If it had 8300 horizontal, and 4200 vertical, the total speed would be the hyponetuse of a triangle made by those, not the sum. sqrt(4200^2 + 8300^2) gives me 9,302 kph, which is a bit more than a third of 26000 (.3578). That squared gives you a bit more than an eighth (.1280) of the heat.

and thats not including the fact that someone else pointed out that it would go with velocity cubed.