The World’s Largest Core Burner

This article was published in Issue 10 of SPARK (Summer 2012), the official newsletter of the UK Pyrotechnics Society. If you’re interested in pyrotechnics please check them out and maybe even join up.

Space Shuttle SRB diagramOne year ago saw the final launch of the NASA’s Space Transportation System, the Space Shuttle, which was retired after 30 years of service. The propulsion system of the Space Shuttles consisted of three main liquid fuel powered engines (liquid hydrogen and oxygen), and a pair of reusable solid rocket boosters (SRBs) which provide the additional thrust required at take-off to escape earth’s gravitational force. These SRBs, despite their immense size and added features, use the same basic principles utilised in the firework rockets we are more familiar with.

The SRBs are a well recognised feature of a Shuttle before launch; the two white tubes straddling the huge external orange liquid fuel tank on which the Shuttle piggybacks during launch. They are manufactured primarily by ATK Launch Systems in Utah, with parts supplied from many other contractors.

Each SRB stands 149ft (45.5m) tall, and weighs 1.3m lbs (590,000kg) before launch, which drops to around 200,000 lb (91,000kg) when fully burnt. The actual motor section is 126ft (38.5m) long and 12ft (3.7m) in diameter. The outer steel casing is built in separate segments, which are assembled at the launch site and, as well as the motor itself, also house the electronics, recovery parachutes, separation gear, and a self-destruct system. They are the largest solid rocket motors ever flown, and were the first designed for reuse. Their modular and reusable nature also means that different configurations have been planned for use in other NASA programs including the (now scrapped) Ares I and Ares V, and the Space Shuttle’s upcoming replacement, the Space Launch System (SLS), due for launch in 2017.

The SRB fuel is a solid mixture referred to as Ammonium Perchlorate Composite Propellant (APCP), with the composition: Ammonium Perchlorate (oxidiser – 69.6%), Aluminium (fuel – 16%), Iron Oxide (catalyst – 0.4%), Polymer binder (PBAN or HTPB – 12.04%), and an Epoxy curing agent (1.96%). Each SRB contains 1m lbs (450,000 kg) of the composition, which is mixed in 600 gallon bowls before being cast into the required segments, just slightly more than is allowed under our 100g rule! The cured propellant has the consistency of the eraser on a pencil, and is a form of synthetic rubber.

STS-134 solid rocket booster segment stacking

Segments of the motor being joined, with the hollow core visible

A hollow core is used in the rocket to increase the surface area of the burning fuel, exactly the same as in black powder core burner rockets. In this case however, the first segment of the motor has an 11-pointed star shaped core, giving a hugely increased surface area for the initial (launch) stage of the burn, with the remaining three segments being a traditional cone shape. As the rocket burns the star softens out to a more circular shape. This configuration gives maximum thrust during the initial launch stage, after which the thrust  drops off to about two thirds to prevent over stress as the Shuttle experiences it’s maximum aerodynamic stress (“Max Q”).

As expected with solid fuel rockets, once ignited they cannot be extinguished, so the SRBs are only ignited as the final step of the launch sequence, as the countdown clock reaches zero. The other three main engines on the Shuttle are started around 6 seconds earlier, and must reach 90% thrust within 3 seconds (as well as some additional checks), otherwise a safety shutdown procedure will automatically end the launch and the SRBs will not be ignited. Ignition of the rockets begins with a PIC capacitive discharge system, charged to 40 volts, which must receive three simultaneous fire signals from the various computers controlling the launch. When initiated the charge starts the chain reaction of ignitions building up to the final ignition of the main composition. A series of NASA Standard Detonators (NSDs) are first to detonate, which ignite a booster charge, which then ignites an ignition initiator, which finally ignite the main propellant! At the same time the pyrotechnic bolts fixing the boosters to the launch platform are also fired, leaving the shuttle completely detached and free to launch.

STS120 Launch

Shuttle launching, with the SRBs providing maximum thrust

Each SRB generate around 3.1m lb (14 MN) of thrust at their maximum (a few seconds after launch), and together provide around 83% of the total lift off thrust for the Space Shuttle. Two minutes into the launch, at around 146,000ft (44.5km) and Mach 4.5 (3,425mph), the rockets are jettisoned from the Shuttle, but continue to rise for around another 75 seconds to their apogee of approx 220,000ft (67.1km). As they start to fall back to earth, parachutes are deployed and they will land in the ocean and around 140 miles from the launch point, and will float upright with around 30 ft sticking out of the water. They are recovered by ship and the major parts will be refurbished and refilled for reuse in another booster rocket.

The simple core burning principle helped launch a total of 135 Space Shuttle missions, which carried numerous satellites, probes and experiments into orbit, launched the Hubble Telescope, and enabled the construction and servicing of the International Space Station. In 1986 a failure of one of the SRBs during launch led to the disintegration of Space Shuttle Challenger, killing all seven crew on board. It was later found that an O-ring used where the segments of the SRB are joined had failed due to cold weather, allowing the exhaust gases of the motor to escape through the side of the casing, and subsequently burn through the casing of the main liquid fuel tank.