How effective would it be to use fusion scoops as an actually effective method of interstellar travel? How quickly could you generate thrust? How much fuel would such a thing need, and would it work in deep space? What sort of procedures would be needed for a starliner using this strategy?
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$\begingroup$ The question of your last sentence is a bit ambiguous. $\endgroup$– BMFJan 23, 2020 at 15:39
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2$\begingroup$ You've asked rather too many questions here. I've just chosen to answer the first one as it makes the others moot, but please keep your questions more focussed in future. $\endgroup$– Starfish PrimeJan 23, 2020 at 15:57
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1$\begingroup$ If "yielding a velocity of 200 km/s after 5 million years" is 'actually an effective method of interstellar travel' then I have some bridges to sell you. $\endgroup$– MazuraJan 24, 2020 at 4:49
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$\begingroup$ @Mazura that's for pushing a whole star around, not a ship. $\endgroup$– thsJan 24, 2020 at 11:00
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1 Answer
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Bussard ramjets have a bunch of major and probably insurmountable problems.
The principle issue is "scoop drag", caused by interaction of the ramscoop's magnetic field with the interstellar medium (ISM). This quite sharply limits the top speed of any ramscoop driven ship... I don't have the figures to hand, but they top out at something like .16c to .2c even with some clever techniques to minimise electron and ion drag, and without those you might only be limited to the speed of a plain old fusion rocket exhaust which can be as low as 0.04c, and who would bother with that? Not that such speeds are to be sneezed at, of course, but it doesn't necessary compete well with the limits of alternatives.
The secondary issue is that there's a minimum speed, or at least a minimum mass flow rate into the scoop in order to develop thrust. This means you need a pretty serious rocket (or non-rocket propulsion alternative) in order to kickstart your ramship. This isn't a showstopper, but it presents an issue if you want to stop somewhere and then start again.
The third issue is the type of fusion you're going to be doing. Most of the stuff you find in space is going to be plain hold protium (1H) and helium (most likely, 4He) neither of which are at all useful for fusion recipes that work under sane temperatures, pressures and timescales.
The fourth issue is ionisation of non-ionised stuff in the ISM. You'll be needing big, powerful lasers to do this usefully, and that will sap power from your already fairly tenuous system. Even with such things (which are likely to be inefficient, and massive heat generators as a result) it is no easy task to ionised the ISM, and if you can't do that efficiently you'll suffer from low fuel flow and potentially greater scoop drag, too.
The fifth-ish issue is that the ISM is not created equal in all places. The Sun, for example, sits inside the local bubble where the density of the ISM is particularly low. This is good news from the point of view of shielding your spacecraft against radiation caused by high-speed travel, but bad news if you want to fuel a ramscoop.
The list probably goes on, but I don't have all the relevant papers to hand.
So, yeah. Basically, Bussard Ramjets are dead.
Some odd alternatives do exist, like Whitmire's catalytic ramjets where the incoming proton stream is used to ignite carbon or neon fusion, a fuel cycle that allows the "catalytic" part (the carbon or neon) to be regenerated so as not to run out. Such reactions are vastly harder to spark off than any of the types of fusion being considered today, though. Alan Bond also suggested the "Ram Augmented Interstellar Rocket", which uses the interstellar medium as reaction mass but not fusion fuel. Variants also use a finite supply of lithium or boron fuel which is fused with incoming ISM protons to generate energy. In all these cases, the designs are still subject to scoop-drag losses there is some scope to go a little faster if their reactions are suitably energetic and clever engineering is used to help mitigate electron and ion drag.
Rocket-driven interstellar travel is a bit of a dead end, to be honest. If you can't manage beam propulsion or at least fusion runways, you may as well stay at home...
answered Jan 23, 2020 at 15:54
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2$\begingroup$ The old figure I recall for the Bussard speed limit was that your can't exceed your exhaust velocity before the scoop drag equals your thrust -- and exhaust velocity for protium-protium fusion, if you can even make it happen, is no better than about .12 c. $\endgroup$ Jan 23, 2020 at 16:54
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1$\begingroup$ @ZeissIkon there's some scope to reduce electron and ion drag with clever (read: even more complex) designs, though there are diminishing returns to be had. Whitmire, Matloff and Fenelly did some work on this back in the day, though I've no idea how well it has held up. WIth sufficiently clever engineering .2c should be achievable, I suspect. $\endgroup$ Jan 23, 2020 at 20:13
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1$\begingroup$ What I recall reading was that you could only exceed .12 c if you could avoid having to capture the incoming hydrogen to fuse it -- which is how Niven portrayed ramships, surrounded by a faint ring of fusion flame. I doubt that's actually possible. $\endgroup$ Jan 23, 2020 at 20:18
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2$\begingroup$ @puppetsock yeah, the whole thing is tragically a dead end. You could get that sort of performance out of a plain fusion rocket without any of the rest of the faff. Beam propulsion or unobtanium is the only way, it seems. $\endgroup$ Jan 23, 2020 at 20:32
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2$\begingroup$ @puppetsock But there'd be no reason to spend 50 years (or much longer, for more distant destinations) in the vicinity of a running fusion drive; you'd accelerate to a point of diminishing returns (8-9% of c seems a reasonable guess) and then coast for most of the journey (it'd be more like 75 years). The only special radiation hazard would be from oncoming interstellar medium, and at that speed, it's minor and easily shielded. $\endgroup$ Jan 24, 2020 at 12:10