The One Man Turbofan

[slittlewing]

Hi monty,

Glad the unit is working well for you, it took quite a time to find that one which is more resilient than the others (Albeit not bulletproof as discussed!!)

Lovely job on the install 💪
Cheers
Scott

[monty]

May 24, 2022 14:09:17 GMT -5 slittlewing said:

Hi monty,

Glad the unit is working well for you, it took quite a time to find that one which is more resilient than the others (Albeit not bulletproof as discussed!!)

Lovely job on the install 💪
Cheers
Scott

Thanks Scott,

Yes, I did kill a few HV units. I even killed one of the final design. Early on I over-volted one because I didn't check the power supply. Somebody had used it since my last time.....stupid rookie mistake. So far I've tested those HV units up to 15V input with no problem. I wouldn't go over that though! They also seem much more resistant to an open circuit on the HV side. They don't seem to break down internally if there is not a spark across the gap. I think because the secondary output is "only" 11kV. That's a lot easier to insulate than 30kV.

I got a little OCD on the ignition box. Next up is the fuel pump....

Monty

[turbotom]

I had some good results with an ignition exciter utilizing three fast SCRs in series as the charge switch. The advantage of this approach is that the switching spark gap / tube is eliminated and that the trigger event is deterministic. On the down side stands the slightly longer breakdown time and the higher complexity since a trigger transformer for the SCRs is required. I controlled my exciter with a microcontroller and used a flyback controller chip to recharge the energy storage caps. The exciter provides 2 Joule of energy at 2kV / 15kV strike and lights off the combustor upon the first snap after fuel has been applied. I'm using modified NGK C7E spark plugs with the ground electrode removed and a stainless steel disc welded to the ground cylinder instead (circular gap). The spark plugs haven't got an integrated resistor and are quite long so it's easy to reach through the combustor liner wall.







P.S. supply voltage is 10~27VDC

[monty]

Tom,

You went even deeper than I dared to go. I mostly just worked on the packaging. I'm not a big electronics guy though...once I figured out I didn't want to use the old pump up the capacitor and dump through a spark gap method, I started looking for alternatives. I doubt my spark is in the multi joule range. I know it's around 11kV and the module draws around .6-.7 amps at 12 Volts. I have no idea what the spark duration, or efficiency of the ignition module is.

Thanks for the info on the spark plug! I'd love to see a picture of the modified version. What gap are you running and what is the spark rate?

Monty

[monty]

Wish I had something exciting to report. Unfortunately my last casting attempt was another failure. Close but no cigar. I'm re-grouping on the casting method. After a lot of research, I've decided to change to lost foam. There are many reasons that the automotive world uses this method almost exclusively for engines these days. The block investment method I've been using is just too much work for large parts like these, among other issues. Too much time and labor! Lost foam should be much less work after the initial hurdles.

I've been getting all of the materials gathered for that. The biggest learning curve for me is going to be the CAM portion. I searched high and low for a 5 axis cam package that I could even afford. It's so daunting, I even considered using a UV resin printer to make molds for polystyrene beads! In the end I've decided I need to get the 5 axis completely operational, and part of that is me obtaining and learning to use 5 axis capable CAM software. It's probably going to be quiet for a few more months while I struggle with that. It's been really hot here this summer, with temps pushing 40 C all the time. Tends to sap my motivation. Fortunately I can do this work in the AC.

Monty

[turboron]

Monty, FreeCad has all the features you need. It is free. Works on Linux or Windows.

Thanks, Ron

[monty]

Jul 26, 2022 9:30:47 GMT -5 turboron said:

Monty, FreeCad has all the features you need. It is free. Works on Linux or Windows.

Thanks, Ron

Ron,

I don't think FreeCad's trajectory planner is up to 5 axis continuous yet. I think it only does 2.5 to 3 at most. I'm watching it with interest, because I want to get away from the windows environment and back to linux eventually. If I ever retire, paying for software all the time probably won't be an option.

For this particular project, I am so heavily invested in Alibre with part models, changing really isn't an option. I currently use Rhino and Alibre. I don't know if I'm just getting old or lazy, but I don't want to learn a new CAD system at this point!

madCAM runs native in Rhino, and I can actually afford the educational version. Even the full professional version is at least somewhat reasonable. It's a bit old school and not as easy to use as many other modern CAM packages, but full 5 axis capability is $10K USD or more for all of those!!!

Monty

[monty]

I am still slowly working on this project.

I have given up being able to get my 5 axis to do actual simultaneous 5 axis work for now. 1.) My machine control doesn't support tool center point control, and neither does anybody else outside OEMs. This means I MUST use CAM to generate my toolpaths and deal with the machine kinematics. Everytime I change a tool, I must re-post the gcode. Just not a viable solution. 2.) Even if I take that path, CAM that can really do this is at least $20k. Not going to happen. Fusion 5 axis is a dumpster fire for so many reasons I don't feel like getting into... So I'm waiting on software and controls to catch up to what I want to accomplish. It'll get there eventually. But for now I'm moving on. I have a really nice 4 axis machine with 5th indexing. Still very useful.

The main part I need the 5 axis continuous for is the fan hub. I've re-designed it to be 3D printed in metal using DMLS. I hope to do that soon. I was hopeful I might be able to make a custom comp wheel eventually rather than guessing from a catalog....but...waiting on the software and control hardware.

The rest of the components can all be lost polymer investment cast for the prototype. I'm giving up on classical investment. If I ever want to make parts in limited production, I'll use lost foam. The foam forms can be made by expanded polystyrene in a 3D printed mold. But that's too much work for a prototype.

However......

Given my recent deep dive into turbine and compressor math, I decided to update my calculations on the fan. I was never comfortable (and I'm still not!) with my calculations around the turbine and NGV. The cycle design works, but I'm not confident the hardware will match the cycle.

Now I at least have a closed solution relating the turbine and fan/compressor. Sure enough, I was asking too much of the turbine. It's on the ragged edge of possibility. To get the inlet conditions for the turbine to behave, I will need to allow swirl in the exit stream. I need to get everything possible from the core flow the turbine will support. While all of this is fresh in my brain, I'm going to update the spreadsheet to deal with allowing exit swirl. I should then at least be able to make the math/physics work. No more work on this project though until I get some data from the turbojet using this turbine. Once I have that, I should be able to update the core design so that I have a little more confidence it will work.

Monty