Jason DavisJun 11, 2014

LightSail Update: Boom Deployment Diagnosis Continues

The LightSail team is continuing to investigate a few problems with the spacecraft’s boom deployment system that postponed a flight system test earlier this month. Last week, I gave a quick explainer on the spacecraft’s TRAC (Triangular Rollable and Collapsible) booms, which extend from LightSail's deployer section when the spacecraft reaches orbit. As the booms emerge, they gently unfold the solar sail from the avionics section.

Here’s a video of the June 2 boom deployment test, during which the LightSail team discovered the problems they are now trying to fix. Right now there are two issues: small stalls during boom deployment and a power anomaly in the motor.

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LightSail boom deployment test LightSail's TRAC booms deploy during a test on June 2, 2014. The triangular booms, consisting of an alloy called elgiloy, crackle and pop as they expand. The slight motor hesitations are believed to be the result of a correctable processor overload.Video: LightSail spacecraft team

There are two things to listen for as the booms extend. First, you’ll hear the motor whirring as it slowly unwinds the TRAC booms. About every ten seconds, you’ll hear a slight pause. This is the problem I mentioned in my previous update: LightSail’s processor appears to be getting overwhelmed as it gathers data for transmission back to Earth, all while trying to control the deployment motor. 

You'll also hear an occasional snap, crackle and pop. Those noises sound alarming (at least, they did to me), but it turns out they are normal. The TRAC booms are made from an alloy called elgiloy, the namesake of the company that produces it. The alloy consists mostly of cobalt, chromium, nickel and iron. As the booms extend from the spacecraft, they flex and pop into their triangular shape, making a lot of noise in the process.

TRAC (Triangular Rollable and Collapsible) boom
TRAC (Triangular Rollable and Collapsible) boom Image: United States Patent and Trademark Office

This week, I spoke with Alex Diaz, who works on the spacecraft’s electronics system. Diaz, as well as project manager Doug Stetson, are reasonably confident they can reconfigure the deployment sequence to avoid overwhelming LightSail’s processor. One possible workaround is recording and storing spacecraft telemetry during boom deployment, but waiting to transmit it back to Earth until after the sequence completes.

The bigger challenge, says Diaz, is figuring out what’s causing a power anomaly in motor. Right now, the motor’s power supply is drawing more power than it should. It isn’t a huge variance—just a couple of amps, in fact—but that’s enough to potentially overheat the circuit and permanently damage the spacecraft. Diaz and the LightSail team are still running tests to pinpoint the cause of the problem.

There’s a possibility that because LightSail has been through so many tests these past few years, its components are aging. The motor could be encountering increased resistance during deployment, which would cause it to draw more power.

This is a good time to mention that there are actually two LightSail spacecraft: LightSail 1 and LightSail 2. Right now, both are slated to fly, but only model 2 will attempt controlled solar sailing. Model A is slated for a shakedown cruise that will test deployment of the sails and operation of the spacecraft. However, its orbit will not take it to a high enough altitude where the acceleration from solar sailing can overcome atmospheric drag. Since LightSail 2 will fly much higher, it has been kept in more pristine condition, while LightSail 1 has borne the brunt of testing.

In the meantime, the full flight system has been tentatively rescheduled for the last week of June. However, that's pending resolution of the power anomaly. We'll stay tuned.

Lightsail 1 artist's concept with Milky Way background
Lightsail 1 artist's concept with Milky Way background Image: Rick Sternbach

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