Rocket propulsion

Many people think that rockets work by the propellant pushing against the air or the launch pad. If this were true, then how would rockets still work in outer space? There is nothing in the vacuum of outer space to push against! Space travel is made possible by the physics of momentum conservation. Read the text aloud Show The need for propulsion
Rocketry and momentum conservationRockets are propelled “forward” when exhaust gases stream out “backward.” Since those gases have mass, they have momentum—momentum that they lacked until they were blasted out of the exhaust nozzle. So the rocket has a forward (or positive) momentum while the exhaust has a backward (or negative) momentum. Since the rocket and exhaust form a closed system, their total momentum must be conserved. Read the text aloud
If the change in momentum is zero, then the total impulses must be zero because impulse is the change in momentum. The exhaust gases were given a negative (or backward) impulse, while the rocket itself received a positive impulse because of the law of conservation of momentum. And that means you can push a rocket forward as long as you have something (such as exhaust gases) to toss in the opposite direction—no launch pad, atmosphere, or other obstacle is required. Read the text aloud Show A note on sign conventions
Rocket acceleration increases as fuel is spent
A typical, modern rocket experiences an increasing, not constant, acceleration. Why? In a rocket, the force of thrust (which is the product of the exhaust velocity and the mass-loss rate, measured in newtons) is more or less constant until the fuel is nearly used up. The mass of the rocket decreases over time as the fuel is used up and expelled out the nozzle. Now combine these two ideas: A constant force divided by a decreasing mass implies an increasing acceleration from Newton’s second law, a = F/m. That is exactly what we see in the first few minutes of a rocket launch. Not only does the rocket gain speed, it does so at an increasing rate. Read the text aloud Show Two-stage systems
A modern cargo-carrying rocket might have a mass of 100,000 kg when empty and 500,000 kg on the launch pad, full of the fuel it will use for the first stage of its journey. Exhaust typically exits such rockets at many thousands of meters per second, with hundreds or thousands of kilograms flowing out each second. Read the text aloud
Free-body diagram for a rocket experiencing upward thrust and downward gravity (weight)Suppose that a single-stage rocket can provide a thrust of 107 N. When empty, it has a mass of 100,000 kg; it can carry 400,000 kg of fuel. What will its acceleration be when its fuel tank is half-empty? (Assume that air resistance can be neglected and that the acceleration due to gravity can be approximated as g = 10 m/s2.) State your answer in both meters per second squared and in “g’s.”
Note: You may wish to construct a free-body diagram similar to that shown here. Be careful: Observe that this diagram shows the mass, weight, and net force for a full fuel tank. Yours should reflect the mass, weight, and net force that correspond to half as much fuel. Show

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