# Thrust in a Water rocket (or any rocket?)

• Swamp Thing
In summary, the conversation revolved around the thrust in a rocket and whether it is solely due to a force acting on a sealed nozzle or if there are other factors at play. Further research showed that the actual force is twice the value predicted by a friend's reasoning and is derived from momentum balance and Bernoulli's theorem. The question then arose if there is a simpler way to explain this, possibly by splitting the force into two terms. The idea of loss of mass was also brought up as a factor in the rocket's momentum.

#### Swamp Thing

I was debating with a friend, who insisted that the thrust in a rocket is merely due to the fact that a force PxA would have acted on a plate that sealed the nozzle. This force would have exactly balanced its counterpart "disk" on the other side of the chamber. After removing the seal, we are left with an unbalanced force PxA.

A bit of internet research - for example, https://www.ohio.edu/mechanical/programming/rocket/nielsen_rocket.pdf
shows that the actual force is twice the value predicted by my friend's naive reasoning. The result is derived from momentum balance and Bernoulli's theorem, but I'm wondering if there is a direct and intuitive way to prove this. I'm looking for something that would be as easy to understand as my friend's logic.

For example, can we split the 2PA into one PA term that corresponds to the "unbalanced disk forces" and another term that can be explained in some simple way?

Swamp Thing said:
For example, can we split the 2PA into one PA term that corresponds to the "unbalanced disk forces" and another term that can be explained in some simple way?
Loss of mass? You increase the total momentum of your rocket and decrease its mass at the same time.

## 1. What is thrust in a water rocket?

Thrust is the force that propels a rocket forward. In a water rocket, this is created by the expulsion of pressurized water and air through a nozzle.

## 2. How is thrust created in a water rocket?

Thrust is created by the principle of action and reaction. When pressurized water and air are expelled through a nozzle, it creates a force in the opposite direction, propelling the rocket forward.

## 3. How does the amount of water affect the thrust in a water rocket?

The amount of water used in a water rocket directly affects the amount of thrust generated. More water means a greater mass to be expelled, resulting in a higher thrust. However, too much water can also cause the rocket to become too heavy and decrease thrust.

## 4. What factors affect the thrust in a water rocket?

Other than the amount of water, factors such as the pressure of the air, the shape and size of the nozzle, and the weight of the rocket itself can also affect the amount of thrust generated. A well-designed and aerodynamic rocket can also increase the efficiency of thrust.

## 5. How is thrust measured and calculated in a water rocket?

Thrust can be measured using a force sensor or by analyzing the acceleration of the rocket. It can be calculated by multiplying the mass of water expelled by the acceleration of the rocket. Other factors such as air pressure and nozzle design may also need to be taken into consideration for a more accurate calculation.