Single Stage to Orbit
A Single-Stage-to-Orbit launcher is a launch system which reaches at least low earth orbit with only a single stage. This definition does not even allow booster stages, which are called one-and-a-half stage launchers (eg. Atlas, R7). In Orbiter the Delta-glider is an example of an SSTO vessel.
- Phil Bono Designs
Current SSTO designs
Why is there no SSTO?
Regardless of the amount of historic SSTO designs, no SSTO design ever reached production stage. The reasons for this are not only economic in nature; there are also substantial engineering problems that must be solved:
- Fuel tanks - The biggest problem for all SSTO designs lies in the ratio between launch mass and payload mass. In the rocket equation, this parameter is part of the mass ratio. Because SSTOs can't drop construction mass during launch, they have to carry the large fuel tanks into orbit, even when they are almost empty. While multistage launchers can simply drop a stage and reduce the construction mass during ascent, a single-stage rocket has to accelerate the whole structure into orbit.
- Engine performance - The engines on an SSTO have to work from launch (at ambient pressure) until reaching orbit in a vacuum. While multistage rockets simply use optimized engines for each phase of the ascent, SSTO designs rely often on complex and often unrealistic engine designs.
- Recovery - An SSTO launcher only makes economic sense when it can be reused. However, making the system reusable increases structure and fuel mass (in other words, total launch mass).
Many past SSTO designs have been multimillion kilogram vehicles, standing taller than the Saturn V.
EADS Bremen is currently developing a Single-Stage-to-Suborbital launch vehicle with a reuseable winged first stage. This approach needs significantly less fuel for ascent and landing. Similar approaches had been pursued by the USA until the mid nineties, but were discontinued as a result of budget cuts.