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The Bussard Ramjet assumes a huge scoop picking interstellar hydrogen, compressing it into plasma, subjecting to nuclear fusion (harvesting the fusion energy of hydrogen) and ejecting the products of fusion at energy higher than what was needed to pick it up. So, free energy and free reaction mass in one.

The whole concept falls apart at the point of "scooping interstellar hydrogen and compressing it" - the scoop would act as a huge parachute, the energy extracted from hydrogen not able to break even with the drag the scoop creates.

But we already have very good sources of energy - a nuclear reactor, RTG, beamed power, even decent fuel cells. It's the reaction mass that hurts us in the tyranny of rocket equation. We don't need scooping the energy. Let's do away with the funnel and fusion.

Let's imagine a ion drive, where instead of a propellant tank, a valve and a nozzle releasing minuscule amounts of gas to be ionized and accelerated, we have an open end, a straightforward intake open to the void. Any interstellar medium particle, if not already ionized, is ionized, then propelled by electric field towards the back of the drive, and ejected at a speed higher than the one it entered at. Regardless of by how much we accelerate the particles, and how many of them we accelerate, we're gaining momentum.

The drive's throughput area would need to be considerably larger than the craft's profile, so that drag from hitting the medium is outweigh by medium that passes through unobstructed and is accelerated instead; and likely the thrust would be minuscule; the area of intakes would need to be huge comparing to the whole craft - likely huge grids of electrodes extending sideways. But are there any other problems I'm not aware of? Is this concept studied? Is it referred anywhere?

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  • $\begingroup$ Possible problems: doesn't the new scheme you're describing need the same sort of inlet (and drag) as the Bussard Ramjet? Without fusion of any collected hydrogen I don't see how it can be an improvement. Also I'm not sure if all types of collected material can be ionised in the same way as an ion drive. (Feel free to disregard this if I've misunderstood something though.) $\endgroup$
    – Andy
    Nov 8, 2016 at 10:55
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    $\begingroup$ @Andy: The whole problem with Bussard ramjet was the idea was to immensely compress the gas through the inlet. In this case no compression occurs - pressure doesn't change. Think a hollow tube vs a funnel. Hydrogen is not collected, just passed through and accelerated in the process. I'm not sure about the process of ionizing the gas - I'm not sure how it's done but from the vague image I got it should be possible to be done in a nearly contactless manner; strip electrons, leave protons/nuclei/cations, accelerate them towards the rear electrode. $\endgroup$
    – SF.
    Nov 8, 2016 at 11:11
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    $\begingroup$ Thanks for the clarification on that (compression etc.) which I didn't know about. By the way Wikipedia for Ion thruster mentions a "Magnetoplasmadynamic thruster" and claims "In a certain configuration, the ambient gas in Low Earth Orbit (LEO) can be used as a propellant" - but sadly no references. This might be a start though. $\endgroup$
    – Andy
    Nov 8, 2016 at 12:06

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The problem you have with this concept is that there simply isn't enough mass out there to accelerate to make this worthwhile. The average density of hydrogen in space is 1 atom per cubic centimeter, which is essentially nothing. Say you had an engine which could impel the hydrogen atoms in 10 cubic meters of space per second, that would be 10,000,000 hydrogen atoms per second, which equates to 1.67 * 10 to the -17 grams of mass per second or .0000000000000000167g. In order to impel those atoms anywhere near fast enough to move your spacecraft it would have to be a very powerful magnetic field, which would mean that it's likely pretty heavy. It would also need fuel - nuclear fission reactors need refueling, you'd use far more nuclear fuel trying to get moving than you would save with this approach.

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    $\begingroup$ Imagining the engine surface of 1000m^2 (large grids extending around the craft), moving at 10km/s, that's 10mln cubic meters per second. 10^-12kg, or 10^-6mg. 1mg/s is the ballpark for typical ion engines. So, a million times weaker than typical ion engines, but never running out of fuel - and the faster it goes, the more fuel it gets. $\endgroup$
    – SF.
    Nov 8, 2016 at 17:19
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    $\begingroup$ Nooo, that's not true. There's 2 types of fuel in your scenario, 1 is the mass being shifted by the magnetic field, but then there's the fuel for the power source creating the field. That will run out. $\endgroup$
    – GdD
    Nov 8, 2016 at 17:27
  • $\begingroup$ Technically true. Realistically, you can build this sort of craft to last a couple hundred years. Also, good solar batteries and a flyby relatively close to the sun, where void is not nearly as empty (and travel speed = amount of medium passing through the engine grows immensely) - that has a potential to give it more of escape speed than anything maybe short of Breakthrough Starshot. And again, with a good speed, amount of fuel grows. $\endgroup$
    – SF.
    Nov 8, 2016 at 17:54

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