# Rocket Engine Thrust Formula Help

• mleesk
In summary, the conversation discusses the derivation of the ROCKET ENGINE THRUST formula, which includes two terms: the sum of momentum change (mass flow) multiplied by exit velocity, and the product of pressure difference and exit area. The variables used are mass flow rate, exit velocity, ambient pressure, exit plane pressure, and exit area. The conversation also mentions the use of F=ma and momentum flow rate equations in the derivation process and emphasizes the importance of checking units.
mleesk
hello,

does anyone know how to derive the ROCKET ENGINE THRUST formula:

T = P/R AV + (P-B)A

i know you need to use F=ma and Momentum flow rate equations to get the first place. but what do you do after that?

Any help would be greatly appreciated.

Thanks,
Michael

Explain your variables. I see you are accounting for pressure difference across the nozzle in the second term, but I want to make sure of the variables in the first term.

The thrust is the sum of the momentum change (mass flow) * (exit velocity) if the nozzle is over or under expanded than you will get extra thrust from the exit area times the pressure difference (Exit Plane Pressure - Ambient Pressure) * (Exit Area)

$$\dot{m} V+({V}_{exit} -{V}_{amb}){A}_{exit}$$

The mass flow is density times velocity times area, by perfect gas law

$$\rho = \frac{P}{RT}$$

$$\frac{P}{RT}V^{2}+({V}_{exit} -{V}_{amb}){A}_{exit}$$

so it looks like there is a T missing and the velocity should be squared. CHECK YOUR UNITS!

Last edited:

## 1. What is the formula for calculating rocket engine thrust?

The formula for calculating rocket engine thrust is: F = m * a, where F is the thrust force, m is the mass flow rate of the propellant, and a is the exhaust velocity.

## 2. How is the thrust force of a rocket engine measured?

The thrust force of a rocket engine is typically measured using a thrust stand, which is a device that supports the rocket and measures the force it produces. The force is usually measured in pounds or newtons.

## 3. What factors affect the thrust force of a rocket engine?

The thrust force of a rocket engine is affected by various factors such as the mass flow rate of the propellant, the exhaust velocity, the nozzle design, and the ambient pressure. The type of propellant and the design of the engine also play a significant role in determining the thrust force.

## 4. How does the exhaust velocity affect the thrust force of a rocket engine?

The exhaust velocity of a rocket engine is directly proportional to the thrust force. This means that a higher exhaust velocity will result in a higher thrust force. This is because the higher the exhaust velocity, the faster the propellant is being ejected from the engine, resulting in a greater force pushing the rocket forward.

## 5. Can the thrust force of a rocket engine be increased?

Yes, the thrust force of a rocket engine can be increased by increasing the mass flow rate of the propellant or by increasing the exhaust velocity. These can be achieved by using more powerful propellants or by optimizing the design of the engine and nozzle. However, there are limitations to how much the thrust force can be increased due to factors such as the size and weight constraints of the rocket.

• Aerospace Engineering
Replies
2
Views
2K
• Aerospace Engineering
Replies
2
Views
2K
• Aerospace Engineering
Replies
2
Views
2K
• Aerospace Engineering
Replies
5
Views
2K
• Aerospace Engineering
Replies
2
Views
1K
• Aerospace Engineering
Replies
24
Views
2K
• Aerospace Engineering
Replies
2
Views
2K
• Introductory Physics Homework Help
Replies
10
Views
650
• Aerospace Engineering
Replies
16
Views
2K
• Aerospace Engineering
Replies
7
Views
2K