"ionized particle moving in your frame can be recovered as energy"
You also recover it's momentum, which means you slow down.
Really you are recovering your own kinetic energy, you are breaking against the interstellar medium, probably very violently and destructively.
That's like saying "a building was moving in the frame of reference of the plane, so it didn't crash - it recovered energy."
A more advanced version of this concept is known as Buzzard ramjet, and it was calculated to be energy negative
Making something aerodynamic minimises your losses, it doesn’t “recover” energy.
If you have a more precise idea and that was an analogy, I recommend giving that idea in a more precise way — I’ve seen my own “what if” ideas wildly misunderstood by my own poor analogies.
>Making something aerodynamic minimises your losses, it doesn’t “recover” energy.
not necessarily. For some parts aerodynamic design means loss minimization, while some parts (like [ram]jet engine inlets or pretty much whole body of a scramjet plane/missile) are designed for ram effect, ie. to recover some of the kinetic energy of the incoming air as the air has velocity in the hull's frame of reference.
The same thing for interstellar medium - the "aerodynamic" design would mean either coming up with some means of smoothly redirecting the incoming medium around the ship (probably by ionization and EM field) and/or recovering the energy and collecting the mass, whereis GGP limits the consideration only to the "breaking against the interstellar medium, probably very violently and destructively."
“ram effect” isn’t something I can Google (I get pages about farming); the closest I’m familiar with given the context would be ram compression of air intakes in engines, but that’s for increasing the mass flow though the engine, not energy recovery.
The only engine I’ve heard of that attempts to use the ISM as a source of fusion fuel is the Bussard ramjet referenced by @ClumsyPilot (which uses an EM field), and as they point out, it’s been demonstrated to be a net-loss not a net-gain. An easy way to intuit this result is that the energy per particle of a fusion reaction is similar to the 14 MeV per particle (0.82-22.4 MeV for the most promising reactions) which I used as a starting point in my calculation.
You’d actually have to decelerate the ISM from the frame of reference of the ship in order to increase the rate of fusion to its maximum, but even then most nucleon impacts do not cause fusion and instead bounce, slowing you instead of helping you even if you can solve the problem of efficiently using the energy of any particles you decelerated at the front of your ship to accelerate them again at the back.
>“ram effect” isn’t something I can Google (I get pages about farming); the closest I’m familiar with given the context would be ram compression of air intakes in engines, but that’s for increasing the mass flow though the engine, not energy recovery.
ramjet inlet is designed to convert kinetic energy of incoming air into a pressure by slowing down that air, and the higher pressure results in higher efficiency of the engine. In case of scramjet a lot of the energy impacted upon the air pushed away by the hull at those speeds is concentrated in the form of high pressure of the shock wave - so the scramjet inlet is designed to scoop that high pressure air, again for high efficiency of the engine.
>the energy per particle of a fusion reaction is similar to the 14 MeV per particle
option 1. completely absorb the momentum of that particle by the hull and slow down accordingly and waste the collision energy as heat/radiation. Option 2. slow down the particle (thus still of course absorbing the whole momentum) in the EM field thus generating some useful energy and using that particle, if a suitable one, later for fusion - thus recovering significant share or even crossing into net-positive a bit (depends on efficiency of the recovery and the fusion processes)
Your option 2 does not work as anything other than a break — it brings the ISM into the same frame as the ship, slowing the ship down in the process. Really useful break, but not useful as more than a break if you can’t beam in arbitrary energy not carried with the ship and if you can beam in every you probably don’t need use the ISM as a break.
Consider: take all the energy from bringing the ISM into the same frame as the ship, use it with 90% efficiency to reaccelerate the same matter out the back of the ship in an ion drive, and you merely roughly double the velocity where drag becomes clearly dominant (I am handwaving the relativity correction as the difference between x8 and x10). 99% efficiency, you’re looking at 0.086 c * 2^2 = 0.34 c, still far short of what you need for time dilation to get you 1000 ly in 15 subjective years.
Also, any engineering solution along the lines of what you suggest would turn existing $1000 high-school-project Farnsworth fusors into useful power output fusion reactors rather than mere switchable neutron sources.
(As an aside, learning about the reality of Bussard ramscoops gave me the headcannon that the ones in Trek only refuel the ships while they park themselves in ionospheres; but Trek is big on the “space is an ocean” trope, so not that important).
Congratulations, you just nerd-sniped me. I just had both the Good Omens quote and the knowledge of how bizarre and alien the Biblical angels are described to be merge together in my imagination as wheels-within-wheels trying to do the gavotte.
You also recover it's momentum, which means you slow down. Really you are recovering your own kinetic energy, you are breaking against the interstellar medium, probably very violently and destructively.
That's like saying "a building was moving in the frame of reference of the plane, so it didn't crash - it recovered energy."
A more advanced version of this concept is known as Buzzard ramjet, and it was calculated to be energy negative
https://en.wikipedia.org/wiki/Bussard_ramjet