Thor to Thorad: Prolific Agena D Boosters
Part Four of a Series Reviewing Thor Family History
by Ed Kyle, Updated 7/14/2016
Thor-Agena D used the "standardized" Agena D upper stage, a
stage designed to fly atop Thor, Atlas, and Titan with minimal changes.
An improved Bell XLR81-BA-9 restartable engine, still producing 7.26
tonnes thrust but now with a more-efficient 300 second specific impulse rating, powered
Agena D. The stage was the same size as Agena B, but weighed less at 6.821 tonnes
loaded and 0.673 tonnes empty. The improved engine efficiency provided up to 265
seconds of burn time, a 25 second increase from Agena B. The two stage rocket could
lift 1.15 tonnes to polar orbit.
Radio command guidance, through a Bell Telephone Laboratories (BTL) 600
system mounted in the Agena, provided position and velocity correction during the last
minute or so of the Thor first stage burn and, on some flights, during the first Agena
burn. Both Thor and Agena were equipped with three-gyro autopilots that responded to
the guidance correction signals, but both could also fly independent of radio command in
"open loop" mode. Radio command guidance improved insertion accuracy
compared to "open loop" guidance.
Agena's gyros were "uncaged" when the Thor verniers shut down,
initiating the upper stage's inertial reference. Agena restarts were usually
triggered by ground command.
Agena D was built around a 1.524 meter diameter cylindrical tank section.
The aluminum section included a forward fuel tank and an aft oxidizer tank
separated by a common bulkhead. The tank section was 2.452 meters long. A fuel
duct ran from the fuel tank through the oxidizer tank to reach the engine. Two
fairings carried cables and pressurization lines around the outside of the tank
Lockheed Agena Assembly Line
A 0.281 meter tall cylindrical forward section, positioned atop the fuel
tank, housed Agena guidance and control systems. It consisted of a tubular welded
aluminum frame covered by beryllium panels. The forward section housed Agena
guidance, communication, and control equipment. These included the radio guidance
system, three-gyro inertial reference package, horizon sensor, velocity meter, sequence
timer, command receivers and decoder, telemetry systems and transmitters, and an optional
Agena's aft section included the Bell engine and a "rack"
composed of a magnesium structure and skin augmented by tubular aluminum braces.
This was part of what was termed the "Agena spaceframe system". The
approximately 2 meter long aft rack held one or more pressurized nitrogen tanks, two sets
of triple cold-gas thruster packs for roll control and on-orbit maneuvering, and up to 450
kg of other, optional, equipment or payloads. Solar panels were mounted to the aft
rack on some missions, for example.
Since Agena began flying before digital computers were small and light
enough to fly on-board, basic ascent-phase "programming" was initially provided
by a motor-driven mechanical 6,000 second timer - think of a more complicated version of a
washing machine timer. It had 72 switches in 12 banks that could be configured to
control 24 separate discrete (on/off) events. Additional timers,
"programmed" by plugging jumpers into a patch panel, could be added for
An optional "orbital programmer" was used to control Agena when
out of ground station range. It consisted of a punch-tape system that could accept
radio commands, store them, and, when run, actuate up to 52 on/off events per orbit.
Four tapes could store 512 orbits worth of commands, enough for about a month's worth of
unique low earth orbit command sets. Later-model orbital programmers used magnetic
Thor-Agena D flew 21 times from 1962 to 1967 from Vandenberg AFB pads
75-1-1, 75-1-2, and 75-3-5, carrying Keyhole 4 and 5 film return spysats, DMSP 3A military
weather satellites, and a variety of electronic intelligence and experimental U.S.Navy
Thrust Augmented Thor Agena B/D
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 combined to produce 69.4 tonnes of liftoff thrust
for about 28 seconds, with tailoff ending at about 40 seconds. The boosters
augmented the upgraded MB-3 Block 3 Thor first stage engine, which itself produced 78
tonnes of liftoff thrust for 150 seconds.
TAT-Agena D, the most-oft flown U.S. Air Force Thor space launch vehicle,
weighed 67.82 tonnes at liftoff and could lift roughly 1.5 tonnes to polar orbit.
The vehicle's busiest year was 1964, when 20 launches occurred from
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. The launch
vehicle required about 25 working days on its pad before launch, all but the last few days
in horizontal position within the slide hanger. The Agena stage was mated
horizontally with Thor about 18 working days before launch. The mated vehicle was
erected to vertical only three days before launch. Strap on motors were installed
only two days before launch.
For the first time, Keyhole 4A imaging satellites equipped with
two film return "buckets" were flown. TAT-Agena D also orbited
electronic intelligence satellites and three NASA payloads (OGO 2 and 4 and
Thorad Agena D
The final U.S. Air Force Thor Agena was the Long Tank Thrust Augmented
Thor (LTTAT)-Agena D, also known as Thorad Agena D. Long Tank Thor was stretched to
21.4 meters in length, with the tapered kerosene tank replaced by constant 2.44 meter
diameter tankage. The stage now weighed 70.354 tonnes loaded, but only 3.715 tonnes
empty. The extra propellant extended the MB-3 Block 3 burn time to 215
The rocket depended on three strap-on Castor 2 boosters to kick it off the
pad. The Castor 2 boosters combined to produce 87.56 tonnes of thrust for 37
seconds. Thorad Agena D weighed 91.625 tonnes at liftoff, stood about 34 meters
tall, and hauled about 2 tonnes to polar orbit.
There were 43 Thorad Agena D launches 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,
signals intelligence satellites, and Nimbus weather satellites.
The Nimbus satellites were powered by SNAP-19 RTGs. On May 18, 1968,
the launch of Nimbus B failed when the Thorad Agena D guidance system malfunctioned.
Unlike earlier SNAP RTGs, SNAP-19 was designed to survive launch vehicle failure.
As a result, the SNAP-19 RTG was salvaged from the Pacific. After recovery,
the RTG was actually refurbished and later flown again on Nimbus 3.
On May 25, 1972, the final Thorad-Agena D lifted off from SLC 3W, carrying
Keyhole 4B-17. The flight, using Thor booster number 571, was the last purpose-built
U.S. Air Force Thor-based orbital launch vehicle, although space launches of converted
Thor IRBM missiles would continue for several more years. Thorad Agena was
supplanted by more capable Titan-based launchers, and the Corona film return satellites
were eventually replaced by satellites that transmit high resolution images by radio
By the time the last Thorad-Agena D flew, Thor stage production for NASA's
Delta launch vehicle had moved from the old Santa Monica Douglas plant to the newer
McDonnell Douglas facility at Huntington Beach, California. Meanwhile, three of the
five Vandenberg AFB Thor-Agena pads that had hosted so many launches were finally
silenced. SLC 2W continued to be used by Delta and SLC 3W was converted for use by
The Thor-Agena family came nearer than any other U.S. launch vehicle to
the mass-produced ideal. From 1962 through 1964, Thor-Agenas flew about twice per
month. They flew from relatively spartan launch sites, employing horizontal assembly and
processing. Launch processing, and launch pad facilities, grew slightly more complex
with the addition of strap-on boosters and with the switch to Long Tank
After 185 launches, and 160 successes during a 14 year span, Thor-Agena
finally ended, but the Thor family was anything but finished.
Thor History Home Page