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About Sonostar Hub

Geodesic dome kit from Sonostar shown with Sonostar's 1/2

 

It started out that I wanted to cover my swimming pool in the wintertime. “Why not build a geodesic dome to keep the heat inside?” This question based on curiosity launched a research quest that spanned almost a year, looking at all manner of domes and learning that there is a huge body of knowledge available today to help non-engineers to build basic domes.

Throughout the research, I kept an eye open for connectors that would take advantage of the attributes of PVC pipe. PVC pipe is relatively lightweight, readily available almost everywhere, inexpensive, and incredibly strong.  However, pvc connectors for building domes were not available anywhere I looked.  A friend in the plastics business recommended that I make an injection mold. To make the mold, I needed to hire an engineer to work up the specifications into a 3D design. We wanted the parts to be efficient, but also to be specifically useful for building domes. Since every dome ultimately needs a cover, we made the center of the hub solid, so that whatever cover was used could be screwed into the hub without compromising the strength of the hub. We also wanted the hubs to be usable in both large and small domes, so we opted for a 10 degree curvature that would make for a top and bottom side. In small domes, the flex of the PVC pipe would be sufficient to make up the difference. In large domes, the flex would work just the other way around.  We chose to start with a hub that would fit 1/2" schedule 40 pvc - probably the most common plastic piping in the world.  The engineer was able to run the design through a load-stress test that showed us that our hub would withstand at least 100 lbs of angular force on each arm – more than enough for a dome where the stress is equally distributed throughout the entire skeleton.

stress test on 5-star hub Standard hubs Trio

Solid prototypes were produced, and after some small corrections were made, we produced a production run of the 5-Star and 6-Star hubs. To make the 4-Star connectors, we used a band saw to slice off two arms of the 6 Star hubs.

Once we had the hubs in hand, there was nothing left to do but build a dome. It was decided that a 24’ diameter Frequency 4 dome would be a good project to start with. Using Desert Domes’ calculator, I  determined the six different sizes of struts needed to make the dome, remembering to subtract 2.28” from each length to account for the hub center. ½” schedule 40 PVC pipe can be ordered in 20’ lengths, so we ordered 50 pieces and set to work cutting. A Frequency 4 Dome takes 250 pieces of pipe + 65 6-Star hubs, 6 5-Star hubs, and 20 4-Star hubs. Cutting the pipe was the most time-consuming portion of the project. Using a pipe cutting tool is a huge time saver, and color-coding the cut pipes with colored electrical tape proved very practical. Next, we found a big grassy area to start building, and we staged all of the parts where we could easily access them. Since we knew the Dome would be 12’ high, be made sure we had a ladder that could reach the top. Using PVC welding cement, the pieces quickly came together, and within a couple of hours, the dome was erected.

What to cover it with? Coincidentally, the military uses a 24’ freight parachute that I was able to buy online at a military surplus website for $50. It was a little tricky to position it in the wind, but once it was on, it was easy to tie down. As we built our PVC dome, we imagined what they could be used for. Greenhouses are obvious, and the PVC pipes are very suitable to that. They could also be used for low-cost shelter in temperate climates, where the covering doesn’t have to be too heavy. In rural areas, a PVC dome could also make an excellent storage shed or garage. We speculated that others might think that PVC is a good way to go, so the idea to start this website was born.

After a couple of years and several hundred sales of domes all over the world, we made some changes to the molds.  First, we made a 4-star mold for the base hubs that was stronger than the 6-Star hubs that had two legs cut off.  It gave the dome a much more finished look.

Several early adopters commented that they would like to dissassemble their domes and reassemble them someplace else.  They told me that they drilled through the hubs and used nuts and bolts to secure them.  This prompted us to change all the molds and to include holes halfway into each arm.  Most people who purchase domes these days use the nut/bolt fastening system, although some still glue, and some use both.

4-star base hub

Other input we recieved was that we should build hubs for bigger domes using bigger diameter pipe.  We experimented with some models using 3-D printing, and found that we could make hubs that fit both 1" pvc pipe on the inside of the arms, and 1 1/2" pvc pipe that fit around the arms on the outside.  The tests for these showed a 5-fold increase in strength, and the opportunity to build much bolder designs.  We called these new connectors 'Megahubs', because they were so much larger than the 1/2" hubs.

6-Star Megahub Drawing  3-D 6-star std and Mega 6-stars 5-6-star megahubs

Sonostar is based in the USA. All the parts are assembled and shipped from here. If you’re a do-it-yourselfer, all you need to order is the hubs. If you’re not, but you still want a dome and you know someone who can assemble it for you, then buy the kit. It will have everything in it you need except for the cover, the primer and welding cement, and any stakes you need to secure it to the ground. All you need to know is the size you want your dome to be (either radius or diameter). We've built into our dome calculator the extra 2.28 inches that the hubs take, so rely on the numbers and trust the geometry.

At the basic level, domes are simply structures that contain space, and what you put in the space depends on the environment you're in. The really cool thing about geodesic domes is the structural efficiency and the symmetry that is so satisfying to look at. We hope you enjoy building your dome, and we love seeing pictures from everyone who uses the Sonostar hardcore geohubs to build them.