Propulsion technology: The rise of the commercialization of space

Between the late-1950s and mid-1960s, American and Soviet aerospace engineers laid the foundation of space exploration. By the late 1970s, they built on this foundation and increased their reach considerably, sending robotic missions throughout the Solar System and the first astronauts to the Moon. And by the late-70s and 80s, they shifted their focus closer to home and developed the means for staying in space longer.

By the time the final decade of the 20th century came about, more space agencies were joining the fray, a new spirit of cooperation emerged between the old superpowers, and NASA was beginning to look further afield. With the success of the Apollo program, the Space Shuttle, and the International Space Station (ISS), mission planners began setting their sights on two main objectives: returning to the Moon and sending astronauts to Mars.

The Next Leap

The first proposal to send missions to the Red Planet was made in 1990 by aerospace engineers Robert Zubrin and David Baker. Titled  "Mars Direct," this plan called for a cost-effective crewed mission architecture, which also became the basis of Zubrin's book, The Case for Mars, released in 1996. 

The basic idea was to use existing technology to send astronauts to Mars and rely on the onsite procurement of resources - or In-Situ Resource Utilization (ISRU) - to reduce the amount of fuel and supplies the crews would need to bring with them. This concept would inform efforts in the mid-2000s to create a new family of launch vehicles and spacecraft to bring crews to Mars. 

As part of the Constellation Program, which lasted from 2004-2010, two next-generation rockets were designed to send payloads and crews beyond Low Earth Orbit (LEO). These were the Ares I Crew Launch Vehicle and the heavier Ares V Cargo Launch Vehicle (CaLV). Before the Constellation Program was canceled in 2009 due to the economic downturn, a single Ares I prototype (Ares I-X) was built and flight-tested without crew (October 28, 2009).

The first stage of the Ares I was powered by a single solid-propellant rocket booster capable of generating 15,000 kN (3.4 million lbf) thrust. The second stage, which would place the next-generation Crew Exploration Vehicle (CEV) into orbit, would rely on two Aerojet Rocketdyne J-2X engines. Like their predecessors used by the Saturn V, these engines burned a combination of liquid hydrogen and liquid oxygen (LH2/LOX) and generated 1,307 kN (294,000 lbf) of thrust each.

With the NASA Authorization Act of 2010, NASA embarked on a new program of crewed exploration that included missions to Mars. Known as the "Journey to Mars" (or "Moon to Mars"), this program incorporated proposals and technology from the Constellation Program and used them to create the Space Launch System (SLS) and Orion Multi-Purpose Crew Vehicle (MPCV).

Like the Ares V design, the SLS consists of a core stage booster and two solid rocket boosters to send payloads and crew to the Moon and Mars. The core stage is equipped with four Aerojet Rocketdyne RS-25 liquid-fueled engines (LH2/LOX) capable of generating up to 2,279 kN (512,300 lbf) of thrust. The solid rocket boosters, the same kind used by the Space Shuttle, generate 15,000 kN (3.3 million lbf) of thrust each.

The maiden launch of the SLS and Orion took place in the early morning hours on Nov 16th, 2022. As the first mission of the Artemis Program (Artemis I), this mission saw an uncrewed Orion spacecraft conduct a circumlunar flight that established a new distance record.

Competitors, old and new

With the fall of the Soviet Union in 1991, the Soviet space program was officially disbanded. Many of its design centers now found themselves in newly-independent states (mainly Ukraine). At the same time, their main launch facility (the Baikonur Cosmodrome) was part of the newly-sovereign state of Kazakhstan. 

By 1992, Roscosmos had emerged as Russia's new space agency, and a period of reorganization and improvisation followed that eventually restored stability. This allowed Russia to continue developing new launch vehicles and engines, the foremost examples being the Proton and Angara rocket families. 

The key to these technologies was the Universal Rocket (UR) concept, a program initiated by the Soviets and carried on by Roscosmos. The idea was to build Universal Rocket Modules (URM) that could be used by different rocket classes (ICBMs and launch vehicles of various types), thus allowing for lower costs, rapid technology transfer, and flexibility. 

During the 1970s, the Proton-K was used to send uncrewed missions on circumlunar flights and to deploy Soviet space stations (Salyut) to orbit. These three-stage rockets were powered by liquid rocket engines that relied on unsymmetrical dimethylhydrazine (UDMH) as a propellant and dinitrogen tetroxide (N2O4) as an oxidizer.

In the post-Cold War era, Russian engineers began working on a new variant (the Proton-M rocket) that made its debut flight in 2001. Like its predecessor, the first stage was powered by six base-mounted RD-275M engines capable of generating a maximum thrust of 10,532 kN (2,368,000 lbf). The second stage was powered by three RD-0210 and one RD-0211, capable of generating a total of 2,399 kN (539,000 lbf). The third stage was powered by a single RD-0212 capable of generating 613.8 kN (138,000 lbf). 

The Angara rocket family benefitted especially from the UR program. These heavy-launch vehicles came in many variants, and all were built by stacking Universal Rocket Modules (URM-1 and URM-2) to create the first and second stages, respectively. These rockets were liquid-fueled and relied on a combination of RP-1 and LOX. 

The Angara's first stage could consist of up to five URM-1 boosters, each of which was equipped with an RD-191 single-nozzle engine capable of 1,920 kN (430,000 lbf). The URM-2 booster relied on an RD-0124A four-nozzle engine capable of generating 294.3 kN (66,200 lbf). A third stage could be adapted as a Briz or KVTK rocket stage, which relied on a single S5.98M or RD-0146D engine (both fueled with N2O4/UDMH). These generated 19.6 kN (4,400 lbf) and 68.6 kN (15,400 lbf), respectively. 

The European Space Agency (ESA) also made many impressive strides during the late 20th century and the early millennium. From 1979 onward, they succeeded in developing multiple Ariane launch vehicles, which led to the multi-stage Ariane 4 (1988-2003) and heavy-launch Ariane 5 (1996-present) that gave Europe independent launch capability.

These rockets took advantage of the new Viking 6 and Vulcain series single-nozzle liquid-fueled engines. Whereas the Viking 6 (like its predecessors) relied on UDMH/N2O4, the Vulcain thruster family used liquid hydrogen and liquid oxygen (LH2/LOX) fuel. The Ariane 4 and 5 also used externally-mounted liquid-fueled or solid rocket boosters.

The first stage of the Ariane 4 can have two to four boosters added, including the solid rocket PAP (Propulseurs d'Appoint à Poudre) capable of generating 650 kN (150,000 lbf); or the liquid-fueled PAL (Propulseurs d'Appoint à Liquide) that used a single Viking 6 capable of generating 752.003 kN (169,057 lbf). 

The first stage is powered by four Viking 5C engines that generated 3,034.1 kN (682,100 lbf) of thrust. The second stage is powered by a single Viking 4B engine capable of generating 720.965 kN (162,079 lbf), while the third stage uses a single HM7-B LH2/LOX that generated 62.703 kN (14,096 lbf) of thrust.

Meanwhile, the Ariane 5 rocket relies on two Etage d'Acceleration à Poudre (EAP) solid rocket boosters that use a combination of ammonium perchlorate, aluminum, and Hydroxyl-terminated polybutadiene (AP/Al/HTPB). Two varieties are used, the P238 and P241, which generate a total thrust of 13,300 kN (3,000,000 lbf) and 14,160 kN (3,180,000 lbf), respectively.

Depending on the variant used, the first stage is powered either by a single Vulcain 1 or two Vulcain 2 engines, generating 1,015 kN (228,000 lbf) and 1,390 kN (310,000 lbf), respectively. The second stage was powered by a single Aestus engine that runs on MMH/N2O4 and generates 27 kN (6,100 lbf), or a single HM7B engine. 

The ESA also unveiled the Vega rocket in 2012, a three-stage launch vehicle that relies entirely on solid hydroxyl-terminated polybutadiene (HTPB). The first stage is capable of generating 2,261 kN (508,000 lbf) thrust, the second stage 871 kN (196,000 lbf), and the third stage thrust260 kN (58,000 lbf). It also has the option of a third-stage booster powered by a single Ukrainian-developed RD-483, a single nozzle, liquid-fueled (UDMH/N2O4) engine generating 2.42 kN (540 lbf) of thrust.

During this time, the India Space and Research Organization (ISRO) introduced a new series of launch vehicles. This included the Polar Satellite Launch Vehicle (PSLV) unveiled in 1993 and the Geosynchronous Satellite Launch Vehicle Mark II (GSLV Mk II). They also unveiled their first liquid-fueled engine, the single-nozzle Vikas thruster that runs on N2O4 and UH25 (75% UDMH; 25% hydrazine hydrate) and can generate 799 kN (179,622 lbf) of thrust. 

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The first stage of the PSLV uses an S139 rocket motor and six strap-on S139 boosters fueled by solid HTPB propellant and generates a total of 4800 kN (1.079 million lbf). The second stage uses a single liquid-fueled Vikas engine. The third stage relied on an S7 rocket motor that produced 240 kN (53,954 lbf), while the fourth stage used two L-2-5 engines capable of 14.66 kN (3,300  lbf). 

The first stage of the GSLV Mk II is also powered by a single S139 rocket motor, with four L40 strap-on boosters powered by a single Vikas, providing a total of 4800 kN (1.079 million lbf). The second stage relies on a single Vikas, while the third relies on a CE-7.5, India's first cryogenic engine that runs on LH2/LOX.

The China National Space Agency (CNSA) also saw some immense growth since the beginning of the 21st century. In 1999, they launched the Shenzhou spacecraft, an updated version of the Russian Soyuz spacecraft. The Long March 3 (CZ-3) series was introduced throughout the 1990s and early 2000s, as were the YF-20 liquid-fueled (N2O4/UDMH) engines that would power this rocket family.

The CZ-3 is a three-stage rocket, the first stage relying on four YF-21B engines that generated 2,961.6 kN (665,800 lbf) of thrust. The second stage is powered by a single YF-24D, or one YF-22D and four YF-23F Vernier engines, producing a maximum thrust of 741.4 kN (166,700 lbf) for the main engine and 47.1 kN (10,600 lbf) for the Vernier engines. The third stage is powered by a single YF-73 liquid-fueled (LH2/LOX) engine that generates 44.43 kN (9,990 lbf) thrust.

The three-stage Long March 3A (CZ-3A) is a slight variation, with the same loadout on the first two stages and two YF-75 (LH2/LOX) thrusters generating a maximum thrust of 167.17 kN (37,580 lbf). The Long March 3B is another variant with the same first and second-stage loadout as the 3A and four YF-25 (N2O4/UDMH) boosters that generate 740.4 kN (166,400 lbf) each.

The Long March 3C has the same specifications as the 3A but has two YF-25 boosters on the first stage. The Long March 4 (CZ-4) began operations in the early 2000s and included the CZ-4B and CZ-4C. The first stage was powered by four YF-21C engines capable of generating 2,961.6 kN (665,800 lbf) of thrust. 

The second stage was powered by one YF-24C or  YF-22C and four YF-23C Vernier thrusters. Whereas the main engine generated 742.04 kN (166,820 lbf), the Vernier engines generated 47.1 kN (10,600 lbf). The first and second stage of the CZ-4C was similarly outfitted, while the third was equipped with two YF-40A engines capable of generating 100.85 kN (22,670 lbf).

In 2016, China conducted the first launch of its Long March 5 (CZ-5) rocket, a two-stage heavy launch vehicle. Similarly, China has also unveiled the new YF-75 and YF-77 engines that run on LH2/LOX. The first stage of the CZ-5 is powered by two YF-77 engines that generate a combined thrust of 1,400 kN (310,000 lbf). The second stage is powered by two YF-75D engines that generate 176.72 kN (39,730 lbf) thrust.

This is augmented by four CZ-5-300 strap-on boosters, each equipped with two YF-100 thrusters that run on RP-1/LOX fuel and generate a total of 9,600 kN (2,200,000 lbf). A third stage is optional, which is powered by two YF-50D engines that generate 13 kN (2,900 lbf) of thrust.

The rise of "NewSpace"

Today, the number of launch vehicles, spacecraft, and options for propulsion has expanded greatly, thanks in large part to the rise of the commercial space (NewSpace) sector. The commercialization of space launch services has led to new rocket designs and propulsion systems. 

When it comes to commercial developers, the undisputed champion is SpaceX. In recent years, the company has become a world leader in rocket retrieval, reusability, and crew-capable launchers and spacecraft. They are also on the cutting edge of engine technology, as demonstrated by their Merlin and Raptor engines.

The most recent iteration of the Merlin, which is used on every member of the Falcon rocket family, is the Merlin 1D. This single nozzle, liquid-fueled (LOX/RP-1) engine can produce a maximum thrust of 981 kN (221,000 lbf). Then there's the Merlin 1D Vacuum+ variant optimized for thrust in space that also burns LOX/RP-1 and produces 934 kN (210,000 lbf) of thrust.

The two-stage Falcon 9 has 9 Merlin 1Ds on its first stage and one Merlin 1D Vacuum+ on its second stage. The Falcon Heavy consists of three Falcon 9 boosters, each of which has 9 Merlin 1D engines, while the second stage relies on a single Merlin 1D Vacuum+.

Then there is SpaceX's Raptor 2 single-nozzle engine, specifically designed for the next-generation Starship and Super-Heavy launch system. This liquid-fueled engine relies on a combination of LOX and liquid methane (LCH4) to generate 2,300 kN (510,000 lbf) thrust. Like its predecessor, there's also a variant optimized for space- the Raptor vacuum - which can generate 3,500 kN (786,830 lbf) of thrust.

The most recent design for the Super-Heavy first-stage booster includes 33 Raptor 2 engines, while the Starship orbital spacecraft relies on 3 Raptor 2s and 3 Raptor Vacuum engines.

Last, there are the Draco thrusters that power SpaceX's Dragon 2 spacecraft, the crewed version of their earlier payload delivery system. This spacecraft has a launch escape system that relies on eight SuperDraco engines, each of which can generate 73 kN (16,400 lbf) of escape thrust. Meanwhile, 16 Draco thrusters provide steering and maneuver capability, each producing 400 N (90 lbf) thrust.

Another major contender is Blue Origin, the commercial space company founded by Amazon founder Jeff Bezos. To date, Blue Origin has provided multiple flights to suborbit using their New Shepard launch vehicle. This reusable single-stage rocket relies on a single liquid-fueled (LH2/LOX) single-nozzle Blue Engine 3 (BE-3) capable of producing 490 kN (110,000 lbf) of thrust.

The partially-reusable New Glenn, still under development, is a two-stage rocket designed to deliver payloads to orbit. The first stage of this vehicle will be powered by seven BE-4 single-nozzle engines that rely on liquid natural gas (LNG) and LOX, each of which can generate 2,400 kN (550,000 lbf) of thrust. The second stage will be powered by a single BE-3U, an upper-stage variant capable of producing 710 kN (160,000 lbf) thrust.

United Launch Alliance (ULA) has been a major player in the commercial space industry for decades. The workhorse of this company is the Delta II rocket, a two-stage rocket that relies on the RS-27A gas-cycle liquid-fueled engine. This consists of a single-nozzle main engine and two Vernier thrusters capable of generating a total thrust of 4,822 kN (1,084,200 lbf).

The upper stage is powered by a single Aerojet AJ10-118K, a liquid-fueled (Aerozine 50/N204) producing 43.40 kN (9,757 lbf) of thrust. The rocket can also carry an optional third stage that relies on a single Star 48B solid propellant engine capable of producing 77.8 kN (17,490 lbf) thrust.

The Delta IV heavy-launch system comes in two versions. First, there's the Delta IV M+, a two-stage rocket that relies on a single Rocketdyne RS-68A engine (LH2/LOX) generating 3,137 kN (710,000 lbf) of thrust and two Orbital ATK solid rocket motors, with a peak thrust of 1,245.5 kN  (280,000 lbf). 

The upper stage relies on an Aerojet Rocketdyne RL10B-2 engine (LH2/LOX) that can produce a maximum thrust of 110 kN (24,750 lbf). The Delta Heavy version also has two additional RS-68A strap-on engines, giving the first stage a total thrust of 9,411 kN (2,115,677 lbf). 

The Atlas V heavy-launch system also comes in two versions. The Atlas V 400 series relies on an RD-180 (RP-1/LOX) duel-nozzle main engine (4,152 kN; 933,406 lbf) and three strap-on AJ-60A solid rocket boosters, producing 1,688.4 kN (379,600 lbf) of force each. Like the Delta IV, the upper stage relies on a single RL10B-2 engine. The Atlas V 500 series relies on up to five AJ-60A boosters.

The most recent addition to the ULA rocket family is the Vulcan heavy-launch vehicle, currently in development. The first stage of this rocket will be powered by two BE-4 engines, while the second stage will be powered by two RL10C engines. The rocket can also be fitted with two, four, or six solid rocket boosters.

The Atlas V 500 series and Vulcan launch vehicle can also carry the Centaur rocket stage, powered by one or two RL10 thrusters capable of generating 99.2 kN (22,300 lbf) each.  

Then there's RocketLab, founded in New Zealand and a rising star in the commercial space industry. The company's current launch vehicle, the Electron, is a two-stage reusable rocket that relies on its liquid-fueled Rutherford engine, which uses RP-1/LOX propellant. The first stage relies on 9 Rutherfords (optimized for sea level), producing a combined peak thrust of 224 kN (56,000 lbf). The second stage relies on a single (vacuum-optimized) Rutherford capable of producing 25.8 kN (5,800 lbf) thrust. 

The two-stage and reusable Neutron rocket (under development) will be powered by the gas-cycle Archimedes engine, which relies on gaseous methane (CH4) and LOX. The first stage will be powered by seven Archimedes engines capable of producing a combined peak thrust of 7,530 kN (1,640,000 lbf). In contrast, the second will be powered by a single Vacuum Archimedes capable of generating 1,110 kN (250,000 lbf) thrust.

Since the turn of the century, the age of modern rocketry and space exploration has come to be defined by two major trends. First, the emergence of several national space agencies has altered the playing field considerably. Whereas the Space Age was characterized by two superpowers engaged in a constant state of one-upmanship, the "New Space Age" is characterized by international cooperation by more than twenty agencies. 

Second, there is the rise of the commercial space industry (aka. NewSpace), which has fostered innovation and inspired a new era of cooperation between government and industry. The cost of going to space has decreased considerably through the development of retrievable and reusable rockets, single-stage-to-orbit rockets, and small launch vehicles that cater to the satellite market. 

This has increased access to space, allowing companies, universities, research institutes, and organizations to send low-cost missions to orbit. At the same time, commercial space companies are able to support space agencies like never before, performing everything from logistical to human-rated missions.

We've come a long way from the early days of Sputnik and the first humans to go into space. And while the launch vehicles we use today are based on the same principles and designs as those unveiled over sixty years ago, the innovation and advancements that have taken place since are undeniable. 

In the coming years and decades, further developments are expected that could revolutionize space exploration forever. These include new forms of propulsion technology that utilize validated technologies and experimental methods that rely on exotic physic.

These concepts promise rapid transit to destinations like the Moon and Mars, to interstellar travel.

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