REAL Space Legos!

So, MAKE Magazine has this on their current cover:

That’s a Lego Mindstorms NXT computer and other Lego pieces on a spacecraft. “Cool!” my labmate and I thought upon seeing this. “Satellites made out of Legos!”

Well, it turns out that the article says this is a picture of a functional satellite prototype made out of Legos by a group at NASA’s Ames Research Center. (The same group that recently launched a spacecraft that used a cell phone for its computer system!) But, you know…why not? Why not make a satellite out of Legos? I think this would be a great idea!

What would it take?

The physical structure of a Lego-brick satellite would have to withstand the rigors of a launch into space. This involves accelerating the satellite and subjecting it to heating from friction as the rocket climbs, among other things. Space Mission Analysis and Design, Third Edition, gives the following “typical values” for acceleration and thermal requirements of satellites in a launch vehicle:

  • Acceleration: 5-7 g, but up to 4,000 g shocks during stage separation and other events.
  • Temperature: 10-35°C (but the inner wall of a Delta II fairing could get up to ~50°C).

The acceleration requirements, though that shock value sounds drastic, may not be too much of a problem. G-hardening is potentially easily accomplished by potting components in epoxy.  Modern cell phones, for instance, are rated to several thousand g‘s so that they work even after you drop them. A good epoxy applied to all the joints in the Lego spacecraft structure, and probably around the whole structure after it’s completed for good measure, could go a long way toward preventing this from happening during launch!

I’m more worried about the thermal requirements. Lego bricks are made out of acrylonitrile butadiene styrene, which seems like it starts getting deformed due to heat at about 65°C. That 50°C Delta II fairing seems a bit close for comfort! Plus, the temperature of some Lego blocks sitting in direct sunlight in space could climb above this value very rapidly – and lots of transitions between daylight and shadow would cause the parts to expand and contract thermally, working the pieces apart if they aren’t well-secured with epoxy. However, the Lego satellite could be wrapped in something like aerogel or MLI blankets to mitigate the thermal challenges. Somewhat.

Another challenge is survivability of the computer system in the space radiation environment. With no atmosphere to absorb radiation, a cosmic ray could hit the spacecraft and trigger a single-event upset, or “bit flip,” that switches the value of a bit from 1 to 0 or vice-versa. This kind of thing happens to spacecraft computers all the time and corrupts data, so spacecraft computers engage in a lot of error-checking. But the same cosmic rays can also burn out a bit, so that the computer can never read its value again – or even burn out a trace in an integrated circuit so that the circuit fails! That sort of thing would definitely be a problem for a Lego spacecraft, and would shorten the life of the computer substantially unless we did some radiation hardening of the NXT. A simple way to harden it would be to encase it in some metal, but that adds mass, which is always at a premium on spacecraft. However, another strategy is to simply accept that the spacecraft will have a short life in orbit!

…Because, after all, what would be the purpose of launching a satellite made of Legos? It would be to show that commercially available materials are sufficient for at least some space applications, without the millions of dollars of investment in robustness and fault tolerance that the spacecraft industry generally demands. If the satellite’s mission can be accomplished in a few days and the lifetime of the craft is a week, then why should all of its components be certified for years of operation in orbit? Perhaps we could, instead, come up with much cheaper – or much riskier – satellite designs. We could try out new materials, new components, and new mechanisms without designing them never to fail. Instead, we accept a few failures as learning experiences, and move ahead with the designs that work.

Legos are, at least, a fun place to start. Perhaps most importantly, they are easy to get into the classroom, so that students can think about building the structure, thermal, power, electrical, and payload systems into a functional satellite – and can re-arrange or re-format those systems at will. But hey – when they’re done, why not launch?!

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