The Perfect Airplane
Fast, green, and quiet. Come on, brainiacs, you can do it.
- By Ed Regis
- Air & Space magazine, September 2009
Reaction Engines Ltd/Adrian Mann
(Page 2 of 4)
This then was the Impossible Airliner, politically correct and guilt-free, which was to say, superfast, quiet, and with zero carbon footprint. Well, not really all that quiet: It would produce “takeoff sideline noise” of more than 100 decibels a quarter-mile away, and it would fly “via North Pole and Bering Straits to avoid supersonic overflight of Eurasian land mass.” But you can’t have everything, even for a one-way ticket price of €3940 (about $5,500).
The question is whether you can have any of it. All of these extremely advanced design concepts rest in large part—if not wholly—on other equally advanced design concepts: materials and technologies that also have to be invented, tested, proven, and then fused with still other vaporware in what engineers commonly referred to as a “highly integrated vehicle concept,” whatever that means.
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.