Single-stage to orbit
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.
Hydrogen Peroxide has been proposed as an oxidiser for SSTOs, due to it's storability.
This is the classic combination suggested for SSTOs, due to it's high specific impulse. However, it suffers from issues with storage (it is a low temperature cryogen), requiring heavy storage tanks. Compounding this problem is it's low density, meaning the tanks have to be much larger to contain the same mass of fuel as a denser fuel suchj as Kerosene or Propane.
The lightest of the Hydrocarbons, Methane possesses certain advantages over Hydrogen which make it an attractive fuel choice. It is much denser, meaning tank size (and so weight) can be lower. Due to it's similar boiling point to LOX, it does not need to be as insulated, eliminating the need for a bulkhead between the Methane and Oxygen tanks.
Propane is a denser fuel than Methane, reducing tank mass further. It's melting point is near to the boiling point of LOX, so they can be stored near the same temperature. This has the advantage of improving the density of the Propane.
Aluminum/LOX monopropellent has been proposed as a potential fuel for an SSTO. While having a low Isp (285s), it has an exceptionally high density, of 1.69g/cc. Due to it being a monopropellent, the operation is simplified, improving safety and reliability.
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
- ISROs Avatar spaceplane - in development
SSTO on other planetary bodies
The Lunar Ascent Module used during the Apollo missions was an expendable SSTO, as it could take off from the Luna surface and reach orbit within a single stage.
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. It may be possible for an early colony to fabricate an SSTO themselves, although without the payload ratios one imported from Terra would have, but probably still enough for the needs of the colony.
Methane (a denser fuel than hydrogen) could be easily produced by Sabatier reaction.