Nuclear Thermal Propulsion
Unlike most things at Beyond NERVA, nuclear thermal propulsion (NTP), otherwise known as the nuclear thermal rocket (NTR), has been built and tested by both the US and the USSR. Both programs looked at a solid core NTR, the simplest type of nuclear rocket. In a nuclear thermal rocket, the heat from a reactor is used to directly heat a propellant, which is pushed out a nozzle at the back of the spacecraft. This propellant isn’t burned, just heated, so there’s no need for the fuel to even be able to burn. However, the lighter the propellant, the more efficient the rocket, since the same amount of energy can make a smaller molecule travel faster than a larger one, and this is proportional to the atomic mass of the molecules that make up the propellant. Because of this, hydrogen is usually chosen since it’s very lightweight, but ammonia, CO2, and others have been proposed over the years as well.
NTRs are usually broken up by the state of matter of the fuel within the reactor: solid, liquid, gas, and some in-betweens like vapor core reactors.
Solid fueled NTRs, like in the rockets tested in Project Rover (US) and the RD041X (USSR), are closest in form to the reactors used on Earth. However, solid fuel has limitations, mainly with how hot it can get before melting down. There’s lots of variants on this, using different shapes and materials for fuels, but in general they aren’t as efficient as a rocket that could reach higher temperatures. They are still significantly better than chemical engines, with the NERVA NRX getting a specific impulse of 825 using H2 propellant, and the Russian RD0411 getting an isp of about 900. There are other designs, like pebblebed reactors, that can do even better: the Project Timberwind rocket was supposed to break 1000 seconds of isp, and still other designs offer improvements here as well.
Recent NASA research into NTP has focused on lowering the enrichment required for the fuel by using a new form of fuel, a ceramic metal or CERMET. This is just one of several different types of fuels that can be used in an NTR-S, which are looked at more on the main page.
Liquid fueled NTRs have been proposed as a way around the temperature limitations of solid fuel: don’t fight melting down the reactor, do it on purpose and plan for it! This is more difficult to pull off, but there are some designs that have been proposed over the years that make this possible. These designs can run much hotter than a solid core NTR, and so are more efficient, but the materials you make the reactor itself out of are going to melt at some point if you make it too hot, so there’s still limits to how good this rocket can be.
Two main types of LNTR exist, the bubbler LNTR (where the propellant passes through the molten nuclear fuel), and the radiator LNTR (where the propellant passes across the surface of the molten fuel only). While no reactor of this type has undergone nuclear testing, both the bubbler and radiator types have been studied in some depth – even though their design and history has largely been forgotten.
To achieve even higher temperatures, sometimes the fuel is turned into a gas, or sometimes a plasma. This is a gas-core nuclear thermal rocket, and there’s two varieties with very different advantages and disadvantages. The first is called the closed cycle gas core NTR, also sometimes called the “nuclear lightbulb.”
This type of engine uses a vortex of a buffer gas to hold fissile material inside a silica bulb, with the wavelengths of energy coming off the nuclear reaction tuned to be absorbed by the propellant, not by the containment structure. This offers far higher specific impulse, as well as (theoretically) far higher fuel burnup due to onboard reprocessing and recycling of the fuel (a necessity for this type of engine.
Sadly, this adds mass and complexity to the design, the two greatest hurdles for this system to overcome.
Open Cycle Gas Fueled NTP
The final concept is often considered the crown jewel of nuclear thermal propulsion, but also by far offers the most challenges, and is also the least developed in terms of both experimentation and development into a working prototype.
This engine again uses a gas or plasma of a fissile material, but uses no solid containment structure between itself and the propellant. Two main containment modes are used: either using gasdynamic pressure, using the flow of the propellant, as well as the inertia of the fuel itself, to contain the fuel; alternatively, electromagnetic containment, using powerful (10+ tesla) magnetic fields, has been proposed as well.
This is the first engine on the list that meets the common impression of nuclear spacecraft: with a specific impulse of over 9000, thrust of up to 3 MN, and a thrust-to-weight ratio of over 30, this is a rocket that can propel truly massive spacecraft very fast across the solar system.
Sadly, due to the extreme challenges of containing the fissile fuel and fission products without a solid barrier, loss rates of fissile fuel are unworkably high for gasdynamic systems (with estimates being well above 90% fuel loss), and electromagnetically confined systems require field strengths beyond what is practical for a spacecraft in the near future.
While I have not had time to cover this system in any depth, Winchell Chung has done extensive research and writeups on the various proposals over the years, available here:
Multi-Modal Nuclear Propulsion
Some NTR designs also incorporate electricity production capability. While this adds mass and complexity to the reactor system, it also eliminates the need for additional systems to provide electrical power for the spacecraft. Many different ways have been proposed in the past, and they’ll be looked at more in depth as time goes on.
Bimodal Nuclear/Electric Propulsion
The most common type of bimodal design is meant to use both thermal and electric propulsion in the same spacecraft using the same reactor. This way, the higher thrust thermal propulsion can be used for major burns, such as leaving or entering orbit around a planet or moon, and the electric propulsion can be used in what would normally be the cruise phase to decrease overall mission time.
Whether this type of rocket engine is a good choice depends on how much mass the electric propulsion system adds, and the challenges of integrating additional cooling capability into the reactor can be daunting. Many different options have been proposed over the years, each addressing these tradeoffs and challenges differently.
Bimodal Thermoelectric NTP
A relatively recent design looks to overcome the thermal limitations of solid fuel to increase the specific impulse of an NTR, by adding a thermal induction heater downstream of the reactor core. This is the Nuclear Thermal Electric Rocket, under development by Dr. Dujarric and his team at ESA. Other designs have looked at using electrically heated thermal rockets run off a nuclear reactor, but often those are tentative designs for reaction control systems on nuclear powered space stations and the like, and have tended to be small systems. However, this sort of bimodalism is very rare.
There is one final type of hybrid NTR to mention, and that’s the Trimodal NTR. This is mainly explored in the TRITON concept, but there have been some other proposals. Here, a relatively typical bimodal NTR with both thermal and electric porpulsion systems has an oxidizer afterburner in the same way that the LANTR has. This allows for a wide range of thrusts and specific impulses, but at the cost of adding the mass and complexity of not just one additional propulsion system, but two, and an additional independent cryogenic storage, pumping, and emission system as well.