Solid rocket precision shutdown

I was reading yet another web site with the old saw that you can't
turn off a solid rocket motor once it has ignited, save for destroying
the motor.

Then I stopped to consider the Minuteman or MX. Both of these
missiles are three-stage solid rockets. Does either need a
precision thrust cutoff from the third stage (or other two)? If
not, how do they achieve precision targetting with what must be
a variable delta-V?

I assume both have some sort of throttleable propulsion on the
bus that delivers the MIRVs. Is this enough to overcome
uncertainties in the boost delta-V?

Minuteman and Peacekeeper use thrust termination ports, IIRC, on the third
solid stage. Both have small liquid post-boost propulsion systems to fine-tune
the bus pointing before RV release.

I remember a missile engineer telling me once that, for an ICBM, an error of 1
m/s in velocity resulted in a 1 km error at the target point.
Matt Bille
)
OPINIONS IN ALL POSTS ARE SOLELY THOSE OF THE AUTHOR

Iain McClatchie wrote:
I was reading yet another web site with the old saw that you can't
turn off a solid rocket motor once it has ignited, save for destroying
the motor.

Then I stopped to consider the Minuteman or MX. Both of these
missiles are three-stage solid rockets. Does either need a
precision thrust cutoff from the third stage (or other two)? If
not, how do they achieve precision targetting with what must be
a variable delta-V?

I assume both have some sort of throttleable propulsion on the
bus that delivers the MIRVs. Is this enough to overcome
uncertainties in the boost delta-V?

The 4th stages use storable liquid propellant.

In article ,
Iain McClatchie wrote:
Then I stopped to consider the Minuteman or MX. Both of these
missiles are three-stage solid rockets. Does either need a
precision thrust cutoff from the third stage (or other two)? ...
I assume both have some sort of throttleable propulsion on the
bus that delivers the MIRVs. Is this enough to overcome
uncertainties in the boost delta-V?

Yes, but the uncertainties are kept down by having thrust termination on
the final stage, so it can be cut off on command. The final stage has
thrust-termination ports up near the top of the motor, which are blown
open to drop the chamber pressure sharply. The outgassing from the fuel
surface continues at full speed momentarily, since it's driven by heat
already transferred into the fuel, but the combustion rate in the gas
drops with the pressure drop... and so if the pressure drops quickly
enough, the flame is blown away from the surface and goes out. (Caution,
I oversimplify a bit.) Done carefully, with attention to details, this
can give a relatively clean and predictable cutoff. And then the little
fourth stage, aka warhead bus, cleans up any remaining error.

And no, alas, you can't apply this to the shuttle SRBs. Solids were
chosen for the shuttle on the assumption that thrust termination was in
fact possible... but shutting off a solid this way is a fairly violent
process, and it turned out that the orbiter and ET are not strong enough
to survive it.
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"Think outside the box -- the box isn't our friend." | Henry Spencer
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