space and that Single Stage to Orbit spaceships are the best way to achieve it. There naturally remains the policy question of how best to develop SSTO capability.
This Council believes that immediate full-scale development of an operational SSTO transport as a major Federal budget line-item would be premature and likely disastrous. While we are convinced that SSTO is the proper approach, we have too little operational experience with SSTO ships for an optimum design. However, further "studies" would add little or nothing to our knowledge base. We do not need "studies". We need new data on SSTO operations, and practical experience with flying fully reusable rocket ships. We are well past the point of learning much from computer simulations. We need to fly something.
Consequently this Council recommends design, construction, and operation of an X-rocket. Our original concept of the SSX, SpaceShip Experimental, now incorporating what has been learned from the design, construction, and flight tests of the DC/X, should be used as a design starting point for a new SSTO X rocket. The design objective should be for the minimum ship capable of achieving orbit and return to Earth.
The X-rocket approach involves the rapid development, construction, and test of experimental flight vehicles in order to gain operational experience and advance the practical usable-in-the-field level of technology. Under the X concept you do the best you can with what you have, rather than waiting for new technology; but you also design for the possible retrofit of new technological developments if they arrive in time.
The rapid pace and flight-vehicle focus of "X" aircraft programs keeps lab bench exercises and internal memos from consuming all resources, and forces the developers to integrate the needed technologies together in a flight vehicle environment. X-aircraft do not have payload or other operational mission requirements imposed, nor are they engineered for mass production. "X" programs by their nature have to be fast and cheap; too much time or money and they tend to bog down in open-ended pursuit of perfection, rather than quickly achieving a significant step toward the ultimate goal. It cannot too often be repeated that X vehicles are not prototypes.
Typical "X" programs take three to four years, and cost in the low hundreds of millions. Because they are not prototypes there is and should be no inherent commitment to a production vehicle followon. Peak funding tends to be late in the second or early in the third year of an X-program as major vehicle components are built and paid for. Peak funding level tends to be 40%-50% of the overall cost of the X program.
Two aircraft X-program examples: The X-29 program cost $340 million and successfully demonstrated forward-swept wings in a fighter-type aircraft. This technology is now available when required by either military or commercial aircraft designers, but there has been no commitment to produce such a fighter. The X-31 program has cost $170 million so far and has demonstrated engine thrust vectoring for extremely low-speed maneuvering, also without any commitment to a production followon.
One rocket X-program example: The DC-X program cost $70 million and demonstrated vertical takeoff and landing of a reusable rocket, plus rapid servicing and reflight by a small ground crew. Moreover, in an unplanned experiment the ship demonstrated recovery from engine failure on takeoff.
This Council recommends continued operation of the DC/X, including retrofit of the tankage with modern lightweight fuel and oxidizer tanks.
We further recommend timely design, construction, and operation of a new Spaceship Experimental. This ship should have a narrow closed-end mission: To fly as high and fast as possible with existing engines while allowing retrofit with upgrades thereof. It should incorporate no fewer than 8 nor more than 16 engines. We recommend the initial design be for the RL-10 engines suitably modified for altitude compensation. The RL-10 has proven reliability and reusability. The purpose of the SSX-2 should be to gain operational experience in flying and servicing single-stage reusable spacecraft; to gain operational and engineering experience in multiple engine reusable rockets; and to mature the technologies needed for single stage to orbit flight, particularly lightweight reusable cryogenic fuel tanks and lightweight low-maintenance reusable thermal protection.
Experience in design, management, and operation of multi-engine reusable rockets in systems without excessive overhead weights for the rocket fuel and control systems is a vital part of this program.
A parallel program should develop an upgraded RL-10 engine with a goal of about 1250 PSI chamber pressure. This engine should be in a configuration that allows retrofit into the SSX-2.
SSX-2 should be designed to achieve orbit. It may be found that details of construction, particularly requirements for high structure safety factors, preclude full orbital capability, particularly with the original RL-10 engines. This is acceptable, but the design configuration should not make orbital flight impossible. One purpose of X programs is to build ships which can be modified through experience. This is known as "over build then nickel and dime up the performance."
We recommend funding at levels of approximately $100 million a year for no more than five years. We do not recommend substantially higher funding. Over funding the program will be as disastrous as under funding. One purpose of reviving the X programs is to gain experience in developing sophisticated new technologies on time and in budget.
This Council supports development of a complementary NASA X-rocket program that would concentrate on advanced engine technology. The Council notes that NASA's plans are at the moment somewhat less advanced than DOD's, and thus peak funding for NASA's complementary X-rocket would occur a year or two after that for DOD's SSX-2, allowing parallel development for both at what would still be overall a modest level of funding.
The real danger to SSTO is not technological, but organizational. X programs must be kept small and lean, and while they must be adequately funded they must not be over funded. Too much money destroys technological development as surely as too little money. The ideal X project would be carried out by a small management team working at a remote base such as Edwards Air Force Base under a program office that has a limited life span. It is important to the future health of this nation that we once again learn how to use X programs as springboards to development of both military and commercial technology, and that we have organizations with successful X program experience. We believe that X programs may be the single most important way that government can invest in future technologies vital to the future of this nation.
Completion of X program development of an experimental Single Stage to Orbit vehicle will allow the potential user communities, both government and commercial, to make intelligent economic decisions about design and procurement of new space vehicles. This Council does not believe that SSTO technologies are mature enough to justify any large multi-billion dollar program. We recommend a small and manageable X program to rapidly and cheaply mature SSTO technology. The goal ought to be an experimental ship that can go to orbit and return twice in a single day.
Optimum Design: In particular, the amount of payload for an SSTO depends on ship size and mass fraction. For technical reasons calculated payloads are in the order of 1% to 4% of the Gross Liftoff Weight (GLOW) of the ship; that is, in a 500,000 pound ship the payload will be between 5,000 and 20,000 pounds. A technical discussion of these matters can be found in Jerry E. Pournelle, "The SSX Concept," Journal of Practical Applications in Space, Volume IV #4 Summer 1993.
Need to Fly: There is also a practical shortage of young aerospace engineers who have participated in building flying hardware. This could be serious in years to come.
X-Rocket: X concepts were presented and discussed at a session of the annual meeting of the American Association for the Advancement of Science, February 19, 1994.
Flight Tests: For a discussion of what has and has not been learned from the DC/X program, see attached Council Report of February 16, 1994
Failure on Takeoff: One major value of X programs is that things happen that you didn’t expect. No one would have planned the near failure of one of the DC/X engines on takeoff (at least not early in the test program) but it happened. Two lessons were learned: that the ship was able to recover with the existing software and engines, and how to avoid that particular accident in future.
For technical analysis of SSX click here.