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  1. Lightbulb
    #1
    The release of Orbiter 2005 may mean this needs to be updated. I'm looking into it now

    An introduction to Orbital mechanics.



    I am hoping this will help kindle some interest in the freeware gem Orbiter. I know that many people are put off by the steep (as in cliff like)learning curve - and it is steep. Orbital mechanics, while relatively simple, are not intuitve to us ground bound types. And as an added layer of confusion it is a subject littered with terms that are completely opaque to the uninitiated - does your eccentric periapsis have a mean anomaly?

    Essential stuff
    Orbiter - you'll need the base package and the texture package.
    OrbiterSound 2.5 - adds much needed sound to the sim, and a few other tweaks.
    Tutorial Scenario #1

    Not so essential stuff

    Hi-Res textures for Earth and the Moon.
    Re-EntryMFD - I find this useful when trying to get back to base...

    Lesson 1 : Into orbit.

    The first lessons will take place on (and hopefully around) the Moon - with no atmosphere to deal with and a fairly low mass, we'll be up and running in no time.
    • Start the tutorial scenario you should be greeted with a view of your first command, GPF-01 - a Shuttle Type A. Not a very sleek beast, is she?
    • Use the use the [arrow keys] in combination with the [control key] to slew your viewpoint around.
    • Once you've had a good look around jump into the cockpit [F1].
    • Now set up the MFD's. [left shift S], [right shift O] and [right shift P]. On the left MFD you now have "Surface MFD" showing, and on the right MFD you have "Orbit MFD".
    • Turn of the HUD and switch it to "Surface" mode - press [H] until it read "Surface" in the top left corner. This HUD mode will be familiar to anyone who had flown a modern jet sim.
    • OK - time for take off. Slowly increase the throttle for the hover thrusters to about 33kN - [NumPad 0] or use the levers on the panel. The shuttle should begin to lift off the ground.
    • Engage the "Horizon Level" autopilot [L] - this autopilot mode will keep the shuttle level.
    • Yaw the nose around to a heading of 090 - use [Numpad 1] and [NumPad 3]
    • Once the shuttle has climbed up to an altitude of 200m (listed both on SurfaceMFD and in the top left corner of the HUD) give the hover thrusters full power - hold down [NumPad 0]. Now the shuttle should leap skywards - things are starting to happen quite fast now.
    • At an altitude of 2000m, give the main thrusters full power (hold down [NumPad +], tap [Control] and then release [NumPad +] - this should lock the throttle at full power), and imediatly close down the hover thrusters (hold down [NumPad .]). The shuttle should continue upwards for quite a while yet, and reach an altitude of around 10km - what we need is a lot of horizontal speed, and that is what the main thrusters are doing right now.
    • Watch OrbitMFD closely. You should see the green curve representing your orbit slowly fatten and extend. Keep an eye on the number labled "Ecc" - this should be dropping towards zero. As soon as it starts to increase again, kill the main thrusters imediatly [NumPad *]
    • Double check that all the thrusters are at zero power. If all has gone to plan, you're now in a very low Lunar orbit - well done!
    • Use [NumPad 2, 4, 6, 8, 0 and .] to roll, pitch and yaw the shuttle around - don't worry about which way you're facing or if you're in a wild tumble...your orbit is stable as long as none of the thrusters are active.
    • The easiest way to stop any rotational motion is to use the "KillRot" autopilot - tap [NumPad 5], and the autopilot will fire the various attitude thrusters such that the shttle stops rotating.
    • Switch the HUD to "Orbit" mode ([H] to cycle modes).OK - time for a little bit of theory - sorry.


    There are four primary directions used to describe your craft's attitude while in orbit.



    Prograde - In the direction of travel
    Retrograde - Opposite to the direction of travel
    Orbit Normal - perpendicular to the orbital plane
    Anti-Normal - the mirror of orbit normal

    Almost all orbital manouvers use one or more of these vectors, and Orbiter provides an autopilot mode for each of them. Tap [ to engage "Prograde mode" - the caft will begin to rotate. Notice that the prograde vector is represented by a cross in a circle on the HUD. Tap ] to engage "Retrograde mode" - the retrograde vector is represented by a cross. These two directions are fairly intuitive.

    Now engage "Orbit Normal mode" - tap the ; key. The shuttle will rotate to 90 degrees away from it's direction of travel. The ' key will engage "Anti-Normal mode" and point you the other way. It can be very hard to figure out which is noraml and which is anti-normal - one way to remember is to use your right hand. The curl of the fingers represents the prograde direction, and the thumb extends to the orbit normal.

    And that's the end of the lesson - next time we'll change the shape of our orbit to suit our needs, and learn what some of those scary numbers mean on OrbitMFD.

  2. Post
    #2
    Great stuff there p00ns =] I always get started - but then slack off after going cross-eyed trying to read the standard info...

    Will download it again!

  3. Post
    #3
    When waiting for Doom 3 I used this to pass the time, Flying from Earth to Mars. Took quite a few goes before I mannaged to be able to stop at the desternation.

  4. Post
    #4
    Awesome, cheers!

    But do the ships come with LASERS?
    My ear canals are very sensitive. They're stainless steel. Took a bullet in Corregidor. Passed straight through.

  5. Post
    #5
    Originally posted by Disco Bacon
    But do the ships come with LASERS?
    *Slap!*

  6. Post
    #6
    You want a space duel, good sir?

    OK, back to the topic
    My ear canals are very sensitive. They're stainless steel. Took a bullet in Corregidor. Passed straight through.

  7. Post
    #7
    Lesson 2 : Climbing up and falling down

    • Point the shuttle prograde - either by steering manually or by using the prograde autopilot.
    • Fire the main thrusters until the number labelled "Ecc" reaches around 0.5 - then kill the engines.


    Right - time for some more theory.

    The graphic for your orbit should now be quite elongated - you can modify the display of OrbitMFD to make it easier to read by using [right shift M] repeatedly.

    When changing the shape of our orbit there are two points of interest...

    Periapsis : The point of closest approach to the body we are orbiting. It is marked with a round filled dot on OrbitMFD.
    Apoapsis : The point where our orbit takes us the furthest from the cental body - marked with a round empty dot.

    And there are two numbers listed on OrbitMFD directly related to these two points.
    • PeD : Periapsis Distance - the distance of the periapsis from the center of the central body in metres.
    • ApD : Apoapsis Distance - I bet you can't guess what this is...


    Trap for beginners : PeD and ApD are measured from the center of the body, not from it's surface. If you don't pay attention, you may create a new crater on the moon...If you want to know the radius of any planet (or moon), [control I] brings up a dialog with all the information you'll ever need. Keep a close eye on the gray circle that represents the central body on OrbitMFD as well.

    Two other number of interest are :
    • PeT : The time in seconds until you next reach the periapsis.
    • ApT : The number of seconds until apoapsis.


    Almost every manouver related to changing the shape of your orbit is performed at one of these two points. OK, let's get out of the classroom for a bit.

    • Coast around to the apoapsis - your current location is marked by the solid radial line of OrbitMFD. You may wish to accelerate time for a bit - use [T] to increase the rate, and [R] to decrease it. Don't exceed 100x.
    • Once at apoapsis, turn prograde and fire the main engines until the number labelled "Ecc" reaches zero (or as close as it will go) - watch the shape of your orbit as you do so.
    • Complete a few orbits, and enjoy the view...we're going back to the classroom soon.


    A summary of what we have done so far : A prograde burn at periapsis increases ApD, and a prograde burn at apoapsis increases PeD. So to climb from a low circular orbit to a high one, we need to make two burns - one to increase ApD to the required value, and one to bring PeD up to the same value.

    Three more numbers of interest on OrbitMFD:
    • Ecc : Eccentricity - descibes the shape of the orbit. A value of 0 indicates a perfectly circular orbit. A value from 0 to 1 indicates an elliptical orbit...higher values are more elliptical. A value greater than 1 means you are no longer in orbit - you aren't coming back.
    • T : Period - the time in seconds to complete one orbit.
    • Vel : Orbital Velocity in metres per second.


    Take a note of your current velocity and period - they should be around 970m/s and 34,000s (that's about 9.5 hours).

    Right - how do we get back down again? It should come as no surprise that we will be doing retrograde burns.
    • Coast around to your apoapsis - actually, we don't really need to wait if our orbit is very nearly circular...every point on our orbit is roughly the same distance from the Moon anyway.)
    • Turn retrograde, and fire the main engines until PeD is 1.770km - don't let it fall any lower...the surface of the Moon is not much lower than that.
    • Coast around to the (new) periapsis, and perform another retrograde burn so that your eccentricity falls to around zero - be careful.
    • Do a few orbits and enjoy the sense of speed...


    Take a look at your new velocity and period - 1656m/s and 6700s (around 1.9 hours).
    Even though to lower our orbit we used retrograde burns (which means we slowed down...), we are now going much faster...this result is very counter intuitive, but will be quite important later on.

    Here endth the lesson.

  8. Thumbs up
    #8
    keep it coming, this is really cool!

  9. Post
    #9
    OK - you asked for it....


    Lesson 3 : Inclined to change?

    • Open MapMFD in the left MFD with [right shift M] - this shows your current position over the surface of the Moon with a small "+" symbol, and the track of your orbit over the surface.


    Notice how our orbit carries us north and south of the equator - the orbital plane is inclined with the equatorial plane at about 45 degrees.



    The two points on our orbit where the two planes intersect are referred to as nodes. The ascending node (where our path goes from south of the equator to north) is marked on OrbitMFD with a filled box, and the descending node is marked with a empty box. The two nodes are connected with a dotted line.

    • Coast around to just before the descending node and engage the anti-normal autopilot.
    • Just before you pass the node, fire the main thrusters and watch your path on MapMFD.
    • Keep burning until the number labelled "Inc" is reduced to 20.


    Note that your eccentricity did not change much (if at all), while the path you describe through the sky has changed dramatically. This manouver is referred to as an orbital plane change and it is used to align your orbital plane with other objects (be it a planet, a spacecraft or a surface base).

    And just to be rigorous...
    • Inc : Inclination - your orbital plane's inclination with the equatorial plane.
    • LAN : Longtitude of Ascending Node - describes in which direction the orbital plane is inclined, and is measured against the background stars and not the surface of the body you are orbiting. I don't find LAN to be of much use in Orbiter, but have included it for the sake of completeness.


    There is another Shuttle Type A in orbit around the Moon, and we are going to try to rendezvous with it. To do so it is essential that you align your orbital plane with it, and there is an MFD for just that purpose - AlignMFD.

    • Open AlignMFD in the right hand MFD - to do so you'll have to bring up the menu first (OrbitMFD will not pass the command to open AlignMFD for some reason) - use [right shift F1].
    • Now you'll see a list of all the MFDs available - use [right shift A] to open AlignMFD.
    • Select a target with [right shift T] and enter the targets name - "SH-02".
    • Coast until it looks something like the picture below.




    This MFD lists both your and the target's inclination and LAN, and shows a stylised graphic of your orbit showing your postion (the green radial line) and the postion of the two nodes where your orbital plane intercects with the targets orbital plane. It is at these nodes where we'll do the plane change.

    Further down it list's the relative inclination between the two orbits (Rinc), and the rate of change of Rinc - it should be zero because you are not firing any thrusters.

    The next two lines list the position of the ascending and descending nodes.

    Tn is the time in seconds until you reach the next node - in this case the ascending node.
    The computer has calculated how long the burn will be, and lists this as Tth.

    • Coast until Tn is around 100 seconds and the engage the anti-normal autopilot.
    • When the box at the bottom of AlignMFD reads "Engage Thrust", give the main thrusters full power.
    • Watch Rate which should turn negative, and Rinc which will start to decrease.
    • Stop the burn when the box at the bottom reads "Kill Thrust" again.


    If all has gone well, Rinc should now be near zero - certainly less than 1.0. If it is not, you may wish to perform another burn at the next node.

    Now you know how to change your orbit's inclination, and how to align it with any other orbit. In summary, plane changes are performed at nodes - when passing a ascending node you burn down (anti-normal) and when you pass a descending node you burn up (normal).

    In the next tutorial we'll try to rendezvous with SH-02, but in the meantime you should:
    • Reopen OrbitMFD in the right hand MFD.
    • Target SH-02 (use [right shift T]) in OrbitMFD - this will display Sh-02's orbit in yellow as well as your own in green.
    • Adjust your orbit using prograde and retrograde burns so that your orbital radius matchs SH-02's. You may wish to quicksave before you try this...[control S].


    Good luck.

  10. Post
    #10
    Great work with this tutorial... Lots of fun... One small problem i had though was trying to do a very very small burn to help get my orbital planes aligned. It would've worked perfectly if i hadn't had time acceleration on.

    So my next question is how do i get out of polar orbit with no fuel?

  11. Post
    #11
    There should be a big red button under a glass cover... labelled "Do not press!"

  12. Post
    #12
    Na it should read "Smash glass and press button to summon 'Poons in emergency"


  13. Post
    #13
    Originally posted by RFI Band
    So my next question is how do i get out of polar orbit with no fuel?
    just wait a few hundred years, itll eventually degrade...
    What do you mean you dont play Aces High?

  14. Post
    #14
    Ah-ha...I thinks it's time for another "Trap for beginners"...

    Trap for Beginners #2 : Time Acceleration - This must be used with care, and there are several ways in which it can get you into trouble - RFI Band has found one of them...
    [i]
    • As a rule, never do any burn with more than 10x acceleration. That fuel tank will empty very fast...
    • Don't use high time accelerations when near planetry bodies...the simulation may not be able to track your path accuaratly and you'll end up deep in interplanetry space...Another danger is a temperory pause as Windoze pages the virtual memory (for example) - the sim sees a big time step, and moves you off into a wild orbit...
    • Never touch any controls at 10,000x accel - even the usually helpful and friendly "Killrot" function will go beserk and spin you ship like a blender on a sugar high...and it will never recover.


    These are all annoying "features" which may be "improved" in later versions of the sim...until then, be careful when compressing the space-time continuum.

    As for getting you out of your pickle...perhaps you could launch a rescue mission in SH-02. If you have FuelMFD or TetherMFD there are lots of possibilities...Or start throwing things out of the airlock in an effort to propel yourself out of trouble...

  15. Lightbulb
    #15
    Ships that pass in the Night

    Hopefully you have managed to shape your orbit to match that of Sh-02 - Our next task is to syncronise our orbit with SH-02, and fly in formation around the Moon.

    I would suggest that you download and fly my saved state for this mission - while there is no reason why your current state is unusable, for this tutorial it is easier if we all start in exactly the same state.

    We are now orbting around the Moon, chasing SH-02 almost exactly - but how do we catch up? The first intuitve impulse is to do a prograde burn - after all, we want to go faster so we catch up, right? But let's examine what happens if you do this. Recall from your flight so far that a prograde burn raises your orbit - giving you a longer path to fly, and slowing you down. If we do a prograde burn we'll actually fall further behind!

    What we need to do is take a short cut around the Moon - we need to fly a shorter path, and fly it quicker...

    ...a retrograde burn.

    • Open SyncMFD in the left hand MFD with [left shift Y]
    • Prepare for the upcoming manouver by engageing the retrograde autopilot.


    SyncMFD shows our orbit and that of Sh-02, the current postions of both craft (our position as a dull green radial, and SH-02's as a yellow radial). The bright green radial is the reference line and this can be switched between several locations.
    • Intersection 1 and 2 - the two points where the orbits cross.
    • Sh periapsis and Sh apoapsis - Our ship's periapsis and apoapsis.
    • Tg periapsis and Tg apoapsis - the target's periapsis and apoapsis.
    • Manual - a refernce line that can be set anywhere with the [shift ,] and [shift .] keys.

    On the right of the MFD are two lists of numbers - Sh ToR and Tg ToR (ToR stands for "Time on Reference"). This numbers show the time in seconds until each craft crosses the refernce line, for the next five orbits. To catch up with SH-02, we to to adjust our orbit so that we cross a reference line at (almost) exactly the same time. The two crossings which are currently the closest are highlighted in yellow.



    • Adjust the reference line to read "Tg periapsis" with [right shift M] - this reference will remain stable as we do our burns.
    • Just as our ship passes the reference line, begin a retrograde burn (use low power, or bursts of power), and cut the power when the ToR for the first orbit matches. Be wary of how low our periapsis gets (watch OrbitMFD)
    • Coast around the Moon...Our craft should start slowly gain on SH-02...
    • As you approach the refernce line again, prepare for a prograde burn.
    • At the reference line, adjust your orbit to match SH-02's one again with a prograde burn.


    Now, if all has gone well, we should be within 20km of SH-02 - at this distance, if we are careful and patient, just burning straigt at SH-02 will get us closer - for the nmost part, common sense reigns again.

    • Switch the HUD to docking mode [H] - this HUD mode highlights the target with a box, and shows the relative velocity markers.
    • Swing the nose around until you are facing SH-02 - the distance to the craft will be shown on the HUD under the box.
    • Now swing the nose to face the relative velocity vector - this is a cross in a circle. Under the marker is the relative velocity - if it is positve you are receeding, and if it is negative you are closing.
    • If the velocity is positive, a burn straight at the vector will cancel out your relative motion with SH-02. If, on the other hand, it is negative a retro burn will be needed - swing the nose around 180 degrees to find the required vector.
    • Perform a short burn to stop your relative motion.
    • Now you are free to close with SH-02 in your own time - point at SH-02 and do a very short burn...over time you may notice the velocity vector start to slide away from SH-02 - this is the subtle effect of your slightly differing orbital motion.
    • If you switch the control thrusters to translation mode [/], your can adjust your motion with the directional controls - toggle between the two modes as needed.
    • Your aim is to close to within 200m and hold position there - as you close it would be useful to slow down using retro rockets, but the shuttle doesn't have any thrusters dedicated to this. Instead the auxillary pods on the side of the shuttle can be rotated to serve as main, hover or retro thrusters - make sure they're in retro mode, and use them to slow down (you'll have to use the buttons and levers on the panel for this.
    • If you're feeling really adventurous, try to dock with SH-02...use [shift D] to bring up the docking MFD. Refer to the manual for detailed intructions on this MFD...if there is demand for it, I will cover this MFD in detail later.


    Next time, we return to base.

  16. Post
    #16
    There's no place like Home

    Time to head back to Brighton Beach, the spaceport on the Moon.

    • Open MapMFD with [left shift M]
    • Adjust the inclination of your orbital plane so that your flight path take you right over Brighton Beach.
    • Reduce your orbital radius to 1765km - this is just 20km above the surface, so be careful.
    • Open CommMFD with [right shift C] - use the mouse to set the first frequency to 116.30kHz, and the second to 132.20kHz.


    Now we are set up to begin our descent into Brighton Beach - we are on the right path and have reduced our hieght above the surface to as low as is comfortable. The nav/comm radio is tuned to the VOR frequency at Brighton Beach, and also to the ILS frequency for landing pad 1 - we'll need these later, and we'll be far to busy to tune them then...

    • Open Re-entryMFD with [left shift E], and select Brighton Beach with [left shift B].
    • Open LandingMFD with [right shift L], and make sure nav 1 is selected - use [right shift N].
    • Point the craft prograde, engage the horizon level autopilot [L], and switch the HUD to surface mode [H].




    The most important displays on Re-entryMFD are at the bottom - Decel req is the constant deceleration that is needed to bring you to a halt exactly over the targeted base. Decel act is your actual decereration created by your thrusters or by the atmosphere - It is your job to keep these two numbers equal.
    At the very bottom of the MFD is a display showing how off course your are - it is very sensitive - try to keep the ball centered, and you'll fly over the targeted base.
    Also shown Is your current altitude and ascent rate.

    • Coast onwards until the required desceleration is around 2.5m/s/s - use [control S] to quick save the mission. It may take a few attempts before you get a feel for this.
    • Engage the auxillary thrusters...change the power setting to match the required deceleration.
    • Use yaw to keep the ball centered.
    • As you begin to slow, your descent rate will increase - use hover thrusters to keep it under control
    • Brighton Beach will come into view at around 70km out - you may want to toggle the panel with [F8] to get a better view.
    • Keep a close eye on the required deceleration, and use the hover thrusters to place your veloctity vector just above Brighton Beach.
    • Once your ground speed is down to 300m/s (which would be about Mach 1 on Earth at sea level...) it's time to begin you final approach into Brighton Beach - switch Nav beacons with [right shift N].
    • Now it's time to see how good a virtual pilot you really are...slow the Shuttle down and bring it to a hover over pad 1, and touch down - this will be very testing - you'll need to toggle between translational and rotational control repeatedly, and keep adjusting the power to the hover thrusters. If you foul it up, don't worry too much - just load the quicksave, and try again.






    That's the final part of this tutorial - hopefully you found it helpful (and interesting). If there is demand for it, I will post a second tutorial based around a trip from the Earth to the Moon, and back again.

  17. Post
    #17
    OK - maybe not quite the last part...

    Here is a scenario created for you to try out you skills on...

    The hapless RFI_Band (sorry ) has become stranded in a wildly eccentric polar orbit after he pressed the clearly labled "Do not press this button" button...take off from Brighton Beach, and hold station with RFI_Band (this is as close to a "rescue" as a can cook up without some serious coding), and then return to base.

  18. Post
    #18
    I would like to point out that I have retried the simulation and got much further.
    But i got a new task for you to attempt windlepoons. How would you rescue a delta-gilder stuck in orbit around Mars with no Fuel (notice my signature).


    Don't ask how i got there.

  19. Post
    #19
    From the earth th the Moon (and back again...)

    Well, here it is - part two of the Orbiter tutorial. And you thought I'd have got bored and given up by now...;) Here's the scenario file

    I'll start this post with a list of what sets Earth apart from the Moon...
    • Mass - It's a lot larger, and so it'll take a lot more energy to overcome it's graviation.
    • Radius - well, it is a bit bigger around the middle, too - this won't effect things that much - but our minimum allowable periapsis distance will quite a bit larger now.
    • Atmosphere - this is the biggie. Before we can get anywhere near the speed needed for an orbit (which is already many times higher than we needed around the Moon), we have to climb above most of this horrible stuff...what self repecting planet would have an atmosphere this thick, anyway?
    • Rotational peroid - The Moon takes around 28 days to turn on it's axis once, so it was hardly noticable. The Earth zips around in 24 hours...and this will have a noticable effect on our orbital path with respect to the ground. And it is important that we launch to with the direction of rotation and not against it...it hard enough to build up the required speed as it is.


    OK - load up the scenario provided, and admire the sleek lines of your new ride - the Delta Glider. Think of it as a super space shuttle - it'll even take off from a runway.

    • Move your view to the cockpit, and slide the panel down so you can see the runway ( use the [arrow keys]) - make sure the HUD is in surface mode.
    • Give the main thrusters full power, and roll down the runway. At around 100m/s airspeed pull the nose up and lift off - raise the undercarridge with [G].
    • Perform a hard turn to a heading of 90 degrees - a launch to the East is with the direction of the Earth's rotation.
    • Pull hard up to an angle of 80 degrees and hold the nose there - and keep those engines running hard!
    • Adjust your pitch according to the chart below...
      Code:
      Alt(km) | Velocity (m/s) | Pitch (degrees)
      ------------------------------------
      0         |               |     80 
      40       |                |    70
      70       |                |    60
      100      |               |    40
      -------+----------------+-----------
               |      3000          | 30
               |      5000          | 20
               |      5500          | 10
               |      7000          | 0
      -------------------------------------
    • Watch your eccentricity - cut the engines when it reaches it's lowest value.



    Hopefully you are now in a stable orbit at around 150km altitude. I'll leave it up to you to use AlignMFD to align your orbit with the Moon's.

    Next time, we start on the four day voyage to the Moon.

  20. Post
    #20
    I'm sorry - it's time for some more theory.

    Hohmann Minimum Energy Transfer
    Unless you have a very advanced spacecraft (like, for instance, Captian Kirk...) it is important to be fuel efficient - and the most efficient route to any destination is the minimum energy transfer.

    The Hohmann transfer (I'll use this term from now on - it's shorter ;) ) is an orbit around the central body (in our case, Earth) with it's periapsis at the source (in our case, Low Earth Orbit or LEO) and the apoapsis at the targets orbit (The Moon, in our case). So (almost) all we need to do is do a prograde burn so that our apoapsis is lifted out to a distance of 386,000km. One other concern is small matter of timing - we have to time our burn so that we arrive at our apoapsis at the same time that the Moon reaches the same point in space.



    TansferMFD is designed for planning this manouver. It allows us to see the effect of any prograde burn at any time in the future (well, up to one orbit of the central body into the future...)

    • Open TransferMFD [shift X]
    • Ensure that the refernce is set to "Earth" - use the button marked "Ref". This is the central body.
    • Make sure the source is set to "GL-01" - it will read as "Self" in the MFD.
    • Set the target to "Moon"


    The MFD shows our orbit in bright green (it's a very small circle in the middle of the display), and our position with a green radial.
    The Moon's orbit is shown in yellow, and it's position with a yellow radial.

    • Turn on the planning mode by pressing the "HTO" button (or [shift X] again)
    • Use the "DV+" button (or [shift +]) to increase how much we will change our speed when we do the burn (or, increase our "delta V". "delta V" is the term used in all orbital mechanics to indicate the size of any burn). A dotted hypothetical orbit will begin to extend outwards from our orbit.
    • Keep increasing the delta V until the planned orbit reaches the Moon's orbit - the display will change.

    Now there are three more lines on the display. The grey radial shows where our closest approach to the Moons orbit is. The yellow dotted radial shows where the Moon will be at this time. The green dotted radial shows where we are planning to do our burn.
    • Adjust the location of our ejection burn with the "EJ+" and "EJ-" buttons (or [shift <] and [shift >] - the aim to to get the grey and the yellow dotted radials to coincide - you will probably have to adjust the delta V a bit to keep the orbits intercecting as you adjust the ejection date.






    In the list of numbers on the left of the display, ther are two of interest - Dv (the planned delta V in m/s) and DTe (the time in seconds until the ejection burn). Note that the DTe assumes we have an ideal spacecraft that can do any delta V instantaneously - our Delta Glider is far from ideal, and will take a significant amount of time to do this delta V, so we need to start the burn before the planned ejection time if we want to stay close to the planned trajectory - roughly one third of it will be before Te, and two thirds after Te.

    • Coast around until DTe reads 100 seconds (I calculated tis number using the TLAR system {TLAR - That Looks About Right} - also known as the "Buck Rodgers" method)
    • At Dte 100, do a full power prograde burn. Watch Dv - it will start to decrease.
    • When Dv reaches zero, cut the power.


    And now we're on our way to the Moon - if your planning was good, and Sir Isaac Newton got his equations right, we should bump into it in about four days time.


  21. Post
    #21
    Question and a problem, the question: is there a keyboard shortcut or a way to configure joystick key for the auxiliary engines on the shuttle-a (I have a hard time with the fine adjustment on the on screen throttle)? The problem is that the second and thrid (but not fourth) scenarios link to 404s.

  22. Smile
    #22
    First of all, welcome to our little corner of the internet.

    Regarding the auxillary thrusters on the Shuttle-A - no, there isn't a short cut key which is a bit of a nuisance - especially when you toggle the panel off with [F8].

    An alternative is to turn 180 degrees and use the main thrusters, but this can cause brain overload...

    And thankyou for pointing out the "404 errors" - I think I have rectified the problem now.

  23. Post
    #23
    Thanks, hi, and I'll try using mains next time, still trying to perfect my rendevous and docking. I did it!!! Completly sucessfull docking on second try (ran out of fuel chasing it the first time) and then landed on my second try (again ran out of fuel after missing three times, second try I overshot once but only barely and was able to land almost immiedietly). Thanks, i've been trying to get into this sim for awile and this has done it.

  24. Lightbulb
    #24
    Another alternative to using the main or auxillary thrusters is to use the RCS thrusters in translational mode - of course they are very weak, but if you need fine control they can't be beaten.

    use [keypad 9] and [keypad 6] for forward and reverse...

    I'm glad you're finding the tutorial useful

  25. Post
    #25
    Shooting for the Moon - the trip out, and the arrival.

    • Coast onwards until you have reached about halfway out to the Moon. Keep an eye on the value labled "G" at the bottom of OrbitMFD.


    The accuracy of the curves describing your course which are shown on OrbitMFD and TransferMFD relies on a big assumption - that the only thing affecting your orbit is the body about which you are orbiting (in this case Earth). Most of the time this is almost true - when you are in a low Earth orbit almost all the forces acting on your craft are due to Earth. But there are (very) small forces from the Moon, the Sun and even Jupiter all acting on your craft and adjusting your orbit away (very, very slightly) from the ideal course indicated in the MFD's. But as you move away from Earth, these extra forces become more and more important. The "G" value gives you a rough idea of how much influence the refence body has. Obviously the simple approximation will not hold very well if we fly near the Moon, which is what we intend to do.

    So how can we tell what is going to happen to our path through space? TransferMFD has a mode which calculates the path taking account all the forces acting on our craft.

    • Turn off the planning mode in TransferMFD [shift X]
    • Engage the numerical mode with [shift M]


    The numerical mode will step forward through time tracing out our course in bright yellow - if our course goes close enough to the Moon, the bright yellow curve will deviate wildly from the elliptical approximation we have been using.



    • Coast onwards until G is just less than 0.5 - at this point the orbit HUD will automatically change from a Earth centered frame of reference to a Sun centered frame of refernce.
    • Change the refernce body in OrbitMFD to the Sun [shift R] - note that it's G value is now higher than Earths.
    • Change OrbitMFD's refernce body to the Moon - note that the Moon has very little influence at the moment (G is around 0.01...), as so the path described in OrbitMFD is not very accurate at all. But as we get closer it will get more an more accurate.
    • Coast onwards until G is around 0.4...by this time the predicted orbit will be fairly accurate, so it is time to do a course correction.
    • Find out what your predicted periapsis distance is - if it is less than 1738 km, you're going to hit the Moon and need to move your course away from the Moon a bit. If it is greater than 3000 km you should probably get a bit closer to the Lunar surface for those extra style points (oh, and it will save a bit of fuel later...)
    • A simple way to adjust your course is to use your translational RCS thrusters. Align your craft prograde, and set the RCS to translational mode. Then use [keypad 1] and [keypad 3] to shift your course towards or away from the Moon...the longer you leave the correction burn, the more fuel it will use.




    • Coast onwards until you reach the periapsis - at this point do a retrograde burn to enter a lunar orbit - keep burning until your eccentricity falls to near zero.
    • Congratulations! You've arrived. And in one piece too, with any luck. Feel free to de-orbit and land at Brighton Beach.


    Next time, we try one of Orbiter's hardest challenges - returning to Earth from the Moon.

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