How Yvonne Brill's Rocket Design Works

Since 88-year-old rocket scientist Yvonne Brill died on Wednesday, the Internet has been in a rage over the obituary the New York Times published. The online version has since been changed, but the print version still has the original lede, which puts Brill’s beef stroganoff before her contributions to rocketry.

Her work was varied – a consequence of her changing jobs as her husband moved through his own professional career – and fascinating, far more so than the post-New York Times kerfluffle. So, rather than adding more noise to the fray, how about we talk about the rocket work Brill actually did? It's all amazing stuff.

One of Brill’s contributions was the development of a system in the 1970s that enables an unmanned spacecraft to stay stationary in orbit. This is a crucial system, as no launch or flight is perfect. Problems with a booster can leave a satellite in the wrong orbit and problems in flight can seriously alter its orientation or path.

Case in point: Apollo 13. It reached orbit just fine, but when an oxygen tank exploded, the force of the escaping gas was enough to change the spacecraft’s flight path and require the crew manually align themselves for reentry. The same thing can happen with robotic spacecraft, but in these cases there’s no crew on board to fix things.

This wasn’t an unknown problem in the early days of space exploration. One of the ways scientists had developed to negate and fix flight problems was to introduce auxiliary propulsion systems into the spacecraft. These could correct errors, execute new orders from the ground, and maintain orientation.

At the time, magnetic torquing, gravity responsive systems, solar wind devices, and decomposable fuels (such as hydrogen peroxide and hydrazine) fed through thrusters were the main auxiliary systems in place. Electrostatic ion thrusters were also around, but they’re typically finicky and usually need a chemical propulsion system as a backup. Having multiple thrust systems of various designs added significant weight to any spaceflight platform.

Brill found a solution: Instead of using a number of different systems, use one single type of propellant for all functions. In 1974, Brill patented the hydrazine resistojet, which is an electrically-heated thruster. Eventually, monopropellant thrusters became standard in the industry, which means Brill was one of the central players to developing satellites than can fly themselves on their own. Oh, and yeah, she could cook.

Brill’s system had a liquid monopropellant distributed through a manifold, a pipe branching into several openings, feeding into a series of thrusters mounted on the spacecraft such that the exhaust would push the spacecraft in the right directions. This NASA video (starting at 7:00) from the 90s explains how it works:

It’s similar to the setup on the Apollo Lunar Module, where small thrusters jutted out at all axes to give astronauts full directional control. But the Apollo spacecraft were manned. In Brill’s robotic system, one set of thrusters was designed to give a lot of thrust (one pound or more) while the other set delivered less (on the order of 0.01 to 0.05 pounds of force). Valves on the thrusters delivered fuel selectively to one of the two sets, or both at the same time for dual-thrust propulsion.

The arrangement was fairly simple, at least as far as rocket science is concerned. As per Brill’s design, propellant in a reservoir is fed through a filter to remove impurities, then into a manifold, one for high and one for low level thrusters. The system is pressurized by nitrogen, which is either housed in the same tank as the propellant or separately. A pressure transducer monitors the gas supply pressure, while a pressure regulator adjusts the propellant flow. Valves can be operated manually, Brill stipulates in her patent, from a ground station via signal communication links

In action, liquid propellant (Brill’s preference was hydrazine), is delivered into a cylindrical catalyst bed by a series of perforated pipes impregnated with a catalyst material. The hydrazine decomposes upon contact with the catalyst at extremely high temperatures and varying pressures depending on the propellant supply pressure. The resulting expansion is thrust that is able to be modified for specific levels of power, remotely from the ground.

Brill was lauded not only for its creation but for foreseeing the beautiful simplicity of using a single propellant in an already complicated business. This patent, first conceived in 1967, was first applied on an RCA spacecraft in 1983, before becoming the go-to design in the satellite industry. Today, satellites using her invention are the backbone of our worldwide communication network. So while Brill may have made a mean beef stroganoff, she also expanded the frontiers of space, and the obituary argument aside, that's what she should be remembered for.