When the flight speed exceeds Mach 5, hypersonic aircraft will encounter high temperatures exceeding 2200°C (4000°F). How to protect the aircraft from the effects of high temperatures? RTX Technology Research Center believes the answer is to make them sweat.
Hypersonic flight is expected to revolutionize the aviation industry unprecedentedly since the sound barrier was broken in 1947. However, going from supersonic to hypersonic speeds has proven to be more challenging than going from subsonic to supersonic speeds.
One of the biggest challenges is the huge amount of heat generated by an aircraft traveling at more than five times the speed of sound. At these temperatures, all but the most exotic materials will melt or become unusable. This means that the precisely designed and machined lines of a hypersonic vehicle, especially the leading edge, can quickly round and deform, completely changing the vehicle's aerodynamics.
The obvious way to avoid this is to cool the skin of the aircraft. Unfortunately, for traditional systems, this means added weight and complexity, which engineers don't particularly like.
As an alternative, RTX, under a DARPA contract, is looking into using the same mechanism we use to cool down — sweating — to cool hypersonic vehicles.
The idea is to install a network of microchannels at the leading edge of a hypersonic vehicle that would deliver fluid to the surface of the skin in a manner similar to human sweat glands. When the liquid reaches the skin's surface, it evaporates, taking away heat. This way, the aircraft maintains sufficient cooling capacity to maintain its aerodynamic performance.
According to John Sharon, project team leader at the RTX Technology Research Center, they used predictive modeling and advanced micro-fabrication technology to create a wedge-shaped test object the size of a credit card. It is first placed over a burner described as a large "cream pudding torch," and then an electric arc is used to heat and expand the gas to high temperatures and high speeds that more closely simulate hypersonic flight conditions.
The next step will be to improve the technology, make the sweat channels smaller, and scale up the test object to the scale of a full-scale hypersonic vehicle. If the technology proves successful, it may also be applicable to other problems, such as protecting gas turbine blades.
"When you fly at more than five times the speed of sound, the temperature rises very quickly in a fraction of a second," Sharon said. "The team members involved in the modeling did a great job estimating how long the test specimens would survive."