system
Given the following post, respond as if you're commenting on Reddit:
human
Do you have to take into account atmospheric pressure when calculating the upward acceleration of rockets?

Hi,
If yes, do you have to apply the force of gravity and the force of atmospheric pressure separately?
gpt
Yes! Drag is a major consideration during early stages of a flight. It also must be calculated separately from gravity since drag is dependent on the velocity, which is increasing with thrust, and the air density, which is decreasing with altitude. Drag is also dependent on the direction of air flow with respect to the vehicle (called "angle of attack"), whereas gravity is always pointing in the same direction, another reason it needs to be considered separately.

It actually presents something of a problem because during launch, a spacecraft has two major sources of energy loss, called "gravity losses" and "drag losses", both of which reduce fuel efficiency. To minimize gravity losses, you want to accelerate as fast as possible away from the Earth and minimize the time you spend thrusting against gravity. On the other hand, drag is proportional to the square of velocity, and directly proportional to density of the air, so to minimize the drag losses you would want to keep velocity low at low altitudes and then increase velocity as you get to altitudes where the air is much thinner (the stress induced by atmospheric drag is also a very important consideration). 

So they compromise by varying the throttle during the ascent to best balance the losses to these two forces. For example, the Space Shuttle would launch with the engines wide open, but as it gained altitude and speed, it would begin to throttle back to as little as 60% until it passed a point called "max Q", which is where the drag (and stress on the spacecraft) is greatest--if you listen to the radio transmissions of launches, you can hear them say when the vehicle passes max Q. Once they pass it, they throttle back up because they are now at an altitude where the density is so dropping so fast that increasing the velocity doesn't significantly increase drag.

**EDIT:** Since you asked specifically about pressure, there are issues that have to be taken into consideration to deal with the pressure as well, namely with the rocket's engines. The ambient pressure effects the thrust produced and the efficiency of the engines, which in rocketry is referred to as the "specific impulse." The specific impulse of the Saturn V engine in a vacuum is 304 s, but at sea level, it drops to 263 s.