I recently read that the Space Launch System core stage, which uses renamed / slightly reworked Space Shuttle Main Engines and therefore requires a hydraulic system to service said engines, will use Shuttle Auxiliary Power Units (APUs) with a helium / hydrogen gas system instead of hydrazine to drive the hydraulic system. They call them CAPUs. ("Core" APUs)
That prompted this question:
Thrust vectoring of the nozzles on the Space Shuttle Solid Rocket Boosters (SRB) was powered by two redundant Hydraulic Power Units (HPU) in each booster. The HPUs were basically Shuttle APUs and hydraulic pumps and were powered by a hydrazine monopropellant system that provided hot gas to spin a turbine.
Now, before the STS-107 Columbia failure, work was in progress to replace the toxic hydrazine fuel system in the HPUs with high-pressure helium bottles that would blow down through the turbine. A plausible solution for the short SRB flight time.
So, will the SLS SRBs use hydrazine to power their hydraulic systems, or a greener alternative?