Wednesday, July 10, 2013

Oribter 2010 beta example scenario, Mission to Mars, and Mars flight handling discussion

Sort of inspirational post on the subject matter of Orbiter 2010 beta.
I couldn't immediately find a glider scenario starting on Mars.  I will hope someday in the future terrain simulators are even better on this point, but for now I leave it to my imagination in so far as realistic looking ground level terrain topography.  Basically I created a scenario modifying the Cape Canaveral SCN file using the following.  Save this as Mars Launch.SCN, or whatever you choose to name it.  Then pull up your Orbiter Launchpad and go.  Its much easier to start from the ground by the way on planets that feasibly have surface terrain, and where lift isn't problematic because of gravitational conditions.  Simply adding the desired planets name in the script placements below.  If you wanted to set up an orbiting start around a given celestial body, you'd need 3 tuple coordinates for the following, RPOS, RVEL, and AROT.  These are Relative Position, Relative Velocity, and Angle of Rotation respectively.
An example earth based rotation would be as follows for example.

  RPOS -1939370.68 176.20 -5993558.24
  RVEL 6666.863 2.877 -2135.915
  AROT -176.52 -83.95 -89.26

Keep in mind these examples I don't believe immediately translated readily from planet to planet.  In the obvious sense, if you relative velocity at the given relative position were applied to Mars, you'd likely find you ecliptic orbit getting out of hand quickly owing to differences in gravitation here.  If you weren't into the hard math in determining exactly orbital configurations another method, is simply creating the scenario, say from launch and then recording this to a data SCN file, where saved it could be used in application to another scenario of the same type.  Fortunately the hard math of this problem hadn't need deal with orbits of the eccentric not equal to zero, portion, but you'd find yourself likely working through polar coordinates and then translating this into relative cartesian for RPOS and RVEL.  While AROT would be computed using something like a three tuple Euler rotation coordinates (representing the ecliptic plane)...I'd check on this to be sure though.

Also there is a useful script function if you implement your Lua console for modifying simulation dates.  This is nice because it simulates all conditions leading to the given input simulation date.

The function for changing date is Julian date format or MJD,
I used thie for nice jumps of approximately 100G  travel distance, say going from Earth to Mars, which translated at my velocity to approximately 30 days travel time...I used with 'unlimted fuel' setting a high ejection velocity cutting much potential travel time relative to the slower more realistic likelihood of travel times expected otherwise.  I also took advantage of Mars relative close proximity to Earth in this case reducing travel times, fastforwarding MJD to a more optimal launch window.  The script function is:

oapi.set_simmjd(MJDvalue)

If you were the less patient type as I am...I frequently run sim accelerations and using the lua console function above to speed things up to work on the nitty gritty stuff.

If you plan the journey to Mars as I did initially starting from the ground.  For interplanetary transfers,  you'll need to make sure to set Reference to 'Sun', Source to 'Earth', and Target to 'Mars'.   Selecting HTO, make sure firstly to give yourself a big enough ejection burn setting DV+ or DV- high enough for the escape intercept...I run a burn at 11k above my existing orbiting velocity starting TAS initially at 18k.   Secondly, you'd want to make sure your projected intercept the gray line coincides the projected position of the planet at intercept.  The planet's projected track is represented by the dashed yellow line, so you need to overlap the gray line with the dashed yellow line using the EJ + or EJ - buttons.  Also you'll want to keep in mind the elapsed time to lauch DTe.  Google provides nice conversion computation tools if you hadn't wanted to hand calculate.  Keep in mind 2,630,000 seconds translates into 1 month.  On the typical starting date, DTe may run high depending on your scenarios MJD and relative planetary alignments.  Obviously, you should leave enough spare time to launch to both get into orbit, and have your glider aligned on the Mar's ecliptic plane relative the sun.  Much of this can be accomplished easily within the course of hours of equivalent sim time.  As a note I'd also mention that DTe does represent the time for the ejection burn, but you also need to make sure that you are accurately in line on the burn itself.  Thus in orbit you may not be exactly aligned for the burn for your given relative position orbiting Earth for the burn.  There is a small color coded line (of the same color as your reference celestial body that you are orbiting...for Earth 'green'), that you will need to pay attention to before performing the burn.  DTe letting lapse is fine for the course of time until having reached your position launch window isn't generally a problem in my opinion...especially given the minor burns for course correction that would generally be likely especially as you are more distant from your given destination.  Visually you can read your launch position as follows:  Once visually your relative position (small line rotating around your celestial reference source line) is in parallel to your dashed launch line (for Earth to Mars the big dashed green line), you preform the burn.  Unfortunately, since your source is set to Earth and not your ship, you will need to do the math computing the amount of ejection velocity, just add your DV to your existing TAS read.  Once the burn is complete optimally you may be in luck to see that the celestial body that you are travelling to is in line (as in this case Mars) is somewhere within your glider's frame of view.  At ejection, your crafts is moving tangential to projected travel path, generally you should be able to see that your relative position (e.g. from Earth to Mars, this would be your relative position to Earth at ejection point is tangentially aligned to your projected travel path) is initially nearly tangential (parallel) to the projected dashed line travel path.  If you see that your relative position from the source celestial body, appears to deviate within fairly short order from this projected path at the ejection point, you may want to perform right away a correction burn to bring your glider in line to this.  This will optimally spare significant course deviation and correction burns later on.   Next, I recommend setting up your Orbit MFD and setting Mars as your source, and make sure your ship is selected for elliptic comparisons.  If you run your Simulator at 100 or 1000 times, you should see a noticeable decline in Periapsis values.  Here indicating that you are roughly moving in the same direction towards in some manner your intended travel destination.  I also recommend if you run the simulator at 1000 times to normal that you avoid keeping your ship's prograde setting on (as this can cause for some odd reasons numeric instabilities in terms of your ship's rotational stability).  Its better to use Kill Rotation at normal sim speeds, and then let the ship drift at higher simulation speeds, and the toggle back to normal speed, repeat the prograde function to bring the ship back in line for the visuals, and then kill Rotation and repeat the process.  Also using the script function below in 5 or 10 day increments I've found helpful, since this exceeds well above 1000 times normal sim time.  Optimally if you hadn't wanted to maximize in terms of course corrections.  These need to be done with the ship rotated normal (perpendicular or 90 degrees) to pro grade motion.  While you can do course corrections for the visually, I've also found using the Orbit MFD a great tool, as you can examine the Periapsis values decline as you near closer to your destination.  Generally speaking, the problem of the approach is like a resolutions problem, if you were 50G outside of your destination, Periapsis may show in some fractional order of this but not highly resolved...in my case something like ten G or somewhere in the proximity), and then nearing closer with a correction burn I'd find this getting closer and closer to the given target.  I've generally found that this fractional proportionality isn't such that at 50G you could expect (at least for myself) to be something like 500k or 10k for periapsis, even if you appear to have your course visually aligned on the planetary body itself.  Thus generally once a maximum stability appears numerically on your Orbit MFD on the correction burn, you've likely generally maximized for your given relative position.  Generally I speed I speed up sim time here, and repeat the process getting nearer, usually working in 5 to 10 day increments.  The nice thing is that Orbit MFD unlike the strictly visual approach I believe accounts for relative planetary motions, so Periapsis values should remain stable over time if you flight motion is ideally in line...that is your elliptic is moving on the same plane and in the same way as the Celestial body itself on the elliptic...in the past, I've fought counter intuitively  in principle neither paying attention to course trajectories so much attempting to chase down planets which should seem like taking two steps backwards relative a step forward because of odd course trajectories.  Repeating this process, eventually you should be declining both in terms of periapsis values and orbital eccentricity.  Once established within reasonable distance of your planet on the order of 500k for periapsis, you should prep for the retrograde burn.  If you are 20k or less on your TAS for flight speed, ideally you can accomplish this at periapsis.  If your ship's velocity is higher then this, you will likely need to drop down at some point prior to reaching periapsis to slow your ship down, ideally within velocity ranging at no more then 20k.  The retrograde burn should be done to minimize orbital eccentricity close to zero.  In orbit, you can see how your relative orbit is relative to say Olympus base toggling to your map.  Here, you flight path ideally will intersect on ecliptic plane that ranges from equatorial to mid latitude regions.  If you need to correct your orbit, do so I believe reducing your Rinc value.  Preparing for Martian entry, is a matter of your ship's path moving within reasonable proximity to the base.  Your ship's projected distance bubble on the map provides excellent visual reference for when to accomplish the retrograde burn for re entry.  Once the bubble coincides visually with the base, you are likely in good range.  You will notice as you perform the burn your orbital path suddenly terminating with a square icon, this is your projected re entry point.  Ideally you'd want to accomplish re entry such to have this point coincide as close as possible to the base itself, while maintaining reasonable angle of attack (i.e., you don't want to skim the surface of Mars like a pebble on a pond, otherwise, you'll just skid on the atmosphere...likewise you don't want to drop like a rock...40 degree angle could be a good rough approximation...) although I've used this more from the visual standpoint at glance using the Orbital MFD measuring the angle of projected path relative planetary orbital altitude minimum).      

Generally speaking Mars is a fraction of the gravity found on Earth, and Martian atmosphere is noticeable different in so far as aerodynamics (being less dense at ground level, and similarly at higher altitudes). On you decent hold the craft level on the surface horizon.  I used a similar angle of attack similar to that of Earth's (40 degrees)...running TAS at 1600.  Velocity picked up to over 2000, and atmospheric frictional bleeds are not as noticeable relative to Earth's, coupled with poorer Earth relative aerodynamic handling.  Optimally it help having your entry lined up here.   Generally I've found enough aerodynamic stability running TAS at around 1k, and ranging 10 to 16k.  The more optimal method that I've found involved using a rotational pendulum swing of the craft (with Level Horizon and Hold Altitude engaged), and the using a combination of retrograde burns (relative to positive translation motion) to burn velocity, and similarly using this method to change the glider's direction.   If you are a noob to Martian flight , I recommend that traditional flight mechanics seemed much to poor in terms of handling otherwise, especially given the more rarefied atmosphere at the altitudes that I mentioned, and flying lower seemed either to slow or risky in terms of reasonable flight handling where I typically encountered more substantial gains and loses in altitude (for instance 10 k in altitude within 20 to 30 seconds...keeping in mind that Martian atmosphere typically runs at a high window of 22km...as opposed to Earth's 50 km where flight mechanics are more noticeable both in terms of wing lift and drag).   I'd recommend using the level horizon function, Hold Altitude, and keeping Kill Rotation on hand as needed.  Generally speaking if you land at Olympus Base, at 50 k < out I recommend stabilizing the craft outside of traditional flight lift physics...meaning use the Hold altitude function.  This forgoes the need for maintaining retrograde (or positive translation) motion of your craft for wing lift.  Secondly there aren't any bases on Mars having landing strips like that found on Earth.  Generally speaking your landing spot is a small launch pad.  Dialing in your NAV from frequencies.  You'd use your VOR/VTOL, and is in the case of station docking operate the craft using linear thrust.  Because linear thrust translation translates into approximately 1 m/s differentials, in terms of acceleration and de acceleration, its important to consider your relative velocity relative to the position to your launchpad.  I ran ultra slow, at a 20 m/s pace 20 km out, and then gradually dropped my relative velocity as needed in keeping to the rate at which I could feasible de accelerate the craft.  My final landing velocity were between 0 and -1 m/s.
Here's the Mars launch SCN file below.


BEGIN_DESC
Launch the delta-glider from Venus and see how it handles under extreme dynamic pressures.


Warning: You must disable the complex flight model for this scenario, otherwise your engines will not work under the enormous atmospheric pressure at the
Venus surface.
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 51982.9068993519
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-01
END_FOCUS

BEGIN_CAMERA
  TARGET GL-01
  MODE Cockpit
  FOV 60.00
END_CAMERA

BEGIN_HUD
  TYPE Surface
END_HUD

BEGIN_MFD Left
  TYPE Surface
END_MFD

BEGIN_MFD Right
  TYPE Orbit
  PROJ Ship
  REF Mars
END_MFD

BEGIN_SHIPS
ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
  ELEMENTS 6734919.2 0.00090 74.51300 169.03400 328.32497 560.71341 51982.906
  AROT 30.00 0.00 50.00
END
Mir
  STATUS Orbiting Earth
  ELEMENTS 6670999.7 0.00060 3.50000 360.00000 0.03290 132.19809 51982.906
  AROT 5.00 10.00 15.00
END
Luna-OB1:Wheel
  STATUS Orbiting Moon
  ELEMENTS 2237990.3 0.00001 89.99950 0.00059 317.37509 574.48880 51982.906
  AROT 0.00 0.00 88.25
  VROT 0 0 10
END
GL-01:DeltaGlider
  STATUS Landed Mars
  POS 15 -7
  HEADING 80
  PRPLEVEL 0:0.200 1:1.000
  NAVFREQ 402 94 0 0
  XPDR 0
  NOSECONE 0 0.0000
  GEAR 1 1.0000
  AIRLOCK 0 0.0000

END
END_SHIPS

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