What it was: The solution NASA envisioned to the difficulty of getting large payloads to anywhere much beyond Earth with mere chemical rockets. Something like a dozen of them would serve as the brute force “trucks” of the American space program beyond Low Earth Orbit.
Details: We’ve already discussed some aspects of the Integrated Program Plan, NASA’s ambitious 1969 proposal to follow up the Apollo Moon landings with a new goal and new technology. The new goal was a manned Mars Mission, but the new technology had two particular pieces that would do the grunt work of building a space station and a Moon base as intermediate steps to the red planet: a reusable orbiting space plane (not yet dubbed the “Space Shuttle”) and the Reusable Nuclear Shuttle (RNS), many of which would have been built. It would have been the space plane’s role to get astronauts and cargo into low Earth orbit, while the RNS would have been used for the “high frontier”, so to speak. If something was going to go higher a few hundred kilometers, it would be offloaded from the spaceplane to an RNS, and then sent on its way—potentially to the Moon, or even beyond.
The RNS was suited for this task and similarly restricted from landing on Earth for one reason: their engines were given oomph by a nuclear reactor, but approaching one too closely at the wrong angle would expose a person to a fatal dose of radiation.
Start with the Nuclear Shuttle’s advantages. A variety of factors affect the power of a rocket, but the dominant number is the specific impulse (ISP) of the propellants it uses (to be precise, it’s a proportional measure of how much propellant the rocket has to use to add or subtract a given amount of velocity, though confusingly its unit is the second). With variations due to several other factors, rocket engines that use UDMH and N2O4 produce a specific impulse in the neighbourhood of 280 seconds, while LOX/LH2 is much more efficient at around 450 seconds (the low density of liquid hydrogen hamstrings it, though, so it’s often only used in upper stages where the rocket is already well underway and moving fast).
Unfortunately, all chemical fuels with a better ISP than that are either fantastically explosive, corrosive, toxic, or some hellacious combination of all three of those characteristics. Even at that, the best known ISP ever obtained (with a tripropellant of lithium, hydrogen, and fluorine) is 542 seconds.
Ultimately this because chemical propellants depend on chemical bonds, and there’s only so much energy you can contain in those. Quite early on rocket engineers realized that a good way to higher ISP was to use a different source of energy. In the absence of real exotics like nuclear fusion and matter/antimatter reactions, nuclear fission was the way to go. Hydrogen heated by a nuclear reactor can have an arbitrarily high ISP; it’s just a matter of how much heat one can get away with before the physical components of the engine are melted away.
When John F. Kennedy made his famous 1961 speech that started the race to the Moon he made a largely-forgotten reference to the Rover nuclear rocket, a contemporary project that was working on a preliminary nuclear-fission powered rocket. This in turn led to successively more advanced nuclear engines with the colourful names KIWI, Phoebus, and Peewee-1. By the end of 1969, NASA had a design for a functional nuclear rocket engine, the NERVA-2.
NERVA-2 would have had a specific impulse of 825 seconds in vacuum, and be able to burn for 20 minutes and produce 399.5 kilonewtons of thrust. Compare this to the J-2, NASA’s comparable workhorse engine (used on the second stage of the Saturn V, among others): it produced 486.2 kN of thrust, but was far less efficient at just 421 seconds of ISP. Accordingly, even though the NERVA-2 was far larger and heavier than the J-2 (having an entire nuclear reactor on board does that), the savings on propellant mass and the mass of the tanks needed to store it would make any spacecraft using one smaller than the same spacecraft based around a J-2.
Getting to the Moon is considerably more difficult than getting to orbit—you need to add another 3 to 4 kilometers per second to your orbital speed—and so the radically reduced fuel consumption of a NERVA-2 engine was very useful. Enter the Reusable Nuclear Shuttle. This was a conceptually simple ship: a single large fuel tank containing LH2 would have a NERVA-2 attached to one end, while the other had a docking adapter that could connect up to a variety of payload containers. Attach your payload, light the engine, and the RNS would push the payload into high orbit, to the Moon, or even beyond. Ideally you’d also put it on a trajectory which would let it return to Earth orbit, as the NERVA-2 was designed for ten round trips before it would be unsafe to light up again.
The disadvantage of the RNS lay in the radiation environment it produced. The rocket’s exhaust was only marginally radioactive and so arguably acceptable to allow on a launchpad, but in the event of a containment breach on the ground or, worse, in the air the engine would have sprayed uranium all over the environment. Even in the heady days of the late 1960s this was considered too risky, so the plan was to launch an RNS on top of a Saturn rocket using conventional fuels—if the Saturn blew up, the reactors were sufficiently ruggedized that they could survive the accident intact and fall into the ocean safely (by 1960s standards anyway).
What was more problematic was the NERVA-2 in orbit. Once the reactor was up and running it needed a great deal of shielding to protect approaching astronauts. As shielding was heavy, the RNS wasn’t going to have much of it. Instead the approach chosen was the have a “shadow shield”, where the propellant tank and any propellant aboard would provide most of the shielding. This meant that humans getting close to an RNS had to approach it from the front at a fairly shallow angle, using the bulk of the RNS to cover them from the reactor. If they approached from the sides or, God forbid, the aft where the engine was located they were assured of radiation sickness or death. Even on top of the RNS, a crew member would get about the recommended annual maximum radiation dose each time the engine fired.
Nevertheless, the advantages of the RNS outweighed the disadvantages in NASA’s collective mind, and the Integrated Program Plan called for it to be the workhorse of the space program beyond Earth orbit. Each would be used up to ten times (with refueling gingerly taking place after each use), after which it would be discarded in a high orbit due to its extreme residual radioactivity. With it, crews and payloads could be sent to the Moon and returned, and ultimately the American manned Mars mission craft envisioned for the early eighties would be perched on top of three of them.
What happened to make it fail: As with much of the IPP, the nuclear shuttle never got built because of a combination of disinterest from the Nixon administration and the falling budgets that that caused. Of all its parts, only the re-usable Space Shuttle and its rocket stack made it off the ground.
The RNS has its own particular story embedded in this larger tale, though. For many years the nuclear rocket engine program had been championed by New Mexico Senator Clinton P. Anderson, as much of the work on NERVA had been done at Los Alamos. Just as NERVA-2 was ready to become operational he became seriously ill and unable to press his case as much as he had in the past. The White House convinced Congress to pull the plug on the nuclear rocket on the grounds that it would be the basis of a manned mission to Mars, a goal about which Congress was quite negative at the time. The plan was that the freed-up funds could be used for the more-practical Boeing 2707, a Mach 2.7 supersonic commercial passenger plane similar to the Concorde or the Soviet Tu-144. Ironically, Anderson had enough clout remaining in the Senate to apparently engineer a 51-46 vote against moving ahead with that project; the House of Representatives soon followed. While the exact maneuvering involved has never been documented, the vote was widely considered retaliation for the cancellation of NERVA.
Regardless, with its funding quickly dwindling despite Congressional efforts to keep it going, NERVA was cancelled on January 5, 1973, and the Reusable Nuclear Shuttle was dead.
What was necessary for it to succeed: Like much of the Integrated Program Plan, the RNS was doomed by the political currents in Washington, within NASA, and in the general public. When it came down to picking something to move forward on NASA picked the Space Shuttle and the hope that one day they would be able to move on to a space station from there. The RNS ranked third (with the Moon base and Mars landing fourth and fifth) on their priority list, and they even tried very hard to claim that without the Space Shuttle they would not be able to get any nuclear shuttles into space. This was not actually true as the initial plan to use NERVA involved an upgraded Saturn rocket, but it was a measure of NASA’s determination to do anything to get the Space Shuttle built.
Ultimately that’s the main route to getting the RNS into the sky. NASA engaged in a great deal of internal debate from 1968 to 1970 over whether to continue with ballistic capsules or move on to a reusable, winged orbiter. Related to this was the debate over whether or not to focus on Earth orbit as a testing ground or push hard into the rest of the solar system. If both debates had gone the other way, a nuclear engine would have been very attractive to planetary mission planners and the money would have been there to continue with NERVA and the RNS–despite Congress’ objections to Mars missions, the presidential Office of Management and Budget had considerable discretion to ignore how it was told to allocate the money it received until a post-Nixon backlash in 1975.
Instead the arguments settled around a winged orbiter and sticking close into Earth unless the mission was unmanned, and we got the space program that we did from 1975 to the first decade of the 21st century. Nuclear rockets were revived for a short while during the days of the Strategic Defense Initiative’s Project Timberwind, but again it never came to anything.
Even if the RNS got built, there’s the possibility that it would have been much restricted in use or even cancelled outright no matter what successes it scored. The Three Mile Island accident in 1979 soured the American public on nuclear power in general, and after the Challenger explosion in 1986 NASA became very leery about dangerous payloads–for example, deciding against the planned Centaur-B booster that was to be orbited aboard STS-61-G later in the same year for the purposes of getting the Galileo probe to Jupiter. While both were specific incidents, they were each the culmination of long-term cultural trends that likely would have choked off the use of the RNS no later than the mid-1980s, and possibly earlier if one of them was involved in an accident.