Well, yes. The link asserts that thrust is developed against a solid object. But in space there is nothing to thrust against. So, any thrust at all must be inherent upon point of launch? A space craft's velocity is dependent upon the energy developed at launch - only? Notwithstanding slingshot effects of the sun and planets?
A rocket engine burns fuel and oxidizer and blasts it to the rear. By Newton's second law and 3rd laws, the action of blowing the exhaust out produces an equal force reaction in the opposite direction: forward. A rocket actually works better in space because there is no friction from the air to resist the rocket motion.
If you want to experience action-reaction first hand, stand in a 100 pound john boat floating loose in the lake and try to broad jump 5 feet to the dock. The boat, being lighter than you, will go flying backwards in reaction to your action of trying to jump forward.
Last edited:
I have seen this before...lmao
It is the second paragraph that seems to contradict the 1st one. The boat will go flying backwards, and the jumper will not go forward. The jumper is thrusting against something with very little resistance, so he can't launch forward, and falls into the water.
I hate to appear daft. I am of course, embarrassed.
If there is only vacuum to push against, how can an object move? Are you saying that the spent propellant creates an object to thrust against?
Newtons third law.
I found this, you figure out what to do with it...lol
Rocket thrust is given by the equation
F=m˙vexit+Ae(P1−P2)
where m˙ is the mass flow rate, vexit is the average exit flow velocity across the exit plane, Ae is the cross-sectional area of the exhaust jet at the exit plane, P1 is the static pressure inside the engine just before the exit plane, and P2 is the ambient static pressure (i.e. atmospheric pressure).
Provided that the nozzle is not overexpanded and flow separation does not occur, Ae remains constant, and the thrust difference is realized primarily from the change in P2. If nozzle is overexpanded to the point that flow separation occurs, however, the exhaust jet area drops as well, causing further losses.
I found this, you figure out what to do with it...lol
Rocket thrust is given by the equation
F=m˙vexit+Ae(P1−P2)
where m˙ is the mass flow rate, vexit is the average exit flow velocity across the exit plane, Ae is the cross-sectional area of the exhaust jet at the exit plane, P1 is the static pressure inside the engine just before the exit plane, and P2 is the ambient static pressure (i.e. atmospheric pressure).
Provided that the nozzle is not overexpanded and flow separation does not occur, Ae remains constant, and the thrust difference is realized primarily from the change in P2. If nozzle is overexpanded to the point that flow separation occurs, however, the exhaust jet area drops as well, causing further losses.
Close, but not quite. The exhaust IS the thrust, and the action/reaction force is at the interface of the two masses. Mass one is the exhaust gas and mass two is the rocket motor and whatever is bolted to it. Mass one times its acceleration equals mass two times its acceleration. The action/reaction forces are equal and opposite.
In the case of you and the light boat, boat mass times boat acceleration equals your mass times your acceleration. The boat being lighter leaves faster, and you only get enough speed to go partway to the dock.
If you stand on a concrete pad and jump, the movement of the pad is negligible (mass of the whole earth) and you get virtually all the acceleration. But the forces at the interface are equal and opposite, per 3rd law.
.Second Law
As acceleration increases, the force increases. The second law says that the acceleration of an object produced by a net (total) applied force is directly related to the magnitude of the force, the same direction as the force, and inversely related to the mass of the object (inverse is a value that is one over another number... the inverse of 2 is 1/2). The second law shows that if you exert the same force on two objects of different mass, you will get different accelerations (changes in motion). The effect (acceleration) on the smaller mass will be greater (more noticeable). The effect of a 10 newton force on a baseball would be much greater than that same force acting on a truck. The difference in effect (acceleration) is entirely due to the difference in their masses.
Third Law
The third law says that for every action (force) there is an equal and opposite reaction (force). Forces are found in pairs. Think about the time you sit in a chair. Your body exerts a force downward and that chair needs to exert an equal force upward or the chair will collapse. It's an issue of symmetry. Acting forces encounter other forces in the opposite direction. There's also the example of shooting a cannonball. When the cannonball is fired through the air (by the explosion), the cannon is pushed backward. The force pushing the ball out was equal to the force pushing the cannon back, but the effect on the cannon is less noticeable because it has a much larger mass. That example is similar to the kick when a gun fires a bullet forward.
The spent propellant is the object. It has mass, and it is accelerated out of the engine by the combustion; Newton's Third tells us that there has to be an equal force in the opposite direction. See https://en.wikipedia.org/wiki/Rocket#Physics for a better explanation.
I appreciate it, Rod Snell and Dave70968. Truly, for many years I have wondered about this. Honestly, if ya'll had not explained it in this way, I would not have seen the thrust in this light.
So, this leads to another question I have pondered: would not the moon be a good place to launch from? I mean, with the weak gravity, escape velocity could be increased significantly. This trip to Mars that will take, what, around a year? Well, if launched from the moon, might it not take a matter of days?
So, this leads to another question I have pondered: would not the moon be a good place to launch from? I mean, with the weak gravity, escape velocity could be increased significantly. This trip to Mars that will take, what, around a year? Well, if launched from the moon, might it not take a matter of days?
So, would a lunar launch pad be efficacious for a quad set of RS-25s?
