X-Andrew-Authenticated-as: 7997;andrew.cmu.edu;Ted Anderson
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Thu, 11 Apr 91 01:28:59 -0400 (EDT)
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Date: Thu, 11 Apr 91 01:28:51 -0400 (EDT)
Subject: SPACE Digest V13 #390
SPACE Digest Volume 13 : Issue 390
Today's Topics:
shuttle radiator experiment
Re: space news from March 4 AW&ST
Re: comsat cancellations and lawsuits
Re: Space Stations, Money, Startrek
Re: SPACE Digest V13 #364
Re: back-up crews
Re: Advancing Launch Technology
Re: Nuclear Rockets
Re: SPACE Digest V13 #378
Re: SPACE Digest V13 #364
Administrivia:
Submissions to the SPACE Digest/sci.space should be mailed to
space+@andrew.cmu.edu. Other mail, esp. [un]subscription requests,
should be sent to space-request+@andrew.cmu.edu, or, if urgent, to
>>Odd. Don't we (i.e. the human species) already know how to live and
>>work in space?
>
>Yes, in the sense that we know some approaches that work. No, in the sense
>of knowing all there is to be known about it, and being able to estimate
>the effectiveness of new approaches before trying them.
>
>>Haven't humans spent up to a year in space with no major problems?
>
>Two humans have done this, without major problems. More data would be
>very welcome, since future efforts are unlikely to precisely duplicate
>the exact set of conditions under which this was done. We have an
>existence proof, but not enough data to confidently draw graphs and
>define the boundaries between acceptable and unacceptable conditions.
>
>>>... One of the things we need to know is what exercise is necessary
>>>and why...
>>
>>Odd. Don't we already know the answer to these questions?
>
>No. We have vague notions at best. What's wanted is enough detailed
>knowledge to guide engineering design of future space activities. That
>we do not have.
>--
I can't believe that after Skylab and Mir we need MORE data! We built
a space shuttle, for heavens sake, from previous data and extrapolation of
learned data. The magnificent thing flew the first time with no unmanned
trials. Data isn't the problem. Vision, imagination and motivation are
the problem. We don't have a clear national goal. I wonder where we would
be now if Kennedy hadn't been killed. We would probably at least have a manned
Martian base by now. We have some very smart professionals out there and Gigabytes
of data they could be drawing on for a real station. I heard that we can't even
recreate Apollo because the blueprints are missing. Thats utterly criminal. Heads
should roll just for that. To think the American taxpayers spent 25 Billion
on a moon program, a quite successful one at that, and we can't recreate it or
come close to doing it again after we have already done it once!
NO SIR We don't need to spend money on FRED to reaquire DATA we already have.
But like I originally stated I would agree to a TAX hike for a REAL STATION.
Doug Phillipson
My thoughts are my own!
------------------------------
Date: 10 Apr 91 15:14:27 GMT
From: agate!bionet!uwm.edu!zaphod.mps.ohio-state.edu!samsung!news.cs.indiana.edu!maytag!watmath!watdragon!watyew!jdnicoll@ucbvax.Berkeley.EDU (James Davis Nicoll)
Subject: Re: SPACE Digest V13 #364
In article <9104052012.AA24364@ucbvax.Berkeley.EDU>, space-request+@ANDREW.CMU.EDU writes:
> >I agree that chemical rockets haven't much future beyond LEO. But I'm
> >rather fond of fission torch rockets myself. I want to get there NOW,
> >not next year. :-)
>
> But where can you go? You certainly won't be using fission in MY back
> lagrange point (oops, I mean yard) |-)
>
> Actually, with Ion systems, since the ISP is so high, you can accelerate for
> the entire trip. Toward your destination at first, and slowing down
> after the half-way point. You'll start out at only a few meters/minute, since
> the amount of mass is so low. But after a day you'll be moving 76400 times
> faster. after a week, 534800 times, and after a year, 2.7e+7 times. (.90c)
> This is all ignoring relativity, of course. As an example, at .9c, a trip to
> Alpha Cent. would appear (th those on earth) to take 4.4 years. But you would
> feel like it only took about 2.5 years. That's 2 times the speed of light!
You might want to consider what mass ratio you think is feasible.
Isp for an ion drive is, uh (flip flip flip) 20,000, so the exhaust velocity
is 200 km/s. The equation you want to use is
M1/M2 = e**V1/Ve
where M1 is the mass of the ship & reaction mass
M2 is the mass of the ship sans reaction mass
V1 is the total change in velocity
Ve is the exhaust velocity
If you play around with this equation, you'll find that once
V1/Ve gets much bigger than 1, M1/M2 gets very large rather quickly.
V1 (km/s) M1/M2
50 1.28
100 1.65
200 2.72
400 7.39
800 54.60
1600 2980.96
3200 8,886,110.52
3200 km/s is a bit over .01 C. That would make it over four
centuries to Alpha Centauri. This does not, of course, show that using
rockets to get very high (tenths of C) delta vees is impossible, but it
is impractical with the system you are talking about.
Pulsed Fission rockets are listed as having an Isp of 50,000
(Exhaust velocity of 500 km/s), so a mass ratio of 7.39 would get you
1000 km/s delta vee. A mass ratio of 8,886,110.52 (somewhat larger
than is now used) would get you 8000 km/s delta vee (about .03 C),
which gets you to Alpha C in ~160 years.
I have no idea what the Ve is for the fission system the other
poster was talking about is.
In any case, you can't just assume that the ion drive gives you
unlimited delta vee (although being able to get delta vees of 400km/s would
make travel within the solar system much less time consuming). 400 km/s
delta vee at an acceleration of .1 m/s**2 gives you 46 days of acceleration.
If you weren't planning on stopping, you would go 5 1/3 AU before running
out of reaction mass. Using half your delta vee, you'd go about 1 1/3 AU
during the boost phase, and the same decelerating (ignoring trivialities
like orbital mechanics). Much faster than we can do, but it still would
take weeks or months to get around within the solar system.
I'm sure other posters could do a better job on this subject than I
just did. While pointing out the no doubt numerous flaws in the above, try
not to char me *too* much :)
James Nicoll
PS: Relativistic equations to calculate mass ratios give higher
values (although the difference is very small for small delta vees)
