Aluminium alloys certainly work (i.e. professionals have flown them in
space), and they can have very good high thermal conductivites, and they
are pretty lightweight. However the temperatures that the alloys start to
misbehave at are really rather low (~150C), and they are really rather
prone to warping and ageing.
Overall, the modelling I've done suggests they work reasonably well at
relatively modest chamber pressures like around 10 bar, but above that
other copper and nickel and even steel based materials seem to have better
properties for handling the higher pressures and hence high thermal fluxes
and that can allow for longer, more efficient nozzles/higher Isp, with far
less chance of significant metallurgical issues.
On 1 May 2016 at 18:11, Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
On Sun, 1 May 2016, Gerald Taylor wrote:
Aluminum has a low melting temperature, a rather low softening
temperature where it loses its temper, low strength and stiffness when not
tempered...
While all of that is true, it's largely counterbalanced by the fact that
aluminum has *very* high thermal conductivity, so it's hard to get local
overheating in a regeneratively-cooled engine. Last I heard, Masten's
flight engines (hundreds of flights, heaven knows how many ground tests)
are all aluminum, and they're not the only folks doing it.
This is also a large part of why Columbia's wing lasted as long as it did.
Despite the severity of the hot-gas leak, the hydraulic systems within the
wing failed before the aluminum structure did.
and if the aluminum oxide surface cracks, would make one heck of a fuel.
Aluminum-oxide cracks generally are self-healing because new oxide forms
instantly on the newly-exposed surface. In practice, aluminum generally
won't ignite until well above its melting point. It's a ferocious fuel
when it gets going, as witness its use in thermite, but getting it going is
tough, as witness the drastic measures needed to ignite thermite.
(Many high-temperature metals are much more vulnerable to such things,
because they don't form oxide layers or their oxides aren't such excellent
protection, so they need deliberately-added protective coatings, and if one
of those cracks, watch out.)
Henry