Re: starship-design: Pellet track

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

L. Parker wrote:
>Just a general observation for everyone to think about...we are doing this 
>backwards...

>Kelly and Isaac were (are) having a conversation from which this is excerpted:
>>>Assuming a .1% percentage, a rocket with a 10:1 thrust/weight ratio
>>>and 1 millions sec Isp would be absorbing 10^12 watts/kg in waste
>>>heat.  Without massive heat rejection systems (which would adversely
>>>affect the thrust/weight ratio), that's going to melt the rocket in
>>>a fraction of a second.

[...]

>There is a great deal more, but the relevant parts have already been stated.

>First, any sort of rocket propulsion capable of propelling a starship needs an
>ISP of about 1 million give or take a few.

Well, the "pellet track ramjet" (I'm finally giving it a name) could
have a much lower Isp.  Assuming a velocity stream acceleration of
merely 100km/s (equivalent to 10,000 secs Isp) and a final velocity of
.5c, each pellet track needs to be 1,500 times the mass of the
starship.

That may sound like a lot, but it compares very favorably to a
traditional rocket--even a 1.8 million sec Isp rocket requires
a mass ratio of 15,000 in order to accelerate to .5 c (I assume
it can be refueled for deceleration en route).

Also, a 1.8 million sec Isp rocket is 50% of the ideal fusion
drive, and just a pipe dream now.  10,000 sec Isp ion drives
exist _today_, even though they are low thrust.

Critically, a lower Isp rocket generates proportionately more
thrust for a given power level.  Assuming the same power
level, a 10,000 sec ramjet would produce 180 times as much
thrust as a 1.8 million sec rocket!

>Second, any such propulsion system is going to generate waste heat.

Yes.

>Third, it is fairly likely that the total thermal gain to the ship
>is going to be significant, NO MATTER WHICH SPECIFIC DRIVE YOU USE.

Yes.

>Fourth, the ship will be subjected to this temperature continuously
>for extremely long periods of time and will therefore also be subject
>to thermal and elastic fatigue.

Uh oh.  Please don't get temperature and heat confused.  High
temperatures aren't necessarily dangerous and relatively low
temperatures aren't necessarily safe.

What's important here is the level of waste heat, and the ship's
capacity to reject this heat.  If it can't reject the heat fast
enough, it will start to heat up and melt/break up/otherwise fail.

Now, even without any special effort, a certain amount of waste
heat will be naturally rejected via heat radiation.  With extra
heat rejection equipment, a great deal of heat may be rejected.

Thus, as long as the power of the waste heat generated is below
that which can be rejected, the ship can operate indefinitely
without melting itself.

Given that the power of the waste heat will be a certain percentage
of the power of the stardrive, this determines the limit on the
power of the stardrive.  In turn, given the stardrive's thrust/power
ratio, this determines a limit on the thrust of the stardrive.

The thrust/power ratio of a rocket is roughly 1/(Isp * gee).
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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