Extended Long Tank Delta
Tenth in a Series Reviewing Thor Family History
by Ed Kyle, Updated 4/9/2010

Delta 95 was the second "Straight Eight" Delta

1972 was a watershed year for Delta.  For the first time, NASA and McDonnell Douglas introduced improvements that were not "hand me downs" from the U.S. Air Force Thor program.  In addition to the previously described nine-strap-on-booster, Universal Boat Tail, and Delta Inertial Guidance System (DIGS) upgrades, Delta flew its first Extended Long Tank first stages, AJ10-118F powered second stages, and Star 37E third stages. 

Before year's end, Delta had been remade, flying for the first time in its soon-familiar "Straight Eight" configuration that featured a continuous eight foot (2.438 meter) diameter vehicle from its new payload fairing down to the base of its Extended Long Tank first stage.  The changes substantially increased Delta's hauling capability.   By the end of 1972, Delta could lift 1,835 kg to LEO or 635 kg to GTO, compared to 1,295 kg and 454 kg a year earlier.  

Extended Long Tank (ELT) Delta flew 93 times from 1972 until 1990.  All but four of the launches were successful. During that time the Extended Long Tank first stage served as the core for five different basic Delta models.

New Model Numbering System         

During 1972, McDonnell Douglas introduced a new four-digit Delta model numbering system.  Table 1 lists the designations used during the Extended Long Tank era.

Table 1:  Delta Model Numbers

First Digit:  First Stage and Strap on Motor Types

0:  Long Tank, MB-3-3 engine, Castor 2 motors (1968)
1:  Extended Long Tank, MB-3-3 engine, Castor 2 motors (1972)
2:  Extended Long Tank, RS-27 engine, Castor 2 motors (1974)
3:  Extended Long Tank, RS-27 engine, Castor 4 motors (1975)
4:  Extended Long Tank, MB-3-3 engine, Castor 4A motors (1989)
5:  Extended Long Tank, RS-27 engine, Castor 4A motors (1989)

Second Digit:  Number of Strap on Motors

Third Digit:  Second Stage Type

0:  AJ10-118F (Aerojet Transtage derivative, 1972)
1:  TR-201 (TRW LM Descent Engine derivative, 1972)
2:  AJ10-118K (Aerojet ITIP engine, 1982)

Fourth Digit:  Third Stage Type

0:  No third stage
3:  Star 37D (TE-364-3, 1968)
4:  Star 37E (TE-364-4, 1972)
5:  Star 48B (TE-M-799, 1989)

Long Tank, Extended 

Diagram of a Delta 2914 Model

Extended Long Tank included a 3.049 meter tank stretch compared to the previous Long Tank stage.  It was 22.5 meters long and weighed 83.749 tonnes fully fueled, a 13.661 tonne increase - all but 0.393 tonnes of which was propellant.  

The small dry mass increase was due to McDonnell Douglas's new isogrid tank walls.  Isogrids were created by machining triangular patterns in aluminum sheets, which were then formed into curves and welded together to form cylinders.   The new design was both lighter and stronger than the previous waffle pattern that had required more internal stringers.   

The new stage was still listed as a "Thor" and given Thor serial numbers on company records, but it now bore little resemblance to the original IRBM.  It was also no longer manufactured in Santa Monica.  During the early 1970s, McDonnell Douglas moved Delta production to its newer Huntington Beach facility.   The old Santa Monica Douglas factory that built about 400 Thors, was closed in stages and, finally, demolished in 1977. 

Extended Long Tank was initially powered by the long-serving 78 tonne thrust MB-3-3 propulsion system, but only eight such "1000 series" Deltas would fly before more powerful 92.987 tonne thrust Rocketdyne RS-27 engines would take over the job beginning in 1974. 

The 1000 Series Deltas

Delta 97, a 1604 model, orbited Explorer 50 from Pad 17B on October 26, 1973.

Delta 90, a Delta 1604 model boosted by six Castor 2 strap on motors, used the first Extended Long Tank.  It orbited Explorer 47 from Cape Canaveral Launch Complex 17B, a pad newly converted to handle ELT, on September 23, 1972.  It was one of only three ELT Deltas fitted with AJ10-118F powered, 1.39 meter diameter second stages.  The 5.428 tonne second stage was stacked atop a tapered interstage as it had been for the 300 and 900 series Long Tank Deltas. 

Delta 90 carried the first Thiokol Star 37E third stage.   Star 37E, also identified as TE-M-364-4, was 1.88 meters long, 0.94 meters in diameter, and produced 6.35 tonnes of thrust for 43.6 seconds.  The motor had a 285.5 second specific impulse.  The spin-stabilized motor was mounted on a spin table attached to the top of the second stage. 

As was the practice with Delta 600 models, three of the Delta 1604 Castor 2 strap-on motors were ground-lit and three were air-lit, ignited nearly 40 seconds after liftoff.  All of the motors were jettisonned only after the second pair had burned out. 

Delta 92, launched with Anik A1 from LC 17B on November 10, 1972, was the first "Straight Eight" Delta.  It was a model 1914 with nine Castor 2 motors and the new second stage.  Although the third digit in the model number, a "1", indicated that the second stage was powered by a TRW TR-201 engine, in fact it used an AJ10-118F engine.  The final "4" indicated that a Star 37E third stage was used.  Delta 1914 could lift 635 kg to GTO.

The second stage, manufactured for the first time by McDonnell Douglas rather than Aerojet, was 5.689 meters tall and 1.394 meters in diameter.  It was fitted with a "mini-skirt" truss structure that both suspended it within an 8-foot (2.438 meter) diameter interstage and provided an attachment for the new 2.438 meter diameter aluminum payload fairing.  The DIGS and other avionics were mounted to the top of the stage.

Like the "900" series, the "1900" series Delta Castor 2 strap-on motors performed a 6-3 staggered start sequence, with six lighting on the ground and six in mid-air just before the first six burned out.  All nine Castor 2 motors continued to be jettisonned at about the same time shortly after the air-lit set burned out. 

Four 1000 series Deltas all with AJ10-118F second stage engines, including two "Straight Eight" models, flew in 1973.  Delta 99, which carried Explorer 51 into orbit from Vandenberg AFB SLC 2W on December 16, 1973, used the final AJ10-118F powered second stage and was the last Delta to fly with the old tapered interstage.

Delta 109, the last white Delta

The final two 1000 series Delta flew in 1975.  Both were "Straight Eight" Deltas with TR-201 second stage engines. 

Delta 109 was unusual for its time because it used only four strap on motors to launch GEOS 3 from SLC 2W on April 9, 1975.  Delta 109 was also the last Delta painted entirely white.  

Delta 112, which orbited OSO 8 from LC 17B on June 21, 1975, was the last 1000-series.  It was a Delta 1914 model with what was expected to be the last flown MB-3-3 first stage engine.  This Delta presented a unique appearance because it had been painted white, but flew with a blue interstage barrel. 
 

Delta 2000

Delta 100, the first 2000 series Delta, only used three Castor 2 motors.  It was the first blue-painted Delta. 

Rocketdyne's RS-27 powered the 2000 series Deltas, which were still boosted by Castor 2 strap on motors using the same firing sequences as before.  RS-27 was essentially a Rocketdyne H-1 engine, repackaged from the large inventory of unflown Saturn IB engines. 

Rocketdyne built 294 H-1 engines for NASA's Saturn I/IB booster programs during the 1959-1968 period.  Only 152 flew on Saturn rockets.  Forty five H-1 engines assigned to five unflown Saturn IB stages were removed and transferred to the Delta program when Saturn/Apollo ended.  Some H-1s had been fired extensively in ground testing and were therefore not available, but dozens more remained in storage, unassigned, in ready-to-fly condition.  These were also transferred to Delta.   Ultimately 83 RS-27 engines would power 2000, 3000, and 5000 series Delta rockets. 

The 2000 series Deltas also used a restartable TRW TR-201 second stage engine that was derived from the Lunar Module descent engine.  The ablatively cooled engine burned Aerozine 50 and nitrogen tetroxide to produce 4.377 tonnes of thrust for up to 315 seconds. 

With Apollo program hand me downs RS-27 and TR-201, Delta could now lift 1,860 kg to LEO or 705 kg to GTO from the Cape and 1,395 kg to sun synchronous orbit from Vandenberg. 

Delta 100, a Delta 2313 model, was the first of 45 Delta 2000s that flew from 1974 to 1981.  Fitted with the first TR-201 second stage engine, it lifted off from Canaverals Pad 17B on January 18, 1974 with Skynet 2A, a military communications satellite for the United Kingdom.   Unfortunately, a short circuit in a second stage electronics package circuit board left the upper stages and satellite in an unstable low orbit (96 x 3,406 km x 37.6 deg) that rapidly decayed.  An investigation revealed that a substandard coating had been used on the circuit board.

As usual, Delta quickly recovered, running up 29 consecutive successes.  The streak included 12 successful launches in in 1975 and 9 in 1976.   Ten more launches followed in each of the following two years. 

This was when NASA's Delta first really hit its stride.  Newly capable Delta launched communication, weather, and science  satellites for the U.S., U.K, Canada, France, Indonesia, and NATO.  Satellites included Westar, SMS, Skynet, NOAA, Symphonie, Landsat, GEOS, Anik, Nimbus, OSO, COS, Explorers 54 and 55, Marisat, and CTS, among others.  The launches took place from Cape Canaveral's LC 17A and LC 17B - LC 17A having been upgraded and returned to service in August 1975 after a four year absence - and from Vandenberg AFB SLC 2W.   

The streak ended on April 20, 1977, when Delta 130, a 2914 model, suffered a failure after the second stage 2 burn.  The third stage was released early, before it had been spun up.  As a result, the stage and its ESA-GEOS 1 payload tumbled out of control. 

Delta 130 was the final Delta 2000 failure.  Seventeen more flew successfully before the 44th and final example of the type, Delta 157, lifted off with NASA's SME and the University of Surrey's UoSat 1 from SLC 2W on October 6, 1981.  

Delta 3000, and Production Shutdown

Delta 151, a 3910 Model, orbited Solarmax from LC17A on February 14, 1980.

The final ELT upgrade was the switch to Castor 4 strap on motors, creating the Delta 3000 series that flew from 1975 until 1989.  Castor 4 weighed twice as much as Castor 2 and produced nearly twice as much thrust over a longer duration.   RCA Global Communications paid a portion of the Castor 4 development cost to allow it to launch its three-axis stabilized 868 kg Satcom communications satellites. 

Delta 118, a Delta 3914, was the first 3000, hauling Satcom 1 to GTO on December 12, 1975.  The heavier strap on motors required use, for the first time, of a staggered jettison sequence.  Only five of the nine motors were ignited on the ground.  After the five motors burned out, they were jettisoned in a staggered 3-2 sequence.  Then the four air-lit motors were started.  2000-series Deltas used a staggered 6-3 start sequence, but jettisoned all nine strap-on motors at once.  

Delta 134, a Delta 3914, failed on September 13, 1977 when one of its Castor 4 motors suffered a casing burn through 54 seconds after liftoff.  This was the last Delta failure until 1986.  During that span, Delta logged 43 consecutive launch successes, setting a new standard for U.S. launch vehicle success.

McDonnell Douglas continued to improve Delta, adding a PAM-D (Star 48B) third stage in 1980 and, beginning in 1982, the Aerojet AJ10-118K engine developed during the Improved Transtage Injector Program (ITIP).  Delta 147, launched on December 17, 1978, used the first Delta Redundant Inertial Measurement System (DRIMS).   DRIMS improved the inertial measurement unit introduced with DIGS, but kept the DIGS guidance computer.  DRIMS added redundancy on all axes of motion. 

Nonetheless, by the mid 1980s Delta appeared to be living on borrowed time.  NASA's Space Shuttle entered service in 1981, and the Agency began shifting all of its Delta and Atlas class payloads to Shuttle.  The Delta production line began grinding to a halt in 1983.  Only four Deltas flew in 1984, and none lifted off in 1985 - the first year without a Delta launch since the program began in 1960. 

When Challenger and crew were lost on January 28, 1986, only six more Delta launch vehicles remained in inventory, and several did not have mission assignments.  The first of these, Delta 178, lifted off on a warm spring evening from Pad 17A on May 3, 1986, carrying GOES G.  Unfortunately, the launch failed 71 seconds later when a short circuit caused the RS-27 main engine to suddenly shut down.  The rocket spun out of control into a fireball within sight of stunned spectators along the Cape's beaches. 

This, the fourth major U.S. launch failure within a few months, had knocked down the most reliable U.S. launch vehicle on a day when NASA most needed a success.  The failure led to instinctive finger pointing, followed by analysis and longer term strategic thinking.  Though it seemed impossible at the time, this dark day would mark a remarkable turning point for both Delta and U.S. orbital space launch capability in general.