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How much did each MarCO 6U cubesat weigh? suggests that the density of a MarCO 6U cubesat might be as low as 0.75 g/cm^3 whereas I think they are usually about twice that.

That led me to speculate that their insides might have been able to fit into a 4U configuration but since the reflectarray antenna needed to be roughly 6U-sized it didn't make sense to try to squeeze it into 4U.

Question: Have there been any square 4U (2x2) cubesats?

Are there any reasons why the 2x2 configuration might be undesirable?

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There is the list of cubesats, including cancelled and planned, that looks much more full than ones in Wikipedia. Two Dragracer cubesats of size 2x2x1.5U are mentioned near the end of this list. These planned to launch in 2020 on board of Electron KS launcher. It looks like that size of 2x2U was simply skipped (and is awaiting for it realization).

Dragracer is mission to demostrate electrodynamic deorbiting of satellites to show, that a small satellite with a deployable tether can safely deorbit in a matter of weeks.

The Dragracer project is a joint project to test Tethers Unlimited's Terminator Tape, a conductive tape designed to produce enough drag to lower the satellite's orbit. Millenium Space provides the bus, integration is provided by TriSept and launch by Rocket Lab.

The mission consists of two basically identical satellites 6U (2×2×1.5U) CubeSats of a joint weight of 25 kg. The satellite with the terminator tape is called Alchemy, while the reference satellite is called Augury. After deployment, the two spacecraft split up and one deploys the 70 m Terminator tether, while the other one decays naturally to provide a direct comparisson. It is expected, that the satellite with Terminator Tape will reenter after about two to four weeks, while the other will remain about eight months in orbit. Millennium will use radar to track the satellites and collect data.

Also is mentioned that 2x2x1.5U is the Raptor class bus, and mass of each cubesat is 12.5 kg. Orbit is planned about 400 km high.

Artistic impression of Dragracer cubesats Image from here.

UPD: Another "square" cubesat with dimensions of 3x3U is ULTP, launched 7 March 2020 on board of Cygnus NG-13 cargo ship inside of Slingshot deployer. Orbit after deploying was 476 x 486 km with inclination of 51.65 degrees. Mass of ULTP is mentioned of 25 kg, that looks too much for 9U. For 3x3x1 dimensions it is unclear is it 300x340.5x100 mm or 300x300x113.5 mm, according to Cal Poly San Luis Obispo cubesat design specification.

Also, it is unclear geometrical size of Dragracer - is it 200x200x170.2 mm or 200x227x150 mm, although 150 mm is not mentioned in Cal Poly SLO specification (it looks like 200x200 on the picture). Of course, some subesats may not comply with this specification, there are examples of 1U cubesats (excuse me, I lost the links), that slightly more than 100x100x113.5 mm. It depends of cubesat deploying method.

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  • $\begingroup$ Excellent and interesting answer. Thank you! I see that you've mentioned mass as well, I wonder if there's anything that can be added to Cubesat mass density (kg/U) statistics? $\endgroup$
    – uhoh
    Oct 29, 2020 at 23:36
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    $\begingroup$ @uhoh I have been apdated my answer by including a mention of the 3x3 square cubesat. $\endgroup$
    – Peter Nazarenko
    Oct 31, 2020 at 13:59
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To add to @Peter Nazrenko's excellent answer...

You ask if "Are there any reasons why the 2x2 configuration might be undesirable?"

I think it is not so much a matter of undesirability as it is a natural evolution of how CubeSats came to be. The CubeSat's fundamental innovation was its containerized deployer, which greatly reduced the risk to the rocket and primary payload.

The initial deployer's by CalPoly were all 3U which naturally led to an abundance of 1U and 3U satellites (with a smattering of 2Us). As rockets started flying with more deployers, it was easiest to just stack 3U deployers next to each other. Then when the success of CubeSats led to the need for larger satellites, it was easiest on the various deployment stacks to just "join" two 3Us to make a 6U dispenser (either 2x3 or 1x6). You could conceivably pack a 4U and a 2U into a single 6U dispenser, but as mentioned before 2Us were not that common to begin with. So the evolution just led naturally from 1 and 3 to 6 and mostly skipped 4U.

Keep in mind that beyond 6U there are a number of proposed standards but not nearly as strictly adhered to as 1, 3 and to a lesser extent 6U. There also some competing standards. For instance the University of Toronto Spaceflight Lab has flown a number of non-CubeSat small cube-shaped satellites. It's X-POD can accomdoate CubeSats but is not strictly compatible with the CubeSat standard (if I recall). They also have an X-POD for what is effectively a 16U.

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