The Delta L, Thor-Delta L, or Thrust-Augmented Long Tank Thor-Delta was a US expendable launch system used to launch the Pioneer E and TETR satellites in 1969 (failed) and HEOS satellite in 1972. It was a member of the Delta family of rockets.
The first launch of the Delta L took place on 27 August 1969, from Launch Complex 17A at the Cape Canaveral Air Force Station. A defective valve caused plumbing in the hydraulics system to rupture and leak fluid, causing first stage engine gimbaling to fail around 2-1/2 minutes into launch. The stage completed its burn successfully, but threw the second stage far off course. Orbital velocity could not be achieved and Range Safety sent the destruct command at T+383 seconds. Neither the Pioneer E nor the TETR payloads achieved orbit. The second, and final, Delta L launch was from Space Launch Complex 2E at the Vandenberg Air Force Base, on 31 January 1972. It successfully placed the HEOS-2 satellite into a highly elliptical orbit.