Calculating Pressure

[SiliconPyro]

I'm trying to figure out how to calculate parachute requirements, so I'm working through this equation. I think I get it, mostly, but I'm lost at calculating pressure. Can someone who knows their math explain how to work through that equation, maybe with a few examples?

Edited September 9, 2014 by norcalairman

[mhoram]

The pressure at an altitude is calculated via the following:

Each planet has two defining factors for the atmosphere. For Kerbin these are:

- Scale height (in Meter): 5000m

- Pressure at sealevel (in atm): 1 atm

For other planets these values are available on their wiki-pages.

You can calculate the pressure within the atmosphere at an altitude of lets say 13km above sealevel by:

pressure_Kerbin(Altitude) = Pressure_At_Sealevel * exp(-Altitude / Scale_Height_Kerbin)

pressure_Kerbin(13km) = 1 atm * exp(-13km / 5km) ~ 0.07427358 atm

(more physics stuff here)

[SiliconPyro]

The pressure at an altitude is calculated via the following:

Each planet has two defining factors for the atmosphere. For Kerbin these are:

- Scale height (in Meter): 5000m

- Pressure at sealevel (in atm): 1 atm

For other planets these values are available on their wiki-pages.

You can calculate the pressure within the atmosphere at an altitude of lets say 13km above sealevel by:

pressure_Kerbin(Altitude) = Pressure_At_Sealevel * exp(-Altitude / Scale_Height_Kerbin)

pressure_Kerbin(13km) = 1 atm * exp(-13km / 5km) ~ 0.07427358 atm

(more physics stuff here)

Okay, I think I'm almost there, but what is "exp"?

[ratchet freak]

Okay, I think I'm almost there, but what is "exp"?

the exponentiation function, in other words "exp(x)" is "e to the power of x" with e=2.71828182845904523536028747135266249775724709369995

[SiliconPyro]

the exponentiation function, in other words "exp(x)" is "e to the power of x" with e=2.71828182845904523536028747135266249775724709369995

Okay, how do I use that in, say, Python? I tried:

1*math.exp(-13/5)

The result was ~ 0.04978706, not ~ 0.07427358. Any idea what I'm doing wrong?

[ratchet freak]

Okay, how do I use that in, say, Python? I tried:

1*math.exp(-13/5)

The result was ~ 0.04978706, not ~ 0.07427358. Any idea what I'm doing wrong?

-13/5 = -3 unless you make one of them a double or float

[SiliconPyro]

-13/5 = -3 unless you make one of them a double or float

No, I didn't and I checked that python was returning -3 for -13/5. And exp(-3) resulted in 0.04978706. So is that correct?

[ratchet freak]

No, I didn't and I checked that python was returning -3 for -13/5. And exp(-3) resulted in 0.04978706. So is that correct?

what I mean is that you should make sure the numbers you are dividing are doubles otherwise they get rounded down (to -infinity) and you lose accuracy,

if you try -13.0/5.0=-2.6 and exp(-13.0/5.0) you will see the correct output (the ~0.07427358 you were expecting)

[SiliconPyro]

what I mean is that you should make sure the numbers you are dividing are doubles otherwise they get rounded down (to -infinity) and you lose accuracy,

if you try -13.0/5.0=-2.6 and exp(-13.0/5.0) you will see the correct output (the ~0.07427358 you were expecting)

Ahhhh! It's been too long since I played with Python. Thank you for your patience, ratchet freak.