Neutral and progressive burn from nozzleless motors

[Maserface]

Do whatever you want

[calebkessinger]

I don't think you need super engineered spindle removal tools. The ones I make work wonderfully with no real cranking force needed to pull the spindle. I usually twist all the motors off of the spindle but some of the 6lb whistles get tight enough I feel like i'm going to pull the workbench over cranking on them.. I grab a puller.. hold it by hand and just spin the push off nut... bam. The spindle falls out! Pretty dang amazing. Others have looked into how to keep the nozzle of a bp rocket from sticking and come up with good practices. IN the 6lb size they press the nozzl.e, spin it off rewax the spindle and then re-assemble it and press the fuel grain. Seems to work.

 

Just various thoughts.

 

Hey.. now that I have a cnc I could fool with my twisting base/ramp design.

[dagabu]

Hey.. now that I have a cnc I could fool with my twisting base/ramp design.

 

Now, THAT sounds like a plan! I even dream about that kind of spindle remover, I envision 4 equal ramps on both of the pieces. Just a 1 degree slope.

[lloyd]

Don't make the two parts of the same metal, Caleb, or you'll have a galling issue.

 

Lloyd

[OldMarine]

How about replaceable Delrin inserts for ramps mating with stainless ramps on the opposing plate?

[NeighborJ]

Maybe it can have removable handles on each plate so no vice or wrenches would be needed.

[OldMarine]

Simple holes with some rods would work. A quarter turn and POP!

Edited March 15, 2017 by OldMarine

[NeighborJ]

Yeah, holes where the rammer removal rods can fit.

[dagabu]

Collaborative Designing, Saweet!

[NeighborJ]

Had another go at this motor today which ended in failure again but it revealed yet another clue. Made another batch of fuel and this time I used 4% wax. When I first removed the spindle a smooth and shinny core was revealed, but after a minute or two cracks started to form. The tell I observed was the location of the cracks, they started exactly in line with the seams of my tube support. So the problem is definitely the split tube support.

I will use the shim stock support next and see if it solves the cracking issues but first I will need to trim it down to 8" so I can work the drifts. I did not doubt Dags opinion that the split support is not ideal but he didn't seem to want to explain why and I'm still not entirely sure why it didn't work.

[calebkessinger]

hmmmm... what is the support made out of again? I missed that part.

if it is secure it should be just fine. but any space for the tube to expand is all bad.

[NeighborJ]

The support was made from carbon steel, tubes are pulpy, and when I put them together they fit so good they don't want to separate even if they aren't tightened down. If they are tightened any more the tubes start to distort and the drifts become tough to insert.

[dagabu]

I'm sorry to disagree with the assessments here of the support. Think about a balloon and its shape. Fill it with air and it becomes 'round', the split support design does not keep the round shape, it pinches the tube in an oblong shape and as it "relaxes", the pressed powder inside pushes against all sides equally but the oblong shape leaves a void area to push against on the narrow width and more pressure on the long width.

 

Not my findings here, all the science was done by others long before me. The more rigid the material for the support, the worse the effect. PVC seems to expand to become round, split black pipe is a poor choice due to its strength and its strength also makes it a greater missile hazard in the case of a compressed ignition, i.e. friction sets off the fuel while pressing a rocket.

 

Dave F solved this problem with the shim stock support or using an aluminum tube or 2 piece support set with the split facing you. If you use a split steel tube, then make sure to face the split toward you when pressing.

[NeighborJ]

Thanks Dave, this is exactly what I've observed with this motor support but lacked the words to describe the idea. Today I've trimmed my shinny new shim stock to length and was planning on pressing another motor but ran out of steam so maybe I'll get to it this weekend.

About the safety of my press. I use a portapower ram with a manual pump on a remote hose. It is bolted to a swing arm on a tree so I can load the motor in the press and then swing it behind the tree to crank on the handle remotely. It was originally mounted to the doorframe of my shed with a heavy sheet of plywood and tin sheeting for protection before it got crushed by a tree. I'm still managing to build without a workshop but I've had to get creative until I find a new workshop solution. And yes of coarse seam was facing me but will no longer use the split tube support.

Edited March 17, 2017 by NeighborJ

[dagabu]

A couple hints on the shim stock:

1. Make sure it is clean

2. Round the end that is on the outside

 

Dave just uses his fingers to tighten the shim stock, mine don't work like that anymore so I use a tool made from hard wood, just smaller than the ID.

[NeighborJ]

When I trimmed the stock to size, I left a tab on the end of the inner spiral so I can hold it with my thumb as I tighten it around the motor. I don't know if it will work well but I figured there is no harm in trying, I can always trim it off if it doesn't work.

[DavidF]

Thanks Dave for saying it better than I could. I think the compressibility of the PVC is an important consideration. The ovalation from a split support along with the uneven compression from the clamps bugged me from the beginning. That said, I made quite a few good rockets with such a system. When NeighborJ mentioned the time spent on his support, I was thinking he clamped it shut and reamed it round on the inside?

 

Every method of tube support has its pros and cons. I like my method the best

[dagabu]

My fav was, is and will be a reamed solid tube of aluminum where the rocket tube is pressed in, loaded and pushed out with the ram. That said, I only use that for small rockets, #2 and up, I use the shim stock.

 

Thanks Dave but you used ovatation, the wife told me that I had to use ovate or ovoidal to describe the shape, I chose oblong since I'm an idiot since I thought that meant something to do with eggs.

[NeighborJ]

DavidF to answer your question, I bored out the nipple to size then split it with a bandsaw. It should have been perfectly round but if I under or over tightened the threads it would be oblong. I could use shims to gauge the exact thickness of the cut but I'm done screwing with it.

[lloyd]

Neighbor,

The commonly-accepted way to do that is to pre-mate two half-thickness flats, each with a thin kerf to aid as a boring guide (if drilling -- if lathe boring, the kerfs can be eliminated). The kerfs align with one-another when the clams are mated.

 

Then you bolt them together, and bore after they're joined.

 

Alternatively, if you have 3D milling capabilities, the bolt holes and 'bore' may be flat-milled, doing the bore with a ball-nose bit. With the appropriate software and fine-enough depth increments on the finishing pass, the bit may be much smaller than the final diameter of the bore, and you'll still get a clean, smooth, and perfectly-round bore.

 

<shrug> It is what it is. If it works satisfactorily for you, then it's OK. If not, you might try the above procedure on your next go-round.

 

Lloyd

Edited March 17, 2017 by lloyd

[calebkessinger]

Could be the whole problem is the pulpy tubes..

They won't take too much pressure from my experience.

[NeighborJ]

You may be right Caleb, I chose the skylighter tubes because I feared that a hard tube wouldn't have enough bite to contain the lengthened pressure spike of this motor design. The pulpy tubes have the bite but that wall spring is what is making the grain walls crush in on itself. It holds up fine where the grain is thick but it's the thin areas which crush back in. I will give it another go with the shim support but if that fails as well then I'll try one of my hard tubes. I just fear that the grain can blow out without a true nozzle and waxed hard tubes.

[lloyd]

Neighbor,

At sufficient pressures, most any pre-granulated comp (including whistle) will 'bite' into the surface of even a hard-walled NEPT tube.

 

You should be fine.

 

Lloyd

[OldMarine]

I've only pressed up to ¾" whistle rockets but I use waxed NEPT tubes and have yet to have a blowout or CATO. I use both a clamshell support I got from Caleb and a split PVC one Steve LaDuke made me and both work great. Caleb's and Steve's tooling vary slightly in rammer and base OD so I choose the set that best fits a particular tube to avoid too much slop or rammers fitting too tightly. I'm enjoying this thread immensely!

Edited March 17, 2017 by OldMarine

[NeighborJ]

I'm glad your enjoying it OM. I've not had issues at all with standard whistle tooling either but if this motor works the way I intend then it will see pressures comparable to optimized sali endburners, those I've made and never quite been able to get as reliable as I want. I had to either enlarge the nozzle or eliminate tube waxing so it can bite enough.

I like exploring the boundaries of what is considered normal practice because it always ends up teaching me new and unexpected things which most conformists are not inclined to know.