Hypersonic Flight & the Dominion of Heaven

A guide to the whims and deadly dangers of the fastest flight there is

Answerable only to God and thermodynamics, hypersonic flight is Very Difficult Indeed. Let’s throw caution to the wind, ramp the throttle up all the way, and find out a little more about the ultimate aeronautical challenge.

1: Hypersonics & me: What is it, and why do it?

The “why” is easy: Who wouldn’t want to fly faster than a speeding bullet? A lot faster! Hit Mach 5 and a flight from Los Angeles to Singapore, which is the Pacific ocean and then some, would be concluded in a little over five hours. You could hypothetically cross the Pacific, conduct business, take a few tourist pictures, and be back in LA the same day, having gone halfway across the planet twice. Nifty.

But what exactly is hypersonic flight, aside from just “really, really fast?”

We should probably start with definitions, and the eerie realm of the hypersonic is not one for exact definitions: “Sonic” gives us a clue, from the Latin word sonus for sound, so we know it has something to do with sound… except that is sort of doesn’t.

The first realm we reach as we accelerate is subsonic, which is easy enough. When air flowing over an aircraft is subsonic, the flow is moving well under the local speed of sound and the pressure waves from the plane propagate upstream into the air ahead, which smoothly parts to let the aircraft through, more or less. It also doesn’t significantly change in density as it flows over the plane, which is why this flow regime is called ‘incompressible flow’. It’s easy and familiar. We live in it.

The next realm is transonic, which is reached by most airliners: In this realm, the airflow is mostly lower than the local speed of sound, but there are regions, such as over the front of the upper wing, where local flow acceleration causes it to graze the sound barrier, locally and just for an instant.

In supersonic flow, -surprise!-, the aircraft is over Mach 1, which is the local speed of sound. As a result, pressure waves do not propagate upstream ahead of the aircraft and air cannot get out of the way. To make up for this, a shockwave forms ahead of objects in the airflow; a knife-thin region where air pressure, temperature and density shoots up as the flow sheds energy and changes direction. This is ‘compressible flow’, and it causes issues: As you accelerate into it, drag coefficients spike upwards and then settle back, the centre of lift moves backwards and there’s the danger -if your design isn’t on point- for errant shockwaves to hit control surfaces and cause a whole heap of trouble. The angle of this shock is pulled backwards with increasing speed, and for this and other reasons we see different designs from aircraft that have to operate in this realm; more swept-back and skinnier. Aerofoils are thinner, more wedge-shaped and have less of the characteristic teardrop profile we associate with them. Odd features appear on the aircraft to deal with shocks and decelerating air into the engines. More on that later.

But what about the hypersonic realm?

The exact point that defines entry into this is not quite clear, but it’s roughly between Mach 5 and 6, or five and six times the local speed of sound. At this stage it’s not just compression, but thermal effects that start to dominate, as the heating of the air produced by increasingly powerful shockwaves as well as viscous skin drag starts to do odd things: It changes variables like (but not limited to) kinematic viscosity, which starts to ramp up with increased heat, and this in turn, along with chemical changes in the composition of the air itself, makes aerodynamics and control a lot more challenging.

It also starts to push against the limits of the materials the aircraft is made of, as surfaces are exposed to the full fury of hypersonic flow but must keep on working, and this brings us onto our next topic.

Propulsion.

2: Hypersonics & me: The engine.

There is one simple way of reaching the hypersonic realm: You can strap a bloody great rocket engine to your aircraft and blast it there by brute force, but not only is this very expensive and inefficient on fuel, but I doubt you want to travel overseas on the tip of a ballistic missile, so let’s limit ourselves to more civilised air-breathing craft.

This is a turbojet, similar to the turbofan, the classic suck-squeeze-bang-blow machine that, -no giggling at the back!- propels most of our large aircraft. Air is decelerated by a tailored intake, and then passes through a compressor that ramps its pressure up many times at which, wonderfully potent now, it enters the combustion chambers, is dramatically heated and then expands again through the turbine, whose motive force then powers the compressors and a fan stage, if there is one. The higher the pressure you can reach, the greater the opportunity for energy reclamation through expansion and the more efficient your engine. So… just keep adding more power?

Sadly life is never that easy. As airspeed increases, the energy it carries goes up by the 2^nd power, and if you are to decelerate this air back down to subsonic speeds, which a turbofan or turbojet needs, you will heat it as that energy is dumped. This is a problem, as the hotter your intake air, the harder it is to compress it and the less efficient your engine. It’s an even bigger problem if you go really fast, as the mere act of compressing it in your turbocompressor will heat it close to the limits of the materials available, which places a ceiling on your compression ratio, and therefore the work that your engine can do. This limits turbojet engine aircraft to speeds slightly exceeding Mach 3.

Damn.

“But wait, Jordan!” I hear someone shout. “Aren’t turbine blades cooled by jets of air? Couldn’t you do that in the compressor?”

Well yes, but only if you have a convenient source of even higher pressure cool air available: This is the compressor, after all. It’s the already the highest pressure bit.

No, instead we must look for other means. See the graph shown below:

As speed goes up, the amount of compression the turbocompressor can do goes down, down down… but the amount done by the intake goes up, up, up. At some point it makes sense to ditch the turbomachinery entirely and just use the intake to do the compression: You’ve created a ramjet.

The ramjet is an elegant and simple solution to the supersonic propulsion problem, and a ramjet powered aircraft can get you to about Mach 5 or 6, which is probably as fast as a hypersonic airliner would ever need to go, so it shows promise. It does however have a significant disadvantage, which is that it’s basically useless below Mach 2-ish, so you need another engine to accelerate it, which is more than a bit wasteful. Hybrid engine solutions are possible and companies such as Hermeus are developing them, but the challenge and expense is high.

Other solutions are needed if you want to go even faster than Mach 6, where stagnation temperatures get so high that even a ramjet solution, which still needs to decelerate air to subsonic, starts to slag itself. This brings you into the bizarre territory of the Scramjet, or supersonic combustion ramjet, a powerplant which attempts to stabilize combustion in a supersonic flow.

This is possible, but it runs at odds with most combustor design, which is based on the principle of recirculating, swirling mixing flow where fuel & air can combine. This is easy in when subsonic, but in a supersonic flow, pressure and material only flows backwards, like a rugby ball, and the technical challenge in keeping such a beast running is very high indeed, as the flow consistently outpaces the flame. As if that wasn’t enough, fuel efficiency starts to suffer as well, as the high enthalpy of incoming air means that the comparative addition to it from combustion is less significant.

A scramjet also trades off some of the ability to throttle and control your engine, meaning that the aircraft may have to follow a constant dynamic pressure path as it accelerates and decelerates; in practice, drawing a giant parabola in the sky. Elegant maybe, and certainly impressive for passengers as they watch the blue of the sky deepen as they touch space and lighten as they descend again, but probably not the kind of thing air traffic control is keen on.

Finally there are the true oddball solutions such as strutjets, which hybridize rockets and scramjets but are undeveloped and not well-proven, and the SABRE engine concept of the recently-deceased Reaction Engines: A spectacularly complex combined cycle hybrid rocket engine that uses a helium precooler to supercool inlet air in front of a turbocompressor, allowing turbomachinery to run from 0 all the way to Mach 5 in one powerplant. Neat, but massively complex and not something you’d necessarily want to rely on in a world where debris can and does enter the front of engines.

So in the harsh, cold air of technical reality, our most realistic bet is probably a hybrid turbofan-ramjet. Mach 5 is plenty.

But wait, there’s more!

3: Hypersonics & me: Keeping it cool.

Your aircraft needs payload, and unless that payload is of the explosive variety it’s presumably a living, breathing human with nuisance requirements like oxygen, cool air and toilet facilities. Maybe a meal or a glass of something bubbly too, and can I get one of those moist blanket things? You know, the ones you rub on your face with the sort of perfumed smell? Thanks!

At the very least, you don’t want to cook people.

For spacecraft re-entering the atmosphere at Mach 25 or so, this is achieved by either ablation, where a heat barrier slowly burns up and abrades away, or by insulation; having a big enough mass of material between you and the world that the trip is over before it’s had a chance to burn your bottom. This is fine for a big spaceship that has to do this for a few minutes, but less ideal for a skinny airliner that needs to do the LA to Singapore route and serve a miniature steak and beverage of choice on the way.

On a normal airliner there is a similar problem, in that compressing the outside air needed for the occupants to breathe also heats it, and because passengers don’t do well in 80 or 90 degrees Celsius you then need to cool it back down using a ram air inlet that exchanges air with the frigid world outside. That’s a perfectly surmountable problem on, say, a Boeing 787.

Great, but this doesn’t work so well in a hypersonic plane, where poking a heat exchanger into the airflow means generating shocks and superheating it, making the problem even worse. -So you need to dump your heat somewhere and you can’t easily use the outside world.

What do we do?

A clue is presented by the SR71 ‘Blackbird’, the superfast cold war spyplane operated by the CIA & the US air force during the cold war. It’s JP7 fuel was designed to work well across a wide thermal range, so it could be delivered to the plane by a tanker in a chilled state and then used as a heat sink, beating the violence of kinetically-delivered skin heating by moving some of that heat around with the fuel itself.

This could be useful for a hypersonic airliner, but probably not with JP7: However if you were to make use of cryogenic fuels such as quick-burning hydrogen, then the ridiculously low temperature of stored liquid hydrogen, minus 253 Celsius (-423 Fahrenheit) would be a convenient way of keeping your passengers chilled out for the entire journey. This brings its own issues, -hydrogen is a nuisance to store, process and use-, but it’s a possible solution at least.

Another could be endothermic fuels, fuels that react endothermically (absorbing heat) at some processing stage within the aircraft. Dehydrogenation or cracking reactions, for example, used in refining, are highly endothermic, converting heavier hydrocarbons into faster-burning lighter ones, and maybe with the right chemical mix and catalyst this could have promise, but it’s a complex, failure-prone long shot and opens you up to the issue of what to do with coking byproducts and similar: Not an easy fix.

Solutions, it seems, abound.

But easy ones are in short supply.

4: Hypersonics & me: Materials.

The all-pervading menace that is shock, radiation & skin-friction heating defines this. Let’s get one thing out of the way early: On the really hot bits, like leading edges and engine intakes, Aluminium has to go, as does carbon-fibre reinforced polymers (CFRP), which is a pity because they’re so useful on current-generation airliners and combine strength, low weight and affordability. Instead, we’ll need to get more exotic…

Titanium alloys are flexible solutions, if expensive, and are reasonably light as well. Ti64 (Titanium-Aluminium) is a dominant alloy inside the front end of most jet engines for a reason, but in the hypersonic realm its limits start to get stretched, and while titanium will get you to the lower reaches of the hypersonic realm, it won’t push you far into it.

Nickels, common on the back-end of aerospace jet engines and throughout the hot turbine stages, have excellent heat resistant properties, but they are both expensive and heavy, so aren’t much of a solution in any sort of bulk. High temperature Inconel alloys are in a similar position, so where else do we look for the magic combination of low weight, high strength and excellent heat resistance?

The X43 hypersonic scramjet demonstrator, which clipped briefly through Mach 9.6, made use of a carbon-carbon weave composite on its leading edge. The X51 hypersonic cruiser, which broke Mach 5 for a rather longer time, made use of a tungsten nose tip, which is exceptionally dense, but a lot of aluminium and steel further back on the structure.

If we want to tick all the boxes, albeit at some expense, we can look further into ceramics: Conventional high temperature ceramics are brittle, but ceramic matrix composites (CMCs), using a ceramic weave of fibres embedded in a ceramic matrix with surface coatings to allow a level of slippage, are a possible route. They have already been trialled in the hot stages of aero-engines, even on low pressure turbine blades, and high temperature formulations are achievable. Still more exotic are ultra high-temperature CMCs (UHTCMCs), which attempt the same with more highly temperature-resistant ceramic materials. Research on these is in the early stages, quality variable and production costs excruciating, though it’s a plausible avenue of development if we want to spend time going Really Really Fast.

So again: A plethora of possible solutions, none of them easy or straightforward.

5: Hypersonics & Me: Why bother?

This is a fair question. Crossing the Pacific in five hours flat, or the Atlantic in two, might sound pretty good, but do we really need it? After all, we’re diurnal creatures that tend to set stock by individual days, and snipping down occasional travel by a few hours might sound like a minor benefit when bundled in a technological extravaganza that makes reusable space travel look easy.

That said, if you set your mind back a century or more you’d probably think the same about crossing the Atlantic in a Boeing 737: Why would you want to go so fast, anyway? Is your time really so valuable?

Maybe it is.

Our world is always speeding up, and what seems like an extravagance one day, like keyless entry, LIDAR controlled autopilot on your car or an electric powertrain, might be pretty commonplace the day after. Sooner than we think it’ll be the turn of driverless cars, opening up vast swathes of time we’re used to wasting that can be turned to something worthwhile. The clock ticks faster and the seconds are suddenly worth more. The same logic could one day dominate supersonic, then hypersonic travel, who knows.

“It’ll be too expensive!” Cry some hypothetical people I just made up. “A rich man’s toy, that’s all.” And like most things it’ll begin that way, but price is an elastic thing; it moves down with innovation, mass manufacture and competition. It cost 3 billion dollars to sequence human DNA for the first time, and that was a million in 2007. It’s six hundred dollars today.

Things change.

“It’ll be bad for the climate!” Cry more hypothetical people. “Can we just learn to live within our means?” -Which, as hypothetical arguments go, is a bit of a death-chant for a slow-dying future where nobody is allowed to do anything in case it angers Mother Gaia, but I digress. It may be that hypersonic flight is no more damaging to the climate than any other form of transport; indeed, if cryogenic hydrogen becomes the fuel of choice, it could be made squeaky-clean.

In the end it comes down to one thing: Do you think we should try it, just because we can?

I think we should. I think that it’s our duty, as the dangerous, curious apes we are, to give it a go, fly high, fly fast and see where it takes us. We’re apes yearning to fly, to spread wings of silver, puncture the nave of heaven…

And buzz an angel.