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1. Without foam: Explosive mix of fuel vapour and air above liquid fuel ignites easily. Once this ignites, a compression wave pressurises the remaining gas, increasing the explosion.
2. With foam: Foam expands to fill space in tank as fuel level drops. Vapour ignition is confined to the area close to spark, stopping explosion.
The explosive-suppressant foam stops fuel tanks from exploding when pierced by bullets. One US plane fitted with it managed to land safely after being shot 19 times in Iraq.