Thor 304 had been erected on the pad a few weeks before launch, and during this step, a connecting pin in the erector broke and sent the rocket falling back down to ground level. Launch crews examined the Thor for damage, but could not find any outward indication of it. The booster passed routine preflight systems checks and liftoff took place at 6:22 PM PST on July 14, 1980.

Shortly after liftoff, the Thor's telemetry system failed, but weather conditions and visibility were excellent, and the booster could be tracked visually for over two minutes. Telemetry from the DMSP satellite indicated that it was operating properly. SECO took place on time, as did the second stage burn. Third stage burn began, and then after about four seconds, all telemetry abruptly ceased.

The initial assumption was that the third stage exploded, and initial Air Force press releases about the launch stated such. However, radar tracking showed that the third stage motor continued operating as it drove the DMSP into the Pacific Ocean.

The investigation board concluded that connectors in the electrical interface between the second and third stage had been jolted loose by the erector mishap several weeks earlier. After second stage burnout, staging failed to take place due to the misaligned connectors, and the third stage motor thus fired with the spent second stage still attached to it. The force from the third stage motor burning managed to tear the second stage free, but in doing so, it ripped out wiring in the base of the third stage, which shorted out the guidance computer and caused it to direct the stage and spacecraft down into the ocean. Much like the DMSP failure in 1976, the incident was attributed to poor program management.^[3]

Thor DSV-2U was used to launch five DMSP satellites between 1976 and 1980.^[1]