The super heavy rocket from SPACE X in the United States has accumulated 29 engines, which makes people's scalp numb. Some rockets choose to use only a few large engines, and China is also building a bigger heart for the Long March 9 Rockets. Why doesn’t our country also build existing engines? Is the bigger the engine, the better, or the more quantity, the better?
[Engines that are piled up with some terrible fears have become a symbol of super heavy rockets]
First declared that this article discusses liquid rocket engine , and solid rocket engine will not be mentioned for the time being.
American space exploration technology company is planning to launch a super heavy rocket. This model is equipped with a total of 29 gray-backed falcon engines. The tail of the rocket looks dense and spectacular.
At the same time, NASA's SLS "space launch system" super heavy rocket was pushed back to the factory at the end of September due to technical failures and various weather reasons. There are only 4 engines in the "Space Launch System", and the tail looks much simpler than that of the super heavy rocket. The specifications of these two rockets vary greatly, with the "Space Launch System" taking off weight of 2,497 tons. The super heavy rocket is accompanied by a "starship", and the take-off weight is about 4,600 tons. But even so, the engine numbers of the two rockets are much different.
[SLS only has 4 nozzles, but the engine is much larger]
On average, the take-off weight of each ton, the number of engines of super heavy rockets is four times that of the "space launch system". Why do the two rockets choose such a big engine layout? Why do heavier rockets use a bunch of small engines? What are the differences in safety and cost between these two options?
In fact, smaller rockets sometimes use many engines. New Zealand Rocket Lab's electronic rocket is a small model with a take-off weight of only 10.5 tons, but its first stage also uses a full nine engines. This engine, called Rutherford, is very small and an adult can hold it in his arms. The China Long March 6 launch vehicle, much larger than the Electronics rocket, has a take-off weight of more than 100 tons, but only uses one YF100 engine in the first stage. The American Lockheed Martin Cosmic God 5 large rocket has a take-off weight of more than 330 tons, and the first stage only uses one RD180 engine.
[The electronic rocket uses 9 engines, but this rocket is actually not as big as some engines]
The size and size of the rocket engine , what kind of engine layout to choose for a rocket is determined by very complex factors.
First, compare the advantages and disadvantages of both large and small engines.
The benefits of using a small number of large engines are of course the structure of the rocket and the fuel supply pipeline are relatively simple. As many engines as there are, they need to be equipped with installation structures and fuel delivery pipelines. This is not too easy for rocket design. When installing the engine, of course, the more numerous it is, the more troublesome it is to install. The installation structure and pipes themselves also take up weight. Although small engines and large engines can achieve a relatively close thrust-to-weight ratio, if both types of weight mentioned above are included, using a large number of smaller engines may bring adverse factors in the overall carrying ratio of the rocket.
[The design of the fuel pipe of the rocket engine is very complicated. Fuel preheating and exhaust gas emissions are particularly troublesome. Many of these fuels are ultra-low temperature liquid and need to be used in hundreds of degrees. Pipeline design needs to be considered] The fewer engines used by
, the theoretically the probability of the engine failure is relatively low. This can be calculated by using a simple probability.
However, the design of large engines is very difficult and the manufacturing process is more complicated.The RS25 engine used by the "Space Launch System" rocket is derived from the main engine of the space shuttle , while the RD180 engine is derived from the RD170 engine of the most powerful rocket in the Soviet era, the Energy. This is a magical tool developed by the two superpowers of the United States and the Soviet Union during the Cold War. Even people like Musk who appear as a pioneer in aerospace entrepreneurship will not dare to touch such high-end products. So large rocket engines are not bad, but they are too rare.
[Large engines have strong performance, but they are more challenging for design and processing]
Of course, as a space powerhouse, this crown jewel must be picked. At present, China is developing a new generation of engines with a thrust of nearly 500 tons for the Long March 9 launch vehicle. Only in this way can it support the Chinese dream of a lunar base.
So are there any disadvantages of large engines?
The disadvantage of large engines is the advantages of small engines. Small engines can be manufactured in large quantities using mature processes. Some very small rocket engines, such as Rutherford, mentioned in the previous article, can be produced using the most advanced metal D printing technology, which not only improves production efficiency and reduces costs, but also realizes some structures that cannot be made by traditional processes. In addition, with the support of modern engineering management and quality control technology, the reliability of rocket engines has been greatly improved. Things like the sudden failure of engines in the early stages of the development of the aerospace industry are rarely seen.
For example, the engine used by Falcon Heavy has reached more than 99.9% per unit, and the overall failure rate is reduced. In the past, the Soviet N-1 rocket had 30 engines in parallel in the first stage, but because the reliability of a single unit was only about 95%, the overall failure rate was very high, resulting in all five launches of the N-1 rocket, which ultimately ruined the Soviet moon landing plan .
[The Long March 9 is not as big as the Long March 5, and the engine needs greater thrust]
Musk's gray-back falcon engine not only manufactures in large quantities and flies frequently, but also achieves more than 10 reuses. Although some abnormalities have occurred by chance, it has never lost Falcon 9 or Heavy Falcon Rocket due to engine failure. According to the calculations of China Aerospace researchers, multiple engines can achieve reliability of more than 100%, which means that some engines fail and can be turned off. The remaining engines ensure the normal flight of the rocket by increasing thrust and balancing torque.
That is to say, if the number of engines is sufficient, the engine can be allowed to fail and make up for it with other engines. This is also an idea to solve the problem.
[Performance is sometimes also a quality]
So here, we can get a preliminary conclusion that the advantages and disadvantages of large and small engines are a dynamic evolution process. Considering only the current historical stage, the parallel connection of multiple small engines is more suitable for companies that intend to carry out commercial flights. What they need is mature and stable rockets and high-intensity and high-frequency flights, allowing certain risks of failure to exist. Using large engines to develop giant rockets is an exploratory behavior of national aerospace agencies. These national-level institutions need to take risks to explore the technological frontiers, so as to bring more technological breakthroughs, and these technological breakthroughs can be transformed into commercial value in the future, which is what we often call the advantages of military-to-civilians.
Once the technology of large engines is fully mature, it can gradually turn to commercialization. Under such a virtuous cycle, it can promote the continuous development of human rocket industry. So, we are engaged in big rocket engines not because it is simple, but because it is difficult.
[After laying a solid foundation, you can seek greater technological breakthroughs]
