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Re: Propellant fluid dynamics at zero-g

>I once read somewhere (cobwebs tearing apart here) that on manned (or maybe
>it was unmanned) missions, just prior to engine firing, either the thrusters
>or the main engine was ramped up with an initial low energy burst in the
>direction of flight.  Maybe this happened a split second before a burn at

Yes, this has been common practice with many launchers for many years.
Small solid-fuel rockets are typically attached to the outer surface
of each stage, some pointing forward and others aft. They are fired in
sequence when staging.

The forward-pointing rockets are called "separation motors" and are
fired when the stage is depleted to back it away from the next
stage. The rearward-pointing rockets are called "ullage motors" and
they act primarily to settle the fuel in the tanks so the engines in
that stage can be ignited. They also help move the upper stages away
from the just-jettisoned lower stage.

On the shuttle, the separation motors are at the top of the SRB cones
and fire mostly sideways. They act to rotate the top of the SRBs away
from the orbiter and external tank.

The separation and ullage motor firings are easily seen during a
launch.  They generate the big puffs of smoke that often seem to
envelop a launcher at staging. They were especially visible on the old
Saturn V launches.

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