It is a disappointment that this mission does not have a RocketCam because
third stage separation is pretty spectacular and very unusual. Most
boosters are boring at this phase, but a high thrust spin-stabilized solid
motor does some strange things. Normal boosters don't have third stages.
Delta II does because, frankly, its gutless. It is the only US booster left
that uses a spin-stabilized stage (Taurus has an option for it, but Taurus
flights are pretty rare, and the spinner is a non-standard service. The
Atlas V that launched New Horizons used a vector stabilized version of this
stage.)

Two or four (I forget which) small tangental solid motors attached to the
spin table fire. These wind the third stage up to 70 RPM in three seconds.
Their operation resembles that of a firework pinwheel. The third stage is
separated immediately so that the friction in the spin table doesn't slow it
down. The second stage is left spinning slowly in the same direction.
After about six minutes of spinning like this, the motor fires. 90 seconds
later, with the motor spent, two tethered weights are deployed. This cause
a rapid increase in the rocket science value of "moment of inertia." Since
angular momentum is preserved, this armstretching exercise slows the spin
rate down. After the weights are let go, the spacecraft separates.

The third stage has a low order intelligence called a "nutation control
system", which is one hydrazine thruster fed from a dinky tank on the tube
between the Phoenix cruise stage and the motor. All it does is cancel any
wobble, which usually results from the spacecraft's propellant sloshing
around. Phoenix's little 80397 tanks carry only 52kg of propellant under
rubber diaphragms. If they're the new "stiff" rubber diaphragms, they won't
slosh much, so the NCS has a boring job for this mission compared to
something like the Mars Global Surveyor or a Star-2 commsat.

I'm letting this message fly with about five minutes remaining in the TV
channel's 'hold'. If it blows up, I'll let you know. Otherwise, this'll be
my last message regarding this ascent.

Terry


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