One of my many back-burner projects is a micro Sawppy: a smaller 3D-printed motorized Mars rover model with a target parts cost under $100. Well, the LEGO group has joined the chat with their $100 NASA Mars Rover Perseverance kit number 42158. This is more complex and expensive than their earlier rover (Curiosity rover #21104), but less than what serious fanatics have created. As someone who grew up playing with LEGO and a fan of Mars rovers, I had to get one.

This product is in the Technic line, which goes beyond cosmetic appearance by incorporating mechanical features. The front of the box depicts a wheel raised by a boulder, implying a functioning rocker-bogie suspension.

The back of the box shows four-wheel steering, with alignment for turning-in-place and for driving in an arc. It also advertises some level of arm articulation. It'll be very interesting to see how that's implemented. I opened the box and got to work. Window-shoppers can download the assembly instruction PDF to see all the details without spending money.

I was not surprised to find the wheels were custom pieces unique to this kit.

Also unique to this kit were a few decal stickers.

Construction started with fairly standard Technic fare. The first thing that really made me think "Huh, haven't seen that before" were these angled shafts. Usually, universal joints were used only for vehicle models with suspensions and had to transmit power through range of suspension motion. This is the first time I recall seeing them used for an interior non-articulating joint.

The first completed bit of rover suspension was the differential bar on top of the rover. A stout triple-beam unit to handle the load. It got me excited to seeing the rest of rocker-bogie suspension implemented in LEGO.

I was surprised to find the rover instrument mast could be built in the stowed travel configuration. No such feature for the rocker-bogie, though, which is fixed in the deployed position. I have ambition to give micro Sawppy a rocker deploy pivot but that'll take development effort I have yet to invest.

A little over four hours later, I completed assembly of the kit. It's pretty cool and absolutely recognizable as Perseverance rover with little buddy Ingenuity. The robot arm articulation geartrain turned out to actuate only the shoulder joint, elbow and wrist joints are posed manually.

As is typical for LEGO, a few extra copies of easy-to-lose pieces are included. If one goes missing, we can still complete the kit.

The cover art boulder is no lie: rocker-bogie suspension geometry is fully functional.

It was also, unfortunately, quite flexible. LEGO pieces have small gaps between them so we can snap them together and take them apart. These small gaps, combined with the natural flexibility of ABS plastic, resulted in a pretty floppy suspension that splayed out as it squatted. On the upside, I've determined the real thing built with carbon-fiber and titanium is also pretty floppy so all good.

A similar problem hampered the corner steering mechanism as well. It's a pretty clever design switching between "center steering" and "arc travel" modes with a single lever in the middle of rover body. But transmitting steering motion all the way out to the corners meant many linkages, each adding their own tiny bit of play. As a result, steering angles for the four corner wheels are more suggestions than commands. But still, pretty cool.

I enjoyed assembling and playing with this kit. LEGO assembly instructions, using techniques polished over decades, were excellent as always. I aspire to one day make Sawppy assembly instructions as clear as LEGO assembly instructions. However, the instructions only describe one thing. The beauty of LEGO is that it enables building countless other ideas of my imagination. It did that very well, until I got into 3D printing.