A hard start happens when unburned propellants accumulate inside a rocket engine before ignition. When the mixture finally catches, it burns all at once, causing an immediate and violent spike in chamber pressure. This shockwave can shatter turbopump blades, rupture plumbing, or completely destroy the engine bell. Engineers carefully sequence valve timing to prevent this pooling of fuel and oxidizer.
Heard on the job
Telemetry shows a massive pressure spike at T-minus zero, so we're looking at a hard start that blew out the nozzle.
Insider tip
A hard start is essentially a bomb detonating inside the engine. On test stands, it is one of the primary reasons for massive, sudden engine failures.