[Motor Purchased] 5 inch diameter rocket and a CTI O3400.

[ColumbiaNX01]

I suggest using all 12 holes in the motor retainer, not just three. That's a lot of motor mass and the T-nut threads can strip.

There are 3 mounting holes in the thrust plate. I attched 3 t-nuts and then glued over them. I am using the same technique GaryT used. Once I am ready to attach the retainer I will glue in all 4 center rings and then glue thrust plate on (I have no reason to remove it). With everything glued in place with the t-nuts its not going anywhere.

 

[Yoehahn]

Sir, I just used 5 minute epoxy to tack fins down in place!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Yes, I will be making fillets where the fin touches the motor tube, fillets where the fin enters the airframe, external fillets using proline 4500. Each fin will have 6 fillets. I am using us compsites epoxy with chop carbon fiber mixed in to make internal fillets. Once that is done I am going to foam the whole fin can!!!!!!!!!!!!!

I still wouldn't have used the 5 min, it started failing me on a 4" DS just on landing. When I tacked on fins, I did em one at a time with the proline and a jig cut out of a cardboard box and haven't had any problems. It takes a lot longer, but this is serious stuff we are dealing with here. 5 min epoxy and CA glue is good up to a J in my opinion (not ideal however). Best of luck with the project

 

[ColumbiaNX01]

Thanks!!!

 

[JohnCoker]

There are 3 mounting holes in the thrust plate. I attched 3 t-nuts and then glued over them. I am using the same technique GaryT used. Once I am ready to attach the retainer I will glue in all 4 center rings and then glue thrust plate on (I have no reason to remove it). With everything glued in place with the t-nuts its not going anywhere.

The problem is that T-nuts threads can strip. I once had a failure when using four of the holes with T-nuts. There must have been a strong jerk when the drogue or main deployed and the M1939 stripped the threads on the four T-nuts. I now use only stainless steel T-nuts from McMaster-Carr, but I'd still recommend more than three.

 

[ColumbiaNX01]

I am using 3 stainless t-nuts from lowes. The 3 t-nuts attach the thrust plate to aft end. Thrust plate is threaded and I use screws provided with the reatainer to attach reatainer to thrust plate. A 5 inch thrust plate with a 98mm motor hole only comes with 3 mounting holes. Do u understand? Also thrust plate is glued on aswell like a center ring.

 

[CarVac]

I am using 3 stainless t-nuts from lowes. The 3 t-nuts attach the thrust plate to aft end. Thrust plate is threaded and I use screws provided with the reatainer to attach reatainer to thrust plate. A 5 inch thrust plate with a 98mm motor hole only comes with 3 mounting holes. Do u understand? Also thrust plate is glued on aswell like a center ring.

John: I think he's using all 12 screws to attach the retainer to the thrust plate, but the thrust plate itself only has 3 (much larger) holes for mounting it to the centering ring.

It should be fine, but I wouldn't use any components from Lowes on a project of this magnitude. Just sayin'.

 

[ColumbiaNX01]

Most hardware like I bolts, quick links, screws, ect... I have purchased at lowes. When I do I get Stainless Steel. So I dont know about the regular hardware. So what is done is done!!! Anyone have a suggestion on how I can improve my build techniques and design please let me know, but what is already built is already built. I cannot go back.

Thanks

 

[dixontj93060]

It should be fine, but I wouldn't use any components from Lowes on a project of this magnitude. Just sayin'.

There is no appreciable difference between standard stainless steel hardware whether purchased at Lowe's, Menards, McMaster-Carr, etc.

 

[CCotner]

you guys do know that, in general, stainless steel is about half of the strength of normal alloy steel? Stainless steel generally isn't strong, just corrosion resistant.

https://www.mcmaster.com/#socket-head-cap-screws/=q1to6r

Click on alloy steel socket head cap screws-170,000PSI
Click on stainless steel socket head cap screws-70,000PSI

 

[dixontj93060]

you guys do know that, in general, stainless steel is about half of the strength of normal alloy steel? Stainless steel generally isn't strong, just corrosion resistant.

https://www.mcmaster.com/#socket-head-cap-screws/=q1to6r

Click on alloy steel socket head cap screws-170,000PSI
Click on stainless steel socket head cap screws-70,000PSI

I wasn't making that point, but certainly true. My point is that standard stock hardware, stainless or not, is pretty much the same anywhere it is sourced.

 

[CCotner]

I wasn't making that point, but certainly true. My point is that standard stock hardware, stainless or not, is pretty much the same anywhere it is sourced.

Yes, agreed, though for critical (meaning margin of safety of less than 100% or so) joints I like to see spec sheets. But for everything in hobby rocketry basically, hardware is hardware.

 

[ColumbiaNX01]

Very good information!!!!!

 

[bobkrech]

op: You'd think they're building an Abrams tank.

Estimated pad weight: ~75 pounds.

Max thrust load: <+1,100 pounds.

Max acceleration: ~+17 G.

Estimated burnout weight: ~50 pounds.

Really early or late 100 G deceleration main deployment: -5,000 pound max shock load.

Well bonded 6 mm 11-ply Aircraft Grade Finnish Birch Bulkheads should be sufficient.

Anything stronger/heavier is either insurance or ballast. :blush:

FWIW (3) x 10-24 screws in flanged retainer in 1/4 ply: ~1800 pounds failure.

Max deceleration: ~-36 G.

https://www.pro38.com/products/pro98/motor/MotorData.php?prodid=21062O3400-P

https://rocketmaterials.org/datastore/hardware/Motor_Retainers/index.php

 

[CCotner]

op: You'd think they're building an Abrams tank.

Estimated pad weight: ~75 pounds.

Max thrust load: <+1,100 pounds.

Max acceleration: ~+17 G.

Estimated burnout weight: ~50 pounds.

Really early or late 100 G deceleration main deployment: -5,000 pound max shock load.

Well bonded 6 mm 11-ply Aircraft Grade Finnish Birch Bulkheads should be sufficient.

Anything stronger/heavier is either insurance or ballast. :blush:

FWIW (3) x 10-24 screws in flanged retainer in 1/4 ply: ~1800 pounds failure.

Max deceleration: ~-36 G.

https://www.pro38.com/products/pro98/motor/MotorData.php?prodid=21062O3400-P

https://rocketmaterials.org/datastore/hardware/Motor_Retainers/index.php

Agreed completely, that's why I said that for virtually all hobby rocketry it didn't matter. Even on Bare Necessities there were only three "critical joints" by my definition.

 

[ColumbiaNX01]

You'd think they're building an Abrams tank.

Estimated pad weight: ~75 pounds.

Max thrust load: <+1,100 pounds.

Max acceleration: ~+17 G.

Estimated burnout weight: ~50 pounds.

Really early or late 100 G deceleration main deployment: -5,000 pound max shock load.

Well bonded 6 mm 11-ply Aircraft Grade Finnish Birch Bulkheads should be sufficient.

Anything stronger/heavier is either insurance or ballast.

FWIW (3) x 10-24 screws in flanged retainer in 1/4 ply: ~1800 pounds failure.

Max deceleration: ~-36 G.

So your saying I am doomed for failure?

 

[bill2654]

With 6 fillets per fin, you are plenty good. And 3 screws @ 1800lbs your motor ain't going nowhere.

 

[ColumbiaNX01]

If it worth anything the 3 screws are not 10-24 they are 10-32!!

 

[H_Rocket]

So your saying I am doomed for failure?

No, Bob is saying - that you are way over designing this (and for the most part we are egging you on). Don't feel singled out, it is a very common thing in the hobby. I know I can raise my hand if asked "have you gone way past common sense in building a rocket?".

The problem is that T-nuts threads can strip. I once had a failure when using four of the holes with T-nuts. There must have been a strong jerk when the drogue or main deployed and the M1939 stripped the threads on the four T-nuts. I now use only stainless steel T-nuts from McMaster-Carr, but I'd still recommend more than three.

This is why I actually put a bead of epoxy down when I use the flanged Aeropacks - and - tie the recovery harness to an eye-bolt at the top of the motor case.

Also, I have decided that going forward, I will use screws and nuts to install the flanged Aeropacks. Of course this kind of necessitates external fin can assembly.

 

[dixontj93060]

The problem is that T-nuts threads can strip. I once had a failure when using four of the holes with T-nuts. There must have been a strong jerk when the drogue or main deployed and the M1939 stripped the threads on the four T-nuts. I now use only stainless steel T-nuts from McMaster-Carr, but I'd still recommend more than three.

View attachment 157965

Why would T-nuts, which typically have twice as much thread length as a standard nut, be more prone to stripping? Seems like the extra thread length would be a benefit, not detriment.

 

[CarVac]

Why would T-nuts, which typically have twice as much thread length as a standard nut, be more prone to stripping? Seems like the extra thread length would be a benefit, not detriment.

Extra thread length is useless. Because bolts (and/or nuts) deform over the length of the interface, extra threads only mean it takes more energy (not more force) to break them: the first thread takes most of the load and if it breaks, the rest will fail by cascade.

 

[dixontj93060]

Extra thread length is useless. Because bolts (and/or nuts) deform over the length of the interface, extra threads only mean it takes more energy (not more force) to break them: the first thread takes most of the load and if it breaks, the rest will fail by cascade.

Oh, so a single thread is good enough?! Ridiculous and wrong. The bottom thread bears the most, but the other threads help. Taken from the Fastenal Thread Design Guide:

The axial distance through which the fully formed threads of both the nut and bolt are in contact is called the length of thread engagement. The depth of thread engagement is the distance the threads overlap in a radial direction. The length of thread engagement is one of the key strength aspects and one of the few which the designer may be able to control.

 

[Rocketjunkie]

Assuming the nut and screw have the same strength, if you have enough (in spec) threads for engagement greater than the diameter of the screw, the screw will break off before the threads strip

 

[bobkrech]

That's the 5 good thread rule which in practical terms means the strength increases up to the depth that equals the diameter of the screw.

example: 5 threads of a 1/4-20 = 1/4"

 

[JohnCoker]

you guys do know that, in general, stainless steel is about half of the strength of normal alloy steel? Stainless steel generally isn't strong, just corrosion resistant.

Very interesting. On the Wikipedia page, it lists "structural steel, ASTM A36" as having a yield strength of 250MPa. McMaster-Carr sells T-nuts in two grades of SS, 316 and 18-8 (plus zinc-plated steel, no designation). This page lists 316 SS as having a yield strength of 205MPa (min). Anyone know what strength measurement is most important here?

All that said, we don't know what material the generic T-nuts are made of, but given my experience I don't think it's a very high grade of steel.

I used to use generic T-nuts (zinc-plated steel from local hardware store) without thinking about it too much. Occasionally I'd get a mis-threaded one and have to chase out the threads with a tap, but generally they seemed fine. When I had the screws tear out the threads, I switched to stainless ones, for three reasons:

• sometimes corrosion resistance is important
• I'd hoped they would be stronger
• I expected them to be of a higher quality

Note that the four T-nuts that the screws pulled out of were generic ones from OSH and the screws were 316 stainless button head screws. The screws suffered no apparent damage so a naive assessment is that the screws were significantly stronger than the T-nuts.

 

[ColumbiaNX01]

View attachment 158060View attachment 158061View attachment 158062

Proline Epoxy 4500 fillets!!!!

 

[JohnCoker]

This is why I actually put a bead of epoxy down when I use the flanged Aeropacks - and - tie the recovery harness to an eye-bolt at the top of the motor case.

I haven't tried using epoxy to attach the flanged retainers, but I agree that retaining from the forward end of the motor is preferable, at least for large motors.

 

[lmt56]

View attachment 158060View attachment 158061View attachment 158062

Proline Epoxy 4500 fillets!!!!

Looks good.

 

[CCotner]

View attachment 158060View attachment 158061View attachment 158062

Proline Epoxy 4500 fillets!!!!

Back on topic for the thread... gorgeous fillets! But it's the inside ones that count, and that are... entertaining, to make. Gotta show us those too! =)

 

[CCotner]

Very interesting. On the Wikipedia page, it lists "structural steel, ASTM A36" as having a yield strength of 250MPa. McMaster-Carr sells T-nuts in two grades of SS, 316 and 18-8 (plus zinc-plated steel, no designation). This page lists 316 SS as having a yield strength of 205MPa (min). Anyone know what strength measurement is most important here?

All that said, we don't know what material the generic T-nuts are made of, but given my experience I don't think it's a very high grade of steel.

Yield strength is a pretty good metric, but when buying from mcmaster or MSC or the like, you can usually get the actual tested tensile and shear yield strengths (in force, not stress) of the hardware you're comparing.

The strength of stainless varies a lot by composition and so does steel; I'm not an expert and I don't want to pretend that I am. However, most of the time when I see bolts and stuff on Mcmaster, the 18-8 stainless is the weakest, the 316 is a little better, the cheap steel screws are about the same or a little wore, and the alloy steel ones (that meet all the ASTM and MIL specs) are 1.5 to 2x stronger

My rule of thumb (that my machinist mentor taught me) is that if it doesn't advertise it's specs (like the generic hardware of note, or buying metal without test reports) it's because the specs are crap. It's entirely possible that generic stainless steel has to be made under greater control than generic steel, and so generic 316 or 304/18-8 is stronger than generic cheap steel.

 

[mikec]

Note that the four T-nuts that the screws pulled out of were generic ones from OSH...

This paper, "Holding strength of T-nuts in solid wood and wood composites", says that for all the tests they did on a variety of woods and T-nut sizes, either the T-nut ripped through the wood material or the T-nut flange folded up -- I don't see any indication that they ever stripped a T-nut. Of course, the paper doesn't go into great detail about where the T-nuts were procured from, and I could imagine that the cheap tin-plated ones are worse than what they tested.

https://www.agriculture.purdue.edu/fnr/faculty/eckelman/documents/d199856a_000.pdf