My little "Cyclops" horizon-cam D-engine launch vehicle (attached pic)
View attachment 342696 flies fine, but after two launches I'm coming to the conclusion that a tube this short (20cm) and narrow (24mm) is just too small for a D-engine's ejection blast.
After chute and shock cord there's only about 2 inches for wadding; chutes are getting melted, shroud lines damaged. The nose pops out fine, but despite using plenty of masking tape for friction-mounting the engine, the engine shotgunned out both times.
Is a tube of this size just too small for D engines, or are there any special tricks for dealing with a D-sized ejection charge in a small tube? I am thinking of drilling two or three 1-cm holes in the tube, just above the engine to discharge some of the blast (covering them with sacrificial tape to keep things aerodynamic on the way up).
Anyone with any ideas/advice on this kind of thing?
Many thanks.
Cool rocket, nice engineering on the nose cone camera.
Regarding vent holes, I'd say absolutely NOT. You want all that hot gas and flames to go forward and blow out nose cone, chute, shock cord, and wadding.
Vents will not only reduce the force, but diverting the hot gas with holes anywhere near the engine will direct it out the sides through the holes, and will burn your tube around the holes. Don't ask me how I know this......
Your tube diameter is not a problem, D's do fine with minimum diameter tubes.
Length of your tube may be a problem, not regarding ejection, but in giving you enough room to LOOSELY place the wadding, chute, and shock cord between the engine and the nose cone (also think about the length of the shoulder of the nose cone, which takes up some tube space.) Estes Cosmic Cobra was a Helicopter nose cone rocket with a really short tube, I always had problems getting it packed with wadding, cord, chute AND these huge rotor blades.
A longer tube would make it much easier for you. You could add an extension to your tube, but looking at your design, looks like vast majority of your effort is on the nose cone, may be more worth your time and effort just to build another rocket body with a longer tube.
For minimum diameter rockets, if friction fit isn't working (usually does for me, I wrap a piece of masking tape around engine and fold it back and forth once so small area of triple tape thickness-- usually does the trick), then external tape works. Looking at your model, looks like fins go all the way to the edge, so you can't wrap around easily. Here's my solution.
First, Put small (say 3 cm long) pieces of clear cellophane tape at the tail on of the rocket between the fins (this will protect your paint job when you remove the USEFUL pieces of tape.)
when ready to launch, you can use either clear or masking tape to place over the butt of engine sticking out the bottom and overlappiing the clear plastic tape between the fins (three pieces for three fins, four for four.) Should not go as far forward as the forward edge of the clear tape you placed above. Press new tape down hard, should form fit to the butt of the engine. These should hold the engine in place. The underlying clear tape allows you to easily peel off the other tape without marring your finish when you're done.
Nothing is failure proof, but this has worked well for me.
Looking forward to seeing more of your designs and videos!