The mishap in which an MX missile dropped several inches in its Wyoming silo last June was caused by the failure of a section of rubber material to remain attached to the fiberglass skirt of the missile stage manufactured by Morton Thiokol Inc. near Brigham City.

That is the conclusion of a hefty incident report prepared by the Air Force. The document, "Report of Missile Accident Investigation _ Peacekeeper ICBM (LGM-118A) Tail Number 84-0552," was signed by Col. Ronald D. Huff of Ellsworth AFB, S.D., who conducted the Air Force investigation.

The mishap destroyed the missile's first stage, manufactured by Thiokol, at a cost of $4,588,000; wrecked the Missile Umbilical Group (a collection of cables that connected the missile with its controllers) and the mechanism that is supposed to retract the umbilical, at a cost of an additional $85,000; and damaged the fourth stage slightly, costing $24,942 to repair.

The report was classified when it was completed in November 1988, but most of it was declassified recently.

The 2-inch-thick report was sent to the Deseret News last month as a result of the paper's request under the Freedom of Information Act.

The incident happened at F.E. Warren Air Force Base, at a silo called Quebec-10, or Q-10, about 40 miles north of Cheyenne, Wyo.

While the missile was out of commission, its 10 nuclear warheads were unusable.

The report says tests and specifications in Thiokol's manufacturing process met all specifications.

Rocky Raab, spokesman for Morton Thiokol, said that because the mishap involved an operational missile, the Air Force has advised Thiokol that it should not comment on the matter.

On June 12, 1988, a malfunction signal was received by the missile combat crew monitoring Q-10. It suggested a failure in the missile's environmental control system, showing that two lines supplying freon coolant were disconnected from the guidance and control system.

Later it was discovered the missile had dropped 6 to 8 inches in the silo when the seal between the skirt and the motor gave way.

A special team from Boeing, Air Force Detachment 60 and an explosive ordnance disposal team from Ellsworth AFB, S.D., removed the warhead and re-entry vehicle. These were undamaged and were returned to the base's weapons storage area.

Then the other stages were pulled up carefully and returned to their manufacturers. Only the Thiokol-built first stage, where the failure occurred, was irreparable.

The 71-foot missile weighs 192,000 pounds, and normally most of the weight is supported on a "skirt" that is bonded to the first stage motor, extending below the nozzle.

"Stage I was discovered to have wrinkles in the external protective material...in the aft skirt area, " the report says.

The missile had been operational on "missile alert" since Jan. 21, 1987. Air Force investigators concluded that none of the activities at the base contributed to the incident.

Studies by Morton Thiokol confirmed that a failure had occurred at the joint where the aft skirt bonds to the pressure vessel, the report says. The pressure vessel is an egg-shaped container that holds the fuel.

"Specifically, the bond failed between the inner shear ply and the aft skirt," the report says.

The shear ply is a piece of rubber that is bound between the pressure vessel and the skirt, spreading the weight evenly over the joint.

The shear ply is manufactured at Morton Thiokol using green rubber. The rubber is laid out, then vulcanized at high temperature and pressure.

"The completed shear plies are then placed in a plastic bag and stored until required for the buildup of a Peacekeeper (MX) stage," the report says.

The bond between the rear skirt and the pressure vessel failed, while that between the forward skirt and the pressure vessel did not.

Teams studying the manufacturing records found that the same batch of epoxy was used in making both of the skirts, so it could not be a failure of the glue itself. Also, a good bond was present on the forward skirt.

But the shear ply for the aft skirt was from a different lot of raw rubber than that used for the forward skirt.

So attention shifted to the shear plies.

"After Thiokol manufactured the plies, there are no quality assurance checks or laboratory analyses required to verify the properties of the completed shear ply before it is applied to the skirt assembly." it adds. "The shear ply used on the aft skirt is known to have sat in ambient conditions longer than other shear plies investigated."

Ben Lloyd, manager of composite materials at Morton Thiokol's Strategic Division, testified about the plies.

On Oct. 27, 1988, Lloyd reported to Hoff, and was also questioned by Lt. David R. Claxton of Vandenberg AFB, Calif., a technical advisor to Huff.

Claxton asked what kind of stresses there might be in the bond under a stacking load when the missile is assembled and sitting upright on the skirts.

That would be about 70 pounds per square inch, Lloyd replied. "A good bond is 1,100 psi," he added.

In other words, the bond should be able to withstand several times the expected stress.

Claxton then asked, "What would you speculate would be the cause of having a good bond on the forward skirt and a bad bond, a poor bond on the aft skirt?"

Lloyd answered, according to the report, "Well, first of all, that is the dilemma, is the fact that we have such a good bond on the forward and such a poor bond on the aft with seemingly the only difference being the shear ply lot on the aft and the shear ply lot on the forward...

"We have determined that there is nothing wrong with the formulation of the shear ply lot on the aft. And, you know, so the question remains as to why, you know, it's made the same time; it turns out that the forward skirt was not made-well, the forward shear ply was not made at the same time as the aft shear ply.

"But the skirt assembly, which has both shear plies in, was made at the same time on the same plaster mandrel, painted with the same resin, gone through the same cure, saw the sasme humidity exposure, whatever. And so it really has caused us to believe that there is something about the rubber shear ply that is different.

"Not that it's a bad material, but it may be a carrier of a bad material, a bad component, bad contaminant. And that, right now, is really what we believe happened, that it was out of the normal somehow; that there was something out of the normal in that shear ply that caused the problem."

Lloyd thought it unlikely one ply would be coated with resin differently than the other.

Col. Huff asked whether there was anything that Morton Thiokol found in the curing process or process of mixing the resin or anything else that would deviate from the normal.

Lloyd said, "We have found...that this shear ply appears to have sat in ambient conditions longer than any of the other shear plies that we have looked at, at least so far."

Huff wanted to know what he meant by ambient conditions.

Lloyd: "In the shop unprotected, you know, in a building, you know, not in a bag or anything like that. Not necessarily out of specification, but, you know, like I say, if you have these specifications and you have limits on times and that kind of thing, from about at least probably 10 or maybe 15 motors that we've looked at so far, it seems to have been the longest in ambient exposure."