Question by Rich W: Would a hybrid Brussard ramjet supplying H for a VASIMR ion interstellar rocket work?
The Hydrogen scooped up by the Brussard ramject would be used as fuel/propellant by a VASIMR ion rocket (instead of fusion). An onboard nuclear module would supply the electricity to generate the magnetic fields and RF waves. The advantage is that we don’t have fusion yet, but we do have the VASIMR ion rockets now.
Best answer:
Answer by Steve meat
Shouldn’t we wait for the fusion ?…i mean whats the hurry?..the universe will still be there. we just don’t want this thing on the bridge with us Mr Scott.
What do you think? Answer below!
One Response
it’s in the realm of myth. lets examine some quantitative matters.
whatever your power source, you’ll need a lot of electricity (several tens of megawatts) to accelerate particles to high velocity. VASIMIR designs seem to like xenon as the propellant; though this probably isnt critical, xenon is already a gas, and is massive, and easy to store.
you’ll need a few tonnes of reaction mass to make it useful for interplanetary missions. so this begs the question, what’s the efficiency needed from the scoop? and is there sufficient particle density in the interplanetary medium to feed this?
“The temperature of the interplanetary medium is approximately 100,000 K, and its density is very low at about 5 particles per cubic centimeter in the vicinity of the Earth; it decreases with increasing distance from the sun, in inverse proportion to the square of the distance.”
i’m guessing there is around 10^30 particles/tonne; because 1 gram of hydrogren is 6×10^23 particles.
but if we scoop 1 cubic km of space, this will contain only 5×10^15 particles; still much less than 1 gram (too low by a factor of 100 million).
if the mouth of the scoop has cross section of 1 km square, and the scoop travels at 20km/s, AND if it’s 100% efficient, it would pick up 1 gram in 5 million seconds = 2 months. so it needs to be much larger, and travel a lot faster.
i dont know how to make something like this
plasma at 100,000K is very hot, and corrosive, so dont let it touch any part of the craft.
if the scoop has a mouth the size of lake Ontario (~20,000 km square)then it could conceivably pick up 1 gram of particles in an hour at the above speed. i’m just not seeing this as significant reaction mass to accelerate the craft, though.