The heat energy required for burning of the rocket casing in flight is obtained at the expense of the rocket itself.

Explanation:
When a rocket is launched into space, it experiences a significant amount of friction as it moves through the Earth's atmosphere. This friction generates heat, which can be destructive to the rocket's casing if not managed properly.

Heat generated by friction:
1. As the rocket moves through the atmosphere, air molecules collide with its surface, causing friction.
2. This friction converts the kinetic energy of the rocket into thermal energy, resulting in an increase in temperature.
3. The temperature continues to rise, causing the rocket casing to heat up.

Burning of the rocket casing:
1. The burning of the rocket casing occurs due to the high temperatures generated by the friction.
2. The heat energy causes the rocket casing to undergo combustion, resulting in the burning and eventual disintegration of the casing material.
3. The burning of the casing is necessary to prevent it from overheating and potentially causing damage to the rocket's internal components.

Expense of heat energy:
The heat energy required for burning is obtained at the expense of the rocket itself.
1. The rocket casing is designed to withstand the high temperatures generated during its flight.
2. The material used in the casing is chosen to have a high heat resistance, allowing it to endure the heat for a certain period.
3. However, as the rocket continues to experience friction and the temperature rises, the casing reaches its limit and starts to burn.
4. This burning process consumes the heat energy, causing the rocket's casing to gradually disintegrate.

Conclusion:
In conclusion, the heat energy required for burning the rocket casing in flight is obtained at the expense of the rocket itself. The friction generated during the rocket's movement through the atmosphere causes the casing to heat up, leading to its combustion and disintegration. It is crucial for the rocket's design and materials to account for this heat generation and manage it effectively to ensure the safety and success of the mission.