1 Foreword

At 20:25 on October 13, 2024, Beijing time, SpaceX of the United States conducted the fifth flight test of the starship system and achieved complete success. During the test flight, the orbital launch integration tower (OLIT-1), known as "mechanical Godzilla", completed the capture and recovery of the first-stage overweight boost (B12) of the starship system for the first time. The recovery process was amazing. This article will focus on the orbital launch integration tower, briefly introduce the unique and crazy recovery scheme of starship system, and sort out its development process and technical design, so as to provide a general food guide for "mechanical Godzilla.

Figure 1 Starship System Fifth Flight Test and Official Mission Chapter

 

2 Gods are born.

The predecessor of the starship system is called BFR system (Big Falcon Rocket). The plan can be traced back to 2005. At that time, SpaceX was established only 3 years ago and had not carried out any rocket launch mission. The plan was only put forward as the company's vision. By 2016, SpaceX's main medium-sized launch vehicle Falcon 9 has been nearly finalized. The recovery and reuse technology has achieved a breakthrough and tends to be stable. BFR, which has been silent for many years, has been renamed Interplanetary Transportation System (ITS) and returned to the public's view. At that time, the mission of ITS system was to colonize Mars, with a take-off mass of more than 10000 tons and a low earth orbit carrying capacity of 300 tons. The rocket body was mainly made of carbon fiber composite materials.

Figure 2 Musk and BFR models

By the end of 2018, the ITS system abandoned the original carbon fiber arrow design and switched to stainless steel arrow materials. Subsequently, the entire system was renamed Starship (StarShip). In 2019, the Starship System's first demonstration prototype, the Starbug (Starhopper), successfully conducted two test flights, demonstrating to the public the technical feasibility of combining a stainless steel arrow body with a Raptor engine. In March 2020, SpaceX released Starship User Manual V1.0, at which point the Starship system still uses the traditional landing leg recovery configuration. The first sub-stage is called Super Heavy booster, and the landing leg design is integrated with the rocket tail. The second stage is called Starship (Starship spacecraft), and the landing leg is telescopic design.

Figure 3 Evolution of the Starship concept

At the end of 2020, there will be a radical change in the way Starship is recycled. Musk announced on Twitter that he would use the robotic arm on the launch tower to catch the rocket. This recovery method is also vividly called "chopstick" recovery. In early 2021, the first orbital launch integration tower (OLIT-1) at Starship Base in Boca Chica, Texas, began tower base construction, completed the first steel frame module construction at the end of April, completed the tower body capping at the end of July, completed the quick disconnect arm hoisting at the end of August, and completed the main capture arm hoisting in mid-to-late October. By the end of 2021, the tower is basically in operational condition and will take about one year to build. After the completion of the basic construction of the orbital launch integrated tower, SpaceX has carried out a series of tests on it, including a 500-ton static water bag bearing test, which also indicates that the tower has practical capacity. With the gradual formation of the orbital launch integration tower, a more vivid and more sci-fi title has begun to spread like wildfire. This is the birth of "Mechanical Godzilla" (Mechazilla).

Fig.4 Assembly process of orbital launch integrated tower

From the completion of the launch tower to the fourth test flight of the starship, the tower mainly undertakes a series of flight preparation and testing work of the starship system, and does not carry out too many test items related to recovery. After the fourth test flight of the Starship achieved significant results, Musk announced that the fifth test flight will attempt to use the tower to recover the first stage of the Starship. Since then, the launch tower began to carry out a lot of modification and testing work with the goal of recycling.

So far, SpaceX has built a complete ground launch recovery support system at the Starship base and has completed five orbital flight test missions. Starship base's second ground system is under construction, and the main body of the orbital launch integrated tower has been stacked and capped. In addition to the Starship base in Texas, SpaceX is also planning the construction of a Starship launch site at Cape Canaveral. The orbital launch integration tower in LC-39A is nearing completion, but the project is suspended due to related planning adjustments.

 

3 a hand.

As the heaviest man-made aircraft in human history, the take-off weight of the Starship system has reached an unprecedented 5000 tons, far exceeding the 3000 tons of the legendary Saturn 5, and even exceeding the full-load displacement of a 054A frigate. Therefore, it is no accident that the complexity of the supporting ground support system has reached a new height. At present, SpaceX has built a complete starship ground support system at the starship base in Boca Chica, Texas, which mainly includes orbital launch pad (OLM), orbital launch integrated tower (OLIT), tank farm (Tank Farm), water-cooled steel plate and supporting facilities. One of the most complex and most innovative, or orbital launch pad and orbital launch integrated tower.

Fig.5 Composition of Starship Ground Support System

The orbital launch pad is also known as the "zeroth stage", and its function is similar to that of a traditional rocket launch pad, mainly providing structural support for the rocket body. A quick disconnect device is provided on the surface of the launcher near the launch tower to provide filling air supply and electrical connection for the overweight booster. The device itself has telescopic and thermal protection capabilities to protect itself from being damaged by the plume. There are guiding devices on both sides of the surface of the launch platform to assist the overweight booster to dock with the launch platform, but because these guiding devices have no thermal protection capability, they were temporarily removed before the fifth test flight. The inner ring of the launch pad is equipped with 20 pinning and releasing support arms. These support arms are used to support the arrow body and can play the role of pinning and releasing, which can ensure the safety and stability of the arrow body during static ignition and formal ignition. At the same time, these support arms are also equipped with engine starting devices to provide starting services for the outermost 20 Raptor engines of the overweight boost. This design greatly simplifies the complexity of the arrow. In addition to the above functions, the launch pad also has functions such as water spray cooling and noise reduction.

Figure 6 Composition of orbital launch pad

Orbital launchers are important, but the most striking of the Starship's ground support systems is the Orbital Launch Integration Tower of Mechanical Godzilla Ben La. The main functions of the orbital launch integrated tower include the vertical assembly of the starship system, the propellant filling of the starship spacecraft, and the recovery of the overweight booster. It consists of three parts: the main body of the tower, the quick disconnect arm and the capture arm system.

The main body of the tower is a steel frame structure. The total height of the tower body is about 145 meters, and the cross section is a square with a side length of 12.5 meters. The tower body is stacked by 8 sections of prefabricated steel frames. The main body of each section of steel frame is composed of hollow square steel columns with a side length of 1.6 meters. The middle is a personnel maintenance elevator. The left rear side of the tower is provided with a personnel maintenance ladder. Main winching pulley block and other structures are installed at the top of the tower, and the main winching driver is installed in a separate equipment room at the root of the tower to control the capture arm system to move up and down along the tower body. Dedicated track modules are laid on the three edge surfaces of the tower facing the launch pad to serve the lifting movement of the capture arm system. A quick disconnect arm and its actuator are installed on the side of the middle of the tower away from the launch pad. The lower part of the tower body is provided with a lifting limit device at a position close to the ground. When the capture arm system is in maintenance or other special conditions, it can be lowered to the limit device to unload the hoisting system. Flame baffles are laid at the lower part of the tower towards the two sides of the launch pad. It is worth noting that before this test flight, a part of the flame plate was added to the top of the original flame plate, and the new flame plate is used to protect the tail flame that swings violently during the overweight boost landing maneuver.

Fig. 7 Composition of Orbital Launch Integrated Tower

A quick disconnect arm is installed in the middle of the tower. The device is mainly connected to the tail panel of the starship spacecraft to provide filling, gas supply and electrical connection for the second stage of the starship system. Its function is similar to that of the quick disconnect device on the orbital launch platform. The main body of the quick disconnect arm can swing in the horizontal direction. In the launch preparation stage, it swings forward to connect with the starship. The filling panel will move back and disconnect at the moment of ignition. The main body of the disconnect arm will swing to the right and rear to avoid flames. At the same time, the protection cover will also It will automatically close to prevent the filling panel from being damaged by the tail flame. The early stage of the quick disconnect arm was also equipped with auxiliary stabilizing tentacles for stabilizing the top of the overweight boost, but this device was later removed.

Fig.8 Comparison of early quick disconnect arm and simplified design

If the orbital launch integration tower is the core of the Starship ground support system, then the capture arm system is undoubtedly the core of the orbital launch integration tower. Since the capture arm system has been improved several times in the past, this article selects the latest state to introduce. The two main capturing arms are coaxially mounted on the lifting platform, and the lifting platform is in contact with the guide rail of the tower body and can slide up and down. One end of the hydraulic actuator is mounted on the lifting platform, and the other end is connected with the main capturing arm. The drive system of the hydraulic actuator is arranged on the lifting platform. The length of the capture arm body is about 36 meters, and the length of the usable capture zone is about 21 meters. During takeoff, the left and right capture arms are deployed to the maximum opening angle to avoid the rocket tail flame, and the opening angle is about 110 ° at this time. During recovery, the two capture arms can be close to an opening angle of 0 ° while the stop device contacts to prevent the body from being pinched off.

Figure 9 Capture arm system composition

The upper inner side of the capture arm is equipped with a recovery guide rail, which in turn is connected to a set of parallelogram mechanisms. Under the action of two jacking cylinders, the recovery guide rail moves up and down under the constraint of the parallelogram mechanism, and the movement stroke is about 0.8 meters. Five buffers are arranged along the lower part of the guide rail. In the recovery preparation phase, the jacking cylinder jacks up the recovery rail. The overweight booster landing hook is brought into contact with the recovery guide rail to drive it down together. During this process, the buffer absorbs the excess mechanical energy to provide a proper mechanical environment for the arrow body.

Fig. 10 Recovery Guide Rail in Jacking State

Because there is a certain control error between the capture arm and the rocket, a lateral buffer block is added to the internal measurement of the recovery guide rail. The early lateral buffer block may have used foam material with soft coating, but it was seriously damaged by the tail flame in the subsequent test flight. Therefore, a new type of metal-coated buffer block was replaced before the test flight, and the buffer block also played an important role in the final recovery process.

Fig. 11 Damaged buffer block and new buffer block in previous test flight

After the overweight booster lands stably, the reset sliders at the front and back of the guide rail move to the middle under the drive of two sets of lead screws. During the movement, the overweight booster landing hook will be pushed to the middle position of the capture arm, completing the fine adjustment of the position of the overweight booster after landing. The lower part of the capture arm is provided with a fine-tuning auxiliary arm. The device has three degrees of freedom and can fine-tune the posture of the arrow body in a small range. With the cooperation of the reset slider and the fine-tuning auxiliary arm, the recovered arrow body can be accurately placed back to the launcher or transfer bracket.

Fig. 12 the fine-tuning auxiliary arm is docking with the starship spacecraft.

The middle part of the capture arm is provided with a rotatable lift pin specially used for stacking starship ships. In the recovery preparation phase, the device will rotate outward to avoid interference with the recovery rail movement. It is also because the current starship spacecraft needs to use such special devices for operation, so the starship spacecraft does not have the ability to rely on tower recovery for the time being. In addition to the above-mentioned main devices, the orbital launch integration tower is also equipped with a large number of sensors and cameras, but there is no direct evidence that computer vision-assisted technology is used to participate in the recovery control of the rocket.

Figure 13. Starship Lift Pins and a large number of cameras (green box)

Corresponding to "mechanical Godzilla", the design of overweight boost is also worth understanding. Compared with Falcon 9, the overweight boost has two major improvements in recovery landing. One is to change the landing mechanism from landing leg to landing hook, and the other is the power distribution design of landing section. Compared with the huge landing leg design of Falcon 9, the two small landing hooks boosted by overweight are inexplicably lovely. The fixed landing hooks simplify the number of actuating mechanisms on the arrow while reducing a large amount of dead weight, and improve the reliability of landing while increasing the carrying coefficient. The landing hook scheme also optimizes the force transmission design of the full arrow and cancels the auxiliary structures such as the cross truss in the Falcon 9 storage box, further reduce dead weight.

Fig. 14 Overweight Boosting Top Structure and Landing Hook Design

On the issue of power distribution in the landing section, Falcon 9 can only use the most central Merlin engine among the nine engines, and it is difficult for a single engine to realize effective control of the roll channel, which introduces more resources for air-conditioning attitude control. At the same time, Falcon 9 cannot realize fine control of class hovering because of the relationship between the limit of engine thrust adjustment and the landing quality of the arrow body, the landing process must ensure that the speed and position are reduced to zero at the same time, resulting in a certain trade-off in the final control accuracy. The Falcon 9 was not designed for vertical landing in its early design, so it can be considered to have certain birth defects. The Starship system is a completely reusable design (vertical), and there is also a large amount of pre-model flight data as a reference in the design process. The overweight boost stage is equipped with 33 raptor engines, 13 of which are used in the approach deceleration section and 3 of which are used in the final landing section. The single thrust of the raptor engine is close to the landing quality of the overweight boost, while the three engines can realize hovering control by throttling to the minimum thrust. This power configuration not only provides efficient roll control capability, but also provides strong redundancy and flexibility. It is also with this system design that the tower recovery vision of the Starship system may become a reality.

 

4 see the sun through the clouds

Since Musk first announced on Twitter at the end of 2020 that he would abandon the landing leg program in favor of the "chopsticks" program, the doubts about the program have not subsided.

SpaceX has accumulated more than 350 successful landing leg recovery experiences on Falcon 9 series launch vehicles, of which 267 have been successfully recovered in succession, with an overall recovery success rate of 96.7. The recovery success rate in 2022 and 2023 is as high as 100, while the first data is only 33.3 in 2015. As the first practical reusable launch vehicle, the Falcon 9 series is not only the absolute main force of SpaceX, but also the object of the global aerospace industry. According to the regular plot, SpaceX's next-generation launch vehicle will undoubtedly continue to advance according to the mature plan, but it is clear that Musk and SpaceX engineers have different ideas.

First of all, in order to realize the vision of "making life multi-planetary", Musk has put forward higher requirements for the reusability of the starship system. Even though Falcon 9 has launched more than 100 times a year, there is still a qualitative gap from the real flight operation. Secondly, on the giant volume of the starship, the design of the landing leg also has great engineering problems. The rocket needs to add a huge amount of dead weight to strengthen the structure of the arrow body and install the giant landing leg. More importantly, these costs will be infinitely amplified in the high-frequency launch of flights. Finally, SpaceX has accumulated a large amount of recovery data and design experience in the recovery and reuse of the Falcon 9, which in turn has been used to optimize the design of the Starship, making the reusable design of the Starship more reasonable, more thorough and more efficient than the Falcon 9, which evolved from the disposable rocket.

On June 6, 2024, SpaceX organized the fourth comprehensive test flight of the starship system. The results of the test flight were exciting. Overweight boost successfully achieved high-precision sea splashdown in the Gulf of Mexico. SpaceX Vice President Gerstenmeyer claimed in an interview that the landing accuracy reached "half centimeter level", and the starship successfully achieved sea splashdown even when the rudder surface burned through, musk then announced that he would try the tower to recover the overweight boost in the next test flight.

On June 26, 2024, the test arrow body numbered B14.1 was installed on the orbital launch pad, and then the first simulated arrow body matching test was started. This was also the first time that the outside world intuitively understood the simulation process of tower recovery. The test was carried out using the left capture arm, including the swing control test of the capture arm and the buffer mechanism test. Due to the problem of control accuracy, the collision between the guide rail and the arrow body made a huge metal impact sound during the test, simulating the shaking and deformation of the arrow visible to the naked eye. After the test, the arrow body appeared obvious scratches, and then the test arrow body was sent back to the factory. After this test, the capture arm began to be maintained.

Fig. 15 First Simulated Arrow Body Matching Test

On August 8, 2024, the B14.1 test arrow body returned to the orbital launch pad for the second simulated arrow body matching test. This test used two capture arms to cooperate. On August 17, the simulated arrow body was returned to the factory after the test.

Fig. 16 Second Simulated Arrow Body Matching Test

After completing the second simulated arrow body matching test, the orbital launch integrated tower began a large-scale renovation project. The renovation project includes installing a complete buffer block for the capture arm, replacing hydraulic actuators, installing and inspecting various sensors, and strengthening the capture arm The main body weld is reinforced, and a new stop device is reinstalled. At the same time, the overweight booster B12 also completed the adaptive modification, the landing fulcrum and auxiliary fulcrum for structural reinforcement. A series of simulated capture action tests were carried out on the modified orbital launch integrated tower at the recovery position near the top of the tower.

Fig. 17 Overweight boost B12 after structural reinforcement

On September 21, 2024, the modified capture arm lifted the B12 to a recovery position near the top of the tower, showing the outside world what the tower would look like after recovery. Now we already know that this is not the last time B12 will appear in a recovery posture. On September 22, the Starship system was stacked, and SpaceX announced that the Starship's fifth test flight was ready, pending regulatory approval. On October 8, SpaceX announced that the test flight was aimed for October 13.

At 20:25 on October 13, 2024, Beijing time, the combination of starship S30 and overweight booster B12 took off from orbit launch pad 1 of starship base, and the take-off process was very smooth. T 2 minutes and 42 seconds, the first and second levels completed the thermal separation, and the overweight boost began to return to the field. T 6 minutes and 30 seconds, 13 Raptor 2 engines started smoothly and began landing deceleration. After 7 seconds, 10 engines in the middle layer were shut down, leaving only the last 3 engines for the final landing maneuver. T 6 minutes and 46 seconds, the tail flame of the engine passes through the capture arm plane, 2 seconds later the tail of the rocket passes through the capture arm being closed, when the middle of the methane tank passes through the capture arm, the buffer block starts to contact with the arrow body successively, T 6 minutes and 56 seconds, the overweight booster landing fulcrum contacts the upper end face of the recovery guide rail, successfully completing the first tower recovery test and setting a new record for rocket recovery. According to the video of the position returning to zero after the overweight boost landing, it can be preliminarily judged that the landing accuracy along the direction of the capture arm is about 0.7 meters. Although it is not as sub-centimeter as previously claimed, considering that this is the main maneuvering direction, the control ability of the overweight boost is evident.

Figure 18 Starship Tower Recovery Process

The success of the fifth test flight tower recovery of the starship swept away all the doubts of the outside world, and declared with indisputable facts that the transportation technology between heaven and earth has entered a new era of reuse, and human beings have stood at the door of the era of interstellar migration.

Figure 19 Overweight Boost Landing

 

5 Twin Towers

At the same time as the fifth test flight of the starship, SpaceX also announced the latest plan, that is, to realize the complete reuse of starships in 2025, to catch up with the Mars orbit window in 2026, to send unmanned starships to Mars, and finally to achieve the vision of producing hundreds of starships annually. In order to achieve the above-mentioned long-term goals and better support the smooth test flight of the Starship system, SpaceX has already begun new operations.

On August 21, 2024, just as the No. 1 launch tower was being modified for the fifth test flight, the second orbital launch integration tower of the Starship Base also completed the hoisting of the tower top, which marked the Starship Base officially entered In the era of twin towers, two "mechanical Godzilla" more than 140 meters high stood side by side on the empty Boca Chica Beach ".

Figure 20 Starship Base Tower 2 capped

With a large amount of engineering experience accumulated from the previous ground support system, the new ground system design has also been significantly improved, and the most obvious improvement related to tower recovery is the new capture arm system. The new capture arm can almost be regarded as a "cost reduction and efficiency enhancement" version of the original capture arm. The total length is shortened from the original 36 meters to 26 meters, the effective capture length is shortened from the original 21 meters to 11 meters, the original two sets of lead screw centering devices on each capture arm are reduced to one set, the jacking cylinder is restored to one set, and the buffers are symmetrically arranged in two sets. The shortened capture arm effectively reduces its own moment of inertia and improves the control accuracy; at the same time, due to the shortened length, the type variable and the amount of shaking in the swing process of the capture arm are also greatly reduced, and the control accuracy is further improved. Of course, it should be noted that the premise of the shortening of the capture arm is the strong landing accuracy of the Starship system. Without this premise, these improvements will not be realistic. Up to the current position, Starship Base Tower 2 is still under intense construction and is expected to be officially put into use in 2025.

Figure 21 Capture arm of the new launch tower

Standing at the time of the fifth test flight of the starship, looking forward to the future development of the starship, there are still many hurdles waiting for breakthroughs in the future. The goal of the starship system is to be fully reusable. At present, the design of the starship spacecraft does not have the capability of tower recovery. Moreover, due to the conflict between the position of the engine and the original landing leg, the new landing leg design has not yet been announced. Perhaps the telescopic landing hook will become an option for the second-level rapid reuse of the starship spacecraft.

Figure 22 The recovered starship encounters starworms, which means that a thousand sails have passed by the side of the sunken boat.

 

6 Summary

Starting from the first orbital flight test of the Starship on April 20, 2023, SpaceX achieved the first successful recovery of the Starship system in more than a year. Behind the repeated failures, accompanied by the steady progress of the project, this is undoubtedly a shining page in the history of human spaceflight. While the starship system is gradually maturing, Musk's crazy dream is slowly becoming a reality, and human civilization may really have come to the eve of the interstellar immigration era. As practitioners of commercial aerospace, after the shock and excitement, what we need is for us to continue to work hard. From the earth to the endless orbit of deep space, from the present to the future of eternal exploration, tracking the development of cutting-edge technology, to create a space power dream, we are already on the way.