Arcjet
Understanding Arcjet Thrusters
When a satellite needs to maintain its orbital position or raise its orbit, it fires small thrusters. Chemical rockets work, but they are inefficient — they require heavy propellant for modest velocity changes. Arcjet thrusters use electrical power (ultimately derived from solar panels) to superheat propellant far beyond chemical flame temperatures, achieving two to three times better fuel efficiency than cold gas or bipropellant systems.
The Temperature Advantage
Thruster exhaust velocity — the key driver of propellant efficiency (specific impulse) — is proportional to the square root of temperature. Chemical combustion is limited to about 3,000–4,000 K by the energy content of the propellants. An arcjet heats propellant to 10,000 K or higher by concentrating electrical arc power directly into the gas flow. This temperature advantage translates to specific impulse (Isp) values of 600–1000 seconds, versus 230 seconds for cold gas and 300 seconds for hydrazine monopropellant thrusters.
RF and Microwave Plasma Thrusters
A related technology, the Microwave Electrothermal Thruster (MET), replaces the arc discharge with a resonant microwave cavity (typically 2.45 GHz) that sustains a plasma discharge inside the propellant flow. The microwave field couples energy directly into the plasma without electrode contact, eliminating erosion. Small METs have been demonstrated at power levels from 10W to 100W, suitable for CubeSat and microsatellite propulsion where thruster lifetime and propellant efficiency are critical constraints.
Key Equations
An Arcjet thruster is a form of electrothermal spacecraft propulsion in which an electrical arc discharge is used to superheat a propellant gas to extremely...
Key specifications:
000 K | 2.45 GHz | 10 W | 100 W
Link budget: C/N = EIRP−FSPL+G/T−10log(kB)
Comparison
| Aspect | Arcjet Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | RF plasma thrusters eliminate electrode... | Application-dep. | Critical | Verify in sim |
| Operating range | Both systems exemplify the direct conver... | Application-dep. | Critical | Verify in sim |
| Performance | Understanding Arcjet Thrusters When a sa... | Application-dep. | Critical | Verify in sim |
| Integration | Chemical rockets work, but they are inef... | Application-dep. | Critical | Verify in sim |
| Trade-off | The Temperature Advantage Thruster exhau... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
What propellants work with arcjets?
Hydrogen achieves the highest specific impulse due to its low molecular weight, but requires cryogenic storage, which is impractical for most satellites. Ammonia and hydrazine are the practical workhorses — they are storable liquids at room temperature, can be decomposed thermally before entering the arc chamber, and achieve Isp values of 600–800 seconds. Hydrazine is also compatible with existing monopropellant thruster infrastructure.
Why isn't arcjet propulsion used on all satellites?
Power constraints. An arcjet requires kilowatts of electrical power to produce millinewtons of thrust — the power levels available on communications satellites (10-20 kW from solar arrays) can support arcjets, but smaller satellites (100W or less) cannot. Small satellites typically use cold gas thrusters, resistojets, or electrospray thrusters that operate at much lower power levels, accepting lower efficiency in exchange for compatibility with limited solar power budgets.
What is specific impulse (Isp) and why does it matter?
Specific impulse measures how efficiently a thruster converts propellant into thrust, expressed in seconds. It is the exhaust velocity divided by standard gravity. Higher Isp means more delta-V (velocity change) per kilogram of propellant consumed. For a geostationary satellite that must maintain its orbital slot for 15 years, the difference between Isp 230s (hydrazine) and Isp 800s (arcjet) is the difference between carrying 200 kg versus 75 kg of propellant — directly translating to revenue-generating payload capacity.