Back in December, British ESA astronaut Tim Peake took two specially augmented Raspberry Pis, called Astro Pis, to the International Space Station (ISS) as part of his six-month mission. These Astro Pis are running experimental Python programs written by school-age students; the results will be downloaded back to Earth and made available online for all to see.
To satisfy the safety requirements that ESA and NASA have for small payloads aboard the ISS, we had to build the Astro Pi flight unit and put it through a rigorous qualification process.
One of the two Astro Pi flight units
At £3000 each, you can see why we only ever made eight of them. Why do they cost so much? Each half of the case is milled out of a solid block of aerospace-grade aluminium using a five-axis CNC mill. The two halves are then bead-blasted to give them a matt surface, then they’re anodised with a special coating to aid thermal radiation. After that, there’s some manual touch-up work, followed by installing the Raspberry Pi hardware and, finally, laser-etching the markings and logos.
That all adds up!
However, to quote from the original blog post where we announced it:
This will not be available to the public to buy because we’re only making a small number of them. We may however, in due course, release an object file so schools with a 3D printer can print one themselves.
With today’s blog post we’re making good on this promise!
The first attempt
Initially we just tried to 3D-print the original CAD files to see how hard it would be. The trouble with 3D printers is that they use hot thermoplastics, which can bend and sag under their own weight.
To avoid this, the printer creates what’s known as scaffolding and rafting to ensure the structural integrity of the object during the printing process. The user has to peel off this support material to get the original object they were trying to print. Any part of the object that overhangs will cause support structure to be built below it to prevent sagging. So the lower part of the flight case, with the grid of pins, came out chock full of the stuff:
Scaffolding and rafting that must be manually removed
After about 20 minutes with a pair of pliers, and accidentally snapping one of the corner pins, we decided this would be too frustrating for most users.
The base with scaffolding and rafting still remaining
The lid was slightly better. It was printed with the outer surface of the case facing downwards, to avoid support structure filling the internal cavity. But this meant that the outer surface came out with rafting all over it, and removing this resulted in a characteristic stringy finish that doesn’t look great.
The lid, printed with outer surface facing down
So we set about modifying the design so that even users with low-end 3D printers would be able to successfully print it, with minimal scaffolding and rafting.
Several attempts later
Many thanks to Ben Martin from Solid Models in Cambridge for running off so many test prints for us, and to Jonathan Wells (who did the original CAD work) for the many tweaks and changes. Our own Creative Producer, Rachel Rayns, contributed lots of 3D printing experience which led to these decisions. It was most definitely an iterative process!
The first change we agreed on was to slice off the heat sink on the base, so that it could be printed in the opposite orientation. That way it would have nothing overhanging to cause support structure to be built between the pins.
The heat sink as a discrete part (click for 3D STL view)
We then sliced off the top of the lid so that it could be printed with the clean side facing upwards, meaning the stringy side would face down.
The lid as a discrete part (click for 3D STL view)
That was a lot nicer looking. So with the lid and heat sink sliced off, it meant the two original middle bits were left as discrete parts.
The middle as a discrete part (click for 3D STL view)
We also removed the pillars between the USB and Ethernet ports because these snapped off easily. Finally, for convenience, we changed the corner bolt enclosures from a sunken captive screw to a straight-through M4 nut-and-bolt design.
The base as a discrete part (click for 3D STL view)
You can use epoxy adhesive (or similar) to join the heat sink to the base and the lid to the middle. When the Raspberry Pi and Sense HAT are installed it’ll end up looking something like this:
The assembled flight unit, still missing a few buttons
To guide you through the assembly process we’ve created a brand new educational resource that covers everything from downloading the STL files and getting the fixtures and fittings you need right through to testing that you’ve wired up the push buttons correctly. Click through and take a look:
The Astro Pi flight case is one of the most desirable cases in the history of the Raspberry Pi. With this resource you will learn how to 3D print your own case and install the Astro Pi hardware inside it.
We’re really looking forward to seeing the cases you make – please show us by tweeting pictures to @Astro_Pi and @Raspberry_Pi.
By far the most exciting benefit of owning an Astro Pi flight unit is the ability to prototype and test code that could be run on the International Space Station. Head over to the Astro Pi website now to get involved in the new coding challenges!
Where are the STL files?
Why are there four files, not two?
We sliced the case into four layers to minimise the amount of scaffolding and rafting that needs to be printed; it also keeps printing time down. The text of the blog post above explains this in more detail.
Can we modify the STL files?
Yes. They are released under the Creative Commons attribution license so you are welcome to modify them. Please note that GitHub has a great STL viewer and also has a 3D file diff, which could be useful for tracking changes.
Can we have the original CAD?
Currently, no. Raspberry Pi needs to retain the ability to be the sole manufacturer of the space-qualified Astro Pi flight unit. You are welcome to reverse-engineer the STL files we’ve released today, though.
How do you fit the hardware inside it?
The educational resource we’ve written covers this in great detail: check it out here.
I don’t have access to a 3D printer, but I really want this case. What can I do?
You may be able to find one at your local hackspace. You can also find local 3D-printing services through the 3D Hubs website.
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