[Spaceships] Air performance
 

I think I must be missing something in evaluating air performance for a "jet fighter" (whether or not space-capable) built with Spaceships.

At TL7 (or above), a "spaceship" that is Streamlined and Winged, and has one Jet Engine (and a fuel tank), has an acceleration of 1G. From p35 of Spaceships, that gives a maximum speed in atmosphere of 2,500 mph.

That seems extremely fast compared with real jet fighters, yet it's the minimum "spec" in terms of the construction. (I guess I could use the "smaller systems" option from Spaceships 7, but is that really what's intended for this situation?)

I imagine I'm missing/misinterpreting something. I realise that getting a precise top speed is below the resolution of this system, but I'd expect to be closer than this.

Re: Air performance
 

It just massively outperforms real aircraft.

For example, the F/A-18 is a TL7 SM+5 (about 33 tons fully loaded) craft and had 4-5 systems worth of fuel tanks. It has a top speed of 1,190 mph and a range of about 1,200 miles. So about half as fast and burning fuel up twice as fast.

Note: Multiple sources gave different numbers and I'm not an expert.

Spaceships could be assuming better fuel, a different configuration or as you said it could just be below the design resolution.

Re: Air performance
 

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.

Re: Air performance
 

I'm not sure whether .1 G is enough for that, but it's important to bear in mind that for real aircraft thrust is heavily dependent on airspeed. Realistically the Mustang at top speed would have far less thrust than it would stationary. Jets can provide thrust at higher speeds where a prop simply wouldn't.

EDIT: Note that the basic jet is described as "a turbo ramjet or scramjet". Those are not what you find on modern jet fighters. The afterburning turbofan from SS7 is closer...

Re: Air performance
 

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

Re: Air performance
 

Most modern fighters are right around 1.0 thrust-to-weight ratio, which would be 1G acceleration. The F/A-18 is listed at 0.96 T:W when fully loaded, pushing up to 1.13 by the time it's burned half its fuel. Many modern fighters are above 1.0 even when fully loaded. It's quite hard to find a modern fighter that has a low enough T:W to need smaller systems instead of just rounding to 1.0.

Re: Air performance
 

And another data point: the SR-71 has a thrust-to-weight ratio (and therefore max acceleration in Gs) of 0.45 and a top speed of 2200 mph.

Re: Air performance
 

The F-15 is famously capable of essentially ballistic flight, not dependent on aerodynamic lift at all. That was used for launching the ASM-135 ASAT.

Re: Air performance
 

There's more to the max speed than T/W ratio and coefficient of drag (important as those are). For instance, for quite a while, top views of the SR-71 were classified and photos were prohibited, because that view allowed people to to measure the angle from the tip of the nose to the engine inlets, and thus the speed at which the shock wave from the nose during supersonic flight would bend back into the engines, putting a cap on the speed. Move the engines inboard a little bit, and the plane would go a little faster. (Assuming there's not something else bad that happens in that case.)

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.

Re: Air performance
 

Most modern fighters are, if not under normal flight conditions then certainly under any one of: Afterburner thrusting, no payload, or partial fuel load.

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.

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.

Re: Air performance
 

Honestly, the formula for peak airspeed in spaceships is complete nonsense, but doing a better job is mostly out of scope.

Re: Air performance
 

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..

Re: Air performance
 

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.)

Re: Air performance
 

Doing even a modestly accurate one is well into "don't try this without a computer" range.

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.)

Re: Air performance
 

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.

Re: Air performance
 

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.

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.

Re: Air performance
 

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.

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.

Re: Air performance
 

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.

Re: Air performance
 

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.

Re: Air performance
 

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.

Re: Air performance
 

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.

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.

Re: Air performance
 

what unit is the speed in?

Re: Air performance
 

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.

Re: Air performance
 

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

Re: Air performance
 

Fantastic, thanks. I'll have a proper look at those, and related Spaceships items on your blog.

Re: Air performance
 

For what it’s 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.

Re: Air performance
 

Err, no. It's true most of the spectacular stuff had been done by 1980, but since then thrust-to-weight ratios and reliability have improved a great deal.

Re: Air performance
 

I meant on the beginning side; turbojets are TL7, not TL6, but the 3e/4e crossover happens right around then.

Re: Air performance
 

To my mind jet engines that work well enough to put into the production aeroplane are one of the signifiers of TL7.

Re: Air performance
 

I've read that the major difference between Vietnam era fighters and modern ones is the huge reduction in maintenance. My dad was a crew chief stationed in Germany around 1960 and said only once did he see the whole squadron operational. That was the Cuban missile crisis and involved ignoring redundant systems being not redundant.

Re: Air performance
 

In 3e yes (1950). In 4e not so much (1940). I'm not sure if anything major other than crude radar marks the 1940 date. IMHO when they changed the TL6 date to 1880 they went too early on that too.

Re: Air performance
 

1940 is decent, working, military radars. Primitive but useful ones date from the 30s.

As for jets, 1940 is just about the point flight-worthy jets started being produced, and the mid-point of the first turboprop's production run.

It's also the point where penicillin was shown to be producible in useful quantities, though mass production didn't start for a couple of years.

Re: Air performance
 

https://en.wikipedia.org/wiki/List_o...f_World_War_II

...has no manned jet aircraft with an "entered service" date before June 1944.

As to the first turboprop's production run......

https://en.wikipedia.org/wiki/Vickers_Viscount

....has the Vickers Viscount not even flying until 1948.

Doesn't look much like 1940 to me.

Re: Air performance
 

I was talking about the engines. You're still building TL7 tech engines even if nobody puts them in a plane. Yes, they came out a little after 1940, but it's a close enough date, IMO. If you push TL7 much later, there will be even more complaints than there already are about how radar is given as TL7, but was introduced in TL6 and so on. Yes, some things come in earlier, and some later, but not everyone seems to go with that.
Again, engine, not aeroplane, though it looks like it never entered true production.

Re: Air performance
 

No, but it does list the Heinkel He 178's first flight as 1939, and three other aircraft with first flights in 1940. This includes the Caproni Campini N.1, which was regarded as the first successful jet aircraft while the He 178 was still a secret. That none of them were adopted by any military isn't really important - the He 178, for instance, was entirely a private venture, not a military project. Also worth pointing out that the turbojet was apparently invented way back in 1930.

Since the question is when the technology is available and not when it was first used for the specific purpose of blowing people up, 1940 sounds like a "good enough" date. Jet engines were invented before the date, and jet-powered aircraft regarded as successful were flying on or before that date.

Re: Air performance
 

I tend to think of TLs as being mostly for organizing gear catalogs and then the Skills for using that gear rather than memorializing theoretical accomplishments.