Things began to heat up in Vietnam by mid-1964, calling for increased reconnaissance. At the time, RC-135M aircraft (RIVET CARD) were operating out of Yokota, but their tasking was against targets from Shanghai to Petropovlovsk. Their interest turned more and more toward Vietnam until that became their primary mission. Later, specially configured RC-135Ms were devoted exclusively to SEA for months at a time. The missions became more frequent, more frenzied, more risky and more demanding.

The RC-135Ms were moved from Japan, as the 82nd Strategic Reconnaissance Squadron, and assigned to the 4252SW at Kadena (Okinawa) with increased tasking against the conflict in Vietnam. Backend crews - linguists fluent in the Vietnamese language - were supplied by USAF Security Service's newly-formed 6990th Security Squadron.  The tasking grew heavier and heavier until the RC-135Ms were flying a daily mission in the Gulf of Tonkin (twelve hours on station); the Vietnamese conflict operations name was "Combat Apple."  That SIGINT (signals intelligence) platform was making such significant contributions to the SEA intelligence effort that it was soon tasked to provide 24 hour-a-day coverage.

The 82SRS/376SW and 6990SS fulfilled that around the clock mission against staggering odds. The RC-135 flight was just over 19 hours for a 12 hour orbit in the Gulf of Tonkin. They had to generate and fly two aircraft a day on 19 hour missions and have a secondary ready for immediate launch (for every mission) in the event the primary couldn't launch or aborted. That doesn't sound too difficult until you realize they only had five aircraft, encountered terrific corrosion problems from the salt spray and high humidity, endured terrible weather both on the ground and in the air, had to move to Clark or Yokota several times a year during typhoon evacuations and overextended the life cycle of every system on the aircraft as well as the airframe itself. In spite of those obstacles, Combat Apple was on station 24 hours a day without break for several years - until the requirement ended.

The Combat Apple orbits were all in the Gulf of Tonkin in the beginning. Those orbits gave the flexibility to set up smaller orbits anywhere in the Gulf, in order to be where the action was. Their refueling orbit was just below the DMZ, enabling them to continue collecting intelligence while refueling.  On several occasions, North Vietnam Air Force (NVAF) MIG-21s would streak out over the Gulf at supersonic speeds and make a pass at the RC-135. Both fuel and fear limited them to only one pass. They would fire everything they had and run for the safety of their AAA and SAM umbrella back home. The RC-135M had no defensive systems and was operating near maximum altitude (especially soon after refueling), limiting its maneuverability. In spite of being such an easy target, no RC-135Ms were lost to MIGs. But after a few very close encounters, fighter escort was ordered for the Combat Apple. The escort came from carriers in the Gulf, in the form of F-4s and F-8s. The fighters orbited above and a few miles inside the RC-135 track. That escort (like too many other operations in that conflict) soon fell into a stable, predictable routine - always leaving the ferret to refuel at the same time and being relieved by new escorts at the same time. The enemy very quickly identified that pattern and took advantage of it. Two MIG-21s would launch, accelerate to supersonic speeds and cross over into the Gulf of Tonkin just as the escorts rendezvoused with their tanker. That gave them a clear shot at the unprotected RC-135, with ample time to get back over their own defenses before Navy fighters could react.

That occurred twice before additional fighter cover was provided. A couple of orbits before the high cover would depart for refueling, the RC-135 would fly to the far southern end of its route. There, two more fighters would join the RC-135, without being detected, and tuck in under its wings. That trio would then set up an orbit just off Haiphong, while the high cover left to refuel on schedule. The MIGs took the bait and no sooner entered the Gulf than they were jumped and flamed by the close cover. They lost several MIGs to that tactic before finally giving up and leaving the RC-135 alone. Once they quit, they never again came out over the Gulf after the Combat Apple.

In 1972 an overland route was established for Combat Apple. The route carried them all over Laos and gave access to much intelligence that was unobtainable from the Gulf of Tonkin routes. It took them over the Ho Chi Minh trail and other troop and supply routes. AAA and SAMs then became a threat in addition to the MIGs, which renewed their attempts at the RC-135, as fighter cover was not provided for the Laotian routes. Weapons, moved into Laos against the RC-135, were quickly detected and struck. The ineffectiveness and high losses of that tactic soon led to it also being abandoned by the enemy. The most intense action against the RC-135 came when it was mistaken for a B-52 by enemy radar operators.  With that second route, the daily routine was for one aircraft to fly a 12-hour orbit over the Gulf and the next RC-135 to fly the overland route.

When you stuff 35 men into the already full RC-135, with all their professional gear, flight gear, lunches and baggage (with the weather, they never knew where they would land), it became quite crowded. That made the ride even more exciting and hairy when taking evasive actions against MIGs or SAMs. But, by far, the most exciting and hairy rides were the result of the torrential rains on Okinawa. Yes, it could be thrilling making takeoffs and landings in zero-zero conditions and penetrating thunderstorms. But the heavy rains caused another unique problem. The RC-135Ms were equipped with thrust reversers, to aid in stopping all its mass. Regardless of all measures taken to prevent it, those rains still found their way into the thrust reversal systems, creating a short in the control circuits and causing the engine to go into reverse. In every case, that occurred shortly after takeoff while climbing out at full power (usually dodging thunderstorms). It caused the RC-135 to yaw violently, lose a great deal of airspeed rapidly and try to roll. It was a wild ride that came with no warning and stained more than a few flight suits, not to mention the Combat Apple's mad dashes into Thailand after being mistaken for a B-52 and chased by MIGs.

With the end of U.S. involvement in the hostilities in SEA, the RC-135 interest increased for a short time. The area was looked at intensely to detect changes in equipment, tactics, etc. The area then took its place with all others in the global commitment of SAC's RC-135s.  At the height of the war against North Vietnam, it is estmated by the Defense Department that nearly 75 percent of the usable intelligence collected in Southeast Asia against North Vietnamese targets was done by the RC-135M Combat Apple.