How to calculate altitude from pressure on Kerbin

[AeroGav]

Hi all,

I've been busily procrastinating on a Spaceplane performance guide.  

I've got to a section where i'm discussing the effect of altitude on jet engine performance, ie. what's the best altitude for penetrating sound barrier, what's best for hitting max airbreathing speed.

Looking at the config file of the rapier,  I can see that at 0.35 atmospheres of pressure, we have a thrust multiplier of 0.5.      At 0.08 , thrust multiplier falls to 0.3 (looks like a sweet spot to me) , then at 0.018 we still have 0.09 (still somewhat decent, we thrust has only fallen half as much as pressure - drag - has , right?). Above that things go south very quickly.

The thing is, is there a way to convert these pressure numbers, expressed in "atmospheres",  into actual altitudes I can use when flying an aircraft? 

The KSP wiki has a table of pressure (in atmospheres) vs altitude (in km), but there are only 11 data points and inconveniently the numbers in the config file fall in between.

It's not a linear relationship of altitude vs pressure so interpolating it won't be very accurate.  My best guesses are at these numbers correspond to 6km (thrust multiplier 0.5) ,  14km (thrust multiplier 0.3) and 23km (thrust multiplier 0.09),  but is there any tool to calculate the numbers properly?

 

[Streetwind]

The "altitude variation" section in the Wikipedia entry for Atmospheric Pressure should have the kind of math you're looking for. Kerbin uses pretty much all Earth values across the board except planetary radius, so you can just grab and use the constants presented there, too.

Hope that helps

[AeroGav]

6 minutes ago, Streetwind said:

The "altitude variation" section in the Wikipedia entry for Atmospheric Pressure should have the kind of math you're looking for. Kerbin uses pretty much all Earth values across the board except planetary radius, so you can just grab and use the constants presented there, too.

Hope that helps

Thanks, but I'm not that good at Maths,  basic algebra is about all i can do without serious hand holding and serious step by step working.  Didn't study it past age 16.

In the meantime, I did it the hard way by flying a plane and making a note of my altitude when my pressure (in pascals) hit the values tagged in the config files.    If anyone is interested -

 

Engine	Altitude	Percent of Sea Level	Percent of Sea Level	Speed Advantage compared with sea level
		Air Density	Power
RAPIER	6.5km	35.000	50.000	1.429
RAPIER	14km	8.000	30.000	3.750
RAPIER	21.5km	0.018	0.090	5.000
WHIPLASH	4.5km	50.000	60.000	1.200
WHIPLASH	10km	16.000	50.000	3.125
WHIPLASH	17km	4.500	17.000	3.778
PANTHER	6.5km	34.000	56.000	1.647
PANTHER	15km	7.07	13.971	1.976

[sardia]

Is your graph saying a rapier is faster at 21km than a whiplash at 17km?

[Stone Blue]

Maybe this thread might be helpful for you:
 

And a little known utility, made by r4m0n, who was one of the FIRST RemoteTech devs:
 

 

[Leafbaron]

Try this equation I extrapolated using excel. y(pressure_atm)=2E^-28x^6-5E^-23x^5+6E^-18x^4-3E^-13x^3+9E^-9x^2-.0002x+1.0012, where x is altitude in meters

[Waxing_Kibbous]

Maybe this can help you?

 

[Snark]

16 hours ago, AeroGav said:

In the meantime, I did it the hard way by flying a plane and making a note of my altitude when my pressure (in pascals) hit the values tagged in the config files.

In other words... you actually flew a research vessel and USED A SCIENCE INSTRUMENT to actually GATHER SCIENCE!

You're livin' the dream, man!