Nuclear Propulsion and Power Generation in Space

Hello world, my name is Dave, and I hope you enjoy the ride.
When we think of rockets and space we think of big billowing flames jetting out the back of a massive rocket, hurling a payload into orbit. We think of small bursts of combusted fuel nudging the space shuttle into a proper orbit. And the case is closed, that is what space travel is, burning of some high energy oxidizer and fuel combo. What if I told you there was a more efficient way?

Nuclear power is not just for generating electricity here on earth. Several different designs have been tested here on Earth with great success. A quick Google search (or Bing, if that tickles your fancy…) of projects like Transit 4a and Cassini will show that America (and a few other countries) have been using Nuclear power in space for a long time. In these cases we are using Radioisotope Thermoelectric  Generation (RTGs), a process where we use the natural decay of some fancy element like Plutonium to generate electricity through the Seebeck effect (turning heat into electricity). These little guys are great for providing power to a satellite or a rover. Even the Curiosity Rover uses this process to generate the juice needed to roll around Mars, although it is technically a different system. The RTGs reduce a probe/craft’s reliance on the sun for solar power because the sun isn’t as bright at the edge of the solar system.
That is all well and good, for sending robots and unmanned probes out to the far reaches of the Universe, but what happens when we want to put people out there? The biggest obstacle we face it Time. Prolonged space travel increases the radiation dose our astronauts receive, which increases the potential for cancer.
So how do we reduce the time spent traveling the solar system?
We go faster.
How do we go faster?
We make a bigger boom at the back of the rocket.
How does one make a bigger boom?
By increasing the Specific Impulse of the rocket, or burning more fuel faster and longer.
That sounds like too much science. Is there an easier way of explaining it?
Funny you should ask. Yes, yes there is. The specific impulse is a measure of the thrust generated per rate of fuel consumption (shown below in Equation 2).
Traditional chemical rockets are governed by the ‘Rocket Equation’ (below in Equation 1). It essentially says that to go faster we need more fuel to lift the weight of the rocket. But then we need more fuel to lift the fuel. And more fuel to lift that fuel….  Thankfully the rocket uses fuel as it flies and most common models jettison the used booster rockets so the weight of the overall system is reduced.

Rocket equation and Specific Impulse
Rocket equation and Specific Impulse

Theory is all well and good, but how does this add up practically? Again there is a bunch of science surrounding the answer, but it comes down to the amount of energy available in the fuel.
Relative energy density for various fuel sources
Relative energy density for various fuel sources

Looking at this fancy figure we can see that Nuclear power has far more energy available than all other fuel sources. Then looking at more science and math we can come to the conclusion that a Nuclear rocket/spacecraft using liquid Hydrogen as the propellant we can pretty much double the specific impulse of a Chemical rocket. The implications of this are literally far reaching. The higher specific impulse means the spacecraft can fly faster for a given fuel load, or fly farther.
This will reduce the time for a Mars transit to a couple months rather than almost a year, which will reduce the dose astronauts receive. But how does one design a super high tech jet fighter  spacecraft? Lucky for us NASA has about 20 something different designs on the books, most of which already have some sort of prototype built and tested. They all operate under the same basic principles; the reactor heats up and gets stupid hot, liquid Hydrogen is pushed through the reactor, the Hydrogen also gets stupid hot and expands to a gas, Hydrogen is then expelled through an itty-bitty nozzle and BAM we have thrust.  This nifty diagram shows the basic anatomy of the most promising design, NERVA.
Schematic of NERVA rocket internals
Schematic of NERVA rocket internals

Now the question that always surrounds nuclear power in all forms is: ‘What happens if this thing gets blown to bits and is scattered to the wind?’ The answer is not a simple one, and there are multiple agencies out there that are responsible for the safe testing and operation of the RTG spacecraft which would also govern the nuclear spacecraft… Long story short, we would need to get the reactor into space pre-criticality, meaning the reactor would have never been brought to the point of fission before its launch. This drops the amount of harmful radioactive particles to almost zero, which is far less than the RTG method that uses an active radioisotope at launch. So, if a nuclear spacecraft were to undergo some catastrophic accident on liftoff, the nuclear fuel would not create a death plume and contaminate the world. But where does that leave us? It seems we still need chemical rockets to get this beast into space safely.
Fortunately for us, the cost of pushing things into space is coming down. SpaceX has their reusable Falcon 9R in the works, and they have demonstrated that a rocket can be guided back down to Earth in a relatively stable manner. Chemical fuel is pretty cheap (around $17/kg) and the fuel quality is increasing based on the fuel mixture used.
Unfortunately the whole nuclear propulsion and power generation in space thing is always on the chopping block. It seems that every few decades NASA gets the funding to push forward on this, but then a few years later they get shut down. Most of the research into this form of propulsion was done in the 60s and 70s. After we put people on the moon America kind of forgot that space is a bit bigger than the distance between here and the moon, so the funding was cut. Then in the 90s the funding was there again, so NASA started up Project Prometheus… but that was killed in the early 00s. Now, with the advent of the private space companies, it seems that someone will look up these old tests and design a core for interplanetary travels. This thing could best be used as a ferry between worlds. Each launch could carry more Hydrogen up to the ferry and thus is born the interplanetary shuttle service.
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