# Question on Big Bertha conversion

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#### NewEntity1

##### Well-Known Member
Now that I have spare parts, I actually opened my Big Bertha package for the first time. I noticed that the motor mount tube in it is very, very long...215mm long (around 8 1/2"). It also has no thrust ring, rellying on just the motor clip and friction in the mount tube itself to prevent the engine from shooting up through the rocket.

The 24mm mount I'm going to use (from fliskits, with those excellent fiberboard centering rings ), is much shorter. However, since it will be a larger motor, I think there should still be enough ejection gasses to fill the larger open volume left by the shorter mount. However, I am a bit concerned about structural rigidity, and CG.

There are two things I can do:
1) Epoxy the entire length of the BT60 body tube, which would increase the structural strength, and also move the CG forward slightly. The added weight won't be a problem, since this is a rocket already designed to fly on 18mm motors, so the 24mm mount should handle the added weight easily.

2) Hold off on this project (yet again), until I can pick up some BT50 tubing, and find a reliable way to shove the 24mm thrust ring waaay down the tube...well, actually that should be easy enough...I'd just install the motor clip first, then push the thrust ring down until it hits the clip

Which do you think is better?

#### sandman

##### Well-Known Member
The Estes Mean Machine is 4 x BT-60's (18" each) long!

That ejects it's parachute just fine.

I use a short stuffer tube in a regular BB anyway.

In other words...build it with the short tube and put a motor block in it.

Only change to make is to add a "leader" attached to the motor mount (Kevlar or I've even used stainless fishing leader) to attach a longer shock cord to.

sandman

#### NewEntity1

##### Well-Known Member

I've built the fliskits 24mm mount for the modified Big Bertha now...its curing at the moment.

I chose to put the rear centering ring in the position suggested in the Estes BB's instructions (6mm from the end) instead of 1" from the end like suggested in the instructions that came with the mount. I'm also periodically flexing the mounting clip to make sure it doesn't cure onto to the surface of the mounting tube...would be kinda hard to get the motor if it did, then

#### NewEntity1

##### Well-Known Member
I have 2 yards of 1/4" nylon shock cord on order from Hobbylink, along with a loc 18" nylon parachute.

I was going to use 3 feet (half) of the nylon shock cord for the Bertha along with the parachute. Would the nylong be able to stand up to the heat of being attached directly to the motor mount when using BP motors, or do I really need kevlar or the stainless steel leader suggested?

If I use stainless steel leader, how do I realiably attach the shock cord to the leader? It won't do me much good to have a fireproof leader if the shock cord separates from it on ejection

#### sandman

##### Well-Known Member
The steel leader I've used has a loop at one end and a clip on the other. I get 'em at the local bait and tackle store.

I attach the clip end perminently at the motor mount and tie the shock cord on the loop.

I've never used nylon shock cord on a low or mid powered rocket. I just buy some 1/4" elestic cord at a fabric stor. It's only about .15/yard. sandman #### NewEntity1 ##### Well-Known Member I decided to take your advice, Sandman. Mostly. I bought some 30lb test steal fishing leader, designed for catfish. I cut off the clip at the end however, and fed the loop through a slit I cut in the centering ring. I then fed a sliver of balsa wood through the loop and pulled the loop up tight against centering ring, locking it in place. I then epoxied the sliver of balsa against the centering ring and the mount tube to hold it in place. What I'm describing might be hard to visualize, so I've including a picture... The harder the lead is pulled, the harder the balse sliver is driven against the centering ring...thus making for a self-tightening connection, and no chances of zippers on the motor mount #### sandman ##### Well-Known Member Well, if it's for catfish she should be plenty strong! Tie a loop in the shock cord and just clip it on. This way you can change the shock cord out after 40 or 50 flights and put a new one on. sandman #### NewEntity1 ##### Well-Known Member I just test fit a spent motor casing, and I have a bit of a problem...the motor clip is up too high. This can be seen in the picture above, as well. I had a bugger of a time getting the casing back out. I may have to abandon this as "Prototype-A" #### sandman ##### Well-Known Member NewEntity1, Lemme see! no pic! OK, don't do anything 'til I see it! sandman #### NewEntity1 ##### Well-Known Member It's the picture in the reply where I first showed the leader attached. It's a jpg and shouldn't be a problem to download. If your having trouble downloading it, I can upload it to my website, and provide a link. At any rate, I decided to sand down the length of the mount tube about 2 mm. This *might* give me enough room to get the motor out. I can't really sand the tube down much further than that though, since I put the rear centering ring so close to the end. LINK to custom mount #### KenParker ##### Well-Known Member Originally posted by NewEntity1 I just test fit a spent motor casing, and I have a bit of a problem...the motor clip is up too high. This can be seen in the picture above, as well. I had a bugger of a time getting the casing back out. I may have to abandon this as "Prototype-A" What do you mean by "the motor clip is up too high"? If you mean that you have the centering ring so close to the end of the tube, and there is no slot in the centering ring to let the motor clip move..... then simply drill a hole in the centering ring where the motor clip goes through it, then use an exacto to square it up, so that you can bend the clip up to get the motor out. #### NewEntity1 ##### Well-Known Member No, thats not what I mean. The clip moves fine. Up too high means the motor doesn't overhand the end. #### sandman ##### Well-Known Member Don't worry about getting the motor tube flush with the centering ring if you have to. THAT's what we call a "Tulanko Tail" anyway! That won't hurt anything. sandman #### NewEntity1 ##### Well-Known Member Well, I don't think I'll need to sand it down that far I think the 2mm I took off will be enough to get the motor off, at least with a pair of plyers Btw...thanks for all the replies, folks. I'll keep ya'll posted on its progress, and try to post more pics, too #### Steward ##### Well-Known Member Careful guys...you scared me for a moment...!!! I'm building a modifed BB myself...and I've been keeping an eye on this thread....LOL #### NewEntity1 ##### Well-Known Member This morning I woke up having thought of an even better way to attach the steel leader...wrap the leader around the mount tube below the top centering ring, then feed the loop built into one end of the leader through the loop built into the other end, then feed the loop up through a notch cut into the top centering ring. It should be a notch in the centering ring that is flush with the mount tube and not a slit...otherwise with this construction method, the leader would end up zippering down the the mount tube anyways. With this aproach, the load on the leader would be distributed clear around the circumference of the motor mount tube. I'll take pictures demonstratiing this idea when I get around to building the Executioner's motor mount . #### sandman ##### Well-Known Member This morning I woke up having thought of an even better way to attach the steel leader...wrap the leader around the mount tube below the top centering ring, then feed the loop built into one end of the leader through the loop built into the other end, then feed the loop up through a notch cut into the top centering ring. It should be a notch in the centering ring that is flush with the mount tube and not a slit Ahhh, yea...that's the way you are supposed to do it...I thought somebody said that already. I always do a wrap or two of the nylon reinforced packing tape ( you know the stuff that's impossible to tear) just under the leader. sandman #### NewEntity1 ##### Well-Known Member Ahhh, yea...that's the way you are supposed to do it...I thought somebody said that already. You probably did, and I just didn't understand ya...I can be a little thick, sometimes Meanwhile...the project is progressing nicely. The motor mount is epoxied into the body tube, and the first two fins are epoxied in place, the second one still curing. I originally tried to just put yellow glue on the fin roots, but since the Bertha uses surface mounting, they kept slipping out of alignment even with the Estes guide-thingie. The epoxy held them in place much better even with the epoxy still wet. Once all four fins are in place and cured, I'm going to add epoxy fin-fillets as well. I took a couple pictures of the progress, but since I lost the software installation disk that came with my camera several years ago (and replacement downloads are no longer available), I have to swap in my old hard-drive each time I want to upload more pictures from the camera. Because of this, I wont be uploading any more pics until tomarrow. There are connectors out there that converts a USB port into an old RS232 port, but I've not seen any that go the other way around. My camera hardware requires an RS232 connection, so I'm stuck with the old software until I can get a new camera #### NewEntity1 ##### Well-Known Member Uh oh Um...guess I can't reach down into the tube and tie the elastic onto the snap swivel, after all Ok...design change: I'm going to cut the body tube down six inches, tie the elastic on... glue a coupler tube in, then re-attach another length of BT60 tube on top (stringing the elastic through it). Hmm, since I'm making this change anyways...I think I'll go ahead and make the rocket taller, attaching a longer length of tube than I cut off. An extra 3" sounds good . I'll also go ahead and attach a second length of elastic to the nosecone, attach *that* to the snap swivel as well, then tie the parachute in between both lengths of elastic. This change will make it easier to get the wadding in, anyways... Does this sound like a reasonable solution? #### TwoWalks ##### Well-Known Member Sounds to me like this rocket is going to have more elastic in it than my shorts. Just kidding: the changes sound like they would work. Will look forward to seeing how this turns out as I have a Big Bertha that I am going to begin building as soon as I decide on a cluster of C's or a single E. #### rocket trike ##### Well-Known Member Two walks I would go with the cluster. They are so cool. I have done a big bertha with a 3 18mm cluster. What a great rocket. I personally say there is no other way to go. Just my 2 cents. #### bmhiii ##### Well-Known Member Is the leader, which has the swivel you want to tie on to, long enough to fish (pun intended) it out the backside thru the motor tube? You can then tie on your elastic and stuff it all back through? bmhiii #### NewEntity1 ##### Well-Known Member That would probably work...I could push the end down with a stick far enough to be able to grab the leader from the other end using a hooked wire. However, I've kinda sold myself on the coupler idea at this point The original Big Bertha mount tube was nine inches long, which would have meant pushing the the wadding down the remaining nine inches of the 18" body tube...however the mount I've put in is only 4" long, which means I have to push the wadding down 14" of body tube before it makes contact with the mount. This is still doable, but I think it will be easier to get the wadding properly arranged without using too much if I cut the tubing into two sections and load the rocket from the middle More importantly, it will make the Bertha stand out at the launches. When they see me crack it open in the center, they'll know this is no ordinary Bertha #### Steward ##### Well-Known Member In following the progress of this project....(I'm also building a modified BB)...the last post throws me....Your talking of pushing the wadding all the way down to the motor mount? I've never done that...always placed it just deep enough to allow room for the recovery items... Is there a reason for that?...(have I missed something somewhere)? Or is it just a personal preference sorta thing? No big deal...just curious...!!! #### NewEntity1 ##### Well-Known Member Your talking of pushing the wadding all the way down to the motor mount? I've never done that...always placed it just deep enough to allow room for the recovery items... No reason at all, except for that being the way I thought it was supposed to be done I only came back to making rockets a month ago after a 20+ year hiatus, so I may have a lot of misconceptions... #### Steward ##### Well-Known Member Like I said...No big deal...just curious. I don't guess it would make any difference...as long as it works...(I've considered a "baffle" on this BB...and still have time to do it)...and nothing melts or burns....Thanks!!! #### TwoWalks ##### Well-Known Member Originally posted by Steward Like I said...No big deal...just curious. I don't guess it would make any difference...as long as it works...(I've considered a "baffle" on this BB...and still have time to do it)...and nothing melts or burns....Thanks!!! Steward, also thinking about a baffle in the BB, about where do you figure it should be placed? Down just far enough to accomodate the recovery system? #### KenParker ##### Well-Known Member I'll throw in my.02 here on a couple of things. As usual, I'm sure that opinions will vary.

As far as what to do with the recovery wadding - whether to force it down into the body tube as far as it will go, or just put it far enough down to allow fitting the recovery device in between it and the base of the nose cone - I usually *gently* push it as far down as it will go. Why? *Most* (not all, there are always exceptions) rockets leave the pad pretty fast, and I figure that if you don't put the wadding and recovery device as far down the body tube as they will go, the liftoff will do it for you. I guess I would rather already have them already there than to have the liftoff forces shift them. Again, just a preference.

I would place the baffle right above the motor mount tube. The baffle's purpose is to filter out the hot particles from the ejection charge. Again, I would rather get those burning particles filtered out as soon as possible. Again, just my preference.

#### TwoWalks

##### Well-Known Member
Originally posted by KenParker
I'll throw in my \$.02 here on a couple of things. As usual, I'm sure that opinions will vary.

I would place the baffle right above the motor mount tube. The baffle's purpose is to filter out the hot particles from the ejection charge. Again, I would rather get those burning particles filtered out as soon as possible. Again, just my preference.
What affect would the baffle have on the center of gravity? Would it have less affect if placed on the CG? As long as its between the motor and the recovery system its reason for being would still apply.

#### Stymye

##### Well-Known Member
annother option,
In my stormcaster I installed a piece of chore boy above the motor tube and glued a paper centering ring above it
sort of a quasi aerotech baffle,it seems less constricting than a centuri style baffle directly above the motor...it's cheap, simple and lightweight .I still put a piece of wadding under the chute for extra protection ,,It's a 3x 18mm cluster so far so good.