Introduction

Quite surprisingly, rockets in their primal form were invented before turbomachinery, even though turbines and pumps are both present in modern launcher engines. However, it is interesting to note that  both can be traced to the same ancestor. In this post we will discuss some of the history and technical evolution of rockets and turbomachinery – and this all starts with an old pigeon.

Rockets

Circa 400BCE, a Greek philosopher and mathematician named Archytas designed a pigeon-like shape made out of wood that was suspended with wires and propelled along these guides using steam demonstrating the action-reaction principle long before Newton formalized it as a rule in Physics. As we know today, the faster and the more steam escapes the pigeon, the faster it goes. Turn this 90 degrees to have the bird face upward, and you have a very basic rocket concept. However, rockets are a lot more complex than this, and do not typically use steam (except in the case of liquid hydrogen + liquid oxygen propellants) as the propelling fluid. 

The next major development in terms of rocket evolution was most likely due to accidents around 100AD when bamboo tubes filled with a simple form of gunpowder were thrown in fires to provide explosions during religious festivals in China – some may not have blown up as intended and instead flew off propelled by the exhaust gases of the burning gunpowder. Based on such results the Chinese began experimenting on gunpowder-filled tubes and eventually attached them to arrows to increase their range when shot from bows before realizing the bows were not actually needed and that these mini-rockets with solid propellants could be fired from long sticks set on stands on the ground. These “arrows of flying fire” were used extensively at the battle of Kai-Keng in 1232 by the Chinese to repel Mongol invaders which marked the first time rockets were used for non-entertaining purposes.

Indeed, medieval and early modern rockets were used militarily as incendiary weapons in sieges with thousands shot at once, more for their impact destruction power as it was the case for the Mysorean rockets. Used against the British several centuries ago, they were fired while attached to swords through a bamboo shaft and could travel almost 1,000 yards with 1 pound of powder. The British Congreve rockets followed suit and eventually changed the stick location from the side to the center of the rocket for added flight stability.

Many technological evolutions came to rockets throughout the 13^th and 15^th centuries, such as: improved propellant (Roger Bacon in England) for increased range; use of launch tubes (Jean Froissart in France) – which was the forerunner of the modern bazooka – for better accuracy; spin stabilization, and so on. The need for a stick was removed thanks to William Hale, who in 1844 modified the rocket design to have a vectored thrust that produces a spinning effect therefore increasing accuracy and significantly enhancing range due to reduced air resistance.

As early as 1898, rockets were considered a possible mean for space exploration. This idea was brought forth by a Russian schoolteacher named Konstantin Tsiolkovsky, who is considered the Father of modern astronautics. Although no actual tests/prototypes were developed, he also suggested using liquid propellants in order to increase range instead of solid propellants used in the past for fireworks and weapons.

The infamous V-2 rocket comes as a famous result of these ideas although the first liquid propellant rocket took flight in 1926 to destroy a nearby cabbage patch after dropping from an altitude of 12.5 m; it was powered by liquid oxygen and gasoline. The V-2 was developed by the Germans during WW2 by the Verein für Raumschiffahrt (Society for Space Travel) and used liquid oxygen and alcohol burning at 1 ton every 7 seconds and could be fitted with a warhead for destruction purposes if desired. Alcohol had been selected as the fuel due to its lower combustion temperature since cooling of the nozzle was an otherwise issue. It is also in this timeframe that the first turbopump was employed.

After the war, rockets were mostly used to study high-altitude conditions (temperature, pressure, cosmic rays, etc.) especially after nuclear tests before being turned toward space exploration recently.

Turbomachinery

About 300 years after the wooden pigeon, the aeolipile came to life by another Greek called Hero of Alexandria (very modest name if you ask me). Once again steam was the working fluid here, and was used to rotate a hollow sphere mounted with pipes in a fire-heated water basin. The liquid water turning to steam rose in the pipes, filled the sphere, and then escaped from it through 2 L-shaped pipes located at diametrically opposite positions. This configuration allowed the aeolipile to rotate by giving it a circumferential thrust. This is essentially how reaction turbines function and it used the same fundamental principle as Archytas’ pigeon.

In about 200 BCE, water wheels started emerging in the Mediterranean region. These are thought to be the first turbomachines to have existed for the purpose of producing mechanical work (typically to grind grains) to eventually turn into practical hydroelectric water turbines in the 1880s. Other forms of primitive turbines include applications like the smoke jack in which a 4-vaned turbine (in Leonardo da Vinci’s design of 1607) was actioned by means of smoke rising in a chimney. The turbine was positioned in the narrowest part of the chimney to utilize the most kinetic energy from the smoke. It was attached to a set of gears which rotated the spitroast on which dinner was being cooked, therefore ensuring an even cook all around.

It was then understood that the force of the machine depended on the draught of the chimney (air flow rate) and the strength of the fire (air temperature) which were fundamental concepts at the time and led to various technological developments, especially in the area of cooling.

It took approximately 200 years between the first mentions of steam turbines between 1629 (Giovanni Branca in Italy) and 1648 (John Wilkins in England) and the birth of the steam turbine as we know it today. The first impulse type turbine was invented by Carl Gustaf de Laval in 1883 which was followed by the first reaction type turbine the following year by Charles Parsons. During the Industrial Revolution, these turbines were used extensively and started replacing other means of power production that had lower efficiencies like wind and water power. Parsons’ turbine originally generated 7.5 kW of electricity and was found easy to scale up (up to 50 MW during Parsons lifetime).

The concept of gas turbine, as we know it, was introduced in 1791 in a patent written by John Barber in England and which was designed to power a horseless carriage. It was not until 1903 however that a Norwegian, Ægidius Elling, built the first gas turbine that could produce a positive net power (11 hp). Since then the race for efficiency has been on the mind of engineers all around the world. As seen in Da Vinci’s observations increasing the turbine inlet temperature leads to high power. However, one needs to ensure that the thermo-structural integrity of the turbine components is not compromised even when operating at temperatures above the metal’s melting point ,therefore requiring various cooling methods. The current gas turbine efficiency record is held by Mitsubishi Heavy Industries with their M501J turbine featuring a 60+% efficiency and having a turbine inlet temperature of 1600C.

Similarities in Technological Innovations/The Modern Rocket

The modern rocket configuration comes from Robert Goddard who in 1912 made three discoveries which improved on solid propellant rockets:

1. Burning of the fuel can be achieved in a small combustion chamber instead of building the entire propellant tanks to withstand high pressures. This lower tank pressures allows for significantly decreasing the mass of the rocket by thinning the walls and therefore increasing its efficiency. The combustion chamber pressure is achieved through pumps (typically one pump for each propellant).
2. Rockets can be staged in tandem. This allows for empty propellant tanks and their engines to be discarded from the rocket, therefore reducing its mass. Each successive stage is typically optimize for its specific operating conditions based on the altitudes at which it will operate. Since gravity is higher at ground level the bigger (and therefore heavier) engines are used for the takeoff phase and a few minutes after this, they would become substantial dead weight if still strapped to the rocket. To date, no single-stage rocket has successfully reached orbit; 2-stage configurations are quite common even though up to 5 stages have been used. Notable efforts are being made for single-stage to orbit rockets like the Reaction Engines Skylon, the DC-X or the Lockheed Martin X-33.
3. The rocket exhaust speed (and therefore its thrust) can be significantly increased by using a De Laval nozzle. This allowed making the transition between 2% efficient engines with subsonic exhaust to 64% efficient engines with hypersonic, highly direct jet exhaust. This allowed an increase in thrust, more than doubling thrust produced.

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In the same way turbomachinery in-between blade channels need to be careful design (including determining the best number of blades) to provide a compromise of efficiency and smooth flow (less friction and less separation – phenomenon in grossly over-expanded nozzles and causes a non-uniform jet around the engine axis therefore leading to a side force being developed and tilting the overall engine), rocket nozzles should feature a flow that is adapted to the ambient conditions such that it is neither grossly over nor under-expanded. Also due to lightweight considerations in most cases, the nozzle geometry is fixed but special apparatus exists (aerospike for instance) that allow modifying the geometry so that the nozzle remains optimal regardless of the ambient pressure at the given altitude.

On 16 March 1926, Robert Goddard launched the world’s first liquid-fueled rocket in Auburn, Massachusetts about 1.5 h drive from SoftInWay’s headquarter, in typical Massachusetts traffic. He also conceptualized having a dedicated area for payload in rockets to carry scientific instruments to orbit.

Cooling innovations in gas turbines have also benefited the rocket industry as exhaust nozzles are quite heavily cooled to prevent melting under the extreme jet temperatures. From the development of superalloys in the 1940s to vacuum induction melting in the next decade to thermal barrier coating (TBCs) in the 1970s, engineers have continued pushing the limits as to how much above the material melting point one can operate at (~200F in the case of TBCs). While gas turbines cooling is typically done using a gas (compressor bled air) in order to be injected into the turbine flow path for maximum power output, it makes more sense for rocket nozzles to be cooled using a liquid due to their higher specific heat capacity and the opportunity to use this latent heat of vaporization. Looking at a typical gas-generator liquid rocket engine cycle, one can see that the liquid propellant (fuel here) uses the hot nozzle as a boiler to provide a gas to the turbine.

Many emerging technologies have been applied successfully to turbomachinery in the recent years, and are being used for rockets as well. One such technology is additive manufacturing (3D printing). It allows manufacturing parts much faster and cheaper than before. Turbomachinery blades, impellers and even bigger parts have already been 3D printed with quite impressive material properties compared to forged manufacturing. As 3D printing can accommodate the size of bigger pieces, complete rocket engine can be printed in a single piece without joins as is the case for the Prime Rocket from Orbex in the UK which stands at 17 m tall (~1/4 of SpaceX’s Falcon 9). The absence of welding and joins allows even higher operating temperatures as these are typical regions of high thermal stresses. Another startup making a lot of noise in this context is Relativity, who can take a traditional 3,000 parts engine and turn it into a 3-piece printed design.