On Apr. 26, 1971, pilot Lt. Col. Thomas Estes and RSO Lt. Col. Dewain Vick flew SR-71 #968 15,000 miles in 10 hours 30 minutes non-stop on a grueling marathon mission to test the endurance of the J58 engines and the Blackbird airframe, but mostly to see how many times they could refuel before the liquid nitrogen gave out
The SR-71, like its predecessors, the Lockheed A-12 and the YF-12A prototype interceptor, is powered by two J58 afterburning turbojet engines, each producing 34,000 lbf (151,240 N) of thrust.
Developed in the late 1950s by Pratt & Whitney’s Aircraft Division of United Aircraft Corp. to satisfy a U.S. Navy requirement, the J58 engine was built to operate at speeds above Mach 3 and at altitudes exceeding 80,000 feet. It was the first engine specifically engineered to run for long durations in afterburner and the first to be flight-qualified at Mach 3 for the U.S. Air Force (USAF).
An engineering marvel, the J58 featured a single-shaft rotor layout and an innovative compressor-bleed bypass system for extreme high-speed flight. Its standout feature was six bypass tubes that routed air from the compressor stage straight into the afterburner, enabling the Blackbird to achieve significantly better fuel efficiency than other afterburning jet engines while running in full afterburner (AB).
On Apr. 26, 1971, pilot Lt. Col. Thomas Estes and RSO Lt. Col. Dewain Vick (who, along with his family, was one of my neighbors at Beale AFB) flew SR-71 #968 on a punishing 15,000-mile mission, completed in 10 hours 30 minutes without stopping. The purpose was to push the limits of the J58 engines and the Blackbird airframe—and, more specifically, to find out how many times they could refuel before the liquid nitrogen supply was exhausted. At the extreme speeds and temperatures at which the Blackbird operates, oxygen becomes dangerously volatile and can spontaneously ignite inside the fuel tanks and lines.
To prevent this, all six fuel tanks are purged with pure nitrogen before they are filled. The Blackbird also carries 260 liters of liquid nitrogen in three dewars [for this particular flight, the SR-71 had only two 106-liter liquid nitrogen dewars; the third 50-liter dewar was added in the mid-1980s]. As the nitrogen is pumped into the fuel tanks, it warms and expands into a gas, topping off the tanks as fuel is burned. Without this nitrogen, the empty space in the fuel tanks would cavitate due to the rising pressure when the aircraft descended to lower altitudes for refueling.
But Crew Chief Floyd Jones explains that the SR-71 does have three LN2 Dewars: two in the nose wheel, and the third in the left chine are just outboard of K bay. The LN2 flows from the Dewar to the coils (heat exchangers) in tanks 1 and 3, which turn the liquid into gaseous nitrogen. The wing tanks are the ones that have the worst leaks. The reason being that during the in-flight refueling, tanks 3 & 6 are empty; during the refueling, the excess hot tanks and the very cold fuel cause the tank sealant to fracture (crack), causing leaks.
Departing from Beale AFB in California, they flew two circuits around the continental United States, routing via Missoula, Montana; east to Bismarck, ND; southeast to Peoria, IL; east to Columbus, OH; southeast to Cape Hatteras, NC; southwest to Gainesville, FL; south to Tampa, FL; west to San Antonio, TX; northwest to El Paso, TX; west to El Centro, CA; and then back to Beale AFB. After completing the second lap, they began a third, turning south at Bismarck toward Santa Fe, NM; then heading west to Las Vegas, NV; and finally returning to Beale (flight plan via Ron Kloetzli). The 15,000-mile, 10.5-hour mission required five in-flight refuelings. Post-flight inspection showed the aircraft had suffered no ill effects from the ordeal. To this day, the SR-71 is still the only aircraft certified to operate in full continuous afterburner.
Colonel Shelton, about his 13-hour flight during the Yom Kippur War, said that after a long flight like this, his adrenaline was hyped up so high that he was unable to sleep even though he was exhausted.
To burn off the surge of adrenaline, he would take a long walk to calm himself. On Sept. 20, 1971, Estes and Vick received the 1972 Harmon International Trophy from President Richard Nixon in recognition of their achievement. They were also honored with the 1971 Mackay Trophy for the same mission.
Check out Habubrat’s Facebook page for further Blackbird photos and stories.
Source: www.Habu.org
Photo by Lockheed Martin via www.Habu.org and U.S. Air Force
