Thor, Able, and Star: Thunder God to Orbit
Part Two of a Series Reviewing Thor Family History
by Ed Kyle, Updated 6/18/2009
is an iconic photograph of the early space age. A
starkly white, futuristically streamlined rocket stands on its Florida launch pad,
illuminated in the humid evening darkness by brilliant spotlights, sheathed in wisps of
liquid oxygen boiloff, surrounded by hardhatted technicians - a rocket waiting to fly
toward the Moon. The year was 1958. The rocket, poised to boost its satellite
payload further from the Earth than any man-made object had previously flown, was named
Before the Moon shot, Thor's first non-IRBM job was to test ICBM
reentry vehicles and heat shields. For this purpose Thor-Able was originally
developed to fly long-range suborbital flights. Such was the importance of this
effort that Thor-Able began flying even before the Thor missile had completed its research
and development flight tests.
Thor-Able was a two-stage rocket created by stacking the Vanguard
space launch vehicle's second stage atop Thor. The second stage, dubbed
"Able" for this application, was a pressure-fed design built by Space Technology
Laboratories. It burned nitric acid and unsymmetrical dimethyl hydrazine (UDMH) in a
single 3.54 tonne thrust Aerojet AJ10 engine. Able was 5.67 meters long, 0.81 meters
in diameter, and weighed about 1.88 tonnes, a substantial increase from Thor's standard
one tonne warhead/reentry vehicle mass.
To convert Thor from a single-stage IRBM into the first stage of
a multistage vehicle, the missile's inertial guidance system was removed, replaced by a
guidance system located in the Able stage. The warhead was replaced by an interstage
First PGRTV Flight
Two-stage Thor-Able flew nine times from Cape Canaveral in
1958-59. The first three flights were part of the Advanced ICBM Reentry Test Vehicle
program to test ablative heat shield materials for an improved General Electric reentry
vehicle for Atlas. The first of these attempts, with Thor 116 on April 24, 1958,
failed when the Thor main engine suffered a turbopump failure at T+146 seconds. The
second attempt, launched by Thor 118 on July 10, 1958, succeeded, flinging the reentry
test vehicle more than 9,700 km downrange and 1,600 km into space. The RTV
successfully reentered and parachuted to the Atlantic. It was the first time that a
second stage had separated, ignited, and flown for the U.S. Air Force. The success
was repeated by Thor 119 with an Able stage that lofted its payload more than 10,100 km
across the Atlantic two weeks later.
The final six two-stage Thor-Able flights were part of the
Precision Guided Reentry Test Vehicle program. These flights used a
"Thor-Able 2" configuration that tested the Titan ICBM radio-inertial guidance
system. The first of these missions failed on January 23, 1959 when an electrical
failure at Thor 128's main engine cutoff prevented ignition and staging of the Able
vehicle. The next five flights, performed monthly from February to June, 1959, were
all successful. The April 8, 1959 flight was notable because the U.S. Navy
successfully recovered the reentry vehicle.
Orbits Explorer 6
Seven three-stage Thor-Able orbital launch attempts were also
performed during 1958-1960. These flights added an Allegheny Ballistics Laborary
X-248 "Altair" solid rocket motor as a third stage to create the "Thor-Able
1" variant. The spin-stabilized motor weighed 238 kg and was 1.83 meters long
and 0.46 meters in diameter. It provided 1.27 tonnes of thrust for about 38 seconds.
The first orbital attempts were part of ARPA's "Operation
Mona", better known as "Pioneer", the name assigned by NASA when it assumed
control from ARPA after the first launch. Pioneer was the first U.S. attempt to
reach the Moon. Pioneer only weighed 38 kg but still maxed out Thor-Able's payload
capacity. The spacecraft carried a simple imager, a radiation detector, a
magnetometer and a microphone to detect micrometeorite hits. Pioneer also was fitted
with a small solid motor intended to perform a lunar orbit insertion
The first Pioneer lifted off on August 17, 1958, but was
lost when Thor 127's LR79 experience a turbopump failure 77 seconds after liftoff.
The flight is usually identified as "Pioneer 0".
Newly formed NASA took over the program before the
second Pioneer attempt, which took place on October 11, 1958. The Air Force actually
ran the launch. This time Thor (No. 130) performed well, but the Able stage
underperformed, leaving Pioneer 1 on a 43 hour, 17 minute long suborbital trajectory with
a 113,854 km apogee. The failure is attributed by various sources as having been a
programming error, a failed accelerometer, a minor pitch deviation, or an improper third
stage separation. Although Pioneer 1 didn't reach the Moon, it performed
unprecedented measurements of the Earth's radiation belts.
Pioneer-2 lifted off on November 8, 1958, but the third
stage failed to ignite and the vehicle only reached 1,600 km, reentering over Africa 45
minutes after launch.
Thor-Able Launches Tiros 1
The final four Thor-Able flights, performed for NASA from Cape
Canaveral between July 1959 and January 1960, orbited Explorer 6, Pioneer 5, and Tiros 1,
the first weather satellite. The Transit 1A navigation satellite launch failed to
reach orbit when the third stage failed.
Explorer 6 was boosted into a highly elliptical Earth orbit to
study the radiation belts. It also transmitted the first photographs taken of Earth
from orbit. Pioneer 5 flew into solar orbit.
Tiros 1, a 120 kg weather satellite launched April 1, 1960, took
tens of thousands of weather photos during several weeks of service. It convincingly
demonstrated the utility of the orbital weather satellite concept.
By April, 1960, the Thor-Vanguard (Thor-Able) combination had
proved its relative reliability and utility to NASA. Among NASA's early, interim
stable of orbital space launchers, which included Juno I, Juno II, Vanguard, and Atlas
Able, only Thor-Able had succeeded more times than it had failed. NASA now planned
to extend the success by fitting a never-flown, but already designed, Vanguard second
stage gas-jet attitude control system to the Able stage, giving it the ability to coast
and restart. The new rocket, intended to fly for only a short time as an
"interim" system while NASA waited for more powerful Atlas Centaur, would be
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" second stage.
Able-Star, the worlds first restartable stage, was a
"wide-body" compared to the Able stage that carried more than twice as much
propellant. Able-Star was 1.4 meters in diameter and 4.52 meters tall. It
weighed 4.497 tonnes loaded and 0.599 tonnes empty. It was a helium pressure fed
stage that used the same propellants as Able. It also used a similar engine, the
AJ10-104, which produced 3.674 tonnes of thrust. The engine could burn for up to 262
A programmed autopilot controlled the Thor first stage, replacing
the original Thor guidance system. The tapered forbody section that had housed
Thor's guidance system was replaced by a stepped interstage adapter, giving Thor-Able-Star
a less-sleek, more business-like appearance than Thor-Able. A lightweight guidance
system on the second stage controlled the Able-Star portion of flight. Nitrogen jets
provided three-axis flight control of the Able-Star stage during coast periods.
Thor-Able-Star flew 19 times during 1960-65, including 11
launches from the Cape and 8 from Vandenberg AFB. Using its second stage restart
capabilty, it could lift more than 230 kg to a 1,000 km x 28.5 deg circular orbit.
Able-Star performed the first in-space restart 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 finalize the 373 x 748
km x 51.28 deg orbit.
Begins Failed Transit 3A Launch from Cape Canaveral LC 17B
Early launches from the Cape 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.
In 1963, Thor-Able-Star made its first flight from Vandenberg
AFB. The first launch took place from Complex 75-1-1, a former Thor IRBM test pad.
Thor-Able-Star would fly from nearby Complex 75-1-2 in 1964. The sites were
later renamed Space Launch Complex (SLC ) 2 East and 2 West, respectively.
All eight Vandenberg launches carried U.S. Navy Transit
navigation satellites, aimed toward near-polar orbits. The first three
Vandenberg-launched Transits used SNAP 9A RTG nuclear power sources, which were loaded
with more Plutonium 238 metal fuel 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, as it had been
designed to do, releasing about 1 kg of Plutonium 238, reportedly more than 20 times as
much Plutonium 238 released during the Chernobyl incident years later, and nearly twice as
much as all of the atmospheric nuclear tests performed since World War II .
Larger amounts of radioactive elements besides Plutonium 238 were, of course, released
during those events. After Transit 5-BN-3, SNAP RTGs were redesigned to contain
nuclear fuel in the event of a launch failure.
The final five Transit-O ("Oscar") missions, all
successful, used solar powered satellites.
Thor Able-Star's early flight record was spotty, with four launch
vehicle failures, two by Thor and two 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
Although August 13, 1965 saw the last flight of Thor-Able-Star,
parts of the Able-Star upper stage would migrate to NASA's ever-improving Delta launch
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