[AR] partly-electric propulsion (was Re: Zubrin,)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 4 Sep 2019 14:37:19 -0400 (EDT)
On Tue, 3 Sep 2019, Christopher Buchanan Shay wrote:
[...] By pumping more (stored electrical) energy through a reaction
system than possible with a standard chemical system, ISP might be
improved over a non-nuclear chemical or nuclear reaction engine.
There are some fundamental problems here, alas.
First, any energy extracted electrically before the propellants hit the
chamber is not available to be released when they burn, so it's not clear
that you are gaining anything by taking it out and then putting it back in
electrically. Conceivably it could be applied more efficiently --
although see next paragraph -- but remember that the simpler alternative
is always to just throw the propellants into the chamber unchanged, and
let the extra energy make the flame hotter, and you have to beat that
somehow. (In particular, in the extreme case where your fuel-cell
reactants are entirely separate from the chamber propellants, the mass
flow of fuel-cell reactants has to be counted in your Isp calculation --
it too is mass expended by your propulsion system.)
Second, ordinary chemical (or nuclear) rocket engines are not all that
inefficient, especially ones that have a high expansion ratio (via either
high chamber pressure or operation in vacuum, plus a long nozzle).
They're not perfect and maybe you can improve on them, but they're pretty
good and it's going to be a struggle.
And third, compared to chemical rocket engines, electrical hardware is
*VERY* heavy for the amount of power it handles. The Apollo anniversary
having just passed, consider the Apollo SM. It had three fuel cells,
total mass about 340kg, maximum total output about 6.9kW (normal operating
max was about 4.2kW, but any two fuel cells could supply that with a bit
of margin, to cover the case of one fuel cell failed). It also had a
rocket engine, Isp 315s and thrust 20.5klbf, total mass just under
300kg... whose jet power (after losses) was about 140 *megawatts*. Yes,
the useful power delivered per hardware kilogram for the rocket engine was
over 2000x that of the fuel cells! Yeah, fuel cells have gotten better
since then, but that engine wasn't a particularly high-performance rocket
engine even then. Chemical rocket engines are amazingly small and light
for the power they handle; getting a useful improvement by doing some of
the power handling electrically is a *very* uphill battle.
...whether it is possible to actively (rather than passively) shape a
reaction engine exhaust jet using acoustic waves. Is it possible to, in
effect, form a virtual (i.e. active acoustic) aerospike?
This is a more interesting idea, I'd say, but it will still be tricky.
Notably, once the exhaust goes past the engine throat, it is supersonic,
so shaping it with sound waves will be a challenge -- it's moving faster
than they are, and attempts to interact with it tend to inject shock waves
into it. Perhaps possible, probably not easy.
Henry
Other related posts: