Re: Why Xenon in ion rockets

On 6 Mar, 22:15, Peter Fairbrother <zenadsl6...@xxxxxxxxx> wrote:
Henry Spencer wrote:
[the Russians] *did* put
considerably more effort into a related concept, the Hall thruster, aka
stationary plasma thruster. (In fact, the two are sometimes confused.)
Hall thrusters have quite different properties, despite some superficial
similarities -- they dodge some of the problems of ion thrusters, at the
cost of having very complex theory that nobody fully understands.

Basic Hall thruster theory is not that complex. The only real difference
between a Hall thruster and an ion thruster is that a magnetically trapped
flow of electrons, typically forming a ring-shaped region, is used in place
of a metal cathode grid. This eliminates grid wear of course, but at the
price that (at the moment) the maximum accelerating voltage (and thus the
Isp) is lower than in an ion engine.

The behaviour of the plasma in the electric and magentic fields can be
complex, as can wall-plasma interactions, but we have the basic theoretical
models for that, and it's not very different in overall complexity to eg the
flow, mixing, chemistry and wall interactions in the combustion chamber of a
chemical rocket engine. Instabilities hurt in both cases, and most of the
peceived difficulty seems to me to be because the Hall thruster is newer and
less researched than the chemical rocket engine.

I mentioned that the maximum accelerating voltage in a Hall thruster is
lower, which might be a good reason to use a light ion, eg hydrogen, as the
exhaust velocity and thus Isp for both Hall and ion engines would be higher
by a factor of 11.5 or so - but the energy required per unit of delivered
impulse would be 131 times larger too.

And here we come to the real reason why xenon is used in both types of
engine - energy supplies are heavy.

Hydrogen engines will usually be heavier too, but this is secondary to the
mass of the energy supply. Say a spacecraft might have 100 kg of xenon
propellent, changing that to 10 kg of hydrogen would save 90 kg - but the
increase in the energy supply needed would far outweigh that.

Ignoring thrust for a moment, which requires power rather than energy and
may or may not be relevant depending on mission, with an unlimited and
weightless energy supply it would make much better sense to use hydrogen if
Isp is important, as the Isp would be higher - but unfortunately we do not
have such energy supplies.

For eg an interstellar ion drive probe using zero-point energy hydrogen is
probably the way to go - but not for station keeping on satellites. Although
I wonder about that sometimes, as they usually only thrust for a small
proportion of their life, and often have basically unlimited energy (but not
power) from solar panels.

For a Hall engine there is an additional possible reason to use a heavy ion
- the difference between the interaction of electrons and ions with the
magnetic field is (roughly speaking) used so that the magnetic field traps
the electrons and not the accelerated ions - this Hall trapping effect
depends on the charge/mass ratio. However as electrons are 1700-odd times
heavier than even a hydrogen ion, it is not in itself a compelling reason to
use Xenon.

--
Peter Fairbrother

Thanks for the description. Given the reduced gridware, would it be
possible to use Oxygen as the reaction mass?

The reason I ask is that if we start to use lunar or NEO materials,
there will be a large surplus of oxygen in orbit.

The other useful feature would be the ability to vary the exhaust
velocity according to the relative supply of energy (power x time) and
reaction matter. For example, electric engines used to transfer
powersats from L1 to GEO would go for very high velocity, low thrust.
An asteroid mover would go for a low velocity, high thrust approach.

.

Relevant Pages

  • Re: Why Xenon in ion rockets
    ... Basic Hall thruster theory is not that complex. ... between a Hall thruster and an ion thruster is that a magnetically trapped ... Isp) is lower than in an ion engine. ... engine - energy supplies are heavy. ...
    (sci.space.policy)
  • Dawn Journal - April 22, 2008
    ... of ion thrusting since its launch nearly 7 months ago. ... software that simplifies operation of the science instruments. ... ready for installation on the spacecraft. ... ion thruster #2 can be pointed. ...
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