Explosive Engine Key to Hypersonic Plane

Tag : Jet Engine Components

The military wants to build a new kind of aircraft that can take off from a runway, fly at over six times the speedof sound, and then come back home. No one is quite sure whether Darpa and the Air Force can pull itoff -- right now, it'd take a combination of a standard plane, arocket, and an "air-breathing" scramjet-powered craft toget it done. But if there's any hope at all of putting this $800million plane in the air, it'll need a whole new kind of engine.
The program to build that engine is called Vulcan . And, in a briefing to potential builders, Darpa recently unveiled new details about the ambitious engine effort .
The key, Darpa thinks, is to combine a plain ol' turbine enginewith a so-called "Constant Volume Combustion," or CVC,engine. What does that mean? Well, in a standard engine, Aresexplains, "combustion takes place at constant pressure, andvolume increases." In a CVC set-up, "combustion takesplace at constant volume, and pressure increases. This results inhigher thermal efficiency and lower specific fuelconsumption." Hopefully.
Vulcan program manager Tom Bussing has been working on one kind of CVC for years . It's called a pulse detonation engine . And rather than burning fuel, it blows the stuff up.
The thing is, no one has ever put one of the PDEs into production,yet. And it's very much an open question whether anyone ever will.Air Force chief scientist Dr. Mark Lewis tells DANGER ROOM:
Honestly, the scientific jury is still out on PDE's and theirderivatives, but this is definitely a worthwhile area for futureresearch. From a basic science standpoint, the thermodynamic cyclethey use would indeed be more efficient than a standard jet enginecycle; a jet burns its fuel and adds its heat at a constantpressure (or nearly so) whereas a PDE adds heat at a constantvolume - very similar to an automobile engine. It can be shown fromfundamental principles that you get more useful work when you addheat at constant volume than at constant pressure, so the basicscience is sound. To that end, the Office of Naval Research has putconsiderable resources into exploring the basic physics; the AirForce and NASA have also invested over the years.
But the devil is in the proverbial details, and that's why this isan important research area. A PDE by its very nature would not be aconstant burning process, rather an intermittent process (think ofa series of detonations, one after the other). That could put extrastress on engine components, makes it difficult to harness thecombustion effects for efficient propulsion, and can also bedifficult to initiate.
A few years ago, one of the most senior NASA propulsion experts dida very detailed analysis, concluded that a good PDE is definitelybetter than a bad jet engine, but he couldn't say much more aboutthe real potential of a PDE. That tells me it is still an importantarea for fundamental work.
If we can get the concept to work practically, It does hold greatpromise for being able to cover a wider range of Mach numbers. AFRL[Air Force Research Laboratory] actually built a small airplane that was powered by a small PDE (at very low speeds); it didn't work especially well, but it wasan important first step.
I'm glad they're doing this work. It's high risk, high payoff, soobviously I wouldn't bet a billion dollar vehicle on it just yet.And I think the folks working this area also understand that.