So, they sold out the big rocket I wanted (Mega Der Red Max) so I decided to print my own!

Long time, no post....  Here's some of what I've been up to in the past few months:

Estes surprised everyone (at least me) when they announced at the end of July that they were selling a 4" diameter Pro Series II™ Mega Der Red Max™ Model Rocket, using a 3-Engine Cluster.  I anguished for a couple of weeks about whether to buy one and they sold out before I decided to buy one.  I should have recognized the "Limit 2" next to the order button meant they hadn't made a lot of them.

After a month or so had passed, I saw someone on The Rocketry Forum discussing 3D printing the Mega DRM so I reached out to "frogglicker" (David Frey) who generously shared the STL files he'd developed.  He designed a custom 2 piece nose cone for me, based on a file I made with OpenRocket.  Estes was selling the 4" body tube used in the Mega DRM so I bought two (just for fun and possible future projects).

OpenRocket simulation of my Mega Der Red Max in flight with three engines.

I started printing the nose cone he provided, then stopped mid print when he emailed to say they were designed for a slightly different sized body tube than the one I had.  He took the one I exported from OpenRocket that was too tall for my printer, divided it in half and added threads to reconnect the two halves.  I test fitted the nose cone and it looks massive!

Test fitting nose cone and a fin on 4 inch body tube

I also printed the screw in base my friend designed, and added in the Featherweight GPS tracker I've designed before because this rocket might go very high with the right combination of engines.

The fins, which are huge, came next.  This is where things got really complicated!  I wanted do print using PETG, which is better for heat resistance than PLA but I haven't used it too much.  However, I kept having problems getting the fins to print without bubbles on the surface.  I tried printing the fins upright but found the fins easily cracked when I tapped the side on a table.  I also had trouble with the horizontal prints not sticking to the bed, so resorted to rubbing a glue stick on the area of the print to help with adhesion; frogglicker also recommended getting a 3D printer adhesive weld from Amazon.

Example of surface bubbling when printing with PETG

This led to many frustrating weeks trying to adjust the print settings for PETG to get the fins to print.  I tried "ironing" to smooth the surface, which didn't work.  I adjusted the speed, the temperature, tried with and without ironing, etc.  In the middle of this, the printer stopped mid print and I ended up having to order a new "print head" (the hot nozzle that extrudes the melted filament) which arrived just in time for Halloween.  I also had to order a Creality Filament Dryer/Filament Storage Box which removes water from PETG so it prints better.  This became more of a problem when dealing with overnight or multi-day prints.  Finally, the first week of November, I was able to print a clean fin by reverting to the default PETG settings, changing the filament temperature to 230 C per the PETG packaging and coating the bed with a glue stick!

Successful printing of 8 mm thick Mega Der Red Max fin

I quickly set up and printed two more fins using these settings (9 hours each print)!  Next, I printed centering rings.  My friend had sent files to print a single 38 mm engine, but I decided to stick with the original Estes style with three 29 mm engines (3 X E16-4 or 3 X F15-4) since I don’t have much experience with the really large engines; I also bought an 8 pack of F15-4’s so am eager to try clustering them.  I designed a centering ring for three engines in Tinkercad then printed three.  

Test fitting a three engine centering ring

One of the centering rings is designed to fit in the notch between halves of the fin tab for extra stability.

I printed screw on engine retainers using black PETG, and printed a black bottom centering ring to keep the black on red look.

I finally ordered a 48" parachute to bring this beast back as gently as possible.  Simulations based on the finished weights show it weighs 50 ounces without engines and will fly to ~400 feet on 3 E16-4 engines.  If I use 3 F15-4 engines it should fly to ~730 feet, and 3 F67-6W (white lightning) engines would take it to 1700 feet!

I've also 3D printed black PETG launch lugs and black rail guides.  I don't have a rail launcher, but my club does so I might get to try the really big engines there.  Now, I need to get busy putting it together.  I need to cut slits in the body for the fin tabs, and 3D printed a fin alignment guide designed for me by frogglicker.  I've also got a 3" diameter Estes PSII Der Big Red Max kit to build, which should fly ~1100 feet on a single F15-4, so I will be able to compare them side-by-side.  I'll try to post some build notes soon and, hopefully, a launch picture soon.