### Help Support The Rocketry Forum:

#### KilroySmith

##### Well-Known Member
This looked like an interesting extension of my skills, so I picked one up.

First thing to note: No instructions, no available OpenRocket models, and an incorrect parts list. Took me an hour of moving parts around on the floor to figure out how everything goes together.

Attached is an initial OpenRocket model. I'm having a couple of problems with this, and hoped someone might have an idea.

1. "Warning: Discontinuity in rocket body diameter". Man, does this error annoy the heck out of me. One of these days, it's going to annoy me enough to go git the source, find out where this message is displayed, and modify it to IDENTIFY THE COMPONENTS that have a discontinuity. I've looked at each section, and everything looks like it should be continuous - anyone have a suggestion?

2. The transition section and the lower body tube should butt up against each other, with the internal coupler providing support. OR doesn't seem to handle this well - it insists on sliding the lower body tube inside the transition. I had to create a "fake body tube" that pushes the lower body tube down to where it belongs. My sense of aesthetics hates this. Anybody have a better solution?

3. No matter what I do, OR refuses to save any engines I configure the rocket with. I configure and sim the rocket, everything's great, then I save, exit, and restart - and the configurations and engines are gone. Any help here?

I expect it'll take a couple of months for me to get enough time to finish this; I'll update the thread with pictures as the build progresses.

Thanks,

/frank

View attachment Stock Rocketry Warehouse Double Shot Two Stage.rkt

#### thomas

##### Well-Known Member
1. I have no idea, this is already there if you only create a tranistion.
2. I think that is the normal behaviour, the shoulder on the transition is like on a nosecone supposed to be a coupler.
If you want to have a bodytube there, take the fake bodytube as a real bodytube and remove the shoulder on the transition.
3. You have to safe as an Openrocket .org file not rocksim .rkt, then the motor configurations will be safed.

#### KilroySmith

##### Well-Known Member
1. I have no idea, this is already there if you only create a tranistion. 2. I think that is the normal behaviour, the shoulder on the transition is like on a nosecone supposed to be a coupler. If you want to have a bodytube there, take the fake bodytube as a real bodytube and remove the shoulder on the transition. 3. You have to safe as an Openrocket .org file not rocksim .rkt, then the motor configurations will be safed.
1. I actually did download the code, and after looking at it I think you're right - it doesn't check if a change in diameter is intentional or not; if the diameter changes, the warning is printed. Maybe the code change will be a bit bigger than I thought.

2. I was trying to build the model the same way as the physical rocket. Your way would work also, but would show a transition that doesn't look like what's in the kit. Still looking for a solution here....

3. Dayyam. Make a bad choice in filenames once, and be confused forever. Thanks for that tip. /frank

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#### dhbarr

##### Amateur Professional
Anytime two adjacent ODs don't line up precisely OR complains. OR doesn't necessarily display the dimension qty's with enough precision to see which component to tweak.

Usually this is because, say, a NC is a few thousandths off from a BT. I usually just punch auto on the NC after everything else is correct.

#### T-Rex

##### Ordinary Average Guy
TRF Supporter
First thing I would suggest is change the file type to ork (the open rocket file type). I have had problems working with rkt (RockSim file type) files in Open Rocket. This may be the problem with your not being able to save motors and configurations.

The discontinuity is in your "fake body tube"
The only reason that OR would put the tube inside the transition is the diameters entered when the design was made. It could be an error in the transition component if you pulled it from a library. As was stated above, it should use the lower shoulder as the connector. If you intend to have the lower shoulder be inside the tube coupling, then adjust the diameter appropriately, or give it a shoulder length of 0.00

I have no problem removing the fake tube, making the lower body tube 34", eliminating the discontinuity and saving with motors with the file in OR format.....

Hope it helps, Enjoy!!

View attachment Stock Rocketry Warehouse Double Shot Two Stage.ork

#### KilroySmith

##### Well-Known Member
I have no problem removing the fake tube, making the lower body tube 34", eliminating the discontinuity and saving with motors with the file in OR format..... Hope it helps, Enjoy!!
Thanks for the help, Terry. Switching to '.ork" solved my saving problems. However, your suggestion on fixing the transition doesn't help - the body tube that came with the kit isn't 34" long, it's 33" long. It doesn't slide inside the transition, nor does the transition slide inside the body tube - the lower shoulder on the transition has an OD precisely the same as the OD of the body tube (thanks for catching that, by the way), and the coupler is supposed to slide inside both of them and hold them together. I know that from a practical standpoint, doing it your way generates pretty much the same results, but if I'm going to do the work to build the model, I'd like to share it with others, and have it be as precise as I can get it. I'm guessing that OR makes the assumption that a transition will always use the shoulder as a coupler, but it's not the case here. I'll update with everyone's suggestions and repost the file as good as I can make it.

#### KilroySmith

##### Well-Known Member
So I've updated the model.

Couldn't solve the placement of the lower body tube and the transition, so the "fake body tube" is still there. Added a couple of configurations (small motors, big motors). The weights of all the components correspond with the pieces that I received; your grams may vary. Note that there are a couple of extra bulkheads (one 54 mm, one 4") in the kit that I haven't placed on the rocket. You can do with them as you see fit. Note also that there are no parachutes, glue, bridles, electronics, etc - this represents only the parts that come in the kit.

I've started on my modifications - adding a vent ring around the Av bay, adding parachutes, electronics, bridles, electronics, batteries, etc. Working on how to separate the booster and sustainer, dual-deploy on both, and add a 2m GPS tracker. /frank

View attachment Stock Rocketry Warehouse Double Shot Two Stage.ork

#### T-Rex

##### Ordinary Average Guy
TRF Supporter
Frank,

Sometimes we have to fool the computer so it will do what we want.....

Enjoy!

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##### Well-Known Member
TRF Supporter
Very cool. A two stage is on my rocket bucket list, so this kit had caught my eye a while back. Good luck with the build!

#### KilroySmith

##### Well-Known Member
OK, so I've spent the last week doing some research on how I'm going to fire this puppy off. Seems to me (I'm open to suggestions, but not from the MARSA-NET proponents) that I'll try:

1. Booster is ignited normally.

2. Primary stage seperation will be drag

2a. Secondary stage seperation will use a MissileWorks PET2 in the interstage to fire a BP charge at burnout+1 sec. Gotta buy one.

2b. Tertiary stage seperation will be the sustainer firing - I'm assuming there'll be some damage if this occurs.

3. Single-deploy Booster chute will be deployed by my existing Stratologger. Backup chute deployment will be from Motor charge.

4. Primary second stage ignition will be an existing MissileWorks RRC3 using the Aux output in the sustainer AVBay, conditioned on expected altitude from OpenRocket to prevent ignition if the rocket is pointed at the Pacific Ocean.

5. Dual Deploy on the sustainer will be handled by the RRC3. Backup drogue deployment will use motor charge, if possible (Delay may not be long enough on big motors).

Failure Scenarios:

1. Cato or other failure in booster - when acceleration drops to zero, PET should fire to seperate stages. Once upward acceleration stops, SL and RRC3 should fire drogues, followed by mains. Should be safe.

2. Stages don't seperate. Sustainer will fire, and probably destroy the top of the booster. If seperation occurs, parachutes will deploy normally, everythings safe. If seperation doesn't occur, SL and RRC3 will fire at apogee of whatever's left of the rocket, and everything should come down under chute. Assuming the booster isn't fully engulfed in flame and burns through the recovery harnesses...

3. Stages seperate, but sustainer doesn't fire. SL and RRC3 fire at appropriate apogees, and everythings safe.

4. Rocket windcocks, and is headed towards the Pacific at burnout. Rocket will not be at appropriate altitude, sustainer won't fire, SL and RRC3 will deploy chutes. Everyone's safe.

5. Rocket is vertical, but doesn't reach simulated altitude. Sustainer doesn't fire, booster/sustainer drag seperate (or BP seperate), and SL and RRC3 deploy chutes. Everyone's safe.

Sound good?

/frank

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#### BDB

##### Absent Minded Professor
2. Primary stage seperation will be drag

2a. Secondary stage seperation will use a MissileWorks PET2 in the interstage to fire a BP charge at burnout+1 sec. Gotta buy one.

2b. Tertiary stage seperation will be the sustainer firing - I'm assuming there'll be some damage if this occurs.
I just want to be sure we are on the same page....you aren't saying that you will lose the poster via drag separation but retain the interstage coupler, are you? I think you are saying that the staging can occur via any of the three mechanisms that you mentioned. If so, it makes sense to me, though I'm not an expert on the staging electronics. (But that's why I'm subscribed to this thread.)

#### KilroySmith

##### Well-Known Member
though I'm not an expert on the staging electronics. (But that's why I'm subscribed to this thread.)
I'm not an expert either, which is why I'm building this rocket. I'm learning quite a bit - and like a hundred engineers who've done this before me, I'm thinking "The available electronics all suck or are too expensive, I'll go build something better". I think I can keep myself from falling over that ledge, however; although an altiduino looks like a really good option.

Yes, your understanding is correct - the booster and sustainer can separate from any of the three causes, and will always seperate at the sustainer/IS junction. The booster will blow the IS to deploy it's chute.

I guess if I trusted drag seperation, I wouldn't need to go buy a $45 PET. But, I don't, so I will. Fortunately, I do trust the SL and the RRC3 (with the motor ejection charge backup), so I don't have to find two more altimeters as backup for those two. It will be ugly if the electronics in the sustainer fail, and the motor ejects only the drogue - that'll keep the sustainer from being lethal but it'll probably bounce pretty good when it hits.... /frank #### MALBAR 70 ##### More Rockets Than Room TRF Supporter I've had this rocket high on my wish list ever since I saw one on the AMW table at LDRS34 . I too am interested in staging electronics, so I'll be following along as well. #### KilroySmith ##### Well-Known Member Well, let's talk about staging electronics, then. What I've learned (and this knowledge is all of several weeks old and untested in the field, so take it with a grain of salt). The Booster and Sustainer are pretty normal single or dual deploy setups. Single deploy would be...interesting... on the sustainer as it's going to be mighty high, so I'm going for dual-deploy there. Something like a Jolly Logic Chute Release would work really well on the Booster when using large motors, because you really only have one place to eject the chute from - the Interstage (IS) to booster coupling. I'm just going to single deploy there for the time being. When I decide to buy a$200 motor, maybe I'll consider buying a Chute Release at the same time.

You have two unusual events that you need to manage - seperating the sustainer and booster, and igniting the sustainer. All the reading I've done says that the best plan for seperation is to do it at burnout - the booster is heavy, which helps in the coast phase, but (especially in this rocket, where the booster is larger diameter) it's also very draggy. Best to get rid of it when it stops providing thrust. I think drag seperation will work fine, because of the increased diameter, but I want to be able to give it a kick if necessary. Igniting the sustainer is a bit tricky - if you've had an unusual boost phase, you probably don't want to light the sustainer. For example, if the rocket weathercocks badly off the rail and is pointed horizontally at booster burnout, lighting the sustainer will probably not be what you want to do. From a safety perspective, if the booster CATOs or has some other issue that ends up with it pointed at the crowd, lighting the sustainer is once again not what you want to do. You also generally want to light the sustainer a small amount of time (maybe 1-10 seconds) after the booster burns out for maximum altitude - after burn-out, it'll "Coast" and continue to climb as it slows down.

Looking at the Altimeters Comparison Guide linked from the "Rocketry Electronics and Software" subforum here, the RRC3, Raven3, Marsa 54, or various Altus Metrum altimeters seem to be the most accessible commercially available solutions for seperation/sustainer ignition. Prices are generally significantly higher than your run-of-the-mill dual-deploy altimeter.

I'm using what I consider to be the bare minimum here. The RRC3 is a normal two channel dual-deploy altimeter, but includes a third "Aux" channel. The third channel can be programmed to fire in a number of conditions - but they are pretty rigidly defined. "Fire on burnout" (what you'd want for seperation), or "Fire 4 seconds after burnout if the rocket is within 10 degrees of vertical, and over 2000 feet" (what you might want for sustainer ignition) aren't available as conditions. What is available is "Fire at 7 seconds after launch if altitude is over 2000 feet". This should work OK, if your simulations correctly predict time and altitude values. In general, you'd want to set the altitude so that if the rocket weathercocks significantly, or if the boost phase isn't perfect, you won't get to the firing altitude and you won't light the sustainer and you won't have to go searching for the rocket in the next county (or won't have to call the EMTs to deal with the spectator who just got center-punched).

The staging electronics can be located in the IS, as I'm planning. You can't place the air start electronics there if you want to light the sustainer after seperation. Alternatively, if you have sufficiently sophisticated electronics (like the Marsa 54 or Altus Metrum), you can locate them in the upper av bay and run wires down to the IS for both seperation and sustainer ignition. In my case, I'll run some wires down the inside of the body tube, through the sustainer fin can, to provide sustainer ignition from the RRC3 in the upper avbay. People have been successful using copper tape down the body tube - either on the inside or the outside - but I'll probably just use 30 ga magnet wire or wire wrap wire; it'll carry the firing current just fine, the half-ohm resistance will nicely current-limit the igniter, and it'll have a lot less impact on the strength of the (motor centering rings->body) glue joint.

Today, it's technically possible to put a 6-DOF gyro/accelerometer on an inexpensive altimeter and have plenty of channels and capability to put all the staging and sustainer ignition duties into the upper av bay for less than \$100 (retail). The 6DOF chip would give you very accurate information about both acceleration (X,Y,Z) and rotation (X, Y, Z) of the rocket, letting you know the precise orientation of the rocket at the precise moment of engine burnout. But, it'll take a few years before a market as small as ours designs and builds a new altimeter with one. I'd love to take the technology from my day job (building tiny custom ASICS with high-speed CPUs and memory), combine it with a baro and one of these cheap 6DOF devices, and build a many-channel multi-stage multi-deploy altimeter that would be small enough to fit in a 13 mm tube. But, that's not to be anytime in the near future.

Anyway, that's what I've learned.

Next up, I learn to wash, sand, and epoxy various fiberglass pieces together.

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#### KilroySmith

##### Well-Known Member
OK, Let's first look at what comes in the kit.

First, the booster bits, laid out more-or-less how they fit into the finished rocket:

Next, the Sustainer:

If you were expecting nuts, bolts, chutes, bridles, etc, you're going to be disappointed. Just a bunch of solid fiberglass pieces, and two very nice anodized aluminum AvBay bulkheads.

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#### KilroySmith

##### Well-Known Member
Ok, so those images didn't work well, let's hope I get this figured out.

First thing was to prepare the bits and pieces. Dragged everything outside, wiped down all the gluing surfaces with Acetone, sanded the Centering Rings and Fins with the palm sander, sanded anything else that would get glued by hand, then wiped everything down again with Acetone. And yes, decided not to touch them again without gloves on.

Note that the fins will not go through the slots in the body tube. I got out a small file and ran it up and down the slots until the fins could be inserted.

So I decided to not try and wedge in an eyebolt into the booster CR, and just epoxy the recovery bridle in. Unlike everyone else, I decided to epoxy the bridle into the fin/body tube junction as an internal fillet, rather than epoxying to the Moor Mount tube. Let me tell you, the motor tube would be a lot easier. Note the two grooves on the outside of the Centering Ring about 120 degrees apart to feed the bridle through:

A view down the body tube with the CR and MM in place showing the bridle, and way too much RocketPoxy:

Now a view from the other side. Fins installed by buttering the tab (forward and inside) with Rocketpoxy, inserting them through the slots in the body tube but pushed all the way to the back of the slot, until the tab contacted the motor mount tube and then slid forward to contact the forward CR. You can see a bit of epoxy peeking out here, along with the (unepoxied) recovery bridle.

I eyeballed the fin insertion, and used the rubber bands to hold them in place overnight while the epoxy cured. Not the most precise approach, but this isn't heart surgery.

For some reason, I didn't take any pictures after putting in small internal fillets on the fin/body and fin/MM joints. But take my word for it, they're there - perhaps not necessary, but we land on rocks out here and I'd rather the whole thing bounce than knock a fin loose.

For Engine retention, I'll just handmake some Kaplow clips. To hold those in, we glue some 8-32 nuts on the inside of the aft CR before epoxying it in place. I used ShoeGoo here rather than Epoxy, because its pretty flexible, holds tenaciously, and its easy to peel off bits that end up where they don't belong when it sets up but before it dries. Note that the ShoeGoo is under the nut, around the nut, and on top of the nut - it's not going anywhere:

And that's where we sit today. The Booster is fully assembled. Next step will be to build the Sustainer, and then the Interstage comes last so I can correctly set the length of the coupler coming out of the transition into the sustainer.

#### Banzai88

##### Lvl 1,Wallet....Destroyed
TRF Supporter
Watching carefully! 2 stage something is on my winter build list, and I've always wanted this rocket.

#### KilroySmith

##### Well-Known Member
Taking a bit of a break here. My next step is to build the sustainer - but the sustainer fins didn't come beveled. Not wanting to spend two hours with the palm sander shaping them, I'm going to spend the next 10 hours building a jig for my tablesaw that'll let me use the disk sander attachment that I just bought to put a bevel on them. But before I could do that, I had to clean the garage so I could FIND the table saw. Today, I got 2/3 of the garage cleaned up - but the table saw is in the other 1/3. I'll try for that part this week.

I'll post pictures when I get set up.

/frank

TRF Supporter
Subscribed!!!

#### kidrobo

##### Active Member
Here is the jig I made up for my Darkstar 2.6 fins. The ledge at the bottom was roughly the same as the fin width. Taped some sandpaper to the back board so the fin wouldn't slip. Used a diamond blade for concrete/brick: https://www.amazon.com/gp/product/B000FUM78I/?tag=skimlinks_replacement-20. Probably took us ~2 hours (with a trip to the hardware store) to put together

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#### KilroySmith

##### Well-Known Member
Great shot of that jig. I love the idea of the sandpaper and those clamps (don't have any in stock, though). I hadn't worked out the clamping mechanism I'm going to use yet, maybe I'll steal yours; it solves a number of problems quite simply that I hadn't worked out to my satisfaction in my head. The visible bevel is a beauty.

#### NateLowrie

##### Well-Known Member
Can you show a quick shot of the jig on the table before feed through? Do you use the rip fence to guide it?

#### kidrobo

##### Active Member
Can you show a quick shot of the jig on the table before feed through? Do you use the rip fence to guide it?
I can try to get a picture, but the jig and saw are at my dad's place.

The base of the jig runs along the fence and the fence was set so the blade was almost touching the jig. The supports that hold the mounting surface doubled as handles to push the jig through. I am thinking about making something out of aluminum that would allow you to set the jig for different fin thicknesses. Just cause.

Mockup without the mounting s

Here is a pic of the fins on the rocket:

#### KilroySmith

##### Well-Known Member
So I had some time this weekend, and made some progress. First, the beveling jig. After reading on here, I decided to sand the bevels in, using a sanding disk in my tablesaw. Bought a "balance blade" from amazon ( https://www.amazon.com/gp/product/B000P4LRMG/?tag=skimlinks_replacement-20 ) and slapped a self-adhesive 120 grit sanding disk on it. Then I built a very square box, 2" tall, and added a 4" face to it. The intention was to attach the fin, one at a time, to the 4" face, set the blade to a 10 degree angle, slide the box up against the sanding disk, and bevel to my heart's content:

Sanding Disk mounted in Ryobi BT3000 Table Saw

Back side of the very square box - the 4"face (in the back in this shot) needs to be at 90.00 degrees to the face up against the sliding fence on the saw.

Fin mounted on the 4" face with screws and washers. It's hard to see in this photo, but I ran the box through the saw to create a 1/8" deep groove at the bottom. That has the effect of creating a line which is perfectly horizontal as far as the saw is concerned so we can get a very even bevel. I lined up the bottom of the fin with the top of the groove.

The box and fin in place on the table saw. Note that the fingers stay on the crosscut fence, so there's no danger of sanding off your knuckles. Depth of sanding of the bevel was eyeballed - press against the saw for a second, pull back and look, press again, etc.

An example fin edge. Note the bevel is slightly wider at the leading and trailing edge which I don't fully understand. It means the fin was slightly concave, putting the leading and trailing edges slightly closer to the disk and sanding a bit more off. Don't know how that could have happened, but this was the worst case of that kind of distortion of all 12 bevels I did (leading and trailing edges, 2 sides, 3 fins).

I didn't take a photo, but the face board ended up with a 8 or 10 holes in it after I also beveled the trailing edges. I figure after 5 or 6 more rockets, I'll have to put a new face board on. One really cool thing is that it was trivial to sand the trailing edges at a sharper angle - 5 degrees. It may not be the most efficient, but it looks cooler than heck.

So now I'm ready to build the sustainer. I'm not going to go into a lot of detail - Similar to the booster, I recleaned everything with Acetone, measured and epoxied the upper CR to the motor tube, put some epoxy in the tube just below the top of the fin slots, slid the motor tube and CR into the sustainer body tube up to the top of the fin slots, forcing a fillet of epoxy on the top side and a thin layer of epoxy under the CR as it scraped the epoxy off the walls. Fins had the root edge buttered, and were inserted through the slots until they hit the motor tube, and were held in place until the RocketPoxy cured.

Motor Tube with upper centering ring. Note the 3/32 Aluminum Tube that extends from below the motor mount all the way up to the top of the sustainer main body tube, using a small groove cut in both CR's. I'll run 4 pieces of 26 gauge Magnet wire through this conduit for the seperation charge, and sustainer igniter. The Aerotech 38 mm DMS tube is just the right diameter to hold the CR square to the motor tube.

Manufactured injector to place epoxy fillets inside the fin box. There are several big problems trying to do that on this rocket:
1. The forward end of the fin box is 10 inches up from the aft end of the body tube.
2. The aft end of the fin tab is about 3 inches up from the aft end of the body tube.
3. There's only about 1/4 inch of clearance between the motor tube and the body tube.
I like putting fillets at the fin/motor tube and fin/body tube junction on the inside of the rocket, but it's nearly impossible to do here. So, I took a 12 gauge hypodermic needle and epoxied a 12" 1/8 inch brass tube to it. I mixed up some Bob Smith 30 minute epoxy, poured it into a 15 ml syringe, and attached my really scary looking 12" needle. With caution and a steady hand, I could rotate the rocket to point one fin straight down, then place epoxy in the upper two fin/motor tube joints, and on each side of the lower fin/body tube joint, rotate the rocket 120 degrees and do it again, the rotate it the final 120 degrees and do it again. The epoxy ran a little bit, but the rotation mostly got the epoxy at each joint to run partially up the fin and partially around the motor tube or body tube, giving a very strong fin connection (we land on rocks out here) which isn't real pretty, but neither you nor I will ever be able to see the sloppiness. Just don't get any epoxy on the body tube or motor tube between the aft end and the aft end of the fin tabs, or you'll have a real hard time getting the rear CR into place.

And that's about where I am. I've made a mistake and not placed any way to attach the recovery harness to the fin box, so I'm going to have to jury-rig something. I'm thinking about drilling a small hole through the upper and lower CR, and threading a Kevlar cord through it. A knot at the aft end, and a loop at the upper end to connect to the recovery harness should be just fine. What do you think?

/frank

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#### Forever_Metal

##### JustAnotherBAR
nice! nice non-standard human thinning agent in pic 9 too :wink:!!

op:

fm

TRF Supporter

NikeMikey

#### KilroySmith

##### Well-Known Member
The problem I mentioned in the last post is kicking my posterior.

The very small ring (1/4") between the motor mount and the body tube on the sustainer is making it hard to fix my mistake. The coupler from the interstage inserts into this space, and nestles up against the aft CR, taking up almost 1/8" of the available space in this ring. The aft CR is recessed about 3" from the aft end of the body tube, making it very difficult to work in that space.

Without fully thinking through the problem, I drilled a couple of 5/32" holes through the two CR's with a 12" bit, and threaded some 1/8" kevlar through the two CR's with the intention of knotting them below the aft CR to provide an anchor for the recovery harness. Unfortunately, 1/8" kevlar with a knot is significantly larger than the 1/8" space that I intended to place it in; not only did the knot not fit into the space (although I probably could have stuffed it in), it was certainly going to prevent the IS coupler from fitting in where it needed to.

I considered using some 1/16" stainless steel aircraft cable with a stop swaged on the end. A 3/32" cable would have been better, but a swaged stop is bigger than the 1/4" space I had available. I was hoping to get the cable with a swaged stop through the aft CR so that the forward CR would provide the anchor point, but when I considered the likelihood of being able to successfully feed the 1/16" cable through a 1/16" hole blind, 10" up the body tube, after feeding it through a hole in the aft CR, I despaired of being able to do so.

I ordered a new CR from Madcow, hoping to attach the recovery harness to it, then slide it down the body tube and over the motor tube and epoxy it into place on the about 1" of motor tube stub sticking above the forward CR. But, looking down the mouth of the body tube, I didn't think I could successfully do that and mount it strongly enough.

So, I'm left with the ugly solution of epoxying the harness to the inside of the body tube, below the AvBay coupler. I decided to install two independent lengths of kevlar, as a backup in case a bad deployment wanted to rip everything out. I took some 2" masking tape, and masked the inside of the body tube where the avbay coupler needed to slide in to prevent accidentally dribbling epoxy on the area and making an even bigger mess. I mixed up some RocketPoxy, saturated the kevlar bridles with the epoxy by laying them out on a piece of aluminum foil, then working the epoxy into the kevlar with my epoxy mixing stick. Once I was satisfied, I transferred the bridles into the body tube and pressed them in place and let it set up for an hour or so. . I mixed up some more epoxy, and tried to lay some smooth fillets on each side to maximize the area bonding the kevlar to the body tube. That didn't go so well cosmetically, but I'm pretty sure both bridles are attached to the body tube with a bond stronger than the kevlar. But you don't get to see it because it's so ugly.

Now it's on to building the Interstage.

#### cerving

##### Owner, Eggtimer Rocketry
TRF Supporter
What I've done for a few rockets is to get a short piece of coupler tubing (about 1C) and a stepped bulkplate, epoxy the bulkplate on, then mount the eye/u-bolt with the shock cord on the inside of the bulkplate (on the coupler side). Put some epoxy inside your body tube, then slide this in and let it cure. You can't use motor eject any more, of course (unless you drill a few holes in the bulkplate), but it's a very strong mount. I've used this for both 29mm MD rockets (Mongoose 29) as well as larger ones (3" RW G3), and it's also an easy way to repair a shock cord that gets torn off the motor mount.

#### KilroySmith

##### Well-Known Member
Oh, I like that one, there's a whole lotta goodness there. Good and strong, a lot easier than my approach, and would look neat and tidy. I'll remember that one for the next time; but it's too late for this one - the mess has been made.

Thanks for the insight.

/frank