Various questions on clustered rocket

[f16fan12]

Hi all,
For my senior project I'm building a rocket (attached Rocksim). It's meant to be a learning experience in some aspects because I haven't scratch built a rocket of this size and power before (though the power is nothing much, just a 29mm and two 24mm). I've built other high power kits with 38mm motor mounts and have had success with those.

The rocket is 4" in diameter with two 34 inch body tubes and a nose cone approximately 16" long. All parts are LOC.

I just have a couple questions to begin with:

1. What would you recommend for the fin thickness? My Binder Designs Excel, which is also 4" but a little shorter than what this will be, has 3/16" fins. Do you think the same thickness will be fine? Also, what's the easiest way with regular tools to cut through wood of that approximate thickness?

2. Clustering... I'm thinking of buying a high power 29mm case because all I have now is the 40-120. I've heard how you're not supposed to cluster motors with different propellant types but can it really be that hard? Ideally I would like to see a blackjack or something like that in the middle with white lightnings on the two 24mm motors, but I'm fine with other combinations that'll look nice. You got any tips on igniting them? I launch with a club, so power isn't the problem.

I was thinking about perhaps trying my hand with electronics, like a timer or something cheap, to maybe airstart the motors. I've never used electronics before, so I'm wondering if it's worth starting to use electronics on a rocket I would very much like to get back in one piece. What do you think?

Thanks a lot.

I appreciate all feedback,
Jesse

 

[heada]

Jesse,

3/16ths ply or 1/4th ply will be plenty strong and easy to work with for a rocket like this. I think 1/8th would work as well, but I'd stick with either 3/16ths or 1/4th. As for whats the easiest way to cut it? Ask 100 people and you'll get 100 answers. My preferred method is a band saw but you can use anything all the way down to a normal hand-saw. The X-acto knife will not cut it (pun intended)

As for clustering different types of motors, its normally shied away from different types of propellant (black powder and APCP) but different formulations of a type (black jack vs blue thunder) are easier to handle. I wouldn't try a BP/APCP combo but for this type of project, I wouldn't hesitate to mix APCP forumations. I would try to stay away from a really fast lighting motor (blue thunder) and a really slow lighting motor (black jack) but using using a medium speed motor (redline or white lighting) with either a fast or slow motor isn't that hard. Air starting the two outboard motors via a timer (or flight computer) is a great idea.

Good luck!

-Aaron

 

[roadkill]

Jesse,

At a quick glance at your design, a few things popped into my
mischievous mind...

- Its big and heavy, if equipped with correct size chute to bring it down nice, you'll get away fine with 1/8" thru the wall fins properly filleted, speed kills - not weight...

- Same reason, those 24mm mounts really are not going to make much difference in anything besides a coolness factor, why not go for a twin 29mm cluster instead...

- Use longer mmts with third ring on the front end to eat up some of that volume, you got quite a bit of bird to pressurize...

- For electronics you have to add an e-bay in your design, or you could keep it simpler with motor ejection well simulated for the right delay. If you have to drill the delays on two or more motors, leave one about a sec longer for a 'backup' charge...

- Post lots of pics...

 

[f16fan12]

Thanks for replying you two.

The chute will probably be 48" topflite. I think that's pretty large even for this size rocket, but I've got a nice field to fly in with MDRA.

The 24mms are I guess for show, because I would like to see a white lightning and two blackjacks, but I'm thinking now a redline and two whites or blacks would also look nice.

The electronics I was thinking about would not be for dual-deployment, but rather for airstarting the 24mms. I don't think I'll do that because I just don't have any experience fitting electronics into rockets.

About how long do you think the motor mounts should be?

Thanks,
Jesse

 

[heada]

I make my mounts so that about 90% of the longest motor I'll use is supported. That works out to be about 4 to 5 inches on a 24mm and between 6 and 10 inches on a 29mm. The added weight of additional tube is very small so I see no reason to keep them short (unless its a short rocket)

-Aaron

 

[Daedalus]

3/16" ply should be fine for the fins.

My latest cluster build Pyrophoria uses a single length of LOC 4" tube and on a 29mm 240ns H120 WS and 6 D11-P motors got to just short of 1400 feet. Given the size I would say your rocket will be somewhat underpowered on a 29mm motor - you certainly won't have many motor options available. I would suggest increasing it to 4 29mm mounts (you can still fly it on 2 29mm motors) or a 38 mm and 6 24mm mounts.

The easiest way to cluster is a central composite and surrounding BP motors lit with quickmatch if you have access to it. For all composites ground starting the core motor and then airstarting the surrounding motors is a good move - you know you have your main motor running and hence there will be an ejection charge.

 

[f16fan12]

Well I wouldn't mind using more power, but I have two problems:

1. I've already got the 3.9" - 1.14" centering rings, and so they're just waiting to get two 24mm holes drilled in them.

2. The motors would be expensive. I only have one RMS 29/40-120 system, and another, not to mention 2 others, would be expensive. I guess I could use single use motors, but those can get rather expensive as well.

Though now I'm thinking...maybe a 3 motor cluster arranged in a line rather than in a triangle would be good. Then I could launch it with one 29mm or two 29mm or three 29mm. Though one would be a stretch I guess.

What do you think?

Thanks.

 

[heada]

If you make the motor mounts 29mm, you can always make a 24mm to 29mm adapter.

I would also move the motors to an in-line configuration so that if all 3 motors lit at the same time, you wouldn't have off-center thrust.

With some inventive thinking, you might be able to make interchangeable motor mounts (take one out, put a different configuration in) I seem to remember seeing a thread on here involving a Saturn 1B that could either be launched on a 24mm or 4 18mm motors and it used an interchangeable motor mount.

-Aaron

 

[new2hpr]

I'm also building a similar size rocket for my L1. It is 4" x 4-5', depending on payload section. I'm starting with a LOC 438 EXL, but adding 6x 24mm mounts around the stock 38mm.

Leaving off the payload and ebay, it's light enough to test out on a G80 or 7x D12's and still get respectable altitude. (Rocksim puts it at around 1000ft, so somewhere below that).

The L1 flight is still wide open as to what combination to fly. Will depend upon if I put a timer/stager down in the fin can or just ground start all. Might just stick to a single H to keep the cert easy, but that woundn't be fun.

It's also set up for dual deploy, but that will wait 'till after L1. I'll stick to motor deploy for now.

-Ken

 

[roadkill]

Jesse,

You could shorten that forward body tube down to 17"
with still plenty of stability margin with 'full stack' load
of motors and save yourself almost 7oz of weight for a bit better performance.
That'd leave you with half a tube to make a nice payload bay...

Ken,

For your L1 flight, just keep it simple and use one single
center motor, cert flights are better off successful than fun.
Most of failures in cert flights are caused by 'too many moving parts' aka unnecessary doodads. Once you got your cert, play all you want...

 

[f16fan12]

How do you all recommend I ignite 1 29mm possibly H-128 and 2 F-21s at the same time? Quickburst isn't allowed to sell igniters anymore, right? And I can't find the Magnelite kit anywhere...

 

[heada]

I got my Magnalite from PML. $25 for enough pyrogen to make a LOT of igniters. I've made over 200 igniters and still have about 1/2 my pyrogen left.

https://secure.consumersinterest.com/pml/components.asp?groupid=24

You can also use the FirstFire igniters from Aerotech but I find that a good hand-dipped magnalite is better.

If you *really* wanted to, you could rig up a "clip whip" to fire a cluster of copperhead igniters as well, but I don't think anyone would recommend that.

-Aaron

 

[f16fan12]

mmm thanks for that link heada. You recommend I buy the wire from PML too, or should I just try getting some from home depot or somewhere?

 

[heada]

You could get the core wire at a home improvement type store but not the required nichrome. You can buy the nichrome online (e-bay almost always has it listed) and the silver solder to make your own. You can make an igniter without nichrome, but you'd have to have some conductive primer (nitrocellulose lacquer), and I find that its just easier to use the pre-wrapped, soldered and ready for dipping igniters from PML if I'm going to make less than 50 or so. At $12 for 12 igniters, its just too easy to buy them from PML. If you plan on making 100s of igniters, buy the nichrome and solder and wrap them yourself (drops the price a lot)

-Aaron

 

[heada]

Oh, and as far as getting them to ignite as the same time, make sure you're using a powerful 12v launch system. A relay with a car/boat battery would be ideal.

This fall, I launched a 7 motor cluster with hand dipped magnelite igniters. All 7 motors lit (6 F42s and 1 G40) The G40 actually came up to pressure about 50 feet above ground.

-Aaron

 

[Flamboyant]

Cool flight idea:

Launch on Black Jack motor (slow liftoff)
Airstart two Blue Thunder motors after about two seconds. (slow liftoff to really fast)

 

[f16fan12]

I'd love to try that if I knew something about electronics...

 

[roadkill]

You guys remember that the reason
why the liftoff is slow is the low impulse
long burning characteristics of the Black Jack
propellant...

Much safer to hit the fast and hard burning
Blue Thunder first, once you get it cooking
kick in the BJ for a long sustainer burn...

...or you might end up with the
'Scud of the Year' award...

 

[heada]

Thats why I had 6 F42 motors and only 1 G40. I knew that the 6 F42 would kick the rocket off the pad and the G40 would give it a little staying power once it was up to speed. I thought about using a G75 as the central motor, but with the 6 F42s I wasn't entirely sure that the G75 would lite before the F42s pulled the rocket kicking and screaming into the air..... 3000 feet and 400 mph

Putting a central H268 and 6 G80s would have broken mach and 1 mile...but it sure would have been expensive.

-Aaron