Understanding the Timing of the Re-Entry Burn in Falcon First Stage

The key to successful re-entry for a returning Falcon first stage starts with the moment the burn initiates. Unlike the duration of the burn, which is predetermined, the timing of the burn's start is crucial to ensure the booster's safe landing. A single-second variance in the start time can significantly impact the fuel reserves, making the landing burn impossible and dooming the booster.

The Precise Timing of the Re-Entry Burn

The burn must commence at a precisely calculated moment to ensure that the boosters enter the atmosphere just as the burn concludes. The re-entry burn works by rapidly accelerating the booster while atmospheric effects are still minimal. As the burn progresses and the booster ascends, the effects of hypersonic entry drag are replaced by the rocket exhaust plume, managing the thermal stress on the booster.

The burn begins with the booster experiencing a brief period of rapid acceleration. As the atmosphere exerts less resistance (owing to the low density at the start), the booster continues to gain speed until it reaches a point where its velocity, though still supersonic, is close enough to terminal velocity to begin the deceleration phase. After the burn ends, the vehicle's velocity begins to drop, eventually slowing to below terminal velocity.

The Role of Terminal Velocity During Re-Entry

The transitional period where the booster is decelerating but still beyond terminal velocity is crucial. Terminal velocity, in this phase, is still extremely supersonic. The grid fins play a significant role in slowing the booster further until the landing burn, which finally brings it to a stop.

Observations from telemetry data reveal that the booster's velocity often continues to change even after the burn has ended. In some cases, the vehicle may briefly slow down, indicating that it was slightly above terminal velocity. Conversely, in other instances, the velocity might increase for a short period, suggesting that the booster was below terminal velocity during the burn.

Handling Horizontal Velocities During Re-Entry

When a returning Falcon first stage is trying to land on a drone ship in the ocean, there is often a significant horizontal velocity component at the start of the re-entry burn. This horizontal velocity must be accounted for in the timing and duration of the burn to ensure the booster lands safely and accurately.

Understanding and optimizing the timing of the re-entry burn is essential for the safe and successful deployment of SpaceX's reusable rocket boosters. Each second counts, and the right calculations can make all the difference between a successful landing and a potential failure.

Conclusion

For a returning Falcon first stage, the decision to end the re-entry burn at the right time is critical. The timing of the burn must be precisely calculated to ensure that the booster enters the atmosphere in a controlled manner, allowing the grid fins to slow it down to a safe landing. By mastering this crucial aspect, SpaceX can continue to achieve its goal of reusable rockets and significantly reduce the cost of space missions.