hard start
An explosion in a rocket engine during startup, usually due to the presence of too much propellant prior to ignition.
hard start: uncontrolled combustion spike at engine ignition
A hard start occurs when a rocket engine ignites with a sudden, violent pressure spike rather than the smooth rise expected from normal combustion. The engine chamber experiences a brief but intense explosion as accumulated propellant ignites all at once or in a severely compressed timeframe, generating shock waves that stress the engine structure. This is distinct from a normal start, where the flame front advances through the combustion chamber in controlled fashion and pressure builds gradually.
Hard starts happen most often when liquid propellant accumulates in the chamber before ignition occurs. This can result from pre-ignition liquid injection, delayed spark or flame initiation, or a misfire that allows propellant to pool. Cryogenic engines like those burning liquid hydrogen and liquid oxygen are particularly vulnerable because low temperatures can condense vapors back to liquid if ignition is delayed. Solid rocket motors can experience hard starts if ignition is inconsistent or if the propellant grain has cracked, creating unintended surfaces where fuel can collect.
The resulting pressure transient can fracture turbopump blades, crack combustion chamber walls, damage injector heads, and rupture engine seals. Even engines that survive the event intact may have their service life shortened by the strain. Hard starts are detected during ground testing through sudden pressure spikes visible on engine telemetry and through post-test inspection for cracks and deformation. Flight engines that experience hard starts are typically removed from service and inspected for structural damage before reuse.
Prevention relies on precise sequencing of ignition systems, reliable propellant injection timing, and maintaining chamber geometry to avoid pooling. Some engines use igniter systems with redundant spark sources to ensure rapid flame establishment. Propellant conditioning, purging procedures, and temperature control help prevent unwanted accumulation in liquid engines. The term arose from the sharp, unpleasant quality of the acoustic signature and mechanical shock, in contrast to a soft or normal start.