At NASA’s White Sands Test Facility, Donald Henderson and his team spend much of their days shooting projectiles at 15,700 miles per hour. Hypervelocity testing done at the Las Cruces, NM center simulates the impact of micrometeoroids and orbital debris on spacecraft shields.
NASA’s White Sands Test Facility, built originally in 1963, has played a critical role in supporting hazardous testing of oxygen systems, command-and-control systems, and thrusters for the agency’s space shuttles.
In a Q&A with Photonics & Imaging Technology, Hypervelocity Test Engineer Donald Henderson explains the critical role that high-speed cameras play in the facility’s test processes.
Photonics & Imaging Tech Briefs: Your team shoots hypervelocity particles at shields. Take us through a typical test. What kinds of technologies are required?
Donald Henderson: We shoot shields that protect spacecraft from particles out in space. We typically take the test article and set it up inside of a chamber. Then, we prepare our gun systems to shoot at it. The gun requires a high-strength steel barrel and what we call a sabot, which is a housing used to carry the projectile down the barrel.
P&IT: How is the shot fired?
Henderson: We get the gun prepared and aligned to the target and insert the launch package: the sabot and the projectile. We typically charge the gun with hydrogen gas and gun powder. The gun powder charge drives a piston down a pump tube, which is filled with the hydrogen gas. The pressure on the hydrogen gets ramped up to about 100,000 psi, and that’s released in the back of the launch package -- accelerated from 0 to 7 or 8 km per second in just a few feet. The sabot gets separated from the projectile, and the projectile is left alone to fly into the target at that point.
P&IT: Why is this testing so important? What kinds of environments are you trying to simulate?
Henderson: The near-Earth space environment, where the International Space Station (ISS) and a lot of the satellites are, gets bombarded by small particles all the time. The space station is impacted, on average, once every orbit. That’s 16 times a day that it can get pummeled by small bits of debris.
We’ve also done testing for the Solar Probe Plus and the James Webb telescope. The Solar Probe Plus is going within 8 and a half helio-diameters [solar radii] from the Sun -- very close to the Sun. The James Webb telescope is going to be at Lagrangian Point L2, in between Earth and Mars. The telescope and probe are going to be impacted by debris traveling at a very high speed -- 7 km per second or so. The scientists were able to take our data and extrapolate up to those various velocities.
P&IT: What have you learned from these tests?
Henderson: NASA keeps a database for different shield configurations so they can know what the probability of penetration is of the test articles. For instance: How strong are the windows that astronauts are looking out of from the ISS? How many times can they be hit and still survive?
P&IT: How can these tests directly impact the day-to-day responsibilities of the astronauts on the ISS?
Henderson: We’ve had cut-glove incidents over the last 10 years or so on the Space Station. The gloves were cut by craters created by hypervelocity debris, which impacted their handles used to traverse around the Space Station. Some of these cut-glove incidents actually unzipped the gloves down to the bladder sections, which can be devastating for the astronauts.
We performed testing with various projectile materials and sizes to identify what size crater could cause the gloves to get caught. Then, we did a series of tests to help astronauts practice to repair the craters.
P&IT: What role does imaging technology play in the tests?
Henderson: It’s critical. We are asked to prove to NASA that our projectile is intact and is the shape that it’s intended to be prior to impact. You can imagine when 100,000 psi of hydrogen gas hits the back of the launch package and starts accelerating it. There are a lot of forces in play on that projectile, and the projectiles can get deformed during the launch. We use imaging to show that the projectile was alone, that there wasn’t secondary debris along with it, and that it was not damaged during the launch operation.
P&IT: How can a projectile’s shape be changed during the test?
Henderson: As they’re flying towards the target, the projectiles might hit some of the laser stations or whatever we might have in the path. If the projectile flies off a little bit, it could create additional debris, deform the projectile, and cause worse damage than expected. Then, you would be making determinations off of an erroneous result that would not benefit the space program.
P&IT: What kinds of cameras are used?
Henderson: They are very-high-speed cameras. We use Phantom cameras [from Vision Research], v711s and v2512s, that can image up to about 1.4 million frames per second. They also have a lot of light sensitivity. When we’re taking pictures inside of our chambers, it’s hard to dump enough light inside of there. We have to have very sensitive cameras that are capable of picking up the images. We also use Sim cameras [from Specialised Imaging], which are capable of operating up to 200 million frames per second.
P&IT: Where is the camera located during the test?
Henderson: The camera is located on what we call the target chamber, where the target is located. The camera is orthogonally placed to some viewports. We’ll have the cameras outside the gun looking in through those viewports.
P&IT: What are you testing now?
Henderson: Currently, we are testing honeycomb structures. Those are used, for instance, in battery boxes. Outside the space station are orbital recharge units (ORUs). The ORUs collect energy from the solar panels and supply power to the space station when the space station is in the dark phase of its orbit around the Earth. NASA is going to be replacing the nickel-hydrogen batteries, which have been up there for 20 years or so, with lithium ion batteries. NASA wanted to test the honeycomb boxes that house these lithium ion batteries.
P&IT: What kinds of test work is being done with the Multipurpose Crew Vehicle (MCV)?
Henderson: We’re testing the ablative material for the MCV, the material used on the outside of the capsule as it’s coming back to Earth. If it’s impacted by something, [astronauts] need to know how much damage is there.
P&IT: What other ISS technologies have been tested at your facility?
Henderson: We have also been testing ISS radiator panels. Radiator panels take the heat that’s absorbed from the solar panels and radiate it back into space. Flow tubes inside these radiators carry anhydrous ammonia, which carries the heat, over a large surface area, back into space. However, having a large surface area means the radiator panels do get impacted from time to time. [NASA is] trying to find the ballistic limit of the tubes, which is critical to know. We started mimicking that impact.
We’re also testing composite overwrapped pressure vessels (PVs). The outside of spacecraft has fuel, oxidizers, and propellants stored and used on their thrusters. The fuel PVs are subject to [debris impact]. We’re doing a test program now to analyze the ballistic limit of those vessels.
P&IT: How is the shield constructed in a way to withstand that kind of impact?
Henderson: Typically, a multi-shock shield, or a Whipple shield, is used. The multi-layered shield essentially, at these velocities, acts a little different than your intuition might tell you about impacts. Here on Earth, when a bullet impacts, you would want a large or thick piece of metal in front of you. That’s pretty impractical on a spacecraft, because the material is heavy, bulky, and tough to get up into space.
The [space shields] basically use the projectile’s energy against itself. At these very high velocities, the particles, when they impact, get shock waves sent through them, and those shock waves can break up the particles. The first layer would break it up into thousands of little pieces, would spread out the kinetic energy into smaller bits, and then the subsequent layers would absorb the impact.
P&IT: What parts of the shield-testing process are being documented through imagery?
Henderson: What’s most important is to show the particle prior to impact, just in front of the first plate, and to show that the particle is in intact, alone, and un-damaged. The secondary image we want is the impact, the shape of the impact, and the amount of debris coming off the front of the first layer. At times, we’re asked to show the impact as it travels through the different layers.
P&IT: What do you think is most exciting about the work that you do?
Henderson: We’re helping missions succeed, and in a small way, we’re helping the International Space Station stay up -- now that NASA has extended the lifespan of the ISS another ten years or so. More and more of these impacts will happen over this time. It’s important that they learn how to mitigate the impacts so the astronauts stay safe while they’re up there.
For more information visit www.nasa.gov/centers/wstf .