gturner wrote:
The IUS on the Shuttle does use trajectory optimization, with each flight planned with months of simulations to make sure a satellites own RCS fuel won't be overly consumed to correct a residual error due to the inexact final impulse (which is accurate to about 0.5%)
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The IUS motors were intentionally undersized so that there never would be overburn. Each IUS stage burn was followed by an IUS RCS burn to make up the shortfall and provide the exact impulse required.