Space tugs revisited (microwaved power)

[wumpus]

Sometime after playing KSP I became convinced that anything that needed to go to Mars should be powered by ion thrusters.  Note that for crewed flights this can involve docking with multiple pre-positioned chemical boosters (positioned via ion tugs) and doing a burn that doesn't require all stages present (sic semper tyrannical rocket equations).

The elephant in the room turned out to be the Van Allen belts and their effects on solar radiation.  Since any tug moving from LEO to escape velocity will spend nearly all its time in the Van Allen belts (and nearly all ion engine craft are simply boosted to escape velocity chemically), a naive solution appears disastrous.

Other solutions include nuclear power (probably a good idea for VASIMR, but that is a completely different solution), and I'm now wondering if beaming power via microwave might be a better solution.  Ideally the power would come from a high orbital satellite (high enough not to worry about being eclipsed at night) but it would presumably start with ground stations.  Fortunately, there exists a surprising amount of research on this (although mostly power point and research/government white papers) going the other way (sending space power to Earth), so most of the issues should be obvious.  Any ideas if microwave antennas can handle the Van Allen belts better than most craft?

[Canopus]

The Van Allen Belts are probably the reason that the proposed Deep Space Transport will be stationed in Lunar orbit allowing Astronauts a fast transit through the radiation using Orion. So i guess the simplest solution is to not fly your ion powered vessel through the belt at all and to service them with spacecraft that minimize their passage time using faster chemical engines.

[DDE]

Highly unlikely, especially because the things most affected - computers and crews - would still be present and just as vulnerable.

You need high thrust, thrust even VASIMR can't provide. Control drums to full reflector, stand by to engage atomic rocket thrust!

[wumpus]

21 minutes ago, DDE said:

Highly unlikely, especially because the things most affected - computers and crews - would still be present and just as vulnerable.

You need high thrust, thrust even VASIMR can't provide. Control drums to full reflector, stand by to engage atomic rocket thrust!

Computers can be shielded [seriously.  How can size be an issue for the things], and crews and ion speeds will never mix (thus the whole point about lifting stages.  With kerbal magic these would simply be fuel tanks, but expect to be limited to fully fueled stages for the foreseeable future).  I think the big question would be the microwave power receivers (may have to be tube based) and ion engines.  Historically, the issue has been solar panels, but there are almost certainly ways of getting around that, but I have to wonder what the next worst issues are.

[tater]

If you are collecting solar (lossy) so you can beam it via microwaves (also lossy), so you can accelerate particles (some loss there) to gain high Isp, but low thrust...

Why not just concentrate the sun on the tug and use sunlight itself to heat a propellant? Solar thermal.

[DerekL1963]

6 hours ago, wumpus said:

Sometime after playing KSP I became convinced that anything that needed to go to Mars should be powered by ion thrusters.

What leads you believe that?
 

[tater]

Maybe it's because ion engines in KSP are grossly OP  .

[Scotius]

5 hours ago, tater said:

Maybe it's because ion engines in KSP are grossly OP  .

Indeed. To gain any reasonable TWR for a manned mission, you would have to stack so many ion engines they would seriously cut into payload fraction. And then you would still have to add power source able to keep them running.

[magnemoe]

6 hours ago, tater said:

If you are collecting solar (lossy) so you can beam it via microwaves (also lossy), so you can accelerate particles (some loss there) to gain high Isp, but low thrust...

Why not just concentrate the sun on the tug and use sunlight itself to heat a propellant? Solar thermal.

Solar thermal has an ISP a bit higher than nuclear thermal its a bit low for an tug who has to return to LEO, Ion or vasimr would work well, issue is power requirements the huge solar panels or heavy reactor will eat much of the benefit. 
I see LEO to GEO as the most useful place for an tug because its an often used transfer making reuse more practical. 

Now for an regular Mars run having beamed power as an extra option would be smart, you use this for the escape burn from LEO, afterward you continue with lower trust using solar or reactor, this is also used for braking into Mars orbit and earth return. 
 

[kerbiloid]

Probably the radiation belts are the thing near gas giants.
I guess, Saturn-Titan colony would use it as a significant-to-main energy source.
(And Jupiter-Ganymede, too , if somebody indeed needs a colony there).

But near the Earth this probably would make more pain than gain.

[Bill Phil]

9 hours ago, tater said:

If you are collecting solar (lossy) so you can beam it via microwaves (also lossy), so you can accelerate particles (some loss there) to gain high Isp, but low thrust...

Why not just concentrate the sun on the tug and use sunlight itself to heat a propellant? Solar thermal.

Because then you suffer similar limitations as Nuclear Thermal. Exhaust velocity depends on temperature, and temperature is limited by materials. The real advantage is that you could potentially get high isp and decent thrust, not too high since we don't want our spaceship to evaporate. Another advantage is high power densities without nuclear technology, which is more of a political issue. In any case you'll need beefy radiators.

[tater]

The point is that for crew missions you can't use a super high Isp, but slowly accelerating tug as you will dwell in the Van Allen belts too long.

[Bill Phil]

5 hours ago, tater said:

The point is that for crew missions you can't use a super high Isp, but slowly accelerating tug as you will dwell in the Van Allen belts too long.

Certainly. Of course, you could just put everything above the belts and use chemical tugs for that leg of the journey... although that does add complexity.

[tater]

Yeah, the OP is about ions though. Ions rock for cargo, obviously.