Soyuz 7K-OK No.1

Soyuz 7K-OK No.1 was an uncrewed Soyuz spacecraft meant to practice a rendezvous with its sister ship, Kosmos 133. That first mission failed, so the rocket was rolled out to the pad on 12 December 1966, with a launch set for 14 December at 11:00 UTC. At ignition, one strap-on booster didn't fire, triggering an automatic shutdown of the core and the other boosters. Ground crews began making the booster safe for removal. About 27 minutes later, without warning, the launch escape system fired. It blasted the descent module free, which landed 400 meters from the pad. But the escape system’s exhaust also set the rocket’s third stage on fire. Flames spread down the side as personnel fled. Minutes after that, the core and strap-ons exploded, wrecking the launch vehicle and heavily damaging Launch Complex 31. One person was killed outright; two more died later while cleaning up.

Investigators had no film of the fire or explosion—tracking cameras had been switched off after the abort. Telemetry showed the Blok D strap-on’s igniter simply failed to activate, a minor fix. The real puzzle was why the escape system triggered. At first, they thought the gantry tower might have bumped the booster when it was moved back into place, but a deeper look revealed a different flaw. During the attempted launch, the booster switched to internal power as normal, which armed the abort sensing system. Earth’s rotation then caused the rate gyros to register an apparent 8-degree tilt 27 minutes after the abort, and the system interpreted that as a flight-path deviation, firing the escape system. The abort sensing system was redesigned to prevent this. On the upside, the escape system worked perfectly, proving it could save a crew—as it would years later during the Soyuz 7K-ST No.16L abort in 1983. One last mystery remained: why the escape system’s exhaust ignited the third stage. The conclusion was that when the descent module separated from the service module, it severed coolant lines in the service module, spilling highly flammable fluid that caught fire on contact with the exhaust.