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Old 03-10-2021, 09:16 PM   #11
Fred Brackin
 
Join Date: Aug 2007
Default Re: Air performance

All numbers about real world aircraft performance are simplifications in one way or other. For example check out Operation Sageburner at the link below.

The Navy was setting records and putting its' thumb in the USAF's eye while they were the only ones with the brand new F-4 Phantom. One of those digital thrusts was setting a new low altitude speed record. That was just over 900 miles per hour at ana ltiude of no more than 125 ft above sea level. I've heard they went as low as 50 ft.

The usual number given for an F-4's top speed is as high as 1600 mph depending on altitude and many other factors. Check the World Records section.

https://airandspace.si.edu/collectio...m_A19690213000

....and if some of those numbers are surprisingly higher than what you find for an F/A-18 the F/A-18 wasn't designed for supersonic sprinting. It's a seldom used capability in combat aircraft and the amount of experience between the design periods for the F-4 and the F/A-18 have shown that..

Speed, engine power and other things depend heavily on many factors. That 2500 is probably a reasonable number for a hypothetical mature TL8 follow on to the TL7 SR-71 that would be using turbo-ramjets or scramjets as the first part of an orbital flight plan.
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Old 03-10-2021, 09:27 PM   #12
Anthony
 
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Default Re: Air performance

Honestly, the formula for peak airspeed in spaceships is complete nonsense, but doing a better job is mostly out of scope.
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Old 03-10-2021, 09:36 PM   #13
Fred Brackin
 
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Default Re: Air performance

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Originally Posted by Anthony View Post
Honestly, the formula for peak airspeed in spaceships is complete nonsense, but doing a better job is mostly out of scope.
Doing a truly accurate formula for winged aircraft with airbreathing engines is too complex for anything except a highly capable supercomputer program using far, far more data than any game system would supply..
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Old 03-10-2021, 10:40 PM   #14
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Default Re: Air performance

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Originally Posted by ericbsmith View Post
But as others have said, Spaceships is extremely optimistic in a number of ways. One thing to consider is what Spaceships considers "streamlined" is actually a radical streamlining only found on a few aircraft, because that level of streamlining is necessary to survive reentry. Most realistic aircraft, including TL7 fighters, don't actually have that level of streamlining.
Reentry vehicles are not particularly streamlined as a rule. They're not trying to minimize aerodynamic drag, if anything rather the opposite. Their aerodynamic design largely focuses on stably maintaining a heat-shield-first attitude.

Where exciting streamlining comes in is mostly for very fast air-breathing craft with aerodynamic control surfaces. (A rocket stack could be considered pretty streamlined too, but it's a kind of boring streamlining since it doesn't really want to interact with the air at all.)
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Old 03-10-2021, 10:56 PM   #15
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Default Re: Air performance

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Originally Posted by Fred Brackin View Post
Doing a truly accurate formula for winged aircraft with airbreathing engines is too complex for anything except a highly capable supercomputer program using far, far more data than any game system would supply..
Doing even a modestly accurate one is well into "don't try this without a computer" range.
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Old 03-10-2021, 11:46 PM   #16
Ulzgoroth
 
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Default Re: Air performance

And even if you could get an accurate max speed, it would become grossly inaccurate once you plugged it into a stat line, since in reality that maximum occurs under particular flying conditions (altitude/air pressure especially) whereas in GURPS it's treated as one number to fit all situations.

(An even slightly accurate aerodynamic flight simulator is another thing that isn't practical without a computer.)
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Old 03-11-2021, 12:04 AM   #17
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Default Re: Air performance

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Originally Posted by Ulzgoroth View Post
And even if you could get an accurate max speed, it would become grossly inaccurate once you plugged it into a stat line, since in reality that maximum occurs under particular flying conditions (altitude/air pressure especially) whereas in GURPS it's treated as one number to fit all situations.
This reminded me of a news story from last year where a 747 traveling NYC to London broke an air speed record for the plane of 825mph and the travel speed record for the trip because it had a 200mph tail wind, allowing it to travel "faster than the speed of sound" relative to the ground, but not relative to the air stream it was in. Most commercial planes travel around 500-600mph normally to maximize fuel economy.
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Old 03-11-2021, 01:06 AM   #18
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Originally Posted by Anaraxes View Post
Spaceships is meant to build spaceships, not aircraft. It's not surprising if pushing it's already optimistic numbers out of its scope leads to slightly wonky results. Though I don't find 2500 mph out of line for a spacecraft that for some reason is forced to operate in atmosphere -- especially when they don't have airbreathing engines and thus limitations like the above.
I appreciate it's out of primary scope, but the numbers are there. They just seem off, especially given that my "design" is so unspecialised.

Quote:
Originally Posted by Fred Brackin View Post
Doing a truly accurate formula for winged aircraft with airbreathing engines is too complex for anything except a highly capable supercomputer program using far, far more data than any game system would supply..
Of course. I'm certainly not expecting any great accuracy. But a different set of numbers in the air speed table wouldn't be any more complex. I'm just a bit surprised that a generic design available at any TL (7+) and any size performs so much better than most specialised real examples.
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Old 03-11-2021, 03:15 AM   #19
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Default Re: Air performance

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Originally Posted by Varyon View Post
I'm pretty certain most modern fighter craft don't have a full 1G of acceleration, so they should absolutely be built with smaller systems. I think you'll still end up with too high of a top speed, however - IIRC top speed scales with the square root of acceleration, so something with 0.1G would have a top speed around Mach 2. That's Move 1/750, while the old P-51D Mustang had Move 3/218.
An F-16 moderately loaded has about 1G of acceleration. So did/does the MiG-19, the world's first supersonic fighter to enter production.

The jet engines in Spaceships have about four times the thrust they should, and about (at TL8) fifteen times the fuel efficiency. Mind you, Spaceships doesn't say whether it's making any allowances for the fuel savings from not flying at 100% power constantly.

As for the top speeds, the given airspeeds in Spaceships might not be unreasonable for a non-airbreathing craft with hypersonic streamlining. Modern supersonic jet fighters are neither of these.
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Old 03-11-2021, 07:28 AM   #20
ericthered
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Default Re: Air performance

Last year I did an in-depth "down shifting" of the stats for aircraft using spaceships on my blog, trying to get stats that more closely match TL8 reality. The tweaks I made that you are interested in are as follows:


Streamlining:

There is a lot more to streamlining that simply streamlined and unstreamlined. I use the following numbers as the "base speed" in the equation for airspeed spaceships gives on page 35.
  • If you're streamlined like a rocket, bullet, space shuttle, or SR-71 blackbird, use the base speed of of 2,500 mph from the book.
  • If you've got decent control surfaces and can meaningfully turn, but are still streamlined like a fighter jet, use 1,500 mph as your base speed.
  • If you're streamlined like a modern car, or under SM+5 and have control surfaces, use 500 mph as your base speed.
  • If you're not streamlined, use 100 mph as your base speed.
Additionally, under 1G, multiply the base speed by the fraction of 1G, not by the square of the fraction of 1G.



Turbofan Engines:
The thrust for Turbofan Jet Engines are twice as high as they should be: individual turbofan jet engines have thrust to weight ratio's of about 5, not 10 (to be honest, even that is a high number, 4 is more typical). The thrust for down shifted turbofan jets are half of their listed values.



The numbers you get aren't perfect, but they are much closer.
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