Achieve aerial U-turns and verify "soft landing", what problems did the Starship

If we say that the first test flight of the "starship" last April, which exploded 3 minutes and 48 seconds into the flight, was rated as "10 points," then what kind of report card has the starship, after more than a year and three test flights, submitted within the "fast failure" mechanism?

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Multiple Engines, Multiple Risks

On June 6th, Beijing time, the starship conducted its fourth test flight in Texas, USA. The most significant achievement of this test flight was the realization of the splashdown of both stages of the rocket, or in other words, the verification of the recoverability of the first and second stages of the starship.

Let's take a closer look at the entire test flight process. At 7 minutes and 20 seconds after launch, the first stage booster "Super Heavy" achieved vertical landing and "braking" ignition, ultimately landing smoothly in the Gulf of Mexico. This effectively means that SpaceX has achieved a soft landing for the starship, albeit on a "virtual platform," which is to say, it landed directly in the sea. About 1 hour and 6 minutes after launch, the second stage spacecraft, the starship, also successively achieved maneuvering inversion and ignition to slow down - whether it achieved landing ignition is still to be determined, and it ultimately splashed down in the Indian Ocean, which means that the second stage of the starship has also achieved controllable re-entry.

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The splashdown of "Super Heavy" in the Gulf of Mexico and the second stage starship spacecraft in the Indian Ocean were the main objectives before the fourth flight of the starship. During the first three test flights, "Super Heavy" fell out of control at high speed into the Gulf of Mexico until it disintegrated; the second stage starship spacecraft either lost contact directly or disintegrated and self-destructed before or after reaching orbit.

SpaceX's starship has been one of the rockets that has attracted much attention in recent years. It stands about 120 meters tall with a diameter of 9 meters, making it a super-large two-stage rocket. The first stage booster "Super Heavy," which is about 69 meters tall, is composed of 33 "Raptor" engines, runs on liquid methane and liquid oxygen, and has a fuel load of about 3,400 tons; the second stage "starship" spacecraft is about 50 meters tall, with a fuel load of 1,200 tons, capable of carrying astronauts or cargo, and has a near-Earth orbit payload capacity of 100 to 150 tons.

Its large size and strong payload capacity need no further elaboration; this time, let's mainly talk about the performance of the record-breaking 33 engines of the starship.The more engines, the higher the requirement for system synergy.

It needs to be clarified that more is not always better when it comes to space rocket engines. Historically, only the Soviet N1 moon rocket in the 1960s had ever been equipped with 30 engines. The more engines there are, the higher the requirement for system synergy, and the risks will increase exponentially. The Soviet N1 failed repeatedly, and after four prototypes were destroyed, the project was abandoned.

The Starship is designed to deliver payloads to the moon and even farther to Mars, and a multi-engine configuration is necessary to provide thrust for such a massive spacecraft. However, during the fourth test flight, not all 33 engines started successfully.

From the live broadcast, it can be seen that one "Raptor" engine did not ignite. Industry insiders have said that redundancy of engines was considered from the beginning of the Starship's design, but if this issue cannot be resolved, next time it might be two or three engines that fail to ignite, "the instability of the engines will affect the reliability of the Starship's mission execution."

During the fourth test flight, one engine failed to ignite.

This is not the first time this kind of problem has occurred. On May 24, SpaceX updated its analysis of the "Starship" third test flight (March 14), which also mentioned engine failure issues. According to the plan, after the first-stage booster "Super Heavy" separated, it was supposed to ignite 13 engines for propulsion, with 6 engines shutting down as instructed during the return, but during the landing ignition, all 13 engines should have ignited, yet the 6 that shut down early could not be restarted, and of the remaining 7, only 2 managed to ignite, which directly led to the Super Heavy not splashing down smoothly.

The failure reason given by SpaceX was the continuous clogging of the engine's liquid oxygen supply filter. To prevent this issue, in the fourth test flight, SpaceX added additional hardware outside the liquid oxygen tank to improve filtration capabilities and made many software improvements, but the engine still had problems during the initial ignition phase. What caused this time?

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Heat shielding remains a challenge.Another issue that warrants attention is that during the Starship's re-entry into the atmosphere and its passage through the plasma, many heat shield tiles were lost, and the flaps were severely damaged, appearing to be directly burned through in the live broadcast. According to the live footage, about 48 minutes after launch, the second-stage spacecraft began a controlled entry into the atmosphere, experiencing the most dangerous phase—flying through the "blackout zone."

When manned spacecraft return to Earth at high speeds, they typically generate intense friction with the atmosphere between 35 and 80 kilometers above the Earth's surface, with temperatures reaching over 2700°C. This causes the gas molecules and the ablated materials on the spacecraft's surface to become ionized. The continuously generated ionized gases envelop the spacecraft, forming a plasma sheath that absorbs, attenuates, refracts, reflects, and scatters electromagnetic waves, leading to abnormal or interrupted radio communication between the spacecraft and the outside world. This is known as the blackout phenomenon, and this period, which lasts 4 to 6 minutes, is also referred to as the blackout zone. The combination of high altitude, high temperature, high speed, high gravitational acceleration, and inability to communicate makes this stage the most critical phase for the spacecraft during re-entry into the atmosphere and a period prone to accidents.

During the spacecraft's traversal of the blackout zone, tracking and measurement can generally only rely on radar and optical equipment. SpaceX utilized Starlink to live broadcast this process for the first time during the third test flight on March 18th of this year. This is also why we were able to see the burnt-through heat tiles in the live broadcast of the fourth test flight.

The TUFROC heat-resistant material equipped on the windward side of the Starship encountered issues during this test flight.

As we mentioned in previous articles, because the Starship's body is made of stainless steel, only the windward side requires a heat shield, while the leeward side does not. The heat shield used on the windward side of the Starship is the TUFROC heat-resistant material (toughened unidirectional fiber-reinforced oxidation-resistant composite), developed by NASA and equipped on the leading edge of the wings of the U.S. Air Force's military vehicle, the X-37B.

The heat-resistant materials commonly used on rocket bodies, whether it's RCC (reinforced carbon-carbon composite), HRSI (high-temperature surface insulation tiles), or ultra-low-density nano-aerogel insulation materials, will be ablated, meaning they continuously thin out in high-temperature environments, making them unsuitable for reusable rockets. The TUFROC heat-resistant material, however, not only provides efficient thermal protection but also has a low ablation rate—this means the material will not easily peel off or be damaged when facing extreme temperatures.

TUFROC is not actually a single material but rather a "multi-element bi-structure" solution. "Multi-element" refers to the variety of materials used, including reinforced lightweight ceramic/carbon insulation material ROCCI and low-density insulation materials (AETB or FRCI). "Bi-structure" refers to the two-layer structure, with the inner layer being AETB or FRCI, which appears as a softer "foam layer," providing good insulation and cushioning properties, helping to reduce the direct transfer of heat; the outer layer is ROCCI, which not only provides insulation but also offers physical protection to prevent external physical damage from affecting the insulation performance of the inner material.

The footage showing the Starship's flaps being burned through can be seen.In theory, this heat shield that resembles a tile should be able to withstand temperatures of at least 1700 degrees Celsius, with a stable structure that can be reused multiple times, a short manufacturing cycle, and low cost, making it an outstanding representative of lightweight, high-strength, and tough thermal protection materials. However, it was still burned through this time. Whether it is an issue with the material itself or due to premature detachment leading to a failure in protection, these are issues that SpaceX needs to focus on.