BACK TO THE DRAWING BOARD: With the time I've got available now (which is not much, with real-world work heating up a lot from now until mid-December), I've taken the opportunity of having 2-3 months until my self-imposed deadline to start selling kits to explore a radical redesign of the entire engine section and fin assemblies. The pics below show progress on this project, with notes following:
This redesign represents coming around in a complete circle to an approach I used in my very first demonstration build, as well as a fundamental change in approach to how the rocket-body parts for the engine section are fabbed. In my original designs, I included the fin fairings as part of the body parts. When I switched over to the large tab-and-slot fin/body joining method, I incorporated the fairings into the fin assembly parts. Here, with the promise of the stronger and more precise peg-and-hole joining method I'm perfecting, I'm exploring going back to the original concept. Also, for lots of reasons, I made the choice when I switched fin designs to go to a body design for the engine section that had four separate outer parts, and three separate internal joining parts. With more experience dealing with overhangs and large, long prints, I've decided to explore reducing the engine section body parts to just two, and incorporating, if possible the internal joining elements directly into the external body part print.
The first pic shows the lower of the two new body parts vertical on it's build platform, The second pic shows the same part tilted to catch shadows to highlight an issue with this approach. These pics were taken when the part was fresh off the printer, and there's been no sanding or other preliminary finishing work.
The downside to how I'm approaching this is that the sharp convex turn from the body to the fairing produces some artifacts in the smoothness of the external surface. You can see that the worst of these issues are limited to the top third of the body shell, where the shell is thinnest, to reduce print time (more about which below). If I stick with this method, this lower engine section part will be the roughest of any of the parts in the kit. Given that the entire surface needs to be puttied, though, this MAY be acceptable -- something I won't know until I do a pass at surface finishing on these parts.
Among the many benefits of this approach is that the exhaust fairings on the bottom sides of the part come out much better, with no need for major clean-up to address outer edge issues that resulted from the fact that they were overhangs supported by sacrificial material. Also, with fewer transitions from one body part to the other, and with more surface continuity without the large indentations of the bigger slots, the over-all contour of the part will require much less third- and fourth-pass sanding to get a smooth contour, a problem that was inherent in the previous beta design.
The last pic shows the status of the redesign project as of this morning. Four of the new two-part fins with panel lines have been printed, and are mated together only with dry-fit of the styrene pegs. No sanding or surface work has been done on them -- they look just as they do when separated from their build bases. The line where they mate to the body is an indented panel line. The lower engine section part has had one 5- to 10-minute workover with a rough grit sanding sponge and a minute or so of "shaving" with an xacto to remove some "strings" on the external surface that occur in the printing process when the printer makes the convex external detour to print the body/fairing contour.
At the stage presented in the last pic, this redesign project looks promising enough to support the substantial effort that will go into bringing it to "final production" level. A major issue is the upper body part. Incorporating the fin fairings into it, accommodating the panel details in it, and incorporating what were previously separate internal "joining collars" are
major challenges. The first attempt at doing this is currently printing. And therein lies a major challenge: That print will take 15 minutes shy of
FIVE HOURS.
Previously, the longest print I've done on anything that might be a production part has been just a little over two hours. Up to now, I'd considered such prints to be very long and "risky" in the sense that, if there was a hardware or design failure that didn't manifest itself until near the end of the print . . . I was
unhappy with the result.
But two things make me willing to risk such a long print. First, the printer is significantly more reliable now than it was previously. Since cranking it up for the first time in February of this year, Makerbot has made some important design upgrades, all of which I've incorporated into my machine now. The printer control software, stepper-motor driver for the extruder, the extruder heater, the plastic reel-and-feed mechanism and the build platform surface and wiring are all much more robust than they were in the first version of the printer I was running at the beginning of the year. Blown prints due to hardware failures are very, very rare now.
Second, I've gotten a
lot better at designing in a way that
WORKS in the printer. The constant, quick feedback loop between design and fabrication I've mentioned before on this subject has resulted in me being much less likely to design into a part something the printer can't faithfully print in a way that works in the end.
All of which means I'm willing to at least see if a near-five-hour print makes sense. If it does, the kit will be much easier to build and have fewer, better-fitting parts. The downside may be that the occasional failed mega-print could mean that my per-kit production time might grow from one to two weeks, since the long prints will only be something I can do 2-4 times per week -- on the weekends. Printing all the other parts fit into before-and-after-work time slots during the week.
