Spacecraft traveling at hypersonic speed produce enormous amounts of heat. To mitigate its harmful effects and ensure survival, engineers rely on effective heat shields.
Since Viking in the 1970s and NASA’s Curiosity rover in 2012, rigid aeroshell heat shields have provided spacecraft with protection during reentry. To successfully send humans to Mars, however, we will require even better shields.
Re-entry
Re-entering space can be an exothermic event that generates intense heat – sometimes exceeding that of the sun’s surface temperature – so interiors of spacecraft must often be protected against it.
To do this effectively, the most widely utilized strategy is an ablative shield – a large, round surface coated with compounds designed to burn off and dissipate heat on reentry, while creating a gassy barrier against it that helps insulate interior spaces from heat damage.
Orion will employ an origami heat shield dubbed Pridwen after King Arthur’s sword to make their test flight safer and cheaper than ever. Pridwen will launch on board an NOAA satellite from Vandenberg Space Force Base in California; should its test flight be successful, an even larger version could potentially be deployed during missions with astronauts.
Thermal Protection
Heat shields come into their own during entry, descent, and landing (EDL), protecting spacecraft from extreme temperatures while helping slow its descent through an atmosphere.
Jeremy Vander Kam is the deputy system manager of NASA’s Orion spacecraft’s thermal protection system and looks forward to learning from its first voyage to Mars in 2024. This mission will put to test both its heat shield and thermal tiles, offering him an opportunity to gain valuable experience.
Over time, rocket scientists have experimented with various forms of reentry heat shielding; two primary methods have proven particularly successful: ablative insulation and refractory insulation.
Orion’s underside tiles are constructed with Avcoat material, first employed by NASA since Apollo missions. Avcoat works by “blowing off” gaseous reaction products onto its surface to form an ablative layer that blocks convective and catalytic heat transfer from entering Orion.
Materials
Heat shields are typically made from ceramic tiles with black surfaces to absorb and dissipate radiant and kinetic heat entering from atmospheric entry, most commonly seen covering the Space Shuttle bottom surfaces and nose. In modern vehicles, reinforced carbon-carbon material may also be utilized on high heat-load points like the nose and wing leading edges for even greater heat reduction.
These tiles are passively cooled by the spacecraft’s thermal systems and dissipate heat through convection; however, they cannot disperse all heat evenly at higher speeds; an increased thickness may be necessary.
University of Manchester team has developed a prototype that employs centrifugal force to stiffen lightweight materials and avoid burnup during reentry, thus significantly reducing weight of future missions to Earth and Mars (where aerodynamic drag must be used to slow down), making long journeys possible, potentially allowing astronauts or rovers to explore organic molecules on planet surfaces while studying how life began.
Design
Travelling at more than seven kilometres per second through any planet’s atmosphere would cause it to incinerate without heat shields absorbing and dissipating the heat generated from friction with its atmosphere on reentry back to Earth.
Current re-entry shields are static, meaning that they do not move relative to the spacecraft they protect. Ceramic tiles often form these shields and cover the surface of Space Shuttle orbiters as an effective barrier; however, this technology adds substantial weight and takes up valuable propellant resources during launch.
A new generation of inflatable aeroshells may provide more protection while simultaneously reducing weight and complexity. One such solution, designed by Pridwen Heat Shield Company and deployed from inside a vehicle into an expandable structure that can reach diameters up to six meters can disperse shockwaves more widely, dissipating impactful re-entry heat onto wider areas of ground surface.