This is due to the trust curve Characteristics need for each individual rocket.
Since you seem to know model rockets, look at the Thrust Curve on a typical Model Rocket engine (A-E class engines). It has a sharp spike at the beginning that quickly drops off to steady, non-changing thrust, that eventually stops as the fuel is used up That spike is important to get the uncontrolled model up to air speed before it leaves the launch rod.
[This spike-then-steady-state thrust is a characteristic of what are called End Burning engines.]
The SRBs are attached to a larger rocket which has Active Steering. If you watch the slow-motion of the Space Shuttle taking off, you can see the nozzles on the shuttle are swiveling. The rocket can actually be steered on the way up! The Space shuttle doesn't need a launch rod -- it can be steered (using a computer) to stay vertical even at the very slow speeds just after it starts to rise.
Because the Space Shuttle is actively steered, the SRBs use a Core Burning engine. A Core Burner is ignited FROM THE TOP, but as the hot gasses race backwards, they ignite the SIDES of the core (in the "donut hole"). This produces a sudden increase in thrust, but then the thrust stays pretty steady WITHOUT THE SPIKE until the engine burns out.
Thus, the SRBs are ignited from the top, but they actually burn from the INSIDE OUT, not the top.
Model rockets in the F-M engine class use the SAME propellant as the SRBs -- Ammonium Perchlorate -- and they are also Core Burners. When you play with these "model" rocket (which can weigh from 10-150 pounds, be up to 20 feet tall, and hit 10,000 feet in seven seconds) you must be VERY carefully to insert the igniter ALL THE WAY to the top, or the engine will fail to achieve full thrust and the rocket will crash.
Since you seem to know model rockets, look at the Thrust Curve on a typical Model Rocket engine (A-E class engines). It has a sharp spike at the beginning that quickly drops off to steady, non-changing thrust, that eventually stops as the fuel is used up That spike is important to get the uncontrolled model up to air speed before it leaves the launch rod.
[This spike-then-steady-state thrust is a characteristic of what are called End Burning engines.]
The SRBs are attached to a larger rocket which has Active Steering. If you watch the slow-motion of the Space Shuttle taking off, you can see the nozzles on the shuttle are swiveling. The rocket can actually be steered on the way up! The Space shuttle doesn't need a launch rod -- it can be steered (using a computer) to stay vertical even at the very slow speeds just after it starts to rise.
Because the Space Shuttle is actively steered, the SRBs use a Core Burning engine. A Core Burner is ignited FROM THE TOP, but as the hot gasses race backwards, they ignite the SIDES of the core (in the "donut hole"). This produces a sudden increase in thrust, but then the thrust stays pretty steady WITHOUT THE SPIKE until the engine burns out.
Thus, the SRBs are ignited from the top, but they actually burn from the INSIDE OUT, not the top.
Model rockets in the F-M engine class use the SAME propellant as the SRBs -- Ammonium Perchlorate -- and they are also Core Burners. When you play with these "model" rocket (which can weigh from 10-150 pounds, be up to 20 feet tall, and hit 10,000 feet in seven seconds) you must be VERY carefully to insert the igniter ALL THE WAY to the top, or the engine will fail to achieve full thrust and the rocket will crash.
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I see nothing to verify your statement that the SRB was designed to burn propellant from the "top down". Check out the link I've provided for detail on the design & operation of the solid rocket booster engines used by the space shuttle.
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Are you shure?To me it seems nonsensical,where would the trust come from unless the burning top would blow out into space everything below it.That of course would cut the mission short.