A rocket fired from its launching pad not only picks up speed but its acceleration also increases significantly as firing continues Why is this so?

Answer 1

The speed of a rocket is dependent on the thrust of the engine, and the weight of the rocket. As the engine runs at a constant thrust, the rocket accelerates - it speeds up.

But it also gets LIGHTER, because the rocket fuel that it is burning was part of the original mass of the rocket. Let's say that the rocket's "dry" weight is 100,000 pounds. The weight of the fuel alone might be twice that, or 200,000 pounds. When the rocket blasts off, it is a 300,000 pound rocket, with an engine big enough to lift a 300,000 pound weight.

One second later, the rocket might have burned 10,000 pounds of fuel. So the same engine that was designed to lift a 300,000 weight is now lifting only 290,000 pounds; it will accelerate even faster. Nine seconds after that, the 200,000 pound fuel load is half gone, and the whole rocket weighs only 200,000 pounds - and you get even MORE acceleration. (But half of your fuel is gone!)

And this hints at the problem with rockets; if I wanted to add another 100,000 pounds of fuel, it would probably take another 100,000 pounds of rocket to handle it. (A bigger engine, and more fuel tanks!) Now I've got a 500,000 pound rocket, but only 300,000 pounds of fuel. And the more fuel I add, the bigger the engine needs to be just to lift the weight of the fuel - and now I'm getting less and less acceleration, because of the weight of the fuel! As I increase the amount of fuel, pretty soon I'm getting no advantage at all, because ALL of the extra fuel is being used up, just to lift the extra fuel!

Answer 2

As the rocket burns fuel, it decreases its mass, leading to a greater acceleration due to the conservation of momentum. Additionally, the force generated by the burning fuel remains constant, whereas the decreasing mass results in a higher acceleration. This phenomenon, known as the rocket equation, explains why acceleration increases significantly as the rocket fires.