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Thread: Why do we not build nuclear powered space ships?

  1. #1 Why do we not build nuclear powered space ships? 
    Time Lord
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    Not sure if this is the right sub-forum for this question, but I'm curious to know. Is it just paranoia that prevents the use of nuclear powered rockets for space travel?


    I'm not thinking like a rocket that uses small nuclear explosions to propel itself. That has some clear problems.

    I'm thinking more along the lines of a rocket that uses a very hot fuel rod to heat propellant as it is leaving the nozzle. The energy-compactness would seem to make to increase the mass-thrust ratio enough to make travel into outer space a lot easier.

    It might even be possible to use an air-breathing method to get into the upper atmosphere, and then use an on-board propellant to go the rest of the way.

    So I'm curious why this option is not being entertained? NASA rocket designers have to be aware of the possibility. Considerable research was put into creating nuclear powered aircraft in the late 1950's, and the essentials of that design would transfer with very little difficulty to a rocket design - or so I would think?


    Nuclear-powered aircraft - Wikipedia, the free encyclopedia


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  3. #2  
    Genius Duck Moderator Dywyddyr's Avatar
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    Nuclear powered aircraft can get away with "blowing" relatively large amounts of air relatively slowly.
    A rocket requires a high-speed exhaust.
    So far the only way found to do that with nuclear is to have an open-cycle system - what is heated and expelled (air or other reaction mass - caesium is one favourite IIRC) ends up irradiated.
    For some reason there's going to be a backlash of opinion about rockets spewing radioactive materials (air or not) into the atmosphere.
    Back in the 50s/ 60s there was a "cruise missile" planned.
    One aspect of it was that the exhaust trail for the entire length of its flight was about as radioactive as the explosion from its warhead.
    This didn't strike most people as a good thing in general.


    Last edited by Dywyddyr; August 23rd, 2014 at 08:07 AM.
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  4. #3  
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    Yes, Project Bifrost is an example of ongoing research.
    Project Bifrost - Nuclear Space Technologies | Icarus Interstellar

    Nuclear powered rockets were explored and have not been used yet, your Wikipedia page has a link included in it to a page on nuclear rocketry.
    Nuclear thermal rocket - Wikipedia, the free encyclopedia

    I don't really know why they never were accepted.
    I think it was concerns about having radioactive material falling back to earth that prevented nuclear rocket from being used for launching spacecraft but it might have been a cost issue instead.
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    nuclear powered rocket is only slightly better then chemical powered rocket in therms of fuel density. Also with nuclear technology there are risk envolved and rocket are not that safe and can explode.
    Using nuclear reactor you only produce heat and temperatures are limited to few hundred of degress C so not enough to produce plasma. You have to convert that heat into electricity using steam turbine (wery heavy machine) and then using electricity and electro magnets to produce stream of fast particles.

    Maybe you can read on Wikipedia about plasma propulsion and ion thrusters.
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  6. #5  
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    Quote Originally Posted by dan hunter View Post
    Yes, Project Bifrost is an example of ongoing research.
    Project Bifrost - Nuclear Space Technologies | Icarus Interstellar

    Nuclear powered rockets were explored and have not been used yet, your Wikipedia page has a link included in it to a page on nuclear rocketry.
    Nuclear thermal rocket - Wikipedia, the free encyclopedia

    I don't really know why they never were accepted.
    I think it was concerns about having radioactive material falling back to earth that prevented nuclear rocket from being used for launching spacecraft but it might have been a cost issue instead.
    They were working on a nuclear rocket called "NERVA" back in the late sixties, which was meant to be used for future Mars missions. However, flagging public interest in the Moon landing missions prompted congress to cut funds to NASA( at the very least, it bolstered those opponents to the program and gave them more sway), which cut the planned number of missions short and stopped production on the Saturn boosters. This left NERVA without the heavy-lifting launch platform it needed to get it into orbit. NASA ended up shifting its resources to unmanned probes and the shuttle program and the NERVA project was cancelled. things might have been different if Johnson had served another term as president. He was a huge advocate for the space program and his influence could have possibly tipped the scales the other way in congress.
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  7. #6  
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    Quote Originally Posted by svizoman View Post
    nuclear powered rocket is only slightly better then chemical powered rocket in therms of fuel density.
    Best specific impulse from chemical rockets is about 450 seconds; nuclear rockets have demonstrated specific impulses of 825 seconds.
    Also with nuclear technology there are risk envolved and rocket are not that safe and can explode.
    True - but so can chemical rockets.
    Using nuclear reactor you only produce heat and temperatures are limited to few hundred of degress C so not enough to produce plasma.
    The NERVA rockets operated at about 2000K (1700C.)
    You have to convert that heat into electricity using steam turbine (wery heavy machine) and then using electricity and electro magnets to produce stream of fast particles.
    That's something very different - a nuclear reactor powering an ion thruster. NERVA used nuclear energy directly to heat propellant. (BTW ion engines have specific impulses of up to 19,000 seconds.)
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    specific impulse is something to be bothered in solid fuel rocket. Liquid fuel rocket can operate till the fuel is gone if the nozzle cooling is sufficient. However due to staging principle if will be unwise to drag all that dead weight if you can dump it and ignite second stage.

    If chemical rocket explode you get huge fireball, on the other hand if the nuclear will explode you' will contaminate area with nuclear radiation.

    1700C is not near hot enough to produce adeqate thrust. You can produce thrust but that is only slightly better that with chemical rocket.

    You want particles being ejected at the back to travel as fast as they can (up to C). Since F=m*a you can calculate why 1700C would not produce enough pressure and thus speed. Ion particles flys up to 1/10 of C
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  9. #8  
    Genius Duck Moderator Dywyddyr's Avatar
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    Quote Originally Posted by svizoman View Post
    specific impulse is something to be bothered in solid fuel rocket.
    Specific impulse is important in ALL rockets. And I don't see where "solid fuel rocket" comes into it.

    Liquid fuel rocket can operate till the fuel is gone if the nozzle cooling is sufficient.
    And solid fuel rockets can't?

    1700C is not near hot enough to produce adeqate thrust.
    Link please.

    You can produce thrust but that is only slightly better that with chemical rocket.
    You want particles being ejected at the back to travel as fast as they can (up to C). Since F=m*a you can calculate why 1700C would not produce enough pressure and thus speed. Ion particles flys up to 1/10 of C
    Wrong. Check the Wiki page on specific impulse to learn what it means.
    Last edited by Dywyddyr; August 8th, 2017 at 09:56 AM.
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  10. #9  
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    Quote Originally Posted by svizoman View Post
    specific impulse is something to be bothered in solid fuel rocket.
    Right. Specific impulse is important to all rockets.
    Liquid fuel rocket can operate till the fuel is gone if the nozzle cooling is sufficient.
    So can solid fuel rockets.
    However due to staging principle if will be unwise to drag all that dead weight if you can dump it and ignite second stage.
    Depends on a great many things. For example, a high enough specific impulse and you don't need a second stage. The Atlas LV-3B used a single stage to get to orbit. although it dropped its outer two engines along the way.
    If chemical rocket explode you get huge fireball, on the other hand if the nuclear will explode you' will contaminate area with nuclear radiation.
    Agreed. Which is why missions that use nuclear rockets typically don't even start operation until they are close to (or in) orbit.
    1700C is not near hot enough to produce adeqate thrust. You can produce thrust but that is only slightly better that with chemical rocket.
    NERVA derivatives have specific impulses of over 900 seconds. The most efficient chemical engine out there (SSME's) are around 450 seconds. So nuclear thermal engines are more than twice as efficient as the most efficient chemical engines.
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  11. #10  
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by svizoman View Post
    specific impulse is something to be bothered in solid fuel rocket.
    Specific impulse is important in ALL rockets. And I don't see where "solid fuel rocket" comes into it.
    you are right, my bad. I thought this is actualy total impulse (force*time). Yeas you are correct specific impulse is WERY important factor.
    What I wanted to say about solid fuel rocket is that the time they burn is mostly determined by the diameter of the engine, of course there are other parameters like what king of fuel is used, etc. Solid fuel rockets have internal gutter and once ignited fuel starts to burn along the whole length of the motor. Time of burn is dependant mostly of diameter or thickness of fuel. Also this type or rocket deliver uncontroled and a bit uneven thrust so they are mostly used as a booster. Liquid fuel rockets either with turbine pumps or compressed nitrogen pumps deliver controled and smooth ride.


    Liquid fuel rocket can operate till the fuel is gone if the nozzle cooling is sufficient.
    And solid fuel rockets can't?
    They can, you are correct. Again I wanted to point out that with solid fuel rockets you don't realy have control over thrust.

    1700C is not near hot enough to produce adeqate thrust.
    Link please.

    Do not have it. Again I believe you need way higher temperatures in order to gain efficiency to a point you can say this is better that chemical rocket engine.


    You can produce thrust but that is only slightly better that with chemical rocket.
    You want particles being ejected at the back to travel as fast as they can (up to C). Since F=m*a you can calculate why 1700C would not produce enough pressure and thus speed. Ion particles flys up to 1/10 of C[/QUOTE]
    Wrong. Check the Wiki page on specific impulse to learn what it means.[/QUOTE]

    I did read on wiki, thanks for that.

    But again Newton's law still exsist and if you want to boost efficiency you need to propel matter with as much velocity as possible. In chemical rocket you both use fuel and oxidiset to give you heat and pressure and to give you mass you can accelerate thus producing thrust.

    But if we want useful engines for let's say Mars mission, we need better economy. Nuclear and chemical here is a no go.
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  12. #11  
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    Quote Originally Posted by svizoman View Post

    But if we want useful engines for let's say Mars mission, we need better economy. Nuclear and chemical here is a no go.
    The ISP of 825 sec projected for the NERVA engine was considered quite up to the needs for a Mars mission during its development. In terms of efficiency, it far out performed chemical rockets. With chemical rockets of the time a trip from LEO to Mars orbit and back to LEO would have required a fuel to payload ratio of ~36. NERVA would have reduced that ratio to ~6.4
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  13. #12  
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    Quote Originally Posted by svizoman View Post
    Also this type or rocket deliver uncontroled and a bit uneven thrust so they are mostly used as a booster. Liquid fuel rockets either with turbine pumps or compressed nitrogen pumps deliver controled and smooth ride.
    Right. But in many cases you don't care about a "smooth ride" - you care about getting to orbit. For example, the (now-cancelled) Ares I was to be a man-rated launcher; the first stage was a single solid rocket booster.
    They can, you are correct. Again I wanted to point out that with solid fuel rockets you don't realy have control over thrust.
    In cases of launchers that doesn't matter too much. As long as the final stage is throttleable, you take all the thrust you can get during liftoff.

    But if we want useful engines for let's say Mars mission, we need better economy. Nuclear and chemical here is a no go.
    Well, given that we could do a Mars mission with chemical rockets, I don't think that's true. It would definitely be easier with high thrust, high specific impulse rockets, but even regular old H2/O2 rockets will get you there (and CH4/O2 rockets will get you back.)
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