What does atmosphereCurve mean?????

[fusedbean89]

ive been making a mod and desided to put a engine in the mod but i came across this

 

  atmosphereCurve
                {
                        key = 0 4200
                        key = 1 100
                        key = 1.2 0.001
                }

what does   atmosphereCurve do and what do the keys do ????

[blowfish]

It's a float curve which maps atmospheric pressure to engine specific impulse.  So for that curve, it will get 4200s I_sp in a vacuum (0 atmospheres) and 100s at sea level (1 atmosphere) (and 0.001s once you get up to 1.2 atm, which in the stock system only happens on EVE).