Liquid fuel vapors will only combust within a certain range of concentrations; too “lean”, they won’t burn, too “rich”, they won’t burn.
Remember “flooded carburetors”? Too rich a mixture.
I would think that the air in any fuel tank would have about as much fuel vaporized into it as physically possible ... therefore, how could it ignite?
Simple really.
The Sikorsky SH-53D flown by the USMC used to employ 500 gallon fiberglass ferry tanks to extend their range. Navy & USAF CH-53s had external tanks and did not need the ferry tanks.
However, after a USMC -53 was blown out of the sky by an exploding ferry tank, use of these tanks was prohibited. The accident investigation board believed that the crew chief removed the fill cap to check fuel level. Static electricity and inadequate grounding procedures most likely ignited the fuel vapors.
Jet fuel has lower vapor pressure than gasoline. Whereas the air over gasoline in a closed tank will become too rich to be explosive, jet fuel in a tank(Kerosine), with its lower vapor pressure, does have an explosive air mix in the tank.
We were investigating the possibility of flying our -53s to Europe and first tried the equivalent flight legs from North Carolina to Homestead, FL to Grand Turk to Roosevelt Roads, PR. We had to get special permission and give additional training to flight crews before we were allowed to use those 500 gall fiberglass ferry tanks.
An “empty” (having previously contained fuel) jet fuel tank will very definitely explode.
Remember “flooded carburetors”? Too rich a mixture. .....
I was a GM mechanic from the late 60s until I joined the national guard exactly 41 years ago today. We used to take a pan of gasoline and throw lit matches into it and they would go out. Then we put a fan on one side of the pan and the vapors would explode.
I suppose you could blow up a diesel fuel tank.....WITH A CUTTING TORCH!