Eli5 how do SLS and Space shuttle SRBs burn for the same time when SLS SRB has an extra segment?

Question

Wouldn’t SLS SRBs burn longer? Or is because the solid fuel burns from the inside out ie. not bottom to top?

In: 0

Answer 1

The cross section of an SRB is a “donut”, not a circle. The entire length of the booster is burning from T=0. Burn time is based on propellant *thickness*, not length. A longer booster is more thrust, not more burn time.