Henry Spencer wrote:

In article ,
Scott Lowther wrote:
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.

If memory serves, the fore annular areas have insulation on them to
prevent (or at least delay) combustion there, but the aft ones don't.

Also, the bore is not exactly cylindrical; it widens gradually toward the
aft end.

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

Increased surface area for ignition at the head end.

Not so much for ignition, as for high thrust at launch. Then the thrust
drops off some as the fins burn down, at around the same time as the SSMEs
throttle back, to reduce acceleration and limit aerodynamic loads on the
orbiter around the time of maximum dynamic pressure. The SRB thrust
gradually builds up again as the bore burns outward, and the SSMEs also
throttle back up once the stack is up in thinner air.

The ill-fated ASRM project was going to have a deeper "thrust bucket"
designed into it, to eliminate the need to fiddle with the SSME throttles.
I don't know exactly what they were going to do for that.

Hi Henry,

You forgot to mention one of the most important parts of SRB thrust profile
design. The burn rate is varied radially.

Although my knowledge in this area is limited, you missed one of the most
important design considerations in vary large solid rockets. After the
"thrust bucket", thrust is allowed to build until a certain point. Then
burn rate is varied to achieve a constant acceleration. This constant
acceleration is then held until the SRB burn out. First stage is gee
limited much the same as second stage is, except that the SRB thrust
profile (burn rate) is used to achieve this.

So, even though the surface area is increasing as the propellent burns, and
the mass of the vehicle is decreasing as both SRB and ET propellant is
burned, thrust is decreasing to maintain a constant acceleration.

Varing the burn rate of the propellent is done through out the SRB thrust
to achieve the desired goals. The "thrust bucket", and gee limiting being
two of the major ones. So as they pour the matched pairs of segements the
actual propellant mixture is varied. Propellant samples are taken along the
way, to be burned to determine exactly what the performance of the matched
set will be.

This aspect of SRB design in many respects is still an art. Many things go
into it that can effect the actual outcome in terms of SRB thrust profile.
I remember a long time ago, the vendor of one of the components of the
propellent was changed (low bidder most likely). Even though this vendor
was producing an equivalent and acceptable product, their manufacturing
process of the compound was different enough to produced a slightly
differnt burn rate in the finished product.

Craig Fink