From: Doug Jones (djonesxcor@qnet.com)
Date: Sat Mar 17 2001 - 15:56:45 MST
Spike Jones wrote:
>
> > Spike Jones wrote:
> > >
> > > So now assume yourself a rocket designer. You are
> > > given all the same constraints as the rocket designers
> > > had in 65, with the exception that you need not return
> > > payloads...Now what does your craft look like? spike
> >
> > Michael Lorrey wrote: Dyna-Soar, Hermes, HL-20, etc.
>
> Heres where Im going with this. Using modern technology,
> if you had to design a craft with all the same constraints they
> had back in 1965, *you would end up with something that
> looks a lot like the space shuttle*. Beats me how the heck
> else you would return big payloads other than a big glider
> with rocket engines.
The biggest difference that was considered at the time was integral vs
expendable propellant tanks. Had they designed a true RLV instead of
the desperate kludge that the shuttle was forced to be by arbitrary
spending caps, the orbiter would have been far larger, but with a lower
area loading on reentry due to the bulk of the empty tanks. This would
have simplified the thermal protection system and lowered the landing
speed dramatically. If it were a true two-stage vehicle, the orbiter
engines could have been optimized for vacuum operation, and the better
performance of both stages would have *reduced* the dry mass of the
system for an equivalent payload.
> If you had to return just the really expensive bits {motors
> and meat}then you might do better, but consider this: since
> a good engineer designs around standard parts, the shuttle
> designers used the main engines already developed for the
> Saturn V first stage. The SatV had 5, the shuttle has 3, but
> they were already developed, so they used em.
Nope, the SSME was a new design- and it was clearly the second-best
design, by the second-best team. Rocketdyne had better clout and
connections than Pratt & Whitney, and the standard rumor is that the
SSME was North American's reward for taking the fall for the Apollo 1
fire. The SSME is another engineering horror story in itself...
> By the same reasoning, if we were to develop a new
> heavy lift reusable, we would likely do the math and recognize
> that we have an external tank, already developed and being
> mass produced, economies of scale, so we can buy these things
> cheeeeapy cheap {as space hardware goes} 33 million bucks
> each. Furthermore, when you actually do the math, the
> strap on solids make sense too, for all the same reasons:
> they are off-the-shelf parts now. Good chance it would
> be cheaper to go ahead and throw away an external
> tank than pay the price to have a totally reusable vehicle.
$33 million for a propellant tank is obscene. For comparison, brand-new
Boeing 737-700 of similar mass *fully equipped* is only $43 to $51
million depending on options. As for the boosters- a pressure-fed
liquid can be developed for a fraction of the cost of purchasing ten
sets of solids, would be safer, higher performance, more reliable,
equally reusable, give the payload a FAR smoother ride (shuttle payloads
must be designed to take 55 gees RMS due to the damn solids), and not
release hundreds of tons of HCL per flight- important if flight rates
are to climb.
Bottom line: shuttle was, is, and always will be a white-collar jobs
program, pure pork. NASA pisses away $14 billion a year, while a real
commercial RLV will require under $1 billion in investment to develop,
and will produce an excellent ROI.
See
Read it and weep.
> These observations lead me to these conclusions: that
> a new heavy lift vehicle in the near future {next 20-30 yrs}
> will look a lot like the shuttle, with external tank and
> solids, and consequently the cost to orbit will not get
> dramatically cheaper in that time frame, which means
> the most promising route for space engineering is to
> make the payloads dramatically more capable, as in
> nanotech. spike
I disagree. Smaller launchers can become airliner-cheap, and the
continued lack of demand for heavy lift launch will see to it that no
non-government vehicle is built for the purpose. Only if space tourism
lights off in a big way would new heavy lifters be developed, and they
would likely be pressure fed liquids to minimize development cost and
schedule risk. LOX-kerosene first stage, LOX-hydrogen second stage,
with the first stage water recovered and the second stage expended or
salvaged in orbit. The price per pound would be higher than a good RLV,
so it would be used only for bulky or high unit mass payloads... and
with clever design, even a luxury space hotel could be designed to be
lofted in pieces small and light enough for an RLV.
I'm putting my money (and time) where my mouth is. We'll see in a few
years if I'm correct.
-- Doug Jones, Rocket Engineer
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