Bad Design
Sorry but anchoring a tube at on end and hoping it'll be stiff enough is just asking for trouble.
Surely a better design would have been to use a tube with a slot in it. The tube could have been clamped to the truss along its entire length. 2 inserts at the nose and tail of the aircraft would ensure that there would be no twisting upon launch, and as long as the inserts we made of a suitable material and the slotted tube well greased there wouldn't be much friction.
And wouldn't a 'blast' plate fixed to the truss behind the rocket motor give it a bit more of a kick on ignition. Yes newton's 3rd but I'm assuming that the mass of the truss, balloon etc is a lot more than the aircraft.