The space-clutter dilemma

The history of the solid rocket engine reaches back to 13th-century China, when soldiers fired gunpowder-filled “fire arrows” at invading Mongols. After many advancements, solid rocket engines—called that because they use a solid, granular fuel—have powered U.S. intercontinental ballistic missiles, launched astronauts into space, and delivered the nation’s most sophisticated satellites into orbit.

Yet the technology always suffered a major drawback: a solid rocket engine can fire only once. This has had special consequences for satellites because, although solid rocket engines can launch them into orbit, once there, satellites must rely on small, liquid-fuel engines for trajectory adjustments.

Liquid-fuel engines can fire as many times as needed. But their compression chambers and volatile fuel make for a higher risk of accidental ignition, especially during the strain of launch. Although that risk is tolerable for larger satellites delivered individually into space, the most ubiquitous satellites are smaller communication or imaging devices that often share one launch to reduce costs. If a single liquid-fuel engine explodes, it jeopardizes the rest. So these small satellites are often sent into orbit without a way to be maneuvered or to be deorbited once they’ve completed their missions.

Once in low Earth orbit (typically below an altitude of 1,200 miles), about 5,000 satellites contend for room, along with 20,000 softball-size or larger pieces of space debris—everything from dropped payloads to spent rockets. Collisions are infrequent (the last was in 2009), but near collisions pose a constant threat.

In January, two defunct satellites missed each other by 154 feet. If they’d collided, the debris could have crashed into other satellites, possibly causing more collisions. This problem grows exponentially as satellites become increasingly affordable and more are placed into orbit.

At Los Alamos National Laboratory, the work of research engineer Nicholas Dallmann and his team has finally solved this problem. Their Restartable Rocket Engine can start, stop, and start again as many times as necessary to help small satellites maneuver safely through space. The Restartable Rocket Engine, which is safe, reliable, and cheap to build, was made possible by three major breakthroughs.

The first concerned the fuel grains. The typical solid rocket combines both a fuel and an oxidizer in its body to use as propellant, but the Restartable Rocket Engine physically separates these two, which reduces the chance of accidental combustion to almost zero.

Next, to ignite the fuel, the Los Alamos team developed an electrolyzer similar to those used on submarines but modified for low-gravity space. This electrolyzer separates water into hydrogen and oxygen gas and lights them with a spark.

The biggest innovation was the ability of the rocket engine to switch on and off. Dallman’s team knew that a rapid change in pressure in the engine body could do this. So they reinvented an aerospike and cowl, the small opening at the end of a rocket body that focuses escaping gasses combusted in the engine that create thrust. The cowl on the Restartable Rocket Engine can expand and contract, like a camera’s shutter, to rapidly drop the pressure in the body and stop combustion. Then it resets with a magnet so the process can begin again.

With those three developments, the Restartable Rocket Engine is safe enough to be mounted on small satellites piggybacking on a shared payload, and durable enough to survive years in space. With this technology, Los Alamos hopes to help society conserve low Earth orbit as a resource for future generations for centuries to come.