NASA Spaceflight's Mars Ascent Vehicle from Northrop Grumman takes shape for Mars Sample Return mission says:
For this, Northrop Grumman will provide a 3 meter tall, two-stage, solid propellant rocket — with the first stage derived from the STAR 20 and and the second stage derived from the STAR 15 motors in Northrop Grumman’s propulsion systems catalog.
The propellant each stage will use has a long flight history for solar system exploration, having been used for the Surveyor lunar landings in the 1960s and on the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft missions to the outer planets and reaches of the solar system.
The STAR 15G rocket motor was designed and qualified during 1997 in two different grain design configurations. The motor design was based on the ASAS 15-in. diameter development motor that was used to evaluate design features and component and material technology in seven tests between December 1988 and June 1991. Northrop Grumman employed its Thiokol Composite Resin technology on this motor, one of several STAR designs to use a wound graphite-epoxy composite case.
The motor’s unique regressive thrust-time profile is an example of propellant grain tailoring to restrict thrust to maintain a low level of acceleration to the payload. An alternative propellant loading of 131 lbm was also tested during qualification.
The specifications in the catalog say:
WEIGHTS, LBM Total loaded (excluding ETA and S&A): 206.6 Propellant (excluding 0.12 lbm of igniter propellant): 175.5
which makes me wonder (without any knowledge) if those two 12 cm long stems that look like valve stems on large truck/tractor tires might be related to ignitors.
I looked at the STAR-20 image and there are no such structures.
Question: What are these two little valve stem-like projections from the Northrop Grumman STAR 15G upper stage rocket motor?
above: Screenshot cropped and amateurishly sharpened from the PDF catalog for the STAR 15G. below: Similar but no sharpening for the STAR 20.