I came across these figures for the Delta II launch of an NOAA-N satellite and something struck me as odd.

Launch Date: 2005 May 11
Launch Time: 10:22:01 UTC/03:22:01 PDT
Launch Site: Vandenberg AFB SLC-2W
Launch Vehicle: Delta II with three strap-on solid motors
Flight Events:

Event T+ (sec)
-------------------------
Liftoff 000.0
Solid motor burnout 060.0
Solid motor separation 099.0
Main engine cutoff 264.2
1/2 separation 272.2
Stage 2 ignition 277.7
Jettison faring 296.0
Sustainer engine cutoff 1 676.0

The above info may be generic and not computed for this specific launch

Orbital Altitude: 870 km
Orbital Inclination: 98.730°
Orbital Period: 102.14 minutes

The SRBs burn out at 60 seconds, but are retained on the vehicle for another 39 seconds. At first sight it seems like nothing short of madness for the main engines to lug along empty SRBs for another 39 seconds. I guess there might be a vibration issue to do with this, but does anybody know the real reason?

Marcus