Editor's Note: This article is a summary of our open source information on Starship at home and abroad, and adds our understanding and analysis. The purpose is to facilitate all interested readers to quickly understand the latest progress of tracking Starship. Limited by the rush of time, there may be some mistakes and omissions, welcome criticism and correction.

The main content of this article includes six parts;
1 Review of the fourth flight
2 Improvements for the fifth flight
3 The fifth flight
4 Five flight issues of concern
5 Chopsticks system and the preliminary introduction of supporting design on the arrow
6 About the next flight

The following is the text

1 Review of the fourth flight

At around 20:50 on June 6, 2024, Beijing time, the SpaceX Starship conducted its fourth orbital-class flight test. The launch site is still at the Boca Chica Starship Base in the Gulf of Mexico, Texas. shortly after launch, the booster rocket successfully landed on the sea surface as planned. The starship flew for about an hour. When entering the atmosphere, the thermal tiles fell and the wing was burned through, but in the end it persisted to land on the sea and completed all the established tasks and goals.

 

2 Improvements for the fifth flight

2.1 Overweight Booster

Boosters B7 and B8 belong to the same batch of overweight boosters, while four-flight B11 and five-flight B12 also belong to the same batch of products, so there is no major change in structure. A few fine-tunes are presented here.

2.1.1 Start tank expansion

Fig. 1 Overweight Secondary Start Tank

Due to the same problem of accidental shutdown after the second start of the engine, SpaceX not only added a giant filter plate at the bottom of the storage tank, but also expanded the small storage tank for the second start;

Headertank (head storage tank/secondary start storage tank) is a special storage tank for secondary start of engines used by SpaceX for starship rockets with huge storage tanks. This storage tank and the main storage tank use multi-way manifolds to provide propellant for raptor engines at different times respectively. The existence of small start storage tank greatly reduces the amount of gas required for pressurization for the pressurization delivery system and ensures the pressurization efficiency, this enables the starship to quickly perform a second start within a short period of time after shutdown, without considering the complex and difficult-to-control problems of arrow body attitude, propellant bottoming and pre-start pressurization.

If the method of replacing the small storage tank with larger volume is adopted for the expansion of the small storage tank in the secondary start, it is relatively difficult for the overweight booster that has been processed, and the main storage tank needs to be cut and replaced and then re-welded. Therefore, the method of bundling multiple additional storage tanks is adopted, and the newly added additional storage tanks can be transported into the booster through manholes for welding.

2.1.2 Added anti-collision structure

Fig. 2 Anti-collision Structure (Stabilizer Arm Lock Slot)

Figure 3 Anti-collision Structure (Landing Hook)

The Wufei Super Weight Plan uses chopsticks for recycling. SpaceX uses B14.1 test storage box for static testing of chopsticks recycling. SpaceX's recycling strategy is that the chopsticks are closed and the booster slides downward until the buffer lifting platform contacts the landing hook. Due to the problems of rocket landing accuracy, corresponding speed of chopsticks and motion inertia of chopsticks, the rocket arrow body collides with chopsticks. B14.1 storage box breaks during the testing process, in order to solve this problem, SpaceX has also added an anti-collision reinforcement beam (longitudinal) to B12 and a black coating (the purpose of the coating may be to reveal the collision area or adapt to friction). This longitudinal reinforcement beam is also designed with a guiding inclined plane to prevent the arrow body structure from being scratched when the rocket slides towards the arm bend of chopsticks.

In addition to the vicinity of the chopsticks, the increased area also includes the upper stable arm locking slot area. The stable arm is a freedom locking device designed to prevent the arrow body from shaking around the suspension point during the movement of the chopsticks after the rocket lands.

2.1.3 Star link antenna shape modification

Fig. 4 Change the Front and Rear Star Chain Antennas

The starship uses a large-scale networking satellite constellation for telemetry data reception throughout the flight of both the first and second stages. The starchain antennas used by the five-flight starship have been iterated many times, and some of the star chain antennas have been modified to improve the design.

 

2.2 Starship Class II

2.2.1 Overall replacement of thermal protection structure

During the reentry process of the leading edge flap of the fourth flight starship, part of the thermal insulation tiles broke and fell off, and the metal structure was burned through. SpaceX then quickly decided to replace the thermal protection structure of the fifth flight starship. During the fourth flight process, four pieces of thermal insulation tiles in the second stage of the starship replaced this new type of thermal insulation tile for testing. Through subsequent tracking, it was found that the starship V2 already in production had adopted this new type of thermal protection scheme, therefore, it belongs to the rapid improvement of the existing scheme.

Figure 5 New thermal insulation tile

The new thermal protection structure adds a layer of ablation layer (black) on the basis of the original two-layer structure. The existing scheme is a three-layer structure, the lowest layer is black ablation paint, the middle layer is white thermal insulation felt, and the outermost layer is a new type of thermal insulation tile. The new type of thermal insulation tile is much thinner than before and is four times the strength of the previous scheme. The three-layer structure is fixed with pins welded on the bulkhead wall, and the pins are unidirectional buckles, after the pins are inserted into the grooves of the thermal insulation tiles, the pins expand to support the thermal insulation tiles, and each thermal insulation tile is fixed with four pins.

Fig. 6 Three-layer thermal protection structure being installed

At the same time, felt for caulking is added to the gap between the thermal insulation tiles in the new plan. The reason is that Musk himself believes that one of the reasons why the right front flap was melted during the four-flight was that the tolerance control of the customized thermal insulation tiles was not good, resulting in too large a gap between the thermal insulation tiles, causing plasma generated during the reentry process to invade the gap and melt the metal structure.

This new scheme does not adopt a three-layer structure in all insulation tiles, but only in key parts with severe heating, and some areas are glued and fixed.

In addition, SpaceX has added several aluminum insulation tiles for testing to further upgrade the thermal protection system. The ultimate goal is to undergo 1427°C reentry heating without any maintenance work. Next flight.

Fig. 7 Partial aluminum insulation tile

2.2.2 Newly added exhaust pipe

The methane tank and the oxygen tank each have a new exhaust line added to the bulkhead, the purpose of which is not yet clear.

Fig. 8 New Exhaust Pipe

 

2.3 chopsticks service tower

In the original plan of Sifei, although the overweight booster splashed down at the seaside, the chopsticks needed to be recovered ectopic in synchronization with the data fed back by the rocket. In fact, the movement speed of the chopsticks was far lower than expected, and there were problems of excessive movement inertia, insufficient stiffness and continuous oscillation after movement. Therefore, it quickly became the focus of improvement after Sifei. This paper briefly introduces some key improvements.

2.3.1 Add lateral buffer energy absorbing block

SpaceX used the test version of the overweight storage tank B14.1 to complete several recovery tests with chopsticks after the fourth flight. During the test, due to the continuous deformation and oscillation of inertia after the chopsticks arrived at the designated place, SpaceX rubbed and collided with B14.1 for many times, causing a huge impact sound on the scene. B14.1 was damaged during the test and longitudinal cracks appeared in the bulkhead.

SpaceX therefore installed a metal energy-absorbing box in the sliding contact area inside the chopsticks, and observed obvious deformation during actual flight.

Fig. 9 Metal energy-absorbing box installed with chopsticks (B14.1 Scratches caused by friction with it)

2.3.2 Add buffer edge strip jacking cylinder

Chopsticks use six gas springs to lift 750mm buffer beams installed on the top of the chopsticks to buffer the vertical impact of the rocket. During the test, it was found that the left and right beams are prone to tilt when lifting, and a single active control lifting cylinder (or cylinder) has limited capacity to lift the buffer beam, adding an additional lifting cylinder.

Fig. 10 Buffer Beam

2.3.3 Grinding weld repair welding reinforcement

SpaceX polished all welding areas of the chopsticks and welded X-shaped stiffeners to improve the welding strength.

Fig. 11 Welding Area

It is worth mentioning that at the peak, there were 9 aerial work vehicles working simultaneously to speed up the welding progress, which is also a common practice for SpaceX to catch up.

Figure 12 9 aerial work vehicles

2.3.4 Replacement of Chopsticks Slewing Cylinder

Within a short time after the fourth flight, SpaceX replaced the telescopic oil cylinders for the left and right arm rotation. During flight, the ground system is required to use the rotation function to adapt to the deviation of the rocket landing area, which puts forward two-way high precision requirements for the rocket control capability and chopstick rotation capability.

Fig. 13 Replacement of Cylinder

2.3.5 Two-tower stacking

If there is an accident on the 5th flight and the recovered starship destroys the launch tower, then spacex will activate the second launch tower. this launch tower has been built for a long time to avoid this situation. the top of the second tower is completed before the 5th flight, and the design of the second tower is also adapted to the height of the starship V3.

Fig. 14 Two-tower standard section module transportation

 

3 The fifth flight

At 20:25 on October 13, 2024, Beijing time, SpaceX launched the Starship heavy launch vehicle at the base of Boca Chica Star City, Texas. This is the fifth orbital stage test flight of the liquid rocket.

The first-stage overweight booster was completed according to the scheduled plan: 33 raptors started, maximum dynamic pressure, inter-stage thermal separation, 13 engines returned for ignition, throwing heat separation ring, 13 landing ignition, chopstick recovery and other key actions, becoming the first successfully recovered full-size starship booster.

The second-class starship spacecraft was also completed according to the scheduled plan: six raptors were turned on, interstage thermal separation, re-entry ignition, re-entry into the atmosphere, landing ignition, sea splashdown and other actions. However, the leading edge flap hinge was burned through during reentry, which is consistent with the four-flight condition. Several explosions and sinkings occurred within a short period of time after the splashdown, but all missions were still completed.

 

3.1 Flight Plan

The trajectory of the starship 3. 4. the five-flight mission is basically the same. The main difference of the five-flight mission is that the rocket takes off from the Boca Chica Starship Base in the Gulf of Mexico. The booster will not splash down in the nearby sea area but will return to the Boca Chica Star City Base and complete the recovery through the chopstick service tower.

Figure 15 Three Flight Trajectory of Starship

Figure 16 Starship Five Fly Trajectory

The fifth flight orbit of Starship II is basically the same as that of the fourth flight. Judging from the speed and altitude provided in real time during live broadcast, Starship II entered an elliptical orbit with an inclination of -15 × 213km × 26.2 degrees. As the perigee altitude of the orbit is lower than sea level, it cannot fly continuously and belongs to suborbital flight.

This does not mean that the starship cannot reach orbit at present, only because SpaceX has decided to avoid uncontrollable de-orbit after entering orbit due to some power system failures (there are engineering challenges of propellant bottom sinking and engine vacuum secondary aerodynamics) during such flight tests, thus adopting the second sub-stage to reduce the filling amount and enter suborbit, which can be selected by ballistic natural de-orbit.

Since the entire flight trajectory covers almost the whole world, an inclination of 26.2 degrees is selected to ensure that the trajectory minimizes crossing densely populated areas.

Figure 17 The flight speed altitude time curve provided by the famous orbitologist Jonathan McDowell according to the live broadcast.

The primary and secondary combinations used in this flight are Starship Number 30(S30) Booster Number 12(B12).

Fig. 18 photos of starship five flying off and chopsticks recovery

 

3.2 development history

3.2.1 S30 Starship

The first segment of S30 was discovered on 14 September 2022;

July 22, 2023 S30 stacking start (nose cone docking load compartment);

18 Aug 2023 S30 end of stacking (tail butt joint);

December 30, 2023 S30 was transferred to the Massey test site for low temperature testing;

April 8, 2024 S30 began to install the engine;

On May 8, 2024, S30 completed the first static ignition (called secondary power system test run in China), which was the first static ignition carried out by Starship without turbine spin-up test;

On May 10, 2024, S30 was pushed back to Gaowan (final assembly plant). According to the experience of Sifei, the overall heat protection structure was replaced (thinner heat insulation board and new ablation layer were replaced) and a vacuum engine was replaced;

The second static ignition was carried out on S30 on July 20, 2024;

On September 23, 2024, S30 and B12 were combined for the first time, and the filling test was carried out, and the wet rehearsal was carried out again on October 7;

9 October 2024 S30 removal from the launch pad and installation of the Flight Termination System (FTS);

11 October 2024 Final stacking completed and ready for flight.

Figure 19 Starship Quiet Point

3.2.2 B12 Overweight Booster

On September 9, 2022, construction of the B12 was confirmed for the first time;

On June 3, 2023, B12 section stacking will begin;

28 August 2023, B12 stacking ends;

On January 24, 2024, the B12 engine will be installed;

On July 9, 2024, B12 was launched from High Bay (final assembly plant) and upgraded to the launch pad;

On July 11, 2024, B12 was tested for the first low temperature filling;

On July 12, 2024, the B12 performed its first engine spin-off test;

On July 15, 2024, B12 carried out the first static ignition of 33 engines (a sub-power system test run);

On September 20, 2024, B12 was raised to recovery height by chopsticks;

On September 21, 2024, B12 installed the thermal separation ring, followed by the first and second stage stacking;

On September 23, 2024, S30 and B12 were combined for the first time, and the filling test was carried out, and the wet rehearsal was carried out again on October 7;

9 October 2024 B12 Installation of the flight abort system (FTS);

11 October 2024 Final stacking completed and ready for flight.

Fig. 20 Overstatic Point

3.2.3 B14.1 Test Tank

Fig. 21 B14.1 Consists of common bottom, column section, front bottom and front skirt

B14.1 is a special test tank. SpaceX used this tank to cooperate with chopsticks for testing, and gained a lot of engineering experience, which eventually led to the success of this recovery. Therefore, this article also summarizes its development history here.

B14.1 has only one upper tank (liquid oxygen tank) and one common bottom. At the beginning of the design, it is used to verify the process and optimize the structure. It is used for the chopstick system upgrade test after the fourth flight of the starship;

It was first discovered on February 17, 2023;

Stacking began on February 28, 2024;

Lifted to chopsticks by jacks on June 21, 2024;

A slap test was conducted on June 26, 2024 (chopstick tower quickly closed and slapped to the rocket bulkhead);

The slap test and damping test of the buffer lifting beam were carried out on June 27, 2024;

From August 7 to August 16, 2024, the rapid closing of chopsticks and the compression test of lifting beam were carried out several times;

Leaving the launch pad on August 17, 2024.

 

3.3 preparation before takeoff

T-50min ~ 48min secondary filling starts and lasts until T-3min20s;

The first-level filling starts at T-40min -34min and lasts until T-2min50s;

T-19min40s the engine starts to pre-cool;

T-40s, there is a hold in the standard process, which is used to wait for the best take-off wind speed to reduce the structural load;

The Starship has the ability to stay in this state for several minutes for decision-making (which is not possible on the Falcon 9); for checking the engine, arrow body attitude, electrical equipment, and pressurization system; at certain times this practice can effectively improve the availability of the launch window and can quickly solve some temporary problems, such as high wind speed;

T-30s, the launch command system released the launch;

T-10s, the flame deflector (water-cooled steel plate nitrogen firefighting) is started;

T-3s enter the engine ignition sequence,

T 2s, the rocket takes off;

 

3.4 flight phase

Figure 22 Flight action curve given by the official

The rocket took off 2 seconds after the 33 engines were started. Since the containment release device was still not activated for this flight, the rocket will leave the launch pad when the thrust-to-weight ratio exceeds 1 and reach the maximum dynamic pressure (Max Q) 1 min02s after liftoff, I .e. the peak value of aerodynamic stress experienced by the rocket;

After 2 minutes and 41 seconds, 30 of the 33 engines in the first stage will be shut down one after another, and the remaining 3 engines will also actively reduce the thrust. While preparing for the inter-stage thermal separation, the requirements of the bottom of the first stage propellant will be met as much as possible, so as to quickly restart the first stage engine for return ignition after separation;

At 2 minutes and 41 seconds, the locking mechanism used for inter-stage separation is unlocked, the three vacuum versions of the secondary engine are started synchronously, and the three sea-level versions of raptors in the secondary inner ring are started after a short time. Due to the need to avoid excessive load on the primary heat shield, the three inner rings actively swing out by 15 degrees at the start time.

Before the second-stage engine is started, the first-stage overweight booster starts the grid rudder to give the pitching moment to the assembly. After separation, the second-stage lost the pitching moment, and the first-stage arrow body swung away under the action of the pitching moment. As the starship's first-stage and second-stage separation surface is located behind the end of the vacuum extension nozzle, the first-stage and second-stage separation is almost instantaneously completed without separation stroke.

At 2 minutes and 48 seconds, the first-level overweight booster started all 13 engines for return ignition; after 53 seconds, the engine was shut down;

In 3 minutes and 43 seconds, the primary overweight booster abandons the thermal separation ring, and then starts the cold air attitude control and opens some main valves of the engine to assist the overweight booster to stay away from the thermal separation ring;

At 6 minutes and 33 seconds, 13 engines were started to land and start. 23 seconds later, all engines were shut down, the overweight booster was captured by chopsticks (SpaceX called mechanical Godzilla), and the recovery was over.

At 8 minutes and 27 seconds, the startup of the second-stage starship ascent ended, and the second-stage starship spacecraft was injected into orbit;

At 48 minutes and 03 seconds, the second-class starship began to re-enter the atmosphere;

1 hour, 05 minutes and 20 seconds, the second-class starship starts landing ignition

1 hour, 05 minutes and 34 seconds, the second-class starship splashed down at sea.

 

3.5 flight results

All flights were in line with expectations, B12 was captured by chopsticks, realizing the first time that humans use ground equipment to recover spacecraft, and the second human orbital booster was successfully recovered! S30 accurately splashed in the predetermined sea area.

Figure 23 Successful recovery

 

4 Five flight issues of concern

Starship system since the project, so far has a complete system capability, this chapter mainly combs the launch of the situation worthy of attention, while introducing the entire Starship project since the establishment of the project, have encountered and solved or temporarily solved some of the problems.

Since it can only be collated through open source information analysis, this article focuses on situations where physical specialties can be directly observed.

4.1 engine system

The Raptor engine had a start-up reliability problem when the star ship first flew. After the first flight, the 20 engines on the outer ring were changed to ground start, and the design of the Raptor engine was optimized with a large number of ground tests. At present, the Raptor engine has been iterated to the third generation test run and the second generation is equipped with arrows.

Fig. 24 Engine Failure

The secondary effects of the Raptor engine explosion include damage to adjacent engine lines, hydraulic systems, and propellant leakage. Improvements to this include providing protective covers for each engine and changing the hydraulic servo system shared by all engines to an independent electric servo system for each engine.

Fig. 25 Engine Protective Cover

During the third and fourth flights of the Starship, the problem of engine secondary start failure essentially comes from the booster delivery system, which will be introduced later.

During this flight, there was a deformation problem in the thrust chamber of 20 raptor engines on the outer ring of the first sub-stage due to aerodynamic heating during landing. Musk's reply was not a big problem, but no solution was provided. The feasible methods include expanding the engine cycle pre-cooling system (different from the self-generated pressurization system, which mainly uses pump pressure to cool the nozzle and pre-combustion chamber areas when the engine is started to avoid structural thermal failure; cyclic pre-cooling is mainly suitable for pre-cooling part of the engine structure with the help of convection generated by the temperature difference on the arrow before the engine is started, to avoid a large amount of propellant gasification during startup, resulting in abnormal startup state) use range and suction intensity, and to maintain the propellant flow in the nozzle cooling channel after the 20 engines in the outer ring are shut down.

Fig. 26 Deformation of Partial Engine Nozzle

4.2 interstage separation

Starship's first-stage booster uses interstage thermal separation, which is an interstage separation technology commonly used in Eastern countries (including Russia and China). It mainly refers to starting the second-stage engine in advance before the first-stage separation unlocking device is started, musk replaced this separation method after the first flight of the starship and added an inter-stage thermal separation ring (later, due to its huge structural weight, he chose to add a special thermal separation ring separation in the first-generation starship. This measure is a temporary measure, and a lightweight thermal separation ring without separation will be designed in the second-generation starship). On this basis, he added a sub-stage to keep three engines on through the whole separation process;

Fig. 27 Interstage Thermal Separation

The advantage of this is that for the second stage, there is no separation and sliding section, impulse is continuously generated, the time of the rocket in the atmosphere is shortened, the gravity loss in the atmosphere is reduced, and the carrying capacity is improved. On the other hand, for the first-stage overweight booster, the inter-stage separation, engine startup and U-turn are reduced as much as possible, and the impact of huge shaking of propellant on the structure in the box is ensured that the main propellant tank is bottomed, it is used to continuously provide "high quality" propellant without gas inclusions to the engine when the secondary start tank is exhausted.

During the third flight of the starship, the measure of early swing of the grid rudder before separation was also added, which reduced the demand for the capability of the air-conditioning attitude control system during separation.

4.3 Attitude Control System (Auxiliary Power System)

After the inter-stage thermal separation was completed during this launch and the return ignition was completed, the weather condition of Boca Chica on that day was excellent, making the ground observation equipment clearly show the state of the overweight booster after the separation of the thermal separation ring for the first time: the booster uses the cold air attitude control system (bull bell structure, installed at the bottom of the storage tank) and opens the main valve of part of the engine, through pulsed low-temperature gas injection, an impulse to move the overweight booster away from the thermal separation ring is provided.

Fig. 28 Automatic cooling control and engine start at the same time

When the engine opens the main valve, it does not ignite. Only the tank pressure makes the propellant gasify through the thrust chamber to generate thrust. This method of generating cold air attitude control by the main engine is unprecedented.

On the other hand, the primary and secondary air-conditioning attitude control engines all use the extraction tank air pillow as the propellant source. During the second flight of the starship, the original methane tank air pillow gas combustion RCS was improved to this scheme, thus avoiding the problem of impurities in the methane tank air pillow gas (from the pressurized transportation system).

4.4 pressurized conveying system

The Raptor engine has been unsuccessfully started or shut down in advance since its second flight. After many analyses, this problem is basically confirmed to come from the design of the self-generated supercharging system. The self-generated supercharging system used by the traditional rocket uses the heat energy generated by the engine to gasify and expand the low-temperature propellant and then guide it back to the storage tank to pressurize the storage tank. The Raptor engine is the only commercially operated full-flow staged cycle engine. When designing this part of the system, the pre-combustion chamber gas (equivalent to turbine exhaust gas or burner gas for traditional open engines) is guided back to the storage tank to provide the large storage tank. The required boost pressure.

Figure 29 Schematic Diagram of Raptor

The advantage of this is to avoid carrying a large number of supplementary pressure cylinders. (Compared with Falcon 9 multiplex rocket, due to the long-term taxiing section and the secondary start of multiple engines, it is often necessary to add a large number of cylinders in the storage tank or compartment. The starship has erased the clear boundary of the compartment in the design, and there is no compartment absolutely used to store enough cylinders for the first time, therefore, the side strip wing is added to store the pressurized gas cylinder);

Fig. 30 Composite Cylinder under Edge Wing

However, this scheme brings great trouble to the pressurized conveying system. The precombustion chamber gas comes from combustion and has complex composition, including combustion products carbon dioxide and water. In the two extremely low temperature media of liquid oxygen and methane, these two gases will freeze into ice and dry ice particles. This part of particles is absolutely redundant for the engine and will block the filter screen or pump inlet, cause engine start failure and early shutdown.

SpaceX has adopted temporary measures for the remaining generation of starship overweight boosters after B7, including adding two to three layers of filter baffles in the storage tank to filter out most of the excess through fine screens of the baffles. The complete solution to this measure will be implemented in the second-generation starship, mainly by changing the source of self-generated pressurized gas to avoid combustion products returning to the storage tank.

Fig. 31 carbon dioxide dry ice accumulation at the bottom of the box (map source_famous youtube blogger CSI Starbase)

The engine starting problems encountered by the booster transportation system during the second flight also include the problem of propellant sinking caused by too fast turning angular velocity and too fast secondary starting after inter-stage separation. The solution is to expand the capacity of the small storage tank for secondary starting, and at the same time change the angular velocity used during the second flight to throw off the primary and secondary separation methods into thermal separation. The filter baffle added in the storage tank can effectively inhibit propellant sloshing and topping at the same time.

Fig. 32 Start the expansion of small storage tank

4.5 structure system

During the third flight of the starship, there was a problem that the load compartment door could not be fully opened and closed. The door cover and frame had poor rigidity and could only be opened in a microgravity environment. During the third flight, reentry was carried out because it could not be closed, which greatly weakened the secondary axial bearing capacity and accelerated the problem of rapid fragmentation of the secondary reentry. This was correspondingly strengthened in the subsequent design of the starship;

Figure 33 Starship Three Fly Open Load Hatch

During the flight of this starship, there was a problem of tearing the side strip of the overweight booster before landing. This is a minor problem and is easier to solve.

4.6 thermal protection system

During the second flight of the starship, combustion occurred in the engine compartment, damaging part of the engine control structure and the engine itself. after the second flight, additional protective covers were added to all engines, which were not only used to prevent the explosion of a single engine from causing chain effects, but also to provide certain thermal protection to the engine components exposed in pneumatic heating. this was a temporary measure. the third generation raptor integrated a large number of cooling flow channels to the exposed outer surfaces of all components, therefore, the three generations of raptors will not need any protective cover, which is the fundamental solution in the future.

The Starship Class II thermal protection system of thermal insulation tiles fell off or failed locally throughout the five flights. The leading edge flap hinge ablation problem occurred in the four and five flights, which were more severe. Fortunately, the flight was not out of control. SpaceX has repeatedly optimized and improved this system in three years. In addition to adding multi-layer thermal protection structure and optimizing the performance measures of each layer, SpaceX has also improved the overall aerodynamic layout. The leading edge flap of the second-generation starship has been slightly moved to the leeward side, improving the flap shape and hinge structure so that it will not produce parts that cannot be retained by the thermal insulation tiles during movement.

Fig. 34 Insulation Tile Falling, Flap Hinge Melted

During this starship flight, a large amount of methane vapor was ejected from the QD panel area of the first-stage overweight booster during landing, which was ignited by the tail flame of the engine and continued to burn during and after landing. The impact and solution of this problem are still uncertain.

4.7 ground system (excluding chopstick device)

During the first flight of the star ship, the impact of the engine tail flame caused the pyrolysis of fireproof concrete and the concrete tornado. The raised particles hit the engine, resulting in the failure of some engines during takeoff. Later, in the subsequent flight, the whole launch pad system was redesigned, the ground steel plate interlayer was added, and a large amount of cooling water was sprayed into the rocket through the steel plate interlayer and pores to absorb the tail flame energy and avoid the launch pad damage.

Fig. 35 Pyrolysis of concrete on launch pad

Since then, Starship has further optimized the launch process and canceled the containment and release link (this link played a great role in Falcon 9 and is also the standard practice of traditional European and American liquid rockets. After the engine is started, it is necessary to stay on the ground for several seconds to complete the ground inspection and confirm the normal operation of the system, and release the rocket, which can avoid flight failure caused by engine start failure, which is especially important in multi-engine rockets), it also increases the take-off speed, ensures that the rocket leaves the launch pad as quickly as possible, and reduces the ablation effect of the tail flame on the launch pad.

Figure 36 Starship Launch Pad Pinch Release System

This choice is an optimal choice for balancing single-engine reliability with multi-engine redundancy brought about by multi-engine system reliability.

The ground system also cooperates with the engine system to realize the design that the outer ring engine starting system is completely placed on the ground.

The fourth flying starship also optimized the propellant filling speed. The main measures include increasing the filling valve, supercooling system capacity and expanding the ground propellant storage system to solve the problem of long launch preparation time.

 

5 Chopsticks system and the preliminary introduction of supporting design on the arrow

Introduction to 5.1 Landing Scheme

This article is only a preliminary introduction in this section, followed by a special article to analyze the composition and function of the chopstick system.

Musk first tweeted on December 31, 2020, announcing that he would use the launch tower arm to catch the overweight booster and use the grid rudder as a landing bearing area. This is the first time SpaceX has announced this recovery plan. Musk later called it chopsticks (Chopsticks) and mechanical Godzilla (Mechazila). Compared with the widely spread statement that chopsticks hold the rocket, the actual image description should be closer to the actual description. Mechanical Godzilla uses arms to catch the rocket through the armpit of the rocket.

Figure 37 Chopsticks Arm

With four years of development and evolution, the whole recovery scheme has become the current state. The landing and mounting area has been changed from the grid rudder to the lug structure (called landing hook or load pin) extending below the grid rudder. A cylindrical metal piece is installed at the end of the lug through a ball hinge bearing to finally contact with chopsticks. The recovery process is that when the launch tower arm is closed, the rocket obliquely approaches the arm bend and decelerates, after reaching the recovery point directly above, it decelerates and slides down to the launch pad vertically. At this time, the chopsticks are completely closed, the load pin finally collides with the buffer beam of the chopsticks in the axial direction, the engine is shut down, the buffer beam is pressed down to absorb energy, and the landing is completed.

 

5.2 Arrow Specially Designed for Chopsticks Recycling

5.2.1 Grid rudder

The grid rudder design of the starship does not adopt the 90-degree spacing layout of the very mature grid rudder of Falcon 9, but is changed to 60-degree and 120-degree spacing. The purpose is to obtain better pitching moment, so that the overweight booster can quickly turn around and return to the launch site after separation. On the other hand, the landing hook can also be located in the middle of two adjacent grid rudders, sharing stiffness with the reinforced areas of the two grid rudders, reduce the weight gain to overcome the concentration;

The grid rudder of starship adopts stainless steel sheet and uses manual welding molding process. Compared with the titanium alloy casting company used by Falcon 9, the actual dimensional tolerance of the product is relatively rough, and the welding deformation of all ribs can be seen by naked eyes. The bearing area of the rudder is basically the same as that of Falcon 9, extending to the top of the storage tank through the extension rod and through the longer force arm in the cabin, convert a large bending moment (relative to the body bulkhead) into a small concentrated force acting on the top of the tank;

Fig. 38 Welding Deformation of Grid Rudder

However, these still cannot make the multi-curved thin shell structure with swept front edge suitable as the bearing point of chopsticks. Musk also changed the design soon after proposing to use the landing hook as the new landing bearing point.

5.2.2 Landing hook

The landing hook is used to replace the structure of Falcon 9 landing leg. It is installed in the bulkhead area on the top of the storage tank and located under the grid rudder. The exposed outer part of the bulkhead can rotate around the bearing point on the top of the tank and can be raised up and down from the outside. Using this method, the storage tank can be introduced as the third bearing point to ensure that the three bearing points directly bear axial pressure instead of bending moment, axial compression is far more suitable for carrying thin shell structures such as rockets than bending moments.

Figure 39 Landing Hook Internal Structure

5.2.3 Stabilizer arm lock groove

After the rocket lands successfully, it needs to be moved to the launching platform by the chopsticks to be fixed. In the process of moving, only the landing hook carries the load, and the freedom of the pitch channel of the rocket is released. The rocket may sway due to too fast moving speed or large inertia. Therefore, the chopsticks introduce the chopstick stabilizing arm, which is a rocker arm structure located directly below the chopsticks, and can lock the freedom of the pitch channel by pressurizing point, the corresponding constraint point on the arrow is the stable arm lock slot.

5.2.4 Enhanced chopstick contact area

When the rocket lands, there will be a working condition of axial movement of chopsticks that have been completely closed. As part of the reinforced cabin of the starship is designed as an outer stringer, and the stable arm lock groove is an obvious outer protrusion, SpaceX has designed a guide beam for anti-scratch during chopstick friction and bulkhead reinforcement in the collision area. This part is painted black, which may be used to reveal and record the collision friction during flight.

Figure 40 Bottom structure is the stabilizing arm

5.2.5 QD panel

A key reason for the design of the chopstick system is the short time required to launch multiple starships for future trips to the moon and Mars, which poses an unprecedented challenge to the speed of overweight boost reuse.

According to the situation of Falcon 9 and the analysis of the post-processing of many commercial space grasshoppers in China, it is difficult for the rocket booster to get the staff close to the rocket to complete the post-processing operations such as propellant discharge in a short time after landing, so the unmanned post-processing process is a necessary condition for high efficiency.

At the beginning of the design, the starship continued the design idea of Falcon 9's unattended launch pad. The existence of chopsticks also ensured that the recovered rocket could quickly re-dock the designated position of the launch pad. Auxiliary equipment for automatic docking and visual recognition was designed on the filling and discharging docking panel, which can help the highly integrated pipeline and the rocket to automatically complete accurate docking and complete unmanned post-processing and filling procedures.

Fig. 41 Ground Side of QD Panel

Figure 42 QD Panel Rocket Side

 

6 About the next flight

SpaceX and FAA (Federal Aviation Administration of the United States) have differences on the granting of starship flight permits. This issue has been mixed with political issues such as the current US presidential election. Musk used social media and Republican power to put pressure on FAA regulatory authorities and successfully obtained the five-flight flight permit several months in advance. FAA also granted the six-flight permit, provided that the six-flight and five-flight processes are completely consistent, this is almost impossible for Musk and SpaceX. At that time, the six-flight authorization will be even more dramatic, but the five-to-six-flight preparation interval will definitely be the fastest since the previous flight. Musk mentioned in his introduction before the 5th flight that SpaceX plans to make the first trip to Mars by 2026, and we also briefly speculate that the 6th flight will achieve several key tasks:

1) Propellant transfer in-orbit demonstration, which is a necessary process for starships to go to Mars, and Sanfei has conducted a test;

2) Starship two into orbit, and the corresponding vacuum raptor in orbit to start off orbit, before various reasons, has not been carried out;

3) It is possible to skip the remaining generation starships and directly conduct the first flight test of the second generation starship, including the new aerodynamic shape and thermal protection system;

4) Release the second-generation star chain through the load compartment door. The second-generation star chain has a very large width and can only be launched through the starship. With the gradual confirmation of the starship's ability to enter orbit, it has become a reasonable choice to carry the test during low-orbit flight. At the same time, the test also includes the load compartment door that had problems before and the star chain adapter structure for ejection separation.

 

7 Conclusion

On October 13, 2024, SpaceX successfully completed the first chopstick (recovery tower) recovery test of Starship booster. This is the first time in human history that ground equipment has been used to recover rocket boosters. Musk and his SpaceX company spent three years turning the creative rocket recovery solution he proposed when he released Twitter in 20 years into a great feat that shocked the world.

The ground belongs to the ground, and the flight belongs to the flight, which is the ideal description of the boundary between the launch and recovery support system and the rocket flight system in the minds of space workers. On the one hand, this means that a landing system that works only during landing does not need to be a dead weight that follows the entire flight; on the other hand, a clear system boundary is a sufficient decoupling of the design parameters that hold the elbow to each other for system design, such as rocket aerodynamic design does not need to consider how a large landing device is embedded in the overall aerodynamic shape, and landing device design does not need to consider the aerodynamic effects of deployment; musk's first principles require all designers to avoid time and effort to solve a problem that should not exist. These designs are all oriented to the most essential characteristics of launch vehicles as vehicles-all for carrying capacity and carrying efficiency.

For China's aerospace practitioners, a clear route is very important. Fortunately, China's leading infrastructure capabilities in the world have laid a solid foundation for high efficiency and low cost for the new chopstick recycling track. The rest is hard work and catching up.