support or COIN (Counter Insurgency Warfare) missions were propeller powered types such as the Cessna O-1 Bird Dog and North American OV-10 Bronco FAC spotters, Douglas A-1 Skyraider, Douglas B-26 Invader, and Douglas AC-47 Spooky gunship. Fairchild C-123 Provider and Lockheed C-130 Hercules transports flew supplies into battlefields such as Khe Sanh. "Fast movers" included the supersonic North American F-100 Super Sabre, while the giant Boeing B-52 Stratofortress would be modified to unload a massive high explosive payload on enemy troop concentrations. The Lockheed AC-130 would become the ultimate gunship, while the AX specification to replace the Skyraider would evolve into the Fairchild Republic A-10 Thunderbolt II. USAF Republic F-105 Thunderchiefs flew the bulk of strike missions against North Vietnam in Operation Rolling Thunder, while carrier-based Douglas A-4 Skyhawks (which could "buddy-buddy" refuel) were flown by the Navy. That first campaign was marred by carefully measured regulations that prohibited attacks against SAM missile sites and fighter bases, and frequent bombing halts, and produced little in political results. Rolling Thunder saw the first combat use of electronic computers aboard PIRAZ ships to display comprehensive real-time aircraft position information for force commanders. Lessons learned were applied to the later Operation Linebacker which employed McDonnell Douglas F-4 Phantom II, B-52 Stratofortresses, swing-wing General Dynamics F-111 Aardvarks, LTV A-7 Corsair IIs and all-weather Grumman A-6 Intruders was more successful in bringing North Vietnam to the negotiating table after a massive ground invasion. North Vietnam effectively combined Soviet and Chinese anti-aircraft artillery, SA-2 guided missiles, and MiG fighters to create the most heavily defended airspace up to that time. US air strikes would combine the use of airborne radar platforms such as the Lockheed EC-121 Warning Star, Boeing KC-135 Stratotankers for air refueling, radar jamming aircraft and specialized "Wild Weasel" units to attack SAM missile sites. Jolly Green Giant helicopter crews escorted by Douglas A-1 Skyraiders ("Sandy"s) would retrieve downed pilots over hostile territory. With the use of "smart" guided bombs late in the war, this would set the model for future US air operations. Experts were surprised when advanced F-105s were shot down in its first encounter against the elderly but nimble MiG-17. Dogfights were thought to be obsolete in the age of missiles, but pilots now needed maneuverability. The McDonnell Douglas F-4 Phantom II was quickly tasked with protecting against MiGs, but lacked a built-in gun when missiles were still very unreliable. Air