Hello All,

Today's 18 mm model rocket motors with their much more energetic "atomic ejection charges" pose a problem for motor-ejecting, classic front motor boost-gliders such as the Estes Falcon (see: www.dars.org/JimZ/k-13.htm ), AMROCS/Semroc Hawk (see: http://www.rocketreviews.com/review...m_hawk.shtml#HS and http://www.semroc.com/Store/scripts...s.asp?SKU=KV-65 ), and Renger Sky Slash (see: www.dars.org/jimz/eirp_12.htm ). At ejection, the modern motors kick themselves out of the models' motor mount tubes with much greater force than the 1960s - 1970s era motors did. When the wildly tumbling spent motor cases strike the gliders' fuselage booms or tail assemblies, these parts are often broken completely off the models.

Although I have not flight-tested it yet (it's still a bit cold for rocket flying here in interior Alaska!), there is a possible solution to this problem. When the motor ejects itself from the glider's forward-located motor mount, the removal of its mass causes the glider's CG (Center of Gravity) to shift rearwards, which changes the model from a ballistic vehicle to a glider. Because the motor's thrust line does not pass through the glider's CG point (it passes *above* the glider's CG point because the motor mount is up on a pylon), the force of the motor's ejection causes the glider's nose to pitch down momentarily, which pitches the rear fuselage and tail assembly *up* where they will be nearer to--if not directly in the way of--the path of the ejecting spent motor case.

Two or three stiff yet springy pieces of small-diameter wire (such as stainless steel music wire) could be epoxied to (or into) the glider's fuselage boom at a 90 degree angle, at or behind the glide CG point. Before installation, these wires would be gently bent so that they would curve backwards (they would look like a very tall Ham Radio whip antenna on a car does when the car is going 80 miles an hour). These curved wires would *not* be in the path of the ejecting motor case (assuming a hypothetical ideal, tumble-free, straight-line ejection), but just below its path. In the real world the ejecting motor case would of course tumble, and it would hit the wires instead of striking the glider's rear fuselage or tail assembly. Because of the wires' elasticity they would absorb much of the impact, and they would also deflect the motor case away from the glider's structure.

If the motor case had a streamer (attached inside its front end using a short length of cotton string glued or taped in place, see http://astrocam.aea6.k12.ia.us/man4.jpg and http://astrocam.aea6.k12.ia.us/hummingbird.html ) in order to make the model "NAR contest-legal," it would be very unlikely to snag on the rearward-curving wires. (Indeed, they would tend to prevent the streamer from catching on the glider's tail assembly.) While these wires would of course generate some drag, it might be possible to arrange them so that their vortices or wakes could produce some beneficial augmentation of lift or yaw stabilizing forces on the glider's tail surfaces.

(Incidentally, the kit instructions for the Semroc Hawk boost-glider include a step for modifying the model to be "NAR contest-legal," which involves cutting 1" off the motor mount tube and taping a streamer to the rear end of the model rocket motor.) There is no need to "butcher" the model in this way so that a streamer-equipped model rocket motor will fit in it. A much simpler way to do it without changing the model at all is shown in the instructions of the Centuri Hummingbird boost-glider kit (see: http://astrocam.aea6.k12.ia.us/man4.jpg and http://astrocam.aea6.k12.ia.us/hummingbird.html ). Simply tape a length of cotton string to a streamer, tape the other end of the string inside the motor case using a square of masking tape (or glue it in place using a square piece of card stock), push a small amount of flameproof wadding down into the front end of the motor (or wrap the rolled-up streamer in one square of flameproof wadding), and then push the streamer down into the front end of the motor.

A roll of 7/8", 1", or 1-1/16" wide fluorescent orange plastic flagging tape contains enough material for scores of motor case recovery streamers. Prior to a launch session, one could prepare all of the streamer-equipped motors at home beforehand. These ejected motors could be left on the flying field during the launch session and be picked up afterwards, and their streamers could then be removed and reused for the next flying session. (An ejected motor lying on the ground with its streamer could even be used as a spot-landing "target marker" for a subsequently-launched model in a streamer, parachute, or glide recovery spot-landing contest!)

__________________
Black Shire--Draft horse in human form, model rocketeer, occasional mystic, and writer, see:
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All of my book proceeds go to the Northcote Heavy Horse Centre www.northcotehorses.com.
NAR #54895 SR

Another modification that I have heard of, but have not tried yet, is to cut a deep scallop into the top of the motor tube from the aft end forward. As the engine is thrust backward at ejection, it meets less friction and less restraint along the top side of the tube, and that is said to cause it to flip out in that direction, away from the glider. I plan on trying it out in one of my upcoming builds.

My old-school gliders don't get damaged by ejection motors - they just don't glide! (And some don't even boost...)

MarkII

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SAM # 0011

That issue also could make the pod rip off at ejection. I like Rob Edmonds’ solution to the too-hard ejection problem. His kits that kick out engines have a 1/4” vent hole in the engine pod so that when he casing starts to move, the vent hole is exposed and a good bit of the ejection gases get vented instead of completely being used to kick the engine out.

Now, I do not know how much the ejection force is reduced, but I figure that must have been a problem with prototypes and adding that must have solved it.

Hmmm, that is a sort of simple R&D project someone could do. A bunch of engines and some sort of system to determine either how hard the "kick" was or how fast the engine ejected out, for a normal closed pod vs pod with vent hole (or different hole configurations/sizes).

The vent hole does not solve the issue of an engine tumbling into the tail, of course, but I thought it was worth mentioning.

- George Gassaway

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The instructions in some the kits and plans for designs that have ejecting engines often include a step for adding a streamer to the spent casing to make it "NAR contest-legal." This has led to a misconception about that type of recovery techniques, a misconception that endures despite persistent efforts to eradicate it. It can especially be confusing to newcomers to the hobby, or to BARS who are just returning after many years. This is the idea that rockets that eject their engines are "illegal" at NAR-sanctioned launches, unless the flier attaches a streamer to the engine casing. There may be a couple of valid reasons for adding streamers to the engines in rockets (gliders and others) that eject them for recovery, but there is nothing in the Model Rocketry Safety Code that requires it. The Safety Code contains the bedrock "rules" for NAR-sactioned launches.

The "NAR contest-legal" statement refers to the use of ejecting-motor gliders in NAR **boost glider competitions.** It has to do with the official "contest" definition of a "boost glider" as stated in the Pink Book (the codified rules for competitions). Basically, a BG is a rocket that separates after boost into two or more sections for recovery, with one section recovering via glide recovery. In order to be a "qualified flight" (IOW, one that counts), all of the separated components must deploy a suitable recovery device. Most of us are familiar with "pop-pod" style boost gliders, and this type is what that rule is basically talking about.

But a glider model such as an Astron Falcon or the Semroc Hawk that ejects its engine in order to transition into the glide is, by this definition, a boost glider, because it separates into two or more parts for purposes of recovery. Therefore, if one wanted fly such a model in a boost glider duration event, the flight would only "count" (or be qualified) if the ejected engine also returned to the ground via a suitable recovery device. (Tumble recovery is apparently not considered to be "suitable," at least for spent engine casings.) Hence the step in the kit's instructions calling for the builder to attach a streamer if the glider is to be used in NAR competitions. That is the only situation in which attaching a streamer to an engine that will be ejected is required. (And even then, technically, it only applies if the participant wants the flight to "count" for purposes of the competition.) Since practically no one uses ejecting-motor gliders in BG events anymore (the pop-pod style is much more competitive), this issue almost never comes up.

You can fly this type of glider, or any other rocket, such as the Mosquito, that is designed to eject its motor during the recovery phase, at NAR-sanctioned Section launches without fear that you are breaking some kind of rule. Individual Sections, and large regional rocket festivals may impose their own "rules of the range," and they may include such a requirement, but that will be a local rule, not a national one. Landowners also always want attendees at any launches to pick up their debris before they leave the field as a condition for permitting the club to use their land, and they may sometimes include picking up ejected rocket cases as a condition. In that case, attaching something like a streamer to the engine can simply help one find it on the ground after the flight.

MarkII

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SAM # 0011

I've been thinking about the issue with Hawks and Falcons breaking the fuselage in front of the tail feathers as I have a Hawk in the build cue.

Sunday I attended a local Free Flight contest and got into a discussion with some my fellow hand launch and catapult glider pilots. Examining the latest generation of birds was somewhat eye-opening for me in that none of the top guys build fuselages of balsa any more. They have all moved on to carbon fiber wrapped tubes for the strength at next to no weight penalty. When I was a competitor, I built my fuselages of straight grained spruce and laminated thin thin thin carbon fiber strips to the sides.

Now with the Hawks and Falcons, the solution may to laminate thin CF sheeting to the sides of the balsa fuselage. I believe (but have no practical experience yet) that the breakage is caused as Black Shire says when the ejection charge drives the nose down. The tail feathers however, resist this movement (upward around the center of gravity) and the fuse break most likely occurs just in front of the horizontal stabilizer.

I will report back on my findings.

__________________
Kit (aka Cranky Kong)

I'm just a roadie for the banned...

Just how often do these fuselage breaks happen?

MarkII

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SAM # 0011

I know this thread deals with boost gliders but I just want to make sure this isn't misunderstood as applying only to boost gliders.......These are excerpts from the US Model Rocket Sporting Code........

9.2 Ejected Motors. No entry in sanctioned competition shall eject its motor or motors in flight in such a manner that the spent motor casing or casings fall freely apart from the model. Ejected motor casings must descend with an attached and fully deployed streamer or parachute. The streamer area must be no less than 10 square centimeters for each gram of jettisoned mass; the parachute area must be no less than 5 square centimeters for each gram of jettisoned mass.

15.2 Separation. Unless specifically allowed by the rules of that event, no timed entry may separate into two or more unattached parts, or eject its motor.

So, for any competition event, the motor cannot free fall (rule 9.2) Additionally, no timed event can eject its motor (rule 15.2). As far as I know, the boost glider duration events are the only timed events that permits the model to separate into two or more parts, with only the glider portion being timed. As Mark has stated, these rules apply to competition events and not sport launching. However, the club hosting the launch may implement additional restrictions as they see fit for both personal and private property protection.

__________________
Don
NAR 53455
"Carpe Diem"

I have heard of clubs not allowing engine ejection (without streamers) because indeed they think it is an NAR “rule” in the Safety Code, instead of realizing it is a contest only rule. So when a club might not allow engines to be ejected, I have to wonder, is that really their own extra limitation they decided to add, or do they think it is the NAR Safety Code they are upholding?

- George Gassaway

I am hardly an expert on the ins and outs of the Sporting Code, but my take on Rule 9.2 and Rule 15.2 is that they are basically insisting that competition entries have engines that are securely retained in the models so that they DON'T eject. I don't know, maybe a model that somehow loses its engine post-deployment could provide an unfair advantage in a PD event, for instance. I don't want to drift off into a discussion about the Pink Book, though.

When Black Shire talked about attaching streamers to engines that are to be ejected during recovery (it was good advice, by the way), I thought that it might be helpful to just add a side note about the non-existent "rule." When I got back to flying rockets a few years ago, I read a lot more about model rocketry than I ever did before, I studied many more plans than I had ever seen before, and most important, I talked (via the internet) about rocketry to a great many more people than I ever had before. And I admit that I, too, picked up that mistaken idea at that time, and held it for awhile until someone on one of the forums wrote something very similar to what I posted above to dispel the myth. (It might have been Fred, and he might have posted it on the oldrockets list.) Since then, I have been seeing the myth being repeated less and less often, but I still come across it from time to time.

To prevent anyone from getting the wrong idea, though, let me add that Black Shire never said anything that would repeat this false notion, and I never tried to imply that he did. I was just adding a kind of footnote that would be somewhat related to the discussion.

MarkII

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SAM # 0011

I thank you all for your replies! While the older motor-ejecting front motor boost-gliders (and rear motor boost-gliders) can't outperform most pop pod boost-gliders, my interest in the older designs is both nostalgic and pragmatic. These are the models that I "grew up with."

Also, I've never enjoyed flying pop pod boost-gliders as much as motor-ejecting ones because with the former, one must always divide one's attention to follow the falling pop pod as well as the glider (especially when flying alone). My father never flew pop pod or parasite boost-gliders (or multi-stage rockets) for this reason. Failing to find a boost-glider's ejected spent motor case doesn't render a model unflyable, and using streamer-equipped motor cases makes it easier to look for expended motors at leisure when cleaning up the flying field after a launching session.

I have heard of the motor mount tube scallop-cut technique as well. It sounds like it should work, although I've never tried it myself. I've never thought that I was "acting illegally" by flying motor-ejecting models, but I'd like to see them be welcome at NAR-sanctioned events (perhaps in "Old Timer" boost-glider events analagous to "Old Timer" F/F [Free Flight] glow engine airplane events).

Vent holes in the motor mount tube not only reduce the motor case's exit velocity (and this would indeed make for a good R & D project), but they could also (depending on their location) serve as "attitude thrusters" to counteract unwanted pitching at ejection.

The sudden tail pitch-up at motor ejection could very well be a contributory (or even primary) cause of fuselage boom and/or tail breakage in some front motor boost-gliders, due to the suddenly-increased aerodynamic loads on the horizontal stabilizer. On my father's Falcon, the ejecting motor case actually did strike the model's tail assembly and break it off on more than one occasion (we could actually hear the impacts). Carbon fiber sheeting would certainly strengthen the area (my father successfully used short lengths of Maple dowel for repairs, as carbon fiber wasn't around then).

__________________
Black Shire--Draft horse in human form, model rocketeer, occasional mystic, and writer, see:
http://www.lulu.com/content/paperba...an-form/8075185
http://www.lulu.com/product/cd/what...of-2%29/6122050
http://www.lulu.com/product/cd/what...of-2%29/6126511
All of my book proceeds go to the Northcote Heavy Horse Centre www.northcotehorses.com.
NAR #54895 SR