"Graduated" burns?

[Zatnikitelman]

I was reading Apollo 13 for the 11th or 12th time in two years P) and noticed something interesting about the various burns executed to correct the trajectory. The burn profile for lack of a better term called for the engine to start at like 10% thrust, then Lovell to go to 40% and I think in one burn, then all the way up to full. Is the cause of these "graduated" burns structural where they have to let the chamber and nozzle build up the pressure or they'll crack something? If so, then why the drop back to 0% at the end of the burn; wouldn't a sudden builddown from operating pressure be just as damaging as a sudden buildup?
Or is this because of some weird burn guidance thing that I just don't know from my years of just hitting NUMPAD + in Orbiter ?

 

[Tommy]

You must be referring to the burns made with the Lunar Lander's engines, as they were the only engines that could be throttled. However, there was combustion instability at certain throttle ranges so throttle positions between 60% and 100% were "forbidden". I suspect the slow throttle-up was because using the LL's engines on the stack was untested, and they wanted to be sure there were no induced rotations or other problems.

 

[Piper]

Being Apollo 13, I think it had to do simply with Lovell getting the "feel" of using the Lunar Lander's engine while still attached to the Command Module, which it was never designed to do.