Wednesday, July 27, 2016

Dr Mark Lewis Speaks at MIT AeroAstro Centennial on Hypersonic Flight





Dr. Lewis talks about why the experience with the X-51, despite what would otherwise be considered only a set of partial successes, was actually quite important both from the technology development and the system point of view: first, the X-51 flew on regular hydrocarbon fuel, not an exotic or cryogenic fuel, such as the hydrogen fuel that other scramjet variants used for hypersonic flight have flown on; second, while the X-51 was launched from the underbelly of a B-52, it was actually flight-scale, that is, an actual missile or ISR (intelligence-surveillance-reconaissance) vehicle would be of roughly the same dimensions; third, it was thermally balanced, in other words, was in no danger of melting - and could have flown [and actually did fly] as long as it had fuel; fourth, it was air-launched, and flew on a B-52 that was originally designed more than 50 years ago - pointing to a possible way of modernizing the warfare abilities of a relatively old aircraft; fifth, unlike previous attempts, the scramjet used in this test demonstrated not only net positive thrust, but positive acceleration, with the hypersonic craft being on a final upward trajectory, thus silencing the many skeptics of scramjet propulsion - this variant was actually designed in 1958!







Dr Lewis also noted that this was a first step, and much more remained to be done to make a missile based on the X-51 operational: the ignition system, cooling,  the booster, controls, guidance & navigation, and warhead aspects would need further development. Furthermore, going up the Mach scale in airbreathing propulsion remains of strong interest, and for example, M=6 is a 'sweetspot' for many military applications, and for two-stage-to-orbit systems, M=11 is of particular interest. At a more fundamental level, of course, there is a greater understanding needed of the physics of hypersonic boundary layers. Overall, a better Test & Evaluation process is also needed.

He also pointed out that alternative modes of achieving objectives, also deserve study - for example, a hypersonic glider, which glides down to earth after being boosted up to high Mach speed by a traditional (non-airbreathing) rocket (as opposed to being thrusted up via is also of interest. Or the approach pioneered by Reaction Engines - the brainchild of Alan Bond - in which a cryogenic system liquefies the air sucked (or breathed) in - and burns the liquid oxygen component of it, together with the cryogenic fuel such as hydrogen that it is carrying - rather like in a conventional rocket engine. This solves the fuel-mixing issues that arise in scramjets, however, the liquid oxygen is not carried  along from the ground, as in conventional rockets, and is obtained by condensing air in flight. This was originally considered as part of the technology scoped out for the National Aerospace Plane, but was negatived then, because condensers (for the air) were considered too heavy and inefficient to serve in the capacity desired. However, Reaction Engines might have an approach that merits further study.