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The Flea is born

A project log for The Space Flea

A Proof-of-Concept design for a tiny Femtosatellite, based on the SunCube concept

OzQubeOzQube 04/14/2016 at 15:260 Comments

Day 1 of the project and a project log already???? I thought I'd describe why I'm doing the project.

Well, Hackaday did post an article about the SunCube recently, and it got me thinking. Is a 3x3x3cm spacecraft feasible?

I looked into the ASU press release, and the specification they published. I have to admit that most of my reactions were initially negative. This is coming from someone who is already well into the build of a slightly larger satellite - OzQube-1. OzQube-1 is a single unit PocketQube satellite, measuring 5x5x5cm - which is still tiny! Throughout the build I've managed to squeeze in a heap of capability into a small space. The satellite is going to take photos from space and transmit them down to receivers on the ground.

"But a SunCube measures only 2cm less" I hear you say. Think about this. Each face of a PocketQube is 25cm sq. vs 9 for a SunCube. So straight off the bat, a SunCube has 1/3 of the potential solar power collecting area. By the time you take into account fasteners etc, you'll run into some serious power constraints.

On my satellite, the smallest solar panel measures 45x50mm. It has 6 solar cells, producing a total of 265mW in space. During my satellites development, I found some tiny solar cells, each measuring 22x7mm. The are the ones I used in my 1 Square Inch of Power project here on Hackaday.io. I have made a panel to fit on the SunCube using 3 of these cells. This should output around 85mW in space. So that's definitely around 1/3 of the power generation capability. ( And to use Ohms Law - at 3.3v, that's about 26mA )

Now 3.3V @ 26mA is actually enough to run an Arduino, which needs 12mA (See here) with a few mA to spare to power sensors or charge batteries.

Then there's the issue of communications. Thanks to $50Sat, we know that people on the ground can receive transmissions from a satellite using a 100mW transmitter (the RFM22B). The problem here is that it draws 85mA when transmitting at full power. Even receiving requires 18mA. So we're going to need some kind of supplemental power to be able to run this radio. We won't be able to run the main processor and the radio at the same time if we were just relying on the solar panels.

Supplemental power can come from a battery or supercap. The cold vacuum of space isn't very nice to LiPo batteries in a number of ways. You can't charge below 0 degrees. The pouches expand and can cause damage to themselves and surrounding hardware. So even though a tiny LiPo will fit in the SunCube, it might not be the best choice. A supercap on the other hand can be charged in freezing temperatures. But it still may suffer when put in a vacuum. ( so testing will be needed)

But with a fully charged supercap, a burst transmission of the radio is possible.

Furthermore, looking at some of the new LoRa radio modules, like the RFM98W, you'll find they have a receive current of only 10.3mA. Combine that with processor sleep modes, it looks like the SunCube may actually be in the black as far as power goes.

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