Altitude and pressure

[MatterBeam]

Hello.

What's the relationship between altitude (meters) and pressure (atm) on Kerbin? How do I know what pressure there is at 12km altitude? If I set my parachutes to open at 0.3atm, at which altitude will they open at?

[regex]

You have a barometer, use it.

[7499275]

You have a barometer, use it.

While that was extremely blunt... I dug this up. Its from the old Aero model so I'm not sure how much it has changed but here ya go. http://forum.kerbalspaceprogram.com/threads/108209-variation-in-atmospheric-pressure-with-altitude

Also there is this http://wiki.kerbalspaceprogram.com/images/thumb/c/cb/Pressures.svg/600px-Pressures.svg.png off the wiki... Not sure how old it is.

[Renegrade]

Other threads have indicated that the model has changed, it's not the simple exponent system it was before, but some three-phase NASA-y thing.

So I'd go with regex's suggestion (although last time I peeked at a barometer, it seemed to be displaying kPa rather than a fraction..) and build a sounding rocket with a barometer on it.

Or a plane.

[regex]

While that was extremely blunt...

I keep hearing that KSP is all about learning and science, and then when I give someone the tool, and the opportunity, to do actual science in a video game I'm told I was being extremely blunt.

Its from the old Aero model so I'm not sure how much it has changed

I could be wrong, but all indications from the Kopernicus mod indicate that Kerbin now has a proper pressure curve instead of using scale height. Therefore, the values have likely changed.

E:

although last time I peeked at a barometer, it seemed to be displaying kPa rather than a fraction..

Well, kPa at 78m on Kerbin is, what, about 100? So just divide the kPa at altitude by 100. At least, that's how I think it should work... Edited May 17, 2015 by regex

[MatterBeam]

I was thinking more along the lines of: how do I calculate atm fraction for use when writing .cfg files for a modded jet engine?

[regex]

I was thinking more along the lines of: how do I calculate atm fraction for use when writing .cfg files for a modded jet engine?

Wrong forum, ask here:

http://forum.kerbalspaceprogram.com/threads/120899-HOWTO-Airbreathing-Engines-in-KSP-1-0

[adinfinitum]

E:Well, kPa at 78m on Kerbin is, what, about 100? So just divide the kPa at altitude by 100. At least, that's how I think it should work...

Kerbin's atmosphere is set up so that at sea level pressure is 1 atm, or 101.325 KPa. So dividing by 100 will get you pretty close to the right altitude

As for OP's question, the easiest way is to just send up a sounding rocket with a chute set to open at 0.3 atm to find what height it deploys, and use the barometer like regex said to find pressures at certain altitudes

[MatterBeam]

Kerbin's atmosphere is set up so that at sea level pressure is 1 atm, or 101.325 KPa. So dividing by 100 will get you pretty close to the right altitude

As for OP's question, the easiest way is to just send up a sounding rocket with a chute set to open at 0.3 atm to find what height it deploys, and use the barometer like regex said to find pressures at certain altitudes

So if I want to theorycraft an engine thrust curve, I'll have to go into the game and empirically test pressures at various altitudes?

[7499275]

Finding pressure at altitude with baramoter can be kinda difficult though unless your rocket can almost hover perfectly still at a given altitude.

[adinfinitum]

If you do go the barometer way, you can take screenshots as you pass through the atmosphere, that'll give you pressure at an exact altitude and pressure. I read somewhere that in KSP the atmosphere is an 80% scale model of earth, but I'm not sure if that was old or new atmo.

I'm not saying its the only way to do it, but it would be the most direct way to find points for a thrust curve.

I don't know what the equation is for the atmospheric pressure, but I'm sure if we keep this on the first page I'm sure someone will come along who does.

[regex]

I don't know what the equation is for the atmospheric pressure

There is no equation, it's a floatCurve. That means that you have to know the points along the curve to properly evaluate the pressure at a given altitude. I'm not sure how to get that out of the game but you might want to look into the Kopernicus mod thread or the thread I linked earlier.

[adinfinitum]

There is no equation, it's a floatCurve. That means that you have to know the points along the curve to properly evaluate the pressure at a given altitude. I'm not sure how to get that out of the game but you might want to look into the Kopernicus mod thread or the thread I linked earlier.

I read through the original post on that thread, and while it was talking about how the floatCurve for the mass flow at a given pressure can be set up, it doesn't say anything about relating altitude to pressure in KSP. Thats the atmCurve that KSP calculates from temperature and altitude, and I think Squad used real world equations for this atmosphere, I just don't know what in game values OP would have to plug in to get a pressure/altitude curve matching that of the game.

[MatterBeam]

I read through the original post on that thread, and while it was talking about how the floatCurve for the mass flow at a given pressure can be set up, it doesn't say anything about relating altitude to pressure in KSP. Thats the atmCurve that KSP calculates from temperature and altitude, and I think Squad used real world equations for this atmosphere, I just don't know what in game values OP would have to plug in to get a pressure/altitude curve matching that of the game.

A bit of research tells me that it's proportional to e^(h/h') where h' is the scale height for KSP. h' = 5000 if I remember correctly.

[adinfinitum]

A bit of research tells me that it's proportional to e^(h/h') where h' is the scale height for KSP. h' = 5000 if I remember correctly.

Is that still true? Nathan talks a bit about it in this thread, but I didn't see a definitive answer

http://forum.kerbalspaceprogram.com/threads/117038-Reverse-engineering-Jet-Engines-how-do-velCurve-and-atmCurve-work

[OhioBob]

Hello.

What's the relationship between altitude (meters) and pressure (atm) on Kerbin? How do I know what pressure there is at 12km altitude? If I set my parachutes to open at 0.3atm, at which altitude will they open at?

There is no longer a simple equation to calculate pressure as there was pre-1.0. I talk about it in the following post:

http://forum.kerbalspaceprogram.com/threads/117069-Great-New-Physics-Thread%21?p=1940443&viewfull=1#post1940443

The following will provide a very close approximation that will suit most purposes. The pressure is accurate to within 0.5% for altitudes below 62.3 km, and much closer than that in most cases. Above 62.3 km it is accurate to within 2%.

[TABLE=class: grid, width: 1000, align: center]

[TR]

[TD=colspan: 3, align: center]Kerbin Standard Atmosphere, 0 to 70 km[/TD]

[/TR]

[TR]

[TD=align: center]Geometric

Altitude, z

(km)[/TD]

[TD=align: center]Kinetic

Temperature, T

(K)[/TD]

[TD=align: center]Pressure, P

(Pa)[/TD]

[/TR]

[TR]

[TD=align: center]0-8.815[/TD]

[TD=align: center]288.15 – 8.11117 × z[/TD]

[TD=align: center]EXP[ –5.6684193E-05 × z³ – 1.9580663E-03 × z² – 0.14836114 × z + 11.5260885 ][/TD]

[/TR]

[TR]

[TD=align: center]8.815-16.050[/TD]

[TD=align: center]216.65[/TD]

[TD=align: center]EXP[ 3.6215104E-06 × z³ – 1.1135486E-04 × z² – 0.19514262 × z + 11.753405 ][/TD]

[/TR]

[TR]

[TD=align: center]16.050-25.729[/TD]

[TD=align: center]196.7513 + 1.23980 × z[/TD]

[TD=align: center]EXP[ 1.8765033E-06 × z³ + 4.0682342E-04 × z² – 0.21018261 × z + 11.868445 ][/TD]

[/TR]

[TR]

[TD=align: center]25.729-37.879[/TD]

[TD=align: center]139.7102 + 3.45679 × z[/TD]

[TD=align: center]EXP[ –4.1690244E-05 × z³ + 5.0761226E-03 × z² – 0.36509550 × z + 13.508117 ][/TD]

[/TR]

[TR]

[TD=align: center]37.879-41.129[/TD]

[TD=align: center]270.65[/TD]

[TD=align: center]EXP[ –3.6083972E-04 × z³ + 0.044146606 × z² – 1.9496301 × z + 34.813685 ][/TD]

[/TR]

[TR]

[TD=align: center]41.129-57.440[/TD]

[TD=align: center]411.8568 – 3.43327 × z[/TD]

[TD=align: center]EXP[ –8.6910710E-07 × z³ – 1.0609374E-03 × z² – 0.062235305 × z + 8.6153226 ][/TD]

[/TR]

[TR]

[TD=align: center]57.440-68.798[/TD]

[TD=align: center]354.7555 – 2.43916 × z[/TD]

[TD=align: center]See below[/TD]

[/TR]

[TR]

[TD=align: center]68.798-70.000[/TD]

[TD=align: center]186.946[/TD]

[TD=align: center]See below[/TD]

[/TR]

[TR]

[TD=align: center]57.440-62.300[/TD]

[TD=align: center]See above[/TD]

[TD=align: center]EXP[ 1.5412356E-05 × z³ – 3.5432535E-03 × z² + 0.058616347 × z + 6.7777398 ][/TD]

[/TR]

[TR]

[TD=align: center]62.300-70.000[/TD]

[TD=align: center]See above[/TD]

[TD=align: center]EXP[ –9.37298921E-04 × z⁵ + 0.304103311 × z⁴ – 39.4630615 × z³

+ 2560.291576 × z² – 83044.59921 × z + 1077314.96395 ][/TD]

[/TR]

[/TABLE]

- - - Updated - - -

A bit of research tells me that it's proportional to e^(h/h') where h' is the scale height for KSP. h' = 5000 if I remember correctly.

That's no longer true since version 1.0. Scale height is no longer a constant.

Edited May 18, 2015 by OhioBob

[cannonball89]

I'm interested in this thread as I've lost several kerbonauts (jeb among them) in the early stages of my first career save in 1.02 due to improper staging having chutes deploy during rentry heating effects. I have tried to save them at the last minute by adjusting the pressure settings of the chutes but I haven't succeeded the two times it happened. Until I figure it out the solution is just to spend an extra 5 seconds in the VAB to add an extra stage for chutes or to double check staging before reentry. Ultimately my fault, but I'd still like to know the mechanics before I lose another brave kerbonaut due to my own staging error.(it will help me sleep at night).

[SAI Peregrinus]

http://forum.kerbalspaceprogram.com/threads/119337

Put a barometer on a sounding rocket, use that to graph pressure vs altitude.

[42_aerobrake_passes]

To answer the question that I suspect the OP may actually be interested in: If you put the slider for the pressure to open at all the way to the right, it will still trigger high enough to slow you down in time as long as you have sufficient chutes for your mass. Somewhere between 3k-4k on Kerbin IIRC. Duna would need more attention to detail.