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New Ion Engine Enters Space Race
Bibek Paudel brings us a BBC report on the development and testing of an new ion engine by a security firm named Qinetiq. The engine will be used in an ESA spacecraft tasked with mapping the Earth's gravitational field from orbit. Only a handful of ion drives have been used for space missions before, some of which we have discussed. Quoting:
"Cryogenic pumps can be heard in the background, whistling away like tiny steam engines. Using helium gas as a coolant, they can bring down the temperature in the vacuum chamber to an incredibly chilly 20 Kelvin (-253C). The pressure, meanwhile, can drop to a millionth of an atmosphere. Ion engines ... make use of the fact that a current flowing across a magnetic field creates an electric field directed sideways to the current. This is used to accelerate a beam of ions (charged atoms) of xenon away from the spacecraft, thereby providing thrust."
It depends on how long you mean. Its certainly still a long way from being able to run an interstellar mission, but for an interplanetary mission it provides a lot of benefits. Lower mass and faster travel times are the primary ones; even though its low thrust its constant thrust so it can build up, particularly on longer missions (think to Jupiter rather than the moon).
Also, the one problem I see with the idea of 'vacuuming' space, beyond the obvious engineering problems, is that in order to use them in a system you'd lose more momentum than you'd gain, at least using engine technology of this sort. Imagine it from the spacecrafts point of reference, all the very rarefied gas is coming towards it at the speed the spacecraft is traveling in the inertial frame. As it captures the gas, it has to slow it down to stationary, and then speed it up and send it back out; in doing so unless the exhaust velocity is faster than the spacecraft velocity, you're going to lose momentum rather than gain it.
Now if you could come up with a way to ionize the gas as it passes and use magnetic fields to accelerate it further (like a swimmer or an air-breathing engine) that would certainly be interesting.
The only thing that matters is VELOCITY, not momentum. Mass flow RATE. At least if the test is "efficiency". That is what we (rocket scientists) call "Specific Impulse" (Isp). When you do the Delta-V equation, it's only
DeltaV = Isp*ln (m1 - m2) if memory serves. If not, someone will fix it for me. Nothing about momentum. The difference in mass is the only factor for a given propellant/engine combo
Whatever you can get out of the poopchute the fastest is the most efficient. Without speaking of the ionization process, hydrogen is prolly the best, being the lightest, BUT it's density is so low that the mass to contain it lowers the return. Recall that Clarke's Discovery had ammonia instead of hydrogen as Sakharov propellant, because it was denser (smaller, lighter tanks). And thus, it didn't leak out after 9 years (2010 - 2001)
Xenon is probably an optimum of mass and density. Plus whatever they said about ionization.