Rand answered my earlier musings. What he’s let on so far (I appreciate that he can’t tell everything he knows) is intriguing. If you take the wing of the Busemann biplane, which is a theoretical model going back to the thirties (twenties?– I took my Liepmann and Roshko home last night and left it there), at zero alpha and at discrete Mach numbers controlled by the geometry it has no wave drag as the shockwaves are completely contained within the two wings. Sadly, it also generates no lift at this condition because of symmetry. I don’t have a picture, but essentially the airfoils are very roughly triangular with the upper one flat on the top and the lower one flat on the bottom, so a chordwise view looks like a 2-D converging-diverging nozzle – or a De Laval nozzle. Interestingly, this nozzle is used mainly to accelerate subsonic to supersonic flow, or to achieve constant flow rates despite fluctuations in back-pressure (pressure downstream of the exit), or as rocket engine nozzles. Here though, the flow is already supersonic before entering.

In the Busemann biplane, the airflow is compressed beneath the upper wing and thus has higher pressures on the bottom that the top, which is lift. The bottom wing would operate in the exact same way, only upside down which leads to no net lift. The secret of lift at zero alpha is then to replace most of the lower wing with a jet of air with higher energy than ambient. While the lower wing would provide a small amount of negative lift, the upper wing due to its much larger area and much greater compression would provide far more positive lift. The question is, will this jet eliminate the shocks from the upper wing in the same way a correctly sized solid lower wing would? I don’t know — and even if theory tells you it’s possible, that doesn’t mean you could actually achieve such a state with real equipment in real life (which Rand is clear about himself).

One of the interesting things is that the wing would operate at a fixed Mach number without shocks. Since you couldn’t vary the angle of attack, the only control of lift at cruise is altitude. Thus you’d fly a particular altitude for a particular weight — once you got to your cruise Mach, the plane would either float up or down until it reached the altitude that its lift equaled its weight. Then it would float steadily upwards during cruise as it burned fuel.

Even if such a wing did work, life isn’t all roses. You have the structural issues of making the wing, especially the lower one which will have to be small, hollow for this high energy air to flow through and out of, and strong enough to take the loads. And you still have the whole rest of the plane. What do you do about the shockwave the nose of the aircraft generates? I know it can be mitigated by high fineness ratio, but not eliminated. I suppose the nose shape could be such that the shock only went upward. If not, this shock also has implications for wing placement – the wing will most likely operate without shocks in a very narrow Mach range. Being the nose shock the Mach number will be lower than in front of it, and unless you can design this wing to also handle have a region through a shockwave, the wing will have to be completely behind the shock. There are also control surfaces to worry about. It’s fine to have your wing produce constant lift, but control surfaces have to be able to vary their forces and moments. You’d get shocks and wave drag off of them. You could use engine thrust vector control, but you’d need it in all three axes.

I think you still have a problem in getting to cruise. This wing wouldn’t be particularly good at subsonic flight, and I’m not sure you could put flaps in the wing without causing problems at cruise. Thrust vector control would help again here, but you sizing the wing for takeoff and climb versus cruise would be a problem. And because the wing design allows for zero wave drag only at certain supersonic Mach numbers, you’d still have to blast your way through the transonic drag wall at high angle of attack. My engineering judgement tells me that such a transport, when all said in done, will have more expensive acquisition and operating costs than current subsonic transports. High enough to outweigh the benefits of faster travel

So what I see is a fairly straightforward science problem — will this semi-solid Busemann biplane wing design eliminate wave drag — coupled with a host of engineering problems. And really, this is the sort of thing NASA should be all over. Start the funding off to assess the science problem first, and then if it looks feasible, start on the engineering problems. Solving engineering problems is what puts the joy into engineering.