An early 1980’s Bell Helicopter artists concept of what would become the V-22 Osprey. The design is largely right, though the canopy is noticeably different. Note the refueling probe above the nose and the Gatling gun projecting from the tip of the nose.
The OPEC oil embargo of the west of 1973-74 and subsequent skyrocketing of petroleum prices made sure that the American SST program, cancelled by Congress in 1971, stayed cancelled. As Concord subsequently showed, an SST in an era of expensive aviation fuel would be an economic disaster.
In the late 1970’s there was a flirtation in the American aviation industry with liquid hydrogen as an alternate fuel for jetliners. LH2 would pose a number of issues, not least of which being the very low density of the stuff; relatively gigantic heavily insulated fuel tanks would be needed. For subsonic jetliner designs, these tanks often took the shape of extremely large fuel tanks on the wings, nearly the size of the aircraft fuselage. This was not much of an option for supersonic transports due to the increased drag. Nevertheless, liquid hydrogen fueled supersonic transports were designed. One such is shown below, a late 1970’s Lockheed design. The liquid hydrogen tanks occupy much of the forward and aft fuselage volume; the passengers are stuck in a relatively short segment in the middle of the double-deck fuselage. There would be no direct connection between the passenger compartment and the cockpit… so at the very least, the likelihood of a hijacking – another feature of air travel in the late 1970’s – would be greatly reduced.
By the 1980s, efforts to wean the west off OPEC petroleum were bearing fruit (or at least looking promising); as a result, the price of oil plummeted. And with cheap oil the imperative to design hydrogen-fueled aircraft largely vanished.
A piece of NASA presentation artwork from 1962 depicting an “Advanced Saturn” (a.k.a. “Saturn C -5” a.k.a. “Saturn V”) with a nuclear third stage instead of the S-IVb stage.
In 1949, the Langley Aeronautical laboratory of the NACA studied external stores (apparently fuel tanks) configurations for the Vought F7U Cutlass. A wide and occasionally unusual range of layouts was considered. As it happened, the Cutlass was a disaster of an airplane, with low powered engines prone to flameout in the rain and landing gear prone to collapse. The Cutlass did not last long and a surprising fraction were destroyed in crashes.
Conventional nuclear thermal rockets such as the NERVA can have a specific impulse of around 900 seconds, about twice what you can get from conventional chemical rocket engines. That’s good, but it’s also really low compared to what could be obtained from nuclear thermal systems. Solid core NTR’s have core temperatures substantially cooler than what you’d see in, say, an SSME, and for good reason: the core would soften and fail if it got much hotter. Thus the reason for the high performance of NTR’s is not due to high temperature, but to low molecular weight of the propellant (pure hydrogen, rather than water vapor for the SSME). But what if the core wasn’t limited to the low temperature of an NTR?
One way to do that is the gas-core engine. Here the uranium is allowed to not only melt, but to vaporize. It it retrained in the engine, typically, by spinning the engine or at leas the vapor. Thus the dense uranium vapor is spun out to the walls of the engine, and the much lighter hydrogen propellant is in the core. The keep the walls of the engine from melting, the hydrogen is first released into the engine from the walls themselves. The hydrogen bubbles up through the seething uranium gas, taking heat from the uranium as it does so.
Another approach is illustrated below, the Coaxial Flow Gaseous Nuclear Rocket. Here, instead of uranium spun to the walls, vaporized plutonium is retained along the centerline of the engine, with hydrogen flowing around it.
In these cases, specific impulses can get in the range of 5,000 seconds. But the problems with these designs were many. Startup and shutdown would have been lengthy and complicated processes. In the best cases, some of the fissionable gas would have escaped, meaning excess would need to be carried. In the coaxial system, it’s not entirely clear just *how* the hydrogen was to keep the plutonium vapor in place.
Another photo (via the NASA HQ History Office) of the Lockheed STAR Clipper. This was an early stage-and-a-half concept with a reusable orbiter and expendable propellant tanks. Vastly more info on this is available in APR issue V3N2.
In 1963, the Atomics International division of North American Aviation studied a terrestrial modification of the SNAP space-based reactor. The SNAP 4, also known as COmpact Multi-Purpose Automatic Controlled Transportable (COMPACT), had a 1 cubic foot core made of uranium zirconium hydride. The heat generated would boil water, which would drive steam turbine generators. The steam would be condensed and returned to the reactor to be boiled. The closed-loop system would in turn be cooled be either an external water source or via air cooling. Electrical power output was expected to be from 300 to 3,000 kilowatts. The core lifetime was to be from 1 to 5 years, with no maintenance required for 12 months at a stretch. The whole package would fit in an envelope 8 feet in diameter and 30 feet long, ranging from 48 to 125 tons depending on application.
The COMPACT system was meant to be a truck, train, ship, barge or aircraft transportable auxiliary or emergency power supply system (for disaster relief and such), or as primary power supply for remote locations. The claim was made that if put into production, electrical cost from the COMPACT system would be comparable to that from deisel-electric generators.
A Lockheed painting of the CL-840, an attack helicopter proposed for the Advanced Aerial Fire Support System contest of 1964-66. This design won, and was built as the AH-56 Cheyenne. Sadly, the design was more advanced than the technologies required to support it, and it was cancelled after only a few prototypes were built.
Another photo (via the NASA HQ History Office) of the Lockheed STAR Clipper. This was an early stage-and-a-half concept with a reusable orbiter and expendable propellant tanks. Vastly more info on this is available in APR issue V3N2.
Someone is selling a McDonnell-Douglas painting (the original actual painting, it seems) of an SST concept:
The aircraft uses a “parasol” wing, which was a concept that enjoyed a bit of popularity in the 1970’s. The idea: at supersonic speeds shock waves shed from the nose of the craft would impinge on the underside of the wing, adding lift and reducing fuel requirements. As memory serves, an added bonus would be that the benefit of area ruling would be in place, but without the need to actually “wasp-waist” the fuselage. Being able to produce a bland cylindrical fuselage would greatly reduce cost and stress on the large pressurized structure.
Such “favorable interference” designs would produced for fighters, SSTs and bombers, from USAF design labs to Boeing to McD to Lockheed and probably others. In time, the idea faded away; the gains in supercruise performance were apparently outweighed by cost and weight.
Note that the positioning of the engines, unusual for an SST, would also serve the favorable interference purpose: shock waves from the inlets would impinge on the wings above.