How did we get to the Moon? Propulsion technology and the space shuttles

They became the workhorses of their respective space programs or would become the basis for future designs. As the Space Age progressed, engineers developed rockets and thrusters that were more powerful and sophisticated and took advantage of new fuels. For example, the Apollo Era (1962-1972) saw the introduction of hydrazine (N2H4) and liquid hydrogen fuel (LH2).

These engines would launch the most powerful launch vehicles in the world - like the Saturn V and Soviet N1(L3) - and send robotic explorers and crews beyond Earth orbit and to the Moon.

The Space Race officially ended during this period and was followed by the Soviet and American space programs settling in for the long haul. This led to long-term objectives being set and new technologies that are still in use today. 

During this decade, more nations joined the race by creating their own boosters, engines, and space programs. These included China, India, and the European Space Agency, all of whom would become increasingly important as the 20th century drew to a close.

Shooting for the Moon

Having sent the first astronauts and cosmonauts to space, the Soviet and American space programs focused on the next great leap. This consisted of building rockets and spacecraft that could accommodate more than one person and reach beyond sub-orbit.

For NASA, this resulted in the Gemini Program (1961-66) and the Voskhod Program (1964-65) in the Soviet Union.

The Gemini Program saw the debut of the two-stage liquid-fueled Titan II Gemini Launch Vehicle (GLV), an adaptation of the Titan II ICBM. The first stage booster relied on a single Aerojet LR-87 two-chamber, two-nozzle, turbopump engine that generated 1,900 kN (430,000 lbf) of thrust. 

The second stage relied on a single Aerojet LR91, a scaled-down version of the LR87 that had only one chamber and generated about 440 kN (100,000 lbf). Both engines relied on a combination of Aerozone 50 (a 50/50 mix of hydrazine and UDMH) and N2O4. 

The Gemini spacecraft was a larger version of the Mercury spacecraft and could accommodate a crew of two. Once again, the engines consisted of retrorockets that were only used during reentry.

However, this time, the designers went with four TE-M-385 Star-13E spherical-case solid-propellant motors, each capable of providing around 11 kN (2,470 lbf) of thrust.

The Soviet Voskhod Program also relied on rockets and spacecraft that were variations of their Vostok predecessors. The rocket (R7 11A57) was another variation on the R7 Semyorka, relying on four strap-on boosters (with RD-107 engines) and two Vernier engines, and a core powered by a single RD-108 and four Verniers.

These provided a combined thrust of around 4922.6 kN (1,107,100 lbf), while the second stage was powered by a single RD-107 and had a maximum thrust of about 294 kN (66,000 lbf).

The Voskhod spacecraft was similarly based on the Vostok but enlarged to accommodate a crew of two to three cosmonauts. Like Vostok, the craft was equipped with cold-gas retrorockets in an exterior module that generated 15.83 kN (3,560 lbf) of thrust.

The spacecraft also had a solid-fuel retrorocket package that provided 117.7 kN (26,460 lbf) of thrust. This allowed for the spacecraft to make a soft landing rather than forcing the crew to parachute during reentry.

These programs were a stepping stone towards the ultimate goal of the American and Soviet space programs: the Race to the Moon! NASA launched the Apollo Program in parallel with Gemini and began developing super-heavy launch vehicles and spacecraft. This would culminate in the three-stage Saturn V rocket and the three-part Apollo spacecraft that would transport the Apollo astronauts to the Moon.

The first stage (S-IC) was powered by five Rocketdyne F-1 engines, which were fueled with RP-1 (kerosene) and LOX and generated 33,000 kN (7,500,000 lbf) of thrust. The second stage (SII)was powered by five Rocketdyne J-2 engines (RP1/LOX) that generated 4,400 kN (1,000,000 lbf) of thrust, while the third stage (S-IVB) relied on a single J-2 engine 890 kN (200,000 lbf).

The Apollo spacecraft, known as the Command and Service Module (CSM), consisted of two separate elements - the Command Module (CM) and the Service Module (SM) - that carried the Apollo Lunar Module (ALM). The Service Module contained the spacecraft's main engine, which provided 97.86 kN (22,000 lbf) of thrust, while the Command Module was equipped with sixteen retrorockets (440 N each). 

The ALM consisted of two elements: the Descent Module (DM) and Ascent Module (AM). The former relies on a single solid-fuel rocket engine that was used to slow the descent, providing 44.04 kN (9,901 lbf) of thrust, while the AM has a single ascent engine that provided 15.57 kN (3,501 lbf).

While the Soviets officially ceded the race to the Moon, their space program pursued the development of super-heavy launch vehicles that could compete with NASA. The fruit of their labors was the N1 (L3) rocket, a three-stage vehicle powered by the new family of Kuznetsov (NK) single nozzle, liquid-fueled (kerosene/LOX) engines. 

The first stage relied on thirty NK-15s (45,400 kN; 10,200,000 lbf), the second had eight (14,040 kN; 3,160,000 lbf), while the third relied on four Kuznetsov NK-21 (1,610 kN; 360,000 lbf). A possible fourth and fifth stage were considered that would be equipped with a single NK-19 (446 kN; 100,000 lbf) and RD-58 engine (83.40 kN; 18,749 lbf) - respectively. 

Only four prototypes were created, all of which suffered catastrophic failures during testing. However, the Soviets also managed to create the Soyuz variant of the R7 rocket, which would become the workhorse of the Soviet-Russian space program and the most widely-used rocket in the world. 

The main difference between the Soyuz and previous R7s was the incorporation of a single RD-110 engine on the second stage of the rocket. Like the RD-108 and 109, this engine had four chambers and nozzles, relied on a mix of kerosene and LOX propellant, and was capable of generating 1,374.00 kN (308,887 lbf) of thrust.

The Space Shuttle Era

With the closing of the Apollo Era, the Space Race was effectively over, leaving the space agencies of the world to contemplate what their next step would be. For both NASA and the Soviets, the main concern was how to adapt to a diminished budget environment. And with the race to the Moon over, the priority shifted to developing technologies that would allow for long-term stays in space.

In the U.S., this resulted in the Space Shuttle Program, a reusable spaceplane and a first-stage booster system that could deliver satellites and payloads to orbit. The five shuttles that made up this crewed program would perform 135 missions before retiring in 2011 and would play a vital role in the creation of the International Space Station (ISS). 

The architecture of the Space Shuttle system included the reusable Space Shuttle Orbiter, an external fuel tank, and two solid rocket boosters. The Shuttle was powered by the Space Shuttle Main Engines (SSMEs), which consisted of three Aerojet Rocketdyne RS-25 thrusters that used a combination of liquid hydrogen (LH2) and LOX to generate a combined 5,250 kN (1.18 million lbf) of thrust. 

The Orbiter also had two AJ10-190 steering thrusters capable of generating 26.7 kilonewtons (6,000 lbf). These made up the Orbital Maneuvering System (OMS) mounted in two separate removable pods on the Orbiter's aft fuselage. The expendable solid rocket boosters were capable of generating 13,000 kN (3 million lbf) each. These would fire alongside the RS-25 to send the vehicle to an altitude of about 150,000 ft (28.4 mi; 46 km) before being jettisoned.

The Soviets attempted to develop their own reusable spaceplane at this time, known as the Buran Shuttle. Like the Space Shuttle, Buran was paired with an expendable launch vehicle to put it into orbit and then relied on its own engines for propulsion and steering. The launch vehicle was the newly-designed Energia GRAU 11K25. 

Like its R7 predecessors, the GRAU 11K25 was essentially a core-stage booster with four strap-on boosters. The core was powered by four RD-0120 single-nozzle liquid-fueled (LOX/LH2) engines capable of generating a maximum of 7,500 kN (1.7 million lbf). Each strap-on engine was equipped with an RD-170, a four-nozzle liquid-fueled (RP-1/LOX) engine. Together, these boosters were capable of generating a maximum thrust of 32,000 kN (7.2 million lbf). 

The orbiter element relied on its Joint Propulsion System for maneuvering in space. This consisted of three liquid-fueled thrusters (LOX/RP-1) and three steering thrusters that used gaseous oxygen as propellant. The program was ultimately canceled due to budget cuts, and only two prototypes were built (which never flew to space). 

But like the Space Shuttle Program, the Buran Program led to new designs and propulsion concepts that would inspire Russian engineers in the post-Soviet era. 

Other nations join the race

During the Apollo Era and after, other space agencies emerged and began developing their own launch vehicles, spacecraft, and engines. Like the Soviet and American space programs, these efforts paralleled the development of nuclear missiles and adapted many of them to create launch vehicles. 

China began its space program during the 1960s, which led to the two-stage Feng Bao-1. The first stage of this rocket was powered by four single-nozzle YF-20A gas-cycle generator engines that burned unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide (N2O4). The second stage was powered by a single YF-22/23, which also used a combination of UDMH and N2O4.

China also developed the three-stage Chang Zhen-1 (Long March-1) vehicle in the late 1960s. The first and second stages were powered by four YF-2A engines and a single YF-2A (respectively), which similarly used nitric acid/UDMH propellant. The third stage was powered by a single GF-02 engine that used solid propellant. 

The two-stage FB-1 generated 3,000 kN (674,400 lbf) of thrust with its first stage, while its second stage generated 761.9 kN (171,282 lbf). The three stages of the CZ-1 generated 1,101.2 kN (247,600 lbf), 320.2 kN (72,000 lbf), and 181 kN (41,000 lbf) respectively.

Between 1968 and 1972, the European Launch Development Organization (ELDO) - the predecessor of the European Space Agency (ESA) - introduced the Europa rocket family. The three-stage variant (Europa I) was powered by two Rolls-Royce RZ.2 liquid-fueled engines (LOX/kerosene) that produced 1,673 kN (376,000 lbf) of combined thrust. 

The second stage was powered by four LRBA Vexin-A (UDMH/N2O4), generating 238 kN (53,504 lbf) of thrust, while the third stage used a single liquid-fuelled Astris engine (N2O4/Aerozene 50), which generated 23.3 kN (5238 lbf). The Europa II variant included a fourth stage, which was powered by a single solid-propellant rocket.

In 1979, the ESA began developing the Ariane rocket family, starting with the three-stage Ariane I. This rocket debuted the Viking rocket engine, which relied on N2O4/UDMH fuel and would become the mainstay of the Arian rocket family. The first stage of the Ariane I was powered by four Viking-5C engines that generated 758 kN (170,405 lbf) of thrust each. 

The second stage used a single Viking-4 engine that generated 720.965 kN (162,079 lbf). The third relied on a single HM7-A engine, which burned a mix of LH2 and LOX and generated 61.674 kN (13,865 lbf) of thrust. 

The Indian Space Research Organization (ISRO) also debuted its first orbital rocket (SLV-3) in 1980, a four-stage all-solid propellant vehicle that could generate a total thrust of 62.200 kN (13,983 lbf). These three national pace agencies - China, India, and the European Union - would all achieve great strides in the ensuing decades, thanks to the early achievements made between the late-60s and mid-80s. 

The period spanning the 1960s to the 1980s was a very auspicious time for space exploration. It began with the Moon Race, which culminated in the Moon Landing, and ended with the creation of the Space Shuttle and the first space stations. When the sun set on this era, many of the technologies it spawned would carry over into the next, when NASA and other space agencies set their sights on revisiting old haunts (the Moon) and new frontiers (Mars).

To be continued..., a fascinating series on the evolution of propulsion technology.