5" Motor (to be)

[Dwatkins]

@ 750psi the force on the forward bulkhead will be 12,600lbs. How much safety do you want.

Dennis

 

[patelldp]

If you are designing a 5" diameter EX motor, shouldn't you know how many bolts you need based on your desired safety factor? Maybe drawing a mathematical hypothesis here and asking for someone to check your work is more appropriate.

Not trying to be a jerk, just trying to instill the desire to learn in others.

 

[DeeRoc29]

With a .188 wall, at 750psi design pressure, the safety factor for the 6061 case is 3.5. I see no point in having a higher safety margin for the closures. For the closures with 10 pins on each end, the factor would be about 7.9.

Again, I'm no engineer so somebody correct me if I'm missing something.

 

[DeeRoc29]

Thinking further, it's not so much the number of pins that I'm concerned with as how they are spaced out around the case. If ten pins are spaced equally around one plane then the center of each pin would be 1.5" apart from the next. Using 1/4" pins, this would leave 1.32" of aluminum in between each hole.

 

[AlphaHybrids]

Thinking further, it's not so much the number of pins that I'm concerned with as how they are spaced out around the case. If ten pins are spaced equally around one plane then the center of each pin would be 1.5" apart from the next. Using 1/4" pins, this would leave 1.32" of aluminum in between each hole.

5" OD circumference = ~15.70" length Next, take out (10 * .25) = 2.5" of circumference and you are left with 15.7-2.5 = 13.2" circumference at .188" thickness (we are assuming here, as this isn't exactly true, but close enough for this calculation) = 2.98 square inches of aluminum holding back 12,600 pounds. That equals a stress ~4230 PSI. What is your yield and ultimate stress on your aluminum?

Edward

 

[DeeRoc29]

I see what you did. Thanks, Edward. I believe yield strength is 35,000 and ultimate strength is 42000 psi for 6061 aluminum.

 

[AlphaHybrids]

You'll probably also want to calculate the stress on the aluminum that the pins are bearing on. You have a hole that is 1/4" diameter, .785" circumference. Ideally, 1/2 of that circumference would be doing the holding. It isn't. You are probably have about 60 degrees that is actually bearing. That is 1/6 the circumference, or .1308". Take .1308 * .188 (not exactly correct, but close enough for this) and you get .0246
square inches per pin. (12,600/10)/.0246 square inches = 51219 psi. Now, there is probably actually more bearing area than 1/6, maybe 1/3. If it is 1/3 then your stress is 1/2. You might want to increase the pins, or do a couple rows.

Edward

 

[MClark]

The pins are way stronger than the case; you need to look at what the tube will fail at. In a perfect world the aluminum is good for about 2000 lbs. per ¼” pin. I do not live in that world.

I have issues with dowel pins, especially if they are not tight. Only three of them are touching in reality until the motor pressurizes and the tube yields to allow the others to touch. If you want to use pins roll pins are better, driven into match drilled holes. They need to be deep enough in the closure to not want to tip, about 1.5 diameters min, 2 is better. Many pins are better than few pins. The cases I have made in 4” using pins (ALL my motor I have flown used snap rings) I used 48x 1/8” roll pins. I do not disassemble pinned motors.

Before I spent a nickel on machining the tube I would make sure it is round. Most cuts offs are pretty bad at the mill cut end.

Mark

 

[DeeRoc29]

Before I spent a nickel on machining the tube I would make sure it is round

My machinist has already recommended this and is planning on truing it up if necessary. He makes rocket motors for a living so I trust him. Oh, and he's not even charging me a nickel

Interesting how many different opinions have surfaced in this thread in support of either snap rings, dowel pins, bolts, and to a lesser degree, threads. Kinda makes one think it doesn't matter...they'll all work just fine...

 

[AlphaHybrids]

They all work fine...when engineered correctly. That is the key point. You really can't just apply any of them and hope they will work.

Edward

 

[DeeRoc29]

"When engineered correctly" - absolutely. It's just that I was sure that I would use dowel pins until Mark's post...now I'm thinking snap rings would be better. In any case, I've learned a lot from this thread, so thanks to all who've posted.

Speaking of snap rings, which of these snap rings would be best for my project. Again, the ID of my case is approx 4.625" and the wall is .188.

Smaller - https://www.mcmaster.com/#cadinlnord/99142a690/=j5g0l4

or

Larger - https://www.mcmaster.com/#cadinlnord/99142a690/=j5g27e

I assume the deeper the snap ring groove, the better. At the Pc I'm expecting, I'd really like to leave at least .125" wall in the case, but that would make the groove only about 1/16th.

 

[Reinhard]

Make the groove at least half the depth of your casing wall thickness. If you overpressure it, and the snap ring won't let go, this will ensure that it rips off one end of your casing instead of splitting it open length wise. This will constrain most of the destructive force of such an event in the rockets longitudinal axis, reducing the risk from flying debris.

Reinhard

 

[MCS]

You also need to calculate the "rip" strength of the tube given the pin location.

The intuition is this.... if you put the pins 1/8" from the end of the tube, they may tear loose. If you put the pins 3" from the end of the tube, the potential to tear loose is much less. Even if the area of two pin configurations is equal, the number of pins and distance between pins is also a factor.

Im don't think I could write down an analytical solution to the question because it involves things like force concentration factors, but you should be able to find a chart somewhere on the Internet or in a MechE textbook.

(sorry my "statics" classes are wayyyyyy too far in the past to even try)

-->MCS


.

 

[AlphaHybrids]

Yes, you want to determine the correct end margin so that you avoid block shear. Typically I make the center of the pin at least 2.5 pin diameters from the end.

Edward

 

[ClayD]

For a neat nozzle, you need to put the GVR Rao calculations into excell.. Have it plot a CSV. and rename it to an SCR. Then start a spline, and list the SCR for the points. then close the spline.

I do this on my nozzles, then i have a lazer cut the shape in mild steel plate, and use the lathe to cut the shape. Not cnc, but poor mans shaped nozzle!

Hard part is, I have a hard time getting the gas dynamics correct enough to make efficiency of the (better shape)

I am interested in the dowl pin method.....
ANYONE have any pictures? if they are taught, could you thread the inside of the pin, and use a screw - pully to remove them?

 

[AlphaHybrids]

I am interested in the dowl pin method.....
ANYONE have any pictures? if they are taught, could you thread the inside of the pin, and use a screw - pully to remove them?

You might have a hard time threading a hard material like dowel pins, but Mcmaster has pins like this. How do you teach the pins? Sit them down and lecture?

Edward

 

[butalane]

For a neat nozzle, you need to put the GVR Rao calculations into excell.. Have it plot a CSV. and rename it to an SCR. Then start a spline, and list the SCR for the points. then close the spline.

I do this on my nozzles, then i have a lazer cut the shape in mild steel plate, and use the lathe to cut the shape. Not cnc, but poor mans shaped nozzle!

Hard part is, I have a hard time getting the gas dynamics correct enough to make efficiency of the (better shape)

I am interested in the dowl pin method.....
ANYONE have any pictures? if they are taught, could you thread the inside of the pin, and use a screw - pully to remove them?

I'll take some pictures tonight.

 

[north boy]

How do you teach the pins? Sit them down and lecture?

Edward

I think he means taught as in tight lol

 

[CarVac]

taut?

 

[ClayD]

You might have a hard time threading a hard material like dowel pins, but Mcmaster has pins like this. How do you teach the pins? Sit them down and lecture?

Edward

Yes, as hard of a time threading as teaching I would assume...

Although touting and professing are my prefered modus operandi...

This is what I originally was planning in my FC and Nozzle Carrier... For the same reasons as Mark stated I believed they may leave a few of the bolts fayed, and would be stressed when the others were forced into contact.

 

[ClayD]

So assuming your using the

what would you use to bore the hole? Obviously, you would have to leave the pin with the threaded portion outside the case wall.