Thursday, 14 October 2021

Nuclear Conversion for Starship

There has been much discussion about converting the SpaceX Starship to use nuclear propulsion. It would allow for a great increase in specific impulse and a massive extension of mission capabilities.

But is it actually worthwhile?

The image above is modified from BocaChicaGal’s photo.

Nuclear thermal rockets do indeed have impressive performance. Their specific impulse is up to three times greater than chemical rockets, they produce comparable amounts of thrust and they could be designed to accept a variety of propellants, from CO2 to ammonia. They can reduce travel times in space, push around much larger payloads and get refuelled with whatever fluid is available at their destination.

Full scale mockup of the NERVA engine.

For these reasons, they are lauded as the best way to accelerate human exploration and expansion into space. 

They are not a new technology either. The idea to use nuclear energy to propel spacecraft dates back to 1944. Serious testing has been done on nuclear rockets, with ground tests in 1955 and functional gigawatt-scale rockets firing for several minutes by the 1970s. The Strategic Defense Initiative rekindled studies into nuclear propulsion with Project Timberwind, a program that ran until 1991 and resulted in modern designs that were even more capable. 

We are now in the midst of another revival of this technology. Millions have been awarded to BWXT, General Atomics and Ultra Safe to restart the development of nuclear rockets. 

Naturally, there have been calls to combine the capabilities of nuclear rockets with the other great aerospace development of our time, which are reusable rockets and their champion; SpaceX’s 9m wide Starship and its SuperHeavy booster. Then, we would acquire the ability to send even heavier payloads to orbit and beyond.

The modern nuclear rocket

The BWXT design.

The design and performance targets for nuclear propulsion has shifted considerably over the last few decades. The initial efforts in the 1970s were straightforward in the desire for maximum power and thrust.

The reactors from Project Rover.

Project Rover, for example, resulted in the Phoebus-2A reactor that managed an output of 4000 MW for 12 minutes during a test run. As a fully developed engine, it would have managed 5000 MW and 825s of specific impulse. It would have held 300 kg of enriched uranium and had a relatively cool core temperature of 2300 K (although the goal was 2500 K). Total mass was 9300 kg, meaning it had an impressive power density of 537 kW/kg.