5" Motor (to be)

[DeeRoc29]

I was browsing through the scrap area at my favorite metal yard a couple weeks ago. This quickly caught my eye. The tube is 5.0" 6061-T6 seamless with .188" wall and is 48.25" long. ID is approx. 4.625". Also picked up some raw 5.5" 6061 round stock for closures/bulkhead. The price was right, so I went for it. I have an acquaintance who has offered to help machine the motor. All he needs from me is a drawing of what I want. This got me thinking...I don't exactly know what I want. I thought I'd post here in hopes of receiving advice from those who've dealt with motors and hardware this size.

The intention here is a 5 grain APCP motor. Preliminary Burnsim predicts max chamber pressure of 500 to 750psi depending on which formula I run. In any case, it's an O motor. :clap:

With that in mind, what would you do in terms of:

1. Aft retention. Threaded closure? Bolted closure? Other?
2. Nozzle. I'm thinking a full diameter graphite nozzle is a bad idea, so maybe an aluminum nozzle carrier with graphite insert? I'd love to see pics or drawings of designs that worked well in the past.
3. Forward bulkhead/retention. Not interested in delay wells. Just simple and functional.
4. Liners/casting tubes. I checked Yazoo mills and they can accommodate the dimensions I need, but it's cardboard, and I'd worry about spirals on the ID of the casting tubes. I could also make my own liner out of FG using a 4.5" aluminum mandrel.

I'd appreciate your thoughts!




6" ruler for scale

 

[tfish]

Derek, Check out this page from John DeMar https://spaceportservices.com/BALLS2011/150mmHardware.html

Tony

 

[DeeRoc29]

Thanks, Tony. I take it from the pics that the the motor was successful.

 

[tfish]

yep...



https://spaceportservices.com/BALLS2011/X4inVideoFrames.html

I like how he did the forward closure..putting the bolt heads inside.

I have some more info..on another computer...that I'll post in a couple of days.

Tony

 

[tfish]

Here is the other item...

https://www.lokiresearch.com/Documents/152mmBrochure.pdf

Tony

 

[MuddAmateurRocketryClub]

I'd copy what CTI does on their 98mm reloads; it looks easy to machine and size for o-rings and the like. As far as nozzles, If I were going to invest in a custom motor case, I'd turn an optimized de Laval Inconel nozzle and get it black oxide coated for some degree of radiative cooling; single-use nozzles strike the machinist in me as a waste. Don't skimp on time spent optimizing the nozzle' replacing the simple cone nozzle on medium-to-large commercial reloads can be a %10-%15 boost in total impulse.

Of course the crazy cool idea you could go with would be to turn down the OD throughout the middle section of the tube (where there aren't internal threads) to a fairly thin size, then wrap uni-directional carbon around it...make it ultra light. But I don't know of that concept ever being actually tested in a hobbyist setting.

 

[butalane]

1. A bolted closure is going likely going to get you a highest max Pc. If you decide to go that route, I would suggest looking into dowl pins instead of bolts. You can over design the bolt retention and make it work but I hate the idea of using something threaded in a shear enviroment.
2. See attatched picture. I've used a design like this in 6" hardware. You'll get far more propellant in the case this way and it pushes the end of the exit cone past the end of the case if you use bolts.
3. Flat bulkhead.

 

[edwinshap1]

@Butalane, what is the darkened rectangle mean?

@Mudd, If the motor were burning for more than a few seconds, and at higher than 10k ft, I would say a de laval nozzle would be optimal, but for lower altitude flights, there's no reason to not go with a 15 degree conical nozzle.

 

[tfish]

Graphite insert?

Tony

 

[DeeRoc29]

Thanks, guys. I appreciate all the advice. Jared, I assume the dark area is the graphite insert.

Butalane, can you elaborate a bit more on the dowel pins? I've not seen that before.

 

[edwinshap1]

Ah, the insert would make sense. Also, I think Butalane's talking about not threading, but having a strong physical connection. I have seen threads under high load get just stressed enough to not twist back out.

 

[FredA]

"elaborate a bit more on the dowel pins"

You are placing the pins/screws in shear....
If threaded, you are pinching the threads.
AND, the material thickness and shear strenght of a screw is much less than a pin.

You only want to insert something to stop the shear action.
Dowel pins are cheap, dimensionally consistent, and strong....ideal for the application.
You also just need a grove to insert them into on the carrier side -- shaped much like an oring seat.
Just push dowel pins through the case wall and into the groove....apply a wrap of tape to keep them from falling out....your DONE.

No threading -- lower precision needed as you just need to keep them all in the same plane, but radial spacing is not critical.

 

[MuddAmateurRocketryClub]

View attachment 91486

This is exactly what I was thinking; the o-ring groove placement at least. Strong and easy to machine. Dowel pins are also an excellent suggestion-check the material you're ordering, alloy steel is usually the strongest in shear by a wide margin over stainless, but most dowel pins Mcmaster sells are stainless. Check with your machinist to make sure they have a good set of over-under reamers; you can probably get it so that you can press two or three pins in by hand and get an arbor press to install the rest (first locating then adding load-bearing support). My concern would be; how are you going to extract the pins afterward? maybe pin the nozzle in and thread the Forward closure, load entirely from the front...just thinking aloud.

Also, Butalane; your inbox is full, apparently.

 

[edwinshap1]

There are also other options. A well milled snap ring can hold a lot of pressure as well, and it will not lower the total strength of the bulkhead/nozzle as there won't be any thinner areas.

 

[THarrison]

I think the most important question is what tools does your machinist friend have available? Does he have a lathe with enough distance between centers to chuck up the 48" case and does he have a steady rest with a 5" through hole?

If yes: I vote snap rings. I have seven O motor cases (4.5" and 6") and used snap rings on all of them. It's super simple to machine and your casing is thick enough to handle your expected pressures with a decent safety margin.

If no: Use pins like butalane suggested. The holes can be indexed on a mill or even done with a hand drill and a paper guide. They take a bit more effort to install/remove than the snap ring, but are capable of containing higher pressures.

As far as liners, some of the cardboard manufacturers will send "samples" to your specs before you presumably (in their mind) order a bunch more. I imagine you don't need a pallet of liners, so inquire about this and you may be able to avoid the minimum order threshold and get a few liner sets. I believe Missileworks used to sell liners for this tube size, so you may want to check with Jim Amos and see if he still has any floating around his shop.

In case it isn't obvious -- In the drawing butalane posted, there are orings around the circumference of the graphite insert as well

-Todd Harrison

 

[bobkrech]

@Butalane, what is the darkened rectangle mean?

@Mudd, If the motor were burning for more than a few seconds, and at higher than 10k ft, I would say a de laval nozzle would be optimal, but for lower altitude flights, there's no reason to not go with a 15 degree conical nozzle.

A de Laval nozzle is simply a convergent-divergent nozzle with choked flow in the throat. Our simple conical nozzles are de Laval nozzles. A parabolic nozzle is simplya shortened form of a de Laval nozzle....

Bob

 

[MuddAmateurRocketryClub]

A de Laval nozzle is simply a convergent-divergent nozzle with choked flow in the throat. Our simple conical nozzles are de Laval nozzles. A parabolic nozzle is simplya shortened form of a de Laval nozzle....

Bob

True, i was being vague. I did not refer to a parabolic nozzle either; I referred to an isentropic minimum-length expansion nozzle. You'd solve for it using MOC probably, 6 or 7 points, input those point locations to solidworks, and make a spline through them, and call it close enough.

 

[FredA]

"press two or three pins in by hand and get an arbor press to install the rest (first locating then adding load-bearing support). My concern would be; how are you going to extract the pins afterward"

There is NO NEED to make these pins tight.
As long as they present at the same loading plane to the carrier, they can be loose enough to fall out!!!
Once pressurized, the shear force will lock them in place.

Clearly you don't want too much slop....but there is no need for line-to-line designs that need presses to assemble and diassemble.

 

[butalane]

There are also other options. A well milled snap ring can hold a lot of pressure as well, and it will not lower the total strength of the bulkhead/nozzle as there won't be any thinner areas.

How is the total strength of the bulkhead or nozzle affected by the retention method? This design would be retained by either a snap ring or a pin ring and is unaffected by the method of retention therefore it is not stronger or weaker in either case.

As Todd said, for reusability it would be wise to put O ring grooves on the OD of the graphite insert, otherwise it could be RTV'd in place. The graphite insert, in this case, begins before the aluminum insert. This design allows the addition of an insulation disk to self center on the graphite and protect the bare aluminum.

True, i was being vague. I did not refer to a parabolic nozzle either; I referred to an isentropic minimum-length expansion nozzle. You'd solve for it using MOC probably, 6 or 7 points, input those point locations to solidworks, and make a spline through them, and call it close enough

Applying the MOC and obtaining a bell shape would involve a lot of assumptions, and while it would save you a bit of mass overall it presents some manufacturability issues. I have seen it done, and this design would be workable with a curved expansion cone, but for a non optimized low pressure motor I'm not sure its worth it. If the OP came up with an initial workable design and then started optimizing it after he might not be overwhelmed by the many optimization considerations on the first iteration of design.

@FredA, yeah exactly! If anyone has ever seen the stress concentrations in an FEA of a point in a loaded system, you understand my suggestion. That being said bolts do work, and you don't need to punch them through the holes to remove them... Fred, how do you remove yours? Punch through? I wasn't considering making them as loose as you described, but definitely loose enough to lightly tap in with a hammer.

 

[MuddAmateurRocketryClub]

@FredA, yeah exactly! If anyone has ever seen the stress concentrations in an FEA of a point in a loaded system, you understand my suggestion. ... I wasn't considering making them as loose as you described, but definitely loose enough to lightly tap in with a hammer.

That's about what I had in mind; maybe .001 of slop in the locating pins and about .0005 to .001 or interference on the load-bearing pins; I just like smooth pressings rather than hammering it together.

You're right, I'm not sure it's worth it for making the bell-shaped nozzle; but if his shop is CNC'ing parts to his CAD files already I'd do it because why not. =p But it is a secondary optimization as you said.

 

[MCS]

>> apply a wrap of tape to keep them from falling out

In minimum diameter configurations won't the tape burn-off??

In non-minimum diameter configurations I like threaded end closures, but I thread the outside of the case, not the inside. As the case pressurizes, it expands INTO the closure. If the threads are cut carefully, and a little sealing compound is used, O-rings can be eliminated.

 

[edwinshap1]

I'd be wary of using a spline in SWK with only 6-7 data points. I ran a parabolic nozzle calculation which worked fine, until I entered it into SWK, it then bent improperly at the ends of the curve. Plus you need a tangential curve coming off the throat. I have designed VK nosecones, but I had a data point stepped every 0.001" on the Y axis, and the auto generated spline worked fine.

 

[CarVac]

You can simply avoid the ends of the spline by drawing the spline longer and trimming the ends.

 

[butalane]

I'd be wary of using a spline in SWK with only 6-7 data points. I ran a parabolic nozzle calculation which worked fine, until I entered it into SWK, it then bent improperly at the ends of the curve. Plus you need a tangential curve coming off the throat. I have designed VK nosecones, but I had a data point stepped every 0.001" on the Y axis, and the auto generated spline worked fine.

I'll try this tonight. It sounds like your spline was underdefined at the ends; depending on the degree of smoothness the nth derivative of the spline segments should be equal at the knots. You cannot expect the spline to be tangential (ie derivatives equal) without defining what the derivatives at those points need to be. In free space these points are unconstrained and the line will travel where it wants.

Consider a spline as a physical entity - that is their origin - and this makes more sense. If you pinned a stiff metal ribbon to a table in 6 places you would have a fairly representative expansion cone, however, only if the first and last pins were on the same hoizontal line as the second and 5th respectively. If they were not, the ribbon would deflect in the region between the 1-2 and 5-6 in an undesirable way.

You're method with the NC works because you're defining the hell out of the curve, you could approximate with straight lines and it would look correct at .01".

I took an advanced CAD class as an elective in graduate school and our first task was to use splines to sign our names in excel. By carrying out the number of derivatives we could smooth the curves and make the letters look like a cursive font and the beginning and ends of the letters to mesh perfectly smoothly so when they were plotted together it looked like the text was one pen stroke.

 

[daveyfire]

I thread the outside of the case, not the inside. As the case pressurizes, it expands INTO the closure. If the threads are cut carefully, and a little sealing compound is used, O-rings can be eliminated.

Be sure to cut a relief groove somewhere, otherwise when the motor blows up, it'll blow the side out of the case, rather than the ends off.

 

[edwinshap1]

Yeah, I would say it was underdefined. I had 10 or so points along the curve, including combustion chamber and converging section. I ended up manually inputting and smoothing the curves with tangent arcs. However, once I changed to a conical diverging section the cross section was a lot easier to create.

That's the parabolic type nozzle on my first attempt, though my diverging arc is hand made.

Conical 15 degree expansion, looks fine and should perform to how I need.


Also, Derek if you need any help with designing CAD files I'd be happy to help.

 

[DeeRoc29]

I appreciate the comments! Regarding my machinist, let's just say he works for a well known aerospace giant. I assume the shop to which he has access has everything.

At this point, I'm leaning towards Butalane's suggestion of dowel pins on both ends. Just seems too damned easy. My metal yard has .25" diameter 4140 round alloy steel rod in stock. Think eight or ten of those puppies equally spaced around the circumference of the motor would do the trick?

Jared, thanks for your offer to help. I'll send you a PM when I get a chance.

Derek

 

[CarVac]

You could stagger them as you go around the tube to get more pin density without making the metal thinner between them.

 

[MuddAmateurRocketryClub]

Especially if you take CarVac's excellent suggestion and use a pair of pin rings that are staggered in angle and offset by some length axially, I suggest you pop for a pack of mcmaster #98381A542 or similar. Since you know the shearing strength of the pin (since these are made for this application specifically) you can model it and calculate exactly how many of what size pin you need. Furthermore, being designed for this application, they are precision ground to a very exact size; since you need a hole diameter tolerance of a few tenth's of a thousandth of an inch at most (which your machinist's reamer set will easily meet) it doesn't make sense to use a rod that probably doesn't meet the same tolerance-you'll have uneven loading on the pins and the aluminum will get deformed around them.

 

[DeeRoc29]

So I haven't had much time to work on this project lately, but speaking with my machinist renewed my enthusiasm. I also conducted a successful static test of a 75mm scaled down version of this motor last weekend. My goal is to have the 5" version tested and ready to fly at LDRS 32. When all is said and done, it should be a 32K Ns O4400.

Regarding the steel alloy dowel pins from McMaster - the website says 1/4" pins break at 10,000 pounds. If I space ten of them equally, would that not be enough? Seems like two rows would be overkill and take away propellant volume. I understand some of you have recommended two rows of pins and I'm appreciative of your experience. I'm not an engineer of any sort, but my gut says one row of pins would be enough.

Derek