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Inter-stellar ion drive

A relativistic rocket engine that ionizes stray hydrogen atoms
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Space is not (quite) a vacuum. There is (on the average) some amount of matter in every cubic meter of space.

Now imagine a starship, something along the lines of the killer cornucopia in the Start Trek episode "The Doomsday Machine". As it travels through inter-stellar space at relativistic speeds (lets say C/10-C/20) it would encounter inter-stellar gas, ionize it and accelerate it using a linear accelerator to 99.995% C or even higher. It would not have to carry expendable fuel that is expelled to provide propulsion. Of course it would need engery to ionize the hydrogen it encounters and accelerate it.

mzellers, Mar 17 2007

like this http://en.wikipedia...wiki/Bussard_ramjet
[jhomrighaus, Mar 17 2007]

[link]






       This idea makes sense. The grab stuff and throw it behind you engine. You could chart your path thru areas where there was more gas. It might be good for a solar sail vehicle in lightless areas far from stars.   

       But you need to buff this up a little, [mz]. Where is the power from? Would your device do fusion on every third atom it encountered and use that to accelerate the rest? Or a big battery? Also, if you have got it, you could bring a little math to lend credibility. Assume a weight for your ship, and figure out how much thrust a hurled H would provide. Then cite a figure for the density of atoms in interstellar space.
bungston, Mar 17 2007
  

       what you are describing is known as a Bussard RamJet. It was first proposed in the 1960s. See link for the details.
jhomrighaus, Mar 17 2007
  

       Not quite - the Bussard ramjet uses hydrogen fusion, what [mz] proposes is a true ion drive rather than a fusion reactor and will work on any matter. Of course 99% of matter in space is hydrogen and as [jhom] points out, you'd probably have to end up fusing some of it for power anyway. I suspect the Bussard ramjet would be much more efficient use of scarce resources, but [+] anyway for a new approach.
wagster, Mar 17 2007
  

       As written the idea is unworkable as hydrogen in its most prevalent form is not in an ionized state and as such would not be able to power an Ion drive. This is a ramjet. [mzellars] has not proposed anything new that I can see. The idea is notably bare of any technical detail of how it actually performs its function, I suspect it even teeters on the edge of bad science.   

       //Of course it would need energy to ionize the hydrogen it encounters and accelerate it.//   

       This is what turns it into a ramjet and not just an ion drive. The bussard uses hydrogen fusion because with out it the ship wont do anything.
jhomrighaus, Mar 17 2007
  

       The idea says "ionize it" and "ionize the hydrogen it encounters". Then [jhom] says "hydrogen . . . is not in an ionized state". I think that's understood.   

       The idea proposes putting a charge on the normally un-ionized interstellar hydrogen--ionizing it--and booting those ionized atoms out the back as reaction mass. Which is new and good to me.   

       The energy needed to ionize atoms is lost. The energy needed to push the atoms is also lost, and probably pretty great. The idea doesn't mention where the energy comes from, but that is okay, it isn't relevant to the idea.   

       The energy doesn't come from the hydrogen atoms, so it isn't a Bussard ramjet. It's more of an interstellar prop plane, or even a paddlewheeler ship. The surrounding medium is used only to push against, and is not used for energy. [+]
baconbrain, Mar 18 2007
  

       If its not a bussard it is an incredibly inefficient ion drive which makes it baked and widely known to exist. Whats really new? essentially the idea here is use hydrogen instead of xenon or whatever other element so you dont have to carry fuel, the problem is the hydrogen is incredibly light(very small mass) that you will need to carry 100s of times more fuel for your power source(as you will need 100s of times more power to generate the same amount of thrust) I suspect that it would net out or even lose in the end. The wonderful thing about the Bussard is that the fuel and the reaction mass are the same thing and a relatively small power source to get things going.
jhomrighaus, Mar 18 2007
  

       Yeah this might work sort of, but the Bussard is better in every way. Why take a good idea and specifically sabotage it for no good reason?   

       This is the equivalent of designing futuristic prop airplanes. We have jet engines. there's no point in improving the performance of the less powerful basic technology. Even if you succeed, your new invention will be obsolete before the first one is ever built.
Smurfsahoy, Mar 18 2007
  

       I agree that it's inefficient, probably. But it saves messing around with fusion, which requires that the interstellar gasses go from vacuum to fusion pressures. Which we can't do yet, even on Earth.   

       As far as concerns the thrust problems problems arising from hydrogen atoms being so light: It seems to me that a certain push from the ion engine (and its magical power source) is going to produce a certain thrust, whether pushing light atoms very fast, or heavier atoms just sorta fast.   

       As far as pointlessness goes, well, this is the Halfbakery. I bunned this for being a new approach. I'll bun an idea for a super-efficient steamship paddlewheel, too.
baconbrain, Mar 18 2007
  

       Thinks... ion-drive boat....
wagster, Mar 18 2007
  

       This Bussard concept is really cool! He didn't think to stockpile half of the hydrogen for when the time comes to hit the breaks though...
placid_turmoil, Mar 19 2007
  

       The energy involved in this would be extremely high. Just to get back the speed you lost from picking up/running into the atom in the first place (assuming incorrectly, but reasonably on a local scale, a zero average velocity for interstellar hydrogen relative to your origin and destination) you would have to accelerate it to your current speed, and then put extra energy into it to gain any forward velocity. To get this much energy you are either carrying some huge batteries, or you are fusing most of your hydrogen. Again, a Bussard Ramjet does it better.
MechE, Mar 19 2007
  

       the problems with a Bussard are essentially the same as for this idea.(drag etc)
jhomrighaus, Mar 19 2007
  

       Except that Bussards attempt to get their energy by fusing the atoms without slowing them down much initially. This idea could do the same, except that in order for the ions to accelerate at all they either need to spend an appreciable time in the accelerator (which is going to produce a very long ship) or have such a high electrical potential on the rails as to be absurd (at least with anything resembling the immediately forseeable state of materials science).
MechE, Mar 19 2007
  

       // This is the equivalent of designing futuristic prop airplanes. We have jet engines. there's no point in improving the performance of the less powerful basic technology. Even if you succeed, your new invention will be obsolete before the first one is ever built. //   

       In addition to [baconbrain]'s defense of pointless and obsolete ideas, I'll defend this one as potentially pointy and solete: New propeller-driven airplanes are designed all the time by serious aircraft companies.
notexactly, Mar 11 2019
  

       This has a boot strapping problem unless the ship could paddle all species in the starting vacuum volume.
wjt, Mar 13 2019
  
      
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