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If we consider nuclear-rocket technology not feasible (whether due to technological or political limitations), the second best options for future transportation in solar system are perhaps solar-electric ships... there are many concepts to use ion-thrusters in combination with photovoltaics in large scale - both historical and more recent.

With respect to thin-film solar cells I have some idea what are the ultimate limits of thrust-to-weight ratio - currently something like 6W/g ... in future it can be in principle developed may be up to 100-400 W/g (10-40% conversion efficiency in 1 micron foil at Earth orbit)

Now what about Ion thrusters? What makes them heavy? I guess current experimental realizations do not try to push thrust-to-weight up hard enough, simply because they don't have enough power to feed them. But in combination with these thin-film solar cell, the bottleneck starts to be the weight of thrusters.

Intuitively I assume classical ion thrusters with grid electrodes can be made lighter than Hall thrusters. While the grids can be made of thin metallic foils or wires, the Hall thrusters need quite heavy electromagnets. But I don't really know the engineering challenges which have to be solved in order to make them lighter.

What I really like with this respect is this concept where the thrusters are distributed over the solar array

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    $\begingroup$ Do you want to account for the solar panels mass ? They consume lots of power. $\endgroup$ – Antzi Oct 2 '16 at 11:47
  • $\begingroup$ I'm guessing that a good answer will mention the need for copper and possibly iron for confinement magnets, and electronics for DC as well as RF power supplies. $\endgroup$ – uhoh Nov 2 '16 at 5:59
  • $\begingroup$ Amtzi - I explainded why I think mass of solar panels could be reduced so much that bottleneck start to be elsewhere $\endgroup$ – Prokop Hapala Nov 2 '16 at 7:53
  • $\begingroup$ It's also worth noting that solar powered ion thrusters loose efficiency as they move further from the sun. I remember reading somewhere that for this reason, currently ion probes are out for anything beyond the asteroid belt. $\endgroup$ – Dragongeek Nov 10 '18 at 0:14
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There are 2 things that limit basically all electric rockets. One is the power supply. An electric rocket is only as good as it’s power supply. Whether it is solar or nuclear, in many designs, the weight of the power generation is so great that it usurps the advantage of needing little propellant. As you pointed out, the increasing performance and light weight of solar cells is encouraging.

The other big limitation is the density of the exhaust stream. Because these are extremely hot, they are naturally of low mass and large volume. So the problem becomes how to compress the exhaust stream so you can have more thrust.

I don’t know that the weight of the engine itself is as much of a problem. As you pointed out a design with magnetic coils (like the VASMIR) is heavier, but the coils enable it to squeeze the exhaust stream to make it more dense, so maybe it cancels out the weight burden.

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  • $\begingroup$ increasing density of exhaust stream is one possibility, other is to increase area of the exhaust "nozzle". What I tried to suggest - if electrostatic ion thruster needs basically just 2 electrode grids ... those grids can be very lightweight (like alluminium foil) ... why not make ion thuster like 10g/m^2 ? ... but in many current electrostatic ion thuster desings there is lot of other heavy stuff besides the grids ... is it really necessary? Or there is just not enough motivation for aggressive weight reduction. $\endgroup$ – Prokop Hapala Oct 3 '16 at 21:44
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    $\begingroup$ Using a very lightweight electrode grid is not possible due to grid erosion. $\endgroup$ – Uwe Oct 24 '16 at 9:05
  • $\begingroup$ I always thought the biggest limitation comes from erosion of the electrode by high energy ions. $\endgroup$ – Aron Nov 2 '16 at 3:51
  • $\begingroup$ Can you support the following sentences: 1) "Because these are extremely hot, they are naturally of low mass and large volume." and 2): "So the problem becomes how to compress the exhaust stream so you can have more thrust." Or at least add links to clearer explanations? $\endgroup$ – uhoh Nov 2 '16 at 5:12
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    $\begingroup$ I suspect that even if you handwaved the mass of the thruster itself down to zero, the limits of chemical, solar or RTG power supply mass would leave you with very poor performance, and I would be surprised if even nuclear fission couldn't deliver 1:1 TWR (i.e. < 10 m/s^2). Someone other than me should run the math. $\endgroup$ – Russell Borogove Nov 2 '16 at 5:20

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