The rocket thruster that changed the world
by Matthew R. Francis
Our culture has two stereotypes for intellectual genius: Albert Einstein and rocket scientists. Ironically, the very impractical Einstein would have been a terrible rocket scientist, a job requiring careful planning and high levels of engineering skill. In many ways, these two stereotypes of genius couldn’t be more different. Yet both theoretical physics and rocket science are creative endeavors, and their best practitioners change the way we explore our Universe.
So it was with rocket engineer Yvonne Brill, who died on March 28, 2013 at age 88. She held the patent on the “dual thrust level monopropellant spacecraft propulsion system,” a complicated name for an extremely important invention. Basically, Brill found a way to use a single type of rocket fuel to boost and control spacecraft while in the vacuum of space. In the early 1970s, she solved a problem inherent to spaceflight by combining several different systems into one, allowing spacecraft to be lighter by carrying less fuel.
Rocket science for all
Fuel weight is a significant problem for a rocket. If you want to get into Earth orbit or beyond, you have to expend a lot of energy, overcoming Earth’s gravity and pushing against air resistance. That requires transporting the weight of the payload (whether it’s people, satellites, or robotic rovers), the rocket itself, and the fuel needed from the surface into space. The heavier the payload and rocket, the more fuel you need – which adds to the total weight of the rocket.
The payload itself needs fuel, whether it’s the Space Shuttle, a communications satellite, or a probe headed for Mars, for a variety of reasons. First, on its way up, a rocket is subject to unpredictable fluctuations in its trajectory, meaning it won’t end up exactly where it’s supposed to. That’s a really bad thing for a satellite: a failed orbit could mean the (very expensive) equipment burns up in Earth’s atmosphere. However, even a successful orbit isn’t forever: Satellites orbit within Earth’s exosphere, the outermost region of the atmosphere where the air is extremely thin but still produces a small drag on anything flying through it.
On the flip side, too much correction is also a bad thing, possibly sending satellites tumbling. The density of the exosphere is too low to provide anything to push against (and the situation is worse in deeper space), so two thrusters acting in opposite directions provide steering for a spacecraft. These thrusters work successively, producing small adjustments until the craft is aimed the right way. The Space Shuttles had six thrusters of this kind because their wings and tail were useful, like a plane’s, in only the atmosphere.
In the Apollo missions and other spacecraft from the same era, multiple systems handled the boosting and correction tasks. You would have one set of rockets for the main thrusters – those that carried the lunar part of the mission away from Earth orbit to the Moon, and those that took Apollo out of lunar orbit back to Earth – and those responsible for less significant steering. Each system had separate fuel types and therefore needed different tanks. Some thruster designs even required multiple chemicals, which had to be mixed in exactly the right proportions to create thrust.
Brill(iant) designBrill’s design eliminated this redundancy and lightened the spacecraft in the process. She also used a type of fuel called hydrazine, which is so reactive you don’t need oxygen or another chemical injection to ignite it. (On Earth, we’ve got lots of oxygen available for making things burn, but in space, you need to carry your own fuel for fire.) Brill’s system pumped liquid hydrazine through an aluminum nozzle. The chemical composition of the nozzle reacted with it, splitting it into smaller molecules and releasing a lot of energy.
The energy from the chemical reactions was released in the form of heat, accelerating the molecules and producing an expanding cloud of gas. Because of the design of the thruster, the gas could expand in only one direction: out the business end of the nozzle, providing the thrust needed to steer the spacecraft. To achieve even higher temperatures, Brill added a heating coil, powered by electricity. For that reason, her system is known as a hydrazine resistojet because the electrical resistance in the coil was what made it heat up.
Thus, Brill’s design allowed a small amount of liquid hydrazine fuel to be transformed into a lot of hot gas, expelled at a fast velocity through a nozzle in a controlled way. The same tank could fuel every thruster, and a computer could mange whole system – an essential feature for a satellite. (For more information and larger historical perspective, see Amy Shira Teitel’s excellent piece for Vice.com.)
When we think about spaceflight, we often focus on the astronauts or even on the spacecraft themselves, to the neglect of the engineers who make it all work. However, not a single one of the great space adventures could exist without engineers like Yvonne Brill. The Viking landers of the 1970s, the satellite communications revolution of the 1980s that continues today, and the Space Shuttles owe much to Brill’s work. Even as researchers look for improved fuels to replace hydrazine (a nastily toxic substance), her thruster concept and design continues to be the basis for spacecraft control.
Rest in peace, Yvonne Brill. Your work may have been rocket science, but honoring you doesn’t take the genius of an Einstein.