Comment by Retric
8 years ago
Bussard Ramjets can be useful for interstellar travel. The net thrust is not great, but for very long and relatively slow trips it let's you power a very large ship without dragging along as much fuel assuming you can get hydrogen only fusion to work.
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?
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