Orbital Mechanics

Question 1

What are the 6 orbital elements?

Answer 1

• Semi-major axis
• Eccentricity
• Inclination
• Right Ascension of Ascending Node (RAAN)
• Argument/Angle of Periapsis
• True Anomaly

Question 2

Define:

Semi-major axis

Answer 2

The semi-major axis is the longest diameter of an ellipse

Unit: km

Question 3

Define:

eccentricity

Answer 3

The deviation of an orbit from being circular.

Unit: scalar, 0 to 1

Question 4

Define:

Ascending node

Answer 4

The point where the orbiting object crosses the equatorial plane from south to north.

Question 5

Define:

RAAN

Answer 5

Right ascension of the ascending node

The angle measured from a fixed reference direction (usually the vernal equinox) to the ascending node of an orbit.

Unit: degrees

Question 6

Define:

Argument/Angle of Periapsis

Answer 6

The angle from the ascending node to the periapsis, showing where the closest approach to the central body occurs within the orbital plane.

Question 7

Define:

True Anomaly

Answer 7

The angle between the periapsis and the current position of the orbiting body, indicating its position along the orbit at a specific time.

Unit: degrees

Question 8

Define:

Hohmann Transfer

Answer 8

Efficient orbital maneuver to move a spacecraft between two circular orbits using two engine burns: one to enter an elliptical transfer orbit and one to circularize at the destination orbit.

Question 9

What two things are conserved in an orbit?

Answer 9

Angular momentum and energy

Question 10

What three fundamental properties can be used to determine an orbit?

Answer 10

Position, velocity, time

Question 11

What’s the difference between the Earth Centered Inertial (ECI) and Earth-centered Earth-Fixed (ECEF) frame?

Answer 11

Earth rotation, omega term

Question 12

If eccentricity = 0, what shape is the orbit?

Answer 12

Circular

Question 13

If 0 < eccentricity < 1, what shape is the orbit?

Answer 13

Elliptical

Question 14

If eccentricity = 1, what shape is the orbit?

Answer 14

Parabolic

Question 15

If eccentricity >1, what shape is the orbit?

Answer 15

Hyperbolic

Question 16

Formula: Tsiolkovsky rocket equation?

Answer 16

Question 17

Formula: the Vis-Viva equation?

Answer 17

Question 18

What is a perturbation?

Answer 18

Deviation of an orbiting object from it’s predicted path.

Question 19

How do perturbations affect an orbit?

Answer 19

• Equatorial bulges cause precession of the node and the perigee
• Atmospheric drag reduces the semi-major axis unless make-up thrust is used

Question 20

What is the patched conic approximation?

Answer 20

A method to simplify a complex multi-body problem into multiple two body problems.

Question 21

Formula: Orbital velocity for a circular orbit

Answer 21

Derive from centrigual acceleration = gravity

Question 22

Formula: Orbital Period

Answer 22

mu = grav. constant
a = semi-major axis

Question 23

Name two types of orbit transfer maneuvers

Answer 23

• Hohmann Transfer
• Bi-elliptic Transfer

Question 24

Formula: delta-V

Answer 24

Question 25

Specific impulse is equal to what engine parameter?

Answer 25

Nozzle exit velocity (m/s)