RE: starship-design: Bussard drive

[Timothy van der Linden <Shealiak@XS4ALL.nl>]

Hello Lee,

>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.

You may stop the particles, but you shouldn't loose too much energy in
doing so, because you have to speed up the particles again. This scenario
is usually to difficult to realize, hence it is best to not slow down the
particles.

>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.

Yes, applied to scooping only.

There is something I didn't think of before: Some of the particles that the
scoop tries to pick up will never reach the intake of the engine, because
the scoop field was just not strong enough (The magnetic scoop field does
not have a sharp edge, hence there will be particles that encounter a weak
field and they will be moved towards the intake, but not quite.). These
particles thus generate drag without contributing as reaction mass or fuel.
The larger the velocity of the ship, the more particles will slip through
the scoopfield and thus the more drag will be created.

So the sharper the edge (the greater the magnetic fieldstrength difference)
the  better the performance of the scoop.

This thus also means that my initial assumption "final velocity equals
exhaust velocity" isn't valid per se, but depends on the efficiency of the
field.

>The ramjet idea does have several difficulties to over come if it is to work
>however:
>
>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.

This is what led me to the thought mentioned by me above. (I'm not sure you
[Lee] realized what I wrote above.)

>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.

We have to burn it partially, since we need to add kinetic energy. That is
unless we have another source of energy on the ship of course (this source
should not be underestimated!).

>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.

Same argument, acceleration in this case means adding energy.

Timothy