Liquid rocket reuse, where to focus next?

Recently, a type of liquid oxygen-kerosene engine independently developed by my country successfully completed repeated flight tests. This marks the first time that China has realized the reuse of liquid rocket propulsion systems, marking the entry of my country's liquid rocket engine reuse technology into the engineering application stage.

Looking around the world, as reusable rocket technology becomes increasingly mature and technical difficulties are gradually overcome, countries have set off a new round of development enthusiasm. So what are the current models of reusable liquid rockets? What will be the future development trend?

Recycling and reuse have many difficulties

As the main power device of space launch vehicles, liquid rocket engines have the characteristics of high performance, strong mission adaptability, high technical difficulty, and long development cycle. They are one of the most complex products on space launch vehicles. Therefore, the reuse of liquid rocket propulsion systems is one of the key technologies that must be broken through to achieve the reuse of space launch vehicles.

Specifically, if liquid rocket engines are to be reused, breakthroughs must be made in key technologies such as multiple starts, low inlet pressure starts, wide range thrust adjustment, status assessment and detection, and health management, rapid and simplified processing, high-temperature component structure fatigue life assessment and life extension, and full-mission complex thermal environment prediction and control. The difficulty of developing these technologies far exceeds that of traditional disposable liquid rocket engines.

Among the world's major aerospace powers, the United States has the strongest scientific research strength and the richest application experience in the field of reusable rocket propulsion. As early as the 1980s, the United States had successfully developed a reusable high-thrust liquid rocket engine and used it as the main engine of the space shuttle.

The main engines of the space shuttle use liquid hydrogen and liquid oxygen propellants. The three engines provide a total thrust of more than 600 tons, and the thrust can be adjusted within the range of 65% to 109%. This design is to allow the space shuttle to obtain greater thrust during ignition and the initial ascent phase, making it easier to fly and accelerate. In the final ascent phase, the main engines will reduce thrust to facilitate precise control of the speed to orbit.

The main engines of the space shuttle use liquid hydrogen and liquid oxygen for propulsion

After the retirement of the space shuttle in 2011, new forces in the US commercial space industry took on the important task of "reuse and return to the earth and the sky". Among them, SpaceX's Falcon 9 rocket revolutionized the recovery and reuse of the first stage of the orbital launch vehicle.

In December 2015, the Falcon 9 rocket successfully recovered the first stage on land for the first time. In March 2017, the Falcon 9 rocket's first stage was reused for the first time. As of September 2022, the rocket has successfully recovered the first stage more than 130 times, and the first stage of a single rocket has been reused up to 14 times. The high launch intensity, reliability and economy are amazing.

The technical basis for the Falcon 9 rocket is nine parallel Merlin 1D engines. This engine is specially designed for reusable rockets, uses liquid oxygen-kerosene propellant, has a single sea level thrust of 87 tons, a specific impulse of 275 seconds, and has multiple ignition capabilities.

Compared with traditional liquid rocket engines, the most special feature of the Merlin 1D engine is that it can achieve thrust adjustment within the range of 39% to 100%. When 9 engines are connected in parallel, the total thrust can be adjusted within the range of 4.3% to 100%, laying a solid power foundation for rocket recovery and reuse.

In addition, the Merlin 1D engine is also used in the Falcon Heavy rocket, with 27 engines connected in parallel forming the first stage, making this rocket the most powerful in service, with a low-Earth orbit carrying capacity of 63 tons. Since 2018, the Falcon Heavy rocket has been successfully launched three times, but the recovery work has not been very smooth, which shows the complexity of liquid rocket power recovery and reuse.

The heroes compete to meet the challenge

Looking across countries, the reuse technology of liquid rocket propulsion systems has attracted many aerospace industry players to join in, and they are willing to challenge greater difficulties and pursue higher targets.

According to public information, a certain type of liquid oxygen-kerosene engine in my country participated in its maiden flight test in 2021 as the main power unit of a certain aircraft. After testing and maintenance, it recently successfully participated in a repeated flight test mission.

In addition, my country has exposed a number of liquid rocket propulsion system recovery plans and reused spacecraft plans, using different propellants and configurations. Some news reports on the progress of engine testing also confirm that my country's liquid rocket engine reuse technology is progressing steadily.

SpaceX is developing the Starship, which has a low-Earth orbit capacity of 160 tons. The project being promoted simultaneously is the liquid oxygen-methane engine code-named "Raptor", whose latest model has a sea-level thrust of 300 tons and a specific impulse of 334 seconds, and can achieve thrust adjustment within the range of 20% to 100%.

The first stage of the "Starship" has 33 Raptor engines in parallel

The most difficult and most promising indicator of the Raptor is the ultra-high pressure in the combustion chamber, which has surpassed the previous best liquid rocket engine in this regard, the Russian RD-180. The ultra-high combustion chamber pressure can give the Raptor a higher thrust-to-weight ratio, and the current version has reached 107:1, and there is still a lot of room for subsequent improvement.

At present, the second stage of "Starship" has completed limited-altitude test flights many times, and the first stage, which is connected in parallel with 33 "Raptors", will soon undergo a comprehensive ignition test, pushing "Starship" to attempt its first orbital launch as soon as possible.

Blue Origin has also successfully developed a reusable rocket. The New Shepard rocket made its first flight and was recovered in November 2015. Before the launch failure on September 12 this year, it had successfully flown 21 times and carried out 6 manned missions. The New Shepard rocket can only fly suborbitally at present, using the BE-3 liquid hydrogen and liquid oxygen engine, while Blue Origin is developing the BE-4 liquid oxygen and methane engine, which has a thrust greater than the Raptor and is planned to be used on the New Glenn rocket to carry out orbital launch operations.

The reusable rockets of the United States have seriously squeezed the market share of traditional rockets of Russia and the European Space Agency, forcing Russia and Europe to also pay attention to the research and development of reusable rockets and their liquid engines.

The Amur rocket is a reusable two-stage medium-sized rocket developed by Russia, and is expected to make its first flight from the Vostochny Cosmodrome in 2026. The first stage of the rocket is designed to be reused 10 times, with five RD-0169 liquid oxygen-methane engines in parallel. When the first stage is reused, the Amur rocket is expected to be able to send 9.5 tons of payload into low-Earth orbit, and when used once, the low-Earth orbit carrying capacity is about 12 tons.

ESA has planned a reusable rocket based on the Ariane 6 rocket. The first stage will use 7 or 9 Prometheus liquid oxygen-methane engines. The rocket configurations under discussion have diameters of 5.4 meters and 4.6 meters respectively, and can be bundled with liquid boosters to further increase the carrying capacity.

New trend of three-legged tripod

The emergence of reusable launch vehicles has profoundly changed the pattern and development direction of the world's space transportation field. Liquid rocket propulsion systems pursue reusability, low cost and high reliability, which has become a consensus among countries in the development of a new generation of liquid rocket engines.

So what propellant will be chosen for future liquid rocket propulsion systems? Judging from the development trend, liquid oxygen-methane seems to be the mainstream choice for the new generation of reusable rocket propulsion. Because liquid oxygen-methane propellant has the advantages of high specific impulse, low cost, clean and environmentally friendly, easy maintenance and use, it is suitable for large-scale production and repeated launches of engines. It is also convenient for long-term storage in space and can effectively reduce the size and mass of the engine. Especially for the increasingly popular round-trip missions between the Earth and Mars in recent years, liquid oxygen-methane engines may benefit from the Mars in-situ resource utilization experiment, and the potential is huge.

However, the advantages of liquid hydrogen and liquid oxygen power, such as high specific impulse and high reliability of liquid oxygen and kerosene power, cannot be ignored and are indispensable in certain missions. Therefore, the technical route of reusable liquid rockets is likely to present a three-legged pattern.

As reusable rockets gradually become mainstream in the future, the research and development, manufacturing, and maintenance of existing liquid rocket propulsion systems are also facing changes. From the perspective of efficiency, the engine may use more high-strength, low-weight new materials and innovate low-cost rapid manufacturing processes, while rapid multiple start performance has also become a new requirement for the engine.

In short, the reuse of liquid rocket propulsion systems has obvious advantages and a bright future. It can not only reduce the cost of space launches and better serve production and life, but also open up new areas of space exploration. (Author: Ben Xun)