Using interstellar medium as propellant for ion propulsion

Question

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?

Answer 1

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.