Jump to content

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


Recommended Posts

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)

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...