Dawn of the Space Age: Past, present, and future of propulsion technology

Since then, there have been further changes, from expendable launch vehicles to space planes and rockets that can be flown multiple times. In these sectors, there's been no shortage of innovation and advancement.

But in other sectors, things have not changed much since the days of the Mercury, Vostok, and Apollo programs. 

In fact, today's space capsules and rockets are very similar in appearance to those used by our grandparents - albeit with hugely updated avionics and navigation systems.

As for propulsion, solid rocket boosters and liquid-fueled engines are still the mainstays of launch vehicles. When it comes down to it, space launches are still beholden to the Rocket Equation.

However, that does not mean that there haven't been some very interesting developments over the years. These include propulsion technologies already in use today, such as ionic propulsion.

And some concepts date back to the early Space Age but were never realized, like nuclear rockets, fusion drives, antimatter engines, and even more exotic ideas.

Once realized, these technologies could make interplanetary (and even interstellar) missions a reality. To understand where the future of spaceflight could be taking us, it is important first to address how we got here. Buckle up because the history of rocket and aeronautical propulsion is a long and fascinating one.

History

The field of rockets, aka. "reaction engines," dates back to 13th century China, where the Song Dynasty used gunpowder-filled tubes to fashion fireworks, artillery, and multiple launch rocket systems (MLRSs).

The technology was spread across Asia and Europe with the Mongol conquests of the mid-13th to early 14th century. Throughout the Middle Ages and Renaissance, rockets evolved considerably as a weapon used in siege warfare.

By the 19th century, the first proposals were made for using rockets for the sake of human spaceflight. One of the first was made by Scottish astronomer and clergyman William Leitch in an essay titled "A Journey Through Space" (1861):

"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 impressions they produce, as compared with that received from the platform of the Earth…."

"The only machine, independent of the atmosphere, we can conceive of, would be one on the principle of the rocket. The rocket rises in the air, not from the resistance offered by the atmosphere to its fiery stream, but from the internal reaction. The velocity would, indeed, be greater in a vacuum than in the atmosphere, and could we dispense with the comfort of breathing air, we might, with such a machine, transcend the boundaries of our globe, and visit other orbs."

In 1896, Russian-Soviet scientist Konstantin Tsiolkovsky, "the father of aeronautics," began developing the theory and designs that would provide the foundation for further advances in spaceflight. This included the "Rocket Equation," which he formalized in 1897 and published in his 1903 paper," "Exploration of Outer Space by Means of Rocket Devices."

The equation established the relationship between the change in the rocket's speed (delta-v), the exhaust velocity of the engine (ve), and the initial and final mass of the rocket (m0, mf).

He also described rockets with steering thrusters, multistage boosters, space stations with airlocks, simulated gravity (rotating stations), and closed-cycle bioregenerative life support systems that would support the crew. His paper also contained a schematic for a rocket that became the basis for all rocket designs.

Early Rockets

During World War II, Germany developed the first liquid-fueled rockets to achieve spaceflight. These were known as the V-2 rocket designed by chief rocket scientist Wernher von Braun.

In June 1944, the V-2 became the first artificial object to cross the official boundary between Earth's atmosphere and space - aka. The Kármán Line (62 mi, 100 km altitude).

After the war, multiple V-2 rockets were seized by Allied and Soviet scientists, who began reverse engineering them as part of their own rocketry programs. The initial purpose was to create ballistic missiles that could deliver nuclear warheads, but these programs eventually led to the creation of the first space-faring rockets.

In the Soviet Union, these efforts led to the R7 Semyorka rocket designed by Soviet lead engineer Sergei Korolev. This rocket made its inaugural flight in 1957 and was the world's first intercontinental ballistic missile and launch vehicle.

The rockets relied on four strap-on boosters, each of which had a four-chamber Glushko RD-107 engine and a core four-chamber RD-108 engine. 

These used a mix of kerosene and liquid oxygen (LOX) as propellants. The combined thrust of this rocket was equal to around 3,904 kilonewtons (kN) or 877,654 pounds thrust (lbf).

In the United States, similar efforts were made under Operation Paperclip, where wartime German rocket scientists, including Werner Von Bruan, were recruited to assist in creating America's space program.

By 1958, these efforts gave rise to the single-stage PGM-11 Redstone rocket that relied on an NAA Rocketdyne 75-110 A liquid-fueled (ethyl alcohol/LOX) engine - which generated around 350 kN (75,000 lbf) of thrust.

These were known as gas-generator cycle engines, where a small trace of fuel is burned in a gas generator, and the resulting hot gas is used to power the propellant pumps.

They combine two propellants, a reaction mass and an oxidizer (LOX), in a reaction chamber where they are ignited and channeled through nozzles to generate thrust. 

The 'Space Race'

In 1957, a new arena of competition opened between the Soviet Union and the United States. Previously, attempts to develop rockets were confined to the "Arms Race" between these two power blocks.

But with the Soviet launch of the first artificial satellite (Sputnik 1) into orbit on October 4, 1957, the "Space Race" officially began. After that, the development of heavier, more elaborate rockets had both military and peaceful applications.

This included heavy launch vehicles and multistage rockets designed to send heavier payloads into space, including the first astronauts and cosmonauts. Once again, the Soviets took an early lead with their Vostok program (1960-1963), which led to the development of the three-stage launch vehicle and spacecraft of the same name.

Also known as the 8K72K (or Vostok K), this rocket was derived from an earlier launch vehicle that carried the first Soviet uncrewed probes to the Moon (as part of the Luna Program).

Like the R7, the first stage booster relied on four strap-on engines, each of which had a four-chamber and nozzle RD-107 gas-generator cycle engine. The core of the first stage relied on a four-chamber RD-108, while the second stage had a single RD-109.

These engines all ran on a combination of LOX and kerosene propellant. Each strap-on booster also had two Vernier engines for maneuvering, while the core had four such thrusters.

The total thrust of the core stage and boosters was equal to about 4,795.4 kN (1,078,050 lbf), while the second stage generated 54.5 kN (12,257 lbf) of thrust.

The Vostok spacecraft consisted of a capsule and an external module that contained cold-gas thrusters and a retro engine. The thrusters provided attitude control while in space, while the TDU-1 (S5.4) main engine was only used for the re-entry braking maneuver and used a mix of TG-02 propellant (a 50/50 mix of triethylamine and xylidine) and AK20F (a mix of 80/20 nitric acid and dinitrogen tetroxide) for propellant. 

This rocket and spacecraft sent cosmonaut Yuri Gagarin to orbit on April 12, 1961, making him the first man to go to space. They also sent the first woman, Valentina Tereshkova, to space on June 16, 1963. These were the first and last flights of the Vostok Program, which flew six missions and cosmonauts to space in total.

These efforts were mirrored by NASA's Mercury Program and the development of the Mercury-Redstone launch vehicle. This rocket relied on an A-7 Rocketdyne, derived from the 75-110 A, and also burned a combination of LOX/ethyl alcohol and could generate 350 kN (78,000 lbf).

The program also benefitted from the new Mercury-Atlas LV-3B rocket, which relied on a Rocketdyne XLR-105-5, a LOX/kerosene gas-cycle engine that generated 363.22 kN (81,655 lbf) of thrust.

The Mercury spacecraft was outfitted with three cool-gas rockets designed to provide attitude control and brake the spacecraft during reentry. These rockets and spacecraft were used to send six of the Mercury 7 astronauts to orbit between 1961 and 1963.

The first flight (Freedom 7) saw Alan Shepard become the first American astronaut to go to space on May 5, 1961, while L. Gordon Cooper flew the final mission (Faith 7) on May 15, 1963.)

Foundations laid

The concepts and designs introduced during the Space Race tell an interesting story. Not only were they derived from research that went back half a century. They would also influence the design of rockets, spacecraft, and engines for over half a century to come.

The Space Age also saw the rise of familiar names in the space industry, such as engine designers Glushko and Rocketdyne.

The adoption of liquid-fueled gas-generator cycle engines for boosters, and the relegating of solid propellant rockets for maneuvering thrusters, was also established during this time.

Today's rockets continue to rely on (for the most part) a combination of kerosene (or some variant thereof) and LOX as an oxidizer to send spacecraft to orbit. Once there, solid rockets, retrorockets, and cold gas thrusters do the rest.

While much of the Soviet and American space programs' early accomplishments were owed to German rocket scientists (like Werner von Braun), other scientists, like Sergei Korolev, Robert Goddard, Hermann Julius Oberth, and Robert Esnault-Pelterie played a major role.

Moreover, these scientists (to varying extents) drew from Tsiolkovsky'sprograms' early designs and the famous Rocket Equation.

Through collaboration between government and industry and competition between national space programs, rocket engineers established a framework in the late 1950s and early 60s that would lead to even more ambitious goals by the late 60s and early 70s.

Even today, as rocket engineers prepare for the next great leap - returning to the Moon and sending the first astronauts to Mars - we are still working within that framework.

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