Push for the planets

For a number of years, the search has been on for smaller, lighter and more efficient propulsors capable of repositioning spacecraft, raising their orbits, and, ultimately, of propelling them far out into the solar system on interplanetary missions. The chemical-propellant thrusters so far used to manoeuvre spacecraft are too heavy to be efficient over long […]

For a number of years, the search has been on for smaller, lighter and more efficient propulsors capable of repositioning spacecraft, raising their orbits, and, ultimately, of propelling them far out into the solar system on interplanetary missions.

The chemical-propellant thrusters so far used to manoeuvre spacecraft are too heavy to be efficient over long distances. On-board propulsion systems sometimes make up over half of a spacecraft’s total mass at launch, according to Dr David Fearn, science adviser for space technology at the Defence Evaluation & Research Agency (Dera).

Along with other scientists in Europe and the US, Fearn and his team at Dera are working on the development of light-weight propulsion systems that employ positively charged atoms, known as ions.

In an ion thruster, the propellant is ionised in an electrical discharge. The ions produced are then extracted from the resulting plasma and accelerated to a very high velocity by an electric field, thus producing an extremely energetic exhaust.

The latest British thrusters, which Dera has at an advanced development stage, use the inert gas xenon as propellant. This is ionised by electron bombardment, the resulting ions being accelerated by the electric field produced between perforated and aligned grids forming the exit to the thruster.

Ion thrusters produce about a hundred million times less thrust than the rockets that put spacecraft into orbit. But they are very efficient, because the velocity of the ejected propellant is an order of magnitude higher than conventional thrusters, at 30 40km/s. This means the spacecraft can carry less fuel, so its payload capacity can be increased, its lifetime can be extended and its launch costs can be cut.

Other benefits include smoother, more easily controlled throttling vital if the spacecraft’s position has to be controlled precisely. The system is also ‘clean’, as it does not use highly toxic, corrosive and potentially explosive chemicals.

The drawbacks of electric propulsion systems are that they are more complex and need a lot of electrical power. They also provide only low levels of thrust, so must operate for longer than chemical drives to reach a given velocity. So they must be particularly reliable, with an operating life of 10,000 15,000 hours.

Dera has been working on ion propulsion for around 30 years, but commercially viable thrusters have only been developed in recent years. They are based on the US Kaufman design, heavily modified in joint research between Dera and the UK Atomic Energy Authority’s Culham Laboratory. Matra Marconi Space (MMS) is also contributing to the research, under an agreement with Dera to jointly identify spacecraft propulsion opportunities.

The first of the thrusters, the 10cm diameter T5, was developed in the late 1980s for tasks which require moderate thrust below around 30 millinewtons (30mN). Its first use will be on the Artemis communications satellite, due for launch in 2000 by the European Space Agency (ESA).

The T5 is part of the UK-10 ion propulsion system, developed and formally qualified for flight by MMS. Once the Artemis satellite is in orbit, the UK-10 will be used for north-south station-keeping.

Seeking other missions

Dera says the T5’s wide throttling range could make it a candidate for ESA’s forthcoming Gravity and Ocean Circulation Explorer mission, where variable atmospheric drag at low altitude will have to be balanced by an appropriate level of thrust.

A more powerful 22cm-diameter T6 is also being developed, to a specification to suit the requirements of a new generation of larger, long-life geostationary communications satellites. Dera is working on the thruster, which has a nominal thrust of 150mN, MMS on the associated power conditioning unit and control system. Prototype testing should start later this year.

Studies are also under way to identify the specification required for a higher-thrust T6, capable of orbit-raising manoeuvres. A more powerful 25cm thruster, producing high-velocity thrust in the 50 500mN range, is at a less advanced stage of development. Fearn says this is best suited to interplanetary missions.

Another near-term use for the T5 thruster could be the SMART-1 satellite, on which ESA will conduct tests to determine ion propulsion’s suitability for interplanetary travel. This low-cost mission to a comet or an asteroid could make use of two T5 thrusters and a spare unit to enhance reliability. Dera is awaiting an invitation to tender from ESA.

The T5 will not be the first ion propulsion system tested in this role. Nasa’s Jet Propulsion Laboratory has completed an 8,000-hour test on a prototype interplanetary thruster to be fitted on the Deep Space 1 (DS1), scheduled for launch in October.

The ion engine will be activated a few weeks after launch to increase the spacecraft’s speed by about 3.6km/s fast enough, if all goes well, to reach asteroid 1992 KD on 28 July 1999 and Comet Borrelly two years later.