Fission rockets could open up the solar system


Although the nascent commercial space industry is focusing almost exclusively on chemical rockets, the inefficiency of chemical propulsion places serious limitations on orbital and extra-orbital activities. The most efficient liquid propellant combination, employing liquid hydrogen and liquid oxygen, requires cryogenic storage and large hydrogen fuel tanks. Fission rockets are substantially more efficient than chemical rockets, are safe to operate, and could be used to send humans to mars. In an interview with Sander Olson, fission propulsion advocate Tabitha Smith argues that fission rockets could be rapidly developed and become the enabling technology for opening up the solar system for human exploration.

Tabitha Smith

Question: The U.S. had a nuclear program in the 1960s called Nuclear Engine for Rocket Vehicle Application (NERVA). Whatever happened to that program?

NERVA began in the early 1960s and continued until the program was cancelled in 1972. It met its goals and flight qualified the Nuclear Thermal Rocket (NTR) but President Nixon decided to pull the funding using NTR for a Mars mission to put it into the Space Shuttle program, using the Shuttle to launch large classified spy satellites into orbit for the US Air Force and other such missions. Some of the original plans using an NTR called for using NERVA rockets to put a human on Mars by 1978 and a permanent lunar colony by 1981. People who study the concept of lunar colonies and Mars missions in depth invariably come to the conclusion that nuclear propulsion (either thermal or electric) and electric propulsion powered by nuclear becomes the only technologically feasible options for propulsion, that have reached a stage beyond “theoretical” and/or that has actually be tested and qualified on a large scale.

Question: You have just participated in the creation of a new company, General Propulsion Sciences, LLC , to develop new forms of space propulsion. How is that going?

GPS, in addition to NTR R&D, does electric and plasma propulsion and also manufacturers microthrusters for cubesats and other small spacecraft. The company is also expanding capabilities to include Hall Thrusters. The aim of GPS is to hasten the furtherance of advanced propulsion R&D in order to enhance the capabilities of spacecraft (from very small spacecraft to very large, man-rated space craft). Although nuclear propulsion won’t garner any profits in the near term, we believe that NTRs offer the only viable way to get to Mars, and that they would be nearly ideal for any missions beyond near earth orbit.

Question: Various proposals for employing NTRs have been discussed. Which one do you support?

The simplest (and most “near term”) mission architecture involves using the NTR as a second stage rocket with a chemical first stage to leave the Earth’s atmosphere; then the NTR, carrying the payload, would activate its engine to take it beyond orbit, off to the particular mission’s destination. In our opinion, and according to research, this is the most logical method, using readily available technology, (rather than using the conventional chemical rockets of today) for sending large payloads (involving people and their habitats) to destinations beyond the Earth-Moon system

Question: Could an active NTR be used, to launch directly from Earth to space?

It could, but due to the expenses involved with open-air testing, expensive potential lawsuits from anti-nuclear activists, and other variables that would subtract money from the mission and force the dollars into the courts, , it is unlikely that a nuclear rocket to orbit will ever be built, at least in the United States. So the plans of the NTR community at large envision using a conventional chemical engine for the first stage, and launching the NTR and/or the parts for an NTR-driven spacecraft in the upper stage into space, away from Earth.

Question: Could you use the Falcon 9 Heavy rocket to deliver an NTR to space in its second stage?

Yes, our NTR can be assembled in orbit, using one or two Space-X Heavys to deliver the parts. Using a modified Falcon 9 Heavy using only chemical propellant, at most only 11 tons could reach the Mars surface, and that would take at least a year. By contrast, an NTR powered spacecraft (delivered to orbit using a Heavy-lift rocket, though ideally the Ares V or SLS) could deliver more payload to the surface of Mars more quickly – in approximately half a year or less. Expedited flight time is more humane – it’s better for the health of astronauts, which depletes rapidly the longer they are exposed to the natural radiation of space, low gravity and bone loss, and psychological harm caused by long-term spaceflight in cramped quarters. Also, an NTR can deliver a larger payload to Mars which means a larger habitat module – so you’re talking about a small capsule with relatively cramped quarters (like the one or two room Orion capsule) versus an actual habitat that an NTR could deliver – like a Bigelow module with three stories.

Question: Is Elon Musk amenable to putting an NTR on the second stage of a Falcon 9 Heavy?

No, at this point Mr. Musk does not currently appear to be amenable to the idea. The concept is clearly feasible from a technical standpoint, and an NTR would expedite Mr. Musk’s dream of sending people to Mars. It is Musk’s job to focus on Space-X’s current goals of building their chemical rockets, in order to satisfy their customers in the space community and NASA’s other immediate missions and needs in Low Earth Orbit (LEO). We (the NTR community) would welcome an opportunity to present a detailed proposal to Mr. Musk on a private-industry partnership mission to Mars, using our NTR engines and spacecraft designs.

Question: What sort of ISP (specific impulse) could an NTR achieve?

Specific impulse refers to the efficiency of a propellant. There are a number of different approaches that could be used for an NTR. In a closed-cycle gas core engine, An NTR could achieve a specific impulse as high as 2,000 seconds. The most efficient chemical fuel combination, liquid hydrogen/liquid oxygen, has a specific impulse of about 450 seconds. It is this dramatic potential performance improvement of nuclear over chemical that justifies the investment.

Question: Some are worried about radioactivity issues with NTRs. Are such concerns justified?

I like to use the barbecue analogy to answer this question.

If you see a picture of an NTR engine compared to a large chemical engine (like the one you’d need to go to Mars), in relative size comparison you’d see the NTR engine looks like a BBQ grill full of coals (the Uranium) and the chemical engine is about the size of a boiler room full of toxic chemicals and a web of metal piping and pumps.

So in the event of an accident, what goes “boom” is the propellant tank – which is used in both cases (or in the chemical rocket’s case, the oxidizer is also there to go “boom”, additionally) – liquid Hydrogen (LH2) is extremely explosive. The resultant explosion of tons of LH2 separates the engines from the failed spacecraft – the boiler room full of pipes, toxic stuff and hazardous chemicals will more readily break apart and rain down upon people, while the BBQ grill will more-than-likely (as the RTG during the Apollo 13 mission) return, intact, with the BBQ coals contained within.

Even if some of the BBQ coals (the Uranium) were to escape the grill (the engine), they wouldn’t be “hot”, because NTRs aren’t designed to begin cooking (the fission process) until they have reached space to begin their mission, off to Mars or wherever their destination may be. The fission process requires a highly orchestrated sequence of control drums, reflectors, and other nuclear instigators, and it’s basically impossible to have criticality without them in the right positions.

Question: Some have argued that fusion rockets should be used to put payloads into orbit and beyond.

Fusion rockets are a theoretical concept that will inevitably be the next advancement in nuclear rocket technology after fission rockets, but any discussion of using fusion for space propulsion is premature. No one has demonstrated a working prototype of a fusion rocket. By contrast, fission rockets have operated successfully for several hours. We know the technology works, and we know how to build it.

Question: Now that the lunar colony has been cancelled, is there any mission for an NTR?

No, the Administration of the United States has not formally given NASA a mission to use NTRs, even though NASA has been directed to develop NTRs within the Advanced Exploration Systems program, and the National Research Council (in Feb. 2012) has deemed NTRs of highest research priority to extend human activities beyond LEO. The commercial space market lacks the profit incentive (due to international UN regulation, forbidding anybody from “claiming” space stuff) to develop a project such as an asteroid mission, a lunar colony, or a trip to Mars. The Government (in particular, the international community collectively) has the resources for a mission to use an NTR driven spacecraft(s) but currently lacks the motivation for necessary political coordination and will. I want to see humans land on Mars in the remaining lifetime of the NERVA and Apollo community, but this will only happen with an NTR.

Question: How much would it cost to develop a fully functional, flight qualified fission rocket prototype?

It depends on the mission, but on average (across the range of missions), an NTR using NERVA technology could be designed, constructed, and tested for a few billion dollars (ranging from several hundred million for a small engine and old NERVA technology to several billion for a modernized, larger spacecraft). Current design techniques make extensive use of computer simulations, which substantially reduces development time. We also have materials that weren’t available in the 1970s for NERVA. I’m confident that with an increase in funding and a mission written on paper we would have an NTR ready to go within five to seven years. This would probably happen after NASA is finished preparing for the SLS and the Orion-focused missions, or if the private space community were able to anticipate the need and prepare ahead of time.

Question: Your company is doing a project called Bifrost for Icarus interstellar. What will Icarus use NTRs for?

NTRs can be used like small boats attached to large
sailing vessels on interstellar missions to scout ahead to a
potentially habitable planet, be used as “life rafts”, or to go
forward to harvest materials for the crew from the surrounding space
environment. There are other nuclear technologies that Bifrost focuses
on, such as surface power and Orion-boost phase.

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