AA 528 Spacecraft Dynamics and Control. Mehran Mesbahi Aeronautics & Astronautics Winter 2017 University of Washington

Transcription

1 AA 528 Spacecraft Dynamics and Control Mehran Mesbahi Aeronautics & Astronautics Winter 2017 University of Washington

2 Spacecraft dynamics and control What is this class all about? what is in the name? Spacecraft: a craft in space Dynamics: a branch of classical mechanics Control: a branch of engineering/mathematics and is just a conjunction Let us start with the craft in space how does it look like?

3 Cassini craft

4 some facts about Cassini size: 30-passenger school bus (about 7 m) 7.5 miles of wire; 58 computers; 26 engineering computers travels 2.2 billion miles to reach Saturn more than 1.5 billion miles during its 4-year tour max planet-relative velocity of 70,000 mph narrow angle camera can read a newspaper headline at a distance of one mile over two trillion bits of data will be returned (equiv. to about 2,400 sets of Britannica)

5 Spacecraft usually has a mission Cassini mission to Saturn one of the most ambitious missions ever launched 18 instruments 27 diverse science investigations Missions are achieved via various subsystems on the spacecraft bus

6 subsystems command and data handling subsystem attitude and articulation subsystem propulsion module subsystems power subsystem radio frequency subsystem antenna subsystem structure subsystem mechanical device subsystem electronic packaging subsystem cabling subsystem temperature control subsystem

7 Cassini and me

8 Cassini in action

9 Let us dissect what Cassini is doing in this clip Spacecraft is on a trajectory: a path tagged with time The trajectory is generated by natural and engineered forces One natural force results in the spacecraft (its center of mass) orbiting Saturn due to Saturn s gravity this is the so-called 2- body problem But the spacecraft might also be under the influence of the Sun (Sun-Saturn-Spacecraft) this is now the three-body problem But the spacecraft might also be so close to Titan that we should consider (Sun-Saturn-Titan-Spacecraft) a four-body problem Regardless, the trajectory of the center of mass follows Newton s law in response to various forces

10 understanding the motion of a point mass under the action of gravity is part orbital/celestial mechanics/astrodynamics what is the big deal about gravity? well- F=ma has a lot of structure/lots of insights/very elegant it is an important dynamics for everybody its understanding is connected to some of the most celebrated figures in science among them

11 typical undergrad course on orbital mechanics covers two body problem Kepler s laws impulsive orbital transfers Hohmann transfer fast transfers plane changes time of flight (mean anomaly) orbital parameters and their geometry ground tracks orbit determination

12 typical undergrad course on orbital mechanics covers interplanetary transfers sphere of influence relative velocities, vector addition/subtraction patched conic approximation (planetary escape, heliocentric transfer, planetary capture) planetary passage and gravity assist intercept problem and fast transfers launch and staging rocket equation/elementary propulsion and may be some attitude dynamics no this is not about the mood of the spacecraft

13 so what is attitude dynamics? understanding the rotational dynamics of bodies under various forces/torques kinematics: underlying frames and frame rotation dynamics: how forces/torques influence this motion attitude dynamics and astrodynamics can often be treated separately; exceptions: rotational dynamics due to gravity gradients, etc., spin synchronization and guess what, we have another celebrity involved.

14 and you see Euler this movie of Cassini

15 so our first order of business will be on attitude kinematics and dynamics frames/different representations of orientation direction cosine matrix Euler angles Quaternions Gibbs parameters Euler s equations moment of inertia, principle axis parallel axis theorem angular momentum vector, energy equations

16 so we talked about spacecraft bus, dynamics, and our last piece is control control is about three things: sensing actuation and whatever goes in between often an algorithm, software, mechanism the purpose of control is to synthesis behavior while also benefiting from the natural behavior

17 some sensors and actuators

18 Cassini in action environment what earth says mission objectives actuation sensing GN&C system

19 and guess what we also meet some other celebrities

20 here are the topics that i plan to discuss kinematics rigid body dynamics gravity 2-body problem orbit determination orbital maneuvers orbital perturbations low thrust trajectories formation flight restricted three-body problem

21 more topics that i plan to discuss yo-yo mechanism attitude stabilization disturbance torques spin/dual spin/gravity gradient stabilization orbit determination attitude linear/nonlinear control spacecraft navigation practical issues if i have time i will also discuss launch phase landing model predictive control

22 course mechanics weekly homework assignment (20% of class grade) due on Wednesdays at 11:45 pm Office hours: Tuesdays 1:00pm-4:00pm midterm (40%) (5th week) project (40%) choose a topic of interest by end of January report (due the last week of class) class presentations (during the last two weeks)