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Those with good memories will recall that some SSTO supporters have been
advocating use of dense propellant combinations like H2O2/kerosene or
LOX/propane, instead of the orthodox LOX/hydrogen. The Isp is lower, and
hence you need a higher mass ratio, but the much greater density makes the
mass ratio easier to achieve. When examined with sophisticated scaling
models, rather than mindless "fixed fraction of propellant mass" ones, the
dry mass goes
down — despite an increase in gross mass — because the
vehicle is smaller.
Well, Mitch Burnside Clapp has done it again.
He's found a big flaw
in a major assumption of the standard argument, and now dense fuels look
even better.
The incorrect assumption is that the total delta-V to reach orbit is the
same for all fuels. It's not. Dense fuels need less. Substantially less.
Consider two SSTOs, one LOX/LH2 and one H2O2/kerosene (I like LOX/propane
myself, but H2O2/kerosene is Mitch's favorite, and it's his discovery...),
with the same GLOM (gross liftoff mass), the same engine thrust (and so
same initial acceleration), and no requirement for G-limiting. Draw a
graph of mass vs. time for both.
Assume for the moment that they have the same total burn time. The curves
(well, lines) start from the same point. The H2O2/kerosene one has to get
rid of more mass, so to reach its final mass in the same amount of time,
the slope of its mass line must be steeper.
Wait a minute. A steeper mass line means that at any time after liftoff,
the H2O2/kerosene SSTO has lower mass than the LOX/LH2 one, and since they
have the same thrust... the H2O2/kerosene SSTO is accelerating faster. If
they have the same total delta-V requirement, that last assumption must be
wrong: the H2O2/kerosene burn time is shorter.
But... the biggest penalty on top of the theoretical delta-V is gravity
losses, and gravity losses are a function of burn time! The H2O2/kerosene
SSTO is accelerating faster, so it has lower gravity losses, and needs
less total delta-V. Moreover, that makes its burn time still shorter, and
its mass line still steeper, so the difference in acceleration is even
larger than it first seems.
Adding G-limiting, which is a practical necessity, changes the details
but not the overall result: the dense-propellant SSTO loses mass faster,
accelerates faster before G-limiting, and so has lower gravity losses.
The bottom line, when all this converges — including a small gain from
lower drag on a more compact vehicle, and a very small bonus from lower
drag making the acceleration still higher — is that a standard orthodox
NASA LOX/LH2 SSTO needs 31000ft/s to reach the space-station orbit, and an
H2O2/kerosene SSTO needs only 29050ft/s.
(In fact, the explanation came after the numbers — when good trajectory
simulations kept coming out with lower delta-Vs for H2O2/kerosene, Mitch
decided he had to understand what was going on.)
Now, consider. The H2O2/kerosene SSTO is operating in a very steep part
of the mass-ratio curve. A 6% saving in delta-V is
not trivial. For
engines with a vacuum Isp of 320, the required mass ratio drops from 20 to
16. Given the aforementioned sophisticated scaling models, at this mass
ratio, the H2O2/kerosene SSTO's payload at the same GLOM is now equal to
that of the LOX/LH2 design.
So the dense-fuel SSTO has lower dry mass, smaller vehicle size, cheaper
and easier-to-handle propellants, and now suffers no GLOM penalty... Just
what was the advantage of LOX/LH2 supposed to be again?