Piercing Through the Hypersonic Barrier

Figure 1: The X-51A Waverider hypersonic vehicle, powered by a Pratt & Whitney Rocketdyne SJY61 scramjet engine, is designed to fly at Mach 6. (U.S. Air Force graphic)

2152 words / 11 minutes read

Introduction

The eternal quest for speed by mankind has led to technological developments from wheeled bullock carts to engine-powered automobiles to rocket-driven space shuttles. Each one has been a major invention, propelling mankind to greater heights of growth and development. Today, we are at the frontier of yet another technological breakthrough in the form of hypersonic flights. At hypersonic speed, any place on earth will just be a couple of hours travel. It can be as good as, one having his afternoon lunch in New York, enjoy a hypersonic flight to Moscow, watch the world cup finals live, and be back home to have dinner with family!!! This is not a fantasy, but a potential possibility in the coming years.

Not only can hypersonics change the global travel time but could easily jeopardize the existing balance of power among the nuclear-armed nations. It is this underlying potential threat that has triggered a new arms race among the major superpowers of the world. If the great space race of the post-world war era helped develop technologies to build satellites and launch mankind to space, a new kind of global race in hypersonics is defining technologies to travel many times faster than the speed of sound.

But what is hypersonics, why is it so sought out for? what is the technology behind it? These and many more questions on hypersonics are what this article tries to address in the coming sections.

Video 1: Speed comparison between DARPA’s HTV-2 hypersonic vehicle, a C-5 transport aircraft, and an F-18 fighter aircraft.

What is hypersonic flight?

Coined by the Caltech aerodynamicist Hsue-Shen Tsien way back in 1946, the word ‘hypersonic’ refers to speeds in excess of five times the speed of sound. In aerodynamic jargon, it means ‘speeds above Mach 5’. This flight regime is not ‘just a bit speedier’ than supersonics, but is altogether a different world with its unique set of challenges and complexities.

To get a better perspective of the different speed regimes, consider this comparison: time to travel a distance of 5800km from New York to Paris on a subsonic passenger aircraft will take about 5 hours 45 minutes, on a supersonic aircraft like Concorde, it would be around 3 hours 30 minutes and on a hypersonic plane it could just be under 1 hour 45 minutes !!!

Apart from speed differences, there is more to these flight regimes than that meets the eye. When compared to subsonic or supersonic flights, the flow physics associated with hypersonics are vastly different and the technical challenges increase exponentially with increase in speed. Though Space Shuttles, reentry capsules, ballistic missiles have reached top speed in excess of Mach 5, no regular commercial flights have been possible to date. The delay is due to the humongous technical challenges arising from the various factors affecting high-speed flight and aerodynamics.

Even before we wade into the technical challenges of hypersonic flights, let’s begin our understanding of the field, by seeing possible ways of reaching such high speeds flights.

An X-51A WaveRider hypersonic flight test vehicle is uploaded to an Air Force Flight Test Center B-52 at Edwards Air Force Base on July 17, 2009
Figure 2: An X-51A WaveRider hypersonic flight test vehicle is uploaded to an Air Force Flight Test Center B-52 at Edwards Air Force Base on July 17, 2009. (U.S. Air Force photo/Chad Bellay)

How to fly at hypersonic speeds?

Well, there are possibly four ways to do that. The first approach can be by piggy-backing on a ballistic missile. Using rockets the vehicle is launched into outer space and it descends back to the ground more or less in an uncontrolled way. This is what typically happens with reentry capsules, where it is more a matter of surviving the atmospheric impact with a lit bit of essential control while landing than a well maneuvered controlled flight.

Another way to fly hypersonic is through what is called as ‘boost-glide’ approach. In this mode, the vehicle zips through the air using rockets, reaches the rim of the atmosphere using solid or liquid-fuel propellant, and then glides back to the earth’s surface. Unfortunately, gliding has a limited range, as, without a power source the drag catches up and brings the vehicle down thereby limiting its flight range. The good part is, this provides a larger level of maneuverability, something which was not possible in the ballistic flight approach.

The third possible way is by what is termed ‘the cruise’ approach. The vehicle has an onboard air-breathing engine, which provides the required power for sustained flight and maneuverability. These engines are similar to turbojets seen in commercial aircraft with additional features like coolers, variable air inlets, and afterburners. The front part of the engine brings down the intake air speed from supersonic to subsonic to make combustion possible. However, a reduction in incoming airspeed results in shoot-up of pressure and temperature. Fortunately, the variation in flow parameters is all within manageable limits till the vehicle speed reaches Mach 3. Beyond that, the turbine blades can no longer take the load and will start to melt out. For this reason, turbojets can only be used as an intermediary power source in hypersonic flights.

Video 2: Boeing’s X-51 WaveRider Test demonstration live video and animation.

For flights in the range of Mach 3 to 6, ramjets are a viable alternative. Unlike turbojets, ramjets do not have any moving parts like compressors, but air compression happens by shock waves generated by a carefully designed engine intake. Air compressed by the forward motion of the aircraft is brought down from supersonic to subsonic speed inside the intake passage, before entering the combustion chamber. The engines perform efficiently till about Mach 6. But beyond that, the heat generated at the intake is nearly equal to the heat generated by combustion in the rear and as a consequence, it’s hardly possible to extract any useful thrust energy.

So, to generate a meaningful amount of thrust, the incoming air should get compressed, but not as much as it happens in ramjets. For this reason for flights beyond Mach 6, scramjets – supersonic combustion ramjets are a better choice. They are similar to ramjets except that, the air is slowed down a bit, but still supersonic in nature. With this arrangement, usable thrust can be generated along with keeping the increase in temperature and pressure within manageable limits.

Hypersonic vehicle engine: Schematic diagram of Scramjet engine
Figure 3: Schematic diagram of Scramjet engine.

However, unfortunately, both ramjet and scramjets cannot operate at low altitudes and at lower speeds, as they need high-speed air as input to kick-start the engines. In theory, building a cruising hypersonic vehicle is possible, but getting up to that speed, staying there, and achieving balanced combustion is the tricky part. As experts in the field often say, combustion in scramjets engines is equivalent to keeping a match lit in a category 5 hurricane !!!

For flights in excess of Mach 10, the vehicle’s flight path takes them virtually into vacuum space, beyond the rim of earth’s atmosphere, thereby making the air-breathing engines completely redundant. As a solution, for this reason for flights beyond Mach 6, hybrid engines — a combination of rocket-powered engines with air-breathing engines are proposed. At lower altitudes, the air-breathing engines become the vehicle’s powerhouse, and at altitudes beyond the atmosphere, rocket-powered engines with their own supply of oxygen propel the vehicle forward. This will ensure sustained energy supply for the entire flight, right from ground zero. However, having multiple power sources will make the plane heavy, expensive, and also less efficient. Nevertheless, they could possibly fulfill the dream objective of one vehicle for flights from ground zero to outer space and back.

Hypersonic weapon: Lockheed Martin High-Speed Strike Weapon (HSSW), a new air-launched weapon system
Figure 4: Lockheed Martin High-Speed Strike Weapon (HSSW), a new air-launched weapon system.

The potential benefits of mastering this flight regime are immense, and hence the sustained decades’ long efforts.

Why the renewed interest and concerns about hypersonics?

Research activity in the hypersonic field has been there since the end of the second world war. Breakthroughs in ballistic missile technology, lunar missions, reentry vehicles, and Space Shuttles have been possible only through extensive research in hypersonics. However, after the end of the cold war, the interest in the field had subsided. Though attempts to build hypersonic vehicles continued at various research centers around the world, it was not persuaded aggressively, as a national program. But surprisingly, in the last few years, there has been a burst of interest and if not outright concern about hypersonic weapons and vehicles. Unofficially, a new arms race among the US, Russia, and China has begun and other countries like the UK, Germany, Australia, India are not far behind.

There are multiple reasons for this renewed excitement. Militarily, hypersonic weapons have the potential to bring in a strategic shift in power. Like nuclear weapons, countries wielding hypersonic weapons in their arsenal can start throwing their military weight in conflict situations. Speedy weapons like these can act as a game-changer in conflict zones by hitting the targets undetected within a couple of hours from the safety of the homeland, located thousands of kilometers away from the conflict zone.

More importantly what is creating worry and fear among the military establishments is the fact that any hypersonic weapon or plane can easily penetrate and overcome the most advanced anti-missile, radars, point-defense system with ease. More than the speed, it is their maneuvering abilities that make them deadly.

Hypersonic weapons are fast but not as fast as ballistic missiles. ICBMs like Minuteman III on top gear flies at Mach 20. But, ballistic missiles cannot be steered, at least not to the level one wishes for. They follow a pretty simple trajectory – first go up, reach outer space and descend down to the target, all in the open sky, offering themselves to be easily picked by satellites and radars. At least theoretically, any effective defense shield should intercept even the fastest of the ICBMs.

DARPA's hypersonic glider, designed to fly at Mach 20
Figure 5: DARPA’s hypersonic glider, designed to fly at Mach 20.

On the other hand, hypersonics weapons flying at around Mach 5 even though slower than ICBMs are highly maneuverable and this is a big asset. Speed, steerability, stratospheric flight path, gives hypersonics the stealth edge, making them hard to detect and extremely difficult to chase and shoot down. On top of that, the ability to do evasive maneuvers makes them highly unpredictable and any intercepting attempts will end up being futile.

Further, more than subsonic or supersonic, hypersonics is a potential weapon even without an ounce of explosives on it. The kinetic energy of the hypersonics itself makes them as deadly as a small tactical nuclear device.

Apart from its capabilities as a lethal weapon, hypersonics has the potential to revolutionize the transportation system. With a speed of one mile per second, they will make any corner in the world reachable within a couple of hours.

Part of the buzz around hypersonics in recent times is also due to crazy futuristic visions of space tourism, colonization of Mars and beyond set by billionaires like Elon Musk and Jeff Bezos. Their rapid strides in space technologies including the building of powerful rockets, reusable space vehicles, etc have caught people’s imagination and have brought back the space exploration excitement of the ’60s and ’70s.

More importantly, engineers, scientists, military establishments now strongly believe that with the current advancements in science and technology, hypersonics is well within reach. More than 10 countries are into active hypersonics research. The number of research papers being published is increasing with each passing year. It is reported that in AIAA International Space Planes and Hypersonics Conference alone more than 300 papers got published last year with China leading the way with 260 papers.

A lot of research work has been done and a lot more remains to be made. Even with the current surge in efforts to master hypersonics by various research institutes around the world, it will take at least another couple of decades to fly the first commercial hypersonic transport plane. A large number of technical challenges which have been tormenting hypersonic flights since the end of world-war-2 have been overcome, but many still remain. The challenges lie in every aspect of hypersonic flight — aerodynamics, airframe structures, control and guidance, propulsion, you name it.  Kevin Bowcutt, Senior Technical Fellow and Chief Scientist of Hypersonics at Boeing Research & Technology was quoted saying  — hypersonics is much harder than rocket science.

Concluding remarks

Hypersonics is one field that demands a high-performance quotient in all relevant disciplines of engineering. Ignore one and the whole hypersonic project ends up disastrous. The vehicle design is not component by component but multi-disciplinary in nature. The aerodynamic design aspect affects the engine performance, which in turn affects the performance of the thermal protection system and controls. So, the design needs to be more holistic, considering all the interdependence and interference effects of various components in the vehicle.

Every breakthrough in technology is taking us a step closer to overcoming the hypersonic barrier. With the present renewed interest, enthusiasm, and determination, it is not far, when hypersonic flights will become the new norm of air travel.

Further Reading

  1. Spiked Blunt Bodies for Hypersonic Flights
  2. Know your mesh for Reentry Capsules
  3. Supersonic Parachutes for Reentry Vehicles
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