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I need to cluster 7 C-motors...

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OZ1SEJ

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I'm in Denmark. I need to launch a rocket weighing 1 kg (approx.) to an altitude of 100 m. Because of import regulations, the most powerful motors I can buy are C motors. The rocket needs to be quite reliable, because the payloads (cansats) aren't mine, and they need to be recovered in fair condition. It must be launched up to eight times in one day, once a year.

What would you do?

I've 3D printed a motor mount for 7 C motors, but I can't shake the feeling that this path opens up rich possibilities for things to go awry. On the other hand, I don't know what else to do. I know this forum discourages people from producing motors themselves, and I'm not sure I would be able to deploy a parachute with the same reliability that commercially available motors provide.

Any good ideas? Cause I don't have many left.

Best regards,
Steen
 

LithosphereRocketry

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I'm in Denmark. I need to launch a rocket weighing 1 kg (approx.) to an altitude of 100 m. Because of import regulations, the most powerful motors I can buy are C motors. The rocket needs to be quite reliable, because the payloads (cansats) aren't mine, and they need to be recovered in fair condition. It must be launched up to eight times in one day, once a year.

What would you do?

I've 3D printed a motor mount for 7 C motors, but I can't shake the feeling that this path opens up rich possibilities for things to go awry. On the other hand, I don't know what else to do. I know this forum discourages people from producing motors themselves, and I'm not sure I would be able to deploy a parachute with the same reliability that commercially available motors provide.

Any good ideas? Cause I don't have many left.

Best regards,
Steen
I'm not a cluster expert but here are my thoughts...

-What kind of current do your igniters take? How big of a battery does it take to fire them reliably?

If 6v can light it easily you should be fine with a car battery, though if you want to be extra safe you could set the battery up at the pad with a relay system. If you haven't seen a relay system before there are plenty of examples on the forum.

If you're lighting BP motors and have access to loose BP, you could look into the flash pan technique.

Sent from my LGL44VL using Rocketry Forum mobile app
 

Zeus-cat

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I have seen people use flash pans for large clusters and they work very well. All the motors ignited. The biggest problem you may have is getting black powder. I do not know how easy it is to get in Denmark.

Essentially, a flash pan is a pan that sits under all of the motors. A small amount of black powder is spread in out in the pan and a single igniter is placed in the black powder. No igniters are put in the motors. When the igniter burns it sets the black powder on fire and it burns very quickly, or flashes as it is called. This fire ignites the motors. I have never used a flash pan myself, so I do not know how much black powder to use.

I do not think smokeless powder will work as it probably won't flash. You need old fashioned black powder.
 

dhbarr

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Oh, and since you're struggling to lift a mass I'm assuming you'll need a fairly short delay.

Stagger them I'd say ( e.g. 1*3s, 2*5s, 4*7s ). This way best case scenario you blow most of your ejection charges into open air ( long kevlar leader ), worst case you blow the body tube but in any case that laundry is coming out to slow the payload :)
 

rharshberger

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Also of concern with flash pans is your 3D printed cluster mount and whether it can take the heat. If you want to shave some weight off build the cluster mount conventionally (unless the 3D printed one is lighter of course) using wood centering rings and cardboard motor tubes.
 

dhbarr

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Can't get the Klima D9's?
 

Zeus-cat

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Also of concern with flash pans is your 3D printed cluster mount and whether it can take the heat. If you want to shave some weight off build the cluster mount conventionally (unless the 3D printed one is lighter of course) using wood centering rings and cardboard motor tubes.
I forgot to address the 3D printed motor mount. Even lighting motors conventionally I would be worried about the heat from 7 motors damaging the motor mount. I would use more traditional materials as I would be very leery of 8 launches in one day on a 3D printed motor mount. And I sincerely hope you are not using Quest motors; the Chinese ones burn very hot. I have had a single C motor melt a plastic motor mount in an Estes rocket. I assume it was styrene plastic which should be much more heat tolerant than 3D plastic.
 

Sooner Boomer

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I forgot to address the 3D printed motor mount. Even lighting motors conventionally I would be worried about the heat from 7 motors damaging the motor mount. I would use more traditional materials as I would be very leery of 8 launches in one day on a 3D printed motor mount. And I sincerely hope you are not using Quest motors; the Chinese ones burn very hot. I have had a single C motor melt a plastic motor mount in an Estes rocket. I assume it was styrene plastic which should be much more heat tolerant than 3D plastic.
Surprisingly, paper can handle heat a lot better than plastic can (of course this depends upon the particular plastic and paper). Plastic will also soften and lose structural integrity under heat, where paper will not. An additional consideration is the glue used in construction, esp. when heavy use is anticipated. Glues will also soften and fail under heat. Epoxies are less heat sensitive than PVA glues, and metal-filled epoxies tops in resistance.
 

markjos

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Interesting challenge.

So I know the other replies have focused on ignition and cluster configuration, etc., but I couldn't help wonder about the performance. Could you reasonably get a 1kg rocket (empty weight, I presume) safely off the pad, and then to 100m on a cluster of 7 C motors? So I threw a very rough design and sim together in OpenRocket, and much to my (naive?) amazement, it's possible - if a bit marginal. 2.6" dia, 33" long, 41 oz with a cluster of 7 C6-5 motors, flies to 323 ft (98m). I'm sure with a bit of tweaking, it's doable. Thrust to weight and rail/rod exit velocity are a bit iffy, but...

This was really just to satisfy my own curiosity, mind you.

Mark
 

dhbarr

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Interesting challenge.

So I know the other replies have focused on ignition and cluster configuration, etc., but I couldn't help wonder about the performance. Could you reasonably get a 1kg rocket (empty weight, I presume) safely off the pad, and then to 100m on a cluster of 7 C motors? So I threw a very rough design and sim together in OpenRocket, and much to my (naive?) amazement, it's possible - if a bit marginal. 2.6" dia, 33" long, 41 oz with a cluster of 7 C6-5 motors, flies to 323 ft (98m). I'm sure with a bit of tweaking, it's doable. Thrust to weight and rail/rod exit velocity are a bit iffy, but...

This was really just to satisfy my own curiosity, mind you.

Mark
I did the exact same thing :).

The Klima c6-5's, most likely. Long rod.
 

OZ1SEJ

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Wow - thank you for all your interesting answers! :)

I was planning on using Estes C6-5 motors. Hopefully I will get a permit for importing those from a British reseller, but nothing is certain yet. I might just as well not get that permit. The Danish Safety Technology Authority requires me to make sure the seller encloses user manual and safety information in Danish, and my British friends have been very forthcoming. Shipping is however extremely expensive, and I can't store more than 5 kg NEQ at a time...

I haven't contacted Klima yet, because in my (very limited!) experience, the Klima igniters malfunction a lot more often than the Estes igniters (no ignition). But it would be interesting if I could persuade them, too, to enclose Danish instructions. Maybe the shipping costs will be lower.

Also thank you for your many suggestions for igniting many motors at once - they look very interesting, and I will most certainly study them in close detail.

I've never heard of the flash pan method before. We're an educational institution, so we can manufacture black powder at our own discretion. We regularly make rocket candy with our students; we just don't put it in a rocket motor, but roll it to a stick and ignite it with a fuse - outside. (Which is why I started considering making the motors ourself in the first place.)

With regard to the 3D printed motor mount: You have a point! I never thought of it, but of course the ABS plastic melts around 120°C. I have a single D motor mount, 3D printed in 2 mm ABS, which has withstood four or five launches so far, and still looks good. Of course, when using seven motors, the walls around the center one will get a lot warmer.

Once again, thank you so much for all your friendly suggestions and relevant questions! This forum rock(et)s! :-D

Best regards,
Steen
 

LithosphereRocketry

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*snip*
With regard to the 3D printed motor mount: You have a point! I never thought of it, but of course the ABS plastic melts around 120°C. I have a single D motor mount, 3D printed in 2 mm ABS, which has withstood four or five launches so far, and still looks good. Of course, when using seven motors, the walls around the center one will get a lot warmer.
*snip*
What I might do for the 3D printed mount is make the motor holes the OD of a conventional MMT and epoxy paper tubes inside them. That said, assuming you are using US certified motors the casing temperature is legally required not to go over 200°C under any conditions, so if you can make a mount that can handle that, you're OK.

However, if you go flashpan, that will put an open flame on the 3D printed part, so then you would want plywood.

Estes igniters misfire a lot too. I've never used Klima but I would assume Estes isn't any better, especially now that they've toned down their pyrogen. If you can find the black-tipped igniters, those work great- white-tipped, not so much.

Whatever you do, we want pictures!
 

RainierWolfcastle

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Given the weight and relatively low launch height along with the difficulty getting larger motors at your location, would it be worth looking at alternate launch methods such as slingshot or an air powered canon? Both take it out the realm of this forum, but might be worth looking at.
 

LithosphereRocketry

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Given the weight and relatively low launch height along with the difficulty getting larger motors at your location, would it be worth looking at alternate launch methods such as slingshot or an air powered canon? Both take it out the realm of this forum, but might be worth looking at.
I see a potato cannon in the OP's future... Recovery might be a challenge, though.

Aside from potato cannons, have you looked at Estes C11's if your limit is a C? They're 24mm in diameter, so you need a 3.1" outer tube, but they have a lot more kick for a heavy payload... How much does your payload actually weigh? And what dimensions does it need?
 

Salvage-1

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If you wish to launch a kilo cansat (say... a can of beans) to 100m with repeatability, you may want to look at a compressed air mortar.
 

CoyoteNumber2

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Steen,

Are you able to obtain nitrous oxide in your country? If so, hybrid rocket motors may be a good option for you.
 

OverTheTop

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I have seen people use 3D printed motor mounts and aft panels. I don't think it is as much of a problem as people think.

Run an experiment with a small rocket with one or two motors to try.

Also, if 3D printing, don't overbuild it. It is only C motors you are flying. If you make the print too thick it will be too heavy and you might be better off going with a more conventional mount system.

Post #3 had a pic of a cluster mount. If you are clever you can possibly leave out part of the tubes to save mass. :wink:
 

Leo

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I haven't contacted Klima yet, because in my (very limited!) experience, the Klima igniters malfunction a lot more often than the Estes igniters (no ignition). But it would be interesting if I could persuade them, too, to enclose Danish instructions. Maybe the shipping costs will be lower.
...
My humble response is that you get in contact with Klima ASAP. It seems to me you haven't been flying Klima in a very long time! A lot has changed. Igniters are 100% reliable and they have special ignition material for cluster rockets!

The instructions come in German, English, French, Spanish and Italien.
 

shreadvector

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Ĉu la instrukcioj venas en Esperanto?


My humble response is that you get in contact with Klima ASAP. It seems to me you haven't been flying Klima in a very long time! A lot has changed. Igniters are 100% reliable and they have special ignition material for cluster rockets!

The instructions come in German, English, French, Spanish and Italien.
 

Raketenolli

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I second Leo. You can make a more heat resistant 7x18 mm MMT from Klima parts (he's currently prototyping larger kits, so he might have a 6x18 oder 7x18 MMT for 75 mm tubes on hand) and also get his D9 engines (probably easier to get them from Germany or even pick them up, depending on where you are in Denmark), which allow you to stagger the ejection charges, as they come plugged and with different delays. I ignite clusters like that with a single igniter ("Brückenanzünder A") and matchtape ("Anzündband") and a bit of black powder in every engine's nozzle, but his "ignition spaghetti" ("Anzündstäbe") can also work very well. He has all of that in stock except loose black powder.

Oliver
 
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