Acoustic shock rattles telescope

| Environmental Testing

James Webb Space Telescope undergoes environmental testing to simulate the rigours of launch

The Webb Telescope spacecraft is undergoing environmental testing with its first failure resulting from acoustic testing.

The spacecraft element of NASA’s James Webb Space Telescope recently completed its first two major launch environmental tests at Northrop Grumman Aerospace Systems in Redondo Beach, California, and will soon undergo further tests to ensure it will handle the rigours of launch and the harsh environment of space.

The spacecraft element’s first test simulated the mechanical shock caused by the separation of the spacecraft’s payload adapter after launch. The second test simulated the extreme sound and resultant vibration of the launch environment.

Detailed inspections of the hardware after the acoustic test showed that fastening hardware that hold the sunshield membrane covers in place had come loose.

“NASA is reviewing options for repair and the next steps in launch environment testing,” said Greg Robinson, Webb’s programme director. “The team is reviewing the test data and hardware configuration and is actively working towards corrective action. We expect to get back to the environmental test flow shortly and continue to move safely and methodically toward mission success.”

Discoveries like this one are not uncommon in the development of a complex and unique spacecraft. “This is an example of why space systems are thoroughly and rigorously tested on the ground to uncover imperfections and fix them prior to launch,” said Robinson.

Webb’s spacecraft element is the observatory’s combined sunshield and spacecraft bus. The spacecraft element and Webb’s combined optical element and science instrument payload will form the complete observatory.

The shock of payload separation

When Webb is launched into space, it must be folded like origami to fit inside its Ariane 5 rocket’s payload fairing, which protects it from the forces and heat of the atmosphere as the rocket accelerates into space.

Inside the fairing, the payload adapter physically attaches Webb to the top of the Ariane 5. The adapter has two halves — one that is permanently attached to Webb and the other that is attached to the second stage of the rocket. When the rocket reaches a specific altitude in Earth’s upper atmosphere, the payload fairing is jettisoned and falls back to Earth. Following this, the first stage of the Ariane 5 expends its fuel and also is jettisoned.

After the second stage of the rocket gives Webb a final nudge to send it on its way to its orbit, the two halves of the payload adapter separate, releasing Webb from the rocket. The release sends a mechanical shock through the observatory.

To simulate this separation on Earth, engineers at Northrop Grumman first suspended the spacecraft element in the air with the payload adapter attached to it. They then remotely released the bottom half of the payload adapter.

The engineers monitored the forces caused by the release to ensure they were within expected values, and high-speed video cameras recorded the separation to make sure it was smooth.

Sound and vibration tests

After completing shock testing, engineers enveloped the spacecraft in a plastic tent and moved it into Northrop Grumman’s Large Acoustic Test Facility. The tent protected the spacecraft from contamination during the move and during the acoustic test.

During the test, engineers subjected the spacecraft element to launch sound frequencies from 25 Hertz to 2,500 Hertz. It was also tested at loudness levels up to 142.5 decibels, about 3 decibels higher than what is expected during launch.

Engineers mounted several microphones inside and outside the tent to monitor the acoustic environment during testing. They also mounted about 500 accelerometers around the spacecraft element to monitor the vibration responses.

Jonathan Newell
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