300 tons of near-Earth delivery? In the era of space exploration, how much more powerful will future rockets be?

Recently, Musk, CEO of SpaceX, announced that Starship will not be recycled and its near-Earth orbit capacity is expected to reach 300 tons. So is this amazing indicator of Starship reliable? What are the factors that currently limit rocket capacity? How much capacity can rockets have in the future?

300 tons is not a fantasy

According to SpaceX's publicity, "Starship" is a heavy-duty space launch vehicle developed to realize grand visions such as "colonizing Mars" and "making humans a multi-planet species." It has a diameter of 9 meters, a height of about 119 meters, and a total take-off weight of about 5,000 tons. It conducted its first flight test on April 20 this year, but unfortunately it was unsuccessful.

Starship ignites and takes off

Currently, the two stages of the Starship use a total of 39 liquid oxygen-methane engines, with a total takeoff thrust of about 7,600 tons. The near-Earth orbit capacity is expected to reach 250 tons in the non-recovery mode and 150 tons in the two-stage recovery and reuse mode. The reason why Musk confidently declared that the near-Earth orbit capacity of the Starship can reach 300 tons, increasing the indicator by 20% at one go, lies in the improvement and upgrade of the entire rocket system, including the engine.

For SpaceX, there is a precedent for increasing rocket capacity by more than 20%, which can be seen from the development history of the currently popular Falcon 9 rocket. The Falcon 9 rocket V1.0 version successfully completed its first flight in 2010, with a low-Earth orbit capacity of 10.5 tons. Later, the V1.1 version with a low-Earth orbit capacity of 13.1 tons and the V1.2 version with a low-Earth orbit capacity of 17.95 tons were successively launched. The Block 5 version, which is currently launched frequently, has a low-Earth orbit capacity of 22.8 tons, and it is still being continuously improved and "tapped". It is not difficult to see that the capacity of each version has increased by more than 20% compared with the previous version.

It can be said that rapid iteration and continuous improvement of technology and products are the "routine" of SpaceX. This is the case with the frequently launched Falcon 9 rocket, not to mention the "Starship" which is still in the development stage. In fact, the "Starship" started with the Big Falcon Rocket that was unveiled in 2016. Since the International Astronautical Congress in 2016, it has gone through more than 10 rounds of program iterations in 7 years, and its core components have been continuously improved and upgraded.

Take the Raptor engine as an example. Since its development started in 2012, it has developed to the third generation and conducted a static ignition test on May 13 this year, with a thrust of up to 269 tons. The second-generation Raptor engine used in the first flight of "Starship" has a thrust of 230 tons. Obviously, the third-generation Raptor engine will strongly support the improvement of "Starship"'s carrying capacity.

Musk revealed that based on the third-generation Raptor engine, the second stage of the "Starship" will be extended by 10 meters, and the number of vacuum version Raptor engines will be increased from 3 to 6. By then, the total height of the extended version of the "Starship" will increase from the current 119 meters to 129 meters, and the total take-off weight is expected to exceed 6,000 tons.

Calculations show that if the extended version of the Starship can achieve a low-Earth orbit capacity of 300 tons with a total takeoff weight of 6,000 tons, the carrying efficiency will be as high as 5%. In comparison, the carrying efficiency of many main rockets in the United States, Russia, Europe and Japan is around 3.5%, and the carrying efficiency of the Falcon 9 rocket is 4.15%.

There are two main reasons why the Starship's carrying efficiency index is so high. First, it uses a high-performance liquid oxygen-methane full-flow regenerative cycle engine, which takes into account multiple aspects of performance such as specific impulse, reuse, operation and maintenance. The vacuum specific impulse is as high as 380 seconds, and the second-generation Raptor engine weighs only 1.6 tons and has a thrust-to-mass ratio of 150. Second, the Starship has achieved a lightweight design through means such as a common bottom tank. The super-heavy booster stage structure coefficient is about 7%, while the current mainstream rocket sub-stage structure coefficient is generally around 10%.

Overcoming the “two major obstacles”

The aerospace industry calls rockets with a capacity of 20 tons to near-Earth orbit large rockets, and rockets with a capacity of more than 100 tons heavy rockets. In order to achieve a significant increase in capacity, it is necessary to break through a large number of key technologies.

According to public information, my country's Long March 5 rocket has made breakthroughs in more than 247 key technologies, represented by 12 major key technologies such as the overall optimization design of large liquid rockets, large-diameter rocket body structure, and 120-ton high-pressure regenerative liquid oxygen-kerosene engine.

Long March 5 carrier rocket

Analyzing the difficulties in improving rocket carrying capacity, the most important ones are the high-thrust high-performance engines and the large-diameter rocket body structure, which can be said to be the "two major obstacles" to improving rocket carrying capacity.

The engine is the "heart" of the launch vehicle, and its importance is self-evident. Generally speaking, under the same design level, the greater the rocket's carrying capacity and the greater the total takeoff weight, the greater the takeoff thrust required from the engine. Currently, there are two main ways for rocket sub-stages to achieve high thrust:

First, the thrust of a single engine is large, so the number of engines can be less, such as the Saturn V rocket, the Energia rocket and the SLS rocket. Among them, the first stage of the US lunar rocket Saturn V uses five F1 liquid oxygen-kerosene engines with a thrust of 690 tons in order to achieve large carrying capacity. In the first few ground tests of the engine, the F1 engine exploded frequently due to unstable combustion. NASA and Rocketdyne Company took great pains to solve the problem of unstable combustion by improving the configuration of the injection disk.

The second is to increase the overall thrust by increasing the number of engines. Although this can reduce the difficulty of developing a single engine, the reliable parallel use of multiple engines has become a key difficulty. The four flight tests of the Soviet lunar rocket N1 all failed, indicating that the integration of multiple engines and the resulting vibration, fuel delivery, control and other issues cannot be ignored. Fortunately, technology is always advancing and developing, and the Falcon series of rockets have successfully realized the engineering application of multiple engines in parallel.

The diameter of the rocket body is a key parameter of the rocket, which determines the rocket's aspect ratio. From the perspective of reducing flight loads and the difficulty of rocket control systems, the rocket's aspect ratio should be as small as possible, that is, a larger diameter rocket body performs better. In addition, a larger diameter rocket body can carry more propellant, providing more space for expanding the rocket's carrying capacity. However, the increase in the diameter of the rocket body will bring about a size effect (also known as a volume effect, which refers to the influence of the geometric size on the properties of metals), which is more sensitive to pressure.

At the same time, the demand for rocket lightweighting is becoming more and more urgent. This places extremely high demands on the design, materials, and processing technology of rocket tanks, shell segments, and other structural parts that are only a few millimeters thick. my country's Long March 5 rocket is commonly known as the "Fat Five" because it has broken through the 5-meter diameter rocket body structure design, manufacturing, and testing technology, achieving a leapfrog development in the diameter of my country's rocket bodies. For heavy rockets, the diameter often starts at 10 meters, and its manufacturing equipment and process, structure transportation, and testing are all greater challenges.

The era of space exploration is "borderless"

The space age began in October 1957, when the Soviet Union launched the first artificial earth satellite. Twelve years later, the Saturn V rocket successfully launched the Apollo 11 spacecraft, marking the first time that humans set foot on an alien planet. However, in the more than 50 years since then, humans have not been able to expand their footprints further. Due to the lack of mission requirements, the Saturn V heavy-duty rocket only carried out 13 launch missions before entering a museum. The Soviet Energia heavy-duty rocket also "failed" due to changes in the situation and missions.

At the beginning of the 21st century, with the proposal of a series of grand space plans and engineering requirements, heavy-duty rockets once again entered the public eye. NASA has successively carried out the development of the Ares V heavy-duty rocket and the SLS heavy-duty rocket. Although the former was unfortunately "aborted" and the latter's technical level had no significant advantage over the Saturn V rocket, the new mission requirements provided new opportunities for the development of heavy-duty rockets.

After that, Starship not only took over the "baton" of the largest rocket entering the engineering stage, but also provided more precedents for research and development design for later generations. In the future, new materials, new concept power, innovative structural design, etc. are expected to help humans deliver larger and heavier payloads to the target orbit.

The biggest good news is that humans have entered the era of space exploration, represented by the construction of giant constellations, large-scale deep space exploration, and 1-hour global transportation. The scale of space mission development and utilization continues to expand, providing a broad space for the application of heavy rockets. Whether it is the deployment plan of tens of thousands of satellites, or the more careful and practical deep space missions such as the moon and Mars, or the new concept logistics ideas such as 1-hour global commercial transportation missions, it is expected to accelerate the development of heavy rockets.

It is not difficult to imagine that, driven by the diversified needs of the era of space travel, the scale of rocket takeoffs will become larger and larger, and the rocket capacity will be raised to a level that our predecessors found incredible, and perhaps it will be "boundless". At present, we cannot accurately predict how much capacity rockets will have in the future, just as our ancestors who sailed across the ocean in small wooden boats hundreds of years ago could not have imagined the huge ships sailing in the vast ocean today. However, heavy rockets will definitely promote the development of near-Earth space in the future and help humans fly further. (Author: Wu Shengbao)