This memory randomly came back to me when I woke up in the middle of the night and it's been bothering me since. When I was a kid in the early 90s, I remember telling my dad about a rocket concept where the rocket would scoop up dust and gas from the space environment around it and use that as a propellant. His response was to make a face and go "cool" and that was the last time I thought about it. I think I might have seen it talked about in a library book and have a vague memory of a cartoonish illustration of a rocket with a funnel on the front.

At the time, the concept exited me, but it felt like a really long stretch to make something like that work. I haven't really thought about it since. Is this concept still bouncing around? It doesn't really sound that viable, but I don't know crazy ideas sometimes work.

Oh, you can do the math and make a more educated opinion than just pure guess. Find space "density", rocket size... figure out how much propellant you actually need and how many millenniums you need the get enough to keep your engines work for a few seconds. It actually might be a fun little article.

https://en.wikipedia.org/wiki/Bussard_ramjet

The "Feasibility" section is pretty much a litany of big problems.

Sounds like you're remembering the Bussard Ramjet, which sucks up hydrogen from the interstellar medium to fuel a fusion reactor. It's technically possible on paper, but not even close to practical for a variety of reasons, among those being that the density of the interstellar medium is too low to provide enough fuel and fusion reactors aren't efficient enough.

A 2021 study found that, while feasible in principle, the practical construction of a useful Bussard ramjet (of the Fishback's design) would require a funnel with a diameter of 4000 kilometers, and thus might be beyond even a civilization of Kardashev type II.

That's just an engineering problem. I'm optimistic. Any day now.

I love the kind of problems for which fusion reactors are not efficient enough!

Why not scoop up junk and expel it behind you at a greater velocity, using whatever energy source you have.

It works better if you have dedicated fuel collectors that act as refuel points

So that's where star trek got the idea of Bussard collectors from.

That is essentially the concept of the ramjet, yes.

I thought ramjet was combustion. In space, you just need inertia?

Both. The concept is that hydrogen gets fused to produce power, which creates a self sustaining loop for the spacecraft. The exhaust is then ejected at greater speeds than the intake velocity. In theory that allows us to sidestep one of the biggest issues of spacecraft, the need to carry your own propellant / energy supply.

Is that Steve Spangler in the photo?

I think you'll like A Deepness in the Dark by Vernor Vinge. Published in 1999, and predominately features such technology.

I'll look it up

yeah it's called a water bottle rocket i think and i've seen one like 2 months ago.

also you can't reach orbit that way. the rocket equation says that you'd need exponentially more fuel the higher you want to go, simplified. precisely, it's a bit more like: m~0~ / m~f~ = exp(Δv/v~e~) where v~e~ is the effective exhaust velocity (how quickly the water is thrown out behind) and Δv is constant 7 km/s for going to orbit, so if v~e~ is sth like 20 m/s you can see how that's difficult. chemical fuel has v~e~ of 3 km/s so Δv/v~e~ is sth around 2.3 and exp(2.3) ≈ 10 so that's doable.