Task Force Shares New Insight into Ariane 6 APU Failure

On July 9, 2024, the Ariane 6 rocket debuted by lifting off from the Guiana Space Centre in Kourou. The maiden flight of Europe’s new heavy-lift launcher successfully deployed several payloads from the industry, space agencies, and universities. However, not everything went according to plan. After deploying most payloads, the upper stage did not perform the third ignition of its Vinci engine.

Maiden liftoff of Ariane 6 from french Guiana. Credits: ESA/M. Pédoussaut — *Maiden liftoff of Ariane 6 from French Guiana. Credits: ESA/M. Pédoussaut*

This burn was needed to deorbit the stage, allowing the deployment of the last two payloads, two reentry capsules. Instead, the stage was passivated, and the reentry capsules could not carry out their missions. During World Space Business Week Steven Rutgers, ArianeSpace COO, announced that the Ariane 6 Launcher Task Force, staffed by personnel from ESA, CNES, Arianespace, and Ariangroup, shed light on the causes of the issue.

The Auxiliary Propulsion Unit

The failure can be traced back to a sub-system called the Auxiliary Propulsion Unit, or APU. It carries out two crucial tasks: propellant pressurization and auxiliary thrust generation. At the heart of the system is a heat exchanger, which heats up liquid hydrogen and liquid oxygen from the main propellant tanks, making them evaporate. Some of this gas is then fed back into the tanks, pressurizing them. This allows the stage to do away with heavy helium bottles, another method of achieving pressurization. While for Ariane 5 they were an acceptable solution, the flexibility demanded of Ariane 6 would have made such a solution impractical.

*A view of an Ariane 6 second stage ready for hot firing tests. To the top right of the orange covers, one of the APU exhaust nozzles is visible. Credits: ESA*

The rest of the gas is fed into a gas generator. Here, combustion creates the heat which drives the abovementioned evaporation of the gases. The exhausts are then made to expand in two small nozzles at the base of the stage, generating up to 300 N of thrust. This way, the APU can complete precise correction maneuvers, help in deploying clusters of satellites, and deorbit the stage at the end of the mission. Crucially, thrust from the APU is also used to settle the propellants in the tanks before reigniting the Vinci engine. This is crucial because in microgravity propellants float all over the tank, risking to starve the fuel inlets.

Failure and solution

After performing two ignitions of the Vinci engine and deploying most payloads, Ariane 6’s upper stage entered a coasting phase. During this phase, the APU was commanded to restart, as it had previously done with success. However, one sensor registered an excessively high temperature, and the flight software shut down the APU. Without the thrust provided by the system, the trajectory began to deviate from the plans. Then, the third ignition of the Vinci engine could not take place as a result of the unavailability of the APU. Unable to perform the deorbit burn, the stage passivated itself, removing possible sources of explosions. This ensures the stage won’t create great quantities of space debris in the future.

Reviewing the data collected during the flight led to the identification of a solution. By changing the ignition preparation sequence of the APU, more favorable conditions can be achieved during startup. This change, which is entirely a software one, should solve the temperature anomaly encountered during the flight. The revised code is already undergoing testing.

Ariane 6

Ariane 6 is a heavy-lift launch vehicle, capable of delivering up to 20600 kg to LEO or 11500 kg to GEO. It employs two stages powered by liquid hydrogen and liquid oxygen and a customizable number of solid boosters. The first stage uses one Vulcain 2.1 engine, an evolution of Ariane 5’s Vulcain 2, while the second stage uses one Vinci engine. The second stage was designed with flexibility in mind, allowing for multiple restarts and great flexibility in the kind of missions that can be carried out. During liftoff, Ariane 6 is assisted by two or four P120C solid rocket motors. The two configurations are dubbed Ariane 62 and Ariane 64, respectively.

Development started in 2014, funded by the European Space Agency. The lead contractor is ArianeGroup, while launches out of the Guyana Space Center are managed by ArianeSpace.

Despite the need to address the problem, Ariane 6 is on track for a second launch. Expected to take place in December, as stated by ArianeSpace CEO Stéphane Israël in July and now confirmed by COO Rutgers, it will carry into orbit the French reconnaissance satellite CSO-3. The release of preliminary investigation results bolsters confidence in its success.