Nuclear thermal propulsion is back!
Let's hope it's here to stay
by Brian Scott Pauls
After efforts that have been on-and-off (but mostly “off”) for more than 60 years, the US is again resuming research into nuclear thermal propulsion (NTP). This is the most practical technology that would allow us to settle the Solar System in the near future.
NASA, in conjunction with the Department of Energy, has awarded three 12-month contracts, “each valued at approximately $5 million”, for the development of nuclear thermal reactor designs “for the specified performance requirements that could aid in deep space exploration.”
If we hadn’t abandoned similar efforts in 1973, we might be on the moons of Jupiter and Saturn right now.
In the science fiction of the 1940’s and 1950’s, authors often assumed spacecraft exploring/settling the Solar System would use NTP. The eponymously named ship in Robert A. Heinlein’s first juvenile, Rocket Ship Galileo, uses this form of propulsion, as does the spaceship in the movie loosely based on that book, Destination Moon. NTP was the technology that drove our dreams of humanity spreading out to the planets, making ours a truly space-faring civilization.
Why is NTP so great? In two words, specific impulse (Isp)—the measure of a rocket’s efficiency.
Whatever a rocket expels out the back, to move itself forward in space, is called working mass. Rockets which create a small amount of thrust for a particular amount of working mass have a low Isp. Rockets which create a larger amount of thrust for the same amount of working mass have a higher Isp. The higher the Isp, the greater the rocket’s efficiency. This directly affects the size of the rockets we can send into space, and how quickly they can reach different destinations once they leave Earth.
Chemical rockets provide only a moderate Isp. The SpaceX Falcon Heavy second stage has an Isp of 348 seconds (the unit used to measure Isp), the Saturn V third stage had an Isp of 421 seconds, and the RS-25 Space Shuttle main engines had an Isp of 452 seconds.
Previous US research into NTP began with Project Rover in 1955, and continued until NERVA ended in 1973, culminating in the NERVA XE engine, which produced an Isp of 710 seconds in tests. Engineers concluded practical NTP would be more than twice as efficient as chemical rockets. Although the NERVA XE was judged ready for flight, it was never used. The program was cancelled due to budget cuts.
The US government revisited theoretical research on NTP in the 80s, the 90s, and 2013. While this research resulted in more efficient designs, none of these projects produced a working rocket engine like the NERVA XE.
The projects just awarded won’t produce working models either, but they could lead to one (or more) if NASA and the DoE consider any of the efforts successful.
Current travel time to Mars, using chemical rockets, is 8-9 months. With new designs targeting an Isp of 900 seconds, NTP could cut that in half.
NASA’s budget cuts in 1973 deprived us of the science fiction future promised by NERVA. We have another chance to get it right.
It’s time to go all the way.