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kuririn

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But Jeff strikes me as being a craftsman.
I don't think he would like taking the easy way out.
Right Jeff?
 

neil_w

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I like the extra details. The whole thing looks really good. Make sure the pylons are strong and sturdily attached.
Thanks! I haven't really looked at erockets' stuff carefully. Yet another supplier I apparently need to give lots of my money to...
Uh, that's a big 10-4.

Erockets has the largest supply of LPR parts anyway. I buy mostly at BMS, but Erockets is still an essential resource.
 

Jeff Lassahn

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But Jeff strikes me as being a craftsman.
I don't think he would like taking the easy way out.
Right Jeff?
I'm not philosophically opposed to taking the easy way out, but I also don't own a 3D printer. So that kind of damps my enthusiasm for custom plastic parts...

As for being a craftsman, I'm more of the guy who goes into a project that's a bit too challenging, gives it a good try for a while, then screws something up by being too quick and not careful enough. My finish and paint jobs are legendary.

Maybe this project will be a kind of penance where I learn to do better...
 

Jeff Lassahn

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Make sure the pylons are strong and sturdily attached.
That's one of my next bits of Designer's Remorse. I just kind of stuck those on the design where they looked good, attachment will be tricky.

Normally I would put a glue fillet at each end, but with the way the core shows through on the nacelles I don't want a big fillet there. So I'm thinking there's a slot in the central nacelle tube which the pylon goes through and anchors to ... I dunno yet, a thing. Then all the other details on the nacelles also mount accurately and strong via some magical build and alignment method that will come to me in a dream or something.
 

mbeels

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Oh, and also, there's almost no space for a parachute.
That looks tricky! What about ducting the ejection charge through either the central core and/or braces around the sphere?

Or sticking an Eggtimer Apogee in the forward section and going with electronics?
 

mooffle

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Jeff, if you're willing to share the design file I'd be happy to draw the technical challenges in the way I would tackle it. I don't thing I can explain it sufficiently without a visual or else I'd try.
 

Jeff Lassahn

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Version3.png

Cleaning up some details and working on decal design.

This version has about 1.5cm added to the rear section to make a bit more room for the parachute. Doesn't look as weird as I was worried it would.

back to fretting about construction internals...

Attached is the ORK file I'm working with, it's highly dependent on the not-yet-released pods feature, so weirdness probably happens if you open it with the stable version of OpenRocket.
 

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neil_w

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It's looking better and better. I *love* the forward extensions on the left and right cage pieces, and the decals are excellent. I'm not sure about that purple band on the nose, but that's minor.

As for the construction, though...

1) I believe there is exactly 0% chance of getting your recovery materials in there. In my opinion you really need to run the motor mount tube through the ping pong ball and vent to the forward portion. In addition to providing plenty of room for the parachute, it'll also shore up the strength of the plasma ball. As shown, with just the cage pieces holding the whole rocket together, I'd be very nervous about it.

Yeah, it won't look quite is awesome with the tube running through there, but I don't think there's a reasonable alternative, unless you want to add multiple additional inches to the rear portion (and then you still have the plasma ball construction to sort out)
1603070575298.png

2) You need to clean up that OR file. Materials and thicknesses are suspect; don't trust the mass and/or CG measurements you're getting yet. I think that the pylons are going to need to be TTW through the tail cone for strength (look at that tiny root edge) and if it were me I'd try to see if I could afford the weight of basswood back there.

Anyway, nothing wrong with a good challenging build. :) This'll be a great rocket if you can pull it off.
 

Jeff Lassahn

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I'm not sure about that purple band on the nose
Me either. It needs something up there, and I was hoping to get something that suggested the embedded dish thingie from the Trek motion picture version Enterprise. Maybe I'll scrap it and do some little port things that suggest comms gear or phasers or whatnot...

I believe there is exactly 0% chance of getting your recovery materials in there.
I'm thinking I've got at least 25%, I'm not giving up just yet...

You need to clean up that OR file.
Eventually. There's no point fiddling with the masses, etc until I've got the construction stuff figured out so the internal bits can be the right shapes and sizes. Then maybe I'll clean that stuff up to get a better CG estimate, or maybe I'll only worry about CP and stuff weight into the nose when it's actually built until the real CG is far enough forward.
As for worrying about the CP, I've got the pylon fin set extending in an odd shape most of the way back on the nacelles, not because I really want to build it that way but because I think OpenRocket's belief that body tubes don't modify the CP is probably bogus for the nacelle tubes with all their air-trapping fiddly bits. So I'll lie to OpenRocket about the geometry I'm planning to build, and it will lie to me about where the CP is, and who knows, maybe great things will happen!
 

mooffle

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Attached is the ORK file I'm working with, it's highly dependent on the not-yet-released pods feature, so weirdness probably happens if you open it with the stable version of OpenRocket.
The file wouldn't even open, but thanks for sharing. I'll try to explain it, though Neil's post above makes the clearest point especially regarding recovery. I'll apologize ahead of time for using ms paint.

If the plasma ball is smaller than the BT you may be able to make your pylons a 'composite' part. See top sketch, the thick red line is a dowel running longer a bit longer into the tubes, the blue pieces are small balsa fins. Then paper over the whole visible part and drill or cut small notches in your interior cones for the rods to pass.

If the plasma ball is bigger then I would just cut a piece of plywood on a scrollsaw like the bottom picture, and allow enough space for your engine mount to pass through the ball as suggested. Then slot the cone or do a built up design as opposed to one piece. The whole thing would be absurdly strong in this case
plasmacore.png
.
 

jqavins

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As a third variant on mooffle's ideas, the arches could go through without bending out to horizontal:
1603125052684.png

As for deployment, I'd give very serious thought to going electronic, where there's plenty of room above the ball.
 

Jeff Lassahn

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A couple of concepts for packing the parachute behind the plasma ball:

1. Build a spool shaped extension onto the front, which acts as both a parachute holder and a coupler.
Screen Shot 2020-10-19 at 3.51.48 PM.png


2. Do as sort of rear ejection setup by extending the motor mount forward and making the rear body tube part of the front.
Screen Shot 2020-10-19 at 3.58.00 PM.png

I'm liking how #1 feels, assuming it can be made rigid enough.
 

neil_w

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In #1, where does the shock cord go? Are you assuming no wadding due to the piston-like arrangement? That rear disk is going to be right up against the ejection charge.

It might work, but it all feels risky to me. Perhaps someone else who has done something vaguely similar will have a better vantage point.
 

Jeff Lassahn

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The shock cord gets wadded up in about 0.5cm of space between the rear disk and the top of the motor mount. Hopefully no wadding, because the rear disk blocks the direct heat from the ejection charge.

This may be the sort of thing where it makes sense to build a scratch rocket to test the mechanisms.
I've also been playing with a version which uses a floating disk rather than a spool, and that one actually flew once:

Too bad it's now October and so it's going to be raining steadily for the next five months or so...
 

jqavins

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The disk protects the parachute; what protects the disk? As close as it'll be to the engine it will be seeing quite a blast (which I think was Neil's point). You might want to pack what little space there is between the disk and engine with dog barf.

I don't have a good feel for the volume in the parachute compartment. How large a 'chute can you get in there?
 

Jeff Lassahn

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The space is about 4.5cm long and 3.3cm diameter. This is comfortably enough for an Estes 12" plastic chute and I'm hopeful it's enough for an 18".

The one flight I've done with a papered disk up against the motor mount, the disk collected some soot but doesn't appear to be eroded. So it can definitely survive several flights, but it might degrade long term. Also since the shock cord is not protected by wadding it will probably fail eventually.

For longer term survival I'm tempted to apply some kevlar cord and aluminum foil or some such. I don't have any experience with rockets that have lived for more than a dozen or so flights, so I may need to up my game if I'm going to build nice things.
 

jqavins

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You might try buttering the hot end with JBWeld. That product's main advantage over other epoxies is its high heat tolerance. And the surface hardness, like any epoxy, will protect against mechanical erosion.
 

neil_w

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You might try buttering the hot end with JBWeld. That product's main advantage over other epoxies is its high heat tolerance. And the surface hardness, like any epoxy, will protect against mechanical erosion.
A good layer of protective epoxy is an excellent recommendation, but I would *think* that any old epoxy would do the job fine. It's only a momentary blast of heat, and it's not structural. My understanding is that JBWeld's heat tolerance is best for motor mounts where you don't want it to soften under sustained heat.

I only say this because if you don't have JBWeld laying around then I don't think it should be necessary to buy it just for this purpose. But I would absolutely coat the disk with epoxy. Same recommendation would apply to the disk plug Jeff was experimenting elsewhere.

Disclaimer: IANAEE*

The shock cord, if jammed into that tiny space, would definitely take a beating. Kevlar is the only thing to even consider there. And it will be bundled up right on top of the motor ejection blast, so it won't be immune to consequences either.

My concern remains to build a beautiful rocket like that (which will take some effort) and go with a really marginal recovery system... well, I would want to be more confident of the thing coming down in one piece. But it's your show, if you think you can make it work, go for it.









*I am not an epoxy expert
 

jqavins

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Good point that the JB is probably not altogether necessary as opposed to other epoxies. Honestly, its use for the sake of heat resistance is what I've read here, and I use it in many applications because it is what I have on hand, and because it has a consistency that I like sometimes: a little bit flowey (unlike putty epoxies) yet not too runny (unlike BSI). It's a nice intermediate, for some purposes.

As for the kevlar, a design that makes replacing it now and then an easy task would be a very good idea.

IANAEEE.
 

Jeff Lassahn

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I think I've got some design details figured out to the point where this thing might be buildable.

I'll use a 3/16" dowel through the ball as the central support for everything.
The parachute will use the spool style design shown above, with a 15" chute. There's plenty of room for a 15", marginal room for an 18".
The ball cage pieces will extend through slots in the cones to connect with the central dowel, which will make them both stronger and easier to position, but makes the cones harder to build.

Nacelle pylons will have fin tabs through the walls on both ends. I'm going to steal the design trick from the Estes USS Enterprise model of putting a slotted insert into the nacelle tube as an additional support for the pylon.

I cleaned up the ORK file to match the design.
Next, I go back to obsessing about how to paint it...
Screen Shot 2020-10-27 at 5.08.49 PM.png
 

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BABAR

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Three ideas.

Run a BT-5 from front to back THRU the ball. Line the inside with a piece from an aluminum can (not fireproof, but will handle the ejection blast okay. Use this as a duct to pass the ejection charge to the forward compartment, where you have room to spare for wadding, baffle, altimeter, chute, shock cord, and a small kitchen sink.

Other idea. If you are 3D printing parts, print two curved tubes the size of your peri-ball struts (hey, I don’t know what to call then, at least they aren’t Ball Sacks like my Interceptor E model build!). I think you have room for a baffle in the rear of the rocket (and if you don’t, just make the rear of the rocket a bit longer until it CAN handle a baffle.). Two tubular ball struts May look cooler than the multiple ones you have now (IMO), and with the baffle cooling the ejection charge a bit, again can duct the ejection gas to the forward compartment.

Final frontier idea. Never seen this done, but theoretically could work. Forward section is a nose cone attached to the outer body tube. This slides over a slightly smaller body tube nearly the full length of the forward section. The slide is REEEEEEAAALLLLY loose, I mean like you can’t even pick up the rocket from the front without pulling the section off. But since it is really looooong, it isn’t gonna move around during boost. Put some weight in the nose cone to give it some kinetic energy potential.

Do REAR EJECT with your motor, with a Nomex streamer. What you want to do is somewhere near apogee (maybe even a little BEFORE, I’d go a bit short on delay) cause a rapid deceleration (or BACKWARD acceleration) of the rear of the rocket relative to the front. The nose and forward body tube keep flying forward, sliding off the tube and hopefully pulling out the chute. Having a very small chute at the very front (last thing packed) May help act as a drogue to pull out the rest of the laundry. It’s kind of like drag separation of a multistage rocket.
 
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