Astro Pi: Mission Update 5 – flight safety testing

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The road to space is long and winding, but the two Astro Pi flight units are almost there! The next thing for us after this is to hand over the final payload to the European Space Agency so it can be loaded onto the Soyuz-45S rocket for launch on December 15th with British ESA Astronaut Tim Peake.

To be allowed on the rocket, you need a flight safety certificate for your device, and these can only be obtained by presenting a whole host of measurements and test results to a panel of experts at ESA ESTEC in Holland.

The expertise and equipment to carry out many of these tests is well outside the capabilities of the Raspberry Pi Foundation, and without the facilities and personnel available through our UK Space partners this would not have been possible – we’ve had to use facilities and partners all over Europe to get the work done.

I’ll list below the tests that were done approximately in chronological order starting from March.

Power integration test

AIRBUS Defence and Space, Bremen, Germany >

Astro Pi on Twitter

Here is how the #AstroPi will get power from @ISS_Research mains. 120 VDC to 120 VAC inverter https://store.griffintechnology.com/power/powerblock-universal-wall-charger-with-charge-sensor … pic.twitter.com/rl63e7An9q

Once in orbit, the Astro Pi will have two ways of getting power. It can use an AC inverter (above) that allows the crew to use all kinds of standard domestic appliances (like a normal USB power block); it’s also able to get power from any laptop USB port.

It is likely that when the Astro Pi is deployed in the Columbus module we will run from an AC inverter, but when we’re in the Cupola module we’ll just draw power from one of the laptops which is also there.

To gain permission to draw power from a laptop like this we needed to do a power integration test, to evaluate that the electrical load doesn’t have any adverse effect on the laptop.

astro_pi_bremen

The most common laptop on the ISS is the IBM Thinkpad T61P (circa 2007 from before Lenovo acquired them – Eben also uses one of these). Pictured above is an identical ground laptop with a special USB current probe connected to an oscilloscope. Note that this was done before we had the aluminium flight case, so you’re just seeing the Sense HAT, Raspberry Pi and camera parts of the whole Astro Pi unit.

The flight hardware was then powered up through the current probe so the oscilloscope could measure current inrush as well as maximum current when using the Astro Pi at max performance. Some diagnostic software was then used to check that there were no adverse affects experienced by the laptop.

Coin Cell Battery

Surrey Satellite Technology, Guildford, UK >

Since the Astro Pi will not be connected to the LAN on the ISS the only means it has of keeping the correct time is with a Real Time Clock (RTC) and a backup battery.

The flight stack up for Astro Pi is as follows:

  1. Raspberry Pi B+
  2. Custom RTC Board (has coin cell holder and push button contacts)
  3. Sense HAT

Batteries on the ISS have a whole host of possible hazards associated with them, and so any battery flown is subject to a stringent set of batch tests.

Astro Pi has a batch of eight Panasonic BR-1225 coin cells which were all tested together. Here is number 5, which is one of the ones that will fly:

battery5topbattery5bot

The test procedure involved visually inspecting the coin cells, measuring their width and size with callipers, testing their voltage output during open circuit and under load followed by exposing them to a vacuum of about 0.6 bar (~450 mmHg) for two hours.

Afterwards the measurements were redone to see if the coin cells had leaked, deformed or become unable to provide power.

Conformal Coating

Surrey Satellite Technology, Guildford, UK >

One of the safety requirements for circuit boards in space flight is that they are coated in a protective layer, rather like nail varnish, called conformal coating. This is a space grade silicone-based liquid that dries to form a hard barrier. In microgravity a metallurgical phenomenon called tin whiskers occurs. These are tiny hairs of metal that grow spontaneously from any metallic surface, especially solder joints.

The hazard here is that these little whiskers break off, float off and become lodged somewhere causing a short circuit. So the conformal coat has two purposes. One is to protect the PCB from any invading whiskers, and the other is to arrest any tin whiskers that may grow, and prevent them breaking free.

sense_hat_conformal

For the Sense HAT (above) we needed to define a number of keep out zones for the coating so as not to compromise the pressure and humidity sensors. The surfaces of the LEDs were not coated to avoid dulling their light too. If you look closely you can see the shiny coating on the HAT; in particular, see the joystick bottom right.

It’s much easier to see on two camera modules:

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ir_conformal

Vibration

AIRBUS Defence and Space, Portsmouth, UK >

Vibe testing is not actually required for safety, but we undertook it anyway as insurance that the payload would survive the vibration environment of launch. The testing involved placing an Astro Pi into some flight equivalent packaging and strapping it down onto a vibe table.

The vibe table is then programmed to simulate the severity of launch conditions on a Soyuz rocket.

Astro Pi on Twitter

Flight hardware about to go through Soyuz launch conditions vibration testing at @AirbusDS @JohnChinner @dave_spice pic.twitter.com/zphG5kpSuR

The tests needed to be done in x, y and z axes. To accomplish this two different vibe tables were employed, one for up and down (z, see above) and one for back and forth (x and y, see below).

Astro Pi Vibe Testing 1

Testing the Astro Pi flight unit, which is packaged inside the foam, on a Vibe table to simulate the Soyuz rocket launch conditions.

After the vibration sequence the Astro Pi was tested to ensure the vibration had not caused any issues, the case was also opened and the interior was inspected to ensure no connections had become loose.

Electromagnetic Compatibility (EMC)

AIRBUS Defence and Space, Portsmouth, UK >

EMC is the study and measurement of unintended electromagnetic signals that could interfere with other electronics. Almost all electronic devices these days undergo EMC testing in order to get CE or FCC markings. The Raspberry Pi B+ and Sense HAT both carry these markings; however their test results were obtained in a home-user setup, with a keyboard, mouse, HDMI monitor and Ethernet all connected.

The Astro Pi flight unit will be used without all of those. So these tests were required to ensure that, when used in this way, the Astro Pi doesn’t cause any problems to other systems on board the ISS (like life support).

John Chinner on Twitter

Trying to detect micro-volt level emissions from @astro_pi using big, sensitive antennas! pic.twitter.com/AwxA8Ood4B

The tests were conducted in a special EMC test chamber. The walls are lined with super-absorbent foam spikes that exclude all electromagnetic signals from coming into the room from the outside.

Astro Pi on Twitter

Now in the EMC chamber! pic.twitter.com/hjeMfozUSn

That way, any electromagnetic signal measured must have originated inside the room.

John Chinner on Twitter

Testing the emissions of @astro_pi above 1GHz. Looking for any signals from it that might interfere with ISS systems. pic.twitter.com/DGIbCEbjZa

A test script was run on the Astro Pi to stress it to maximum performance while a series of antennae were used to examine different ranges of the electromagnetic spectrum.

John Chinner on Twitter

This antenna is looking for any minuscule radio signals emitted from @astro_pi between 30 and 200MHz. Test passed! pic.twitter.com/D8j8dOTFMm

A set of electromagnetic susceptibility tests was also conducted to evaluate how the Astro Pi would behave when experiencing strong magnetic fields.

John Chinner on Twitter

Testing @astro_pi against AC magnetic fields produced by this coil. Still working perfectly. pic.twitter.com/O4RcCiHqba

No issues were found, and all tests passed.

Off Gassing

ESA ESTEC, Noordwijk, Holland >

The off-gassing test is done to ensure the payload does not give off any dangerous fumes that might be harmful to the crew.

The test involves placing the payload into a bell jar and pumping out all of the air. Synthetic air of known properties is then pumped in, and the whole jar is held at 50 degrees Celsius for 72 hours. Afterwards the synthetic air, plus any gasses released by the payload, are pumped out and analysed using a mass spectrometer.

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If you look closely, you can also see some Raspberry Pi SD cards in there. The test needed to be representative of the entire payload, so it’s one of the flight units plus five SD cards. The resulting measurements were then just doubled to account for two Astro Pi units with ten SD cards.

Thermal Capacity

Raspberry Pi, Cambridge, UK

This test needed to demonstrate that no touchable surface of the Astro Pi flight case would ever reach or exceed 45 degrees Celsius.

In microgravity the process of convection doesn’t occur, so the case was designed with thermal conduction in mind. Each of the square pins on the base can dissipate about 0.1 watts of heat. We also wanted to avoid any fans as these cause EMC headaches and other problems for safety (moving parts).

David Honess on Twitter

Thermal testing of the @astro_pi flight hardware to ensure it never reaches or exceeds touch temperature (45C) pic.twitter.com/eFGRXXg5Xe

We used five temperature probes connected to another Raspberry Pi for the data logging. Four of the probes were placed in contact with the surface of the aluminium case using small thermal pads and kapton tape (HDMI side, base by the camera, SD card slot side and top side). One was used to monitor ambient temperature some distance away. The Astro Pi was then placed inside a small box to simulate the reduced airflow on board the ISS and was then stressed to maximum performance for four days.

The results showed that an equilibrium was reached fairly quickly where the only input into the system was the fluctuation of ambient temperature.

Sharp edges inspection

ESA ESTEC, Noordwijk, Holland >

This test was almost a formality, but was done so ESA could verify there were no sharp edges that could cause harm to the crew. The test was done using a special piece of fabric that was dragged over the surface of the flight case. If it snags then the test is failed, but thankfully we passed without issue first time.

Astro Pi on Twitter

Our final test was done this morning! The sharp edges inspection. This was done with the final flight articles. pic.twitter.com/VaEbIppWfI

The test is important because a crew member with a cut or infected hand is a serious problem in orbit.

Experiment Sequence Test

ESA-EAC, European Astronaut Centre, Cologne, Germany >

The experiment sequence test is a full end-to-end reproduction of everything that Tim Peake will do on orbit. It was done in a replica of the ISS Columbus module on the ground.

On orbit they have step by step procedures that the crew follow and these tests are an opportunity to improve and refine them. There is a procedure for deploying the Astro Pi, one for powering it from the ISS mains, and another for powering via laptop power. There is one for fault finding and diagnostics and also one for getting files off the Astro Pi for downlink to Earth.

Astro Pi on Twitter

We’re at @esa’s European Astronaut Centre today testing #AstroPi flight procedures ready for @astro_timpeake to use pic.twitter.com/gcR5QRDweW

The tests used a surrogate crew to play the role of Tim Peake. Each procedure was run, and any anomalies or problems that caused a deviation from the procedure were noted.

Astro Pi on Twitter

The Astro Pi looks very much at home in the Columbus mock up! Testing all going well so far. pic.twitter.com/M3emHTC8JO

The Astro Pi will run a Python program called the MCP (master control program*) and this oversees the running of the competition winning code from the students. It is designed to monitor how long each has run for, and ensures that each receives the allotted run time, despite the Astro Pi being, potentially, rebooted multiple times from single event upsets due to the radiation environment on the ISS.

There were a couple of minor issues found, and we’re required to repeat one of the tests again in September. But otherwise everything worked successfully.

All the test reports are then combined into a Flight Safety Data Pack (FSDP). This also includes a flammability assessment which is an examination of all materials used in the payload and their risk of being a flame propagation path on the ISS. The main heavy lifting with the FSDP documentation was done by Surrey Satellite Technology, whom we’re eternally grateful to.

Thanks for reading if you made it this far! Next mission update will be after we’ve handed over the final payload.