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A asked this before, but what size engine should I actually use?

  • 29mm

    Votes: 3 42.9%
  • 38mm with adapter

    Votes: 4 57.1%
  • Other?

    Votes: 0 0.0%

  • Total voters
    7

Oscar S.

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Hi! I have been doing low-power rocketry for years, but I recently got interested in doing high-power rocketry, so after reading a book and talking to very helpful people on the forum I think I have a pretty good and detailed idea of what I need to do to build my level 1 rocket. It will be my first completely scratch build, and it should fly well on medium-power rocket motors as well as H motors. I created this thread so that I could share my experiences with this project and get some helpful advice with building techniques.

The rocket will be about 4' long, and I will do most of my sourcing from LOC/Precision, although I plan on getting the parachute, shock cord and motor retainer elsewhere, and I will have to make some parts myself. It will have a 29mm motor mount and the body tube in the lower section will be 2.14", and it will widen to 2.56" in the nose and payload section. I have attached the OR file for the rocket so you can see more specifically what I have in mind. :)

I also have a detailed parts list linked here, I have actually used the PBS system to organize it. If it is confusing, there is a video about it linked here. It is a bit overkill for a project like this, but it is practice for me for other occasions. To make things simpler I have highlighted all the actual parts and left comments about how I will get them.

Just to clarify, I do not have most of the parts yet as you can see in the parts list, and I will have to save up to get the parts. I am hoping to get the first order in and get started with actually building within a few months.:D In the meantime, it would be great if some of you guys could give some ideas of what order I should do things in and how I should put things together(especially for the transition, that was not covered in my book). Thanks!
HY6fb1B.png
 

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Nytrunner

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It's a (slightly) upscale ventris! I like it
This size and weight of rocket will fly really well on 29mm motors, and will absolutely disappear on 38's over 2grain.

It looks like you've got the booster tube up inside the transition pressing on a bulkhead. Since that transfers the thrust, the transition can be purely cosmetic like layered paper shroud with epoxy, or 3d printing. (Mike at mac performance produces good 3d tailcones, I bet he could, make one to your dimensions)

Build order? I'm bad about that, so I usually do things as I have time for them.
Here's a general flow (heck, open up a sheet of estes instructions and pretend the parts are bigger!:cool:)
-fix centering rings to mmt, except rear if you want to do internal fin tab gluing or place a rail button back there(will shock cord attach here or elsewhere?
-prep fins if needed (cut, round, bevel, etc...)
-when glue is drying in one section, go work on another like the nose or payload
-install mmt in booster
-work on transition pieces
-install rear centering ring and retainer
-fin fillets
-recovery layout
-misc throughout

Go fly!
 

Oscar S.

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It's a (slightly) upscale ventris! I like it
This size and weight of rocket will fly really well on 29mm motors, and will absolutely disappear on 38's over 2grain.

It looks like you've got the booster tube up inside the transition pressing on a bulkhead. Since that transfers the thrust, the transition can be purely cosmetic like layered paper shroud with epoxy, or 3d printing. (Mike at mac performance produces good 3d tailcones, I bet he could, make one to your dimensions)

Build order? I'm bad about that, so I usually do things as I have time for them.
Here's a general flow (heck, open up a sheet of estes instructions and pretend the parts are bigger!:cool:)
-fix centering rings to mmt, except rear if you want to do internal fin tab gluing or place a rail button back there(will shock cord attach here or elsewhere?
-prep fins if needed (cut, round, bevel, etc...)
-when glue is drying in one section, go work on another like the nose or payload
-install mmt in booster
-work on transition pieces
-install rear centering ring and retainer
-fin fillets
-recovery layout
-misc throughout

Go fly!
The way I plan on doing the transition is putting the booster tube inside the larger tube with centering rings as if it is a motor mount, then gluing on a 3D printed shroud. I have been experimenting with 3D printing and CAD lately, so I can probably figure out how to do that bit myself. :)

I am not quite sure what the best way to attach my shock cord is. The book I have, Make: High-Power Rockets does it by pushing it under the forward centering ring and wrapping it around the motor mount. Another question: What type of shock cord would be best for this rocket, and where would you recommend getting it? Thanks!
 

Steve Shannon

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I build the aft most portion first (what some call the booster or fin can). In fact I build it all relative to the correct position of the motor retainer, even though that’s the last part installed.
Because you’re using Loc tubes and presumably plywood fins and centering rings, everywhere I say epoxy, you can substitute a high quality wood glue.
First, make sure everything fits just right.
I place the retainer in place temporarily, then the aft centering ring, then use the fins as spacers so I can mark the location of the forward centering ring.
I usually epoxy my shock cord to the MMT, notch the forward centering ring to pass over it, put epoxy around the MMT and then slide the forward centering ring down to the marked location. I leave a fillet under the forward centering ring to help withstand the forces on the motor mount tube. If you’re building using Stu Barrett‘s zipperless design and electronic deployment you don’t have to worry about the shock cord, notch, etc.
After the epoxy cures, figure out where you want your rail buttons (don’t even consider using launch lugs.) You may want to add backer tabs to the centering rings like Kris Hull did in his L3 build.
Then I drop the shock cord through the MMT, smear epoxy into the body tube at the front of the fin slots and slide the assembly into place so the forward centering ring just clears the fin slots. Put the aft centering ring on dry and use it to align the MMT in the body tube. Let the epoxy cure again.
Remove the aft centering ring and pull the shock cord out of the MMT so it hangs out the front. Tape or rubber band it to the BT so it’s not in your way.
Attach your rail buttons. Everyone will give you different advice for where. I put one just ahead of the aft centering ring and near the center of gravity.
Epoxy in the fins. Do whatever internal fillets you like. I fillet where they’re glued to the MMT and where they pass through the BT.
Epoxy in the aft centering ring.
Fillet the fins on the outside.
Finally, JB Weld the motor retainer in place. It should fit right up to the bottom surface of the aft centering ring.
 

Andrew_ASC

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C756278C-5B96-4525-98B2-22CB8F32D2A5.jpeg
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I prefer 38mm. You can always adapt down but never up. The Aerotech H130W is hazmat free H motor, for RMS 38/240 casing.
The RMS 38/360 has hazmat free I180 load. Most of the Loki reloads are also hazmat free.

Onebadhawk makes some nice Kevlar harnesses for recovery in 3/16” size.
 

Bat-mite

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Can't vote as there is no right answer. You have a simulation file. Try all kinds of 29 and 38mm motors in it. See what is stable or not stable. See what puts the altitude appropriate to your field. See what gives you the most available motors to use. Run sims using high and low winds, look at the flight duration.

If you multiply the descent time by the cross wind speed, you'll see how far your rocket could drift under chute. Find motors that keep you on your field.
 

Oscar S.

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I build the aft most portion first (what some call the booster or fin can). In fact I build it all relative to the correct position of the motor retainer, even though that’s the last part installed.
Because you’re using Loc tubes and presumably plywood fins and centering rings, everywhere I say epoxy, you can substitute a high quality wood glue.
First, make sure everything fits just right.
I place the retainer in place temporarily, then the aft centering ring, then use the fins as spacers so I can mark the location of the forward centering ring.
I usually epoxy my shock cord to the MMT, notch the forward centering ring to pass over it, put epoxy around the MMT and then slide the forward centering ring down to the marked location. I leave a fillet under the forward centering ring to help withstand the forces on the motor mount tube. If you’re building using Stu Barrett‘s zipperless design and electronic deployment you don’t have to worry about the shock cord, notch, etc.
After the epoxy cures, figure out where you want your rail buttons (don’t even consider using launch lugs.) You may want to add backer tabs to the centering rings like Kris Hull did in his L3 build.
Then I drop the shock cord through the MMT, smear epoxy into the body tube at the front of the fin slots and slide the assembly into place so the forward centering ring just clears the fin slots. Put the aft centering ring on dry and use it to align the MMT in the body tube. Let the epoxy cure again.
Remove the aft centering ring and pull the shock cord out of the MMT so it hangs out the front. Tape or rubber band it to the BT so it’s not in your way.
Attach your rail buttons. Everyone will give you different advice for where. I put one just ahead of the aft centering ring and near the center of gravity.
Epoxy in the fins. Do whatever internal fillets you like. I fillet where they’re glued to the MMT and where they pass through the BT.
Epoxy in the aft centering ring.
Fillet the fins on the outside.
Finally, JB Weld the motor retainer in place. It should fit right up to the bottom surface of the aft centering ring.
Ok, sounds like I should practice working with epoxy and making fillets first. I will probably do that this weekend, though I have to clean up my workspace first. :rolleyes: I don't think I have fast drying epoxy or JB Weld, so I will have to acquire those.

Also, I have the materials to make the fins, and I have the shape figured out. Would it be a bad idea to cut them out before I have the tubes, or would it be okay to cut and sand them now, and cut the fin tabs more precisely later? Does anyone have a technique for hand-cutting fins more precisely?
 

Oscar S.

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Can't vote as there is no right answer. You have a simulation file. Try all kinds of 29 and 38mm motors in it. See what is stable or not stable. See what puts the altitude appropriate to your field. See what gives you the most available motors to use. Run sims using high and low winds, look at the flight duration.

If you multiply the descent time by the cross wind speed, you'll see how far your rocket could drift under chute. Find motors that keep you on your field.
I experimented with the OR file, and it seems 38mm is the way to go. I can easily put in an adapter and only fly it on 29mm motors for my first flights and maybe even my cert flight, but it would be future-proofing it. Just a question, at what high speed would it be bad to fly a cardboard/plywood rocket?
 
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Oscar S.

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View attachment 372535 View attachment 372534 View attachment 372533 I prefer 38mm. You can always adapt down but never up. The Aerotech H130W is hazmat free H motor, for RMS 38/240 casing.
The RMS 38/360 has hazmat free I180 load. Most of the Loki reloads are also hazmat free.

Onebadhawk makes some nice Kevlar harnesses for recovery in 3/16” size.
Thanks for telling me about Onebadhawk, I think that's what I will use.:) I think I'll get the 3/16" tubular kevlar harness with 3 loops because the swivel seems nice. However, the 15 feet seem like a lot. Should I just cut the loops? I will have to do that for at least one end so that I can actually mount it on the motor mount like planned unless I figure out some other way to mount the shock cord.
 
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Steve Shannon

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Don’t cut the loops. Instead, use something like a quick link which can be opened and then attached to the loop.
Also, as far as making fins, I would stack pieces of plywood and cut them out all at once using the saw of your choice or even order them and the tube slot cutting from Loc.
 

Oscar S.

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Don’t cut the loops. Instead, use something like a quick link which can be opened and then attached to the loop.
Also, as far as making fins, I would stack pieces of plywood and cut them out all at once using the saw of your choice or even order them and the tube slot cutting from Loc.
Ok, that sounds like a good way to do it. Just to clarify though, my concern was that the harness would be too long and would weigh too much.
 

Oscar S.

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Quick links are great. Steve knows what he is talking about.
I know, and I plan on using them, especially to attach to the payload/nose section. My concern is that 15 feet seems a bit long and therefore heavy for such a small rocket like this and that the loop will make it difficult to attach at the motor mount.
 

Nytrunner

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I experimented with the OR file, and it seems 38mm is the way to go. I can easily put in an adapter and only fly it on 29mm motors for my first flights and maybe even my cert flight, but it would be future-proofing it. Just a question, at what high speed would it be bad to fly a cardboard/plywood rocket?


LOC parts are solid. The speed limiter will likely be your fins. (If you dont want to spend time figuring out epoxy first, a bottle of Titebond II wood glue will do you fine. The paper and wood will fail before either glue choice)

What material and thickness are the fins? (Phones dont like ork files ;) )
 

Nytrunner

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I know, and I plan on using them, especially to attach to the payload/nose section. My concern is that 15 feet seems a bit long and therefore heavy for such a small rocket like this and that the loop will make it difficult to attach at the motor mount.

Using a nylon harness, you could probably go down to 10 or 12 ft. It just downstairs pack as tight as kevlar (and also zippers less)

Is it possible to keep the chute and cord up in the payload section? That way you can anchor it to one of the transition bulkheads, and the nose.
 

Oscar S.

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Using a nylon harness, you could probably go down to 10 or 12 ft. It just downstairs pack as tight as kevlar (and also zippers less)

Is it possible to keep the chute and cord up in the payload section? That way you can anchor it to one of the transition bulkheads, and the nose.
I was actually thinking of using THIS. It would probably pack, and it is kevlar so no need for a sleeve. How short should I go with that?
 

Nytrunner

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I was actually thinking of using THIS. It would probably pack, and it is kevlar so no need for a sleeve. How short should I go with that?

Thatll be fine most likely. When you contact Onebadhawk, Teddy will also letyou know if he has any other suggestion for your sizerocket.

Kevlar doesn't stretch like nylon or elastic. It has to be longer so the rocket sections can lose some energy before hitting full extension of the cord.

Just one of the many "right" options for rocketry lol, there isnt just one
(I personally prefer nylon, but that's me)
 

Oscar S.

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Thatll be fine most likely. When you contact Onebadhawk, Teddy will also letyou know if he has any other suggestion for your sizerocket.

Kevlar doesn't stretch like nylon or elastic. It has to be longer so the rocket sections can lose some energy before hitting full extension of the cord.

Just one of the many "right" options for rocketry lol, there isnt just one
(I personally prefer nylon, but that's me)
Thanks! I chose kevlar because I couldn't find nylon in the size I needed, and the fire-resistance of kevlar seems nice. So, assuming I chose kevlar, what would be a good length for a 4-foot rocket like this? Is 15 feet just right or not enough to prevent zippering?
 

Oscar S.

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I was just looking at my OR sim... and it seems that with a lot of the 38mm motors the stability drops below 1cal. What is the best way to be able to add and remove weights from the nose, depending on the motor I'm using? I was thinking put in a chunk of motor mount with centering rings and put a bulkhead at the end. Then you would, before putting it in the nose, put a bolt through the bulkhead and epoxy it in place. Then you could add and remove washers and then keep them in place with a nut.
 
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Andrew_ASC

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15ft of harness isn’t too long. Heck I would go with 20-25ft on an L-1. I tried running 7ft of shock cord in a short L-1 scale missile once and the nosecone that has a pound of lead in it jerked off at attachment point the body tube crashed to ground and the nosecone floated down under chute. An experienced flyer recommended I use 20ft of shock cord. Since following his advice I had no problems since. There is a rule of thumb about 2-3 rocket lengths. I found out the thumb rule didn’t always work and longer is better with Kevlar. There are a lot of forces on ejection of nosecone.

You can add some lead shot and epoxy mixture to the nose to account for the range of motors you want to fly. When you pick motors try to make sure the rail exit velocity exceeds 45fps on a 6-8ft rail.
 

Nytrunner

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I was just looking at my OR sim... and it seems that with a lot of the 38mm motors the stability drops below 1cal. What is the best way to be able to add and remove weights from the nose, depending on the motor I'm using? I was thinking put in a chunk of motor mount with centering rings and put a bulkhead at the end. Then you would, before putting it in the nose, put a bolt through the bulkhead and epoxy it in place. Then you could add and remove washers and then keep them in place with a nut.

I do something very similar to what you describe. Ring last the shoulder of the nose with threaded inserts. Removable bulkhead with eyebolt on the aft side for the shock cord. The LOC removable his weight system is a larger scale of what you describe, and John Coker has a similar method as well.

You'll find that just installing that setup will add some weight to the nose itself and help out your stability.
 

Oscar S.

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15ft of harness isn’t too long. Heck I would go with 20-25ft on an L-1. I tried running 7ft of shock cord in a short L-1 scale missile once and the nosecone that has a pound of lead in it jerked off at attachment point the body tube crashed to ground and the nosecone floated down under chute. An experienced flyer recommended I use 20ft of shock cord. Since following his advice I had no problems since. There is a rule of thumb about 2-3 rocket lengths. I found out the thumb rule didn’t always work and longer is better with Kevlar. There are a lot of forces on ejection of nosecone.

You can add some lead shot and epoxy mixture to the nose to account for the range of motors you want to fly. When you pick motors try to make sure the rail exit velocity exceeds 45fps on a 6-8ft rail.
Okay, thanks! It looks like the only problems I'll have with speed leaving the pad is with small F and E motors.
 

Oscar S.

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I do something very similar to what you describe. Ring last the shoulder of the nose with threaded inserts. Removable bulkhead with eyebolt on the aft side for the shock cord. The LOC removable his weight system is a larger scale of what you describe, and John Coker has a similar method as well.

You'll find that just installing that setup will add some weight to the nose itself and help out your stability.
Ok, I put the design in OR and you're right, it is generally more stable even without extra weight. Heres what I imagine it looking like:
HY7Yb2X.png

One question: How do I figure out how big to make the centering rings in the nose? I let OR calculate automatically for these, but IDK if I can trust the measurement it gives me.
 

boatgeek

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If it will fly on 38mm motors, it probably won’t fly well on E or low thrust F motors. It’s just really hard to cover that much of the alphabet.

For maximum speed, I’ve seen 54mm LOC tubing and nosecone fly to Mach 1.5 with no trouble on a J.

If you really want to fly E and F you probably need to lighten it up a bit and go with a 29mm motor mount.

How high is your club’s waiver?

[edit] If you just buy two standard centering rings, you can slot them in to where they fit and then cut the tube to suit. You could also just make the fins a bit bigger and live with being a little over stable (and weathercocking) on small motors instead of adding nose weight.
 

Oscar S.

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If it will fly on 38mm motors, it probably won’t fly well on E or low thrust F motors. It’s just really hard to cover that much of the alphabet.

For maximum speed, I’ve seen 54mm LOC tubing and nosecone fly to Mach 1.5 with no trouble on a J.

If you really want to fly E and F you probably need to lighten it up a bit and go with a 29mm motor mount.

How high is your club’s waiver?

[edit] If you just buy two standard centering rings, you can slot them in to where they fit and then cut the tube to suit. You could also just make the fins a bit bigger and live with being a little over stable (and weathercocking) on small motors instead of adding nose weight.
I don't think my design was ever meant to fly E or F motors. G motors work perfectly fine, though. When I launch it will probably be at MDRA, which is relatively nearby to where I live. I will probably use the LOC motor mount adapter and fly 29mm motors on my first one or two times flying.
Later, If I want an only mid-power rocket, I'll probably get the Estes black star voyager kit.
 
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Nytrunner

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Ok, I put the design in OR and you're right, it is generally more stable even without extra weight. Heres what I imagine it looking like:
HY7Yb2X.png

One question: How do I figure out how big to make the centering rings in the nose? I let OR calculate automatically for these, but IDK if I can trust the measurement it gives me.

Depends on how big a tube you stick in there. No need for a forward ring, just run the tube till it touches the cone (with some epoxy to stick it)
(That's the thing about plastic parts, wood glue won't work there)

You're putting a lot of thought into it, It'll be great to see it come together!
 

Oscar S.

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Depends on how big a tube you stick in there. No need for a forward ring, just run the tube till it touches the cone (with some epoxy to stick it)
(That's the thing about plastic parts, wood glue won't work there)

You're putting a lot of thought into it, It'll be great to see it come together!
I was thinking about getting LOC/Precision's 6" length of 29mm motor mount tubing. I think you're right that it would be simpler. In OR, it looks like it would just barely fit, and if it doesn't quite work I can move the other centering ring up.

I think that I'll be able to start work on the fins this weekend, and I will be sure to take pictures along the way. :)
 

Oscar S.

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Alright, this might be a bit premature, but I have no idea what to call the rocket! Maybe Titan after Saturn's moon, but it isn't really a Titan-like rocket. Does anyone have Ideas? :D
 
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