Ad Astra: The Past, Present, and Future of Rockets!

An in-depth look at the origins of rocketry, how they've allowed us to go to space, and what the future could hold for them.
Matthew S. Williams

Sixty years ago, cosmonaut Yuri Gagarin became the first man to go to space. Just three years prior, the Soviets launched Sputnik I, the first artificial satellite to go to space. Within two years, ten more men and the first woman (cosmonaut Valentina Tereshkova) would join them by going to orbit as well.

Within a decade of Gagarin's historic flight, several more astronauts/cosmonauts would go to space, twelve would walk on the Moon, and dozens of robotic spacecraft would be sent to explore Mercury, Venus, Mars, and beyond. The Space Age was officially in full swing, and things have never been the same.

At every juncture in this tremendous leap was the science of rocketry. While the term "rocket science" is synonymous with genius, it actually has some pretty humble origins. In the Middle Ages, rockets were basically tubes packed with gunpowder designed to terrorize enemies with their horrible combination of noise and explosive force.

Today, rockets are responsible for deploying everything from telecommunication and internet satellites to astronauts and space stations. Beyond Earth orbit, they are used to send robotic explorer missions to every planet in the Solar System. Looking ahead, they may be the key to our "interplanetary" future or be replaced altogether!

Such is the nature of rockets. They are a delivery vehicle, and they are delivering our species to new a whole new phase of development. What we do with them once we get there (reuse, repurpose, or toss aside) remains to be seen.

Early Rockets

The use of gunpowder rockets likely goes back to the Song Dynasty of 13th century China, and the idea may have then been exported to Europe and the Middle East by the Mongol invasions in the mid-13th century. Henceforth, rockets would be used by militaries for various purposes, such as laying siege to fortifications and walled cities, as well as for fireworks. 

The name "rocket" is derived from the Italian word rocchetta ("little spindle"), which referred to their similarity in shape to the device used to hold the thread from a spinning wheel. The term was adopted into the French roquette by the mid-16th century and began appearing in English texts by the early 17th century.

Ad Astra: The Past, Present, and Future of Rockets!
Illustration of the "fire chariot" from the Wǔbèi Zhì (1621). Source: Taiwania Justo/Wikimedia Commons

By the late 18th century, the Kingdom of Mysore (present-day southern India) developed the "Mysorean rocket," which the British adopted by the early 19th century. Using compressed gunpowder and iron cases, these designs increased the range of military rockets from 100 to 2000 yards (~90 to 1830 meters).

In 1861, the Scottish astronomer, mathematician, and church minister William Leitch became the first to propose using rockets for the sake of space travel. In a book titled "God's Glory in the Heavens," he elucidated on the belief that humanity's ultimate destiny lay in space:

"Let us, however, attempt to escape from the narrow confines of our globe, and see it, as others see it, from a different point of view. Let us take a nearer survey of other orbs and systems, and see what impression they produce, as compared with that received from the platform of the Earth. But what vehicle can we avail ourselves of for our excursion?... The only machine, independent of the atmosphere, we can conceive of, would be one of the principle of the rocket."

Tsiolkovsky's "Rocket Equation"

It was also during the 19th century that scientists began distilling the fundamental principles of rocketry into a mathematical formula in earnest. This would come to be known as the "rocket equation," or ideal rocket equation, which actually had several authors - all of whom are believed to have derived it independently of one another.

The first recorded example was British mathematician William Moore, who published the equation in a study (1810) and then as part of a book titled "A Treatise on the Motion of Rockets" (1813). However, it would be Konstantin Tsiolkovsky, the Russian/Soviet physicist and "father" of the Soviet space program, who would be credited with the equation.

In 1903, he published a treatise titled "Exploration of Outer Space with Reaction Machines," in which he argued that the development of the rocket would allow humans to become a space-faring species. Not only did he present a mathematical formula for how a rocket engine would work, but his schematics became the basis of modern rocket designs. As he described it:

"Visualize the following projective: an elongated metal chamber (the shape of least resistance) equipped with electric light, oxygen, and means of absorbing carbon dioxide, odors, and other animal secretions; a chamber, in short, designed to protect not only various physical instruments but also a human pilot...

"The chamber is partly occupied by a large store of substance which, on being mixed, immediately form an explosive mass. This mixture, on exploding in a controlled and failure uniform manner at a chosen point, flows in the form of hot gases through tubes with flared ends, shaped like a cornucopia or a trumpet. These tubes are arranged lengthwise along the walls of the chamber.

"At the narrow end of the tube the explosives are mixed: this is where the dense, burning gases are obtained. After undergoing intense rarefaction and cooling, the gases explode outward into space at a tremendous relative velocity at the other, flared end of the tube. Clearly, under definite conditions, such a projective will ascend like a rocket."

Ad Astra: The Past, Present, and Future of Rockets!
Evolution of Tsiolkovsky's rocket designs. Source: Museum of Flight

American physicist Robert Goddard would also independently develop a rocket equation in 1912, when he began researching rocketry for spaceflight applications. This was followed by French engineer Robert Esnault-Pelterie and German-Austrian physicist Hermann Oberth deriving the same equation in 1913 and 1920, respectively.

Together, Tsiolkovsky, Goddard, Esnault-Pelterie, and Oberth are regarded as the "fathers" of modern rocketry and astronautics. This honorific is bestowed upon each of them because they had all conducted this research independent of one another and played a central role in the development of astronautics in their respective nations.

At its core, the rocket equation is a simple matter of calculus and can be expressed as:

Δv = ve 1n m0/mf = Ispg0 1n m0/mf

Where Δv (delta-vee) is the maximum change in velocity, ve is the effective velocity of the exhaust mass, 1n is the standard logarithmic function, Isp is the efficiency in which propellant is converted to exhaust (aka. the specific impulse with regard to time), g0 is the standard gravity, m0 is the initial total mass (including propellant), mf is the final total mass (once all the propellant is consumed).

This equation (and variations thereof), along with Tsiolkovsky's design specifications, would inform the development of modern rockets throughout the remainder of the 20th century — and still do today!

The Birth of Modern Rockets

In 1926, Goddard built the first modern rocket by switching from solid to liquid propellant and attaching a supersonic de Laval nozzle to a high-pressure combustion chamber. These nozzles turn fuel exhaust into highly directed jets of gas, drastically increasing engine efficiency and thrust, accelerating the rocket to hypersonic speeds.

During World War II, rockets advanced considerably as a result of being used as artillery. Examples include the Soviet Katyusha and the American T34 Calliope rocket launcher. But the most impressive was the Vergeltungswaffe-2 (V-2), the world's first guided ballistic missile. This weapon was developed by German rocket scientist Wernher von Braun, who was inspired by Oberth.

With the defeat of Nazi Germany, both the western Allies and Soviets captured a large number of rocket scientists (many of whom came voluntarily) and considerable amounts of research. With tensions mounting between the two powers after the war, both sides began leveraging the technology they had acquired to develop their own ballistic missiles.

The primary purpose of this research was to develop rocket systems that could deliver nuclear warheads, which were also being developed by both sides (the "Arms Race"). It did not take long for the Americans and Soviets to recognize the potential for high-altitude scientific research and space exploration as well, which led to a parallel "Space Race."

Aside from giving them access to space for military purposes (such as deploying spy satellites and nuclear weapons in orbit), the Americans and Soviets were also motivated by the desire for prestige. In the Cold War atmosphere, which was as much about ideology as weapons, it was felt that whoever "got their first" would score a massive propaganda victory over the other. 

Ad Astra: The Past, Present, and Future of Rockets!
A Soviet R-7 adapted to launch the Vostok spacecraft. Source: Sergei Arssenev/Wikimedia Commons

Space-Worthy Rockets

For the duration of the Space Race, all developments in rocketry were ultimately tied to the development of ballistic missiles. This was true not only for the United States and the Soviet Union, but all other nations that would establish space programs of their own.

In the United States, space-related research and development in the late 1940s and early '50s were overseen by the National Advisory Committee for Aeronautics (NACA) and consisted primarily of high-altitude flights with supersonic aircraft.

Meanwhile, the Soviet Union pursued research into space under the leadership of Sergei Korolev (1907–1966), who remained their chief designer until his death. With the assistance of the German rocket scientist Helmut Gröttrup, the Soviets began developing their own version of the V-2 rocket, which resulted in the R-1 in 1951.

Like the V-2, the R-1 was a single-stage rocket that relied on a single RD100 engine (an adaptation of the V-2 engine) that employed ethanol as a fuel and liquid oxygen (LOX) as an oxidizer. This design was rejected by Korolev, however, who wanted a ballistic missile with greater range and capability.

These efforts led to the development of the R-7 Semyorka by 1957, a two-stage ballistic missile capable of reaching targets over a distance of 5000 mi (8,000 km). The first stage relied on a core RD-108 engine and four strap-on boosters equipped with an RD-107 engine (fueled by LOX and kerosene), while the second stage relied on a single RD-108.

While the R-7 would quickly be replaced by more sophisticated intercontinental ballistic missiles (ICBMs), it would remain the workhorse of the Soviet (and later Russian) space programs and go through many variations. Using this rocket, the Soviets obtained an early lead in the Space Race and managed to "get there first" twice before the Americans. In fact, R-7 derived rockets were still in use 50 years later and helped assemble the ISS.

On October 4th, 1957, the Soviets launched the first artificial satellite to space, known as Sputnik-1 (Russian for "fellow traveler," or satellite in the astronomical sense). For 22 days, Sputnik-1 transmitted a simple radio signal and completed 144 orbits, then remained defunct until it burned up in Earth's atmosphere exactly three months after launch.

In response, Eisenhower signed the National Aeronautics and Space Act on July 28th, 1958. This Act created NASA, which took over NACA's research and was tasked with developing America's space program. With the assistance of von Braun, NASA developed the single-stage Redstone ballistic missile, which would later be adapted into a launch vehicle known as the Mercury-Redstone.

The Apollo Era (Human Spaceflight)

With artificial satellites now in space, the US and Soviet Union focused on developing crew-capable spacecraft that could send the first astronauts/cosmonauts to space. These would invariably involve larger, more powerful vehicles that relied on liquid oxidizers and various forms of combustible propellants to generate more thrust.

The Mercury-Redstone would be the first NASA booster capable of sending astronauts to space. This single-stage rocket eventually relied on a Rocketdyne A-7 engine and was capable of delivering a crew capsule to suborbital altitudes. The rocket was successfully flight-tested in November of 1960, and NASA appeared poised to send the first astronaut to space.

Unfortunately, the Soviets got their first once again! With the success of the Sputnik program, the Soviet Union set its sights on crewed missions, which resulted in the Vostok program. For the sake of this program, the Soviets developed the Vostok space capsule, which would launch atop an R-7 modified to carry it (Vostok-K).

On April 12th, 1961, Yuri Gagarin became the first man to go to space as part of the Vostok-1 mission. This was followed by Valentina Tereshkova (the first woman) going to space aboard Vostok-6 in 1963. This led NASA to expedite Project Mercury, which would send seven astronauts to space (the "Mercury Seven") between May 5th, 1961, to May 15th, 1963.

These missions relied on the Mercury-Redstone and more powerful Atlas-Mercury, a variant on the Atlas ICBM that launched the last four crewed missions. The Atlas-Mercury rocket was a "stage-and-a-half" vehicle, consisting of two external boosters that relied on a Rocketdyne XLR-89-5 engine and a core stage equipped with a Rocketdyne XLR-105-5.

NASA followed up with Project Gemini (1961-66), a crewed spaceflight program designed to develop techniques, technology, and expertise that would later be used to land astronauts on the Moon. For these missions, NASA adopted the two-stage Titan II rocket, which consisted of a first stage that relied on two LR-87-AJ7 engines and a second stage that used a single LR-91-AJ7 engine.

Between 1957 and 1967, NASA also began work on the Saturn family of rockets that would eventually result in the three-stage Saturn V. With a first-stage equipped with five Rocketdyne F-1 engines, a second stage equipped with five Rocketdyne J-2, and a third stage equipped with one J-2, the Saturn V was the most powerful rocket ever built.

It was this rocket that would take the Apollo astronauts to the Moon. The Apollo Program, which was authorized in 1960, sent a total of twelve astronauts to the Moon between 1969 and 1972. The first was the Apollo 11 mission, where astronauts Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon on July 20th, 1969.

The Soviets also attempted to take the next leap in crewed space exploration. This resulted in the Voskhod program, the redesigned Vostok spacecraft (crews of 2 to 3), and the more powerful two-stage Voskhod rocket. Also derived from the R-7 booster, the Voskhod was based on the earlier Molniya rocket, which had a more powerful upper stage equipped with an RD-0107 engine.

This was followed by the Soyuz program in 1963, which led to the development of the three-stage variant of the R-7. The Soyuz rocket would rely on four strap-on boosters with RD-107 engines, a first stage equipped with an RD-108, and a second stage equipped with an RD-0110.

The Soviets also attempted to develop a lunar rocket known as the N1 - L3, a five-stage rocket that had 30 NK-15 engines (first stage), 8 NK-15V engines (second), 4 NK-21 engines (third), and one NK-19 engine (fourth). By 1974, budget issues, the death of Korolev, and a series of failed launch attempts led the Soviets to abandon the N1 and their plans for a crewed mission to the Moon.

Ad Astra: The Past, Present, and Future of Rockets!

First orbital flight of India's GSLV Mk III. Source: ISRO/Wikimedia

Other Nations Join the Space Race

In between all of these developments, a number of other nations began their own space programs. For instance, China was also motivated by the Soviet's success with Sputnik to develop their own launch vehicles and capacity. Between 1958 and 1960, this led to the development of sounding rockets adapted from the Soviet R-2.

By 1967, China began to pursue a crewed space program as well. This led to the three-stage Chang Zhen-1 (CZ-1, Long March-1) in 1970 and the two-stage Feng Bao-1 rocket in 1972. Whereas the CZ-1 used four YF-2A engines (first stage), a single YF-2 (second), and a single GF-02 (third), the FB-1 used four YF-20A engines (first stage) and one YF-22/23 engine (second). 

The development of these, and other, rockets were spurred on by China's efforts to create its own ICBMs. With the death of Chairman Mao Zedong, progress in China's space program stalled until the 1980s, at which point, work resumed, and more rockets were added to the Long March family.

India followed a similar path towards a crewed spaceflight program. In 1962, Prime Minister Jawaharlal Nehru ordered the creation of the Indian National Committee for Space Research (INCOSPAR), which would later become the Indian Space Research Organization (ISRO).

However, the organization would rely on the Soviets to launch their first satellites to space until 1980. It was at this point that the first Indian-made rocket was created, the Satellite Launch Vehicle-3 (SLV-3), which relied on a single solid-propellant engine.

By the 1990s, the ISRO unveiled their  Polar Satellite Launch Vehicle (PSLV), a four-stage launch vehicle that relied on 6 solid rocket boosters, a single S139 engine (first stage), a single Vikas engine (second), a solid rocket motor (third), and two PS4 engines (fourth).

In 2001, India unveiled the three-stage Geosynchronous Satellite Launch Vehicle (GSLV), powered by four liquid-propellant strap-on boosters, a first stage powered by a single S139 engine, a second stage powered by a Vikas, and a third stage powered by a CE-7.5 cryogenic engine.

The nations of Europe were also moved to action by the onset of the Space Race between the two global superpowers. By the 1960s, this led to the formation of the European Space Research Organization (ESRO), which would be reformed into the European Space Agency (ESA) in 1975.

The ESRO's early efforts to develop a satellite launcher resulted in the creation of the three-stage Europa rocket. The first stage of this launch vehicle was powered by two Rolls-Royce RZ-2 engines, the second stage relied on a four-chambered engine, and the third was powered by a liquid-propellant main engine and two attitude-adjustment (aka. vernier) engines.

The program failed to produce a working launch vehicle, but after 1979, the ESA used the Europa program to inform the development of the Ariane rocket family. These consisted of the two-stage Ariane 1-3 rockets (1979-1989), which were powered by four liquid-fueled Viking engines (first stage) and a single Viking (second).

Ad Astra: The Past, Present, and Future of Rockets!
The Ariane rocket family, visualized. Source: ESA

The Ariane 2 featured an elongated second stage, while the Ariane 3  carried two additional solid rocket motors to boost its performance. Its third stage used a cryogenically fuelled HM7B engine, burning liquid hydrogen in liquid oxygen. On some flights, a Mage 2 kick motor was flown as a fourth stage. In 1988, the ESA unveiled the three-stage Ariane 4, which had a number of variants. It incorporated four Viking 2B engines (first stage), one Viking 4B (second), and one HM-7B (third).

The Space Shuttle Era

With the closing of the Apollo Era, the United States and the Soviet Union began contemplating what would come next. With both nations having proven that they could send personnel to space, their focus transitioned to developing technologies that would make space more accessible and long-duration stays possible.

For NASA, these efforts led to the creation of the Space Shuttle, which consisted of the reusable Space Shuttle Orbiter (SSO) vehicle, two recoverable solid-propellant rocket boosters (SRBs), and an expendable external fuel tank (ET). The SSO was equipped with three Aerojet Rocketdyne RS-25 engines, which would fire in tandem with the boosters to reach space.

During the launch, the SRBs would provide close to 75% of the total thrust, then break off and fall away after exhausting their solid propellant. They would then deploy parachutes to make a soft landing in the ocean, where they would be recovered for later use. The ET, meanwhile, would replenish the SSO's engines and detach to burn up in orbit.

The first shuttle, Enterprise, was unveiled in 1976 and served as a testbed for the technology, though it never flew to orbit, being launched from a modified Boeing 747. In total, five fully orbital shuttles were built between 1976 and 1991, which included the space shuttle Columbia, Challenger, Discovery, Atlantis, and Endeavour. Before being retired in 2011, two shuttles would be lost, the Challenger in 1986 and Columbia in 2003.

The Soviets also built a reusable spacecraft to compete with the Space Shuttle, which yielded the Buran ("Snowstorm"). The orbiter element of this system relied on thrusters that were intended for orbital maneuvers only. The Energia heavy launch system, which was solely responsible for reaching orbit, relied on four RD-170 strap-on boosters and four RD-0120 engines in the central block.

Unfortunately, the program ran out of funds after a single flight in 1988, and the program was canceled after 1991. Thereafter, the Russian space agency (Roscosmos) would continue to rely on its Soyuz rockets and other modifications of the R-7 to provide launch services. After the retirement of the Space Shuttle, this included NASA astronauts destined for the International Space Station (ISS).

During this same time, space programs in other countries around the world advanced and matured. In 1996, China unveiled the three-stage Long March 3, which relied on four external boosters (YF-25), four YF-21C engines in its first stage, a single YF-24E (or a YF-22E main and YF-23F vernier engine) for its second stage, and two YF-75 for the third.

Ad Astra: The Past, Present, and Future of Rockets!
The five Orbital Vehicles of the Space Shuttle Program. Source: NASA

An optional fourth stage could be attached: the Yuanzheng reusable upper stage, powered by a single YD-50D engine. Between 1988 and 2006, the Chinese introduced their Long March 4, a three-stage rocket that relied on the same engines as the Long March 3 for the first stage. The second and third stages were upgraded with a single YF-24C and two YF-40s.

Also, in 1996, the ESA premiered the Ariane 5 heavy-launch vehicle. This vehicle consisted of a core stage that featured the new Vulcain engine and two solid rocket boosters, each powered by a solid-propellant EAP P238 engine.

Between 2016 and 2019, China conducted a series of launches with their Long March 5 rocket, a two-stage launch vehicle that is central to their future plans in space. Powering this heavy rocket are four boosters equipped with two cryogenic-propellant YF-100 engines apiece, a first stage with two YF-77s, a second stage with YF-75Ds, and an optional Yuanzhenge powered by two YF-50Ds.

The Space Shuttle Era Ends

In 2004, the Bush administration unveiled its "Vision for Space Exploration" that included (among other things) the retirement of the Space Shuttle by 2011 and the creation of a new family of heavy launch vehicles. This led to the Constellation Program (2005-2010), which produced designs for two new rockets - the Ares I and Ares V - in order to return to the Moon by 2020.

The Ares I was a two-stage rocket intended to launch crews to orbit the Moon and beyond. The design called for a first stage that relied on a solid-propellant rocket booster and a second stage relied on two Rocketdyne J-2X engines. A prototype, the Ares I-X, successfully launched from the Kennedy Space Center on October 28th, 2009.

The Ares V was the cargo launcher of the program, consisting of a two-stage rocket with two solid rocket boosters - the same type used by the Space Shuttle. The first stage was to be equipped with 5 or 6 Aerodyne Rocketjet RS-68B engines or 5 RS-25s (also used by the Space Shuttle).

However, the program was canceled in February of 2010 by the Obama administration, due to the global financial crisis taking place at the time. By April, the Obama administration had announced a new policy, in the form of the 2010 NASA Authorization Act, which mandated the retirement of the Space Shuttle by 2011, and greenlighted the development of the Space Launch System (SLS).

The design for the SLS was informed by the Ares rockets and called for a heavy launch vehicle capable of sending both cargo and crews to space. It would consist of a two-stage rocket with two solid rocket boosters, four RS-25s, and a second stage propelled by either an Aerodyne Rocketjet RL10 engine.

Once complete, the SLS will be the most powerful launch vehicle in the world since the Saturn V. However, it is already looking at some pretty stiff competition from other space agencies and a new class of contenders - the commercial space sector!

Ad Astra: The Past, Present, and Future of Rockets!
Heavy launch systems, compared. Source: Thorenn/Wikimedia Commons

NewSpace Ascendant

If there's one thing that has come to characterize the modern space age, it's the way commercial space companies (aka. NewSpace) have made their presence felt. While commercial manufacturers like Lockheed Martin, Boeing, and Northrop Grumman have been involved since the early days of the Space Age, these were primarily defense and aviation companies that executed government contracts.

These days, NewSpace has come to exist as a standalone industry dedicated to space exploration that provides launch services to government agencies, private companies, research institutes, and other contractors. Among them are commercial leaders like Blue Origin, SpaceX, and United Launch Alliance (ULA).

In all cases, these companies were founded with the vision of increasing access to space by leveraging technological advancements - reusable vehicles, new materials, new fabrication processes, etc. - to reduce the costs associated with launching payloads and crews to space.

Blue Origin was one of the first, founded by Amazon founder and CEO Jeff Bezos in 2000. To date, the company has developed only one operational launch system, known as the New Shepard. This single-stage reusable vehicle relies on a single LOX/LH2 engine - the Blue Engine-3 (BE-3) - to make suborbital flights.

This will be followed by the New Glenn, a two-stage partly reusable orbital launch vehicle that will consist of a reusable first stage with seven BE-4 engines powered by liquid oxygen and liquid natural gas (LOX/LNG) and an expendable second stage with two re-ignitable BE-3U engines.

SpaceX, created in 2001 by Paypal and Tesla founder Elon Musk, has made considerable progress by comparison. Between 2010 and 2021, SpaceX has successfully tested multiple launch systems, made them commercially available, and secured contracts with space agencies and major corporations to launch payloads and even astronauts (restoring domestic launch capability to the US for the first time since 2011).

Among these are the Falcon 9, a two-stage launch vehicle that debuted in 2010 and became the world's first orbital-class reusable rocket. Its reusable first stage features nine Merlin engines, which are fueled by rocket-grade kerosene (RP-1) and liquid oxygen, while the second stage is powered by a single Merlin engine optimized for vacuum.

This was followed by the Falcon Heavy in 2018, a two-stage partly-reusable heavy launch vehicle. The design incorporates two Falcon 9 first stages acting as strap-on boosters that connect to a structurally augmented Falcon 9 core stage - all of which are reusable. These are powered by nine Merlin engines each, while the second stage relies on a single Merlin optimized for vacuum.

Then there is the Starship super-heavy launch system, which is currently undergoing development and flight testing in South Texas. This rocket will be the first fully reusable and most powerful launch system ever built, consisting of the Starship reusable spacecraft and the Super Heavy first stage.

The Starship and Super Heavy will rely on 28 and 41 of the company's new Raptor engines (respectively), which run on a combination of liquid methane and liquid oxygen (CH4/LOX). The system will also rely on orbital refueling, where a separate tanker vehicle rendezvous' with the Starship in orbit and provide it with the propellant it needs

In 2006, ULA was created from a joint venture between Lockheed Martin Space and Boeing, Defense, Space, and Security. Today, their fleet consists of the Atlas V, the fifth iteration of the rocket that played a vital role in the Mercury and Gemini missions. This expendable rocket consists of two stages and can carry up to five solid strap-on boosters.

The first and second stages are powered by an RD-180 and an RL10-1 engine, which rely on RP-1/LOX and LH2/LOX propellant, respectively. The ULA also maintains a fleet of Delta II and Delta IV Heavy rockets, the former having retired in 2018. Nevertheless, this two-stage rocket completed 155 missions, with a streak of 100 successful launches in a row after 1997.

The first stage employed a single RS-27/RS-27A (RP-1/LOX) and up to 9 solid strap-on boosters, the second stage an AJ10-118K (N2O4/Aerozine 50), and an optional third stage a single solid-propellant Star 48B. The first stage of the Delta IV will rely on a single RS-68/RS-68A engine, as well as four medium GEM 60 or two heavy CBC boosters. The second stage will rely on a single RL10B-2 engine, and both it and the first stage will burn LH2/LOX.

Before 2021 is over, the ULA plans to conduct a maiden flight with their new Vulcan Centaur heavy-lift system. This two-stage rocket will be partially reusable and consist of a first stage that relies on a Blue Origin BE-4 engine and up to six GEM-63XL strap-on boosters. The second stage will consist of the ULA's new Centaur V vehicle, which is powered by two RL-10 engines.

Since their inception, each of these companies has played a major role in the advancement of launch vehicle technology and the gradual reduction of launch costs. And they are hardly alone! Other advancements include single-state-to-orbit (SSTO) rockets - like the Romanian company ArcaSpace's Haas 2 expendable rockets, which rely on the company's Executor aerospike engine (fueled by a LOX/Kerosene mix).

There's also New Zealand-based startup Rocket Lab, which specializes in the creation of lightweight launch vehicles built using carbon composites for the small satellites market. This consists of the two-stage Electron rocket, which relies on 9 of the company's LOX/Kerosene Rutherford engines for the first stage and one for the second.

The Rutherford engine is a major innovation in itself, being the world's first rocket to incorporate 3D-printed elements and an electric-pump-fed rocket engine. By 2024, the company plans to unveil its two-stage heavy-lift Neutron rocket, consisting of a reusable first stage and expendable second stage, both of which will also be powered by Rutherford engines.

The Future

What does the future hold for rockets? The answer depends on what kinds of technological developments happen and when as this century unfolds. For instance, we could be just a few years (decades at most) away from nuclear rockets - i.e., ones that rely on nuclear reactors to heat liquid hydrogen or other propellants.

Rockets could also be used to provide intercontinental flights in the near future, which is something Elon Musk has promised (and China claims to be exploring as well). According to his proposals, the Starship and Super Heavy would offer point-to-point flights between sea platforms located offshore from major cities.

But if recent developments are any indication, the future of rocketry is likely to be one where declining costs, reusable vehicles, and advances in fuel and engine technology open up entirely new opportunities for exploration, research, commercial activities (like asteroid mining), and tourism in space.

At the same time, considerable research is directed towards replacing rockets with horizontal takeoff and landing (HTOL) vehicles - aka. reusable spaceplanes. There's even the possibility of sending payloads and crews to orbit using a Space Elevator, a tensile structure that extends from the Earth's surface to a station in orbit (and is kept rigid by a counterweight).

With technologies like these readily available, rockets could very well fall into disuse, at least on Earth. If humans do become "interplanetary" - a Space Elevator would certainly help in that regard - rockets could find a second life on other celestial bodies, where the lower gravity makes it a lot easier and cheaper to launch things to space.

Who knows what the future will hold? All we know for certain is that rockets got us this far, and are expected to take us a lot further very soon!

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