kaplow@crvax1.DEC (Have you kicked your cat today?) (01/30/86)
I'm still in a daze from yesterdays news. I have followed the space
program from its beginnings and can't remember anything like this,
including the Apollo 1 fire 19 years ago.
To answer the question, I have been involved in model rockets for
over 20 years, and am very familiar with temperature induced problems in
solid propellants. The black powder motors do suffer from temperature
problems, occuring when the motor is fired at a colder temperature than
it was stored at. This is usually the result of storing the motor at a
very high temperature, like the trunk of a car on a hot summer day, but
can also be caused by firing a motor on a cold day, as was the case in
Florida yesterday. These motors are very different in design and
construction from the SRBs.
Some of the new high power motors use almost exactly the same
propellant, save a few trace additives, as the SRBs. They consist of
about 15% rubber propellant (poly something or other) and 85% ammonium
perchlorate oxidizer. These seem to be immune to any storage problems.
The propellant itself is a rubber like material, thus temperature
changes do not cause cracking or bond separation. Hard shock also has no
effect on this material. Over a very long time, surface oxidation will
cause some deterioration, but this requires an unsealed motor and
several years to happen.
One of the advantages of this propellant is that it will not
explode. Many solid propellants burn faster as chamber pressure
increases, causing a chain reaction leading to an explosion. The AP
propellant actually burns slower as pressure increases, regulating
itself. In fact, these motors are often hard to ignite, needing to be
rapidly pressurized or they will extinguish themselves. Any casing
rupture usually extinguishes the propellant, or at least causes it to
burn slowly without producing any thrust. Scraps of this propellant
material will burn in an ash tray if lit with a match, but no more
violently than a similar hunk of rubber.
These model rocket motors, like the SRBs are usually ported down the
center, giving a large initial burn area. They burn radially out toward
the casing, thus the unburnt propellant is the insulation that prevents
the casing from burning thru. A void in the cast propellant, or a crack
between the seperately cast segments in the motor, could cause one part
of the motor to burn all of its propellant down to the metal wall sooner
than the rest of the motor. There was some concern about this on a
flight a year or so, which they found after recovering the spent SRB.
The first replay I saw of the disaster was on a 5" screen, and it
looked like one of the SRBs might have burned thru and started the chain
of events. Later replays last night show either a leak in the rear area
of the ET, or around the 3 liquid engines, just after throttle up. Once
this trail ignited, the whole ET went with it. The fact that the two
SRBs continued burning while tumbling leads me to believe that the SRBs
performed properly. I guess we will have to wait for NASA to analyze all
of the data and find the cause, as the spokesman yesterday made it
pretty clear that they were not going to give the media any more footage
or information until the investigation is completed.
Bob Kaplow
Digital Equipment Corp.
Arlington Heights, IL
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