# Are there two action-reaction pairs operating in a rocket

I'm confused about the action-reaction pair in a rocket. is it

1 The action of the explosion of gases in the combustion chamber on the rocket, and the reaction of the rocket on the gases, or

2 The acceleration of the exhaust by the rocket nozzle(action) and the acceleration of the rocket by the exhaust(reaction).

Which of the action-reaction pairs is it,the 1st or 2nd, or is it both? If it's both then does that mean that there are two action-reaction pairs in rocket propulsion?

• It's basically 2. The force exerted by the rocket on the exhaust gases is equal in magnitude and opposite in direction to the force exerted by the exhaust gases on the rocket. Dec 5, 2018 at 16:58

Which of the action-reaction pairs is it,the 1st or 2nd, or is it both? If it's both then does that mean that there are two action-reaction pairs in rocket propulsion?

Both.

To see the first, consider an inflated balloon. Within it, there are a large number of molecules bumping around, into each other, and into the sides of the balloon. Every time they hit the side they impart momentum, but within some microscopically short time, so does another molecule on the other side going the other direction. So macroscopically, there's nothing except for a net outward force.

Now poke a hole in the balloon. Suddenly the molecules going "that way" no longer deposit their momentum on the balloon, but their opposite partners on the other side of the balloon still do. So now you suddenly have a net force opposite the hole. And because the molecules randomize on the order of nanoseconds, this process continues until it deflates.

The second side is where rockets are not like balloons. More specifically, the combustion chamber of a rocket engine is similar to the balloon, it's roughly spherical-ish and has a hole in one end. However, rockets also have a nozzle, or engine bell, on the other side of the opening:

The bell has the duty of directing the escaping molecules toward the rear. Recall that the gasses randomize their motion on the order of nanometers. So as soon as the molecules going out the hole get past it they're starting to randomize their motion again. So think about one molecule that gets bumped so it starts travelling away at 45 degrees. At some point it is going to hit the bell, so what you want to do is shape the bell so that when a molecule going 45 degrees hits it, the side of the bell is 45 degrees, so it reflects off and is now going straight rearward. That way you get the maximum amount of momentum forward. Now if you consider the angles that do that perfect reflection for any given angle of molecule leaving the combustion chamber, you get a bell.

Ideally you want to produce a flow of gas that is entirely to the rear. However, those molecules are also bumping in the air around it, so some of that energy is being lost to the atmosphere. This is why engines are invariably more efficient in space. Additionally, if you consider the effect of the air on the flow, it tends to randomize it, meaning there will be more sideways component. This means the angles of the bells will have to change to account for this, which is why the engine bell on a rocket designed for low altitude is wider than the ones for high altitude. There are some designs that change their geometry to get the most at any altitude, but they have not been widely used operationally.

Simplify your problem, imagine you are floating in space and you have a squirt gun with water. When the water molecules get squirted out other water molecules are pushing on them to get them out, those water molecules are pushing against the plastic in the gun which eventually pushes you.