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From: Bruce Dunn <>
Subject: Re: Solid Propellant vs. Liquid Propellant Boosters
Date: Mon, 29 May 2000 03:06:09 GMT

Both solid propellant rockets and liquid propellant rockets require the
development of large pressure vessels (the tanks for the liquid
propellants, and the whole rocket for solid propellants).  The
engineering for the liquid containing pressure vessel is considerably
simpler.  The liquid tanks are under regulated pressure, while the
casing of the solid rocket is hammered by pressure spikes from
combustion instability.  The liquid tanks have the propellant in benign
contact with the load carrying structure, while solid rockets require
insulation to keep hot gases from burning through the walls.  Liquid
tanks are in one piece, while in large sizes solids have jointed
pressure vessels.

Having filled a liquid propellant tank, there is nothing much to
inspect.  Having filled a solid propellant rocket, one worries about
voids and cracks in the propellant, batch to batch variation in the
mechanical and burning properties of the propellant, the internal
bonding of the propellant to insulation material and inhibitor coatings,
the effect of the ambient temperature on propellant mechanical and
burning properties, the age and prior storage conditions of the
propellant grain etc.

Engineering of the tanks of large, pressure fed liquid propellant
rockets is actually considerably simpler than that of pump fed liquid
propellant rockets.  With higher pressures, the walls are thicker and
the structure is much stiffer due to internal pressure - this sharply
reduces the need for clever internal structures to provide stiffness and
carry loads (think of the tank as a blimp, rather than a dirigible).  In
spite of the relatively more difficult engineering in making pressurized
tanks for pump fed rockets, in-flight failure of liquid propellant tanks
on pump fed rockets is almost unheard of.  On the other hand, in-flight
failures of the pressure vessel of solids on various vehicles have
generated an entire gallery of pictures of fireballs in the sky.

Against this, it must be admitted that liquid propellant rockets have a
variety of subsystems not present on solids, any one of which can
represent a failure point.  The complexity and number of such systems
however is markedly less on pressure fed rockets than on pump fed
systems.  The relative reliability of pressure fed and pump fed liquid
rockets can be judged for example by comparing the excellent reliability
record of the pressure fed second stage of the Delta II, the OMS system
of the shuttle, and the various pressure fed engines in the Apollo with
the repeated failures of Russian, European, Japanese and American pump
fed upper stages.

Dr. Bruce Dunn
General Astronautics Canada, Vancouver B.C.
Reliable, low-cost transportation to low Earth orbit and beyond

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