Trouvé dans FPSPACE : pourquoi un étage unique est beaucoup moins probable qu'une fusée à deux étages:
The problem with SSTO in pure rocket mode is that the payload is only 1% of the takeoff weight, so if there are any weight problems (ice on the outside of cryotanks etc.), it won't leave the ground or will fail to achieve orbit. Air breathing SSTO concepts (aerobic = air breathing) are based on the fact that even though LOX makes up 70% of the weight of the Space Shuttle, and 50% of the Shuttles weight is the LOX used to get to Mach 3 and 100,000ft, so why not use the atmospheric air to get to Mach 3 and 100,000ft and save 50% of the takeoff weight? It turns out that the air breathing engine would have to have a thrust to weight ratio AT 100,000ft of 50:1, which if you read the degradation in thrust specs in the back of your SR-71 book [dear fpspace readers, he is aware of my rather extensive collection of books relating to this vehicle and other hypersonic and near-hypersonic developments], you will find the engines drop from 50,000LB thrust at sea level to about 2000LB at 75,000ft.
So at sea level you would need an engine with over 1000:1 thrust to weight to have 50:1 at 100,000ft and no one has ever built any engine with a thrust to weight greater than 200:1 (the LOX/Flourine engine if memory serves). Now if you go to TSTO, contrary to what some people have said, they have already been built in pure rocket mode (ATLAS rockets, allegedly a stage and a half, really a TSTO; in the early days they really did put the booster in orbit) and the air breathing versions of TSTO look very feasible.
An alternative technology, Liquid Air Cycle Engines, and the variations where the air has the nitrogen separated out and dumped overboard keeping only the LOX, have a long history of attempted development, but even recently declassified documents appear to only refer to an implementation as a TSTO, not an SSTO. The breakthrough everybody keeps refering to is either a fuel with an energy density greater than hydrogen (nitrogen compounds with more than 5 nitrogens per molecule?), or a way to beam energy, or my favorite, nuclear isomer propulsion.
There is an operational launch vehicle which has an
air-breathing first stage, the Pegasus. While in theory the concept
is sound a combination of the relatively small amount of payload it
puts into orbit, the fixed operating costs, and marketing decisions
has resulted in the Pegasus becoming the most expensive launch
vehicle in the world in terms of dollars per pound of payload.
