WARNING!! You use this at your own risk! Structural integrity of 3D printed parts is hard to predict so use this with caution and wear protective gear (especially safety glasses) to protect yourself from debris should any of the parts fail.
Update 30/06/2017 Added a thread-less version of the nozzle in STEP format so you can make your own threads if you have a different bottle type. The cavity is 30mm wide and 13mm deep at the outer edges (there is a 2mm chamfer in the bottom so at the nozze it is 15mm deep.
Update 28/6/2017 Forgot to add the base structure (called housing)! :D sorry, now it's added!
This is a really improvised experiment to create a fun water rocket launch pad and nozzle. I wanted to use the compressor or a bike wheel pump without gardena connectors.
The release mechanism uses a rotating "locking ring" where you can connect a line for remote triggering. The system only works with the attached nozzle.
To build the air pressure connector, cut of the air valve from an old deflated bike tyre. I use the type with a car tyre valve for ease of use.
The water connector has a restriction valve to block air pressure from going into the water hose but I haven't tried this one yet as my prototype didn't work and I have changed it to the version that is now online.
The base has 3 "arms", one for air pressure, one for water supply and the third is prepared for whatever use might arise, why not connect your soda stream CO2 bottle? or I don't know. :) just drill out the hole in the middle and add your creation. There is already a channel inside so you only need to drill around a cm.
The system uses a number of different O-rings, I bought a box of different sizes and I'm not 100% sure which is the best everywhere so I suggest you buy a bunch of different sizes and try. The one between the base and the nozzle is the most critical and it depends on how much pressure you are going to use, which one will fit. To tight = no liftoff.
Regarding fins The nozzle has fastening with 3mm wide gap for fins. This is to allow a sheet of depron or similar material to use as fins. I did not include STL for fins as the shape of the ones in the picture is totally wrong, they are too small and do not make the rocket fly straight. You can try to make fins of different designs and see what works best and make sure they are not too strong, the idea is that the fins should break before the attachment in the nozzle. If you have fins made from a strong material, use for example a wood pin to lock it in place.
This is highly experimental, but I decided to release it once I got the prototype working and it does for air pressure, the water connector is in progress. You have to attach an external valve to control the water flow. The principal should be to first fill water, then air pressure, then pull the line to release.
Print nozzle and connectors with 100% infill if you use PLA. When I printed the first prototype housing I used 20% infill and 6 perimeters and 0,35mm layer height to make a quick print, this did not work, the housing was not air tight!! When I print the next prototype I will use more perimeters and 0,2mm layer heights. I have also made the water channel narrower to make it easier to get it tight.
All improvised using Fusion 360, made to resemble full scale rocket motor nozzle and a futuristic launch pad.