# Why is the combustor volume of a jet engine decided by altitude?

I have read that the combustor volume of a jet engine is decided by the maximum altitude up to which that aircraft will fly. I'm getting confused by that, because the maximum mass flow rate will occur at sea level so the combustor volume should be decided based on sea level. Can anyone explain how the volume is decided?

• Are you sure it's about measuring the volume? The volume of a combustor is constant. But the higher you fly, the larger the engine must be. So, the max. altitude determines what volume is needed. Dec 5, 2014 at 20:46
• @sweber Did you misread the question? The question seems to be saying exactly what you suggest: that the design of the combustor depends on the desired maximum altitude. Dec 5, 2014 at 20:59
• I edited the question for clarity but I hope I didn't confuse things. I understood it to mean: "combustor volume is determined by the intended service ceiling of the aircraft, but maximum combustor volume is required at sea level, so if the combustor is big enough for sea level then it's big enough for any altitude and therefore the service ceiling shouldn't matter anyway". But since I know nothing about jet engines I didn't want to edit it so radically. Maybe the OP can comment. Dec 5, 2014 at 21:21

It is less the volume than the length which limits the maximum altitude. The cross section of the combustor is determined by the volume flow coming from the compressor and the desired flow speed in the combustor, so this dimension is given. To enable operation at lower pressure, the length of the combustor must be sufficient, so a longer combustor will have a higher volume. That is why you have read that volume determines maximum operating altitude.

## Background

The factors which make ignition easier are

• pressure
• temperature, and
• spark energy of the ignition system

Ignition is made harder by

• mean fuel droplet size (bigger droplets take longer to evaporate)
• flow speed in the combustor, and
• higher turbulence intensity.

For combustion to take place, first the fuel droplets injected into the air stream need to evaporate. This is made easier by higher temperature and pressure, and the lower both are, the more residence time of the fuel-air mixture in the combustor is required for good combustion. Longer combustors have higher pressure losses and weigh more, so engine designers try to limit their length.

With increasing altitude, the pressure in the combustor drops because the compressor will simply lift the pressure at it's inlet by a given multiple. Therefore, combustor pressure drops in sync with ambient pressure, and the required length for sufficient combustion goes up.