Reusable Rockets Don’t Just Fly, They Sweat
In the pursuit of a rocket that can fly more than once, perspiration might just be the answer.
Written by Nicole Imeson
William Matthews, a fourth-year doctoral student, is leading the development of testing rigs to evaluate the material’s effectiveness in how well it sweats gas and how well that gas insulates a spacecraft. Photo: Emily Oswald/Texas A&M Engineering
IMAGINE A ROCKET THAT SWEATS to survive atmospheric re-entry, releasing a fine gas through its porous skin, like perspiration, to shield itself from searing heat. As part of a U.S. Air Force project led by Canopy Aerospace and supported by Texas A&M University, a team of researchers has developed such a transpiration-cooled thermal protection system using porous 3D-printed ceramic materials.
To accomplish this, the team built a cavity behind the spacecraft’s outer surface and pressurized it with coolant gas. The gas seeps through strategically placed pores, forming a layer around the vehicle that offers a reusable, actively controlled solution to withstand the extreme temperatures of spacecraft re-entry.
CONDITIONS OF REENTRY
“Transpiration cooling created a self-renewing gas layer. This layer made the vehicle ‘sweat,’ insulating it from extreme heat and enabling reuse with a simple coolant refill, paving the way for reusable spacecraft,” explained Hassan Saad Ifti, assistant professor of aerospace engineering at Texas A&M University.
As a spacecraft travels through different atmospheric stages, shifting from low-density upper atmosphere to denser lower atmosphere, cooling demands change dramatically. Researchers have studied the effects of varied coolant cavity pressure to adjust the coolant flow rate: During peak heating periods, increasing the cavity pressure pushes out more coolant, enhancing the cooling effect; Conversely, when heating is less severe, the pressure can be reduced to conserve coolant.
Advanced manufacturing techniques let researchers engineer porous materials with controlled pore size and strategic placement across the surface. They positioned and spaced the pores to direct coolant toward regions with higher predicted heat loads.
“The pressure differential across the porous material dictated how much gas passed through. When we increased the pressure, more gas flowed out, meeting higher cooling demands under extreme external conditions,” Ifti explained.
Transpiration cooling offers dual protection against both intense heat and oxidation. The coolant provides three main effects: convective heat transfer, where the coolant absorbed and carried heat away; film insulation, where the gas formed a protective barrier between the vehicle and hot airflow; and oxidation protection, where the coolant shielded the surface from oxygen exposure.
In addition to the convective barrier, transpiration cooling delays the onset of turbulence by thickening the boundary layer, stabilizing it, and dampening disturbances, cutting surface heat transfer by nearly 10 times. The coolant injection also modifies velocity gradients within the boundary layer, suppressing a major source of instability. The resulting coolant film acts as a thermal buffer, with fluid properties that stabilize the interface between the spacecraft and the atmosphere.
From left: Hassan Saad Ifti, Ivett Leyva, and William Matthews stand in front of one of the hypersonics testing tunnels at the National Aerothermochemistry and Hypersonics Laboratory. Photo: Emily Oswald/Texas A&M Engineering
“We’re focused on determining the precise amount of coolant gas needed and identifying the optimal gas to minimize coolant while maximizing protection. This research involves detailed mixing physics and wind tunnel experiments.”
Hassan Saad Ifti, assistant professor of aerospace engineering at Texas A&M University
IDENTIFYING AND OPTIMIZING
Ultra-High-Temperature Ceramics (UHTCs) withstand extreme temperatures in demanding spacecraft applications, making them ideal candidates for porous transpiration cooling material. However, manufacturing UHTCs has been a challenge for industry given their naturally high toughness. Traditional manufacturing methods have not been able to tailor porosity or shape, which are critical to designing a transpiration cooling interface.
Canopy Aerospace identified this opportunity to apply its expertise in 3D-printing UHTCs for NASA and other commercial companies. With the Texas A&M team’s research into cooling requirements on hand, Canopy’s state-of-the-art hypersonic test facilities recently constructed as part of the Bush Combat Development Center provided the ideal testing grounds.
As part of their investigation, the researchers explored nitrogen (N₂) and helium (He) along with other fluids as primary coolants. They identified helium as a more efficient coolant than nitrogen because of its greater heat capacity and higher volumetric flow rate, which could produce a more substantial protective film. However, due to its low molar mass, helium required about twice the storage volume for the same mass as nitrogen, limiting potential payload capacity.
“We’re focused on determining the precise amount of coolant gas needed and identifying the optimal gas to minimize coolant while maximizing protection. This research involves detailed mixing physics and wind tunnel experiments,” Ifti explained.
As for spacecraft materials, ablators and ceramic tiles are two common types used for thermal protection systems (TPS). Ablators dissipate heat by melting, vaporizing, or decomposing, and consuming the protective material during high-heat events, which renders them single-use. Conversely, ceramic tiles insulate a spacecraft’s structure by blocking heat transfer with their low thermal conductivity and high-temperature resistance, enabling reuse. However, their brittle nature makes them vulnerable to damage. As a result, technicians must inspect, repair, or replace individual tiles and components—a time-consuming task—between missions to maintain TPS integrity.
Transpiration cooling promised to redefine spacecraft reusability. Using advanced materials like porous UHTCs and precise additive manufacturing, researchers envision rockets that can refuel coolant and then fly again. As researchers continue fine-tuning the physics, materials, and gas dynamics behind this system, the future of spaceflight is moving closer to becoming reusable and resilient, sweating its way to sustainability.
Nicole Imeson is an engineer and writer in Calgary, Alta.
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