Sound of Freedom over Hanoi: how two SR-71s Generated Tri-Sonic Booms to Free US POWs in North Vietnam

By Dario Leone
Sep 7 2019
Share this article

To ensure that the POWs directly involved in Operation Thunderhead were fully aware of the CinC’s approval of the plan, a unique covert signal was devised – two SR-71 tri-sonic booms, exactly 15 seconds apart, would ring out over Hanoi…

US aviators first became prisoners of war (POWs) of the North Vietnamese in 1964. History would later bestow upon several of them the dubious honour of being the longest-held US captives of any conflict to date.

On Jul. 18, 1965, Commander Jerry Denton was shot down and captured by the North Vietnamese while flying an A-6A Intruder from USS Independence. Fiercely adhering to the six articles within the POWs’ Code of Conduct, he was tortured for ten months, repeatedly beaten and forced to appear in a propaganda broadcast. Pretending to be blinded by the TV crew’s spotlights, Denton began blinking, seemingly random spasms and tics. In fact the brave pilot blinked out in Morse code ‘T-O-R-T-U-R-E’. As told by Paul F Crickmore in his book Lockheed Blackbird: Beyond the Secret Missions (Revised Edition), the film, made by a Japanese crew, was broadcast on American television on May 17, 1966, and was the first confirmation that US POWs were being subjected to atrocities. When the North Vietnamese realized what Denton had done, he was severely beaten in his cell for days.

This print is available in multiple sizes from – CLICK HERE TO GET YOURS. SR-71A Blackbird 61-7972 “Skunkworks”

On Sep. 9, 1965, while flying A-4E 151134 off the deck of USS Oriskany, Cdr James Stockdale was shot down and imprisoned in Hoa Lo, Hanoi. Originally built by the French when Vietnam was one of its colonies, this notorious prison was renamed ‘the Hanoi Hilton’ by its US inmates. Stockdale was a senior POW and like Denton conformed rigidly to the Code of Conduct; he was singled out for regular beatings.

Notwithstanding, he played a key role in establishing a highly effective covert system of inter-prisoner communication, even when individuals were placed in solitary confinement, and became head of the prisoner escape committee at Hoa Lo. He used ‘double-speak’ in letters home to his wife and she realized he was sending her coded information about the POW camp and its conditions, and took the letters to Cdr Donn T. Burrows at the Office of Naval Intelligence (ONI). She bravely agreed to play a vital role in establishing a covert two-way communications link via the couple’s letters. This delivered important information about the condition of Hoa Lo’s inmates and other intelligence to the ONI, and enabled it to get information back to Stockdale.

When Capts John Dramesi and Edwin Atterberry managed to escape from the Zoo Annex, a POW camp near Hanoi, on May 10, 1969, only to be recaptured 12 hours later some three miles away, the repercussions for all the POWs were extreme. Convinced that others must have been involved in supporting the breakout, the prison authorities unleashed an orgy of unbridled sadistic cruelty. The two escapees were tortured and beaten for 38 days, Atterberry dying as a result. The beatings were then extended to many of the POWs incarcerated in a dozen facilities throughout North Vietnam. As a result, captured senior officers implemented a slight revision to the Code of Conduct, whereby further escape attempts wouldn’t be sanctioned unless external help was available, thereby ensuring a greater chance of overall success.

On Jun. 10, 1970, a feasibility study group was convened by the Special Assistant for Counterinsurgency and Special Activities (SALSA), code named Polar Circle, to look into the possibility of springing the 61 POWs believed to be at the Son Tay camp, located about 23 miles West of Hanoi. A training, planning and development group code named Ivory Coast was established on Aug. 8 to review reconnaissance imagery provided by Teledyne-Ryan Buffalo Hunter reconnaissance drones and SR-71s. The low-flying drones were used sparingly over the target area for fear of alerting the North Vietnamese to the possibility of a raid. However, flying at extreme altitude and using its Tech or panoramic cameras, the SR-71 was an ideal platform for obtaining high-resolution photos of the camp without drawing unwanted attention.

In the last ten days before the planned raid, intense reconnaissance efforts were conducted, but every attempt was thwarted by poor weather. Continuous cloud cover concealed Son Tay from the Habu’s high-altitude cameras, and two drones never returned. Nevertheless, Operation Kingpin, the bold raid to rescue the POWs, was mounted in the morning darkness of Nov. 21, 1970. It employed 56 US special forces troops, deployed in five HH-53 Super Jolly Green Giant helicopters, and included 23 other direct support aircraft.

However, after an eight-minute firefight in which between 50-100 North Vietnamese Army guards were killed, it was discovered that the inmates had been moved to another camp (Dong Hoi, 15 miles further east) a few days prior to the raid. Various reasons for the move have been given, including contamination of the camp’s well due to flooding and the beginning of a major relocation of all POWs from outlying camps to those located in and around Hanoi.

There’s little doubt that the lack of detailed, timely photo-intelligence was a key contribution to Kingpin failing to achieve its primary objective, as was excessive compartmentalization, which denied the planners access to a ‘bigger intelligence picture’. However, most returning POWs after the war reported that as a result of the Son Tay raid, their treatment and conditions improved.

By 1972, it has been reported that covert communication into Hoa Lo had become possible by the limited use of a radio, infiltrated into the prison by the CIA. A plan formulated by Capt John Dramesi to escape with a fellow POW between Jun. 1 and 15, steal a boat and travel down the Red River to the Gulf of Tonkin, where they could be rescued by a team of US special forces was communicated to the camp’s senior ranking officer (SRO). Once approved, it was communicated to the ONI and then escalated to the very top of the chain of command – the commander-in-chief, President Richard Nixon, who also approved the audacious plan.

This model is available from AirModels – CLICK HERE TO GET YOURS.

In April 1972, the submarine USS Grayback (LPSS 574), under the command of Cdr John Chamberlain, slipped its moorings at Subic Bay, Philippines and put to sea. On board was US Navy SEAL Team One and Underwater Demolition Team 11 (UDT-11). The plan, Operation Thunderhead, developed by Lt Cdr Edwin Towers, the officer in charge (OIC), involved the SEAL team infiltrating an island in the Red River estuary using underwater swimmer delivery vehicles (SDVs). From there they’d provide surveillance, which would be enhanced by up to four flights per day of a wider area conducted by HH-3As from Detachment 110, Helicopter Support Squadron 7 (HC-7), from USS Midway. To cover Dramesi’s escape plan, the operation was to be run from May 29 to Jun. 19.

To ensure that the POWs directly involved in Operation Thunderhead were fully aware of the CinC’s approval of the plan, a unique covert signal was devised – two tri-sonic booms, exactly 15 seconds apart, would ring out over Hanoi on May 2 and 4. To generate this ‘sound of freedom’, the services of two SR-71s would be required. However, since the mission was so critical, a third airborne spare would also be launched as a hedge against any last minute mechanical problems that might interfere with the signal’s timing.

At precisely 14:16 on May 2, 1972, Maj Bob Spencer and Maj Butch Sheffield were airborne in ‘979, followed just two minutes later by Lt Col Darryl Cobb and Maj Reggie Blackwell in ‘980. At 15:22 Maj Dave Fruehauf and Maj Gil Martinez launched in ‘968. The missions recovered back into Kadena with ‘979 having logged 4.8 hours, ‘980 clocking up 3.8 and ‘968 clocking 4.1.

On May 4, Majs Tom Pugh and Ronnie Rice approached Hanoi in aircraft ‘968 at 75,000ft from the south, while Majs Bob Spencer and Butch Sheffield maintained 80,000ft in ‘980 and flew across the target from the south-east. Meanwhile Lt Col Darrel Cobb and Maj Reggie Blackwell were the airborne spare in ‘978; they were to cross the ‘Hilton’ at 70,000ft from the west should either of the primary aircraft have to abort. When the pre-arranged code word was transmitted, Darryl broke off his run short of the target area.

Both missions were termed ‘entirely successful’ and accomplished their objective within the very tight time constraints, while reconnaissance gathering was of secondary importance to the signal. Unknown to the planners on the ‘outside’, however, there was a major issue with the SRO in Dramesi’s room, who’d become aware of the plan. Despite the escape committee having determined that the plan had a 90 per cent chance of success, he’d gone to the camp SRO and reminded him of the repercussions felt by the rest of the POWs after Dramesi’s and Atterberry’s earlier escape attempt. The appeal caused the camp SRO to reverse his earlier decision. He withdrew permission to conduct the escape and ordered the escape committee to stand down. Since there weren’t the means to communicate this decision quickly enough, Operation Thunderhead was launched regardless. It was eventually called off after a number of mishaps, one of which resulted in the death of Navy SEAL Lt Melvin Spencer Dry.

Sound of Freedom over Hanoi: how two SR-71s Generated Tri-Sonic Booms to Free US POWs in North Vietnam

Lockheed Blackbird: Beyond the Secret Missions (Revised Edition) is published by Osprey Publishing and is available to order here.

Photo credit: U.S. Air Force

Share this article

Dario Leone

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.
Share this article

Share this article
Back to top