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.

As a 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.

Although 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.

Two more 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.

The 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.

The 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.

The 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.

The 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.

The most 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.

While 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.

Work on 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).

Centaur 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.

NASA's 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).

The 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.

The 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.

In 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.

The one 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.

The 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.