Mumbo Jumbo build thread (or, the making of a BT 80 LPR rocket)

[Balltip]

This is about a build that I made during the past weeks.

Quite a few of you will say ”Hey! That one looks pretty much like the Optima!”
You will be very right

But this one is not entirely an Optima, and it is my biggest mod rock so far. And my Mumbo Jumbo (couldn’t think of another name, so what the h…. hrmmm, sorry) flew for the first time this evening – a successful flight!
So what is this? Well, it is nothing but a 3FNC BT80 rocket.

“But it is mine, all mine!” < Said in a rather crazy voice.

Thing is that most of the construction techniques used on this one I have learned here on TRF. So in a way this is also posted because I wanted to say “Thank You All!” for sharing your experience and knowledge! I hope that by posting this, I might be able to show a few newcomers a few things that they might not have seen yet. If so, then I am happy!

 

[Balltip]

The Mumbo Jumbo began as a jumble of parts from Red Arrow Hobbies, two 14.2” BT 80 tubes and a pointy nose cone. I had decided from the start that I wanted a baffle on this one, so what you see on the picture is not only the BT’s and nose cone but also a part of the baffle system. More about the baffle in a little while.

 

[Balltip]

I began construction by making four centering rings out of 1mm lite ply. To make those I used my lathe – a bit of over kill but hey… I got it standing in my shop, so why not?

 

[Balltip]

I wanted to use a stuffer tube for this rocket, as well as a baffle. So I used a full length of BT50 for the motor mount and cut it to length afterwards. And one thing that I do on all of my scratch built rockets (maybe a weird thing – I dunno) is that I hook the engine retention clip FORWARD of the thrust ring. Idea is that the clip will have something more substantial to hold on to than only the BT once the ejection charge goes off. It adds about 0.1 gram of weight or so, and it feels a bit safer. I don’t use pre made hooks but make mine from window wiper stiffeners so it is easy to make them a tad longer. Save your old window wipers, tear them apart and use those springy steel thingies for motor hooks!

 

[Balltip]

And now; the baffle. It was made from one of the centering rings, and six pieces of BT 5. All were glued in place using white glue (the only glue used for this build).

 

[Balltip]

Having read that ejection charges chew a bit extra on BT’s when using a baffle, I decided to try to make my rocket to last a little longer than it might do otherwise. So I placed the baffle inside the joiner tube. Here you see the geometry of the whole thing before it was glued in place in the lower BT.
I also painted all surfaces that will make contact with the ejection charge with heat proof paint. I really have no idea if it will make the baffle last longer, but I did it anyway. The stuffer tube got an inside coating of white glue. Again, I don’t know if it will work, but it can’t hurt.

 

[Balltip]

Here the baffle is in place in the lower BT.

 

[Balltip]

After assembly of the lower half of the rocket I glued the BT’s together. Pic taken down the upper BT. Yeah… a bit too much white glue. But I think it won’t come loose

EDIT: Oooops, wrong pic...
This picture shows the motor tube glued in place. Picture is taken down the lower BT before the baffle is ready. Dang! And here I really tried not to mess up this build thread! Oh well.

 

[Balltip]

The fins got some special treatment. I glued them in place using “the rivet method”. As I have come to understand it this should add some strength as the glue flows down into the holes, making a sort of “glue rivets”. Was definitely worth a try!
All fins on this rocket was made from 1mm lite ply, same stuff as the centering rings.

 

[Balltip]

And here she is. No decals yet, but one flight down!
I got to fly her for the first time tonite. Sorry, no flight pics! We scrambled to the launch field as we saw that the wind suddenly died down. We were in a hurry so I forgot the camera. After flying the Mumbo Jumbo as the finale after a few other rockets, it was dark.

But back to the launch field… Loaded her with a D12-5 and counted down…
And she took off perfectly! Not very high, but high enough to make use of all of that 5 second delay. As she arched over the ejection charge went off and out came the chute. Beautifully! I also made the chute myself. It is about 80cm across (about 32 inces) and it made the rocket come down in a sweet, slow descent. My fingers grazed it as it came down but I did not manage to catch it! Darn! Oh well, it was really nice anyway.

And yeah, this rocket really is mine, all mine!

 

[Balltip]

So, as the last post right now, let me add that I am very happy to have found a place where I can ask things, get answers, and learn a LOT about making rockets!

Thank you, all!

Per

 

[Stormbringer]

cool looking rocket and an interesting baffle.

 

[BARX2]

Balltip, thanks for posting this build thread. I will use your baffle system on my scratch built Executioner/camroc.

 

[by basher]

Originally posted by Balltip
...I hook the engine retention clip FORWARD of the thrust ring. Idea is that the clip will have something more substantial to hold on to than only the BT once the ejection charge goes off. ...

AMEN, brother!! It has ALWAYS bugged me that Centuri or Estes never did this with their kits. You put in a nice, strong bulkhead to handle thrust, then add a glorified paper clip thru the bt with the wall for it's only support.

Nice build-up and may the Mumbo Jumbo continue to serve you/us well.

 

[Balltip]

Thanks for the comments, fellas!

There is one thing that I have beaten myself up over after finishing this one, and that is that I did not make the motor mount long enough to accept an E engine. Okay, I could always friction fot one if I like, but anyway... All it would have taken was a longer engine hook! Oh well, live and learn...

Good thing is I already have another order placed for BT's and a few nose cones!

Per