Rocket Foamie: SkyFun

I'm basically lazy. I like to reuse exiting engineering instead of coming up with my own.

This is an R/C glider conversion. The original airframe is from here: https://www.hobbyking.com/hobbyking/store/__9614__SkyFun_Jet_w_Brushless_Motor_Plug_Fly.html . The modification is reverseable--I can change between electric and rocket motor mount in the field. Being able to fly the model on electric power before launching it on a rocket gives me confidence in the model's trim.

Initial flights are on SU Aerotech E15-PW. Subsequent modifications allow the use of 32mm RMS case, and the model flies on F16 and G12 reloads.

My intention is to ultimately fly this model on an I49N for my L1 cert.

Ari.

Some photos of the 24mm motor mount.

Even though that is a long burning motor, it is still a substantial jump in impulse. I'd be worried about shredding it on launch. Definitely try it with a higher impulse G motor first, and consider using an H for your Cert flight. Of course, the general recommendation is to stay as simple as possible for your certification flights....

Thank you for your feedback R4K. As I'm saying, L1 cert is my ultimate intention with this airframe. I have intermediate goals for it as well. My original decision to use RMS-R/C-32 hardware limits the choices. I have no long-term interest in 29mm hardware, though I do plan to fly a number of SU 29mm motors in this airframe (F27, F50, possibly G77). I'm doing this as part of an R&D project for a UAV design, and we ultimately need to go to L2 to meet our specs, so I'm reluctant to buy 29mm cases I'm going to use only once.

I worry less about total impulse and more about peak thrust in a glider application. As long as the airframe and motor mount stay together, I'm pretty confident I can get it back safely. The end-burning I49 never produces more than 15lbs of thrust over its entire 7.5 second burn. All H reloads I'm aware of produce more thrust than that. H97 gives an almost constant 20+lbs over 1.5 seconds.

As for keeping it simple... I come from R/C background. This airframe is about as simple as they get for me. For my first L1 attempt this past May, I build a LOC H45 kit, with the intention of keeping it simple for the cert flight. After a beautiful launch and deployment, the rocket drifts out of sight and I fail to recover it. The lesson I draw from that experience is to stay with what's simple for me. Since I have little long-term interest in "traditional" rockets, I'm reluctant to invest money and learning effort into sophisticated recovery systems that I'd use except for certification flights.

Ari.

Are you concerned about the forward CG shift you get when the motor burns out?

iter said:

For my first L1 attempt this past May, I build a LOC H45 kit, with the intention of keeping it simple for the cert flight. After a beautiful launch and deployment, the rocket drifts out of sight and I fail to recover it. The lesson I draw from that experience is to stay with what's simple for me.

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The lesson I draw from that is to use a smaller motor or a bigger field.

Seriously, I think it's fine to cert the way you want. You might consider the CTI H54 -- ~3-second burn, peak thrust about 20 lbs but it tails off pretty quickly. If you fly with a club there's a good chance you can borrow motor hardware.

This is a nice project and easy to implement. Possibly, this could be used as a parasite glider on a model rocket. Something like a Estes original orbital transport.

Thank you for your encouragement guys!

@Adrian: Deltas have a fairly wide CG range on account of a wide MAC. So far, I'm OK trimming the plane so the CG shift is within the stable envelope. This may even work with the i49 relaod. The propellant is heavier than my previous flights, but the motor is longer--so its CG is closer to the plane's CG, and the CG shift is manageable. I'm also thinking of moving the casing back by a couple of inches after burnout to compensate for spent propellant. I like the increase in stability, but dislike the increase in complexity. As I'm sure everyone here understands, with the motor aft of the CG, the CG is moving the wrong way: from low CG on boost to nose-heavy on glide.

@mikec: I loose that LOC rocket on an H123 motor; few L1 motors are smaller. Thank you for bringing H54 to my attention. I'm much more familiar with the AT catalog.

@aerostadt: My motivation behind building an RCRG rather than a boost glider is that I want to have only one thing that I need to track coming down! My first experience with R/C gliders is with Edmonds Arcie II. Having to track the booster while flying the glider down is distracting for me. (I eventually shorten Arcie's booster to the length of a 24mm motor and glue it permanently in place at the front of the glider. It's now an engine pod, and the whole thing comes down as one unit)

SkyFun crashes over the weekend while flying under propeller power.

While testing the autopilot (https://www.diydrones.com/profiles/blogs/ardupilot-mega-home-page), I lose control of the model and it goes in nose-first.

I look at ordering a replacement airframe, but decide against it. This is an opportunity to implement lessons form this airframe.

"There is great value in disaster. All our mistakes are burned up. Thank God we can start anew." -- Thomas Edison

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Before I trash this airframe, I'm attaching a photo of the 32mm mount. As you can see, it's beefier than the 24mm mount. The square carbon fiber tube it sits on in 8" long (it doubles as the launch lug). It goes into a wood pocket I build into the fuselage to distribute the load across a larger area. In the photo, you can see where the motor mount separated from the rest of the fuselage in my crash--this could not happen with the rocket motor mount.

Ari.

I'm building a simpler, leaner version of this glider. I'm using SkyFun wings, but none of the rest of it. I'm replacing foam strip ailerons with balsa. I'm scaling down my 38mm plan and going with 29mm hardware.

Downscaling the motor is a structural consideration. The maximum thickness of this wing is 33mm, less than the motor tube OD. This means I can have spar caps that go all the way across the wing, above and below the motor tube, instead of butt-joining spars to the tube.

The 1 1/8" dowel in the picture represents an H97 motor. Blue tape on the left wing shows acceptable CG range. Most of this motor is in front of the CG, so CG moves down from boost to glide, which is exactly what I want!

29mm tube is too small for an autopilot. This is one other simplification from the previous design. I still want the glider to circle back to launch point autonomously, but I'm keeping that idea for the next iteration.

Ari.

My high-tech wing root routing device. A slightly undersize dowel with 80-grit sanding belt and carpenter's glue.

Ari.

Coring complete.

Ari.

iter said:

@mikec: I loose that LOC rocket on an H123 motor; few L1 motors are smaller. Thank you for bringing H54 to my attention. I'm much more familiar with the AT catalog.

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I'm not following you. The H123 is in the smallest 38mm Aerotech case for an H - but the 29mm options include the H128, H165 or H238 all with only between 165Ns and 177Ns vs. 224Ns for the H123...

The H73 (same case as the H123) is another option - it only has about 185Ns.

As for CTI the H133, H90, H175, H164, H410, H54, and H87 all offer < 170Ns in 29mm.

Of all of these I agree that the H54 is the best glider option. In the old days the Ellis H48 and H50 might have been options also but these are much higher overall impulse.

iter said:

Coring complete.

Ari.

Click to expand...

I like the new plan, and how the thrust will go through the CG to reduce pitching moments, and the forward/aft CG location can be optimized with different motors. Looks like a good way to go.

@UV: I see your point. I'm going the glider route, but I see that it's possible to have a shorter L1 flight on that rocket.

I have two difficulties with CTI motors. One is that my local vendor only carries AT motors, the other is that as a consequence I'm much more familiar with the AT catalog. It's possible to overcome both issues--it's just that there's a little more friction. The H54 does look very promising as a glider motor.

@Adrian: I can see a whole line of gliders that go this way. For a 12% arifoil, the center cord must be ~10x motor diameter, so for a 24mm glider, the length is 10"; for a 54mm glider, the length is 22"; and so on.

Ari.

I spread the first glue on this project.

The spaceship-looking tubes on the sides of the motor tube are guides for servo wires. A pencil line marks the front of the motor casing and servo wires must enter the tube forward of that.

I like to use Gorilla glue on foam. It expands to a density that is similar to foam and forms a continuous bond this way. While the expansion fills minor cavities, it also like to ooze out of the glue line. I use masking tape to contain the oozing. The tape forms a sort of mold that the glue follows as it hardens.

Ari.

Snaking servo wires through my little guide tubes proves a challenge. I take plugs off the wires as is my original intention, but end up having to use 24ga copper wire to pull them through the bends in the pipes. I also discover that the body tube is too short and the servo wires end before exiting it. I cut 1.5" off the front of the BT.

I reattach servo plugs and glue on a 5/16 launch lug. On the previous iteration of this design, my 1/4" lugs feel very stiff on a 1/4" rod and I end up using shorter, 3/16" rods. With a fatter lug, I can use 6' launch rods. I debate putting the lug on the top or the bottom. Top is appealing since I can launch at lower angles and the model just hangs on the rod. Bottom is appealing in terms of using only one fin and gluing it directly to the body tube rather than another piece of foam. Shaping the fin to fit BT is simpler than shaping two to fit the airfoil (I use the same dowel I use for the wings). Single fin, bottom lug and vertical-only launch win

I suppose I can still lower the launch rod and fly off inverted. This can be a neat trick (kind of like the Space Shuttle roll) but something I'm going to reserve until after the first flight.

Ari.

I make "solarfilm hinges" out of zagi tape. The ailerons and their linkages are on.

I use fiberglass strapping tape to reinforce the wing--leading edge, trailing edge and ~25% MAC. I do this top and bottom. The tape is easy to apply and gives almost as much strength as gluing spar caps into the foam.

Ari.

NICE! Looks good. Cant wait to see how it comes out.

Thank you for your encouragement!

My plan is to fly this model the LUNAR launch at Moffett this coming Saturday.

Ari.

The complete article. Top and bottom in different colors to aid orientation.

CG shift from full E15 to an empty E15 casing is about 1/4" (maybe 5/16"): well within CG envelope.

Plan to fly this on Saturday.

Nervous about first flight on a new model.

Ari.

Looks good. Have you tried hand launch gliding it yet?

Last night the wind blows downhill on the little local slope. I decide against launching into sink.

Ari.

A few tosses off a shallow slope confirm that the thing is controllable in glide.

Big day tomorrow.

Ari.

The flight today is a success. About 90 seconds duration on an Aerotech E15-PW.

[video=youtube;ZDEJ7V6TwwI]https://www.youtube.com/watch?v=ZDEJ7V6TwwI[/video]

My flight testing plan. I'm taking the advice of incremental increases in total impulse to heart.

Top to bottom:

E15-W (40 Ns): complete as of 6/23/12
F27-R (50 Ns)
F50-T (70 Ns)
G77-R (105 Ns)
H79-J (200 Ns) : L1 cert

So was that loop under boost on purpose?
kj

In a broader sense, I hold that intentions equal results. If I have a safe landing, or a crash, or some other result, I hold that that is my intention. This belief helps me take responsibility for my results--and learn more about my intentions. It reframes my reflection from "where did I go right/wrong/sideways" to "what about this outcome do I find desirable enough to seek it."

On the boost, I notice the glider arching backwards. I push the nose over, and notice that my elevator response is higher than my expectation: the glider is doing a quarter outside loop. I decide to go with it rather than fight it, and end up pleasing the crowd.

Ari.

Oh I hear ya- any flight that doesn't end in me walking to the car to get a bag to put the parts in is a good flight. I also understand the need to pilot an RCRG for part of the flight, as the pilot we're in the loop and have the luxury of being able to exert control on the model. Sounds like dialing in the boost is still happening- adjusting boost trim, setting up flight modes or DR at least to tone down control response, etc.

kj

My local club has launches only once a month. We get to fly at a spectacular field (NASA Aims/Moffett Field) but one flight a month make testing very slow. Between dry summers and California politics, rockets are hard in the San Francisco bay area.

I'm adding an electric prop option to SkyDart Mk 4. This is sort of a universal electric-to-29mm adapter :=) SU G78 for scale. I imagine using this motor mount for testing and trimming other RCRGs as well.

Waiting for the wind to die down around sunset for a quick flight.

Ari.