Single-stage to orbit
From NewMars
A single-stage to orbit vehicle is one that must reach orbit with just one stage. Up to this date, no functional vehicle has been built, only technology demonstrators.
The idea behind SSTO was to reduce high costs by building a large reusable vehicle, thus reducing the complexity of the vehicle and its maintenance. But this has a big disadvantage: the vehicle needs to lift its entire structure into space, a problem that multi-staged rockets don't have. But, with the advances in composite materials, building a fully functional SSTO seems feasible.
Because of the way it is built, a paradox appears: contrary to most rockets, for a SSTO a dense fuel (kerosene or propane for example) it's more performant than liquid hydrogen. This happens because dense fuels can be stored at room temperature (no need for insulation), and have a higher density. Thus, the mass of the fuel itself might be high enough compared to the mass of the tank and insulation to make it more efficient.
The efficiency of the fuel being critical for a SSTO, it can be further increased by using a better oxidizer, like ozone (O3) or tetraoxygen (O4).
A problem with a SSTO vehicle is that it needs to use a single engine for all altitudes. The engine must have different bell shapes depending on altitude, otherwise it won't have high efficiency. Multi-staged launch vehicles don't have this problem because the bell shape of the engines on each stage is adapted to the altitude they work in. The current solution to this problem seems to be using an aerospike engine.
Prototypes and demonstrators:
- JAXA's on-going experiments with a sub-scale SSTO craft, the Reusable Test Vehicule or RVT
- McDonnell Douglas DC-X - cancelled
- Lockheed Martin X-33 - cancelled
- Roton - cancelled
SSTO on Mars
For future pioneers of Mars, using a SSTO for orbital access is probably the best design. Such a vehicle would be fully reusable, easy to maintain and operate. Because of the lower Marsian gravity, it should be more efficient than on Earth. Also, because of the much lower atmospheric pressure on Mars, it should be easier to design an efficient bell shape for the engine.
Methane (a denser fuel than hydrogen) could be easily produced by Sabatier reaction.
