Just 2 Questions on Drag

[Spaced Out]

If you had a camera  pointing directly down the side of a rocket, and the engines were gimballed enough so when performing the initial pitch maneuver or flight adjustments the camera could see the edge of the engine, wouldn't that mean by gimballing it it creates slight drag and puts it very very very slightly off course, or would it be indetectable by the flight and mission control computers? Also this is theoretical because I don't think most engines have that enough gimbal capability to do that.

Also, while any in atmosphere adjustment is being done, wouldn't the change in the direction and new way the air is hitting as it adjusts push the rocket very very slightly off course, or am I totally wrong and it doesn't work like that? Or would it be undetectable by the flight and mission control computers?

[Nibb31]

Usually, the engines are placed so that gimballing keeps them out of the airflow.

[Green Baron]

The rocket has a guidance system and any aerodynamic effects is of course computed and deviations applied. Easy reading.

In marine navigation a course correction consists of two maneuvers: to-course and on-course. The first is a little more than necessary to get back on course, the second rectifies the overcorrection to get back in line with the course. Very easy ... should even be told to certain warship watches to avoid crashes with merchant ships. *sticktongue*

[K^2]

5 minutes ago, Green Baron said:

In marine navigation a course correction consists of two maneuvers: to-course and on-course. The first is a little more than necessary to get back on course, the second rectifies the overcorrection to get back in line with the course. Very easy ... should even be told to certain warship watches to avoid crashes with merchant ships. *sticktongue*

One of the simplest schemes for course corrections in real control loops is known as PID controller. The 'P' and 'D' portions of the acronym are effectively the to-course and on-course corrections respectively. The difference is that they are computed continuously, so that you never switch from one to the other.

The 'I' is the integral portion which corrects for any continuous action trying to move you off course. Such as, if there is a constant current trying to carry you off course. It's a bit like having an experience helmsman who doesn't take the ship fully on-course to counter that current and not require another to-course correction later.

So while computers let us do all of this with more precision and more frequent updates, the basic ideas haven't changed much.

[IncongruousGoat]

Well, (historically, at least) spacecraft guidance didn't bother trying to adjust for the effects of the atmosphere. Guidance programs used a preprogrammed pitch v. time table up until they'd mostly left the atmosphere, only then switching to some closed-loop guidance program and correcting for any deviations that may have occurred during ascent. Trying to actually account for aerodynamics in real time is really, really hard when you've got restricted computing capabilities, and most of the time it just isn't worth it.

[Pand5461]

@Spaced Out short answer is: of course there are some deviations but we know how to deal with them.

Long answer: modern launchers have onboard guidance systems that detect the perturbations and recalculate the attitude program needed to get to desired orbit on the go (so-called closed-loop control). Switch to closed-loop control usually happens when rocket leaves the thick part of atmosphere, as @IncongruousGoat already mentioned. One reason for that is that onboard computers have very limited capabilities to properly optimize atmospheric flight. Another is that large perturbations of reference trajectory early in flight will likely result in mission failure anyways, and small ones can be easily adjusted for later. So it's just much more reasonable to calculate the reference atmospheric trajectory and follow that as close as possible.

In the early space era, I believe, a number of trajectories were precomputed for missions where precision was crucial (like shooting the Moon), accounting for possible deviations. Rocket was then guided from ground stations based on which trajectory it's currently on. Sometimes the precision was just not that important, so just a simple precomputed pitch-time program for the whole ascent was enough.