On Sat, 24 Feb 2018 13:30:30 -0800, "shadow at shadowgard.com (via tml
list)" <xxxxxx@simplelists.com> wrote:

>On 24 Feb 2018 at 13:16, Jeff Zeitlin wrote:

>> So, Drake more-or-less got it right with his plasma thrusters (using
>> any room-temperature liquid as 'working fluid') for getting through
>> the atmosphere, and powering the ship through a 'fusion bottle'.

>Well, hydrogen is best because you want the lowest possible molecular
>weight in the exhaust (basically, the lighter the particles the
>higher the exhaust velocity for a given energy input, and thus the
>higher the thrust)

>Water is ok, but given the molecular weight is 18 vs 2, you lose
>efficiency.

Well, as a good author, Drake didn't as-you-know-bob the details; he just
sorta acted like it was something that "everybody knows". Especially as
said details simply weren't relevant to the stories. Since they have
seemingly unlimited power, I can imagine that they refine the 'working
fluid' into various pure elements, and use the lightest first, discarding
(or perhaps selling?) whatever's unused whenever they make planetfall and
refuel. Again, it's not specified, so we just don't know - but when I write
this supplement (see: copious spare time), I can write it that way for the
gearhead types who insist on knowing.

>> (That, of course, leads to the question of whether his "high drive",
>> pretty much the same thing but done with a matter/antimatter reaction,
>> is plausible for an exoatmospheric reaction drive. And ignores the
>> question of where the antimatter comes from.)

>As Dr. Robert Forward pointed out in several places, an antimatter
>rocket has an "interesting" property. Depending on the mix of
>matter/antimatter you get different exhaust velocities. Which means
>that a mass ratio of 4 works for the whole range of thrusts!

"mass ratio of 4"? Pointers to a layman's explanation?

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