Actually, I imagine they only work at relatively low velocities. I.e., yes they need to be going a certain minimum velocity to work at all, but I think you'll have a hard time getting anywhere near lightspeed with them.
what changes from the straight-up rocket scenario that is instead of having a suitcase of baseballs, you're just sliding along the ice by yourself, baseballs happen to be scattered about, and you're picking them up and throwing them as soon as you encounter them.
Problem number one is that the stuff you're picking up slows you down (i.e., the scoop will have drag), and if you can't throw it away at least as fast as you caught it, your velocity can't increase, which then means you can never get going faster than your throw (exhaust) velocity w.r.t. the cloud of baseballs.
The solar wind moves outwards, so that helps a bit both with leaving the system and with braking on arrival, but it's only 500km/s, less than 0.2% of the speed of light.
which means you badly need the ion-engine or gamma-laser that's throwing shit at or near the speed of light. You also need energy to power it (along with the scoop). If we're relying on antimatter that we've brought with us, then we're mostly back to rocket economics (The Suck).
But if you think you're going to fuse the hydrogen you're collecting to get the energy you need, then you need to know that most interstellar hydrogen is protium (i.e., no neutrons at all), which tends not to fuse very well (hint: How often do you see He2 in the wild?) and thus sucks for generating energy when you don't have massive quantities of the stuff and stellar gravity at your disposal. Non-stellar fusion reactors really need deuterium or tritium which tend to be much less abundant.
. . . unless it's possible for an artificial reactor to use the carbon cycle, the much-higher-yield process that the stars much bigger than our sun use. On the other hand, there's probably a reason you only see that process in the big stars, e.g., you need lots of compression. So... I'll believe that when I see it.
It also turns out estimates of the density of interstellar hydrogen are quite a bit lower than what was current when Bussard did his original paper. And we're most likely completely screwed for intergalactic travel.
no subject
what changes from the straight-up rocket scenario that is instead of having a suitcase of baseballs, you're just sliding along the ice by yourself, baseballs happen to be scattered about, and you're picking them up and throwing them as soon as you encounter them.
Problem number one is that the stuff you're picking up slows you down (i.e., the scoop will have drag), and if you can't throw it away at least as fast as you caught it, your velocity can't increase, which then means you can never get going faster than your throw (exhaust) velocity w.r.t. the cloud of baseballs.
The solar wind moves outwards, so that helps a bit both with leaving the system and with braking on arrival, but it's only 500km/s, less than 0.2% of the speed of light.
which means you badly need the ion-engine or gamma-laser that's throwing shit at or near the speed of light. You also need energy to power it (along with the scoop). If we're relying on antimatter that we've brought with us, then we're mostly back to rocket economics (The Suck).
But if you think you're going to fuse the hydrogen you're collecting to get the energy you need, then you need to know that most interstellar hydrogen is protium (i.e., no neutrons at all), which tends not to fuse very well (hint: How often do you see He2 in the wild?) and thus sucks for generating energy when you don't have massive quantities of the stuff and stellar gravity at your disposal. Non-stellar fusion reactors really need deuterium or tritium which tend to be much less abundant.
. . . unless it's possible for an artificial reactor to use the carbon cycle, the much-higher-yield process that the stars much bigger than our sun use. On the other hand, there's probably a reason you only see that process in the big stars, e.g., you need lots of compression. So... I'll believe that when I see it.
It also turns out estimates of the density of interstellar hydrogen are quite a bit lower than what was current when Bussard did his original paper. And we're most likely completely screwed for intergalactic travel.
So I'm not really optimistic about this at all.