Woo hoo! New patent ...

Bob Martin wrote:

Yup. Not to give away details that I don't know how public they are, but
some of the rocket motors I've worked around have little more than a
featureless cylindrical bore... and some are astonishingly complex
series of very organic, flowing fins. It's all a matter of the thrust
profiles you're looking for.

The SRB's for the shuttle have a smooth cylindrical bore,

Not *exactly*. The propellant is segmented, with gaps between the
segments; the propellant burns not only along the cylindrical inner
surface, but also on the fore and aft annular areas of the segments.
This give a reasonably neutral burning profile - i.e. constant thrust.
If it was truly a smooth cylindrical bore, surface area would increase
with time, and thrust would go up... squishing the crew.


except for the
upper part (which has a bunch of fins on it).

Increased surface area for ignition at the head end.

--
Scott Lowther, Engineer

"Any statement by Edward Wright that starts with 'You seem to think
that...' is wrong. Always. It's a law of Usenet, like Godwin's."
- Jorge R. Frank, 11 Nov 2002