Thrust Plates: When and Why?
- Thread starter Banzai88
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Yes, it is dead simple, but here you go. (The inside chamfer is to clear the flange on the outside of the Slimline where it hits the aft CR.)
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Neverrrrr 
Braden
Braden
tjsnakez
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Who needs thrust plates
Peaceful_Warrior
Member
I’m not sure if this helps anyone or not, but I’ve been building a custom 2” dia. rocket to fly up to “F” 29mm motors. I wanted to try a scratch build, so my objective was to use as many found items as possible. I designed and built a kind of thrust plate for the rocket by incorporating some old aluminum camera parts I had on hand. I used the motor core and fins from an old Estes Pro-Series 2 Booster kit. I then used JB Weld to epoxy all the components together. I think the custom “push plate” helps provide strength to the motor attachment because I made it to be inserted into the aft end of the rocket body in such a way that it butts against the bottom end of the rocket body. Being aluminum it should also help to protect the aft end of the rocket body tube from any damage caused by a hard parachute landing. It also serves an aesthetic purpose, as it makes for a nice trim for the aft end.
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The Mac Performance weighs 1.8-1.9 oz.
The SC Precision weighs 4.8 oz.
This made a difference for me. I love the SC precision, but for larger motors in the EAC Viper (build in my signature), the additional 4 oz. at the very tail of the rocket led me to use the Mac Performance. I'll save the SC Precision for a longer rocket.
BDB
Absent Minded Professor
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Contrary to my post from last year, I ultimately went with the thrust plate from Mac Precision. It's great!
kramer714
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Lemme understand this, are we worried about the motor pushing through the rocket? I use thrust plates for other reasons usually because i like the closeout and it is easy to make on the lathe, simplifies motor attachment, makes the aft end of the rocket very durable, looks cool.... having said that, just doing the math...
Suppose you have one of the new Aeroroni M10,000 75mm motor you know the 1 second burn M one....... uh huh.
And you accidentally forgot to add centering rings, you know like Ikea furniture you always end up with extra parts. (centering rings dont make good coasters, the coffee spills through the hole in the center)
And your fins only have a 6 inch chord, 3 of them, because you didn't have enough material to make 4, keeping the drag low for this bad boy, you are with me there....
Quick math, that works out to be 2,300 lbs of peak thrust (put the nose against the brick wall cant get higher than that kinda thrust)
so you have 3 fins, 6 inches long each, and like a doughnut they have 2 sides, so that works out to be...
3 fins x 2 sides X 6 inches = 36 inches of fillet.
Thrust per fillet inch is 2,300/36 or 53 lbs per inch of fillet. That is a 5 minute epoxy kind of number (if you mix it right... dont get me started...)
So if you use an actual motor say a M6400 with 7,500 N max thrust = 1,700 lbs of thrust. And lets say your fins are 10 inches long (still three of them) that drops the Thrust per fillet inch down to 28 lbs per inch....
Now add 2 marginal centering rings and assume they ONLY touch the body tube and the motor tube, no fin contact. They have a circumference of 12 inches each of bond-line. Opposite direction (perpendicular) but think about it, and assume you only fillet the 'top of these'. 1,700 lbs of thrust / 24 inches= 70 lbs of thrust / inch of bond-line.
I know the loads dont transfer evenly, but if you assume (3) 10 inch fins and (2)centering rings, assume the centering rings are only filleted on one side, you get. 60 inch of fin fillet and 24 inches of centering ring fillet = 84 inches of fillets to hold 1,700 lbs of thrust. That is 20 lbs of thrust per inch of fillet.
whew, so lets look at reality, if you had 2 centering rings and 3 fins BOTH with a reasonable bond....... yeah do you really need a thrust ring for load transfer do you?
Just saying....
Suppose you have one of the new Aeroroni M10,000 75mm motor you know the 1 second burn M one....... uh huh.
And you accidentally forgot to add centering rings, you know like Ikea furniture you always end up with extra parts. (centering rings dont make good coasters, the coffee spills through the hole in the center)
And your fins only have a 6 inch chord, 3 of them, because you didn't have enough material to make 4, keeping the drag low for this bad boy, you are with me there....
Quick math, that works out to be 2,300 lbs of peak thrust (put the nose against the brick wall cant get higher than that kinda thrust)
so you have 3 fins, 6 inches long each, and like a doughnut they have 2 sides, so that works out to be...
3 fins x 2 sides X 6 inches = 36 inches of fillet.
Thrust per fillet inch is 2,300/36 or 53 lbs per inch of fillet. That is a 5 minute epoxy kind of number (if you mix it right... dont get me started...)
So if you use an actual motor say a M6400 with 7,500 N max thrust = 1,700 lbs of thrust. And lets say your fins are 10 inches long (still three of them) that drops the Thrust per fillet inch down to 28 lbs per inch....
Now add 2 marginal centering rings and assume they ONLY touch the body tube and the motor tube, no fin contact. They have a circumference of 12 inches each of bond-line. Opposite direction (perpendicular) but think about it, and assume you only fillet the 'top of these'. 1,700 lbs of thrust / 24 inches= 70 lbs of thrust / inch of bond-line.
I know the loads dont transfer evenly, but if you assume (3) 10 inch fins and (2)centering rings, assume the centering rings are only filleted on one side, you get. 60 inch of fin fillet and 24 inches of centering ring fillet = 84 inches of fillets to hold 1,700 lbs of thrust. That is 20 lbs of thrust per inch of fillet.
whew, so lets look at reality, if you had 2 centering rings and 3 fins BOTH with a reasonable bond....... yeah do you really need a thrust ring for load transfer do you?
Just saying....
Last edited:
I use them , just cause I'm cool
cbrarick
Wildman CT
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reasonable. They are one of the best rocketry bling. Totally unnecessary, CG pushed the wrong way but waay cool to look at.I use them , just cause I'm cool
Especially when looking up at the aft end during boost!
ghuber
Greg Huber
I fly a ton of tube fins, so there are no through the wall fins to transfer load to the body tube, and apart from my L3, I just lasercut a plywood thrust plate glued to an interior diameter centering ring of the same material (my intuition is that this prevents bending of the thrust plate under load). My motor mounts are mostly removable, which I've learned to love for ease of cleaning very dirty fiberglass rockets after landing among onions at URFF. For J/K motors I've used 1/4" plywood, and for smaller rockets I use 1/8" plywood. For my L3 I used the aluminum SC Precision thrust plate (5 inch to 75mm) and it is very pretty and seems indestructible: https://scpconcepts.com/rocketry/thrust-plates/
I’d like to share how I am doing thrust rings integral to interchangeable motor mounts. I have motor mounts that are interchangeable in a 3” rocket for 29mm, 3x29mm cluster, 38mm and 54mm. The thrust ring component itself looks like this:
It is 3D printed with PAHT-CF filament which is high temperature nylon with embedded carbon fibers. It is extremely tough and light weight (even with an infill of 50%). I am using #8-32 brass threaded inserts for the top bosses as well as the side holes which are used to mount the ring into a fiberglass body tube using stainless steel bolts. There are three mounting bolts and the other holes are tooling holes for a fixture to assure all motor mount are concentric and in-line when assembled. On the backside view, you can see a step which is there as a stop for a cardboard motor tube that is glued in. The large bosses are for mounting a 1/16” thick aluminum retaining plate to hold the motor(s) in during motor ejection (which I no longer use so the plate is somewhat superfluous).
The attached shows the complete motor mount and multiple views showing how it all works. The wide centering “ring” is printed with PETG and a very low infill so it is extremely light. Being wide like that means it is perfectly concentric and cannot be tilted. The rectangular slots are for clearing the fin tabs as the motor mount is slid into the rocket. The rounded features are to clear the rivet nuts for rail buttons.
I have flown this configuration multiple times including a 3x29mm cluster and single motors up to an I195. This weekend I will fly it with a 38mm J350. Launch plans included 54mm K535 and L1000.
I have ground tested this approach and calculated forces/strength as best I can and, so far so good. Note: I wouldn’t use this on a cardboard body tube unless it was epoxied in.
It is 3D printed with PAHT-CF filament which is high temperature nylon with embedded carbon fibers. It is extremely tough and light weight (even with an infill of 50%). I am using #8-32 brass threaded inserts for the top bosses as well as the side holes which are used to mount the ring into a fiberglass body tube using stainless steel bolts. There are three mounting bolts and the other holes are tooling holes for a fixture to assure all motor mount are concentric and in-line when assembled. On the backside view, you can see a step which is there as a stop for a cardboard motor tube that is glued in. The large bosses are for mounting a 1/16” thick aluminum retaining plate to hold the motor(s) in during motor ejection (which I no longer use so the plate is somewhat superfluous).
The attached shows the complete motor mount and multiple views showing how it all works. The wide centering “ring” is printed with PETG and a very low infill so it is extremely light. Being wide like that means it is perfectly concentric and cannot be tilted. The rectangular slots are for clearing the fin tabs as the motor mount is slid into the rocket. The rounded features are to clear the rivet nuts for rail buttons.
I have flown this configuration multiple times including a 3x29mm cluster and single motors up to an I195. This weekend I will fly it with a 38mm J350. Launch plans included 54mm K535 and L1000.
I have ground tested this approach and calculated forces/strength as best I can and, so far so good. Note: I wouldn’t use this on a cardboard body tube unless it was epoxied in.
