Forces on a rocket

Problems:
1. For a model rocket, how is thrust different from lift?

In a model Rocket Thrust is the force that opposes Weight, lift, unlike airplanes, is the force used to stabilize the flight.

2. The aerodynamic force of lift is generated by which parts of the model rocket?

The aerodynamic force of lift is generated by the wings or stabilizers on the rocket.

3. Drag is aerodynamic friction with one source being the interaction between the moving rocket’s solid surface and what substance?

The air.

4. As the speed of a rocket increases what forces also increase?

As speed increases, the drag and thrust also increase.

5. In order for a rocket to continuously accelerate, what forces must remain unbalanced? Describe the relationship between these forces.

In order to continuously accelerate, the forces of drag and thrust must remain unbalanced. Because thrust is the force that produces acceleration but if drag balances it out then there will be no acceleration (thrust>drag)

6. If the forces become balanced, describe the motion of the rocket that results.

If the forces become balanced then the rocket will remain at constant velocity without acceleration but thanks to weight it will eventually start to slow down and stall.

7. Describe the similarities and differences between the forces of lift, drag, and weight.

Traditionally lift was used to overcome drag; however in rockets thrust is the force that overcomes drag. Drag is produced by friction of the air on the rocket’s surface, in a rocket the drag to lift ratio is much greater than in airplanes. Weight is the force of gravity upon the rocket and it is overcome by thrust.

8. How is the weight and mass of an object determined mathematically?

Weight = Weight (Newtons) x Force of Gravity

Mass = x

9. Describe how both the mass and the weight of a rocket would vary from the moment of launch until a landing on the Moon.
a. Launch Phase

During the launch phase the rocket is going to experience an increase in weight because as it climbs due to the thrust produced, the force of gravity(which is the same case if you are in an elevator that is moving up) causes for the weight to increase. But it also depends on the rate of consumption of fuel because if the fuel is consummed at a high rate then it is actually going to lose weight and mass.

b. Coast Phase to Moon

By the time the rocket is coasting to the moon it has used up all the fuel and therefor, the weight is only affected by the fact that there is virtually no gravity but mass stays the same.

c. Landing Phase

At the landing phase, if the rockets ignites a motor(s) to avoid crashing itis going to lose more weight but also gain it beacuse of the gravity of the moon but it is very negligible, mass stays the same(unless it actually burns fuel).

10. What is the center of gravity of an object and how can it be determined?

The CG of an object is the average location of the weight of an object. And the easiest way to determine it is by balancing the object.

11. If you could choose any shape for a rocket, which shape has the highest and which has the lowest coefficient of drag (CD), and which is the most desirable for a real rocket?

The coefficient of drag has a lot to do with the area that opposes the wind flow, in rockets with large CDs they have a flat design with a lot of surface opposing the wind flow (a box shape for example or a bottle). Rockets with low CDs have a shape that is similar to the one of a pencil, because the surface area does not directly oppose the windflow and instead cuts through it. This design is preffered because in rockets and airplanes, the goal is to have as little drag as possible and a suface/shape that doesn't obstruct the wind flow is ideal.

12. The drag coefficient represents the complex interaction of what three factors on the drag produced by an object moving through a medium.

(1)
$$Cd = D / (.5*r*V^2*A)$$

The three factors are:

• r - Density of the air
• V - Velocity
• A - Reference Area

13. What two air characteristics are considered by an aerodynamicists when calculating the drag coefficient?

The two air Characteristcs are the density of the air and the velocity of the air

14. How slow does an aircraft have to move in order for the air to remain, for all practical purposes, uncompressible?

when an aircraft is moving it allways causes the air to compress due to the fact that it is moving through air, therefor in order to keep it uncompress an airplane must not move at all or at a very slow speed where air compression doesn't occur or is very negligible

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page revision: 9, last edited: 31 Jan 2008 05:35