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Old 03-10-2021, 01:10 PM   #1
pgb
 
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Default [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.
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Old 03-10-2021, 01:47 PM   #2
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Default 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.
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Old 03-10-2021, 02:02 PM   #3
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Default 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.
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Old 03-10-2021, 02:13 PM   #4
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Default Re: Air performance

Quote:
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.
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...
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Old 03-10-2021, 04:29 PM   #5
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Default Re: Air performance

Some fighters are capable of extended flight straight upwards, so logically these must have at least 1G of acceleration.
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Old 03-10-2021, 06:35 PM   #6
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Default Re: Air performance

Quote:
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.
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.
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Old 03-10-2021, 06:36 PM   #7
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Default 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.
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Old 03-10-2021, 06:55 PM   #8
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Default Re: Air performance

Quote:
Originally Posted by Phoenix_Dragon View Post
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.
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.
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Old 03-10-2021, 07:19 PM   #9
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Default 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.
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Old 03-10-2021, 08:09 PM   #10
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Default Re: Air performance

Quote:
Originally Posted by RyanW View Post
The F-15 is famously capable of essentially ballistic flight
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.
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