The explosion at Hanwha Aerospace's Daejeon plant has put the safety management of processes handling rocket solid fuel (solid propellant) under scrutiny. Experts said that because solid fuel is hard to control once ignited, controlling ignition sources and minimizing worker exposure are the keys to safety management.
The explosion on the morning of the 1st at Hanwha Aerospace's Daejeon plant is presumed to have occurred during the process of cleaning pipes and tools contaminated with rocket solid fuel. Hanwha believes the accident happened while wiping off the viscous solid fuel with water and detergent.
This is not the first solid fuel–related accident at Hanwha Aerospace's Daejeon plant. In 2018, an explosion occurred while filling a rocket motor with solid fuel, killing five and injuring four. In 2019, an explosion and fire broke out in the "demolding" workspace that removes solid fuel from a rocket motor, killing three.
Including this accident, the Daejeon plant of Hanwha Aerospace has suffered three major casualties across different processes—filling, demolding, and cleaning. The death toll stands at 13.
◇ "Solid propellant doesn't ignite easily, but once it does, it keeps burning"
In the rocket field, "fuel" generally refers to propellant. Propellant is a collective term for substances that generate the force that pushes a rocket forward. A rocket expels hot gases produced by burning propellant out the back and moves forward by the reaction.
Propellant consists of a combustible component and an oxidizer that supports combustion. Liquid-propellant rockets store these two components separately and mix and burn them in a combustion chamber. In contrast, solid propellant has the two components mixed together from the start in a single mass. During manufacturing, it is injected in a viscous, dough-like state and then hardens into a solid.
Aerospace engineering researcher A, who has experience developing solid propellant, said, "Fires in wood or oil go out if you cut off the oxygen supply, but solid propellant contains an oxidizer inside that supplies the oxygen needed for combustion," adding, "Once ignited, the material itself continues reacting, so it is hard to extinguish."
They added, "However, solid propellant requires heat energy above a certain threshold to ignite, so small shocks or light friction do not easily set it on fire," and "In the workplace, electrostatic discharge, sparks from metal tools, strong impacts, and heat generated by repeated friction can serve as ignition sources."
The danger can be greater if a solid propellant reaction occurs in a confined space. A said, "In an open space, if it catches fire, it may just burn out, but if solid propellant burns inside piping or a container, gas is generated and internal pressure builds," adding, "It can lead not to a simple fire but to explosive damage."
◇ Propellant accidents repeated overseas as well… "Reduce material, personnel, and exposure time"
There have been major accidents overseas while handling solid propellant and its raw materials. In 1988, at the PEPCON plant in Henderson, Nevada, a fire and explosion occurred while about 4,500 tons of ammonium perchlorate—used as an oxidizer in solid propellant—were in storage.
According to a report by the U.S. Fire Administration, two people were killed and about 372 were injured, with property damage estimated at more than $100 million (about 151.6 billion won). There were seven explosions in total, and two large ones were recorded on seismographs as magnitudes 3.0 and 3.5, respectively. Damage was particularly severe within a 1.5-mile (about 2.4 km) radius, and building damage was reported as far as 10 miles (about 16 km) away.
The 2003 accident at the Alcântara Launch Center in Brazil is also cited as an example showing the dangers of solid propellant. Three days before launch, during final checks of the VLS-1 V03 satellite launch vehicle, one of the first-stage solid-propellant boosters ignited unexpectedly, and the rocket and launch pad were destroyed by explosion and fire, killing 21 technicians. A Brazilian investigation committee considered electrostatic discharge inside the first-stage booster igniter the most likely cause.
Overseas agencies emphasize the principle of reducing worker exposure to mitigate such risks. The National Aeronautics and Space Administration (NASA) explosives and propellant safety standard stipulates as a basic principle of explosives work that the number of people entering hazardous areas should be minimized, the amount of explosives required for the task should be minimized, and the time people are exposed to hazard should be kept as short as possible.
In solid propellant safety assessments, procedures to identify hazards in advance are treated as important alongside measures to reduce worker exposure. Roberto Bubbico, a professor at Sapienza University of Rome in Italy, and others noted, "Hazard analysis must identify all ignition sources that could accidentally trigger propellant combustion," and "Based on this, preventive and protective measures appropriate to each production stage should be established."
References
Chemical Engineering Transactions (2012), DOI: https://doi.org/10.3303/CET1226008