[AR] Re: Way OT question: degerate matter thrusters?
- From: Lloyd Droppers <ldroppers@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 18 Feb 2015 13:22:24 -0800
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> 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
>
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