[AR] Re: Way OT question: degerate matter thrusters?
- From: "Monroe L. King Jr." <monroe@xxxxxxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Thu, 19 Feb 2015 02:27:37 -0700
If compression of that level where possible I can think of a gillion
better ways than compressing air for propulsion.
Illogical
Monroe
> -------- Original Message --------
> Subject: [AR] Re: Way OT question: degerate matter thrusters?
> From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
> Date: Thu, February 19, 2015 1:46 am
> To: arocket@xxxxxxxxxxxxx
>
>
> On 18/02/15 21:22, Lloyd Droppers wrote:
> > 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.
>
> It would take a whole passel of energy to force matter into
> near-degenerate condition. If you then let it expand then the repulsion
> between particles would cause them to gain great velocity. We are
> potentially talking about a good fraction of the speed of light here.
>
> So yes, I think it would work, and it would make a good sci-fi drive.
> But as to how it could be done in practice ...
>
>
> Note that in an ordinary gas the particles do not repel each other, so
> as the gas expands and the distance between the particles increases they
> do not gain energy. As the particles are on average moving away from
> each other, when they bounce off each other their speed drops a little,
> and this causes cooling.
>
> Expanding from the near-degenerate state, where increasing the
> seperation of the particles releases mind-boggling amounts of energy,
> the increased energy would completely overwhelm the cooling effect.
>
> Suddenly releasing the compression would result in a release of energy
> which would be more powerful than a similarly-sized atom bomb going off.
> Depending on exact conditions, it could be considerably more powerful.
>
> -- Peter Fairbrother
> >
> > 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____
> >
> >
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