Onboard propulsion

Draft

Launch and Propulsion covers how thrusters work. This page covers everything around them: the tanks, feed system, propellant management and layout that turn an engine into a subsystem.

Architecture by mission

Type	$I_{sp}$	Thrust	Typical use
Cold gas	60 to 80 s	mN to N	CubeSats, contamination-sensitive proximity operations
Monopropellant hydrazine	220 to 235 s	0.5 to 500 N	Attitude control, small manoeuvres, the long-standing default
Bipropellant	300 to 325 s	10 to 500 N	Apogee insertion and large manoeuvres
Green monopropellant	235 to 250 s	0.5 to 100 N	AF-M315E and LMP-103S, replacing hydrazine to remove handling hazards
Electric	1500 to 4500 s	mN	Station keeping, orbit raising, primary propulsion

Many spacecraft carry two systems: a chemical system for time-critical manoeuvres and attitude control authority, and an electric system for the delta-v-intensive work. That combination is now standard on commercial geostationary platforms, where the chemical stage lifts perigee out of the radiation belts quickly and the electric system finishes the transfer.

Hydrazine’s replacement is being driven by ground operations rather than performance. Hydrazine is acutely toxic and carcinogenic, requiring self-contained protective suits for fuelling, which adds cost and schedule to every launch campaign. Green monopropellants offer slightly better performance and density and can be handled with ordinary protective equipment.

Pressurisation

Propellant must be pushed to the thrusters, and the two ways of doing that trade simplicity against performance.

Blowdown loads the tank with propellant and a fixed charge of pressurant gas, usually helium or nitrogen, in the same volume. As propellant is consumed the gas expands and pressure falls, so thrust and specific impulse decline through the mission. Blowdown ratios of 3:1 or 4:1 are typical, meaning beginning-of-life pressure is three or four times the end-of-life value.

The system is extremely simple, with no regulator and no separate pressurant tank, and it is the right answer when varying thrust is acceptable. The penalty is that thruster performance is a function of how much propellant remains, which complicates manoeuvre planning.

Regulated systems carry pressurant in a separate high-pressure tank and feed it through a regulator that holds constant tank pressure. Thrust and specific impulse stay constant throughout. The cost is a high-pressure tank, a regulator, and additional valves, all of which are mass and additional failure modes. Large bipropellant systems are almost always regulated, because a 30% thrust decline through an apogee burn is unacceptable.

Propellant management in microgravity

On the ground, gravity holds liquid at the bottom of a tank and the outlet is placed there. In microgravity there is no bottom. Surface tension governs, and liquid will wet the walls, form a bubble anywhere, or float as a mass with the pressurant gas surrounding it.

Feeding gas into a thruster instead of liquid causes anything from a performance shortfall to a destroyed engine, so every tank needs a propellant management device that guarantees liquid at the outlet.

Surface tension vanes: Radial vanes that use capillary forces to draw liquid toward the outlet. No moving parts, no life limit, and the most common approach.
Sponges and traps: A capillary structure near the outlet that retains a liquid reservoir even when the bulk propellant has moved away, guaranteeing supply through a settling burn.
Elastomeric diaphragms: A flexible membrane physically separates pressurant from propellant and expels liquid positively. Complete expulsion and simple behaviour, but limited by material compatibility and cycle life.
Settling burns: Fire small thrusters to accelerate the vehicle and settle propellant to one end before a main burn. Costs propellant, and is used where a PMD is impractical, notably on large cryogenic stages.

Cryogenic propellants add boil-off to this problem and are the reason long-term in-space cryogenic storage remains difficult. See Refueling and propellant depots.

Gauging

Knowing how much propellant remains is surprisingly hard, and the uncertainty has direct operational consequences.

Method	Principle	Accuracy	Degrades when
Bookkeeping	Integrate commanded thruster on-time times modelled flow rate	±2 to 5%	Errors accumulate monotonically over the mission
PVT	Measure pressurant pressure, volume and temperature, infer the gas volume and hence the liquid volume	±2 to 5% early, worse late	The tank empties, since the inference is a small difference between large numbers
Thermal gauging	Apply a known heat pulse and measure the temperature response, which depends on the thermal mass present	±1 to 3%	Requires a quiescent thermal state

Every method degrades toward end of life, which is exactly when the answer matters most. A geostationary operator deciding whether to attempt another year of station keeping or to commit the remaining propellant to a graveyard manoeuvre is working with an uncertainty of several per cent on a quantity that is nearly exhausted.

That uncertainty is why disposal reserves are conservative, and why some satellites are retired with usable propellant still aboard. The alternative, running the tanks dry and being unable to perform disposal, leaves a derelict in the operational belt permanently. See Station keeping.

Layout and plume impingement

Thruster placement is a configuration problem with three competing pulls.

Torque authority wants thrusters far from the centre of mass, at the extremities, because moment arm is what produces torque. Translational manoeuvres want thrust through the centre of mass, so that a translation does not also produce a rotation that the attitude system must then cancel. And plume impingement wants thrusters pointed away from everything.

A thruster plume in vacuum expands enormously, far beyond the nozzle cone, and carries both force and contamination. Three consequences follow:

Effect	Consequence
Impingement force	A plume striking a solar array produces an unintended force and torque, partly cancelling the intended manoeuvre
Heating	Local heating of an impinged surface, which the thermal model must account for
Contamination	Combustion products deposit on optics, radiators and solar cells, degrading them permanently
Electrical interaction	Electric thruster plumes are plasma and can couple into the spacecraft’s electrical ground

Contamination is the one that ends missions rather than merely degrading a manoeuvre. Deposits on a radiator raise its solar absorptivity, which is the degradation mechanism that makes the end-of-life case the hot case in thermal control. Plume analysis is consequently a real discipline, and thruster canting away from sensitive surfaces, at the cost of some cosine efficiency, is routine.

Feed system

Two components in that chain exist purely for fault containment. Latch valves isolate a branch so that a single leaking thruster valve cannot empty the tank, which is a credible and otherwise mission-ending failure. Filters protect thruster valve seats from particulates, since a particle lodged in a seat holds a thruster open, producing a continuous unwanted torque that the attitude system fights until the propellant is gone.

Both are cheap insurance against failures that have actually happened, which is the usual reason a component is in a flight design.

References

Sutton, G. P. and Biblarz, O. Rocket Propulsion Elements, 9th ed., Wiley, 2017, chapters 6 and 11.
Jaekle, D. E. “Propellant Management Device Conceptual Design and Analysis”, a series of AIAA papers covering vanes, sponges, traps and galleries, 1991 onward.
Dandaleix, L. et al. “Flight Validation of the Thermal Propellant Gauging Method Used at CNES”, AIAA 2005-3773.
Boyd, I. D. “Review of Hall Thruster Plume Modeling”, Journal of Spacecraft and Rockets, 38:381, 2001.
Spores, R. A. et al. “GPIM AF-M315E Propulsion System”, AIAA 2013-3849, on green monopropellant flight qualification.