Atlas-C Able #9 | Pioneer P-1 Exploded
Channel: Indonesian Mapping Mira
Date: 16/09/2020

Pioneer P-1 was a failed mission in the Pioneer program. The spacecraft was a 1-meter diameter sphere with a propulsion module, and was to carry a TV camera and magnetic field sensor. It was to be spin-stabilized and was known as a 'paddlewheel' spacecraft. 

The spacecraft was intended for launch on an Atlas C-Able rocket, but this vehicle was destroyed on 24 September 1959 in an explosion on its launch pad during a pre-launch static firing. The P-1 spacecraft and an Able IV space engine were not present on the launch vehicle when it exploded, and were later used on the Pioneer P-3 mission.
Source: https://en.wikipedia.org/wiki/Pioneer_P-1

At 10:12 AM EST on September 24, the PFRF test for 9C was initiated. Following a normal engine start, a fire erupted in the thrust section. After 2.5 seconds of engine operation, an automatic cutoff command was issued to the propulsion system. A LOX-fed fire quickly burned out of control and was too intense for pad fire extinguishing facilities to handle. About 37 seconds after the test began, the Atlas began to lean over and fall towards the umbilical tower, exploding in a gigantic fireball that completely leveled LC-12. Both umbilical towers and the service tower were knocked over, a one-ton piece of the latter being thrown 500 feet from the pad, and the concrete launch stand caved in. The pad was put out of use for the next six months.

Investigators concluded that the disaster was due to the above-mentioned configuration change on the Atlas C, in addition to several weight-saving modifications unique to Missile 9C. When the Atlas was assembled at Convair, workmen attached a helium vent line to a port near the bottom of the RP-1 tank, below the anti-slosh baffles. On the Atlas B missiles, the vernier helium tank was mounted in a higher location resulting in a different fuel tank attachment point above the baffles. Helium pressure gas from the vernier propellant tanks leaked into the sustainer RP-1 turbopump, leading to cavitation which caused propellant unloading, pump overspeed, and rupture of low pressure LOX ducting. This then caused the fire that led to vehicle destruction. The reason for the LOX ducting rupturing was not clear, but likely the sudden pressure change from the engine shutdown or the sustainer turbopump blades rubbing against the pump casing. The accident was ultimately ruled to be the result of poor engineering judgement in attaching the vernier helium vent line to the bottom of the RP-1 tank. This was not the first occurrence of the failure mode; Missile 6C had exploded on the test stand at Sycamore Canyon the previous March due to the vernier start tanks being connected incorrectly.
Source: https://en.wikipedia.org/wiki/SM-65C_Atlas