Will the burn string on a glider still work if I spit the motor?

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Rktman

Eric
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I've been wondering for a long time whether the burn string on a glider will still release if I intentionally kick the motor out on a streamer? One of the disadvantages of RGs is the weight of that spent motor. Getting rid of it at apogee would really help. Anyone ever try this?
 

Ez2cDave

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I've been wondering for a long time whether the burn string on a glider will still release if I intentionally kick the motor out on a streamer? One of the disadvantages of RGs is the weight of that spent motor. Getting rid of it at apogee would really help. Anyone ever try this?
Hi, Eric !

If ANY part separates from a Rocket-Boosted Glider, it is no longer considered to be a "Rocket Glider" . . . It has now "morphed" into a "Boost Glider" ( parts separate or are otherwise jettisoned ).

To answer the "Burn-String" question . . . Yes, the Burn-String will still release, since it is burned through by either the motor Ejection Charge or some electronic ignition means ( such as an altimeter ).

That last sentence just gave me an idea . . . Fly only with Booster engines and use an Altimeter to deploy variable-geometry components or separate a Pop-Pod at Apogee, rather than relying on a preset Delay Train . . . Hmm !

Dave F.
 

Rktman

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Since this would only be for sport flying, it's primarily to drop dead weight. I'm working on a scratch built swing wing and didn't want reliability issues with the burn string.
 

Ez2cDave

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Since this would only be for sport flying, it's primarily to drop dead weight. I'm working on a scratch built swing wing and didn't want reliability issues with the burn string.
Eric,

The Burn String should function normally, even if ejecting the motor casing.

Personally, I like to use Waxed Dental Floss for Burn Strings, as it melts / burns through very easily, yet it is very strong.

Dave F.
 

caveduck

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I use fine rod-winding thread. Mostly because it works, I have a ton of it accumulated over the years, and it comes in fun colors...It also makes good competition chute shroud lines. If I were intentionally ejecting a motor though, I'd probably just physically release the string when the casing leaves the party. There is some risk of not burning the string because you lose the "hibachi effect". Built a lot of swing-wing RG's back in the day, they were fun - started with the Jon Robbins (in)famous Groundhog.
 

georgegassaway

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I would be concerned that the burn string, would not burn when the engine ejects. Instead, I'd work up a way for the motor to hold the string in place when the motor is in the mount, and the string free to slide out or otherwise be released once the casing is no longer there. But I'd rather just keep the engine in it for simpicity/reliaiblity.

Take note that most Rocket Rocket Glider type designs are all designed for the burned out engine mass to be up front as noseweight. You don't want to eject the casing, only to have to add noseweight of the equivalent mass. I did one swing-wing boost glider, with a pop-pod. Just one. And it needed a lot of noseweight.

I'll also note that for nearly all models I fly that use burning "string", I use elastic thread. So I can pull it taut for boost, and when it burns it "snaps" itself out of the way.

Ironically, I would expect greater reliability issues with NOT using a burn thread, than using a system depending on the engine casing ejecting itself, where the thread may end up getting hung up on something (or not burning if burning is tried).

And that's not even in the top 2 of swing-wing problems. #1 is deployment strength (as in not enough), and #2 is hinge weakness/damage. Also a lurking problem is engine exhaust burning the string/ thread during launch.

Here is a thread I created 10 years ago that describes some methods for strong hinges (yet allowing the wings to be removed easily), for better rubber band leverage, and the method I used to solve the problem of engine exhaust burning the thread (wings are locked by a music wire pin on boost, that is pulled out to unlock them once a burn thread allows a rubber band and unburnable cord to pull the pin out).


Below - 6 foot span swingwings, 1972 , and 2008.

 
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Rktman

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Curious if @BABAR has used burn string rotor retention in some of his helirocs that ALSO ejected their motors?
 
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BABAR

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Hey Eric,

actually, for years nearly ALL of my helis RELEASED the motors. I say “released” rather than “spit” because most were tubeless, no separate motor mount. I use two #16 rubber bands for motor retention AND ROTOR retention.

I believe you can reliably use motor eject for burn strings with one proviso, put the string RIGHT IN FRONT of the motor casing. The reason is to eject you need a (mostly) closed tube. If the tube is “mostly closed” and the motor ejects, the motor ejection gases aren’t going to travel very far up the (closed) tube, they will go part way up, get compressed, and decompress out the back when the motor ejects. But they will definitely travel up the tube part way. How much? I dunno, but I DO know if it’s like a cm in front of the motor, it’ll burn it!

I’ve kind of found my niche In my designs, if I was using the old style with string I think I would swap it out for elastic cords, they burn well and stay taut and self retract.

hope this helps!
You’ve already seen this, but included if others are interested.
 
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BABAR

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I've been wondering for a long time whether the burn string on a glider will still release if I intentionally kick the motor out on a streamer? One of the disadvantages of RGs is the weight of that spent motor. Getting rid of it at apogee would really help. Anyone ever try this?
Btw, some may call it heresy, but depending on the field rules, there is nothing wrong with kicking a 13 mm or 18 mm casing WITHOUT a streamer. As you know, there are kit rockets designed exactly this way. (semroc Hawk, Astron Falcon, Starlight Sparrow), You should look for and try to retrieve the casing if you can find it, if you make and effort and can’t find it, it’s biodegradable and not gonna hurt anything. I have been launching at a local park for a decade without any complaints. My newer designs incorporate a streamer because I am hoping to get broader acceptance on the forum (but no one seems all that thrilled, lol!). Places like Sod Farms with special harvesting equipment that could be damaged by casings may have rules against this. YMMV
 
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Rktman

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Btw, some may call it heresy, but depending on the field rules, there is nothing wrong with kicking a 13 mm or 18 mm casing WITHOUT a streamer. As you know, there are kit rockets designed exactly this way. (semroc Hawk, Astron Falcon, Starlight Sparrow), You should look for and try to retrieve the casing if you can find it, if you make and effort and can’t find it, it’s biodegradable and not gonna hurt anything. I have been launching at a local park for a decade without any complaints. My newer designs incorporate a streamer because I am hoping to get broader acceptance on the forum (but no one seems all that thrilled, lol!). Places like Sod Farms with special harvesting equipment that could be damaged by casings may have rules against this. YMMV
True. Working on an experimental glider that I need to dump as much nose weight off as possible, so eliminating the streamer will really reduce the amount of tail weight needed to compensate. To clarify, keeping the streamer would mean nesting an 18mm motor tube (with streamer) inside a 24mm tube, along with two additional CRs.

Sans streamer, all I need is a short piece of motor tube and a single CR. Ditching the motor will get rid of approx. 11.12 grams (close to half an oz.) of dead weight on the glider's front end. (Normally this wouldn't be a good thing since you want more of the weight of a glider to be up front, but I have a swept-wing design, and the CG is further back than with a traditional glider. That requires weight- biasing the aft end).
 

BABAR

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True. Working on an experimental glider that I need to dump as much nose weight off as possible, so eliminating the streamer will really reduce the amount of tail weight needed to compensate. To clarify, keeping the streamer would mean nesting an 18mm motor tube (with streamer) inside a 24mm tube, along with two additional CRs.

Sans streamer, all I need is a short piece of motor tube and a single CR. Ditching the motor will get rid of approx. 11.12 grams (close to half an oz.) of dead weight on the glider's front end. (Normally this wouldn't be a good thing since you want more of the weight of a glider to be up front, but I have a swept-wing design, and the CG is further back than with a traditional glider. That requires weight- biasing the aft end).
Added plus, an 18mm tube and nose cone not only weighs less than 24 but should be much less drag
 
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