[d_silin]

From what I heard from industry people, overheating is an unsolved problem for VASIMR. It is 60-70% efficient, meaning that 60kW of thermal power has to go somewhere.

Waste heat management for 200MW system in space is firmly outside the realm of present-day technology.

[extrapickles]

I’m assuming you mean 60MW of heat, which would likely be worth recycling, especially if it’s not low grade heat. If you then are left with say 30MW of unrecoverable heat, than its only ~500x what the current largest space thermal system can handle (the one on the ISS).

It really depends on the mass budget they have for that speed for if anything new needs to be discovered.

[Florin_Andrei]

> It is 60-70% efficient

Funny - in a lot of other technology sectors, that would count as pretty good efficiency.

[rbanffy]

True, but radiating 30-40% of your waste heat isn't that easy.

[RandallBrown]

Could you use the waste heat to boil something and then spray it out the back as extra reaction mass?

[Symmetry]

That's how nuclear thermal rockets work. Boiling and spraying is going to be much, much higher thrust than what you're getting out of the electric drive but the spray will be going much slower so the amount of boilable propellant you carry with you will end up being the limiting factor in how fast you can do. And at that point you might as well forget about the electric part.

Now, since you you can be boiling pure hydrogen rather than the water that comes out of a chemical rocket you can make the propellant faster. But you're limited in temperature by what your reactor can take without melting so that only buys you a power of 2 or so in efficiency.

I wrote a series about how the different types of rockets compare starting here: http://hopefullyintersting.blogspot.com/2015/03/rockets-some...

[alextingle]

That's just reducing the effective exhaust velocity of your engine. Rocket equation says you will run out of propellent [your "something"] faster than you would like.

[ryanmarsh]

Yes, but what? You have to lift that mass to orbit. What mass boils off with greater energy than, say, mass that combusts?

[TheOtherHobbes]

You don't necessarily need to lift that mass to orbit for interstellar flight.

https://en.wikipedia.org/wiki/Bussard_ramjet

[adwi]

Any reasonable ways to recapture?