First time minimum diameter build.

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Ccolvin968

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Long time no post!
It's been really busy with life in general.

I'm planning on starting a 38mm carbon fiber minimum diameter build in a couple weeks.
Is there anything special I need to know about doing a MD build?
Is building a CF rocket any different than building a FG one?
My first two HP rockets were through wall fin mounting, so I was confident in those.
Haven't failed me yet!

I'll be using aeropoxy for the construction of this kit so epoxy strength isn't a major concern.
How or where do I place the rail buttons since I can't screw them into the lower booster section?
Can I just aeropoxy those on?

Looking forward to getting this built and flying next year!
It'll only be about a pound completely built. :-D
Should be an easy Mach killer. :)

Thanks for any help on my first MD build!
 
Is this the Madcow CF Mongoose 38 that is a literal steal right now? Unbelievable price.

I have never built CF before, but lots here have. You will get results. A big thing to think about in MD is retention. I'll be following....
 
To stay true to form with a MD you would not use rail buttons or any type of rail guide, although if your not going for any records I would say do whatever you feel comfortable with. The low profile universal rail guide is a good choice. I have several MD rockets, including the original Mongoose 38 and none have rail buttons. I have used either a club tower or the fly-away-rail guides in both the 38 and 54mm config. I know some have reported issues with the rail guides, which should not be dismissed so quickly, however I have not had an issue and still have full confidence in the technology (currently looking for some wood to knock).

Other considerations you will have with this build is motor retention, which you are going to get a whole wack of opinions on. These will vary from friction-fit to traditional retainers and internal MD retainers like an Aeropack (which is what I use), and then there are the custom homemade retainers. If you have access to a 3D printer you might want to look around the forum. I was looking at a 38mm MD GoDevil build that the OP did up a really nice simple 3D printed internal retainer. Plus and minuses of these vary. Friction-fit is the KISS method so simple low-to no cost and a lot of guys swear by it, I have never been comfortable but I wouldn't dismiss it so fast. Traditional retainers are relatively low cost and allow any size hardware to fit, without the need to use extension rods or something that is removable and relocatable. However traditional retainers have a bit of external bulk that goes against the premise of MD. Again this is or isn't an issue depending on the flyer. Internal MD retainer is reliable and holds true for MD, but it is the least flexible without a bit of thought, which equals more work and it may require the addition of extension rods so you're caring a bit of extra mass with smaller or shorter hardware.

Another relative challenge for a 38mm MD is electronics. You will want to go with electronics vs. motor eject, IMO, and you will want a tracker. Packing that into a 38mm air-frame is very doable but it will require a bit of extra thought vs. say a 54mm or larger air-frame. Again lots of threads will show you some really great ideas, and today there are some good 3D printed solutions for both. I used a 3D printed sled for the altimeter section on my Mongoose 38, and went with an EggFinder mini in the nosecone for tracking. Again lots of discussion around this setup. Even to the point where if you were going for max performance you may consider more of non-traditional single split HED config.

One of the more controversial discussions will be fin attachment, specifically T2T or not. Again this is personal and I would do first what is aligned with your goals and or just what you want to do. With a 38mm there certainly is enough flyers who have successfully launched with the bare min, I am not one of them. I have done T2T on all my MD builds, mainly becasue it is aligned with long term goals of launching something that will really requires it, and it does not make any sense to me that the first time I am using a technique is on a build where it will really count. Therefore part of why I have done this is to gain practical hands on as well as to refine my technique a bit, any with T2T you will change up your method with each layup, certainly for the first x number of attempts.

At the end of the day there are a lot of considerations with MD vs. a traditional rocket and not just in fin mounting, but first and foremost your decisions are yours so have some fun with it.
 
Motor selection matters more on MD than most places. For instance, you could choose top retention, use an EX tailcone closure, and shave 4-6" of airframe off with a certain set of AT choices. CTI & Loki have different requirements in 38mm as well. So are you wanting to go high, fast, other?
 
Thanks mpitfield. I appreciate you taking the time to type all of that up. That's a lot of things to consider.

DHbarr: I want this to be my fast rocket. I have a 4" Tomach that will hit 20,000 easy. (my clubs waiver limit.)
I plan on using the typical motor closure from aeropack.
 
Thanks mpitfield. I appreciate you taking the time to type all of that up. That's a lot of things to consider.

DHbarr: I want this to be my fast rocket. I have a 4" Tomach that will hit 20,000 easy. (my clubs waiver limit.)
I plan on using the typical motor closure from aeropack.
Fastest top speed or fastest acceleration off the pad? Weight is the enemy in either case, should be lots of fun. What motor are you leaning toward, and are you married to the Aeropack? Losing that mass aft means less nose weight required, all things being equal.
 
Thanks mpitfield. I appreciate you taking the time to type all of that up. That's a lot of things to consider.

DHbarr: I want this to be my fast rocket. I have a 4" Tomach that will hit 20,000 easy. (my clubs waiver limit.)
I plan on using the typical motor closure from aeropack.

Aeropack! I hope you mean the internal retainer version.

Drag is the Devil, I tells ya! The Devil!
 
Unlike the through the wall mounting kits, you use fin alignment jigs, and you tape off near the fillets. When you make a fillet, you can leave a slight gap near end of fin root at first to remove the fin jig, without epoxing the fin jig to airframe, or that's how some kits explain it. Scratch built two HPR MD multistages with L-1 motors for university project, one was CF other was Fiberglass. Jim Jarvis has excellent visual tutorials if you want to tip to tip wrap well. We didn't think it was needed below Mach 1.5 but your opinions may vary. You can sort of scuff up the fin root end with 60 grit sandpaper and also the area the fin will attach too on body tube. Masking tape off the fin part you don't want epoxied. You can do additional fillets as needed after fins are initially bonded to airframe straight. Other people may have way better tips, that's how noobs did it.

The CF material will wet sand very nicely to a glass smooth finish with 1200 grit sandpaper. You may spend more money with laminates or lubes for finishing if you wish too.
CF is not pleasant to sand large quantities of usually a yard or two of 38mm tube indoors is bad idea in poor ventilation, you will enjoy outdoors ,or if you must do indoors then just grab a harbor freight paint respirator its much better experience, try to have a water supply running. CF dust tends to coat everything and stick to bottom of shoes, waxed tile floors, and you're clothes. The university lab was indoors at first, we started wearing paint respirators after day 1 of a week, the janitors complained of black dust coating everything, and they quickly changed that decision so we took airframes off campus and sanded outside for our own sanity. (We had zero experience with the stuff on scratch builds at first. A kit should have so little sanding you won't notice it too bad.)

Fiberglass had similar sanding techniques to CF, however it was much less messy and bothersome. It tracked well with GPS trackers where with CF you'll want a hollow nose to track with due to it blocks RF signals. Try to limit your G load under 60 G's for trackers and electronics. If you use a fast propellant like WARP9 or VMAX you will likely not have a tracker signal during boost or could have disconnected batteries in flight and a failed recovery plus at worst ripping fins off.

You can friction fit the casings to airframe without buying those aeropack retainers by using masking tape or 3M tape. Spiral wind it from end to front in layers may help.
Depending on how much Mach your airframe has 1.7+ you may want to call Aeropoxy about the stuff you have and ask what Tg value it loses tensile strength at. It's very strong stuff room temp to slightly over a hundred fifty F, (Depends on type), then it just drops strength real fast. Rocketpoxy, proline, and cotronics were also better at thermal loads but cost more money.
You should worry about aero heating slightly for the epoxy choice. You really need to figure out your performance goals first and motor selection. Faster will cost more money.
 
Thanks Andrew. I guess friction fit might be my way to go... Not exactly sure how I feel about it though.
It would sure as hell save weight and drag... I'm not awful worried about $25 otherwise. Lol.
I don't want to use the internal motor retainer... I like having my motor eject as a backup.
Just double checked. I have Rocketpoxy... Not Aeropoxy. My bad!

DHbarr: Fastest top speed... Although I want to have the option to switch motors in and out as I wish.

CI: I was thinking about the slip on one... But after reading your comment, I agree with you.
I don't want that much excess drag.

I'll mess around with some friction fitting before I commit to that, but I saw a guy at my last launch hit 2 674 with a friction fit motor.
It literally pealed the paint off the nose.

Do we think Mach 3 is doable?
Not on the first flight and not every flight.
I'd want to be above Mach 1 for each flight though.

No idea for motor selection yet.
I'm a go with the flow and build it for whatever motor I feel like sticking in it. Lol
 
Loki k1127, just gonna' leave that there. Or any other loki 38/1200, should be able to edge out m2ish.

I'll submit m3's not achievable off the shelf in 38mm single stage with anything that's ever been certed.
 
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Make sure you build it to fit the Loki 38-1200 case. Those are the nastiest motors available in 38mm. I got Mach 2.3 on the K627. I was going for altitude, there are other options for that case that should crank up the acceleration and speed. I doubt I'll get Mach 3 though. I think I remember the best sim being in the Mach 2.7 range for the Loki K1127 in my GoDevil.
 
Somewhere in the 2.5 to 2.7 range is about the most you'd get with a K627 and a stock rocket. The J1026 sims in the 2.25 range.
 
Just got that CF Mongoose from Madcow for the literal steal of a price at $99! :D
 
Sold out at 9:10, maybe sooner (stepped away from my desk).
 
I also picked up a 38mm Mongoose . As expected, it was sold out before 7:08 AM PT. I was debating buying this the last few days. Alas, I couldn't pass up what looks like a great deal on something I was thinking about building before the sale was announced. I also picked up a couple of Madcow 6" chute protectors and a 38mm AeroPack Min Diam Retainer with Bypass Holes so that I can use motor eject for flights that don't coast more than 14 seconds or so.
I'm looking forward to building it. I'm thinking dual deploy with an Eggfinder Mini or perhaps the new Featherweight Tracker.
 
I got one too. I saw the same thing as Kevin, gone by 7:08. You don't have to be faster than the bear...

I see a drag race coming Kevin.

Mike
 
I got one too. I saw the same thing as Kevin, gone by 7:08. You don't have to be faster than the bear...

I see a drag race coming Kevin.

Mike

Sounds like fun!
How about we go with the hard hitting H550ST? I'm guessing that would give one of the fastest kicks off the pad since it's using the light 38/360 case.

I was also thinking that a slow long burn like an 38/360 I49 would be nice in this rocket as well. Would make for a longer run off the starting line!

I'll reserve the J motors for light wind days!

I'll be working to figure out how to integrate the motor retainer and shock cord anchor to enable flying AeroTech 38mm RMS loads for their 360 through 1080 case sizes. I have the 360, 720 and 1080 cases.. and don't see myself buying any others. I'll likely pick up the Reload Adapter System, and that threaded perforated closure for use with the RAS and motor retention.
 
Congrats, guys! Very, very tempting, but I am not looking to get into MD and mach-busting right now. Curious to see what's coming for the next three Fridays.
 
I also picked up a 38mm Mongoose . As expected, it was sold out before 7:08 AM PT. I was debating buying this the last few days. Alas, I couldn't pass up what looks like a great deal on something I was thinking about building before the sale was announced. I also picked up a couple of Madcow 6" chute protectors and a 38mm AeroPack Min Diam Retainer with Bypass Holes so that I can use motor eject for flights that don't coast more than 14 seconds or so.
I'm looking forward to building it. I'm thinking dual deploy with an Eggfinder Mini or perhaps the new Featherweight Tracker.

I'm new to the MD arena. Can someone explain in simple terms how the 38mmAeroPack Min Diam Retainer and the Aerotech Forward Closure adaptor or the CTI forward closure adapter work? I understand how the AeroPack Retainer is placed in the tube but I can't seem to find the Aerotech one listed anywhere and I don't understand how the CTI one attaches to the motors. Sorry if this sounds ignorant but like I said, I've never done a MD build before.
 
I'm new to the MD arena. Can someone explain in simple terms how the 38mmAeroPack Min Diam Retainer and the Aerotech Forward Closure adaptor or the CTI forward closure adapter work? I understand how the AeroPack Retainer is placed in the tube but I can't seem to find the Aerotech one listed anywhere and I don't understand how the CTI one attaches to the motors. Sorry if this sounds ignorant but like I said, I've never done a MD build before.
AT is just a tapped plugged forward; or a RAS floater with the open tapped passthrough forward.

CTI adapter just screws in where the powder would go, IIRC.
 
I'm new to the MD arena. Can someone explain in simple terms how the 38mmAeroPack Min Diam Retainer and the Aerotech Forward Closure adaptor or the CTI forward closure adapter work? I understand how the AeroPack Retainer is placed in the tube but I can't seem to find the Aerotech one listed anywhere and I don't understand how the CTI one attaches to the motors. Sorry if this sounds ignorant but like I said, I've never done a MD build before.

As per jhbarr the adapter just screws into the end of the CTI charge cup. Below are pics of my 54 and 38mm adapters, which as you can see by the last pic vary in depth and the plug that fits into the hole in the base of the charge-well. I found the easiest way to screw them in was using a socket wrench with a 22mm socket, otherwise you can misalign them very easily.


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As per jhbarr the adapter just screws into the end of the CTI charge cup. Below are pics of my 54 and 38mm adapters, which as you can see by the last pic vary in depth and the plug that fits into the hole in the base of the charge-well. I found the easiest way to screw them in was using a socket wrench with a 22mm socket, otherwise you can misalign them very easily.


View attachment 331341View attachment 331342View attachment 331343View attachment 331344View attachment 331345View attachment 331346

I was not aware that there were threads inside the CTI charge cap. I assume you must remove the BP since I don't see where there is an opening that would allow it to work with the Aeropack adapter with bypass holes for motor eject.
 
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