Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Could a fission powered rocket reach C?

  1. Jun 17, 2004 #1
    OK, this is a question probably asked and answered before, but I've been wondering about this for years.....

    Alkatran made a comment in the space-buoy thread about how mass changes with speed in SR. As you approach the speed of light, your mass increases towards infinity, and it would take more and more fuel to keep you accelerating at a sensible pace, so you could never get to light speed.

    It seems like a fair enough point, if you're using chemical thrusters of some sort, but what about this...

    In a fission reactor, heavy elements are broken down into lighter ones, but the sum of the mass of the lighter elements/particles is less than the mass of the original. The absent mass has been converted into energy, and that energy is eventually harvested as a power source.

    Is this OK so far? It's been a while...

    So when something goes fast and hence increases its mass, where does that extra mass reside? Would the energy released from fission be increased by the same fraction that the mass increased?

    If so, then doesn't it stand to reason that if you power a space shuttle using a fission engine, then the amount of energy liberated by the fission reaction will increase at the same rate as the mass of the shuttle, making the relativity of mass a rather irrelevant point?

    So... could a fission powered rocket reach the speed of light?
    Hope that made sence - what do you think?
  2. jcsd
  3. Jun 17, 2004 #2
    one of the problems you'll be facing is any acceleration in space requires you to eject mass out opposite the direction you're moving in.

    with a fission engine, what would you be thrusting with?
  4. Jun 17, 2004 #3
    Yes, you're right, there is an issue there, and I don't really think you could reach c using this method. But in principle, you could expell anything out of the back, and its mass would also be rising with your approach to c. The only practical problem is about how much expellable matter you can carry.

    What I'm talking about is the possibility of approaching c much more freely and getting much closer than using a conventional approach. If you consider that the your extreme speed is helping you just as much as it is weighing you down, doesn't this just boil down to a non-relativistic problem of trying to reach an infinate speed? i.e. You could never carry enough fuel or spend enough time to manage it?
  5. Jun 17, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Okay, remembering that the increase in "mass" is as measured by the frame which the ship is moving with respect to, you also have to take into account the time dilation and length contraction effects.

    As pointed out. the only way to get your ship to move is through the action-reaction principle.(throw something out the back). The faster you throw it the more forward thrust you get. IOW, the higher your exhaust velocity, the better you will accelerate.

    So in effect, to an outside observer, as your velocity increases, time dilation and length contraction combine to reduce your exhaust velocity, dropping your thrust. Th ecloser you approach c, the faster this drop off in thrust. You never get that extra push you need to reach c.
  6. Jun 17, 2004 #5
    Well there goes the patent then...
  7. Jun 17, 2004 #6
    In rocket engine design there is a concept called specific impulse which is a measure of force per mass flow rate which comes out in unts of seconds. The equation gives a good description of rocket engine parameters. Force divided by mass flow rate (the same mass generating the force) is maximized with a minimum mass flow. Therefore one starts looking for lightest massive particles. Nukes may sound nice but there are limitations. Remember what haoppened to Dr. Spock. ALso, radiation shielding adds much.

    Confining the design to throwing stuff out the back end with the necessity for rather large fuel requirements gives clues for scooping up fuel as you go. Huge wide diameter H2 scoops are a thought. And while your on the subject take a look around at possible target locations for travel in v < c ships. Training, flight time to and return, orbit time at target, calculating value and utility of the trip in the first instance all severely narrow the options on human space travel.

    Paying homage to Special Relativity will guarantee a rather boring space adventure future for the wandering types. If we don't cast off the shackles of SR we can't get there from here. Meaning those that preach v < c for eternity aren't doing your space travel plans any good, especially if you listen to their admonitions that "you can't do it".

    Just listen to those that are restricting and emotionally denying v > c dynamics on theoretical grounds and avoid them like you would avoid the plague.
  8. Jun 17, 2004 #7
    the stuff you heated up with the fission process.
  9. Jun 17, 2004 #8
    AT the speeds you are considering .95c seems as good as .999c. The return on investment decreases drastically as one appraoches c, so your basic design will always be a few pecentage points less.

    Look around the universe. The planet earth isn't the only energy source in town.

    If yo reall aren't in a hurry, you can build a thruster that maintains a steady constant thrust that builds up speed gradually.
  10. Jun 17, 2004 #9


    User Avatar
    Science Advisor

    It helps any, Drew, I had a similar idea myself some years ago using antimatter. The concept was basically the same; the reaction converts matter to energy, as we approach lightspeed the mass of the matter becomes nearly infinite, producing the infinite energy required. Unfortunately, it's that " nearly infinite" that did me in. You see, to get that fuel of infinite mass that we would need to convert to get our infinite amount of energy, we would have to already be traveling at lightspeed. Anything short of that, and we would have a "nearly" infinite mass, which we could convert to "nearly" infinite energy (which would be "nearly" enough).
  11. Jun 17, 2004 #10

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    LOL, you make it sound as of casting off the shackles of SR were an option. It isn't. You can't just circumvent nature with a paradigm shift.

    But you can't do it. That statement is uniquivocally backed up by every experiment ever done with particle accelerators.

    And in that case, you'll be sitting around wondering why you can never get that rocket to budge past the v=c line on the speedometer, and never getting a full explanation for it.
  12. Jun 17, 2004 #11

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Actually, you could, even with the effects described by SR. To the rocketeer, the distance from port to destination would be contracted, and with enough speed it would certainly be possible to reach any destination in a reasonable time frame as measured by the rocketeer. But the catch is that a trip that takes 2 months in the ship's frame could take hundreds of years from an Earth-bound point of view.
  13. Jun 18, 2004 #12
    Thasss watt eye me eeen: I would like to return to the planet earth with my current favorite squeeze in a recognizable shape.
  14. Jun 19, 2004 #13
    i don't think you can use "increased mass" as fuel to propel yourself in any case.

    keep in mind the ship's entire mass will have increased as well, not just your fuel supply. in the end you'll be evening out just trying to use that increased fuel mass to propel your increased mass ship
  15. Jun 19, 2004 #14
    This of course, ram1924, if you have designed the thrust system or velocity generator, or space iteration force processes using SRT you have designed in a guaranteed failed space exploration bureauocracy.

    Think about the consequences ram1024. The closest star is about 5 light from earth. This means that any travelers at v ~ c will take a minimum of 10 years to complete the round trip. Combine this with all the necessay education and training, acceleration, deceleration, orbiting and observation limitations etc, that drives th mission duration into the minimum 20 year range vey soon, The closest star will soon become boring, what ever the findings there.

    It is the farthest stars that intrigue us all. The very serious damage that SR theorists provide to an effective massive space dynamic is the robotiic repeated SR theory that limits the maximum velocity for all of mankind and mutes ithe collective magination and creative efforts.

    If you want to travel to the stars every SR theorist should involve themselves with an intensive personal program to blow SR theory out of existence. They should make the finest search that one is humanly capable of providing in the search for a valued replacment for SR.

    Who will raise their hand in an unambiguously forceful manner and tell the childen in this world that they will never travel to the stars.

    The quote that should be on every theorists, engineer's, technician's, space curiious' and space traveler's mind is that uttered clearly with an unmistakeable confidence that, "once our enemies are found we must then, cut 'em out and kill 'em".
    Gen. Colin Powell Staff Meeting in Desert Storm.
  16. Jun 19, 2004 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    specific impulse (isp) is exhaust velocity divided by g (acceleration of gravity at the surface of the Earth), a convenient concept when dealing with rockets designed to lift payloads into orbit. Not as convenient when talking about interstellar craft.

    Basically what we are dealing with is the rocket equation:

    [tex]V_{f}= V_{e}ln(MR)[/tex]

    Vf is the final velocity of your rocket.
    Ve is the exhaust velocity.
    MR is the mass ratio, or the mass of the fueled ship divided by the mass of the ship when unfueled.

    The higher the exhaust velocity, the greater final velocity you attain for any given fuel mass. Higher exhaust velocity comes from having a high energy to mass ratio in your fuel. Nuclear reactions liberate larger amounts of energy per unit mass than chemical fuels by far, fusion even much more so than fission. But even fusion doesn't allow us to attain more than a small fraction of c for any reasonable fuel/ship mass ratios.
    While the Bussard ramjet is an interesting concept, It also has its limitations. As you scoop up that fuel, you encounter drag on your ship, which reduces the acceleration you get from thrust. Even if you were to cause the hydrogen to undergo fusion, there will be a point where the drag will equal the thrust and you will get no more acceleration. The max velocity attainable would be less than 0.1 c. Some models even predict that the minumum velocity that a ramjet would need to attain in order to operate is greater than the maxiumum velocity the ramjet can maintain. Meaning that it is a wash from the get go. And that is with a nuclear reaction, trying to burn the H2 in a chemical reaction would br futile in terms of generatinf any postive thrust.
    The universe is not here for your benefit and is not under any obligation to behave by rules of your liking. The universe has limits and we are bound to them. Now we have two choices about those limits:

    We can whine and moan about them and try to wish them away.
    We can probe and push these limits until we best understand them and get the most out of working within them.

    And maybe, just maybe, someday a loophole or way around will be found to the c limit. If it does happen, it will be by the second manner, and it will be by building on SR not by tearing it down.
  17. Jun 21, 2004 #16
    Completely right. SR is here to stay, like it or not. What we need is something like warp drive or similar.. something that goes around SR.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Could a fission powered rocket reach C?
  1. Water Powered Rockets (Replies: 12)