Post by Earl Colby Pottingerhttp://media.armadilloaerospace.com/2004_06_15/perfectBoostedHop.mpg
An actual pressure fed rocket with almost no moving parts doing a
powered landing. It is my considered opinion that this is The Right
Way To Do It. Build a big, simple booster that lofts a high
performance upper stage all the way out of the atmosphere, then
returns to land on the same pad it took off from. At first flance it
sounds like an inefficient staging strategy, since the upper stage
requires nearly SSTO dV, but removing the requirment of boosting
through the atmosphere (optimize only for vaccuum boost and reentry)
does still simplify the problem quite a bit, and the operational and
testing aspects are great.
Not an especially bad idea. Of especially consideration
are, on the plus side, greatly eased aerodynamic
considerations on the launch vehicle as well as perfomance
improvement on of the near-SSTO main engine in vacuum.
On the minus side though there are large gravity losses,
vehicle sizing issues, and characteristic minimum boost
times to deal with. Which make up for some of the
disadvantages. At the minimum it's a workable interim
solution that makes progression toward lean, mean, SSTO
launching machines slightly easier. I guess this would be
a "bigger dumber stage" concept.
Let me expand on some of the issues here. First is
aerodynamics, modern launch vehicles travel at supersonic
and even hypersonic speeds during launch. This places
constraints on payloads and especially payload packaging,
which can steal from raw payload capability by requiring
strong payload attachments and aerodynamic fairings, which
take up mass. This also brings into play a great deal of
vibrational energy, which the payload has to survive once
and only once to get to orbit but never after. Which
requires designing and testing the spacecraft carefully
to make sure it will survive a launch, things that usually
do not come free, or even cheap. However, a "pop-up and
boost" launch vehicle would have a much different flight
profile. Theoretically the portion of the pop-up flight
within the atmosphere could be at rather unimpressive
speeds, such as low mach numbers, or even sub-sonic. At
mach 1 it takes only a few minutes to climb out of the bulk
of the atmosphere. Raw gee forces will almost necessarily
still be a concern, as the pop-up booster still has to
avoid throwing away all its fuel to gravity and the near
SSTO still has to climb into orbit before reentering.
Nevertheless, steady gee forces are much, much easier to
deal with mechanically than vibrations and lurches, and
with liquid propulsion systems on all stages it's possible
for a pop-up booster to give satellites or other cargo an
incredibly gentle ride. This is no small issue, because
engineering fragile spacecraft to be able to survive the
rigors of launch is a huge cost sink, this could
potentially make low-cost satellites much more feasible.
Working only in low pressure or vacuum can provide
substantial performance improvements for a rocket engine.
Roughly about a 10% improvement in Isp for LOX/Kero
engines and up to a 25% improvement in Isp for LOX/LH2
engines. For Kerosene engines, this is enough of an
improvement to bring the delta V/Isp side of the equation
into overlap with the achievable dry mass fraction side,
with a bit of margin as well. Additionally, depending on
how much time the pop-up booster gives the launcher to get
into orbit the necessary thrust to weight ratios could be
lowered as well, allowing for lower engine weight and
easing some throttling issues (if you start off with a
thrust/weight of less than 1 gee then the inevitable
problems of high gees or very deep throttling in a LOX/Kero
booster near burnout become much less of a concern).
With those two factors added together, if we *had* a bigger,
dumber, pop-up booster now we could quite easily put SSTO
capable vehicles on them using very prosaic engineering
(60s vintage). With a few tweaks (such as modern
electronics, more finely honed structures, modern alloys,
and composite structures) we could almost certainly build in
enough margin to add the bits that would make it reusible
(like TPS).
The downsides are worth considering though. First, gravity
losses are going to be big, and pretty much all the
propellant used by the pop-up booster will at least appear
to be entirely wasted in terms of getting to orbit. The
good news is that this is a propellant issue, and propellant
is dirt cheap compared to orbital launch costs, so it's a
non-issue with respect to operations today, but won't be
competitive in the long term when ground launched SSTOs
can operate at low multiples of propellant cost (may we some
day be blessed with such a "problem"). Second, the real
concern is the sheer size of the booster. SSTOs are almost
inevitably rather bulky with high GLOWs. The pop-up booster
has to be big enough to take that beast a couple hundred km
straight up. And that means it has to be absolutely
gargantuan. The thrust on it ought to be quite impressive.
The good news though is that it can be made very low tech,
and metal, or even concrete, and propellants are cheap.
As a concept design, imagine something along the lines of
a Saturn-V first stage with a fixed payload bay and crew
compartment(s) above the tanks, as well as TPS and landing
systems and such like here and there, with slightly fewer,
lower thrust engines. Now imagine this connected to the
mother of all brick sh*t-houses with a small horde of the
most powerful engines in the world on the underside. I'd
imagine the pop-up booster on this beast would have a
"payload bay / shroud" more similar to an aircraft hangar
than anything else. It would be impressive at the very
least.