Soyuz Post #2: Why the Soyuz is hard to design!
Published onApril 22, 2021

Where are all the kits?

There are reasons why the Soyuz—one of the most beloved rockets of space geeks everywhere—doesn’t have commercial kits readily available. I mean, with FIFTY YEARS of pent up demand, companies should be clamoring to bring one to market! Sure… there are static models that you can buy on eBay or Amazon. But, making a flying Soyuz model is HARD! Like really, Really, REALLY hard-core hard (especially if you are trying to make an accurate 1:100 scale version). Let me list FOUR reasons why…

#1: There are almost no straight sections

The Soyuz is curvy! Even in that “straight” midsection, the dimensions are constantly changing. And those boosters! Oy vey! Full of compound curves. Although that makes the rocket look amazing, it’s also a scratch builder’s nightmare. Other scale rockets (like the Saturn V) have large sections that can be approximated using paper tubes. This is a godsend since paper is lightweight and can withstand heat. Although plastic (or resin) is terrific for making small detail parts (like fairings and conduits), it is a terrible material for large structural elements because 1) it is heavy and 2) it tends to melt or breakdown when exposed to hot ejection gases.

So if the goal is to make an extremely accurate 1:100 Soyuz that will not melt, BOTH paper tubes and plastic/resin need to be utilized. The paper will provide the sturctural support, protection against the heat, and weight savings. The plastic will provide the “skin” for all the curvy section and tiny details. Obviously, we want to keep the amount of plastic/resin to a minimum since it is quite heavy, so the non-paper parts need to be made as thin and light as possible while maintaining structural integrity.

#2: Thin-walled detailed parts are difficult to manufacture

There are TWO recent technologies that make the 1:100 Soyuz possible. The first is SLA (stereolithography) 3D printing. This process not only produces exquisitely detailed resin parts (with virtually no discernable layer lines), but can also produce very precise thin-walled parts with tolerances down to 0.05 of a millimeter. Because of various manufacturing limitations, many of these components simply cannot be made with traditional plastic molding techniques.

We live in exciting times!


#3: The interstage is nearly impossible to mould traditionally

But thin-walled parts are not its only trick! SLA printing can also produce delicate-looking parts that are surprisingly strong. Consider the Soyuz interstage—although it is intricate and tiny (only about the diameter of a quarter!), this part is remarkably strudy. At first glance, this interstage has no business being this solid. But all those interlacing triangles spread out the forces (that’s why triangles are frequently used in designing bridges). The Soyuz engineers knew what they were doing!

Again, manufacturing this type of structure simply wasn’t possible before the advent of SLA printers..


#4: The design of the Soyuz is inherently heavy

In the cross-section above, notice that the vast majority of the rocket is hollow. But no matter what weight saving methods are used, there is no way to get around simple physics—the design of the Soyuz, with its four large strap-on boosters, makes it a heavier model. Without engines or parachute installed, the 1:100 Soyuz weighs in at about 174 grams. For a rocket that is only 20.5 inches tall, that’s pretty hefty. Unfortunately, the most powerful traditional block powder 18mm engine (Estes C6-3) is only rated for a maximum weight of 113 grams.

But did that stop me from strapping on “C” motors? No way!! I made two launches with Estes C6-3 engines. In both flights, the initial assent was slow and arching because there was not enough thrust off the launch pad. And because they did not gain enough altitude, the parachute deployed only about five feet before impact. Both rockets sustained damage to the boosters.

Remember where I mentioned that there were TWO technologies that make the 1:100 Soyuz possible? Well, here’s the second one: composite 18mm D motors! Specifically, the Quest D20-4. Instead of black powder, this motor uses a composite propellant (similar to what was used in the Space Shuttle) that packs more energy per square inch. This engine has a weight rating up to 340 g and it is small enough to fit into an 18 mm BT-20 tube (traditional D engines require a 24 mm tube BT-50 tube).

Ta-da! Magic!!

All the ingredients come together!

Theoretically, all the technological pieces are now available for an exceptionally accurate 1:100 Soyuz model rocket that can fly. I have some Quest D20-4 motors coming in the mail. Wish me luck!

There are still many more steps that need to happen before a kit can come to market! But for me flight testing is probably the most stressful. After hundreds of hours designing and tinkering, it would be nice to know that I produced an actual working rocket!! Obviously, I did launch a very rudimentary “test of concept” model before spending hundreds of hours in a CAD program. But, as with all thing, as the design became more detailed and complicated, it became heavier and veered away from that initial prototype. So, if this doesn’t work… well… anyone want to grab a beer with me? Actually, I may need two….


  1. David Bauer

    Does it require nose weight?

    • davekoo

      Yes some but not much. Because the nose cone has more resin, it acts as it’s own nose weight. If you look at the cross section diagram of the Soyuz above, the pink crosshatched areas are resin. Hope that helps!

  2. Rick Randol

    Did you try the Estes C5-3? It has a Max liftoff weight of 227 grams. Good luck. Looks amazing so far.

    • davekoo

      Thanks Rick for the tip!! Just ordered 6 motors from eRocket…

    • davekoo

      Thank you. The photo was an early prototype. The final product will have better fit and more scale details!

  3. John

    Nice! Love your N-1 and I’ll jump on this one as soon as It’s available.

    • davekoo

      Thank you John. Finally had some time this week to work on the Soyuz and get it flight ready for a Quest D engine!


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