THOR: FLOWN VARIANTS
by Ed Kyle
Delta 31, a Sleek Thor-Delta C, Orbited NASA's
Explorer 28 (IMP C) from Cape Kennedy LC 17B on May 29, 1965.
"Baseball Card" type details of most of the
flown Thor family variants are provided in the following links, listed in approximate
In 1955 the U.S. began Thor IRBM development as a
stop-gap until Atlas could enter service. Thor would use Atlas-derived propulson and
warhead and an existing inertial guidance system. It would have 2,400 km range to reach
the Soviet Union from Western Europe. Douglas Aircraft Company won the SM-75 Thor contract
in December 1955. The company delivered its first Thor from Santa Monica, California to
Cape Canaveral on October 26, 1956.
The early R&D Thors, named Douglas Missile 18, or DM-18, were powered by 135,000 lbf
Rockeydyne MB-1 engines and topped by dummy nose cones. Thor 101 blew up on its LC 17B
launch pad on January 25, 1957. This was followed by three more failures. Finally, on
September 29, 1957, still less than two years after the program began, Thor 105 flew a
successful long-range flight from the Cape. One month later, Thor 109 flew the first
full-range flight down the Atlantic Missile Range.
All-inertial guidance system test flights began in December, 1957. Reentry vehicle test
flights began in February 1958 with Thor 120, which also debuted a new, less-tapered
Thor DM-18A, the operational variant without fins, numbered Thor 138 and higher, began
flying in 1958. The first crew training launch from Vandenberg, which was also the first
long-range launch from that West Coast base, took place on December 16, 1958, when Thor
151 flew successfully over the Pacific Ocean.
Thor became operational in Great Britain during December 1959. There, 60 Thors were
deployed at four former airfield bases. A total of 1,000 personnel manned each base. Thors
were stored horizontally in retractable steel shelters on an erector-launcher mount.
Missiles could be launched within 15 minutes of an order, in theory.
Thors were retired from IRBM duty in August 1963 and returned to the United States where
nearly all would be assigned to other duties.
2. Thor Able
"Thor-Able" (orginally proposed as
"Thor-Vanguard") was a Thor first stage topped by Vanguard second, and in some
cases, third stages. The second ("Able") stage was a pressure-fed nitric
acid/UDMH stage powered by an Aerojet AJ-10 series engine. The third stage was a
spin-stabilized Allegany Ballistics Lab X-248 "Altair" series solid motor.
The rocket, in two-stage form, was created by Ramo-Wooldridge Space Technology
Laboratories (as the prime contractor) to test ICBM ablative reentry vehicle techology
before Atlas and Titan were ready. In 1958 it flung GE Advanced Reentry Test Vehicles more
than 10,000 km to prove the ablative technology. It was the first time that a big USAF
liquid fueled rocket had staged successfully. The two-stage rocket was controlled by an
autopilot rather than an active guidance system.
Someone at STL quickly realized that Thor-Able could be turned into an ICBM (briefly named
"Thoric") - that it would be able to carry the lighter RVs and warheads soon to
appear. The idea was rapidly quashed by the chain of command, which also removed STL from
the Thor-Able prime contractor role in favor of Douglas.
Thor's first orbital attempts were part of ARPA's "Operation Mona", better known
as "Pioneer" - the name assigned by NASA when it assumed control after the first
launch. Pioneer was the first U.S. attempt to reach the Moon. Adding radio guidance to the
Able second stage and an ABL X-248 third stage created the "Thor-Able 1" variant
to do the attempt. Three attempts in 1958 failed to reach the moon, but did manage to loft
Pioneer 1 to record altitude where it collected data on the Van Allen belts.
"Thor-Able 2" was a two-stage variant with BTL guidance in the Able stage that
boosted GE "RVX-1" (or "Precisely Guided Reentry Test Vehicle") scaled
ablative reentry vehicles on multiple ICBM-distance flights in 1959. RVX-1 looked a lot
like the Mark 3 and 4 RVs that later topped Atlas and Titan ICBMs. The Navy managed to
recover some of these, proving the design.
Thor-Able 2 with a third stage orbited NASA TIROS 1, the first weather satellite, in 1960.
Thor-Able's 3 and 4 orbited NASA's Explorer 6 and Pioneer 5 in 1959 and 1960,
respectfully. These used an improved AJ10-101A second stage engine.
Thor-Able served as the starting point for NASA's highly successful Thor-Delta, which
first flew in 1960.
At the dawn of the Space Age the highest priority U.S.
space program was U.S. Air Force Weapons System 117L, also known as the Advanced
Reconnaissance System. One component of WS-117L was the "Corona" photographic
"spy" satellite, which returned exposed film in small "satellite recovery
For the program, Lockheed Missiles and Space Company developed a new upper stage named
"Hustler", later renamed "Agena". Agena A was more than a rocket
stage, it was an orbiting platform for Corona - a spacecraft in its own right. It was
powered by a Bell 8048 UDMH/IRFNA turbopump-fed engine. The engine was not restartable, so
after separating from Thor the stage would coast for a couple of minutes to apogee before
beginning its burn. Agena's forward section housed an inertial guidance system that used
horizon sensors to provide updates. Cold-gas thrusters located in its aft section provided
Fifteen Thor-Agena A launches occurred between February 28, 1959 and September 13, 1960.
All launched toward near-polar orbits from converted Thor IRBM pads 75-3-4 and 75-3-5 at
Vandenberg AFB. The missions were given the "Discoverer" cover name. Discoverer
was said to be a scientific research effort, but it was actually a Corona development
Development was hard-won. One Agena A stage was destroyed even before the first flight in
the "Discoverer Zero" pad accident on January 21, 1959. A sneak circuit
triggered during a pre-launch test caused the stage to fire its ullage rockets. Agena 1019
was subsequently scrapped, but the Thor 160 booster was refurbished and used on the
Discoverer 12 flight in 1960.
Six of the 15 Thor-Agena A launches failed to reach
orbit, and a seventh launch failed to achieve the proper orbit due to a guidance system
failure. Brand-new Agena was most-often the culprit. Not until Discoverer 13, flown in
August 1960, would a capsule be orbited and successfully recovered - the first man-made
object recovered from space. Discoverer 14 returned film containing images taken by the
spacecraft's Keyhole camera. The images covered 1.5 million square miles of Warsaw Pact
territory and revealed the presence of 64 previously unknown airfields, 26 surface to air
missile sites, and a previously unknown launch center at Plesetsk.
4. Thor DM-18C
Thor DM-18C was a special test vehicle that was powered
by an upgraded Rocketdyne MB-3 Block 2 (LR-79-NA-11) engine that produced 165,000 pounds
thrust. A GE low drag fairing topped the missile, covering the standard GE Mark 2 reentry
vehicle. DM-18C tested the improved, higher thrust engine and demonstrated modest range
improvement. Three launches, all from Cape Canaveral LC 18B, took place during
January-February, 1960 (Thors 256, 259, and 263). All were successful.
MB-3 Block 2 would soon begin to power Thor space launch variants.
In 1960, ARPA and the U.S. Air Force began flying
Thor-Able-Star, originally named "Thor-Epsilon" which used the Aerojet General
"Able-Star" pressure-fed hypergolic second stage powered by an AJ10-104 series
engine. Developed for primary customer the U.S. Navy, Able-Star was the worlds first
re-startable stage. It was a fat version of the Able stage, carrying more than twice as
Thor's tapered guidance section was replaced by a stepped interstage adapter. MB-3 Block 1
150 Klbf engines apparently powered the initial Thor boosters, with MB-3-2 165 Klbf
engines replacing them at some point. A lightweight STL guidance system topped the second
stage. Nitrogen jets provided three-axis flight control of the stage during coast periods.
Thor-Able-Star flew 19 times during 1960-65, including 11 launches from the Cape and 8
from Vandenberg AFB. It performed the first in-space stage restart during its first flight
on April 13, 1960. After the stage completed its initial 258 second burn, it and its
Transit 1B payload coasted for 19 minutes before the stage performed a second, 13 second
long burn to raise the orbit.
Early flights orbited the U.S. Navy's initial Transit (navigation) and U.S. Army's Courier
(communications) satellites, along with Solrad/GRAB electronic intelligence radar signal
"spy" satellites flown piggyback with Transit. The final two of six
Cape-launched Transit satellites were powered by SNAP 3B nuclear power sources
(radio-isotope thermoelectric generators or "RTGs") - the first time that RTGs
were launched into orbit. Thor-Able-Star also orbited ANNA 1B (Army, Navy, NASA, Air
Force) a satellite that carried beacons for use in ground surveying.
Thor-Able-Star flew from Vandenberg AFB Complexes 75-1-1 and 75-1-2 during 1963-65. The
sites were later renamed Space Launch Complex (SLC ) 2 East and 2 West, respectively. All
eight Vandenberg launches carried Transit navigation satellites, aimed toward near-polar
orbits. They used SNAP 9A RTGs, which were loaded with more Plutonium 238 than the SNAP 3B
RTGs. The SNAP 9A design was discontinued after the Transit 5-BN-3 flight failed to reach
orbit due to an Able-Star stage failure. The RTG disintegrated in the atmosphere releasing
about 1 kg of Plutonium 238. The final five Transit-O ("Oscar") missions, all
successful from a launch vehicle perspective, used solar powered satellites.
Thor Able-Star's early flight record was spotty, with five launch vehicle failures, two by
Thor and three by Able-Star, during the first 10 flights. But only one of the final nine
Thor Able-Stars, the one that carried the last nuclear powered Transit, failed, and the
Thor first stage itself flew successfully during the final 14 flights.
Although August 13, 1965 saw the last flight of Thor-Able-Star, the basic stage structure
would subsequently migrate to NASA's ever-improving Delta launch vehicle.
NASAs Milton Rosen coined the rocket name
"Delta". He chose Delta because it would be the fourth Thor-based
space launch vehicle after Thor Able, Thor Able-Star, and Thor Agena. Rosen had led the
Naval Research Laboratorys Viking sounding rocket and Vanguard satellite programs.
He transferred to NASA in October 1958 along with 157 NRL Vanguard program employees to
form the Agencys new Goddard Space Flight Center.
Thor-Delta was an adaptation of Thor-Able 2 (originally "Thor-Vanguard"). It was
meant to serve as an "interim" launch vehicle for NASA, until larger Atlas
launch vehicles (Atlas Vega and Atlas Centaur were the plan at the time) were brought on
line. A cold gas-jet attitude control system was added to the previous Able second stage
to create Delta. With this system, Delta could coast and reorient itself in space after
its pressure-fed AJ-10-118 engine had performed its burn. This improved the accuracy of
solid fuel third stage spin-up insertions. Both upper stages were tweaked and weight was
shaved from Thor itself. A Bell Telephone Laboratories BTL-300 radio guidance system was
added in an equipment compartment atop the second stage to control the vehicle. Typical
missions saw the second stage coast for several minutes after its burn before aiming and
spin/separating the ABL X-248-A5 third stage.
Goddard ordered 12 Thor-Deltas from Douglas Aircraft and the other stage contractors in
April 1959. The first, carrying Echo 1 from Cape Canaverals Complex 17A on May 13,
1960, failed. A re-try in August with Echo 1A succeeded. So did the third, and fourth, and
so on until all of the final 11 were successful. They orbited five Tiros, two Explorer,
one Orbiting Solar Observatory, Ariel 1 the first U.K./U.S. satellite, Echo 1A, and
Telstar 1 the famous experimental AT&T active repeater communications satellite. With
such unprecedented success, NASA removed the "interim" label and ordered more,
improve Thor-Deltas, about which more later.
More-capable Thor Agena B began flying on October 26,
1960. It launched 43 times, failing eight times, during its five-years of service. It used
an upgraded DM-21 Thor first stage powered by an MB-3 Block 2 (initially) or Block 3
engine that produced 165 to 170 Klbs of liftoff thrust. The original Thor guidance section
was replaced by a shorter, lighter adapter section. The Agena B second stage, which was
powered by the restartable 16 Klbf thrust Bell 8081 (initially) or 8096 engine engine, was
60 inches in diameter and weighed about 14,770 lbs fueled. It carried two times more
propellant than Agena A and used a more powerful, more efficient engine that also gained
payload capability by being able to restart. Like Agena A, Agena B was a spacecraft in its
own right, able to maintain attitude while operating on batteries for up to two weeks.
The improved launch vehicle orbited Keyhole 2 through 5 (mostly) film return spysats and
three "Ferret" electronic intelligence satellites. It also flew several times
for NASA, orbiting second generation weathersat Nimbus 1, Canadian ionespheric satellites
Alouette 1 and 2, and NASA's Echo 2 and Explorer 31. Alouette 1, the first non-U.S. or
U.S.S.R. built satellite, operated for a then-unheard-of decade.
Most Thor Agena B missions, which either went to LEO or
to slightly elliptical orbits with low perigees, used a single restart for a total of two
Agena burns. The first burn was usually something like 234-ish seconds duration, while the
second burn at first apogee would only last a few seconds. Agena B was the second stage to
restart in orbit (Able Star was first), but it was the first turbopump powered stage to
Thor Agena B flew from three VAFB pads, flying 17 times in 1961 and 18 times in 1962.
Most of the NASA launches took place a year or two after Thor Agena B was done with its
Pentagon work. The NASA launchers appeared to use Agenas that were similar to the three
Ferret launchers, with longer cylindrical equipment sections rather than the partially
tapered sections used by the KH Agenas.
8. Thor DSV-2D
Two Thor DSV-2D vehicles launched suborbital "Big
Shot" missions from Cape Canaveral in 1962. They boosted 135 foot diameter Echo
balloons above the atmosphere to test balloon deployment methods for subsequent planned
orbital launches. This was NASA's Applications Vertical Test Program (AVT), better known
as "Big Shot". A cylindrical Equipment Section, topped by a DA-92 shroud built
by Douglas, were added to the now-standard DM-21 Thor, which was also used as the first
stage for Thor Ablestar and Thor Agena B. After the boost phase, the shroud and a balloon
deployment canister were jettisoned at an altitude of about 250 nmi. During the coast to
1,000 nmi, the balloon inflation experiment was performed while TV and film camera's in
the Equipment Section recorded the result. During the first launch, the balloon ruptured
during inflation. The second launch produced a successful inflation.
9. Thor DSV-2E
DSV-2E Thors were DM-18A IRBM Thors modified to perform
live exoatmospheric thermonuclear warhead launches from Johnston Island in the Pacific
Ocean as part of Operation Fishbowl during 1962. The Thors carried instrumented
"pods" attached to the side of their propulsion sections. The pods were released
at intervals during the boost phase to gain differential separation from the exploding W49
or W50 warhead. After reentering, the contaminated pods floated beneath parachutes to the
Pacific and were recovered. The Thors flew from a pair of tactical launchers (Launch
Emplacements 1 and 2) set up near one corner of the tiny island. During launches, most
Johnston Island personnel had to be evacuated to ships standing offshore.
The tests taught the U.S. about the effects of exoatmospheric nuclear explosions, both on
orbiting satellites and on ground-based communications and power systems. The project
produced dazzling nuclear effects, but it also suffered a series of disastrous failures.
There were eight DSV-2E launches, seven with live warheads. Four of the seven
"live" launches failed.
The first Fishbowl launch was a successful R&D flight with no warhead. The second
launch, carrying an active warhead, was "lost" by a defective range safety
tracking radar and had to be destroyed 10 minutes after liftoff. Three subsequent Thors,
all carrying nuclear warheads, suffered propulsion system failures and had to be destroyed
by range safety. Two of those destructions occurred downrange, a minute or more into
flight, dropping some radioactive contamination on and near Johnston Island. The third
failure, on July 25, 1962, was a true Cold War disaster.
Thor 180, the missile for that "Bluegill Prime" shot attempt, was fitted with a
W50 thermonuclear warhead capable of producing a 400 kiloton explosion. A propellant valve
stuck at ignition, causing a leak that fed a rapidly expanding fireball that enveloped
Thor on its launch pad. The range safety officer fired the destruct system, destroying the
Thor, the warhead, and the launch emplacement, which burned for some time, contaminating
the island. Despite several subsequent cleanup efforts, Johnston Atoll, managed by the
U.S. Fish and Wildlife Service since 2000, is still affected.
In the end, Operation Fishbowl only produced three successful high altitude explosions.
One of these, Starfish Prime on July 9, 1962, was a 1.4 megaton explosion, created by a
W49 warhead at an altitude of 400 kilometers. It created a fireball and artificial aurora
visible in Hawaii, along with an electromagnetic pulse that disrupted power and
communications. It also pumped enough radiation into the Van Allen belts to destroy or
seriously degrade seven orbiting satellites.
Two attempts took place during the midst of the Cuban Missile Crises, on October 16 and
26, 1962. The latter Bluegill Triple Prime shot, which detonated a W50 warhead at 48 km,
almost unbelievably took place while SAC was at DEFCON 2.
The final Fishbowl launch carried the "Kingfish" 400 kiloton warhead up to its
98 km detonation altitude. Kingfish was one of the last above-ground U.S. nuclear tests,
because the U.S. and the Soviet Union signed an atmospheric test ban treaty shortly
Thor-Agena D used a "standardized" Agena D
upper stage that was designed to fly atop Thor, Atlas, and Titan with minimal changes.
Agena D used an improved Bell 8096 restartable engine, still producing 7.26 tonnes thrust
but now with higher specific impulse. It also used a Bell Telephone Laboratories (BTL) 600
radio guidance system.
Thor-Agena D flew 21 times during 1962 to 1967 from Vandenberg AFB pads 75-1-1, 75-1-2,
75-3-4 and 75-3-5, carrying Keyhole 4 and 5 film return spysats, DSAP Block 1 military
weather satellites, Poppy electronic intelligence satellites, and experimental U.S.Navy
satellites among others. Four launches failed, and a fifth placed Poppy 1A/1B in a
too-high orbit when Agena failed to cut off as planned. The satellites still functioned,
but the orbit limited data collection.
A, B, C, C1
NASA/Goddard couldn't ignore the success of the original
twelve Thor-Deltas, so it ordered more. These featured incremental improvements,
introduced pretty much "one at a time". The method worked. Only 2 of these 24
Deltas would fail outright.
Delta A introduced MB-3 Block 2 engines that produced 170 klbf liftoff thrust. Delta A,
which flew twice in 1962, also featured a shorter interstage between Thor and Able to
shave weight. The improved Delta actually stood 4 to 5 feet shorter than the original
Delta B, which flew nine times during 1962-64, introduced a 36 inch second stage stretch
and improved AJ-10 engine performance. This variant launched Explorer 17, TIROS 7 and 8,
Relay 1 and 2, Telstar 2, and Syncom 1 and 2. Syncom 1 was the first launch to GTO, though
the satellite was lost during its apogee motor firing. Syncom 2 succeeded, reaching an
inclined geosynchronous orbit.
Delta C added a more powerful ABL X-258 Altair 2 third stage during its 11
flight, 1963-67 run. It orbited more Explorers and a string of early solar observatories
and weather satellites. Remarkably, Delta boosted the TIROS 9 and 10 and ESSA 1
weathersats into near sun synchronous orbits - from Cape Canaveral, Florida! The flight
paths doglegged south, crossing Cuba and Panama before the third stage fired over the
equator just northwest of South America to complete the insertion. (Delta would not fly
from Vandenberg AFB until 1966.)
Delta C1 added an even more potent United Technologies FW-4D third stage motor during its
two launches. Delta 64, the 38th and final Thor-Delta with the 32 inch diameter
Vanguard-derived second stage, orbited NASAs fifth Orbiting Solar Observatory (OSO
5) on January 22, 1969. This was the final Thor-Delta to fly without strap-on solid
motors. The old Vangaurd stage scored its 37th consecutive "Delta" orbital
success on that flight, a relic of the early Space Age surviving to fly even after NASA
had launched astronauts atop massive Saturn V boosters a few miles up the Florida coast.
Thor-Agena was beefed up with the addition of three
Thiokol Castor 1 solid rocket motors beginning in 1963. These "Thrust Augmented
Thor" (or TAT) boosters lifted two Agena B, and 61 Agena D, stages with payloads
toward orbit from 1963 until 1968. The Castor 1 motors, derived from the Sergeant missile
motor, nearly doubled the liftoff thrust compared to Thor-Agena. The solids burned for
about 40 seconds, with the last dozen seconds comprising a tailoff. They were jettisoned
at T+65 seconds to reach a safe drop zone. The boosters augmented the upgraded MB-3 Block
3 Thor first stage engine, which itself burned for nearly 150 seconds.
TAT-Agena D would become the most-oft flown U.S. Air Force Thor space launch vehicle. It
could lift roughly 1.5 tonnes to polar orbit, including the Agena stage. The vehicle's
busiest year was 1964, when 20 launches occurred from Vandenberg.
Five pads, 75-1-1, 75-1-2, 75-3-4, 75-3-5, and PA-1-1, a former Atlas Agena pad on the
U.S. Navy test facility at Point Arguello (incorporated into Vandenberg as South
Vandenberg after 1964), handled TAT Agena D launches.
For the first time, Keyhole 4A imaging satellites equipped with two film return
"buckets" were flown, accounting for the majority of launches. TAT-Agena D also
orbited electronic intelligence satellites, the Quill 1 experimental radar mapping
satellite, and three NASA payloads (OGO 2 and 4 and PAGEOS). TAT-Agena B orbited NASA's
13. Thor ASSET
Thors lofted six ASSET (Aerothermodynamic/elastic
Structural Systems Environmental Tests) lifting body reentry experiments on suborbital
flights from Cape Canaveral/Cape Kennedy during 1963-65. These were U.S. Air Force
missions that evaluated reusable, maneuverable, re-entry vehicle designs that might be
able to fly to a precise landing point on earth. McDonnell Aircraft of St. Louis built the
ASSET vehicles. The original reason for the program was to support X-20 development, but
ASSET continued after X-20 was cancelled. One of the ASSET reentry vehicles was recovered
after parachuting to an ocean landing. Two other recoveries were attempted. Significant
telemetry hauls were made even when the vehicles were not recovered.
Single-stage Thors, retired UK IRBMs returned to Tulsa, Oklahoma for refurbishment,
performed three of the flights as "DSV-2F" variants. Two-stage vehicles that
also used retired Thor first stages topped by what were essentially Delta B type second
stages performed the other three flights, as "DSV-2G/Delta" vehicles. The Delta
second stage was only partially loaded with propellant for a relatively short 50-60 second
burn. The only mission failure occurred during the first DSV-2G launch when the second
stage failed to ignite. The final ASSET launch on February 22, 1965 was the last U.S. Air
Force Thor flown from Cape Canaveral.
Thor DSV-2J vehicles were retired Thor DM-18A IRBMs that
were refurbished for use in ASAT Program 437 and its follow-ons. A total of 17 of these
Thors performed suborbital flights between 1964 and 1975. The launches were from two
Johnston Island launch emplacements originally built for Operation Fishbowl.
Program 437 Thors were designed to pass within 3 nmi of an orbiting satellite where the
missile's 1.44 mT W49 warhead would destroy the satellite. Test launches were performed
with dummy warheads against orbiting U.S. upper stages and satellites. A total of nine
launches took place. During the initial tests, two Thors would be counted down
simultaneously to ensure that at least one would meet the short launch window. (A sizable
staff was required. Personnel rotated between Johnston Island and a training pad at
Vandenberg AFB.) The ASAT system stood active watch from 1964 until 1970 (longer than
Thors had stood IRBM duty), then was placed on disassembled standby until 1975.
Program 437AP (Alternate Payload) carried a camera system rather than a warhead, to
photograph orbiting satellites. The camera and film reentry vehicle was adapted from
Corona KH-4. Only four launches took place during 1965-66, two of which apparently
succeeded in photographing orbiting U.S. objects.
After the ASAT program stood down, two one-off launches took place in 1970. The first, for
Program 922 (former 437Y) was an ABM sensor test against a Minuteman 2 RV launched toward
Kwajalein. The Thor shut down 6 seconds early, preventing an intercept, which would not
have occurred regardless because, first, the payload and Thor collided after separation
and, second, the Minuteman RV failed to separate from its upper stage! I haven't seen any
photos of this Thor, but it was said to have a separable maneuvering payload.
The second launch, for the High Altitude Program (HAP), was completely successful. This
Thor's heavy payload, which created a simulated nuclear explosion for an on-board X-ray
detector to sense, was housed in an Agena-like shroud, which required installation of a
special service tower at LE-2. (I wonder if the tower might also have supported the 922
launch.) A reentry pod returned film or data or both.
After four years of non activity, Johnston Island hosted two final Thor launches in 1975.
These BMDTTP (Ballistic Missile Defense Test Target Program) launches served as targets
for Kwajalein ABM radars. I haven't seen any photos of these vehicles, but I have read one
report that they were in their IRBM configuration. They were the final suborbital Thor
15. Delta D (TAD)
Goddard Center adopted solid motor thrust augmentation
to Delta about 1.5 years after it was proven by the U.S. Air Force TAT-Agena D. Adding
three Castor 1 SRMs to the DSV-3C Delta C model created "Delta D" (DSV-3D), also
known as "Thrust Augmented Delta" (TAD). The engine skirt and engine section
were slightly modified to support the SRM loads. TAD jumped off its pad with a nearly 2.37
thrust to weight ratio. This variant only flew twice, but both launches were historic.
Delta 25 boosted Syncom 3 to GTO on August 19, 1964. The extra boost allowed for a 16.5
deg GTO. Syncom 3's own apogee kick motor and thrusters then had enough energy to boost
itself into the first-ever geostationary orbit. The satellite was positioned above the
International Date Line, where it relayed coverage of the 1964 Tokyo Olympics.
Delta 30 performed a similar launch, on April 6, 1965, for "Early Bird", the
first commercial communications satellite. Early Bird, or Intelsat 1, was similar to the
first three Syncoms, but provided 240 telephone circuit equivalent service (or one TV
channel). This garbage-can size satellite substantially increased trans-Atlantic circuit
capacity, decisively changing how long-range telecommunication service would subsequently
be provided. Early Bird functioned for four years before being retired.
Thor Burner 1 (originally "Thor Altair"), was
developed to orbit Defense Meterological Satellite Program missions (originally Defense
Satellite or Systems Application Program) to support NRO Corona missions. The early DSAP
satellites, built by RCA, were derived from the original TIROS spinners. Scout, intended
to launch these satellites, was still struggling at the time (three of five DSAP launches
failed), so plans were made to top refurbished retired Thor IRBMs with Scout
"Altair" fourth stages and payload "heat shields". The move was costly
for LTV and NASA, which saw eight planned Scout launches canceled.
Two types of motors and heat shields ended up flying atop Thor Burner 1. The first two, in
1965, used Lockheed/Grand Central Rocket Co. MG-18 motors and 25.7 inch diameter heat
shields from the two already built Scouts assigned to the program. The final four in
1965-66 used UTC FW-4S Altair 3 motors and 34 inch diameter heat shields.
Douglas Aircraft replaced the Thor guidance section with a shorter, lighter set of
adapters and swapped inertial for lighter BTL radio guidance. The company also added a
cold gas attitude control system atop Thor to provide stability during the coast to apogee
and the proper attitude for second stage spin-up and separation. I'm looking for details
on this system, which might only have been used by the FW-4S vehicles that went to higher
Launches took place from VAFB 4300 B6 (former 75-2-6, later SLC 10W) to boost the tiny
satellites into sun synchronous orbits. SAC's 4300 support squadron performed the
launches. Four of the six launches were successful. The payload heat shield failed to
separate during the first launch and the second stage motor failed to start during the
Subsequent "Thor Burner 2" types continued the DMSP launches until 1980, as we
17. Delta E-H (TAID)
After Thrust Augmented Delta, the next logical step was
to take advantage of all that thrust by increasing the second stage mass. That step, taken
in 1965, created Thrust Augmented Improved Delta (TAID). Douglas borrowed the Able Star
tanks, stretched them, and added an improved AJ10-118E engine to create the second stage.
This roughly doubled the burn time compared to the previous Vanguard-based Delta stage,
increasing payload to orbit. The TAID second stages performed a single burn, then provided
attitude control during a coast prior to spin-up and separation of the third stage.
Both Castor 1 and Castor 2 motors could be used with the TAID series. Castor 2 provided
slightly less liftoff thrust but burned slightly longer. An MB-3 Block 3 Rocketdyne engine
powered Improved Delta's Thor first stage. MB-3-3 improved reliability and a bit more
Third stage options included the ABL-258 and FW-4D spin-stabilized solid motors. With
ABL-258, Improved Delta was called "Delta E". With FW-4D it was "Delta
E1". The rocket was identified as "Delta G" when no third stage was used.
"Delta J", with a Star 37D third stage motor, flew once. (Unflown types included
"Delta F/F1", which was Delta E/E1 without SRMs, and "Delta H", which
was Delta G without SRMs.) An Agena-style 65 inch diameter payload fairing topped the
rocket, providing much more internal volume than earlier Delta shrouds.
Twenty six TAID launches took place during 1965-71. Every single one succeeded. Launches
took place from Cape Canaveral and Vandenberg AFB, marking Delta's introduction to the
Payloads included Intelsat 2 communication satellites, NASA Explorer and Pioneer
satellites, ESSA weather satellites, HEOS 1, ISIS 1-2, and Biosat 1-2. The Intelsats
separated into GTO and raised themselves into GEO. The Pioneers went into solar orbits.
Most of the Explorers were launched into highly elliptical Earth orbits. Explorer 35
(IMP-E) inserted itself into lunar orbit after a precise TAID launch. Delta boosted HEOS 1
into a 440 x 230,000 km x 28.3 deg orbit. The satellite's apogee kick motor subsequently
raised its perigee to 6,800 km.
TAID was the first Delta to sport the soon-familiar "Delta" triangle logo.
In 1961, Douglas Aircraft proposed a series of Thor
upgrades for space launch. The "Thor Advanced" (Thorad) concept called for a
constant diameter airframe that would eliminate the tapered IRBM LOX tank and increase
propellant capacity. Three Sergeant solid motors would augment thrust. Before Thorad would
fly, Douglas would add the solids to a standard Thor to create "Thorad Junior"
(better known as TAT Agena-D). Replacing the MB-3-3 engine with a higher thrust Saturn H-1
engine would create "Thorad B". These proposals were eventually realized.
Thorad-Agena D (also Long Tank Thrust Augmented Thor-Agena D), had the constant 96 inch
diameter airframe and a 14 foot stretch. Stage weight increased 45% to nearly 70 tonnes
and burn time increased to 218 seconds MECO/227 seconds VECO. Three Castor 2 SRMs roughly
doubled the liftoff thrust provided by the first stage engine. A new adapter section
topped the stage, allowing continued use of the existing transition section and Agena D
adapter. This resulted in a distinctive three-step taper between the first and second
There were 43 Thorad Agena D launches, with three failures, from Vandenberg AFB between
1966 and 1972. Launches took place from SLC 1W, 1E, 2E, and 3W, the former 75-3-4, 75-3-5,
75-1-1, and PA-1-1. Payloads included double bucket Keyhole 4A and 4B satellites, Poppy
and Strawman signals intelligence satellites, Nimbus weather satellites, OGO 8, and the
remarkable SERT 2.
The Nimbus satellites were powered by SNAP-19 RTGs. On May 18, 1968, Nimbus B was lost
when Thorad's control system failed about two minutes into the flight. Unlike earlier SNAP
RTGs, SNAP-19 was designed to survive launch vehicle failure, and it did. The RTG was
eventually salvaged from the Pacific and its nuclear material reused.
Two Thorad-Agena D variants are listed. SLV-2G (1966-71) used a DSV-2L first stage. SLV-2H
(1969-72) used a DSV-2L-1A first stage. The stages were identical in external appearance.
It is possible that the "1A" was only added to differentiate Thorad-Agena D from
Thorad-Delta (which used the DSV-2L-1B stage). At the time, newly merged McDonnell Douglas
was shifting manufacturing from the original Santa Monica Thor factory to Huntington
Beach, where the Thorad stages came off a common production line.
Thor 571, launched May 25, 1972, was the final Thorad-Agena D and the last U.S. Air Force
Thor-based stage manufactured, although space launches of converted Thor IRBM missiles
would continue for several more years.
In March 1964, the DMSP program office approved plans to
develop a more powerful Thor Burner 2 launch vehicle that still used repatriated Thor
Burner 2 used a Thiokol Star 37B motor (TE-M-364-2, a modified Surveyor retro-rocket
motor) to power the "Burner 2" second stage. Boeing's Burner 2 stage was built
around the Star 37B. It had a strap-down inertial guidance system and a 3-axis reaction
control system, allowing it to coast without spin stabilization. Four 10 kgf hot-gas
hydrogen peroxide thrusters performed stage separation, provided pitch and yaw reaction
control thrust during the Star 37B motor firing, and completed a vernier maneuver
immediately after the Star 37B burn. Eight 1 kgf gaseous nitrogen cold-gas thrusters on
the stage provided pitch-yaw-roll attitude control during coast and performed spacecraft
spinup and post-spacecraft separation maneuvers. Thor Burner 2 was topped by a new
Goodyear conical phenolic shroud that enclosed the upper stage and payload. The unpainted
fairing was distinctively vermilion (orange-red) in color.
Thor Burner 2 flew 12 times from September 16, 1966 to June 8, 1971, carrying 10 DMSP
Block 4 and 5A satellites and performing a pair of U.S. Air Force Space Test Program (STP)
missions. All 12 launches were successful. One of the STP missions, flown on June 29,
1967, used a Star 13A apogee kick motor to insert Aurora 1 and SECOR 9 into a 3,792 x
3,947 km x 90.1 deg polar orbit.
Thor Burner 2A added a third stage and a modified fairing to the Thor Burner 2 design. A
Star 26B motor served as the third stage motor. The 3-axis Burner 2A control bus was built
around the Star 26B. A Star 37B motor served as the second stage, with the 3rd stage bus
providing guidance and control during its burn. The shroud was extended by the addition of
a cylindrical section. Thor-Burner 2A performed eight launches with DMSP Block 5B and 5C
satellites between October 14, 1971 and February 19, 1976.
The final launch failed because the Thor was not loaded with enough kerosene fuel, causing
Thor to burn out a few seconds early. The upper stages performed their burns, but the end
result was insufficient velocity to maintain a stable orbit. A old mixture ratio
typographical error on the LR79 main engine certification testing data sheet was deemed
responsible for the improper fuel load.
20. Long Tank
In 1968, NASA gained access to the stretched Long Tank
Thor stage already proven as an Agena booster. Long Tank Thor served as the first stage
for Delta Models L, M, N, M6, and N6, which all used the TAID second stage. L, M, and N
used a trio of Castor 2 strap-on solid motors to augment liftoff thrust. M6 and N6 used
six SRMs with three ground-lit and three air-lit. All motors jettisonned in sets of three
beginning after T+90 seconds.
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 motor. Delta N did not have a
third stage. Deltas M6 and N6 were the six SRM versions (also called "Super
Six") of Deltas M and N.
Delta 58, an "N" carrying the Tiros 17 weather satellite from Vandenberg on
August 16, 1968, was the first Long Tank Thor Delta (also called "Thorad Delta"
or "Long Tank Thrust Augmented Thor Delta"). Altogether, there were twenty-four
L, M, N, M6, and N6 flights, with four failures, during 1968-72. They launched eight
Intelsat 3, two Skynet 1, two NATO 2, two ESSA, two OSO, and three ITOS/NOAA satellites.
Single launches included HEOS 2, IMP I (Explorer 43), Biosat 3, and TD-1A.
Delta 59, the first "M" with Intelsat 3-1, failed on September 18, 1968 from
Cape Kennedy. The rocket suffered a pitch rate gyro failure that became noticeable about
20 seconds after liftoff. It began to break up at T+102 seconds. The range safety officer
sent a destruct command 6 seconds later.
Delta 71, another M, 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
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
MB-3-3 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.
Delta 86, an N6, 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.
Delta 85, an N, nearly failed after 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 low earth 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
Two transitional Long Tank Thor Delta models flew in 1972-73. Delta 300 used three Castor
2 strap on boosters and a modified second stage powered by a more-powerful AJ10-118F
engine derived from the Titan 3 Aerojet Improved Transtage Engine Program (ITIP). 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 (six ground-lit) and
the same second stage. Both models used the Universal Boat Tail. Neither flew with a third
stage. Both used the Delta Inertial Guidance System (DIGS). Prior Deltas had 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. Successes included NOAA 2 and 3, ERTS 1, and Nimbus 5.
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.
During seven years of service, 72 Long Tank Thor launches occurred, boosting 29 Delta and
43 Agena missions. Long Tank served as the basis for the follow-on Extended Long Tank
Extended Long Tank Delta
Extended Long Tank (ELT) Delta flew 93 times during 1972-1990, succeeding 89 times and
orbiting all manner of important payloads.
Extended Long Tank included a roughly 120 inch first stage tank stretch compared to the
Long Tank stage. It flew with both MB-3-3 and RS-27 engines. Rocketdyne's RS-27 was
essentially a repackaged H-1 engine salvaged from the large inventory of unflown Saturn IB
engines. A total of 83 RS-27 engines flew.
Three different second stages flew atop ELT Deltas.
The first was the AJ10-118F powered stage that was a holdover from Long Tank Delta. It was
topped by the 65 inch diameter Agena shroud.
The second was powered by the TR-201 TRW Lunar Module descent engine derivative. This
stage was the first to be suspended within an extended interstage cylinder, through the
use of a "Miniskirt". The design allowed use of a 96 inch diameter payload
fairing. It was informally named "Straight Eight" because, for the first time,
the entire launch vehicle had a constant eight foot diameter.
The third was the AJ10-118K powered "ITIP" (Improved Transtage Injector Program)
stage that began flying in 1982. This stage used fatter tanks originally developed for
Japan's N-2 launch vehicle. Today's Delta 2 second stage is similar.
Spin-stablized Star 37D, 37E, and 48B third stage motors flew atop ELT Deltas aimed beyond
LEO. During the Shuttle era when Delta served as an STS backup, NASA carded Delta
3910/PAM-D and Delta 3920/PAM-D variants. The PAM stage, a Star 48B spin-stable solid
motor, was considered part of the "payload" just as it was on Shuttle.
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.
ELT Delta's used the four-number model identification system.
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)
22. Thor Star 37/Star 37/ISS
Thor Burner 2A could not lift DMSP 5D1, a much upgraded,
heavier military weather satellite. Two Star 37 upper stage motors were needed to reach
orbit. Thus, the final five Thor LV-2F flights, launched between September 11, 1976 and
July 14, 1980, flew as Thor Star 37/Star 37/ISS launch vehicles. "ISS" stood for
"Integrated Stage System", a hydrazine-based propulsion system on the satellite
that provided 3-axis control during the solid motor burns and a final trim burn. A Star
37XE motor served as the second stage while a Star 37S-ISS acted as the third stage. A
longer payload fairing with a blunter nose housed both stages and the payload. The
upgraded launch vehicle could lift roughly 500 kg to the DMSP sun synchronous orbit.
The first four launches were good, but the July 14, 1980 finale was a disheartening
failure. Refurbished IRBM Thor 304 flew true, and the first Star 37 burn looked good, but
at Stage 3 startup all telemetry was lost. It was subsequently determined that connectors
between the second and third stages had not disconnected due to a misalignment. When the
Star 37S motor ignited, the wiring harness was jerked out of the third stage and
satellite, killing the flight control system. The stage pitched down and failed to
generate sufficient orbital velocity. It turned out that an incident during launch vehicle
erection - a broken pin that caused the rocket to suddenly drop a few centimeters - had
most likely caused the connector misalignment.
Thor 304 was the final Thor IRBM to fly, and the final launch from Space Launch Complex 10
23. N-1, N-2,
From 1975 through 1992, it was possible to see
"Delta" lookalike rockets liftoff from Tanegashima, Japan. A total of 24
Thor-based rockets, assembled in Japan under license from the U.S., flew for the National
Space Development Agency of Japan (NASDA). There were three variants, all unique to Japan.
N-1 ("N" stood for "Nippon"), launched from 1975 through 1982, was an
MB-3-3 powered Long Tank Thor Delta with three Castor 2 boosters. An MHI built LE-3
pressure-fed hypergolic engine powered the second stage. A Star 37N solid motor served as
the third stage. An Agena shroud topped the rocket. N-1 could lift 1.2 tonnes to LEO or
0.36 tonnes to GTO. It flew 7 times with one failure. Notable successes included Japan's
first geostationary orbit launch, of Kiku 2 (ETS-2) on February 23, 1977. The lone failure
occurred on February 6, 1979 when the fifth N-1's Star 37N third stage collided with its
Experimental Communications Satellite (ECS-A) satellite payload shortly after spacecraft
N-2, which flew 8 times during 1981-87, used an MB-3-3 powered Extended Long Tank Thor
stage augmented by nine Castor 2 strap on motors. The second stage was powered by a
restartable Aerojet AJ10-118FJ pressure-fed engine (after NASDA's planned LE-4 engine
stumbled during development). The stage used new fatter tanks that would later be adopted
by NASA's 3920 series and later Deltas, including today's Delta 2. Star 37E served as a
third stage motor. N-2 used DIGS inertial guidance. It could lift 2 tonnes to LEO or 0.73
tonnes to GTO.
H-1, which flew 9 times during 1986-92, introduced a new NASDA-developed common bulkhead
liquid hydrogen fueled second stage that was powered by a brand new NASDA-developed LE-5
engine built by MHI and IHI. The rocket was controlled, for the first time, by an inertial
guidance system developed in Japan. H-1 could lift 2.25 tonnes to LEO or 1.1 tonnes to
For a time during the post-Challenger accident period, McDonnell Douglas considered
adopting Japan's upper stage, or at least the LE-5 upper stage engine, for use on U.S.
Delta launch vehicles.
During the mid-1980s, McDonnell Douglas shut down the long-running Delta production line
as NASA moved payloads to Space Shuttle. The 1986 Challenger disaster changed everything.
Soon, the U.S. Air Force was asking for an expendable launcher that could orbit the GPS
satellites originally slated for STS. McDonnell Douglas won the resulting Medium Launch
Vehicle competition over General Dynamics (Atlas K), Martin Marietta (Titan 3 Commercial),
and Hughes (Jarvis).
After pondering use of Japan's H-1 liquid hydrogen upper stage, McDonnell Douglas pulled
an early 1980s proposal off the shelf to create "Delta 2". The rocket was built
around a stretched "Extra Extended Long Tank" first stage that was 148 inches
longer than the "Extended Long Tank" version. It carried 96 tonnes of
propellant, a 16 tonne increase.
The first, interim Delta 6000 series vehicles used RS-27 engines and upgraded steel-case
Thiokol Castor 4A strap on motors. The ultimate Delta 7000 series rockets used new RS-27A
engines that were more efficient in vacuum and new GEM-40 Graphite Epoxy Motors developed
by Hercules. All versions used the existing Aerojet AJ10-118K ITIP-powered second stage.
Star 48B served as the third stage for GPS missions. A new 9.5 foot diameter standard
fairing housed most payloads. The old 8-foot fairing flew a few times. McDonnell-Douglas
also developed a 10 foot diameter metal shroud based on the company's Titan 3C fairing.
Delta 2 initially used the existing DRIMS guidance and control system, but on December 30,
1995 Delta 230 became the first to use the Redundant Inertial Flight Control Assembly
(RIFCA). RIFCA, built around six ring laser gyroscopes and six accelerometers, provided
triple redundant guidance, flight control and mission sequencing functions.
The first Delta 2, a 6925 identified as Delta 184, orbited GPS-2 1 from Cape Canveral LC
17A on February 14, 1989. Seventeen 6000-series Deltas flew. Delta 212, the last in 1992,
was the final flight of a Saturn H-1 derived RS-27 engine. Delta 201, the first 7925,
orbited the first of the heavier GPS-2A satellites on November 26, 1990. All told, Delta 2
performed 49 GPS launches with one failure.
In December 1994, NASA requested bids for a Medium Light Expendable Launch Vehicle
(Med-Lite). McDonnell-Douglas's offered Delta 732X and 742X, which used three and four
GEM-40 boosters respectfully. In 1997 a new 10 foot diameter composite payload fairing
(10C) began flying for Iridium. A stretched 10L version was developed for NASA beginning
in 2002, ending use of the Titan-derived fairing.
From 1989 through 2011, Delta 2 was the most often-flown, versatile, productive, and
reliable U.S. launch vehicle. It became the longest-lived essentially unchanged U.S.
launcher, even though its ownership and production sites moved twice. It flew from three
launch pads at two launch sites. It was common to see multiple Deltas stacked
simultaneously. 151 Delta 2 rockets flew by the end of 2011 with two failures, making
Delta 2 one of the most successful orbital launchers in history. In addition to its
bread-and-butter GPS work, Delta 2 orbited commercial, non-U.S. government, and NASA
For NASA, Delta 2 did something no previous Thor/Delta had done - it reached into deep
space, to Mars, and to asteriods. The list of payloads includes Mars Pathfinder, Odyssey,
Spirit, Deep Impact, and many others. Commercial payloads included Iridium and Globalstar
"little LEO" satellites.
In 2003, Delta 7920H and 7925H began flying. They used powerful GEM-46 solid rocket motors
reassigned from the terminated Delta 3 program (about which more soon). They launched the
Opportunity Mars rover, the SIRTF/Spitzer space telescope, the planet Mercury orbiter
MESSENGER, asteroid Vesta and Ceres orbiter Dawn, gamma ray telescope GLAST, and lunar
orbiters GRAIL A and B.
Although Delta 2's Cape launch site was closed after 2011, the Delta 2 story is not over.
Two more launches from Vandeberg AFB SLC 2W remain on tap. If both succeed - never a given
- the rocket's consecutive success string would reach 100.
25. Delta 3
During the post-Challenger era, McDonnell Douglas
studied liquid hydrogen fueled upper stages for its Delta launch vehicle. In 1986, it
briefly considered Japan's H-1 LH2/LOX upper stage for the U.S. Air Force Medium Launch
Vehicle (MLV) program before deciding on the "Delta 2" approach. Two years
later, it proposed a new LH2/LOX upper stage to be built by Martin Marietta for the MLV-2
program (won by GD Atlas 2). The company continued to study the idea until, on May 10,
1995, it announced that it would develop "Delta 3" using more than $200 million
of its own funds, with a planned first launch in 1998.
Soon, Delta 3 held contracts for 18 launches through 2002, including NASA/NOAA GOES N, O,
and P and five ICO Global Communications launches.
Delta 3 used a 4 meter diameter "Delta Cryogenic Upper Stage" (DCUS) and
more-powerful Alliant 46 inch diameter Graphite Epoxy Motors (GEM-46) to lift the heavier
stage and payload. The first stage had a shorter but fatter kerosene fuel tank (4 meters
rather than 2.4 meters diameter) so that Delta 3 would fit within the Delta 2 service
tower. A Pratt & Whitney RL10B-2 engine with a large Snecma-built extendible nozzle
powered DCUS. At liftoff, the boosters and RS-27A main engine would together produce more
than 1 million pounds of thrust. The rocket could lift 3.8 tonnes to GTO.
Delta 3 was assembled in Pueblo, Colorado. Japan's Mitsubishi Heavy Industries made the 4
meter diameter second stage liquid hydrogen tank and first stage kerosene tank using tank
tooling from its H-2 stage.
On December 15, 1996, McDonnell Douglas and Boeing announced their intention to merge
under the Boeing name. The merger was consummated on July 1, 1997. The merger would have
decisive consequences for the Delta 3 program, though that was not initially apparent.
A relatively smooth development program was followed by a troubled flight program. Delta
259, the first Delta 3, launched from Cape Canaveral SLC 17B (the only pad rebuilt for the
type) on August 27, 1998 with the Galaxy 10 communications satellite. At about T+50
seconds, the rocket began to suffer 4 Hertz roll oscillations, using up the GEM-46 TVC
hydraulic fluid. The rocket pitched over and broke apart at T+72 seconds. Flawed roll
control equations were found to be the cause.
The second Delta 3, Delta 269, launched with the Orion 3 communications satellite on May
5, 1999. This time the flight proceeded flawlessly through the first RL10B-2 burn, pushing
the stage and payload into a parking orbit. After a coast period, the RL10B-2 engine
restarted for a planned 162 second burn, but it shut down after only 3.4 seconds,
stranding Orion 3 in LEO. An investigation found that the RL10B-2 engine's combustion
chamber had burst during the restart due to defective brazing of a welded reinforcing
strip. Pratt & Whitney subsequently modified its brazing process and its inspection
Delta 3 finally succeeded on August 23, 2000 when Delta 280 launched the 4,348 kg DM-F3
mass simulator to subsynchronous transfer orbit. By then, however, Boeing was committed to
its Sea Launch commercial launch partnership and had begun to develop Delta 4 for the EELV
program. In 2000, Boeing bought Hughes Space & Communications, the satellite builder
that held the bulk of the Delta 3 backlog. Then the commercial satellite market collapsed,
leaving Boeing deeply overextended. Something had to give, and the first of those
somethings was Delta 3, which was quietly shut down after Delta 280.
In the end, Delta 3's primary accomplishment was to prove RL10B-2 in flight for Delta 4,
and to prove DCUS, which was the first all-new high energy upper stage developed in the
U.S. since the 1960s. The Delta 4 Medium "Delta Cryogenic Second Stage" was
largely derived from DCUS.