Throw-weight is normally calculated using an optimal ballistic trajectory from one point on the surface of the Earth to another. An optimal trajectory maximizes the total payload (throw-weight) using the available impulse of the missile. By reducing the payload weight, different trajectories can be selected which either extends the nominal range, or decreases the total time in flight. A depressed trajectory is a non-optimal, lower and flatter trajectory which takes less time between launch and impact, but with a necessarily lower throw-weight. The primary reasons to choose a depressed trajectory are either to evade anti-ballistic missile systems, by reducing the time available to shoot down the attacking vehicle (especially during the vulnerable burn-phase against space-based ABM systems), or in a nuclear first-strike scenario. An alternate, non-military, purpose for a depressed trajectory is in conjunction with the space plane concept with use of air-breathing engines, which requires the ballistic missile to remain sufficiently low inside the atmosphere for air-breathing engines to function.