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RE: starship-design: Bussard drive
> You may want to reconsider the following:
In the particular instance of ramjets, you are correct that you have to be
able to accelerate the reaction mass to greater than its original velocity
relative to the ship. In addition, you must also do so without slowing the
reaction mass down on its way through, that problem has been realized for
quite some time. But I was speaking of reaction mass propulsion in general.
After looking back at the original statement, I see it was applied during a
conversation about ramjets and was probably meant to apply to them
specifically. But the way it was worded made it appear to apply to reaction
mass thrust in general...a patently false idea.
The ramjet idea does have several difficulties to over come if it is to work
1) A solid scoop will never work. Even if a thousand or ten thousand meter
scoop could be designed to withstand up to one tenth of a g of acceleration,
as the velocity increased, the mass of the matter impacting it would go up
proportionately. It would soon grow beyond any reasonable engineering.
2) If the scoop is immaterial, i.e. a field of some sort, then the same
argument still applies although a little differently. As speed increases,
the field strength must be increased without increasing the field area in
order to prevent more and more of the matter from slipping through before it
can be deflected far enough to bring it to the ship.
3) As you point out, all of this deflection must be done without generating
so much drag from slowing down the fuel that we can't get enough thrust out
of burning the fuel.
What I would suggest, is that IF we can generate a field sufficiently strong
to catch ionized hydrogen, then we can probably generate one strong enough
to function as a sort of inside out accelerator and simply magnetically (or
whatever) propel the hydrogen backward without even bothering to burn it.
The second possibility is that the field itself may be able to accelerate
the matter as it is deflected inward, before it is burned thereby gaining
additional thrust rather than drag.