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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.
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.
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.
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
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.
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.
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.
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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