As technologies jet aircraft, rockets and nuclear energy emerged in the mid-20th century out of the terrible strife of World War II. Not an auspicious beginning, certainly enough, but commercial aviation depends on jet aircraft; industrialized countries derive significant fractions of their electrical power from nuclear plants and conventional rockets have carried thousands of satellites and even hundreds of astronauts into space. Yet despite parallel development and success, in application these technologies have hardly intertwined. There has been limited use of nuclear energy in aerospace applications thus far when compared with expectations of researchers, forecasters and science fiction writers of the 1950s and 60s watching their then seeming exponential growth. In fact I was among those typing out on my Smith-Corona in those years stories and scenarios based on nuclear powered spacecraft expected up ahead; but decades later it is computational, communications and information technologies related to that typewriter which have been revolutionized. The nuclear and aerospace technologies seem locked in near steady-state; they seem to lack such quicksilver interactions since the 1970s, though now and then nuclear rockets show signs of renaissance.
The starkly stated case for nuclear energy in aerospace applications is based on the limitations of conventional chemical combustion. If a rocket needs to achieve 7 miles per second velocity to escape from the Earth and consuming a pound per second of chemical propellants in rocket engines offers about 465 lbs of thrust at best, the attraction of nuclear fission rockets already demonstrated is that they can offer double the thrust or more. Other nuclear systems could offer ten or a hundred times more thrust per pound of propellant or, as we shall discuss further, “exponential” gains. But there are other serious technical and safety considerations to contend with.
It is not for lack of plans that nuclear energy has not transformed aeronautics and astronautics. At the end of World War II, plans for nuclear energy abounded and met with success in the electric power industry and the propulsion of naval ships such as submarines, aircraft carriers, icebreakers and destroyers. The virtue of the nuclear submarine, for example, was that it could cruise for months without refueling, converting nuclear energy to mechanical or electrical energy so efficiently that submarines cruised under north polar ice for months without surfacing. In this application, however, a nuclear engine provided about one pound of thrust for about 150 pounds of nuclear power plant. The submarine, buoyant in water, used its propellers literally to overcome drag. Drag increases with the square of velocity and submarines underwater would top out at perhaps 30 knots.
Airplanes meet less drag resistance than do submarines in moving through their medium of air, but to fly they must also be built much less densely for lift to overcome weight. Kerosene burning commercial jet engines with their large turbofans have thrust to weight ratios of about 5 or perhaps 6 at best. If lift equals weight and the lift to drag ratio is about 15, then an aircraft taking off at full power needs an engine that can produce a 1/15th or more of its weight in thrust. If an engine can produce 5 times its weight in thrust, that’s 75 times as much thrust as it needs to propel itself. In the case of a Boeing 747, a 400 ton aircraft, four 4-ton engines provide propulsion allowing cruises across the ocean at subsonic speeds in the stratosphere. Aside from that 16 ton assignment for engines, the aircraft is loaded with structures, passengers and cargo – and 100-tons of fuel that will be burned in flight.
Clearly, if nuclear jet engines were to replace jets that burned hydrocarbons, the thrust produced per pound of engine would have to greatly improve over submarine engines. If the lift to drag ratio of 15 still applies, then at the very worst, the nuclear aircraft engine would have one-fifteenth of its installation weight in thrust; more, if the aircraft has any other structures, payload or crew. A reasonable guess would be one-fifth its installed weight. Yet the nuclear reactor would have to be shielded with lighter materials and the power cycle would have to be more efficient. The central nuclear reactor core that would turn propellers or gas turbines would have to fit within the weight confines previously allotted for hydrocarbon fuels on board such an aircraft – or else displace payloads. What’s more, the safety concerns of operating a conventional aircraft are serious enough, especially in the event of a crash. Would risks associated with nuclear fuel spills and radiation exposure ever warrant use of such aircraft? Would its very exhaust be a hazard, exposed to a nuclear energy process?
In the 1950s, a large test aircraft, a converted B-36 bomber, demonstrated the capability to carry an activated nuclear reactor, but it did not use its power to run any engines. There were other related programs for nuclear aircraft, many of which few details were made public. Beside the airframe projects that anticipated large aircraft, there was related research to provide large engines. Much of this research was later applied to development of the engines that powered the early jumbo jets, the Boeing 747 and the air force C-5 cargo plane.
Now let’s take a look at rockets.
Chemical rockets generally do not cruise like airplanes. They burn their propellants (fuel and oxidizer) very quickly to attain changes in velocity and then they coast until the next time a maneuver is required and change their velocity again. These maneuvers are characterized by a formula called the rocket or Tsiolkovsky equation, named for the early 20th century Russian pioneer in astronautics. This relationship relates rocket mass before and after a maneuver to change a velocity (Delta-V) with the efficiency of the propulsion system, the specific impulse (Isp), a relation we described above in terms of pounds of thrust obtained for pounds of propellant burned per second. When these units are used the remaining units after algebraic cancellations are “seconds”. Algebraically the formula can take the following form.
Mass final/ Mass initial = exponent (-Delta-V/ gravitational constant and Isp)
Let us take the example of a chemical engine such as might have powered a single stage to orbit spacecraft based on burning hydrogen and oxygen. The rocket has a Delta-V budget of 30,000-fps to reach orbit and its Isp is 455 seconds. If the vehicle weighs one million pounds at take-off, at the end of its burn it would weigh 128,825 lbs, meaning propellant would have been 87% of its take-off weight. Now suppose this rocket ship simply used hydrogen heated by a nuclear reactor as its exhaust product, possessing ( Isp = 900 seconds our estimate). Its final weight or mass would be 354,860 lbs or almost three times higher.
Remarkably, while nuclear aircraft were not demonstrated with prototypes such as X-planes other than a power plant ferrying converted B-36 bomber, a couple of nuclear rocket engines were in test programs from the late 1950s to the early 1970s. An account of these programs is provided by James A. Dewar (To the End of the Solar System, The Story of the Nuclear Rocket, 2nd Edition, Apogee Books, 2007).
At face value this sounds very attractive until we confront issues of rocket failures on a launch pad in Florida or somewhere down range. Perhaps the breakthrough of nuclear energy into astronautics can best be obtained by carrying nuclear power into orbit – and far away from Earth. But even when orbit is reached, risk is not reduced entirely. The Soviet satellite program already employed nuclear power to operate relatively powerful naval ocean reconnaissance radars on orbit and one of these, Kosmos 954, failed and re-entered the Earth’s atmosphere in January of 1978, scattering debris over northern Canadian territories and provinces.
Concerns were high about contaminants even though the nuclear device supplied only about 2 kilowatts of power. Both the American and Russian space programs have developed 100 kilowatt space nuclear reactors that could be used either for power or propulsive applications. Propulsive in this case could be electric propulsion similar to solar electric power devices already flown in interplanetary space. Electric propulsion in space is based principally on accelerating ionized particles of propellants such as mercury, cesium or argon through an electric field chamber and “nozzle” to velocities higher than can be obtained with chemical combustion. As a 1980s figure of merit was the 30-cm diameter thruster that accelerated mercury to 30 km/sec velocities, about the same as the Earth’s velocity in orbit around the sun, equating to specific impulse of 2800 seconds. A six engine array of such thrusters could provide about a Newton of thrust from a 25 kilowatt power source such as a solar array.
You can see where this might be going – or how far. For solar system exploration, such a system would be limited to regions close to the sun and not eclipsed by other bodies. Back in 1980 solar arrays had efficiencies of less than 10% based on solar flux intensities of 1370 watts per square meter. If, for example, a solar array generated 125 watts per square meter, then 25-kilowatts meant a 200 square meter ( 2150 square feet) array to run an engine that provided less than a pound of thrust. To duplicate a 100-megawatt nuclear power source, adequate for a human mission to Mars, a solar array 4000 times larger would be required. But the benefit with either power source would be space propulsion with a thrust of thousands of pounds. Currently there are efforts afoot to build such spacecraft and the VASIMIR rocket engine developed by former astronaut Chiang-Diaz is a 21st century example of the state of the art. The VASIMIR engine has been suggested as an alternative spacecraft approach for dispatching a human expedition to Mars. With a nuclear power source and prolonged burns at moderate levels of thrust, the one way trip time to Mars could be to one half or one third from its nominal 240 days, considerably lowering human exposure to space radiation and absence of gravity. It would likely make travel to Mars more than a one time ultimate human adventure if the expenses foreseen for chemically propelled missions could be reduced as well.
I hear the faint ring of a carriage return. It sounds like the ghost of a Smith Corona portable typewriter.