This is a 1/10 scale model of the J2 rocket engine by Rocketdyne that powered Stage 2 and 3 of the Saturn V rocket. While the F1 engine was the workhorse of the missions and provided an unprecedented 1.5 million pounds of thrust each (7.5 million pounds of thrust total!), the J2 was amazing in its own right and had the unique responsibility of needing to restart in a vacuum, thereby getting the vehicle into lunar orbit, and ultimately returning it to Earth.

Background
I became interested in this project because my father Roger was a test manager for the J2 in SACTO and at the AEDC test facility in Tullahoma, Tennessee in the 60s. We have had an ongoing love of the project, but recently I became aware of Paul Fischer’s incredible scale model of the Saturn V Rocket and Gantry. Thus, my dad and I embarked on a 5 month build. Around the time that we were building the small version of the J2 engine, he mentioned that he would love to have a larger more detailed model of the engine. This started me on a search for a printable model which didn’t exist, although I did find several models that were fairly detailed, just not printable.

What I did find was a site hosted by Chris Pennello, whose grandfather Julian was also involved in the testing of the J2 in SACTO. His site contained original design and section drawings of many of the parts of the engine (an amazing document) and also detailed pictures of a scale model of the engine. Also turns out that his Grandpa and my dad worked together, and knew each other. Very cool.

This all led to learning Fusion 360 and ultimately creating a more detailed model than was available. Admittedly it is not perfect, but it is definitely cool, and evokes the spirit of the amazing engine. The best part was talking through every aspect of the engine with Dad while I designed it, and getting his thumbs up on the final product. Again, there are missing parts, like all the control lines that were just too difficult to reproduce at a scale that would fit on my Prusa MK3S+. I may come back and bend up some small tubing some day to complete it, but then again, maybe not.

Project notes
The most challenging aspect of this build is assembly and alignment. For the first version of this I printed the thrust chamber in two parts with separate intake and exhaust manifolds, but it proved too difficult to align everything properly.

In the second version, I added alignment marks within the chamber, but also created an alignment jig for the injector manifold and the turbopumps. This allowed us to get these key parts into final position before gluing, and allowed us to then attach the various ducts to the manifolds. In addition, we printed the manifolds integrated into the chamber, so that we didn’t have to align these parts on the fly.

Here is a rough set of steps we took:

1. Print all the parts. Most can be positioned to print without supports. For example, we split the thrust chamber into rings so that it would print without supports.
   • For thrust chamber ring 1, make sure to turn off skirt loops in print settings so it will fit on the print bed.
2. Begin gluing thrust chamber sections 1-12 together, making sure that the piping on the chamber wall aligns between layers (Fig 1)
3. Once you reach the manifold sections, look inside the chamber for alignment notches. You will need align the rings from here on out so that the injector on top aligns properly with the tapered manifolds (Fig 4)
4. Attach “top of bell” and “combustion chamber” sections, again aligning internal notches for alignment (Fig 4)
5. Glue the “Injector plate and mounts” on top of the assembled thrust chamber, making sure to align the notch on the chamber with the notch on the bottom of the injector plate (Fig 2 & 3)
6. The inlet flange for the injector manifold comes in 2 pieces; align the notches at the bottom of the inlet and injector and glue them together (Fig 5)
7. Glue the injector manifold to the top of the thrust chamber using the positioning jig (it keys off of one of the mounting posts on the thrust chamber wall and also off of the intake flange of the injector manifold) (Fig 6)
   • IMPORTANT: Make sure that the positioning jig is not warped along its backbone. Mine was slightly, so I just glued a small piece of angle iron to the back of it to straighten it up. If you don’t do this, then your pump positions will be slightly off and propagation of errors will become an issue later
8. Insert the Oxidizer and Fuel turbopumps into the jig (they should snap into the holder arms on either side. Once you have this in place it establishes the positions of the critical components. (Fig 7)
9. Attach the high pressure oxidizer duct between the injector and oxidizer turbopump; use the notches on the bottom for alignment (Fig 8) Attach tension brackets and rods. Note you will need to split the lower bracket so you can slip it around the duct.
10. Attach mounting hardware for the ox turbopump
11. Attach high pressure fuel duct and main fuel valve between the fuel turbopump and the intake manifold (Fig 9) Attach tension brackets and rods as you did with the ox duct.
12. Attach mounting hardware for the fuel turbopump (Fig 10)
13. At this point you should be able to remove the positioning jig. Feel free to snip the arms if needed but it should lift straight up (Fig 11)
14. Attach the fuel hot gas duct to the bottom of the fuel turbopump (Fig 12)
15. Attach the heat exchanger duct to the bottom of the oxidizer turbopump (Fig 13)
16. Attach the hot gas transfer duct between the ox and fuel turbopumps (Fig 14)
17. Attach the helium regulator to the thrust chamber under the ox duct. Note there is a notch in the back that fits under the mounting ring on the thrust chamber (Fig 15)
18. Attach the gas generator (GG) to the fuel turbopump and thrust chamber (Fig 16)
19. Attach the main oxidizer valve to the valve body between the injector and ox duct (Fig 17)
20. Attach the hot gas bypass valve to the hot gas duct (Fig 18)
21. Attach the electrical control package (ECP) to the thrust chamber (Fig 19)
22. Attach the primary flight instrumentation package  to the thrust chamber (Fig 20)
23. Attach the propellant utilization valve (P/U valve) to the ox turbopump and high pressure ox duct (Fig 21)
24. Attach the oxidizer bleed valve to the bottom of the high pressure ox duct (Fig 22)
25. Attach the fuel bleed valve to the top of the high pressure fuel duct (Fig 23)
26. Attach the secondary instrumentation package to the rear of the thrust chamber under the hot gas transfer duct (Fig 24). We just called it this because the name of this package is not in any of the documentation we found, and couldn’t remember what it was. If anyone knows what it’s called, please let me know.
27. Before attaching the start tank, we need to attach the 3 valves to it in the correct positions so that the mounting struts will align properly. Use the “valve placement jig” to do so (Fig 25). Take care to only glue the valves to the sphere, not to the jig. Remove the jig. 
28. Now using the struts, attach the start tank to the thrust chamber as shown(Fig 26 & 27)
29. Assemble the inlet ducts and attach them to the tops of the turbopumps (Fig 28)

At this point the model is essentially complete. As mentioned there are several parts that are missing. We decided to leave the gimbal mount off mostly because he wanted to see the injector torus better. None of the various control and bleed lines are included. We preferred to see the components better so left them off. I would probably bend up craft tubing to add them anyway, so they’re not included here.

Tips:

• Connecting the ducts to the manifolds on the thrust chamber can be a little challenging, so multiple sets of hands can definitely help
• For the tension rods on the high pressure ducts, I used 3/32” aluminum tubing from Hobby Lobby. 
• For the inlet duct brackets, I just glued them together, didn’t use any actual connecting hardware. Looks fine and would have been a lot of tedious work!
• Finally, I got a inexpensive airbrush and used Vallejo aluminum enamel paint to paint everything. They worked perfect straight out of the bottle; no need to thin.
• Some of the ducts were split so they could be printed easily without supports. You can of course fuse them together in your slicer if you’d rather print them as one piece. For the ones I did split, I used plastic putty to lessen the gap. You can still see them but not nearly as severe.

I hope you enjoy building this as much as we did!