By Nicole Petrucci
As part of our series on the Third Offset Initiative, the author presents a sound argument for the development of hypersonic technology and potential applications for future use. The following article is edited from The Phonics of Hypersonics: Technical Thresholds, Operational Concepts, and Strategic Implications published by Air University in June 2016. This first part of the excerpt will discuss the physics behind hypersonics while the second part of the excerpt will focus on military applications.
Hypersonic technology takes speed to another level. Hypersonics, defined as speeds greater than Mach 5, represent the logical progression of aviation and speed from the Wright Flyer to the bi-plane to the jet engine to the rocket. Each step in this evolution produced many capabilities, and the promise of higher and faster has always been prevalent. With that evolution in speed and capability, however, comes a natural evolution in strategy and doctrine. Just as the advent of supersonic technology changed how the United States in general, and the USAF in particular, executed air supremacy, so hypersonic flight may change how the USAF looks at its entire mission set in a revolutionary way. Hypersonics could allow a diverse set of strategies to combat irregular and asymmetric threats as well as entrenched and layered air defenses. The application of hypersonic technology could change the United States’ views strategic deterrence, forward presence, and military action.
The USAF officially recognizes three principal missions where it intends to utilize and develop hypersonic technology: (1) hypersonic cruise (for applications on cruise missiles, reconnaissance, transport, and strike aircraft); (2) reentry from orbit, and (3) as a high-speed accelerator, a reusable booster or single-stage-to-orbit. This article will discuss the first two missions. The second mission comprises slowing high-speed vehicles while the others concern a highly efficient propulsion system.
Physics of Hypersonics
Speed above Mach 5 is a boundary problem for aircraft, just as Mach 1 was a boundary problem for design and propulsion in the 1940s. Normal aerodynamics ignores the compressibility of air. Below Mach 1, neglecting the compressibility effects is possible because they are minimal, and airplanes fly well enough. Above Mach 5, ignoring the compressibility of air is no longer possible. The air itself has distinct regions and characteristics. At less than 500 mph, the atmosphere is obedient and easily defined. As the speed increases, there are abrupt discontinuities.
One of those discontinuities exists regarding air pressure at Mach 1. Aerodynamic drag intensifies rapidly at Mach 1, which requires greater thrust and more streamlined airframes. Below Mach 1, the pressures an airplane generates as it moves through the air are small relative to the ambient pressure in the atmosphere. At Mach 1.2, the dynamic pressure is equal to the atmospheric pressure; at Mach 5, it is 25 times greater. While this quandary of pressure may seem intense, it is not even the real problem. Mach 5+ airflow causes such extreme temperatures it alters the chemistry of the air around the vehicle. Molecular bonds vibrate at frequencies that influence the forces acting on objects moving through the air; and beyond Mach 5, aerodynamic heating causes molecules to break apart and produce electrically charged plasma around the aircraft. When contemplating, it is important to conceive the scope, importance, and challenges of aerodynamic heating. Control and dissipation are literally a trillion-dollar problem. The first supersonic designs
were profoundly different from subsonic aircraft. The pressures and temperatures of the supersonic regime required upgrades from wood and fabric to aluminum and steel. The Bell X-1 was radically different from anything designed prior. It broke through the sound barrier and signaled a fast-paced national research effort to keep going faster. As speed increases through Mach 2, aluminum loses its usability. By Mach 2.2, aluminum is useless.
Now think of the difference between a Mach 1 aircraft and a Mach 3 aircraft, the Bell X-1 and the SR-71 Blackbird. The Blackbird was a true Mach 3 airplane, unique in design from even a Mach 2 airplane, such as the F-104 Starfighter or F-15 Eagle. Mach 3 speeds in the lower atmosphere produced so much heat that the plane would have melted if made of aluminum and steel. The Blackbird was made of titanium and had to incorporate heat sinks into its design. When sitting on the ground the airframe had gaps, but at Mach 3, thermal expansion sealed the panels. Mach 3 is only truly achievable where the atmosphere is thin, 85,000 feet, where the atmospheric pressure is 1/5th that of sea level. Flight at these pressures allows 80% less friction, but still produces temperatures exceeding 600°F. The SR-71 returned from missions with rivets missing, panels delaminated, and parts fused together.
The only aircraft that comes close to Mach 3 today are the Mikoyan MiG-31 Foxhound and MiG-25 Foxbat. While both have a thrust-to-drag ratio capable of speeds exceeding Mach 3, both are speed-limited by the Russian Air Force to Mach 2.8, which is a temperature redline. Above Mach 2.8, the airframes and engines begin to melt. Radars once tracked a MiG-25 at Mach 3.2, but the damage to the engines was beyond repair. Russian fighter aircraft have not flown above Mach 3 since. The inability of contemporary aircraft to withstand intense aerodynamic heating is the reason why the air forces of today do not operate at or above Mach 3.
Step up from Mach 3 to Mach 5 and the temperature doubles again. No turbine engine ever developed or designed would survive. This region is at the limit of what any known metal or alloy can withstand without exotic materials like ceramics. NASA found that the X-15 absorbed eight times more heat at Mach 6 than it did at Mach 3. Scientists developed new fields of science to build new structures and designs, incorporating the leading edge of chemistry, physics, thermodynamics, fluid mechanics, and more. The problem was daunting, but not insurmountable.
The hypersonic cruise missile can give the advantage to the 2020 USAF much like the cruise missile gave to the 1980 USAF. A hypersonic CM (HCM) is the solution to re-creating that capability gap and reinstituting the dominance of Global Strike. HCMs could attack the newest S-300 systems directly, with impunity. The HCM will boost to 200,000 feet and cruise towards the target from 1,000 miles away at Mach 6. Close in, the missile will dive over from the upper stratosphere and slam into its target from directly overhead at five kilometers per second. Air defenses will not be able to engage this missile with anything resembling consistency, whether the systems detect the missiles or not. HCMs could re-assert the dominance of US forward presence in the face of continued A2/AD proliferation. It is a not a lasting solution, however, it simply repeats previous solutions for the same problem. The HCM will attack the SAM directly, until the day it is good enough to engage the HCM, and then the HCM will overwhelm the system. It is not a new idea; it is just a faster old idea. Defeating A2/AD systems is necessary to promote AirSea Battle, and HCMs are the solution to continued Global Strike in the short term. This technology fits into existing paradigms concerning strategies and doctrines; enabling the US to continue forward-presence missions, permitting an offensive style of warfare, and is cheaper and faster than a revolutionary technology. It does all of this without upsetting foundational doctrine and styles of warfare. HCM is the perfect fit for the United States today, assuming the HCM incorporates into existing operational concepts, and supports the bomber similar to how the CM supports current bombers. The CM compensated for the lack of penetration ability of the 1980s B-52 against Soviet defenses. The HCM will compensate for the lack of penetration ability of the 2020 USAF against A2/AD defenses. The CM allowed a blending of quantity and quality, with an emphasis on quantity, and the HCM will do the same.
The hypersonic conversation has occurred to this point without the inclusion of nuclear weapons, to the detriment of the development of national policy. Acting as if a new bomber, a new cruise missile, or any combination of those can develop without having any impact on strategic stability is laughable. To move down the acquisitions path of both systems ignoring nuclear policy is both naive and dangerous. Twice the United States contemplated reducing the nuclear triad to a dual force by eliminating the bomber portion. Twice the US came to a conscious decision that the air-breathing leg of the triad remained necessary. Therefore, for the purposes of exploring the policy implications of an HCM, we must assume the US will continue to be committed to the air-breathing nuclear force.
Today that force consists of the B-2 penetrating bomber dropping nuclear bombs and the B-52 standoff bomber launching nuclear CMs. The B-1 is no longer considered a nuclear bomber due to past treaty limitations, not through any deficiencies in capability. While the exact methods employed and dispositions of these bombers in any nuclear engagement are classified, the quantity of aircraft and missiles suggests the B-52 armed with the ALCM plays a much larger role in this dynamic than the B-2 dropping gravity nuclear bombs. Regardless of the warhead inside, the CM is vulnerable to modern air defenses. The United States saw this problem as early as the mid-80s and created a new subsonic CM to deliver nuclear warheads, the Advanced Cruise Missile (ACM). It attempted to incorporate stealth technology, allowing it to penetrate where the ALCM would not, removing the ALCMs vulnerability, and reinstituting the capability gap to the offense. It would have been the answer to all the United States’ problems except that the ACM was a disaster. It had accuracy issues, maintainability issues, reliability issues, carrier limitations, and was a general disappointment. To be blunt, while the USAF had every confidence it would not be shot down; it had equal confidence it would not hit its target. Therefore, the problem of delivering air-breathing nuclear weapons remains.
The solution today is the same as it was in the late 1970s: a combination of a penetrating bomber and a standoff bomber delivering a CM that can directly defeat modern air defenses. Therefore, to assume that the United States will not arm HCMs with nuclear warheads to replace vulnerable ALCMs similarly armed is not valid unless the United States intends to do away with its air-breathing nuclear force entirely. The other alternative is that the future air-breathing leg of the triad exists for political reasons only, similar to the fielding of the Snark, and serves no valid military purpose.
The United States should concurrently field an autonomous, precision-guided nuclear and non-nuclear version of the long-range HCM. The US should also prioritize the development and fielding of long-range HCMs over short-range HCMs. The longer-ranged missiles provide strategic effects, contribute to extended deterrence, and are easier to develop. Any air-launched HCM will require a booster of some sort. These boosters are physically large and heavy, prohibiting their carriage on smaller tactical fighters. The size and weight of any air-launched HCM requires a bomber. Even if the USAF develops short-range HCMs, they will still have to be launched from bombers until the technology matures and it is physically possible to fit HCMs onto smaller aircraft. Shorter-ranged missiles will no doubt be more numerous, and the US will have to make a quantity-quality decision on their application. In the short-term, short-range sub-sonic CMs may prove to be good enough for tactical situations, and it may take some time before a hypersonic short-range CM may be necessary. The land-attack variant of a long-range HCM should have a higher priority over any anti-ship HCM, for the same reasons. Also, the technological advances needed for an HCM to find, fix, and engage moving targets such as ships is a generation beyond a simpler land-attack variant. At the same time, the US should pursue development of submarine-and-surface-ship-launched HCMs for land attack and anti-ship operations, but again these should be prioritized after an air-launched version. The US should not pursue development of land-based HCMs because it would invalidate the Intermediate Nuclear Forces Treaty. The diplomatic advantages of this treaty outweigh the military advantages of having a land-based HCM. The air- and sea-launched versions will provide the required strategic and tactical capabilities.
Prompt Global Strike
For the past few decades, the United States has been searching for the capability to reach any target on the planet within an hour without relying on forward-based forces. This is the essence of a program that has become known as Prompt Global Strike (PGS).
Hypersonic boost-glide RVs have been the focus of PGS concepts in many nations, including the United States. It seems clear that boost-glide RVs, with the specific intention of being utilized in a PGS system, is what Air Force Research Laboratory (AFRL) meant when it identified space reentry vehicles as an area of continued hypersonic research for the USAF. Boost-glide weapons are particularly attractive now because the technology exists to turn these weapons into precision-guided weapons. One of the many reasons why ICBMs were fitted with nuclear warheads was because of their inaccuracies. The larger warhead was necessary to make up for the inaccuracy of the weapon. Today, a boost-glide RV can be expected to have precision accuracy, the same as a laser-guided bomb. This is what makes boost-glide viable today when it was not viable in the 1960s.
There are many reasons why the US is not contemplating a PGS system for the full spectrum of military operations. First, an ICBM attack has never been ordered in the history of war. ICBMs are forever branded with a nuclear stigma, and other nations would note the launch of such weapons and assume they were nuclear. The potential for miscalculation or misinterpretation is high. Secondly, ICBMs are very expensive, and it does not make economic sense to shoot one or more at any target that is not of the utmost national urgency. Third, PGS is not intended to destroy Russian nuclear weapons, or affect strategic stability with Russia. Finally, while PGS has all the appearances of something that is strategically desirable for the United States, it has refused to categorize PGS in a strategic context.
While the Department of Defense has made suggestions, it has never fully enunciated why it wants PGS. Instead, the US seems to be pursuing the technology for technology’s sake. These missions are merely suggestions of possible use. They are not the actual scenarios for PGS design. The Department of Defense has not made public statements with any specificity. This is a problem because different missions have different weapon requirements. The Pentagon is preparing to choose the best PGS system to buy and field, but the best system depends on the scenario. For example, the Ballistic Missile Defense (BMD) system has a specific purpose: defeat limited North Korean missiles that threaten the United States. It is not designed to defeat a mass missile attack from Russia or any short-range missile attack anywhere. It was designed for that specific mission, fielded to optimize that mission and exists within that strategic context. In fact, the US has made strides to assure Russia that BMD does not impair its strategic stability. PGS does not have a clear purpose and needs one for evaluation against the US strategic context. At the same time, Russia and China are developing PGS systems as part of their strategic context.
Despite its shortcomings, fielding a PGS system could redefine how the United States views strategic effects with military weapons. The United States has been on a multi-decade journey to create a conventional alternative to nuclear weapons; non-nuclear weapons that can create strategic effects. What the hypersonic cruise missile will do for this policy shift is small compared to what PGS would similarly accomplish. In this vein, hypersonic technology, when fielded in a fully global PGS system, can revolutionize Global Strike doctrine. It also has the capability to revolutionize how the United States views the defense of the homeland. Recall that offensive warfare, far from its borders, is critical and integral to all of US foreign policy. If the US can field a PGS system, it is only a matter of time until someone else can field one. In that case, an adversary would not need a global Navy or an intercontinental amphibious invasion force. A truly global PGS system in the hands of an adversary may change the very foundation of how America thinks about defense.
Technology can shape strategy and doctrine by enabling or defeating capabilities not previously capable. The US has a long history of believing that its technological prowess is a guarantee of victory, while promoting technology for technology’s sake absent a strategic context. Hypersonic technology is a long time in the making, and because of the slow burn of this capability, the US has the time to fully contemplate the strategic ramifications before fielding such technology, allowing it to pursue what is in its interest and dropping what is not. By developing hypersonics in these three ways, the US can achieve more of what every military organization has always wanted.
Nicole Petrucci is a Space Operations Officer in the United States Air Force. She is the creator and author of many space-related and non-kinetic tactics and techniques used by operators in the Department of Defense and is a recognized expert in the space control, space operations, missile warning, and strategic deterrence fields.
The views expressed are those of the author and do not necessarily reflect the official policy or position of the Department of the Air Force or the US government.