What is the upper stage of a rocket? Is it on top of the rocket? Click here to learn more about the "space shuttle"

The rocket upper stage is a device we often hear about. The main working position of the upper stage is in the space above the von Karman line, but its mission is not space application, but to send artificial satellites or other spacecraft to the predetermined working orbit. Therefore, although many upper stages do not look like rocket stages, this type of product is still developed and used by rocket manufacturing units.

Rocket upper stages sound like a large category, but they come in all shapes and sizes. Some look like the stages of a launch vehicle, while others look like a round pancake with a nozzle. Some are several stories high, while others are about the size of a human body. What are their characteristics?

The Centaur upper stage looks similar to the rocket stage.

What is the rocket upper stage used for?

The purpose of the upper stage is mainly to improve the performance and mission adaptability of the basic stage vehicle, that is, the launch vehicle. Compared with the traditional launch mode, the combined launch mode of the basic stage rocket + the upper stage is adopted. The upper stage and the satellite are first sent to a low orbit, and then the rocket separates. The upper stage sends the satellite to a medium or high orbit working orbit, or other orbits, which simplifies the flight process of the launch vehicle and reduces the pressure of ground measurement and control.

The upper stage of the Electron rocket separates from the second stage

In interstellar exploration activities, upper stages are also needed to enable probes or spacecraft to transfer from Earth orbit to interstellar orbit. During the Apollo program, the lunar landing assembly was sent into the lunar orbit using a large upper stage.

Russia exhibits a combined model of the Fregate upper stage and Mars rover

The upper stage can also make full use of the capabilities of the launch vehicle to carry some secondary payloads and small satellites in addition to the main satellite. This is especially important for large and medium-sized launch vehicles. After using the upper stage, it is easier to launch multiple satellites with one rocket, effectively improving the launch efficiency and reducing the number of launches. This is of great value today when launch activities are becoming increasingly frequent and launch sites are operating at a high intensity. The upper stage itself can also carry some space technology test equipment to give full play to its value.

What is the difference between the upper stage and the launch vehicle?

The upper stage is basically a complete space vehicle, including complete power, guidance and navigation control, thermal control, measurement and control, and structure systems. Unlike the launch vehicle, the main flight environment of the upper stage is the Earth's upper atmosphere and outer space. There is basically no atmospheric pressure outside, but there is a relatively harsh radiation environment. Such an environment determines that the design characteristics of the upper stage subsystems are different from those of the launch vehicle.

The upper stage engine can use a low-pressure combustion chamber and nozzle with an optimal vacuum expansion ratio. The upper stage is generally used to complete orbital entry missions, such as Earth transfer orbit and interplanetary transfer orbit, so their working time may be only a few hours or several days. Some upper stages are connected to satellites for a long time to provide them with full-life orbit change maneuvers, which may take several months to fly.

This determines the fuel selection of the propulsion system. The upper stage that works for a short time can use cryogenic fuel and solid fuel. For example, the Centaur uses a liquid hydrogen expander cycle engine, and the HM7B upper stage of the Ariane 5ECA uses a gas generator-driven turbopump. The solid fuel upper stage engine has the simplest structure, such as the EPKM upper stage and CZ-2C/FP upper stage developed by China.

The long-duration upper stage engines use hypergolic propellants, such as the classic Agena, and the Delta-K or Ariane 5ES second stage, which all use a combination of UDMH and nitric acid. Directly supplying fuel with high-pressure gas can meet the engine combustion requirements, without the need for a turbopump. This also determines that the design ideas of the fuel supply system are very different.

Agena B upper stage on display at the museum

In addition to the solid propellant upper stage, all others must consider in-orbit shutdown and restart. This brings about the problem of propellant management and fuel delivery for a long time in orbit, and it is also necessary to solve the problem of how to suppress the inclusion of gas in the propellant. In addition, in outer space, the temperature environment is more severe than in the atmosphere, and factors such as the deep cold space background, solar radiation, earth infrared radiation, and earth reflection must be considered. The engine's own plume and thermal radiation during operation, and the thermal backwash after shutdown will have complex effects on the engine itself and the fuel supply system, so the main components of the engine have strict requirements for temperature control.

Is it better to be more stable or more maneuverable?

The attitude control of the upper stage includes two aspects: attitude stabilization and attitude maneuvering. The former is the process of maintaining the existing attitude, and the latter is the redirection process. A few upper stages use a spin-stabilized control scheme, but most of them now use three-axis stabilization. According to the research of domestic scholars, the following four factors should be considered in the design of the attitude control of the upper stage. The first is the balance between the minimum propellant consumption and the shortest control time, and the best solution should be selected. The second is the layout and optimization of the attitude control thrusters, which should not be too many or too few, and should ensure precise control and simplify the structure. The third is self-repair technology. If a thruster or a part fails, the flight mission should be completed through the coordination of other thrusters. The fourth factor is related to the entry of multiple satellites into orbit. After some satellites are separated, the mass distribution characteristics of the upper stage and the remaining satellites have changed, and the attitude control mode must be re-established. This is particularly important today when small satellites are popular.

The upper stage generally does not need to consider the aerodynamic shape, so, including the Russian Fregate and China's expedition series upper stage, there is no fairing. However, some large upper stages need to be installed on the top of the launch vehicle, exposing the outer shell, passing through the entire atmosphere, so they also use a longer cylindrical outer surface, which looks the same as the rocket stage.

Fregate upper stage launches OneWeb satellite

So far, humans have developed more than 20 types of upper stages, and no more than 10 are in use. With the normalization of the multi-satellite launch mode, especially with the increasing scale and variety of constellation construction, upper stages will play a greater role in future space launch activities.

Today's rocket upper stages are responsible for multi-star and multi-orbit space transportation missions. Looking to the future, the upper stages may evolve into another new concept of spacecraft, that is, a space tugboat.

At present, China has fully mastered the development technology of the upper stage and has carried out successful flights. With the further development of interstellar exploration, manned spaceflight and commercial spaceflight, the upper stage may develop in two directions: large upper stages are used for the orbital insertion of interstellar probes and large manned spaceflight facilities, and small upper stages are used for the orbital insertion of small satellites and constellation deployment. It will become one of the main carriers for commercial space flight.