Scramjet Engines on the Long Road to Mach 5

When Charles “Chuck” Yeager reached a speed of Mach 1.06 while flying the Bell X-1 Glamorous Glennis in 1947, he became the first man to fly faster than the speed of sound in controlled level flight. Specifying that he reached supersonic speed “in controlled level flight” might seem superfluous, but it’s actually a very important distinction. There had been several unconfirmed claims that aircraft had hit or even exceeded Mach 1 during the Second World War, but it had always been during a steep dive and generally resulted in the loss of the aircraft and its pilot. Yeager’s accomplishment wasn’t just going faster than sound, but doing it in a controlled and sustained flight that ended with a safe landing.

Chuck Yeager and his Bell X-1

In that way, the current status of hypersonic flight is not entirely unlike that of supersonic flight prior to 1947. We have missiles which travel at or above Mach 5, the start of the hypersonic regime, and spacecraft returning from orbit such as the Space Shuttle can attain speeds as high as Mach 25 while diving through the atmosphere. But neither example meets that same requirement of “controlled level flight” that Yeager achieved 72 years ago. Until a vehicle can accelerate up to Mach 5, sustain that speed for a useful period of time, and then land intact (with or without a human occupant), we can’t say that we’ve truly mastered hypersonic flight.

So why, nearly a century after we broke the sound barrier, are we still without practical hypersonic aircraft? One of the biggest issues historically has been the material the vehicle is made out of. The Lockheed SR-71 “Blackbird” struggled with the intense heat generated by flying at Mach 3, which ultimately required it to be constructed from an expensive and temperamental combination of titanium and polymer composites. A craft which flies at Mach 5 or beyond is subjected to even harsher conditions, and it has taken decades for material science to rise to the challenge.

With modern composites and the benefit of advanced computer simulations, we’re closing in on solving the physical aspects of surviving sustained hypersonic flight. With the recent announcement that Russia has put their Avangard hypersonic glider into production, small scale vehicles traveling at high Mach numbers for extended periods of time are now a reality. Saying it’s a solved problem isn’t quite accurate; the American hypersonic glider program has been plagued with issues related to the vehicle coming apart under the stress of Mach 20 flight, which heats the craft’s surface to temperatures in excess of 1,900 C (~3,500 F). But we’re getting closer, and it’s no longer the insurmountable problem it seemed a few decades ago.

Today, the biggest remaining challenge is propelling a hypersonic vehicle in level flight for a useful period of time. The most promising solution is the scramjet, an engine that relies on the speed of the vehicle itself to compress incoming air for combustion. They’re mechanically very simple, and the physics behind it have been known since about the time Yeager was climbing into the cockpit of the X-1. Unfortunately the road towards constructing, much less testing, a full scale hypersonic scramjet aircraft has been a long and hard one.

A Tight Squeeze

In a conventional turbojet engine, an axial compressor is used to increase the pressure and temperature of ambient air as it enters the engine. This hot compressed air is then combined with atomized fuel and ignited in the combustion chamber, which causes it to expand and get even hotter. These hot gasses exit through the engine’s exhaust nozzle as a high velocity jet, but not before passing through a turbine which generates the power to run the compressor. It takes a delicate balance to get a turbojet engine running, and the multitude of rotors and stators which make up the compressor and turbine stages must be constructed to exacting specifications and of the highest strength materials. Turbojets are also limited to a maximum speed of around Mach 3; any faster and the engine simply can’t keep up with the pressure of the air entering the inlet.

In comparison, a scramjet engine in its most basic form doesn’t require any moving parts at all. Air moving through the engine still goes through the same three stages of compression, combustion, and expansion; but the difference is that the air entering the engine is moving so fast that the geometry of the inlet is enough to compress it to the point it’s ready for the combustion stage. With no compressor to power, the engine doesn’t need a turbine stage either, so the expanding gasses are free to leave the nozzle immediately. Since the air doesn’t need to be slowed down while moving through a scramjet, such engines are theoretically capable of operating at speeds up to Mach 24.

Derivative of CC BY-SA 3.0 artwork by GreyTrafalgar

Like its supersonic counterpart the ramjet engine, scramjets are sometimes referred to as “flying stovepipes”, as they’re quite literally hollow tubes in which air and fuel are combined to produce thrust. It’s a design that’s so incredibly simple, at least in theory, that it almost seems too good to be true. So then why are we still struggling to develop a practical version?

Getting Up to Speed

The problem is that a scramjet engine doesn’t actually work until it’s physically moving at near hypersonic speeds. Any slower than Mach 4 or so, and the incoming air isn’t moving fast enough for it to become compressed inside the engine’s inlet. Accordingly, testing of scramjet engines thus far has been largely limited to mounting them to the front of conventional rockets in a one-time test that ends with the destruction of the engine. It’s a slow and expensive way to develop an engine, and has played a big part in holding practical scramjet development back.

NASA X-43

So while scramjet technology was being studied as early as the 1950’s, it wasn’t until 1991 that one was successfully tested by the Soviet Union. Even then, it was a fairly limited proof of concept. It would be over a decade later, in 2004, that NASA really made serious headway towards a practical scramjet-powered vehicle with the X-43.

This unmanned aircraft was mated to a modified version of a Pegasus rocket and launched from the bottom of a B-52 bomber, much like commercial air launched orbital vehicles. Upon separation from the booster rocket, the X-43 fired its own scramjet engine for ten seconds to accelerate up to Mach 9.6. The program was a complete success, and the X-43 still holds the record as the fastest aircraft ever flown.

State of the Art

Even though its been fifteen years since the X-43 made its last flight, the cutting edge of hypersonic scramjet development really hasn’t progressed much. Plans by the United States to build an aircraft that combined the low-speed performance of a turbojet with the Mach 3+ capabilities of ramjet and scramjet engines were canceled in 2008; meaning testing still relies on complicated and expensive air launch programs.

In the United States, the direct successor to the X-43 program is the Boeing X-51 Waverider. Development on the X-51 started in 2005, just a year after the X-43 made its record breaking Mach 9.6 flight. In fact, the X-51 uses an engine that was originally intended for a later variant of the X-43 that was canceled in favor of developing a newer vehicle.

Boeing X-51 Waverider mounted to the wing of a B-52

The X-51 first flew in 2010, but due to a number of subsequent failures it didn’t have a fully successful test until 2013. On that flight it was able to maintain a speed of Mach 5.1 until the engine’s fuel was depleted (approximately 210 seconds), after which the vehicle splashed down into the Pacific Ocean. It might not have flown faster than its predecessor, but the X-51 demonstrated it could fly for longer.

China is also reportedly working on several scramjet powered vehicles, potentially even a spacecraft which uses a hybrid rocket-scramjet propulsion system. Unfortunately there’s little public information about these programs, outside the handful of test flights that have been reported by Chinese media. Most recently Chinese media reported on the successful flight of the “Starry Sky-2”, generally believed to be analogous in design to the X-51, in August of 2018. Officials claim the vehicle attained a maximum speed of Mach 6, and flew under power for over 400 seconds. If these claims are accurate, it would have bested its American counterpart by a considerable margin.

The Future

Lockheed Martin concept art for the SR-72

For a hypersonic aircraft to be truly practical, it will need to be able to lift off under its own power and smoothly transition to its hypersonic engine while in the air. Lockheed Martin has proposed such a system, which they call the turbine-based combined cycle (TBCC), for their next-generation SR-72 reconnaissance aircraft. Comprised of a turbojet engine and ramjet which share a common inlet and exhaust nozzle, it’s an evolution of the concept used in the SR-71’s engines.

While it’s debatable if the SR-72 as envisioned will actually get built, Lockheed Martin has ready been pushing ahead with the TBCC engine technology as a stand-alone project. It’s even rumored that they have built and flown a small unmanned aircraft for flight testing. But even in the most optimistic of timelines, this research won’t produce a workable vehicle any earlier than the late 2020’s.

Excepting some military black project which the public doesn’t know about, a practical aircraft capable of reaching Mach 5+ under its own power by 2030 seems plausible. It took 44 years to go from the Wright Flyer to Glamorous Glennis, and it will be at least 80 years from that point until a practical hypersonic aircraft takes to the skies. Considering that we’re still tackling the finer points of practical supersonic aircraft and the relative complexity of the accomplishments, history will likely look back on this as a rational and necessary progression.

58 thoughts on “Scramjet Engines on the Long Road to Mach 5

    1. You may, the engine is actually called SABRE, and the variant for airplanes rather than orbital rockets is called Scimitar.

      But it won’t come to anything. The whole concept is being carefully ignored stateside because it wasn’t invented in the US, so it “doesn’t exist”. In other words, if you don’t have the IP, there’s no point in having it because it’s not yours to sell. The ESA on their part is run by the French and they won’t have the Brits take any glory, so the EU publi funding for developing it is practically nil. It lays solely in the hands of the UK to do anything with it, but they got burned by the Concorde and are twice shy to make another one.

      It exists on paper, and in individual proof of concept parts, but nobody’s put a whole engine together to see if it does anything. The Chinese hypersonic hybrid engine however is likely to be a copy of the SABRE/Scimitar.

      1. It looks like a hybrid Turbine/Ram by a guy I knew who was a US air force pilot in grad school who became an air force researcher – 1981 or so. The last I saw of him was in an interview in which he was denying that the “contrails with donuts” were from some fictional program called Aurora. But they did look like what he expected from his engine back in those many long discussions about the physics.

        1. It’s a mixture of everything. It’s fundamentally either a rocket motor OR a turbojet with a bypass ramjet. The main point is the cooling system that is designed to operate at gigawatt levels, so it can cool the intake air to -150 C in 100 milliseconds and that makes the compressor stage unusually effective.

          The point is that the SABRE/Scimitar achieves a thrust to weight ratio of 14 while a scramjet does about 2. It’s very light compared to any other hypersonic engine, and it operates all the way from ground level to the vacuum of space.

          The Indians have similiar plans, with the modification that they’re trying to develop a heat exchanger that can liquefy the air in the intake, so while the engine is burning liquid H2, it’s also collecting the excess LOX into a tank along the way, so you can take off with empty oxidizer tanks. The advantages are obvious.

  1. “Until a vehicle can accelerate up to Mach 5, sustain that speed for a useful period of time, and then land intact (with or without a human occupant), we can’t say that we’ve truly mastered hypersonic flight.”

    Anyone else remember the X-15? It met those requirements and then some, having reached Mach 7 at times.

    1. Per Wikipedia, “Jules Bergman titled his book on the program Ninety Seconds to Space to describe the total powered flight time of the aircraft.”

      It’s hard to argue that 90 seconds qualifies as “a useful period of time”.

      The X-15 also operated across the borderline between atmospheric flying and sub-orbital space flight.

      1. I don’t see a lot of difference between the X-15 and the STS other than flight path, the later is much faster, can carry more personnel and payload, and takes off without a secondary support vehicle (ignoring the world’s largest RATO assist). And the shuttle was mentioned early in the discussion.

          1. Rockets aren’t the same tech as a scram jet. Albeit I’m thinking they condense more energy in a smaller package and this is another… huh… wonder what industry funds this and what is the real need that can’t be done in a more cost effective way.

            Now in regards to the B-52 and other launched systems… Amazing considering the never before done feat ever in the history of intellectual thinking and applications of all time (which is rather modern in the evolutionary/creation (I mean… dude… words were created… then written) scale)… of having a system capability and performance with a still ongoing life cycle. Crazy… when nations could have a dual in more rational reasonable ways to perform mass lethal force prosecutions and mass casualties with fair warning to upgrade society to be skewed by the next cycle of deviant malicious intent filth… to be cleaned again hopefully for a greener more times of peace World future. High tech pan troglodyte with push the bonobos to the front of the mob ways and means if you ask me.

      2. Not really, you’ll never have a practical rocket powered plane for a number of reasons. Burning through its propellant in a minute and a half is a good one, but the 100 foot tongue of flame behind it probably won’t be too popular at the airport either.

        To be fair the X-1 and X-15 had basically the same downsides, but at least the X-1 project rapidly (relatively) opened the doors to “regular” supersonic flight with the data it collected. The X-15 flew ~20 years after the X-1, but we still haven’t seen the technology become practical, for military or otherwise.

    1. Nothing like some good old hyperbolic writing.
      My latest favorite example was local Nitwitness News & Weather, repeatedly, reporting a smidgen over 4 inches of snow as “nearly half a foot”.

    1. I think it’s a matter of different tools for different uses. Satellites are useful, but can’t provide up-to-the-second info like aircraft/drones can. ICBM’s are useful, but can’t be recalled like bombers can. Signal intelligence is useful, but human intelligence (i.e., spies) can provide info that sig-int alone can’t.

    2. Who told you that?
      Aside from the aerial combat implications, scramjets are a decent way to get into space, even if you still need a booster to get fully into LEO or an appropriate transfer orbit.

    3. I was kind of wondering the same.

      It’s going to be kind of hard to use for spying when you are moving so quickly you pass over the target before you have a chance to collect any intel.

      Bombs can be delivered by rocket with no pesky pilot to have to devote resources to keeping alive.

      There’s no way it will scale up to hold enough passengers to be a commercial flight option.

      Would it be good for billionaires’ private jets? Maybe but they would only use it on the longer trips b/c otherwise they would arrive before it is even up to speed. How big is that market though anyway?

      1. Concorde died through lack of customers, so an even more cutting-edge, more expensive, even faster passenger plane isn’t going to get off the ground. Spying, though, it’s not how fast you do the actual spying, it’s being able to get away in a hurry if you need to.

        Bombs, you’re right. For sheer tonnage it’s cheaper to use aircraft, but since computers got small enough and smart enough to do the navigating for missiles, there’s no need to have a human doing the job.

        1. Concorde died because re-engineering it after the Paris crash would have cost more than the small profits that British Airways were making. As far as anyone knows Air France never made a profit, but BA were (and not lacking for passengers either).
          Being banned from most airports and routes because of noise concerns about sonic booms didn’t help either.

        2. Having lived on its flight path, Concorde was appallingly loud even in subsonic flight. It died because it was a massive inconvenience for 100ks of people just so a few 100 could get to New York and back in a day instead of Skyping.

    4. It’s difficult to use a satellite for traveling. Of course with today’s time wasting airport security it will not make much difference if you fly Mach 2 or Mach 5 afterwards. :-(

  2. Jet engines do not increase the pressure inside of the combustion chamber. The pressure gets lower and lower on every stage of compression. This is simply because fluids like air won’t flow from an area of low pressure to one of high pressure.

    1. Not true at all. Each stage of an axial compressor increases the compression by about 1.2 or so. A centrifugal compressor ratio is about 8:1 per stage. Compression is necessary to improve the overall efficiency of the combustion process.
      Your statement about fluid flow would be true for a simple tube, but compressors add energy to the air in the form of pressure, temperature and kinetic energy.

      1. THANK YOU JN… I was about to lose it after compressionless-compressors and a couple, ~”that’s right, and furthermore…” And a piston engine compresses as much as it can manage to suck in… ignoring turbos, etc. Really, I want to send a few of you back to 3rd grade to re-read your Encyclopdia Britænica. In fact, I think a New Standard would do the trick. A new type of compressor, that doesn’t waste time by actually compressing… *I* need to decomoress, reading some of that.

        1. Without the combustion happening, the average pressure of a naturally aspirated car engine is atmospheric pressure. It doesn’t compress air, it merely pumps it around and the aim of the engine designers is to minimize the pressure drop at the intake as well as the back-pressure at the exhaust. The ideal engine would run with zero pressure difference between the intake and the exhaust manifolds.

          With the throttle closed at idle, even the compression cycle may be merely returning the air inside the cylinder back to atmospheric pressure, because the vacuum against the closed throttle means there’s little air getting inside the cylinder.

          But, as stated above, this is just pedantry. Of course a compressor compresses – the pedantry is about confusing relative compression with absolute pressure.

        2. I’m not a big fan of internet hearsay; so, here’s a more in-depth description of engine compressors for the curious (Pages 1-39 to 1-41):
          https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/FAA-H-8083-32-AMT-Powerplant-Vol-1.pdf

          Unfortunately, I think there is much confusion here because people cannot clearly indicate where they would measure pressure differentials and what useful information that would yield. For example, if you look up the term engine pressure ratio, it would tell you where the pressure taps are located and that it indicates the engine’s thrust.

    2. In a supersonic flow condition, air can flow from lower to higher pressure because the pressure wave cannot propagate upstream. The air is moving less like a fluid, and more like a hail of shotgun shot. There’s not enough time for the “pellets” to bounce back in the direction of travel, so rather than sending a pressure wave forwards, it compresses into a shock front and the air upstream hasn’t got any idea what’s going on until it hits the shockwave.

      Scramjet engines utilize this very principle by setting up a sort of traffic jam inside the tube, which, given subsonic conditions would propagate back upstream and block the intake and push most of the oncoming air around the engine, but under supersonic conditions the air simply gets compressed and expands again out the other end. Add fuel, ignite, and you got yourself a jet.

      This is why scramjets only work when they’re already well supersonic, and why they don’t make practical engines for airplanes or space planes. You need a second set of engines (or rockets) to first get up to mach 1+ and once you’re there, the scramjets only provide a thrust-to-weight ratio of about 2, which means your entire plane is just engines and fuel tanks with some wings lashed on. There’s no meaningful capacity left over for payload.

      1. This is also one way to explain why a rocket nozzle works: the de Laval nozzle expands in a way that the flow becomes supersonic, and in a supersonic flow the pressure again cannot travel upstream. Seen from the combustion chamber side, anything after the constriction nozzle looks like a hard vacuum because there’s no pressure coming back up from there and the difference in pressure between the bottom and the top of the rocket engine is greater. The reaction force is thereby increased.

        This is counter-intuitive because the first instinct is to think that the rocket is pushing against something – against the atmosphere – but this is not the case. The lower the pressure at the exhaust, the greater the thrust. This becomes apparent with another thought experiment: if you plug the engine exhaust entirely, the back pressure is maximized, but then there’s no difference in pressure between the top and the bottom of the combustion chamber, so there’s no net force in any direction – the rocket doesn’t go.

  3. “Excepting some military black project which the public doesn’t know about, a practical aircraft capable of reaching Mach 5+ under its own power by 2030 seems plausible.”

    Missing a negative?

  4. A fairly important distinction has been omitted here. I think it’s important to define and clarify scramjet and its relation to a ramjet. This is all discussed in great detail on pages on wikipedia and the NASA website.

    But in short: a “ramjet” uses an inlet to both compress the incoming air and slow it to subsonic speed before combustion.

    A “scramjet” is called that because the “sc” stands for “supersonic combustion”. The incoming air isn’t slowed to subsonic speed, and combustion takes place in supersonic air. That last item is no mean feat. And lots of other complications come from trying to go that fast–just like all the problems faced (and solved) by the people that made the SR-71 go Mach 3+.

    Ramjet technology is pretty mature and robust. Scramjets are problematic because of all the needs to control supersonic combustion and build and engine and airframe that can tolerate the conditions they operate in.

  5. Scramjets and hypersonic flight are being pursued by many governments because of missile technology. Hypersonic missiles could plausibly attack and disable an aircraft carrier, which are currently the state-of-the-art in projecting military power.

    1. Alarming, but not alarming enough. Try this: With aircraft which can rapidly deploy from even the most distant airfields on Earth to the theater, aircraft carriers, and whole naval force, will not be disabled, but obsoleted!

  6. Considering how SR-71 “didn’t exist” for years of its service, I suspect the “slow progress” of this research is basically a lie, and we already have those airplanes flying over us since quite a while. Call me a conspiration theorist.

  7. For many years I was following a program from, I think, Aerojet/Rocketdyne, called an aeroturboramjet. Then it disspeared.
    I’ve always wondered if it didn’t work and got shelved or if it “went black”.

  8. I’ve been watching this and hypersonic ICBM R&D for years. Ramjet stuff is super classified and what little is public is very behind.. The US is way behind Russia on hypersonic and SAM tracking, so probably this too..

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