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Old 04-30-2018, 10:10 AM   #11
Flyndaran
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Originally Posted by a humble lich View Post
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A far as the effect on humans, I doubt there is much research of the long term health effects of living in a high pressure atmosphere. I would be interested to know what the effect of living at 2 atmospheres for decades would be. Is is possible to have some for of acclimation to high pressures like our bodies do for low pressures?
While frustrating from a scientific point of view, it can be a boon for GMs. It means that you're much freer with your imagination with less risk of players saying that you're wrong or unrealistic.

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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Old 04-30-2018, 10:34 AM   #12
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Or look at it this way -- the whole point of a turbocharger or supercharger is to compress the air going into the engine, so there's more in the cylinders. A higher pressure atmosphere means everyone has a free supercharger installed.
A turbocharger increases the pressure, but increases the oxygen partial pressure in proportion. At 2 atmospheres of pressure, normal air is going to start causing mild oxygen toxicity effects. A dense but breathable atmosphere probably has lower proportional pressure. You have more air, but most of the excess is inert.

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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Old 04-30-2018, 12:38 PM   #13
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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A turbocharger increases the pressure, but increases the oxygen partial pressure in proportion. At 2 atmospheres of pressure, normal air is going to start causing mild oxygen toxicity effects. A dense but breathable atmosphere probably has lower proportional pressure. You have more air, but most of the excess is inert.

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.
After looking up a few things about the chemistry of nitrous oxide boosters, I'm currently tempted to houserule that in this station's atmosphere, every engine automagically has the equivalent of a nitrous oxide burner with unlimited duration (Vehicles p84), which provides a 20% power boost for any given engine, at a cost of 20% higher fuel consumption. If I go this way, I'll probably have to handwave something about breakdowns only being an issue if the engine wasn't designed for the station's air mix, or if an engine switches between station-rim air and Earth-standard air, or something of the sort.

(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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Old 04-30-2018, 03:11 PM   #14
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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... 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.
Are you sure you have that the right way round?
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Old 04-30-2018, 03:27 PM   #15
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Are you sure you have that the right way round?
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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Old 04-30-2018, 03:53 PM   #16
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Vehicles Expansion 2 gives a formula for stall speed in different gravities and pressures, and I'm guessing that in most Vehicles formulas, Loaded Weight refers to "GURPS Vehicles Weight" multiplied by the local gravity... but I'm not quite sure how I should adjust drag for pressure. Anyone know what math is involved?
Multiply parasitic drag by atmospheric pressure; divide lift-induced drag by atmospheric pressure. You should probably go directly to aerodynamics rather than trying to work your way through Vehicles, though; using Vehicles will not make the math a lot harder than basic drag calculations, and will significantly reduce accuracy.
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Old 04-30-2018, 04:03 PM   #17
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Default 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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Old 04-30-2018, 04:46 PM   #18
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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A turbocharger increases the pressure, but increases the oxygen partial pressure in proportion.
Yes. I was focusing just on the pressure effect. If the OP wants Earth-normal humans to breath this atmosphere (and preserve an otherwise Earthlike environment), he'll have to reduce the proportion of oxygen. And that will, of course, mean less fuel per unit of air -- you really want the oxygen -- and thus less power that if everything other than the pressure remained equal.

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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Old 04-30-2018, 04:54 PM   #19
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Originally Posted by Anaraxes View Post
Yes. I was focusing just on the pressure effect. If the OP wants Earth-normal humans to breath this atmosphere (and preserve an otherwise Earthlike environment), he'll have to reduce the proportion of oxygen. And that will, of course, mean less fuel per unit of air
Less fuel per unit mass. Per unit volume is the same, though if you're using a compressor you need more pressure. The big worry is that the presence of neutral gases significantly affects combustion (the neutral gases absorb some of the heat of the reaction; if there's too much, there isn't enough left over for a self sustaining reaction), possibly meaning your fuel won't ignite at all.

Fuel cells will be fine, though.
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