The more power we feed an ion thruster, the more thrust it generates. In theory, if we could give it, let's say 10 gigawatts of power, it could generate pretty high thrust. In the example I calculated it would be 60 kN of thrust. Ignoring the practical problems like heat and general damage to the thrusters, this should increase the thrust while keeping the weight of the engine the same. However, according to an answer to my previous question, the thrusters have very small thrust to weight ratios. (In theory, how much thrust could an ion thruster generate?) Why is that? Would the increased power input also increse the ratio, or is there some other part of the equation that increases the weight, keeping the ratio low?
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7$\begingroup$ The thrust-to-weight ratio of the thruster is almost irrelevant; the mass of the power source dominates the mass of the thruster itself. $\endgroup$– Russell BorogoveJul 13, 2019 at 13:53
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$\begingroup$ You can't just decide to ignore the practical problems. $\endgroup$– Loren PechtelJul 13, 2019 at 23:28
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1$\begingroup$ @LorenPechtel But you can always ignore some practical problems in a well-written Stack Exchange question in order to isolate and better understand a particular issue or phenomenon. $\endgroup$– uhohJul 14, 2019 at 20:46
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$\begingroup$ @RussellBorogove unless you engine includes a big post-accelerator (see also) in which case the mass may not be so trivial. $\endgroup$– uhohJul 14, 2019 at 20:48
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1$\begingroup$ The source of 10 gigawatts of power would add so much mass to the craft you'd end up with TWR even worse. Besides, up there, you're not in a race. You can take sweet time to accelerate so low TWR isn't a big problem. What you really want is performance - specific impulse. So squeeze the amount of power you have into the least amount of propellant you can, to accelerate that propellant as much as you can, and if you can squeeze it into even less propellant - great! Thrust will drop some more but you get even more performance! $\endgroup$– SF.Jul 15, 2019 at 10:47
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1 Answer
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Why is the thrust to weight ratio of ion thrusters so low?
Because (as the OP notes) the thrust is so low.
- Why is the thrust so low?
Because the pressure/density of the plasm is so low.
- Why is the pressure/density so low?
In order to give the electrons a long enough mean free path to build up tens of eV of energy before they hit their first atom.
- Why do electrons need to build up so much energy?
So that they can efficiently ionize the first atom they hit with a high probability.
- Why else is the pressure/density so low?
So that the ions, once ionized, are not likely to hit any neutral atoms or worse yet recombine with any free electrons along their acceleration path; so that they maintain a relatively ballistic trajectory through the plasma and main acceleration gap and out of the engine.
Current, working designs for engines need to operate at low pressure to run as efficiently as they do. If you try to raise the pressure substantially, you'd have to dramatically increase the power used to produce and maintain the plasma.
It's really just diminishing returns for the power used to produce the plasma, and the power used to accelerate the plasma is trivially small in comparison.
Another way to increase the thrust would be to do some post-acceleration of the ions. However that's also somewhat of a diminishing returns scenario. According to this answer to If specific impulse is directly related to exhaust velocity, would a ion post-accelerator improve the Isp of a propulsion system?:
$$v = \sqrt{2mE} $$
The square root relationship means increasing the energy from 100 keV to 100 MeV (factor of a thousand) increases the momentum and therefor the thrust only by a factor of 31.6
That's certainly quite a lot, but the added mass will be quite a lot as well, so the impact on the thrust to weight ratio (the original question) will be a lot less!
