• drdede
In summary, the discussion centers around the behavior of a rocket with a perfect dynamic fulcrum and center of balance, assuming it never rotates in flight and has continuous and stable thrust. It is determined that the rocket's acceleration would depend on its thrust to weight ratio, and if the thrust were greater than the square root of two times its weight, it would accelerate upwards at a shallower angle than 45 degrees. It is also noted that as the mass of the rocket decreases, the component of resultant force caused by gravity also decreases, causing the rocket's trajectory to curve upwards. The misconceptions of the rocket oscillating or traveling in a straight line are corrected, and it is acknowledged that the solution may vary depending on assumptions made such as

#### drdede

I have a question about rockets. Consider a rocket, with a perfect dynamic fulcrum and center of balance (lets say its a magic rocket ship maybe.) Anyway, this rocket never rotates in flight, and has continuous and stable thrust through the flight. Would it continually bounce up and down or continually fly upwards?

http://xs139.xs.to/xs139/09206/rocket472.jpg [Broken]

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You lost me...

Basically if you shot a rocket at a 45 degree angle would it fly up then fall down due to gravity (accelerating downwards even though there is no change in engine thrust)

If the direction it was pointing was constant (after launch, it remained pointed at 45 degrees, regardless of flight path angle), its acceleration would depend on its thrust to weight ratio. If the thrust were greater than the square root of two times its weight (~1.414 times its weight), it would accelerate upwards along a line, at a shallower angle than 45 degrees. If its thrust were less than that, it would be unable to get off the ground.

cjl said:
If the direction it was pointing was constant (after launch, it remained pointed at 45 degrees, regardless of flight path angle), its acceleration would depend on its thrust to weight ratio. If the thrust were greater than the square root of two times its weight (~1.414 times its weight), it would accelerate upwards along a line, at a shallower angle than 45 degrees. If its thrust were less than that, it would be unable to get off the ground.

Cool, so it wouldn't fall down. I thought that was the case but unfortunately that is the harder one to simulate.

It isn't that hard: you have two force vectors, just add them together into one. That's the direction the rocket will accelerate in.

It would fall down when the thrust ran out. It would fall in an arc...

drdede: I think there's some misconceptions happening here, but I can't pin them down. Can you elaborate on your scenario and how you see it playing out from beginning to end?

If the thrust is constant, and it is carrying the fuel it is burning, then the thrust to mass ratio is constantly improving, and the acceleration is constantly increasing.

Bob S said:
If the thrust is constant, and it is carrying the fuel it is burning, then the thrust to mass ratio is constantly improving, and the acceleration is constantly increasing.

Yes! Moreover, since the mass is decreasing, the component of the resultant force on the rocket caused by gravity is decreasing. So, even though the angle of the rocket is fixed, the rocket's trajectory will curve upwards.

So neither picture is correct. The rocket won't oscillate (why did you think it would?), nor will it travel in a straight line. Of course, there are still a bunch of unvoiced assumptions here. Is the curvature of the Earth significant, for example.

That is true. A lot of the solution is dependent on which assumptions you make. In my post for example, I was assuming effectively constant mass, which is clearly not the case for most rockets.

## 1. How do rockets work?

Rockets work by using a propulsion system, typically a liquid or solid fuel, to create thrust and propel the rocket into the air. The thrust is generated by the Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. The burning of fuel releases hot gases, which are then expelled out of the bottom of the rocket, creating an opposite and equal force that propels the rocket upwards.

## 2. What is the purpose of rockets?

Rockets have various purposes, including satellite launch, space exploration, and military defense. They are also used in commercial industries for telecommunications, weather forecasting, and navigation systems. Rockets have also been used for scientific research and experiments, such as studying the Earth's atmosphere and studying other planets.

## 3. How do rockets overcome Earth's gravity?

Rockets overcome Earth's gravity by achieving enough thrust to reach escape velocity, which is the minimum speed needed to break free from Earth's gravitational pull. This is usually achieved by the rocket's propulsion system, which continuously expels hot gases and creates enough force to overcome the pull of gravity. Once the rocket reaches escape velocity, it can travel into space without being pulled back to Earth.

## 4. What is the difference between a rocket and a missile?

A rocket is a vehicle that is designed to be launched into space and is used for various purposes, such as satellite launch and space exploration. A missile, on the other hand, is designed to be launched and guided towards a specific target, typically for military purposes. Missiles also tend to have more complex guidance systems compared to rockets, which are primarily focused on achieving propulsion and reaching a desired altitude.

## 5. How are rockets tested before launch?

Rockets undergo various tests before launch to ensure their safety and success. These tests include static firing, where the rocket's engines are ignited while the rocket is held in place, and simulated flights, where the rocket is put through similar conditions to what it will experience during launch. Engineers also use computer simulations and models to test various aspects of the rocket, such as aerodynamics and structural integrity, before conducting physical tests.