Torque, forces, momenta

Question 1

three examples of continuous spacecraft mechanisms

Answer 1

• antenna pointing and tracking
• solar array pointing and tracking
• attitude control

Question 2

three examples of single shot mechanisms

Answer 2

• launch vehicle seperation
• solar array retention and deployment
• contamination cover removal

Question 3

How do products of inertia relate to symmettry and anti-symmetry axis?

Answer 3

• For a symmetry axis the products of inertia are always 0 in that plane
• Since the positive quadrants of the plane don’t balance the negative quadrants of the plane in terms of x.y.da the axis is not a principal axis and PoI will be non-zero

Question 4

Asides from the reaction wheel, what other sources of internal torque are there?

Answer 4

Any movement of on-board objects such as the fuel it contains, astronauts moving etc

Question 5

What is meant by a pure angular momentum in the context of reaction wheels? When might they not be pure?

Answer 5

Pure is when the wheels are aligned with the principal axis thus only having one angular momentum component.

• When they are angled, which is the case with a redundant wheel, the component clips each plane.

Question 6

Three examples of external torque

Answer 6

• thruster fired that’s not aligned with the spacecraft’s CoM
• solar radiation
• aerodynamic drag

Question 7

Describe what happens in the two cases:
- torque applied parallel to angular momentum, H
- torque applied perpendicular to H

Answer 7

• change in magnitude of H but not direction
• angular momentum direction changes and not magnitude (motorbike analogy)

Question 8

What happens to the rotational kinetic energy and the angluar momentum when fluid sloshes about within the spacecraft

Answer 8

• Kinetic energy converted into heat, thus net energy of the spacecraft decreases
• however angular momentum is conserved and so momentum lost by the fluid is gained elsewhere on the spacecraft
• long term consequence is that spacecraft will rotate about its axis of maximum inertia

Question 9

Describe the two profiles used for rotating a solar array 90 degrees via some mechanism?

Answer 9

1: accelerate the rate of rotation up to 45 degrees and then decelerate for the ramaining 45 at the same rate. this means net angular displacement is 90 and the angular velocity returns to 0 at 90.
2: same but with constant angular speed (0 torque) for the middle third of the event

Question 10

What are the four points on the output speed versus output torque

Answer 10

• no-load speed (0 torque)
• operating point 2 (torque required to overcome frictional forces)
• operating point 1 (rpm needed to provide torque corresponding to the required angular acceleration)
• stall torque (maximum load, rpm=0 and maximum torque output point)

Question 11

What is meant by a 100 % stall torque margin?

Answer 11

• there are frictional forces which must be overcome to do work
• however other effects such as cold welds increase the magnitude of these frictional forces
• so a rule of thumb in spacecraft design is to determine your stall torque and then double it to allow for contingencies in torque requirements
• one shot devices can work well with just a 10% stall torque marign

Question 12

What are the effects of a gear-train on the motor’s torque?

Answer 12

It will multiply the torque, but it will increase mass and friction