04-30-2018, 10:10 AM | #11 | |
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Join Date: Oct 2004
Location: Forest Grove, Beaverton, Oregon
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Re: [Vehicles] Low-grav, high-pressure aircraft
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Gurps 3rd. Blue Planet has some interesting information on diving and subsequent pressures. But the only danger not already mentioned occurs when repeatedly cycling between high pressure and sea level. Nasty things to certain bones, IIRC.
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04-30-2018, 10:34 AM | #12 | |
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Join Date: Sep 2004
Location: Southeast NC
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Re: [Vehicles] Low-grav, high-pressure aircraft
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So assuming the air is .3 partial atmospheres of oxygen, an engine would - all else equal - use about 43% more fuel. It will likely need a lower compression ratio, as the cylinders and valves will also have to contain twice the overpressure. All together, I'm not sure what the effect would be, but I don't think it would be anywhere near linear.
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RyanW - Actually one normal sized guy in three tiny trenchcoats. |
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04-30-2018, 12:38 PM | #13 | |
Join Date: Nov 2004
Location: Niagara, Canada
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Re: [Vehicles] Low-grav, high-pressure aircraft
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(Which just might come into play, as according to the original source material, the station's large air-circulation pumps only manage to keep a pressure of around 1 bar near the hub, half that at the rim. Which might come as something of a surprise to someone flying up there in a machine whose engine's performance depends on a full 0.3 bars of oxygen.)
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04-30-2018, 03:11 PM | #14 |
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Join Date: Oct 2010
Location: Cambridge, UK
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Re: [Vehicles] Low-grav, high-pressure aircraft
Are you sure you have that the right way round?
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04-30-2018, 03:27 PM | #15 |
Join Date: Nov 2004
Location: Niagara, Canada
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Re: [Vehicles] Low-grav, high-pressure aircraft
Sorry, bad phrasing: "the one-bar atmosphere at the hub is half the density of the atmosphere at the rim".
I also think I've found a way to give myself a lot less work. I expect most of the engines will be alcohol-burners, which reduces engine power; and the extra oxygen, treated like a continuous nitrous boost, increases power; and the numbers are both close enough and hazy enough that I can just say the two factors usually just cancel each other out. (Until somebody decides to pony up the cash to buy x10-cost custom-brewed petroproducts, and an engine that can burn them; or things otherwise head out of the everyday.) Next up - making some guesstimates for the wingspan of a 40ish-kilogram sapient squirrel-ish griffon-analogue. And deciding which biomes to include in the six eternal-sunlight and six eternal-twilight regions (each region being ~83,000 km^2). And trying to reconcile my desires for a "Dungeon Crawl Classics" level of ignorance about as many aspects of the world as possible with the existence of at least one decent university. And so on and so forth. All that fun worldbuilding stuff. I'll probably be able to manage on my own for all that... well, except maybe for the sqriffon's wingspan - anyone care to chip in on that one? :)
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04-30-2018, 03:53 PM | #16 | |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: [Vehicles] Low-grav, high-pressure aircraft
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04-30-2018, 04:03 PM | #17 |
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Join Date: Sep 2004
Location: Southeast NC
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Re: [Vehicles] Low-grav, high-pressure aircraft
I looked at the numbers, and the pressure seems to be pretty much entirely dependent on the oxygen partial pressure - with X% more oxygen you burn X% more fuel producing X% more heat which creates X% more pressure, which provides X% more power output (roughly speaking, but reality rarely behaves as nice as numbers on paper). The really heavy parts of an engine are heavy primarily to deal with those stresses, so this increase in performance will necessarily require a tougher (and heavier) engine to cope.
I suspect the power to weight ratio will stay about the same, or very slightly improve due to the extra convection the dense atmosphere provides allows you to have a proportionally smaller cooling system. The engine (particularly the intake system) might need a different design, but I don't see anything that screams vastly improved performance, pound-for-pound.
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RyanW - Actually one normal sized guy in three tiny trenchcoats. Last edited by RyanW; 04-30-2018 at 04:07 PM. |
04-30-2018, 04:46 PM | #18 | |
Join Date: Sep 2007
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Re: [Vehicles] Low-grav, high-pressure aircraft
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I'm not enough of a mechanical engineer to know what really drives the limits on compression. I'm pretty sure it's not material properties, though, but think rather it's "knocking", which in turn depends on the octane rating of the gasoline. Usually gas engines are somewhere around 9-12:1, up to maybe around 14:1, so nowhere near the pressure you could build a cast-iron (or aluminum) cylinder to hold. Geometry of the engine probably places its own limits on the ratio as well, because you'll only want to make the thing so wide or high or tall for a given application. But then they get into all sorts of funky tricks with valve timing and asymmetric stroke cycles, too, so it's not as simple as volume of the cylinder with the piston at the bottom and at the top. |
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04-30-2018, 04:54 PM | #19 | |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: [Vehicles] Low-grav, high-pressure aircraft
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Fuel cells will be fine, though. |
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gravity, ornithopter, pressure, vehicles |
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