Comment by PaulHoule
8 years ago
People who have looked at particular instances of fusion-powered ramjets have found that they don't produce enough thrust to overcome drag:
https://en.wikipedia.org/wiki/Bussard_ramjet
More recent thinking on the concept has centered around magsails which turn the drag into a good thing. Decelerating a starship is an even tougher problem than accelerating one, and magsails are a great choice for that. (And might even be able to get a speed of 0.2% of light for departure on the solar wind)
You need fuel for more than just propulsion. A hybrid engine that provides trust to offset the drag while also powering a ship is very viable.
Remember, drag is a function of relative speeds. A hypothetical example with zero velocity would allow you to gather fuel without any drag.
Now for a very large and 'slow' generation ship you need a lot of energy to keep the crew alive, able to manufacture repair parts, keep the lights on etc. Now, say you want need 1 ounce of fuel per hour that does not seem bad but if your talking a 100,000+ year trip that's 54+ million pounds.
Sure, that kind of trip does not seem appealing, but remember taking 4x the mass at 1/2 the speed takes the same energy. Further you are going to want to bootstrap a civilization at the other end which means outside of grey goo taking a lot of stuff. With the added benefit of being able to go somewhere else.
PS: You also get more energy from hydrogen the further up the chain you go. A multi stage reactor that's spitting out lead provides more energy.
lead is not the endpoint of fusion, iron is.
if you expect to take a 100,000 year trip you should expect to live off the land and mine Kuiper belt objects and rouge planets. And figure that once people have lived 10,000 years under those conditions they probably won't find anything interesting about terrestrial planets.
> A hypothetical example with zero velocity would allow you to gather fuel without any drag. //
What do you mean by this, zero velocity within an atmosphere won't gather anything?
Space is not an absolute vacuum which is why this works in the first place. If your ship sits in the interstellar medium so it's not being pushed around on net then by definition it's drag is zero. However, it's possible to collect some non zero amount of hydrogen and thus energy by putting a high vacuum pump to empty space. Efficiency left as an exercise to the reader. But, it would operate the same way as a vacuum pump inside an atmosphere, just vastly slower with random particle motion providing a continuous stream of new particles.
Rest of the idea:
The Ramjet works by collecting hydrogen and Helium from a large area because you have a high relative velocity to the medium which also imposes drag. Think filter feeding whales. So you are collecting linearly more matter and thus energy per unit time with increased speed. However, drag is a function of matter collected AND relative speed so something like velocity ^3.
This suggests there is some point where you get less energy from collecting that you lose in drag. But, this also means below some speed you get more.