Spacecraft electric propulsion uses solar array power to ionize and accelerate inert gas propellants like Xenon (no, not from Superman’s home planet) for propulsion by Hall and gridded ion thrusters that gradually speed the spacecraft along its journey.
EP systems possess very high specific impulse, or the amount by which a change in speed alters per unit of propellant used, making them the optimal solution for long-duration missions.
What is an EP thruster?
Like electric cars using solar energy to recharge their batteries, spacecraft use solar energy to power innovative EP thrusters. When electrons from this energy come together with electrical current, they’re then sent directly to an EP thruster that uses its fuel (typically xenon in current systems) to produce ions which accelerate towards their intended destination and form an exhaust plume to propel its spacecraft in that direction.
This process may be slower than using chemicals, but is offset by the lower force of gravity in space. Maxar’s first all-EP spacecraft EUTELSAT 7C successfully reached orbit after more than 100 days of continuous thrusting.
Performance measurements require mounting the thruster on an inverted pendulum stand with flexures that redirect known forces into horizontal forces on the stand, which a Linear Variable Displacement Transducer (LVDT) measures and converts into an analyzable voltage signal proportional to thrust. Aerospace employs various diagnostic tools for thruster characterization including time-resolved ion energy distribution functions using retarding potential analyzers (RPAs). These devices feature three or four electrostatically biased meshed grids and collectors that measure energy distribution functions from thrusters for thruster characterization purposes.
Hall thruster
At this type of thruster, neutral propellant gas (usually xenon) diffuses into a thruster channel where it is ionized by colliding with high-energy electrons circulating in it and bearing positive charges of one. Once created, this plasma is then accelerated through its second mesh to produce high-velocity ion jets which propel forward movement of the spacecraft.
Hall thrusters differ from conventional ion thrusters by operating in a quasi-neutral plasma, thus eliminating space-charge build up as an issue and thus being capable of higher current and thrust densities at discharge voltages ranging from hundreds of volts up to several kilovolts.
One major challenge associated with Hall thrusters, however, is limiting erosion caused by rapid ion flux. To address this, NASA Glenn researchers have devised innovative techniques for increasing performance using advanced boron nitride ceramics in Hall thrusters.
Ion thruster
SSEP thrusters offer superior performance, durability and fuel efficiency – perfect for many tasks including precision positioning and orbit maintenance of small satellites (Picosats).
Propeller gas atoms are supplied to a hollow cathode unit, often duplicated for redundancy. It is then enclosed by a magnetized discharge chamber featuring an external cathode with an electric field which attracts electrons from external cathodes and accelerates them toward the center of the chamber, where they collide with propellant gas molecules, ionizing them.
Ions are then trapped by grids containing internal mesh of electrodes, known as grids, and accelerated at high exhaust velocity for steady thrust rather than sudden bursts of acceleration that characterize chemical propulsion systems. NASA’s Dawn mission to the Moon utilizes this propulsion method while Hayabusa from Japan visits Asteroid 25143 Itokawa while ESA Bepicolombo heads towards Mercury while LISA Pathfinder studies low frequency gravitational waves – these systems also make an ideal fit for Cubesat missions!
Plasma thruster
Plasma thrusters utilize many of the same technologies found in Hall thrusters and gridded ion engines, but with higher electrical costs. Their exhaust produces high velocity air flow for thrust.
Plasma thrusters use propellant gas (typically xenon) as fuel in an engine, where it is then ionized by an electric field and electron flux from an anode anode to accelerate and counteract negative ions, canceling out thrust and producing neutral plasma that can then be used to maneuver spacecraft.
To travel quickly to Mars in an acceptable timeframe, plasma thrusters need to generate massive thrust. Glenn researchers have miniaturized all necessary components necessary for designing high-performance EP thrusters for both Space Science Exploration Pilot (SSEP) and commercial missions requiring long life performance with fuel efficiency as the goal. Northrup Grumman currently works under research license with Glenn in developing customer satellite systems incorporating this cutting edge technology.