See also parts I, II, IV, V, and VI.
So besides burning stuff, how else can we make our rockets move? Well, one probe that's been in the news a lot recently is Dawn which recently went into orbit around Ceres and showed us those two funny bright spots on it. One of the nifty things about Dawn is that it uses an ion drive for propulsion.
Ion drives, just one sort of electric drive out there, work by using electric fields to accelerate atoms very quickly out the back of the rocket. Since they decouple the energy used to accelerate the propellant from the propellant itself there really isn't any firm limit on how fast the propellant goes besides how much electrical power you have available to shove them. A typical ion thruster might have a ve 42,000 m/s, almost 10 times higher than the best you can get from a chemical rocket.
Unfortunately that electrical power is a bit of a sticking point. It has to come from somewhere and that somewhere is going to be relatively heavy compared to a chemical rocket with the same thrust. For instance, the Space Shuttle engines had a combined thrust of 5,580 kN. Remember that for a rocket power is half the thrust times the ve so the space shuttle is putting out 12.4 GW of power which is comparable to the biggest power stations in the world. Since the Shuttle engines just had to put the hydrogen and oxygen together, burn them, and only let the hot gasses leave in one direction it was possible to make them fairly light at just 10 metric tons all together. Just the turbines of an electric plant that can generate 12 GW would be much, much heavier than that. And a typical Ion Drive with a ve 10 times higher and so requiring 120 GW for that thrust would be totally unfeasible.
All that means that ion drives tend to be low thrust. When you're getting into orbit you need to accelerate quickly just to overcome gravity. If your acceleration is at 9.8 m/s2 or less you won't be able to do anything at all. But if you make it to orbit then there isn't nearly as much of a hurry. If you're spending a week traveling to the moon like Apollo 11 you can probably afford to spend a day accelerating. And if you're going to be taking multiple years getting to the asteroid belt like Dawn did then you can take a long time indeed to get up to cruising speed.
How fast will a typical ion engine accelerate? Well, lets say we want to go from low Earth orbit to low Mars orbit and back without anything complicated like aerobraking, which should take about 20,000 m/s of Δv all told. With a ion drive with a ve of 42,000 m/s that means that we'll need 38% of our mass to be propellant. That's pretty good for a trip like this. Lets say we're willing to use another 38% on the engine mass, how fast can we go? Well Dawn's engine (named NEXT) weighs 5kg, puts out .236 N of thrust, and takes in 6.9 kW of power. If we calculate how much power that thrust/ve combo ought to give us it comes to 5.0 kW, so the rocket is 72% efficient which isn't that bad. So if we took that 38% mass and filled it full of NEXTs the rocket would accelerate at .018 m/s2. At that rate all our acceleration and deceleration would be over in 12 days, which is pretty small considering an efficient trajectory will take eight and a half months traveling each direction.
Sadly we do have to think about power. For each kg of NEXTs we use we need 1.38 kW or power. Space grade photovoltaics have a power to mass ratio of 77 W/kg so we'd need 18 kg of solar panels for every kg of thruster, increasing our acceleration/deceleration times by a factor of 19. That would mean acceleration periods equal to half the normal trip time.
Can we do better? Well, NASA has a nuclear generator for use in space that they've tested out some that gives 195 W/kg, which means only 7 kg of generator for every kg of thruster which means 3 months of acceleration which is still bad but less so.
Of course if you're willing to have a lower ve you can have more thrust at the cost of using more fuel. And if you want to go to the outer system it might make sense to try to use a higher ve engine since your trip is going to take so long anyways that you can afford to spend longer getting up to speed. There's actually one sort of electric thruster, VASIMR, that can alter its ve in flight instead of having a fixed ve for the particular design.
I expect that a lot of future missions to places further than the moon are going to use these sort of rockets in one form or another. We've put a lot of effort into making chemical rockets as efficient as possible but various sorts of electric rockets are fairly undeveloped and I hope that there's a lot of room for improvement.
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