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Clipperride

Does KSP Module Mach/Altitude ?

Question

At sea level, Mach 3 is around 1021m/s but by the time you get to 60,000ft or 18,288m it's down to around 885m/s.  Does KSP model this change when it comes to the performance of jet engines that vary thrust depending on your velocity?

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Yes, the engine thrust curves are scaled by Mach number, and Mach number is scaled by atmospheric density and temp -- all that is indeed modeled.

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Yes.

You can see this by opening the Aero GUI (Alt-F12 on PC, Physics, Aero) and watching the Mach nr change live as your plane climbs. Having the rightclick menu of your engine pinned open at the same time will also show you how the thrust develops, and you will notice that planes that struggle to get to Mach 1 at sea level can often punch through quite easily at higher elevations.

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One interesting consequence of this is that it's actually possible to get RAPIERs going faster ASL than they will in the upper atmosphere, because of the way their thrust curves are structured :v

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6 hours ago, Clipperride said:

KSP is so detailed in so many ways!

Yes, it makes me annoyed when people moan about how "simplified" KSP's aero model is.

 

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8 minutes ago, bewing said:

Yes, it makes me annoyed when people moan about how "simplified" KSP's aero model is.

 

Well, it is. There's lots of stuff it doesn't model. It's a lot better than it used to be, though, back when you could get yourself to orbit by moving control surfaces and full and empty tanks fell at the same speed :v

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6 hours ago, foamyesque said:

Well, it is. There's lots of stuff it doesn't model. It's a lot better than it used to be, though, back when you could get yourself to orbit by moving control surfaces and full and empty tanks fell at the same speed :v

Shouldn't empty and full tanks (assuming they are the same size) fall at the same speed?  I'm thinking about Galileos' proof by dropping balls of the same shape but different mass from the Leaning Tower of Pisa and, of course, the Apollo 15 "Hammer and Feather" experiment.  The tanks would have the same acceleration, but different momentum.

 

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16 minutes ago, Clipperride said:

Shouldn't empty and full tanks (assuming they are the same size) fall at the same speed?

In a vacuum, yes. In atmosphere, they should have different terminal velocities. (If they can actually reach terminal velocity, which depends on air density.)

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28 minutes ago, bewing said:

In a vacuum, yes. In atmosphere, they should have different terminal velocities. (If they can actually reach terminal velocity, which depends on air density.)

 

Well, and on how far they're being dropped. :P

46 minutes ago, Clipperride said:

Shouldn't empty and full tanks (assuming they are the same size) fall at the same speed?  I'm thinking about Galileos' proof by dropping balls of the same shape but different mass from the Leaning Tower of Pisa and, of course, the Apollo 15 "Hammer and Feather" experiment.  The tanks would have the same acceleration, but different momentum.

 

 

As @bewing says, that's true in a vacuum (a consequence of the masses cancelling: the force of gravity increases proportionate to the mass of an object but the force required to achieve a given acceleration also increases proportionate to mass), but in an atmosphere there's drag to consider -- and drag doesn't care about mass, it cares about areas and how they're arranged. There's a concept called "ballistic coefficient" that describes, effectively, how much effect drag has on something. Consider a beach ball; now consider an equivalent spherical rock. One not only has a much higher top speed when it's falling, it has a higher acceleration getting there.

 

But in early KSP, everything fell like it was in vacuum.

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6 hours ago, foamyesque said:

.... Consider a beach ball; now consider an equivalent spherical rock. One not only has a much higher top speed when it's falling, it has a higher acceleration getting there.

Didn't Galileos' experiment show that two objects of equal area but different mass (he used also used spheres in his experiments as they present the same profile regardless of orientation) do accelerate at the same rate as they head for their different terminal velocities?

"Four hundred years ago--or so the story goes--Galileo Galilei started dropping things off the Leaning Tower of Pisa: Cannon balls, musket balls, gold, silver and wood. He might have expected the heavier objects to fall faster. Not so. They all hit the ground at the same time, and so he made a big discovery: gravity accelerates all objects at the same rate, regardless of their mass or composition" at least until you get up to the size of moon's and planets. See -

https://science.nasa.gov/science-news/science-at-nasa/2004/06may_lunarranging 

Not that it's especially important to the modelling of the speed of sound at altitude! :wink:

 

Edited by Clipperride

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Yes, but he was working in laminar flow conditions -- very low speeds with no significant drag. foamyesque is right that when drag is a significant factor, then gravitational acceleration in a fall will be a nonlinear function of time, and will always be lower for a less massive object of the same size/shape/orientation -- because the gravitational force will be less for the one, but the opposing drag force will be the same between the two objects at a particular speed.

 

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Ahhh, I see why I was at cross purposes with @foamyesque above.  And that explains why Galileos' experiment was at such odds with our subjective expectations of how things behave. Cheers

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