Alan Pretre no@spam wrote:
"Ian Stirling" wrote in message
...
How are the SRB's throttled?

Geometry.
If you have a simple solid rocket with a cylindrical hole up the middle
and it burns from the inside to the outside, then the thrust generally
increases as the area of burning fuel increases.

Making funny shapes in the core to begin with (star shaped hole, or other
shapes or varying the composition) can all be used to trim the thrust
profile.

The SRBs cannot be throttled.

Thanks. I thought perhaps they diluted the fuel in places with nonfuel
matter. Does the burning in the cylindrical hole happen all along the
length of the SRB or does it work its way from one end to the other?

The SRB burns from the inside out.
One advantage of this method is that the fuel insulates the casing until
the last of it burns, so you only have to have insulation on the inside
that can cope with the very end of the burn and any asymmetries in burning,
rather than that which can cope with (at the nozzle end) several minutes.

There is a complex "starter", which IIRC is basically a small rocket
engine in the nose of the SRB pointing down the core, engineered for
utter reliability.

You can have combinations, where the core burns outwards from the middle
but only half-way up the rocket and then transitions to end burning, so
that you get a large burst of thrust at the beginning then a long sustained
burn.

(also, the propellant does not burn fast enough to allow a "end burner" of
the SRBs size to generate enough thrust.)

--
http://inquisitor.i.am/ | | Ian Stirling.
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"The theory of everything falls out trivially." -- Etherman, sci.physics kook.