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Old 03-11-2021, 08:36 AM   #21
Varyon
 
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Default Re: Air performance

Quote:
Originally Posted by ravenfish View Post
Some fighters are capable of extended flight straight upwards, so logically these must have at least 1G of acceleration.
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Originally Posted by Phoenix_Dragon View Post
Most modern fighters are right around 1.0 thrust-to-weight ratio, which would be 1G acceleration.
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Originally Posted by Rupert View Post
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.
I sit corrected, then. GURPS stats for fighter jets are largely missing (apparently there are a few in a Pyramid issue I don't have), and given they usually rely on catapults to launch from aircraft carriers, I assumed they would have less than 1G thrust.
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Old 03-11-2021, 09:12 AM   #22
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Default Re: Air performance

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Originally Posted by Varyon View Post
I sit corrected, then. GURPS stats for fighter jets are largely missing (apparently there are a few in a Pyramid issue I don't have), and given they usually rely on catapults to launch from aircraft carriers, I assumed they would have less than 1G thrust.
An F-14 on take-off would have about 0.9G acceleration with full afterburners, which I don't think they generally did for a carrier launch, and without afterburners the thrust/weight ratio was more like 0.5. The F/A-18 is much the same.
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Old 03-11-2021, 09:15 AM   #23
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Default Re: Air performance

Quote:
Originally Posted by Varyon View Post
I sit corrected, then. GURPS stats for fighter jets are largely missing (apparently there are a few in a Pyramid issue I don't have), and given they usually rely on catapults to launch from aircraft carriers, I assumed they would have less than 1G thrust.
An F-16 with nothing but a couple of hundred lb air-to-air missiles loaded has a take off weight roughly equal to it's maximum thrust _with_ the sfterburner in use. No afterburner means 2/3rds of that thrust.

Launch catapults for carrier aircraft are more about the stall speed rather than thrust. If your airplane isn't travelling faster than its' stall speed you have inadequate lift and probably impaired control as well. Airspeed also affects thrust as most aircraft don't get maximum power at zero airspeed. You need something like the enormous turbofan on a Harrier to do that.
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Old 03-11-2021, 09:16 AM   #24
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Default Re: Air performance

Quote:
Originally Posted by Varyon View Post
I sit corrected, then. GURPS stats for fighter jets are largely missing (apparently there are a few in a Pyramid issue I don't have), and given they usually rely on catapults to launch from aircraft carriers, I assumed they would have less than 1G thrust.
Two things to note relating to that last:
1) You need more than 1G of thrust to launch vertically, and even more to launch directly into the air from a standing start at a lower angle than that. Carrier aircraft launch with at most a modest incline. Thus they need to benefit from aerodynamic lift.
1b) If you rely on aerodynamic control surfaces you have no flight control at very low airspeeds.

2) Jet engines may not achieve their best thrust at rest. Though the thrust stats you see for them seem likely to come from ground testing.
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Old 03-11-2021, 09:19 AM   #25
the-red-scare
 
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Default Re: Air performance

Edit: these may be in error, see my next post.

If it’s of any interest, these are the formulas I came up with for Spaceships air vehicles based on the (also not perfect) VE2 formulas and some reasonable assumptions about shape and area. It ended up simplifying down to only needing length in yards and Accel. in G to calculate. Note this is the length from the hull size chart, not the adjusted length based on streamlining — I was aiming for ease of use, so there are conversion factors already built in.

Stall Speed = square root of length * 36
Top Speed = square root of (length * Accel. * 600,000)

Again, these give results consistent with VE2, which is not actually physically accurate but is good enough to be plausible for a game.

Last edited by the-red-scare; 03-11-2021 at 11:25 AM.
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Old 03-11-2021, 09:42 AM   #26
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Default Re: Air performance

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Originally Posted by the-red-scare View Post
If itís of any interest, these are the formulas I came up with for Spaceships air vehicles based on the (also not perfect) VE2 formulas and some reasonable assumptions about shape and area. It ended up simplifying down to only needing length in yards and Accel. in G to calculate. Note this is the length from the hull size chart, not the adjusted length based on streamlining ó I was aiming for ease of use, so there are conversion factors already built in.

Stall Speed = square root of length * 36
Top Speed = square root of (length * Accel. * 600,000)

Again, these give results consistent with VE2, which is not actually physically accurate but is good enough to be plausible for a game.

what unit is the speed in?
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Old 03-11-2021, 10:13 AM   #27
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Default Re: Air performance

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Originally Posted by ericthered View Post
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.
You should probably also cap the speed of a craft using a turbofan at something subsonic, seeing as they seem to be high-bypass engines. Afterburning turbofans can be assumed to be low-bypass and thus capable of the 200 mi/h speed given as a turbojet cap.

Both turbojets and turbofans should give 0.25G thrust per system (and as you note, that's a little generous, though less so recently).

At TL8 turbofans should use two systems of fuel per hour at full power. Consumption is roughly linear with power unless the engine is running very low. Cruising speed in GURPS is 0.8% of top speed and fuel consumption per unit of time can be assumed to be 50%, for simplicity.

For a TL8 turbojet, fuel consumption should be three system per hour at full power.

At TL7 both types should use four systems per hour (and turbofans should probably cost the same as turbojets at TL7).

Yes, this means turbojets are simply worse than turbofans at TL8+, except for extremely high speed applications. This reflects the real world where they've been replaced with high-bypass turbofans for slow speed use, and low-bypass turbofans (usually with afterburners) for high speed uses.

This is very rough, but closer to reality than the thrust/weight ratios Spaceships gives, and with more realistic fuel consumption. Of course this also means they're very fuel hungry and will eat up a lot of your plane's mass if said plane is high-performance.

One thing to consider - if the rules are to be fixed and be an improvement, they need to work for slower aircraft as well.

A test would be how they work for a Boeing 747. A modern version has a max-TO weight of just under 500 tons, and four engines of 66,500 lbs thrust each, for a maximum acceleration of about 0.25G. Their total weight is about 4% of the aircraft's, close enough to one system (especially once we include the weight of their mountings, etc.).

A 747's maximum speed is about 580 mph, though that's a never-exceed speed and it might be capable of more if you don't care about your pilot's licence, the airworthiness or the plane, or its continued flight. As it cruises at ~550 mph, I'd hope the design rules would claim about 690 mph top speed. Of course, in RL this is right in the transonic speed range and an airframe designed for high-subsonic speed could not be expected to safely reach this speed.

Anyway, with 0.25G acceleration, if speed is linear with thrust the base speed needs to be ~2,800 mph. If based on the square root of acceleration, 1,400 mph.
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Old 03-11-2021, 11:23 AM   #28
the-red-scare
 
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Default Re: Air performance

Quote:
Originally Posted by ericthered View Post
what unit is the speed in?
Should be mph. I really should have kept better notes… and I actually think I copied an obsolete attempt.

The raw materials to use the VE2 formula are:

***

Aerial Drag = Sa/5

Sa = length in yards^2 * 9 sf (this is derived from the armor scaling for a streamlined ship)

Sl is whatever you want, but Space 3e and Traveller always used “very good” which is 5

***

Top Speed = square root of (7500 * (Amt/Adr))

Amt = Accel. * Lwt. * 2000 lb (self-explanatory)

***

SM +6 fighter with 1G acceleration:

Adr = 3600/5 = 720

Top Speed = sqrt(7500 * (200000/720)) = 1,400 mph
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Old 03-11-2021, 03:47 PM   #29
pgb
 
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Default Re: Air performance

Quote:
Originally Posted by ericthered View Post
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.
Fantastic, thanks. I'll have a proper look at those, and related Spaceships items on your blog.
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Old 03-12-2021, 01:25 PM   #30
the-red-scare
 
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Default Re: Air performance

For what itís worth, if you convert the VE2 jet engines with the optional fuel realism rules, this is what you get:

TL6 turbojet: 0.15G, 10 minutes per tank
TL7 turbojet: 0.3G, 10 minutes per tank
TL7 turbofan: 0.25G, 20 minutes per tank
TL8 turbofan: 0.5G, 20 minutes per tank

Afterburners (TL7+) multiply thrust by 1.5 and duration by 0.3 for turbojets, and thrust by 1.65 and duration by 0.25 for turbofans.

These are 3e TLs, but pretty much all production jet development occurred during 4e TL7. So you might call the turbojets TL7 and TL8, and the turbofans TL7 and TL9 with a 0.3-0.4G TL8 intermediate version.
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