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posted by janrinok on Sunday February 04, @12:54PM   Printer-friendly

https://physicsworld.com/a/nuclear-powered-spacecraft-why-dreams-of-atomic-rockets-are-back-on/

Launching rockets into space with atomic bombs is a crazy idea that was thankfully discarded many decades ago. But as Richard Corfield discovers, the potential of using the energy from nuclear-powered engines to drive space travel is back on NASA’s agenda

In 1914 H G Wells published The World Set Free, a novel based on the notion that radium might one day power spaceships. Wells, who was familiar with the work of physicists such as Ernest Rutherford, knew that radium could produce heat and envisaged it being used to turn a turbine. The book might have been a work of fiction, but The World Set Free correctly foresaw the potential of what one might call “atomic spaceships”.

The idea of using nuclear energy for space travel took hold in the 1950s when the public – having witnessed the horrors of Hiroshima and Nagasaki – gradually became convinced of the utility of nuclear power for peaceful purposes. Thanks to programmes such as America’s Atoms for Peace, people began to see that nuclear power could be used for energy and transport. But perhaps the most radical application lay in spaceflight.

Among the strongest proponents of nuclear-powered space travel was the eminent mathematical physicist Freeman Dyson. In 1958 he took a year’s sabbatical from the Institute of Advanced Study in Princeton to work at General Atomics in San Diego on a project code-named Orion. The brainchild of Ted Taylor – a physicist who had worked on the Manhattan atomic-bomb project at Las Alamos – Project Orion aimed to build a 4000-tonne spaceship that would use 2600 nuclear bombs to propel it into space.

[...] Despite Project Orion ending, the lure of nuclear propulsion never really went away (see box “Nuclear space travel: a brief history”) and is now enjoying something of a resurgence. Rather than using atomic bombs, however, the idea is to transfer the energy from a nuclear fission reactor to a propellant fuel, which would be heated to roughly 2500 K and ejected via a nozzle in a process called “nuclear thermal propulsion” (NTP). Alternatively, the fission energy could ionize a gas that would be fired out of the back of the spacecraft – what’s known as “nuclear electric propulsion” (NEP).

So, is nuclear-powered space travel a realistic prospect and, if so, which technology will win out? [...] A nuclear spacecraft would instead use fission energy to heat a fuel (figure 1) – most likely cryogenically stored liquid hydrogen, which has a low molecular mass and high heat of combustion. “Nuclear propulsion, either electric or thermal, could extract more energy from a given mass of fuel than is possible via combustion-based propulsion,” says Dale Thomas, a former associate director at NASA’s Marshall Space Flight Center, now at the University of Alabama in Huntsville.

Thomas says that today’s most efficient chemical propulsion systems can achieve a specific impulse of about 465 seconds. NTP, in contrast, can have a specific impulse of almost 900 seconds due to the higher power density of nuclear reactions. Combined with a much higher thrust-to-weight ratio, NTP could get a rocket to Mars in just 500 days, rather than 900.

“The thrust-to-weight ratio is crucial because it determines the spacecraft’s ability to accelerate, which is especially critical during key mission phases, like escaping Earth’s gravity or manoeuvring in deep space,” says Mauro Augelli, head of launch systems at the UK Space Agency. “The specific impulse, on the other hand, is a measure of how effectively a rocket uses its propellant.”


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  • (Score: 1, Insightful) by Anonymous Coward on Monday February 05, @10:08AM (5 children)

    by Anonymous Coward on Monday February 05, @10:08AM (#1343084)

    Even if one has an unlimited amount of energy, how does one get thrust?

    I have to have mass to eject at considerable speed. Nuclear can only help me so much. I will run out of ejecta. Ion drive? Where do I get all the electrons from? It's not like I can "ground it to Earth" anymore.

    • (Score: 1) by khallow on Monday February 05, @04:23PM (4 children)

      by khallow (3766) Subscriber Badge on Monday February 05, @04:23PM (#1343150) Journal

      Even if one has an unlimited amount of energy, how does one get thrust?

      If you're not limited by the need for a certain thrust to weight ratio or mission duration (two very common restrictions though), then light is good enough. It's extremely inefficient (thrust would be power generated divided by the speed of light), but would generate arbitrarily high delta-v for a vehicle with genuine unlimited amounts of energy.

      • (Score: 1) by anubi on Tuesday February 06, @11:22AM (3 children)

        by anubi (2828) on Tuesday February 06, @11:22AM (#1343318) Journal

        I am puzzled here.

        Does a flashlight generate thrust?

        At all?

        https://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html [ucr.edu]

        Maybe it does. You made me look for it. I learned something new. Thanks for the comment!

        I thought at one time, it was photon mass that made a radiometer work...then I finally understood how it worked. Thermal. Radiant energy absorbtion. And also finally understood why it rotated in the direction it did, when my concepts of conservation of inertia was the exact opposite of my observation. I was considering the radiometer like a pelton wheel or an anemometer - the three cup spinning wind speed measurement thingie. Wrong!

        A few years ago, a friend of mine got me interested in the generation of thrust that did not involve ejection of mass. He had several concepts of inertial trickery. One of them I even wrote a c++ program to compute the inertia over the convoluted path of the inertial mass.

        Yup...closed loop integral calculated to zero.

        He was wanting me to actually build the thing. However my computer showed me I would only have made a terribly expensive shaker.

        --
        "Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
        • (Score: 2, Informative) by khallow on Tuesday February 06, @07:27PM (2 children)

          by khallow (3766) Subscriber Badge on Tuesday February 06, @07:27PM (#1343358) Journal

          Does a flashlight generate thrust?

          At all?

          Absolutely. Photons have momentum even though they have zero mass. Just remember that you're dividing your power by 3*10^8 m/s (the momentum of a photon (it's magnitude) is p=E/c where E is the energy and c is the speed of light). 300 megawatts yields a newton of force. You wouldn't be using this to get off a planet's surface (you'd probably get orders of magnitude more thrust from the vaporizing crust than you would from the light itself), but in space small amounts of thrust over long enough periods of time adds up.

          • (Score: 1) by anubi on Wednesday February 07, @12:28AM (1 child)

            by anubi (2828) on Wednesday February 07, @12:28AM (#1343425) Journal

            Thank you for that. I had no idea that possible and was only in the realm of science fiction movies. This is the first time I have read anything that explained it in a way I could understand it.

            I hope you are working in a place they appreciates your insight as to what makes things work.

            --
            "Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
            • (Score: 1) by khallow on Wednesday February 07, @02:08AM

              by khallow (3766) Subscriber Badge on Wednesday February 07, @02:08AM (#1343437) Journal
              I thought about a means to use this as propulsion. One possibility is to take a radioactive isotope that decays via beta particles like tritium, nickel 63, or strontium 90 and put it in a glass rod with reflective backing and phosphorescent coating (something like a old TV tube which generates light by being struck with electrons). The electrons from the decay hit the coating and convert to photons which then emit in one direction, propelling the craft (which consists of little else) along. With this, you can get decades to a few centuries of consistent propulsion with good energy density and about as good a thrust to mass ratio as you'll get with this sort of design FWIW. Alpha emitters (like Plutonium 238) are better in energy density, but I have some worry that they'll damage a phosphorescent coating rather quickly.

              The catch is that you can do better with solar sails near the Sun, which also use photon propulsion but in a more effective way. At Earth's distance from the Sun, photon density is roughly 1.3 kW per meter squared. And you get twice the momentum transfer from a particle reflecting off a mirror than if it were emitted from the vehicle. That's because in the first case, you're reversing the direction of a photon (like catching the photon and reemitting it, both generating the same momentum transfer). So for starters, the solar sails effective acts as if it were a flashlight emitting 2.6 kW per square meter of mirror. A ten by ten square kilometer sail would generate the 1 Newton of force without requiring any power from the spacecraft.

              It goes up dramatically as you get closer to the Sun. For example, it is thought that we could bring a solar sail to a million km away from the Sun without it melting. That two orders of magnitude closer distance means four orders of magnitude greater force. The same 10 km by 10 km sail can generate somewhere near 10k Newtons of force (less because it's not quite 100 times closer and not quite 1 Newton of force at Earth radius). That's a lot of acceleration and I've seen studies that indicate it's enough to accelerate a heavily optimized solar sail to 0.01 c with most of the velocity accumulated happening before the sail reaches the orbit of Mercury.
  • (Score: 2) by turgid on Tuesday February 06, @08:17AM

    by turgid (4318) Subscriber Badge on Tuesday February 06, @08:17AM (#1343296) Journal

    The details of the reactor design must be very interesting. All reactors need cooling after they're shut down to remove the decay heat. That must be quite a challenge on a spacecraft. The only way to get rid of that heat is by radiation, which is very inefficient, unless you have an infinite supply of coolant, in which case you can just dump it out the back (for thrust too).

    Thinking about it, you could start the reactor up to some kind of nominal full power level for maximum thrust, then shut it down after a short period of time and generate thrust from the decay heat, timing it so that the thrust you need tails off over time in the right way to give you just the thrust you need.

    The concern would be that you ran out of coolant before the decay heat got low enough to be dissipated purely by radiation. I imagine a nuclear-powered spacecraft would have to have enormous radiators for such an occurrence to avoid a recriticality and or a meltdown. Perhaps in a dire emergency, the rector could be jettisoned into deep space?

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