Flight Dynamics

Overview: Here you'll learn all about orbits and how satellites and other objects revolve around the earth. You'll learn about concepts like planetary motion, apogee and perigee, and angles of inclination. Also, you'll learn what "attitude" is and about the Attitude Control System on a spacecraft.

Introduction

Video of rocket launch;
Video of Space Station;
Video of satellite;
Video of unmanned rocket launch;
Rocket Cam video during of ascent;
Video of free flying satellite;
till picture of Space Station and free flying satellite appear sequentially during audio);

"Whenever NASA plans a mission, whether it is for the Space Station or any orbiting satellite, scientists and engineers must plan it's orbit and attitude, the pointing direction, and how to achieve them!"

Short video segments of Space Station and free-flying satellite;

"To find their way in space, scientist and engineers must follow the laws of motion that are known as Flight Dynamics."

Logo of the word "Orbital Mechanics" appears;

Motion of Celestial Bodies

Pictures of ancient astronomers and their depictions of planets and comets in the sky;

"Long before NASA launched its first space mission, objects in our solar system like planets and comets were watched by astronomers who wondered how they moved."

Animation of the path of Mars as seen by an astronomer on earth;

"To those astronomers, the paths of certain objects in the night sky, like Mars shown here, seemed like a person wandering back and forth. So they called those object 'planets' which means 'wanderer'. For a long time, the astronomers wondered why the wandering planets moved the way they did."

Animation of the Earth and Mars (view point: above Earth and Mars orbits);

"From their vantage point on Earth the path of Mars seemed complicated. However, it is actually fairly simple as can be seen in this representation of Earth and Mars. As the Earth, represented in black, orbits around the Sun faster than Mars, shown in red, it occasionally catches up to Mars and passes it. To an observer on Earth, Mars appears to slow down, move backwards and move forward again."

Screen snap of the solar system (continued view point: showing inner planets);

"It is easy for us to see from this imaginary vantage point the inner planets Mercury, Venus, Earth, and Mars move in paths around the Sun that are almost circles."

Screen snap of the solar system (view point: Outside solar sytem);

"If we move our vantage point to view the outer planets, Jupiter, Saturn, Neptune, Uranus, and Pluto we also see paths of the planets are not flat like a desktop. Rather, their paths move in up and down directions as well as back and forth in all three dimensions of space."

Kepler and Newton

Picture of Johannes Kepler; Diagrams associated with Kepler's 3 Laws of Orbital Motion;

"Early in the 17th century, after years of studying measurements of the motion of the planet Mars, a brilliant mathematician named Johannes Kepler was able to conclude that the shape of an orbit of a planet orbiting the Sun is an ellipse. This conclusion is known as Kepler's first law of law of orbital motion. Eventually Kepler established a total of three laws that described how planets moved around the Sun. His second law showed that a planet would travel at different speeds at different locations on the ellipse. His third law gave a mathematical relationship for the distance of a planet from the sun and the amount of time it takes the planet to complete one orbit."

16) Picture of Kepler; Picture of Isaac Newton; Graphic of equations for Universal Law of Gravitation;

"Although Kepler's three laws describe how a planet moves, they do not explain why they move in the elliptical paths we call orbits. It was not until the later part of the 17th century that the famous Physicist Sir Isaac Newton applied his Universal Law of Gravitation to prove that Kepler's laws were true. The explanation for the motions that Kepler described with his three laws, Newton found to be in the force of gravity that exists between all objects."

17) Pictures other Kepler and Newton and scientist and engineers and graphics of equations and diagrams;

"Since the time of Kepler and Newton, other mathematicians and scientists have continued to study and develop the laws and equations that describe the motion of objects in space. Today, NASA scientists and engineers call these tools Orbital Mechanics and use them to plan their missions."

Description of Orbits

Single satellite orbiting the earth in an elliptical orbit (viewpoint: askew and moving toward the top);

"To learn more about the way NASA uses orbital mechanics to plan missions, lets study the orbit of a single spacecraft orbiting the earth."

Single satellite orbiting the earth in an elliptical orbit (viewpoint: looking straight down on the orbit);

"Just as Kepler showed for the planet Mars, we see the spacecraft orbit is the shape of an ellipse."

Diagram of an ellipse;

"In this diagram of an ellipse, the first thing we notice is its oblong shape. While a circle has one center point, an ellipse has two points called 'focus points' or 'foci'. In fact, a circle is a special case of an ellipse where the two foci coincide."

Single satellite orbiting the earth in an elliptical orbit (viewpoint: looking straight down on the orbit);

"Because of the work of Kepler, orbital mechanics tells us that the center of the earth is located at one of the focus points of the elliptical orbit of our satellite."

Multiple satellites orbiting the earth orbits ranging from circular to highly elliptical);

"Regardless of the number of spacecraft orbiting the earth or the exact shape of the ellipse of a spacecraft's orbit, the center of the earth is always at one focus."

Diagram of orbits in previous mpeg video;

"The shapes of the elliptical orbits of spacecraft can differ greatly. Some of the orbit shapes are very oblong and others only moderately so. Some are even the shape of a circle, which is just a special case of an ellipse with focus points that are located at the same spot. The measure of how oblong an orbit is, is called 'eccentricity'. The more oblong the orbit the higher its eccentricity."

Single satellite orbiting the earth in an elliptical orbit (viewpoint: looking straight down on the orbit);

"Another way NASA scientist and engineers can describe the shape of an elliptical orbit of an earth-orbiting satellite is to describe the distances to the point in the orbit where the satellite is closest to Earth, called the perigee, and the point where it is farthest from the Earth, called apogee."

Diagram of single satellite orbit with apogee and perigee labeled;

"The height of apogee and the height of perigee are measured in kilometers or sometimes in nautical miles. A nautical mile is slightly longer than the "statute mile' used to measure the distance between places on land."

Video Space Station as viewed from Shuttle;

"The height of apogee of the Space Station orbit is roughly 354 kilometers or about 191 nautical miles. The height of perigee is approximately 341 kilometers or about 184 nautical miles."

Video of free flying satellite;

"Other spacecraft fly in orbits whose height of apogee and perigee are several thousand kilometers."

Single satellite orbiting the earth in an elliptical orbit (viewpoint: looking straight down on the orbit);

"The time it take a spacecraft to travel around its orbit from perigee to perigee is called the 'orbital period'."

Video of free flying shuttle;

"When in space, the orbital period of the space shuttle was about 90 minutes. In order to complete an orbit in 90 minutes, the Space Shuttle travelled at an average speed of about 17,500 miles per hour or about 5 miles per second."

Single satellite orbiting the earth in an elliptical orbit (viewpoint: looking straight down on the orbit);

"As we watch our imaginary spacecraft move around its orbit we notice that it moves fastest at perigee and slowest at apogee. Kepler had observed this phenomena when studying Mars and recorded this observation as the second of his three laws."

Single satellite orbiting the earth in an elliptical orbit (viewpoint: moving toward the side);

"When we take a look at our model orbit from a different vantage point, we can see that the orbit is tilted or 'inclined' off of the equator. The angle between the orbit path and equator is called the 'angle of inclination' or just "inclination'."

Single satellite orbiting the earth with 0 deg inclination (viewpoint: moving toward the side);

"A satellite in an orbit with a zero-degree inclination flies directly above the earth's equator at all times. These orbits are called "equatorial orbits"."

Single satellite orbiting the earth with a 51.6 deg inclination (viewpoint: moving toward the side of the ascending node and then the descending node);

"The Space Station's orbit has in inclination of 51.6 degrees. The Space Station therefore flies over more of the earth's surface than a satellite in an equatorial orbit and can observe much more of it. The point at which the Space Station or any other satellite crosses the equator when moving in a northerly direction is called the 'ascending node'. On the opposite side of the orbit, where a satellite crosses the equator while moving in a southerly direction, is the 'descending node'."

Single satellite orbiting the earth with a 90 deg inclination viewpoint: moving toward the side);

"Some satellite orbits have inclinations that are 90 degrees. These orbits are called 'polar orbits' because they pass over the north and the south poles of the earth."

Choosing an orbit for a space mission

Video of free flying satellite and space shuttle;

"Before launching a mission to space, the scientist and engineers at NASA must decide what kind of orbit the spacecraft must fly in to accomplish its mission objectives."

Video of Shuttle approaching the Space Station;

"In order for the Space Shuttle to reach the Space Station, NASA had to launch the Shuttle in an orbit that is very close to the orbit of the Space Station. That is, the height of apogee and perigee, the inclination, and the location of the ascending and descending node of the Shuttle's orbit must be very close to those of the Space Station's orbit. After launch, the Space Shuttle made several adjustments to its orbit until it can rendezvous and dock with the Space Station."

Video of crew moving supplies into the Space Station;

"After the Shuttle docked with the Space Station, the crews could re-supply the station with food water and new scientific instruments. After the completion of the shuttle program in early 2012, NASA has been relying on Russia's Soyuz spacecraft to ferry the crews to and from the station."

Video of free-flying satellite;

"The orbit of unmanned satellites depends upon what kind of job the satellites do as well as to re-supply the station."

Single satellite in orbit with 0 deg inclination and period = 24 hours;

"Many communication satellites, such as the NASA Tracking and Data Relay satellites for example, fly in very high circular orbits above the equator. The period of the orbit matches the rotation rate of the earth so that the satellite flies above the same point of the surface of the Earth at all times. These orbits are called 'geosynchronous' orbits."

Aura satellite orbiting the earth with a 98.2 deg inclination with observation cone;

"Many scientific satellites like this one called 'Aura' fly in polar orbits so their instruments can cover the entire surface of the earth over time. Aura uses scientific instruments to study earth's atmosphere to determine its health and to understand how it changes."

LEO orbiting the earth while adjusting attitude;

"There are two major parts to Flight Dynamics, orbit determination and control, and attitude. Now that we have discussed the orbit of a spacecraft, lets discuss the basics of the attitude of a spacecraft."

Picture showing the difference between the attitude and altitude of an object;

"The attitude of a satellite is a term that simply means the orientation of the spacecraft and its rate of rotation. Keep in mind that the term altitude is the height of an object, and different from attitude which is the orientation of an object. In order to conduct and observe scientific experiments, and send the results back to Earth, precise attitude must be known."

Diagram of the three components of the attitude control system;

"The Attitude Control System is what controls the attitude on the satellite. There are three main components to the ACS sensors, actuators, and algorithms. Lets discuss each component."

Imagery of satellites adjusting their attitude using different kinds of sensors;

"Every ACS uses sensors to measure and detect the satellites current attitude. There are many different kinds of sensors. Gyroscopes are instruments inside the spacecraft that can sense rotation, Horizon sensors detect light from the Earths horizon. And sensors like the Sun Sensor measure the Sun's angle and Star Trackers measure angular separations among observed stars."

Imagery of satellites adjusting their attitude using different types of actuators;

"An actuator is a device that directly controls attitude. The most common ACS actuator is a thruster, which uses small bursts of fuel to change the attitude of a spacecraft. Momentum Wheels are rotors that use an electric motor to spin the satellite in the opposite direction. And Control Moment Gyroscopes are special types of gyroscopes that can be tilted to control attitude as well."

Two diagrams of a driving car algorithm, analogous to the rotation of satellite and thruster algorithm;

"Sets of sensor data, along with methematical algorithms, are used to determine the current attitude, which is compared to the desired mission attitude. If these two sets of orientation angles differ, actuators are activated to correct the attitude. ACS algorithms are step by step procedures to control attitude. A simple example of an algorithm is driving a car. If you're going too slow, press the accelerator. And if youre going too fast, press the brake. This is a basic algorithm similar to how an ACS controls attitude on a satellite."

Diagram of three ACS components working together, then showing satellites viewing earth horizon;

"Here, we see a basic diagram of the ACS sensors, actuators, and algorithms working together. The Attitude Control System is critical in a spacecraft for successfully completing its mission. Lets look at some examples of different missions utilizing their Attitude Control Systems."

Generic videos of each satellite adjusting its attitude;

"The Swift satellite uses the ACS to view Gamma Ray Bursts. The TDRS satellite uses the ACS to maintain its precise angle to increase the time other satellites can communicate to the ground. Hubble controls its attitude to point towards different galaxies and stars. And the Cassini satellite uses thrusters to adjust attitude when it's near Saturn."

Conclusion video of ACS, generic satellite imagery;

"This section has covered the basics of spacecraft attitude and its control system. In conjunction with orbit determination and control, the flight dynamics functions are carried out to support the overall mission objectives."

Short animations of satellite models;

"Whatever the mission, whatever the orbit, NASA scientists and engineers use the same laws and tools of flight dynamics to maintain the orbit and attitude they need to accomplish their mission."

Fly It!: Introduction

This is the Fly It! Module of the Orbital Mechanics training for the Space Operations Learning Center (SOLC). We have prepared a mission for you to accomplish using what you've learned from the Flight Training section. Are you ready?

The crew of the International Space Station (ISS) has run out of all of their supplies and are in danger. They need your help!

Your mission is to rescue the crew by getting supplies to them using the Space Shuttle. Here are your objectives...

Phase: Load Supplies Onto the Space Shuttle
Phase: Launch at the Correct Angle of Inclination and Altitude
Phase:Dock the Space Shuttle to the Space Station

Phase 1 - Supplies are Loaded onto the Shuttle and are ready to be delivered to the crew of the International Space Station.

Phase 2 - The correct Angle of Inclination is 51.6 Degrees and the correct Altitude at which to launch the space shuttle is 191 Miles.

If you don't remember the correct answers, please look through the "Apogee and Perigee" and "Orbital Inclination" sections of the Flight Training at the main Orbital Mechanics page.

Phase 3 - The space shuttle is maneuvered and docked to the International Space Station within the fuel constraint. The supplies loaded in Phase 1 have been successfully delivered to the International Space Station and the crew has been saved.

Conclusion - We hope you have learned a lot about Orbital Mechanics. Please visit the Space Operations Learning Center home page for more training modules and a lot more for you to learn.

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