Lying down or standing up? There are many ways to transport a rocket

Not long ago, a video of the Long March 7 rocket carrying the Tianzhou 6 cargo spacecraft being transferred at the Wenchang Space Launch Site once again attracted everyone's attention. After the rocket completes the final assembly and testing, it is transferred from the final assembly and testing plant to the launch station. The process is rocket transfer or transfer. In fact, the common transfer methods of rockets are "lying" transfer and "standing" transfer. So, why do similar launch vehicles develop completely different transfer methods? What are their respective advantages and disadvantages?

Low horizontal transfer costs

The "lying" transfer of launch vehicles is scientifically known as horizontal transfer. The Soviet Union first invented the horizontal transfer method of rockets. Russia continues to develop it today and it is the mainstream transfer method in the country. It has also been used in other countries and is a common transfer method.

When the rocket is transported horizontally, the launch system design is relatively simple. We often see Russian trains pulling rocket erection vehicles and Soyuz rockets to the launch site, which is full of steampunk historical weight. The Proton is also transported horizontally by train, and even the unprecedented heavy-lift Energia rocket in the Soviet era also uses horizontal transport.

Soyuz rocket transported by train

The rocket will be assembled horizontally in the assembly and test plant, and then horizontally transferred to the launch station, after which the erection mechanism will perform the erection operation, turning the horizontal rocket and payload assembly into a vertical state to wait for testing and launch. The Saturn V, once the world's largest rocket, is only 10 meters in diameter. The common Soyuz rocket adopts a parallel bundled design, with a core stage diameter of 2.95 meters and a maximum diameter of 10.3 meters. Therefore, for most rockets that only need to be transported horizontally, a height of more than ten meters in the assembly plant is enough.

If the rocket is transported vertically, it will be necessary to build a general assembly and test building that is tens or even hundreds of meters high, which will cost too much. Vertical lifting of launch vehicles is not only troublesome and laborious, but also brings more dangers to high-altitude operations. The horizontal assembly method that is matched with horizontal transportation facilitates rapid assembly, and the expansion of the factory and the addition of assembly lines are conducive to the rapid launch of rockets. The Soviets chose the horizontal transportation method when they first started to develop launch vehicles, avoiding the problem of vertical lifting of rockets, which was a very reasonable choice.

After the end of the Cold War, the Soviet Union's horizontal transfer technology was favored by the United States. For example, the US Delta IV rocket and Falcon 9 rocket both use horizontal transfer, the purpose of which is to reduce the cost of rocket testing and launching on the ground.

Vertical transfer is more reliable

In addition to "lying" transportation, rockets can also be transported vertically by "standing up". Vertical transportation of rockets naturally requires vertical assembly, which requires a tall assembly and test building. Despite this, the "three verticals", namely vertical assembly, vertical testing, and vertical transportation, are still widely used. The United States has long been dominated by the "three verticals" model. Influenced by it, the main launch vehicles of the European Space Agency and Japan also adopt the "three verticals" model. Rockets in some countries, especially Europe and Japan, generally use the vertical transportation model. my country's Long March 2F, Long March 5 and Long March 7 rockets are also vertical transportation models. The main reason is that the effect of vertical transportation is better.

The Shenzhou XIII spacecraft-rocket combination is transferred to the launch area

As we all know, launch vehicles are launched vertically, and vertically transported rockets are assembled vertically. After the vertical assembly of the rocket is completed, it remains in a vertical state until launch. The vertically transported rocket only needs to undergo a comprehensive test after assembly, and its pre-launch test can be greatly simplified. Although horizontal assembly is convenient, the rocket is prone to unbalanced force in the lateral direction, and after the transfer is completed, there is a vertical erection process at the launch station. The rocket changes from horizontal load-bearing to vertical load-bearing, and the change in the overall force state will affect the state of the rocket. After the horizontal assembly is completed, the test is comprehensive and complete, and a large number of tests must be carried out after the erection to check the changes in the state of the rocket before and after the erection.

In order to improve the rocket's carrying capacity, scientific researchers are pursuing the ultimate goal of reducing the structural weight. Considering the rocket's longitudinal load-bearing requirements during flight is already a headache. If a horizontal transfer method is used, it is also necessary to consider strengthening the lateral strength. Under the same technical level, the structural weight will increase rather than decrease, and the carrying capacity will be compromised.

More importantly, delicate satellites are extremely sensitive to their placement. Some satellites have oversized lightweight folding antennas that cannot be changed at all. After the rocket and satellite are assembled, they can only be transported vertically to keep the satellite status unchanged to the greatest extent possible. In short, although vertical transport requires supporting vertical assembly and the construction cost of the vertical assembly building is also high, vertical transport ensures that the status of the rocket and satellite remains unchanged to the greatest extent possible, and improves the reliability of the rocket and satellite.

There will be many ways to transport rockets in the future

Horizontal transfer saves money, vertical transfer is reliable, is there any way to combine the advantages of the two? The US space industry has made some innovative attempts. Although the US Delta IV rocket is horizontally transferred, you can see from the transfer photos that it is very different from the horizontal transfer of Russian rockets such as the Soyuz: the horizontally transferred Delta IV rocket is not complete, with only the "headless" first and second stages of the rocket. Its fairing/payload assembly is docked with the propulsion stage by vertical lifting after the "headless" rocket arrives at the launch area and is erected. This hybrid mode has the advantages of low horizontal assembly costs and unchanged vertical assembly state of the payload, which may become the future development direction. Now the US Falcon Heavy rocket is building a huge vertical assembly tower to launch high-value military satellites to achieve vertical assembly of large precision military satellites.

Both "three horizontal" and "three vertical" are rocket test and launch modes that were born in the last century. With the development of reusable space launch vehicles, rockets may no longer need to be transported in the future. In the 1990s, the United States planned to develop a Delta Clipper reuse spacecraft, a single-stage orbital rocket that launches vertically and returns vertically. From the design concept, there is no problem of assembly and transportation.

Of course, the DC-X, the verification aircraft of the "Triangle Clipper", was abandoned halfway, and the subsequent research and development ended in vain, but it did point out a promising route. Now, SpaceX is developing the "Starship" transportation system. As a two-stage orbital fully reusable rocket, its first-stage "Super Heavy" and second-stage "Starship" will accurately reverse the rocket to land, hover near the launch tower, and be clamped and recovered by the robotic arm Mechazilla on the launch tower. "Super Heavy" and "Starship" will be assembled vertically directly on the launch tower with the help of the Mechazilla robotic arm. In other words, although the design of the "Starship" system is more complicated, it also does not have the problem of transportation. It may be a general trend to cancel the transportation of rockets in the future. (Author: Zhang Xuesong)