Steve asks:
>You give a TL8 LH rocket a specific impulse of about 428. Why? The
>SSME has a specific impulse of 450+.
The SSME has an Isp(vaccum) of about 454, though they're hoping to hit
460 by the late 1990's. However, at sea level, that's closer to 390-400.
IMHO, without explaining the whole specific impulse variation with altitude
theory, the only reasonable way is to average and simplify.
Maybe I should state that thrust goes up 5 tons for all liquid rockets
operating purely in vaccum. (this is off the top of my head... let me
analyze it a bit first 8-) That should help correct things a bit.
>The resistojet has a specific impulse of 18 here. Again, why? There is
>not really any upper limit to such with enough power, and according to my
>steam tables, the steam at 232C should have a specific impulse approaching
>190, depending on the nozzle design.
I was presuming a really low-tech and low-heat resistojet. I can't imagine
wanting to use one at a higher TL (well, ok, maybe, but the chart can't
get infinitely big 8-). However, you have a point, which I'll explain in
a bit...
>Your fusion rocket (experimental) has an Isp of 51,400+. Your TL9 version
>has an Isp of 50,100+. Why does the production version have LOWER Isp
>than the experimental? And why are the performances so low? Admittedly,
>a fusion rocket is pretty open-ended as far as performance, but I have
>seen studies that concluded that a fusion rocket could have an Isp in
>the range of 1,000,000 or more.
Oops. 8-) You're right, the specific impulse ought to go up a bit at TL9.
As for why did I set the values that low... well, I've talked to several
people who do things like design theoretical fusion engines, and they
don't think that specific impulse of fusion rockets will get higher than
about that (50,000) for any near-term (i.e. one we can understand, if not
build, now) designs. Those values of 1,000,000 were from physicists, not
engineers. 8-)
>And why are you using liquid Xenon and such for your ion drives, instead
>of cadmium, which has a density of 8.65?
Because last time I checked, the people working with testing ion drives
were having reliability problems with the ones that didn't use noble
gases. I don't want to presume using something that may or may not
work.
>For that matter, why do you assign a density of 1.0 for rocks? Very few
>rocks float, even fewer have neutral bouyancy. Most seem to have a
>density in the range of 2-3.
You need to stack round rocks in non round fuel hoppers, and more importantly,
rocks don't pack that well unless you make them into cubes or such.
If someone wants to complain about this, well, if you think you can pack them
in more, go right ahead 8-)
>[email removed]
Ok, now here's the one question that I've been avoiding talking about:
"why didn't I do anything about putting Ion drives in?"
Well, there's a good reason for this. In real ion and MPD drives, there
is a strong variation in specific impulse and thrust among the various
drive options. More importantly, the specific impulse and thrust of a
particular engine type varies with sort of inversely... you can get
higher thrust at lower Isp and visa versa. There are power supply weight
considerations that require a careful look at the MegaTraveller power
supply options before any reasonably accurate simulation can be done
in these rules.
WHile mostly applying to the Ion and MPD engines, this also will
affect things like the resistojet, which could theoretically get a whole
lot hotter, the mass driver, etc. I need to go back and re-read several
J.Propulsion and J.Spacecraft papers to take a look at how the tradeoffs
work in detail. Until I've done that, I am not willing to make a guess
as to how to accurately simulate these items in MegaTraveller... 8-)
- -george william herbert
[email removed] [email removed]