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Teach Orbital Maneuvering
with KerbalEdu

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Theory

Orbits and Docking

This lesson works as an introduction to orbital mechanics and tutorial for orbital encounters. The scenario is for students who have some previous experience of KerbalEdu.

Teacher Resources

  • Theory: Students learn about trajectories, Kepler's Laws, conics and more.
  • Play: Students plot an encounter and orbital rendezvous with added possibility to train docking procedures.
  • Share & Discuss: These tasks focus on orbits and orbital mechanics.

Gemini 8

  • Eighth flight in the Gemini program (NASA's second human spaceflight program)
  • Launched March 16, 1966
  • First orbital docking with Agena Target Vehicle.
  • The flight ended with an emergency landing after thruster malfunction.

Gameplay screenshot

Teacher Resources

Which spacecraft is faster?

What is an orbit? How would you define one?

Show Notes

Orbits are all about falling.

Gameplay screenshot

Teacher Resources

The strength of gravity is ‘inversely proportional to the square of the distance between them.’ Meaning the craft closer to planet needs higher speed so it goes past the ground before it falls.

READ:

Trajectory

  • Trajectory is a flight path of a moving object.
  • The uninterrupted trajectory is called an orbit.
  • The lowest point of orbit is called Periapsis (Pe) and the highest Apoapsis (Ap).
  • Orbital eccentricity determines deviation from the perfect circle. 0= circular, 1= parabolic escape orbit.

Gameplay screenshot

Teacher Resources

Mathematically orbits are known as ‘conic sections.' In conic mathematics, the section is a curve which is obtained from an intersection of a plane and a cone. In astrophysics, orbits form based on gravity. Orbits are elliptic due to the nature of gravity.

Note: In Kerbal Space Program ‘Patched conics’ (Note: accessed through the cheat menu by pressing Alt-F12) offer simplification on gravity sources by turning so called ‘n-body problem’ (several gravity sources) to multiple two-body problems.

READ:

Image Source: Conics, Wikimedia Commons.

Kepler's Laws

  1. The orbit of a planet is an ellipse with the Sun at one of the two foci.
  2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

Gameplay screenshot

Teacher Resources

In astronomy, Kepler's laws of planetary motion are three scientific laws describing the motion of planets around the Sun. According to Kepler’s first law, all closed orbits are in the shape of the ellipse. Circular orbits are cases where both the higher and lower point of an orbit are at the same distance from the center. An exactly circular orbit is almost impossible to attain in practice.

Example: The International space station has a perigee of 400.2 km and an apogee of 409.5 km, making it practically a circular orbit.
Tell the students: Periapsis (Perigee, Pe) is the point of closest orbit, and Apoapsis (Apogee, Ap) the farthest.

Orbital eccentricity is a parameter which determines the amount of orbit deviation from a perfect circle (0 being circular, values between 0 and 1 is an elliptical orbit, 1 being a parabolic escape orbit, and 1+ being hyperbolic orbit). For example the orbits of comets have a high eccentricity (highly elliptical orbits).”

READ:

Image Source: Parabolic curves, Wikimedia Commons.

Movement and Orbit

  • Periapsis: The point in the path of an orbiting body at which it is nearest to the body that it orbits.
  • Apoapsis: The point at which an orbiting object is farthest away from the body it is orbiting.

Gameplay screenshot

Teacher Resources

‘Ascending Node’ and ‘Descending Node’ are parts of orbit where the orbit crosses the reference plane (equator). ‘Inclination’ is the number of degrees that tells how much the orbit differs from equatorial orbit (90 degrees being polar orbit).

READ:

Image Source: Key terms, Kerbal Space Program Wiki.

Play

Lesson Goal

Jebediah has forgotten his snacks on Kerbin. Luckily his friend is ready to share snacks, but he’s on a different orbit. Students need to guide Jeb's ship to make a rendezvous with the Snackship on the higher orbit.

Teacher Resources

Note: The goal of this challenge is to use orbital nodes to change orbits, observe orbital speeds and manage them to get an encounter with the Snackship.

  • Completing the encounter would mean good understanding of Newtonian physics and management of the maneuver node.
  • Managing to dock would mean excellent control and understanding of Newtonian mechanics!

The mission itself requires some gameplay skills. Using different keys to RCS thrust and tracking movement of the instruments is not a task that can be described as ‘easy.' The minimum goal for students is to get the close encounter with the Snackship. This is because fine-tuning the docking might take time.

How to Play

Gameplay screenshot

KerbalEdu Mission Library

Gameplay screenshot

Teacher Resources

Open the mission from the KerbalEdu Mission Library. Do this by opening up the browser and type (or click on the link):

http://kerbaledu.com/kerbalmissionlibrary/

Use search to find the correct mission or share this direct link with the students:

http://kerbaledu.com/kerbalmissionlibrary/node/78

Kerbal Mission Library allows users to launch missions right from the web without the need to register or log in. You can see the ‘Launch in KerbalEdu’-button on the page.

Controls Chart

Gameplay screenshot

Teacher Resources

Controls:

  • Kerbal craft is directed with ‘W’ ‘A’ ‘S’ ‘D’ keys. If the craft responds too fast user, the can press
  • ‘Caps Lock‘ to make controls slower.
  • ‘Q’/’E’ roll the craft
  • ‘F’ / ‘T’- enables/toggles SAS (Keeps craft pointed to that direction)
  • ‘Space’ - Detach stage/launch rocket. The stages are shown in the left part of the screen, and they can be arranged with the mouse.
  • ‘M’ - Toggle Orbital map on/off
  • ‘F5’ Quicksave
  • ‘F9’ Quickload
  • ‘Esc’ exit menu
  • ‘R’ - RCS on/off. While RCS is on it’ll augment translations (WASD) of the craft. However the user can enable RCS to apply thrust to different directions:
  • ‘H’ key forward, ‘N’ backward, ‘I’ down, ‘J’ left, ‘K’ up, ‘L‘ down

Image Source: Key bindings, Github (TriggerAu).

Maneuver Nodes

Gameplay screenshot

Teacher Resources

“A maneuver node grants the opportunity to design an orbit before using the engines. The node is created by clicking on the current orbit on the map (press to access) and selecting ‘Add Maneuver.'

To edit a maneuver node, click and drag the orbital controls away from the center.

The predicted orbit is seen as an orange dashed line. When the node has been created, you also need to follow its orders by positioning your ship towards the blue maneuver symbol and using the engines at the right level of power at the right time. Doing either of these incorrectly will typically destroy your well thought-out plans.”

Note: Completing this lesson without using the maneuver nodes is impossible. The Teacher should walk through the basic functions of maneuver node with the students before advancing to the lesson challenge.

Maneuver nodes are an important tool in the game and the teacher should provide enough information to get the students figure the rest out by themselves. If the teacher wants to simulate something like the Gemini or Apollo programs in KSP the students should learn the necessary gameplay skills to back it up. However, as project work, grouping up and selecting a mission controller, engineer and the pilot for the team works best.

Note: In the bottom right example picture the maneuver symbol and the retrograde symbol are in the same position because the picture depicts the required burn.

Additionally, you can use SAS to keep the craft pointed to the maneuver node. This can be achieved by clicking maneuver node icon in SAS mode options that are located at the left side of the Navball.

READ:

Walkthrough: Orbital Maneuvering 101

  • Your goal is to meet the Snackship in orbit with your spaceship.
  • Kerbal spaceship starts from a circular orbit that has same inclination as the Snackship, making the encounter easier.

Rendezvous maneuver has four steps:

  1. Coasting closer
  2. Plotting the interception
  3. Encounter
  4. Approaching and docking.

Gameplay screenshot

Teacher Resources

Explain the process involved in the rendezvous. Use the Orbits & Docking mission from the Mission Library to avoid launching and adjusting the orbit-phase of the game.

  1. Go to the map screen (‘M’) and select ‘Snackship’ as the target by clicking it. Now the game actually displays points of the orbit that intersect with the Snackship.

Jeb’s craft is in a lower orbit than ‘Snackship’. This is actually a good thing because his orbital speed is actually higher than those above him meaning Jeb is gaining on the Snackship.

  1. Students should create a maneuver node just before Jeb’s ship is about to pass under the Snackship. Making maneuver nodes for rendezvous requires a lot of trial and error. However, as maneuver nodes are getting familiar, you can plan more complex tasks for students.

READ:

Orbital Maneuvering (Pt. 2)

  • Your goal is to meet the Snackship in orbit with your spaceship.

Gameplay screenshot

Teacher Resources

Explain to the students that in this picture the player is creating a maneuver node by pulling green icons that are prograde and retrograde. The first intersection of the Snackship orbit passes ahead of the target (orange markers) but because its orbit is now a bit higher (= duality travelling slower and Snackship taking over) the second intersect (purple markers) are right on the target.

Note: From the example picture students can see that the second intersect happens at the good distance of 3,7 km between craft and the relative speed of 65 m/s, meaning the total delta-v needed to match the orbit of the Snackship.

  1. Add time acceleration and wait several orbits until the spaceship is trailing a bit behind the Snackship.
  2. Experiment with the maneuver node trying to get a close encounter. Students who repeat the process see intersect markers closing towards each other.

Note: You can tell the students to test making a maneuver node several times and adjust it for the encounter. Not every solution works or is practical (hard burns would mean that the craft doesn’t have enough fuel for the encounter). Adjust prograde and retrograde markers carefully to see the encounter indicators change (Indicators tell separation with target - optionally this should be below 5,0 km). Kerbal takes into account the second orbit as well so you can see if you’re closing in on target. If you pass the target, just raise Jeb’s ship orbit higher and let Snackship close. After all there are several styles to complete the encounter but every one of them requires grasping basics of the orbital dynamics.

Image Source: Retrograde and Prograde markers, Kerbal Space Program Wiki.

Orbital Maneuvering (Pt. 3)

  • Your goal is to meet the Snackship in orbit with your spaceship.

Gameplay screenshot

Gameplay screenshot

Teacher Resources

Continue supporting the students with their attempts, giving advice if needed. Restart the scenario if necessary. Example picture shows the correct alignment to get grade A Interception. When the kerbal ship is in danger of passing snacks, raise orbit above it to let the Snackship catch up.

5. Perform the encounter maneuver when the separation is below 5 km. For this, the Snackship should be selected as the target from the map screen. At this point the navball will switch to target mode and display relative velocity with the target. The rest is up to you!

Press F5 to save the game and toggle soft controls with Caps Lock.

Steps for the textbook encounter:

  1. Aim towards the retrograde symbol (purple Y-symbol) of navball and burn until your relative speed is zero.
  2. Use RCS Thrusters from here on out. (‘R’ to enable RCS and ‘H’ to thrust forward). Aim for the target prograde (purple round symbol) and burn for 1-2 m/s. Aiming towards the target is also possible but means that the craft will pass behind the target necessitating more course corrections. As you get to a suitable distance, turn the face to the retrograde symbol again and burn enough to stop.
  3. Repeat steps 1-3 until you’re within 100m of the Snackship. Cut the relative speed to zero.

6. Docking (optional): Click the snacks docking port on the screen to change navball relative to port. (Press ‘Set as target’) Remember that the user can enable SAS (toggle ‘T’) to keep craft stable and switch it off when magnetic docking collars start to connect.
The goal is to approach slowly and directly to the docking port and allow the magnetic forces to make the final adjustment. If you are in danger to drift past, stop the craft completely and try again.

Note: RCS is controlled with: ‘H’ - Forward ‘N’ - Backwards ‘I’ - Down ‘J’ - Left ‘K’ - Up ‘L’-right. As this can be tricky the WASD-keys can be changed to direct RCS like in EVA-mode.

Some players prefer alternative controls. If you have ‘docking mode’ engaged (round symbol in left corner of the screen, below the green rocket) you can use WASD controls like in EVA mode. ‘Shift’ - up ‘Control’ - Down ‘A’ - Left ‘D’- Right ‘W’- up ‘S’ - Down.

Note: The lesson goal is to achieve a close encounter. While the docking is an interesting exercise it shouldn’t be mandatory for all students in the class because the need for actual gameplay skills and fair understanding of orbital mechanics. Usually it takes one or two hours to actually get the first docking right so teacher shouldn’t be alarmed if students don’t get it right at first.

For the lesson purposes the actual rendezvous maneuver and the time it actually takes is an important discovery.

Suggested mods:

Docking Port Alignment Indicator is a handy tool that allows docking with an intuitive gauge. While not mandatory it should be used for any project work that includes lots of high precision docking. The mods and addons are Installed in the /GameData folder.

READ:

Share & Discuss

Share & Discuss

  • How many close encounters did you get?
  • How many orbits did you have to wait before the craft would have passed the Snackship?

Tasks after Playing

  • What kind of shape are orbits in nature?
Show Notes

Orbits are ellipsoids in nature.

  • Which one moves faster, the object in higher orbit or lower orbit?
Show Notes

The object in higher orbit moves faster.

  • How do you define which way to burn? Prograde or Retrograde? How does burning to these directions change the craft’s speed and orbit?
Show Notes

Burning towards retrograde will slow the craft down while burning towards progrades will speed it up. Speeding up will rise the apoapsis of
the orbit or raise the periapsis when it’s done at the apoapsis. Retrograde burn in apoapsis would lower the periapsis and retrograde burn in periapsis would lower the apoapsis.

Teacher Resources

Evaluation

Throughout the lesson, the students are producing hypotheses and measurements. Evaluate their work as individuals but also as members of a group. What kind of role did they assume in their group? Did they share the workload evenly? Did they find the solution in an organized manner or by randomly trying everything? Did they come up with the plausible theory of why the solution worked?