LPR laundry shelves, and maybe baffles

[neil_w]

Lately I've been fretting over shock cords and such. Historically I've been doing the standard Kevlar to the engine mount, elastic tied to that just below the end of the BT to avoid zippering.

My IRIS-T is longer and skinnier than my usual, and also is a bit tail heavy. And so I was thinking about a laundry shelf, but just can't wrap my head around all the details.

First off, it's not really a laundry shelf. The parachute is attached close to the nose cone, so it can't really slide down the tube. The shelf would be more for holding the dog barf (not important) and the shock cord, which is actually a significant weight item, and a good thing to keep forward in this case.

But how do I implement a shelf that (a) I can fabricate myself, (b) can keep the shock cord from falling back through it, and (c) has enough open air space to avoid excessive back pressure on the ejection charge?

This is a BT55 rocket with a 24mm mount. I've heard anecdotally that you should have equivalent open space to the cross section of the motor, but many baffles seem to have less space than that. Should the kevlar be attached to the shelf? That means that there's no way to get to it if there's a problem (or likewise, no way to change the elastic unless the Kevlar is extended beyond the end of the BT.)

I'm just flummoxed by this whole thing. Is a shelf a stupid idea in a low power rocket? Is there some other reasonable way to keep the weight of the shock cord forward during flight? What is *really* the rule for how much air space needs to be provided for the ejection charge?

These seem like dumb questions but I've sort of tied myself into a knot trying to think it through. Thanks for any input.

 

[JJSR]

I would think something like this, I just threw it together, so don't punish me for errors

 

[markg]

I haven't put in a shelf for the purpose of holding the laundry or shock cord forward, but a baffle seems like it would do the job for you and you'd be able to skip the dog barf.

On rockets that size I've been putting in a baffle, attaching the kevlar directly to the baffle and having it extend a couple of inches out of the tube. Tie the elastic to that and you are good to go. On larger rockets I put in a loop of kevlar so that I can inspect/replace it if necessary.

cheers - mark

 

[rharshberger]

On small diameter rockets baffles are nice, I don't start using my shelf/recovery anchor points until I get to BT60 and up. The upper picture is a BT70 anchor and shelf on the right side, the lower picture is a version for a 3" Estes airframe rocket (T300) and it has holes similar to a baffle. The sections of coupler are to allow the shelf to be pushed into position and remain square to the tube, and to act as a re-enforcement against pulling out of the airframe.

 

[kuririn]

Many of the TLP kits use a "parachute compartment disc", as they call it, to keep the recovery gear from sliding back down the body and adversely affecting the center of gravity. They are basically centering rings, with a hole in the middle. I agree with Markg, why not go the extra yard and put in a baffle. Accomplishes the same thing and you don't need the dog barf/wadding. Adds up after a while.
Also, while your parachutes are tied to the nose cone, many modellers like to tie it about a third of the way down the shock cord. This minimizes chute tangles with the nose cone from weaving in and out of the shroud lines during ejection and descent. Something I learned from Chris' blog.

 

[neil_w]

The sections of coupler are to allow the shelf to be pushed into position and remain square to the tube, and to act as a re-enforcement against pulling out of the airframe.

Yeah, I was definitely thinking of that if I'm installing a simple disc of some kind in there.

Many of the TLP kits use a "parachute compartment disc", as they call it, to keep the recovery gear from sliding back down the body and adversely affecting the center of gravity. They are basically centering rings, with a hole in the middle.

How much hole should be in the middle to accommodate a 24mm ejection charge? I have 2055 and 5055 light plywood rings on hand. I guess the rim around the edge is enough to keep the ball of barf and the shock cord resting on it from sliding down?

I agree with Markg, why not go the extra yard and put in a baffle. Accomplishes the same thing and you don't need the dog barf/wadding. Adds up after a while.

In hindsight I agree. However, I don't have a baffle on-hand, no way to cut the rings. Next time I make a mail order (I just stocked up, so won't be for a while) I'll get some baffles or baffle-making supplies to keep on hand. I've never used a baffle before so I guess there's a bit of inertia keeping me from trying them (plus the fears of inadequate airspace; I *have* heard of baffles getting blown out).

Also, while your parachutes are tied to the nose cone, many modellers like to tie it about a third of the way down the shock cord. This minimizes chute tangles with the nose cone from weaving in and out of the shroud lines during ejection and descent. Something I learned from Chris' blog.

I generally do that as well, although not necessarily a full third of the way down. The way I do it would still keep the chute from sliding below the front half of the rocket. The shock cord on the other hand...

 

[kuririn]

How much hole should be in the middle to accommodate a 24mm ejection charge? I have 2055 and 5055 light plywood rings on hand. I guess the rim around the edge is enough to keep the ball of barf and the shock cord resting on it from sliding down?

Don't know if there's a science to calculate the optimum hole size for that. If so, it's way over my head. TLP also uses rings that have multiple smaller holes for the shelf, like a baffle. BTW excellent work on your Iris-T. Sorry to hear about the finishing problems, looking forward to a successful refinishing.

 

[neil_w]

BTW excellent work on your Iris-T. Sorry to hear about the finishing problems, looking forward to a successful refinishing.

Thanks, I think it'll come out good. About to go pick up my new can of paint (was just delivered to store).

 

[dr wogz]

another thought, it to make an 'X' out to wood (such as 1/8" x 1/4" balsa) to fit inside the tube for your shelf..