# Design & component advice needed for 38mm min dia scratch

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#### Viperfixr

##### Born Again Rocketeer
TRF Supporter
I would really like to scratch build a 38mm minimum diameter rocket that will be able to break 10k AGL and Mach 1 on an AT J570, or even higher & faster on the new 38/1320 motor AT just released (perhaps at LDRS 29??). I've scratch built some, but I am not a rocket design expert. First design questions:

- Is there a fin area rule? How do you know how large (or small) to make the fins? Rocksim stability shows the attached design green, but I am wondering how far I could shrink the fins to save drag.​

Now, a couple component choice questions. It would be easy to buy carbon fiber tubes and be done with it. Like most of you, I am trying to maximize the bank for the $$buck. If I can still smash 10k/M1 on a J570 and have a solid/dependable rocket, that would be my choice (not trying to set records). Using Rocksim, it seems about 41oz empty weight seems to be the J570 optimum mass--yields over 11k altitude. However, making that light weight in a dual deploy rocket is tougher in design that I thought after playing in RS. What do you all think about these choices? - Body Tube Choice: needs to take up to 100G's and Mach flight. A) CF/Kevlar + FG sleeves over phenolic, B) Hawk Mt/GLR FG tubes, C) ARR Blue Tube 2.0 (with or without CF/Kevlar sleeve). ARR makes a 38mm ebay, which seems like a nice match.​ - Fins: custom or self-cut seems to be the only way to get the shape I like--similar to a PR Mongoose. A) Hawk Mountain/PML/GLR G10 customs (not bad$$$wise), B) cut my own from CF laminate (big $$'s)​ - Motor retention, I am debating between safety wire through a hole in the back of two fins through the aft closure ring, or a GLR slimline (which doesn't work for a snap ring motor and has a drag/weight penalty).​ - Recovery harness, I am planning on a plugged & tapped AT forward closure with an eye bolt on the top side.​ Many of you have tons of experience here, and I am all ears--please give your 0.02 or more. In the end I know this is my decision, and I am likely to get as many opinions as responses, but I value your thoughts. This project excites me, and seems like a great bang for the reload$$$'s rocket--can't wait to build it.

Attached is my draft project--thank you for your time and inputs.

View attachment HiAlt38.rkt

#### Handeman

##### Well-Known Member
TRF Supporter
I don't have rocksim so I can't see your plans, but I think you may be overdoing things a bit. The first question I have, are you looking for high speed, 1.5 mach+, or altitude 10k+? A long burn (moonburner) motor can get you the altitude without the high speed. If you want both, then you've narrowed your design choices quite a bit.

The J570 is a good motor for altitude, although the Pro38 1008J420-15A might be better. You might want to consider using 38mm MMT as body tube (its thicker then standard BT) and attach the fins with a good epoxy fillet. If you use fins that are only a BT diameter span and plenty stiff, I'm not convinced you have to glass it.

Then again, if you want to do the fiberglass or carbon fiber tubes, they will definately hold up, so go with that. They will cost you more to build, but the chance of a shread will be almost eliminated.

#### hardinlw

##### Well-Known Member
I flew a Hawk Mt Basilisk at LDRS28. This is an all FG rocket with 1/16" G10 fins. The rocket is 2" (54mm) OD with a 38mm MMT. I used a slimline retainer with a Loki J-1000 which is a snap-ring engine. With dual deploy and redundant altimeters, it weighted 6.5 pounds ready to fly. That's with a 44" (I think) main from Fruity Chutes and a streamer for the drogue. The main just barely fit. It hit 10,250ft and was just slightly supersonic.

#### cjl

##### Well-Known Member
I would really like to scratch build a 38mm minimum diameter rocket that will be able to break 10k AGL and Mach 1 on an AT J570, or even higher & faster on the new 38/1320 motor AT just released (perhaps at LDRS 29??). I've scratch built some, but I am not a rocket design expert. First design questions:

- Is there a fin area rule? How do you know how large (or small) to make the fins? Rocksim stability shows the attached design green, but I am wondering how far I could shrink the fins to save drag.​
Nothing specific. Typically with my designs, I try to make sure that in Rocksim, both the Barrowman and Rocksim methods predict at least 0.9-1 caliber of stability. If this is true with a design, it is almost guaranteed to be stable.
Now, a couple component choice questions. It would be easy to buy carbon fiber tubes and be done with it. Like most of you, I am trying to maximize the bank for the $$buck. If I can still smash 10k/M1 on a J570 and have a solid/dependable rocket, that would be my choice (not trying to set records). Using Rocksim, it seems about 41oz empty weight seems to be the J570 optimum mass--yields over 11k altitude. However, making that light weight in a dual deploy rocket is tougher in design that I thought after playing in RS. What do you all think about these choices? - Body Tube Choice: needs to take up to 100G's and Mach flight. A) CF/Kevlar + FG sleeves over phenolic, B) Hawk Mt/GLR FG tubes, C) ARR Blue Tube 2.0 (with or without CF/Kevlar sleeve). ARR makes a 38mm ebay, which seems like a nice match.​ Honestly, standard PML phenolic works, fiberglass works, blue tube (unglassed even) would work. The body tube on a small diameter machbuster isn't under as much stress as some people think. Just make sure the couplers are snug and it can't wobble side to side and you should be OK. You'll also want to make sure it can take a pretty hard hit, since in most cases, a 38mm machbuster will have a smallish chute and come in fairly hard (to minimize the recovery hike). - Fins: custom or self-cut seems to be the only way to get the shape I like--similar to a PR Mongoose. A) Hawk Mountain/PML/GLR G10 customs (not bad$$$wise), B) cut my own from CF laminate (big $$'s)​ Either way would work, honestly. For the shape you have there, I'd probably go with 0.093" G10, and then add a layer of T2T glass just to make sure they're nice and solid. You don't need CF unless you're trying to absolutely optimize everything. - Motor retention, I am debating between safety wire through a hole in the back of two fins through the aft closure ring, or a GLR slimline (which doesn't work for a snap ring motor and has a drag/weight penalty).​ If you're going for altitude, don't do the slimline. I've flown some MD rockets before, and I've always just friction fitted. It works fine, but is a bit of a pain. Otherwise, you can do the wire retention, which should be plenty. - Recovery harness, I am planning on a plugged & tapped AT forward closure with an eye bolt on the top side.​ That should work, although it makes flying CTI somewhat more difficult. Many of you have tons of experience here, and I am all ears--please give your 0.02 or more. In the end I know this is my decision, and I am likely to get as many opinions as responses, but I value your thoughts. This project excites me, and seems like a great bang for the reload$$$'s rocket--can't wait to build it.

Attached is my draft project--thank you for your time and inputs.
One comment - 38mm minimum diameter rockets are fun, but they take a large recovery area and vanish quickly. Here's a good video demonstrating an I600 altitude shot to 16,000 feet:

They're a lot of fun, but make sure you've got the area to get it back. Oh, and you might consider the new Cesaroni J530 Imax if you are going for absolute altitude. More powerful and a slightly longer burn than the AT J570.

#### Viperfixr

##### Born Again Rocketeer
TRF Supporter
I flew a Hawk Mt Basilisk at LDRS28. This is an all FG rocket with 1/16" G10 fins. The rocket is 2" (54mm) OD with a 38mm MMT. I used a slimline retainer with a Loki J-1000 which is a snap-ring engine. With dual deploy and redundant altimeters, it weighted 6.5 pounds ready to fly. That's with a 44" (I think) main from Fruity Chutes and a streamer for the drogue. The main just barely fit. It hit 10,250ft and was just slightly supersonic.
Wow--nice flight.

Ref comment above, yes, I might be overdoing it a bit--I usually do. I am thinking of Blue Tube 2.0 without any FG/CF sleeving--sounds like a perfect no fuss tube at a good price. Perhaps this is the right middle ground (without over-doing it)?

Also ref comment above, yes, I am trying to exceed Mach 1 and 10k. Getting closer to over these goals is just gravy--just for fun--not trying to set records.

#### rocketace

##### Well-Known Member
I went through the same problems a while back, except I also wanted to make it strong enough for H999 & I1299 which would give mine 200+ G's.

For retention I decided to go for safety and use the slimline and covered it with the T2T so its hardly noticeable. For the fins I made a shape that I liked and trusted RockSim then decided to get custom C10 cut form GL and they came our awesome. For the airframe I went the custom rout and made my own carbon fiber (3 layers) so it could be as light as possible yet still hold up under 200G's. Right now im just using a nose cone form GL, but when I get time, Im going to make a custom Von-Karman nose cone for it.

The only observation I see from your design, is that if your going for absolute MAX speed and/or altitude a Von-Karman or Ogive will be better, but if not go with the look that you want

Pics are at: http://web.me.com/tennisace1/Site/Kritter.html

#### bob jablonski

##### Well-Known Member
I used phenolic slotted and G10 fins and a 1" wide pcs of fiberglass fin reinforcement with a Vulcan I500. It held togeather as far as anyone could see it. Never did find it to verify.
Mr. Bob
Starlight dude

#### bobkrech

##### Well-Known Member
Don't overbuild and make a lead sled. A plain fiberboard tube or blue tube possibly with a wrap of 1.5 oz FG cloth is plenty strong.

You might look into using mil-surplus flare parachutes. They are very strong and light weight.

http://www.alwaysreadyrocketry.com/Products/chutes.html 24" square or 36" round for main.

http://aeroconsystems.com/chutes/index.htm 23" square or 36" round for main.

The 36" rounds only weigh 1 ounce with bag and only occupy ~1.5" length in the airframe. For dual deploy, split the airframe at apogee and pop the main at 300'-400' agl.

Bob

#### Menkar

##### Active Member
Hi,

I looked at your RockSim file and found that your fins (at least on my machine) have a weight of 0.043g :y: I switch to carbon fiber and the weight goes up to 105g, that make more sense.

I have a piece of G10 3/32" (0.093") more or less the size of your fins and it weight 45g.

Otherwise your rocket look really nice, I would like to fly that high.

Good luck.

André

#### New Ocean

##### Well-Known Member
As on the other forum, why 100gs? Does a J570 really hit that?

#### dixontj93060

##### Well-Known Member
Maybe not 100G, but getting there at 70+ gees. Example attached is one minimum diameter design I haven't yet built which is not particularly high performance.
As on the other forum, why 100gs? Does a J570 really hit that?

#### Viperfixr

##### Born Again Rocketeer
TRF Supporter
As on the other forum, why 100gs? Does a J570 really hit that?
A 45oz rocket with a I1299 will pull a hair over 95Gs when I plug it all into RS9. It sounds like a gruesome flight--would love to have a Mach buster, altitude maker and a G machine too.

Does anyone know of a way to purchase a true VK nosecone in 38mm?

I am hedging on whether or not to go with plain Blue Tube or CF sleeved flexible phenolic. Any thoughts?

#### dixontj93060

##### Well-Known Member
A 45oz rocket with a I1299 will pull a hair over 95Gs when I plug it all into RS9. It sounds like a gruesome flight--would love to have a Mach buster, altitude maker and a G machine too.
Wow, yeah, I never sim'ed a Warp-9 reload (I like to see my flights), but here's another minimum diameter design which I have started building based on a LOC Weasel (mod'ed for Contrail hybrids really), but 100+ gees on both H and I motors. I knew there was a reason I bought that 250G Parrot.

Wow, yeah, I never sim'ed a Warp-9 reload (I like to see my flights), but here's another minimum diameter design which I have started building based on a LOC Weasel (mod'ed for Contrail hybrids really), but 100+ gees on both H and I motors. I knew there was a reason I bought that 250G Parrot.
The peak G reading in the Rocksim H999 file is a test stand artifact, but check out the I1299. Now that's a cannon shot.

#### New Ocean

##### Well-Known Member
I would glass it, but a phenolic MMT would work at that point.

#### Handeman

##### Well-Known Member
TRF Supporter
You really may not have to go that heavy on the glass and certainly not CF. A Loc tube with a light e-bay and nose cone should hold up to a lot.

My L1 rocket was built from 2" ID thick wall mailing tube, 6mm plywood from Micheals craft store, assembled with Elmers Carpenter Glue and held up fine to an I1299N load at 70Gs, 460 mph and 4028 ft. I really think it would have held up to twice the thrust.

#### bobkrech

##### Well-Known Member
A 45oz rocket with a I1299 will pull a hair over 95Gs when I plug it all into RS9. It sounds like a gruesome flight--would love to have a Mach buster, altitude maker and a G machine too.

Does anyone know of a way to purchase a true VK nosecone in 38mm?

I am hedging on whether or not to go with plain Blue Tube or CF sleeved flexible phenolic. Any thoughts?
You will need to reinforce the airframe to prevent a shred as I recommended above because fiberboard tubing alone is not stiff enough to prevent column buckling failure.

My friend Boris Katan and his son Paul http://www.bpasa.com/ launched sent up the X-16, a LOC Weasel powered by a Cesaroni 5 grain I540-16 White Thunder at NERRF 3 in 2007. The rocket is shown below.

It was a 32oz minimum diameter rocket on a full I, and simmed to reach mach 1.8 a second. The the front half of the airframe shredded by buckling failure (folding over) at 0.7 sec (the red puff in the picture and video), which RockSim predicts was at 550 ft and 1050 MPH as did my independent calculations. X-16 mach buster video (1.5MB)

I would not worry about the nose cone shape as far as drag is concerned. Your rail buttons will contribute more to drag than the NC shape so I would use a pop off rail button design to minimize drag.

Bob

#### cherokeej

##### Well-Known Member
What you're looking for is pretty easy, but not with paper. You need very stiff fins, AND a very stiff tube. Check TAP Plastics for a deal on their uni-directional carbon, used correctly, it will stiffen the fins and improve their attachment.

Eliminate the drag of rail buttons by launching from a tower.

Use the motor case as the airframe. Make a short, all carbon fincan, to slip down to the base of the case. Friction fit it. Make a short forward body tube, all carbon, with a shoulder inside, to sit against the forward closure. Leave just enough room for a good length of Kevlar shock cord, a charge, and an RDF transmitter, then the b/h of the av bay. Make the bay as short as possible, with just enough tube sticking forward of the av bay for the nosecone shoulder. The main is inside the nosecone. Minimum length, minimum diameter, optimized mass with added weight epoxied into the forward end of the nosecone. If'n ur gonna be a bear, be a Grizzly.

#### cjl

##### Well-Known Member
In some ways, you're better off using a full length tube than you are going the motor-as-airframe route. You can get a smoother transition to the fincan, and probably lower overall drag (especially supersonic drag) that way. That video I posted above is of a rocket that's about as optimized as I've seen.

Here's a picture of what you should aim for if your goal is absolute peak altitude.

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#### Viperfixr

##### Born Again Rocketeer
TRF Supporter
Excellent inputs...I was not optimizing length at all--plan was nearly 68" tall with nosecone! AT J510 is 23" long, and the ebay will be 6 inches long. Adding room for recovery harnesses and parachute--say 32" for the bottom and 10" for the top with a 5:1 conical nosecone...about 48" without scrimping too much. I saw Curt VD's rocket was somewhere around 29" total--wow!

#### cherokeej

##### Well-Known Member
I think Curt popped his streamer at the top. And it took him several tries. First few times, it took a left out of the tower. That thing was ridiculous. Not much more than postage stamp fins on a carbon tube, wet-sanded with 1600 grit, and a nosecone. Talk about a bullet. He finally got it to fly straight, though.

cjl is correct. You can eliminate some drag by making a full length a/f tube. But it's a trade. The short fincan concept allows switching long motor cases, without flying the added length every time, and the added mass and required length of an a/f coupler. But his more conventional design allows you to fly short motors too. There's an idea... Build two rockets.

That pic he posted sure is pretty. Bet that puppy moves out on just about anything you wanna feed it, let alone a J570. :eyepop:

James

That pic he posted sure is pretty. Bet that puppy moves out on just about anything you wanna feed it, let alone a J570. :eyepop:

James
Thanks. That's this one:

I flew it twice on an I600 last year. First time it went a little under 16kft, but the main shook out at apogee and it drifted 5 miles, out of our waiver cylinder. The second time it went a little over 16kft, but the main deployment charge was insufficient and it came in hard. Next season I'll try out the back end with a new nosecone on a CTI I216. It will also use a new deployment concept I tried out at the end of last season that provides dual deployment with a single airframe break.

#### GregGleason

##### Well-Known Member
Does anyone know of a way to purchase a true VK nosecone in 38mm?
You can PM me and I can give you a name/email who can turn you a CNC von Karman wood plug, and at a reasonable price. He turned one for me (base OD of 2.7" and fineness ratio of 5:1) and it came out well.

Greg

#### cjl

##### Well-Known Member
I think Curt popped his streamer at the top. And it took him several tries. First few times, it took a left out of the tower. That thing was ridiculous. Not much more than postage stamp fins on a carbon tube, wet-sanded with 1600 grit, and a nosecone. Talk about a bullet. He finally got it to fly straight, though.

cjl is correct. You can eliminate some drag by making a full length a/f tube. But it's a trade. The short fincan concept allows switching long motor cases, without flying the added length every time, and the added mass and required length of an a/f coupler. But his more conventional design allows you to fly short motors too. There's an idea... Build two rockets.

That pic he posted sure is pretty. Bet that puppy moves out on just about anything you wanna feed it, let alone a J570. :eyepop:

James
That's Adrian's I600 altitude rocket, as he posted - it's an awesome rocket to see. It's also impressive to see how he crammed dual deployment and an I600 into a rocket that's that short.

Here's liftoff on an I600R:

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