Hi,
I've been looking recently at ion thrusters and I'm interested to know how their thrust per unit mass compares to that of traditional chemical engines. What kind of thrust does a typical chemical engine generate and what kind of thrust can an ion thruster generate? I understand that the thrust from ion thrusters is much lower but that they have a high Isp, but I have no information on any actual experimental values.
Any help is greatly appreciated!
From my memory I've seen claims that chemical engines have 4 km/s exhaust velocity and that nuclear thermal rockets can double that. But the ion thruster of the space probe Rosetta has an exchaust velocity of 30 km/s! And the efficiency per mass fuel is proportional to this exhaust velocity. I hope those who really knows this stuff can develop this answer.
I would like to add the following related question:
How can the thrust (mass exhausted per unit of time) be much increased with ion engines?
One requirement is electric energy. It should be possible to solve with a nuclear reactor like a bigger version of the Russian TOPAZ. And because Mercury is both hard to reach and near the Sun, maybe solar power could be enough there. Are there other important design problems with building a high thrust ion engine?
since when does rosetta have an ion thruster?
but to somehow answer the OP question:
thrust:
chemical (the best used on real rockets, from my memory)
- solid: 12 MN (sssrb)
- liquid: 8 MN (rd-170)
ion
here are some numbers for ion thrusters - for start sufficient, i would say:
http://en.wikipedia.org/wiki/Ion_thruster#Comparisonslets say under 1 N, usually
NASA's gridded electrostatic ion thrusters:
NEXT: 6.9kW, 4100s Isp, 236mN thrust, 58.2kg for thruster, power supply, gimbal, and xenon distributor (but no tank)
NSTAR: 2.3kW, 3100s Isp, 92mN thrust, 49.4kg total weight (including 20.5kg lumped-together dry mass of tank & distributor for 82kg xenon, unlike above figure)
Hall Effect thrusters from one provider:
Snecma PPS 500: 0.5kW, 1330s Isp, 30mN thrust, 4kg for complete subsystem
Snecma PPS 1350-G: 1.5kW, 1660s Isp, 90mN thrust, 29kg for complete system incl tank for 80kg
Snecma PPS 20,000: 20kW, 2500s Isp, 1050mN thrust
At these power densities, the mass of solar panels, batteries, and electrical subsystems is going to be comparable to the mass of the thrusters. As far as I can tell, ideally power level should scale with the square of specific impulse, for a given amount of thrust(4kw at 2000isp buys the same thrust as 1kw at 1000isp), but often lower-impulse Hall thrusters (designed for stationkeeping small GSO satellites) are relatively *thermally* inefficient, causing power needs to increase. They're also relatively shorter-lived, the smaller you go.
but to somehow answer the OP question:
thrust:
chemical (the best used on real rockets, from my memory)
- solid: 12 MN (sssrb)
- liquid: 8 MN (rd-170)
ion
here are some numbers for ion thrusters - for start sufficient, i would say:
http://en.wikipedia.org/wiki/Ion_thruster#Comparisons
lets say under 1 N, usually
Thanks, that's just the sort of thing I was looking for! I'm assuming those values are no per kilo!