My very own Go Devil 38 build

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Quick links are bulky and heavy, but the extra weight will probably improve your maximum altitude (though not max velocity).

I've got one of these under construction, also, for flight at NXRS in June. I skipped the internal retainer and am opting for a combination of friction fit and aluminum tape at the aft closure to secure the motor. I'm then using a threaded forward closure as the shock cord anchor... and thus avoid your latest concern about how to replace the shock cord when it wears out.

thinking about friction fit makes me wonder if this rocket would be a decent candidate for motor ejection... not something I've tried before, but might be interesting.

-e
 
For those of you using the Shapeways NC sled, how are you attaching the NC bulkhead to the NC? Looks like the sled is set up to have an eyebolt through the sled and bulkhead, but I don't see attachment to the NC coupler...
 
emckee said:
For those of you using the Shapeways NC sled, how are you attaching the NC bulkhead to the NC? Looks like the sled is set up to have an eyebolt through the sled and bulkhead, but I don't see attachment to the NC coupler...
Click to expand...

Two holes through the side of the shoulder allow installation of countersunk screws through the nose cone shoulder to the uprights of the sled, clamping it all together. If you look closely at the 3D rendering, you'll see small light blue dots in the triangular portion of the uprights just up from the base. That's where the holes would go through for your securing hardware. With ejection charges sized right, there's very little stress on them as 'pins'. I plan on putting in threaded inserts to be able to use this sled in a variety of rockets that I have that share similar space in the nose cone.
 
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I received my sled from Shapeways. Tom Keith is right. The nylon is a little flexible and I am disappointed. The webside said strong and flexible. Launching it will show if it is OK or not. I think it will be fine. I can fix the stiffness with full length stiffeners for my next creation. The battery bay makes it locally very stiff. I will just extend the walls full length to make it stiff.

I noticed the ring at the front has a slight gap to the inside of the nosecone so I will add blue tape or something to make it snug.
Here is the sled.
IMG_3495.jpgIMG_3496.jpg

The fit to the nosecone shoulder is very satisfyingly snug. I made the hole for the eyebolt and nut long on purpose so I could get a box end wrench in there and tighten the nut. I learned to do that designing parts for rockets and airplanes at work.
IMG_3505.jpgIMG_3536.jpg
 
I buggered the hole. Rats! I wanted to use #6-32 set screws to hold the nose on. They have tiny hex recesses in the top. I didn't use a tap for the thread and I should have. As I was driving it in I rounded the hex recess. I had to drill it out. Rats! Did I tell you I buggered the hole? I buggered the hole!

I went to 1/4-20 set screws instead. It is nice and solid on the good side and should work OK on the bad side. This is the bad hole.
IMG_3539.jpg

The idea was that I would drive the set screws progressively further in to 1) allow the nosecone to be removed from the forward body tube and 2) allow the sled to be removed from the nose. Here are the steps.

Set screw is a bit proud of the surface and is holding the nose to the forward body tube.
IMG_3540.jpg

Here I drove the set screw in far enough to allow the nose to come off.
IMG_3538.jpg

Here you can see I drove the set screws in far enough to allow the sled to come out of the shoulder.
IMG_3536.jpgIMG_3537.jpg

By the way...did I tell you I buggered the hole? I buggered the hole!

This is what a buggered hole looks like.

IMG_3541.jpg

Now that's buggered!
 

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They do offer PLA but it wasn't offered for my part. There are design rules for the various plastics. I suspect my part didn't pass an automated check against the PLA design rules so PLA wasn't offered as a material choice. For my next part I'll read and adhere to those rules.
 
Gents,
I am at that stage. How much BP for the two charges? It is a 38mm. The available volume in the sustainer is 5.5 inches long. There is a 15 ft shock cord and a 12 in chute.

In the forward body it is 6.7 inches. There is a 10 ft shock cord and a 30 in chute.

The formula in Modern High Powered Rocketry 2 says:
BP (grams) = 0.006*D_compartment^2*L_Compartment

That comes out to 0.074 g for the sustainer and 0.090 grams for the forward compartment.

Does that sound right to you?

Tom
 
TomSmith58 said:
Gents,
I am at that stage. How much BP for the two charges? It is a 38mm. The available volume in the sustainer is 5.5 inches long. There is a 15 ft shock cord and a 12 in chute.

In the forward body it is 6.7 inches. There is a 10 ft shock cord and a 30 in chute.

The formula in Modern High Powered Rocketry 2 says:
BP (grams) = 0.006*D_compartment^2*L_Compartment

That comes out to 0.074 g for the sustainer and 0.090 grams for the forward compartment.

Does that sound right to you?

Tom
Click to expand...

How are you packing your charges? I have found a lot of those calculators to be inaccurate but a good starting point. The last 38 MD rocket I flew used 0.8 g charges, I believe the numbers you are getting are extremely small.
 
I am using Wildman paper tube charge cups. They have an e-match that looks to be hot glued into the end. The tubes are about 2.5 in long and I plan to cut them shorter once packed.

I remember charges in the 0.8-1.0 g range on a 54mm. Smaller charges seems the right direction but I agree those numbers seem seedy lily small.
 
TomSmith58 said:
I am using Wildman paper tube charge cups. They have an e-match that looks to be hot glued into the end. The tubes are about 2.5 in long and I plan to cut them shorter once packed.

I remember charges in the 0.8-1.0 g range on a 54mm. Smaller charges seems the right direction but I agree those numbers seem seedy lily small.
Click to expand...

IIRC, MP2 book was solving for 15 psi in the bay. For a small diameter tube, especially if you are using shear pins, you will want more shear force and thus higher psi.
 
TomSmith58 said:
I made the hole for the eyebolt and nut long on purpose so I could get a box end wrench in there and tighten the nut.
View attachment 291615
Click to expand...

Good on ya for that one. If I had a $1 for every time I had to go upstairs and throat punch one of my mechanical engineers for designing a part that had to be teleported into where it goes, or that it was impossible to get tools and fasteners on, I could have retired last year!

I'm a stickler for getting rid of tag ends on fasteners, so I've cut the stud on my eye bolt to give 2 threads above the nylon insert. I also decided to use one of the 1/8 thick bulkheads with a 1/16 milled edge to give extra sealing and stiffness to the base. Also, it made for a perfect friction fit into the very top of the nose cone. RW coupler piece length for the nose cones varies a LOT in length (something I hope that MadCow standardizes) from one kit to another. Very frustrating, but using the bulkhead seemed to work for the 3 rockets that I want to use this in.

What's the 'as designed' bottom surface to center of the side mounting holes? As delivered, my part measures 13/32, and I'm debating on sanding the annoying 1/32. I think a fair amount of it is the strange fuzzy coating. Does this come in naked nylon?

Bummer on the auto rejection of the PLA as a production material. I think that this sled design will work in general, but I hope that whatever you have to do to get it as PLA print doesn't compromise that functionality.

* I was able to tape my favorite battery into the sled, and I think that'll work for now. As an addition, please think about putting pads lower on the uprights for a pcb screw switch, as it'll give a little more positive indication that 'power is applied' than the magnetic switch before the GPS gets a lock and starts down linking to the base station. The access hole would come through the nose cone shoulder, 90 degrees out from the side screws.

Also, I think that there's a place for a naked perch support matching product. That is, the anchor base and uprights to hold the cross screws that go through the coupler tube/cone shoulder so that you can fly the bird without the GPS and an empty cone. Could also be used as an anchor sled for nose weight.
 
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I use 0.3g in the 4" payload and 0.6g in the booster of my darkstar mini dd. Thats with a 29mm motor. 1 2-56 shear pin in the nose.
 
dixontj93060 said:
IIRC, MP2 book was solving for 15 psi in the bay. For a small diameter tube, especially if you are using shear pins, you will want more shear force and thus higher psi.
Click to expand...

That is the truth. Higher pressure is your friend here. I would think a minimum of 0.5 to upwards of 1. Really, this where you ought to ground test to size the charges.
 
Banzai88 said:
Good on ya for that one. If I had a $1 for every time I had to go upstairs and throat punch one of my mechanical engineers for designing a part that had to be teleported into where it goes, or that it was impossible to get tools and fasteners on, I could have retired last year!

I'm a stickler for getting rid of tag ends on fasteners, so I've cut the stud on my eye bolt to give 2 threads above the nylon insert. I also decided to use one of the 1/8 thick bulkheads with a 1/16 milled edge to give extra sealing and stiffness to the base. Also, it made for a perfect friction fit into the very top of the nose cone. RW coupler piece length for the nose cones varies a LOT in length (something I hope that MadCow standardizes) from one kit to another. Very frustrating, but using the bulkhead seemed to work for the 3 rockets that I want to use this in.

What's the 'as designed' bottom surface to center of the side mounting holes? As delivered, my part measures 13/32, and I'm debating on sanding the annoying 1/32. I think a fair amount of it is the strange fuzzy coating. Does this come in naked nylon?

Bummer on the auto rejection of the PLA as a production material. I think that this sled design will work in general, but I hope that whatever you have to do to get it as PLA print doesn't compromise that functionality.

* I was able to tape my favorite battery into the sled, and I think that'll work for now. As an addition, please think about putting pads lower on the uprights for a pcb screw switch, as it'll give a little more positive indication that 'power is applied' than the magnetic switch before the GPS gets a lock and starts down linking to the base station. The access hole would come through the nose cone shoulder, 90 degrees out from the side screws.

Also, I think that there's a place for a naked perch support matching product. That is, the anchor base and uprights to hold the cross screws that go through the coupler tube/cone shoulder so that you can fly the bird without the GPS and an empty cone. Could also be used as an anchor sled for nose weight.
Click to expand...

Tom,
I think the answer to your question about the side hole to bottom surface dimension is about 0.4 inches. What I intended was 0.5 inches from the bottom of the bulkplate. I found that my bulkplate didn't slip inside the shoulder. I measured the part same as you and convinced myself that it was 0.40 inches and went with that. The way I buggered my hole (did I mention I buggered the hole?) I guess it didn't matter, at least on that side. The other side came out very nice.

As for design iterations to accommodate different switches that will have to wait. My day job is really demanding. I spent all my spare time this weekend on my rocket and I should have been working.

I think a much tougher purpose built PLA design would work for mounting ballast. In fact, I am in favor of casting my own lead so what I would do is create a cylinder of lead with a pocket down the middle that accommodates the socket needed to turn the nut. I would use the eyebolt an nut to hold the ballast in.
 
Gents,
I was reading someone else's post and he said he had vent holes in his main and sustainer compartments. If I had that it would make testing my ejection charges much simpler. Right how I have to route the wire through my coupler and out the vent hole there. If I had a vent hole in the side of my main compartment for instance, I could just route the wire straight out and it would be much easier to set up.

I am using two 2-56 shear pins per compartment. I have two going through the forebody into the coupler and two going through the sustainer into the coupler. Do I need vent holes? Would they hurt deplyment? Would I have to go back and re-size my ejection charges?
 
You NEED vent holes. This rocket will be fast. You don't want to build pressure inside. The vent holes only need to be 1/16" or 5/64" so getting the wires through will be tight.
 
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"By the way...did I tell you I buggered the hole? I buggered the hole!"

Good God, man. Drill with the pointy end of the cordless screwdriver, not the battery end.

Signed, All my holes suck.
 
Thanks for biting in my joke Igotnothing. All the rest of you guys are humorless blocks of wood.
 
ksaves2 said:
Is that what happened on the flight? Kurt
Click to expand...


Kurt,
It's toast. On the first flight. I used an Aerotech I140. Nothing deployed. I recovered the broken sustainer section and the coupler with the RRC3. The drogue charge was not burned. I will diagnose it later.

This is after 20 inches of digging. What I saw above ground was the last 10 inches of rocket. The first 40 inches were burried.

IMG_3700.jpgIMG_3705.jpg
 
TomSmith58 said:
Kurt,
It's toast. On the first flight. I used an Aerotech I140. Nothing deployed. I recovered the broken sustainer section and the coupler with the RRC3. The drogue charge was not burned. I will diagnose it later.

This is after 20 inches of digging. What I saw above ground was the last 10 inches of rocket. The first 40 inches were burried.
Click to expand...

Oh my gosh. Well my last 4 flights were not very good either. The tracking thread with the Formula 54 and the failure of an inadequate motor ejection charge and then a motor burn through I've previously recounted. It's pretty stupid that I've never
"over charged" a rocket and blown the shockcord to bits and I still undercharge. (I've even ground tested and use the same size charge, same powder and still had issues.) I had two 38mm motor powered WACs have problems one with an inadequate ejection charge which is a total loss and the other that had a recovery system tangle that the Jolly Logic Chute Release couldn't deploy. Two out of the three rockets are going to fly again, the F54 and the Yellow WAC are repaired. My white colored WAC was garbage can material.

Did the tracker help you locate the rocket? How did it behave on descent? On my ballistic recovery, I only received 2 or 3 packets just before it hit. That was it. Nothing received at altitude. Did yours work differently or were you able to visually track only? Kurt
 
My tracker faithfully took me to the last known coordinate which was the launch rail. I had 9 plus satellites so great GPS. The GPS unit was in the nose and I could not recover it. I bet it's smashed.

I am just now sorting all the tools I packed back to their proper place to make room to examine my RRC3.
 
I will. However the motor performed as expected. I am pretty sure I miss used the RRC3.
 
TomSmith58 said:
I will. However the motor performed as expected. I am pretty sure I miss used the RRC3.
Click to expand...

Yeah, my burn through with the H250 I filed the report as I had the paperwork with the ID numbers on it so it makes more sense. On my inadequate ejection charges it was the drivers fault with using measured charges. Testing with a plastic canister
to apply to a motor well doesn't appear to be valid. I ground tested twice with good results, put the measured amount in the motor well and the shearpins didn't shear at altitude. Me thinks some of the powder went through the hole in the well and "blew"
backwards. I learned my lesson if using an electronic canister test (ie. ematch) for an apogee motor ejection test, one needs to add quite a bit extra powder to the motor well as opposed to the canister.
How much? I'd say 30% more.

Kurt
 

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