How to build sensors with rugged enough sensor protection to withstand extreme heat of hypersonic flight

March 25, 2021
A hypersonic missile must withstand temperatures at leading edges like wings and nose that are hotter than a blast furnace designed to melt steel.

THE MIL & AERO COMMENTARY – One of the toughest design challenges of next-generation hypersonic munitions is developing navigation, guidance, sensors, and communications subsystems that are rugged enough to operate through the extreme heat, shock, and vibration of hypersonic flight.

Fortunately design trends are heading in the right direction with a U.S. military research program called High Enthalpy Aperture Technology (HEAT).

Hypersonics experts at the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., have chosen three U.S. technology companies to develop rugged RF radomes and infrared windows able to withstand the environmental extremes that future hypersonic missiles and aircraft must endure.

Hypersonic flight is unlike almost any other kind, as structures fly through the air faster than five times the speed of sound. How fast is MACH 5? It's 3,836 miles per hour; that's faster than one mile per second, and when it comes to future generations of hypersonic vehicles, MACH 5 will be at the slow end.

Related: The electronics design challenges of hypersonic flight

Experts are talking about future munitions that eventually could travel at speeds approaching MACH 20. That's 15,345 miles per hour, or more than four miles per second.

At those speeds the atmosphere at or near sea level imposes tremendous drag and friction on the structure of a missile, and creates a fiery environment akin to a spacecraft re-entering the Earth's atmosphere. Remember the 2003 disintegration of the Space Shuttle Columbia? It burned up on re-entry because some heat shielding failed, and killed seven crew members. A hypersonic munition must operate through even hotter conditions from firing to impact.

A missile traveling at MACH 5 must withstand temperatures hotter than 1,800 degrees Celsius (3,272 degrees Fahrenheit) at the leading edges like nose cones and wings. That's about 30 percent hotter than a blast furnace designed to melt steel.

It's one thing for a missile body to withstand hypersonic flight without burning up. It's quite another for sensitive navigation and guidance systems, electro-optical sensors, and radar systems to operate reliably in such difficult conditions.

Related: GE Global Research to develop ways to enable sensors to withstand the shock and heat of hypersonic flight

That's where the DARPA HEAT program comes in. HEAT seeks to demonstrate new materials and design approaches to enable RF and infrared apertures on hypersonic missiles and aircraft to withstand extremes in heat and dynamic pressure.

So far three companies are developing materials to shield sensors from heat and vibration as part of the HEAT program: the General Electric GE Global Research Division in Niskayuna, N.Y.; the Lockheed Martin Corp. Missiles and Fire Control segment in Orlando, Fla.; and the Georgia Tech Research Corp. in Atlanta.

Lockheed Martin won a $2.5 million HEAT contract on 11 Feb., Georgia Tech won an $8.3 million HEAT contract on 3 Feb., and GE Global Research won a $7.5 million contract on 4 March.

High-speed aerospace systems like hypersonics require specialized RF radomes and IR windows to protect sensitive electronics from the heat of high-speed flight while providing transparency for radar and RF communications transceivers, as well as infrared sensors used for guidance, communications, and target detection.

Related: Extreme Environment Sensors project seeks flight control instruments for hypersonic aircraft and missiles

These aperture materials must withstand extreme thermal, mechanical, and chemical environments during hypersonic flight that can limit their performance. The problem isn't just heat. Shock waves, for example, shock waves can impose wavefront distortions and boresight errors on guidance electronics.

The three companies are considering solutions that may involve affordable and manufacturable means of controlling thermo-optical and elastic-optical effects; maintaining desired transmission amplitude and bandwidth; and reducing thermal deformation, mismatch, and radiation.

The HEAT program is a four-year, two-phase effort, which is divided into three technical areas: integrated RF aperture materials; infrared aperture materials; and next-generation aperture materials.

Related: Military researchers ask industry to develop rugged materials for hypersonic radomes and infrared windows

Lockheed Martin, Georgia Tech, and GE Global Research experts are looking into new materials that combine metals, ceramics, and coatings for high-performance structures, as well as new computational capabilities necessary to develop these materials.

The program's first phase, in progress now, is developing integrated aperture materials, and the future second phase will involve ground testing.

So it's clear that hypersonics isn't just about speed; it's about enabling electronic and electro-optical sensors to operate reliably in some of the harshest environments known. The HEAT program will take a big step in ensuring the success of future hypersonic munitions programs.

About the Author

John Keller | Editor-in-Chief

John Keller is the Editor-in-Chief, Military & Aerospace Electronics Magazine--provides extensive coverage and analysis of enabling electronics and optoelectronic technologies in military, space and commercial aviation applications. John has been a member of the Military & Aerospace Electronics staff since 1989 and chief editor since 1995.

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