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Thread: alternatives to rocket fuel

  1. #1 alternatives to rocket fuel 
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    for later deep space missions, i have been thinking that our current generation fuel may not be enough. You need something that either last a long time, or ouy can mine off asteroids and planets. any ideas?


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  3. #2 alternatives to rocket fuel 
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    Leapfrogging from asteroid to asteroid, even if these contained a suitable source of fuel would be in inefficient process if propulsion is based on action-reaction forces (throwing mass out behind to accelerate forward). The space vessel would waste fuel decelerating to a prospective asteroid as well as accelerating after leaving it.

    I guess the process would be simplified if these fuel rich asteroids could just be gobbled up as the space vessel encountered them.

    Some people propose "solar sails" and I guess carbon nanotube constructions would be a good technology to look into. However this approach would have difficulty outside the solar system (which is where possibly the best action is).

    Unlike boats that can sail into the wind (vector of forces wrt the boats angle of approach to the wind, sea and the rudder) a space vessel would have difficulty returning based on solar winds. It might only have tickets available for a one way journey.

    I have also read that some people suggest a "hydrogen ramjet" approach. Even at 100 atoms per cm^3 a space vessel traveling sufficiently fast could scoop up a usable quantity and preferably attempt a fusion reaction with them.

    Long deep space journeys could last hundreds of years so perhaps sending biological explorers is problematic; artificial replicas would be far better suited and possibly better hardened to radiation. However, even so, the time experienced by the occupants could slow down appreciably if the vessel's speed approached c; from their experience it might only take a century (based on their experiential acceleration of 1 g) for them to get close to the speed of light (although 1,000's of years to external observers). Even so, the energy acquired by the space vessel (through relativistic mass increase) would have to come from somewhere.

    Even nuclear reactors are unlikely to be adequate; the fuel will eventually expend and if, say a laser or ion based propulsion system is used, mass loss will occur. Over time the space vessel would be out of fuel.

    So I agree fuzzymechy that something has to be done about refueling on the fly. The hydrogen ramjet approach appears to have merit for deep space treks. Carbon nanotube technology, required for strong, low mass collection funnels is in progress and the technology may not be that far away.

    However the best approach is to devise a propulsion system that does not expend mass. The conventional rocket action-reaction approach, come such a day, may well be obsolete.

    If interested, people might like to consider the following "impulse engine" concept;



    Although simple rectilinear motion would not result in a net accelerating force, combined with a rotation at the same rate might overcome this - would this approach avert the mass loss hassle?


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  4. #3 Re: alternatives to rocket fuel 
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    ? That impulse engine... um it's a vibrator right? I do hope I'm wrong.



    Quote Originally Posted by Vaedrah
    I guess the process would be simplified if these fuel rich asteroids could just be gobbled up as the space vessel encountered them.

    ...

    the best approach is to devise a propulsion system that does not expend mass.
    Hauling an enormous mass of fuel along is so wasteful. Imagine if cars towed fuel tanks as heavy as the car.

    I mentioned a solution earlier on these forums. Maybe somebody will like it.

    Sling fuel out in advance of mission. That costs relatively little from orbit, started years ahead of time. The lightweight ship catches up to fuel and gobbles as it goes.

    You will want to lay out fuel at a frequency/velocity/volume such that the mission ship gets fed steadily. Ideally the ship accelerates at 1G to halfway, then reverses acceleration to break at 1G the remainder. Fuel though, unlike frail humans, can be accelerated much more. Anyway I imagine the first phase of a mission would start launching fuel even before the ship had left the drafting board - a decade later perhaps it would set off to catch up.


    ***

    The 1G acceleration limit really sucks the zing out of space travel. The only way over that is to accelerate with G itself.
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  5. #4  
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    Of course a constant 1G of acceleration has the novel effect of obviating the need to create any artificial gravity. I'm betting that it would still "feel" like 1G of constant acceleration to the crew & passengers even once relativistic effects started lessening the amount of the real speed increases.

    Of course, I could be misinterpreting relativity here....


    As for slowing down, the best way will probably always be the way Nasa does it. Just sling shot your ship around planets in a very well planned out path, and let their gravity slow you down. (Actually Nasa usually uses this approach to speed up their exploration ships up so they can get out to Saturn/Jupiter/Neptune/etc, not slow them)
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  6. #5  
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    From memory, passengers that experience 1 G acceleration will eventually approach the speed of light and their time, relative to the remaining people will "slow down". I remember integrating this effect once wrt to the accelerated passengers frame of reference and it seemed that they might age say years whilst the outside universe aged towards .

    I'm betting that it would still "feel" like 1G of constant acceleration to the crew & passengers even once relativistic effects started lessening the amount of the real speed increases.
    However the energy still has to come from somewhere corresponding to the mass increase of the space vehicle and its passengers. I guess no one likes energy infinities.
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  7. #6  
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    Quote Originally Posted by Vaedrah
    passengers that experience 1 G acceleration will eventually approach the speed of light
    Eventually. .
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  8. #7  
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    Perhaps someone can confirm if the acceleration could be held constant to the passengers, would they experience a finite time to reach the speed of light whilst those left behind would experience an infinite time (i.e. without concern for the energy infinity).

    Despite the energy infinity concern, time dilation should certainly slow time down for the travelers as the space vessel increases velocity at constant acceleration, say at G. This sounds like an integral to solve with "external time" set from to . Will the integral for internal time, based on the familiar time dilation formula



    converge to a bounded result?

    If so, what will happen to the moving travelers when their clocks tick past a finite, bounded ?
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    Perhaps someone can confirm if the acceleration could be held constant to the passengers, would they experience a finite time to reach the speed of light whilst those left behind would experience an infinite time (i.e. without concern for the energy infinity).

    Despite the energy infinity concern, time dilation should certainly slow time down for the travelers as the space vessel increases velocity at constant acceleration, say at G. This sounds like an integral to solve with "external time" set from to . Will the integral for internal time, based on the familiar time dilation formula



    converge to a bounded result?

    If so, what will happen to the moving travelers when their moving clocks tick past a finite, bounded ?
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  10. #9  
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    Formulas for a ship under constant acceleration (as measured from by the ship) are as follows:





    As requested:

    T is the time as measured by the non-accelerated observer
    c is the speed of light.
    a is the acceleration as measured from the ship
    t is the time measured from the ship (accelerating observer)
    d is the distance traveled by the ship according to the non-accelerated observer

    The units can be in MKS, CGS, FPS or natural units (where c=1), just as long as you are consistent.
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  11. #10  
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    Gentlemen,
    I am delighted we are getting technical here. I applaud it. On the other hand if one of my employees handed in a technical document with formulae and no listing of what the variables were and no mention of units they would get a very severe dressing down. Some of us can see at a glance what is what - that is not the case for everyone. Let's be professional, please.
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  12. #11 alternatives to rocket fuel 
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    Thank you Janus. I though I'd try your equations on MathCAD. Also, so that I don't get a "dressing down" by OphioLite I thought I'd better specify the units!



    Now assuming (and it's a big assumption) that I haven't made a slip up in this rush job, we can see the relationship between the accelerated traveler's frame of reference and the time experienced by those left behind, both expressed in years



    Note: The y axis is plotted in log time, x is linear

    If true, the traveler could well outlive his or her children!

    Also, how far will those left behind see the accelerated traveler's space vessel go. This time, distance is shown in light years for the traveler's time experienced in years



    It looks about right - distance and time in light years should converge - the initial difference only occurs at the beginning of the journey.

    Further, if the equations are correct and I have entered the conversions correctly, there may little difficulty going anywhere in space, at least from the time perspective of an accelerated traveler.

    (I guess a photon propulsion system would be used, e.g. laser powered by a nuclear reactor)
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  13. #12  
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    There are a few stars as near us as 5 light years. Presumably these are our first destinations? Now, given a pathetically slow 1G round trip of four "legs" (accelerate half-way, brake half-way), would the clocks be significantly off on return to Earth? Apparently not!
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  14. #13  
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    The fuel requirement for such a craft can be found by:




    Where all variables remain the same as in the last post with the addition of

    Ve is the exhaust velocity of the rocket
    MR is the mass ratio of the rocket or



    Where Mrocket is the mass of the rocket and Mfuel is the mass of the fuel.

    The second equation is more revealing if solved for MR, which will give you the fuel requirements to travel a given distance with a given acceleration and given exhaust velocity.

    This gives us:



    Example:
    To travel to Alpha Centauri at 1g acceleration with an exhaust velocity of 0.1c would take a mass ratio of 622858. IOW it would take 622,859 kg of fuel for every 1 kg of rocket.
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  15. #14  
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    Interesting. However if the traveler had 100 years worth of uranium for the vessel's reactor, then the traveler would have enough fuel to go as far, practically as she or he wanted to. The fuel would only have to last 100 years and weigh enough to accomplish this. This weight might only be a few kg.

    After all, a laser propulsion system would produce the same acceleration regardless of the traveler's velocity. The traveler would accelerate at 1 g based on photon propulsion over this example period of 100 years to the traveler.
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  16. #15  
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    Quote Originally Posted by Vaedrah
    Interesting. However if the traveler had 100 years worth of uranium for the vessel's reactor, then the traveler would have enough fuel to go as far, practically as she or he wanted to. The fuel would only have to last 100 years and weigh enough to accomplish this. This weight might only be a few kg.

    After all, a laser propulsion system would produce the same acceleration regardless of the traveler's velocity. The traveler would accelerate at 1 g based on photon propulsion over this example period of 100 years to the traveler.
    Not even close. 1 kg of U235 will release 6.48e13 joules of energy through fission and converts about 0.1% of its mass to energy. Ergo, 99.9% of its mass is dead weight.

    A trip to Alpha Centauri at 1g will take 2.3 years ship time. You will reach Alpha Centauri moving at 0.982894347c

    To accelerate 0.999 kg up to 0.982894347c takes 4.06e16 joules, or 6146 times the energy 1 kg of Uranium releases. Uranium can't even provide the energy needed to accelerate it own mass (let alone a spaceship)at 1g for 2.3 years or to Alpha Centauri .

    In fact, could only provide enough energy to accelerate itself at 1g for 13 hrs to reach a speed of a little under 4% of c

    And that is assuming that 100% of the energy released actually is applied to that acceleration and none is lost.
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  17. #16  
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    Quote Originally Posted by Vaedrah

    After all, a laser propulsion system would produce the same acceleration regardless of the traveler's velocity. The traveler would accelerate at 1 g based on photon propulsion over this example period of 100 years to the traveler.

    Even with a photonic drive and complete matter to energy conversion, you would need a MR of 29,787 to operate for just 10 yrs ship time at 1g acceleration.
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  18. #17 alternatives to rocket fuel 
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    Thanks Janus. I can see that the energy requirements as estimated from the perspective of the observers left behind would result in your predictions. This energy should also be the same as calculated from the accelerated travelers frame of reference.

    I also had a play with 100 % conversion from matter+anti matter conversion and a (gamma ray) laser with 100 % conversion and it seemed that any amount of fuel of weight x, say, would be used up in 0.951 years based on an acceleration force experienced by the (massless) traveler + "mass of fuel" at .

    There might be some additional benefit from the reduction of mass during its conversion to photons, but not enough to make the journey into a mere Sunday outing!

    So it would appear that the original post represented a realistic proposal; harvesting mass from space would be preferable to carrying "dead mass" on a space vessel that would be used up before a single light year had been traveled. Some sort of interstellar ramjet harvesting space borne hydrogen and dust might be the ticket.

    Either that or we need wormholes, direct conversion of travelers to photons or just watch star trek on DVD.
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  19. #18  
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    G'day from the land of ozzzzz

    Look into Z-pinch experiments for jets based on focus fusion.

    Not only the production of jets but the ultimate speed of light travel.
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  20. #19  
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    Thanks Harry. I had a quick look

    The Z-pinch is an application of the Lorentz force, in which a current-carrying conductor in a magnetic field experiences a force. One example of the Lorentz force is that, if two parallel wires are carrying current in the same direction, the wires will be pulled toward each other. The Z-pinch uses this effect: the entire plasma can be thought of as many current-carrying wires, all carrying current in the same direction, and they are all pulled toward each other by the Lorentz force, thus the plasma contracts.
    From Wikipedia

    http://en.wikipedia.org/wiki/Z-pinch



    I guess your suggestion is to use a magnetic confinement process to allow fusion and thereby directly convert mass to thrust from photon momentum (energy). Although as yet unsuccessful on earth based experiments, i.e. instability and negative net energy gain, a fusion process might be suitable for space flight. The conversion efficiency would still be low, based on differential mass conversion rather than absolute mass conversion (as in matter+antimatter) but if the mass is space-harvested then this might not be problematic.

    The same problem exists as with the asteroid harvesting idea - harvesting mass requires deceleration to pick it up or if additional mass is accumulated continuously this will cause some deceleration of the space vessel.

    As pretty as Tesla sparks may seem, the stars may still have the last twinkle of a laugh.
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  21. #20 Re: alternatives to rocket fuel 
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    Did nobody read my suggestion:
    Quote Originally Posted by Pong
    Sling fuel out in advance of mission.
    I.e. Pac-man chasing slightly slower ghosts.
    ?

    Even the fuel goodies may have little boosters for tweaking up to the ship.
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    G'day from the land of ozzz


    Vaedrah

    Now that you took the first step.

    Read more on Z-pinch experiments and Z-pinch in the formation of astrophysic jets.

    Plasma properties.
    Technical overview I
    http://www.plasmacosmology.net/tech.html

    Z-pinch astrophysics jets
    SAO/NASA Astrophysics Data System (ADS)
    http://adsabs.harvard.edu/cgi-bin/nph-basic_connect

    Plasma Cosmology
    http://www.matter-antimatter.com/plasma_cosmology.htm

    This property is one of the most important of all properties. It holds the key to the recyling process that may explain the workings of the universe.
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  23. #22  
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    Yes Pong. Slinging fuel out in advance of a vessel's trajectory still needs energy for this to occur. If the fuel can be "slung out" in advance, then the vessel, equally, and logically, could have been "slung out" also.
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  24. #23  
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    Thanks for the url's Harry. I like anything that's conjectural and poses questions to established viewpoints. I'll get back to you tomorrow.
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  25. #24  
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    Quote Originally Posted by Vaedrah
    Slinging fuel out in advance of a vessel's trajectory still needs energy for this to occur.
    Naturally. Off the moon or some convenient site, where energy is cheap and you can mount a brute force accelerator. We'd be accelerating durable packets much less massive than the mission vessel. Rough handling. They might burn some fuel too on way to intercept, if we hadn't started fueling long before the mission.

    Quote Originally Posted by Vaedrah
    If the fuel can be "slung out" in advance, then the vessel, equally, and logically, could have been "slung out" also.
    Won't it take longer to run 'round in circles before setting off? People are stuck in that vessel growing impatient.

    Besides the vessel is likely to be enormous and ungainly, while a lifeless packet of fuel needn't be... shouldn't be. I don't think it logical at all to accelerate the vessel in place.
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  26. #25  
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    Quote Originally Posted by Pong
    Naturally. Off the moon or some convenient site, where energy is cheap and you can mount a brute force accelerator. We'd be accelerating durable packets much less massive than the mission vessel. Rough handling. They might burn some fuel too on way to intercept, if we hadn't started fueling long before the mission.
    I agree that this seems like a good idea. And as an added bonus, you can use this sort of system to slow down upon reaching your destination without burning any fuel. If you have masses that were launched ahead of the ship that are moving much slower than the ship, the ship will slow down thanks to conservation of momentum when it captures them. Of course, you need a way to capture the fuel without jolting the ship to much, which might be very tricky to actually engineer.
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  27. #26  
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    Quote Originally Posted by Vaedrah
    Yes Pong. Slinging fuel out in advance of a vessel's trajectory still needs energy for this to occur. If the fuel can be "slung out" in advance, then the vessel, equally, and logically, could have been "slung out" also.
    If you do this sort of scheme right, your ship can end up going MUCH faster than the maximum speed that you can get with your fuel launcher. Suppose you want to go 58 million km (about the distance from the earth to mars) and your launcher can launch things at 5 km/sec. At that rate it would take about 135 days to drift to your destination. But if your ship can gain 2 km/sec of velocity from each fuel packet and you launch 20 fuel packets, you can reach your destination in just 17 days.

    Also, most likely the fuel is being launched from some sort of ground-built accelerator that imparts all the velocity on the fuel packet in a fraction of a second. Humans couldn't survive that.
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    Quote Originally Posted by Scifor Refugee
    Of course, you need a way to capture the fuel without jolting the ship to much, which might be very tricky to actually engineer.
    I suggest the fuel packets have their own little thrusters to correct course and tweak up to the mission vessel for gentle docking. In this way the vessel just makes continuous bee-line acceleration.

    We get fuel for return trip by swinging fuel around the destination's gravity well. U-turn. Planning and timing is everything, but I think we're up to that.

    A busy route (far future!) would eventually appear as stream of fuel packets traveling in opposite directions at various speeds. Again, it is an individual packet's job to tweak up to vessels and dock as needed.
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  29. #28  
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    I see a few interesting concerns here, and I love to make lists:

    1) - C = approximately 300 million meters per second. (give or take). Acceleration of gravity is 10 meters per second ^ 2. So, even if we ignored relativity (which of course can't be ignored) we'd still be looking at 30 million seconds. (There are 3,153,600 seconds in a year)

    2) - Lasers, as a means of propulsion, probably don't convert even a 100th of the energy into momentum. They're great in terms of mass/thrust ratio, but you need almost infinite energy.

    3) - As for re-fueling.... You have to remember that any new propellant you pick up has to already be moving as fast as you are, or it will slow you down to pick it up. Same goes for additional uranium, or etc.


    That said: I envision a huge, gigantic propellant tank crammed full of something that can be ejected by a single, small, ion drive. And then a comparatively very small ship, with some kind of nuclear reactor on it.

    Once you've ejected half the mass in the propellant tank, you should be traveling at almost 1/2 the speed the ion drive is propelling those particles to go at. Then you eject half of the remaining fuel again, then half again, then half again, and so on.

    If your ship is small enough compared to the amount of propellant you had when you started, you might get a pretty good speed going. For the first leg of the trip,
    maybe we could fire propellant at the ship from an ion cannon located on the moon, and have the ship accelerate by catching it.
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  30. #29  
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    Quote Originally Posted by kojax
    Once you've ejected half the mass in the propellant tank, you should be traveling at almost 1/2 the speed the ion drive is propelling those particles to go at. Then you eject half of the remaining fuel again, then half again, then half again, and so on.
    Burning half the fuel is more like 70% of the speed of the of the rocket exhaust. The formula is exhaust speed * ln(mass before burn/mass after burn).

    Also, you dropped a zero from the number of seconds in year. There are about 31 million seconds/year, so it's only about 1 year to reach the speed of light at 1 G.
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  31. #30 Re: alternatives to rocket fuel 
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    Quote Originally Posted by Vaedrah
    Some sort of interstellar ramjet harvesting space borne hydrogen and dust might be the ticket.
    The downside of the Bussard Ramjet is that at some point, the drag created on your ship by collecting your fuel equals the thrust you can get from fusion of the fuel.

    Estimates I seen put the top theoretical velocity of such a ship at 10% of c, and in practice less.
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  32. #31  
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    Quote Originally Posted by Scifor Refugee
    Quote Originally Posted by kojax
    Once you've ejected half the mass in the propellant tank, you should be traveling at almost 1/2 the speed the ion drive is propelling those particles to go at. Then you eject half of the remaining fuel again, then half again, then half again, and so on.
    Burning half the fuel is more like 70% of the speed of the of the rocket exhaust. The formula is exhaust speed * ln(mass before burn/mass after burn).

    Also, you dropped a zero from the number of seconds in year. There are about 31 million seconds/year, so it's only about 1 year to reach the speed of light at 1 G.
    Oh. You're right. Going back in the calculation I had 360 seconds in an hour instead of 3600.

    With the rocket fuel, I guess I was just assuming that ejecting half of your mass would result in both halves going the same speed in opposite directions. I guess it matters whether you do it in increments or not?
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    Quote Originally Posted by kojax
    With the rocket fuel, I guess I was just assuming that ejecting half of your mass would result in both halves going the same speed in opposite directions. I guess it matters whether you do it in increments or not?
    Remember that the last bit of fuel burnt is trying to accelerate less mass than the first bit of fuel burnt (because the ship weighs less and less as it burns through more and more fuel), so it results in more acceleration.
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    Yeah, I was going over it again, and now I get why. I was using rules for collision, but forgetting, like you say: The ejected mass doesn't count against anything because it was ejected.
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