How do jet engines even work at very high altitudes?

I know that some jet fighters have a service ceiling of 65,000 feet, the F-15 for instance. Altitude.org tells me that "At 65000ft, the standard barometric pressure is 6 kPa (47 mmHg). This means that there is 6% of the oxygen available at sea level." Ok, so there is a little oxygen to help the combustion mode. I suppose there is some equation where the mixture of fuel to that oxygen ration allows the engines to still work. However, if you go to the SR-71's service ceiling of 75,000 feet that brings you down to 4% oxygen. So when reading on some aircraft though it seems they were also designed to reach 97,000 to 100,000 feet - which brings it down to 1%. Is that all that is needed to provide combustion or is some other fuel brought along to help out?

a reply to: Fools

www.quora.com...

a reply to: DAVID64

I think the comments section is close to an answer:

"It's true that density of air decreases as altitude increases, the jet thrust depends upon mass flow rate, in extremely simplified terms it is Thrust = Mass_flow_rate times exhaust_velocity. Mass flow rate is density x velocity x cross sectional area.

So when air is rarer the aircraft needs to fly faster in order to ingest enough air to maintain that mass flow rate.

But there is upper limit to both altitude and speed for every turbine based jet engine which varies as per design like Mig-25/31; both flying with jet engine can fly as high as 20 kms and at speeds mach 2.8 - 3.2 for short period of time after which engine will be severely damaged."

Jet engines compress the air making it more dense, so the fuel can burn.

Do forget that air is also easier to push through at higher altitude.

a reply to: Fools

See DAVID64

www.quora.com...

JP-7 mixture with an Oxidizer .....

tri-ethyl borane (TEB), which burns at a high temperature when it is oxidized (combined with air).

Zaphod58 will be by. But read DAVID64.

These could be fired separately or together. JP-7 (Jet Propellant 7, MIL-T-38219) is a jet fuel developed by the U.S. Air Force for use in supersonic aircraft because of its high flashpoint and thermal stability. It is the fuel used in the Pratt & Whitney J58 engines, used in the Lockheed SR-71 Blackbird.

www.456fis.org...

originally posted by: Bigburgh
a reply to: Fools

See DAVID64

www.quora.com...

JP-7 mixture with an Oxidizer .....

tri-ethyl borane (TEB), which burns at a high temperature when it is oxidized (combined with air).

Zaphod58 will be by. But read DAVID64.

These could be fired separately or together. JP-7 (Jet Propellant 7, MIL-T-38219) is a jet fuel developed by the U.S. Air Force for use in supersonic aircraft because of its high flashpoint and thermal stability. It is the fuel used in the Pratt & Whitney J58 engines, used in the Lockheed SR-71 Blackbird.

www.456fis.org...

Thanks, the absolute record is a Mig 25. Seems it went up to 123,520 ft. At that altitude there is ZERO oxygen. So it must also have used a fuel that needed almost no oxygen.

a reply to: Fools

On an earlier attempt, he reached 118,000 feet, and was down to 46 mph. The engines flamed out and he returned to lower altitudes on a ballistic trajectory.

Jet engines don't work at much above 80,000 feet due to the low oxygen content. They will need an additive in the fuel to oxygenate the fuel, which means a rocket system or something similar.

a reply to: Fools that was done using a ballistic approach, max throttle and aim for space the aircrafts mass will pull it up higher.
the airbreathing highest sustained flight is still the sr-71(rs-71) thought i read somewhere a couple of the u2 were hinted at getting higher (max ceiling classified).

Pretty sure the SR-71 would have used low flash point fuel due to the temperature outside. They actually maintained a temperature of 350* and it had a very low flash point. It would not ignite even if a match hit the fuel, for example. It acted as an additional coolant source as well. Something like 11,000 gallons worth of the fuel needed and it had to be warmed up before it could be used. It basically would get so hot on the outside, they needed a fuel that would/could stay cool inside. Humans are incredible sometimes.

On topic for "Air Density".... There is a great YT video from NASA that provides a "You are Riding It", high definition video and audio of the solid rocket motors separating from the shuttle, (this occurs at about 150,000 ft. ). Each booster rocket has 2 HD cameras, and the video being a few hours long, does liftoff to orbit and re-entry to ocean, from all of the camera's separately.

When they separate, they spin end over end for a while. Since it is now an empty cannister, when the wind hits it at the nozzle , is is like blowing wind across the lip of a bottle, and sounds just like that , except much louder. So even at 150,000 feet, there is still air up there: N,CO2 and O2, just not much of it. As the the drogue chutes kick in and it decends, you can hear the increase in wind vortex. It is a cool video, enjoy. Ride the boosters



BRW: NASA also has the same video from the main tank, as well as the shuttle cockpit itself.

Usually jet engines lose thrust with altitude. The fancy >1 thrust to weight ratios of modern fighters don't actually exist at the altitudes that they usually operate at. There's enough pressure to sustain combustion, but thrust will be reduced. Aircraft drag will also be reduced which why fighters can fly so quickly.

NASA has an engine simulator here:

www.grc.nasa.gov...

a reply to: Fools

The real key is that a jet engine compresses the ambient air, putting a larger mass of oxygen into the burner box than would be available otherwise. The military engine I am most familiar with is the General Electric J-79, 2 of which powered my F-4s. It has a 17 stage compressor section with variable stator vanes. The ratio of fuel to air in a jet engine is about 15 parts fuel to 1 part air, so if there is less air, you use less fuel, which is why airliners fly high, plus we jet pilots like to look down (way down) on others.

a reply to: SR1TX

The way aircraft warm up fuel for combustion is by running the engine oil through the fuel tanks in piping, this cools the oil and heats the fuel making it easier to combust.

a reply to: Fools

The front of the jet engine is an intake, the force through the air pushes against that air and at higher speed's more air pressure exists and is pushed into the intake, this mean's simply put that there is enough air to fuel the combustion and it also mean's that the higher you go the faster you can go and also the faster you have to go, this is were the limitations on the engine and the air frame + aerodynamic principles (strength and shape of the air craft tailored to those altitudes and air pressures) come to bare more than the low air pressure.

Generally speaking if there is enough air pressure to provide enough lift at a given speed then there is also enough air pressure to provide enough oxygen to the jet intake but of course more efficient fuel and engine design would help, at higher altitudes pulsed jet engine's (the Aurora etc) with a high rate of combustion cycle would likely be far more efficient using the jet's speed and intake to scoop air in and pressurize it then efficiently mix it with a highly reactive fuel and detonate it providing small burst explosions instead of a normal jet's constant explosion of fuel and oxygen.

originally posted by: Bigburgh
a reply to: Fools

See DAVID64

www.quora.com...

JP-7 mixture with an Oxidizer .....

tri-ethyl borane (TEB), which burns at a high temperature when it is oxidized (combined with air).

Zaphod58 will be by. But read DAVID64.

These could be fired separately or together. JP-7 (Jet Propellant 7, MIL-T-38219) is a jet fuel developed by the U.S. Air Force for use in supersonic aircraft because of its high flashpoint and thermal stability. It is the fuel used in the Pratt & Whitney J58 engines, used in the Lockheed SR-71 Blackbird.

www.456fis.org...

Triethyl Borane is a pyrophoric compound used to start the engines and light the afterburner. JP-7 has a high flashpoint and the borane is used in lieu of a spark igniter. Running a jet engine on a borane isn't a good idea as the product of combustion is the oxide B2O3 which plays havoc with turbine blades.
The JP-7 was used as a heat sink to cool the structure and electronics of the SR-71.

originally posted by: Jinn82
a reply to: SR1TX

The way aircraft warm up fuel for combustion is by running the engine oil through the fuel tanks in piping, this cools the oil and heats the fuel making it easier to combust.

The real reason for this heat exchange arrangement is that at 41,000 feet of altitude, the outside air temperature will be 70 below zero F, turning kerosene (jet fuel) into the consistency of Jello, which has trouble flowing through the fuel lines. Warming it thins it considerably. It also sweves as an oil cooler for the engine oil.

I know it must have been done but can you compress oxygen to liquid, and if you only need 7% oxygen last time I went scuba diving diving I had 30 minutes in one tank which was definitely not 100% oxygen, why cant we have a oxygen injection to go very high for a while?

originally posted by: F4guy
a reply to: Fools

The real key is that a jet engine compresses the ambient air, putting a larger mass of oxygen into the burner box than would be available otherwise. The military engine I am most familiar with is the General Electric J-79, 2 of which powered my F-4s. It has a 17 stage compressor section with variable stator vanes. The ratio of fuel to air in a jet engine is about 15 parts fuel to 1 part air, so if there is less air, you use less fuel, which is why airliners fly high, plus we jet pilots like to look down (way down) on others.

Your fuel/air ratio seems to be backwards. Isn't it 15 parts air to 1 part fuel? I know the JT-8 looks like it's burning coal when running but wow! At your ratio, the jet would be more efficient at low altitudes which ain't the case...just my observation.

a reply to: C0bzz

ughh i tried for a hour to get that to work, no luck even after reading the FAQ