Inside solid rocket boosters

Ethan Wong

June 14, 2024

The Space Shuttle and the Artemis SLS rocket are similar in many different ways. In addition to both vehicles' RS-25 engines, they are also propelled by solid rocket boosters, carrying humans to new adventures in space. Before their implementation onto the SLS rocket, solid rocket boosters engraved their name into the aerospace hall of fame through their time on the Space Shuttle. As stated by NASA, these boosters provided almost 70% of the total thrust that lifted the Space Shuttle–generating 2,650,000 lbs of thrust each–triumphing the 1.2 million lbs of thrust created by the three RS-25 engines attached to the Shuttle. Although now contracted under Northrop Grumman for the Artemis program, these Thiokol-manufactured boosters weighted over 1.3 million pounds with 85% of its total weight being the 1 million lbs of propellant used for launch. 

I like to think of the solid rocket booster as a stiff pool noodle. The outer shell of the solid rocket boosters are made of steel and the inside contains a layer of asbestos-type rubber that acts as an insulator, preventing the steel from overheating (similar to ablative cooling in rocket combustion chambers and nozzles). Similar to the texture of un-hardened concrete, around 1 million lbs of propellant are spread evenly throughout the inner tube; unlike traditional rocket propellants, the booster utilized aluminum powder for fuel, ammonium perchlorate (NH4 ClO4) for an oxidizer, and iron oxide (Fe2O3), which is used as a catalyst for the propellant by increasing the efficiency of the burning (helped speed up chemical reaction). The middle is left open (again, like a pool noodle) and serves as a combustion chamber-type environment. 

Think about the pool noodle again. Now imagine cutting it up into pieces (the edges being flat). Similarly to this, the booster was assembled in sections by joints and pressure-sealed by O-rings. The top and bottom of these cylindrical pieces had a layer of inhibitor on them to prevent anything from escaping the interior. These O-rings are crucial to the SRB despite their size, as the Challenger mission failure was due to a damaged O-ring caused by cold temperatures, which led to a gas leak after takeoff.     

Image Courtesy of NASA

Now what actually starts the motor? Near the top of the booster is the NASA Standard Detonator and it acts like a giant domino chain. A few days before launch, a safety pin is removed because once this igniter is triggered, there is no stopping the booster. To start, once the safety pin has been removed, a motor with a shaft can turn and align channels leading to the standard initiator. Caused by an electric spark, this helps the initiator burn which then causes the giant booster igniter to start burning. This process happens extremely fast and will burn for over 2 minutes before being detached from the Space Shuttle.   

And if the importance of these boosters hasn’t been stressed enough through their sheer impressiveness and importance, they are the only part of the Space Shuttle that is held down to the base. Four bolts hold down each solid rocket booster to the launch pad and are broken at liftoff. Without these bolts, the Space Shuttle must rely on its weight to stand, as no other security features are holding it to the ground (disregarding umbilical anchors attached on sides). The booster is also capable of steering using its gimballed nozzles, and it is also equipped with parachutes near the top next to the guidance system.

Despite the end of the Space Shuttle era, we will still get to see the solid rocket boosters in action with the SLS rocket, and also other rockets such as the Atlas V (which has five of them). Standing 117 feet tall, and now producing upwards of 3.3 million pounds of thrust for 126 seconds of air time, these outstanding pieces of engineering will surely continue to impact the field of rocketry until the birth of nuclear engines.