Are you still carpooling or group-fighting? Let's ride on a rocket together.

In the era of rapid development of the sharing economy, everyone can "share" almost everything: "carpooling", "playing together", "group buying", and all kinds of "share" economy have occupied everyone's life. In the final analysis, the "share" economy is to use the principle of "small profits but quick turnover" to enable buyers to get lower prices on the one hand and sellers to get more profits on the other hand. However, have you heard of "group buying" to launch rockets? Why do satellites need to "share" cars to go into space? What are the benefits of doing so? Let's take a look.

Satellite launch requires carpooling: There are many benefits to riding a rocket in a group

On July 9, 2021, at the Taiyuan Satellite Launch Center, the Long March 6 carrier rocket sent the "Zhongzi" 02 satellite into the predetermined orbit, and the launch was a complete success. This is also the second time that the Long March 6 has carried out a domestic commercial launch mission this year. Not long ago, the Long March 6 successfully launched multiple commercial satellites for domestic and foreign users into space through a one-shot multi-satellite method.

The traditional method of launching satellites is to launch one satellite with one rocket. Preparing for a rocket launch requires tens of millions of yuan and takes several years, which is a considerable amount of work. However, if multiple satellites are put together and several satellites are sent into orbit simultaneously or one after another using one carrier rocket, the cost of launching satellites can be greatly reduced.

The "multiple satellites in one rocket" technology was born for this application. Starting from September 20, 1981, my country launched three scientific experimental satellites with the "Storm 1" rocket, becoming the fourth country in the world to master the technology of launching multiple satellites in one rocket. At present, the "multiple satellites in one rocket" satellite launch method is also very popular internationally. On January 24, 2021, the "Falcon 9" rocket of the US Space Exploration Technology Company carried 143 satellites into space, setting a record for the number of satellites launched in a single launch. The previous record was held by India's PSLV-C37 Polar Satellite Launch Vehicle, which sent 104 satellites into the predetermined orbit in February 2017.

In theory, the bigger the rocket and the stronger its carrying capacity, the stronger its "multiple satellites with one rocket" capability. However, for satellite launches, especially commercial launches, the bigger the rocket's carrying capacity, the better. With the development of microelectronics technology, satellites are getting smaller and smaller, and various cubic satellite and microsatellite concepts are popular.

Taking 2019 as an example, my country launched about 80 satellites, of which microsatellites accounted for 67.5%. In the next 5-10 years, my country's demand for launching commercial small satellites will exceed 4,000. These satellites are often only tens of kilograms or even several kilograms. It takes hundreds of "big rockets" with a full payload of several tons to carry out the launch mission, which will prolong the "carpooling" time. Even if the full payload is assembled, there will be a problem of insufficient space. The miniaturization and rapidity of rockets have also become the main trend of commercial satellite launches.

The "courier" in the new generation of rockets: Long March 6

As satellites become cheaper and launch missions become more time-sensitive, the price requirements for single transportation are also getting lower and lower. As an emerging force that cannot be ignored in the international commercial launch market, my country is increasingly in need of a new type of rocket that can launch a variety of satellite payloads at a low cost. The Long March 6 is my country's new generation of carrier rockets to meet such needs.

Long March 6 and its derivative models are mainly used to replace Long March 2CD and Long March 4 series launch vehicles. When measurement and control conditions permit, the orbital capacity of its main market - SSO (solar geostationary orbit, almost running on a polar orbit, with an orbital altitude of about 700km) reaches 1 ton, which solves the problem of excess capacity for launching small satellites in my country without small launch vehicles. When only domestic measurement and control is used, 500 kg of capacity can also meet a considerable part of the SSO payload. This also means that when launching satellites with lighter payloads, there is no need to call the Yuanwang survey ship, relay satellites and overseas measurement and control stations to complete the telemetry mission, which greatly reduces the telemetry cost in the launch mission.

As a pioneer in the commercial launch of my country's new generation of carrier rockets, it has embarked on the commercial road of "multiple satellites in one launch" since it was put into operation. It looks beautiful to "scatter" multiple satellites at a time, but it is not easy to do. The key technology that needs to be solved is to prevent the collision between satellites and rockets and between satellites when a large number of satellites are released, and to consider the compatibility of satellites of different sizes and different operating orbits. To this end, the Long March 6 uses a serialized and standardized multi-satellite launch interface device for the first time, which can adjust the attitude with high precision and realize the forward orbit control of the rocket's last stage, "multi-satellite orbit period control method" and other technologies to ensure that the separation speed and direction of the satellites are different, and the orbital periods between multiple satellites and between satellites and the rocket's last stage are pulled apart and fly farther and farther, ensuring the safe distance between stars and between satellites and rockets.

So far, the Long March 6 has successfully carried out six commercial launch missions. Since its first launch on September 20, 2015, it has sent 20 microsatellites into space orbit, setting a new record for my country's launch of multiple satellites with one rocket. It has since carried out five commercial launch missions, all of which were in the "multiple satellites with one rocket" launch mode, with a success rate of 100%.

Launch costs: Shipping costs reduced immediately? Start with fuel

Compared with the rocket itself, there is another not insignificant expense in the rocket launch mission – the fuel cost.

In line with the principle of "fly meat is meat", the cheaper the rocket fuel, the better. Before the launch of the Long March 6, my country's commercial launch vehicles mainly used unsymmetrical dimethyl hydrazine/nitrogen tetroxide (oxidizer), which is a relatively expensive fuel: the international average price is about $300/kg. Taking the Long March 3B launch vehicle, one of the main commercial launch vehicles in my country, as an example, its commercial launch promotion quotation is about $60 million, and the rocket ex-factory price is about $15 million. The fuel used by the rocket (the first and second stages are unsymmetrical dimethyl hydrazine/nitrogen tetroxide) costs about $3 million, accounting for about 5% of the launch quotation and 16% of the rocket cost (including fuel).

On the other hand, hydrazine-based fuels are highly toxic, so special care must be taken during storage and refueling. Landed rocket debris can also pollute the environment. Therefore, with environmental protection requirements becoming increasingly stringent, Long March 6 has abandoned traditional hydrazine fuel and adopted a cheap and worry-free fuel engine: the YF-100/YF-77 liquid oxygen/kerosene rocket engine as the first and second stage engines of the rocket. As one of the members of the new generation of launch vehicles, the rocket engine equipped with Long March 6 was designed with modularization in mind. Its YF-100 is the booster of Long March 5, and the YF-77 is common to the second stage rocket of Long March 7. The high degree of versatility reduces the types of new generation rocket engines, increases production and spreads costs. The common point of this generation of rocket engines is that the "materials" are clean, worry-free and cheap.

Compared with UDMH, the coal-based aerospace kerosene used in the YF-100/YF-77 engine relies on my country's abundant coal resources, is cheaper, and has a lower external dependence. With the easy-to-prepare liquid oxygen, the cost per kilogram of fuel can be reduced from hundreds of dollars to several dollars. Even considering the loss of cryogenic fuel during storage and refueling, the fuel cost can still be reduced by more than 2/3; another outstanding feature of kerosene/liquid oxygen fuel is that it is non-toxic and harmless, more "friendly" to the launch site and personnel, and only produces water and carbon dioxide when burned. In addition, because kerosene/liquid oxygen is extremely less corrosive than traditional hydrazine fuel, the rocket engine has the ability to be tested multiple times, and the engine of the Long March 6 rocket has the potential to be reused in the future.

International prices of various liquid rocket propellants. The aerospace kerosene used by Long March 6 has similar specifications to RP-1 fuel and is cheaper than nitrogen tetroxide/unsymmetrical dimethylhydrazine (UDMH).

Cheap and worry-free fuel is only the first step in saving money for the Long March 6. The Long March 6 has also made great efforts to save fuel. The first and second stage engines of the Long March 6 both use high-pressure regenerative cycle (also known as staged combustion cycle) technology. Since the main advantage of the high-pressure regenerative cycle is that all gas and heat are discharged through the combustion chamber, the Long March 6 rocket engine has achieved a combustion efficiency of 98% in the test run; and the high pressure tolerance of the combustion chamber allows nozzles with a larger expansion ratio to be used on the engine, thereby increasing the specific gravity of the rocket engine and making full use of every drop of fuel in the rocket.

For fast delivery, “trip leveling” is also the best option?

In addition to cheap launch, rapid launch is also a major attraction of the Long March 6 carrier rocket.

The launch preparation period of Long March 6 has been greatly shortened through the "three-level technology" of horizontal overall test, horizontal overall satellite-rocket docking, and horizontal overall transportation erection launch. Correspondingly, whether it is my country's Long March 11, the United States' "Minotaur", Japan's "Epsilon" and other solid rockets known for their rapid launch capabilities, the launch preparation period takes 7-16 days, while Long March 6 only needs 7 days at most to deliver customer payloads to space.

With the continuous launch of the Long March 6, the curtain has been raised for the intensive use of my country's new generation of carrier rockets in commercial spaceflight. It is conceivable that in the near future, the Long March 6 rocket will usher in a relatively busy period of fulfilling international and domestic commercial orders, and will repeatedly carry out the launch mission of multiple satellites with one rocket to meet the launch needs of various customers.