Re: Polythene tanks?

From: Peter Fairbrother (zenadsl6186_at_zen.co.uk)
Date: 01/31/05 

To: sci-space-tech@moderators.isc.org
Date: Mon, 31 Jan 2005 15:44:58 +0000

David Summers wrote:
 
> Long skinny tanks can also cause problems during boost, since they have
> to hold up under the pressure of the fuel being accelerated at multiple
> Gs. For example, using 60 Psi as the desired base pressure during a 3
> G burn, you can only have a column of LOX about 10 meters high.

I was more concerned about the much larger LH2 tank, where the low density
sadly means that you can't usefully use that extra pressure to feed the
engines, even if you put the tank on a long stalk in front of the engines :(

But the tanks aren't particularly skinny anyway, the LOX tank is a sphere,
and the LH2 tank is 4.2 m diameter by 10.5 m long; it's just that at 4.68 m
external width the booster's fuselage is skinny compared to a 747 at 6.50 m,
or an A-380 at 7.14 m.

> I think you are severely underestimating the problems you will have
> with hypersonic flight and heating, but I don't really know of any good
> references.

For a cargo flight there isn't any hypersonic flight, except perhaps the
reentry of the booster stage, which reenters at about mach 7 - does that
count as hypersonic? It'll slow down pretty fast. Normally it will be empty
and quite fluffy then, though it will be built to survive a
heavier-but-still-fluffy reentry in case the second stage fails to seperate.

The people carrier version of the second stage also re-enters, and does so
from orbit; but it starts as a fairly standard shuttle-like reentry and
landing vehicle with minimum wings and 2 small jet engines to provide some
loiter time and go-around capability, but little (cross-) range. It is built
for passengers not payload, with safety the foremost consideration and
weight a distant second.

The wings might be folded during reentry, I haven't decided. It also might
instead be an unusual shape (a model of which hopefully first flies at the
end of next week, if all goes to plan and my flu clears up). "Unusual" is
about the only word to describe it, it functions as both a hypersonic
reentry capsule and a slow speed aircraft, a bit like a lifting body but not
at all really.

[Roughly, the second stages both weigh 60 tons fuelled, of which 46 tons is
LOX/LH2 propellant (8.36 tons of LH2 and 37.6 tons of LOX), 3 tons is tanks,
2 tons is main engines, thrust structure, instruments/computers and
manouvering/RCS. The cargo version has a payload of 9 tons, plus the empty
tanks and any remaining propellant can also be considered to be payload.

The people carrier has a payload of one ton, leaving 8 tons for habitable
structure, crew, deorbiting engines, TPS, wings, jets, fuel and landing
gear;

plus air supply, insulation, temperature control system, lights, batteries,
seats, washable plastic decorative interior; lifejackets, bobbles, safety
gear, emergency air and batteries, bulletproof lockable cockpit door; vomit
bags and vaccuum, but no toilets, passengers are only aboard for about 1
hour] 

-- 
Peter Fairbrother
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