Final answer:
The starter motor on a multi-spool turbojet engine typically turns the N2 spool, enabling engine ignition. Jet turbines are designed to break apart in a way that conserves angular momentum, preventing transfer to the aircraft wing and avoiding potential damage.
Step-by-step explanation:
The starter motor on a multi-spool turbojet engine typically turns the N2 (secondary) spool, which is connected to the high-pressure compressor and the high-pressure turbine. The reason for this is that the N2 spool has less mass and inertia compared to the N1 (primary) spool, allowing the starter motor to accelerate it more easily to the necessary speed to commence the engine ignition process.
Concerning the conservation of angular momentum, if a jet turbine were to seize suddenly, it is designed to disintegrate in a controlled manner. The fragments fly outwards, following a perpendicular trajectory to the axis of rotation. Hence, the angular momentum of the spinning turbine is conserved in the radial velocity of the parts as they move outward. This avoids any significant transfer of angular momentum to the rest of the aircraft structure, like the wing, which could potentially result in structural damage. The concept aligns with the laws of conservation of momentum, a principle in physics that states that in a closed system, the total momentum remains constant if no external forces act upon it.