Frictions exerted during atmospheric reentry are enough to rend spacecraft into comets of glowing slag if not properly mitigated — that’s a good thing, when intentional, but otherwise nearly always very bad. The Space Shuttle, when it was still in service, was designed to hit the outermost edges of Earth’s atmosphere traveling at around Mach 25 (~17,000 MPH), then ride a wave of superheated plasma — generated because frictional forces are so great that they literally tear the surrounding air apart at the molecular level — down into the atmosphere until aerodynamic surfaces regain their effectiveness.
“Utilizing atmospheric drag is the most mass-efficient method to slow down a spacecraft,” NASA notes. To survive those intense 3000-degree F temperatures, the Shuttle relied on layers of ablative heat shielding tiles that would melt and slough off, carrying extra heat away with them, but for tomorrow’s reusable spacecraft, NASA has something better in mind, something inflatable.
NASA has scheduled a launch window beginning November 9th for the LOFTID mission. It will fly out of Vandenberg Space Force Base aboard a ULA Atlas rocket, alongside a new NOAA “polar weather satellite.” After the satellite separates from the Atlas rocket’s upper stage, the LOFTID will unfurl and inflate in low earth orbit ahead of its reentry.
“One of the biggest differences is before we were doing suborbital tests, coming in at roughly 5,600 miles per hour or 2.5 kilometers per second, which is already difficult,” Steve Hughes, LOFTID aeroshell lead at NASA's Langley Research Center said in a press release. “But with LOFTID, we’ll be coming in at nearly 18,000 miles per hour, or 8 kilometers per second. That is about three times as fast, but that means nine times more energy.”
The LOFTID heatshield offers four layers of protection against all that energy. The outermost layer is made from ceramic and silicon carbide yarn woven into cloth on the same sorts of industrial weavers that make denim. The second and third layers are two kinds of insulation, they’re there to protect the fourth layer — the actual inflatable bits. Everything is stacked into a series of concentric rings — themselves constructed from a woven polymer ten times stronger than steel by weight — that will help guide the shield’s expansion.
NASA has been developing Hypersonic Inflatable Aerodynamic Decelerator (HIAD) technology for more than a decade. LOFTID (Low-Earth Orbit Flight Test of an Inflatable Decelerator) is the latest iteration of that tech, a new kind of heat shield that potentially avoids many of the issues NASA has with the current generation of rigid aeroshells. These hard shields have a hard limit on their size, dictated by the diameter of the rocket’s shroud. Soft aeroshells don’t face that limitation and can be extended far past the shroud’s edge, enabling NASA to protect larger and heavier payloads as they enter atmo.
This is especially important to our future solar system exploration plans, because the other issue with current heat shields is that they only work in Earth’s atmosphere. You try to set something the size of the Space Shuttle down on the surface of Mars and that exercise is going to end with your spacecraft a very long streak smeared across the Red Planet — or one very short crater if you’re especially unlucky. Mars’ atmosphere simply isn’t thick enough to generate sufficient friction against modern-sized heat shields to safely slow the Shuttle’s descent. So, NASA is testing out an inflatable one that is.
When it begins its descent, LOFTID will be traveling at more than 25 times the speed of sound. NASA hopes that by the end, LOFTID will be crawling along at a relatively pokey 609 MPH. Throughout its flight, the test shield’s onboard data recorder will transmit the most pertinent sensor and video data while storing as much as possible onboard in an ejectable recorder. Should everything go according to plan, the LOFTID shield will slow sufficiently to deploy a landing chute before setting down in the Pacific Ocean ahead of retrieval by the ULA.