Minimum diameter motor retention

[terryg]

I am finally getting around to finishing a 38mm carbon fiber mongoose that i purchased years ago. I want to use a minimum diameter motor retainer for it and fly both Aerotech and Loki cases with it. Aerotech cases have 5/16" threads and Loki cases use 1/4" threads. Is there an easy way you folks have used to adapt from one to another?

 

[SpaceX76]

https://www.ebay.com/itm/1632203246...nlc6iTc5XntNhsaetK-rbsQ5ttsxn89RoC38kQAvD_BwE


I don't know how strong theses are but it is probably the easiest solution, or something similar. Assuming you are doing a pugged motor with an AreoPack

 

[NateB]

I thought about the same and then realized with the snap ring case it would be difficult to thread and unthread onto a retainer. I'm in the slow process of building a minimum diameter Darkstar Mini and decided to pull the retainer all together, attach the shock cord to the eyebolt on the motor, and use tape for retention. If the tape would fail, the shock cord will keep everything together.

 

[DEmery]

attach the shock cord to the eyebolt on the motor, and use tape for retention. If the tape would fail, the shock cord will keep everything together.

This is what I do.

 

[Art Upton]

Dr. Kosdan used masking tape on an O motor in the desert, I know it's old school but if done correctly it works.

Also, can you put in an Eyebolt forward closure on the motor? I know you can get AT ones that are threaded for a bolt.

 

[manixFan]

I have flown many MD 38mm rockets using a variety of retention methods:

• If I want to use a threaded rod with a heavier case, I'll glue bulkheads on both sides of a short, reinforced coupler tube with a hole large enough for the largest diameter rod and far enough forward to allow for the longest motor I'll fly (typically the Loki K627), then cut threaded rod just long enough to just pass through the upper bulkhead and securely hold a nut with a lock washer. The recovery harness also attaches to the bulkheads
• If I don't want to glue in a bulkhead, I'll use the technique Adrian (Featherweight) showed awhile back, using two pressure test plugs to grip the inside of the body tube
• For lightweight cases (like CTI), I'll use friction fit (masking tape on motor) with aluminum tape around the bottom (leave an unpainted 1/2" strip on the body tube for the tape to grab)

Sometimes with CTI cases I'll remove the ejection charge, scuff up the well, and JB weld a 1/4" nut to hold a threaded rod. I use aluminum rod and nuts to keep weight to a minimum. You'll have to cut a rod to the right length for each different motor case, but with aluminum rod it's very easy to do.

Good luck,


Tony

PS: I would like to meet the person at Aerotech who picked the 5/16th bolt size for the 38mm closure and give them a stern taking to. There is only one reload I'll fly in AT cases because of that - the J510W.

Follow up on 5/16" – I haven't tried this yet, but it be easier to use than a male-male adapter:

https://www.oreillyauto.com/detail/c/russell/russell-5-16-x-1-4-inch-bushing-reducer/rus0/661563

 

[manixFan]

To follow on to my above post, below is a photo of the retainer method I got from Adrian Adamson. It takes two of the pressure test fittings to get the right parts, but it's very easy to use and can obviously be moved from rocket to rocket since it stays with the motor. The motor shown is the Aerotech with the 5/16th bolt size. I wouldn't use it with a cardboard tube rocket unless fiberglassed. (EDIT) So far, I've only used it with 38mm MD rockets and I don't know if parts are available for larger diameters.

To use, you tighten the bolt and compress the large rubber washer until it just barely fits into the motor tube. Slide the motor into the rocket and then turn to compress the washer against the sides of the motor tube. Once tight, it is basically impossible to move the motor. I've thought about also attaching the recovery harness to the bolt as well, but haven't yet. But I will try it on my next 38mm MD build.


Tony

design credit to Adrian Adamson of Featherweight – @Adrian A

 

[WoShuGui]

attach the shock cord to the eyebolt on the motor, and use tape for retention. If the tape would fail, the shock cord will keep everything together.

Exactly…

 

[Highpowerrocketyahoo]

Dr. Kosdan used masking tape on an O motor in the desert, I know it's old school but if done correctly it works.

Also, can you put in an Eyebolt forward closure on the motor? I know you can get AT ones that are threaded for a bolt.

This. Done properly, very effective.

 

[manixFan]

This. Done properly, very effective.

I wholeheartedly agree. Masking tape friction fit is all I did for many years when I started flying MD rockets. But in the last few years, RSOs have been more and more reluctant to approve it for flight. But if I show up with a nice, shiny band of aluminum tape around the end, I get a lot less pushback. I also learned the hard way that the tape I used decided it liked the paint more than the rocket did, so now I leave an unpainted band at the bottom.


Tony

 

[Highpowerrocketyahoo]

I wholeheartedly agree. Masking tape friction fit is all I did for many years when I started flying MD rockets. But in the last few years, RSOs have been more and more reluctant to approve it for flight. But if I show up with a nice, shiny band of aluminum tape around the end, I get a lot less pushback. I also learned the hard way that the tape I used decided it liked the paint more than the rocket did, so now I leave an unpainted band at the bottom.


Tony

As an RSO you can look at a rocket and tell if someone knows what they are doing. Ask a couple questions and feel good knowing they know how to friction fit the right way.

One of the most important thibgs an experienced flyer can do is mentor someone on the spot. If there is mutual respect most people will take this well.

An RSO asked me today if I was comfortable with the fit of my nosecone on my HOJO. I thought about it for a sec, reevaluated and sanded it a bit and came back and had a great flight. He was respectful, and right.

Rocketry is a great hobby because of the great people.

 

[Conway Stevens]

I have zero issues friction fitting any motor 29mm up to 98mm O motors. I always connect my harness to the motor so it's not going anywhere anyways. But done right completely acceptable.

I have made multiple minimum diameter retainers /anchor points for various sized rockets. Usually I've used a small section of coupler. 1 or 2 inches long. Then I use an av bay stepped bulk plate that fits the coupler. I use a short section of all thread in the forward closure of the motor attach the coupler with bulkplate to the top with nut, washer and lock washer. Slide the assembly inside the body of the rocket. Measure where it's at and drill 3 equally spaced holed around the body through the coupler section. Then I pull out the motor with coupler install rivet nut inserts that are low profile into the holes of the coupler. file them flush reinstall line up the holes. Install set screws. Now tighten the nut onto the all thread and it pulls the motor tight. Through the bulk he'd in the retainer I drill a hole make a loop of kevlar and install through the hole and knot it to not pull out. Now you have an attachment point.

 

[bjphoenix]

I was thinking about this subject and thinking it should be easy, but any motor retainer that goes over the back end of the engine would stick out far enough to defeat the purpose of minimum diameter.
I have an Aerotech 38mm motor and I'm thinking you put it in the end of the 38mm airframe and let the end closure bear on the end of the airframe, you don't even need an engine block in the front.
So I was wondering if you could take a piece of thin kevlar, put a loop in the end of it, put that loop over the nozzle, let the 2 kevlar leads go forward through the tube and attach them to something near the front end of the airframe. I guess you would need just enough gap between the Aerotech motor and the airframe for pieces of kevlar to fit in there. I should check my 38mm mounts to see how much gap there is. Actually I suppose a person could use a piece of stainless steel fishing leader for the same thing.

 

[Conway Stevens]

I was thinking about this subject and thinking it should be easy, but any motor retainer that goes over the back end of the engine would stick out far enough to defeat the purpose of minimum diameter.
I have an Aerotech 38mm motor and I'm thinking you put it in the end of the 38mm airframe and let the end closure bear on the end of the airframe, you don't even need an engine block in the front.
So I was wondering if you could take a piece of thin kevlar, put a loop in the end of it, put that loop over the nozzle, let the 2 kevlar leads go forward through the tube and attach them to something near the front end of the airframe. I guess you would need just enough gap between the Aerotech motor and the airframe for pieces of kevlar to fit in there. I should check my 38mm mounts to see how much gap there is. Actually I suppose a person could use a piece of stainless steel fishing leader for the same thing.

Minimum diameter retainers are not done at the rear/aft closure of the motor. They are done at the forward closure on top of the motor. Inside the body tube just above the motor.

 

[James Owen]

Apogee components has a video on minimum diameter motor retention with the laserLoc, they mentioned a few methods. The MD I’m working on right now will use one of the methods they mentioned.

 

[bad_idea]

So I was wondering if you could take a piece of thin kevlar, put a loop in the end of it, put that loop over the nozzle, let the 2 kevlar leads go forward through the tube and attach them to something near the front end of the airframe. I guess you would need just enough gap between the Aerotech motor and the airframe for pieces of kevlar to fit in there. I should check my 38mm mounts to see how much gap there is. Actually I suppose a person could use a piece of stainless steel fishing leader for the same thing.

I've seen steel rope buried in the fin fillets used to loop around the thrust ring of a motor with a bit of tension. I think Apogee has a video about that method. Someone at some launch I attended in the past couple of years was doing that, with two loops, one on either side of the motor on a four finned rocket. Not sure I'd trust that method much when motor eject was used on cases that didn't have something like the wrench notches of Aerotech's Hobbyline cases. Also not sure why I'd use that over friction fit. I'd trust it more with a loop long enough to go around the nozzle I suppose, but that could restrict motor choices a lot.

A kevlar loop around the nozzle, I've seen several times in low-power rockets though not in minimum diameter. I think I've always seen it going up another tube alongside the motor tube, but there's probably enough slack in most tubing to squeeze some thin kevlar alongside a motor case. I'd protect that kevlar ahead of the motor with something if flying with motor eject. Edit to add: I'd only use this on small, light rockets with small motors. Otherwise I'd go with friction fit or one of the other methods mentioned here.

 

[Reinhard]

To follow on to my above post, below is a photo of the retainer method I got from Adrian Adamson. It takes two of the pressure test fittings to get the right parts, but it's very easy to use and can obviously be moved from rocket to rocket since it stays with the motor. The motor shown is the Aerotech with the 5/16th bolt size. I wouldn't use it with a cardboard tube rocket unless fiberglassed. (EDIT) So far, I've only used it with 38mm MD rockets and I don't know if parts are available for larger diameters.

To use, you tighten the bolt and compress the large rubber washer until it just barely fits into the motor tube. Slide the motor into the rocket and then turn to compress the washer against the sides of the motor tube. Once tight, it is basically impossible to move the motor. I've thought about also attaching the recovery harness to the bolt as well, but haven't yet. But I will try it on my next 38mm MD build.


Tony

design credit to Adrian Adamson of Featherweight – @Adrian A
View attachment 584206

Custom sizes can easily be made with a 3d printer and o-rings.

This particular design relied on 4 smaller bolts to clamp the halves together and was a bit inconvenient to assemble/disassemble within a 75mm airframe, but it kept the motor safely retained.




Reinhard

 

[SolarYellow]

I could see doing safety wire through holes at the trailing edge root of composite fins and around the nozzle. (Wouldn't trust wood fins to not pull out over the long haul.) Would be the most miniscule aero mess on a part that's already very messy. But clean safety wire is sexy. It would be especially clean if the rear closure had notches in the perimeter, like AT Hobbyline cases. Of course, the two notches 180 degrees apart kinda points to four fins, which some people don't like.

 

[Sooner Boomer]

At some point in the distant future, I'm going to build a 54mm min. dia. (based on what I call my Design #2). The plan is to embed a standoff threaded for either #6 or #8 in the aft end of the fin root. The standoffs will probably get some gentle reshaping with a file, and a touch of epoxy putty to blend them into the fin root and make them more aerodynamic/appealing. A machine screw with a small washer should be sufficient for retention. Currently, I'm using #6 standoffs in both my 29mm and 38mm powered design #2. The standoff just happens to be exactly the right size to fill the gap between the outside of a 38mm motor tube and the inside of a 54mm body tube.

 

[terryg]

I am going to go with friction fit (at least initially). It will be cheaper and it also allows for shock cord maintenance.

 

[terryg]

How do you recomend friction fitting minimum diameter motors? (some thing I have not done in many, many years LOL).

 

[Jmhepworth]

I use a wrap of aluminum tape to tape the aft end of the motor case (the aft closure, really) to the aft end of the rocket.

 

[Buckeye]

I use a wrap of aluminum tape to tape the aft end of the motor case (the aft closure, really) to the aft end of the rocket.

This^

Just wrapping the hell out of the motor case with masking tape and cramming it into the rocket will not always contain the motor. You need the wrap.

 

[bclark989]

Probably the same thing everyone else is doing, but I made a handy MSPaint diagram!

1. Tape over the motor casing near the bottom, but forward of the thrust ring if there is one.
2. Install the case so it fits very snugly inside the air frame. If it isn't a huge pain to install, use more tape!
3. Once its in, wrap a piece of tape that overlaps the aft end of the air frame and the thrust ring. If you are using a research case that doesn't have a ring, you can either make one (with more tape!) or pinch the tape onto the smaller OD of the case (because it doesn't need to be pretty, it just needs to work). Some folks just skip this step, as the internal friction fit should be super tight, and you should also be anchoring your harness to the forward closure of the motor case, so even if it comes out, it isn't coming down separately.

 

[kjhambrick]

my $0.02 worth ...

bclark989's method always worked for me -- as long as there was at least 1/4 to 3/8 inch of airframe aft of the fins so I could neatly wrap the airframe + motor with several layers of tape.

-- kjh