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If our regular huge power plants on the ground could be used to power part of a rocket launch, launch mass would be smaller since fuel would not have to be carried on board. I wonder what are the main weaknesses and problems with this? How could it be done, if at all?

One limit of electric ion engines seems to be that it requires quite a huge electric power plant to match the effect generated by the explosion in a chemical rocket engine. But with a reusable first stage ion electric engine which is physically connected to the electric grid on the ground, I imagine that one could turn up the power until the cable glows and electric effect is not the limit anymore.

Another limit might be how large an ion engine can be in terms of gas mass flow per second, or how many small ones that can be bunched together. Maybe lifting the cable is a problem, even if it is put on the ground/sea surface under the planned launch trajectory, because one would have to reach a substantial fraction of the escape speed in order for this to be worthwhile.

Are there other electric rocket engine types which would work better than ion engines when very high electric effect is available?

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    $\begingroup$ For a booster stage the thrust to weight ratio needs to be greater than 1. In other words if the spacecraft weighs more than the thrust, it's not going to get off the ground. While ion engines have great ISP, they have very weak thrust. $\endgroup$ – HopDavid Oct 31 '14 at 13:47
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    $\begingroup$ One data point you need is the weight of the cable. See britned.com/About%20Us/Construction: 44 kg/m for a 1 GW DC undersea cable. A cable for a rockets can be lighter (less insulation) but needs two conductors instead of one. Ballpark 50 tons/km would not be a bad start. $\endgroup$ – Hobbes Oct 31 '14 at 14:09
  • $\begingroup$ @Hobbes Ouch, that's heavy! But maybe it could be made lighter? Undersea cables don't have that incentive. It's after all not as far away as the tether materials needed by a space elevator. Your link is informative: "We used over 30 special cable laying vessels and a range of support vessels..." to lay 250 km undersea cable. Floating it on the surface might or might not be easier. $\endgroup$ – LocalFluff Oct 31 '14 at 14:21
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    $\begingroup$ The main cause of cable weight is the conductor. Copper is heavy, and there's no way to reduce the weight while still carrying the same current. $\endgroup$ – Hobbes Oct 31 '14 at 16:00
  • $\begingroup$ @Hobbes Don't hollow cables conduct more per unit of mass? That the electrons tend towards the surfaces of conductors. But even if conduction maps directly to mass of metal, what's the problem if one can simply turn up the power? Surely, a cable can carry enough current to lift itself from Earth gravity. $\endgroup$ – LocalFluff Nov 1 '14 at 4:54
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1) Nobody has built an ion engine that can lift from Earth. Your system might work to take off from a place like Phobos but nothing bigger. (And it wouldn't work very well even there.)

2) The strongest wires out there will take you no farther than the stratosphere before they snap under their own weight. If you the strongest cables we can build to support the wires we can get into space--but getting up there is easy compared to building up the speed to get into orbit.

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  • $\begingroup$ Re 1), "lifters" could be considered such a thing: en.wikipedia.org/wiki/Ionocraft but your points are still completely correct. $\endgroup$ – pericynthion Oct 31 '14 at 22:27
  • $\begingroup$ @pericynthion I would not call such craft ion engines. They're based on electric charge, not upon expelling ions. $\endgroup$ – Loren Pechtel Nov 1 '14 at 0:47
  • $\begingroup$ +1. Thank you for highlighting often neglected points. T/W (thrust to weight) ratio of ion engines. And tensile strength and density of tethers. A massive electric cable throughout an elevator adds to stress but contributes little tensile strength. A climber's power source is often neglected when painting rosey pictures of elevators. $\endgroup$ – HopDavid Nov 1 '14 at 15:08
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Laser propulsion may be a better option if unlimited power and imaginary technology were available.

That is, a land based, ground powered laser is aimed at the base of the rocket, and the heat from the laser provides the energy to heat the propellant that is exhausted to generate thrust.

Of course this has not been demonstrated to any serious level but there are small projects considering it.

There are issues in building powerful enough lasers, that can aim accurately enough over the time duration of a rocket flight. Upper stages will of course still be needed, since the first stage MIGHT stay in line of sight long enough to work, but the upper stages will be out of sight pretty quickly.

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  • $\begingroup$ Maybe cable electric power is the best option during the slower vertical part of a rocket launch? While laser/microwave propulsion from Earth stations have, I think, mostly been proposed for long term and very long term travel. Imaginary technology is NOT assumed here. Just the question if a concentration of today's power grid, with a cable, would enable rocket launches. $\endgroup$ – LocalFluff Oct 31 '14 at 14:56
  • $\begingroup$ Note geoffc suggested lasers as a way to impart thermal energy. Once again, ion engines do not have the thrust to weight ratio to climb out of a steep gravity well. $\endgroup$ – HopDavid Nov 1 '14 at 15:21

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