ISP of compressed air - Larry Niven idea

John Stoffel wrote in message ...
(Gordon D. Pusch) writes:

John Stoffel writes:

DEFINITELY Unobtainium, and one of the many, many Niven physics
bloopers where he ROYALLY screwed the pooch. (It's pretty obvious
why he dropped out of astrophysics; his former Ph.D. advisor must
_wince_ every time he hears about some of Niven's screw-ups...)

Sure, but if you've got GP Hulls which are also Unobtanium, then hey,
it's an interesting question.

If you had materials with a large enough tensile strength to hold
almost-degnerate gas, or even gas compressed to the point that its
effective I_sp significantly exceeded H2/O2, then you wouldn't
_NEED_ rockets --- you could quite easily build untapered
Beanstalks.

That's the other question I have about knownspace, you'd figure that
the puppeteers would have had a thriving business selling impervious
(except to antimatter) bean stalks to various planets. Talk about a
markup!

John

I read somewhere that Niven regretted inventing the Stasis Field,
because they're just too useful. Any material, even a GP hull, is
nothing next to a stasis field. You could build Ringworld out of an
inflatable balloon, turn it into a stasis field, and hey presto..(How
much would it weigh with a wall thickness of 0.1mm?)

(Jake McGuire) wrote in message . com...
Michael J Wise wrote in message ...
If you compress air to near-degeneracy, will it react to form nitrogen
oxides?

And it STILL has an ISP of 1.
It's not going anywhere.

I can't claim to be able to calculate the Isp of this hypothetical
rocket, but given that basically-equivalent cold-gas thrusters have
specific impulses in the 70 second range, I'm fairly confident that
it's not 1.

Yes, a cold gas thruster can give an exhaust velocity of up to 2x the
speed of sound in that gas. 70 seconds of Isp sounds about right.
70sec of Isp into 10km/s of deltaV gives a mass ratio of about 1.6
million to one.

HOWEVER:

Back in 2001, the NY Times reported on metasable nitrogen crystals:
http://www.nytimes.com/2001/05/22/science/22NITR.html

Nitrogen forms a solid at about 2 million atmospheres that is stable
at normal atmospheric pressure as long as you keep it chilled below
75K. There is an impressive amount of stored energy in these nitrogen
polymers, and a metastable nitrogen monopropellant would have an Isp
greater than 400 sec.

If a means for producing metastable nitrogen can be found that would
cost less than about $20/kg, rocket designers would rush to build
engines that would use it as a propellant. The hot nitrogen in the
exhaust would not be completely pollution free, since it would form
nitrogen oxides when it reacts with normal air.

It just goes to show you, that sometimes you can be right by accident.