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Old 04-29-2018, 12:16 PM   #1
DataPacRat
 
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Default [Vehicles] Low-grav, high-pressure aircraft

I'm toying with a setting resembling John Varley's "Gaea": a weirdly-living wheel-type space station, 1300 km in diameter and 250 km wide. ( http://ammonra.org/gaea/ has some nice pictures.) On the interior "surface", the centripetal acceleration is close enough to 0.2 gravities, and the pressure close enough to 2 atmospheres to just use those numbers.

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?


(I plan to use a Hard Science/no-superscience lens, and a biotech-oriented untransistorized steampunk approach wherever feasible. Eg, making anything made of metal more expensive - probably a blanket x10 cost, without going into details of monopolies and imports. Similarly, hydrogen is going to be a lot cheaper than helium as a lifting gas; and if anyone does put together a combustion engine, alcohol's going to be a cheaper fuel than petro-products, given how easy it is to distill on demand. My first goal once I have the right formulas is to try to put together stats for an ornithopter for a SizeMod -1, ST 10 pilot, made of wood, using a muscle engine; and to try and get it as light as possible.)
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Old 04-29-2018, 12:40 PM   #2
Fred Brackin
 
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Originally Posted by DataPacRat View Post
I'm not quite sure how I should adjust drag for pressure.
Double pressure equals double drag. You're pushing twice as many air molecules out of your way.

Halve 1/2D of all physical weapons too.
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Old 04-29-2018, 01:05 PM   #3
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Originally Posted by Fred Brackin View Post
Double pressure equals double drag. You're pushing twice as many air molecules out of your way.
Simple and intuitive; I can work with that. I guess the square-roots from VEx2 made me nervous about more complicated effects that don't actually apply. :)

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Halve 1/2D of all physical weapons too.
Fair enough. And according to GURPS Space, at 1/5th G, Max ranges will be 5 times standard.



And I also get to figure out whether a double-pressure (but still generally human-breathable) atmosphere counts as Dense or Very Dense; and whether I should come up with some quickie system for people used to planetary gravity trying to compensate for the station's Coriolis effects and vice versa; and where on the station the cosmic rays will have the least blockage; and so on. Anyone who's got any thoughts on anything of the sort, feel free to weigh in.
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Old 04-29-2018, 03:21 PM   #4
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Originally Posted by DataPacRat View Post
Fair enough. And according to GURPS Space, at 1/5th G, Max ranges will be 5 times standard.
That would only be the case if the atmosphere was the same. The same reduction to range due to pressure should probably apply here, cutting that in half. Still, 2.5x range seems very generous; even with the light gravity, that's some thick air.

Quote:
And I also get to figure out whether a double-pressure (but still generally human-breathable) atmosphere counts as Dense or Very Dense
2 atmospheres is well into Very Dense (Which is 1.51+). So long as the oxygen content is low enough, it's fairly unpleasant and might lead to health complications (The -1 HT can make various health effects worse, so your population will have notably more problems with health), but it wouldn't be immediately dangerous. If it's just regular earth air at double pressure, then you're probably in trouble.
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Old 04-29-2018, 06:28 PM   #5
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Originally Posted by Phoenix_Dragon View Post
That would only be the case if the atmosphere was the same. The same reduction to range due to pressure should probably apply here, cutting that in half. Still, 2.5x range seems very generous; even with the light gravity, that's some thick air.
So noted.

Quote:
2 atmospheres is well into Very Dense (Which is 1.51+). So long as the oxygen content is low enough, it's fairly unpleasant and might lead to health complications (The -1 HT can make various health effects worse, so your population will have notably more problems with health), but it wouldn't be immediately dangerous. If it's just regular earth air at double pressure, then you're probably in trouble.
After looking up various references, such as https://history.nasa.gov/conghand/fig15d3.gif , my current notes are for the oxygen content to be around a third of a bar - giant dragonfly territory, but still able to be tolerated by humans indefinitely. It's closer towards minor nitrogen narcosis than standard atmosphere, but slightly easier drunkenness is actually thematic.

... Mind you, I've now just realized that I don't even know where to begin looking up how any of the stats of combustion engines (alcohol-burning or otherwise) would change in such a setting. Anyone able to chip in?
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Old 04-29-2018, 07:42 PM   #6
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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Originally Posted by DataPacRat View Post
After looking up various references, such as https://history.nasa.gov/conghand/fig15d3.gif , my current notes are for the oxygen content to be around a third of a bar - giant dragonfly territory, but still able to be tolerated by humans indefinitely. It's closer towards minor nitrogen narcosis than standard atmosphere, but slightly easier drunkenness is actually thematic.
If this is supposed to be for normal humans, I'd be just as concerned about the general effect on the population's health simply due to the increased pressure, even independent of the oxygen content. In GURPS terms, -1 HT to the entire population means a fairly sizable increase in health problems. For example, it cuts the population's resistance to a mild flu (B443) by a third, and resistance to severe instances of the flu (Bio 114) by half. Contagion is similarly affected (Casual interaction with an infected person is almost half again as likely to result in contracting the illness). Not to mention poorer endurance, slower recovery from injury or illness, increased risk of long-term health effects, etc.

Mind, I'm not entirely certain how accurate -1 HT due to high pressure is, but given that you have to move twice the mass of air, it sounds reasonable to me. Species that evolved there would do fine, but ones that developed in 1-atmosphere environments would probably find it at least somewhat unpleasant without a pressure suit.

(No clue about your other questions, though)
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Old 04-30-2018, 02:36 AM   #7
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

I've just finished cobbling together a first draft of a muscle-powered ornithopter that actually gets off the ground. It still needs some tweaking - depending on how drag works, the max airspeed is so close to the stall speed that climbing is ridiculously slow - but I'm about to fall asleep, so I'll post what I have so far in hopes of eliciting some feedback.

-----8<-----

** Gaean Flyer
* Muscle-powered ornithopter. TL6ish. Metal items x10 normal cost.
* Air pressure twice normal, local gravity one fifth normal.
* Pilot and personal gear: ST 10, 100 lbs.

** Wings, Standard:
* TL7 ornithopter drivetrain, 0.2 kW: thrust 0.4 lbs, lift 0.8 lbs, 1.2 lbs, 0.008 cf total (0.004 cf left wing, 0.004 cf right wing), $40
* Access Space: 0.008 cf total (0.004 left wing, 0.004 right wing)
* Left Wing Volume: 0.008 cf
* Left Wing Surface Area: 0.75 sf
* Right Wing Volume: 0.008 cf
* Right Wing Surface Area: 0.75 sf

** Body:
* TL6 bicycle-style Muscle Engine, for ST 10: 0.2 kW, 5 lbs, 0.25 cf, $5
* Access Space: 0.25 cf
* Exposed Cramped Crew Station: 20 lbs, 10 cf, $100
* Pilot & gear: 100 lbs
* Body Volume: (10.5 cf, *1.25 for streamlining, *1.025 for retractable wheels) 13.45 cf
* Body Surface Area: 40 sf

** Wheels: Retractable (into body), 3
* Wheels Volume: 0.05 * BodyVol = 0.05 * 13.45 = 0.6725 cf
* Wheels Surface Area: 5 sf

** Structure:
* Total Area: 46.5 sf
* Streamlining: Very Good
* Lifting Body
* Frame: TL6, Super-Light, Very Cheap (wood), Wings, Very Good Streamlining, Lifting Body: Wt 46.5 * 8 * 0.1 * 2 = 74.4 lbs, 46.5 * $10 * 0.1 * 0.2 * 10 * 2 * 1.2 = $223.20
* Folding Wings: 0.05 * 1.5 / 46.5 * 74.4 = 0.12 lbs, $3
* Armor: DR 1 nonrigid: 0.06 lbs * 46.5 = 2.79 lbs, $13.95

** Stats:
* Empty Weight: 1.2 + 5 + 20 + 74.4 + 0.12 + 2.79 = 103.51 lbs
* Loaded Weight: 203.51 lbs
* Loaded Weight in Gaea: 40.702 lbs
* Price: 40 + 5 + 100 + 223.20 + 3 + 13.95 = $385.15
* Hit Points: Body: 41.5 * 1.5 / 10 = 6 hp. Each wing hp: 0.75 * 1.5 / 10 = 1
* Health: (200 * 6 / 203.51) + 5 = 10.9, maxed to 6

* Lift Area: 0.75 + 0.75 + (46.5*.3) = 15.45 sf
* Stall Speed in Gaea (from VEx2): StreamlineFactor * (LoadedWeight/LiftArea)^0.5 * 0.6324555 mph = 7.7 * (40.702 / 14.45)^.5 * 0.6324555 = 8.17 mph
* aThrust: 0.4 lbs
* aDrag: (46.5 / 5) + 10 = 19.3
* aDrag if double pressure doubles drag: 38.6
* aSpeed: (7500 * 0.4 / 19.3)^.5 = 12.47 mph
* aSpeed if double pressure doubles drag: (7500 * 0.4 / 38.6)^.5 = 8.82 mph
* aAccel: (0.4 / 203.51) * 20 = 0.04 mph/s
* aMR: (2 / 203.51) * 6 * 30 = 1.77 rounded to 2 gravities
* aSR: 1
* aDecel: 2*4 = 8 mph/s

* gSpeed: ((0.4/4) / (203.51/2000))^.5 * 16 = 15.86 rounded to 16 mph
* gAccel: 16 mph / 16 * .8 = 0.8 mph/s
* gDecel: 10 mph/s
* gSR: 2
* gMR: 1.25 (0.5 when wings unfolded)
* gContactArea: WheelArea/50 = 5/50 = 0.1
* groundPressure: 40.702 / 0.1 = 407.02, Very Low IV, off-road speed 1/2 = 7.83 rounded to 8 mph
* Takeoff Run: (8.17 * 8.17) / (4 * 0.8) = 20.85 yards
* Landing Run: (8.17 * 8.17) / (4 * 10) = 1.67 yards

(Grassy areas use off-road speed, which is slightly below stall speed. However, going down a slope can increase speed above max ground speed; as long as the flyer is going down a slope of at least a couple of degrees, it can take off from grassy areas.)
(Given that gAccel is higher than aAccel, it's probably better to adjust the flaps to stay on the ground until max airspeed is reached.)
(Climb rate: ... very low. Whole design probably needs tweaking.)

----->8-----
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Old 04-30-2018, 03:20 AM   #8
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

If people have lived there long enough (1000 years is more then long enough here in Earth) they would have adapted physiologically somewhat to the pressure and O2 levels. We have at least three separate high altitude, four low temperature and one diving adapted populations here on Earth.
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Old 04-30-2018, 06:01 AM   #9
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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... Mind you, I've now just realized that I don't even know where to begin looking up how any of the stats of combustion engines (alcohol-burning or otherwise) would change in such a setting. Anyone able to chip in?
I've thought about similar sorts of high air pressure worlds and how flyers would work. I don't see that air pressure would have a huge effect on combustion engines. The physics of how much power various heat engines put out shouldn't depend on the base air pressure. There would be some design differences (such as cooling is easier as the denser air can carry heat away faster), but I see these as minor. One thing to consider is how the air pressure effects the thrust produced by the drivetrain.

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?
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Old 04-30-2018, 09:43 AM   #10
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Default Re: [Vehicles] Low-grav, high-pressure aircraft

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how any of the stats of combustion engines (alcohol-burning or otherwise) would change in such a setting.
One way to look at an IC engine is not as a power source, but as an air pump. The purpose of the engine is to move as much air through itself as possible. The more air you move through, the more fuel you can mix with that air and burn. That powers your air pump, and coincidentally even gives you extra power you can use for other purposes.

In short, more air pressure and a higher oxygen partial pressure means more fuel you can burn with that air in the same displacement of engine. The power increase will be linear with the increased amount of air, as will the fuel consumption. The weight of the engine itself stays the same, so you've got a better power-to-weight ratio. Or you design for the same power output, but then need a smaller and lighter engine to do that job.

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.

(You've also got higher pressure resisting your attempt to push out the exhaust, so more restriction than would be Earth-normal. So you might derate the power increase a little bit to account for that.)
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