Scratch Scale Jayhawk

[dedleytedley]

I'd like to build a scale flying Jayhawk based on a BT-80 main tube. I'd like to power it with an unusual cluster, a D-12+C-6. From looking at the thrust/impulse diagrams from Estes it appears that the C motor is approxiamately one/half as powerful as the D motor in terms of both thrust and impulse. So, what I propose is to mount the motors such that the C nozzle is twice the distance from the vertical C.G. as the D nozzle. I think that this will apply roughly equal vectors to the rocket. Does anyone have any experience with this type of cluster or have an opinion on it's feasibility?
Thx Ted

 

[TheRocketNerd]

I have apprehensions about your plan of staggering the asymmetrically thrusting motors. The thrust differential would still be present, the C motor would simply be supplying it's lower thrust to a different part of the rocket by the sounds of your plan. Having said that (not meaning to rain on your parade--this is just my opinion as well)... I have an Estes BT-80 kit of the Jayhawk that I bought built and in need of repair. I can get you some pics and/or measurements if you can't find them on EMRR, YORF or JimZ's. Good luck and post some pictures as you go along.

 

[Micromeister]

From the little experience I've had with varing size motors within the same cluster.
I'd suggest having to motor tube Point to the loaded liftoff CG of the model. This should negate the effect of the unequal thrust by applying each motors thurst to the centerline of the vehicle. I've only done this with A10's and C6's in the past but it did work, and kept the model moving in the proper direction when either motor was intentionally left unlit during flight testing.
Hope this helps a bit.

 

[dedleytedley]

Thanks for the replies. To Microneister: I think what you mean is to cant the motors such that the direction of the thrust is angled from the vertical towards the C.G. I can visualize the forces of thrust like weight on a lever with the fulcrum being the rockets' VERTICAL center of gravity. Half as much force at twice the distance from the fulcrum should be equal to a given force and distance. If the thrust is off-axis doesn't that increase the pitch/yaw forces on the rocket?
I'm more concerned about the differences in thrust over the time of the burn. The C motor has both an earlier and shorter thrust peak before the steady burn phase. The D motor is going to be pushing harder for those crucial few hundredth's of a second, accordingly I think it should be a little less than half the distance to the centerline than the C. I don't expect an arrow straight flight from the model but I would like to avoid a power prang. I've found that rockets with a large wing area like this one are prone to do that in the wind.
Ted

 

[NAR29996]

You might consider building a boilerplate in a smaller size to help you work out the bugs first.

 

[dedleytedley]

I would consider building a smaller version if I could get the motor mount rings. I'll be making the rings with a Hobbico circle cutter and I've found that I can't get it to cut very small circles accurately. I make my rings from heavy bond paper(an old reference book) and laminate multiple layers together with epoxy until it is about as thick as fiberboard rings. It will cut 25mm holes fairly well but the 20 mm holes are as small as I can cut with it. I certainly won't be doing any fancy finishing work until it flies successfully. Ted

 

[Micromeister]

Thanks for the replies. To Microneister: I think what you mean is to cant the motors such that the direction of the thrust is angled from the vertical towards the C.G. I can visualize the forces of thrust like weight on a lever with the fulcrum being the rockets' VERTICAL center of gravity. Half as much force at twice the distance from the fulcrum should be equal to a given force and distance. If the thrust is off-axis doesn't that increase the pitch/yaw forces on the rocket?
I'm more concerned about the differences in thrust over the time of the burn. The C motor has both an earlier and shorter thrust peak before the steady burn phase. The D motor is going to be pushing harder for those crucial few hundredth's of a second, accordingly I think it should be a little less than half the distance to the centerline than the C. I don't expect an arrow straight flight from the model but I would like to avoid a power prang. I've found that rockets with a large wing area like this one are prone to do that in the wind.
Ted

Ted:
I think it might be helpful to look at a few offset thrust motored models, such as the Dueces wild. While the Duece's Thrust line isn't quite to the CG of the model but its a good illistration of an extreme application of the principle. Lots of builders have been flying clustered models with motors canted to the centerline CG for a very long time,particularly scale models to prevent precisely the type power prang you discribed. I know it's a little difficult to visulize this but I can testify first hand it does work with very unbalanced thurst motor combinations. The method can be very helpful with stacked two motor models where the motors are mounted opposite the plane of the major wings. By Applying thrust to the CG of the model it may not always lift in a straight-up vertical line but does still accent vertically on a diagonal, sort of side slipping motion. The closer your motors are to pointing to the CG of the model and the further forward that point is on the vehicle, the better the flight goes with all motors thrusting or just one Motor distance from the centerline of the model is now not a major factor as we are transferring the thrust line to the center of mass for the model.
Hope this helps a little.

 

[dedleytedley]

Fair enough Microneister.I'll give it a try. I've been having trouble cutting the 18mm engine hole in the motor mount rings anyway. I was apprehensive making the motors canted because of the difficulty with cutting oval holes. Also with the different liftoff weights of the motors figuring the vertical balance line will be more difficult.C6x2 is more weight than one D12.
What do you think of a motor mount made from Styrofoam SM? The tailcone will be 2.6 to 2.04 with a length of 5 inches in cardstock and rings style. I think I could drill the motor mount tube holes in a block of the SM and shape it to fit inside the 2.04 tube. The angles for the motor tubes shouldn't be too far from the vertical and could be achieved by cutting the base of the block to the corresponding angle, then using a drill press. With well cut holes and the proper glue will StyrofoamSM(the pink hard foam) stand up to use? I would protect the foam from hot gases with paper. Ted

 

[JoeG]

Always wanted to do an R/C glider recovery with a Jayhawk.

Let us know how your testing goes.

 

[Micromeister]

Fair enough Microneister.I'll give it a try. I've been having trouble cutting the 18mm engine hole in the motor mount rings anyway. I was apprehensive making the motors canted because of the difficulty with cutting oval holes. Also with the different liftoff weights of the motors figuring the vertical balance line will be more difficult.C6x2 is more weight than one D12.
What do you think of a motor mount made from Styrofoam SM? The tailcone will be 2.6 to 2.04 with a length of 5 inches in cardstock and rings style. I think I could drill the motor mount tube holes in a block of the SM and shape it to fit inside the 2.04 tube. The angles for the motor tubes shouldn't be too far from the vertical and could be achieved by cutting the base of the block to the corresponding angle, then using a drill press. With well cut holes and the proper glue will StyrofoamSM(the pink hard foam) stand up to use? I would protect the foam from hot gases with paper. Ted

Sounds like a plan: I'd use BT-50 and BT-20 tubes inside the SM foam anyway as an insulator. Why not turn the SM to fit or BE the tailcone unless you don't have a lathe or really want to use a Cardstock tailcone. Either way should work fine. Personally it'd be simpler to setup the canted MMT using rings and the motor tubes then the SM but whatever is easier for your building style and the Styrofoam will add a little rigidity to that long tailcone

 

[g_boxwood]

Great subject Ted!

The Jayhawk is on my lilst as well, still not sure what size actually...

I'll be following your build with interest!

 

[dedleytedley]

I've been trying to find C.P. data for this rocket. EMRR lists 19.50 ? inches from the nose for the Madcow Rocketry version(BT-80). However in a review on the same site a builder says that the instructions state to set the C.G. at 19 inches. Does anyone out there fly this rocket and if yes where is the C.G. located on yours? With an overall length of 31.84 inches a C.P. of 19.5 inches seems about right but that would require a C.G. at about 16 inches or less for stability?
To Microneister: A Tailcone width of 2 inches will not allow both motors to point to the C.G.. Do you think I should angle both motors as close as I can to C.G. or point the D-12 at the C.G. and have the C-6 close to vertical?
I don't have a lathe or experience turning pieces, so for me making the mount fit inside the tailcone will be easier. Also I find with an extra wrap of paper epoxied on I get a very strong tailcone. Ted

 

[dedleytedley]

I'm hoping for some responses so BUMP.

 

[g_boxwood]

Ted, don't worry too much about it, a 1/2" difference is not anything you can't overcome with some nose ballast.

When you finish the model and weight it you'll find your CG and act as appropriate to get proper stability.

Do not cant the motors. It's not really necessary.

 

[Micromeister]

I'm hoping for some responses so BUMP.

I'd use 19" to start.
Looking at the model layout g_Boxwood is probly right, you could get away without canting the motors..
Using BT-80 (2.6"od) tubing as the main frame for this model, your boattail rear flat works out to 1.8" making this a 1:5.2 Scale model with a multiplier of .200. Even at that (1.8") there is plenty of room to cant the motors if you want to. Assuming your using .021" thick (Standard Estes Style BT-80) tubing or cardstock, based on your drawing heres a quick Full size sketch with BT-50 and BT-20 motor tubes. pay no attention to the NC shape but the length is correct.
hope this helps a little.

PS: I was just thinking about this model again: if it were me building I think I'd use a standard PNC-80K Styrene Plastic Estes Nose cone. The shape is about right and would allow adding the Aerospike if desired (Many JayHawks didn't use one) tho the PNC-80K is only 8.15" long to the shoulder. To be scale you'll need to add 1.27" (1-9/32") to the BT-80 Body tube length to make up the difference, possibly adding a break line at that point indicating where the nose starts. I don't think you'd get dinged to badly points wise with this kind of modification as long as the transitions (overall outline) remain the pretty close. This would give you Plenty of room for extra noseweight to get that CP CG ralationship where it belongs.

View attachment BT-80 JayHawk 24 x 36_06-10-09.pdf

 

[dedleytedley]

Hi Micromeister; Thanks for your interest and efforts. I'm going to use an ogive nosecone from TLP for the build and I am going to add the nose spike. My tailcone will be a little out of scale as I'll be using 2 inch tubing for the core. Accordingly the length of the tailcone will be shortened by 1/8 inch. That will allow a little more room for canting the motors.
I'm also considering making the nosecone fins to pivot freely. I came across an article a while back by a fellow who built a high-power version of the Python 4 with the nose fins on dowels that allowed them to move so that they didn't affect the stability.
The flaps on the rear of the big fins could be built so they are adjustable to counter any potential arcing during flight, though I'm unsure at what angle they need to be set. The big fins will be laminate construction, a 5/32 core with a 1 mm layer on either side from balsa and epoxy. This will give a thickness of almost 5mm and hopefully the strength to survive recovery.
I have all the parts except for the mmt and tailcone wraps so I guess it's time to start building. Ted

 

[dedleytedley]

The build is now well under way. I started with the laminating balsa for the fins to the rough shape required. As was planned in the previous post, they were made from a 5/32 core with 1mm strips epoxied on either side at an angle to the core. The next step was making the fin templates in cardstock. I then cut the main fins from the laminate and cut the finlets from 1/8 plywood.
I used the shroud calculator from EMRR for the dimensions. The radii are 441.5mm and 567.1mm with an angle of 21.5 degrees. A thumbtack was used at the origin and I wound a length of fine stainless wire around it. The wire was stretched to the required distance and wrapped around the shaft of a mechanical pencil to mark the arcs. I added 5mm to the arcs for the overlap tab. After cutting it out it wasn't large enough to wrap around the engine mounting tube so I made another a little larger and this time the fit was perfect. The wrap was glued and the next step is to cut the 2.04 tube for the engine mount.
Cutting the engine mount from styrofoam SM didn't work very well. The holes for the engine tubes were just too ragged so I'm making them from 1/8 plywood. With any luck I'll be flying it this summer. Ted

 

[dedleytedley]

Work continues on the Jayhawk. The tailcone assembly is finished and installed in the B.T. An extra wrap of cardstock was glued to the tailcone with 20 minute epoxy. The wiring tunnel was cut from a length of BT-20 and fitted to the airframe. Small pieces of dowel were glued along the length of the BT to serve as attachment points for the wiring tunnel. The fins were shaped to fit the airframe and the mortises cut in the winglets.
In the meantime the cluster mmt was built. Mmt rings were laid out and cut from 1/8 ply the hard way. The 18 and 24 mm motor tubes were glued in and the whole thing filleted with epoxy.
The nosecone was drilled transversely to accept a length of 1/8 launch lug. This was epoxied on the inside of the nosecone. A hole was cut in the tip of the NC for 1/4 dowel. The end of the dowel was shaped to fit on the launch lug across the interior of the NC. Again this was epoxied to the LL crosspiece with the dowel extending out the tip. Another round of glue on the tip smoothed out the joint. The dowel was cut to scale length.
I want to sand the correct scale profiles on the fins but I can't tell from the images I have if they are straight bevels or ogives on the leading edges.Can anyone help? Ted

 

[Pantherjon]

Lookin good!

 

[dedleytedley]

Thanks Pantherjon. I did some more searching and found these images. It looks like the finlets are a straight bevel on the leading edge and airfoils in the rear. The fins leading edge is still a mystery. Ted

 

[dedleytedley]

I got the fins shaped tonight. I wasn't satisfied with the fit of the fins on the Bt so I sanded them until no daylight came through. From the pics I have the fins leading edge is a sharp ogive and the finlets are flat beveled sharply on the front, less sharp flat bevel on the rear and square edged on the outer chord. I sanded the fins accordingly and added a sanded in elevator on the rear.

 

[dedleytedley]

Progress continues...I poked about 40 pinholes in the airframe along the fin lines to serve as glue rivets. Then glued the fins on with 20 min epoxy and later filleted them with the same. The pic below shows my attachment method.
Got a brainwave for the launch lugs, hide them in the wiring tunnel! Even with the centering ring glued into the BT-20 wiring tunnel there is a free passage along the airframe. I made a short standoff from fin scrap and sanded it to about 3 degrees(tailcone angle). A shallow groove was sanded into the tailcone/BT joint and 3/16 LL s were glued on the standoff and BT at 16 inches.
The wiring tunnel was glued in place along the dowels. The seams are 90 percent sealed, a little glue touchup and filleting won't be necessary.
I planned, cut and fit the wiring tunnel tailcone wrap. Ted

 

[dedleytedley]

A little more yet tonight. I drilled out the center of a balsa BT-20 nosecone and cut it one inch from the shoulder. I then cut it along the length a little past center. Then wrapped sandpaper on a BT-80 coupler and sanded it to fit the BT. The NC fit into the wiring tunnel Ted

 

[dedleytedley]

The construction is all but finished. I glued the finlets on and cut the NC fins from 2mm acrylic. They were sanded to fit the NC profile and beveled on the edges. Slots were cut in them to fit the 1/8 dowel snug enough to stay without gluing(for now). The NC fins swing freely as I hoped they would. They will be glued on after the airframe painting is finished. All that remains is to install the baffle and shockcord and about 20 hours in filling, sanding and painting. Ted

 

[dedleytedley]

My Jayhawk finally flew! There hasn't been good flying weather on most sundays since I finished construction but at last there was a calm day. I've been worried about crashing this one so I decided to use spin stabilization to counteract instability caused by the asymmetrical cluster. I cut some small flaps from a stiff but thin plastic package and taped them in the elevator positions. It was loaded with a D12-3+C6-3 and had a 24" nylon chute attached to the baffle. Both motors fired and it rose, spinning before it left the rod. It rose fast and straight leaving a tightly twisted smoke trail then arced a bit and then coned a couple of times. Ejection was just after apogee and the nosecone, parachute and ...baffle:y: separated from the hurtling airframe. It fluttered a bit coming in slowing it a little and fortunately only a finlet was broken off at impact. It's repairable with some epoxy and some filler. Somewhere in the construction process I forgot to glue in the baffle.
Having survived it's maiden flight it is now worthy of filling and paint. I'm going to paint it with one of the AQM-37C paint schemes but I haven't decided on which of these photos. What do you think? Ted

 

[dedleytedley]

Bump

 

[stickershock23]

Looking good!

I dont think the amount of offset you have will cause too much problem for stability. I know it's to late now, but pointing them at the CP a little would have helped out.

 

[dedleytedley]

I've done a little more work to it. I cut 5/8ths off the top of the bt and added 1 3/8ths bt to the nosecone and a section of coupler. This brings it closer to the exact scale height of 31.88 inches to the top of the NC. This gives the line around the body forward of the wirng tunnel. I will draw on the line rearward. The pitot tube was solid 1.4 inch dowel sharpened to a point. To prevent disqualification as a hazard I cut the spike and replaced the center section with a paper tube. I can smell the spot putty solvent right now. Ted

 

[dedleytedley]

The finishing of the rocket has continued in fits and starts but the painting is close to finished. I started filling with microfill and found it very tough to apply. It just wouldn't stick when wet to the lightly primed surfaces. Sanding was followed by three rounds of priming, filling with glazing putty and sanding. A final coat of primer was sanded with a fine sponge and then the first coat of red Armorcote rust paint was applied. More sanding with 400 grit then a heavy coat of red. A final sand was done with 1200 grit and the last coat of red went on. I neglected to mention that I made cardstock covers for the nosecone fins and filled and sanded them after gluing. Ted

 

[dedleytedley]

Here are a couple more pics of the paint. Ted