Hypersonic flight: Threat or opportunity?

If there's one word that keeps military planners from sleeping at night, it's "hypersonic." The ability to fly at more than five times the speed of sound holds the promise of getting to the antipodes in less time than it takes to drive across town. However, it also makes possible deadly weapons that can penetrate any defenses currently available while delivering the destructive force of a tactical nuclear weapon. So what is hypersonic flight and is its development an opportunity or a threat?

If you've been following defense or aerospace news in recent years, you may have noticed that there's been significant buzz about hypersonics. The USA, China, Russia, Germany, Britain, India, Australia, and even Indonesia have been conducting research into various aspects and problems of flying at Mach 5 (3,800 mph, 6,125 km/h) and above. Much of this is basic research, but mixed into news feeds are reports of growing concern about hypersonic developments by America, Russia, and China for military purposes.

Why these reports should worry military analysts is fairly obvious. At the moment, defense systems operate at supersonic speeds and below. Fighter planes, bombers, missiles, anti-missiles, and artillery shells all fly at less than Mach 5. Even the fastest manned jet plane ever built, the now-retired SR-71, could only manage Mach 3.5. The only weapons that do count as hypersonic are ballistic missiles that fly out into space before returning to Earth, which makes them a very special case.

The fear is that if someone develops an aircraft or missile that can travel over Mach 5, it will have an effect similar that which jet engines had on military aircraft. When the first fighter jets appeared in the skies over Europe in the last year of the Second World War, the effect was dramatic. When Nazi Germany's Me 262 blasted by the Allies' Spitfires and Mustangs like they were standing still, it was like being passed by the future. Today, the worry is that hypersonic weapons will be able to penetrate current defenses with the ease of a red-hot battering ram encountering a butter fortress.

Power plays

The development of a practical hypersonic weapon would clearly destabilize global security, a fact compounded by the mystery shrouding many countries' hypersonic programs.

For example, Russia's Zircon hypersonic cruise missile program keeps popping up in the news. Designed by NPO Mashinostroyeniya and allegedly in production since 2012, it's been pushed fairly aggressively by state-run Russian media and has been the subject of much speculation in the West.

Some reports claim that Zircon has already been fully developed and tested, and is now being widely deployed with the Russian armed forces. According to these reports, it has rendered the Royal Navy's Sea Ceptor surface to air missile system obsolete before it's even been cleared for service. This is because this version of Zircon can supposedly travel at Mach 8 (6,090 mph/9,800 km/h) and has a range of 620 mi (1,000 km) with 72 of the hypersonic missiles already being installed in a pair of Kirov-class battlecruisers, as well as on destroyers, submarines, and bombers, with the Russian Army set to get theirs by 2020.

However, other accounts claim Zircon has a speed of only Mach 5, a range of 250 mi (400 km), and hasn't even been properly tested yet – much less deployed. That may seem like a large discrepancy, but the Russian government has long had a propensity for combining bluff with boasting.

As to where Russia really is with regard to hypersonics might be provided by India, which is currently developing BrahMos-II – a hypersonic version of its BrahMos missile that can reach Mach 7 or 8. What's interesting is that a joint project with Russia's NPO Mashinostroeyenia is aimed at developing a suitable hypersonic fuel. To some analysts, this joint work indicates that Russia's hypersonic technology may not be as advanced as previously thought if they're going back to basic research.

It's a similar story with China's WU-14 missile, also known as the DF-ZF, which is a weapon designed to be deployed on the edge of space by a ballistic missile before gliding back to Earth at Mach 10 (7,613 mph, 12,250 km/h). It's been flight-tested seven times from 2014 to 2017 at the Taiyuan Satellite Launch Center in Shanxi Province and is claimed to be nuclear-capable and able to be fitted to ICBMs to give it a range of up to 7,500 mi (12,000 km).

This is all possible, but China has been boasting of having a ship-killing hypersonic ballistic missile for over 10 years, with little evidence to back up the claims. While the flight tests of the WU-14 have been tracked by the West, it's more likely designed as a short-range ship killer armed with a conventional warhead rather than a strategic weapon.

Whatever the state of Russian or Chinese hypersonics, the United States is taking them seriously enough to spend US$75 million on hypersonic defense.

"We must push the boundaries of technology in every area," says Air Force Chief of Staff General David L Goldfein. "Our adversaries aren't standing still. They are looking for every advantage they can get."

The US has a robust hypersonic research program, mainly operating under the government's Prompt Global Strike (PGS) umbrella with the intention of developing an Advanced Hypersonic Weapon as well as anti-hypersonic defenses. The main goal of PGS is to produce a hypersonic precision-guided conventional weapon capable of carrying out airstrike anywhere in the world within one hour of launching from American territory.

The idea is that such a hypersonic weapon could potentially replace 30 percent of the US nuclear arsenal. This would not only have a considerable impact on nuclear deterrent strategy, but would also allow the US to attack strategic targets in rogue states with less risk of sparking a larger war. This is because the trajectory of a hypersonic conventional missile would be very different from that of a nuclear-armed ballistic missile. To date, no system has been officially approved, though the previous US administration did issue contradictory statements with United States Secretary of Defense Robert Gates claiming in 2010 that the US did have a Prompt Global Strike capability.

Meanwhile, the US is working with Australia on the Hypersonic International Flight Research Experimentation (HIFiRE) program, which has had several successful hypersonic flights, including one that reached Mach 7.5. In addition, Lockheed Martin is working on a hypersonic Tactical Boost Glide (TBG) vehicle under a US$147 million DARPA contract.

What is hypersonic flight?

So what exactly is hypersonic flight, why is it so important, and why all the interest in it now?

The first thing to understand is that hypersonics isn't just a souped up version of supersonics. It's as different as supersonics is from regular flight, only the challenges are far greater and the solutions more elusive. In fact, it's a classic example of the law of diminishing returns.

The term "hypersonic" was coined in 1946 by Caltech aerodynamic engineer Hsue-shen Tsien and refers to speeds above Mach 5, where a new series of aerodynamic properties come together and dominate the problems of flight. Mach 1 is the speed of sound at sea level and a temperature of 15° C (59° F). However, this is not the case all the time in the real world. The speed of sound changes with differences in air pressure and temperature, though the air behaves the same regardless of what that speed is. Engineers need to take into account the variable nature of Mach, but, for our purposes, we'll be working with the standard definition of Mach.

Speeds under Mach 0.8 (609 mph, 980 km/h) are called "subsonic." That may seem odd, but speeds between Mach 0.8 and Mach 1.2 (913 mph, 1,470 km/h) are "transonic," that is, they straddle "sonic," which is Mach 1 (761 mph, 1,225 km/h). Mach 1.2 to Mach 5 (3,806 mph, 6,125 km/h) are supersonic, while speeds over Mach 5 are hypersonic. Speeds above Mach 10 (7,613 mph, 12,250 km/h) are high hypersonic and those above Mach 25 (19,000 mph, 30,600 km/h) are the ultrasonic velocities only reached by space vehicles.

For a real world idea of what these speeds mean, a subsonic airliner can travel from New York to London in about 5 hr 30 min and a supersonic plane can do it in 3 hr 10 min. A hypersonic airliner could cover the same distance in 1 hr 35 min. But there's much more to these categories than numbers. They affect the most basic of aerospace engineering problems.

They used to say you can make a tea tray fly if you put enough power behind it, and, at subsonic speeds, "they" were right. It's the reason drones can look nothing like a conventional airplane and why hobby shops sell flying models of the very non-aerodynamic Millennium Falcon.

But as one approaches the speed of sound, things change. That's why there's a transonic category. During the Second World War, for example, some prop-driven and jet fighter planes came close to Mach 1 without reaching it. However, the tips of their propellers or turbines would exceed the speed of sound, causing all sorts of trouble as turbulence threatened to tear them apart.

Sonic is where we encounter the famous sound barrier. At Mach 1, the air doesn't have time to get out of the way of the speeding aircraft and a shock wave starts to build up in front of it, causing drag to increase dramatically. This drag is so severe that, at one time, the sound barrier was regarded as a literal barrier that aircraft might never be able to penetrate.

That changed on October 14, 1947 when the rocket-powered Bell Aircraft X-1 piloted by Captain Charles "Chuck" Yeager pegged the speedometer at Mach 1.06, becoming the first manned aircraft to exceed the speed of sound in level flight and, as the X-1 outran its own shockwave, creating the first aerial sonic boom .

In the wake of this feat, engineers discovered that the problems of supersonic flight were largely a matter of design. By adding features like swept wings and extreme streamlining, supersonic flight soon became practical, culminating in the supersonic Concorde and Tupolev airliners that entered service in the 1970s.

But hypersonic flight is on a whole different level of difficulty. It isn't like supersonic. There's isn't a barrier, but Mach 5 isn't an arbitrary number. This is the speed at which a number of factors that affect high-speed flight come together and dominate the aerodynamics. The most important of these is heat – a fact that wasn't immediately obvious in the history of hypersonic flight.

The first hypersonic flight

While the first supersonic flight used a rocket plane, the first hypersonic flight used a rocket. Specifically, it was a V-2 rocket built by Nazi Germany during the Second World War. The first ballistic missile, this "superweapon" was used against Britain and Holland, but after the war, hundreds of captive rockets and the engineers who designed them were brought to the US to spark the American rocket program.

On February 24, 1949 at White Sands, New Mexico, a V-2 was launched into space with a WAC Corporal rocket in its nose that acted as a second stage. After reaching an altitude of 250 mi (400 km) on a suborbital trajectory, the Corporal fell back to Earth. During its plunge, radar tracked it as it reached Mach 5 before burning up in the atmosphere, leaving only a rain of charred debris to hit the ground at the end of its flight.

The military applications of hypersonic flight weren't lost on strategic planners. In fact, the Germans were already thinking along those lines as far back as 1938 when Eugen Sänger and his later wife, the mathematician Irene Bredt, developed plans for the Silbervogel, better known as the Sänger bomber.

This conceptual design for a hypersonic space launch vehicle would have used a rocket-powered sled to replace a liquid-fueled, rocket-powered suborbital bomber.

Designed to attack the United States from Europe, it would have carried an 8,800 lb (4,400 kg) bomb to be dropped on New York or other targets before flying on to land in Japan after a total journey of up to 15,000 mi (24,000 km)

The Sänger was never built, but Sänger and Bredt went to work for the French government after the war and their designs had a huge effect on later hypersonic and space projects. In fact, they drew so much interest that the Soviet dictator Joseph Stalin tried to kidnap the pair, but was unsuccessful.

One important aspect of postwar research was the discovery that hypersonics were key to developing both spacecraft and intercontinental ballistic missiles. A returning spacecraft or warhead hits the atmosphere at Mach 25 (19,000 mph, 30,627 km/h) or higher and would be destroyed in short order, but in the 1950s, scientists and engineers developed new materials and technology, like ablative heat shields, that not only protect the payload from the high temperatures produced by reentry, but can also allow it to be steered to reach its target.

The first spaceplanes

However, the real goal was controlled hypersonic flight. This was achieved in 1959 when the first of three North American X-15 rocket-propelled aircraft took to the skies. Operated by the US Air Force and NASA until 1968, the X-15 was an alternative to NASA's approach of sending astronauts into orbit atop ballistic missiles. Instead, the X-15 program produced the world's first operational spaceplane that could fly into space under its own power, then return to Earth like a glider – all under the control of the pilot.

During 199 flights, the X-15s were carried aloft under the wing of a mothership (usually a modified B-52 bomber) from Edwards Air Force Base in California. Dropped from an altitude of 45,000 ft (13,700 m), the 50-ft (16-m) long craft then fired its Reaction Motors XLR99-RM-2 rocket engine, generating 70,400 lb of thrust and sending it into a suborbital trajectory.

This caused it to climb at 60,000 ft/min (18,288 m/min) until the X-15 reached altitudes of up to 67 mi (108 km), which is beyond the defined start of space. At this altitude, the atmosphere is a near-vacuum and the airfoils are useless, so attitude control for the spaceplane was courtesy of a set of reaction jets like any other spacecraft.

Technically, these flights were more than trips into space. All eight pilots were later awarded astronaut wings and one pilot, someone named Neil Armstrong, went on to become the first man to set foot on the Moon.

The X-15 still holds the record for the highest altitude manned flight, and it also holds the record for the fastest. In October 1967, William J. Knight hit Mach 6.72 at an altitude of 102,100 ft (31,120 m). The only people who have gone faster have been in Space Shuttles and orbital capsules.

"More important than records, however, were the X-15's probing of hypersonic aerodynamic performance and heating rates, research into structural behavior during high heating and high flight loads, study of hypersonic stability and control during exit from and reentry of the atmosphere, and examination of pilot performance and physiology," said a NASA report on the program.

A similar thing could be said about the X-15's successor, the Boeing X-20 Dyna-Soar. Developed for the US Air Force, this was intended to be a fully operational spaceplane that could act as a reconnaissance platform, bomber, orbital rescue ship, satellite maintenance unit, and space interceptor. The program ran from October 1957 to December 1963 and cost US$5.16 billion in 2017 dollars, before being cancelled while the first Dynasoar was being built.

If the X-15 was a spaceplane, Dyna-soar was a full-blown spaceship capable of operating not just on the edge of space, but to distances as far from Earth as 26,000 mi (42,164 km). The single-seater, 35.3-ft (10.77-m)-long delta-winged craft with a 21-ft (6.3-m) wingspan would be launched atop a rocket booster assisted by its onboard transtage rocket engine with 72,000 lb of thrust.

Once in space, the Dyna-soar was designed to either go into orbit or travel over long distances across the globe at hypersonic speeds using a maneuver called atmosphere skipping. That is, the craft would skim above the atmosphere before hitting it at a shallow angle. Instead of plunging down into the atmosphere, the Dyna-soar would skip off the air and back into space like a stone skipping off the surface of a pond.

The Dyna-soar program was the center of a massive research effort and generated significant data, but despite its success, it was cancelled before flight testing began due to arguments over what type of booster to use, the lack of a clear mission, and NASA favoring its capsule-based manned vehicles.

Going hypersonic

The goal of the X-15 and Dyna-soar was to get into orbit, but their more valuable contribution was to broaden understanding of hypersonics and the problems of flying at such speeds. They demonstrated that the real problems weren't about getting into space, but dealing with the problems of hypersonic speeds, and much of the design and test data found its way into later Space Shuttle and hypersonic projects.

Today, there are basically three ways of going hypersonic. The first is ballistic flight where the vehicle is launched from a rocket like the V-2 and flies with little or no control once it enters the atmosphere. All returning space vehicles hit these speeds and for them it's less a matter of true flight than surviving impact with the atmosphere and possibly controlling where you land. For our purposes, we're setting these to one side because they are essentially nuclear ballistic missiles with more steerable conventional warheads, or manned spacecraft like Orion that have a slightly aerodynamic shape giving some small control over flight path.

The next version is called "boost glide," which includes the X-15 and Dyna-soar. In this, the vehicle is accelerated using a solid or liquid-fuel rocket, then glides at hypersonic speed. This was how the X-15 worked. The Dyna-soar was a much more sophisticated version thanks to its skipping maneuver. The drawback is that these supergliders have only a limited range due to all the drag they have to deal with.

But the holy grail of hypersonics is "cruise." That is, a hypersonic vehicle with an engine that allows sustained flight with longer range and maneuverability.

These engines could be air-breathing turbojets similar to those used by the supersonic SR-71 Blackbird of the Cold War. These jet engines work on the same principle as those installed in subsonic aircraft, except that they have precoolers, variable air inlets, and an afterburner. The core turbojet engine is buried in the middle of all of this plumbing and the reason is that the incoming air needs to be slowed to subsonic before it can enter the combustion chamber. This causes the air to rise in pressure and temperature, all of which has to be compensated for. Unfortunately, turbojets start to struggle after reaching Mach 3, so they are more auxiliary engines than the main drive for a hypersonic craft.

At speeds of over Mach 3.5 and up to Mach 6, a ramjet engine becomes a viable alternative. A ramjet is essentially a jet engine without moving parts. Instead of a turbine, the air is compressed by the forward motion of the engine. This is because at high enough speed, the turbines in the compressor become redundant.

There's an inlet that slows the air down to subsonic, raising the pressure and temperature, which means that the ramjet must be already moving at Mach 3 to work. Otherwise, it's just an empty pipe squirting aviation fuel. The upper limit is because as the speed increases, the air becomes more compressed, the engine gets hotter, then stops working altogether.

At speeds above Mach 6, scramjets become the engine of choice and have been the focus of most modern hypersonic cruise research. A scramjet is, essentially, a ramjet where the air isn't slowed to subsonic. Since the air is only slowed down a bit, but remains supersonic, the pressure and temperature are raised, but not enough to damage the engine.

According to the Queensland University in Australia, a scramjet might be able to reach Mach 14 (10,658 mph, 17,151 km/h). By this point, it becomes hard to burn fuel and drag becomes too high.

To go even faster, a hypersonic vehicle must fly so high that it is now, for all intents and purposes, a spacecraft. This means using hybrid rocket engines that combine both air breathing jets and rocket engines. In the lower atmosphere and lower speeds, it acts like a jet, but at high hypersonic speeds on the edge of space, it switches to an internal supply of liquid oxygen to burn its fuel and becomes a rocket. Such hybrids are less efficient than jets, but can operate over all ranges of speeds up to sending the vehicle into orbit.

Rockwell X-30

Hypersonic research has been dominated by the Americans since the 1980s with a mixture of ambitious and often abandoned projects. One of the biggest was the Rockwell X-30 of the 1980s that President Ronald Reagan called "a new Orient Express." This advanced technology demonstrator project for the National AeroSpace Plane (NASP) program was intended to be not only a passenger spaceplane, but also a single-stage-to-orbit (SSTO) spacecraft to replace the Space Shuttle.

The X-30 went through a number of design iterations, but at its most advanced it had engines burning a mixture of air and liquid hydrogen that could push it to Mach 25. Meanwhile, its lifting body design allowed it to ride its own hypersonic shock wave.

The project was cancelled shortly after the end of the Cold War because the upscaling the prototype unmanned design to turn it into a manned vehicle was too complicated and expensive, and there were all manner of problems with the scramjet engine. However, there was also conjecture that it may have been another American challenge-and-shame-the-Soviets-into-bankruptcy programs that were cancelled after the fall of Communism.

Boeing X-43

The Boeing X-43 of the late '90s and early 2000s was the next big project. An unmanned experimental hypersonic aircraft with multiple planned scale variations, it's the fastest aircraft on record at approximately Mach 9.6. This technology demonstrator was rocket boosted and used a scramjet to sustain flight.

Boeing X-51

Another record holder is the Boeing X-51, also known as the WaveRider because it was designed specifically to exploit its hypersonic shock wave to maintain it in flight. The unmanned research scramjet aircraft first flew in 2010 and reached a speed of over Mach 5 on May 1, 2013 for 210 seconds – the longest duration powered hypersonic flight.

What has changed?

So what do these X-planes and records tell us? Why the renewed interest and even outright concern about hypersonics? What's changed? And why wasn't there such interest ten or 20 years ago?

Part of the reason is that scientists, engineers, and military types are becoming more aware of the advantages of hypersonic vehicles and weapons. For one thing, they are FAST. If such hypersonic weapons or warplanes were available today, they could overcome even the most advanced radar, anti-missiles, and fighters.

True, the same could and was said about the V2 and its descendants, which could punch through any defense of the day. But ICBMs can't be steered – at least, not very much. Hypersonics, on the other hand, are very steerable and that is a big advance, even if they are much slower than a reentering ICBM warhead.

Obviously, being fast, steerable, and in the stratosphere make hypersonics hard to see, hard to chase, and devilishly hard to hit. When one is incoming, there's little time to acquire it and just as little to lock on and engage. Worse, its ability to jink about makes it unpredictable, so calculating an intercept course is tricky.

This speed also translates into range. Even if a hypersonic missile can only fly for a thousand seconds, that's a thousand miles. This makes hypersonics very attractive because it makes them a perfect standoff weapon. Targets could be engaged from the homeland against spots on the other side of the globe in less than a couple of hours and older platforms, including subsonic bombers or surface ships, can remain on frontline service for decades to come with little modification.

In addition, the kinetic force of a hypersonic projectile can give it the punch of a small tactical nuclear weapon without an ounce of explosives aboard. Small wonder the US Air Force called its '80s project to drop tungsten arrows from space "rods from God." Zeus didn't have such thunderbolts.

This is of particular interest to the US, which is concerned with Anti-Access/Area Denial (A2AD). That is, the growing ability to keep American and allied armed forces at bay with increasingly sophisticated weapons and the need to penetrate their offensive/defensive screens. Hypersonics could be the key to this – especially in dealing with rogue states where the alternative is a nuclear strike or a full-scale military operation.

A similar thinking is behind much of the Russian and Chinese interest. Stealing a hypersonic march on the West would allow them to cancel out much of NATO and its allies' technological advantage, leaving whole regions open to a strategic shift in power.

Breaking down technical barriers

The other reason for renewed interest is that solutions to many of the technical challenges that have hounded hypersonic flight since the 1940s are within reach.

One of the biggest problems is that of altitude. Specifically, how to maintain controlled, sustained flight at lower altitudes below the edge of space. Many people envisage a hypersonic missile skimming the ocean waves like a Harpoon, but that isn't going to happen soon. The problems of dealing with such dense air are simply too massive. However, breakthroughs in Computational Fluid Dynamics, which uses complex computer models to figure out how air acts and flows around objects at hypersonic speeds, may help with this.

Another way around is simply to fly at extremely high altitudes, and the faster the higher. This solves many problems, but at velocities above Mach 8, hypersonic craft grow less efficient. Beyond Mach 20, they are in the orbital range and operating in the vacuum of space, so the higher speed once again becomes an asset.

The question is, how to get to those speeds. Despite many years of engine research, the short term solution is to use rockets to accelerate glider bodies. But this is very limiting, so proper engines are a nettle that sooner or later will need to be grasped firmly. This won't be easy, because in hypersonic vehicles the engine must be intimately integrated into the airframe to optimize aerodynamics on a mission specific basis, with very little margin of error in achieving precise control.

Solid or liquid rocket engines aren't too hard to deal with because they just need to burn for a matter of seconds to get up to speed, but air-breathing engines are another matter, and they're crucial if sustained hypersonic flight is to be achieved. They can, in theory, create a cruising hypersonic vehicle, but getting up to speed and staying there is tricky.

The general trend for a cruise hypersonic is to either rely on multiple engines, such as a solid fuel rocket for initial acceleration, then a scramjet for running; a combination of turbojets, ramjets, and scramjets; or a hybrid engine that can reconfigure itself in flight.

It's likely that these engines will increasingly depend on hydrogen. This is due to a number of factors. One of more basic is that fuels like aviation kerosene and methane are inadequate above Mach 5, while hydrogen has a very high power to weight ratio.

In addition, hydrogen absorbs more heat than other fuels and cryogenic hydrogen makes for an excellent coolant to precooling incoming air and keep the engine from melting. This is the case with the British company Reaction Engines Limited's Skylon spaceplane, which has gone one better by including a helium loop precooler for greater efficiency. Based on Lan Bond's work on the HOTL space launcher of the 1980s, Skylon is a single-stage-to-orbit spaceplane that's designed to take off from a purpose-built runway, accelerate to speeds of Mach 5.4 in the atmosphere, then boost to Mach 25 in space to carry 11 tonnes (24,000 lb) of payload into low-Earth orbit. Afterwards, it would re-enter the atmosphere and land like a conventional aircraft.

The general idea isn't new, but what is new and has garnered the attention of ESA, the British government and the US Air Force is Skylon's Synergetic Air-Breathing Rocket Engine (SABRE), which is an air-breathing jet and a rocket in the same engine.

Key to this is a revolutionary system that cools down the air as it enters the engine. It does this by using liquid hydrogen that acts as both fuel and coolant. At hypersonic speeds, the air is coming into the engine at 25 times more force than that of a Category 5 hurricane and the heat is like something blasting out of a cutting torch.

By passing the incoming air over a series of heat exchangers using cryogenic hydrogen fuel as a coolant, the air is cooled from 1,000° C (1,832° F) to minus 150° C (minus 302° F) in 1/100th of a second. Previously, this sort of heat exchanger was the size of a factory, but the SABRE uses one that's small and light enough to be installed inside the hybrid engine that burns air until is reaches an altitude of 28.5 km (17.7 mi), then switches to internal liquid oxygen tanks.

And it isn't just in engines that heat is a problem. For every part of the hypersonic vehicle, heat is the major consideration. When flying through the vacuum of space, this problem does not arise, but in the atmosphere, it's like swimming through molten glass.

Heat at high speeds is produced in three ways. First, there's the heat caused by surface friction between the aircraft and the air as it rushes past. At higher speeds, heat from compression becomes a factor as the air in front of the vehicle bunches up at the nose and on the flight surfaces as the vehicle approaches supersonic speed. At hypersonic speed, this compression becomes so great that the air becomes a glowing plasma and the vehicle heats up as if someone had shoved it into an electric kiln.

This plasma heating dominates at hypersonic speeds and it also affects the onboard electronics and even the structural integrity of the airframe. This heating becomes so intense that these thermal loads become mechanical loads. In other words, all that thermal expansion is like the vehicle's parts are being squeezed or stretched by mechanical stresses. Worse, at these temperatures, the molecules of the air dissociate and reform into much more corrosive chemicals that quickly eat away at the engine and hull.

Due to this heat problem, hypersonic vehicles are either restricted to very short flight times or must include active cooling systems in their designs to protect vital systems – especially electronics. The vehicle and engines also need to be made of titanium, superalloys, ceramics, or composites, or have exotic coatings, like nanoscale tiling to withstand the heat or avoid corrosion while maintaining the strength of room-temperature aviation aluminum at very high temperature, with a minimum of distortion.

On top of this, because hypersonic speeds allow for so little reaction time and because a loss of control for even a microsecond can lead to disaster, hypersonic vehicles require autonomous flight systems with very short decision times and very small margin of error. They need computers that can handle up to 50 million instructions per second with a reliability of up to one billion hours of operation per failure.

Are hypersonics unstoppable?

These are all challenges that hypersonic vehicles and weapons face, but there have been great advances in material science and engine design, as well as a better understanding of hypersonic aerodynamics, in recent years. We may not see truly practical hypersonic weapons until the 2030s or '40s, but when they do appear, will they be as truly unstoppable as some futurists claim?

Anyone who can give a definitive answer could get very rich in very short order, but we can still make some observations about the effectiveness of hypersonic weapons. Perhaps the most obvious one is that arms races are never one sided. They always come down to a competition between offense and defense. Build a sword and a shield will follow. Build a bomber and here comes an interceptor.

True, if deployed today, an effective hypersonic missile would sweep the field. Current radars, anti-missiles, and point defenses wouldn't stand the chance. But by the time the first missiles come online, they will undoubtedly be up against faster, more powerful radars with greater range; faster computers; new satellite monitors; jamming systems; lasers; cybernetic warfare measures; railguns; and autonomous platforms armed with hypersonic anti-missiles.

Also, don't assume that older weapons will simply been written off. If they can do the job, it's surprising how long a system can stick around. Warships still have deck guns, rifles still have bayonets, and the history of fighter aircraft has engineers removing machine guns from fighter planes every generation, only to put them back in again.

The same will no doubt be true in an age of hypersonics. Even if, for example, a hypersonic anti-ship hypersonic missile is fielded that can take out a supercarrier, it may mean that the job of the supercarrier will change. It may have to operate farther from the front line than now, be deployed only in scenarios where the carrier's side has established air supremacy, or it may become more of a peacetime weapon designed to handle low intensity conflicts, and threats to freedom of navigation.

What's next in hypersonics?

So what will the next 20 years see in the field of hypersonics? As far as weapons are concerned, despite reports coming out of Russia and China, reliable information indicates that the United States seems to be the furthest along the road toward building a hypersonic cruise missile. America and its international partners appear to be sorting out the problems of structural thermal stability and protecting electronics against heat, devising a cruise engine, and also have the computers and software to deal with the problems of control. Meanwhile, other powers might be able to field a simpler rocket-boosted glider missile, which may allow for faster deployment, but only of a short range and less sophisticated system.

Whoever is ahead, the challenges that face hypersonics need a lot of work across the board and advances in, for example, control systems won't do much good if the engines aren't up to scratch or if the heat protection is less than is should be. Hypersonics is one of those all-or-nothing branches of engineering. Ignore one problem, and it will negate all the other successes.

We've been concentrating on military hypersonics, but the technology has applications in many other fields besides weaponry. This is important to bear in mind because it's very often in the civilian field that the real breakthroughs are made. In 1897, Sir Charles Algernon Parsons made the Royal Navy look slightly ridiculous in the year of Queen Victoria's diamond jubilee when he buzzed through the Fleet review off Portsmouth in his private yacht Turbina at 34 knots (39 mph, 63 km/h), leaving the world's most powerful warships nowhere thanks to his newly developed marine steam turbine engine.

Hypersonic weapons could and probably will face similar surprises from other sectors. Like supersonics, hypersonics might find its success in the field of reconnaissance. The SR-71 Blackbird set the speed record for a manned air-breathing jet that still stands today and Lockheed Martin is working on a successor, the SR-72, that can go hypersonic.

Based on the company's HTV-2 hypersonic demonstrator that flew at Mach 20, the SR-72 uses an engine that is actually two engines in one. Each engine shares combined inlets and nozzles connected to two very different powerplants as a way to significantly reduce drag.

The upper engine is a turbojet that accelerates it to Mach 3. Then a lower dual-mode ramjet takes over and accelerates the plane to Mach 6. The significance of this design is that Lockheed collaborated for seven years with Aerojet Rocketdyne on how to use an off-the-shelf turbine that could be incorporated into a hypersonic jet system.

The idea is that the pilot-optional SR-72 will be a reconnaissance platform that, like its predecessor, will be able to scoot in, take high-resolution images and carry out electronics monitoring at long distance, then head for home before hostile forces can react.

Another field where hypersonics might steal a march is in satellite launches. Skylon is already being developed with this in mind and DARPA's XS-1 program aims to create a self-contained vertical launch vehicle the size of a business jet that uses cryogenic liquid oxygen and hydrogen propellants to power an updated version of the Space Shuttle's Aerojet Rocketdyne AR-22 engine.

When fully developed the XS-1 demonstrator, called the Phantom Express, would lift off vertically and go into a suborbital trajectory at hypersonic speeds. After it reaches the edge of space, it would launch a small, expendable second stage, which would deliver the payload to orbit. Meanwhile, the Phantom would bank and return to the spaceport, where it would land like a conventional aircraft. The ultimate goal would be to fly 10 missions in as many days while carrying a 3,000 lb payload to orbit.

Boarding call?

Then there's passenger service, which is far from an also ran. In many respects, it's much harder to make a regular service passenger aircraft and the success greater if you can pull it off. In the 1970s, a time when most fighter aircraft could only go supersonic for brief moments using their afterburners, the Anglo-French Concorde passenger plane couldn't just sprint supersonic, it cruised for hours at speed and did so on regular daily flights.

To this day, Concorde holds the record for supersonic flight, clocking up more hours flying above the speed of sound than all other aircraft combined. This, among many examples, shows how the demands for speed and reliability in passenger systems can produce as much progress as any list of Pentagon requirements.

In fact, major successes could come earlier in the non-weapons fields with, perhaps, recon planes in the 2020s, passenger craft in the 2030s, and weapon systems in the 2040s.

The sting in the tail

But whichever approach becomes practical first, there is a true sting in the tail. Much more than subsonic and supersonic, anything hypersonic is a potential weapon's platform. At the start of the First World War, airplanes were a joke and were relegated to little more an acting as spotters for the artillery. When supersonic flight came along, it had the obvious and immediate advantages of speed and altitude, but military applications were a specialized field. Few would see Concorde as a weapons platform, and it would be difficult to see how it could have been turned into one.

Hypersonics are different. All that speed and all that inertia turns any research platform, recon unit, or passenger aircraft into a potential kinetic weapon. They don't need high explosives to destroy a target. All they have to do is hit it. In other words, any hypersonic vehicle is an intrinsic weapon given the proper modifications.

So is hypersonic flight a promise or a threat? Like so much else in our modern age, the answer depends heavily on who is using it and why. A spade can dig holes or bludgeon someone to death. A railway train can carry holidaymakers or troops. A life-giving medicine, like nitroglycerin, can be a deadly weapon. In the end, the good or evil of a technology isn't in the tool, but in the heart of who is wielding it.

One thing is for certain. Hypersonic flight is no longer a pipe dream. Every returning spacecraft must traverse that barrier and with each passing year scientists and engineers come a step closer to taming the challenges that stand in the way of turning hypersonic flight from the study of spectacular test craft into a practical means of travel. Whether hypersonics becomes a way to travel between London Sydney between lunch and tea time, or a new weapon that changes the balance of power in the 21st century is up to us.