Low-Earth orbit, home to the International Space Station and countless satellites, has become a racetrack filled with space junk, skyrocketing the possibilities for collisions. A new lab facility at the University of Illinois Urbana-Champaign is addressing this dangerous highway. It runs experiments on the materials used on the outermost surface of spacecraft that provide protection from impact, because even tiny specks of dust traveling at orbital speeds can create major damage.

The star of the new facility is a two-stage, light-gas gun. It sequentially compresses helium and then hydrogen to propel several-millimeter-size projectiles through a tube at speeds of up to 7.5 kilometers per second to impact test materials mounted at the end of this 35-foot projectile launcher. By way of comparison, a bullet fired from a high-power rifle travels a little over 1 kilometer per second.

“The particular projectile speed of 7.5 kilometers per second is important,” said Ioannis Chasiotis, a professor in the Department of Aerospace Engineering in The Grainger College of Engineering at Illinois. “It is the velocity of spacecraft and orbital space debris in a typical low-Earth orbit, so this two-stage light-gas gun helps us simulate the damage that would occur in such a collision. Tiny flecks of paint, when traveling at that speed, have been known to crack a window on the International Space Station.”

He said NASA and other agencies track pieces of debris larger than 10 centimeters in size. If the International Space Station is about to collide with one, they get a notification to reposition and avoid it.

“What can’t be tracked are smaller pieces of space debris, a few millimeters in size, like the paint fleck. There are an estimated 150 million of them on orbit. There’s also no easy way to sweep them up, so we need to focus on protection.”

Like bumpers on cars, two plates with a gap between them are mounted on the exterior of the most sensitive components of a spacecraft. The gap is essential because the projectile debris, forming upon impact and perforation of the first plate, expands a lot which reduces the severity of impact to the second plate. In Chasiotis’ facility, the plates are made from a combination of high-performance materials, composites with carbon fiber, Kevlar fiber, metals and ceramics. The projectile, made from aluminum or steel, penetrates the first plate, and its extensive debris cloud is contained by the second plate.

“The key is determining what combination and sequence of materials will stop a projectile with the least amount of mass. We can run tests knowing the size of the projectile and set its velocity to learn how individual materials respond to the impact. Then we can use computational tools to design for the material sequence, the thickness and the total weight.
“For example, one thing we’ve learned is at a velocity of 6.5 kilometers per second, aluminum is defeated. Due to the high pressure and heat developed during impact, the aluminum projectiles melted while passing through the first plate, leaving what looked like a powder on the surface of the second plate.”

Chasiotis said that this one-of-a-kind facility of a two-stage light-gas gun with high-velocity/high-pressure capabilities, is also equipped with high-speed optical cameras that record the impact event.

“Because we can’t possibly test every material and every velocity, we take the data from the cameras and validate our computer simulations. We also have tools to measure the pressure that is exercised on the impacted surface. We calibrate the simulations first using the experimental results, which makes our simulation results more meaningful.”

Up to this point, Chasiotis said they have been running the gas gun at velocities close to 7.5 kilometers per second in a vacuum to mimic what happens in space, but they can introduce other gases, such as nitrogen and oxygen.

“The facility can also be used to test at lower-impact velocities, such as in hypersonic applications for which we can simulate the Earth’s atmosphere. Dust particles and ice in the atmosphere can be detrimental when impacting a vehicle travelling at high Mach speeds.”
He also mentioned applications such as the dust plume that forms when a spacecraft lands on the moon.

“The lunar regolith plume is moving at really high velocities, maybe up to a couple of kilometers per second. This fine dust plume can hit the lander itself or other vehicles that are located hundreds of meters away from the landing location and cause potentially severe surface damage.”

The facility was created with a grant from the U.S. Air Force for space-related applications, but Chasiotis is also seeking collaborations with other researchers or companies in the civilian space sector who have interest in material impact testing in the range of 1 to 7 kilometers per second.