Space Plane 4X R/C Rocket Glider is now an "Ex Space Plane" :(

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georgegassaway

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In 2008, I built a 4X upscale of the third Estes kit, and first Boost Glider kit, the Astron Space Plane. One incentive was NARAM-50 was coming up, 50th anniversary of the NAR. I built the 4X to use R/C, mixed elevons. Powered by 32mm reloads, the F13 or G12.

Info on my website about the R/C 2X and 4X, including history of the original kit:

https://georgesrockets.com/GRP/GLIDERS/SpacePlane/Spaceplane.html

It flew very nicely. A slow boost on a the 8 second burn of a G12. Easy to control the boost. For glide, the dihedral and swept wings made it handle VERY nicely as an R/C model.

Flew it a few times before NARAM-50 to test it out. Than flew it a few times at NARAM-50. Posed with Vern Estes, holding a golden Astron Scout.

tGRfOpC.jpg


Had not flown it since then, until this past Saturday. I had replaced the old 72 mHz receiver with a Spektrum 2.4 gHz receiver, since I now use a Spektrum Tx.


UEQR5Pm.jpg


Did a test flight on an F13, the trim was pretty good to begin with, so the boost was fine and the glide went well. A second boost on a G12, went very well, nice long burn, then nose over just at burnout to go into the glide, and a nice glide back.

YucTE5e.jpg


So, everything was great, time to do another flight. Took off, and about 2 seconds into the boost, the exhaust sounds went from "ssssss" to "SSSHHHHHH", a much louder sound, which is never supposed to happen with an endburner like that, the sound getting louder. I knew instantly what was happening: The composite propellant grain had a void or air bubble in it. That means that when the flame front got to it, it caused a much larger surface area which not only increased the thrust but the increased pressure caused the propellant to burn at an even faster rate (a characteristic of AP propellants). So, suddenly the thrust was at least doubled, if not more than that.

It got moving noticeably quicker, for about 1 second, then the wings shredded into several pieces. A good R/C RBG is built to be somewhat stronger than it needs to be, but not excessively overbuilt (heavy) since that will hurt the glide. I guesstimate the model was about 50% stronger than it needed to be. But if the thrust doubled, well, aerodynamic forces increase with the square of the velocity, so if for example it boosted twice as fast, it would have four times more aerodynamic forces acting on it as on a normal flight.

The red BT-80 "fuselage" fell to the ground sideways, with part of one wing on it so it did not hit too hard and survived, including the radio gear. But everything else is toast. Literally part of one wing is "toasted" as the monokote let it dangle long enough for the G12 exhaust to scorch it. Now, since the main wing was nothing but 1/4" balsa (nice light balsa), I could rebuild it in 2-3 days if I wanted to (and had suitable light balsa, not easy to find). But, not anytime soon.

Back to the G12 reload with the void in the propellant. This was a very old reload, from 1996, so anyone who has "newer" F13/G12 reloads, chances are low that you would have this problem, not the same batch. But age has nothing to do with it. That disaster was waiting to happen from the moment in 1996 that the propellant was cast and cured with the void in it. The only other time I had a 32mm reload have a problem with a void in it, was 1999 when another G12 had a void and shredded the wing of my Big Bird-3 six-foot span R/C R/G. Fortunately I was able to rebuild it. But that incident also that is why I knew the instant the sound got louder with the Space Plane 4X, exactly what was happening and that disaster was likely to happen, and about a second later it did.

- George Gassaway

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George, I had this same thing happen to my 6' tall inteR/Ceptor about 2 years ago. Same sound change, sudden accelleration, about two seconds in then a fuselage buckling. I assumed like you that it was a void issue. The reload was not very old, maybe 6 months(so 2.5 years ago). Since the case was not damaged, there wasn't much I could do with aerotech. I stopped using them and started using the 29mm G-25 and redesigned my larger models to use them and the faster burn rate.

Frank
 
An update. Here is a video of the flight and shred.

[video=youtube;p3RQD07oDBU]https://www.youtube.com/watch?v=p3RQD07oDBU[/video]
 
Wow - you really can see the thrust change and hear the tone change - very interesting - thanks for posting.
 
I timed how long the G12 burned, according to the sound on the video. I get right at 5 seconds. The G12 normally burns for about 8.5 seconds. The thrust was normal for about the first second, and then the flame front entered the void bubble and the thrust shot way up and the burn rate also increased due to the higher pressure. Once the "void" bubble got consumed and the flame front got a bit more back to normal in shape (dome-like), then the thrust would have dropped to normal levels and normal burn rate.

Also actually, that reload was from a batch in 1996 where Aerotech had temporarily stopped using a burn catalyst that made the propellant burn faster, so without it the thrust was lower and burn time longer (about 11 seconds or so). So, for the burn time to be about 5 seconds rather than about 11, given that the first second or so was normal, and the last part of the burn would have eventually gotten down to normal, shows that it consumed a LOT of propellant for a brief time, producing a heck of a lot of thrust. So now I'm thinking it must have been at least triple the thrust, perhaps 4X. So, for a brief time the G12 was more like a G36 to G48.

BTW - last night I did recall another case of a G12 with a void in the propellant. I was flying my 1/10 R/C X-1 scale model. It was heavy, and needed take off at 45 degrees since a vertical liftoff was too slow to be safe. Well, on one boost in 1999, it took off at 45 degrees, and was going slow, but fine, then it suddenly had a lot more sound and accelerated a lot more. But fortunately, it survived. The stubby wings were built out of 1/16" sheeted balsa over ribs, very stiff and strong. And I had built it with the capability of using some higher thrust engines (within reason) , I even flew it on an F16 BlackJack reload once.

- George Gassaway
 
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Bummer.
I saw you fly it at NARAM 50....think you brought it in once with the wrong model setting in the TX ....(ailerons might have been reversed IIRC)...but in true "Right Stuff" fashion - you landed it fine anyway.
 
Bummer.
I saw you fly it at NARAM 50....think you brought it in once with the wrong model setting in the TX ....(ailerons might have been reversed IIRC)...but in true "Right Stuff" fashion - you landed it fine anyway.

Wow, great memory. What happened was, first I flew the 2X model on a C6. Then quickly changed the model memory in the transmitter for the 4X. But.... I forgot to click "OK", to change it to the 4X. So it flew with the 2X model memory, which had the aileron control opposite of the 4X. That is why the boost rolled a lot.... it kept rolling (say) right, I kept correcting left, and no good. When it had done a very slight right roll at launch and it turned out that my "left" to correct it was forcing it to roll more to the right. Fortunately it kept vertical. When it burned out and I went to glide, and it turned the opposite way for glide, then everything "clicked", and I knew I had a reversed aileron problem (that finally made me realize the cause of the boost roll problem).

I didn't exactly fly it so well, the natural instinct to turn one way was hard to try to do in opposite form. So, I nudged it to end up in a very wide open area a few hundred feet away, rather than a closer landing. So my best piloting aspect of that.... was a simple choice to be darned careful, setting it up to land in a big area farther off, as opposed to trying to land it closer with a big risk of a mistake.

- George Gassaway
 
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You would think that with the video, AT would be able to help out...
 
You know George, I remembered I did have a second case of this with a G-12 about a year before my first one, on a lightweight large bomarc depron glider, same thing...lifted off, then sudden increase in sound and speed and then airframe failure....
 
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