Fire Blaster

Discussions on mid/high powered model rockets using F powered motors and above.

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Aquaman33
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Fire Blaster

Postby Aquaman33 » Sun Sep 15, 2019 8:57 pm

To cut a long story short I was given the opportunity to buy some cheap 29mm Estes Pro Series F and G motors, but I don't own a rocket with a 29mm motor mount. So thought I would build one.

The rocket will be based on a BT60 (42mm) body tube and a 200mm long nose cone, because I had those in the back of a drawer. So not a minimum diameter rocket, but a fairly small diameter relative to the motor.

The 2 motors that I want to use in this rocket are an F26-6FJ and a G40-7W. So I loaded the body tube and nose cone into Open Rocket and then played around with the fins until I came up with something that was stable and needed a 6 (F motor) or 7 (G motor) second delay.

The result was this:

Image

Open Rocket says it will fly to 1,700 feet on the F motor and 2,300 feet on the G motor. Plenty high enough for me, as its a fairly small rocket and won't have a tracking device.

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Re: Fire Blaster

Postby Aquaman33 » Sun Sep 15, 2019 9:10 pm

I started construction with the 3d printed parts, which are centering rings and the motor retainer.

I designed the centering rings in SketchUp. I wanted to include fin slots so the through-the-wall fins would be very secure and easily aligned.

This is the forward centering ring with a hole for the shock cord. The rear centering ring is the same except it doesn't have a shock cord hole.

Image

Image

Next I downloaded a motor retainer design from Thingiverse:

Image

Image

It can be found here https://www.thingiverse.com/thing:3691731

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Re: Fire Blaster

Postby Aquaman33 » Mon Sep 16, 2019 10:02 am

My son asked if he could come up with the name and colour scheme for this rocket. So after a bit of a brain storm he came up with the name "Fire Blaster".....and he wants flames for the paintwork!

I originally printed the motor retainer in black, but my son suggested we reprint it in orange to go with the fire theme. So we will use the one of the left.

Image

I had some reservations about a 3d printed motor retainer, especially with a G motor, but it seems that lots of people around the world are using them without any issues. I think a total failure is unlikely and if it does start to show signs of damage I can easily print a new outer piece.

According to the Thingiverse designer they have been successfully used with Cesaroni 3 grain H motors.

I printed it slowly with the thinnest layers possible and 100% infill. So it feels really solid.

I forgot to photograph the centering rings before I installed them, but you haven't missed much.

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Re: Fire Blaster

Postby air.command » Mon Sep 16, 2019 9:37 pm

Looking Great Derek! I will be very interested to see how those retainers hold up, but like you said, you can always print a new one. It may be harder to unscrew if it deforms somewhat?
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Aquaman33
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Re: Fire Blaster

Postby Aquaman33 » Tue Sep 17, 2019 10:15 am

air.command wrote:Looking Great Derek! I will be very interested to see how those retainers hold up, but like you said, you can always print a new one. It may be harder to unscrew if it deforms somewhat?


Yes, agree it could get tricky to unscrew, but I am hoping the cardboard motor mount tube provides sufficient insulation.

I understand that some people are successfully flying 3d printed motor mount tubes. But I didn't go that far.

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Re: Fire Blaster

Postby Aquaman33 » Sun Sep 22, 2019 9:01 pm

I ran out of time with this build, so failed to take photos of most stages.

NSWRA's high powered launch is 1 week away, so I just had to get it done.

I cut 5 fins from balsa and then vacuum bag laminated them with carbon fiber.
Image

After sanding I kept the 4 best looking fins. They feel very strong, but light. Here is one prior to sanding and clean up.
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I designed and 3d printed these launch rod guides. They are designed to fit perfectly on the body tube and were epoxied in place.
Image
Image

To reduce the chance of a zipper I:
- Used a Kevlar shock cord 5 times the length of the rocket
- Added 2 layers of shrink tubing at the point where the Kevlar exits the body tube
- Wicked some thin CA glue into the top of the body tube
I have never had a zipper with this technique.
Image

I drilled the nose cone as I don't trust the regular attachment point.
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The process for the paint job was:
1. Mask off carbon fins.
2. Spray body tube white.
3. Mask of lower half of body tube in flame design.
4. Spray top half of body tube yellow.
5. Spray flames light orange.
6. Spray lower half of flames dark orange.
7. Spray tips of flames cherry red.
8. Clear coat.

This was a lot of work, but for my first attempt at flames I am happy with the result. I wish I had taken the orange further up the rocket, but you live and learn.
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Back end finished with motor retainer installed.
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I think it looks better with a black motor retainer, but my son wanted orange and its something a bit different.

The fin attachment into the 3d printed centering rings is incredibly strong. I think the fins would snap before they come away from the body tube/motor mount.

And finally the weigh-in without a chute or motor, just under 150g. This means it could comfortably fly on a D motor, but I only plan to fly it on F and G motors.
Image

Depending which motor I use the Thrust to Weight ratio should be somewhere between 9:1 and 27:1.

The CG is a little further back than I planned, but I have opened up the hole in the nose cone so I can add some fishing shot and epoxy if required.

Looking forward to launching it at the weekend.

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Re: Fire Blaster

Postby Legendontour » Mon Sep 23, 2019 5:31 am

Looks great. Can’t wait to see launch pics


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Re: Fire Blaster

Postby OverTheTop » Mon Sep 23, 2019 7:56 am

Nice work!
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Re: Fire Blaster

Postby air.command » Mon Sep 23, 2019 9:03 am

That turned out really well! Can't wait to see it fly. :D What brand of clear coat are you using?
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Aquaman33
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Re: Fire Blaster

Postby Aquaman33 » Mon Sep 23, 2019 10:11 am

air.command wrote:That turned out really well! Can't wait to see it fly. :D What brand of clear coat are you using?


Thanks George.

I just used this cheap clear coat that I had in the garage.

Image

But I wouldn't recommend it. It dried a bit yellow, so doesn't look great on the white paint. Also it remained tacky for a long time.

But it looks good on the carbon fiber.

I was a bit conflicted with this rocket because I wanted it to look okay, but I didn't want to invest too much time in it in case it gets lost. It's a small rocket that will fly high.

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Re: Fire Blaster

Postby Aquaman33 » Mon Sep 30, 2019 10:18 pm

Aquaman33 wrote:
To reduce the chance of a zipper I:
- Used a Kevlar shock cord 5 times the length of the rocket
- Added 2 layers of shrink tubing at the point where the Kevlar exits the body tube
- Wicked some thin CA glue into the top of the body tube
I have never had a zipper with this technique.
Image

I drilled the nose cone as I don't trust the regular attachment point.
Image



I could have just let this thread go quietly dormant, but I think we should share our failures and hopefully others learn from our mistakes. :oops:

I don't really believe in the concept of tempting fate, but for some reason I did hesitate before I typed the words "I have never had a zipper with this technique." Words I would live to regret.

NSWRA had its high power launch on Sunday and I decided to launch Fire Blaster on an F26 to give it a shake down before trying the G40.

It left the pad beautifully in big cloud of dark smoke from the Fast Jack propellant. But unfortunately drag separation caused the nose cone to eject as soon as the motor burnt out. The ejection at high speed caused a massive zipper down the body tube and ripped the back end out of the nose cone. It happened so quickly/violently that the parachute didn't eject until the ejection charged fired "as planned" a few seconds later.

I had added a little weight to the nose cone, which increases the chances of drag separation.

I checked the fit of the nose cone numerous times before launch and I thought it was a good snug fit, but it obviously wasn't tight enough.

A pic of the damage to the body tube:
Image

And thanks to Ray who found the nose cone:
Image

Good job I didn't just rely on that little plastic loop. :D

I still stand by my techniques for preventing zippers. I believe they work well if your delay is a little too short or a little too long. But they clearly can't cope with separation 2 seconds after ignition!

The altimeter shows that despite drag separation the rocket flew to 1,004 feet. So I guess it would be closer to 2,000 feet if the nose cone had remained in place until the ejection charge fired.....and the G40 would have been well over 2,000.

The fins are well secured to the motor mount, so I may take the Dremel to it and reuse the fins and motor mount in a rebuild. Maybe Fire Blaster II will fly one day. The good news is the 3d printed motor retainer worked great and shows no signs of heat damage.

Oh well, onwards and upwards. :D

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Re: Fire Blaster

Postby air.command » Tue Oct 01, 2019 11:42 am

Thanks for posting the update. Yup the deploy was sure eventful, that is one heck of a zipper. :( I've wondered if a few wraps of carbon fiber tow around the top of the tube would help with zippers. It was good to see the printed retainer work well. What are your plans for repairing the rocket?
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Aquaman33
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Re: Fire Blaster

Postby Aquaman33 » Tue Oct 01, 2019 12:16 pm

air.command wrote:Thanks for posting the update. Yup the deploy was sure eventful, that is one heck of a zipper. :( I've wondered if a few wraps of carbon fiber tow around the top of the tube would help with zippers. It was good to see the printed retainer work well. What are your plans for repairing the rocket?


Yes carbon tow would definitely strengthen the top of the tube, but I still think it would have sustained some damage.

I think the ratio of nose cone weight to total rocket weight was a contributing factor. The rocket was quite light, and the nose cone relatively heavy. So when the back end of the rocket slowed the nose cone had enough mass and momentum to keep going.

I have some long couplers that fit this size tubing, so a may attach one of those to the nose cone in the next version. That will provide a much better friction fit.

I will recycle the back end into something new one day, but I want to focus on some larger projects in the short term.


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