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Pure Bollocks Issue 22_016
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* F E A T U R E S *
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Ion Propulsion Systems
Keeping a satellite in a predetermined orbit has so far meant using
miniature rocket engines, requiring a bulky payload of chemical fuel. Now
there is an alternative.
Electric, or ion, propulsion can provide a small but continued thrust to a
satellite once it is outside the atmosphere.
However, chemical rocket fuel will continue to form much of the launch
mass and cost of putting the satellite into orbit - no other way of
propelling an earth bound vehicle into orbital velocity is known.
The main application for electric propulsion is for the generation
of the small thrust required to keep geostationary communication
satellites in their correct position above the equator. Another possible
use is in thrusters which compensate for the atmsopheric drag imposed on
satellites orbiting at low altitudes.
A far more exotic application for electric propulsion is the provision
of a constant small thrust to spacecraft for periods of up to ten years.
Such a thrust could accelerate them to a very high velocity towards the
stars. Electric propulsion is regarded as essential for the economical
acceleration of vehicles from earth orbit into deep space.
Most commercial satellites are destined for geostationary orbits
above the equator where they handle telephone data, tv, dbs and other
traffic. Although the satellites are well above the atmosphere, their
positions drift with time in north/south directions owing to the
gravitational pull of the sun and moon.
The shape and mass distribution of the earth (which is not perfectly
spherical) also causes east/west drift of geostationary satellites.
The drifts result in a satellite following a 'figure of eight' pattern.
It would be expensive to arrange for a ground station dish antenna to follow
this motion. Even the radiation pressure due to sunlight causes some drift,
but this is only slight unless a satellite has large solar panels.
These effects act continuosly and build up with time, although they are
relatively small (corresponding to typical forces in the mNewton range). If
the effects of the sun and moon always acted in the same direction, they
would accelerate a satellite to a velocity of about 50 metres/s over a year.
This figure can be used as a measure of the total amount of propellant
required. Initial positioning of the satellite requires the
equivalent of about 60 metres/s thrust only once before it is used.
Sunlight and the gravitational and magnetic fields of the earth tend
to cause geostationary satellites to rotate so that their aerials no longer
point at receiving stations on the earth. Thrusters are required on each
satellite to corect this rotation.
The drift and rotation of geostationary satellites can be corrected
by using small jets of gas - usually nitrogen, hydrogen and ammonia derived
from hydrazine. The amount of propellant required rapidly decreases as the
exhaust velocity from the jet increases.
Hydrazine produces jet velocities of little more than 2km/s. This
implies that about one fifth of the initial satellite must be hydrazine with
a consequent increase in launch cost.
Once the supply of hydrazine is exhausted, the life of an expensive
satellite is at an end (typically ten years).
Heavy spacecraft can benefit from the use of a bipropellant system in
which two fuels - usually nitrogen tetroxide and monomethyl hydrazine -
combine to produce a jet velocity of about 3km/s.
In this case the intial weight of propellant is reduced to about one
seventh of the weight of the spacecraft, but the rocket system is heavier and
reliability of bipropellant systems has not been established.
The bipropellant system could be used for the next generation of heavy
geostationary craft, but further improvement can be obtained by
accelerating the propellant to much higher velocities using a source of energy
seperate from the propellant.
Ion thrust engines may offer a solution to this problem. In such
engines, ions are accelerated to extremely high velocities. The ejection of
even a fairly high mass of material in the form of high velocity ions can
provide a small thrust over a long period.
A considerable impulse is built up. The propellant material from which
the ions are formed is used slowly in such systems, so its weigth does not
increase the rocket power needed at launch.
If electric power from solar cells is used to accelerate heavy ions up to
a velocity of 30 to 40km/s, the mass of propellant can be 1/60 to 1/80 of the
total satellite mass. Thus the propellant weigth required per year of
satellite life is 0.1 to 0.l2% of the initial mass.
Although the mass of the propulsion system is added to the satellite
mass, the use of ion propulsion allows a big reduction in the total mass of
propellant.
Use of ion propulsion could save 280 to 300kg in the mass of a typical
two tonne satellite, such as those planned for communications systems
planned until the end of the century.
This means the communications equipment payload could be nearly
doubled and the satellite could bring in far more revenue during its working
life.
Savings of up 25% in satellite mass could be obtained if ion propulsion
is used for the initial positioning (taking about one month) or about 17% if
a chemical propellant system is used for rapid initial positioning.
In most ion thrust engines, electrons bombard atoms of the propellant
to remove electrons and form positive ions. In the system used at the
British Culham establishment, electrons from the cathode strike atoms of the
propellant gas which are pulled through the two grids by a field of 1 to
1.5kv.
The ions are ejected into space and the spacecraft is thrust forward by
recoil conservation of momentum. Electrons must also be ejected into space
to prevent the craft accumulating an excess of negative charge.
The system is placed in a weak magnetic field so the electrons follow a
much longer path between the electrodes. This increases the probability of
ionisation of the propellant atoms.
The field also protects the anode from damage from energetic ion
bombardment. Baffles protect the cathode and control gas flow.
Position maintaining thrusters of this type require a power of a few
hundred watts, which can be obtained from the solar cells. It is a fraction
of the power available from large solar arrays used on modern communication
satellites.
As the equipment draws power from the spacecraft rather than chemical
reactions, higher exhaust ion velocities could be achieved by using
higher accelerating voltages. This would reduce the intitial mass of
propellant required, but would involve a heavier propulsion system.
It is sensible to use heavier ion engines in large spacecraft and keep a
balance between the mass of propellant and that of the propulsion system.
Communication satellites rely on chemical rockets rather than ion
thrusters. One reason for this is the relatively low mass (up to little
more than 1 tonne) of such satellites.
It is only in heavy satellites that the increased payloads could
generate extra revenue to make ion propulsion economical. Considerable
investment is required to develop ion propulsion systems to commercial
status and this must be recouped from communications users.
Satellites of the past have had barely enough power to meet propulsion
as well as other demands. Designers have kept to well tried chemical
propellant systems.
Ion propulsion will be especially attractive for future satellites,
which must be relocated to different longitudes from time to time as the
system requirements change. Similarly, use of electric propulsion makes
it feasable to keep replacement spacecraft in orbit to provide cover for
faulty craft.
Chemically propelled craft use too much fuel to maintain a standby
position for a long period. Electically propelled craft can be moved
economically from geostationary orbit at the end of their life to make room
for replacement craft. This is becoming more important as geostationary
orbits become more crowded.
Mercury has been used as the propellant in most ion thruster work
because of its high density and easy storage. Unfortunately it amalgamates
rapidly with many of the metals used in spacecraft construction. It can
attack many structures in a spacecraft, including the solar panels and
electrical connections. Mercury must be heated to convert it into a
vapour before it is introduced into the ion engine.
Any mercury condensation could result in the shorting of high voltage
insulation and consequent damage. Mercury may solidify, if not heated,
during an eclipse. It is not easy to manage this dense liquid in zero
gravity.
These problems have lead to a search for more suitable
propellants. Caesium vapour, the heaviest of the alkali metal atoms, has
been tried, mainly in France. But caesium is a reactive metal, so it
is not surprising it damaged parts of the satellites. Krypton and argon have
also been tested.
Current work is concentrating on xenon, the heaviest of the inert
gases, to replace mercury as a propellant. Xenon will not react with
spacecraft materials and does not condense on any of the craft's components.
Unlike mercury, no power is required to vapourise Xenon becuase it is already
a gas.
The Atomic Energy Authority's Culham laboratory is currently working in
association with the new British Space Centre. Work is centred on the use
of Xenon propellant for 100mm diamteter thrusters.
Culham expects satellite test flights to take place in 1989 followed by
commercial exploitation soon after.
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