In article ,
Charles Talleyrand wrote:
The solid rocket boosters do not produce maximum thrust at all times.
In part this is to avoid excess aerodynamic forces, and in part this is to
avoid excess G forces. I understand these things.
[...]
Shifting the thrust from the main engines to the SRBs would result in
an increase in payload since it increases the staging effect. The only
limits I can think of being the throttleability of the main engines and
the maximum thrust capacity of the SRBs.


As fuel is burned, the total weight of the stack is reduced, so if you
keep the thrust constant you will get ever increasing accelation. The
construction of the SRBs is such that at launch you get max thrust and
as the fuel is consumed your thrust is reduced.

Essentially with SRBs, the exposed surface area of the fuel determines the
rate with which it is burned and hence the thrust (I am ignoring fuel
mix issues here, although the composition of the fuel is also varied
to control thrust). By shaping the fuel as spikes facing towards the
the vertical axis of the booster you initially have large surface area (and
high thrust). As fuel is consumed the spikes flatten, reducing surface
area, hence rate of burn, hence thrust.

The problem with the SRBs is that the thrust profile is determined when
they are manufactured (using the shape and composition of the fuel). So
their behaviour cannot be altered once they are fired (apart from
limited vectoring). The SMEs are used to adapt the thrust profile to
the prevailing circustances. Of course the wide difference in thrust
between the SRBs and the SMEs, limits this margin.

**vp