Seventh in a Series Reviewing Thor Family History
by Ed Kyle, Updated 12/14/2017
Delta 22, a Thor-Delta B with a Bulbous Fairing, Orbited
Tiros 8 from LC 17B on December 21, 1963
With 325 launches by the end of 2008, Thor-Delta (commonly known
as just Delta) is the most oft-flown U.S. orbital launch vehicle family.
With only 14 failures among those 325 flights, Thor-Delta has also proved to be the
most-reliable U.S. orbital launcher. By some measures, it is the worlds most
It wasnt supposed to be so long-lived. The original
Thor-Delta was only expected to fly for two years as an interim solution to
NASAs launch needs.
When the final tally is made, Lockheeds Agena upper stage
will have flown more times than Delta, but not using the same first stage.
Agena was boosted by three different launch vehicles (Thor, Atlas, and Titan).
Delta has used Thor-derived boosters since its inception.
Delta first stages have, since the late 1960s, evolved
considerably from the original Thor design, to the point that they are no longer
officially called "Thor" stages. The careful management of that evolution
and of upgrades to the Delta upper stages, performed in a systematic, step-by-step,
risk-lowering process, is a big part of the Delta success story.
Delta II, the final Thor-Delta family variation, was
developed for the U.S. Air Force. In the beginning, though, Delta was NASAs
An Early Thor-Delta with a Low Drag Fairing
One of the first things that the National Aeronautics and Space
Administration (NASA) did after it was created on October 1, 1958 was to assess its
orbital launch needs. At first, NASA used hand me downs from the U.S.
Army (Juno I and II), Navy (Vanguard), and Air Force (Thor-Able), with mixed results.
In January 1959, NASA announced plans for its own launch fleet.
The Agency would develop two new unmanned launchers - Atlas-Vega and Atlas-Centaur
and two big boosters, Saturn and Nova, for manned missions. Vega, a
kerosene/LOX stage powered by a modified Vanguard first stage engine, was quickly shelved
in favor of Agena, being developed, unbeknownst to NASA when it performed its study,
by the Air Force.
NASA also decided to create an interim launch vehicle
based on Thor-Able that would fly only until Centaur and Vega/Agena were ready.
NASAs Milton Rosen, Director of Launch Vehicles and Propulsion in NASAs Office
of Manned Space Flight at NASA Headquarters, named the launcher. He chose
Delta because it would be the fourth Thor-based space launch vehicle after
Thor Able, Thor Able-Star, and Thor Agena.
Milton Rosen had led the Naval Research Laboratorys Viking
sounding rocket program from 1947 to 1955. After that, he was technical director of
Project Vanguard, the NRL scientific satellite program that started where Viking left
off. He transferred to NASA in October 1958, as did 157 NRL employees with the
entire Vanguard program, to form the Agencys new Goddard Space Flight Center.
group at Goddard that was headed by William Schindler oversaw the Thor-Delta program.
Thor-Delta was, in many ways, an improved Vanguard. It
replaced Vanguards first stage with the much more powerful Thor, but kept the top
two Vanguard stages, albeit with modifications. The design followed Rosens
hard-learned philosophy of adopting existing systems with minimum modification to minimize
Thor-Delta B Drawing
The 49.34 tonne Thor DM-18A first stage was powered by a 68 tonne
thrust turbopump-fed Rocketdyne MB-3 Block 1 main engine augmented by two 0.45 tonne
thrust roll control verniers, all burning kerosene/LOX. The mostly-aluminum stage
was 2.44 meters in diameter and 18.42 meters long. The first stage was manfactured
by Douglas Aircraft in Santa Monica, California at the same factory that built Thor for
IRBMs and for launching Agena/Corona missions.
A pressure-fed Aerojet AJ-10-118 3.49 tonne thrust engine, fueled
by unsymmetrical dimethyl hydrazine (UDMH) and inhibited red fuming nitric acid (IRFNA),
powered the 4.472 tonne second stage. The slender stage, also built by Aerojet, was
5.88 meters long and only 0.813 meters in diameter. The second stage used
common-bulkhead tanks, with a cylindrical helium pressurant tank positioned between a
forward UDMH fuel tank and the aft IRFNA oxidizer tank. The tanks were made from
stainless steel to combat the corrosive effects of the propellants.
The third stage, mounted to a spin table on top of the second
stage, was an Allegheny Ballistics Laboratory (ABL) X-248 spin-stabilized
Altair solid motor. Altair, a 1.83 meter long, 0.46 meter diameter
fiberglass-case motor that only weighed 227 kg, could provide 1.27 tonnes of thrust for 38
Tiros 2 and ABL X-248 Third Stage Visible without Fairing
During RF Testing of Thor-Delta 3 on Pad 17A in Late 1960.
The third stage motor and payload were both housed within a
payload fairing. There were two fairing types. One was a low-drag type with a
constant 0.813 meter diameter and a conical nose. The other was a
bulbous fairing used for wider diameter payloads.
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 AJ-10 engine had performed its burn. This improved the
accuracy of third stage spin-up insertions.
A Bell Telephone Laboratories BTL-300 radio guidance system was
housed in an equipment compartment atop the second stage. This system, which
received commands generated by ground-based radars and computers, controlled steering
during second stage flight and during the latter half, roughly, of first stage
flight. The system also controlled the second stage cutoff. In addition to the
BTL-300, programmer autopilots provided coarse trajectory control during the
first two stages of flight.
After second stage burnout, and after the stage coasted to an
appropriate point in space, small solid motors would fire to spin up the table, giving the
third stage gyroscopic stabilization. The third stage could, as a result, fly
dumb, without need for a guidance or flight control system, during its
burn. After spin-up, the stage would be released and, after a delay of 15.5 seconds
to allow the stages to separate from each other, would ignite.
Thor-Delta could lift 295 kg to low earth orbit, or 45 kg to
geosynchronous transfer orbit. The rocket was 31 meters tall and weighed 54 tonnes
The First Twelve
Delta 1 without Fairing During RF Testing on Pad 17A
NASAs Goddard Space Flight Center ordered 12 Thor-Deltas
from Douglas Aircraft in April 1959. The first, carrying Echo 1, was launched from Cape
Canaverals Complex 17A on May 13, 1960. Unfortunately, the second stage
suffered an attitude control system failure due to an electrical short circuit.
Delta 1, like so many other early orbital launch vehicles, failed.
The second Delta flew on August 12, 1960. It successfully
lifted Echo 1A, a giant inflatable mylar balloon designed to reflect radio signals, into a
642 km x 47 deg near-circular orbit. Echo 1A was the first communications satellite,
albeit a passive, experimental one.
The third Delta, carrying Tiros 2, flew successfully. So
did the fourth with Explorer 10, and the fifth with Tiros 3. Indeed, all of the
final 11 original Thor-Delta vehicles, flown during 1960-62, succeeded. 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
This was unheard of success during an era when U.S. orbital
launch vehicles were struggling to reach orbit half the time. The success, coupled
with delays and cost overruns in the Centaur program and greater than expected demand for
Delta-class launches, prompted Goddard to order 14 more Thor-Deltas, the first of many
Delta A-D Follow-Ons
Delta 21, a Delta C, Launched NASA's Explorer 18 from Pad 17B on
November 27, 1963
Follow-on Deltas incorporated improvements. Delta A, B, and
C models used MB-3 Block 2 engines that produced 77.1 tonnes of liftoff thrust.
Delta B incorporated a 0.91 meter second stage stretch. Delta C, which could lift
410 kg to LEO, added a more powerful ABL X-258 Altair 2 third stage to the
"A" and "B" improvements. On two occasions, Delta C used an even
more potent United Technologies FW-4D third stage motor that allowed it to lift as much as
600 kg to LEO.
These vehicles extended the Delta success streak to 22 before
Delta 24 finally failed to orbit NASA's Beacon Explorer A on March 19, 1964. (The
Delta B X-248 third stage burn ended, suddenly and probably catastrophically, about
half-way through its planned duration.) During the success streak, Delta boosted
more Explorer, Tiros, and communication satellites.
The communication satellites included Syncom 1 and 2, which were
boosted for the first time into GTO. Syncom 1s apogee kick motor failed, but
Syncom 2, launched by Delta 20 on July 26, 1963, worked, becoming the first geosynchronous
satellite. It would serve as the prototype for numerous subsequent Hughes-built
Although Syncom 2 was geosynchronous, it was not geostationary.
Syncom 2s Thor-Delta B launcher simply could not lift enough mass to GTO.
The satellite had to be placed in a slightly inclined orbit so that it traced out a
Figure-8 pattern above the Earth rather than appearing to hang motionless
above the equator. To solve this problem, the more-powerful Thor-Delta D was
Delta 30, the last Delta D, or Thrust Augmented Delta,
Launches Early Bird from LC 17A on April 6, 1965
Three Thiokol TX-33-62 (Castor 1) strap-on motors, each producing
24.44 tonnes of liftoff thrust, were added to the Thor booster to create Delta D
the Thrust Augmented Delta sometimes identified as "TAD".
Delta D could lift more than 100 kg to GTO.
Thor thrust augmentation was not a new innovation. Nearly
two dozen Thor Agenas had flown with Castor 1 boosters by the time Delta 25, the first
Delta D, lifted off from Cape Canaveral with Syncom 3 on August 19, 1964.
Delta 25s three fixed-nozzle boosters ignited at liftoff
and burned for 27 seconds. They were jettisoned 70 seconds after liftoff. The
Thor main engine burned for 148.7 seconds, boosting the vehicle to an altitude of 112 km.
The AJ-10-118 second stage engine ignited 151 seconds after liftoff, the payload
fairing falling away shortly after ignition, and burned for 161.5 seconds, boosting the
vehicle to a roughly 362 x 1,126 km x 28.7 deg parking orbit.
During the coast, the Delta second stage yawed to the left by 38
degrees to establish an inclination-reducing vector to the third stage burn. 26
minutes 8 seconds after liftoff, as the vehicle crossed the equator, the X-258 third stage
spun up, separated, and ignited for its 22.6 second burn. The burn boosted 68 kg
Syncom 3 into a roughly 1,126 x 35,780 km x 16.5 deg. GTO. The satellites own
JPL SR-12-1 kick motor burned at third apogee the next day to lift Syncom 3 into
One more Delta D would fly, as Delta 30. It orbited
Intelsat 1-1 on April 6, 1965. Intelsat 1-1, also known as Early Bird
and similar to the Syncoms, was the first commercial geostationary communications
satellite. It was placed above the Atlantic Ocean to provide 240 channel
trans-Atlantic telephone service for Communications Satellite Corporation.
Thor-"Vanguard" Fly Out
Delta 38, a Delta C1 with a UTC FW-4D third stage, Launched
Explorer 32 from Pad 17B on May 25, 1966
Delta D was quickly supplanted by more capable Thor-Delta
variants with fatter upper stages and, eventually, stretched Thors augmented
by more powerful strap on boosters, but the original Vanguard-based Delta second stage
flew on for several more years on Delta C.
Only one Delta C failed. It was Delta 33, carrying OSO-C
from Canaveral on August 25, 1966. First and second stage phases of flight were nominal, but the X-258 third stage motor ignited early, while it was still attached to the second stage and being spun-up. The X-258 "burned itself free" from the second stage, but since it was not spin-stable, it wobbled during its burn and was unable to impart adequate orbital velocity.
The failure was caused by a malfunction of a six-second time delay squib (a miniature explosive) that was supposed to ignite the third stage motor only after it had been spun up and separated from the second stage. In this event, the motor ignited 0.61 seconds after spin-up started, after the motor and payload had completed about one turn.
The squib type that failed was flying for the first time on Delta 33.
The new, more powerful squib design was implemented due to concerns about the near-underperformance of the squib it replaced during the successful Delta 32 mission. An investigation showed that the Delta 33 squib could suffer "blow-by" of the time-delay train, but only when ignited while being "spun-up". The "improved" Delta 33 squib design had been vibration tested, but not spin-tested. Future squibs would undergo spin acceleration testing. Delta 33 taught a tough, but valuable, lesson to its Goddard
The other twelve Delta C vehicles succeeded, including the last,
a Delta C1 topped by the UTC FW-4D third stage motor. It orbited
NASAs fifth Orbiting Solar Observatory (OSO 5) on January 22, 1969. This, the
64th Delta mission, was the final Thor-Delta to fly without strap-on solid motors.
Delta 64 was the 38th and final Thor-Delta to fly with the 0.813
meter diameter Vanguard-derived second stage. Of these, only three failed to reach
orbit. Two of the failures involved the third stage. Deltas Thor first
stage flew true every time. The skinny second stage, derived from a Vanguard program
commonly linked by the general public with explosive disaster, only suffered one failure
as Delta- and that during the very first flight. In the end, the old
Vanguard stage survived to fly even after NASA had launched astronauts atop massive Saturn
V boosters a few miles up the Florida coast.
By 1969, NASA had long-removed its interim label from
the Thor-Delta program. Delta had moved far beyond interim. It had
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