Skinny spindle motor issues

[NeighborJ]

I've had this 4# homemade spindle for 6 months. It had issues when I first started making motors on it but I had taken certain unconventional steps to make it work properly and they work but I've been catching a little flack for my unusual building techniques. I would like to take a fresh look at this motor in order to find a proper fix and get this thing flying rite.

This is a skinny spindle design which I use for whistle, nozzled bp, nozzleless bp ,zippers w/ and w/o nozzles. The only type I am having issues with is the nozzled bp motor. It has a tendency to change coarse at 10-20' and then straighten out for the rest of the burn. I've tried extra guidance sticks, different fusing methods and a variety of different methods to prevent this from happening. All have failed except for one. Analysis of slow motion video revealed that the flames are projected to one side for the first 10' of burn then straightens out. This appears to me that the core is not igniting properly and fully before liftoff. I've tried different bp formulas, blackmatch, and fast fuse which is my preferred ignition method but I have the same issues. I make sure the spindle is centered and tru.

The one "fix" which has worked was to lengthen the throat of the nozzle which ensures the gasses are all ejected strait regardless of incomplete ignition. I really don't know what else I can do to make this work properly.

The video I'm sharing is of this motor using a fuel which is too hot. I was hoping it would help with complete ignition so ignore the Cato but the flight trajectory is typical of this motor.

[Maserface]

More details on the spindle please

[NeighborJ]

Of coarse master face I was waiting until someone asked. Also included is a spent motor cross section showing fusing method and spindle. I Am trying to keep an open mind because my best efforts have failed.

[dagabu]

Can you move the ignition further up the core? Skinny spindles can lend themselves to top lighting if a slower propellant is used. The top lighting will ensure 100% grain ignition before hitting the nozzle.

 

One way to cheat in top lighting a rocket is to wrap a piece if visco in masking tape very tightly, leaving the last 1/4" exposed and run that all the way up the nozzle to light the top. Make sure to wrap enough layers to keep ALL of the fire contained in the tape.

[NeighborJ]

I'm willing to try anything Dag, that fuse method with the tape had created Catos years ago on sugar motors so I had abandoned the idea but this motor doesn't have a convergent lip for the tape to wadd up against so it should help.

I was thinking if the nozzle forming rammer is too steep it could create a disturbance in the force. Mine is 45 degrees so if I change it to 30? It may have a positive effect.

[dagabu]

Hmmm, that oak rammer tells a story all by itself. Oak is a tattle-tail for pressure. Even flame hardened oak will deform in the soft pockets easily at 2000 LPI and the one shown shows a clean end. I would get some aluminum rod and try that instead. You will get MUCH better compaction from Al.

 

Whats the ID of that rammer? I may have one in my box I can send you. What is the motor size?

 

In answer to your question, no, 45° is not too great of and angle but you do lose about 20% of your pressing force over using a 30° angle. Keep in mind that the clay will go where you send it, if you press it with an angled rammer, the pressure goes out to the sides and not so much, down. Also, get a flat faced rammer for ALL of your BP pressing, don't use the nozzle former to press the BP, only the clay.

 

Manufacturers cut their costs by eliminating this extra rammer, IMHO, it is essential to making a good rocket a great rocket.

[NeighborJ]

I'm using the oak for now because this motors issues are not ironed out yet. When I can get a consistent working model I am going to turn a more permanent AL rammer. I thought my description of the 30 degree taper would be misunderstood, I was thinking of a more obtuse angle, so I guess I should have said 60 degree. This should direct the gas flow running down it to utilize the short throat more effectively. Mabe? And yes I use 4 rammers, the convergent , two hollow flat and one solid . Edited August 24, 2016 by NeighborJ

[NeighborJ]

OK so after an entire day testing I've taken one step forward and two steps back. After adjusting the nozzle rammer to the new 6Odegree angle it no longer kicks wildly to the side at takeoff. But now the nozzle is shorter and the throat is shorter this means the spindle is basically longer by 1/2". I thought I could compensate for this with the fuel mixture but it still Catos at 60-30-10 screen mix fuel and it is too close to the chuffing zone to make it any richer.

At this point the spindle works well with a variety of other motors and I do not want to change the length but it needs shortened, widened or the long choke in order to work properly. Unless I can find any other way to make this work I will likely fabricate a separate spindle for this one motor tomorrow.

PS. The throat is now being made at 3/16" long is this appropriate?

Edited August 24, 2016 by NeighborJ

[calebkessinger]

Dave.

Some of us use the convergent rammers for everything and haven't ever witnessed a difference or problem.

I'm thinking you hit the nail on the head with the wooden rammer. Hard to put 6 or 7 thousand pounds of force on one of those.

 

J,

 

some motors just need MORE pressure. You might never get a successful flight out of an under-pressed design. Some configurations need 7 to 9 thousand lbs of loading force and more.

[calebkessinger]

I and others have regularly pressed the hottest bp we can make with a nozzle on our skinny spindle stuff. Without the nozzle it will work with the hottest sali whistle a guy can make.

 

Increment size, pressure, tube support and strength. all of these are huge variables.

[NeighborJ]

I'm sorry guys, sometimes I just need to pick my arguments. These Catos are split tubes or at least most of them. I have the cheapo tubes from skylighter so to press at 7-9k psi will do no good. I do have home made tubes which are far superior and those are the ones I will use the aluminum on. One assumption Dag made was that I was using a press. These test motors are rammed and without a support.

Anyway I've gotten this motor to work, it fly's strait and no Cato. Normally when a motor has this kind of issue a few increments of slower fuel can be in order but in this case the fuel is already as slow as it can get so I dug into my bag of tricks and pulled out my top secret delay formula and filled the last 1/2" of spindle with delay.

I'm not ignoring your suggestions on the rammers I'm going to the metal supplier tomorrow to get the AL and at that time I will switch over to the press and support. But there is nothing I can do about it tonight.

The main goal was to get this thing flying strait, mission accomplished even thou I don't see why the 60 degree worked better.

Caleb I have no idea how this spindle compares to yours, I never looked at any specs. I played with it until it worked. So it may be skinnier and if so that would explain why it works for all other motors but this one.

[Maserface]

That's a lot skinnier of a spindle than Calebs, his is designed to have a similar Kn value to a traditional black powder skyrocket tool set.

 

I finally watched the video and I think I have an understanding of what's going on. One problem with skinny spindles is that the initial Kn is very low, this means that some propellants might struggle to get the rocket off the pad, much less in a straight line. The problem quickly corrects itself as the burn progresses because the burning area gets larger, and the nozzle stays the same (ideally).

 

What I think is happening to your rockets is they have just enough initial thrust to get off the pad, but not enough to maintain a straight flight. Half a second later, they really start to move because again, the Kn value is increasing.

 

When you increased the throat depth you decreased the Kn even more, because there was less propellant surface area, this must have kept the rocket on the pad just long enough that the burn could progress to a point where it had enough thrust to fly straight. A wider spindle, shorter spindle, or slower propellant would have done the same. A longer spindle or faster propellant would also fix the problem, because your initial Kn would be higher, or you would generate enough thrust from faster propellant to fly straight with a low initial Kn

Edited August 25, 2016 by Maserface

[NeighborJ]

Alright Dag I've made the new tooling but the aluminum in that size was not available so I was forced to make them out of 316 stainless. Pita but it makes a much nicer product. I'd still like to make one more rammer for the tapered spindle tip but what I have should work for testing. I will also switch over to my good tubes now because I refuse to waste time fabbing a tube support for junk tubes which I will never buy again. If Phil carried 4# tubes I'd just buy some.

I've done some research about my spindle dimensions and found them to be a perfectly scaled up version of master faces slim pickins spindle. It only varies in shape but length to dia ratio is the same. Since he did Caleb's scale for his tools I'm willing to bet they are comparable. If he said I should be able to run hot bp with a nozzle then who am I to argue. We will see if the press will make this possible.

One concern I have is my press, it is an 8000lb ram so it will not produce that force against a 1-1/4" ram. It may be able to get around 6525psi, and this will need to be enough because I'm not dumping any more money into this project. I'm allergic to nickle so I need to take some benadryl and take a shower before I can make the last tool

Edited August 25, 2016 by NeighborJ

[calebkessinger]

Great explanation maser.

Some of us never think about those things.

[otto]

Threads like THIS is why I can't get enough of APC. I don't have an immediate use for this information yet it is at my fingertips. Thanks everybody!

[NeighborJ]

Ultimately I think I am DUN trying to make this spindle work in this tube size with a nozzle. The choke is simply too small to handle the burning surface area of this size tube, no matter the construction technique or fuel.

Masterface, the side jetting of gasses was the cause of the slanted takeoff. Your theory holds merit but I have slower takeoffs with endburners and they fly strait. Dag was absolutely correct about the wooden nozzle former causing an asymmetrical convergent cone, it was this mechanical rutting which caused the side jet effect and not a side light. This was proven to me once I replaced the wood with stainless 45 degree rammer. Flights all were later strait all the way to the Cato. Which brings me to the next issue. The spindle to nozzle ratio is 8.75-1 which turns out to be the exact ratio of the slim pickins whistle spindle it is a proven design. The problem here occurs when I take into account the choke to tube ratio. This setup is more suited for an end burner. The case pressure is just too much for any pyro tube to handle at least not once it nears the end of its progressive burn.

This design is faulty, it works for nozzleless rockets but I suspect it creates a more regressive burn once the core opens up beyond a certain point, at least with bp, whistle? that could be a different story but I have no way to prove or disprove these theory's. So the only way to get maximum efficiency from this spindle is to use it in a smaller tube,8oz-3# or scale it up for a more appropriate choke size.

The whole reason I picked this motor for research is the fact that it obviously had issues and it is these extreme end of the spectrum designs which have the most to teach me about motor design. These are problems which never occur on standard tooling. I get frustrated trying to find the words to describe to you all what I am seeing and that frustration is more so when what I've written is misunderstood. If any of this is not making sense please let me know and mabe we can look at this problem from a different perspective.

[Maserface]

Maybe, but explain to me how a longer throat would solve the side jetting problem? If anything I would expect the mechanical rutting to be worse with a longer throat.

[NeighborJ]

Mabe rutting is not the proper word, the convergent former was soft on one side so when it was compressed the one side was longer. The gas would follow the rut on the convergent and shoot out more on the opposite side of the divergent. The long choke gave the gas enough room to straiten back out. If it was just a slow takeoff the problem would have been fixed by the dozens of variations of guidance sticks I tried.

[Maserface]

Maybe, I am NOT a rocket scientist, but I have a hard time with that theory. I have seen rockets with no divergent/convergent and hand drilled nozzles fly straight., and ive seen rockets made on professional tooling arc and jet just like yours-

[NeighborJ]

I'm not saying that other things can't have the same effect. But the new stainless former fixed the problem, every takeoff was perfectly tru after that. Believe me I wanted to be wrong because I wanted to say to Dag, "see you had me make new tooling for nothing" but I can't. So I'll eat my crow while it's warm because it sure beats eating it when it's cold.

PS the hand drilled nozzles would have the same choke as an overfilled nozzle, plenty of throat to keep everything laminar.

Edited August 26, 2016 by NeighborJ

[Maserface]

Other than the stainless drift, every other detail is the same as the ones that failed?

[NeighborJ]

Exactly the same I was even able to use the single stick again with no difference.

[Maserface]

So same loading pressure, convergent angles, propellant, tubes, all that, as the ones that flew crooked? I know youve been through several iterations of designs here, just want to make sure I follow.

 

Still questioning how the longer nozzle and long convergent (wood) fixed it, if it is indeed "side spitted gas"

[NeighborJ]

I drew some pictures. The steeper former only worked because it had fresh wood that didn't crush but that convergent doesn't streamline and the gasses crash into each other. I suspect that this side spit phenomena can happen to even the best tooling if the tube is loose or the nozzle former does not fit tight in the tube. The spindle is not centered inside the former causing one side to be thicker then the other.

Edited August 26, 2016 by NeighborJ

[dagabu]

Dave.

Some of us use the convergent rammers for everything and haven't ever witnessed a difference or problem.

I'm thinking you hit the nail on the head with the wooden rammer. Hard to put 6 or 7 thousand pounds of force on one of those.

 

J,

 

some motors just need MORE pressure. You might never get a successful flight out of an under-pressed design. Some configurations need 7 to 9 thousand lbs of loading force and more.

 

I get what you are saying but using an angled rammer will cause you to use more force to get the same compaction BUT the hole in the drift makes for a smaller surface making up for that lost compaction. It is true that in order to be competitive, you can't throw in extra rammers and lose that money or time and yes, I understand that it does not present a problem on the face of it (see what I did there?) but you cannot put the same force on that comp without adding a lot more force on the walls. A flat face will give better compaction, and with J speaking of using a really steep angle of 60°, 80% of the actual force is being pressed out to the walls and not down. A flat rammer is almost a necessity at this point.