The A2, for example, was to be powered by four Scimitar engines, a unique and new dual-mode design that incorporated a built-in heat exchanger to keep the turbines from melting at hypersonic intake temperatures. The engine needed two modes because the A2 would pass through two distinct flight regimes: The first included take off, acceleration to Mach 2.5, and landing. For that, the Scimitar would work like a conventional jet engine, with turbines compressing the intake air, mixing it with fuel, and igniting the mixture to produce thrust. But operation at hypersonic speeds caused the temperature inside the intake to reach as high as 1,800 degrees Fahrenheit—a death sentence to turbine blades. Hence the need for precooling, which the Scimitar would accomplish in two ways: through the low temperature of the liquid hydrogen entering the combustion chamber, and by means of a built-in heat exchanger that directed precooled gaseous helium into a diffuser that, like an air conditioner’s evaporator coils, reduced the temperature of the air passing through it.
Of course you pay a price for all this. “Heat exchangers tend to be heavy and complicated, and they can leak,” says Schetz. In its description of the system, Reaction Engines wrote: “The incorporation of lightweight heat exchangers in the main thermodynamic cycles of these engines is a new feature to aerospace propulsion.” In other words, the precoolers also had to be filed under the category of “stuff to come.” Still, the company at least had a heat- exchanger test facility in place by December 2005 at the Culham Science Center in Oxfordshire, England, and has built a number of prototype precooler modules. And Richard Varvill, the company’s technical director and chief designer, had an answer to Schetz’s weight objection.
“The weight issue we’re addressing by having very thin precooler walls and small-diameter tubes,” says Varvill. “The tubes are about a millimeter diameter, made of a certain nickel-based alloy. The mass target for the heat exchanger is one and a quarter tons. They are heavy; they certainly add weight. But if you push the engineering to its limit, you can get an acceptable weight.”
There was a reason for all the complication of the precooled, dual-mode engines. Their main advantage, says Varvill, “is that they are good from rest to hypersonic speeds, whereas alternatives such as scramjets and so forth are not capable of that. So to get to Mach 5, you’d have to have two different engines on the same vehicle. And that certainly has major weight and cost implications.”
The precooler, then, was key to the appeal of the Scimitar engine concept, and therefore of the A2 itself. Varvill’s optimism that the device will work is based on the amount of theoretical modeling and experimental testing the company has already done. “We’re now going to the next level, which is to actually make a precooler [that] will be running in front of a jet engine in a couple of years’ time,” he says.
As for the A2’s zero carbon footprint, that too rested on the success of future technological developments, in this case a method of producing large amounts of hydrogen cleanly and greenly.
But if the A2 is not quite quiet and not quite green, at least there are nearer-term alternative technologies being developed and tested—the Quiet Supersonic Platform, for instance, which originated in 2000 as a Defense Advanced Research Projects Agency program. Its objective was to reduce the sound of a sonic boom to the point that supersonic flight over populated areas would become unobjectionable. To that end, DARPA contracted with Northrop Grumman, which proposed modifying the nose section on one of its F-5E fighters, thereby shaping the sonic boom into one less disruptive to the ear. (The classic sonic boom is a pressure wave with two sharp peaks in rapid succession, like a capital “N.” A shaped wave would have the first peak looking more like the lowercase version: n.)
Northrop Grumman conducted its Shaped Sonic Boom Demonstration out of Palmdale, California, on August 27, 2003, with mixed results. According to the bevy of microphones in place near Harper Lake (an hour’s drive northeast of Edwards Air Force Base), the resulting boom was measurably less intense than that of an unmodified jet of the same type that flew through the same airspace moments later (see “The Boom Stops Here,” Oct./Nov. 2005).
On the other hand, the reduction in intensity wasn’t perceptible to a mere human. “I was out there and listened with my own ears,” says an experienced NASA sonic boom listener (who wishes to remain anonymous to preserve his relationships with NASA contractors), “and to tell you the truth, I couldn’t hear all that much difference.”
Progress in the boom-busting business has been slow and incremental. But even if the sonic boom could be changed from a loud clap to the sound of rolling thunder (nobody thinks it can be eliminated entirely), the public’s stomach for even soft sonic booms is an unknown and is further subject to the vagaries of politics.