The cause of the failure of the large-scale "H3" Rocket No. 8, which failed after launch last December, was discovered to be delamination inside the adapter that mounts the satellite onto the vehicle. This delamination, which occurred during manufacturing, expanded due to the impact of flight. The Japan Aerospace Exploration Agency (JAXA) aims to implement countermeasures and resume launches as soon as possible. JAXA reported the investigation results and measures to a subcommittee of the Ministry of Education, Culture, Sports, Science and Technology, and they were approved.
From JAXA report materials
On December 22 of last year, Unit 8 was launched from the Tanegashima Space Center (Kagoshima Prefecture) carrying the government's Quasi-Zenith Satellite "Michibiki" No. 5. However, an abnormality occurred during the combustion of the second-stage engine, and it failed because the satellite could not be placed into the designated orbit. Flight data showed that 3 minutes and 45 seconds after launch, immediately after the "fairing" (the cover for the satellite mounting section at the top of the rocket) was separated, the section mounting the satellite on top of the second-stage vehicle was damaged.
To determine the cause, researchers inspected the already-manufactured payload support structure. They found that in the areas where components were bonded together during manufacturing, the gap between the carbon fiber reinforced plastic (CFRP) surface and the aluminum interior had delaminated (peeled) more than expected. To reduce weight, the aluminum interior has a "honeycomb" structure, which is a series of small, hollow chambers.
Results from reproduction experiments showed that when the payload support structure and other components were heated to dry before being bonded with adhesive, some parts reached temperatures higher than specified. The adhesive strength in those high-temperature areas weakened, and the air inside the honeycomb structure heated up and expanded, causing delamination between the CFRP and the aluminum. When the vehicle reached a vacuum during launch, the delamination expanded due to the pressure difference between the inside and outside. It is believed that the impact of the fairing separation caused this delamination to become the starting point for the destruction to spread.
From JAXA report materials
As a result, the payload support structure buckled and fell toward the second-stage fuel tank along with the satellite. This destroyed the piping of the fuel tank, causing the pressure inside the tank to drop and leading to abnormal combustion in the engine. Based on various data and reviews, JAXA concluded that it is extremely likely that this process was the primary factor in the failure of No. 8. While it is unlikely that other factors were involved, they will continue to verify this in the future.
From JAXA report materials
For the assembly of the payload support structure, the previous "H2A" rocket, which ended operations last June, used a method of fixing parts with bolts. In the H3, this was changed to a method of bonding with CFRP components to reduce weight and cost. The adhesive strength of the bonding was designed to sufficiently withstand the pressure differences that occur during flight. Manufacturing and inspections were carried out according to regulations, but the delamination caused by high temperatures during manufacturing, decreased adhesive strength, and the influence of air inside the honeycomb was not anticipated during the H3 development stage.
As measures to prevent recurrence, two methods for assembling the payload support structure were considered: (1) A method to repair the delaminated or weakly bonded parts of already-manufactured adapters, and (2) A method to add improved reinforcement measures to the bolt fixation used in the H2A. It was confirmed that both methods provide sufficient strength.
JAXA will adopt the bolt fixation method (2) for future launches for the time being. However, because the next launch, Unit 6, is a test vehicle, it will adopt the repair method (1). This will help verify the correctness of the cause investigation and assist in data acquisition. Note that the adapter used when carrying the "HTV-X" supply vehicle to the International Space Station (ISS) has a different design and is not subject to these measures.
From JAXA report materials
On the 13th of this month, JAXA Project Manager Makoto Arita, who reported to the subcommittee, stated: "This kind of complex combination of factors was a phenomenon we were experiencing for the first time. We have been operating under the slogan 'Test as you fly' (testing under the same conditions as actual flight), but in reality, shock tests in a vacuum are difficult and there were parts where we could not do so. We want to apply these lessons to the future."
The H3 is a two-stage liquid-fuel rocket. It is the common successor to the H2A and the enhanced "H2B," which ended in 2020, and was developed by JAXA and Mitsubishi Heavy Industries. The government positions it as a flagship rocket along with the small solid-fuel "Epsilon." The first unit failed in March 2023 when the second-stage engine failed to ignite due to an electrical abnormality resulting in the loss of an earth observation satellite. After taking measures, it had been successful five consecutive times up to Unit 7 last October.
Launching Unit 6 in the "minimum configuration" without boosters
Unit 6, to be launched this fiscal year, will be in the "30 Configuration," the smallest vehicle setup that does not include solid rocket boosters. This is the first time for this configuration in a domestic large-scale rocket, but because development took time, Units 7 and 8 with boosters were launched first.
In the combustion test for Unit 6 last July, a problem occurred where the pressure in the fuel tank did not rise sufficiently. The cause was that in the system for one of the three engines, a tank pressure gas valve was not installed to reduce costs. Measures were taken to adjust the gas flow rate, and a second combustion test was conducted on the 15th of March. JAXA confirmed that the measures were effective and stated that the verification is largely complete.
Unit 6 will be a test vehicle for the 30 Configuration and will carry a metal dummy weight and six ultra-small satellites instead of a large satellite. It is also positioned as a "redemption flight" after the failure of Unit 8, and its success or failure is drawing close attention.
Provided by JAXA
Original article was provided by the Science Portal and has been translated by Science Japan.