Nuclear Thermal Rockets, an old propulsion system that may be the future for space exploration

To achieve any measure of space travel there is one tool that has always been indispensable, rockets. Rockets have been the primary tool for sending spacecraft into orbit and accelerating them beyond Earth orbit to other planets in the solar system, and for a few craft, on their way out to the rest of the galaxy.

Despite the amazing advances in rocket technology since the days of Apollo, NASA still is forced to rely on chemical combustion to propel vehicles off Earth and to space destinations in a relatively short time period. Chemical rocket engines, while producing a lot of thrust, are highly inefficient and very dangerous as several rocket accidents in the past have proven. Electrical propulsion is a useful alternative for long-term small probes due to its high efficiency, but it produces very low thrust and is not useful for shorter-term manned missions.

Many theoretical concepts for high efficiency and high thrust propulsion offer a tantalizing view for how space travel in the future might work, but for now such concepts are technically unfeasible. There is, however, an option in between the future and now that uses existing technology. The best part is, it is an old idea.

Nuclear thermal rockets, or NTRs for short, are rocket engines that utilize a nuclear fission reactor to heat propellant instead of igniting combustible propellants. The advantages include much higher specific impulses due to a higher range of exhaust velocities that chemical rockets can’t achieve due to limits of the combustible fuels. The idea is surprisingly simple; take a nuclear reactor like the ones used for power generation today, but instead of using it to heat water into steam for power turbines, heat propellant instead and run it out of a rocket nozzle for thrust. This is the simplest form of NTR, which is called a solid core NTR. In fact, it is so simple it has already been done, just not in space.


Basic NERVA engine

In 1955, the Atomic Energy Commission started Project Rover, aimed at the development of engines utilizing nuclear technologies, which were in their prime in the 1950s in America. Four basic designs came from this and 20 rockets were tested, but the AEC work was intended to study the reactor design itself for rocket use, rather than actually build a rocket. In 1961, NASA began the Nuclear Engine for Rocket Vehicle Applications program, or NERVA for short, to formalize the entry of nuclear thermal engines into space exploration. In fact, it was President Kennedy’s hope that Project Rover and the NERVA program would be the next step after Apollo, stating such in his famous speech to a joint session of Congress establishing the goal of landing a man on the moon.

Directly comparing the performance of two different rocket systems is not simple however. There are ways in which chemical propulsion is better than nuclear and vice versa. The most basic form of solid core NTR provides much better specific impulse, a measure of how efficient a rocket is (think gas mileage), but doesn’t have comparable thrust. It also takes a lot of time to warm up a nuclear rocket and cool it down between firings, putting stress on the system. The best way around this is what is called a bimodal NTR, which uses the reactor to both provide rocket thrust and supply power to the spacecraft at the same time. The reactor is started up once and when rocket firings are done it is cooled down to regular operating levels and a Brayton power conversion system is used to supply the spacecraft with power. This employs a different working fluid through a turbine and a radiator to cool it. Thus the reactor only needs to be started up and shut off once per mission.


tritonBraytonTbPratt & Whitney Triton engine design. The large radiator at the top dissipates heat from the reactor.

An even better option is the trimodal NTR conceptualized by Pratt & Whitney. This takes the bimodal concept and adds another NTR concept referred to as LANTR, or LOX-augmented NTR, to make the Triton engine. The LANTR mode allows for more thrust by injecting liquid oxygen into the nozzle to act as an afterburner. This design then allows for a ship to have high thrust, high specific impulse, or power generation from one engine depending on the setting.

There are even more ambitious ideas for NTRs including liquid core and gas core engines, but they have never been built beyond the conceptual stage and present several new challenges among which is a high tendency of releasing radioactive elements into the exhaust. Solid core NTRs keep the radioactive elements away from the propellant, thus making them safer. However, all solid core tests such as NERVA resulted in engines with a thrust to weight ratio lower than one, meaning it could not lift a rocket off Earth.

The Discovery One from 2001: A Space Odyssey is said by Arthur C. Clarke to have Gas Core NTRs

This leads to the obvious fact that despite Kennedy’s high hopes and NASA’s research, nuclear engines never did get used for actual spacecraft. There is a complicated set of reasons for this including cost factor, various issues and most importantly public opinion. The growing public dissatisfaction with nuclear weapons and nuclear power by proxy as a result of the Cold War arms race and later accidents like Chernobyl made it a lot less likely that people would like the idea of a nuclear powered rocket flying, even if it could be safe. Today, nuclear weapon treaties forbid nuclear weapons in space, thus making ideas like Project Orion, which used full nuclear bombs for propulsion infeasible. Such treaties do not disallow nuclear reactors like what NERVA used however.

NASA has always wanted their vehicles to be safe and not cause harm to anyone. As such, the biggest issue with these engines is radiation. Fears of radioactive material dispersed into the atmosphere, or a nuclear explosion happening are common. However, despite the horrible accidents that have plagued nuclear reactors before, they are more safe than many realize and as stated above can be done so that no radioactive material leaves the nozzle. A nuclear explosion is highly unlikely since reactors are not designed to act like nuclear bombs and are more controlled. This aside though, the simplest option is to not use them in the atmosphere at all and make nuclear engines only for use in space, while using chemical engines to get to orbit. The only worry is a sub-orbital structural failure, but designs for the reactors are very robust, leaving it unlikely for radioactive material to be spread. As for fears of the reactor irradiating astronauts, there are ways of shielding them, but studies have shown that the shorter travel times NTRs allow result in less radiation exposure by passengers due to them spending less time in space exposed to cosmic radiation.

Continued research is still being done, in the 1970s a small nuclear engine was designed for possible use with the space shuttle in place of the Space Shuttle Main Engines. The design provided a theoretical specific impulse of 975 seconds, much greater than the 363 – 452 seconds of the SSME for only slightly less of the SSME mass fraction. It was clearly not chosen for the space shuttle however. Continued research under Project Timberwind as part of the Strategic Defense Initiative was done between 1987 and 1991, and in 2012 Icarus Interstellar and General Propulsion Sciences began a development project known as Project Bifrost to develop an NTR system for interplanetary missions.


This is not a reality yet, but it is a possible reality in the near future.

While it hasn’t been used yet despite all the research behind it, nuclear propulsion represents the next inevitable phase of rocket technology for space exploration and it can help humanity to unlock the solar system. With more research and funding NASA can help to improve this technology and make it safer. If you think NASA should continue to develop new innovative propulsion technologies like this, let Congress know:

For more information on Nuclear Thermal Rockets, check out the pages below:–Nuclear_Thermal

The Circle of NASA – From Jobs to Technology & Back


With the Space Shuttle retired and the US economy in its current state, many people are wondering how both the economy and the space industry can possibly move forward.

What they aren’t realizing, however, is that the two are, or at least can be, directly related.

The simple fact is that NASA creates jobs. How? Well, it happens with a one-two punch: A new NASA program or project creates jobs all around and spinoffs create even more.

The following is information taken from studies conducted to investigate the true relationship of NASA expenditures and economic growth.

A study by Midwest Research Institute (MRI) showed that the relationship between Research & Development (R&D) expenditures and technology-induced increases in GNP were directly mutually beneficial. Each dollar spent on R&D returns an average of slightly over seven dollars in GNP over an eighteen-year period following the expenditure. Assuming that NASA’s R&D expenditures produce the same economic payoff as the average R&D expenditure, MRI concluded that a total gain of $181 billion resulted from the $25 billion (1958) spent on civilian space R&D during the 1959-69 period, with $52 billion of that coming in through 1970 and the rest continuing to stimulate benefits through 1987.

A second econometric investigation of the relationship between NASA expenditures and the U.S. economy was conducted by Chase Econometric Associates. This study consisted of two phases. The first phase used a University of Maryland input-output model to analyze short-run economic impact of NASA R&D expenditures. Using an example of $1 billion being proportionately transferred to NASA from other non-defense programs, Chase estimated that the transfer would increase manufacturing output in 1975 by 0.1 percent, or $153 billion (measured in 1971 dollars), and would increase 1975 manufacturing employment by 20,000 workers.

The second phase of Chase’s study analyzed the long-term economic impact of NASA R&D expenditures. Using a production function which related NASA R&D expenditures to the productivity growth rate in the U.S. economy from 1960 to 1974, Chase concluded that society’s rate of return on NASA R&D expenditures was 43 percent.

Lastly, the Space Division of Rockwell International conducted a third study of the macroeconomic impact of NASA R&D programs involving the relationship between NASA’s Space Shuttle program and employment in the state of California. Using an econometric model developed at UCLA, Rockwell estimated that the Space Shuttle program generated an employment multiplier of 2.8; that is, direct Shuttle employment of 95,300 man-years in California produced an increase of 266,000 man-years in total employment.

It’s a lot to take in, but the basic story is this: If NASA wants to create a new spacecraft, people are needed from almost every sector of the science and technology industry to create each and every part of that spacecraft. And after all that, even more jobs can be created as these new products are applied to technology we use right here on Earth.

The result is called a “NASA Spinoff,” and here’s a little bit about them:

“A NASA spinoff is a technology, originally developed to meet NASA mission needs, that has been transferred to the public and now provides benefits for the Nation and world as a commercial product or service. NASA spinoffs enhance many aspects of daily life, including health and medicine, transportation, public safety, consumer goods, energy and environment, information technology, and industrial productivity. These spinoffs are transferred to the public through various NASA partnerships including licensing, funding agreements, assistance from NASA experts, the use of NASA facilities, and other collaborations between the Agency, private industry, other government agencies, and academia. As of 2012, NASA has documented nearly 1,800 spinoff technologies in the annual NASA Spinoff publication.”

There’s a good chance you don’t realize just how many products out there are NASA spinoffs. Memory foam? Spinoff. Infrared ear thermometers? Spinoff. Freeze-dried food? Spinoff. Here are some more NASA discoveries that have discreetly changed your life:

As our motto goes, all it takes is one penny to launch this nation. Let’s get a Penny4NASA.

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The Destiny of a Species

We are living in an intensely important time period, as a species. It is probably the most important time period in all of human history.

Let me expand on that for a minute: right now, so many important revolutions are occurring – in engineering, technology, politics, biology, democracy and virtually every category of human thought on the planet. Humanity as we know it is changing right before our collective eyes. The reach and power that our minds enjoy has never seen such staggering potential before. Anything we dream is increasingly in the realm of possibility. This is happening as a result of many thousands of years of innovations building on one another and centuries of successful efforts to understand and conquer the world around us. New ideas are infusing old processes, previously crazy ideas are given consideration and this self-enriching ride is accelerating exponentially as time goes on. Our abilities to peer into the nature of the universe is also increasing in step, and such an unprecedented view of the structure of the physical universe allows us untold powers and opportunities. It is as if we are heading toward a world of abundance, as Peter Diamandis indicated not too long ago.

If you sit and think about it for a few minutes, you may start to feel euphoric. It is amazing to be on this ride at this time and this place. However, this process we are undergoing is not without its growing pains. We are radically changing our planet in some not-so-kind ways, which is perhaps a sign that we have more wisdom to attain yet. Also, we don’t really know if this explosion of progress will last any particular span of time. The universe is mighty indifferent to our concerns just as it failed to consider the long-term planning of the dinosaurs. So, this ride…it might not last forever. It can end. Indeed, many civilizations of humans have risen to prominence and then faded away to unequal measures of obscurity, and lost many of the contributions they toiled to create. As our ideas slowly collect together, through trial and error, and enrich the whole of humanity, we would be fools to think this will last forever without careful forethought and long-term planning.

That’s where NASA comes in. Careful forethought and long-term planning is what NASA represents. For the first time in our history, we are aware of the dangers of our own sun, the potentially fatal meeting of heavenly bodies that occasionally resets Earth’s progression of life back millions of years, and the short-sighted outbursts of violence and exploitation we humans periodically engage in. NASA has helped us to realize for the first time that we are not trapped on Earth, that we do not have to be victims of ourselves or an indifferent universe. We can do something. Indeed, we should do something.

But, “what can I do?” you might ask; “I am small. The universe is immense.” As I mentioned above, we aren’t so helpless now. Many of the things we work to do in our lives touch many others in ways we can scarcely imagine. And this affects not just contemporary humans, but humans that will exist in the future, as well. In this way, we are all connected. Know that each of us are caught up in an intense epic. It is a story of such immensity and antiquity that, at times, it is bewildering to grasp. We are part of a human lineage of which each individual is an integrated part. Our roles in this here and now may seem boring, or complacent, or mundane, or insignificant. This is not the case. This is FAR from the case. We belong to a great span of human creatures, and a rich, unique web of life – something that exists nowhere else. We are part of a genetic tradition that has overcome every single obstacle that has ever threatened it and explored realms other organisms still cannot fathom. You are a participant to a reality that has never happened before.

Creatures like you hunted down massive ice age monsters, using intelligence and teamwork to beat the odds. Creatures like you forged the first cities, created the first roads and monumental architecture, wrote the first books, blazed those first trails and laid the groundwork for future human civilization. Creatures like you have created a global, instantaneous computer network. Creatures just like you have walked on the surface of the moon.

Importantly, we contribute to the endeavor of human civilization and the long-term survival of life whether we realize the scope of our involvement or not. Even though Neil Armstrong was the first human to plant a 9 1/2 size boot print on the moon, hundreds of thousands of humans worked to put him there. It was a collective effort. And the same will be said of our effort to establish a permanent presence off of our home world. Returning to the moon, visiting our first asteroid, building cities on Mars…it begins with you, with us working together to enrich future generations of humans and guaranteeing the existence of life in an indifferent universe. It begins with a penny for NASA.