Common bottom tank: Helping rockets reduce weight and increase efficiency to improve performance

On July 26, the common bottom of my country's first 3.35-meter-diameter hydrogen-oxygen common bottom tank successfully passed a series of static tests and will be used in the universal hydrogen-oxygen final stage of a new generation of medium-sized launch vehicles. What is a rocket common bottom tank? What are the benefits of using this structure for the tank? What technical challenges do researchers need to overcome?

Many styles are exquisite

Common-bottom tanks are often used in domestic and foreign launch vehicles. As the name suggests, it means that two tanks have a common bottom, called a common bottom. Generally speaking, it is a hemispherical or semi-ellipsoidal structure. Through the common bottom structure, the upper and lower rocket tanks can be connected into one.

According to the shape of the common bottom, the common bottom tank can be divided into convex type, concave type and special-shaped structure.

Among them, the convex type means that the common bottom is arched upward, while the concave type is the opposite. Both the convex type and the concave type are common forms of the current common bottom structure. The specific direction in which the common bottom is arched is mainly determined by the pressure of the upper and lower tanks during operation. Generally speaking, the common bottom should be arched toward the tank with lower pressure.

The upper convex common bottom was used earlier. Its "clients" include the Atlas D rocket that launched the first manned spacecraft of the United States, the Japanese N-1 medium rocket, the Long March 1 rocket that successfully launched the Dongfanghong-1 satellite, the Long March 3A series, the Long March 6, the Long March 8 and other rockets.

The concave common bottom is also not inferior in this respect. Its users include the Falcon 9 reusable rocket that has been very popular in recent years, the Soviet Union's first three-stage rocket Tornado 3 with fully automatic launch technology, the Proton rocket that was once the "leader" in the international space launch market, the ESA Ariane 5 rocket that was just retired, and the Indian PSLV rocket.

As for the special-shaped common bottom tank, it is relatively rare. The precedent of its application is the Ariane 5 rocket. Its liquid hydrogen and liquid oxygen powered second stage was originally planned to design the liquid oxygen tank and liquid hydrogen tank separately, and the liquid oxygen tank was suspended under the liquid hydrogen tank. Later, the plan was improved, and they adopted the special-shaped common bottom tank design with different diameters.

Extra large diameter common bottom tank is being transported

From the perspective of propellant type, common bottom tanks can be divided into two categories: normal temperature and low temperature. From the perspective of structural form, common bottom tanks can be divided into single-layer and double-layer.

There is no fixed rule for choosing which common bottom structure for a rocket. It is generally related to the type of propellant: normal temperature propellant can use a single-layer common bottom tank, while low temperature propellant is more suitable for a double-layer vacuum or honeycomb, foam or other sandwich-type common bottom tank due to the temperature difference between the propellants in the two tanks.

For example, the second stage of the Cyclone 3 rocket uses nitrogen tetroxide/UDMH propellant, and the second stage of the Japanese N-1 rocket uses nitrogen tetroxide/Hydrazine-50 propellant, both of which use single-layer common bottom tanks. The second and third stages of the US Saturn V heavy rocket both use liquid oxygen and liquid hydrogen propellant, so they use double-layer common bottom tanks.

Design and manufacturing are challenging

The tank of a liquid rocket must contain an oxidizer and a fuel, which in turn supplies the engine and generates thrust, just like gasoline and oxygen in the air burn to propel a car. Normally, the oxidizer and fuel can be installed in two independently designed tanks, called split tanks. In contrast, a common bottom tank needs to take into account the performance of both tanks, which is technically difficult in both design and manufacturing.

The common bottom is the key to the design of a common bottom storage tank. It must be able to provide both heat insulation and load-bearing capacity, which is arguably the primary challenge in the design.

Rocket propellants are divided into normal temperature propellants and cryogenic propellants. When the rocket is flying, the corresponding engines and pipelines need to work normally at the appropriate temperature. Especially for the cryogenic propellant common bottom tank, there is a temperature difference between the propellants in the two tanks, and the common bottom needs to "isolate" this temperature difference.

In addition to heat insulation, the common bottom must withstand the combined effects of temperature loads caused by temperature differences, the weight of the propellant in the tank above, the boost pressure inside the tank, and the loads caused by the rocket thrust and its own weight. These all require rocket designers to determine reasonable structural parameters so that the common bottom tank can withstand internal and external pressure loads and temperature loads and meet structural strength and stability requirements.

In addition, the production and manufacturing of the common bottom tank is also more challenging. The common bottom cannot be broken. Once the propellant leaks, there will be a risk of explosion. Therefore, the processing and manufacturing of the common bottom has high requirements for welding quality, material selection and weld control. During the welding process, the structure is very easy to deform, and it is necessary to ensure that the local deformation of the tank bottom is not too large.

Especially for low-temperature common-bottom storage tanks, the common bottom needs to be made into a double layer and have a heat-insulating function. If the heat-insulating structure adopts a vacuum double-layer structure, it must be evacuated, which is extremely difficult in terms of manufacturing and testing. If sandwich composite materials such as honeycomb and foam are used, the quality of the composite material directly affects the bearing capacity of the common bottom structure. Considering that composite fiber is prone to defects such as bubbles and debonding, researchers must overcome technical difficulties such as composite material preparation, molding and bonding.

In particular, large-diameter common-bottom tanks place higher demands on production and manufacturing capabilities, requiring larger processing equipment and mature processing techniques to ensure that the product matches the design parameters, thereby solving problems such as precise assembly and load transfer.

High performance in reducing weight and shortening

The common bottom tank has unique advantages and is of great help in improving the performance of launch vehicles, at least in three aspects.

First, the use of a common bottom storage tank can save the "dead weight" of the bottom of the storage tank.

Secondly, when the tanks adopt a non-common bottom design, each tank must withstand the internal pressure. After achieving a common bottom, the pressure difference between the upper and lower tanks can be used to offset their respective internal pressure loads, further reducing the mass of the tank bottom.

Thirdly, the common bottom tank means that the design of the inter-tank sections is eliminated, thus saving the mass of this part of the structure.

It is not difficult to see that the common bottom tank improves the efficiency of the rocket structure, helps reduce the weight of the rocket, allows the rocket to carry more payload when taking off, and improves the carrying capacity. At the same time, the length of the rocket is shortened, the slenderness ratio is optimized, the "elasticity" is reduced, and the rocket flight is more stable.

It is worth mentioning that in recent years, newly developed launch vehicles abroad, such as "Vulcan-Centaur" and "Starship", mostly use concave internal transport common bottom tanks. The layout of its propellant delivery system is simpler, which is conducive to reducing the unusable amount of propellant, further helping the rocket to reduce weight, and at the same time being able to transport more payloads to the sky. In addition, because the delivery pipe is located inside the common bottom tank, the structural protrusions on the outer surface of the rocket are reduced, optimizing the aerodynamic shape of the rocket.

In short, the common bottom tank combines two tanks into one, making the rocket lighter, more stable, more powerful and better performing. With the improvement of design, manufacturing and process, the common bottom tank will be more and more widely used in rocket design. The "small" change brings many benefits, and the common bottom tank is a good helper for improving rocket performance. (Author: Zhuang Fangfang, Image source: China Academy of Launch Vehicle Technology, Reviewer: Jiang Fan, Deputy Director of the Science and Technology Committee of China Aerospace Science and Technology Corporation)