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A communications satellite does not stay put on its own. Small errors from launch, the pull of the Sun and Moon, and the need to keep its antennas pointed all mean it must fire tiny thrusters for years. Every kilogram of propellant for those thrusters is a kilogram that cannot be equipment. Yvonne Brill found a way to get more out of each kilogram. This post covers who she was, what her thruster does, the physics behind it, a worked example with the rocket equation, common mistakes, and what you can take from her way of working.
Who was Yvonne Brill?
The National Inventors Hall of Fame, which inducted her in 2010, gives her life as 30 December 1924 to 27 March 2013. She was born Yvonne Claeys in Winnipeg, Canada, to parents who had come from Belgium, and was the first in her family to go to college. The University of Manitoba did not then allow women to enrol in engineering, so she studied mathematics and graduated at the top of her class in 1945.
She then joined Douglas Aircraft in California. The Hall of Fame says she was believed to be the only woman in the United States working as a rocket scientist at the time. She studied in her spare time for a master's degree in chemistry from the University of Southern California, awarded in 1951. In 1966 she joined RCA Astro Electronics as a senior engineer in propulsion systems.
The National Science and Technology Medals Foundation says her early studies helped set industry performance standards for rocket propellants and needed thermodynamic data up to 5,000 kelvin, beyond the published tables, so her team extended them. She worked on propulsion for Tiros, the first weather satellite, and for Nova, which the foundation describes as a series of rockets that were part of the Apollo Moon missions.
What problem does a satellite thruster solve?
Brill's US patent No. 3,807,657, filed on 31 January 1972 and granted on 30 April 1974 to RCA, sets out the job. Spacecraft need auxiliary propulsion to correct errors left by launch, to reach and keep their orbit or geostationary position, and to control their orientation for power, temperature and other systems.
Her design used hydrazine, a monopropellant that breaks down over a catalyst into hot gas without needing a separate oxidiser. One set of thrusters gave larger thrusts of about 1 to 5 pounds by catalytic decomposition alone. A second set gave thrusts "in the millipound range" by passing the decomposed gas over electrical heating elements, called resistojets, before it left the nozzle.
Why does heating the exhaust help?
A rocket pushes itself forward by throwing gas backward, so the faster the gas leaves, the more momentum each kilogram carries. Kinetic theory gives a clue to why heat helps: the rms speed of gas molecules is √(3RT/M), which rises with temperature T and falls with molar mass M. Heating the gas with electricity from the satellite's solar panels raises its exit speed without burning extra propellant.
The medals foundation says the thruster kept satellites truer to their orbits and used less fuel, so spacecraft could carry more equipment and stay in service longer, and that hundreds of these thrusters have been launched. Her 2010 National Medal of Technology and Innovation was "for innovation in rocket propulsion systems for geosynchronous and low earth orbit communication satellites".
Worked example: how much propellant does a hotter exhaust save?
These are sample numbers for a classroom comparison, not data for any real satellite. A satellite has a mass of 1,000 kg and needs a total velocity change of 500 m/s over its life, say 50 m/s a year for ten years.
The rocket equation gives the propellant used: m = M(1 − e^(−Δv/vₑ)), where M is the starting mass and vₑ is the exhaust speed.
Unheated exhaust, vₑ = 2,200 m/s. Δv/vₑ = 500/2,200 = 0.227. e^(−0.227) = 0.797. Propellant = 1,000 × (1 − 0.797) = 203 kg.
Heated exhaust, vₑ = 3,000 m/s. Δv/vₑ = 0.167. e^(−0.167) = 0.846. Propellant = 1,000 × 0.154 = 154 kg.
Saving. About 50 kg, which could be fuel for extra years or mass for more equipment.
What the heating had to do. If the gas stayed the same, speed scales with √T, so a 3,000/2,200 = 1.36 times faster exhaust needs about 1.36² = 1.86 times the absolute temperature.
The thrust is tiny. One millipound is about 4.4 mN, and on a 1,000 kg satellite that gives an acceleration of about 4.4 × 10⁻⁶ m/s². That is fine for gentle corrections that can run for minutes or hours.
What mistakes do students make with problems like this?
Subtracting the wrong mass. The rocket equation compares starting and final mass; propellant is the difference.
Thinking small thrust means useless. A small force over a long time still gives the needed change in velocity.
Using Celsius in √T. Use kelvin.
Forgetting that momentum is conserved. The satellite gains forward momentum equal to the backward momentum of the gas.
The JEE (Main) 2026 syllabus covers conservation of linear momentum in Unit 3, satellite motion in Unit 6 and rms speed in Unit 9. The rocket equation is beyond the syllabus; our post on Konstantin Tsiolkovsky derives it. For orbit problems, see our guide to gravitation and satellite problems.
If kinetic theory is the gap, the Fermi Concept Library has every concept in physics with its own lesson, so you can revise rms speed before returning to rockets.
What can a student take from Brill's way of working?
Find another door. Turned away from engineering, she studied mathematics and chemistry and became a rocket scientist anyway.
Do the groundwork others skip. Extending property tables to 5,000 K was slow work that made later designs possible.
Small gains add up. A thruster measured in millipounds changed how long satellites could work.
Next step: redo the worked example for a 2,000 kg satellite needing 800 m/s, with both exhaust speeds.
