IN THE NAME OF

ALLAH

THE MOST BENEFICENT

THE MOST MERCIFUL

SPACERAFT

DYNAMICS AND CONTROL

qsmzeeshan@yahoo.com ; 0321-9595510

DR. QASIM ZEESHANBE, MECHANICAL ENGINEERING

NATIONAL UNIVERSITY OF SCIENCE AND TECHNOLOGY, NUST, PAKISTAN, 2000

MS, FLIGHT VEHICLE DESIGNBEIJING UNIVERSITY OF AERONAUTICS AND ASTRONAUTICS, BUAA, P.R.CHINA, 2006

PhD, FLIGHT VEHICLE DESIGNBEIJING UNIVERSITY OF AERONAUTICS AND ASTRONAUTICS, BUAA, P.R.CHINA, 2009

SPACECRAFT

ATTITUDE

DYNAMICS

AND CONTROL

SPACECRAFT

DYNAMICS AND

CONTROL

DISTURBANCE

TORQUESLECTURE #

SPACECRAFT

DYNAMICS AND

CONTROL

EXTERNAL

DISTURBANCE

TORQUESLECTURE #

External Disturbance Torques

Orbital Altitude

To

rqu

e

Solar

Press.

Drag

Gravity

Magnetic

LEO GEO

NOTE: The magnitudes of the torques is

dependent on the spacecraft design.

SPACECRAFT

DYNAMICS AND

CONTROL

INTERNAL

DISTURBANCE

TORQUESLECTURE #

Internal Disturbing Torques

Examples

Uncertainty in S/C Center of Gravity (typically 1-3 cm)

Thruster Misalignment (typically 0.1 – 0.5 )

Thruster Mismatch (typically ~5%)

Rotating Machinery

Liquid Sloshing (e.g. propellant)

Flexible structures

Crew Movement

Disturbing Torques

FrT

IHT

DISTURBANCE

TORQUES

GRAVITY GRADIENTLECTURE #

Gravity Gradient Torque

Earth Sensor uses the

principle of gravity gradient

sensing, which consists in

the very slight change in

gravity as you move away

from the center of the earth.

When an elongated mass is

placed in this environment, it

will tend to align itself with

this field, since one part of

the mass is pulled more

strongly than the other. By

measuring this

displacement, of the mass

trying to align itself, it is

possible to know where the

earth is !

Gravity Gradient Torque

Diagram showing effect of

gravity gradient on a proof

mass. (

a) the proof mass inside the

satellite is perpendicular to

the earth. Both sides of the

mass being the same

distance from the earth,

there is no detectable

effect.

b) if the satellite rotates by a

given amount, the mass

inside it is of course

rotated in the same way.

c) Since now one side is

closer to the earth and

sees more pull, an

additional rotation (torque)

will occur as shown in (c).

Gravity Gradient Torque

2sin2

33 yzg II

RT

where:

verticalfromaway deviation maximum

inertia of moments mass S/C ,

radiusorbit

parameter nalgravitatio sEarth'

gradientgravity maximum

zy

g

II

R

T

z

y

DISTURBANCE

TORQUES

MAGNETIC TORQUELECTURE #

Magnetic Torque

where:BxmTm

meters radiusorbit

m tesla10 7.96moment magnetic sEarth'

poles theabove pointsfor 2

equator theabove pointsfor

field magnetic sEarth' ofstrength

mAmp dipole magnetic residual S/C

torqueedisturbanc magnetic

315

3

3

2

R

M

R

M

R

M

B

m

Tm

*Note value of m depends on S/C size and whether on-board compensation is used

- values can range from 0.1 to 20 Amp-m2

- m = 1 for typical small, uncompensated S/C

DISTURBANCE TORQUES

AERODYNAMIC TORQUELECTURE #

Aerodynamic Torque

where:

gpaa ccFT

2

2

1AvCF D

gravity ofcenter C

pressure catmospheri ofcenter C

velocity

area sectional-cross

2.5 - 2 are valuesS/C typical drag oft coefficien

density catmospheri

torqueedisturbanc caerodynami

g

pa

v

A

C

T

D

a

DISTURBANCE

TORQUES

SOLAR PRESSURELECTURE #

Solar Pressure Torque

where:

gpssrp ccFT

iAc

FF s

s cos1

angle incidencesun

S/Cfor 0.6 value typical1,0factor ereflectanc

surface dilluminate of area

light of speed

m

Wdensity flux solar

gravity ofcenter c

pressureradiation solar ofcenter

torqueedisturbanc presureradiation solar

2

g

i

A

c

F

c

T

s

s

ps

srp

FireSat

Example

Disturbing Torques

All of these disturbing torques can

also be used to control the satellite

Gravity Gradient Boom

Aero-fins

Magnetic Torque Rods

Solar Sails

Spin Stabilized Satellites

Gravity Gradient Stabilization

Gravitational attraction:

f = μm/r2

Top: f1 > f2 ⇒ torque is out of the page

Bottom: f1 > f2 ⇒ torque is into the page

In both cases, the torque is a restoring torque,

tending to make the satellite swing like a

pendulum

Gravity Gradient Stabilization

In the 60s was viewed as “free”

attitude control

• In general, “G2” is not accurate

enough, spacecraft can even flip over

• Not really free, because of boom mass

• However, OrbComm and TechSat 21

use gravity gradient with flexible solar

panels on an extensible wrapper

around the boom

• The Moon is gravity-gradient

stabilized; Lagrange (1736-1813)

showed this

• In the 60s was viewed as “free”

attitude control

• In general, “G2” is not accurate

enough, spacecraft Tech SAT 21

SPACECRAFT DYNAMICS

AND CONTROLATTITUDE DETERMINATION AND CONTROL SYSTEM

ADCS

LECTURE #

Attitude Determination and

Control System

Attitude Determination and Control Subsystem (ADCS)

Attitude control is the exercise of control over the orientation of an object with respect to an inertial frame of reference or another entity (the celestial sphere, certain fields, nearby objects, etc.).

Controlling vehicle attitude requires sensors to measure vehicle attitude, actuators to apply the torques needed to re-orient the vehicle to a desired attitude, and algorithms to command the actuators based on

(1) sensor measurements of the current attitude

(2) specification of a desired attitude.

The integrated field that studies the combination of sensors, actuators

and algorithms is called "Guidance, Navigation and Control" (GNC)

Attitude Determination and Control

System

Torques

Control

actuator

On-board

computer

Ground

control

Attitude

sensor

Attitude

Controller Actuator Spacecraft

Sensors

Commands

Attitude

Torques

Measured

attitude

Error signal

Attitude Determination and

Control System

Attitude Determination and Control Subsystem (ADCS)

Stabilizes the vehicle

Orients vehicle in desired directions

Senses the orientation of the vehicle relative to reference (e.g. inertial) points

Determination: Sensors

Control: Actuators

Controls attitude despite external disturbance torques acting on spacecraft

Attitude Determination and Control

System

Attitude Determination and

Control System

ADCS Design Requirements and Constraints

Pointing Accuracy (Knowledge vs. Control)

Drives Sensor Accuracy Required

Drives Actuator Accuracy Required

Rate Requirements (e.g. Slew)

Stationkeeping Requirements

Disturbing Environment

Mass and Volume

Power

Reliability

Cost and Schedule

ADCS Design

ADCS Design

ADCS Design

ADCS Design

ADCS Design

THANK YOU FOR YOUR INTEREST