Now why is that? Couldn’t they have taken them on a barge? (And if they did, would they have needed to make the rocket in a whole series of segments each as long as a railcar? Has there ever been another rocket that travelled on rail cars in segments?)
Nope, because instead of giving the contract to a company anywhere on the coast or a navigable river, they gave it to Morton Thiokol in one of the 16 US states that are entirely landlocked Utah. Now what else is in Utah? Orrin Hatch, the powerful senator who spent a good bit of his career shilling for that company.
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Always wondered why the rocket was segmented. Now I know! It was political.
Well, not quite. Turns out stacked donuts of solid fuel had a more useful performance profile than other designs. The performance of a solid rocket fuel is a function of the combusting area. As solid fuel combusts, it erodes. If the erosion significantly increases or decreases the area combusting, thrust and chamber pressure change and if this reaches an inconvenient balance the booster might not produce enough thrust during a critical phase in the mission profile, or it might shut down or blow up. However as the 3 exposed faces of the cylindrical segment erode the surface area remains relatively constant and/or predictable so performance can be made optimal for the mission profile (in this case roughly constant thrust until burnout near the end of the gravity turn, with the SSME used to compensate).
The political benefit was just gravy...however we should take notice that a gravy train is needed for the successful funding of a federal project. Thus elevating such consideration to an engineering concern, provision of gravy, HHOS.