Solid rocket boosters (SRBs) are very expensive pieces of kit. In fact, they cost NASA around USD 25 (GBP 15.7) million to manufacture from scratch, so a cost-efficient recovery and refurbishment system was implemented in the Eighties that’s still used today.
Most shuttle flights follow a predictable path across the Atlantic Ocean as they ascend into space. So, once the SRBs have expended 500,000 kilograms (1.1 million pounds) of solid rocket fuel, they separate from the rocket and begin their 72-kilometre (45-mile), parachute-assisted descent back down to Earth.
Seven minutes after liftoff the SRBs will splash down into the ocean, around 260 kilometres (160 miles) from the launch pad and float helplessly in the water. That’s when NASA’s SRB recovery ships are deployed.
The Liberty Star and Freedom Star can recover one SRB each. Nearly everything can be salvaged from these boosters and both vessels have specialist tools for the job.
The four parachutes are first detached and wound on to a reel, before the crew can focus on the SRB itself. Then a diver-operated plug (DOP) is deployed and installed into the nozzle to help remove the water inside the booster. Air is then pumped from the ship, through the DOP and into the SRB, displacing the water. As more seawater is pushed out of the booster casing, it rises vertically until it topples onto its side, at which point it’s ready to be towed back home to Cape Canaveral, FL, for dismantling.
Each SRB comprises a solid rocket motor, plus thrust vector control, structural, separation, recovery, electrical and instrumentation subsystems. The motor itself is loaded with solid propellant alongside its ignition hardware, which is mixed in three 2,730-litre (600-gallon) bowls and cast in special buildings, before being transported nearly 4,000 kilometres (2,500 miles) to its East Coast destination.
The Liberty Star and Freedom Star are designed by NASA specifically for the task of recovering boosters. They were built in a dry dock near Jacksonville, FL, in 1980 and 1981, are nearly 54 metres (180 feet) in length and displace over 1,000 tons of water. Twin engines provide 2,163 kilowatts (2,900 horsepower) to the propellers, but, curiously, also to two thrusters, one of which is a 317-kilowatt (425-horsepower) water jet thruster designed to protect manatees indigenous to the river where the ships are based. The vessels are crewed by ten mariners, nine SRB retrieval experts plus a supervisor, one NASA representative and several independent observers.