The SRB Gets a Makeover

"There was a lot of light and a lot of rumbling and vibration... and then after about two minutes, when the solid rocket boosters separated, the ride got a lot smoother" - Ellen Ochoa


I was pondering this question the other day: what's the longest you've ever waited for something to happen? As an avid golfer who spent his whole childhood watching Tiger Woods on TV, I think the 11-year wait (2008-2019) I endured for Tiger to complete his comeback and win his 15th major definitely takes the cake. But like sports, the world of aerospace demands extreme patience, and we space fans are nothing if not patient - I feel like I've been waiting forever for NASA's Space Launch System to launch! I first heard of SLS back in 2013, when my family visited Kennedy Space Center. By then the Space Shuttle had been gone for two years, and NASA was promising that its replacement would be ready by 2017. "Four more years? How in the world am I supposed to wait another four years to see this thing fly??"

Of course we all know now that the 2017 launch date proved impossibly optimistic. But now almost nine years after my family vacation, at long last SLS is fully stacked at the Cape and getting ready for its Artemis 1 mission! In eager anticipation, I've been trying to get up to speed with all the components of SLS, and the upgrades to the solid rocket boosters caught my eye, particularly because my knowledge of SRBs is more sparse (since SpaceX doesn't use solid propellants)

The fully stacked Artemis 1 Space Launch System

Like much of SLS's architecture, the two solid rocket boosters attached to the core stage were congressionally mandated to make use of legacy Space Shuttle hardware. The original Space Shuttle SRBs were made by Thiokol, which was later acquired by Orbital ATK, which in turn was acquired by Northrop Grumman in 2018, meaning NG is in charge of the SLS SRBs. The shuttle SRBs by Thiokol had the distinction of being the first-ever solid propellant rocket to be used on a human-rated launch vehicle; supplying over 3 million pounds of thrust each, they were the most powerful SRBs ever designed and provided 85% of the Space Shuttle's thrust at liftoff, with the rest coming from the Space Shuttle's liquid hydrogen RS-25 engines. To start us off, here are a few features that are the same between the original Space Shuttle and new SLS SRBs: 

  • Overall structure: Nose cone and forward skirt (which contains avionics), multiple center segments containing propellant, aft segment containing the nozzle and thrust vector control
  • Propellant: PBAN-APCP. This intimidating looking acronym is shorthand for a unique blend of ammonium perchlorate (which serves as the oxidizer), aluminum powder (fuel), polybutadiene acrylonitrile (fuel, also acts as a binder), iron oxide as a catalyst, and epoxy
  • Grain geometry: 11-sided star, which provided high thrust right after ignition but throttled down as the shuttle passed max-Q
  • Ignition system: Pyrotechnic devices using NASA standard detonators, which can only be fired after a manual safety pin has been removed prior to launch, and the flight computer confirms the orbiter's RS-25 main engines have achieved proper thrust and there are no other problems
  • Segment cases: D6AC high-strength low-allow steel
  • Burn time: Just over 2 minutes
An exploded diagram of the Space Shuttle SRB

Fast forward a few decades, and the souped up Artemis SRBs now stand vertical at Kennedy Space Center! It's kind of neat that even after Northrop Grumman took over Thiokol, they've maintained the same booster manufacturing facility in Promontory, Utah. The completed SRB segments are still shipped via rail from Utah all the way to Florida! And yet today's SRBs can take advantage of modern improvements - here are the primary upgrades that have been made to the SRBs flying on SLS:

  • Additional propellant: The original shuttle SRBs had four main segments full of propellant, but these upgraded boosters now have five in order to provide SLS 20% greater thrust and 24% greater total impulse
  • New insulation: The new SRBs got rid of the old asbestos-laden insulation, which not only eliminates the hazardous material, but also provides cost and weight savings
  • Expendable configuration: Unlike the shuttle's SRBs which were intended to be reused, the entire SLS stack is single-use, including the SRBs. If that sounds more like a downgrade to you than an improvement, well... yeah I'm with you there. But I'll spare you my rant, the big-wigs in Congress are set on SLS's expendable design, so that ship has sailed
  • Other upgrades: Modernized avionics, new nozzle design, improved non-destructive evaluation techniques during testing

They may not be cheap, but they're certainly huge

Finally, a few months ago NASA Spaceflight released a really great article which goes in-depth into the future design changes that Northrop Grumman is working on for next generation of SRBs. The upcoming Artemis 1 is launching on a Block 1 SLS, but NASA intends to develop a Block 2 variant down the road that will make use of a further-enhanced SRB known as BOLE (Booster Obsolescence and Life Extension). Here's a summary of the future-state improvements planned on BOLE: 

  • New propellant formula: Still uses ammonium perchlorate as the oxidizer (the APCP of the original formula), but replaces PBAN fuel with a different solid called hydroxyl-terminated polybutadiene (HTPB) which can handle higher strain levels and can be more densely packed. HTPB is already used on some existing solid rocket boosters, like the Indian PSLV
  • Composite cases: Replaces the current steel casings used since shuttle. Composite technology has advanced significantly over the past few decades and will translate to 30% weight savings
  • Thrust vector control: Existing hydrazine powered TVC is highly toxic and will be replaced with a new battery-powered system, leveraging the work Northrop Grumman was doing on its proposed OmegA rocket
  • Better mating to SLS: The shuttle-derived SRBs always required some reverse engineering to adapt them to the new SLS architecture. BOLE has attach points that are better suited to the SLS booster, as well as a revamped separation system
  • Other structural improvements: Combined nose cone and frustrum, optimized forward and aft skirts, redesigned nozzle

Pretty neat CAD imaging released by Northrop Grumman showing the redesigned attach points to SLS

If you want to read the whole article, you can check it out here!

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