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Gliese 581 d and e. Recent discovery indicates we should send in Kevin Costner

124

Posts

  • tbloxhamtbloxham Registered User regular
    edited April 2009
    So? What is it is more power than our world uses now?
    Build giant solar panels near mercury have them power antimatter factories. Big enough soler arrays and a large number of factories can give you a very large amount of anti matter.

    Nope, for the 100 kg probe its all the energy which arrives on the earth for 4 minutes given a 100% efficient antimatter factory. For a 20 tonne spaceship, its more energy than has ever been used in the history of mankind.

    Your puny weapons are useless against me
  • AeolusdallasAeolusdallas Registered User regular
    edited April 2009
    Scalfin wrote: »
    Scalfin wrote: »
    Scalfin wrote: »
    NotACrook wrote: »
    Kagera wrote: »

    Because once you piss it our you're fucked.

    There's no pissing in hell. You feel like you've got to go, but you just can't squeeze anything out.

    I'm sure there's a chapter in the old testament about it.

    There's no hell in Judaism, so it's either new or one of those parts Christians added after the fact.


    Is there any way to accelerate at the rates we're talking about w/o being turned to mush? If not, maybe the best thing to do is to accelerate at 1g for 10 light years and decelerate at 1g for the next ten.
    1 g of acceleration, if you could sustain it, would get you to 0.5c in less than half a year. Acceleration isn't really the critical problem (although high accelerations are necessary to get the most out of certain gravity assist maneuvers).

    How long would it take if we accelerated at 1g for half the voyage and then slowed back down over the second half (I'm assuming that speeds will stay low enough that force will still equal acceleration)?
    They won't stay low enough. The Gliese 581 system is 20 LY away, so there's a minimum trip time of 20 years. If you could sustain 1 g of acceleration for even 1 full year you'd hit the speed of light.

    Of course, it's impossible to sustain an acceleration like that, because the energy cost increases toward infinity (which occurs at the speed of light).

    Is there any way to calculate how long it would take w/ one g of force, then? Basically I want to know how long it would take if all artificial gravity was supplied by the engines.

    go to the atomic rockets site
    http://www.projectrho.com/rocket/
    they have a lot of information like that there.

  • AeolusdallasAeolusdallas Registered User regular
    edited April 2009
    tbloxham wrote: »
    So? What is it is more power than our world uses now?
    Build giant solar panels near mercury have them power antimatter factories. Big enough soler arrays and a large number of factories can give you a very large amount of anti matter.

    Nope, for the 100 kg probe its all the energy which arrives on the earth for 4 minutes given a 100% efficient antimatter factory. For a 20 tonne spaceship, its more energy than has ever been used in the history of mankind.
    I mean so what if it is more power than we have ever used? Give a big enough soler array and enough factories we could still do it. Not that I am suggesting that we could currently afford to do so.

  • Pi-r8Pi-r8 Registered User regular
    edited April 2009
    tbloxham wrote: »
    So? What is it is more power than our world uses now?
    Build giant solar panels near mercury have them power antimatter factories. Big enough soler arrays and a large number of factories can give you a very large amount of anti matter.

    Nope, for the 100 kg probe its all the energy which arrives on the earth for 4 minutes given a 100% efficient antimatter factory. For a 20 tonne spaceship, its more energy than has ever been used in the history of mankind.
    I mean so what if it is more power than we have ever used? Give a big enough soler array and enough factories we could still do it. Not that I am suggesting that we could currently afford to do so.

    Building giant solar panels near mercury would probably require even more power. Not to mention the "antimatter factories".

  • durandal4532durandal4532 Registered User regular
    edited April 2009
    Well, essentially at the moment we can maybe possibly get a probe there in reasonable time, which would be pretty awesome. But all the options for getting us there seem to require such an energy expenditure that we're better off building some local infrastructure first. Which means moon-base, mars-base, and then on to figuring out how to get quintillions of Joules of solar energy easily.

  • AeolusdallasAeolusdallas Registered User regular
    edited April 2009
    Pi-r8 wrote: »
    tbloxham wrote: »
    So? What is it is more power than our world uses now?
    Build giant solar panels near mercury have them power antimatter factories. Big enough soler arrays and a large number of factories can give you a very large amount of anti matter.

    Nope, for the 100 kg probe its all the energy which arrives on the earth for 4 minutes given a 100% efficient antimatter factory. For a 20 tonne spaceship, its more energy than has ever been used in the history of mankind.
    I mean so what if it is more power than we have ever used? Give a big enough soler array and enough factories we could still do it. Not that I am suggesting that we could currently afford to do so.

    Building giant solar panels near mercury would probably require even more power. Not to mention the "antimatter factories".
    Of course but they could be built over a long period of time. My point is that saying "it would use more power than we have used before" not an automatic argument winner. Once/if we build a civilization that spans our solar system then such project may become feasible.

  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    tbloxham wrote: »
    tbloxham wrote:
    edit - even an antimatter ship tops out at 0.6 c, thats the point where making the fuel go that fast means you get no energy back from the fuel and you may as well not have brought it.
    What?

    By 'anti-matter ship' I mean a ship fueled by antimatter. If you bring along a kilogram of antimatter and a kilogram of matter with you, and your ship is 25% efficient at turning reaction energy into kinetic energy by the time you get to 0.6 c you have spent more energy speeding up that antimatter than you are going to get back by blowing it up. Effectively you'd have gone faster without it.
    Oh, I see what you mean. It doesn't actually work that way, though. There is no circumstance under which bringing more fuel will actually cause you to go slower once you've burned it all (as would be implied by your argument). Any increment of fuel added can be thought of as an increment in initial velocity instead (so a ship with an extra kilogram of fuel to start with is equivalent to a ship going as fast as that kilogram of fuel will accelerate you to when burned). Since you can never slow down by firing your engine (provided it's pointed the right way), we can conclude that any increment of fuel added increases the final speed.

    I can't find the flaw in your reasoning at the moment (and it is really starting to bother me; I've been typing this post for an hour and a half), but your conclusion is false.

    MWO User Name: Gorn Arming
    StarCraft II User Name: DeadMenRise
  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    Ha, I've got it!

    A given velocity increment is worth more kinetic energy at higher speed; consequently, when you are going, say, 100 m/s, and accelerate fuel combustion products out the back of your ship at 10 m/s, your fuel has lost 950 J of energy per kilogram from the perspective of someone here on the Earth (in whose reference frame your spaceship's velocity was measured). Your spacecraft has to get that much energy back for conservation of energy to not have been violated. The faster you go, the more energy is added. You get a partial refund on the deposit you made to accelerate that fuel, basically.

    The closer you get to c, of course, the more energy you get back from a given increment of velocity change in your exhaust—but the cost of a given increment of acceleration is even more, so you still run into a barrier. But it's not at 0.8c, or at any given number less than c, even for a fueled vehicle.

    MWO User Name: Gorn Arming
    StarCraft II User Name: DeadMenRise
  • Phoenix-DPhoenix-D Registered User regular
    edited April 2009
    ..what?

    You're going 100m/s. The fuel is ALSO going 100m/s. Whether you're at rest or moving it comes out relative to you at 10 m/s.

    And KE increases fairly linearly, IIRC. It does become very hard to accelerate further as you near c, but that's because of time dilation, not losing effective energy (you're accelerating at the same rate by your clock, but according to the rest of the universe your clocks are running slower)

  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    Phoenix-D wrote: »
    ..what?

    You're going 100m/s. The fuel is ALSO going 100m/s. Whether you're at rest or moving it comes out relative to you at 10 m/s.
    Right. The kinetic energy will appear vastly different in different reference frames, though.
    Phoenix-D wrote:
    And KE increases fairly linearly, IIRC.
    KE = (1/2)*m*V^2

    So, not linear. And it gets even weirder when you throw in relativity (the above is the Newtonian approximation).
    Phoenix-D wrote:
    It does become very hard to accelerate further as you near c, but that's because of time dilation, not losing effective energy (you're accelerating at the same rate by your clock, but according to the rest of the universe your clocks are running slower)
    No, you've got it partly wrong. You are indeed accelerating at the same rate by your clock, but this is exactly equivalent to an increased energy cost. Remember, energy is force*distance.

    MWO User Name: Gorn Arming
    StarCraft II User Name: DeadMenRise
  • Phoenix-DPhoenix-D Registered User regular
    edited April 2009
    Don't ask which formula I was using for KE. It was pretty hilariously wrong. :P

    If you by your clock are accelerating at a steady rate- the force*distance seems like it should be the same. (to your point of view)

  • ElJeffeElJeffe Super Moderator, Moderator, ClubPA mod
    edited April 2009
    Also keep in mind that, from your own perspective, if you're assuming the destination to be a fixed distance away, it can appear that you're traveling at an arbitrarily large velocity. Given that you know you have to go, say, 20 light years, you could theoretically make the trip in (what seems to you to be) one second. If you do all these calculations in the proper reference frames then nothing is ever going faster than c and all is well, but it can create the illusion of potentially infinite velocity.

    The discussions you guys are having are, at times, largely ignoring relativity in such a way that it makes more sense to think of it not as reaching c, but rather reaching an infinite velocity. The discussion's going to need to be a bit more rigid if we want to talk about limitations in any concrete manner.

    Maddie: "I named my feet. The left one is flip and the right one is flop. Oh, and also I named my flip-flops."

    I make tweet.
  • tbloxhamtbloxham Registered User regular
    edited April 2009
    Scalfin wrote: »
    Scalfin wrote: »
    Scalfin wrote: »
    NotACrook wrote: »
    Kagera wrote: »

    Because once you piss it our you're fucked.

    There's no pissing in hell. You feel like you've got to go, but you just can't squeeze anything out.

    I'm sure there's a chapter in the old testament about it.

    There's no hell in Judaism, so it's either new or one of those parts Christians added after the fact.


    Is there any way to accelerate at the rates we're talking about w/o being turned to mush? If not, maybe the best thing to do is to accelerate at 1g for 10 light years and decelerate at 1g for the next ten.
    1 g of acceleration, if you could sustain it, would get you to 0.5c in less than half a year. Acceleration isn't really the critical problem (although high accelerations are necessary to get the most out of certain gravity assist maneuvers).

    How long would it take if we accelerated at 1g for half the voyage and then slowed back down over the second half (I'm assuming that speeds will stay low enough that force will still equal acceleration)?
    They won't stay low enough. The Gliese 581 system is 20 LY away, so there's a minimum trip time of 20 years. If you could sustain 1 g of acceleration for even 1 full year you'd hit the speed of light.

    Of course, it's impossible to sustain an acceleration like that, because the energy cost increases toward infinity (which occurs at the speed of light).

    Is there any way to calculate how long it would take w/ one g of force, then? Basically I want to know how long it would take if all artificial gravity was supplied by the engines.

    go to the atomic rockets site
    http://www.projectrho.com/rocket/
    they have a lot of information like that there.

    That site is almost all for in solar system stuff, you can see that they haven't used the relativistic corrections which you have to do for any kind of interstellar mission. In the solar system 1 g is indeed not very hard acceleration and you would wish you could accelerate harder. In an interstellar mission the force output of the engines is pretty much unimportant above a certain threshold, effectively your ship will be at max speed well before halfway.

    Your puny weapons are useless against me
  • tbloxhamtbloxham Registered User regular
    edited April 2009
    ElJeffe wrote: »
    Also keep in mind that, from your own perspective, if you're assuming the destination to be a fixed distance away, it can appear that you're traveling at an arbitrarily large velocity. Given that you know you have to go, say, 20 light years, you could theoretically make the trip in (what seems to you to be) one second. If you do all these calculations in the proper reference frames then nothing is ever going faster than c and all is well, but it can create the illusion of potentially infinite velocity.

    The discussions you guys are having are, at times, largely ignoring relativity in such a way that it makes more sense to think of it not as reaching c, but rather reaching an infinite velocity. The discussion's going to need to be a bit more rigid if we want to talk about limitations in any concrete manner.

    Hey, I'm not ignoring relativity, thats why everything in my posts is going so grindingly slowly despite requiring such ridiculous levels of supertech :)

    Your puny weapons are useless against me
  • [Tycho?][Tycho?] Registered User regular
    edited April 2009
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    ragesig.jpg

  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    Phoenix-D wrote:
    If you by your clock are accelerating at a steady rate- the force*distance seems like it should be the same. (to your point of view)
    It is the same, in your reference frame.

    I'm not sure where you think we disagree.
    ElJeffe wrote: »
    Also keep in mind that, from your own perspective, if you're assuming the destination to be a fixed distance away, it can appear that you're traveling at an arbitrarily large velocity. Given that you know you have to go, say, 20 light years, you could theoretically make the trip in (what seems to you to be) one second. If you do all these calculations in the proper reference frames then nothing is ever going faster than c and all is well, but it can create the illusion of potentially infinite velocity.

    The discussions you guys are having are, at times, largely ignoring relativity in such a way that it makes more sense to think of it not as reaching c, but rather reaching an infinite velocity. The discussion's going to need to be a bit more rigid if we want to talk about limitations in any concrete manner.
    We're not ignoring relativity (I'm not, at least), it's just that we don't have any way of getting to the very high velocities where it starts to really make a difference.

    MWO User Name: Gorn Arming
    StarCraft II User Name: DeadMenRise
  • JebusUDJebusUD Registered User regular
    edited April 2009
    [Tycho?] wrote: »
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    I don't think a "light sail" would work. Unless you are saying that the lasers are independent of the craft. In which case it would only work for a short distance and then likely have the light blocked or diffused by somthing.

    Otherwise it would just sit in place, because the light emitted from the laser would push back and then be canceled out by the light hitting the said.

    You haven't given me a reason to steer clear of you!
  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    JebusUD wrote: »
    [Tycho?] wrote: »
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    I don't think a "light sail" would work. Unless you are saying that the lasers are independent of the craft. In which case it would only work for a short distance and then likely have the light blocked or diffused by somthing.

    Otherwise it would just sit in place, because the light emitted from the laser would push back and then be canceled out by the light hitting the said.
    The lasers are in space; probably solar-powered and therefore close to the sun. They focus light on the sail to propel it. The light is obscured somewhat by dust and it does spread out some from the original beam dimensions, but the idea is that you project enough power (and have a large enough source emitter) that these factors still leave you with a lot of force on the sail.

    MWO User Name: Gorn Arming
    StarCraft II User Name: DeadMenRise
  • AJAlkaline40AJAlkaline40 __BANNED USERS
    edited April 2009
    JebusUD wrote: »
    [Tycho?] wrote: »
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    I don't think a "light sail" would work. Unless you are saying that the lasers are independent of the craft. In which case it would only work for a short distance and then likely have the light blocked or diffused by somthing.

    Otherwise it would just sit in place, because the light emitted from the laser would push back and then be canceled out by the light hitting the said.
    The lasers are in space; probably solar-powered and therefore close to the sun. They focus light on the sail to propel it. The light is obscured somewhat by dust and it does spread out some from the original beam dimensions, but the idea is that you project enough power (and have a large enough source emitter) that these factors still leave you with a lot of force on the sail.

    That sounds like it would be difficult to control the rate of acceleration finely, though, and how does the breaking mechanism work?

    idiot.jpg
  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    JebusUD wrote: »
    [Tycho?] wrote: »
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    I don't think a "light sail" would work. Unless you are saying that the lasers are independent of the craft. In which case it would only work for a short distance and then likely have the light blocked or diffused by somthing.

    Otherwise it would just sit in place, because the light emitted from the laser would push back and then be canceled out by the light hitting the said.
    The lasers are in space; probably solar-powered and therefore close to the sun. They focus light on the sail to propel it. The light is obscured somewhat by dust and it does spread out some from the original beam dimensions, but the idea is that you project enough power (and have a large enough source emitter) that these factors still leave you with a lot of force on the sail.

    That sounds like it would be difficult to control the rate of acceleration finely, though, and how does the breaking mechanism work?
    Why would it be difficult to control the acceleration? You can fine-tune the beam intensity or the number of beams converging on the target. I'd say it's actually more controllable than your average liquid-fueled rocket.

    Braking is accomplished by use of a magnetic sail once you reach your destination star system. There are other ways to do it as well, but a magnetic sail is the usual proposal.

    MWO User Name: Gorn Arming
    StarCraft II User Name: DeadMenRise
  • Salvation122Salvation122 Registered User regular
    edited April 2009
    tbloxham wrote: »
    tbloxham wrote:
    edit - even an antimatter ship tops out at 0.6 c, thats the point where making the fuel go that fast means you get no energy back from the fuel and you may as well not have brought it.
    What?

    By 'anti-matter ship' I mean a ship fueled by antimatter. If you bring along a kilogram of antimatter and a kilogram of matter with you, and your ship is 25% efficient at turning reaction energy into kinetic energy by the time you get to 0.6 c you have spent more energy speeding up that antimatter than you are going to get back by blowing it up. Effectively you'd have gone faster without it.

    Okay, so, I don't really know dick about theoretical physics, but couldn't you use the same principles that govern a Bussard ramjet to scoop up the matter-half of your reaction fuel? That'd hugely cut down on your mass.

    sig.png
  • EvigilantEvigilant Registered User regular
    edited April 2009
    JebusUD wrote: »
    [Tycho?] wrote: »
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    I don't think a "light sail" would work. Unless you are saying that the lasers are independent of the craft. In which case it would only work for a short distance and then likely have the light blocked or diffused by somthing.

    Otherwise it would just sit in place, because the light emitted from the laser would push back and then be canceled out by the light hitting the said.
    The lasers are in space; probably solar-powered and therefore close to the sun. They focus light on the sail to propel it. The light is obscured somewhat by dust and it does spread out some from the original beam dimensions, but the idea is that you project enough power (and have a large enough source emitter) that these factors still leave you with a lot of force on the sail.

    That sounds like it would be difficult to control the rate of acceleration finely, though, and how does the breaking mechanism work?
    Why would it be difficult to control the acceleration? You can fine-tune the beam intensity or the number of beams converging on the target. I'd say it's actually more controllable than your average liquid-fueled rocket.

    Braking is accomplished by use of a magnetic sail once you reach your destination star system. There are other ways to do it as well, but a magnetic sail is the usual proposal.

    Why not send out probes to drop a "route", basically stationary platforms in space that shoot laser beams till the next spot or as far as they remain viable. Then said craft would be pushed by these lasers until reaching it's destination. Obviously the initial setup is slow (and costly), but wouldn't the next trips then be much more efficient and less costly?

    "Though force can protect in emergency, only justice, fairness, consideration and cooperation can finally lead men to the dawn of eternal peace." - General Dwight D. Eisenhower
    Google+ Profile Origin: 13Evigilant Steam: Evigilant
  • Salvation122Salvation122 Registered User regular
    edited April 2009
    JebusUD wrote: »
    [Tycho?] wrote: »
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    I don't think a "light sail" would work. Unless you are saying that the lasers are independent of the craft. In which case it would only work for a short distance and then likely have the light blocked or diffused by somthing.

    Otherwise it would just sit in place, because the light emitted from the laser would push back and then be canceled out by the light hitting the said.
    The lasers are in space; probably solar-powered and therefore close to the sun. They focus light on the sail to propel it. The light is obscured somewhat by dust and it does spread out some from the original beam dimensions, but the idea is that you project enough power (and have a large enough source emitter) that these factors still leave you with a lot of force on the sail.

    That sounds like it would be difficult to control the rate of acceleration finely, though, and how does the breaking mechanism work?
    Why would it be difficult to control the acceleration? You can fine-tune the beam intensity or the number of beams converging on the target. I'd say it's actually more controllable than your average liquid-fueled rocket.

    Braking is accomplished by use of a magnetic sail once you reach your destination star system. There are other ways to do it as well, but a magnetic sail is the usual proposal.

    I was under the impression that braking for these sorts of systems was generally done by orbital capture, which is extremely fuckin' slow but utterly dependable (assuming you do the math right.)

    sig.png
  • tbloxhamtbloxham Registered User regular
    edited April 2009
    tbloxham wrote: »
    tbloxham wrote:
    edit - even an antimatter ship tops out at 0.6 c, thats the point where making the fuel go that fast means you get no energy back from the fuel and you may as well not have brought it.
    What?

    By 'anti-matter ship' I mean a ship fueled by antimatter. If you bring along a kilogram of antimatter and a kilogram of matter with you, and your ship is 25% efficient at turning reaction energy into kinetic energy by the time you get to 0.6 c you have spent more energy speeding up that antimatter than you are going to get back by blowing it up. Effectively you'd have gone faster without it.

    Okay, so, I don't really know dick about theoretical physics, but couldn't you use the same principles that govern a Bussard ramjet to scoop up the matter-half of your reaction fuel? That'd hugely cut down on your mass.

    Hmm, that might help a bit. But remember that I'm using the relativistic calculations here, so even things that seem hugely helpful only give you a tiny scrap more speed. Using a ramscoop effectively lets you carry just the antimatter, and thus doubles your total fuel mass. Now, the calculations for this are very complicated to do exactly, but you can get a 'theoretical maximum' from using the equation for the relativistic kinetic energy of a kg of fuel travelling at the target speed and then dividing that by the possible liberated KE from that fuel. When the amount of energy required to accelerate that fuel to a given speed is greater than the energy released by that fuel, you have your maximum speed, since no amount of extra fuel of that sort will make you go any faster. You just accelerate for longer at a slower rate.

    Relativistically..


    KE = (mc^2)/(1-v^2/c^2)^0.5 - mc^2

    liberated KE = ship efficiency * fraction of ship which is fuel * fuel to energy conversion factor * mc^2

    Using a ship efficiency of 0.25, and 90% of the ship being fuel, and a fuel to energy conversion factor of 2 (assuming we can somehow scoop up the mass with our ramscoop for free, it will actually be less than this) our speed tops out at 72% c.

    Your puny weapons are useless against me
  • tbloxhamtbloxham Registered User regular
    edited April 2009
    Ha, I've got it!

    A given velocity increment is worth more kinetic energy at higher speed; consequently, when you are going, say, 100 m/s, and accelerate fuel combustion products out the back of your ship at 10 m/s, your fuel has lost 950 J of energy per kilogram from the perspective of someone here on the Earth (in whose reference frame your spaceship's velocity was measured). Your spacecraft has to get that much energy back for conservation of energy to not have been violated. The faster you go, the more energy is added. You get a partial refund on the deposit you made to accelerate that fuel, basically.

    The closer you get to c, of course, the more energy you get back from a given increment of velocity change in your exhaust—but the cost of a given increment of acceleration is even more, so you still run into a barrier. But it's not at 0.8c, or at any given number less than c, even for a fueled vehicle.

    It absolutely is. Look up the relativistic kinetic energy of an object on wikipedia, or look up at my previous post where I laid it out. As soon as the amount of energy required to accelerate that fuel to that speed is greater than the amount of energy you can liberate from the fuel then you would have been better off building a lighter spacecraft and not bringing the fuel. If the fuel requires more energy to accelerate than it gives back then its not fuel, its simply dead weight. True once the fuel is there you may as well burn the fuel, but it wont do any good since you will never get to that maximum speed powered by that type of fuel, you'll just get exponentially closer and closer as your fuel gets more and more useless. I suppose yes, no piece of fuel is ever totally useless, but you'd soon get to the point where you would be burning as much energy for the last m/s as you did for the 2e8 m/s before it.

    edit - oh, and the reason you dont get your deposit back, is that you need to do the work on the fuel to shoot it out the back. To you you would think nothing had happened, but from earth it would look like you were shooting out your fuel really slowly and at a hugely decreased rate. You never get a single erg of that kinetic energy used to accelerate the fuel back, its all wasted.

    Your puny weapons are useless against me
  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    tbloxham wrote: »
    Ha, I've got it!

    A given velocity increment is worth more kinetic energy at higher speed; consequently, when you are going, say, 100 m/s, and accelerate fuel combustion products out the back of your ship at 10 m/s, your fuel has lost 950 J of energy per kilogram from the perspective of someone here on the Earth (in whose reference frame your spaceship's velocity was measured). Your spacecraft has to get that much energy back for conservation of energy to not have been violated. The faster you go, the more energy is added. You get a partial refund on the deposit you made to accelerate that fuel, basically.

    The closer you get to c, of course, the more energy you get back from a given increment of velocity change in your exhaust—but the cost of a given increment of acceleration is even more, so you still run into a barrier. But it's not at 0.8c, or at any given number less than c, even for a fueled vehicle.

    It absolutely is. Look up the relativistic kinetic energy of an object on wikipedia, or look up at my previous post where I laid it out. As soon as the amount of energy required to accelerate that fuel to that speed is greater than the amount of energy you can liberate from the fuel then you would have been better off building a lighter spacecraft and not bringing the fuel. If the fuel requires more energy to accelerate than it gives back then its not fuel, its simply dead weight. True once the fuel is there you may as well burn the fuel, but it wont do any good since you will never get to that maximum speed powered by that type of fuel, you'll just get exponentially closer and closer as your fuel gets more and more useless. I suppose yes, no piece of fuel is ever totally useless, but you'd soon get to the point where you would be burning as much energy for the last m/s as you did for the 2e8 m/s before it.

    edit - oh, and the reason you dont get your deposit back, is that you need to do the work on the fuel to shoot it out the back. To you you would think nothing had happened, but from earth it would look like you were shooting out your fuel really slowly and at a hugely decreased rate. You never get a single erg of that kinetic energy used to accelerate the fuel back, its all wasted.
    There's still a problem with your line of reasoning. First, we know that the exit velocity, as measured in the reference frame of the spacecraft, is constant for a given type of engine operating under given conditions. That is, it doesn't vary with velocity. (If it did, that would imply the existence of absolute speed and privileged reference frames, both of which flatly contradict relativity.) Consequently, from the spacecraft's perspective, it will undergo constant acceleration. At very high relativistic speeds, time dilation comes into play, and does impose a velocity limit since time for you is running much more slowly relative to someone who is moving at nearly c relative to you. But the phenomenon you are postulating applies, at varying speeds for varying fuel enthalpies, regardless of relativistic effects. So let us ignore them for the moment.

    Say you are near your "maximum velocity". In fact, say you have reached it, or come infinitesimally close. Your claim is that burning a mass of fuel will not accelerate you (or will accelerate you a negligible amount) because the energy will be contained in the exhaust's kinetic energy exclusively. So, the exhaust plume travels away from you at (whatever your exhaust speed is for this type of rocket), and you don't accelerate at all. This is impossible, however, because it violates conservation of momentum. Whatever momentum is given to the exhaust plume must be recovered with an opposite sign in the spacecraft.

    Looked at another way, you are essentially telling our spacecraft crew that they will fire their engines, a plume of exhaust will erupt out the back of their ship, and... nothing will happen. If this were true, they'd be able to infer their "absolute" speed as the speed at which this happens (or the speed at which it begins to have a measurable effect; I think the point is clear). But again, the notion of an "absolute" speed contradicts relativity, so again we have arrived at an impossibility.

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  • ElJeffeElJeffe Super Moderator, Moderator, ClubPA mod
    edited April 2009
    The idea isn't that you'll have fuel on your ship that, when you burn it, will not accelerate you to a faster velocity than that at which you're already traveling. You're correct in that, once you have the fuel on board, you're better off burning it.

    The idea is that you're not better off having the fuel on board to begin with. Instead of thinking of that moment 2 years into the mission when you're firing your rockets, think of the entire trip. Your acceleration across that entire trip will be lower because of all that fuel you brought on. As such, the cumulative effect of all that fuel tops out at a given velocity.

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  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    ElJeffe wrote: »
    The idea isn't that you'll have fuel on your ship that, when you burn it, will not accelerate you to a faster velocity than that at which you're already traveling. You're correct in that, once you have the fuel on board, you're better off burning it.

    The idea is that you're not better off having the fuel on board to begin with. Instead of thinking of that moment 2 years into the mission when you're firing your rockets, think of the entire trip. Your acceleration across that entire trip will be lower because of all that fuel you brought on. As such, the cumulative effect of all that fuel tops out at a given velocity.
    I understand his argument. I was looking at its implications later on to explain why it is wrong. For this scheme to work, you need privileged reference frames and you will end up violating conservation of momentum.

    From your perspective (on the spaceship), a given quantity of fuel burned will produce a given force (and an acceleration proportional to your fuel mass). You are correct in that the fuel required to gain an additional velocity increment at the end of your burn is larger the higher that end-of-burn velocity is, but there is no "hard limit". If you start with more fuel, you have a lower acceleration to begin with. The cost of obtaining a given velocity increases exponentially (and faster than exponentially at relativistic speeds), but it does not increase towards an asymptote (except at c, of course).

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  • fjafjanfjafjan Registered User
    edited April 2009
    Seriously, the way easiest way to get to this planet is to focus on stopping aging. After we've got that pretty much down so we can live untill 500 without dying from out bodies shitting out over time, an 80 year trip isn't that big of a problem. And really, it's pretty unlikely that there will be an upper bound to our natural lives at that point.

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  • AJAlkaline40AJAlkaline40 __BANNED USERS
    edited April 2009
    fjafjan wrote: »
    Seriously, the way easiest way to get to this planet is to focus on stopping aging. After we've got that pretty much down so we can live untill 500 without dying from out bodies shitting out over time, an 80 year trip isn't that big of a problem. And really, it's pretty unlikely that there will be an upper bound to our natural lives at that point.

    Agreed.

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  • ElJeffeElJeffe Super Moderator, Moderator, ClubPA mod
    edited April 2009
    From your perspective (on the spaceship), a given quantity of fuel burned will produce a given force (and an acceleration proportional to your fuel mass). You are correct in that the fuel required to gain an additional velocity increment at the end of your burn is larger the higher that end-of-burn velocity is, but there is no "hard limit". If you start with more fuel, you have a lower acceleration to begin with. The cost of obtaining a given velocity increases exponentially (and faster than exponentially at relativistic speeds), but it does not increase towards an asymptote (except at c, of course).

    Can you prove mathematically that this is the case? Because I don't think you can.

    Meanwhile, tbloxham spelled out the precise equations you need to calculate the maximum speed for a bit of fuel. Look:

    A) For a given chunk of fuel mass, it will take a given amount of energy to accelerate it to some speed, V. Call that energy K.
    B) For a given chunk of fuel mass, there will be some maximum amount of energy you can get out of it, given the nature of your propulsion system. Call that energy P.
    C) If, for a given V, K > P, you cannot reach that velocity. It is impossible.

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  • [Tycho?][Tycho?] Registered User regular
    edited April 2009
    JebusUD wrote: »
    [Tycho?] wrote: »
    I always like the light sail; set up some big fucking ass lasers, and shoot them at the mirrored behind of your craft. Remember, these lasers are big.

    Throw on a bussard ramjet as a brake, and collect fuel while you're at it.

    I don't think a "light sail" would work. Unless you are saying that the lasers are independent of the craft. In which case it would only work for a short distance and then likely have the light blocked or diffused by somthing.

    Otherwise it would just sit in place, because the light emitted from the laser would push back and then be canceled out by the light hitting the said.
    The lasers are in space; probably solar-powered and therefore close to the sun. They focus light on the sail to propel it. The light is obscured somewhat by dust and it does spread out some from the original beam dimensions, but the idea is that you project enough power (and have a large enough source emitter) that these factors still leave you with a lot of force on the sail.

    That sounds like it would be difficult to control the rate of acceleration finely, though, and how does the breaking mechanism work?
    Why would it be difficult to control the acceleration? You can fine-tune the beam intensity or the number of beams converging on the target. I'd say it's actually more controllable than your average liquid-fueled rocket.

    Braking is accomplished by use of a magnetic sail once you reach your destination star system. There are other ways to do it as well, but a magnetic sail is the usual proposal.

    If you're using a magnetic sail you might as well make it a ram jet. I believe I read something a few years ago that said Bussard ram jets were not practical because the amount of drag they caused in the interstellar medium was larger than the thrust you'd get out of burning the hydrogen that you collected. But if you're breaking, then you want all the drag you can. Collect fuel while your at it; fuse it and throw it back out the front of your craft to break even faster, or just collect it for use later.

    ragesig.jpg

  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    ElJeffe wrote: »
    From your perspective (on the spaceship), a given quantity of fuel burned will produce a given force (and an acceleration proportional to your fuel mass). You are correct in that the fuel required to gain an additional velocity increment at the end of your burn is larger the higher that end-of-burn velocity is, but there is no "hard limit". If you start with more fuel, you have a lower acceleration to begin with. The cost of obtaining a given velocity increases exponentially (and faster than exponentially at relativistic speeds), but it does not increase towards an asymptote (except at c, of course).

    Can you prove mathematically that this is the case? Because I don't think you can.

    Meanwhile, tbloxham spelled out the precise equations you need to calculate the maximum speed for a bit of fuel. Look:

    A) For a given chunk of fuel mass, it will take a given amount of energy to accelerate it to some speed, V. Call that energy K.
    B) For a given chunk of fuel mass, there will be some maximum amount of energy you can get out of it, given the nature of your propulsion system. Call that energy P.
    C) If, for a given V, K > P, you cannot reach that velocity. It is impossible.
    B is false. The "maximum amount of energy you can get out of it" is only constant in your reference frame.

    Look at it this way: if you are at this hypothetical speed limit (or very close), what will happen when you turn on your engines? Disregard the amount of fuel it took you to get there; say you are within 0.0001 km/s or whatever of your engine's "maximum speed". What happens when you turn it on? We know for certain that events from the spacecraft's perspective proceed as normal, because there are no privileged reference frames. So, your engine won't mysteriously cease to function. Given that, how can you satisfy conservation of momentum without accelerating? You cannot. Things change a little bit when time dilation is thrown in, but that has nothing to do with the limit you and tbloxham are proposing.

    If this is all too hard to follow, I can dig through the equations later tonight, I think I have to get to a meeting now.

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  • zakkielzakkiel Registered User
    edited April 2009
    ElJeffe wrote: »
    From your perspective (on the spaceship), a given quantity of fuel burned will produce a given force (and an acceleration proportional to your fuel mass). You are correct in that the fuel required to gain an additional velocity increment at the end of your burn is larger the higher that end-of-burn velocity is, but there is no "hard limit". If you start with more fuel, you have a lower acceleration to begin with. The cost of obtaining a given velocity increases exponentially (and faster than exponentially at relativistic speeds), but it does not increase towards an asymptote (except at c, of course).

    Can you prove mathematically that this is the case? Because I don't think you can.

    Meanwhile, tbloxham spelled out the precise equations you need to calculate the maximum speed for a bit of fuel. Look:

    A) For a given chunk of fuel mass, it will take a given amount of energy to accelerate it to some speed, V. Call that energy K.
    B) For a given chunk of fuel mass, there will be some maximum amount of energy you can get out of it, given the nature of your propulsion system. Call that energy P.
    C) If, for a given V, K > P, you cannot reach that velocity. It is impossible.
    B is false. The "maximum amount of energy you can get out of it" is only constant in your reference frame.

    Look at it this way: if you are at this hypothetical speed limit (or very close), what will happen when you turn on your engines? Disregard the amount of fuel it took you to get there; say you are within 0.0001 km/s or whatever of your engine's "maximum speed". What happens when you turn it on? We know for certain that events from the spacecraft's perspective proceed as normal, because there are no privileged reference frames. So, your engine won't mysteriously cease to function. Given that, how can you satisfy conservation of momentum without accelerating? You cannot. Things change a little bit when time dilation is thrown in, but that has nothing to do with the limit you and tbloxham are proposing.

    If this is all too hard to follow, I can dig through the equations later tonight, I think I have to get to a meeting now.

    No, B is correct; you only get a certain amount of energy out of a given quantity of fuel, regardless of the speed you're going when you burn it (in the non-relativistic approximation).

    The real answer to El Jeffe is that in fact tbloxham has not provided an equation. The relevant equation is the rocket equation, delta v = v.exh * ln( m0/m1). Logarythmic equations have no horizontal asymptote. Problem solved.

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  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    zakkiel wrote: »
    ElJeffe wrote: »
    From your perspective (on the spaceship), a given quantity of fuel burned will produce a given force (and an acceleration proportional to your fuel mass). You are correct in that the fuel required to gain an additional velocity increment at the end of your burn is larger the higher that end-of-burn velocity is, but there is no "hard limit". If you start with more fuel, you have a lower acceleration to begin with. The cost of obtaining a given velocity increases exponentially (and faster than exponentially at relativistic speeds), but it does not increase towards an asymptote (except at c, of course).

    Can you prove mathematically that this is the case? Because I don't think you can.

    Meanwhile, tbloxham spelled out the precise equations you need to calculate the maximum speed for a bit of fuel. Look:

    A) For a given chunk of fuel mass, it will take a given amount of energy to accelerate it to some speed, V. Call that energy K.
    B) For a given chunk of fuel mass, there will be some maximum amount of energy you can get out of it, given the nature of your propulsion system. Call that energy P.
    C) If, for a given V, K > P, you cannot reach that velocity. It is impossible.
    B is false. The "maximum amount of energy you can get out of it" is only constant in your reference frame.

    Look at it this way: if you are at this hypothetical speed limit (or very close), what will happen when you turn on your engines? Disregard the amount of fuel it took you to get there; say you are within 0.0001 km/s or whatever of your engine's "maximum speed". What happens when you turn it on? We know for certain that events from the spacecraft's perspective proceed as normal, because there are no privileged reference frames. So, your engine won't mysteriously cease to function. Given that, how can you satisfy conservation of momentum without accelerating? You cannot. Things change a little bit when time dilation is thrown in, but that has nothing to do with the limit you and tbloxham are proposing.

    If this is all too hard to follow, I can dig through the equations later tonight, I think I have to get to a meeting now.

    No, B is correct; you only get a certain amount of energy out of a given quantity of fuel, regardless of the speed you're going when you burn it (in the non-relativistic approximation).

    The real answer to El Jeffe is that in fact tbloxham has not provided an equation. The relevant equation is the rocket equation, delta v = v.exh * ln( m0/m1). Logarythmic equations have no horizontal asymptote. Problem solved.
    B is correct in the reference frame of the spacecraft, which is what I said. It is not correct in the reference frame of the Earth, which is what ElJeffe is claiming (or postulating, I guess). Look at it in terms of velocity difference: from the perspective of Earth, that "1000 m/s" or whatever that your engine spits out in the exhaust represents a much greater total change in kinetic energy when you're already traveling at high speeds. Power is (thrust*velocity), after all, and thrust is constant in our situation. (And of course, at relativistic speeds, your engine ceases to spit out a constant exit velocity at all, at least as meaured from the Earth. But that doesn't affect the hypothetical velocity limit we're arguing about here.)

    As for the rocket equation—I don't think it's really an answer; it seems to be the question in fact. The accuracy of that equation is what everyone here is debating. I believe it is valid; ElJeffe and tbloxham do not. I am glad that you and I seem to agree though.

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  • zakkielzakkiel Registered User
    edited April 2009
    B is correct in the reference frame of the spacecraft, which is what I said. It is not correct in the reference frame of the Earth, which is what ElJeffe is claiming (or postulating, I guess). Look at it in terms of velocity difference: from the perspective of Earth, that "1000 m/s" or whatever that your engine spits out in the exhaust represents a much greater total change in kinetic energy when you're already traveling at high speeds. Power is (thrust*velocity), after all, and thrust is constant in our situation. (And of course, at relativistic speeds, your engine ceases to spit out a constant exit velocity at all, at least as meaured from the Earth. But that doesn't affect the hypothetical velocity limit we're arguing about here.)

    As for the rocket equation—I don't think it's really an answer; it seems to be the question in fact. The accuracy of that equation is what everyone here is debating. I believe it is valid; ElJeffe and tbloxham do not. I am glad that you and I seem to agree though.

    We're saying the same thing in different ways. The chemical or nuclear or antimatter reaction will yield the same energy; you're including the additional energy the exhaust will impart to the kinetic energy of the ship. The reason that the additional energy the ship gets from the reaction is greater at higher velocities is that the fuel itself has already been accelerated, and so you get the benefit of the kinetic energy already in the fuel. When you blow it up, the impulse of the particles against the ship will include both the energy from the fuel reaction and the energy from the forward movement the fuel already had.

    I don't think tbloxham and El Jeffe are intentionally opposing the rocket equation. I certainly hope not, anyway! I think they both just realized that you get diminishing returns from increasing amounts of fuel and assumed there had to be an asymptote somewhere.

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  • CycloneRangerCycloneRanger Registered User regular
    edited April 2009
    zakkiel wrote: »
    B is correct in the reference frame of the spacecraft, which is what I said. It is not correct in the reference frame of the Earth, which is what ElJeffe is claiming (or postulating, I guess). Look at it in terms of velocity difference: from the perspective of Earth, that "1000 m/s" or whatever that your engine spits out in the exhaust represents a much greater total change in kinetic energy when you're already traveling at high speeds. Power is (thrust*velocity), after all, and thrust is constant in our situation. (And of course, at relativistic speeds, your engine ceases to spit out a constant exit velocity at all, at least as meaured from the Earth. But that doesn't affect the hypothetical velocity limit we're arguing about here.)

    As for the rocket equation—I don't think it's really an answer; it seems to be the question in fact. The accuracy of that equation is what everyone here is debating. I believe it is valid; ElJeffe and tbloxham do not. I am glad that you and I seem to agree though.

    We're saying the same thing in different ways. The chemical or nuclear or antimatter reaction will yield the same energy; you're including the additional energy the exhaust will impart to the kinetic energy of the ship. The reason that the additional energy the ship gets from the reaction is greater at higher velocities is that the fuel itself has already been accelerated, and so you get the benefit of the kinetic energy already in the fuel. When you blow it up, the impulse of the particles against the ship will include both the energy from the fuel reaction and the energy from the forward movement the fuel already had.
    Ah, I see. We are agreed.
    zakkiel wrote:
    I don't think tbloxham and El Jeffe are intentionally opposing the rocket equation. I certainly hope not, anyway! I think they both just realized that you get diminishing returns from increasing amounts of fuel and assumed there had to be an asymptote somewhere.
    Seems likely.

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  • tbloxhamtbloxham Registered User regular
    edited April 2009
    ElJeffe wrote: »
    From your perspective (on the spaceship), a given quantity of fuel burned will produce a given force (and an acceleration proportional to your fuel mass). You are correct in that the fuel required to gain an additional velocity increment at the end of your burn is larger the higher that end-of-burn velocity is, but there is no "hard limit". If you start with more fuel, you have a lower acceleration to begin with. The cost of obtaining a given velocity increases exponentially (and faster than exponentially at relativistic speeds), but it does not increase towards an asymptote (except at c, of course).

    Can you prove mathematically that this is the case? Because I don't think you can.

    Meanwhile, tbloxham spelled out the precise equations you need to calculate the maximum speed for a bit of fuel. Look:

    A) For a given chunk of fuel mass, it will take a given amount of energy to accelerate it to some speed, V. Call that energy K.
    B) For a given chunk of fuel mass, there will be some maximum amount of energy you can get out of it, given the nature of your propulsion system. Call that energy P.
    C) If, for a given V, K > P, you cannot reach that velocity. It is impossible.
    B is false. The "maximum amount of energy you can get out of it" is only constant in your reference frame.

    Look at it this way: if you are at this hypothetical speed limit (or very close), what will happen when you turn on your engines? Disregard the amount of fuel it took you to get there; say you are within 0.0001 km/s or whatever of your engine's "maximum speed". What happens when you turn it on? We know for certain that events from the spacecraft's perspective proceed as normal, because there are no privileged reference frames. So, your engine won't mysteriously cease to function. Given that, how can you satisfy conservation of momentum without accelerating? You cannot. Things change a little bit when time dilation is thrown in, but that has nothing to do with the limit you and tbloxham are proposing.

    If this is all too hard to follow, I can dig through the equations later tonight, I think I have to get to a meeting now.

    Hmm, I guess I am wrong as I just wrote a little program to check it. The effect of the diminishing mass of the ship has an effect that I'm not seeing in the basic assumptions.

    I still think that a ship with 10J/kg energy density at the start won't go faster than 4.5 m/s and so on. I do not see how a fuel, being accelerated by the same fuel which is then discarded can end up with a greater energy density at the end of Kinetic energy than it had at the beginning in 'chemical' energy when viewed from the reference frame where it began.

    edit - OK, having done the same thing again my calculations are correct for a ship which doesn't change significantly in mass, and I see how continually reducing mass will help (obviously the last drop of fuel is the most efficient at transfering KE to the payload) but I still don't quite see how the KE is 'concentrated'.

    edit - And it really is definately happening, tomorrow I must sit down and solve these equations to figure it out. Effectively it seems that if the ship mass reduces to near zero (99% mass loss) then the basic Chemical Energy density at start = kinetic energy density at end equation is out by about a factor of 2.

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  • override367override367 Registered User regular
    edited April 2009
    Kagera wrote: »
    Sometimes I wonder where we would be if U.S.'s military budget was entirely devoted to NASA...

    I can't wait when we get to Mars, though. That should happen during my lifetime as well.

    Pfft, getting there is boring.

    Setting up a colony would be cool.

    If half the US military budget of the last 50 years had been devoted to Nasa we would probably have several thriving moon colonies, and even if not, the advances in many of the sciences would be impossible to comprehend. The advances in alternate energy alone are hard to imagine.
    HamHamJ wrote: »
    Time Dilation-
    -does very little for us back home waiting for them to get there.

    You're thinking too small damnit. We figure out a way to dialate time around the entire earth, sure it might fuck with the weather having seasons go buy in like 20 minutes rather than months, and the spaceship crew would still take 80 years to get there, but at least we won't have to wait long.


    On a more serious note, I think humanity should make things like space elevators and asteroid mining an actual priority, as a species. While getting to another solar system is sci-fi maybe forever (although I'd like to think that someday, if we're still around, we figure out a way to do things that we can't even conceive of right now), intra-solar system activities are all quite possible. If we can figure out an economical way to harvest asteroids and other raw materials from within our solar system, we've effectively put an end to scarcity for a great many materials.

    If the human race is still progressing a hundred years from now, inter solar system colonies and bases are an inevitability.

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  • electricitylikesmeelectricitylikesme Registered User regular
    edited April 2009
    I want undersea cities before space elevators and moon bases.

    Though I'd totally live on the moon, but not under the sea.

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