Spray-On Stealth Ceramic Could Make Faster, Tougher Spy Jets

A new skin for stealth aircraft could make radar-invisible aircraft faster, more resilient, and even sneakier in the skies, by addressing one of their lingering design weaknesses. Jets like the B-2 Spirit are run by the US Air Force for clandestine reconnoissance missions, tapping their ability to evade the majority of radar tracking. However the high-tech skin that gives them those talents can be, in itself, a liability.

Stealth aircraft aren't "invisible" to the naked eye, and they aren't completely slippery to radar either, but they're considerably tougher to spot than regular aircraft. To do that, they use radar-absorbent polymers, which soak up the energy that a radar system sends out.

Radar works by bouncing those waves of energy off other objects, and then capturing the return reflection. The polymer skin can instead capture 70- to 80-percent of the energy, rather than bouncing it back, and thus present a much tougher target to spot. Problem is, the polymers themselves can be easily abraded by tougher environmental conditions like moisture, and they decompose at temperatures over around 480 degrees Fahrenheit.

It's a reality that effectively caps the maximum speed stealth aircraft can run at, and the length of time they can be airborne, researchers at NC State University point out. On the one hand, supersonic aircraft build up huge quantities of heat from the friction of air striking areas like the wings. Meanwhile, jet exhaust temperatures are well outside the comfort zone of the stealth polymers, and so aircraft are forced to adopt longer exhaust nozzles which make them heavier and less fuel efficient. To work around that, the US Air Force undertakes complex mid-air refueling in order to keep jets like the B-2 Spirit airborne for extended times.

A team at the university set about to figure out an alternative system, to replace the polymer coating with a new ceramic material. Led by Chengying "Cheryl" Xu, the group developed a ceramic that's not only tougher and able to withstand greater heat and rougher environmental conditions, but also actually performs better in radar-avoiding than the existing polymers.

It can absorb 90-percent of the energy, in fact, or even more, and retains those characteristics at temperatures of 3,272 Fahrenheit or down to -148 F. At the same time, it's water-resistant and sturdier than sand, far improving on the resilience factor.

Jets coated with it could therefore be used in more situations, which currently might ground existing stealth aircraft because the risk to the polymer coating is too great. They could fly faster, too, given there'd be less danger to the stealth coating from increased heat from friction, and potentially further distances as the plane designs could be more aerodynamically efficient as the jet exhausts wouldn't need to be distanced from the anti-radar treatment.

Adding to the improvements is the relative ease of application. The polymer-derived SiOC ceramic skin can be sprayed on as a liquid precursor, the researchers explain, and then reacts with ambient air to oxidize into the solid state. That process takes 1-2 days.

Lab testing of the ceramic material has been promising, though there's obviously a considerable step from there to building an actual stealth jet. The US Air Force Office of Scientific Research has already begun funding the project, and the NCSU team is hoping to work with aircraft manufacturers to develop potential designs that would tap the coating's advantages.