I have a question I could use some help on. I've been designing a very early colony ship (pre-FTL) for my uni and it seems to me like a catalyzed RAIR (a Bussard ramjet, but you only use the hydrogen you suck in as propellant so you don't have to slow it down), would probably be ideal. But I've noticed one major "elephant in the room" problem that nobody ever seems to mention about Bussard ramjets: the power requirement of the scoop and the ionizing laser.
Basically, to propell a 1000 ton ship at 1 G you need a magnetic scoop hundreds of kilometers across, and you have to ionize the hydrogen in your path (a few grams a second at most, but very dispersed) with some sort of giant laser or UV lamp. I imagine this will take a lot of energy, but how much? If it's in the GW, maybe even low TW range it might be manageable. But if it's in the tens or hundreds of TW, I just don't see being able to fit the generators on a ship that isn't so gigantic you might as well just go with some type of conventional rocket (especially since you need a conventional rocket to get the ramscoop to 18,000 km/s before it starts to work).
The best thing I could find was an estimate from OA that the ionizing laser would need 45,000 TW, which would probably be a deal-breaker in favor of some other system (because I don't see being able to mount that kind of generator on anything smaller than an asteroid). But I have no idea what assumptions they were using on how big the scoop was and besides, OA is probably less reliable than Wikipedia if you want to learn about hard science.
Does anybody have information on this? Or, failing that, does anybody know how to calculate how much energy you'd need to ionize a few grams of hydrogen spread randomly over an area of say 100,000 km^2 and suck it in toward a central point with a magnetic scoop field?
Can anyone help me with the power needs for a Bussard ramjet
Moderator: Alyrium Denryle
Hmm, could somebody maybe check this back of the napkin calculation for me?
Hmm, maybe we could try calculating the ratio of empty space to hydrogen and finding an estimate that way. Let's see, according to Atomic Rocket you need a scoop 10^18 meters across to catch 1 gram (I'm assuming they mean in the local bubble). Bussard used a scoop of 10,000 km^2 or 10^10 m^2, so it would have been collecting between 10-150 nanograms per second. At 1312 KJ/gram that's .13-.013 watts.Qeveren;3571918 wrote:I'm not sure how to calculate this, really. The first ionization energy of neutral hydrogen is 1312 kJ/mol (or kJ/gram, more or less), which corresponds to near ultraviolet light around 92nm. But it's scattered over such an enormous area, you'd end up having to saturate the volume with enough UV photons to catch them all. Ummm... I'm guessing this is going to end up involving the cross-sectional area of hydrogen atoms. I'm not entirely sure. Anyone else?
Now hydrogen atoms have a diameter of 50 picometers, and they average .07 per cm^3 in the local bubble. That means if you're spraying energy randomly over the area only around 1 X 10^-8% of it will actually hit a hydrogen molecule. So the energy required is 1 X 10^10 times that necessary to ionize 10-150 nanograms of hydrogen. That's 1.3-13 GW.
That should be manageable, but I'm not sure if a few nanograms of propellant a second is realistic. My own delta V calculations suggest you need to feed about .7 kg through the engine every second to propell a 1000 ton ship at 1 G with the exhaust velocity of proton-proton fusion, which yields 9.2 X 10^15 watts for the ionization laser. Or, using the technological assumptions of my uni, a power plant massing about a hundred million tons. Yowzie! If it's like that I'm much better of using an antimatter-fusion hybrid drive - the fuel it needs to accelerate a 1000 ton payload up to .5 c will be much lighter that that monstrosity of a ship!