2013-04-05 Update: This post is now out of date. To see the most recent version of the thruster, see this post.
I've been working on designing a magnetically-coupled, pressure compensated, and ducted thruster pod. here's a picture of the current design:
I'll discuss the details more later in the post, but here's the important part: The design is ready for a production build and some in-water testing, but I'm looking for some help manufacturing parts. If you are interested in helping to produce a set of the machined parts, please let me know. The particularly tricky part, if you have not guessed already, is the nozzle, which needs to have a precise fit against the prop maximize efficiency. Anyhow, if you have some good machine tools, a good 3D printer, or are an ace machinist and want to give it a shot, get in touch!
And now, on to the details!
- The overall length is approximately 124 mm, and the maximum diameter is approximately 85 mm.
I've used an H105 hydrofoil as the surface of revolution in the shroud. In theory, it offers better performance (less cavation) than the usual Kort nozzle, but in reality, I've guessed on so many things - like the angle of attack, that it's hard to predict if it'll be better or worse. My solution was to plan for adjustments after the fact, which brings us to:
There are two versions of the nozzle. There's a single-piece version and a split version. The two piece nozzle is held together with three 2 mm steel pins, which can be removed, and allow other shrouds to be attached. If possible, I'd like to get a minimum of one split version and three single-piece versions to play with - this will let me make side-by-side comparisons during normal use and swap in other nozzle designs if for comparison testing.
For any aspiring hydrodynamicists out there, professional advice on the bulb and nozzle design would be appreciated!
I'm still experimenting with the magnet mounts in the coupling. I haven't figured out the best way to make them yet (preoccupied with other problems), but all that really matters is that they hold the right number of magnets. (and that they fit, but who's asking . . .)
There's a thrust bearing in the back to ensure that everything works smoothly.
Magnets: I use ten .250" x .250" x .100" magnets for the motor mag mount, and ten .125" x .125" x .125" magnets for the prop mag mount. Initial tests indicate that the coupling is plenty strong, though I have yet to do any in-water testing. I procured the magnets from Magcraft via Amazon.
I use the E-flite spinner nut that's designed to work with the prop. The major benefit of this is that there's no trouble with aligning the prop on the shaft, and that the nut has a nice tapered shape, improving the hydrodynamics.
Right now, there is no geometry locking the nozzle to the rear end cap. Until I've had some time to play with it, I plan on using rubber cement (or something else that can be removed if I feel like checking on the bearings). In the long run, (v2) I'm going to either put in a thread or some snap-locking geometry - depending on how I end up using it.
The final assembly will be filled with light mineral oil (food grade), and put in a vacuum chamber to degas it before it is sealed. This should render the motors corrosion proof and pretty much immune to pressure. A membrane (red in the picture) allows for some compression and expansion due to temperature differences or slight compresability.
If you've read this far, you deserve some reward for your work, so here are some pictures of parts that I've built to start testing out this design:
A motor with half of a magnetic coupling
The other half of the coupling. you can see the thrust bearing that prevents reverse thrust from pushing the prop out of position.
The two assembled sub-assemblies. It looks great, now all I need is the hard parts . . .
As I said earlier, if you are interested in making some of the more complicated pieces, send me a note - I'll send you the CAD and we'll see what happens.