New 102mm project - Chimichunga-I

[snrkl]

Newly minted L1 and I am looking for the next bigger project - a 102mm (4") airframe and entering the world of "Real Dual Deploy"...

Having problems finding a 4" kit in Australia that isn't majorly expensive to ship, so a discussion with Blake from AusRocketry was: "design it and we will make a kit for you..." so the design was kicked off almost immediately..

Design goals:
1) 102mm airframe
2) Dual Deploy capable
3) 29mm motor mount (not in a rush to move to >=38mm.. plenty of time for that on the path to L2)
4) Need to be able to carry it solo...
5) Visually striking
6) Must be Deadpool themed... (It's my partner's turn to pick the theme and she picked Deadpool - I think it is awesome, so why would I argue?!?!)
7) As I am moving into Dual Deploy, I want to be able to see the entire flight..
8) Not interested in FG yet - I want to leverage current build skills and focus the "learning" on the DD elements that will be new.

ORK with decals is on this dropbox link (45MB - so no attachment here) https://www.dropbox.com/s/8bn0pvj5uyhyhhq/Chimchanga-I.ork?dl=0 and has the added advantage of updating whenever I save it..
ORK sans decals is attached here: View attachment Chimchanga-I_no_decals.ork



Never one to shy away from a good looking OR render, here is initially planned art (complete with fabric textures - I have been looking for an excuse to figure out how to paint that):





Current thoughts are that I design to fly on motors up to an I200 (Which *rough* sims to 2100ft on this first revision before material realities are applied, which sounds about right)

Now, I have never built an ebay or DD rocket before, so input / feedback welcome.

Design is currently cardboard and 3mm Birch ply for fins and CRs - If BlueTube is available, I might try that instead.. Need to see what AusRocketry has / can get...

Thoughts?

 

[blackjack2564]

Nice project.
Only thing I would change is motor mount to 38mm.
You can always adapt down. A project of this size is very limited by only 29mm motors. There are many more small 38's with a variety of thrust profiles that would keep you from "outgrowing" this rocket.

38's will also closer approximate your goal of Dual deploy & this could also be used as a L-2 very easily.
I know many fliers who regret not making a MM 38mm.
Very few ended up happy with the decision to go with 29.

Good luck what ever you decide. Have fun!

 

[Tim51]

Really love the design - those fins look great, and 4" is an attractive size. I'm speaking from far less experience than Mr Hendricksen here but my own bears out exactly the point he makes above about the 38mm allowing you to get more out of the Dual Deploy set up you're building. Obviously conditions and fields vary but just as anecdotal evidence: I found with a single deploy 4" fibre board rocket I could get just over 3000' on baby I motors (I195s, I236 etc), followed by long walks and on occasion tree climbing. So I started exploring DD. I found the same booster with stock DD additions/ components solved the walk/tree issue, but the added mass of AV bay and extra chute etc meant the rocket would only hit around 1900' on the same baby I motors - an altitude that in most situations didn't necessitate Dual Deploy, and could be reached with SD configuration on an H motor. Anyway that's just my 0.02 GBP / 0.02 AUD worth (or whatever the exchange rate currenlty is :wink. Good luck with the build!

 

[markkoelsch]

Go 38 mm- to not do so would be a mistake.


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[qquake2k]

Love the design and the tail cone. What kind of retention are you thinking?

 

[Reinhard]

Another thing to consider: A bigger diameter doesn't necessarily equate to more impulse; it also offers more variety - especially if one is into long burn times. For example, compare the 54mm I115W to the 29mm I200W.

Reinhard

 

[tsai]

If you go 38mm, which I also think you should, I believe the 3mm ply fins would be a little thin. With a 38mm mmt, their is an opportunity for some much larger motors later down the road and dealing with the Mach transitions.

 

[Handeman]

If you go 38mm, which I also think you should, I believe the 3mm ply fins would be a little thin. With a 38mm mmt, their is an opportunity for some much larger motors later down the road and dealing with the Mach transitions.

I agree that thicker fins might be in order, but not so much because of Mach transitions. Using 38mm motors in a 4" rocket, the speeds just won't get up there, especially with the heavier DD setup with av-bay. If he goes with 54mm MMT right away (recommended) then Mach is a possibility. With that said, the fin design does slope back toward the tail and could require thicker fins to survive a hard landing with a heavier rocket. Fin damage with that shape is more likely than with a delta design. A large main deployed at low altitude would be a way to compensate.

 

[Steve Shannon]

I also recommend going to a 54mm motor mount and making an adapter for 38mm. If you want to fly 29mm motors in this make a 54/29 adapter rather than stacking adapters. The reason is to avoid the extra weight in the aft end.

 

[snrkl]

Hehe, you guys crack me up...

In 24hours it’s gone from “I’m not in a rush to go big - 29mm is plenty fine” to 38mm then to a 54mm MMT...

I’m betting there will be something bigger suggested soon.

I’m calling the ball and saying I’ve decided to install a 38mm MMT and that’s that..

There will ALWAYS be something bigger available, and when I get there, I’ll build another rocket. I don’t mind - I quite like building them...

The Dropbox linked ORK from post #1 is now 38mm.

I’m interested in thoughts on the ebay design - as I said I haven’t done one before.

Also, need to look into the tailcone as to whether I get a tailcone retainer or build one and use some other retention method.


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[LithosphereRocketry]

That's what I would do. No need to overdo it.

Can you get Loki motors down under? 38mm G's, H's and some I's are hazmat free- not sure if the regulations are the same, but I'd guess they're similar.

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[qquake2k]

Hehe, you guys crack me up...

In 24hours it’s gone from “I’m not in a rush to go big - 29mm is plenty fine” to 38mm then to a 54mm MMT...

These guys are enablers. They used to give me guff about my small motor mounts. I've only got two rockets with 54mm holes. One isn't finished (I've been working on it for years). The other I've flown a few times on 54mm motors, but the last flight was on a 38mm I600. Keep in mind that the larger the diameter, the more expensive the reloads and hardware.

But, having said all that, these guys are the reason I got my L2, and I'm glad I did.

 

[bill_s]

If sticking with 29 mm is part of keeping it light, it makes sense, otherwise not. Less than 3.3 pounds/1.5 kg with motor (also the Class 1 limit here, no waiver req'd). I have one but it grew through rebuilds and I keep to a motor budget and isn't suitable for high flights for multiple reasons. Even then, with the current motor selection shortage I don't think I'd set out to make a rocket that big without more choices. Many of the longer 29 mm motors are too high of thrust.

 

[snrkl]

These guys are enablers.

Oh, I know! It makes me laugh...

But, having said all that, these guys are the reason I got my L2, and I'm glad I did.

Yeah - me too... well, L1 anyway...

If sticking with 29 mm is part of keeping it light, it makes sense, otherwise not. Less than 3.3 pounds/1.5 kg with motor (also the Class 1 limit here, no waiver req'd). I have one but it grew through rebuilds and I keep to a motor budget and isn't suitable for high flights for multiple reasons. Even then, with the current motor selection shortage I don't think I'd set out to make a rocket that big without more choices. Many of the longer 29 mm motors are too high of thrust.

I have decided to up to 38mm and no further, for this next build anyway.. I think 3mm fins should be fine, especially given the way that I fillet...


For me, the joy is not just the flying, but the process.. (The flight for me is just the test at the end that says: "you did it right!")


I tend to build something that is new for me, fly it a few times, then straight away go: "What's next"...


I am not really the kind of person to fly the same rockets again and again...


Once I fly 38mm dual deploys and get comfortable with that, I *KNOW* I will want to do something bigger..

 

[Nytrunner]

102mm......so 4"? Sorry, couldn't resist, that's not a standard size number I was used to seeing.

You'll have enough size, and the long 29's have plenty of thrust, so why not stick a chimichanga or burrito in a bag tied to the shock cord? Extra incentive/reward for getting it back!

 

[Kallahan11]

I'm really upset with the poor advise on this forum to use a 38mm or 54mm motor mount, it's four inches stick a 98mm in there!

I feel you on the not launching the same rocket over and over, much more so on my HP stuff, I don't think I have more than 4 flights on any one HP rocket.

 

[snrkl]

102mm......so 4"? Sorry, couldn't resist, that's not a standard size number I was used to seeing.

It’s funny - I almost started the post with ‘4”’ in the topic but figured that with my signature it would be hypocritical!


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[Nytrunner]

It’s funny - I almost started the post with ‘4”’ in the topic but figured that with my signature it would be hypocritical!


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You're being acculturated!

I've always wanted to visit Australia, now I just need to see if I can time it to see Thunda Down Under

 

[Bat-mite]

It’s funny - I almost started the post with ‘4”’ in the topic but figured that with my signature it would be hypocritical!


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All the vendors I know refer to 4" kits as 98mm. I know that comes to 3.86", but apparently all the tube sizes are based on MilSpec.

 

[Steve Shannon]

All the vendors I know refer to 4" kits as 98mm. I know that comes to 3.86", but apparently all the tube sizes are based on MilSpec.

High Power Rocket body tubes have traditionally been referred to by the size of motor they accept. A 98 mm body tube accepts a 98mm motor. The PML tube sizes were nearly a standard for a long time. The PML 98 mm body tube is 3.900 inside diameter (99 mm, but that allows 98 mm cases to fit nicely.) The thickness of a PML 98 mm tube is approximately 0.062, so the OD is 4.024” or 102.21 mm.
I just put a digital caliper on a CTI motor and it measured 98.43 mm.

 

[snrkl]

You're being acculturated!

I've always wanted to visit Australia, now I just need to see if I can time it to see Thunda Down Under

You and me both - although this is *one* event that the travel costs might be a little worse for your end of the world than mine...


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[Andrew_ASC]

Hehe, you guys crack me up...

I’m calling the ball and saying I’ve decided to install a 38mm MMT and that’s that..

The Dropbox linked ORK from post #1 is now 38mm

You can still Loki 38-1200 it with a K load! If you won't go bigger diameter go longer!!! LOL. Then it'll be you'll rip the fins off...

 

[Bat-mite]

High Power Rocket body tubes have traditionally been referred to by the size of motor they accept. A 98 mm body tube accepts a 98mm motor. The PML tube sizes were nearly a standard for a long time. The PML 98 mm body tube is 3.900 inside diameter (99 mm, but that allows 98 mm cases to fit nicely.) The thickness of a PML 98 mm tube is approximately 0.062, so the OD is 4.024” or 102.21 mm.
I just put a digital caliper on a CTI motor and it measured 98.43 mm.

Makes sense. So maybe it was the motor sizes that were MilSpec. I'm sure I read somewhere that the oddball sizing (38mm instead f 40mm, 54mm instead of 55mm, 98mm instead of 100mm, etc.) was due to military standards of some sort.

 

[Steve Shannon]

Makes sense. So maybe it was the motor sizes that were MilSpec. I'm sure I read somewhere that the oddball sizing (38mm instead f 40mm, 54mm instead of 55mm, 98mm instead of 100mm, etc.) was due to military standards of some sort.

I heard long ago that our motor sizes were simply based on commonly available DOM metal tubing sizes. I don’t know if that’s true.


Steve Shannon

 

[snrkl]

Quick simple question - what is the collective thinking on which section drogue and main live in?

My thinking was drogue in the AFT section and main in the FWD section, such that any motor eject could be used as a backup drogue event, but I was wondering what others choose when building?

Is there recovery ordering reasons to choose one over the other? Is one less prone to tangling once the uncertainty of deployment while falling is taken into account?

 

[Speaknoevil]

4" dia. is tiny.

 

[Nytrunner]

Quick simple question - what is the collective thinking on which section drogue and main live in?

My thinking was drogue in the AFT section and main in the FWD section, such that any motor eject could be used as a backup drogue event, but I was wondering what others choose when building?

Is there recovery ordering reasons to choose one over the other? Is one less prone to tangling once the uncertainty of deployment while falling is taken into account?

Drogue aft of the ebay with main in the payload is often referred to as "traditional dual deployment"

That being said, as long as you walk out your recovery scheme properly, you can do whatever you can make!

 

[Bat-mite]

The logic behind "drogue in the aft" is that you break the rocket at the mid-point, and both ends point down, suspended by the drogue in the middle. When the main fires, the NC goes downward until the chute comes out. Parts stay separated.

If the NC comes off at apogee, now you have a lopsided suspension. When the main charge blows, you are blowing the payload tube back up toward the drogue, possibly causing entanglement.

 

[Nytrunner]

If the NC comes off at apogee, now you have a lopsided suspension. When the main charge blows, you are blowing the payload tube back up toward the drogue, possibly causing entanglement.

This my second point of "walk out the recovery properly"

 

[snrkl]

The logic behind "drogue in the aft" is that you break the rocket at the mid-point, and both ends point down, suspended by the drogue in the middle. When the main fires, the NC goes downward until the chute comes out. Parts stay separated.

If the NC comes off at apogee, now you have a lopsided suspension. When the main charge blows, you are blowing the payload tube back up toward the drogue, possibly causing entanglement.

Thank you - this information is VERY helpful to me..

I would also assume that this is the reason I see REALLY long fincan recovery harnesses with shorter leads to the fwd section before the main event fires, so that the fwd and aft sections of the rocket are not banging into each other causing damage and interfering when the main deployment goes?