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# Drag&Lift, Rocket/Jet thrust: dependent to pressure or density?

## Question

While exploring the ksp atmosphere, found out that it's important to know drag/lift/thrust profile for each height. So what really affects those profiles, pressure or density?

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18 minutes ago, Abastro said:

Thanks!

What about drag&lift? Anyone know whether it is dependent on drag or lift?

Drag and lift are functions of density (and speed squared).

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As far as I know, all engines in KSP scale their thrusts with pressure, that includes rocket and jet engines.

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Drag and Lift

Drag and lift in KSP are proprtional to dynamic pressure, which is equal to 0.5 * density * velocity2 .  Note that it will also change based on your orientation, and that wings have their own lift and drag curves which are functions of mach number (which are defined in physics.cfg)

Rocket engine Thrust

Rocket engine thrust changes with static atmospheric pressure (interpolating linearly between the min and max thrust)

Jet Engine Thrust

Jet engine thrust changes with static pressure and mach number.  Both are defined by curves in the part's config file.  For pressure, you will get 100% thrust at sea level and 0% thrust in a vacuum, but at 50% atmosphere you will get somewhat more than 50% thrust (exactly how much varies from engine to engine).  For mach number, thrust generally grows up to a point then starts falling off.  Where that point is depends on the engine.

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PV = nRT, density = 1/V   -- so pressure and density are basically the same thing, at a constant temp.

And the temperature profile of the atmosphere is hardcoded.

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22 minutes ago, bewing said:

PV = nRT, density = 1/V   -- so pressure and density are basically the same thing, at a constant temp.

And the temperature profile of the atmosphere is hardcoded.

Yeah, I know that. Though I want to know that whether they affected by pressure profile or density profile.

(afaik density = Nm/V = nm where n is number density, but those details are trivial in this case)

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10 minutes ago, Abastro said:

Yeah, I know that. Though I want to know that whether they affected by pressure profile or density profile.

(afaik density = Nm/V = nm where n is number density, but those details are trivial in this case)

Since the two profiles are linearly proportional to each other, I don't see how there is any practical difference.

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5 minutes ago, bewing said:

Since the two profiles are linearly proportional to each other, I don't see how there is any practical difference.

The problem is that the temperature is not constant although it's hardcoded. It makes the two different.

For example, kerbin's ground level temperature is over 280K. The temperature of the low stratosphere is under 210K afaik, which is only three fourths of that of ground level. This means that the pressure decreases 4/3 times faster than the density does while gaining altitude in troposphere.

So, I think this does matter.

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Hmmm -- OK, you've convinced me. But obviously, I don't know the answer to the question, then.

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13 hours ago, Gaarst said:

As far as I know, all engines in KSP scale their thrusts with pressure, that includes rocket and jet engines.

Thanks!

What about drag&lift? Anyone know whether it is dependent on drag or lift?

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1 hour ago, Gaarst said:

Drag and lift are functions of density (and speed squared).

Thanks, again!

Then I think it's better to fly in troposphere/mesosphere with rocket-propelled spaceplane, and in stratosphere with jet-propelled spaceplane.

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41 minutes ago, blowfish said:

Drag and Lift

Drag and lift in KSP are proprtional to dynamic pressure, which is equal to 0.5 * density * velocity2 .  Note that it will also change based on your orientation, and that wings have their own lift and drag curves which are functions of mach number (which are defined in physics.cfg)

Rocket engine Thrust

Rocket engine thrust changes with static atmospheric pressure (interpolating linearly between the min and max thrust)

Jet Engine Thrust

Jet engine thrust changes with static pressure and mach number.  Both are defined by curves in the part's config file.  For pressure, you will get 100% thrust at sea level and 0% thrust in a vacuum, but at 50% atmosphere you will get somewhat more than 50% thrust (exactly how much varies from engine to engine).  For mach number, thrust generally grows up to a point then starts falling off.  Where that point is depends on the engine.

Thanks for the detailed explanation.

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