🚀 go-pugleaf

In article <J85swB3w165w@tradent.wimsey.bc.ca> lord@tradent.wimsey.bc.ca (Jason Cooper) writes:
>> Building the ramscoop itself is the easy part (difficult though it is).
>> Getting the hydrogen to *do* something useful, once collected, is hard.
>> Using it as reaction mass for an antimatter-powered jet engine is going
>> to be much easier than trying to burn it raw.
>
>Ah, but that would defeat the entire purpose of the ramjet itself!

No, not really.  Getting reaction mass from the interstellar medium is
still a *lot* better than having to carry it yourself.  This is, after
all, how existing jet engines work.  It's not as good as the ideal of
cruising forever without a fuel tank, but it still could be very useful,
and it's much more practical.

>...carrying around a mass of fuel equal to what you are going to tak 
>in in the scoop...

Nope, note that I said "reaction mass".  Until you start getting up to
relativistic speeds, much the most efficient way to use antimatter is
to heat a large quantity of reaction mass with a small quantity of
antimatter.

>... How's a carbon-catalyzed 
>reaction sound for getting there (except at the lower end, where we'll 
>have to use some conventional engine to attain a speed at which THAT 
>reaction is possible)?

The problem with sneaky reactions to get around the glacially slow
fusion rate of ordinary hydrogen is "why doesn't that happen in stars?".
The mere fact that stars (which do contain carbon) are billions of years
old indicates that carbon alone is not going to solve your problems.  To
get a reaction many orders of magnitude faster than the natural ones,
you must use conditions or catalysts which do *not* occur in nature.

Incidentally, to avoid running into the fabled ramscoop speed limit,
you must *not* use an engine design that converts all the kinetic energy
of the incoming gas into heat.  Otherwise your thrust will drop to zero
when the incoming gas temperature reaches the exhaust temperature of
your fusion reaction.  As with chemical jet engines, kinetic heating
of the gas stream is your *enemy*, not your friend, because the reaction
you're using for power produces a fixed final temperature, not a fixed
temperature rise.  The simplest way around this is the Bussard equivalent
of the scramjet:  don't slow the gas down much, just heat it as it goes
past.  Alas, this has the same sort of problem that scramjets do:  to
make a relativistic-combustion ramjet work, whatever heating reaction you
use must be fast FAST *FAST*, because it must happen within microseconds
at the very most.
-- 
"God willing... we shall return."       | Henry Spencer @ U of Toronto Zoology
       -Gene Cernan, the Moon, Dec 1972 |  henry@zoo.toronto.edu  utzoo!henry