A working prototype nuclear bomber, AWESOME!
I'm confused, do you mean the M-50? It isn't nuclear-powered, it's a bomber meant to be able to deliver a nuclear strike, which American intelligence got confused about.
Or, do you mean the US XB-72, which I gather
is nuclear powered?
I certainly think the latter is a white elephant militarily speaking. A nuke bomber--any kind of nuclear powered airplane really-is necessarily a big one, since the shielding requirement (if one has any intention of the flight crew living long!) is severe, but does not scale up linearly with the power, so making a bigger plane with a much bigger power plant but the shielding mass rising only in a small proportion is the way to make it work. Such a big plane, even if it delivers fantastic performance (which means more power, a greater power plant in proportion, hence the "break-even" mass with adequate shielding is even higher) is going to be just about impossible to make stealthy; used in an all-out war against a strong opponent, the latter will presumably have a good system of early warning, target tracking and SAMs that will eat it for lunch. Meanwhile it is a massive investment; having enough of them to overwhelm Soviet air defenses and get some of them through to the targets would cost a staggering lot. OTL, bombers since WWII have mainly been useful against Third-World nations that can't credibly retaliate against the USA nor mount a top-rate air defense; for such missions this monster would be clear overkill, though I suppose if the USAF had managed to make the investment and shepherd it through the testing process, and somehow finagle DoD, the President and Congress into approving it for service, once the things are actually in inventory it would indeed be used in Vietnam or some such places. The question is, would it be developed at all?
ITTL the embarrassing intelligence failure does have this effect but I don't think Delta Force is wrong to characterize it as overall a victory for the Soviets; the huge amounts of money doubtless involved had to have starved OTL successful and useful programs, or alternately bloated the DoD budget thus creating overall budget pressure on the US Government in general and setting up the Pentagon for future cutbacks, probably in programs they'd miss.
The question is, does this "kewl" nuke plane pave the way for something really useful down the line?
I suppose that, having started down the road of gigantism, someone in the Pentagon would argue for a transport derivative, one that would probably dwarf the OTL C-5, but have the virtue of not requiring jet fuel to operate.
Actually, many OTL nuke plane proposals I've seen fall back, for various reasons, on a dual-power arrangement that does require some conventional jet fuel--for safety reasons in particular, many suggest the plane should take off and land with the reactor shut down, because crashes are most likely in those crucial phases and a dormant reactor is apparently somewhat safer to contain if it goes down. Also, takeoff/landing are phases of flight requiring higher thrust than cruising; using the nuke only for the latter means requiring less peak power, and a pretty much fixed power, so the nuclear reactor can be smaller and operate at just one optimized setting. To be sure this savings is offset both by the fuel reserves and the alternative power plant! (One proposal I've seen, in a 1970s RAND Corporation study, asserted that the main engines could be designed to either burn fuel or be run off of reactor heat, which means just one set of engines, though each engine is obviously more elaborate and hence heavier). A non-nuclear alternative propulsion system also allows the plane some emergency range and landing reserve should it be necessary to shut down the reactor in flight. Having conventional jet fuel aboard and some of held in emergency reserve, it can be used as shielding allowing some savings on the fixed shielding mass; the reserve, without which the shielding would then be inadequate, would only be used up in an emergency that involved shutting down the main reactor, thus reducing the radiation output.
So really, unless the designers can plausibly answer these doubts, they won't eliminate fuel consumption completely. The argument would hinge on whether they'd save a substantial amount of fuel; size alone would have a tendency in that direction per ton of payload; long-range flights would justify it more than short-range, and a really big plane with highly specialized maintenance requirements argues against using it to reach near-front-line forward bases and for connecting a few widely separated and deeply defended, highly developed logistic bases, from which cargo would be re-deployed to short range forward-base planes. So that's another level of cost to offset the case for it.
(ITTL of course the XB-72, whatever it is, is flying, much of the R&D is done for good or ill, and it's a question of whether doubling down is a case of recouping on the investment or good money after bad.)
Another mission the RAND study considered, besides its primary focus on alternative fuels for a nominal really big transport plane (in the 1000 ton range) is very long loiter airborne stationing. Say for an AWACs type plane, or to station some kind of attack plane in place analogous to an aircraft carrier. Cruise missiles or even ballistic missiles might be based on it.
If the XB-72 is supersonic, I guess they'd go for something with performance comparable to the OTL XB-70 "Valkyrie" bomber, designed to cruise at Mach 3 or about a kilometer per second.
Which, combined with gigantism, suggests yet another mission that tempts the Strangelovian side of my schizophrenic mind--well actually this is much more innocuous and peace-loving than the above stuff--high speed, high altitude rocket launches of high mass.
Mach 3, which I take as the practical upper limit of any airbreathing propulsion system demonstrated to reliably work as of yet OTL (and so even if ITTL things work out better and earlier, it seems unlikely we could do better in the 1970s anyway) is as I say a kilometer per second, or about 1/8 orbital velocity. Combined with the very altitudes such a fast plane can achieve (indeed in a sense must achieve--OTL supersonic planes cannot operate at their top speeds in the lower atmosphere, between the very high aerodynamic forces involved and the very high thermal heating--stratospheric air is much thinner and much cooler) a rocket to put a given payload into a given orbit can be substantially smaller and somewhat simpler than one launched from the ground. The catch is the airplane has to be big enough to lift the whole rocket, payload fuel and all, as well as itself. Given the aerodynamic situation, said rocket has to be carried horizontal; with cryogenic liquid propellants one might have the option of carrying them in separate fuel tanks aboard the launch plane and loading them into the rocket itself just at the last minute, but that requires doubling the total tank volume and while we might save on both insulation and related equipment mass on the rocket itself, the tanks on the plane still have to be efficient shapes for fuel storage and well-insulated to prevent propellant boil-off. Nor am I sure how fast really large masses of liquid oxygen and perhaps liquid hydrogen can be pumped from plane to rocket.
One help there is that I gather that a fairly recent attempt by NASA or the Air Force to develop and demonstrate a Scramjet came to grief when the high-speed flyer failed to safely deploy from the top of its launch plane; this suggests to me it might be safer and more effective to mount the rocket stage on the
bottom of the launch plane, and deploy it by dropping it, rather than fire it from the top, as I'd normally imagined it working. The plane, flying just a bit below its speed and altitude ceiling, goes for a final surge of acceleration and pitches up into a climb to convert some forward speed to upward, following ideally the arc of a circle; if as it approaches its limit of thrust while climbing it simply releases the rocket, the lift on the wing though dropping is now acting on a greatly reduced mass so the launch plane pulls away sharply; with the rocket free-falling on a ballistic path upward into ever-thinning and already sparse air, it can wait some time for the launcher to get good and far away before firing.
So if the rocket is below the main airplane body, and fuel is stored in the latter, loading it in is a matter of opening stopcocks and letting it drop into place. It will still take some time as presumably there are a limited number of connecting pipes of limited cross-section; hydrogen in particular is very low density and hence bulky and hence needs more time, and its extreme cold is such that the rocket tank and the connecting hoses still need major insulation even for a storage time of just minutes (especially considering this is happening at high-speed, hence high-temperature, supersonic cruise). Liquid oxygen is both much denser and higher temperature, so it seems much more doable. A supersonic air launch system thus might be restricted to oxy-hydrocarbon as the maximum energy fuel, giving ISP in the 300+ range versus 400+ for hydrogen, which goes far to pretty much neutralize the advantage of launching from high speed and altitude. But then again hydrogen fuel, while very tempting and increasingly standard, has always been challenging and requires much more fuel tank volume as well as more sophisticated engines, which have hitherto offset their advantages. Perhaps then we should look at supersonic air launch as a way of making simpler, cheaper, more compact oxy-jet fuel rockets competitive with oxy-hydrogen? But the future challenge of getting all the benefits for maximum payload to orbit will still beckon!
Now, how big does a suitable airplane have to be? My best guess is, at least equal to the mass of the rocket launched! The rockets still have to mass something like ten times or more the mass orbited. Good, high-capacity airplanes typically mass empty somewhere between half to perhaps down to a third their total take-off weight; in turn the net useful lift available has to be divided between fuel and payload. So more realistically, considering that a pure nuke plane does not require fuel but does have a very heavy power plant that is in the ballpark of the fuel mass of a conventional equivalent engine set's requirements, we need somewhere between 3-4 times the rocket mass for all-up takeoff weight, meaning the plane itself is 2-3 times the rocket mass. Considering how advanced this plane has to be to get to Mach 3, and that its aerodynamics during the crucial takeoff phase are compromised to its requirement to operate at such high speeds, we should probably err on the cautious side. So supposing a 50-ton rocket can, from such a launch, orbit 5 tons, the plane itself has to weigh in at 150 tonnes, of which I guess a third or more is nuclear power plant. Total take off weight 200 tonnes, which is about 2/3 of a Boeing 747, but double the mass of a Concorde or B-70 IIRC.
Actually that might be too
small for a nuclear plant with adequate shielding. Especially considering that supersonic flight is very power-intensive so we'd need a lot more power than a similar-sized subsonic plane would.
Meanwhile and come to think of it, for a relatively short-duration mission like this--take off, climb and accelerate to Mach 3, climb sharply and drop the rocket, then fly back to base--we'd probably do better not to use a nuke; using conventional fuel means the all-up mass is dropping as we fly and the plane is significantly lighter when the rocket is dropped off, so the airframe itself can in turn be lighter, thus saving on more mass while drop-off acceleration is higher. A nuke system means the mass is practically constant and we are hauling around power not immediately available nor needed, which compounds the all-up takeoff weight, hence thrust, hence requires still bigger engines and so on.
Nuclear power for aircraft thus strikes me as being most useful for subsonic cruise then, for long range or long loiter time. The former may or may not be useful depending on the relative costs of nuclear power versus conventional fuel, and the cost of the nukes has to factor in safety measures, and is a big investment that has to last years, while both state of the art of conventional engines and their fuel costs fluctuate, the former presumably only up, the latter unforeseeably up and down, so the uncertainty means conservative figuring has to be against the best case for conventional, which is the worst case for nukes.
The latter is the only application for which nukes stand out as clearly, unambiguously, advantageous. Giant AWACs, conceivably loitering airborne aircraft carriers or missile launchers. Maybe naval patrol craft. And we have to need giant ones!
