ION ENGINES

By Michael Martin-Smith

Oct 24, 1998 saw the debut of an entirely new, though long theorised, new propulsion system for spacecraft - the ion engine. The first of NASA's New Millennium space missions, aimed at testing, in flight conditions, new systems for the coming century at low costs and risk, was launched by a Delta 2 rocket, on a mission to visit Asteroid 19092KD, in July 1999, and, if all is still well, to follow on to Comet Borelly in 2002. The craft, Deep Space 1, is to verify 12 new technologies, including solar-electric xenon gas ion propulsion, developed by NASA Lewis, self regulating navigation systems, and solar power concentrators, among others. The aim is to reduce costs, size and complexity of future interplanetary robot missions with less risk to major scientific objectives. Following launch, the telemetry and solar panels are already being proven, while first test of the ion engine begins in 2 weeks' time ( Nov 7 or so). This article sets out to explain ion engines and their potential for the future.

The performance of rocket engines is typically governed by the Rocket Equation formula

Vf =Ve*2.3 Log10*(M1 /M0),

where:

This, in effect, is the payload plus the structural mass. It is this relationship which governs the performance of all present and hitherto imagined forms of rocket propulsion. All combinations of fuels, electric ion engines, mass driver systems, fusion, fission, or anti-matter engines, are methods of increasing either the mass ejected per second, or its speed of ejection . Another quantity, the Specific Impulse, is a measure of the amount of thrust to be derived per kilogram of any given fuel - typically, in a Space Shuttle Main engine, this is about 460 seconds - the best that can be done with conventional liquid fuelled engines. Nuclear engines, in which a fuel is heated by an atomic reactor rather than a chemical reaction, could top 900-1000 seconds specific impulse, during tests in the early 1970's.

Ion engines operate in a far more leisurely fashion than chemical combustion, over periods of months or even years, at very low levels of thrust. The energy source is divorced from the propellant, and consists of DC electricity, supplied classically by solar photoelectric panels, or, more futuristically, nuclear-thermionic generation. The propellant is passed through a high voltage current, and electro- magnetic field, so that its atoms are ionised, or stripped of their electron shells; being charged particles, they can then be accelerated by an intense electric field at speeds of up to 40 kilometres per second. The result is a specific impulse of up to 5,000 seconds, 10 times that of the best chemical systems, sustained over thousands of hours. The catch is that the actual thrust, instead of thousands of Newtons, is, typically 18-70 mN in the most developed version, or 260 mN in the bigger brother now being developed. Thus, ion engines are unsuitable for lift off, where giant thrusts are required to achieve rapid acceleration away from Earth's high gravity. In the vacuum of Space, away from 1 G, however, we have all the time in the world, and a slow gentle sustained push can achieve great things. Just as, without friction, a boy can push a large ship away from a pier-side with his thumb if he perseveres, so the cumulative thrust of an ion engine, over weeks, builds up impressive velocity.

Research and Development of ion engines has proceeded in the United Kingdom in two phases; the first, from the 1960's to 1975, resulted in a prototype engine capable of 10 mN, in which mercury was the propellent. This was proposed for a station-keeping role in the large research communications satellite, Olympus, but for funding reasons did not acquire sufficient laboratory life testing to be ready for the job. In the mid 1980's, work resumed on a new version, in which the inert gas xenon was substituted for mercury, in order to avoid risks of contaminating satellites' instruments. The UK-10, with an ion beam of 10 cms diameter, is a light weight focussed ion beam engine with a 10 centimetre diameter, and was developed by the Defence Research Agency at Farnborough, and Matra Marconi Space, with a grant from ESA, and has now had over two thousand hours of "life testing", in daily 3 hour pulses, in preparation for space qualification on board the ESA ARTEMIS satellite. This was due for launch in 1998, but, for budgetary reasons, is unlikely to fly before the year 2002. UK-10 can achieve up to 70 mN, although it is most efficient at 25 mN.

Efficiency and thrust are determined by ionisation voltage from anode to cathode, and by propellant feed rate. Typically, ionisation voltages of over 40 volts lead to erosion of the thrust chamber and reduced life span, while fuel utilization rates of 6 milligrams per second produce 25 mN, within a field of 1100 volts. Applications being considered are orbit maintenance and station keeping on satellites; in allowing 10 times less fuel for this function, considerable gains in mass and orbital lifespan can be achieved. An example is a low mass/low cost Earth resources /mapping satellite proposal in which a 1-2 metre resolution camera can be orbited at 300 kilometres, with a lifespan of 5 years, in a 270 kilogram bus. Launch costs could thus be greatly reduced by use of a Pegasus XL launcher. Much cheaper, if more gradual, boosts from LEO to geostationary orbit could be achieved with ion engines, where a 3 month drift into high orbit is not critical. Even more dramatically, the 370 kg Ulysses solar polar satellite, launched in 1989, required an Inertial Upper(*1) Stage / PAM(*2) combination weighing 20 tons, to take it, via planetary flybys(*3), out of Earth's orbit to the region of Jupiter, before passing over the Sun. With an ion engine cluster, the same spacecraft would have required 2.4 tons of engine/propellant, and could thus have been launched by an Ariane 4 ! It has been calculated that, if the International Space Station were to use ion engines for orbit maintenance rather than chemical rockets, about 1 ton of fuel per year would be needed rather than more than 10 tons as now envisaged - a considerable cost saving, one would think.

In 1997, the Hughes PanAmSat 5 satellite used, for the first time, a small xenon ion engine for orbit maintenance. The larger UK-25 ion engine has achieved 260mN in tests to date, and could probably be stretched to 500 mN. These could be used to send fast probes beyond the outer solar system, to near interstellar space, with Delta class rockets, at costs within the reach of Discovery class missions now being flown to Mars and the asteroids, while the NASA Lewis engine now working on DS-1 has an ion beam of 30 cms diameter, and has been life-tested in laboratory conditions for 8,000 running hours ( nearly one year!) - thus proving the concept of a long duration gentle propulsion to build up speed.

The USA and UK thus have flight ready ion engine families, which will see increasing service in the next century's space industry. The first use "in anger" of a UK ion engine, Together with the German RITA ion system) could be on board the ESA 's advanced Artemis experimental optical communications satellite, while an other early use for a smaller ( 5-6 milliNewtons thrust) ion engine, in development by the UK and Russia, could see service on board an exciting amateur astronomical satellite planned for flight in 2-3 years - none other than the Humble Space Telescope ( funding permitting!!). A similar small ion thruster is to be used on the 370kg Muses-C Japanese/NASA sample/return probe to the asteroid Nereus, due for launch in 2002, with return from Nereus to Earth due in 2006. Thus, within 5 years, we shall see the introduction of a major advance in space propulsion technology, which, for decades has been the stuff of science fiction. In operating at 10 times the fuel efficiency of conventional chemical rockets, smaller, faster, or more longlived spacecraft will become possible, and, ipso facto, costs lowered and possibilities for space exploration increased!

(Acknowledgements are due to the British Interplanetary Society, 27/9 South Lambeth Rd, SW8 1SZ, for permission to use the May 1996 JBIS volume on Electric Propulsion Systems as a reference source.)

-- Michael Martin-Smith

NOTES:

(*1) Inertial Upper Stage is a two stage solid rocket booster, attached to Ulysses( also used with Galileo and some comsats on earlier Shuttle flights) wh ich are used to propel space probes into more distant orbits once spriong launched out of the cargo bay. It means such payloads can be launched from the Shuttle, although the shuttle itself only goes as far as Low Earth Orbit. IUS stands for Inertial Upper Stage.

(*2) PAM = Payload Assist Module, and is a smaller third solid rocket booster stage. Thus Ulysses needed a three stage boost, as well as Jupiter's gravity assist! Nowadays, these stages are very rarely used, and the Shuttle's work is almost all human centred in LEO, and now with the Space station construction.

(*3) Planetary flyby is the use of a planet's gravitational field to add/subtract velocity from a passing space probe. Used to great effect with Voyager and Galileo. Ulysses was designed to do a po lar orbit of the Sun to look at the magnetic phenomena at extreme polar latitudes on the Sun- never seen from earth. This requires an enormous change in velocity- so the craft had to go out to Jupiter, and , by flyby as above, be shifted into a North-Sout h ploar orbit rather than an equatorial ( ecliptic ) orbit. It did not approach the sun very closely- but passed over the poles at about 150 milion kilometres. This 3 D perspective cannot be obtained from Earth.The Sun itself was not used for flyby. \par }}

Torna alla Home Page