From: firstname.lastname@example.org (Henry Spencer)
Subject: Re: Another new propulsion concept
Date: Sun, 3 Jan 1999 02:39:55 GMT
In article <email@example.com>,
>...I know that the biggest limitation of the ion drive is its
Well, let's be precise. Its *specific impulse* -- impulse per unit of
propellant used -- is quite high, and that is its primary virtue. What's
low is its *thrust*, that is, impulse per unit of time. That's indeed a
handicap, sometimes a serious one.
>I don't know for sure, but I suspect this is due at least
>partly to the fact that it's basically solar powered.
Well, there are several factors involved, but one major difficulty is
indeed that the power source (be it solar or nuclear) is rather heavy per
unit of power output.
>If you had a
>more powerful electrical source, could you make an ion drive with
>enough impulse to be useful for near-earth propulsion?
Possibly. The idea has been suggested, particularly in connection with
beamed power. There are some tricky problems with ion-rocket design for
seriously high thrust, but they might be solvable.
>I was wondering about the possibility of using a gas turbine generator
>or maybe a fuel cell to generate electricity, then use the exhaust
>from the generator (which would be water vapor if you use LH2 and LOX)
>in an ion drive.
Unfortunately, this does not work very well. You don't accomplish much by
introducing electricity as an intermediate step here, aside from adding a
lot of heavy equipment. There's not much point in taking energy out of
the exhaust only to put it back in again.
For specific impulse, the problem with using a chemical reaction as the
power source is that you use up far more propellant generating the power
than you need for the ion rocket's exhaust, and those kilograms have to be
carried, and they count against the specific impulse of the overall
system. Using a thruster which can accelerate (say) a gram of propellant
per minute to very high velocities is pointless if you must also burn ten
kilograms of generator fuel per minute to supply the power.
As for thrust, note that because kinetic energy rises with the square of
velocity, while impulse rises only with the velocity, you get the highest
thrust for a given amount of power by using the *lowest* possible exhaust
velocity. That is, it's counterproductive to accelerate only a small
fraction of the total mass consumed.
>...This is because the conventional rocket is extremely
>inefficient due to the amount of wasted energy in the form of waste
No, sorry, this is wrong. Conventional rocket engines are quite efficient
at converting the energy of a chemical reaction into the kinetic energy of
the exhaust jet; it is not possible to do a lot better. Good chemical
rockets with long high-expansion nozzles (for use in vacuum) convert well
over 90% of the exhaust's heat into kinetic energy. In fact, people
designing such rockets using LOX/LH2 have to consider inefficiencies
arising from condensation of water in the exhaust, which tells you that
they're getting so much energy out that the exhaust is falling below
100degC (indeed, lower, because the pressure at that point is below 1atm).
Rocket exhausts *look* hot partly because you see them in atmosphere.
When they slam into motionless air, much of the kinetic energy gets turned
back into heat.
It's difficult to get a turbogenerator system much above 40% efficiency,
and usually it's less than that. Fuel cells can do better, but they are
The good old days | Henry Spencer firstname.lastname@example.org
weren't. | (aka email@example.com)
From: "Bruce P. Dunn" <firstname.lastname@example.org>
Subject: Re: Another new propulsion concept
Date: Sat, 02 Jan 1999 22:01:51 GMT
> This is because the conventional rocket is extremely
> inefficient due to the amount of wasted energy in the form of waste
Actually, a rocket engine operating at a high expansion ratio in a
vacuum puts very little of the derived chemical energy into waste heat.
A regeneratively cooled engine and nozzle are not particularly hot on
the outside, and radiate very little of the energy derived from the
combustion process. The rocket nozzle can be thought of as a device for
converting heat energy (random molecular motion) into kinetic energy
(directed molecular motion). The hot gases which come through the
throat of the engine cool as they expand in the nozzle. The energy
"derived" from this loss of heat goes into accelerating the gases so
that at the nozzle exit plane, they have a much higher velocity than at
the engine throat.
With high expansion ratios, exhaust gas temperature can be as low as
several hundred K. For example, a number of designs for advanced, space
based hydrogen/oxygen engines have used expansion ratios of around
1000. This is sufficient to reduce the temperature of the exhaust gases
from approximately 3500 K in the chamber to under 600 K at the nozzle
exit plane. This is better than 80% conversion of heat into exhaust
velocity, in a light weight device. No device generating electricity
from these same reactants will have a conversion efficiency this high.
Even disregarding losses in an ion engine itself, burning hydrogen and
oxygen to make electricity to accelerate the combustion products
electrically is a non-starter.
Dr. Bruce Dunn
General Astronautics Canada, Vancouver B.C.
http://www.genastro.com/ | 800-577-1117 | 604-876-7640
Reliable, low-cost transportation to low Earth orbit and beyond