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I have been looking at examples of large solid fuel rocket boosters or first stages used for space launch, such as P80, the Space Shuttle SRB, the various versions of Graphite-Epoxy-Motor, the solid fuel booster for Ariane 5 or the first stages in the Minotaur family. It seems that (max thrust)/(weight full of fuel) is typically in the range of 2-3, and I can't find any reaching a thrust/weight of 4 or higher. With the specific impulse of these rocket motors, this corresponds to a burn time in the order of 2 minutes.

In contrast, artillery rockets, and air-to-air and surface-surface to air missiles often burn out in a few seconds, reaching an acceleration of tens of G's. However, these are much smaller, and tend to have a smaller propellant fraction than those used for space launces.

High initial acceleration can limit gravity losses, admittedly at the cost of higher dynamic pressure, but still I would have thought it is beneficial to get higher thrust out of these rocket engines , particularly the side strap boosters.

Now the question: Is there some practical limitation to thrust to weight when solid fueled rockets become larger, or is the higher thrust simply not that useful?

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    $\begingroup$ Probably physical and practical reasons. While weight grows with teh cube of the rocket motor (assuming it's scaled in every dimenasion which it is in practical terms) the surface area (where the chemical reaction and therefore the conversion to thrus happens) only grows with the square. Additionally, you don't want to rip the rocket you attach the bossters to to shreds on the pad, so you need to carefully consider, how much force the busters can apply. $\endgroup$
    – TrySCE2AUX
    Commented Nov 2, 2022 at 9:26
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    $\begingroup$ I think that ^3 vs ^2 is true for end to end burners. But not if you drill a hole though the fuel so it burns inside out. Nor when you add more complex shapes (e.g see the pictures of shapes at nakka-rocketry.net/th_grain.html) $\endgroup$
    – Hennes
    Commented Nov 2, 2022 at 14:25
  • $\begingroup$ @Hennes But wouldn't the propellant thickness from sidewall to the hole scale with diameter (assuming same hole shape) in the same way as burn time scales with propellant length in end-burners? In any case, these considerations assume the burn rate is the limiting factor, I am not sure it is. $\endgroup$ Commented Nov 2, 2022 at 18:33
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    $\begingroup$ @TrySCE2AUX Both sounds very reasonable. However, the boosters given as examples range from 20 to 500 tonnes of propellant, but have similar thrust/weight, which is not really consistent with a simple square-cubed law being the cause. I lean towards the structural limitations, but don't have any good source to back it up. $\endgroup$ Commented Nov 2, 2022 at 18:44
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    $\begingroup$ @Hennes There's a big reason for high launch acceleration in artillery rockets--they are unguided. The faster they accelerate the less time they spend at low speed and thus the less they drift off course due to the atmosphere. $\endgroup$ Commented Jan 23, 2023 at 3:42

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Here are some comments on different potential limitations.

The flow out the nozzle of a solid rocket motor is roughly proportional to chamber pressure and throat area.

Propellant burn rate

To maintain chamber pressure the propellant has to burn at the appropriate rate. The burn rate can be adjusted by grain size and cavity geometry, and it doesn't seem that any of the motors listed in the question have reached the limit of the obtainable burn rate of propellant. In other words, achievable burn rate is unlikely to limit thrust.

Chamber pressure

Increasing chamber pressure increases thrust but carries a weight penalty, because the fuel must all be stored within a vessel that can hold that pressure. This puts a practical limits on the chamber pressure. On the other hand, specific impulse at sea level is improved with higher chamber pressure.

Nozzle area

For adequate efficiency, the flow should be expanded in a nozzle with nozzle exit area several times that of the throat area. For example the space shuttle SRB has expansion ratio of 7.5, which brings the nozzle exit pressure close to ambient pressure at sea level.

For the SRB, the nozzle is about as wide as the booster body. This means that to reach higher thrust from a booster of that length (about 45 meters) with similar pressure and ISP, you would need a nozzle wider than the booster body. That might be a limitation in some cases. Interestingly, the full length version of AJ-260, the largest solid rocket motor ever built, was to be substantially shorter than the SRB. On the other hand the 5-segment booster of the SLS is taller.

Structural limitations

Another consideration is simply the structural strength needed to absorb the forces of a rocket motor. This applies to the booster itself, its attachment points and the upper stages that it is propelling. There is a balance to be struck between minimizing gravity losses and adding structural weight to absorb higher forces.

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SLS booster weight 1,6 pounds. Thrust 3,6 million pounds thrus to weight is 2,25? It is reasonable to assume solid rocketboosters are close to eachother. Also would belive NASA made it effecient as possible at current technology. Adding more boosters will increase total thrust to weight. You could add booster to the structual integrity of the rocket would be at risk, but then you'd utilize staging and not light every booster at the same time, as it would be a bad idea.

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    Commented Jan 22, 2023 at 22:26

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