Back in the glory days before Zubrin pointed out the potential issues with drag, the Bussard Ramscoop concept enjoyed a huge degree of popular interest. Just suck up the interstellar hydrogen, run it through nuclear fusion, and voila! You can power your way between the stars forever.
The thing is, though, that a physical scoop big enough to suck in enough hydrogen would weigh a gazillion tons and bankrupt any civilization that tried to build it. All the practical designs called for either a magnetic or an electrostatic scoop that would draw in the ionized hydrogen from the interstellar medium. Since about 50% of the interstellar medium is already conveniently ionized for us, this seemed like an ideal solution.
But think about the charge balance situation.
You’re sucking in hydrogen ions – that is, positively-charged protons. You’re running those protons through nuclear fusion. Know what you get when you fuse two protons together? A deuteron and a positron. (You also get a positron if you fuse a proton with a carbon catalyst in the CNO cycle.) You’re not sucking in any electrons to annihilate that positron with.
So what do you do with that positron? Do you just dump it into the exhaust? Or do you – and I hate to use this phrase, because of its association with Treknobabble – reverse the polarity of your scoop from time to time, so as to suck in free electrons instead of protons, and use those to annihilate the positrons? What did the designers of the time come up with as a solution?
(And, for that matter, what about at the end of your braking phase, when you’re going so slow that you have to burn rocket engines to finish coming to a stop at your destination? If you were sucking in neutral hydrogen, you could store it during the earlier part of the braking phase, and use it for fuel at the end – but you’re sucking in positively-charged protons, and storing those without storing an equal number of electrons is going to give you one hell of a sting when you shuffle your feet on the carpet and touch a doorknob.)
Seems to me you’d just shoot it out the back- doesn’t that positron have some speed coming out of the reaction? If it does, then it would add to your acceleration.
Yeah, the positrons would hopefully stream out the back (thanks to magnetic fields directing the traffic from the scoop). Eventually each would hit some random hydrogen atom and turn it into an ionized atom. It would be nice if you could point this positron beam toward the front - electrons and positrons are very low mass, so it’s not a major slowdown to your scoop, but it would create more ionized fuel source for you.
Yes, what is the problem with shooting them out the back? I take it that the interstellar medium is not massively positively charged, so if the hydrogen is ionized there have got to be a lot of electrons wandering about out there too. Won’t the excess positrons find them eventually and annihilate into gamma rays?
Slowing down is no problem at all with a ramscoop: You don’t need to store up fuel for it; you just keep on scooping. Except your scoop is still pointed forward, but now your engine is, too, so you get to take advantage of the thrust and the drag.
(1) How do you redirect your engine thrust forward, without interfering with your scoop? And even if you can, won’t you eventually start scooping up your own exhaust?
(2) Ramscoops hardly work at all below about 1-6% of the speed of light. When outbound, you have to expend onboard propellant to get up to this speed. And eventually, while you’re braking, your speed is going to get down below this level. To slow down from that point to the standing-stop you’re going to need at your destination, the braking from your scoop (and the engine output from your scoop) will no longer be sufficient. You’re going to have to burn on-board hydrogen, just like you did when accelerating to ram-scooping speed in the first place. It would be very nice if, during your high-speed braking phase, you could refill your fuel tanks from the interstellar medium you were scooping up, so that you didn’t have to carry enough fuel for both the initial acceleration phase and the final braking phase from the get-go. BUT, if you’re sucking in only ionized hydrogen, you’ll get a HUGE electric charge imbalance very, very quickly. Won’t you need to acquire an equal number of electrons somehow?
Actually … now that I think about it … if you’re using a magnetic scoop, wouldn’t the same magnetic field that draws in ionized hydrogen also draw in free electrons?
And of the few that do, such as Poul Anderson in Tau Zero, it often involves some Handwavium. (The scoop in Tau Zero worked by “seizing atoms by their dipoles – no ionization needed”, which as far as we know is impossible in the real world.)
