Long Tank Thor-Delta
Ninth in a Series Reviewing Thor Family History
by Ed Kyle, Updated 4/8/2010

Delta 59, the second Long Tank Thor-Delta, failed to orbit Intelsat 3-1 from Canaveral's Pad 17A on 9/18/1968, ending a string of 25 consecutive Delta successes. Note the reconfigured Umbilical Tower.      

Thrust Augmented Improved Delta (TAID) had adopted strap on boosters previously proven by the U.S. Air Force Thor-Agena.  In 1968, NASA again gained access to a Thor Agena improvement when it added a stretched first stage, Long Tank Thor, to the Delta program. 

Long Tank Thor first flew in 1966 with an Agena second stage.  "Long Tank" replaced the previous tapered Thor kerosene tank with a cylindrical tank, and stretched the length of both the upper kerosene tank and the lower liquid oxygen (LOX) tank.  The stage was now 22.53 meters in length and a constant 2.439 meters in diameter.  It weighed 70.354 tonnes loaded, about 21 tonnes more than the old Thor stage it replaced, but its dry mass only increased by 0.372 tonnes.  An MB-3-3 engine still powered the stage.

Use of Long Tank Thor created Delta Models L, M, and N.  All three used a trio of Castor 2 strap-on solid motors to augment liftoff thrust.  All three were equipped with the 6 tonne Improved Delta second stage, little changed from TAID, powered by the 3.52 tonne thrust AJ10-118E engine.
 

Delta L, M, and N

Delta 70, a 2.5 stage "N", orbited Biosat 3 from Pad 17A on 6/28/1969.

Delta L used the FW-4D third stage motor.  Delta M, the most often-flown Long Tank Thor Delta model,  used the more powerful Star 37D third stage, which weighed 0.718 tonnes and produced 4.59 tonnes of thrust.  Delta N did not have a third stage. 

Delta M could boost 356 kg to GTO.  Delta N could lift 998 kg to LEO from the Cape or 680 kg to a sun synchronous orbit from Vandenberg AFB.  

Delta 58, an "N" carrying the Tiros 17 weather satellite from Vandenberg on August 16, 1968, was the first Long Tank Thor Delta.  Altogether, there were 20 "L", "M", and "N" flights.  Delta M flew 12 times, Delta N lifted off 6 times, and Delta L launched twice.  Three of the launches, by Deltas 59, 71, and 73, failed.

Delta 59, the second Long Tank Thor Delta and the first "M" model, failed on September 18, 1968, ending a then-record run of 25 consecutive Delta successes.  The rocket, carrying the first Intelsat 3 series communications satellite from Cape Canaveral, suffered a pitch rate gyro failure that became noticeable about 20 seconds after liftoff.  The rocket began to break up at T+102 seconds.  The range safety officer sent a destruct command 6 seconds later.

Delta 71 left Intelsat 3-5 in a useless orbit on July 25, 1969 when its Star 37D third stage motor either suffered a motor case rupture or a nozzle failure during its burn.

Delta 73, the first Delta L, failed on August 27, 1969 when it attempted to launch Pioneer E from the Cape.  This time the culprit was an unstable high pressure relief valve in the first stage power pack.  Pressure fluctuations caused a line to rupture and leak hydraulic oil.  First stage main engine gimbal control was lost 213 seconds after liftoff, during the latter portion of the first stage burn.  The second stage separated and ignited, but was too far off course to make up the lost velocity.  Range safety sent a destruct command at T+ 8 minutes 3 seconds. 

Additional Long Tank Thor Delta successes in 1968 included Delta 62 from Vandenberg with Tiros 18 on December 15 and Delta 63 from the Cape with Intelsat 3-3 on December 18.  There were five successful Long Tank Thor Delta launches in 1969.   They included three successful Intelsat 3 launches, a Delta "N" launch of NASA's OSO 6 from the Cape, and a Delta "M" launch that sent British milsatcom Skynet 1A into GTO from the Cape.

Delta 88, which orbited TD-1A for Europe, was the last Delta N and the final launch from VAFB SLC 2E.  Delta 88 used the first "Universal Boat Tail"

Nine subsequent successful Long Tank Thor Delta launches spanned 1970 and much of 1971.  The run included six GTO comsats and two NASA science satellites from the Cape, and six near-polar orbit weather satellites from Vandenberg.

The last of these successes, by Delta 85, was nearly a failure.   Delta 85 was a 2.5 stage "N" launched from the Cape on September 29, 1971 with OSO 7.  During the AJ10-118E second stage engine's second burn, the stage suffered a control system failure, caused by a nitrogen pressure leak that cascaded into a main engine gimbal thrust vector control (TVC) hydraulic pressure decay.  (The hydraulic pump was run by pressurized nitrogen gas during the coast phase prior to the burn.)  The stage tumbled, but it and OSO-7 still managed to achieve a usable orbit.  Ground crews stablized OSO 7 after it separated, a "save" that allowed it to perform its mission.

Delta 88, launched on March 12, 1972 with Europe's TD-1A science satellite. , was the last "N" and the final launch from Vandenberg AFB SLC 2E.  Delta 88 used a transitional Long Tank stage equipped with the first "Universal Boat Tail" - a beefed up aft thrust structure equipped with mounting points for nine solid motors.  The change was part of the transition from Long Tank to Extended Long Tank Delta that began in 1972. 

Delta M6 and N6

Delta 83 was an "M6" with six strap-on Castor 2 motors.

In 1970, Delta M6 and N6 models, similar to M and N, respectively, but fitted for the first time with six Castor 2 strap on motors, began to fly.  These "Super Six" models could lift 1,295 kg to LEO or 454 kg to GTO from the Cape and 975 kg to sun synchronous orbit from Vandenberg AFB, a substantial increase from the early Thor-Deltas. 

The initial six-Castor-motor Deltas ignited three boosters on the pad and three in the air 38 seconds after liftoff, just before the ground lit motors burned out. All six motors jettisoned after the second set burned out, with the three ground lit motors going at T+90 seconds and the three air lit motors at 95 seconds.

Three 2.5 stage "N6" Deltas launched from Vandenberg AFB with Improved Tiros weather satellites.  The last, Delta 86, failed on October 21, 1971 when its second stage suffered an oxidizer leak.  The stage tumbled out of control after its attitude control system fought the side thrust from the leak.  The control system finally used up its supply of control gas.    

The single "M6" carried NASA's Explorer 43 into a highly elliptical Earth orbit from Canaveral Pad 17A on March 13, 1971.

The additional strap on motors eliminated, for the first time in several years, the placement of the "Delta" logo at the base of the rocket on the thrust section.  Delta 76, the first "N6", was the first to fly without the bottom logo.  It would be two years before Delta 87, the last Long Tank to fly without the Universal Boat Tail, would be the final Delta to carry the lower logo. 
   

Delta 300 and 900

Delta 91, a Delta 300 model, orbited NOAA 2 from VAFB SLC 2W on 10/15/1972.

Two additional transitional Long Tank Thor Delta models flew in 1972-73.  These were identified using a new model number scheme introduced by McDonnell Douglas in 1972.  The "Delta 300" series used three Castor 2 strap on boosters and a modified second stage powered by a more-powerful AJ10-118F engine.  AJ10-118F, which produced 4.08 tonnes of thrust, was derived from the Titan 3 Aerojet Transtage engine.  It burned nitrogen tetroxide and Aerozine 50 (a 50-50 mix of UDMH and hydrazine) rather than the previous nitric acid/UDMH.  "Delta 900" used nine Castor 2 boosters and the same second stage.  Both models used the Universal Boat Tail.  Neither flew with a third stage. 

The "900" series Deltas used a 6-3 staggered start sequence for their Castor 2 strap-on motors (six ground-lit and three air-lit).  All nine were jettisoned together after the final three burned out. 

Delta 900 could lift 1,685 kg to LEO from the Cape or 1,225 kg to sun synchronous orbit from Vandenberg AFB.  Delta 300 launched payloads weighing nearly 370 kg into 1450 km sun synchronous orbits from Vandenberg, but was likely capable of lifting close to 700 kg to lower altitude sun sync orbits.

Delta 89, the first Delta 900, was the first Delta controlled by the Delta Inertial Guidance System (DIGS).  Prior Deltas had used radio-inertial guidance.  DIGS consisted of a Delco Electronics guidance computer and a Hamilton Standard inertial measurement unit that used three strapdown integrating gyroscopes and three strapdown linear accelerometers.  DIGS also performed preflight and flight control sequencing.   

Delta 89, which orbited Landsat 1 from Vandenberg SLC 2W on July 23, 1972, was the first Delta identified by the McDonnell Douglas four-digit model number system.  In this case, the number "9" signified the use of nine strap-on motors, the first time nine boosters had powered a Delta.  The first "0" was for the AJ10-118F second stage.  The second "0" indicated that no third stage was used.

Three Delta 300 and two Delta 900 launches took place from Vandenberg AFB.  One (Delta 96) failed to orbit ITOS E from Vandenberg on July 16, 1973 when a hydraulic pump failed 270 seconds after the second stage ignited.  The pump failure led to loss of thrust vector control.  Once again, a Delta second stage tumbled out of control, but this time there was no recovery.

Long Tank Thor ended service as an Air Force Agena launcher on May 25, 1972.  The final Long Tank Thor Delta launch occurred several months later on November 6, 1973 when Delta 98, a Delta 300 model, orbited NOAA 3 from Vandenberg's still-active SLC 2W. 

Long Tank Summary

During five years of service, 29 Long Tank Thors had flown for the Delta program - in addition to 43 Long Tanks flown with Agena upper stages for the U.S. Air Force.  Five of the Long Tank Thor Deltas had faltered, including two failures during the first stage portion of flight, a disappointing failure rate for Delta.  The program would respond, pushing Delta toward higher success rates as it transitioned to a new first stage, Extended Long Tank, that arrived on the scene in 1972.

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