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Space Future has been on something of a hiatus of late. With the concept of Space Tourism steadily increasing in acceptance, and the advances of commercial space, much of our purpose could be said to be achieved. But this industry is still nascent, and there's much to do. this space.
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G Hudson, August 1989, "Phoenix M: A Small SSTO Launch Vehicle for Commercial Space Transport Missions", August 1989.
Also downloadable from m a small ssto launch vehicle for commercial space transport missions.shtml

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Phoenix M: A Small SSTO Launch Vehicle for Commercial Space Transport Missions
A lox-hydrogen single-stage-to-orbit launch vehicle which is fully reusable offers the lowest possible cost per flight of any chemical rocket technology, including conceptual air breathing systems. We propose the Phoenix type of SSTO for a second-generation commercial space mission launcher.
The Phoenix M Space Transportation System

The Phoenix M vehicle system is a fully reusable, vertically-launched and vertically-recovered single-stage-to-orbit space ( VTOL SSTO) transport system. More than a launch vehicle, it can be refueled on orbit by other vehicles of its class to provide an orbit-to-orbit capability, and it can act as temporary on-orbit spacecraft bus. The propellants are liquid oxygen and hydrogen, and the structure is composite. The Phoenix M can be launched by a crew of fewer than a five people in less than two hours from the hanger to orbit. Cost per pound in low earth polar orbit will be well under $1 million, using modern aircraft costing. Unit cost of the vehicle will be comparable to modern commercial or military transport aircraft on a per pound basis.

How is this possible? We believe the technology for the Phoenix M is largely in hand. By the use of a few innovations, detailed below, the SSTO, once considered a difficult to achieve concept, can become virtually an accomplished fact.

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. We have discovered two new ways by which the sensitivity of the SSTO can be reduced.

  • First, we chose to employ a rocket engine which we have designated the Aeroplug . This engine, when properly designed, can be lighter than conventional rocket engines (In part because it does not carry the weight of the nozzle) and can provide more performance during the ascent to orbit. At the same time it operates 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 uses all composite airframe and tank structure, to be built by Scaled Composites, Inc. ( Burt Rutan). This innovation permits 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 permits a transport system to be fielded which can place several thousand pounds Into orbit for under 155K pounds launch weight. Further, the empty weight of the Phoenix M is under 10,000 pounds. Since the cost of a project is generally assumed to be proportional to the empty weight of the vehicle, there is an obvious advantage to be gained by developing a small vehicle. Operationally, the vertical launch feature requires very minimal facilities. The vertical landing feature of the Phoenix M permits compact launch sites and emergency landing without long runways.

Keys to Feasibility

Three major challenges face the SSTO designer: propulsion, weight and thermal protection. In addition, for vertically launched and recovered SSTOs, the landing mechanism needs to be demonstrated.


Single-stage orbital flight requires high performance from lox-hydrogen rocket engines. One of the highest performing engine systems available is the Aeroplug (also known as the aerospike or plug nozzle) which we propose to use on all Phoenix launch vehicles. This type of engine integrates with the SSTO VTOL concept.

Subscale tests of this engine design can 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 have 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 is 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, we have 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.

Reference 1 discusses a "though experiment" which makes an SSTO out of a proven piece of flight hardware. In summary, we show that a Saturn V third stage, with a Shuttle engine in place of the standard J-2 rocket engine, can attain orbit as a single-stage with a payload of a few tons. This may 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.

We have found a means by which we can adapt the conventional composite structure of modern small aircraft to the airframe requirements of the Phoenix M. This significant innovation will dramatically shorten the development time of the vehicle.

Thermal Protection.

One novel feature of the Phoenix M concept is 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, has the attribute of permitting low-cost conventional aircraft-style construction throughout the Phoenix M. In addition, the concept allows all-weather launch and landing operations, which are 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 should become routine and reliable. Computer simulation of the approach and landing will be necessary to address critic's concerns.


The final requirement for a commercial launch system is 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 has been adopted by the SDIO Single-Stage-to-Orbit program. Combining this method of development, test and operation with low-cost fabrication technology pioneered by Butt Rutan will result in a dramatically reduced cost to produce and operate the vehicle.

A Test Program

PacAm is planning an internal effort to address each of the issues above. First, the propulsion system of the vehicle will be modeled and tested In the wind tunnel using cold flow. Second, a detailed vehicle design will be done to baseline the structural and subsystem masses. Third, the landing system will be demonstrated by flight of a full-scale mockup. Finally, a full dynamic and control simulation from launch to landing will be performed.

The Development Program

We estimate 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.

  1. Gary C Hudson, 1987-1990, 'A Single-Stage-to-Orbit Thought Experiment'
G Hudson, August 1989, "Phoenix M: A Small SSTO Launch Vehicle for Commercial Space Transport Missions", August 1989.
Also downloadable from m a small ssto launch vehicle for commercial space transport missions.shtml

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