Measuring a mere ten centimetres across and emitting a faint blue glow as it runs, the Field Emission Electric Propulsion (FEEP) engine produces an average thrust equivalent to the force of a single falling hair. But despite its low power, FEEP’s thrust range and controllability are far superior to more forceful thrusters, holding the key to future success of an ambitious ESA science mission.
“Most propulsion systems are employed to get a vehicle from A to B,” explains Davide Nicolini of ESA’s Scientific Projects Department, in charge of the FEEP project. “But with FEEP the aim is to maintain a spacecraft in a fixed position, compensating for even the tiniest forces perturbing it to an accuracy that no other engine design can match.” Observing how objects behave when separated from all outside influences is a long-time ambition of physicists, but it is impossible to achieve within Earth’s gravity field. So a next-decade mission called LISA Pathfinder will fly 1.5 million km to an area in space called Lagrange Point 1 (L1), where the Sun and Earth’s gravities cancel each other out, so that the behaviour of a pair of free-floating test masses can be precisely monitored. However, to detach the experiment fully from the rest of the Universe there will still be some remaining perturbations to overcome, most notably the slight but continuous pressure of sunlight itself.
Which is where FEEP comes in. It operates on the same basic principle as other ion engines flown aboard ESA’s SMART-1 Moon mission and other spacecraft: the application of an electric field serves to accelerate electrically-charged atoms (known as ions), producing thrust. But while the thrust of other ion engines is measured in millinewtons, FEEP’s performance is assessed in terms of micronewtons – a unit one thousand times smaller. The engine has a thrust range of 0.1 – 150 micronewtons, with a resolution capability better than 0.1 micronewtons in a time response of 190 milliseconds or better. For propellant, FEEP employs the liquid metal caesium. Through capillary action the metal flows between a pair of metal surfaces that end in a razor-sharp slit. It has a gap measuring just one micron across – a hundred times narrower than the thickness of a human hair. Surface tension holds the caesium in place at the mouth of the slit, until an electric field is generated. This causes tiny cones to form in the liquid metal which have positive ions shooting from their tips to create thrust.
A total of three sets of four FEEP thrusters clustered together will be mounted on the hull of LISA (Laser Interferometer Space Antenna) Pathfinder. Operating together with a separate NASA-designed propulsion system, the FEEP thrusters should yield directional control at least two orders of magnitude more accurate than any spacecraft before it, down to a millionth of a millimetre. This month’s ESTEC tests are intended to qualify the FEEP development model before the construction of the final flight hardware begins.
