I'm not a physicist so I won't talk much about the properties of
degenerate matter. Rather, I will stay with the cold, hard science of
single stage to orbit cold gas thrusters made of space elevator fiber.
The basics of this are the rocket equation DV=Isp*ln(m_initial/m_final).
You said orbit, so lets say LEO DV=9200 m/s. And if we just use N2
instead of air (it's 75% of air, and you probably don't want Oxygen at
that high of a pressure) and assume a simple rocket at 300K Chamber
temperature (I don't know how to design a degenerate Air to room
temperature heat exchanger, but I'll assume it is possible) and a gamma
of 1.4 gives a 1.7 max CF from RPE so that gives you a 75s Isp. This
means that your initial mass would have to be 270000x larger than your
initial mass so that is a tiny payload, but it is physically possible.
If you just need to get to space before your other engines can kick in,
that is 1500 m/s DV. This is only a 7.7x initial mass to final mass
ratio - about the same mass ratio as a single stage rocket is today.
Now, let's run some numbers for real world best case Air rockets. First,
what pressure do you want to be at? As it turns out, super high
pressures don't help your density that much and, more importantly, they
are worse for your pressure to density ratio which is maximum at the
supercritical transition. But let's say 5000psi for some reasonable
density. And lets assume an ideal sphere that uses all of the N2 and no
payload or mass other than the tank.
With Al 6061-T6 you can get 390 m/s, maybe up to 50,000 ft
With Maraging Steel 350 (the best strength to weight metal) you get 770
m/s, probably just over 100 kft 1/3rd the way to space
With a tank of pure spectra 2000 (the best strength to weight thing I
know of) you get 2295 m/s - We made it! Now to make a perfect spherical
structure with no binder.
But with something that could make a space elevator (~ 10x better than
the spectra) you get 4100 m/s - a two stage vehicle and you might make orbit
Now if we used hydrogen at 10000 psi instead of nitrogen, we get a 290s
Isp, but at a much lower density it still improves performance and you
can get to 9500 m/s - Single stage to orbit!
Sorry, I was nerd sniped and thought I should share. All joking aside,
cold gas thrusters are useful, but I think Niven was probably just using
a literary Deus Ex Machina when he talked about degenerate matter.
Lloyd
On Wed, Feb 18, 2015 at 11:35 AM, Galejs, Robert - 1007 - MITLL
<galejs@xxxxxxxxxx <mailto:galejs@xxxxxxxxxx>> wrote:
I know this is way off-topic, but it has always had me wondering and
it seems like Arocket has the appropriate knowledge base to address
this (or, at least wildly speculate)…____
__ __
In some of Larry Niven’s sci-fi stories, he imagines rocket
thrusters (between the ground and orbit) based on super-compressed
air (supposedly “nearly degenerate matter”). Would such thrusters
theoretically work, or are there some thermodynamic (or other
physics) limitations that come into play?____
__ __
Thanks,____
__ __
Robert____