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Chemical fueled plasma rocket

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peter-b:
As you are probably all aware, one of the barriers to using electric propulsion for launch is that it's not feasible to supply the necessary amount of electrical power needed for liftoff thrust.

Aaron Knoll, one of my colleagues in our propulsion research group, recently gave a presentation at which he demonstrated the theoretical basis for a self-sustaining (i.e. no external power source) cold plasma rocket, fueled by Al/O2 with a Isp of ~450s. By reacting the oxidiser and aluminium in a plasma, he has shown that in theory some of the energy given off by the reaction can be harnessed as electrical current and used to sustain the plasma and accelerate the exhaust. His results seem to show that this allows most of the energy released to be converted to exhaust kinetic energy rather than heat.

As usual, I'd be happy to pass on questions from the peanut gallery.

Edits: Correct facts, redact commercially sensitive material.

sanman:
Hey, what a coincidence - I was just discussing with someone the idea of a hybrid plasma-combustion rocket, and how to power it using the combustion, without a nuclear reactor or large solar panels (a la VASIMR)

So I came here to start a thread on this idea, and here I found your post. :D

Yeah, shouldn't it in principle be possible to harness even the waste heat from a combustion rocket, and use it to power a plasma thruster at the same time? You'd want to be converting waste energy that would otherwise be lost, or else the introduction of another conversion step would be pointless, I suppose.

So then perhaps that high speed plasma thrust could be injected into the combustion exhaust to, umm, increase its laminarity, if you can imagine that?
We know that injecting a high-speed laminar plasma flow into a chaotic slipstream can help to increase the laminarity of that slipstream flow, to reduce the turbulence losses for an aircraft fuselage.
So shouldn't the same be possible with a rocket engine?

Anyway, do you have a link to your friend's idea or paper?
I am trying to discuss this idea with others.

sanman:
Okay, so some further thoughts.

Is this electrical current generated from the Al/H2O reaction going to be carried on wires to some sort of field coil that accelerates the plasma?

So won't there be resistance losses? Wouldn't it be better to use  cryo-cooled wiring and coils, for better conductivity and power transmission? In which case, wouldn't you want to try a cryo-propellant to make extra use of it for the cryo-cooling?

I assume that all sorts of rocket combustion exhaust products come out as near-plasma, in which case why restrict yourself to Al/H2O?
Why is its energy of reaction particularly convertible to electricity, as compared to other combustion reactions?

KelvinZero:
Hi, newbie question,

What form does the waste heat of a rocket take if it is not exhaust kinetic energy? Is this kinetic energy which is not moving exactly in the desired direction of thrust or is it vibration or rotation of the molecules etc, or energy released as photons?

sanman:
I'd assume it's all of that. Kinetic translational energy not parallel to the thrust vector, rotational, radiative, anything rubbing or colliding with or eroding the containing surfaces of the rocket to heat it. Any energy that isn't productive is waste, I guess - how much of that you can harvest is another matter.

Maybe if somehow you used your cryo-propellant to supercool superconductive magnets, to allow magnetic fields to be set up so that if your exhaust molecules crossed the field lines, that a Meissner effect current would be produced.

I know the thermoelectric effect isn't very efficient at producing current, but in relation to the copiously intense heat of the exhaust against nozzle and combustion chamber surfaces, perhaps it could still generate a significant quantity of current.

Perhaps if your plasma plume/torch extended from the combustion chamber and through the nozzle, then it would help the combustion exhaust gases escape more efficiently?

I dunno, just speculating.

I can't help but recall that time that one of the Apollo missions got hit by lightning which traveled up the exhaust contrail of the rocket. Electricity is blazing fast, even compared to the speed of rocket exhaust.

Would there be any hazard if your SSTO plasma rocket plume was happening anywhere near the Earth's ionosphere?

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