What is difference between push and pull

[GeorgePauley]

I keep seeing forum entries saying things like "push retrograde, pull prograde". What exactly does this mean? I can see how rockets (or jets) push a vehicle, but darned if I can figure out how to "pull" a vehicle.

[Red Iron Crown]

It's referring to how the marker on the navball reacts to thrusting near that direction. When thrusting near prograde, the prograde marker will be "pulled" toward the direction the ship is pointing. When thrusting near retrograde, the retrograde marker will be "pushed" away from the direction the ship is pointing.

I prefer the similar mnemonic "Pull prograde, repel retrograde".

[Pecan]

There are two meanings - the obvious one not being the one you want ...

i) 'Pull' vehicles, usually space-tugs or similar have the engines attached radially towards the front, with the mass of the payload/structure behind and between them. This means the force is applied ahead of a potentially-wobbly load and it stretches straight. Engines at the back, conversely, encourage wobbles like trying to push a piece of string.

This is a 'pull' tractor:

Tractor Medium Delivering Fuel Module To Mun Station

ii) "Push retrograde, pull prograde" refers to manoeuvres to adjust your direction of movement (prograde), as shown on the navball, towards a target also shown there. I'm afraid I don't have a link to a picture handy but if your prograde is to the left of the target, for instance, you want to burn the other side (to the right) of the target to bring them together - hence you are 'pulling' the prograde market towards your vehicle's heading. On the other hand, when reversing and braking towards a target, if the retrograde (backwards) marker is to the left of the target you want to point your heading even more towards the left and burn there to 'push' the retrograde marker towards the target.

[mckamx]

There are two meanings - the obvious one not being the one you want ...

i) 'Pull' vehicles, usually space-tugs or similar have the engines attached radially towards the front, with the mass of the payload/structure behind and between them. This means the force is applied ahead of a potentially-wobbly load and it stretches straight. Engines at the back, conversely, encourage wobbles like trying to push a piece of string.

See https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy for problems with the "pull is more stable" hypothesis.

[Red Iron Crown]

See https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy for problems with the "pull is more stable" hypothesis.

I don't think it's really a fallacy in KSP because joints are so flexible. Payload behind the propulsion tends to straighten out behind it, payload in front of the propulsion tends to bend further and further. Compare the stability of a tractor trailer moving forward compared to one moving in reverse.

[Sharpy]

I bet the best way to understand the "push retrograde/pull prograde" is to launch the Docking Tutorial in KSP (as it gives you a ship with full RCS), making a rendezvous with your target, then turning it roughly towards prograde or retrograde marker (roughly, not precisely!) and then playing with RCS controls. Press R to activate RCS, then use the keys H, N, J, K, L, I to control various thrusters. In particular observe how the prograde and retrograde markers behave: when you accelerate, the prograde marker moves towards the reticle (is getting pulled by it). The opposite happens to the retrograde marker - it's pushed away. And IJKL just move it up, down, left, right.

This is very useful for docking, as if you manage to overlap the prograde marker with the target marker, you're on the perfect course to dock.

[Pecan]

See https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy for problems with the "pull is more stable" hypothesis.

As RIC said, the pendulum fallacy is not about the same at all. Pull is more stable in KSP because if you pull a piece of string it straightens out rather than collapsing. The pendulum fallacy, on the other hand, refers to directional stability; the fallacy being that the 'train' of parts behind the engine will help it point in the right direction.

[GeorgePauley]

Interesting. The example using the RCS makes sense. But I never really think about pushing or pulling the have marker. I just think about rotating my ship along one of the axis. Is there some sort of benefit to using the push-pull metaphor?

[Pecan]

Like any mnemonic, it's only useful to the extent it works for you.

Once you've practiced docking a few times the chances are you won't need to think it through at all, it becomes pretty instinctive quite quickly - however hard it is to learn in the first place ^^.