You can pre-design for variable thrust by using different cross sections. Solid fuel rockets cannot be turned off once they're lit, but that is ok for first stage because you'll burn it to the end.
Solid fuel engine can have higher thrust but quicker burn time so you'll typically have them as boosters and first stage to get the extra kick at the beginning and get the aircraft up to speed quickly.
For the pic above, the as the rocket burns throughout its length from centre outwards, the amount of thrust at any given instance in time is related to the available exposed area (you know how salt melts faster broken up than in a clump?). The total energy contained is determined by the volume of reactants.
Solid fuel engine can have higher thrust but quicker burn time so you'll typically have them as boosters and first stage to get the extra kick at the beginning and get the aircraft up to speed quickly.
For the pic above, the as the rocket burns throughout its length from centre outwards, the amount of thrust at any given instance in time is related to the available exposed area (you know how salt melts faster broken up than in a clump?). The total energy contained is determined by the volume of reactants.
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