Base
Design requirements for the Base
The Base must:
– Hold the Core stable in a vertical position
– Hold the rocket on the launcher during pressurization
– Provide a triggered rocket release
– Provide containment in case of pressurization-related failure of the Core
Base parts overview
The Base subassembly is composed of several parts
The "Base" is the large triangular tripod (blue-green). The Core fits into the center hole.
The Clamp or "Tower" Section is:
Six Clamps (green)
A hexagonal Collar (purple)
A U-shaped Launch Pin (reddish-purple)
Three rubber bands
The Legs Section is:
Three Legs (orange)
Three Pegs (red, at ends of Legs in grass picture)
All parts except the rubber bands and launch cord are 3D-printed. There are options to use PVC pipes for Legs to save filament. The Base is compatible with all Cores.
Clamp/Tower Section
Clamp mechanism
This mechanism is a 3D-printed incarnation of the ingenious Clark cable-tie system that is widely used in launcher designs.
Six Clamps (orange) hold the rocket to the Base by hooking onto the bottle's flange, as in the first two pictures showing an Alpha prototype. When the Launch Cord (green, extending off the left) is pulled, it sets off a sequence of events:
Launch Pin (green, attached to the cord) pulls out to the left
Rubber bands pull Collar (green) downward
Collar slides down off the Clamps (orange)
Axial (upward) forces applied by the bottle to the slanted clamp mating surfaces pushes Clamps outward, which pivot on the Base and release the bottle
Bottle launches upward and Core pushes Base down downward
The third picture shows the current Version 1.4 model, which incorporates feedback from people around the world and stress-testing in the laboratory (i.e., soccer field). It has improved strength, reliability, and compatibility with all bottles.
Clamp design
The Clamp is 40mm long, 10.4mm wide, and 9.3mm thick. The interface to the bottle measures only square millimeters; in this area, the six clamps must hold over 50 pounds/200 newtons of force and yet smoothly release. This was the design challenge.
Version 1.44 CAD drawings are shown. The first picture shows the Clamp on edge; the left is the top where the bottle flange mates. A 115-degree clamping angle translates axial launch forces to a radial Clamp-displacement force, which is held in check by the Collar.
The second picture shows various other bevels to the bottle end; these provide clearance for the bottle neck and various fin assemblies. Longitudinal ridges in the middle provide stiffness. The curved end on the right interfaces with the Base.
The third picture shows the outside of the Clamp. The center ridge provides stiffness and alignment into notches in the Collar. This prevents tangential displacement of the Clamp at higher pressures.
The fourth picture shows a slicer preview with cutaway, showing the orientation of the layers. Long extrusions match the direction of maximum tension, and distraction forces across layer lines are minimized.
The fifth picture shows a cross-section of the Base's "tower" section where Clamps interface. The Core sits in the central round bevel. Clamps slot in from the outside, right underneath. Functionally, this means that pressurization forces are contained in a small area and not handled by the rest of the Base. The Base only sees compressive forces; all tension is handled by the Clamps.
Collar design
Pictured is Collar v1.42. It has three hooks for holding the rubber bands that provide the drop-away downforce to release the Clamps when the Launch Pin is pulled. It also has three wider cradles for the Legs when they are stowed. These cradles, along with the hooks, provide a handy area to pull up on the Collar when locking the rocket in place.
The Collar is marked with “PIN” location to help with assembly, since its symmetry makes it easy to accidentally rotate it by 60 degrees.
The inside of the Collar has grooves which mate with the ridges on the outside of the Clamps to resist tangential displacement.
Launch Pin design
The Launch Pin holds up the Collar until it is time for launch. Internal grooves add stiffness to the prongs.
In early testing, users had a hard time finding the insertion location for the Launch Pin. After addition of a large embossed arrow marked "PIN" and a beveled entrance, this is rarely an issue.
Legs Section
Leg construction: 3D-printed vs PVC pipe
The launcher exclusively used PVC pipe legs through Version 1.2 due to ease of construction following established precedents. This configuration was inspired by RaketFued's PodPad design. It's economical with materials as PVC pipes are cheap and cutting a pipe is faster than printing a Leg, if you have the tools.
The first picture shows the standard Leg build, with a 3D-printed Foot with a loop for anchoring on one end and 3D-printed Peg that could be stowed in the other.
This works well and I still highly recommend it if you live in an area with PVC pipe with 21.4-5mm outer diameters. This includes ASTM Schedule 40 PVC pipe in North America and 21.5mm overflow pipe in the UK. I am unclear where else in the world uses this pipe.
Unfortunately, these are not universal pipes. I initially attempted to create versions for 20mm OD metric pipes, but this requires modification to the Base and Carrier as well. It became a sprawling system that I could barely keep track of myself. Once I needed a spreadsheet to keep track of combinations, I recognized the project needed a universal Leg. This meant 3D printing.
The second picture shows various development versions of 3D-printed Legs in PLA and PETG to optimize fit, strength, and printing reliability.
The third picture shows the near-final design. Legs have a place to store the Pegs and fit in the Carrier, which was modified slightly to accommodate the triangular profile. They fit in the unmodified Base and have been stress-tested extensively. This provides a region-independent option, which when paired with Core B, result in a globally-accessible universal launcher.
Leg angle
The Base's Leg sockets are placed at 45 degrees from the vertical. This provides a balance between stability, printability, and robustness.
Alpha and Beta versions (first picture) used a wider 60 degree leg angle in hopes of increasing stability and avoiding launcher tip-overs.
Unfortunately, 60-degree legs required 60 degree overhangs on 3D prints, which not all printers handle well without supports. Also, the torque force on the leg supports on launch was severe with such a long lever arm.
In any case, tip-overs can only be prevented by consistent use of Pegs or other ground-anchoring devices. The second picture shows a Version 1.2 launcher anchored with yellow 3D-printed Pegs.
Pegs
Anchoring to the ground is critical for safety, but easily overlooked. Without anchoring, it is possible for the launcher to tip over on attempted launch and point in the direction of the cord pull, resulting in basically a log to the face.
Integrating Pegs into the design increases the chance of anchoring more than any number of warnings or exhortations.
Pegs fit into 3D-printed Legs and PVC pipes that are compatible for use as Legs. Several versions are available for different internal pipe diameters. They are designed to slide into grass and hold firmly. A groove provides a finger hold and a hole is available for loops of cord, though I haven't had to use it.
Design excursions
Lest it appear that the design process is a linear path from concept to execution, I present some meandering excursions that were a part of getting to the current design.
Mini launchers
In Beta testing, I tried to make the design even more portable by shrinking the Base's Leg sockets. Unfortunately, the lever arm becomes too long and the Leg makes quick work of the Base even with clever reinforcements. While it's possible that revisiting this design with current experience may succeed in isolation, many current aspects of the design depend on exact dimensions of the existing Base.
Every kind of Clamp
I tried to make a Clamp for every soda bottle neck finish in circulation today, and some that aren't. I tried this using data I could find online and through the generous help of people on Printables. Then I tried to make those Clamps compatible with all the Cores I was making (and testing). And then I wanted to make the One Clamp that was universal, robust, and tolerant of printer variation. I made a lot of clamps. I think the current Clamp fits the task. But it may need just one more tweak....
Endless permutations
This was the state of confusion in July 2023 which prompted development of universal Legs. Attempted compatibility with four pipe types, four bottles, and five Cores. It was a mess that raised the barrier to entry.
This was fixed with coordinated modifications to Clamps (to make a universal Clamp), revising Core C (for reliability and changing O-ring height for compatibility), deprecating Core A (which would require a complex re-design for compatibility with the universal Clamp), adding a 3D-printed Leg, and deprecating metric 20mm Base/Legs/Carriers.
Summary
The triangular Base design has been stable since Version 1.2, with modifications to the Clamps and Collar for Version 1.4. It's a robust design and can be further extended.
Next section: Pressure and Safety
