Quick Debunk: What do rockets 'push off of'?

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What do rockets 'push off of''?

You gotta be kidding me right?  We learned this stuff in 10th grade.

The short answer is 'the same thing as everything else, mass'.

Flat Earthers will often cite an airplane as "pushing off the air" but this is simply not true.  They do PUSH THE AIR itself, but not OFF of it.  When the airplane propeller blades hit air molecules two forces are produced, one pushes the air backwards and the other pushes the airplane forward -- these are EQUAL and opposite forces.  It is the fact that the airplane has imparted a Force unto the air molecules in one direction that it is PUSHED in the other by this very action.

Neither do submarines "push off" the water, their propellers hit the water and there are two resulting forces -- one pushes the water back and the other pushes the submarine forward.

Let's talk about Newton's 3rd Law -- ALL PROPER FORCES occur in equal and opposite pairs.

And here is a simple experiment you can do that will demonstrate that it is NOT "pushing off the air" that is important but rather the velocity imparted to some other mass.

Sit in a rolling chair on a smooth, level surface.  Now wave your arms around wildly.  That is your control experiment.  The amount you move as you do this is your baseline.

Now throw a small but dense object (let's say a one foot wide, 50 lbs exercise ball) you will be pushed backwards MORE THAN if you throw a less massive but greater surface area object (say a 2' square board) -- this demonstrates that it is NOT the air that pushed you back.  In fact, try fanning the 2' square… did you even move backwards hardly at all compared to throwing the heavy ball?

If you cannot understand this as a thought experiment then ACTUALLY TRY IT.

If you study this relationship carefully you can find the exact amount which air resistance contributes (close to zero) and what is the contribution from the mass itself. And you'll find that F=ma (Force = Mass times Acceleration) is that contribution from the acceleration of the mass.

If we were pushing OFF OF the air only then we could push harder with a larger surface area but that very clearly is NOT what we observe happens.  The force with which we are pushed backwards has to do with the force we impart to the mass.  If you move the ball back and forth without letting go you'll move a lot more than you did waving just your arms but if you do the same with the 2' board you'll hardly move at all more than just waving your arms around caused -- and the amount you do move will be proportional to that extra mass plus a tiny bit from the surface area.

Indeed, you can do this in a vacuum and observe the equal and opposite reactions from ANY proper force.  A simple construct would be a rubber band device on wheels shooting some iron shot.  You should also notice a velocity difference - greater mass requires a greater force to go the same velocity. So the iron shot might go a little ways but the rubber band thingy might 'shoot off' like a rocket.

Ah ha! A CLUE!

In a rocket you have a fuel (like kerosene) *AND* an oxidizer (like liquid oxygen in a tank). These are combined and ignited in the rocket engine which releases enormous amounts of heat and produces very high kinetic energy in the exhaust gases. These exhausted gases expand and bump into the rocket nozzle which is cleverly designed to capture as much of that energy as possible by deflecting that gas OUT of the nozzle. The gases go out the back and, with equal and opposite force, the rocket is pushed the other direction. Side-to-side forces cancel out and reduce efficiency.

The F-1 engines Apollo used burned 1,738 pounds of RP-1 (refined kerosene) and 3,945 pounds of oxygen per second creating 1,500,000 lbf (6.7 MN) and 5 of these engines burned at once. With the exhaust gases at 5000 mph. The engine itself is 19 feet tall, these things are huge. The chamber pressure was 70 bar or 1015 pounds per square inch -- and all that was expelled out of the back end, pushing the rocket upwards.

The gas has a much smaller mass but rocket exhaust gases reach supersonic speeds. And with equal but opposite force it pushes against the engine nozzle as it is ejected out.

PS. Please NOTE that putting an object in front of a vacuum cleaner, which is creating a massive airflow, is NOT the same as testing IN a complete vacuum and thus invalidates the test.

PPS. YES, rockets CAN burn in a vacuum because they have their OWN oxidizer.