Nuclear rockets but not nuclear rockets
In the late 1970's NASA was working on nuclear rockets. They concluded that nuclear energy was the only way to bridge the gap between requirement and achievement. They even made a working prototype engine called Nerva and conducted some preliminary tests. A file photograph from testing of Nerva can be seen below.
NASA then decided to stop Nerva after realizing the hazards behind Nuclear energy and how much catastrophic it could be if things leaned towards the worst. The project was dropped immediately. Nasa started using cryogenic, semi cryogenic and standard solid, liquid boosters for all of their missions, yet there was a small gap that had not yet been bridged. The gap was the fact that there was no reliable propulsive device that could take us beyond Mars or Jupiter. A lot of space probes had gone there without the need for recovery. But if we wanted to increase the longevity of the mission or required something more reliable, electric propulsion was the answer.
That brings me to the title of the article. Why does it say "Nuclear rockets but not nuclear rockets"? What did I mean by that? Did I have some hidden agenda or a witty anecdote perhaps?
The truth is a little more complicated. Since the use of nuclear energy as a direct propulsive source was altogether cumbersome and risky, NASA took an indirect route. They decided that a Nuclear reactor could be used as a power source for electric/plasma thrusters. This decision is brilliant in the sense that plasma thrusters are an ideal candidate for deep space propulsion and no chemical rocket could beat that in terms of specific impulse or the exit velocity or propulsive efficiency. But the plasma thrusters always had a more significant power requirement. Standard space-qualified solar panels or solar sails might not and most certainly will not be able to satisfy this power requirement. But the power output of nuclear reactors could do this. They could keep all the systems alive while allowing the plasma thrusters to do their job. They could even be used to preheat the propellents by circulating them around the wall of nuclear reactors, thus reducing the heat and increasing the enthalpy of the fuel. It is a well-known fact that preheated fuels are far easier to ionize, which makes it a win-win situation and reduces the weight of the overall mission.
The image shown above is an MPD thruster with a magnetic nozzle at the exit. These are very efficient at orbit correction, and interplanetary manoeuvres provided their power requirement is satisfied. Since Nuclear rockets can build a bridge between this gap, there is very little to worry about the deployment of these thrusters in space. MPD thrusters are not the only class of plasma thrusters. But I specifically brought them into the discussion because they lie under my umbrella of research. In future, I will be able to share more details about the specifics of my work. But it is cool to know that we started from an idea of actually using Nuclear Bombs as propulsive devices to something Not Exactly Nuclear Bombs. This leads me to conclude that the future of space exploration will see Nuclear rockets but not exactly Nuclear Rockets.
Update: We wrote an article out of my research work on MPD thrusters. The title of the article is "A new mathematical model for studying fully ionized plasma flows in MPD thrusters". It is an interesting read.