Spacecraft that ‘sweat’? These researchers think that could be the key for reusable hypersonic craft

An illustration of NASA’s Orion spacecraft reentering Earth’s atmosphere. Credit: NASA

---

Just as sweat cools the human body, a proposed transpiration method for reentering hypersonic vehicles has shown promise in early testing, according to findings presented last month by a Texas A&M University team at the International Symposium on Shock Waves in Australia.

Most capsules and missiles are designed to survive their punishing hypersonic trek through Earth’s atmosphere with a heatshield or coating of thermal protective material that ablates, or burns away, during reentry. Instead, the Texas A&M researchers and their project partners at Colorado-based Canopy Aerospace created a coating in which cooling gas is dispersed to dissipate the heat — a method they believe could make spacecraft and future hypersonic passenger vehicles rapidly reusable, like today’s airliners, and allow missiles to be built with cheaper thermal materials that have lower heat tolerances.

For the concept to work, the cooling gas must seep evenly over the vehicle’s exterior, like a shower head tuned to a fine misting spray, said Hassan Saad Ifti, the professor leading the Texas A&M team, which also includes professor Ivett Leyva and Ph.D. students William Matthews and Megan Sieve. They described the results of their early testing in a paper presented at the symposium, “Flow Characterisation of 3D-Printed Porous Silicon Carbide for Transpiration Cooling.”

A series of studies dating back to 1971 has shown that when certain pressurized gases are forced through a porous material to a surface encountering hypersonic air flows, those gases can form a thin layer near the surface to insulate it and cool it, carrying heat away. Now, funded by a $1.7 million grant from the U.S. Air Force, Ifti and his teams are refining their concept, with testing on samples of 3D-printed silicon carbide made by Canopy, which manufactures thermal protection materials for spacecraft and missiles. The end goal is to build and test gas-cooled material on a hypersonic flight before the project wraps up in 2027.

 

Through 3D printing, Canopy can alter the structure of the various samples to adjust how much gas can flow through each one. That porosity depends on the amount of 10- to 20-micron-wide channels in a given sample — about one-third to one-quarter the diameter of human sweat pores. Under an electron microscope, these channels look like cracked mud in a dried-up lake.

“They can make specific areas of the thermal material more permeable to allow more gas to flow through, such as a leading edge of a wing that will be blasted by more heat than other surfaces,” Ifti said.

If the “sweating” materials works as intended, Canopy’s executives see a potential market in the commercial space companies building small fleets of reusable capsules or rockets for applications including on-orbit manufacturing, space tugs and space tourism, said Will Dickson, Canopy’s chief commercial officer. For the downward-facing surfaces of a spacecraft that absorb the full brunt of reentry conditions, there is currently no material that can be reused without first undergoing extensive refurbishment, Dickson said.

“The challenge of the business model is that: ‘Hey, if I have to change the aeroskin every single time it flies, that’s not reasonable,’” he says. “Everybody would love to aspire to commercial jet level of reusability where you landed, refueled, sent it up again. That’s the big unlock, this reusability.”

That’s not to say this sweating method doesn’t have some potential downsides. Operators would have to contend with some extra weight, Dickson said, potentially in the form of tanks, valves and metal components for containing and controlling the pressurized gas. Reducing that weight is a top goal for Canopy and the Texas A&M researchers as they refine their concept.

Part of the solution could come from a new wrinkle described in the symposium paper: Additional channels 3D-printed part-way into the thermal material, through which the gas would flow before seeping through the remaining distance to the surface via the microscopic cracks.

To test the idea, Canopy 3D-printed two types of silicon carbide disks, each 5 millimeters thick with the 10- to 20-micro-wide crack-like channels throughout. One of the disk types also had 1-millimeter-wide, 4.5-mm-long vertical shafts.

To compare how the addition of the shafts changed the flow of gas, the researchers affixed the disks to a pressure chamber, like a lid on a pot, then piped pressurized air into the chamber. The gas flow velocity at the surface of each disk was measured by a miniature hot-wire anemometer moving in a grid pattern to detect the amount of air movement by how much the heated wire was cooled.

They found that the straight shafts allowed the gas to flow with less pressure while still seeping evenly across the vehicle’s surface, Ifti said. For a hypersonic craft, lower-pressure cooling gas could be carried by lighter, thinner-walled tanks, which also helps address the weight issue. Another potential benefit to this approach, he added, is that lower-pressure gas could also be more easily controlled, so the amount of cooling could be increased for certain phases of flight or for certain areas of the vehicle. As another plus, lower-pressure gas would put less stress on the material.

Starting in October, Ifti’s team plans to test its sweating 3D-printed material in Texas A&M’s three hypersonic wind tunnels, which produce a range of the air pressures, velocities and heating encountered during hypersonic flight. Like the test setup for the channeled disks, gas will seep from a pressure chamber through 3D-printed silicon carbide coupons of various shapes into the hypersonic flows of the wind tunnels to compare the cooling of argon, helium, nitrogen and mixtures of the gases. The gas will flow perpendicularly through flat-plate samples, and flow head-on through nose-cone-shaped samples.

A visible-light high-speed camera will read the brightness of pressure-sensitive paint on the surface of the samples to determine how well the gas is sticking to the surfaces as the hypersonic air blows over them. Then, in follow-up experiments, the team plans to read the surface temperatures with infrared camera and heat-flux sensors.

In addition to testing the level of protection their material provides against damaging hot hypersonic gases, they also plan to assess how well the coolant gases prevent the plasma created at extreme hypersonic velocities from forming on the surfaces. Because plasma eats away at the surfaces of hypersonic vehicles, it can alter the aerodynamics of a sharp nose or a thin leading edge on a wing, Ifti said.

Consider, for example, a future version of the Concorde supersonic airliners or the fictional hypersonic plane depicted in the “Top Gun: Maverick” movie, he said.

Such designs “would probably have extreme heating on the nose, especially with vehicles that are not reentry vehicles but in-the-atmosphere flight vehicles where you have to be aerodynamically efficient,” he said. “They’re not going to have blunt noses like the space shuttle.”