Gunpowder don't burn faster in closed space. Confinement in closed space increase the explosive power of gunpowder. That effect is more pronounced for slower burning gunpowder like old fashion black powder, for modern faster burning gunpowder the effect is less but still there. How fast the gunpowder burns in more related to surface area ratio. For example, more finely ground gunpowder will burn faster. Think about a piece of wood, if you chop it into smaller pieces, it will burn up faster because the surface area is bigger.I know that gunpowder burn faster in closed space. Does this apply to fuel of missiles?