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Old 03-11-2020, 01:05 PM
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Rktman Rktman is offline
Mad for modrocs
Join Date: May 2019
Location: Chapel Hill, NC
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Default Ending the “Red Baron” Curse

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 YORF 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 who 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 (see fig #1). 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.

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.

[IMG]Fig 1[/IMG]

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.
[IMG]fig 2[/IMG]

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 (see fig 3).
[IMG]Fig 3[/IMG]

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 (see fig 4).

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.
[IMG]Fig 4[/IMG]

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.
[IMG]Fig 5[/IMG]

As you can see from fig #6 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.
[IMG]Fig 6[/IMG]

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.

Fig 7 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.
[IMG]Fig 7[/IMG]

So that's it, a fairly simple system but dependable and effective. My hope is that you can use this system as a starting point and improve on it and share it here to benefit all BG fans.
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