• ### Kinematics & Horrific Trigonometry

When modelling the latch in OpenSCAD I included the kinematic model - essentially the movement of the latch is a parameter, and all parts move as if they were bolted together and to the appropriate pivot points. Why include this? The most vain reason is that it allowed me to see how the latch worked in motion - and to see if this was something that was interesting enough to spend the time and effort on. A side effect of this was creating some of the animated images you can see in other posts. A more practical reason is that it allowed me to test the model for interference - any point in the motion where one part wants to be in the same place as another part - obviously this would cause a problem.

Anyway, it's finished now, so I wanted to share a little bit of the process of creating the kinematic model. What it came down to was maths. Hideous, horrifying quantities of trigonometry. The YouTube channel ThisOldTony describes trigonometry as "shop math" - math for the machine shop - and while that's accurate, trigonometry really does feel like a blunt instrument at times. Is there a better way to do this? Probably, but I'm afraid I never learnt it...

• ### 3D Printing & Design References

I've finished designing a version of the latch that can be 3D printed, which is now underway. I'm sure I'll find some tolerance that is too tight, or some part interference, but that's just the way 3D printing goes in my experience...

In the meantime I wanted to share a couple of useful resources that I found online while researching this project. First, This page has a copy of a diagram of the Apollo hatch that shows a bit of detail how the hinges attach to the capsule - something that is missing in a lot of images online: http://heroicrelics.org/info/csm/cm-hatch.html

Next, this article from the Journal of Spacecraft and Rockets titled "Review of Seal Designs on the Apollo Spacecraft" mainly focusses on the design of the hatch seals, but has some fascinating detail: https://www.researchgate.net/publication/245438272_Review_of_Seal_Designs_on_the_Apollo_Spacecraft

Lastly, this article by Lance Lininger and Kyle Gotthelf titled "Design and Test of the Orion Crew Module Side Hatch" focusses on the hatch design for the Orion spacecraft. There's a bit of comparison to the Apollo designs, but there's also a wealth of information including how the hatch lever gearbox works, and interesting problems like how the flexing of the hatch door under pressure affects the rods that open and close the latches. https://www.esmats.eu/amspapers/pastpapers/pdfs/2020/lininger.pdf

• ### Hinges!

And here's the matching hinge. The NASA design uses a bewildering 6-bar linkage, and there's still a lot of work to do on this to fine-tune the lengths and angles, but the kinematics actually work in OpenSCAD