Engineers created the A2 with the failures of its doomed supersonic predecessor, the Concorde, very much in mind. Reaction Engines’s technical director, Richard Varvill, and his colleagues believe that the Concorde was phased out because of a couple major limitations. First, it couldn’t fly far enough. “The range was inadequate to do trans-Pacific routes, which is where a lot of the potential market is thought to be for a supersonic transport,” Varvill explains. Second, the Concorde’s engines were efficient only at its Mach-2 cruising speed, which meant that when it was poking along overland at Mach 0.9 to avoid producing sonic booms, it got horrible gas mileage. “The [A2] engine has two modes because we’re very conscious of the Concorde experience,” he says.
Those two modes–a combination of turbojet and ramjet propulsion systems–would both make the A2 efficient at slower speeds and give it incredible speed capabilities. (Engineers didn’t include windows in the design because only space-shuttle windows, which are too heavy for use in an airliner, can withstand the heat the A2 would encounter.) In the A2’s first mode, its four Scimitar engines send incoming air through bypass ducts to turbines. These turbines produce thrust much like today’s conventional jet engines–by using the turbine to compress incoming air and then mixing it with fuel to achieve combustion–and that’s enough to get the jet in the air and up to Mach 2.5. Once it reaches Mach 2.5, the A2 switches into its second mode and does the job it was built for. Incoming air is rerouted directly to the engine’s core. Now that the plane is traveling at supersonic speed, the air gets rammed through the engine with enough pressure to sustain combustion at speeds of up to Mach 5.
A combination turbofan/ramjet. Hokay.
If I understand this properly, the idea is to fly fast subsonic over land to avoid breaking windows, and then to go like a bat out of hell over the water. When I look at that design, I have to wonder how they can really get the range, with all of the drag that is implied from those huge delta wings, not to mention the wave drag at Mach 5. I also wonder where they put the hydrogen–that stuff is very fluffy, and needs large tanks. It’s probably not wet wing (it would be very structurally inefficient), which is why the fuselage must be so huge, to provide enough volume in there for it.
Sorry, but I don’t think that this will be economically viable. As is discussed in comments and the article, hydrogen is not an energy source–it’s an energy storage method, and it’s unclear how they’ll generate it without a greenhouse footprint. Moreover, it’s not as “green” as claimed, because dihydrogen monoxide itself is a greenhouse gas. I’ll bet that this thing has to fly at sixty thousand feet or more to get itself sufficiently out of the atmosphere to mitigate the drag problem, and that’s not a place where you want to be injecting a lot of water.
This concept doesn’t learn the true lessons of Concorde: like Shuttle, a lot of people have learned lessons from Concorde, but the wrong ones. The correct lesson is that we need to get rid of shock waves and drag. Once we do that, we’ll be able to cruise at reasonable speeds (say, Mach 2.5) everywhere, over both land and water, so we won’t have to build the vehicle out of exotic materials and eliminate windows. We’ll also be able to have fast transcontinental trips (two hours coast to coast) which is another huge market that this concept doesn’t address at all. Finally, it has to do it with a reasonable lift/drag ratio, so that ticket prices will be affordable. And I think that the fuel issue is superfluous–Jet A will be just fine for the planet, as long as fuel consumption is reasonable, which makes the vehicle design much easier, with much more dense fuel.
Fortunately, I’ve been working for over a decade with a company that thinks it knows how to do this, and I’m hoping that we’ll be able to start to move forward on it very soon.
[Via Clark Lindsey]
[Update in the late afternoon]
In response to the question in comments, there’s not much publicly available on the web about shock-free supersonics, but here’s a piece I wrote a few years ago on the subject.