Knowledge Zone · Card 5 of 13
Gravity and drag losses
Not all the Δv you spend becomes speed. Subtract the speed you gained from the Δv you burned and the difference is losses — fuel paid to gravity for the time you spent climbing slowly, and to the air for the time you spent going fast through it.
The reference Scout ascent spends 9.7 km/s to end up moving at 7.9 km/s: about 1.9 km/s of losses. Turn too late and you hold the rocket up on its engine, which is expensive. Turn too early and you drive it sideways through thick air, which is expensive in the other direction. The cheapest ascent is the one that runs out of atmosphere and runs out of vertical at about the same moment.