Sather,
May I back-up one post? There you speak of force transfer. If I understand your argument two forces transfer thrust to the airframe:
1. Circumferential joint of thrustplate to airframe.
2. Thrustplate pushing against fintabs pushing against airframe.
What of the force transferred from motor tube to fintab to airframe. That is without thrustplate to fintab contact. Is that substantially equivalent to 2 above?
Feckless Counsel
In full disclosure, I'm not an engineer, so I try to blog in general and may not always use the correct terms. So, I hope this factual and accurate. Thrust is the force I am trying to distribute, (although another compressive force on the aft ring would be from ground contact in recovery, hopefully under parachute). Thrust pushes with great vigor against the thrust plate (aft ring), and from there to the rest of the rocket through individual-rocket-specific structures and joints. My goal is to spread this sometimes intense point force (compression) from the thrust plate to the mass of the rocket through physical structure, relieving the load on individual epoxy joints. Also, by relieving stress, it reduces cycle fatigue and the chance of subsequent failure of the joint.
So, if I understand your question correctly, you inquire about the force being distributed through the motor tube to the fin tangs, and then to the rest of the rocket, independent of thrust plate to fin tang contact. Yup, that certainly is another path the force can follow. I do have a few rockets (4" and 5.5" Polecat Jayhawks come to mind) that use that method exclusively, since a relatively flimsy fiberglass tailcone separates the aft ring (thrust plate) from the aft body tube ring which touches the fin tangs. This works well, but also keep in mind the significant length of the fin tang to motor tube joint helps here, too. On minimum diameter rockets,
all of the force of thrust is carried up the motor/body tube, with the surface-mounted fins hanging on for dear life.
In the typical TTW high power rocket, thrust force (compression) is passed from the aft ring (thrust plate) directly to the tang for TTW fins, then on to the body, etc.
In addition to that, thrust is passed from the motor tube to the body tube through fillets at each centering ring. IMHO, the more paths we can distribute / spread out the forces along, the better. Glassing, fillets, tangs, the appropriate use of all-thread (for tensive force distribution), are all attempts to spread forces over larger areas, minimizing stress and failure at a point. Epoxy joints fail, so I prefer not to rely on them exclusively.
Especially at the extremes... i.e. really heavy fins. For example, from my
10" Jayhawk thread,
..... install ribs along the aft section of motor tube, between the aft centering ring (#1) and the tailcone ring (#0). These ribs brace the tailcone ring against the aft centering ring, relieving the tailcone ring-to-motor tube joint, and the relatively fragile tailcone itself, of compression loads during boost. The goal is to transfer a majority of the force of thrust (at the baseplate of the Aero Pack motor retention body) to the wing and body tubes (where all the weight is) via the ring #1-to-wing tang joint (pushing), with a minority carried by the motor tube-to-wing tang joint (pulling).
In my recycling container
JayCoke Zero, I used a stuffer tube as an internal core structure. I had a design flaw that was pointed out to me early in the build, (when still possible to fix). This rocket uses high thrust motors (L3150, M3400), and the failure of a few epoxy joints could allow the core to break free under thrust. To eliminate that failure mode, I physically tied the heavy wings into the core tube via physical structure, not relying entirely on epoxy to carry the load.
Small revision taking place here before cutting the wings. In the original design, the main wings go "thru the wall" to the core (stuffer) tube, and sandwiched between the large centering rings, but only epoxy and glass tape would hold the wings, rings, and bottle to the core tube itself. It was pointed out to me (thanks, Tim) that there is a potential for the core tube to break free under thrust (particularly if using Vmax propellant), leaving the bottle and wings on the pad. This, of course, would be bad. So, I am re-designing the wing tang to continue "thru the wall" into the stuffer tube and up to the motor tube. This will put part of the tang between core rings, so that the weight of the wings will be carried by the thrust plate (core ring #1) to the wing tang directly, not relying on epoxy to carry the load. It should also help on landing, as the swept winglets of the Jayhawk are most likely to touch down first (assuming nominal parachute deployment), so the weight / stress of the remainder of the vehicle will be carried on the tangs and not in the epoxy joint of the wing root.
