On Tue, 2 Jul 2002, Pierce Nichols wrote:
> The concept is overall way cool -- here's a launcher that's small
> enough to put all of vehicle, support gear, and propellant on a single
> trailer small enough to tow behind my truck with room to spare. Of course,
> it has ASAT weapon written all over it ...
Not on close inspection. An ASAT wouldn't need nearly as much delta-V.
Anyone who goes to the trouble of achieving orbital-class mass ratios is
not interested in ASAT applications.
> Remember that fooling around with your aspect ratio also affects
> your structural fraction. The higher your aspect ratio, the more material
> you need to wrap around your fuel.
Not necessarily. Randall's design was pressure-fed, which means that tank
pressurization will be the overwhelmingly dominant load on the tank walls.
Given that, the tank mass is *not* a function of aspect ratio -- wall mass
per unit volume of a cylindrical pressure vessel (simplifying, neglecting
end caps) is 2*pressure*walldensity/allowablestress. Note, there's
nothing in there that depends on geometry! A longer, thinner cylinder has
lower stresses, i.e. needs thinner walls, and that exactly cancels out the
greater area.
End caps, anisotropic materials, and varying thicknesses complicate
analysis but don't change the bottom line a lot. (Notably, with
composites, a sphere has no significant wall-mass advantage over a
cylinder.)
Henry Spencer
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