Your spacecraft is falling from the skies at an initial speed of Mach 25. Your reentry heat shield, that has to survive a 7,800 degrees Celsius (14,072° F) plasma shock, is a finely tuned hi-tech amalgam of refractory metals and carbides and reinforced carbon-carbon ablation materials. Care to replace your mighty heat shield with a balloon? Not likely! But that is exactly what NASA is considering.
This summer, the third in a series of NASA suborbital test flights will attempt to demonstrate the feasibility of inflatable spacecraft. The Inflatable Reentry Vehicle Experiment (IRVE-3) is scheduled for a suborbital test flight from the Wallops Flight Facility on Virginia's Eastern Shore later this northern summer. Part of the Hypersonic Inflatable Aerodynamic Decelerator (HIAD) project within NASA's Office of the Chief Technologist's Game Changing Development (GCD) Program, IRVE-3 is one of NASA's many projects to develop new technologies to advance space travel.
Why a balloon? Balloons – even those designed to withstand the rigors of reentry from orbit – are small and light. Past use of aerodynamic decelerators required that the rigid structures fit within the launch rocket. In contrast, IRVE-3, while three meters (9.8 ft) in diameter when inflated, packs into a 56 cm (22 in) diameter cylinder, which is three meters long. The complete payload will be launched by a Black Brant sounding rocket.
During the flight, the tubes will be inflated, thereby stretching a thermal protection system (TPS) blanket that covers the tubes. The result is an aeroshell with a heat shield that will protect an active payload returning to Earth, while having the advantage of being much easier to launch to orbit. Also, with a total mass of about 100 kg (220 lb), the inflatable aeroshell is far lighter than a rigid aeroshell.
The flight plan for the IRVE-3 test includes climbing to a maximum altitude of 462 km (287 miles), at which point the IRVE-3 separates from the Black Brant, and is inflated. The inflated reentry vehicle and its payload will then fall back to Earth, reaching a downward velocity in excess of 2.5 km/s (1.5 miles per second) before the atmosphere becomes sufficiently dense for drag braking to start.
Testing has been a major part of this development effort from the beginning. Although the most visually impressive test was that shown above to test the thermal and dynamic response of the inflated reentry vehicle, perhaps the most important was a complete systems test of the IRVE-3 and its payload in near-vacuum conditions.
This test confirmed that the IRVE-3 release and inflation mechanisms functioned properly, as did the data acquisition and transmission electronics. "There are an awful lot of complex systems packed inside the payload on IRVE-3," said Robert Dillman, the chief engineer for IRVE-3. "When it works, it looks simple and that's a good thing. But there are a lot of internal parts that have to work together in order to make that simple function achievable."
If flight tests continue to be satisfactory, one potential application for inflatable reentry vehicles other than planetary probes is emergency evacuation of the International Space Station and other future manned orbital stations. Individual reentry survival mechanisms were first proposed in the early 1960s with the MOOSE (Man Out Of Space Easiest) concept from General Electric.
Although GE performed preliminary testing on some of the components of the MOOSE system, neither NASA or the U.S. Air Force was interested – probably rightly so. Modern techniques and materials developed during the IRVE development and testing program could well make the issuing of inflatable survival vehicles a standard part of space flight contingency planning in the future.
Here's a video simulation of the upcoming test.