AIR FORCE Magazine game-changing weapons? these tofield nation—be first other some Will theUS—or By Mark J.Lewis By Mark press the shooter-to-target timeline. Hawaii. Hypersonic weapons could com- at the Pacific Missile Range Facility in a sounding rocket—launchesaboard An experimental hypersonic — / March 2016 Hallion and retired Maj. Gen. Curtis M. technology readiness study. Advisory Board completed a classified to the Future,” the Air Force Scientific advantage.” the ability to do so creates a significant to operate at the fastest possible speed, and while “we may not always desire catchphrase “speed is life” is often true, and procedures.” The paper said that the force posture, to tactics, techniques, core missions, “from investments, to effect on how USAF approaches its hypersonics will have a revolutionary to the Future” noted that the “leap” to of sound, or above Mach 5. “A Call in excess of about five times the speed autonomy, and directed energy. nanotechnology, unmanned systems, the report identified, the others being five key game-changing technologies adversaries.” Hypersonics was one of and respond faster than our potential urged theserviceto“continue toadapt Chief of Staff Gen. Mark A. Welsh III Air Force: A Call to the Future.” In it, earlier, USAFreleased“America’s such high-speedweapons. A year that have highlighted the utility of one in a series of USAF publications are rapidly accelerating toward reality. enemy’s territory—weapons that today strike quickly at targets deep within an stoppable high-speedweaponsthatcould captures the great promise of nearly un- Though fi ctional for now, the vignette view of the service two decades hence. Chief of Staff, the document presents a the Secretary of the Air Force and the Space Conference. Endorsed by both released by USAF at AFA’s 2015 Air & View of the Air Force in 2035,” a paper “Air Force Future Operating Concept: A their targets.” the hypersonic missiles streaked into munitions. Now intheterminalphase, opportunity to target the high-speed jammers, had missed the one fleeting saturated by the formation of decoy- enemy [integrated air defense system], missiles until it was too late. … The not detecttheapproachinghypersonic Former Air Force Historian RichardP. Coincident withthereleaseof“A Call Hypersonics generallyreferstofl ight This most recent document is just The vignette above was taken from … The enemy did 47

Air Force Research Laboratory photo Bedke have also recently written about PROGRESS AND SETBACKS fl ight test of a new hypersonic engine the utility of hypersonic weapons for The idea of a hypersonic weapon is design suffered a frustrating loss of fl ight AFA’s Mitchell Institute for Aerospace not new. In fact, high-speed military telemetry. Similarly, the US Army’s Studies, in “Hypersonic Weapons and systems have been a dream of technolo- Advanced Hypersonic Weapon, an un- US National Security: A 21st Cen- gists since at least the 1930s, when powered hypersonic glide vehicle, had tury Breakthrough.” Hallion and Bedke German engineer Eugen Sänger and his a successful fl ight in the Pacifi c Missile highlighted the main advantage of this wife, Irene Bredt, did studies of vehicle Range Facility in 2011, followed by a technology: that it can counter the so- concepts capable of ocean-spanning launch failure three years later. called “tyranny of distance” associated flights using skipping trajectories. Some of these setbacks, such as HyFly with global reach. Such weapons would The earliest man-made self-propelled and the AHW, had nothing to do with compress the shooter-to-target timeline, hypersonic object was the two-stage the soundness of the hypersonic tech- allowing for the prosecution of fl eet- American “Bumper” rocket, assembled nologies being studied. Other failures, ing targets or providing more decision from a captured German V-2 with a US such as the DARPA fl ights, yielded in- time before engagement. Hypersonic Army WAC Corporal sounding rocket creased understanding of aerodynamics weapons, they wrote, would enhance as an upper stage, which reached a and materials. Despite this, even some joint operations, would be able to ad- speed of 2.3 kilometers per second enthusiastic supporters of high-speed dress a variety of targets, and would be (5,145 mph) on Feb. 24, 1949. technology have quipped, “Hyperson- deployable from a variety of platforms. After those early steps came some more ics is the future—and always will be.” To accomplish this, Hallion and successes but also a long list of failed hyper- That view is now changing, due not Bedke call for a national strategy that sonic programs and unrealized promises. only to tangible fl ight successes and

A B-52 carries the X-51 under its before its fi rst fl ight in May 2010 at Edwards AFB, Calif. On the pro- gram’s last fl ight three years later, the Waverider reached Mach 5.1, demon- strating the feasibility of air-breathing scramjet propulsion. includes continued research and de- In the early 1960s, USAF began a concrete investments, but also to a shift velopment, maintenance, and support project to build a hypersonic vehicle of ambitions from expensive vehicle for testing infrastructure. They also called the Aerospace Plane, canceled concepts to more practical military call for new investments in a future after the Air Force Scientifi c Advisory systems. A key milestone among these workforce. Board identifi ed “many clearly infeasible was the Air Force’s X-51 program, led The credibility of these various reports factors” in the program. by the Air Force Research Laboratory in has been enhanced by recent success- The roster of abandoned hypersonic concert with the Air Force Test Center ful fl ight efforts, especially the Air programs has grown since, including the and industry partners. The fi rst X-51 Force’s X-51 program and the series 1980s X-30 National Aerospace Plane vehicle fl ew on May 26, 2010, and the of Hypersonic International Flight Re- (NASP), the Navy’s HyFly test program last on May 1, 2013. Though neither search Experimentation (HIFiRE) trials (discontinued after three failed tests), the second nor third fl ights achieved conducted jointly with Australia. Taken and the Defense Advanced Research program objectives, the fi rst achieved with other well-publicized activities Project Agency’s HTV-2 hypersonic nearly all of its objectives and the fourth around the world, there is a renewed glider (canceled after two fl ights). fl ight was a complete success. sense that hypersonic weapons are not Even ongoing hypersonic efforts have Launched from beneath the wing of a only plausible but seemingly inevitable. had setbacks. The August 2015 HIFiRE B-52 bomber, each Boeing-built X-51 was

48 AIR FORCE Magazine / March 2016 initially powered by a solid rocket motor RAMJETS TO entering the engine, the air decelerates derived from an Army Tactical Missile Hypersonic scramjets were almost to low relative velocity, resulting in System (ATACMS) booster, intended six decades in the making. In the late a corresponding rise in pressure and to carry the test vehicle to 4.8 times the 1950s, Richard J. Weber and John S. temperature. That hot compressed air speed of sound, then separate and allow MacKay wrote their landmark National moves into a combustor where fuel is the X-51’s main engine to take over. Advisory Committee for Aeronautics injected and mixed. The combination On its last fl ight, the X-51’s hypersonic (NACA) Technical Note 4386 explor- ignites and adds heat energy, result- “scramjet” engine, built by Rocketdyne, ing a concept for a new type of aircraft ing in hot gas accelerating through a accelerated the craft for 210 seconds, to engine—one that could burn fuel in nozzle to create net thrust. Because a fi nal speed of Mach 5.1 before its fuel air moving at high Mach numbers. the air is compressed by the ram- was exhausted and the vehicle coasted to Working at the Lewis Flight Propulsion ming effect of the engine’s motion, a a planned crash into the Pacifi c. Laboratory (the precursor to today’s ramjet doesn’t require a mechanical In addition to proving the propulsion NASA John H. Glenn Research Center compressor ahead of the combustor. technology, the X-51 fl ights represented at Lewis Field in Cleveland) Weber and Without a compressor, there’s no need a triumph of Air Force-led research and MacKay looked at ways to increase the for turbines, so a ramjet has no primary design where numerical simulation, flight speed of a conventional ramjet moving parts. using the latest in computational codes engine. Though the NACA researchers Because of their dependence on their for design and analysis, was combined didn’t know it at the time, engineers own motion through the air, ramjets with state-of-the-art ground test and in the Soviet Union were working on work poorly at low speeds and can’t real-world fl ight experience. the same problems. produce any thrust at all when standing

USAF photo by Greg L. Davis

The scramjets that powered the X-51 A ramjet is the mechanically simplest still. As a result, ramjets are gener- represent an attractive alternative to type of jet engine. The development of ally reserved for supersonic flight, rockets, the current propulsion choice the modern ramjet dates from the 1920s, beyond Mach 1, the speed of sound. for air-to-air or intercontinental missiles. though the basic concept was proposed That means they must be accelerated Rockets produce high thrust and can fl y over 100 years ago by the Frenchman to operational speed by another kind outside the atmosphere, but because they René Lorin. By the late 1940s prototype of engine, such as a rocket motor (in must carry all of their fuel and oxidizer ramjet engines were being tested and the case of the Bomarc missile) or a with them, they’re ill-suited for high- fl own in Europe and the United States gas turbine engine. speed cruise within the atmosphere. For and were later used to power the Soviet Ramjets also perform poorly at long-range, high-speed cruise within the Union’s Burya cruise missile, the US very high speeds, above about Mach atmosphere, it makes more sense that the Navy’s Gorgon IV missile, and USAF’s 4, although the exact limits depend engine be an “air-breather”—gathering Bomarc interceptor. on a number of factors. There are two oxygen as it fl ies to burn with the car- The ramjet derives its name from primary reasons for this performance ried fuel. It’s the same principle used the basic operating principle where loss. First, any craft fl ying faster than the by all modern jet engines, which gather air is forced into an inlet by the en- speed of sound generates shock waves— oxygen through the inlet. gine’s own motion through the air. On sudden increases in local temperature

AIR FORCE Magazine / March 2016 49 and pressure that create the well-known through the entire engine flow path? By spend only one or two thousandths of a sonic boom. Shock waves waste energy, keeping the air moving at supersonic second in the combustor before exiting adding drag on an airplane but also speeds, inside the engine, thrust could through the nozzle. Even if combustion causing a loss of energy in the ramjet be produced all the way up the Mach were possible, the very process of add- inlet that ultimately reduces thrust. This scale. The resulting engine type has the ing heat to fast-moving air results in energy loss becomes increasingly severe appropriate name of supersonic combus- significant energy losses as compared at higher speeds. tion ramjet, or “scramjet.” to ramjets. Another problem has to do with the Such an engine makes a whole new Despite efforts beginning in the 1960s, temperatures associated with the ram realm of atmospheric flight possible, including the construction of various scramjet test articles (notably, the pio- A hypersonic cruise missile engine neering work of Antonio Ferri), these used in a ground test as part of HyFly. challenges and others delayed the practi- cal development of a scramjet-powered vehicle for more than 45 years. The HyShot research team at Australia’s University of Queensland flew what is generally credited as the first scramjet in July 2002 on the nose of a sounding rocket, though its thrust was less than the Photo courtesy of DARPA/ONR/NASA Langley Research Center Center Research Langley DARPA/ONR/NASA of courtesy Photo

overall drag. HyShot was soon followed by two successful flights of NASA’s highly sophisticated X-43 vehicle, prov- ing once and for all that scramjet thrust could be greater than vehicle drag. Though impressive accomplishments and important steps along the way, both the Queensland work and NASA’s X-43 compression effect. At extremely high with a corresponding list of possible were powered by scramjets that burned speeds the temperature of the air as it missions and vehicle applications. hydrogen and could only operate for a slows down in the inlet can be so high However, actually building a practi- few seconds. that it’s above the temperature at which cal scramjet proved quite difficult. The Air Force’s X-51 took scramjets fuel burns. When that happens, combustion For example, trying to burn fuel in a further by burning a more easily handled stops and there can be no energy addition supersonic stream has been likened jet fuel for almost 3.5 minutes in flight, inside the engine—hence, no thrust. to lighting a match in a hurricane. albeit at a more moderate flight speed. Weber and MacKay asked a simple There is precious little time to inject It was the defining breakthrough that question: What would happen if the the fuel, mix it with the air, and burn may lead the way to practical hypersonic air that enters a ramjet at high Mach it to completion. missiles. number doesn’t slow down much but In a reasonably sized scramjet, the air USAF continues to invest in hyper- instead continues to move at high speeds entering the front of the engine would sonics, including activities at the Air

50 AIR FORCE Magazine / March 2016 Force Research Laboratory and at wind lead in X-43 and the HyBoLt—Hy- China’s People’s Liberation Army has tunnel facilities at the Arnold Engineer- personic Boundary Layer Transition— boasted of a rocket-powered hypersonic ing Development Complex at Arnold aerodynamics experiment. Although missile apparently designed to attack AFB, Tenn. There is signifi cant work hypersonics funding at NASA has been aircraft carriers. Russian leaders, in- being done—on increased thrust, better decreasing in the last few years, the cluding Deputy Prime Minister Dmitry mixing, ignition, and fuels—to explore agency recently committed to expand- Rogozin, have voiced strong support for ways to improve scramjet perfor- ing its research efforts for fundamental continued hypersonics development, and mance. In combination with industry science and to providing ongoing test Russia has announced a joint program partners, USAF researchers are also and modeling support. with India to develop a hypersonic looking to combine scramjets with Despite these successes, there’s a long successor to the BrahMos supersonic other en gine types for an expanded way to go to achieving fully operational cruise missile. flight envelope. weapons systems. Advanced guidance The push for practical hypersonic Lessons learned on the X-51 are systems, sensors, and warheads will be weapons has been construed by some being applied to solving operational required to make practical weapons. Fig- as a new arms race, focused on speed. and scaling issues. At the same time, uring out how to integrate these weapons As Hallion and Bedke have warned, the Air Force Offi ce of Scientifi c Re- with existing or future platforms will hypersonic weapon technology is “ripe search sponsors ongoing university be a challenge. for exploitation as a theater and global programs to expand the fundamental Two DARPA programs, funded in strike game changer” but it’s not yet clear understanding of high-speed aerody- part with USAF dollars, are attempt- “whether America will own that advan-

US Air Force Flight Test Center Public Affairs photo A model of the National Aerospace Plane is readied for a wind tunnel test. Like HyFly, the program contributed to advances in hypersonic fl ight technology, though it was canceled.

The X-51 at Edwards before its fi rst fl ight. The idea of hypersonic weapons goes back to at least the 1930s. NASA photo namics, develop new modeling and ing to demonstrate technologies for a tage fi rst.” Though the US is investing simulation techniques, and invent realistic weapon, but critics note that in hypersonics and their maturation, “it new high-temperature materials and at their current pace the fi rst planned is not on a guaranteed path to near-term instrumentation. fl ights won’t occur until the end of this success.” As the authors noted, there’s Working with the Department of decade, years after the X-51. still no fi rm national commitment to a Defense Test Resource Management Of particular note is the DARPA Tacti- disciplined plan tackling the remain- Center and AEDC, the Offi ce of Scien- cal Boost Glide program. Instead of using ing hypersonic challenges, let alone a tifi c Research has also been developing scramjets, TBG is an unpowered hyper- plan to develop and acquire high-speed programs that allow undergraduate sonic glider that will be rocket-boosted weapons even if planned demonstrations and graduate students to participate to a high . TBG builds on are successful. in hypersonic testing, with the goal of lessons learned from DARPA’s HTV-2 The US has clearly established itself replenishing the workforce that special- and may offer an attractive alternative as the early leader in the hypersonics izes in hypersonics. concept for a hypersonic missile. fi eld, but it remains to be seen whether The Air Force is partnering with The US is hardly alone in develop- the fi rst practical hypersonic weapons NASA, too. Historically, the civilian ing hypersonic systems. Russia, India, will bear the markings of the US or the space agency has been an important and China have been active in the fi eld. insignia of some other nation. ✪ contributor and developed many as- pects of scramjet propulsion. NASA Mark J. Lewis was Chief Scientist of the Air Force from 2004 to 2008. He is cur- engineers played a key role in programs rently the director of the Institute for Defense Analyses’ Science & Technology such as X-51, HIFiRE, and took the Policy Institute. This is his fi rst article for Air Force Magazine.

AIR FORCE Magazine / March 2016 51