I've been simulating single stage to orbit (SSTO) launch to low earth
orbit (LEO) in orbiter space flight simulator for a little while now.
Consider your basic space shuttle main engine (SSME) powered single
stage to orbit (SSTO) rocket. Hydrogen is the most powerful chemical
rocket fuel known (excluding exotics). To reach low earth orbit with a
cryogenic fuel of this nature, a mass ratio of 10 to 1 is required (the
10/1 rule). That is 10 parts of fuel and oxidizer to one part rocket.
After accelerating to a stable orbit roughly 1% of the fuel is remaining
(more or less, depending upon the ascent trajectory profile, the launch
latitude, and final orbit inclination and altitude). That's roughly 100
parts gross liftoff weight to 1 part residual fuel (the 100/1 rule) or
10 parts empty weight to 1 part residual fuel (yet another 10/1 rule).
Thus the usable payload delivered to an orbital station or spaceport is
roughly 1% of the gross liftoff weight, and 10% of vehicle empty weight.
In this case this is fuel which can be immediately converted into energy
and water (via a fuel cell), and water that can then be reconverted back
(using solar energy) into propellant and oxygen. This isn't very much.
However that's the reality of climbing the gravity well of Planet Earth.
In order to increase this payload, the obvious solution is converting
the rocket itself into payload. In this scheme the engine is removed
from the vehicle (roughly 20 percent of empty weight) and returned to
Earth in a cleverly designed nose cone engine carrier, and the tankage,
the oxygen, hydrogen, pressurization, residual fuel tanks and the RCS -
reaction control system, is then immediately pressed into service as
payload for infrastructure in constructing the space station or orbital
spaceport itself. Thus, the usable payload fraction is then increased by
a factor of seven (7) or so, dependent upon the amount of equipment or
infrastructure necessary to successfully reenter and recover a seven
thousand pound space shuttle main engine (SSME) from low earth orbit.
For a reasonably designed single space shuttle main engine powered
single stage to orbit launch vehicle, this represents 3500 pounds of
residual fuel and 25,000 pounds of infrastructure. This is not trivial.
Plus, you get the engine back. Adding some GEM-60s improves the numbers.
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