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From: Henry Spencer <>
Subject: Re: nonrigid tanks
Date: Fri, 13 Mar 1998 14:56:04 GMT

In article <>,
David Cornell  <> wrote:
>Could an inflatible tank be made to fold up like an accordian when it
>was empty so it could be stowed for later reuse, rather than discarded?

Perhaps possible, but it would add a lot of complications to the tank
design.  Tanks designed to hold typical rocket-tank pressures are not all
*that* thin, and I'd be worried about cracking at the bends, especially
if the thing was reused repeatedly.

>SSTOs will require very large fuel tanks.  It's never been clear to me
>what the tradeoffs are between integrating the tanks into the orbiter versus
>using an expendible tank, other than the obvious cost of buying and
>attaching a new tank for every flight in the latter case.  Can anyone
>explain it to me?

An expendable tank gains you two things.  First, it doesn't have to be
built as tough as a reusable tank, since it only gets used once.  Second,
it greatly reduces the surface area that needs thermal protection during
reentry.  However, it has two matching disadvantages...

First, there's no way to test-fly it, since it's one-shot -- it has to
work perfectly the first time.  That gets seriously expensive, because it
means putting a lot of effort into making *sure* it will work the first
time.  Things that can be tested are cheaper and easier to work with in
the long run.

Second, reducing the surface area during reentry is not necessarily a
good idea.  When you're trying to do your decelerating at high altitudes
before getting down into thicker air, spreading the mass over a larger
surface area is often a good idea.  Likewise if you're trying to glide
to a landing.  One reason why shuttle reentry and landing are a headache
is that the orbiter is a densely packed mass of equipment; some nice big
empty tanks would considerably reduce the thermal loading on the tiles
and the aerodynamic loading on the wings.
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         |

From: (Henry Spencer)
Subject: Re: Why not X-33?
Date: Wed, 9 Aug 2000 19:45:27 GMT

In article <>,
Thomas Kalbfus <> wrote:
>It seems very important to them to have it all in one stage.

Remember that X-33 is a research vehicle.  Nobody needs to do research
into building two-stage launchers; that's fairly well understood.

>What's wrong with a drop tank anyway.

1. It does cost something, although not a whole lot.

2. It has to fall somewhere, which means you need a clear area downrange.
   (A big one, when you consider all the abort options.)

3. Tank jettison is an all-or-nothing operation which must succeed and
   cannot be worked up to gradually.

4. Tank defects cannot be worked out in testing, because each tank flies
   for the very first time.

5. After tank jettison, you have a dense, heavy vehicle which makes a
   severe reentry.  Having the bird be mostly empty tanks at reentry time
   is a big advantage.

6. You need major structure and plumbing joints at the orbiter-tank
   boundary, which are heavy, are a fruitful source of leaks (Remember
   when the whole shuttle fleet was grounded for month after month with
   hydrogen leaks?  That's where they were...), and require doors in a
   high-temperature surface.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

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