RE: starship-design: Pellet track

["L. Parker" <lparker@cacaphony.net>]

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.

> 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. If it were up to me, I would
be using something capable of at least 500 gravities continuous acceleration
with no inertial force at all (hey, I want to get there, NOW). Unfortunately,
this is the real world and even drives with an ISP of 1 million sec are 
a matter of fiction...BTW, how are you going to refuel? <G>

> 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.

I don't consider a fusion drive to be ideal unless you can figure out how 
to overcome inertia, in which case, to quote someone else recently, a 
flashlight will do just fine.


> 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. 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...

> 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. A similar effect is observed at low 
temperatures although the cause is a little different. I didn't even get 
into the RADIATION aspect of it, but that only makes it even worse.

> 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. 

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

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.

Lee

                                                          (o o)
------------------------------------------------------oOO--(_)--OOo---------
HOW TO COOK AN EGG -- Physics Edition

"If you tie one of these eggs to the end of a string and whirl it round 
rapidly, and suddenly stop, the movement may perhaps be converted into heat, 
and then . . ." 

"And then the egg will be cooked?" 

"Yes, if the rotation has been swift enough. But how do you get the stoppage 
without breaking the egg?" 

-- Jules Verne, The School for Crusoes