Thursday, August 16, 2012

Third X-51A Scramjet Test Not Successful

Update 5-4-13:

X-51A test 4 (the final one) was successful.   4 minutes burn followed by 2 minutes coasting-down flight.  Hydrocarbon-fueled scramjet burn at Mach 5.1.  Compare that to ASALM-PTV flight test 1 of 7:  accidental acceleration to Mach 6 on hydrocarbon fuel at only around 20,000 feet.  ASALM-PTV was an ordinary ramjet,  not a scramjet.  This was done back in 1980.
Original article:
Various news stories have given the results of three X-51A Waverider tests so far, of 4 vehicles built. The third ended in failure quite recently. So did the second last year (2011). The year before (2010), the first test was a 140-second long success, accelerating through Mach 5 after rocket boost.

The vehicle is a “waverider”, meaning it rides on the pressure of its own bow shock acting on its belly for lift. It is rocket boosted to about Mach 4.5, then “takes over” with a JP-7-fueled supersonic-combustion ramjet (scramjet) engine. It is designed to test scramjet technology in the Mach 4.5 to 6.5 range, according to Boeing, who built it as a joint venture with Pratt and Whitney Rocketdyne for USAF.

Earlier Scramjet Flights

The previous scramjet test effort reported in the news was NASA’s X-43A, of which 3 were built. That vehicle was hydrogen-fueled.

The first test in 2001 ended in failure. The second test in 2004 demonstrated thrust enough to climb slightly at a little under Mach 6.8, for about 10 or 11 seconds. The third and final test , also in 2004, demonstrated scramjet acceleration at about Mach 9.8, also for about 10 or 11 seconds.

It was that Mach 6.8 test in 2004 that broke the previous speed record of Mach 6 held by an aircraft powered by airbreathing propulsion. The third test’s speed record of Mach 9.8 still stands.

Scramjet Technology

Scramjet is a very difficult technology to “tame”. It has been undergoing serious ground tests of various kinds since the early 1960’s. Very careful attention must be paid to balancing the engine and inlet with appropriate ducting. The scramjet takeover speed is necessarily very high: at least Mach 3.5 or 4, which requires an enormous rocket boost. Another problem is the extreme friction heating with external and internal airflow at those speeds. Yet another is the enormous difficulty of injecting and mixing the fuel into the supersonic inlet airstream, without inducing huge shock wave and flow-separation losses.

One of the selling points made for investigating this kind of propulsion is very high speed missile applications, such as “the ability to carry out a military strike anywhere in the world in less than 60 minutes”. Another might be propulsion for a space plane. But, as the two series of flight tests so clearly demonstrate, this technology is still very far from being ready for application.

Some History

Back about 1980, a technology-demonstrator flight test vehicle called ASALM-PTV accidentally accelerated to Mach 6 at 20,000 feet on its very first flight test. This was due to a fuel control “failure” that was nothing but a stupid assembly error, there was nothing really wrong with the design. It was only designed to cruise at Mach 4, and to power-dive at Mach 5. The other 6 flights were perfect. Its design mission was as a cruise missile effectively invulnerable to defense: subsonic launch, supersonic climb to 80,000 feet (24 km), pullover and accelerate to a Mach 4 cruise, then suddenly dive at Mach 5 onto target.

ASALM was not scramjet at all, it was just an ordinary subsonic combustion ramjet, with part of its technological roots dating all the way back to World War 2. It had a supersonic inlet, a large but mild-expansion nozzle, and a dump combustor for its flame stabilization. It was fueled with RJ-5, a synthetic strongly resembling kerosene.

Unlike the two scramjets, ASALM had an “integral booster” packaged entirely within its engine, not a huge booster stage out behind, to be dropped off. The takeover Mach number with ASALM was Mach 2.5, so the booster could be much smaller in any event.

It was not a waverider, but it did fly on supersonic body lift without any wings. ASALM had a very clean, low-drag "dart" shape, which is a part of how it accidentally reached Mach 6 in that runaway test flight.

I got to work on several related technology projects associated with ASALM, and to participate in the engineering done around the booster inside that combustor. A lot of the same technology went into other ramjet engines I worked on.

The NASA X-43A guys named ASALM as the setter of the record they finally broke in 2004, but they didn't know what ASALM was, or what kind of engine it had. I guess they were just too young: ASALM was well before their time.


USAF's design mission for that scramjet missile technology could be done easier, cheaper, and "right now" by marrying existing ICBM technology with existing ramjet cruise missile technology (like ASALM). Put your supersonic ramjet cruise missile inside a re-entry shroud, and stick that on top of an ordinary ICBM. Flight time is 17 minutes to the other side of the world, then you cruise to target in the Mach 3 to 4 range, at around 60-80,000 feet, and finally you dive onto your target at around Mach 5 to 6. If you are attacking fixed geographic coordinates, there is not time for simple inertial guidance to drift.

As I said before, simple. Easy. Cheap.

My question to USAF is: why cruise hypersonic down in the air with all that friction heat and shock loss nonsense, when you don't have to? Actually, the very same question applies to supersonic/hypersonic transport aircraft proposals, too.


Update 9-11-13:  The 4th and final X-51 flight was successful. 


  1. Thanks for the info on ASALM-PTV; that's interesting.

    I think you are mixing two different use cases. There is a useful capability to be had from a fast cruise missile that tactical aircraft can employ, and there is a useful capability to be had in something that can be launched from the US and strike anywhere in the world quickly. The per round cost of these two capabilities should be drastically different.

    Yes it's tough to beat the efficiency of a nice high parabola, but shock or viscous losses are certainly tradeable against other requirements.

  2. jstults:

    Thanks for the comment. I really do appreciate it when people comment or rate the things I have posted.

    With some commercial launchers near $2500/lb at max load right now, and $1000/lb soon with the Spacex Falcon-Heavy, I think a revisit of the all-solid ICBM might show very low costs to do exoatmospheric transfer around the globe in scant minutes.

    Re-entry from barely-suborbital speeds is not that hard anymore. A shroud around the payload made of silica-phenolic ablater and some internal metal structure ought to be really cheap.

    Depending upon the size of launcher, there is room for a lot of supersonic plain ramjet cruise missiles as payload. And you do not need a booster, just shed the shroud and light the ramjet between Mach 2 and Mach 3, after entry is over.

    ASALM's launch weight was 2500 pounds, and that included about 700 pounds of booster propellant, and an ejectable booster nozzle, and an inlet port cover, none of which would be needed in the scenario I envision. So we're looking at slightly under 1 metric ton of ramjet to be rocketed to the other side of the world, per supersonic cruise missile. ASALM was to carry either 1000-pound conventional or nuclear warheads.

    Atlas-V-552 launches 25 metric tons, and Falcon-9 10-13 metric tons, depending upon which engine, all the way to orbit, both at $2500/lb. So, those are capable of sending 10 to 25 ASALM-class vehicles in a single shot anywhere in the world in under 20 minutes.

    Even the little Falcon-1 sends 1 metric ton to orbit rather cheaply.

    I think this mode would be rather hard to beat, considering just how difficult and expensive it is to treat the friction heating, and just overcoming drag to cruise, at hypersonic speeds in the atmosphere.


  3. Gary;

    The biggest "issue" with this idea is the same one every "ballistic" delivery system (including specifically any of the "conventional" ICBM concepts) has:

    There is a great difficulty in telling one of these from one carrying an nuclear weapon. For the most part there are SOME "clues" in the trajectory but those don't appear until very late in the flight and by that time you've already passed the point where you have to make the decision to retaliate or not.

    Sure it COULD be exactly what "they" (people who launched it) is, but are you ready to take the chance that it ISN'T and find out too late it is the first action of a preemptive strike?
    The "smart" answer is no...

    Scramjets have always seemed to me to be an answer looking for a question since they really DON'T help a lot with fast international transport, (same issues you've mentioned regarding staying in the atmosphere for long periods of time at high speed) or space launch. Nice to have for quick, super-long distance all-most invulnerable strikes of course, but inevitably you are going to run into the same problem as a "ballistic" strike: It could still have a nuke on it and be aimed at YOU so what do you do when it comes over the horizon?


    1. Randy

      The risk of mis-identification is very high, yes. Mitigating this is that there is one reentering object (shround containing missiles), not a cloud of warheads and decoys. Plus, everybody tracking it can see where it's going (so exactly where you send it is a very important pre-launch decision).

      I'd think the Russians and the Chinese could tell that one aimed into the sea just east of N Korea is not really aimed at either of them. But, you're right, there is huge risk in using such a weapon.

      I think scramjet might make sense in a tactical missile role, such as USN's anti-anti-ship missile problem, which has been insoluble since the advent of the Kh-31 long ago. Sunburn and Yakhonts are the current huge threats to ships. No defense for them exists.