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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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