From: Vincent Cate (vince@offshore.ai) Subject: Re: Double-Layer Ion Thruster Newsgroups: sci.space.tech Date: 2003-07-14 00:08:40 PST manofsan@yahoo.com (sanman) wrote in message news:... > It's been pointed out that ion thrusters are more efficient on Isp, > but they produce less thrust -- too little to be of use for > launching to orbit. By a factor of how much are ion thrusters too > weak for achieving escape velocity? > By how many times would the above system have to be improved to be > used for SSTO? > > I presume that nuclear power can be used to augment the > thrust/efficiency of electric propulsion. Would it be enough? Saying that ion thrusters are too weak for achieving escape velocity is not really accurate, since once in space they are really good at getting high velocities. The problem is just getting off Earth. The Russian SPT-290 has 1.5 Newtons of thrust and mass of 23 Kg. With F=MA and A=9.8 m/s^2 it would take 23*9.8 = 225.4 Newtons to lift off. So we need about 150 times more thrust, not counting power supply. It takes 30 kw to run this thruster. It would be really amazing if you could get a 30 kw power source down to 23 Kg, so that the total mass was 46 Kg. But even in this case we would need 300 times more thrust. But suppose that our thruster magically could do 300 times more thrust and still weigh only 23 Kg, we would still need 300 times more power. That would mean getting 9 megawatts in 23 Kg. This is just not going to happen anytime soon. :-) So high ISP thrusters (like ion drives and Hall Thrusters) are not going to help with SSTO. It just takes too much power. High ISP thrusters could help with getting to orbit by reboosting a tether which lifts a single stage to tether rocket (SSTT). The mass of the solar power and thrusters actually helps the tether store momentum/energy, so here the high mass/thrust ratio does not bother you. You want a bunch of mass anyway. The SSTT would only need to get to like 5 km/sec. It is far easier to make a reusable rocket that only goes to 5 km/sec because of the needed delta-V and the lower reentry heat problem. This seems a very workable idea. We at spacetethers.com are working on it. -- Vince