DD Design Questions

Shop deals on ebay
Wildman Rocketry
rocket sok
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
SPRock

SPRock

Member
Joined
Oct 9, 2019
Messages
13
Reaction score
4
Hello! I'm new here, I guess you could call me a BAR since I got rid of all my rockets after high-school a decade ago and picked it back up again recently.
Anyways, I had some questions about a dual deployment rocket I'm building to get some experience with it. This will be a silly 7 foot, 3" rocket with a 38mm motor mount. Unrelated, has anyone ever done a Mean Machine upscale?

Specs:
3" blue tube, two sections (4' and 2') with 4 fins, 20" long 38mm MMT, "normal" av-bay, etc.
upload_2019-10-9_21-6-37.png
(drogue parachute missing in sim, main chute in lower for motor deploy, some parts out of place, just ignore :p)

Question, is zipper-less still something people do? This would be three 2' sections. I would think the advantage here is being able to access the shock-cord mount easily plus more positive drogue deployment without running wires down the tube. Although it does weigh a bit more with the extra coupler.
upload_2019-10-9_21-6-21.png
Any objections to 1500# test Kevlar cord? I've had good luck with Kevlar previously. However, I might spring for a nice nylon harness for the main from OneBadHawk.
 
You want your main up in the payload. Have your drogue down low for motor deploy. Or if your doing dual deploy, just let the electronics do the job and do away with motor delay, less chance of zippering too. You can leave your delay long for motor backup if you feel you need it.

1500# kevlar should be fine. 3/8", 1/2", and 5/8" tubular nylon works well too. Use kevlar need ejection charges, or a sleeve of nomex. People been using knots for thousands of years (albeit not necessarily in rocketry) so just learn good knot tying and you won't need fancy sewn loops.

I wouldn't sweat the little weight added from one extra coupler.

A rocket that big, put a 54mm in it. Adapt down for 38 then when you're more comfortable with it you can step up to 54. I have never built a rocket and wished I would've put a smaller motor mount in it.;)

Welcome to the forum and welcome back to rocketry.
Where you at?
 
MikeyDSlagle said:
You want your main up in the payload. Have your drogue down low for motor deploy. Or if your doing dual deploy, just let the electronics do the job and do away with motor delay, less chance of zippering too. You can leave your delay long for motor backup if you feel you need it.

1500# kevlar should be fine. 3/8", 1/2", and 5/8" tubular nylon works well too. Use kevlar need ejection charges, or a sleeve of nomex. People been using knots for thousands of years (albeit not necessarily in rocketry) so just learn good knot tying and you won't need fancy sewn loops.

I wouldn't sweat the little weight added from one extra coupler.

A rocket that big, put a 54mm in it. Adapt down for 38 then when you're more comfortable with it you can step up to 54. I have never built a rocket and wished I would've put a smaller motor mount in it.;)

Welcome to the forum and welcome back to rocketry.
Where you at?
Click to expand...
Yeah no worries, I know where to put the chutes, I was just playing around seeing where things would go.
I already have/made many of the parts for this rocket including fins, tube, retainer, etc, but I wouldn't be opposed to just building another one, eh hehe! Although swapping for a 54mm mount would only require me cutting down my fins and lasering new rings...

I'm in the Texas panhandle.
 
SPRock said:
Yeah no worries, I know where to put the chutes, I was just playing around seeing where things would go.
I already have/made many of the parts for this rocket including fins, tube, retainer, etc, but I wouldn't be opposed to just building another one, eh hehe! Although swapping for a 54mm mount would only require me cutting down my fins and lasering new rings...

I'm in the Texas panhandle.
Click to expand...
Ahh, Texas...I'm centrally located in the boot to your East. Many folks here from Texas. I'm heading to Todd Mission Saturday for RenFest.

What's the weight on your rocket?

Zipperless design or not, your second design with two couplers has benefits. One being what you mentioned: accessing the shock chord mount. Another would be if your booster gets damaged it is easily replaced. I have two rockets I've had to cut the fin can off and add a coupler and new piece of airframe.
 
MikeyDSlagle said:
Ahh, Texas...I'm centrally located in the boot to your East. Many folks here from Texas. I'm heading to Todd Mission Saturday for RenFest.

What's the weight on your rocket?

Zipperless design or not, your second design with two couplers has benefits. One being what you mentioned: accessing the shock chord mount. Another would be if your booster gets damaged it is easily replaced. I have two rockets I've had to cut the fin can off and add a coupler and new piece of airframe.
Click to expand...
Rough estimate, around 4.5-5 lbs but I think that's on the high end. I'm gonna stick with 38mm for now since it'll satisfy me through level 1 and into level 2 with J motors, until I get more comfortable with dual deployment. Then I can just build a new booster with 54 mm. Although by then I'll probably want to build a 4 inch rocket with a 75 mm mount haha!
 
Something to think about with a long rocket like that is that it's overstable and will want to weathercock a lot. So your deployment is going to be at high velocity even with altimeter ejection.
 
micro said:
Something to think about with a long rocket like that is that it's overstable and will want to weathercock a lot. So your deployment is going to be at high velocity even with altimeter ejection.
Click to expand...
Yeah, I am well aware. I remember launching my Mean Machine in a little too much wind and it went almost straight sideways. Every launch after that was calm weather only!
That's part of the reason why I'm considering the more zipper resistant design. Even just using Blue Tube should help in that regard.
 
micro said:
Something to think about with a long rocket like that is that it's overstable
Click to expand...

Not necessarily. A long aspect ratio (L/D>20 ) rocket requires a longer stability margin as the CO shifts more with the flight angle. what may be "overstable" for a shorter rocket, may be just what the super-roc needs. Aim for CG-CP separation of 10-12% of body length.
 
Nytrunner said:
Not necessarily. A long aspect ratio (L/D>20 ) rocket requires a longer stability margin as the CO shifts more with the flight angle. what may be "overstable" for a shorter rocket, may be just what the super-roc needs. Aim for CG-CP separation of 10-12% of body length.
Click to expand...
I remember reading about this phenomenon in a few places. TVM, I believe, talks about it in a newsletter as well. At the moment the fins are a bit big, but they make the look I want. I may have to cut them a bit shorter once I put it all together and measure CG. Easy enough to do with a table saw.
 
Well, I just weighed the thing - minus recovery hardware (parachutes, harnesses and electronics) - and it's just shy of 4 pounds. Should be right around 5 pounds all said and done.
I'm waiting on my harnesses from OneBadHawk, and electronics from Eggtimer and I should be good to go, minus motor goodies. Oh, and I need to find some 4FG black powder around here somewhere...
 
MikeyDSlagle said:
3FG will work fine if you can't find 4F
Click to expand...
Thanks Mikey, I think the problem will be finding any black powder around here. We have the big sports stores but only like two or three local shops and I've never seen anyone with BP rifles at the ranges.
 
SPRock said:
Thanks Mikey, I think the problem will be finding any black powder around here. We have the big sports stores but only like two or three local shops and I've never seen anyone with BP rifles at the ranges.
Click to expand...

I started out with Pyrodex P (pistol variant), as I hate the sulfur smell of spent black powder, but some of my club brethren told me I was crazy to compromise on the one thing you need to bring your rocket down safely. Ultimately I agreed with them and have a few pounds (meaning more than a lifetime supply) of Goex FFFFg (if you just lived closer, I'd give you a few ounces!). BP is night and day better. I believe you mentioned you are in TX - are you close to Arlington by chance? Schuetzen Powder is located there, and while I have no idea if they sell to the public it could be worth giving them a call to ask. If they aren't close, they would no doubt know anyone that sells their BP.

If you absolutely can't find any, Pyrodex P is more available at the big box stores (Cabelas) and would work, but ground test with it to get a comfort level of charge size and repeatability. Pyrodex P needs to be packed pretty tightly, so use wadding in your charge container to ensure it can't move around in flight.

<edit> I didn't see your launch timing, so if you have time you can order black powder. Both Goex and Schuetzen have dealer locators on their websites just to double-check if there's a nearby dealer. If you order, there's a hazmat fee but in my opinion NOT having to worry about a successful deployment is worth a little $.
 
Last edited:
g.pitts said:
I started out with Pyrodex P (pistol variant), as I hate the sulfur smell of spent black powder, but some of my club brethren told me I was crazy to compromise on the one thing you need to bring your rocket down safely. Ultimately I agreed with them and have a few pounds (meaning more than a lifetime supply) of Goex FFFFg (if you just lived closer, I'd give you a few ounces!). BP is night and day better. I believe you mentioned you are in TX - are you close to Arlington by chance? Schuetzen Powder is located there, and while I have no idea if they sell to the public it could be worth giving them a call to ask. If they aren't close, they would no doubt know anyone that sells their BP.

If you absolutely can't find any, Pyrodex P is more available at the big box stores (Cabelas) and would work, but ground test with it to get a comfort level of charge size and repeatability. Pyrodex P needs to be packed pretty tightly, so use wadding in your charge container to ensure it can't move around in flight.

<edit> I didn't see your launch timing, so if you have time you can order black powder. Both Goex and Schuetzen have dealer locators on their websites just to double-check if there's a nearby dealer. If you order, there's a hazmat fee but in my opinion NOT having to worry about a successful deployment is worth a little $.
Click to expand...
Arlington is about a five hour drive from me. I'm in the panhandle. I'll try asking the local shooters groups, if someone has seen some around here they'll know, I hope.
 
SPRock said:
Arlington is about a five hour drive from me. I'm in the panhandle. I'll try asking the local shooters groups, if someone has seen some around here they'll know, I hope.
Click to expand...
If you have a gun club nearby, give them a call. There are a lot of BP shooters out there. Tell the person at the gun club what you’re looking for (“I need a small amount of 3g or 4g black powder” and they will get the word out). Someone at your launch will almost certainly have black powder, and it’s a generous community.
 
g.pitts said:
If you have a gun club nearby, give them a call. There are a lot of BP shooters out there. Tell the person at the gun club what you’re looking for (“I need a small amount of 3g or 4g black powder” and they will get the word out). Someone at your launch will almost certainly have black powder, and it’s a generous community.
Click to expand...
I'm part of a few shooter's groups, there's not really any gun clubs around here. But as I said, I've never seen anyone shoot BP. I've seen substitutes a few times (Triple 7 and such).
 
I've been using Triple Seven 3F exclusively for years, it works great as long as you pack it tightly and seal it well. In the case of charge wells, I put seven pieces of criss-crossing masking tape over the top of the well and one around the perimeter. For straw charges, I cut the straw to length and fill the straw with Triple Seven, and wrap masking tape around the ends 4x, then the circumference 4x, then repeat this until there are about 8 layers of tape around the straw. If you do it right the straw shouldn't "give" when you squeeze it. They're 100% reliable.
 
cerving said:
I've been using Triple Seven 3F exclusively for years, it works great as long as you pack it tightly and seal it well. In the case of charge wells, I put seven pieces of criss-crossing masking tape over the top of the well and one around the perimeter. For straw charges, I cut the straw to length and fill the straw with Triple Seven, and wrap masking tape around the ends 4x, then the circumference 4x, then repeat this until there are about 8 layers of tape around the straw. If you do it right the straw shouldn't "give" when you squeeze it. They're 100% reliable.
Click to expand...
Good info, thanks Cris! If I can't find any BP I'll probably try it. My Cabela's has plenty of Triple 7 and other BP substitutes.
 

Similar threads

Lt72884
chute arrangement for dual deploy using a h550 or j270 motor
2
Replies
41
Views
1K
Banzai88
Banzai88
cornbeef007
Kevlar Shock Cord Damage
Replies
17
Views
858
rharshberger
rharshberger
SuperNova-Rocketry
2 parachutes exiting from the nose cone
Replies
15
Views
631
SolarYellow
SolarYellow
J
Motor Tube Length Question
Replies
8
Views
559
JamesBenMoore
J
haon151
L2 Rocket Design Questions
2 3
Replies
80
Views
5K
Glasspack
Glasspack

Latest posts

Back
Top