Contrail science.

"Grindintosecond" (Grindintosecond)
12/30/2014 at 20:08 • Filed to: planelopnik

Brainy people will tell you how hot jet exhaust condenses the moisture in the air to make contrails but just how hot and how cold are we talking about?

This picture is from 34,000 feet. The contrail ahead is the path of another jet's climb to level flight at 36,000. The first thing to think about is being a kid. Thats right, on the playground in the cold playing with your mouth open watching your breath drift off in misty haze and disappear. Thats just like jet engine exhaust but to create gigantic trails like this we need a huge ammount of heat to find and condense the very scarce and limited moisture from the thin air. How hot?

In cruise at 34,000 feet we saw an outside air temperature of about -51 Celsius (-60f). The engine exhaust temperature we indicated was 770 celcius (nearly 1500f). Thats a temperature difference of, well, I cant divide -60 by +1400. Its huge. Now, were burning basicly 11gallons of kerosene a minute as well so the expansion of hot gasses out the compressing tailpipe at 1400 degrese creates something like a blowtorch as big as your car.

Those are the forces at play.

Picture credit is mine. (No contrails were found or harmed during this article)

DISCUSSION (21)

Arch Duke Maxyenko, Shit Talk Extraordinaire > Grindintosecond
12/30/2014 at 20:11

Damn chemtrails, big government is trying to kill us all with their sprays!

/ tips foil hat

deekster_caddy > Grindintosecond
12/30/2014 at 20:24

Maths and stuff. The difference is just subtraction: 770-(-51) or 770 + 51 = 821C, or ~1500-(-60) or 1500+60=1560F.

(double negatives cancel, '~' means approximately)

How did you get the measurements?

Grindintosecond > deekster_caddy
12/30/2014 at 20:29

I was flying the plane. I was trying to get a delta difference multiple to represent. Another measurement to concidet is exhaust temp during climb powet. Nearly 900c sometimes more for some other planes (non afterburning of course)

HammerheadFistpunch > Grindintosecond
12/30/2014 at 20:37

Nope its mind controlling chemicals...you can't fool me!

RallyWrench > Grindintosecond
12/30/2014 at 20:40

11 gallons a minute sounds really, really efficient to me considering the speed of travel and weight being held aloft.

Tohru > Grindintosecond
12/30/2014 at 20:58

I hope you had permission to fly the plane, and not just reenacting GTA5.

Grindintosecond > RallyWrench
12/30/2014 at 21:03

It's sort of 75% of a 737. Ill convert it to the hourly burn rate we use, something like 4200 pounds per hour, or sixty honda civic gas tanks. But when a fuel burn is looked at, and compared to the number ofo people on board, some of the larger planes can be even more efficient. I might burn 20,000 pounds of fuel in 4 hours, but divided by the people on board thats about 10 gallons per hour per person. More than what your car burns per hour? Well, let's look at the 350 miles in that hour that person just moved for that 10 gallons of gas. That's pretty darn efficient and, again, some larger planes are more so.

turning to the economics, they call it the cost per seat mile. cost per hour to operate/number of seats/miles traveled. A small regional jet is around 12-13 cents, large regional around 10. A 737 is about 7, Airbus 6.

Grindintosecond > Tohru
12/30/2014 at 21:04

shhhhh.....

norskracer98-ExploringTheOutback > Grindintosecond
12/30/2014 at 21:05

CHEMTRAILS!!! FOR THERE IS PROOF, I MUST SPREAD TRUTH.

*Removes tinfoil hat*

Actually pretty neat.

Grindintosecond > norskracer98-ExploringTheOutback
12/30/2014 at 21:15

Yeah it was that day. Watching other traffic during sunset was like a contrail airshow.

doodon2whls > Grindintosecond
12/30/2014 at 21:20

Don't forget that the Hydrocarbon combustion process introduces it's own mass-fraction of H2O into the atmosphere... So some portion of that Kerosene is released as H2O which contributes to the condensate plume...

Kinja'd!!!

Because I am lazy and not a ChemEng, source .

Drakkon- Most Glorious and Upright Person of Genius > Grindintosecond
12/30/2014 at 21:38

You also need to mention the quantity of water created by combustion. I don't know the chemistry of kerosene, but many fossil-fuels have 25% water content in the exhaust.

When we flew a 747 to europe, you could watch the pounds of fuel counter click down. it was more than 11 pounds per minute, I assure you.

Grindintosecond > Drakkon- Most Glorious and Upright Person of Genius
12/30/2014 at 21:59

it was probably 17....per engine...at least!

RallyWrench > Grindintosecond
12/30/2014 at 22:53

That's really cool, thanks for the details.

You can tell a Finn but you can't tell him much > doodon2whls
12/30/2014 at 23:08

Came here to point out that combustion dumps a lot of water into the air. I see you beat me to it. From Wikipedia I found this approximation of the kerosene combustion reaction:

2 C 12 H 26 ( l ) + 37 O 2 ( g ) 24 CO 2 ( g ) + 26 H 2 O( g ); H = -7513 kJ

I'm too lazy to look up molar masses on my phone, but suffice to say for every pound of kerosene burned, several pounds of water are produced.

You can tell a Finn but you can't tell him much > doodon2whls
12/31/2014 at 11:06

This was bugging me so I came back and did the math. By my really sketchy recollection of chemistry and some crude calculations I come up with kerosene making ~1.4 lbs of water for every pound of kerosene burned. At 11 gpm and assuming 7 lbs/gallon density for kerosene that puts about 108 lbs/min of water coming out the back of the engine.

I used 170.33 g/mol for kerosene and 18.02 g/mol for water. From the combustion equation each mole of kerosene makes 13 moles of water.

doodon2whls > You can tell a Finn but you can't tell him much
12/31/2014 at 11:10

Science!

;-)

Happy New Year, Finn...

You can tell a Finn but you can't tell him much > doodon2whls
12/31/2014 at 14:55

Happy New Year to you too.

MFEJAL grey because who knows... > You can tell a Finn but you can't tell him much
01/01/2015 at 15:18

wow, I was impressed with the fuel consumption ratio, but now, with the water, well, I'm in awe. Like some kind of witchcraft. LOL

Kinja'd!!!

You can tell a Finn but you can't tell him much > MFEJAL grey because who knows...
01/02/2015 at 11:44

No witchcraft, just science. There are two things I remember from chemistry. Perfect combustion combines fuel and oxygen and forms CO2 and H2O. When you mix an acid and a base you get a "salt" and water.

Kinja'd!!!

MFEJAL grey because who knows... > You can tell a Finn but you can't tell him much
01/02/2015 at 13:21

Allow me to remind you I'm in Texas. We can explain EVERYTHING with witchcraft. Because science is not widely accepted.