concept

Thermally thin fuel

A material thin enough that when it is heated, its whole thickness warms together, with no meaningful temperature gradient between the exposed face and the back. The conventional test is a Biot number below 0.1. The practical consequence is a change of scaling law: time to ignition goes linearly with areal thermal mass ρcδ, instead of quadratically with thermal inertia as it does in thick solids. Leaves, paper and flight membranes all fall in this class - which is why thin things do not merely burn a little sooner than thick ones, they fail almost immediately under radiant heat.