More Bah!
So I visited a Guardian site, gathering materials from sentinels and destructible panels, decoding obelisks by carting artifacts around (seriously, whose idea was that? I spend half my time driving back and fore to an artifact stockpile swapping SRV cargo around. Madness.)
Haven't figured out how to get blueprints yet, so back to Google (seriously, whose idea was it to omit detailed instructions as if this game catered to actual real life scavengers and explorers - realism can go too far)
So why more bah? Self inflicted.
Moral of the story is to not take a Cobra III as the scavenging pick-up truck as it's utility sockets are mounted on the belly of the beast and as such are no use at shooting down Sentinel missiles unless you park it on high ground overlooking the ruins with its nose hanging out over a precipice - I don't know if this will work as that's just a theory, but I will try it tomorrow sometime....
Currently enjoying the SRV being bounced on its roof with each missile hit, though I have discovered that racing under the Sentinels when they fire reduces the damage you take and the Sentinels take a beating from their own missiles. Hiding behind obelisks playing hide and seek also helps. On the positive side, synthesising premium ammo means they only get one salvo off before the plasma repeaters take them down.
So, trusty Cobra III? Bah!