Air performance

[Fred Brackin]

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.

[Ulzgoroth]

Originally Posted by ericbsmith

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.)

[Rupert]

Originally Posted by Varyon

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.

[ericthered]

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.

[Varyon]

Originally Posted by ravenfish

Some fighters are capable of extended flight straight upwards, so logically these must have at least 1G of acceleration.

Originally Posted by Phoenix_Dragon

Most modern fighters are right around 1.0 thrust-to-weight ratio, which would be 1G acceleration.

Originally Posted by Rupert

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.

[Rupert]

Originally Posted by Varyon

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.

[Fred Brackin]

Originally Posted by Varyon

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.

[Ulzgoroth]

Originally Posted by Varyon

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.

[the-red-scare]

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.

[Rupert]

Originally Posted by ericthered

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.