Delta Iridium upper stage - was Re: Unexpected satellites and other ramblings

Philip Chien (
Mon, 19 May 1997 01:18:40 -0400

Ron Lee wrote:

>>Also noticed in the most recent JSR that the delta booster that launched
>>the Iridiums went into a higher orbit after the fuel depletion burn.
>>Seems to me that a burn to lower the perigee would have made more
>>sense.  Let it burn up instead of staying in orbit where it "might"
>>create more debris.

>Can a discarded Delta booster be commanded/programmed to turn itself around
>and fire a retro burn?

not only can a Delta second stage turn around, it can do pirouttes, aim
towards a particular target, go in to gravity gradient mode, etc.

Due to sloshing in the Iridium fuel tanks the Delta second stage used more
of its Nitrogen attitude control gas than anticipated.  Consequently the
stage was facing the wrong direction when it did its depletion burn.  The
situation is being corrected with a software change for future Iridium
launches.  But yes, the original plan was for the Delta second stage to
deplete its propellants and place itself in to a slightly lower orbit and
eventually have a natural decay.

McDonnell Douglas officials told me that the five Iridium were the three
physical spacecraft originally planned for launch in January, plus the next
two production spacecraft.  The original 1,2,3 serial number Iridiums were
prototypes which will not be launched.

Philip Chien [M1959.05.31/31.145//]