Nuclear Electric Drive

In the previous edition, I covered the Nuclear Thermal Rocket - a simple and reliable form of nuclear propulsion. The fuel efficiency of the NTR is double than that of an ordinary chemical rocket but the energy density of uranium is hundred times greater than the energy density of chemical fuel (such as hydrogen). We should be asking the question - why? The answer lies in the design of the NTR - it is a "thermal" rocket - it ejects its own coolant into space to generate thrust, this is good if you want to build a lightweight engine, but it is not efficient.

To take advantage of the real energy density value of nuclear fuel, we must first generate electricity from it. There are two common ways to accomplish this task.

The first way is to take a batch of uranium and stick a special electrode on it (heat-source) and stick another identical electrode on a cold-surface of the spacecraft (heat-sink). Now if we connect the two electrodes with a wire, we get a potential difference - this works like a battery and is called a Radio-isotope Thermal Generator (RTG). RTG's have been used before and will be used for decades into the future, powering old spacecraft like the Voyager and new soon-to-fly spacecraft like the Titan Moon exploration octocopter - Dragonfly. RTG technology works great on small probes but does not scale-up well. So, if we want to push large cargo-ships, we must look for yet another way to extract electricity.

The second method is to have a fully-fledged nuclear power plant onboard the spacecraft. Coolant that passes through the nuclear core does not get ejected like in an NTR, instead, the coolant first passes through a turbogenerator or a sterling engine to generate mechanical power (converted to electricity via an alternator). The coolant is then recirculated back to the reactor through a set of radiators. This is how you get a Nuclear Electric Drive (NED). It is a large, heavy, and complex machine and produces much less thrust than a thermal rocket, but it is significantly more efficient. As a result, a spacecraft powered by an NED has low acceleration, but it can operate for years while transporting heavy cargo. Since the NED utilizes plasma thrusters (also known as "ion engines"), it too can work on a variety of fuels, some of which are easier to store than liquid hydrogen (typically noble gases are used like Argon, Neon, or Xenon).

Ultimately, harnessing nuclear power, specifically in the form of fission, has many benefits for a space-faring civilization like ours. However, there is no single engine design that works optimally for all applications. Each design is ideal for a specific application. I can certainly see a future where all nuclear propulsion designs work hand-in-hand, a "nuclear trifecta": (1) small probes are powered by compact Radio-isotope Thermal Generators (RTG), (2) to minimize exposure to space-based radiation we utilize Nuclear Thermal Rockets (NTR) for quick passenger transport, and (3) Nuclear Electric Drives (NED) are used to slowly but efficiently haul heavy-cargo within the inner and outer solar system.

#nuclear #electric #propulsion #NEP #drive #NED #radio #isotope #thermal #generator #fission #atomic #powerplant #energy #power #electricity