Wouldn’t SLS SRBs burn longer? Or is because the solid fuel burns from the inside out ie. not bottom to top?
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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.
https://youtu.be/Eis3A2Ll9_E and other YouTube videos. Some search terms might include something as simple as ‘solid rocket’ or ‘solid rocket booster’.
Yes, they burn from the inside out. By selecting different shapes of the cavity, the burn profile can be controlled.
https://www.grc.nasa.gov/www/k-12/airplane/srockth.html
https://www.nakka-rocketry.net/th_grain.html has pictures of several cross sections compared to their thrust profiles.
https://en.wikipedia.org/wiki/Solid-propellant_rocket should link to the article specifically for the Space Shuttle and SLS ones but gives an overview.
It’s all about surface area of the propellant in what is basically a hollow cylinder. Only what’s on that inside surface is currently burning, so control the surface area and you control how fast and thus how long it burns.
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