The Solid Rocket Boosters (SRBs) used by the Space Shuttle and SLS have a skirt near the bottom.
What is it for? Is it only to support the stack?
Main points about the SRB aft skirt are that they:
bear the entire weight of the launch vehicle, prelaunch
had (SLS SRB does not) four hold-down/support posts distributed the loads between the SRB and the mobile launcher platform (MLP); They contained nuts that detonated at SRB ignition.
after launch, the Thrust Vector Control system (rock and tilt) inside the skirt aid the steering of the stack during first stage ascent
contain Aft Booster Separation Motors (4) used in SRB separation
(SLS SRB (not STS) aft skirt)
The cone- shaped aft skirt reacts the aft loads between the SRB and the mobile launcher platform. The four aft separation motors are mounted on the skirt. The aft section contains avionics, a thrust vector control system that consists of two auxiliary power units and hydraulic pumps, hydraulic systems and a nozzle extension jettison system.
The aft motors are located unsymmetrically which causes a small roll moment. The BSM's provide the needed side force to separate the boosters from the external tank at booster burnout.
Shuttle Boosters to sport APU fuel pump safety redesign from STS-134
There are two self-contained, independent HPUs (Hydraulic Power Units) on each SRB. Each HPU consists of an Auxiliary Power Unit (APU), Fuel Supply Module (FSM), hydraulic pump, hydraulic reservoir and hydraulic fluid manifold assembly.
The HPU components are mounted on the aft skirt between the rock and tilt actuators – which are part of the Thrust Vector Control system that drives the SRB nozzle to aid the steering of the stack during first stage ascent.
The APUs and their fuel systems are isolated from each other, with each FSM (tank) containing 22 pounds of hydrazine.
The fuel tank is pressurized with gaseous nitrogen at 400 psi, which provides the force to expel (positive expulsion) the fuel from the tank to the fuel distribution line, maintaining a positive fuel supply to the APU throughout its operation.
The APU turbine assembly provides mechanical power to the APU gearbox, which drives the APU fuel pump, hydraulic pump and lube oil pump.
The turbine exhaust of each APU flows over the exterior of the gas generator, cooling it, and is then directed overboard through an exhaust duct.
AFT SKIRT CONFIGURATION
The skirt is essentially a truncated conical shell reinforced by internal rings and stringers. Four hold-down/support posts which distribute the prelaunch loads to the mobile launch platform are mounted integrally to the skirt.
The aft skirts of the SRB's provide the prelaunch support for the space Shuttle vehicle and are flight structured. They have been primarily designed for the flight loads (prelaunch) . The structure must also withstand the water impact loads. The SRB vehicle on re-entry impacts the water aft end first.
As it was for the shuttle, the aft SRB skirts will bear the entire weight of the launch vehicle. Each skirt slides over four posts to hold the vehicle steady. To keep the shuttle steady while it twanged, these posts used to contain explosive bolts (nuts) that fired when the vehicle pitched back to center—right at the moment of SRB ignition.
Each SRB aft skirt houses a complex thrust vector control system (TVC), which mechanically directs the nozzle at the end of the booster to steer the shuttle during its initial ascent. The TVC consists of two hydraulic gimbal servo-actuators; each independently powered by its own auxiliary power unit (APU) and hydraulic system.
Hold Down post integration with aft skirt:
Aft skirt detail:
These link shave more info regarding TVC:
Re: Modification of single SRB for use in the Ares I-X test flight in 2009:
Aft Skirt — The aft skirt, like the motor segments, uses existing Space Shuttle hardware. The Ground Systems team at KSC is removing the thermal protection system ... so that additional ballast, booster deceleration motors (BDMs), and booster tumble motors (BTMs) can be installed.
The aft skirt for Ares I-X, when operational, will include eight BDMs and four BTMs.
The aft skirt, while Shuttle-legacy hardware, will have hardware added to meet the requirements of Ares I-X.
Regards mention of ballast for Ares I-X only (Note, not SLS nor STS):
Thrust Oscillation Mitigation Approach
Aft Skirt Reaction Mass Actuators
NASA Releases Plan For Ares I Vibration Problem
Since last fall NASA has been faced with the news that vibrations up to 0.5 G inherent in the design of its solid rocket boosters will make the new Ares I a rough ride.
To deal with the few seconds of highest vibration the crew will experience at about 115 seconds into the flight, an active “tuned mass absorption” system consisting of 16 100-lb. weights arranged in a ring around the aft skirt of the first stage will sense motion and move the masses to cancel out vibration, bringing it down to 0.25 G.
All modifications would be limited to the Ares I rocket itself to avoid changes to the Orion capsule design that would mean having to drag the added weight of those systems all the way to the Moon and back. Still, the weight added to Ares I for the active and passive dampening systems will reduce the payload capacity by about 1,200-1,400 lbs.