General Discussion > Advanced Concepts
Chemical fueled plasma rocket
peter-b:
As I understand it, conventional rocket engines work by heating a gas to a very high temperature, which pressurizes it. Recall the ideal gas law pv = RT -- if you keep the volume constant, then increasing temperature must increase pressure.
The pressure difference between the pressure of the gas inside the engine and the exterior is what drives the acceleration of the gas and, by Newton II, the corresponding acceleration of the engine. The use of a nozzle increases the efficiency of this process, but you don't necessarily need one (consider Estes-style hobby rocket motors, which don't use a nozzle).
In a chemical rocket engine, the heat is provided by a highly exothermic chemical reaction, and the working fluid used is formed from the reaction products. In a nuclear thermal rocket, the heat is provided by nuclear fission, and the working fluid is often H2 in order to maximise Isp.
The important thing to note is that a conventional rocket engine is a heat engine (just like a petrol motor in a car or a boiler and turbine in a power station) and thus is limited to at most ~60% conversion efficiency. That means that 40% of the energy obtained from burning your fuel disappears as unusable heat in the working fluid.
The idea behind a plasma rocket is that instead of using a chemical reaction to heat a gas and then a pressure gradient to accelerate it, the chemical reaction would be used to ionise a gas (and generate electrical current as a byproduct) and a voltage gradient would be used to accelerate it. Apparently this can result in the exhaust being colder and faster -- i.e. higher Isp and higher efficiency.
sanman: I can't speculate w.r.t. lightning strikes etc, sorry. You're pretty much spot on with your description of the forms that waste energy takes. But the vast majority of it is just heat, i.e. molecular vibration.
Bolting thermoelectric generators onto a conventional rocket would be counterproductive. Thermoelectric generators are heat engines, and modern rocket motors are actually really good heat engines already -- they very closely approach the theoretical limits for heat engine efficiency. In the best case scenario, all you would be doing would be to reduce your T/W ratio.
He hasn't put any papers out yet, sorry. As soon as anything's published I'll post here.
Al/H2O something we discussed, over a beer, a few weeks ago (the original proposal was Al/O2). It's got a few advantages as probably the cheapest conceivable fuel mix. It's cheap, it's non-hazardous, it's trivially and losslessly storable, and it's dense.
We're getting to the limit of my understanding of what's going on here (I'm a computer jockey who works on remote sensing, not a plasma physicist). I'll pass on some of these queries next time I see him.
But to be clear, retrofitting conventional rocket engines is pointless.
Edit: Correct facts, redact commercially sensitive material.
Jim Davis:
--- Quote from: peter-b on 10/15/2011 11:01 pm ---...fueled by Al/H2O with a Isp of 450-500s.
--- End quote ---
If I understand you correctly the chemical reaction will be:
2Al + 3H2O -> Al2O2 + 3H2
The heats of formation are:
Al 0 kJ/mol
H2O -285.83 KJ/mol
Al2O3 -1675.7 kJ/mol
H2 0 kJ/mol
The net heat released is thus:
3 mol * (-285,830 J/mol) - 1 mol * (-1,675,700 J/mol) = 818,210 J
The masses of the reactants (and products, of course) are:
2 mol * (0.027 kg/mol) + 3 mol * (0.018 kg/mol) = 0.108 kg
The specific heat of reaction is thus 818,210 J / 0.108 kg = 7,576,000 J/kg
Assuming 100 % conversion to exhaust kinetic energy:
Isp = sqrt (2 * 7,576,000 J/kg) / 9.8 m/s2 = 397 s
So this doesn't pass muster with me.
peter-b:
--- Quote from: Jim Davis on 10/16/2011 05:38 pm ---So this doesn't pass muster with me.
--- End quote ---
I back-of-enveloped similar numbers last week, and they looked a bit low, but I assumed that I'd made a mistake somewhere. Hmm, oh well, I'll point it out next time I see him. ;-)
sanman:
Under what conditions can plasma thrust be used? Only outside of the atmosphere, right?
What altitude is the earliest you can use it?
How do you harvest the electrical energy, and use it to accelerate the plasma? I'm assuming you use an accelerative electric field, via a coil. And so you have wires connecting that coil to electrodes which collect the current from the reaction.
If you're talking about scaling this up to hundreds of gigawatts like you said, then isn't that going to mean a whole lot of power losses thru your wiring? Like I said, wouldn't you want some kind of superconductive wiring via cryogenic cooling? And therefore, wouldn't you want your chemical reaction to at least be semi-cryogenic, so that you can use the cryogenic reagent(s) to cool the superconductive wiring?
Not trying to derail your Al/H2O idea from this thread, but suppose it were possible to use a (semi-)cryogenic chemical reaction and harvest the energy mainly as electrical current. Then the cryogenically-cooled wiring and field coil could be made superconductive.
We know that superconductive magnets - including magnetic field coils - have field lines that don't converge. So this could be used to create a perfect magnetic nozzle, couldn't it? Any exhaust particle deviating from the thrust vector would generate a counter-acting current or field force that corrects the exhaust particle back to the thrust vector.
Are there any known designs for ion-thrusters or plasma thrusters which make use of superconductivity?
gospacex:
--- Quote from: KelvinZero on 10/16/2011 04:17 am ---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?
--- End quote ---
In rocket engines where nozzle and combustion chamber are regeneratively cooled, almost all "waste" heat is already collected and put to use.
Temperature (i.e. kinetic energy of _random_ molecular motion) of the exhaust of high-performance rocket engine is rather low. IIRC for LH/LOX engines this temperature can fall below 100 Celsius.
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