Clustered Laserhawk - Questions

[JCRL]

I have a NCR Laserhawk kit that I would like to make into a clustered rocket by adding 24mm motors to the kits (non-functional) side pods.

The side pods have an interior diameter of 46mm, sort of a weird size for buying premade centering rings, so I need to make my own. Any suggestions for a the right size for a holesaw to make cuts?

A second question I have, is should I put recovery devices in the side pods or just tether the nose cones and let the main 36" chute do the work?

 

[ebruce1361]

I frequently cut my own centering rings for odd sized tubes. For the 25mm hole to accommodate the motor and tube, carefully determine the center of your centering ring, drill a small hole at that point, then use a taper drill bit to widen the hole. I have found that hole saws are generally too coarse for use in rockets, and will just make a mess of the wood, whereas the tapered bits allow you to slowly and carefully widen a hole with much cleaner results. You'll likely still have to sand the edges of the center hole, but that's better than having your piece spontaneously rip in half because a tooth on the hole saw decided to bind. From there, use a jig saw or band saw to cut just a bit bigger than the ring, and then sand to the appropriate size. Make sure you use a good compass and ruler to get the hole centered in the ring, and to give you accurate edges of the ring to sand down to.

As for recovery, the booster pods will weigh more with motors in them, but I would think that the main parachute could still handle it all. If you tether the nose cones, you will get burn marks on the side of the main body when the boosters fire their ejection charges. If you fly with a club and they permit it, don't include any motor retention. You'll still want your thrust ring of course, but you could just eject the casings out the back. If ejecting the casings isn't an option, maybe setup a rear-ejection for the booster pods and make sure they are loaded with motors with longer delays than your core motor. That way, the motor mounts can pop out the back on a kevlar cord to vent the ejection pressure, but do so after the rocket is done with the thrust phase. It would really suck to pop the motor mounts out the back and have them dangle right into the core motor's flame and do who knows what all to the aerodynamics in the tail with them flopping around.

 

[BABAR]

I have had good results for centering rings for 18mm and 24mm motors with foam board.

For the 18mm motors (BT20 tube) draw a square a bit larger than the body tube internal diameter. I mark the center of the square and use an 18mm hole drill bit to make the center hole (yup, it’s a bit too small, but it is easy to sand it up to 20mm for a snug fit. Put another way, “it is easier to make a small hole bigger than a big hole smaller.”

I then use a template with the 20 mm inner hole cut out and the outer diameter matched to the internal diameter of the rocket body to draw the outline of the outer diameter of the ring on the foam board, with the template centered on the predrilled hole on the foam.

This will sound crazy, but I use a metal straight edge to cut tangential cuts along the margins of the outer line. It really doesn’t take that many “straight” cuts to cut the ring out, IMO it is much easier than trying to make a curved cut directly with a straight blade.

I have done 24mm BT-50 to BT-80 with this (also to Pringle’s cans, but you need to rough up the internal side of the cans.). Knock wood, I haven’t had a single centering ring fail.

For those of use without fancy power tools, foam board centering rings are a lot easier to make, and certainly a lot lighter, than plywood. Probably would need some reinforcement if you went for 29mm motors......

 

[BABAR]

I like @ebruce1361 idea of just ejecting the casing out the back. They will tumble recover and aren’t going to hurt anybody. I do this routinely with my helicopter and airbrake rockets. It is a courtesy to field owner to try to find the casings and disposed of them, but I don’t think it is a huge deal if you can’t find them. Non composite black powder motors are completely biodegradable.

Assuming your main chute is sufficient to recover the rocket, another option is to put Kevlar shock cords on the nose cones of the pods and just do “nose blow” of the cones with no streamer or chute to relieve the pressure, just need to make sure the main fires before the boosters.

If you don’t like vent holes on the sides of the boosters, you can cut slots of holes in your centering rings that will vent the ejection charges backward, between the motor mount and the outer tubing. This is the most cosmetic venting solution.

 

[ebruce1361]

If you don’t like vent holes on the sides of the boosters, you can cut slots of holes in your centering rings that will vent the ejection charges backward, between the motor mount and the outer tubing. This is the most cosmetic venting solution.

I didn't even think about venting the pressure without moving parts. That's a good idea! Just make sure your holes are big enough to sufficiently let the pressure out while not being so big that your centering rings aren't strong enough to hold the motor mount.

To that end, you might be able to do the same thing without any centering rings. You could glue narrow balsa or basswood strips lengthwise on the motor tubes which then get inserted into the booster pods. It's like how the Estes Ram-Jet is built with the long strakes between the core tube and the outer body tubes, but in this case the outer tube is sealed and the motor tube is only as long as it needs to be for the motor. The motors will fire their ejection charges into an empty, sealed tube which just blows the gasses past the motor tube and out the back.

 

[BABAR]

To that end, you might be able to do the same thing without any centering rings. You could glue narrow balsa or basswood strips lengthwise on the motor tubes which then get inserted into the booster pods. It's like how the Estes Ram-Jet is built with the long strakes between the core tube and the outer body tubes, but in this case the outer tube is sealed and the motor tube is only as long as it needs to be for the motor. The motors will fire their ejection charges into an empty, sealed tube which just blows the gasses past the motor tube and out the back.

Brilliant.

Will also need to cost inside of nose cone with JB Weld or do something else to protect it from heat of ejection blast.

 

[Wallace]

I can show you some nice non-seperating side pod booster zippers from a 2 stage set up. Had a pair of 29mm E's with an F15 core as a 1st (or booster ) stage. Probably 25ish pound kevlar and some 1 1/2" x 10" space blanket streamers just tied on the kevlar with granny knots. Everyone thought the mylar would shred. Mylar, nosecones and kevlar were all intact and in perfectly flyable condition. Heavy wall cardboard tubes didn't fair so well. Moral of the story is: Use caution when deploying even tiny streamers at speed..

 

[Wallace]

Was a beautiful flight. Think those were E 16-0s, an F 15-0 and an F 15-8? in the second stage. Never saw the sustainer again though, just dissapeared. Cheap thrills, gonna cobble another sustainer and re-visit that one, hopefully this spring/summer.