Rocket Lab mission lost in the Paschen of the moment

'Rare conditions' the culprit for an arc that shorted out the power supply

A Rocket Lab update on what caused the failure of its "We Will Never Desert You" mission identifies an electrical arc under Paschen's Law as the most likely culprit.

It was all going so well for the company's Electron rocket during the September 19 mission. The first stage did operate normally; Max Q – the period of maximum dynamic pressure on the vehicle – was passed, and stage separation occurred.

However, at the 151-second mark, the voltage from the second-stage power supply system for the engine's controllers dropped sharply, resulting in a total loss of power and a failure to reach orbital velocity.

According to Rocket Lab, the problem was an electrical arc that occurred within the power supply system. This shorted the battery packs that provide power to the second stage of the vehicle and thus resulted in the surprise shutdown and loss of mission.

The second stage uses three lithium-polymer high voltage batteries (HVBs), and the Rutherford engine is relatively novel, using an electric fuel pump to send propellant into the engine rather than turbo-pumps. During the second stage burn, two HVBs power the pumps and are ejected upon depletion. The third then takes over.

Rocket Lab reckons a combination of conditions occurred simultaneously, resulting in a threshold dictated by Paschen's Law – an equation that gives the voltage needed to start an electric arc – to be reached.

The conditions included a small build-up of helium and nitrogen gas in the interstage, a fault in the insulation of the high-voltage loom within the power supply system, and a particular voltage waveform. While individually, none of the conditions would have caused the loss of mission, the combination in a low-pressure environment resulted in an electrical arc and loss of power.

It all sounds a lot like Reason's Swiss Cheese Model, where the holes in metaphorical slices of Swiss cheese representing weaknesses in individual systems can momentarily align, resulting in an accident.

Rocket Lab's plan is to improve testing on the ground and change the design of the frame section holding the batteries to allow it to maintain enough pressure through launch to stage separation to reduce the chances of arcs forming.

While the company insists that the combination of conditions was a rare event, it has not had a lot of luck regarding the second stage of its launch vehicle. A dodgy electrical connection resulted in failure during the second stage burn for 2020's "Pics Or It Didn't Happen" flight, and 2021's "Running Out Of Toes" mission ended prematurely due to an issue with the engine controller.

Yet another problem with the second stage is not a great look for reliability, particularly compared to other launch providers like SpaceX.

Rocket Lab has already received permission from the Federal Aviation Administration to resume launches, and the window for its next mission from the company's New Zealand Launch Complex 1 opens on November 28 and extends into December. ®

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