An RL10B-2 upper stage engine like the one used on Delta 3. Photo: Pratt & Whitney

A faulty engine joint that went unrecognized during inspections was what brought down Boeing's Delta 3 rocket a failed launch in May, investigators say in a newly released report.

Boeing has concluded the rocket's liquid-fueled upper stage engine, the RL10B-2 built by Pratt & Whitney, suffered a 67-square-inch, diamond-shaped breach of its combustion chamber. The explosion rendered the engine useless and forced the deployment of the $145 million Orion 3 satellite into a worthless orbit.

The interium report details to possible reasons for the engine failure: Either poor manufacturing of the engine chamber or poor manufacturing along with unexpected structural forces on the engine during launch or during engine start. Additional testing is under way to determine which scenario was ultimately reasonable.

The hour-glass-shaped combustion chamber is comprised of four sections with strengthener strips brazed, or soldered, over the seams. The ruptured occurred near the reinforcement strip known as No. 91.

The brazing technique used to attach the strips, started in 1997, is now considered faulty because it can leave voids, or gaps, that make the joints weakened and susceptible to breaking from the stresses of launch.

The investigation found inspection records of the failed engine that indicated the quality of the brazing by engine maker Pratt & Whitney did not meet requirements.

A drawing from Boeing's report showing the engine and location of breach. Photo: Boeing

"In some areas, braze coverage was found to be as low as 20 percent per linear inch: a factor of four below drawing requirements," the report said.

However, Pratt & Whitney workers did not recognize there was a problem because of poor translation of brazing coverage requirements from design engineers to the screening criteria used by quality inspectors.

"The engineering design requirement called out a braze coverage requirement of 80 percent per linear inch. The drawing specified only an 80 percent requirement which was interpreted by the product inspectors as '80 percent coverage averaged over the entire length of the reinforcement strip. ' "

Making the situation worse, inspectors only used X-ray inspections at the time to check the brazed joints. "X-ray inspections may not show everything," Boeing spokesman Walt Rice has said.

As a result, engineers this summer developed an ultrasonic inspection technique to search for gaps in the joints. Those inspections "give a clearer and better picture" of how structurally sound the joints really are, Rice said.

Boeing has now refocused its work from investigating the failure to getting the Delta 3 fixed and ready for its next launch early in 2000 carrying a mobile communications satellite for the ICO constellation.