On Mon, 1 Nov 2004, Alexander Mikhailov wrote:
> there is a somewhat theoretical question. It is often
> assumed that for bigger (liquid) rockets it's easier
> to get the good mass ratio than for smaller one.
Correct, and in general it's true.
> Usually one says that the mass of, say, tanks - a
> major contributor to the rocket dry weight - is
> proportional to the second power of their size...
Only very sloppy people say that. :-) Tank loads are primarily pressure
loads -- even in pump-fed systems, at least with orthodox designs -- and
pressure-vessel dry mass scales with volume, not surface area. As you
note, wall thickness has to grow as the pressure vessel grows.
> So, where do the savings come from? Is it the case
> only for tanks with small inner pressure, so their
> wall thickness is determined by other considerations?
There is some of that. Notably, "minimum gauge" issues -- how thin a
sheet the material comes in, and also possible constraints imposed by the
difficulty of handling large but very thin sheets -- often dictate the
wall thickness for small low-pressure tanks.
A more subtle issue is that it's often easier to apply sophisticated
manufacturing techniques to larger systems.
> It's also said that if an engine uses a higher chamber
> pressure, it has smaller and therefore lighter chamber.
Again, only quite naive people say that. Chambers and nozzles are
pressure vessels, and higher pressure drives up the wall thickness.
Higher-pressure engines actually tend to be heavier, because all sorts of
supporting plumbing gets more difficult to make for high pressures.
What *is* true, mind you, is that high-pressure engines tend to be more
compact. This can matter in systems with volume constraints; notably, the
SSMEs must fit within the shuttle orbiter's body cross-section to avoid
heating problems during reentry.
Henry Spencer
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