Diameter difference between Body Tube ID and MMT OD

[jqavins]

Actually, the motor tube would never "see" a motor's maximum thrust, unless the rocket was restrained from moving.

Dave F.

All the thrust the motor creates bears on the MMT. The only load path to the rest of the rocket is through the MMT.

 

[Ez2cDave]

All the thrust the motor creates bears on the MMT. The only load path to the rest of the rocket is through the MMT.

But the MMT would not see the maximum force of the motor, since the rocket is moving. If the rocket could not move, then the MMT would see maximum force.

Dave F.

 

[jqavins]

The rocket only moves because the motor thrust is applied to it, and the thrust is applied to it only through the MMT. At the interface between the thrust ring and the base of the MMT. If the rocket is held still, then the force is transmitted through the MMT, the CRs, and the BT to whatever is clamping it. If the rocket is accelerating then the force is transmitted through the MMT and the CRs to the BT to induce acceleration. If the rocket has reached terminal velocity as is still moving, the force is transmitted through the MMT, the CRs, and the BT to the air. It doesn't matter where the force path ends, it always started with the MMT.

Turn the problem horizontal and consider a jet plane. Does the engine push any harder on its support pylon if the plane is braked than if it is free to accelerate down the runway, or is in mid flight?

 

[lakeroadster]

If the rocket is bench tested, held via the nose cone or the body tube, that will be significantly more force on the centering rings than they will ever see during the flight. Well, unless the rocket is so heavy that the engine cannot lift it off the pad.

 

[Ez2cDave]

The rocket only moves because the motor thrust is applied to it, and the thrust is applied to it only through the MMT. At the interface between the thrust ring and the base of the MMT. If the rocket is held still, then the force is transmitted through the MMT, the CRs, and the BT to whatever is clamping it. If the rocket is accelerating then the force is transmitted through the MMT and the CRs to the BT to induce acceleration. If the rocket has reached terminal velocity as is still moving, the force is transmitted through the MMT, the CRs, and the BT to the air. It doesn't matter where the force path ends, it always started with the MMT.

Turn the problem horizontal and consider a jet plane. Does the engine push any harder on its support pylon if the plane is braked than if it is free to accelerate down the runway, or is in mid flight?

The rocket airframe never "sees" the maximum force of a motor, because it is free to move.

Hypothetical Example :

(1) A 6ft long section of glassed 98mm MMT tubing has fins glued to it ( tip to tip FG ) and it has a nose cone.
(2) A Cesaroni M4770 is installed and it is launched, goes through Mach, reaches terminal velocity, and is recovered undamaged.
(3) Then, the nosecone is removed ( Nose Block installed to replicate the Nose Cone shoulder ) and the rocket is restrained, horizontally ( only permitted to slide forward and back, freely ).
(4) The restraining device is wheeled over to a solid concrete wall and the open end of the airframe is butted up against it.
(5) Another Cesaroni M4770 is installed and the horizontal rocket, unable to move forward or accelerate, is ignited.

Which time does the airframe "see" the maximum force of the motor ?

If the airframe, in normal flight operation, always "sees" the maximum force of the motor, everything should look the same right ?

The appearance of the rocket in the second scenario is easily predicted . . . CRUSHED !

Dave F.

 

[jqavins]

In the first scenario, the rocket will accelerate upward, very very fast. How fast? Force over mass, if we neglect drag for a moment. What force? Why, the motor thrust, of course. All that thrust is used to accelerate the rocket. How does it get there? When exhaust gas exits the nozzle to create thrust, does it envelope the rocket in a force bubble that accelerates it? No. The motor pushes the rocket. All the thrust generated by the motor is pushing on the rocket.

If that M4770 provides enough thrust crush the tube against a wall then you should expect it, on a launch stand, to leave the rocket behind as it blasts through and out the top.

Just draw the free body diagram. I'm done.

 

[Ez2cDave]

In the first scenario, the rocket will accelerate upward, very very fast. How fast? Force over mass, if we neglect drag for a moment. What force? Why, the motor thrust, of course. All that thrust is used to accelerate the rocket. How does it get there? When exhaust gas exits the nozzle to create thrust, does it envelope the rocket in a force bubble that accelerates it? No. The motor pushes the rocket. All the thrust generated by the motor is pushing on the rocket.

If that M4770 provides enough thrust crush the tube against a wall then you should expect it, on a launch stand, to leave the rocket behind as it blasts through and out the top.

Just draw the free body diagram. I'm done.

The very fact that the rocket can freely move forward and accelerate, clearly shows that "maximum force" was not applied ( 100% ) to the airframe, during flight. However where the rocket cannot move forward, then "maximum force" is applied ( 100% ) to the airframe, destroying it.

Dave F.

 

[bjphoenix]

Turn the problem horizontal and consider a jet plane. Does the engine push any harder on its support pylon if the plane is braked than if it is free to accelerate down the runway, or is in mid flight?

Yes it does.
When the system is accelerating some of the thrust of the motor is accelerating the mass of the motor, the rest of the thrust of the motor is used to accelerate the mass of the rocket. That thrust is pushing on the motor mount. Unless the motor has zero mass, less than 100% of the thrust is available to push on the motor mount.
When the system is static all of the thrust of the motor is pushing on the motor mount.

 

[T-Rex]

I like all those ideas but the one that got me the most is probably the last one. Speaking of the last option, would it be a good idea to use a quicklink to attach the heavier cord to the middle of the other?

I tie a loop in the middle of the first cord and the end of the second cord. The 2 can be connected together using a quick link or by looping the 2 loops together (I don't know how to explain it any better) The knot gurus will be here in a few minutes to mention that these are the wrong knots because they decrease the strength of the cord, but I have yet to have a failure.