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.
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 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.
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.