How the Atom Can Split Martian Travel Time
The promise of nuclear rocketry
Once you're in Earth’s orbit you're halfway to anywhere. To bridge the other half, in today’s “New Space” era, engineers are dusting off a decidedly old-space idea: Nuclear Rockets. Indeed, Nuclear Thermal Propulsion offers a compelling means of making travel to Mars and other far-flung destinations faster, cheaper, and safer.
Background
Space travel is changing. Companies like SpaceX, Blue Origin, Axiom…etc are finding ways of reducing the cost of access to, and living in, space. As we discussed here, the era of the reusable rocket, once a distant dream, has arrived. Thus far, however, we have only achieved partially reusable rocket systems.
We have yet to master the holy grail of space access, that is, fully and rapidly reusable rocket systems, though SpaceX is well on the way. Indeed, by 2030, rocket launches could resemble modern airline flights. They launch, land, undergo inspections/refueling, and launch again with minimal refurbishment.
This is great for access to Earth’s orbit, and perhaps the Moon, but not much else. To go to any other destinations, we need some form of “transfer” stage. Ideally, the transfer stage would be a rocket parked in orbit that could be reused/refueled by visiting spacecraft dozens of times. In the vacuum of space, the key would be designing something that utilized fuel extremely efficiently. Nuclear Thermal Propulsion, or NTP, fits the bill.
How Nuclear Thermal Rockets Work
Chemical rockets work by mixing a fuel (often kerosene, hydrogen, or methane) with an oxidizer (oxygen) and igniting it in an engine that compresses the reaction and directs the resulting high-pressure gas out of the nozzle, pushing the craft forward.
NTP works in much the same way, but forgoes the oxidizer and uses nuclear energy, courtesy of a small nuclear fission reactor, to heat the fuel (often hydrogen) instead. The superheated fuel expands, shooting out the nozzle, and providing forward thrust.
Nuclear rockets cannot, and would not, be used on Earth, their thrust-to-weight ratio is too low and the safety concerns, are too high. Instead, they would be launched into orbit and used solely to transfer spacecraft to distant locations. They are well suited for this role because of their raw efficiency.
Rocket engine efficiency is measured in terms of Specific Impulse, or ISP. Traditional rocket engines typically have an ISP of 250 to 300, but the most efficient engines can achieve as high as ~450. But nuclear rockets could easily breach an ISP of 900, making them at least twice as efficient as the best chemical rockets possible.
Proven Technology
NTP is not a pipe dream. On the contrary, NTP research was quite active in the United States from the 1950s into the early 1970s. Several prototype demonstration engines were test fired, proving that the technology, although immature, is certainly viable.
NTP technology is an ideal candidate for government support due to insufficient private incentives to develop the technology. With the emergence of new, larger rockets, and cheaper access to space, NTP research has gained renewed importance. DARPA, the organization that gave us the internet, recently launched the Demonstration Rocket for Agile Cislunar Operations or (DRACO) Program.
The DRACO program, it is hoped, will finally take nuclear propulsion beyond the ground testing stage, lofting a demonstration engine into orbit around 2025 to prove the technology in flight.
Safety
As with anything with the word “nuclear” in the name, safety concerns abound. But despite the negative connotations, nuclear energy is by and large, safe. New technology, such as improved Tristructural isotropic (TRISO) particle fuel designs, promise to make nuclear reactors safer than ever before.
TRISO isn’t necessarily a new idea either. Essentially, TRISO fuel consists of poppy-seed-sized particles made of a uranium, carbon, and oxygen fuel “kernel.” The kernel is encapsulated by three layers of carbon and ceramic materials that serve as a self-containment system. These layers of material prevent the release of the fissile material under all conditions. That is, they cannot melt in the reactor should something go wrong.
Indeed, new technology will enable TRISO fuel to safely be brought to temperatures seven times higher than conventional nuclear reactors, with no meltdown risk. This is key to the efficiency and safety of future NTP engines.
Implications
Should we challenge ourselves to revisit and perfect NTP technology, the benefits cannot be understated. With their high efficiency, they could reduce the travel time to Mars by up to 25%, shaving months off a journey that exposes astronauts to harmful cosmic radiation. NTP could also open new launch windows that are less dependent on orbital dynamics, and render the possibility of previously untenable abort modes, should something go wrong aboard the spacecraft during transit.
With the emergence of fully and rapidly reusable rockets, access to space is becoming orders of magnitude cheaper and easier. The future of humanity, however, cannot be sustained in orbit, or even on the Moon. At the very least, we will need to make Martian travel a reality. This reality becomes much easier with new propulsion technologies that can take us there quicker, safer, and more frequently.
The main safety concern with this would be in getting the nuclear fuel to the space craft. Initially, at least, it would have to be transferred from earth. Full containment is very heavy. A nuclear fuel container, as used for road transport weighs 20 tonnes not including the fuel. Uranium is also one of the densest materials on earth, with a density of 19.1 g/cm^2.
How would they mitigate a disaster during launch and the subsequent potential nuclear material dispersal?