Maneuvering in Space

1. Maneuvering in Space

2. Maneuvering in Space • Spacecraft Ground Tracks • Tracking the spacecraft • Simple Orbit Changes • Moving From One Orbit to Another • Using the Hohmann Transfer Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

3. Tracking the Spacecraft • Tracking the Spacecraft • Non-rotating Earth ground tracks • Rotating Earth ground tracks • Common orbit types Ground tracks for a trip by car and air from San Francisco to Omaha. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space SECTION 5.1

4. Non-rotating EarthGreat Circles • A great circle is any circle that slices through the center of a sphere. • Lines of latitude are NOT great circles because they do not include the center of the Earth (except the equator). • Lines of longitude are great circles. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

5. Orbiting Around a Soda Can • On top we have an orbit around a soda can. • If we draw a line on the soda can directly below the orbit we’d get a ground track. • If we cut the soda can in half and laid it flat, the shape of the ground track is as shown in the lower figure. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

6. Non-Rotating Earth • Here’s what a ground track would look like for a non-rotating Earth if we stretch the Earth onto a flat-map projection. • Notice that the ground track is made by a spacecraft in orbit around Earth—this orbit is a great circle. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

7. Rotating Earth • Here the orbit is red and it’s a great circle that includes the center of the Earth. • The Earth rotates once per day or 360º in 24 hours. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

8. Add Earth’s Rotation • This is a typical low Earth orbit. • The “map” moves eastward so the second orbit ground track looks like it moved to the west. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

9. Geostationary and Geosynchronous Orbit Ground Tracks Geosynchronous orbit with inclination greater than zero results in a closed ground track Geosynchronous orbit with inclination equal to zero results in a point ground track (Geostationary) Both have an orbital period of 24 hours Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

10. Simple Orbit ChangesMoving from one orbit to another • Moving from one orbit to another, engineers and astronauts need to develop procedures for all orbital maneuvers. • When we maneuver in space, we need to factor fuel into the process. Photograph of Gemini 6A Command Module rendezvousing with the Gemini 7 Command Module. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space SECTION 5.2

11. Simple Orbit ChangesMoving from one orbit to another (cont’d) • In 1925 a German engineer, Walter Hohmann, thought of a fuel-efficient way to transfer between orbits—the Hohmann Transfer. • The Hohmann Transfer uses an elliptical transfer orbit tangent to the initial and final orbits. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

12. Simple Orbit ChangesMoving from one orbit to another (cont’d) • Cars on racetracks: getting off • Effort needed to exit depends on the off-ramp’s location and orientation • If the off-ramp is tangent to the track, exiting is easy—just straighten the wheel. • If the off-ramp is perpendicular to the track, exiting requires slowing down a lot, and maybe even stopping, to negotiate the turn. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

13. Hohmann Transfers: Racetrack Example • Tangential exit requires changing only the velocity’s magnitude—just hitting the brakes or accelerating. • Perpendicular exit requires changing the velocity’s magnitude and direction. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

14. Simple Orbit ChangesUsing the Hohmann Transfer • Hohmann Transfers apply racetrack principle to orbits: use tangential “off-ramps” to use less energy. • A spacecraft changes its energy by increasing or decreasing its velocity— firing rocket engines. • Velocity changes must be tangent to the initial and final orbits. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

15. Simple Orbit ChangesUsing the Hohmann Transfer (cont’d) • Adding or subtracting velocity changes the orbit’s energy. • Increasing velocity adds energy to the orbit. • Decreasing velocity removes energy from the orbit. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

16. Hohmann Transfer Example (cont’d) • Without the second velocity change, the spacecraft will remain in the transfer orbit indefinitely. • Total velocity change for the maneuver is the sum of the two velocity changes. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

17. Hohmann Transfer Example • Transfer requires two steps: • Increase velocity: change smaller circular orbit’s energy enough for the spacecraft to enter the larger transfer orbit. • Increase velocity: change the transfer orbit’s energy enough for the spacecraft to enter the larger circular orbit. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

18. Hohmann Transfer Example (cont’d) • Step 1: Increase velocity(change in velocity1)to enter the transfer orbit. • Step 2: Increase velocity(change in velocity2)to enter a final, larger circular orbit. V = velocity V orbit 1 = velocity in small circular orbit V orbit 2 = velocity in larger circular orbit Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

19. Summary • Spacecraft Ground Tracks • Tracking the Spacecraft • Simple Orbit Changes • Moving From One Orbit to Another • Using the Hohmann Transfer Unit 2, Chapter 5, Lesson 5: Maneuvering in Space

20. Next • Now that we have tools to maneuver in Earth’s neighborhood, next time we’ll discuss interplanetary travel. Unit 2, Chapter 5, Lesson 5: Maneuvering in Space