On Apr. 26, 1971, pilot Lt. Col. Thomas Estes and RSO Lt. Col. Dewain Vick (he and his family were one of my neighbors at Beale AFB) 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. At Blackbird speeds and temperatures, oxygen becomes explosive and can spontaneously ignite in the tanks and fuel lines. In order to prevent this, all 6 fuel tanks are purged with pure nitrogen after being filled.
The Blackbird later carries 260 liters of liquid nitrogen in 3 dewars [for this flight the SR-71 only had 2 – 106-liter liquid nitrogen dewars. The 3rd 50-liter dewar was added in the mid 1980’s].
This nitrogen expands into its gaseous form as it is pumped into the fuel tanks to top them off as fuel is consumed. Without the nitrogen, the empty fuel tanks would cavitate from the increased pressure when returning to lower altitudes to refuel. This process involves pumping nitrogen into the tanks while allowing air to vent out, which removes oxygen from the fuel and makes it chemically inactive.
This greatly reduces the risk of explosion in extreme heat and pressure conditions.
Linda Sheffield