Ending the “Red Baron” Curse

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Rktman

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Red Baron: “The condition in which a boost glider's motor pod and recovery system gets tangled with the glider portion, causing the glider to spiral down to the ground like a WWI airplane which has just been shot down”.

I wanted to share this solution on TRF for those of you who are new to the sport, a recent BAR (“Born Again Rocketeer”), or not an NAR member, where my article appeared in their recent March/April Sport Rocketry publication. Since the Forum reaches a lot of rocketeers who may not be NAR members that can benefit from NAR's extensive library of info, I felt it was a good alternative to reach other boost glider enthusiasts with a way to avoid this aggravating problem. I tried to be as clear as possible with descriptions, and will leave it open for any questions you might have.

First, let me say that I did try other solutions like shorter shock cords, longer pods, and anchoring shock cords externally. Didn't always work. I also wanted to avoid anything complicated to build or having a system with more pieces to recover. What I came up with is the Rear-Eject Pop Pod. The idea is so simple and effective that it’s somehow been overlooked. Don’t put your recovery gear directly in the path of your glider. Instead eject it in the opposite direction, away and out of the path of your glider. Over the last 16 months I’ve made 57 flights and counting without a single instance of the “Curse”.

Basically it's a tube nested within a tube, for example an 18mm (BT-20) motor pod inside a 24mm (BT-50) tube, or a 13mm (BT-5) inside an 18mm. Essentially it’s an engine mount that serves as a sliding “piston”, which is harnessed by a shock cord to the permanently glued-in-place-nose cone. I use at least 250# Kevlar. Use a length of braided steel fishing leader between where it’s anchored around the motor pod and the Kevlar line. I’ve noticed that with many of my rockets, heat fatigue can cause the Kevlar cord to eventually burn through near where it’s anchored around the forward CR. This will prevent that from happening since the steel leader is far more heat resistant.

Rear Eject Pop Pod-downsized.jpg


Let me mention at this point that this system is not meant for any competition BGs where every fractional gram of weight saving is a priority. However, if you value not DQing due to your recovery gear fouling your glider vs a few extra grams of weight at launch, or if you’re a sport flier like myself, then this technique will be well worth an extra 20 minutes of your build time.

The number of centering rings you use is up to you, though I favor using 3 (forward, aft, and one to anchor the top of the engine hook to strengthen the area where it enters the engine mount tube). Install an engine block (you can omit the engine hook and the 3rd CR if you prefer to use a friction fit motor, but in the interest of guaranteeing everything works as it should, I’d recommend not omitting it).

Sand the centering rings if needed so that the “piston” unit slides freely inside the external pop pod tube.

To limit the sliding engine mount’s forward travel, insert another engine block/thrust ring inside the outer main pop pod’s tube.

I limit the exposed section of the Kevlar shock cord to 12 - 18 inches for A – C motors—more than that may cause the ejected motor and tube to lose too much momentum to ensure that it pulls the pop pod free of the glider. Note: I haven’t used this method with larger motors as most front-engine gliders will rarely employ anything larger than a D motor. As such my tests/prototypes were limited to the A – C motor classes.

Important: Attach a segment of flame-proof elastic as a “bungee” setup between two points on the Kevlar cord on the end that connects to the nose cone to absorb the ejection shock. This will relieve some of the initial violence of the ejection charge while still providing a positive connection once the elastic stretches and allows the cord to pull directly on the nose cone. Don’t omit this step — I found out the hard way how essential this is.
Bungee-attachment-style.jpg

On its 3rd test flight, the attachment loop on my plastic nose cone snapped. Luckily there was enough inertia to pull the pop pod free of the glider but it made me aware of how thin and fragile this loop can be on many lightweight plastic nose cones. Instead drill two small holes in the base of the nose cone and knot the shock cord through this, then lock it in with some epoxy. If you use a balsa nose cone, strengthen the base of the shoulder where the screw eye goes in with a generous amount of thin CA, or insert a hardwood dowel into the balsa first and attach the screw eye into that.

On the aft end, the shock cord is attached to steel fishing leader wire epoxied around the motor tube and threaded underneath the centering ring.
tethered piston.JPG


Retrofitting existing pods

An alternate configuration can be used for retrofitting existing pop pods. It involves using a coupler between two same-diameter body tubes and a stuffer or boom tube nested inside the coupler to deliver the ejection pressure forward while providing a shielded space to attach and store your streamer or small chute. For example, for an Apogee Condor kit I had, I used a 3” extension piece of BT-20 and a 13mm - 18mm centering ring sanded down to fit inside the coupler. Then a piece of 13mm (BT-5) tube with a standard BT-5 – BT-20 centering ring on its forward end was then inserted into the coupler’s modified centering ring.

A length of steel fishing leader was anchored to the forward end of the BT-5 stuffer tube. The rest of the setup then followed the same procedure of attaching the bungee elastic/kevlar cord combination to the nose cone.
Retrofit Pop Pod.jpg

It’s important to note that the new pod section and attached hardware needs to be added to the aft end of the existing pod. In other words, trying to incorporate the pylon and hook as part of the new section won’t give you good results. Trust me, I’ve tried it.

If there are concerns about launch stability from the added-on aft segment, you can either extend the pop pod’s length or add some weight to the forward end. In my example I chose the latter as it was the fastest and easiest to accomplish (and adding more pod length to the forward end would still have added weight anyway). To do this in my Condor example, I simply found the CG location on the stock pop pod before modifications and added clay to the front of the modified pod till it balanced in the same spot. It only required 3.35 grams (0.118 ounces) which is really not much.
condor pod-modified-downsized.jpg

As you can see from the next pic there is only a marginal difference in overall length (the original tube is at the top, the modified version is below it). It amounted to just +1.0” in overall length because I relocated the pylon/hook, shortened the original tube, and added the necessary sliding portion to the aft end.
condor pod comparison-downsized.jpg

If you have a large glider that uses 24mm motors, the correspondingly larger parts are easily available. However, I have concerns about retrofitting a 13mm engine pod. Using a Micromaxx BT-2 as a stuffer tube could cause problems. The small, launch lug-sized tube may be too small in diameter to handle the ejection charge pressure. Better to build a new 13mm/18mm rear eject pop pod than trying to retrofit it.

The last photo shows some of my rear-eject pods that are swapped between gliders with the same pylon hook design. The top pod has the nose cone removed to show the shock absorbing “bungee” setup.

RE pods-downsized.jpg
So that's it, a fairly simple system but dependable and effective. My hope is that readers can use this system as a starting point and improve on it, then share what they come up with here to benefit all BG fans.
 

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Alan R

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I love it when people think up new ways to do things, and actually build and engineer the ideas.
I like thinking of new things but I'm really bad at persevering through the engineering build phase
Kudos to you
 

Rktman

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Isn't it interesting what aggravation and frustration can drive a guy to do?o_O
 

BABAR

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Love it!

Questions:

Why three centering rings on motor mount? Why not use a single forward ring to both center the mount and hold the forward end of motor hook in place?

Do you require ANY length of motor mount forward of the second of two rings?

Can you decrease weight by using modified engine blocks and thrust rings rather than the formal purchased blue motor blocks and green thrust rings? For a BT-20 engine block, a 4 mm slice of BT-20, with a bite out of it so it rolls up nicely inside the tube just forward off the nose end of the engine hook. For the BT-50 thrust ring, you can do the same with a 4 mm slice of BT-50. Cheaper and less weight.

Do you even need a streamer? This would be the opposite of “Nose-Blow” recovery, where simply blowing off the nose cone makes the pod unstable so it tumbles down.

Advantage is less weight, also less drift.

Disadvantage is a streamer is easier to see if it lands in the bushes.

Of course, if you go this route you have to NAME it, certainly invites some humorous antonyms to “nose-blow”!
 

BABAR

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For your coupler version, using rear eject tumble recovery without the streamer makes it a cinch. Attach your steel leader inside the coupler, ditch the stuffer streamer pod assembly, just blow the motor mount out the back and let it tumble recover. Just make sure to keep using that fluorescent orange so you can find the thing.

You COULD also use a longer Kevlar cord attached to the front of the wire leader, and attach a small streamer to the Kevlar cord.

Packing is as follows

Length of Kevlar a few inches longer than the body tube
Streamer rolled up attached to Kevlar.
Dog barf wadding (less likely to bind)
Remaining short length of Kevlar and your wire leader
Coupler with motor.

At ejection, motor force WILL initially smush the wadding and streamer forward, while VERY forcefully kicking the motor and mount and coupler out the back.

Hopefully that force will be enough to pull the smush up wadding and streamer out of the forward end of the pod.

Even if it doesn’t, the system is unstable and should tumble recover.
 

Rktman

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Questions:

Why three centering rings on motor mount? Why not use a single forward ring to both center the mount and hold the forward end of motor hook in place?

Do you require ANY length of motor mount forward of the second of two rings?

Can you decrease weight by using modified engine blocks and thrust rings rather than the formal purchased blue motor blocks and green thrust rings? For a BT-20 engine block, a 4 mm slice of BT-20, with a bite out of it so it rolls up nicely inside the tube just forward off the nose end of the engine hook. For the BT-50 thrust ring, you can do the same with a 4 mm slice of BT-50. Cheaper and less weight.

Do you even need a streamer? This would be the opposite of “Nose-Blow” recovery, where simply blowing off the nose cone makes the pod unstable so it tumbles down.

Advantage is less weight, also less drift.

Disadvantage is a streamer is easier to see if it lands in the bushes.

Of course, if you go this route you have to NAME it, certainly invites some humorous antonyms to “nose-blow”!
To respond to your last comment first regarding a name: Bwah-hahaha! Hilarious, good one (the alternate antonyms kept rolling through my head all day long).
Now then, about the 3 CRs--just a habit to build sturdy or redundant, the middle one could be omitted in the interest of saving a few grams.

As for the minimum MMT length, no additional length is needed forward of the front CR, but since the minimum width of most streamers is 2", I need to allow at least that amount shielded by the CRs for the streamer or a small 'chute. Also the aft CR can't be flush with the back to make more room or the motor hook won't bend enough to allow motors to be installed.

As for homemade thrust rings, my priority was something fast and easy to assemble, but I like the idea. It's certainly worth it for anyone using the system for competition where shaving off every gram (or fraction thereof) is of more concern then to a sport flyer.

Streamers...well, yes, makes the pods easier to spot in tall grass/weeds, but more than that it really helps avoid bent and dented pods or damaged pylons. The way I see it, the weight of a streamer is negligible (especially the mylar or crepe ones) vs the benefits. If used for NAR competition, I don't think they require streamers or 'chutes so anyone considering this system could certainly omit it I suppose.

Really like all your great ideas, you really put some thought into this and it was what I hoped for, that other heads might evolve this system into something even better.
 
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BABAR

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Cool.

I’d dump the motor hook. Move the aft centering ring forward to allow you to put one wrap of cello tape around the tail end of the motor mount. Minimum would be 1/2 width of cello tape. This will “seal” the end so when you put tape around it to hold a motor, it won’t rip/delaminate the paper tube when you apply outer tape in next sentence

Then use external tape (cello or masking) wrap, 1/2 over the motor and other half over your outer TAPED surface. Burnished down well, I have yet to have this fail. The tape in the sentence above allows piece to be easily removed after flight.

I use marker tape from the hardware store for streamers, it is about 1 inch wide. Mylar or crepe probably more efficient/effective.

If you are looking to save weight, using a 1 inch streamer, you can actually use a much shorter motor mount, basically 1 inch plus the width of the forward and rear centering rings if you can live with friction fit, or add 1/2 width of cello tape at the tail end for external tape motor retention (note: this external tape wrap will actually be recessed inside the outer tube on boost, so no extra drag.) Seems like a good balance for a sport model optimized for both practical use and performance.
 

Rktman

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For your coupler version, using rear eject tumble recovery without the streamer makes it a cinch. Attach your steel leader inside the coupler, ditch the stuffer streamer pod assembly, just blow the motor mount out the back and let it tumble recover. Just make sure to keep using that fluorescent orange so you can find the thing.

You COULD also use a longer Kevlar cord attached to the front of the wire leader, and attach a small streamer to the Kevlar cord.

Packing is as follows

Length of Kevlar a few inches longer than the body tube
Streamer rolled up attached to Kevlar.
Dog barf wadding (less likely to bind)
Remaining short length of Kevlar and your wire leader
Coupler with motor.

At ejection, motor force WILL initially smush the wadding and streamer forward, while VERY forcefully kicking the motor and mount and coupler out the back.

Hopefully that force will be enough to pull the smush up wadding and streamer out of the forward end of the pod.

Even if it doesn’t, the system is unstable and should tumble recover.
I like the idea of omitting the stuffer tube but have some concerns over the survivability of the streamer, even with a lot of wadding. Certainly deserves experimenting with though.

If the NAR doesn't require a 'chute or streamer for BG competition and allows the pod to tumble recover, a contestant could just omit it I suppose.
 
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Rktman

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Cool.

I’d dump the motor hook. Move the aft centering ring forward to allow you to put one wrap of cello tape around the tail end of the motor mount. Minimum would be 1/2 width of cello tape. This will “seal” the end so when you put tape around it to hold a motor, it won’t rip/delaminate the paper tube when you apply outer tape in next sentence

Then use external tape (cello or masking) wrap, 1/2 over the motor and other half over your outer TAPED surface. Burnished down well, I have yet to have this fail. The tape in the sentence above allows piece to be easily removed after flight.

I use marker tape from the hardware store for streamers, it is about 1 inch wide. Mylar or crepe probably more efficient/effective.

If you are looking to save weight, using a 1 inch streamer, you can actually use a much shorter motor mount, basically 1 inch plus the width of the forward and rear centering rings if you can live with friction fit, or add 1/2 width of cello tape at the tail end for external tape motor retention (note: this external tape wrap will actually be recessed inside the outer tube on boost, so no extra drag.) Seems like a good balance for a sport model optimized for both practical use and performance.
You lost me. A - C motors are 2.75" L so wouldn't a minimum MMT be at least 2.5" long if the motor protruded 0.25" out the aft end to accommodate the thrust ring?
 

BABAR

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You lost me. A - C motors are 2.75" L so wouldn't a minimum MMT be at least 2.5" long if the motor protruded 0.25" out the aft end to accommodate the thrust ring?
Who says you are limited to only 0.25 inches coming out the back end?

Motor mount requirements:

1. Hold the motor in alignment with the rocket
2. Don’t let motor shoot through into the rocket
3. Retain the motor at ejection

Mark a used 18mm casing 1” from the forward end.

Stick it in a BT-20 up to the mark

you have 1 3/4” sticking out the back.

Put a single wrap of cello tape around the interface, 1/2 of tape width over the outer border of the BT-20 and 1/2 of tape width over the motor casing. Burnish the tape wrap down hard

1. There is no waggle (the motor is perfectly aligned)

2. You probably should add a forward thrust ring, which will add about 4 mm to the length. The tape itself MIGHT work without one, but you may find, at least with cello tape, that if you push really hard you CAN push the motor in.

3. Try to quickly pull the motor out by hand. You can’t (at least I can’t ). You have secure motor retention at the cost of two wraps of cello tape in weight (one of those is the protector layer you put on and leave on. If you put hard steady pressure on it you can get it out, but that’s not what an ejection charge does

BTW, you can use this on your helicopter motor mounts as well.

You can’t do this easily with regular rockets because you don’t have ready access to the mount if it is recessed in the rocket body. For rear eject pods and helis you absolutely have easy access to the shortened mount/extruding motor casing interface.
 
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BABAR

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oh yeah, it works great with boost gliders like the Hawk or Falcon that intentionally eject the motor.
 

BABAR

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by dropping from 3 rings to 2 rings, you also reduce the friction for rear ejection. You can also shave off a layer or 2 from the rings as you want the pod to be relatively loose. Pods that slide out easily also are unlikely to eject the motor from the pod, as if it slides out easily the path of least resistance should be ejecting the pod/motor combo, rather than the motor itself. I guess having a shorter mount also decreases the inertia of the mount itself, but I suspect the difference of shortening the mount by 1.5” and ditching the motor hook is negligible in this regard, but definitely a plus in performance.
 

Rktman

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Love it!

Questions:

Why three centering rings on motor mount? Why not use a single forward ring to both center the mount and hold the forward end of motor hook in place?

Do you require ANY length of motor mount forward of the second of two rings?

Can you decrease weight by using modified engine blocks and thrust rings rather than the formal purchased blue motor blocks and green thrust rings? For a BT-20 engine block, a 4 mm slice of BT-20, with a bite out of it so it rolls up nicely inside the tube just forward off the nose end of the engine hook. For the BT-50 thrust ring, you can do the same with a 4 mm slice of BT-50. Cheaper and less weight.

Do you even need a streamer? This would be the opposite of “Nose-Blow” recovery, where simply blowing off the nose cone makes the pod unstable so it tumbles down.

Advantage is less weight, also less drift.

Disadvantage is a streamer is easier to see if it lands in the bushes.

Of course, if you go this route you have to NAME it, certainly invites some humorous antonyms to “nose-blow”!
Re: ditching the streamer and using tumble recovery, I talked to Dan Wolf, Contest Board National Chairman at NAR, and it's apparently a grey area since the rulebook doesn't specifically say you MUST have one. However, he said that if the contest RSO feels the pop pod is falling too fast or heavy end down (ballistically more or less) he may DQ the flight. His advice was to test your pod to see whether it tumbles safely or not without a recovery device.

Given that, I suppose contestants entering a NAR BG competition could build their pop pods as light as possible (sans extra thrust ring, motor hook etc. as you suggested) and ensure it tumble recovers, and just carry backup pods in the possible event the primary one becomes unusable. Speaking just for myself, though, as a sport flier whose priority isn't max performance for competition, I prefer my pop pods not end up bent and bashed after every flight.
 

BABAR

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Anything not worth doing at all isn't worth doing well.

Phrased differently, if the minimum wasn't good enough it wouldn't be the minimum. ( for some reason this philosophy was somewhat unpopular with my commanding officers when I was a Cadet!)

Therefore an option would be to put a very small streamer on your pop pod I typically use plastic Hazard tape , I think it's about 1 inch wide and comes in pink and fluorescent orange . It is NOT flameproof.

Estes Tercel is a nifty glider, but putting a parachute in the pod instead of a streamer was, IMO, a waste of a chute, added unneeded mass, and made it less reliable.

At a ten to one optimal streamer length, for my streamer tape that's one inch by 10 in. It's very light, takes up very little space, and if you fold it rather than roll it around the pod rather than roll it deploys nicely. allows for a very short pop pod length.

If you place streamer right on middle of the Pod, it will keep the Pod horizontal as it falls there for preventing ballistic recovery and the drag of the horizontally falling pod itself will also slow descent.

this technique is also used with an external shock cord mount on streamer duration competition rockets.
 

Aeronerd

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Glad I found this thread. I think it is a great pop pod idea because the first kits I built after "rediscovering" the sport last year were an E-Z Fly glider and a Semroc Bluejay glider and they always seem to get tangled up in their shock cord. I think this could solve my tangling problem so I'm looking forward to building replacement pods using this technique. Or maybe just retrofit what I have like in your alternate method.
 
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Rktman

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Just wanted to reiterate the importance of keeping the CG location of the rear-eject pod the same as the original kit pod, whether retrofitting or fabricating a new rear-eject pod. The original pod’s length and balance point (with an unfired motor installed) was chosen by its designer to ensure a stable vertical launch and minimize or eliminate looping or weathercocking on ascent.

This becomes even more important with gliders with pre-canted forward canard wings. If designing from scratch, the launch CG of glider and pod with an unfired motor installed should fall at or near the center of lift of the canard.
 
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