ASE Tech Elective: EAS 4412Dynamics and Control of Space Vehicles

Mrinal Kumar, Assistant Prof.Mechanical and Aerospace Engineering

SYLLABUS…

EAS 4510: Keplerian Mechanics --- Considers motion of two particles moving under the influence of gravity...

EAS 4412: Zoom into orbit… study the rotational motion of the spacecraft as a rigid three dimensional object

Relationship between 4510 and 4412

What is Spacecraft Attitude?

“Orientation of the spacecraft with respect to some suitable inertial reference frame”

For rigid spacecraft, orientation of spacecraft orientation of its body-frame

ADCS: Attitude DETERMINATION and CONTROL System

SENSORS ACTUATORS

An Illustrative Example*

* Taken from “Spacecraft Attitude and Control: a Practical Engineering Approach”, by Marcel J. Sidi, Cambridge University Press, 2006. Chapter 1: Introduction

Spacecraft: Geostationary communications satellite, e.g. U.S. Intelsat V, European Kopernikus

Main structural parts: • Central body: ~ cube-like structure with cross-section 1.5 × 2.0 m• Solar arrays in N-S direction, with panel dimensions 1.5 × 7.0 m• Antenna tower directed toward Earth, carrying various communication payloads, e.g.

global and beacon horns, communication feed systems, hemi/zone and spot reflectors, TM/TC antenna.

• Sensors, e.g. sun sensors and Controllers, e.g. reaction thrusters; placed over the central body and panels.

(Broad) Types of (Geocentric) Spacecraft

The earliest ones

Explorer I: USA, 1958

Sputnik I: Russia, 1957 Artemis (ESA): GEO

Orbcomm: LEO

Communication (commsats) Remote Sensing

HST: Space observing

Landsat: Earth observing

Why do we need ADCS?

1. Prevent spacecraft tumble!

2. Point antennas in the right direction

3. Point payload in the right direction, e.g. cameras

4. Align solar panels perpendicular to the sun, sensitive equipment away from sun

5. Space operations, e.g. docking…

einstein.stanford.edu

Commonly used Attitude Control Methods

PASSIVE ACTIVE

Gravity Gradient Spin stabilization

Momentum wheels

Control moment gyroscopes

Magnetic Torqrods

Thrusters

Spacecraft: Geostationary communications satellite, e.g. U.S. Intelsat V, European Kopernikus

Typical Attitude and Orbit and Control System Hardware (AOCS)**:

• Reaction bipropellant thrust system, consisting of one 420-N thruster for orbit transfer and two independent (one redundant) low-thrust systems consisting of eight 10-N thrusters each

• Two momentum wheels (one redundant) of 35 N-m-sec each• Two infrared horizon sensors (one redundant)• Four fine sun sensors (two redundant)• Twelve coarse sun sensors for safety reasons (six redundant)• Two three-axis coarse rate gyros; and,• Two three-axis integrating gyros

Redundancy ensures system reliability!

An Illustrative Example*: ADCS Hardware

* Taken from “Spacecraft Attitude and Control: a Practical Engineering Approach”, by Marcel J. Sidi, Cambridge University Press, 2006. Chapter 1: Introduction** Notice the difference between ADCS and AOCS

Reference Frames

ECI:(Earth Centered Inertial)

Reference Frames

ECEF:(Earth Centered Earth Fixed)

Reference Frames

TRF:(Topological)

(~zenith)

(~east)(~north)

Reference Frames

ORF: Orbital Frame

(~velocity)

(~normal)

(~nadir)

Reference Frames

BRF: Body Frame

(~roll)

(~pitch)

(~yaw)