If ludicrously powerful rocket engines are what you're after, science fiction tells us there's no better place to look than nuclear energy. The funny thing about nuclear rockets being they're not strictly science fiction and haven't been since the late 1960s. That's right, folks. No matter if hardly anyone remembers it today, NASA's first nuclear fission engine, the Los Alamos NERVA, proved nuclear-powered spacecraft propulsion was possible on a scale large enough to carry humans.
Alas, the NERVA project was all but thrown in the trash by the Nixon administration. One which quickly became openly hostile to the American space program after the prime objective of beating the Soviets to the Moon was achieved in 1969. But that doesn't mean the same technology couldn't become vital to the next generation of nuclear rocket engines.
But how does a nuclear space rocket work in the first place? Well, it's surprisingly straightforward when you dissect the nuts and bolts. Most who paid attention in science class know the difference between nuclear fission and fusion. I.e., the difference between splitting radioactive atoms apart vs smashing them together and siphoning off the excess thermal energy to convert to electricity or propulsion, depending on the context.
While fusion's made leaps and bounds of progress in the 21st century, it will be a fission engine that will become NASA and DARPA's focal point for the next generation of nuclear engines. With this in mind, we can deduce quite a bit about what this future engine might look like. In truth, the thing could look a fair bit similar to the nuclear engines of old.
In its most basic form, the NERVA engine worked on most of the same working principles as a typical rocket engine. A normal rocket uses a mixture of combustible fuel and oxidizer fed into a combustion chamber via a series of turbopumps and ignited. The resulting explosion is directed out an exhaust nozzle to produce very high propulsive thrust. A nuclear fission engine is much the same.
In a fission engine, liquid propellent, usually cryogenic hydrogen, is still present. The key difference is that the combustion chamber now houses a fission reactor core. This core consists of radioactive isotopes, typically of uranium 235 or 238, which are mixed with a neutron moderator like graphite to keep the fission reaction from going from critical to supercritical.
This especially spicy mixture is fed into a special machine which forms it into fuel rods through which hydrogen fuel can flow through and make contact with fissile material. Once the hydrogen fuel meets the fissile core, the reactor itself does most of the rest of the work itself. The radiation-induced combustion of hydrogen fuel is much more powerful than traditional rocket fuels on their own, as Los Alamos Laboratories in New Mexico discovered for themselves with project NERVA.
As for what type of vehicle this novel engine could be applied to, that's a bit more up in the air for the time being. Artist concepts courtesy of DARPA appears to show a rocket's upper stage carrying what might be an Orion spacecraft like the one launched on Artemis I back in December. This is the most logical application of a nuclear rocket engine, as modern superheavy-lift rockets like the SLS are more than adept at heavy lifting from the Earth's surface with their own chemical first and second stages.
With this in mind, it's clear the name of the game here is making interplanetary travel just that much faster, more reliable, and with the benefit of modern safety precautions, potentially safer as well. With the future of NASA's Artemis program mostly unknown after the first five missions, it's anyone's guess if NASA/DARPA's fission rocket will have any bearing on a potential mission to Mars or the Asteroid Belt in the not-so-distant future.
Lockheed Martin has expressed in the past that they already deem the Orion Crew Module as capable of going to Mars with the right equipment behind it. It's still going to be some time before they'll be able to prove that statement definitively. But after witnessing how Artemis I's Orion capsule circumnavigated the Moon on the first time out and made it look easy, we have no reasons whatsoever to doubt these claims.
Block 1B and Block 2, respectively. As neither of these upgrade packages is set in stone yet, the addition of a nuclear second or third stage is still well within the realm of possibility.
As of right now, expect the first prototype engines to be revealed sometime in late 2025 or early 2026, based on the first estimated test date sometime in 2027. We're hoping it goes according to plans better than NERVA did 60-ish years ago. One thing's for sure, space travel enthusiasts the world over are still brooding at Richard Nixon for axing the Apollo program and NERVA by association. This time around, he's not around to stop it.
No word yet on what role DARPA will play in this whole operation. But this being a special interest research group for the military, most of that information is likely to be highly classified. But rest assured. The extra support is no doubt welcome by NASA. At least this time, they have the Pentagon in their corner. Did we really just end on a math joke?