The Nuclear Option - to Power Spacecraft

Eric Berger, the space correspondent at Ars Technica takes a dive into the complexities of interplanetary missions, and especially powering the missions to Mars, or the outer planets.  The overriding problem in such missions is that the mission must carry everything they can anticipate needing.  The biggest problem is the fuel they need to get there.

His conclusion is that nuclear engines are the only realistic way to do the mission.  Even when Mars is at its closest, and the missions launch at the optimum time to get there in the shortest travel times, the fuel requirements are astounding.  Think 1,000 to 4,000 metric tons of propellant (include the oxidizer) if they use chemical-burning engines.

If that’s difficult to visualize, consider this. When upgraded to its Block 1B configuration, NASA’s Space Launch System rocket will have a carrying capacity of 105 tons to low-Earth orbit. NASA expects to launch this rocket once a year, and its cost will likely be around $2 billion for flight. So to get enough fuel into orbit for a Mars mission would require at least 10 launches of the SLS rocket, or about a decade and $20 billion. Just for the fuel.

The 105 tons to orbit is less than 1/10 of the lowest estimate for fuel required.  Bear in mind this is the biggest rocket NASA has come up with.  You might recall that there's currently a program in which NASA contracted with SpaceX to work on refueling in space.  It's not something that can currently be done and nothing has been tested. 

When faced with a decade of getting fuel into low earth orbit and $20 billion in costs, it's time to re-think how you're planning your missions.  NASA requested that the National Academies of Sciences, Engineering, and Medicine study the various options for mission to Mars in 2039.  They've issued a report that looks at the options for nuclear power and considers them the optimum solution.

The committee was not asked to recommend a particular technology, each of which rely on nuclear reactions but work differently. Nuclear thermal propulsion (NTP) involves a rocket engine in which a nuclear reactor replaces the combustion chamber and burns liquid hydrogen as a fuel. Nuclear electric propulsion (NEP) converts heat from a fission reactor to electrical power, like a power plant on Earth, and then uses this energy to produce thrust by accelerating an ionized propellant, such as xenon.

"If you look at the committee's recommendations for NTP, we felt that an aggressive program, built on the foundational work that's been accomplished recently, could get us there," Braun said of the Mars 2039 goal. "For NEP, we felt that it was unclear if such a program could get us there, but we did not conclude that it could not get us there."

Nuclear propulsion requires much less propellant, although for any mission far enough/long enough propellant is one of the major concerns.  Eric Berger uses a "planning number" of 500 metric tons, less than the lowest estimate for chemical propellants.  

This, of course, is not a new discovery of a surprise problem.  It has been known by anyone who has "done the math" to look at how to design rockets for deep space missions.  In the early days of the space program, the 1950s and '60s, there was a lot of work in this area, and programs like NERVA (Nuclear Engines for Rocket Vehicular Applications) were extensively explored. 

This is an artist's conception of an NTP mission to Mars from NERVA in the '60s. NASA image.

I think everyone concedes it can be done, it's more a question of political will and other priorities competing for the same funding.  Bobby Braun, mentioned earlier is the director for planetary science at the Jet Propulsion Laboratory and co-chair of the committee that wrote the nuclear report.  Braun thinks this is the kind of thing NASA was created for, and a billion or so a year would get the results needed to realize nuclear propulsion.

An interesting question, then, is what about SpaceX and Starship? 

The project seeks to address the problem of needing a lot of chemical propellant by developing a low-cost, reusable launch system. SpaceX engineers know it will take a lot of fuel to reach Mars, but they believe the problem is solvable if Starship can be built to fly often and for relatively little money. The basic concept is to launch a Starship to orbit with empty tanks and transfer fuel launched by other Starships in low-Earth orbit before a single vehicle flies to Mars.
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That's not to say Starship cannot work. However, it does illustrate the challenge of mounting a mission to Mars with chemical-only propulsion. To use traditional propulsion, one needs to push the boundaries of reuse and heavy lift rockets to extreme limits—which is precisely what SpaceX is trying to do with its fully reusable launch system.

OK, so they can get to Mars; now how do they get back, or is it a one-way mission?  The answer is to make their own methane and oxygen on Mars, and they're studying that, too.  This is a place where the lower gravity and thinner atmosphere of Mars help the mission.