The Evolution of Hypersonics and Its Impact on the Future of Warfare, Part I

02/24 /2011 – In January 2011, Second Line of Defense sat down with Professor Lewis to discuss the current status and dynamics of hypersonics.  Mark J. Lewis is chairman of Clark School’s Department of Aerospace Engineering at the University of Maryland, College Park, and President of the American Institute of Aeronautics and Astronautics. He was the chief scientist of the U.S. Air Force from 2004 to 2008.

For the X-15 Test Flight (Credit: the X-51 Test Flight

SLD: Can you give us an update on how the hypersonics programs are going?

Professor Lewis: We’ve just come off a phenomenal year in hypersonics punctuated by three major flights and a host of other smaller, but in some ways, just as significant efforts.  The three major flights were the X-51, the HTV-2 and the X-37.  Each of those was important in its own way.

SLD: Before you review each of the programs might you clarify what is a flight test program and what do you have to do to achieve success?

Lewis: I’m delighted you asked that question because that was actually something I focused a lot of attention on when I was working for Air Force Secretary Mike Wynne, the whole question of what is “flight test?”

And I argue that flight test is experimentation; it means that you put a vehicle in the air to learn, to explore the frontiers of science and technology. When you do that it should always be with an eye towards how you apply that technology to a realistic, practical, operational system.

I think it is important to draw a distinction between “flight test” and “flight demonstration”.  There were lots of people I’d run into in the Pentagon and outside of the Beltway who wanted to do flight “demonstration” – in my definition, demonstration means I’m simply trying to prove something that I already know.  To me is a generally worthless thing to do.  If you already know it, you don’t have to prove it! If it works, no one really cares, you already knew the answer; and even worse, if you fail, you have just fallen flat on your face.

The contrast is what I think of as flight test, where I know that there are things that I know, and I know that there are things that I don’t know, or at least that I’m not sure about, so I build a vehicle that captures the best of my understanding and the best of my technology. The goal is to push the frontier.

I think the ultimate example of that was the X-15 rocket plane.  That program ran through the 1960’s.  It was a hypersonic aircraft, rocket-powered, dropped from a B-52.  There were 199 flights of the X-15, and every single one was designed to gather data, to learn more about the system, to learn about the flight regime, to understand how we build a realistic vehicle that flies faster than about five times the speed of sound.

X-15 (Credit:

X-51 (Credit:

The X-15 was never intended as an operational vehicle, but the physics that we learned, the engineering that we learned, the problems that we solved, were absolutely instrumental in a range of other programs, including manned space flight activities leading up to the space shuttle.

SLD: Could you explain what hypersonics is exactly?

Lewis: Hypersonics is generally considered to be flight in excess of about five times the speed of sound, or Mach five.  There’s no hard and fast definition by the way, so the definition is a bit fuzzy.  Unlike when flow goes from subsonic to supersonic and the physics actually changes very dramatically, as the flow goes from supersonic to hypersonic the changes are somewhat more subtle.

Interestingly, in the Russian language, there is no word for hypersonic; they just refer to high supersonic speeds. At hypersonic speeds, the surfaces of most vehicles under typical flight conditions become so hot that the chemistry of the air can start to become important.  That’s the reason we use the term hypersonic. Also, most of us know that a vehicle flying faster than the speed of sound generates a shock wave in front – that’s a sudden jump in pressure and temperature – but at hypersonic speeds, that shock wave is pressed very, very close to the surface of the vehicle, and that changes the aerodynamics considerably as compared to lower speeds.

SLD: Could you describe then the characteristics of hypersonic vehicles?

Lewis: I think about hypersonic vehicles inhabiting two general categories.

The first category includes vehicles that are meant to be decelerators; that is, they are designed to slow down.  For example, spacecraft coming back from orbit, including the Space Shuttle, an Apollo capsule, a spacecraft entering the atmosphere of Mars, all of those are designed to be slowing down on their way to a planet’s surface from space.   They’re all hypersonic though; when the Space Shuttle first enters the atmosphere on its way back home, it’s flying at about 24 times the speed of sound. That’s clearly hypersonic flight, but the goal is to slow down.

Then there are vehicles at the other end of the spectrum.  They’re the ones that are more difficult to design and build, and the ones for which we have much less experience. Those are vehicles that are designed to either cruise at constant speed or accelerate – speed up – as they go through the atmosphere.

Each of those vehicles types has its own set of challenges.  The biggest problem that we run into for all vehicles is that at hypersonic speeds, as I mentioned previously, heating becomes very important, and the sharper your vehicle’s leading edges are, the sharper the surfaces are, the hotter they get.

The general rule of thumb on a hypersonic vehicle is that, if we want to prevent the leading surfaces from melting, we make those surfaces big and thick and blunt.  You know that the heat shield on an Apollo spacecraft was a big, blunt, round object.  The leading edges on the Space Shuttle wings are also rather thick and blunt.  Having thick and blunt leading edges also means that there’s lots of drag, which really is not a difficulty for something like a Space Shuttle.  Remember, the space shuttle wants to slow down on its way back from space.

Launch of HTV-2 (Credit:

Launch of HTV-2 (Credit:



Now, if we want to build an accelerating vehicle, or one that’s just going to cruise, says a missile or an airplane, we need to have low drag. That means we have to build it with sharp leading edges, and those are going to get hot at hypersonic speeds.

So when we talk about hypersonics today, implicit in that definition is hypersonics applied to things that can accelerate, things that can spend a lot of time in the atmosphere, which in turn means slender, low drag shapes. That’s the technology frontier that we’re working on right now.

Think about what we could do with that type of vehicle, what I term a “low drag, high lift”, hypersonic vehicle.  There are three main categories for this sort of craft.

The first is the weapons category, including high-speed cruise missiles and maneuvering re-entry vehicles for long-range strike.

The second category is airplanes. This would include a high-speed reconnaissance airplane, perhaps a penetrating ISR platform, sometimes called the “SR-72”.  That craft might be designed to perform an SR-71 type mission, but do it at much higher speed to be less vulnerable.

And the third category is access to space, the category of hypersonic vehicles that might fly into space more like an airplane and less like a rocket. I call that the holy grail of hypersonics because if we can do that, if we can build a vehicle that works that way, we’ve suddenly opened up space to be very much more responsive and more accessible.  Imagine being able to fly into space with something that operates more like an airplane and less like a rocket. We wouldn’t have to spend something like the 4,000 man-months I takes today to prepare the Space Shuttle for launch.  We might instead be able to fly up to orbit on something that is maintained with the ease and accessibility of an airplane.

SLD: What is your sense of the realistic path to progress in the hypersonic area?

Lewis: In my mind, very clearly, the first step in developing hypersonic systems is the weapons application.  It’s the lowest hanging fruit.  It is, frankly, the least technologically challenging, and I think it’s also the biggest short-term payoff.  This includes high-speed weapons, high-speed cruise missiles, high-speed maneuvering re-entry systems that give us responsive long-range strike.  That’s where I see the bulk of our research investment being made today.

Next, the high speed reconnaissance airplane also has many attractive applications, most notably as a gap filler if we lose space assets or to give us ISR capability when space is not available. We can learn about building such an airplane from our experiences developing the weapons systems, since the physics will be similar.

I’ll also mention that there have been a lot of studies into the application of hypersonic systems, and really the most significant of those came in the year 2000, a study called “Why and Whither Hypersonics.”  It was done by the Air Force’s Scientific Advisory Board, and it was based on a question posed by Secretary Whit Peters, whom, I think, frankly, might have thought that the Air Force was spending too much money in hypersonics.  The study asked the very pointed question of whether hypersonics was a rat hole that money was being dumped down, from which nothing would ever emerge.

The Scientific Advisory Board did an extensive, exhaustive study, complete with a red team that questioned every result, and they came back with a very positive recommendation on what hypersonic technology could do for the Air Force.

Since that time, there have been a number of studies, including several National Academy reports, a number of other Scientific Advisory Board studies, and all have come back and said, “Look, hypersonics can be a real game changer.”  If we can fly somewhere at speeds of Mach 5, 6, 7, 8, or more, that is, if we can reach reasonably long  distances in very short periods of time, that has very important implications in modern warfare.

Modern warfare is about doing things quickly.  It’s about achieving fast effects, getting results quickly.  If you want to affect something quickly, I can think of basically three options.

The first option is that you have ubiquitous presence.  That means you’ve got an asset anywhere you need it.  That asset might be unmanned, and frankly, that’s a lot of what remotely piloted aircraft are enabling for us – having small assets available and re-locatable at a moment’s notice.  Of course, ubiquitous presence is only good in a limited area; we obviously can’t have ubiquitous presence at every location around the globe, but that’s one part of the solution that is already changing warfare.

The second option for doing things quickly is to operate at the speed of light.  For my aerodynamics friends, the speed of light is about a million times faster than the speed of sound. Operating at light speed means using directed energy systems and/or cyber systems, which are among the other things that Mr. Wynne championed when he was Secretary of the Air Force.  And of course, there’s a lot of development underway right now in directed energy systems, and lots of corresponding questions about how we ultimately would deploy them, as well as how we would ultimately use cyber systems.

If you don’t have the first two available, or if they cannot deliver the desired result, a third option is that you get to where you want to go as fast as you possibly can. That’s the advantage of hypersonics. This could be to perform reconnaissance of some sort, do some sensing, or to deliver weapons on a target.  In order to do that, we need to master the technology required to fly at hypersonic speeds.

Hypersonics would also give us a degree of invulnerability.  We know that the application of stealth technologies has been a tremendous game-changer, but that stealth advantage won’t last forever. I would argue that the next step beyond stealth is speed.

There are complicated trade-offs there.  Obviously, when we fly faster we’ll get hotter.  That makes the vehicle more observable, but the combination of both stealth and speed in some overall way is a very attractive combination for future systems.

If we look at those applications, we can ask, “what are the technical challenges,” and one I’ve already posed is that when we fly faster we tend to get hotter, so first it’s a challenge of materials.  It’s also a challenge in aerodynamic design; this is a realm of aerodynamics where there are still some very basic questions to which we don’t have complete answers.  In some cases these are questions that we can answer satisfactorily at lower speeds, but we can’t answer them at higher speeds.

But the biggest challenge, perhaps, is that of propulsion.  If we want to fly through the atmosphere at hypersonic speed, we will need a very special type of engine or a category of engines that will enable us to do that.  And so that’s properly where the bulk of our technology investments are being made.


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