The Delta 4 rocket’s cryogenic upper stage engine experienced a fuel leak that caused a low-thrust condition. Four-and-a-half minutes into the launch, after the first stage had shut down and separated, the trouble began as the RL10B-2 engine on the upper stage extended its nozzle and fired to life.
When the powerplant was igniting and reached its peak chamber pressure, a leak started above the narrow throat portion of the thrust chamber, setting off a chain of nail-biting events over the next three hours as the vehicle made its climb to the GPS constellation. The Delta 4 made autonomous adjustments, however. The onboard inertial guidance and flight control systems compensated for the lower thrust conditions. Its closed loop guidance system measured the decreased thrust in real time and revised the trajectory and burn durations to ensure the mission succeeded. The GPS IIF-3 satellite was delivered to the correct orbit as planned.
United Launch Alliance has begun an investigation into the incident.