Mach 20 and beyond? DARPA program to develop hypersonic flight

DARPA equates the tactical advantages of hypersonic flight to the development of stealth technology

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Is it possible to develop an aircraft that can fly - and survive -- at over 20-times the speed of sound? It has been an idea that has met with only minimal success over the years but that may change.  Next month the big idea guys at the Defense Advanced Research Projects Agency will hold a briefing to detail exactly what it expects out of a new program that will seek to develop hypersonic aircraft or missile technology.

DARPA says the goal of its Integrated Hypersonics program is "to develop, mature, and test next- generation technologies needed for global-range, maneuverable, hypersonic flight at Mach 20 and above for missions ranging from space access to survivable, time-critical transport to conventional prompt global strike. The program seeks technological advances in the areas of: next generation aero-configurations; thermal protection systems (and hot structures; precision guidance, navigation, and control; enhanced range and data collection methods; and advanced propulsion concepts."

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The program is designed to address technical challenges and improve understanding of long-range hypersonic flight through an initial full-scale baseline test of an existing hypersonic test vehicle, followed by a series of subscale flight tests, innovative ground-based testing, expanded modeling and simulation, and advanced analytic methods, culminating in a test flight of a full-scale hypersonic X-plane (HX) in 2016.

"History is rife with examples of different designs for 'flying vehicles' and approaches to the traditional commercial flight we all take for granted today," explained Gabriel. "For an entirely new type of flight-extreme hypersonic-diverse solutions, approaches and perspectives informed by the knowledge gained from DoD's previous efforts are critical to achieving our goals" said acting DARPA director, Kaigham Gabriel in a statement.

DARPA equates the development of hypersonic equipment to the development of stealth technology in the 1970s and 1980s.  The strategic advantage once provided by stealth technology is threatened as other nations' abilities in stealth and counter-stealth improve. "Restoring that battle space advantage requires advanced speed, reach and range. Hypersonic technologies have the potential to provide the dominance once afforded by stealth to support a range of varied future national security missions," DARPA said.  

There are a ton of technological issues to be addressed, one of the biggest being the heat generated by extreme speeds. 

At Mach 20, vehicles flying inside the atmosphere experience intense heat, exceeding 3,500 degrees Fahrenheit, which is hotter than a blast furnace capable of melting steel, as well as extreme pressure on the shell of the aircraft, DARPA stated.  The thermal protection materials and hot structures technology area aims to advance understanding of high-temperature material characteristics to withstand both high thermal and structural loads. Another goal is to build structural designs and manufacturing processes to enable faster production of high-speed aeroshells, DARPA stated.

DARPA knows the risks first-hand.  In a report this spring DARPA noted that is experimental Hypersonic Technology Vehicle (HTV-2), lost significant portions of its outer skin and became uncontrollable after three minutes of sustained Mach 20 speed in a flight last August.  That was the conclusion of an independent engineering review board (ERB) investigating the cause of what DARPA calls a "flight anomaly" in the second test flight of the HTV-2.

From that ERB report: "The flight successfully demonstrated stable aerodynamically-controlled flight at speeds up to Mach 20 (twenty times the speed of sound) for nearly three minutes. Approximately nine minutes into the test flight, the vehicle experienced a series of shocks culminating in an anomaly, which prompted the autonomous flight safety system to use the vehicle's aerodynamic systems to make a controlled descent and splashdown into the ocean.

Based on state-of-the-art models, ground testing of high-temperature materials and understanding of thermal effects in other more well-known flight regimes, a gradual wearing away of the vehicle's skin as it reached stress tolerance limits was expected. However, larger than anticipated portions of the vehicle's skin peeled from the aerostructure. The resulting gaps created strong, impulsive shock waves around the vehicle as it travelled nearly 13,000 miles per hour, causing the vehicle to roll abruptly. Based on knowledge gained from the first flight in 2010 and incorporated into the second flight, the vehicle's aerodynamic stability allowed it to right itself successfully after several shockwave-induced rolls. Eventually, however, the severity of the continued disturbances finally exceeded the vehicle's ability to recover."

"The initial shockwave disturbances experienced during second flight, from which the vehicle was able to recover and continue controlled flight, exceeded by more than 100 times what the vehicle was designed to withstand," Gabriel said last spring.   

DARPA will host an Integrated Hypersonics Proposers' Day on August 14,  at its Arlington, VA headquarters to detail the technical areas for which it will ultimately contract.

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