Re: starship-design: Pellet track

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

L. Parker wrote:
>Isaac,
>> 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.

>I will grant that there is a difference between ISP and acceleration and 
>that it is possible to get more acceleration from a lower ISP engine but 
>only at the cost of more propellant mass.

Yes, but more propellant mass is, in and of itself, not a problem.
It's a question of how _much_ more propellant is needed.

Because normal rockets have exponentially growing propellant needs,
you just can't afford to have an Isp too much less than delta-v/gee.

A ramjet's propellant needs don't grow so badly, so a _much_ lower
Isp is practical.

>> 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).

>I wasn't picking 1 million as an ISP for any particular reason other than
>the fact that it had already been mentioned.

The reason has to do with the rocket equation and the desire for
relativistic speeds.

>> 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!

>Isaac, I'm surprised at you, everyone knows that there is a difference 
>between thrust and ISP. Just because a particular drive can generate more 
>thrust is irrelevant and you know it.

Umm...I stand by exactly everything I wrote.  Perhaps you misread.

The amount of thrust one can generate _is_ significant.  It determines
how long it takes for you to reach your cruising speed, and this can
have a dramatic effect on how long it takes to reach your destination.

For instance, suppose you have an antimatter rocket capable of .001
gees acceleration.  To reach .5c, it would take 500 years and a
125 light year acceleration run.  Obviously, you won't be cruising
at .5c for a trip to Alpha Centauri.

>HOW much mass did you have to expend
>to do that is the question, unless you can increase the ISP SIGNIFICANTLY, 
>there isn't enough mass in the universe to get even one starship up to 
>near light speed. If you don't want to take my word for it I will be happy 
>to furnish you with a link to NASA's Basic Spaceflight 101 web page...

A ramjet design does _not_ follow the rocket equation.

>> 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.

>I wasn't doing any such thing. I said THERMAL and ELASTIC. Ask any nuclear 
>engineer what happens to  metals that are exposed to high temperatures for 
>long periods of time, they get BRITTLE.

Oh, that's what you are worried about?  Not a serious problem.
Structural components need not be exposed, and exposed surfaces
can be cooled with cooling systems.  It really doesn't matter
whether they get brittle, and if it really becomes a problem
one can always back it up with multiple layers.

>> 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.

>Not true, you can't reject heat uphill, against a gradient. What 
>will happen is it will radiate alright, INTO the ship. 

Umm...without any special effort, the ship will conduct and convect
heat away from any hot spots to the outer surfaces.  It won't be
terrifically effective, but a certain amount of waste heat will
be radiated into space naturally.

Barring some coincidence resulting in vacuum insulative space
all around the hot spots, conductiong and convection will be
more significant than radiation in transfering heat from one
part of the ship to another (assuming no special effort).

>>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.

>IBID

What does IBID mean?

>>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.

>Well, that is true, which was my point, the maximum output of the stardrive
>is a function of the efficiency of the stardrive.

However, there are a great many factors along the way.  The
thrust/power ratio, in particular, can be very favorably manipulated
in a ramjet design.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
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