(sanman) wrote in message
. com...
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