RE: starship-design: Bussard drive

["N. Lindberg" <nlindber@u.washington.edu>]

Timothy, Lee, 
	I like the point about the "sharpness" of the field affecting
efficiency.  However, I question the basic premise of a non-burning scoop.
In all of the discussions on the list about propulsion (about 90%, I'd
say) the final question is always the weight/energy ratio of the fuel.  
Even if your reaction mass _is_ gathered, if you carry 5000 times your dry
weight in fuel, you havent really gained much since the acceleration of
the ship is only dependent on how much wattage is put our the back.  
Scooping then simply becomes a way to add extra weight and complexity to
the ship. In order for scooping to be a good idea, the interstellar
hydrogen should be burned enough to at least make up for the added weight
and drag of the scoop, and then some.  How feasible this is
engineering-wise I can't say. 
Best Regards,
Nels Lindberg
                               

On Thu, 24 Sep 1998, Timothy van der Linden wrote:

> Hello Lee,
> 
<SNIP> 
> 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