The first upper stage to fly
successfully was a cluster of small solid-propellant Baby Sergeant rockets
mounted inside a cylindrical tub that was spun by an electric motor atop a
Jupiter C/Juno I rocket.
The Baby Sergeant pushed the first U.S. satellite, Explorer 1, into orbit
on January 31, 1958.
result, some of the most advanced rocket research has focused on upper
stages. Although there is no strict definition of an “upper stage,” it
usually refers to the third and fourth (if any) stages of a rocket, fired
at high altitude. Because upper stages are rarely visible from the ground
and leave no long firetrails to see, they attract little attention and are
the unsung workhorses of the space age.
the former Soviet Union launched the first space vehicles, the R-7 rockets
that propelled them to space were so powerful that they did not need an
upper stage. Thus, the first upper stage to fly successfully was a cluster
of small solid-propellant Baby Sergeant rockets mounted inside a
cylindrical tub that was spun by an electric motor atop a Jupiter C/Juno I
rocket. The Baby Sergeant pushed the first U.S. satellite, Explorer 1,
into orbit. Despite this early success, the Sergeant's spin-stabilization
system was not very useful because the satellite had to keep spinning
rapidly in orbit, or slow down, and neither approach was satisfactory.
of the United States' early launch vehicles, the Thor-Able and the
Vanguard, also used small solid-propellant upper stages. A version of the
Soviet R-7 used to launch the Luna-1 space probe to the Moon in January
1959, also used an upper stage called the Block-E.
first upper stage to be designed—although not the first to fly—was the
Agena. It was a key part of the U.S. Air Force's WS-117L reconnaissance
satellite program started in 1956. Originally called the Hustler, Agena
was not only an upper stage but also a spacecraft, intended to carry a
satellite payload into orbit and remain attached to it, providing power
and pointing it in the proper direction.
Agena was adapted to a number of different payloads. It first carried the
CORONA reconnaissance satellite (which flew with the cover name
Discoverer) and later the Midas missile warning satellite and various
Samos spy satellite payloads in the early 1960s. It had a five-foot
diameter so that it could fit atop the Thor and Atlas rockets, and used
red fuming nitric acid and unsymmetrical dimethylhydrazine for propellant.
Agena A upper stage.
Agena quickly grew bigger and added capabilities, including the ability to
restart its engine in space. Because the propellant in the tanks would
float away from the rocket engine in weightlessness, the Agena was
equipped with small solid-propellant rockets at the rear called “ullage”
rockets. These fired briefly and pushed the vehicle forward, and the
propellant sloshed back against the rocket engine so it could fire.
Agena D, which entered service in 1962, became the standard Agena vehicle
and was equipped with connection points, called the “aft rack” just
forward of the engine exhaust bell and aft of the tank, which allowed
additional equipment like solar panels and even small ejectable satellites
to be carried. The Agena D carried numerous payloads into orbit, including
the KH-7 and KH-8 GAMBIT spy satellites, and pushed NASA Ranger and Lunar
Orbiter space probes to the Moon.
visible use of the Agena came during the Gemini human spaceflight program,
when six Gemini spacecraft rendezvoused with their Gemini Agena Target
Vehicles to simulate the techniques necessary for a lunar mission. During
two of these missions, Agenas restarted their engines in space to push the
Gemini spacecraft and their crews to much higher orbits. Agena proved so
successful as an upper stage that more than 380 were built and the upper
stage remained in use until the mid-1980s.
Agena and smaller upper stages were launching the first payloads into
space, NASA undertook an ambitious program to develop the most powerful
upper stage ever built, the Centaur. Because increased performance for
upper stages has such a great impact on the weight of payload that can be
launched, NASA and the Air Force sought to use the most powerful
propellant combination possible—liquid oxygen and liquid hydrogen, known
as cryogenic propellants because they must be stored at very cold
temperatures. Centaur was needed to push very heavy payloads to the Moon
and Mars, and as such, was designated a project of “Highest National
Priority” by President Kennedy on November 28, 1962.
Centaur progressed for several years, overcoming numerous design
challenges associated with the extreme cold of the liquid hydrogen. On May
8, 1962, the first Centaur was launched atop an Atlas rocket from Cape
Canaveral, but it failed when its insulation ripped off. This prompted an
extensive redesign of the vehicle and in November 1963, the second Centaur
was launched, this time successfully. Centaur then went on to launch seven
Surveyor landers to the Moon.
Atlas Agena launching Mariner IV
interplanetary probe (Nov. 28, 1964).
was also adapted for the larger Titan 3 and 4 launch vehicles, launching
the Viking spacecraft to Mars and the Voyager space probes out to Jupiter,
Saturn and beyond. A Centaur version (designated as Centaur G Prime or
Shuttle Centaur) was also built to fly aboard the Space Shuttle but was
cancelled after the Challenger disaster due to safety concerns over
launching humans in the same vehicle as a hydrogen-filled Centaur. Updated
Centaurs continue to be used aboard the Titan 4 and the Atlas.
experience with the Centaur contributed to the development of the Saturn
S-IVB stage used aboard both the Saturn IB and the Saturn V. The S-IVB was
used in the demanding task of pushing the heavy Lunar
Module-Command/Service Module stack to the moon. The Delta III rocket,
introduced in the late 1990s, used the first new cryogenic upper stage
produced in the United States since the 1960s.
Different rockets often use the same upper stages. Atlas and Titan have
both shared the Centaur upper stage. Thor, Atlas and Titan have all shared
the Agena. The Space Shuttle has also used upper stages originally
designed for other rockets. The Russian Proton's Block-D fourth stage was
originally developed for use as a lunar descent stage for the Soviet N-1
crewed lunar mission. And many different rockets have used smaller
solid-propellant upper stages.
Atlas Centaur launching Surveyor 5
towards the Moon (Sept. 8, 1967).
United States also developed numerous solid propellant upper stages, the
most notable being the Payload Assist Module, or PAM-D, used atop the
Delta and aboard the Space Shuttle. The largest solid-propellant upper
stage is the Inertial Upper Stage (IUS) used for some of its planetary and
military spacecraft, launched from both the Space Shuttle and the Titan 4.
Martin Company (now Lockheed Martin) developed the liquid-fuelled Transtage
for the U.S. Air Force's Titan III rocket. It could place multiple
satellites into different orbits, but suffered persistent reliability
problems. Although arguably not an upper stage in the true sense, the
highly capable Ablestar second stage used on the Delta II rocket often
entered orbit with its payload.
contrast to the United States, the Soviet Union did not spend significant
effort developing cryogenic fuels in the early 1960s, postponing their
development until the late 1960s. This had a deleterious effect upon the
Soviet space program, restricting the size of payloads that they could
launch to high Earth orbit. The Soviets tended to use the same propellant
combinations for their third and fourth stages as they did for their first
and second stages—usually liquid oxygen and kerosene for rockets such as
the Zenit and the R-7/Molniya.
exception is the Block-D on the Russian Proton rocket (which is now
designated the Block-DM and used to launch commercial satellites). A new
Russian upper stage for the Proton-K, the Breeze-M, uses storable nitrogen
tetroxide oxidizer and UDMH fuel and made its first flight in June 2000.
upper stage used on the Russian R-7/Soyuz rocket has probably been the
most heavily used upper stage rocket of all and continues to evolve. The
restartable Ikar upper stage made its first flight on a commercial Soyuz
rocket (Starsem Soyuz) in 1999, followed by the introduction a year later
of the restartable Fregat upper stage with significantly more propellant
capacity than the Ikar.
Arianespace also developed a cryogenic upper stage for its Ariane rockets.
The HM7B engine, originally developed for the Ariane 1 and first qualified
in 1979, was improved over the years and successfully powered the third
stage of the Ariane 4 for dozens of commercial communications satellite
customers. The Ariane 5 initially flew with a storable propellant third
stage using nitrogen tetroxide and monomethyl hydrazine powering its
pressure-fed Aestus engine. This upper stage proved insufficient for the
increasingly heavy satellites, and Arianespace began developing a new
cryogenic engine named Vinci to power the Ariane 5's upper stage starting