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 UUCP: {allegra,decvax,ihnp4,ucbvax}!decwrl!dec-rhea!dec-crvax1!kaplow ARPA: KAPLOW%CRVAX1.DEC@decwrl.DEC.COM