Absolutely:
- 2018 F-22 contrails 1.jpg (304.87 KiB) Viewed 1550 times
- 2018 F-22 contrails 2.jpg (419.96 KiB) Viewed 1550 times
Contrails are nothing you choose to emit; they are the involuntary consequence of burning jet fuel. Northrop tried adding chemicals to the B-2’s fuel to reduce contrails, but they ultimately gave up.
Interestingly, burning one liter of jet fuel
produces more than a liter of water (due to the added oxygen). Let’s look at the concrete computation because that’s where TFXplorer uses lots of guesswork and I want to move it to a solid base.
This site has the chemical reaction:
C₁₂H₂₆ + 18½ O₂ → 12 CO₂ + 13 H₂O
Real jet fuel has other chemicals added (mind control etc.), but we stick to the ideal model here 🙂
We know about the amount of fuel burnt per second from our engine API, which, in turn, computes it based upon the thrust-specific fuel consumption of the engine model.
We further know about the meteorological conditions from TFXplorer’s atmosphere model. These include the amount of water vapor currently in the air (kg/m³) and the water vapor capacity of the air (kg/m³). They are functions of air density, temperature, etc. and the latter tells us how much water can be absorbed by air before the remainder condenses.
Knowing about the fuel consumption of the engine (kg/s) and ultimately its water production (kg/s) via the reaction formula above, we want to compute the volume of air that is mixed with the additional water (m³). This is the speed of the plane (m/s) multiplied by the frame time (s) and the cross section of the contrail (m²).
Now we can say that, during the last frame, the plane left X m³ of contrail, and that it can absorb Y kg of water vapor, and that our engine actually emits Z kg over the same volume.
max(Z - Y, 0) / X gets us the excess water density (kg/m³).
And here’s where it starts to break apart. Excess water density is fine, but how much excess makes a clearly visible contrail? Is a single gram per m³ enough? Or does it take a few dozen? Does temperature come into play (ice crystals vs. water droplets)? This is where I need to add a magic coefficient again …