The Development Characteristics and Trends of Heavy Launch Vehicles

QIN Tong, RONG Yi, QIN Xudong, ZHANG Zhi

1 Beijing Institute of Astronautical Systems Engineering, Beijing 100076

2 China Academy of Launch Vehicle Technology, Beijing 100076

Abstract: Heavy launch vehicles represent the ability of a country to enter space and utilize space resources. In recent years, with the growth in human space exploration, the major aerospace powers and companies in the world are planning to develop heavy launch vehicles. This study analyzes the development of heavy launch vehicles in the world, reviews the characteristics of China's heavy launch vehicle serial configuration, and then proposes common points and development trends of future heavy launch vehicles in the world.

Key words: heavy launch vehicle, development characteristics, trends

1 lNTRODUCTlON

Launch vehicles are aerospace vehicles that are responsible for delivering payloads such as satellites, manned spacecraft, space stations, space probes, into predetermined orbits. There are various classification methods for launch vehicles, such as single-stage, multi-stage launch vehicles(according to the stages), solid and liquid launch vehicles(according to the type of propellants). Another classification is in accordance to the carrying capacity, which is divided into small, medium, large and heavy (super heavy)launch vehicles. At present, there is no uniform quantitative standard for this kind of classification. In this paper,launch vehicles with a take-off weight greater than 2000 t and a Low Earth Orbit (LEO) capacity more than 100 t is defi ned as a heavy launch vehicle.[1]

Since the 1960s, the United States and Russia (the former Soviet Union) have carried out the development of heavy launch vehicles. The United States developed the famous Saturn V and space shuttles, while Russia developed the N-1 rocket (unsuccessful flight) and the Energia heavy launch vehicle. These heavy launch vehicles greatly improved the ability for man to enter space. With their help, man has landed on the moon, achieved numerous round-trips into space. The heavy launch vehicles played an important role in human development and the utilization of outer space resources.

At the beginning of the 21st century, the US government proposed Project Constellation to return to the moon and decided to develop the Ares 1 manned launch vehicle and the Ares 5 heavy launch vehicle. Although President Obama terminated Project Constellation in 2009, the pace of developing heavy launch vehicles in the United States has not stopped. On September 14, 2011, NASA officially announced a new generation of heavy launch vehicle under the Space Launch System(SLS) development plan.

In recent years, in addition to the SLS plan, China has begun its demonstration for a heavy launch vehicle, which will have a serialized configuration and expected to make its maiden flight before 2030. Russia also announced to develop a LEO 130 t heavy launch vehicle for launch around 2030.Moreover, the US commercial space company SpaceX announced plans to develop its super heavy launch vehicle, Big Falcon Rocket (BFR), before 2025. It can be seen that heavy launch vehicles have become a hot topic in thefield of aerospace.[2]

The development of heavy launch vehicles have their characteristics and trends, including diameter selection, power selection, mission planning, etc. This paper analyzes the development of heavy launch vehicles in the world, summarizes the characteristics of heavy launch vehicles and development trends, and provides reference for the future development of China's heavy launch vehicle.

2 DEVELOPMENT OF FORElGN HEAVY LAUNCH VEHlCLES

So far, only the United States and Russia (the former Soviet Union) have developed heavy launch vehicles in the world, and other space nations such as Japan and India have not proposed heavy launch vehicle development plans. The heavy launch vehicles that have been put into use or planned are as follows in proposed flight order: Saturn V (United States), N-1 rocket(Soviet Union), Space Shuttle (United States), Energia rocket(Soviet Union), Ares 5 (United States), SLS (United States) and BFR of SpaceX (United States).

The above heavy launch vehicles, except for the SLS and BFR are under development, the other models are retired. Although Russia has made no substantial progress in developing new heavy launch vehicles, it has proposed a variety of heavy launch vehicle schemes based on existing technologies and facilities.

2.1 Saturn V

Saturn V was a manned heavy launch vehicle used for the Apollo Program. This launch vehicle with the highest quality and capacity (Figure 1) in the Saturn series was developed in 1957 and successfully achieved its first flight in November 1967. During the development of Saturn V, NASA successively developed two intermediate launch vehicles, the Saturn 1 and Saturn 1B, and used these two to conduct considerable flight verifications on key technologies and systems for Saturn V.

Figure 1 Saturn V (Source from NASA)

Saturn V was a tandem three-stage launch vehicle where the diameter of thefirst and second stages were both 10.06 m and the third stage diameter was 6.6 m. The total length of Saturn V was 110.6 m with a take-off weight of 2,946 t, and a carrying capacity to the moon of about 47 t. Saturn V conducted 13 launches from November 1967 to May 1973, all of them were successful.

2.2 N-1 Rocket

N-1 rocket was a heavy launch vehicle planned by the former Soviet Union to carry people to the moon.The rocket had a tandem five-stage configuration with a total length of about 105 m, a take-off weight of 3,080 t and take-off thrust of 4,620 t. The LEO launch capacity of N-1 was designed to be about 100 t.The rocket used liquid oxygen and kerosene propellants for all stages. Thefirst stage Block A was composed of 30 NK-33 engines. The layout of engines had at its center 6 units and 24 units around the circumference. The second stage Block B,the third stage Block C, the fourth stage Block G and thefifth stage Block D were equipped with eight, four, one, and one liquid oxygen-kerosene engines respectively. From the first stage to the last stage, as the number of engines and the quantity of propellant decreased, the diameter of the launch vehicle decreased too. The whole rocket was tapered with a maximum diameter of 17 m, as shown in Figure 2.

Figure 2 N-1 rocket (Source from Internet)

However all the 4 flights of N-1 ended in failure. After the Apollo Program where the United States sent thefirst person to land on the moon, the Soviet Union terminated the development of the N-1 rocket and canceled the manned mission to the moon.

2.3 Space Shuttle

The space shuttle was thefirst reusable launch vehicle in the world developed by the United States after the Apollo Program. The space shuttle consisted of three parts: one orbiter,one external tank and two solid boosters. The orbiter adopted 3 main engines with liquid hydrogen and oxygen propellants.The vacuum thrust of a single engine was about 213 t. The propellants were stored in the external tank. After the main engines were shut down, the external tank was separated and fell into the ocean. The external tank was 47 m in length and 8.38 m in diameter. Its structural weight was about 33.5 t and the mass afterfilling propellants was about 743 t. Two solid boosters were arranged with the diameter of 3.7 m and the mass of 586 t. The thrust of a single booster was about 1,315 t. The space shuttle took off with a mass of about 2,041 t, and the total mass into orbit including the orbiter was more than 100 t, of which the payload was about 30 t. During the program, the space shuttle conducted 135 flights and suffering two major accidents. It was retired in 2011.

Figure 3 Space shuttle (Source from Internet)

Figure 4 Energia rocket (Source from Internet)

Figure 5 Conceptual diagram of Ares 5(Source from NASA)

2.4 Energia Rocket

The Energia rocket was a general-purpose heavy launch vehicle developed by the Soviet Union. It was used to launch the Buran orbiter and other payloads for different missions into other orbits.The Energia rocket was afirst-half stage configuration and an all liquid propellant launch vehicle. The total length of Energia was about 59 m and the take-off weight and thrust were about 2,220 t and 3,616 t, respectively. Its LEO carrying capacity was about 100 t. The core stage was 58.7 m in length and 7.77 m in diameter. The rocket had 4 strap-on boosters, each of which were 37.7 m long and 3.9 m in diameter. The Energia rocket is shown in Figure 4.Although the launch vehicle was only used twice, its large thrust RD-170 liquid oxygen/kerosene engine has been produced in a number of models, which have been widely utilized including the RD-171 (thrust 740 t), RD-180 (thrust 390 t) and RD-191 (thrust 210 t) engines, and became the main power system for the Proton,the Atlas 5 and the Angara launch vehicles.

2.5 Ares 5

Ares 5 was a heavy launch cargo vehicle developed in the United States for Project Constellation. It adopted a two-stage liquid hydrogen and oxygen core with two large strapped thrust solid boosters (see Figure 5). The diameter of thefirst and second core stages were both 10 m. Thefirst stage used 6 RS-68B hydrogen-oxygen engines and the thrust of a single engine was 318 t. The second stage used 1 J-2X hydrogen-oxygen engine and the thrust was 108 t.The solid booster had a diameter of 3.7 m, with a single thrust of 1,720 t. The Ares 5 rocket had a takeoff mass of 3,705 t and a full length of about 110 m. The designed LEO carrying capacity was 160 t and the LTO carrying capacity was 63 t. The development of Ares 5 was terminated in 2009.

2.6 Space Launch System (SLS)

While the US Obama government terminated Project Constellation and the development of the Ares 5, it proposed a long-term exploration goal for manned landing on asteroids and Mars in 2025 and 2030 respectively, and returning safely. The goal required the development of the SLS heavy launch vehicle series with a maximum carrying capacity of 130 t to LEO. Under NASA's plan, the development of the SLS could be divided into three steps:

Thefirst step: Based on mature technologies of the space shuttle, such as tanks and engines, and with the newly developed upper stage of the Delta 4 launch vehicle, SLS Block 1 will have a LEO carrying capacity of 70 t. By developing SLS Block 1, a new core stage and the complete system can be verified. The initial scientific experimental load launch missions will also provide support for further lunar exploration and other far-Earth explorations.

The second step: Based on the SLS Block 1 design and keeping the solid booster, SLS Block 1B will utilize a new type of exploration upper stage. The SLS Block 1B has a LEO carrying capacity of 105 t and is designed for the manned asteroid landing mission.

The third step: By using advanced liquid boosters and keeping the exploration upper stage, SLS Block 2 will be developed with a 10-m diameter payload fairing and adapter. The LEO carrying capacity of SLS Block 2 will increase to 130 t and the launch vehicle will be able to execute deep space exploration missions and far-Earth manned exploration missions.

2.7 Big Falcon Rocket (BFR )

At the IAC 2017, SpaceX announced its Mars immigration program and its corresponding carrier BFR[3], which can land vertically and both thefirst and second stages can be reused.The entire rocket consists of a booster part (first stage) and a spacecraft part (second stage). It can carry a payload of up to 150 t into LEO and transport a payload of 50 t beyond the Earth orbit. With a diameter of 9.14 m and a height of 106 m, the BFR will utilize 31 Raptor liquid oxygen and methane engines. The full mission take-off mass exceeds 4,000 t and the takeoff thrust will be 5,377 t. The spacecraft part uses 7 Raptor engines and it is large enough to accommodate 40 passengers.It can also supplement the propellant in orbit and continue to perform tasks far from the LEO.

3 CHARACTERlSTlCS OF CHlNA'S HEAVY LAUNCH VEHlCLE

Figure 8 Conceptual diagram of China's heavy launch vehicles

China's heavy launch vehicles (LM-9) will be built in a series of configurations in accordance with the principle of generalization, serialization, and combination.[4]The largest type will have a three-half stage configuration with the maximum core stage diameter about 10 m and 4 strap-on 5-m diameter boosters. The total length of China's heavy launch vehicle will be more than 100 m. By reducing the number of boosters and adjusting the number of first stage engines, two-boost and tandem configurations can be formed. The corresponding structures of the three configurations will be identical and can be modularly combined. The carrying capacity of the series of LM-9 launch vehicles will cover 50 t to 140 t into LEO and 15 t to 50 t into Lunar Transfer Orbit (LTO). They will be able to conduct a mission to Mars with sample return, manned moon landing, manned Mars landing and deep space exploration beyond the solar system. China's heavy launch vehicles will also be able to provide important support for space infrastructure construction (such as space solar power station).[5,6]

4 DEVELOPMENT CHARACTERlSTlCS AND TRENDS OF HEAVY LAUNCH VEHlCLE

The United States and Russia (the former Soviet Union)had developed multi-type heavy launch vehicles (Table 1) and now the United States and China begin to carry out the development and demonstration of heavy launch vehicles (Table 2).With the evolution of heavy launch vehicle, it can be seen that countries have different technical options in the development of heavy launch vehicles according to their own technical base and goals.

Analysis of the development trends of the above-mentioned heavy launch vehicles can be summarized under the followingfive aspects.

1) The heavy launch vehicle is always an important symbol for a space power

To enhance the ability to enter space and to use space resources is always a space power's goal. A heavy launch vehicle represents the level of a country entering into space.Since the 1960s, two recognized space powers in the world,the United States and Russia (the former Soviet Union), have never given up the development of heavy-duty launch vehicles and wanted the ability to launch a payload of more than 100 t into LEO.

On December 11, 2017, the US President Trump signed thefirst space administration directive after his inauguration,officially instructing NASA to return to the moon. It also hasplans for the development of the SLS heavy launch vehicle.The US media also believes that this is an important signal to consolidate the status of the US as a “space leader.” The serial configuration of SLS, in addition to the implementation of the return to the moon mission, will also serve NASA's Deep Space Gate program, Mars sampling return plan, asteroid exploration and capture plan, and manned landings on Mars planned in 2033.

Table 1 The main parameters of retired heavy launch vehicles

Table 2 The main parameter of heavy launch vehicles under development

Although Russia's economic development has been sluggish in recent years, the development of heavy launch vehicles has never been abandoned. The government clearly announced that Russia would develop a launch vehicle with a LEO carrying capacity of more than 50 t before 2030 and a heavy launch vehicle with a LEO carrying capacity of 130 -180 t after 2030 for manned lunar landing and for a permanent lunar station. It can be seen that the heavy launch vehicle is a symbol of national strength and a guarantee for accomplishing significant missions in deep space. Thus, each space power has always been paying great attention to developing heavy launch vehicles.

2) Heavy launch vehicles are developed for specific missions and multi-missions

Although the new generation of heavy launch vehicles such as SLS series, are mostly expendable launch vehicles like the Saturn V and N-1 rockets, the emphasis is on multi-tasking adaptability, economy and sustainability (expansion of capabilities). Saturn V was advanced in performance, but it was only designed for the manned lunar landing. Hence its development was not sustainable, and after the Apollo Program, the Saturn V was retired. After the space shuttle, the new US space development plans focus on expendable launch vehicles, emphasizing technological advancement, multi-task adaptability and sustainability.

China's heavy launch vehicles adopt a serial configuration. By changing the number of boosters, they can achieve a range of carrying capacities to satisfy various tasks such as Mars sample return, asteroid detection and manned lunar exploration. Although the serial configuration is different from that of SLS which uses an advanced upper stage and two advanced boosters to improve the carrying capacity, the core stage structures of China's heavy launch vehicles are completely consistent, which is more conducive to production.

3) Heavy launch vehicles always adopt large-diameter structures

The diameter is one of the most important factors to design a launch vehicle. The core stage diameter determines the size and capability of a launch vehicle as can bee seen from the statistical analysis of the diameter and carrying capacity of more than 10 large and heavy launch vehicles in the world. As a result, it can be seen from the Figure 9 that there is a near linear ratio correlation between the rocket carrying capacity and the diameter of the launch vehicle. Because of this relationship, although heavy launch vehicles in different countries have different numbers of engines, a prominent common feature is using large-diameter structures.

Figure 9 The relationship between the carrying capacity of large and heavy launch vehicles and the diameter of the structures

The diameter of the core stage also affects the length of a launch vehicle. From the development history of the heavy launch vehicle, the total length is generally from 100 m to 120 m. If the launch vehicle is too long, it is very difficult for assembly, transportation and launch installation. Therefore, to achieve a LEO capacity of about 100 t, it is necessary to use a large diameter structure for heavy launch vehicles.

4) The power system selection of a heavy launch vehicle needs to match one country's own capability

The power system is a key part and thefirst problem to be solved when designing a launch vehicle. For power systems of heavy launch vehicles in the world, except the N-1 rocket using full liquid oxygen/kerosene engines and BFR using full liquid oxygen/methane engines, others have selected from the liquid hydrogen/liquid oxygen engines plus liquid oxygen/kerosene engines and the “liquid hydrogen/liquid oxygen engines plus solid engines.” Due to the high energy and high impulse, liquid hydrogen and liquid oxygen propellants are the preferred propellants for the upper stage of a heavy launch vehicle. The key point to power system selection is the comparative analysis of liquid oxygen/kerosene engine and solid engine. Considering current heavy launch vehicles have strap-on boosters, the above problems can also be mitigated using liquid boosters(liquid oxygen/kerosene) or using solid boosters.

Overall, launch vehicles that use liquid or solid booster configurations have their own characteristics. Either of them can achieve the aimed performance. For the mainstream launch vehicles in the world, the numbers of configurations using liquid boosters and solid boosters are basically the same.Countries generally choose a development path based on their respective technological advantages and specific conditions.

5) The overall plan of a heavy launch vehicle should fully consider one country's own characteristics

Heavy launch vehicles represent the highest level of launch vehicle technology in a country or a company. Therefore, the general scheme of a country's or a company's heavy launch vehicle should fully consider its own characteristics.

Taking the US SLS heavy launch vehicle as an example.The solid boosters, long tanks and engines draw on the mature technology of the space shuttle. The basic configuration can be developed only by improving the current requirements, and the extended configuration also utilizes the mature technology of other launch vehicles in the United States. China's heavy launch vehicles have a series of configurations with different numbers of boosters. This idea comes from the Long March 3A series of launch vehicles. SpaceX's BFR also inherits its vertical take-off and landing and multi-engine combination technology in the Falcon 9 rocket, and utilizes a full liquid oxygen/methane configuration based on its ultimate goal of manned landing on Mars. Therefore, the overall plan of a heavy launch vehicle must conform to the technical development path of the country or the company, and it is necessary to fully apply its own technical advantages, thus reduce development risks and development costs.

5 CONCLUSlONS

Heavy launch vehicles have the characteristics of large carrying capacity and high efficiency. As demands for human space exploration increase, development of heavy launch vehicles has become one of the hot topics of aerospace in the world today. This paper draws the following conclusions by summarizing the development characteristics and trends of the world's heavy launch vehicles:

1) Heavy launch vehicles are an important manifestation of a country's comprehensive strength, and it is an important symbol for a space power.

2) Heavy launch vehicles generally use large-diameter structures, which are difficult to manufacture. They depend on a country's industrial foundation.

3) Looking back on the evolution of heavy launch vehicles, it can be seen that different countries have different characteristics in selecting technical routes according to their own technical foundations and goals. The early heavy launch vehicles mainly emphasized satisfying specific tasks. The present heavy launch vehicles mainly emphasize the adaptability and sustainability of multi-task assignments and develop a series of configurations.

4) The development of heavy launch vehicles should focus on technology inheritance and costs according to their national conditions, technological advantages and foundations.