Type: space transportation system
Significant date: 1989
The Pacific American Launch Systems Phoenix M vehicle system was a fully reusable, vertically-launched and vertically-recovered single-stage-to-orbit space (VTOL SSTO) transport system. More than a launch vehicle, it could be refueled on orbit by other vehicles of its class to provide an orbit-to-orbit capability, and it could act as temporary on-orbit spacecraft bus. The propellants were liquid oxygen and hydrogen, and the structure was composite. The Phoenix M could be launched by a crew of fewer than a five people in less than two hours from the hangar to orbit. Cost per pound in low earth polar orbit was to be well under $1 million, using modern aircraft costing. Unit cost of the vehicle would be comparable to modern commercial or military transport aircraft on a per pound basis.
SSTO vehicles require lightweight structures (90% or more of their takeoff mass must be propellant) and high performance engines. Less than optimal performance in either of these two areas can reduce or even eliminate the payload of the vehicle. The Pacific American team had discovered two new ways by which the sensitivity of the SSTO could be reduced, and the SSTO, once considered a difficult to achieve concept, suddenly seemed to become virtually an accomplished fact.
First, they chose to employ a rocket engine designated the Aeroplug. This engine, when properly designed, could be lighter than conventional rocket engines (In part because it did not carry the weight of the nozzle) and could provide more performance during the ascent to orbit. At the same time it operated at a chamber pressure one sixth that of the Space Shuttle engine, significantly reducing cost and development time while Increasing operating life and safety. Second, the Phoenix M used all composite airframe and tank structure, to be built by Scaled Composites, Inc. This innovation allowed the production of a smaller vehicle than was thought to be possible to build heretofore, thus reducing development risk.
Combining these ideas with a simple vertically launched (no wings) vehicle permitted a transport system to be fielded which could place several thousand pounds into orbit for under 155 thousand pounds launch weight. Further, the empty weight of the Phoenix M was under 10,000 pounds. Since the cost of a project is generally assumed to be proportional to the empty weight of the vehicle, there was an obvious advantage to be gained by developing a small vehicle. Operationally, the vertical launch feature required very minimal facilities. The vertical landing feature of the Phoenix M permitted compact launch sites and emergency landing without long runways.
Three major challenges faced the SSTO designer: propulsion, weight and thermal protection. In addition, for vertically launched and recovered SSTOs, the landing mechanism needed to be demonstrated. Single-stage orbital flight requires high performance from lox-hydrogen rocket engines. One of the highest performing engine systems available was the Aeroplug (also known as the Aerospike or plug nozzle) which was proposed for use on all Phoenix launch vehicles. This type of engine integrated with the SSTO VTOL concept. Subscale tests of this engine design could be conducted using cold-gas flow in wind tunnels, as well as with hot gas from a monopropellant gas generator. A number of sub and full-scale tests of the general class of engines had already been performed during the 1960s and early 1970s.
A high ratio of propellant mass to gross loaded mass less payload, which is known as the "mass fraction" of the launch vehicle, is important for an SSTO to be a success. In the absence of actually building and flying the vehicle, the only ways to determine mass fraction and thus empty weight were to be through a very detailed hardware design or by the use of analogy to previous flight hardware. Parametric weight estimates must not be used due to the uncertairrty associated with their application.
Generally, program developers found that a realistic mass fraction figure for modern SSTO VTOL rockets lies between 0.88 and 0.91. This produces "growth factors" (the ratio between gross liftoff mass and payload) In the area of 35 to 80. Thus, for a 2,000 pound payload, the gross weight at liftoff should be about 150,000 pounds. Also, the smaller the vehicle is, the higher the proportion of empty weight is taken up by items such as avionics, which do not scale between larger to smaller vehicles (In contrast to other weights such as engines and tanks). Therefore, the smaller an SSTO is, the higher the growth factor becomes, until the payload disappears.
Studies demonstrated that a Saturn V third stage, with a Shuttle engine in place of the standard J-2 rocket engine, could attain orbit as a single-stage with a payload of a few tons. This could be accomplished without the use of advanced structures, propulsion or electronics. Straightforward application of modern technology in these three areas would produce an SSTO which could re-enter, land, and be reused. Engineers found a means by which they could adapt the conventional composite structure of modern small aircraft to the airframe requirements of the Phoenix M. This significant innovation was to dramatically shorten the development time of the vehicle.
One novel feature of the Phoenix M concept was the use of active cooling to protect the vehicle's composite structure from the heat generated during re-entry. This technique, which is weight competitive with other advanced thermal protection schemes, had the attribute of permitting low-cost conventional aircraft-style construction throughout the Phoenix M. In addition, the concept allowed all-weather launch and landing operations, which were not possible with most ceramic-based thermal protection systems.
The use of rocket braking for landing an SSTO vertically often excites concern. In reality, the control problem is no different that that faced during a vertical launch of any rocket. (Small SSTOs might employ gliding parachutes for initial recovery efforts.) Through the use of redundancy in the Phoenix M, the landing would become routine and reliable. Computer simulation of the approach and landing would be necessary to address critic's concerns.
The final requirement for a commercial launch system was aircraft-style intact abort capability. Simply sated, this requires the vehicle to terminate flight at any time during the launch and still recover the vehicle and the payload undamaged. This approach also contributes to greatly reduced development costs, and was adopted by the SDIO Single-Stage-to-Orbit program. Combining this method of development, test and operation with low-cost fabrication technology pioneered by Burt Rutan would result in a dramatically reduced cost to produce and operate the vehicle.
PacAm planned an internal effort to address each of the issues above. First, the propulsion system of the vehicle would be modeled and tested In the wind tunnel using cold flow. Second, a detailed vehicle design would be done to baseline the structural and subsystem masses. Third, the landing system would be demonstrated by flight of a full-scale mockup. Finally, a full dynamic and control simulation from launch to landing would be performed. They estimated that the full development effort would cost about $25 million, and require three years. The effort would produce two vehicles and would certify the vehicle for flight under accepted government and Industry rules.
The Phoenix M sounded like an incredibly promising program, yet it never went past the early stages of development. However, its bold concept and innovative design philosophy went to good use in other VTOL SSTO programs that Rutan's team worked on after that: the McDonnell Douglas DC-X, and especially the Rotary Rocket Roton, developed by the same Gary C. Hudson that proposed the Phoenix M.
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