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Author Topic: I need some serious Interplanetary Help  (Read 38896 times)

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Offline GXE3

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23 December 2007, 22:08:14
I am tired of being restricted to flight on Earth. I want to travel to other planets. I know how to sync and dock with space
stations and I know how to go the moon....kind of. I also know how to do a proper and accurate reentry. But I don't know how
to get off of our Home Planet. I need some IMFD help. I've seen all of the tutorials on the IMFD website and none of them
have helped me.

Help.
Someone please!!


-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #1 - 24 December 2007, 09:42:39
IMFD is actually very simple to use... once you got the hang of it. For some reason this process took me a lot of
time :sage:

OK, where do we start? First, ignore for the moment offsets and 8-shaped Apollo-style Moon trajectories. These are
nice capabilities of IMFD but unrelated to the interplanetary travel. Also I don't know about you but I can learn very
little if I am presented with a procedure which I should follow without understanding why I am doing this or that. For
that reason I would rather try to explain what various parts of IMFD do.

IMFD consists of a number of programs, which help you solve different problems.

First, a very important principle: All programs, except Map work with a single body affecting your trajectory. This
means that you should select a proper REF (this is often done automatically by the IMFD). For travel from Earth
to another planet, Sun will be the reference. For a travel between two moons of Jupiter, Jupiter is the reference body.
In other words, the reference should be the body, which influences the trajectories of bodies between you are
travelling most. Next, the source (Src) and target (TGT) must be bodies orbiting the reference body. If
you are not orbiting the Sun independently of the Earth (e.g., if you are landed or in Earth's orbit), do not select
yourself ( "x" ) as a source. Select the Earth instead. If you want to travel from another planet to a moon of Jupiter,
do not select the moon as TGT as it travels around the Sun together with Jupiter. Select Jupiter.

As you are programming, you probably know what a tree data structure is. If we build a tree with the Sun as a root
node and parent-child relation is "child orbits around or is landed on parent", then planets will be children of the Sun
(sounds poetic, doesn't it ;)), and moons will be children of their respective planet. Your spaceship may be in a
different position in this tree - you may be orbiting the Sun far from any planet (child of the Sun node), or
orbiting/landed on a planet (child of the planet node), or orbiting/landed on a moon (child of the moon node). Now if
you want to travel to node B, find the nearest node up the tree (I am assuming the root is at the top), which is an
ancestor of both your ship and your target. This will be your reference. The two nodes immediately below it - the one
on the route from your ship up to the reference and the other on the route from the target up to the reference - will
be Src and TGT.

Now about IMFD programs.

* Surface Launch - helps you launch in the right direction (and in the right time if you want to wait and save fuel). For
trips from a planet to one of its moons, this is the first program you would use. For interplanetary travel or travel
between two moons ( "Rescue Mission on Io" ) first a course must be set up. If we remember the tree discussed
above, the launch program solves the problem how to change from "landed on A" mode to "orbiting A" mode.

* Map - all other IMFD programs assume the influence of a single body, so their solutions are approximate. The Map
program shows a much more precise estimation of your actual trajectory under the influence of multiple bodies. The
Map program itself does not calculate a manoeuvre, but it has a what-if mode (Plan), which allows you to see
how your trajectory will look after a manoeuvre performed by one of the other IMFD programs. Often it is convenient
to run the Map program on a separate MFD screen. This requires OpMode of the MFD where Map will be running
to be set to shared with the ID of the other MFD. For example, open IMFD on both left and right MFD screens of DGIV.
Press the upper-left MNU button if IMFD shows its boot screen. Then note the ID of each MFD in the upper-right
corner of the screen. They should be 0 for the left and 1 for the right MFD. Then in the left MFD select OpMode
and enter MFD ID to be 1. This will make both IMFD share their data.

* Orbit-Eject - this program is usually used for interplanetary travel (or travel between moons) as a next step after
Surface Launch. Again, first a course must be set up. Related to the tree, orbit-eject solves the problem how to
change your mode from "orbiting A" to "orbiting A's parent" (go up the tree).

* Course consists of the following sub-programs:
- Target Intercept - allows you to find a trajectory from body A to body B at any time (at the expense of fuel usage, if
the time of ejection is not appropriate or requested arrival time is too close to the ejection time). Related to the tree,
calculates a travel between two nodes with the same parent.
- Tangential Transfer - solves the same problem as Target Intercept, only in the most fuel-efficient way. Unlike the
standard Transfer MFD, delta-V is computed automatically, you only vary the time of ejection (TEj) to find a target
intercept.
- Planet Approach - helps you adjust your flyby trajectory (the periapsis altitude and the inclination of your orbit)
when you are approaching a planet.
- Orbit Insert - calculates a burn near the periapsis that will turn your flyby trajectory into a circular one (or elliptical
with the specified eccentricity, but this would be rarely needed). Related to the tree, orbit insert solves the problem
how to change your mode from "orbiting B's parent" to "orbiting B" (go down the tree).
- Delta Velocity - allows to perform a custom burn. The source must be some external data, for example calculated by
another MFD or historical data for a mission you are recreating in Orbiter.

* Base Approach - although a separate program, it is very similar to Course / Planet Approach. Allows you to target a
base on the surface of the reference body.

* Orbital - contains three simple tools:
- Circularize - allows to convert your current orbit to circular. The radius of the circular orbit will be close (not exactly
equal due to the burn taking some time) to your radius at the time of the burn.
- Velocity Match - same as DGIV "zero relative speed" autopilot.
- Find Target - helps you orient your ship to point at the specified target. Similar to using DGIV antenna and D9
display mode.

* Sling-shot - the weakest part of IMFD, unless I am missing something. Calculates a sling-shot based on your
current trajectory
relative to the reference body, which means it cannot be used for planning a trip. And when you
are already near, say, Jupiter, it may turn out no slingshot to the course you want to continue with is possible.

More later.



Post Edited ( 12-27-07 09:02 )


Offline GXE3

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Reply #2 - 24 December 2007, 20:15:54
Thank you for what you have written so far.
I wasn't expecting someone to actually reply by writing an essay. All the other tutorials just gave me instructions on what to do to get to the moon or to get to mars. I am one of those people who needs to know about the programs.
Thanks again for what you have written so far.



Post Edited ( 12-24-07 20:18 )

-GXE3
A 15 year old.
Long Live Java


Offline Tachyon

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Reply #3 - 25 December 2007, 17:21:35
Yes - Pirx - very well written... printing out now ....


My god - it's full of stars !

Offline Pirx

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Reply #4 - 26 December 2007, 16:21:49
Thank you for the feedback! It greatly helps me overcome my laziness and go on :)

OK, now let's see how we use IMFD in practice. You may start with DGIV "Mission Scenery / [hard long] Venus Weather" but instead of Venus we will be going to Mars. Not that it is any different, I just find a trip to Mars to be more exciting. Also there is a surface base and two moons you may visit.

So our initial configuration is
Code: [Select]
Sun
  Earth
    DGIV (landed on Earth)
  Mars

We want to achieve this
Code: [Select]
Sun
  Earth
  Mars
    DGIV (orbiting Mars)

Obviously, the major leg of the trip will be between Earth and Mars orbiting the Sun. So trip planning starts with

1. Course - planning (Target Intercept or Tangential Transfer).
TGT: Mars
REF: Sun
Src: Earth
In other words, we are solving the problem "how to modify the trajectory of an object sharing an orbit around the Sun with the Earth so that the new orbit intersects with Mars's orbit exactly when Mars is at the intersection".

1.1. Target Intercept
The Target Intercept program will always offer you a solution, though it is not guaranteed to be the one requiring optimal delta-V. Most of the time it isn't. You have two parameters to adjust: TEj - the time of ejection (departure from the Earth) and TIn - the time of Mars intercept. I would recommend to leave the other parameters at their default values. In particular, it seems to me there is a bug in IMFD 5.1 with all modes, except off-plane transfer (edit: the bug is fixed in IMFD 5.1g - thanks, Jarmo!).

You can accept the initial solution, which requires some unrealistic (for the current technology, but not for DGIV) delta-V, or find a more efficient solution by playing with TEj and TIn. For example, TEj = 0, and TIn = 54.15M (millions of seconds; if you use "Set", it is important "M" to be capital - mega and not lowercase - milli). A course with a lower initial velocity usually also results in slower fly-by of the target, which makes orbit insertion or aerobraking at the time of arrival easier. As for the almost 21 months required for the trip, well you have time acceleration, and about your crew - it's their job ;)

1.2. Tangential Transfer
Here you have a single value to adjust - TEj. You goal is to bring the white line (your position when your orbit touches Mars's orbit) near the dashed yellow line (Mars's position at the same time) and then to minimize Dis. Actually, both indicators - lines and Dis(tance) - show you the same thing, only I find the lines more useful during rough adjustment and Dis better for the final fine adjustment. For "Venus Weather" a good TEj is around 43.60M. If you choose tangential transfer, you will have to wait for the launch window. Convert TEj to days by dividing by 24 * 60 * 60 = 86400. Add the result to the current MJD and turn on maximum time acceleration. Meanwhile you may put DGIV in safe mode. Or use the scenario editor to change the date closer to the launch.

2. Surface Launch
REF: Earth
Op-mode: Course-program

Your first goal is to launch into orbit around the Earth. Before the launch the important parameters to watch are Hed (heading) and Time. Heading shows you into which direction you should launch if launching at the current moment. Time shows you after how many seconds the launch direction will be as close to east as possible. By launching eastwards you use Earth's rotation to your advantage.

After the launch watch Hed and EIn. Hed is the direction your ship should be oriented at. EIn is the "error" of your current orbital plane compared to the desired one. Early in the ascent pay attention only to Hed. At later stages try to make EIn close to zero by pointing your nose slightly to the left or right of the velocity indicator (the cross inside a circle).

Why the orbital plane matters? Because you want your orbit around the Earth to be in the same plane into which you will leave Earth's sphere of influence in the next step. You probably already know how "expensive" are significant changes of orbital plane inclination in low Earth orbit.

Optional: If you prefer to launch using the DGIV ascent auto-pilot, it is a good idea to create an ascent profile for achieving an orbit with lower altitude than PRO903SPEC.txt. For example, 150-175 km. This only matters for fuel economy:
* launching into a lower orbit requires less fuel
* the ejection burn is more efficient (Oberth effect) closer to the planet

Code: [Select]
Sun
  Earth
    DGIV (orbiting Earth)
  Mars

3. Orbit-Eject

REF: Earth

If you launched in the correct plane (low EIn), you are ready to leave the Earth. In "Orbit-Eject" program change "Higher Orbit" to "Course" and "Realtime" to "Off-axis". "Course" means that Orbit-Eject program should calculate ejection based on Earth-Mars course  set up in step 1. If you want, you can play with TEj to minimize dV. I would recommend auto-burn (AB) for the ejection. Whenever you cannot see the IMFD button I am referring to, press PG on the left of MFD, which changes all buttons on the right. Auto-burn will wait for the correct ejection time, orient the ship and perform the burn. You are now on a hyperbolic orbit leaving the Earth.

Code: [Select]
Sun
  Earth
  DGIV (orbiting Sun)
  Mars

4. Course - correction

TGT: Mars
REF: Sun
Src: Self ( "x" )

After the Ejection-Burn auto-pilot bids you a nice voyage, you can go back to course program and change Src to self ( "x" ). You are now an (mostly) independent body orbiting the Sun. Normally, the ejection burn takes care your course after you leave Earth's influence to be the one set up in step 1. That was the whole point of setting "Course" as data source for the Orbit-Eject program. Though usually course corrections will be needed:
* The ejection burn may not have been performed perfectly.
* The Course program only considers the Sun to influence your orbit. The Orbit-Eject program only considers the Earth. This is a good approximation, but still an approximation.

Before the first course correction wait for the influence of the Earth to diminish. For example, G below 0.01 in Orbit MFD. Course correction is performed from within the Course program using auto-burn (AB). I strongly recommend course corrections to be done in "realtime" mode. Now, why is it sufficient to just activate auto-burn to achieve a course correction? Let's remember that in step 1 we configured the course program to solve the problem "how to modify Earth's trajectory (which is also ours before we leave it) for a Mars intercept". Now we have changed Src to ourselves. So it becomes "how to modify my trajectory for a Mars intercept", which is exactly what a course correction is about. Those obsessed with fuel economy may try to fiddle with TIn - it may turn out a smaller dV will be needed for an intercept slightly earlier or later than originally planned.

Optional: It is often possible to reach your target with a single course correction by using the Map program.
Open Map on one MFD and Course on the other. On Map select REF: Sun, TGT: Mars, Cnt: r-Mars. What are these p-s and r-s anyway? Cnt: p-Mars (Mars periapsis) means "center the map at your position at the time your ship is closer to Mars", r-Mars (periapsis ref.) means "center the map at Mars's position at the time your ship is closer to it". If you use p-Mars, it will be Mars that will jump around your trajectory during the adjustment. I find it more intuitive your trajectory to move relative to Mars, hence the r-Mars recommendation.
Now instead of AB, use burn vector view (BV) of the Course program. Orient your ship so the cross-hair is in the center of the circles. Start the engines with a low thrust. Monitor PeD (periapsis Distance) on the Map. Stop the burn when it reaches its minimum and starts increasing. What is the idea? Course program generally suggests good direction of the burn but would sometimes overburn or underburn because only the influence of the Sun is taken into consideration. With the Map program you see a very accurate prediction of your actual trajectory influenced by multiple bodies. It is also possible as a next step to orient the ship pro-grade and start experimenting with translational thrusters and monitoring the effect on the Map. Why turn pro-grade before playing with thrusters? Because in time you will probably develop "intuiton" how each of them affects your trajectory and, of course, it is much easier to develop this intuition if the orientation of your ship is always the same relative to its orbit.

To be continued...



Post Edited ( 05-10-08 06:29 )


Offline GXE3

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Reply #5 - 26 December 2007, 21:31:02
This is the best tutorial I have read for the imfd. Please continue!!!

PS: when you finish writing, do you mind if I edit and compile this into a tutorial and put in on my website (credit will be
given to you of course!!!). If you don't want me to I understand, but this is great.


-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #6 - 27 December 2007, 07:56:28
Thank you for the kind words. It is far from perfect and I already had to correct something: It seems there is no
general rule whether you should turn left or right during Surface Launch to correct positive/negative EIn. More
probably Saturday.


Offline Tachyon

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Reply #7 - 28 December 2007, 17:58:25
:applause:  :wor:

I see this getting sticky'd


My god - it's full of stars !

Offline Pirx

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Reply #8 - 29 December 2007, 17:07:01
A couple more words about the course correction:

You cannot use Tangential Transfer directly for course adjustments. Also, the trajectory calculated by it requires a
plane change but no assist or auto-burn is available. Which means you can use this program in two ways:

1. Use Tangential Transfer to find a launch window, then enter TEj and TIn into the Target Intercept program.
2. Launch using Tangential Transfer and later switch to Target Intercept for a course correction.

In the second case you may make your first correction at the node between the source and the target planes, where a plane change burn is required anyway. The nodes are displayed by Tangential Transfer as on Orbit MFD - two squares, one filled and the other hollow, connected with a dashed line.

Regardless of which Course program you were using, check your orbit when you are 60%-70% (roughly) through with
your journey. Even if you have achieved a negative PeA (a collision course) on Map at the initial correction, it is still
a good idea to check. Map is much more accurate than the two-body programs, but not infinitely so.


5. Course / Planet Approach

REF: Mars (Important! Otherwise, you will be calculating an approach to the Sun, which we leave to the
conscience of the writers of Sunshine (2007) script)
TGT: Equator ( "l" )
Src: Self ( "x" )

You use Planet Approach to tune your flyby of Mars for aerobraking or orbit insertion. The farther you are from the
planet, the less dV needed but also the solution is less accurate. Remember that Planet Approach, like every program
except Map, only takes a single gravity source into account, in this case Mars. But both your and Mars's trajectories
are influenced by the Sun. The bigger the distance between you and your target are, the bigger is the difference in
the direction (mainly) and the amount of Sun's gravity. Some tutorials recommend to start the adjustments 2-3 times
the target's sphere of influence (SOI) away. You can turn on SOI on the Map ( "Soi" ). Alternatively, I have found out
that switching one MFD to Orbit with REF: Mars and watching for G to approach 0.10 also works well. Usually more
than one correction will be required as the solution of Planet Approach becomes more accurate as you get closer to
your destination.

With Planet Approach you adjust EqI - the equatorial inclination of your orbit and PeA - the periapsis altitude.

If you want to enter into an equatorial orbit, you want EqI as close to zero as possible. Depending on the orientation
of planet's axis of rotation relative to your trajectory, you may not be able to get too close to an equatorial orbit
during the approach - see Min EqI, Max EqI on the MFD.

Advanced: If EqI is too big, a long plane alignment burn will be required to correct it after the orbit insertion. It
makes sense in this case to adjust EqI not to be minimal but for one node of your orbit to be near the periapsis. Then during orbit insertion stop the burn while the orbit is still highly eccentric ellipse and perform the plane alignment at the opposite node where your orbital speed will be low.

On the other hand, if you plan to land at a base, the orbital inclination should be no less than the base's latitude
(sign of either does not matter) and, if possible, close to it. See also "Base Approach" below.

PeA should be close to the radius of the target orbit (slightly higher) for orbit insertion or inside the atmosphere for
aerobraking.


6. Base Approach

REF: Mars
TGT: A base, for example "Olympus"
Ref: Self ( "x" )

Base Approach program can be used instead of Planet Approach. It has two modes:


6.1. Base Approach for orbit insertion

This is very similar to Orbit Insert, only instead of EqI you specify
- A base (or geographic coordinates).
- The number of revolutions (Num) around the reference body completed before your ship's trajectory passes above the target base. An orbit insertion at altitude Alt is assumed.

Useful for orbit insertion using the engines or aerobraking with skipping (the first entry into the atmosphere -
aerocapture, the second one - landing).


6.2. Base Approach for direct reentry

Allows to specify the height of the reentry interface (Alt), the reentry angle (ReA), and the distance of the reentry point from the target, in degrees (Ant, to convert to a linear distance use: D = Ant * (Pi / 180) * PlanetRadius). These values depend on the planet. After they are set up, start increasing Hint and watch for the solutions found by IMFD. Some may require too high delta-V, in some cases there might be no solution. Then you should go for insertion into an orbit that will pass over the target (6.1).

Also be careful with direct reentries, DGIV is not very "comfortable" with high reentry speeds (compared to circular low orbit speed).

Needless to say, you use either Planet Approach or Base Approach, not both.


7. Course / Orbit Insert

REF: Mars

It is really easy. Just set the Eccentricity (see above why you may want an eccentric elliptical orbit)/Apoapsis/Major
Axis/Orbit Period to what you need and hit "AB". IMFD will perform a retro-burn starting slightly before the periapsis.
The periapsis will decrease compared to the one for the hyperbolic fly-by orbit due to the burn taking a finite time (as opposed to an instantaneous change in the velocity).

Code: [Select]
Sun
  Earth
  Mars
    DGIV (orbiting Mars)



Post Edited ( 12-31-07 06:27 )


Offline GXE3

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Reply #9 - 29 December 2007, 23:01:52
How would you travel from the Earth to the moon using IMFD?


-GXE3
A 15 year old.
Long Live Java


Offline ar81

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Reply #10 - 30 December 2007, 02:46:40
To go from Earth to moon, align orbits with moon, using Map MFD and Align planes MFD.
The use planar intercept.


Offline Pirx

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Reply #11 - 30 December 2007, 09:13:53
Quote
GXE3 wrote:
How would you travel from the Earth to the moon using IMFD?

We are coming to that :) Of course, what ar81 described also works but I see no IMFD in it.

Regarding your previous question: It is OK for me if these posts are collected as some sort of a "tutorial" though I still
find it more like an essay or a brain dump ;)

The "tree structure" concept I talked about might be too complex for some people. I would try another way:

Let's assign a rank to each body, with Sun having rank 1, planets - rank 2, moons - rank 3. The rank of a
spaceship/station is the rank of the body it is orbiting +1 or +2 if landed. Course / Target Intercept and Course
Tangential Transfer only work with bodies of equal rank. When intercepting a target body you enter its sphere of
gravity influence temporarily changing your rank. You use Course / Planet Approach to adjust your trajectory or
Course / Orbit Insert to make this temporary rank permanent. By performing Surface Launch and Orbit Eject you get a
higher rank (lower number).

Now how this relates to trips to the Moon: If you are landed you cannot use Course / Target Intercept as your rank
(4) differs from that of the Moon (3). In this case you start with

Surface Launch
OpMode: Lunar Off-plane
REF: Earth
TGT: Moon

Off-plane means that you will not be travelling in Moon's orbital plane as would be the case if you only use the
standard Orbiter MFDs. You must enter the expected arrival time in TIn. Realistic is 3-3.5d (days). Then Surface
Launch calculates the plane of your orbit from 3 points: the center of the Earth, the center of the Moon at the time
of the intercept
(that's why it is important to specify TIn) and your position. You can launch immediately in the
direction Hed or wait Time for a better (eastwards) launch direction. If you decide to wait, you must add
Time to TIn. In turn this may change Time requiring additional correction. It is not important to get
it perfect, just make sure TIn - Time leaves enough time for the trip (> 250k seconds if you plan to set up a free return
trajectory).

When you reach orbit you have the same rank as the Moon and now can switch to

Course/ Target Intercept
REF: Earth
TGT: Moon
Src: Self

In Target Intercept set TIn to what Surface Launch currently shows. Change "Realtime" to "Off-axis" and
(optional) find the best place to perform the burn using TEj. If you are just going to the Moon (same for visiting
Phobos or Deimos from Mars's surface), you are finished. Course correction and approach to the Moon work just as
for planets.

If you want an 8-shaped free return trajectory, there is something more to be done. Using MOD button switch
to the page containing "Offset Disabled" and change it to "Velocity Frame". The process is explained in details in other
guides so I will be brief. You aim for the Moon ;) but are intentionally missing by 5-15 thousands of kilometres. For a comparison, Moon's diameter is ~3500 km. You still get close enough so that the Moon starts bending your trajectory. A picture worth a thousand words:
http://orbit.m6.net/Forum/default.aspx?g=posts&m=156563#156563

You won't see the actual trajectory in Course, you must use another IMFD shared with the one running Course / Target Intercept. Open Map on the other IMFD:

REF: Earth
TGT: Moon
Cnt: r-Moon
Plan: enabled (shows you the hypothetical trajectory after Target Intercept burn is performed)
Int: enabled (shows the position of Moon's center and your ship at periapsis)

Then you start tuning on the first IMFD the amount of the offset (Rad, a good starting value is 10M meters) and its direction (Lon, Lat) and watch the effect on the second IMFD (Map).



Post Edited ( 12-31-07 06:33 )


Offline GXE3

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Reply #12 - 22 February 2008, 23:31:40
It has been a LONG time since I replied to this topic, but how would you travel from the moon to Earth. I've wacked my head
100 times trying to figure this out.


-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #13 - 23 February 2008, 07:56:08
It's actually quite simple ;)

Code: [Select]
Earth
  Moon
    Ship (landed)

you want this

Code: [Select]
Earth
  Moon
  Ship (landed)

So you must execute Surface Launch and Orbit Eject to become an independent body orbiting the Earth and then perform a reentry. The first slightly tricky question is what program to use as a data source for Surface Launch and Orbit Eject. It can be either Course / Planet Approach or, more typically, Base Approach.

REF: Earth
TGT: Target base for Base Approach or Equator ("l") for Planet Approach.
Src: Moon

So IMFD is solving the problem "how to bring an object sharing its trajectory with the Moon to the Earth". Or, if you prefer something more dramatic, "how to smash Florida with the Moon".

If the ship is capable of a direct reentry (in my opinion, DGIV and XRs are not), you can use the "Approach for: Reentry" mode. Otherwise, use "Orbit Insert" and specify altitude within the atmosphere (roughly 60k, depends on the ship class and reentry velocity). During the first contact with the atmosphere you will loose some speed but not enough to stay in it. Not even enough to reach a close to circular orbit. You can check that on AeroBrakeMFD, Graph/Map, the next projection after the map - see if the predicted velocity stays above the "circular" line.

Next you have two options - wait for the next entry into the atmosphere or use your engines to circularize your orbit. Of course, you can even perform Course / Orbit Insert without the initial aerobraking if you have enough fuel.



Post Edited ( 02-23-08 08:35 )


Offline Pirx

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Reply #14 - 23 February 2008, 08:32:33
Now, what is the problem with the direct reentry from the Moon or on return from an interplanetary trip? After a retro-burn from a LEO (typically -90 m/s) your trajectory still has a very similar curvature to that of the surface of the Earth, so you reach your (hypothetical) perigee after a long trip through the atmosphere and you can bleed off speed without getting too hot.

But with a highly eccentric or a hyperbolic orbit it is quite different. Your speed near the Earth is much higher than the speed of a low circular orbit. Consequently, the curvature of the trajectory is much lower (closer to a straight line) than the surface of the Earth. This means that you reach your perigee much more quickly, after a short trip through the atmosphere and after you pass it your altitude starts increasing and the dynamic pressure and deceleration are rapidly decreasing.

As an oversimplification we can imagine the trajectory to be a straight line and the path through the atmosphere to be a chord ( http://en.wikipedia.org/wiki/Image:Chord_in_mathematics.png ). If you want a longer path through the atmosphere, you need to bring the middle of the chord lower (closer to the center). This means that the maximum dynamic pressure and heat will increase and may become fatal if your perigee is too low.

What else can you do to stay longer in the atmosphere? You can direct the lift towards the surface by flying inverted (belly up). But this:
1. Is too risky to be realistic as at some point (after your speed becomes below circular) you want to restore your normal orientation and use the lift to slow your descent. Performing a 180 degree roll while keeping the AoA (in order not to get a hot wind blowing in the cockpit) can be a bit difficult.
2. Is not supported by the DGIV reentry autopilot because you need a bank angle close to 180 degrees.



Post Edited ( 02-23-08 08:34 )


Offline bealieu

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Reply #15 - 23 February 2008, 10:46:45
or - if your file wa in the form of a 'winzip' file usually it will include a help file(ie. readme or a pdf txt doc.


Offline James.Denholm

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Reply #16 - 23 February 2008, 13:23:09
Thank you so, so much Pirx...

*gets down on knees and worships a scrap of paper with four letters on it*


-------------------------------------
The etiquette of a cigarette, vinaigrette mixed with anisette, the silhouette of a clarinet, is but a stockinet in a landaulette.
http://en.wiktionary.org/wiki/Wiktionary:Rhymes

Offline GXE3

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Reply #17 - 24 February 2008, 06:33:47
THANKYOU!!!!
:wor:

I have learned so much from this topic. I have some concerns though.

1) When I used the IMFD Auto-burn program to go to mars, the "have a nice voyage" message appeared, but the engines didn't
stop burning. Am I supposed to cut off the engines or is it supposed to continue burning? I thought the Auto-burn program
would cut off the engines, so I kept burning & waiting for the engines to stop.....but they didn't. I had to go into the
scenario editor about 5 times to fill the propellant of the DGIV because of this. On the sixth time, I cut off the engines by
myself, but I didn't reach mars.  Did I do something wrong?  

2)Can you clarify on the "Target Intercept" and "Tangential Transfer" course programs. I understand that the target intercept
program provides you with a solution no matter what, but I'm having trouble understanding the Tangential Transfer. How do you
know when to launch?

3)Can you go to the sun? If you can, how?

Thanks again. You're a big help. I WANT THIS TOPIC STICKY'D.


-GXE3
A 15 year old.
Long Live Java


Offline Ursus

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Reply #18 - 24 February 2008, 11:20:08
Quote
GXE3 wrote:
THANKYOU!!!!
:wor:

I have learned so much from this topic. I have some concerns though.

1) When I used the IMFD Auto-burn program to go to mars, the "have a nice voyage" message appeared, but the engines
didn't
stop burning. Am I supposed to cut off the engines or is it supposed to continue burning? I thought the Auto-burn program
would cut off the engines, so I kept burning & waiting for the engines to stop.....but they didn't. I had to go into the
scenario editor about 5 times to fill the propellant of the DGIV because of this. On the sixth time, I cut off the engines by
myself, but I didn't reach mars.  Did I do something wrong?  


Hmm.... going from LEO to Mars in a DGIV, you shouldn't still be burning by the time you reach the edge of Earth's SOI,
unless you've planned a really course.

But... from playing with a vessel in which such inefficient transfers are common... If you leave the SOI while the auto-burn
is still going, you need to cut the engine, set the source in the course program to your vessel, then continue the burn using
the course program.


-----
Occasionally-visiting Grumpy Old Bear

Offline Pirx

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Reply #19 - 24 February 2008, 11:54:46
1) The initial burn should be done in the Orbit-Eject program with "Course" selected as data source (first parameter).
Also, every time you use Auto-Burn in a program supporting change of source, make sure Src is set to your ship ("x").
Such programs are: Course, BaseApproach, Sling-shot. For example, if you attempt a "course correction" in Target
Intercept with Earth still selected as Src and realtime mode, then IMFD will burn and burn and burn hoping that finally
that Earth (Src) will start moving towards Mars :badsmile:

2) Both Target Intercept and Tangential Transfer course programs have a parameter TEj (time of ejection). So you can
select a different departure time with either. But with Tangential Transfer this is the only parameter you can vary so
you are going to change it more often. TEj is the number of seconds since the current moment. Convert it to hours by
dividing by 3600, to days by dividing by 86400. Or just accelerate time and watch TEj decreasing.

3) Going to the Sun starting landed on the Earth is generally the same as returning from the Moon, only one level up
in the hierarchy. So you use Course / Planet Approach with
REF: Sun
TGT: Equator ("l") or Ecliptic ("e")
Src: Earth

Choose the desired flyby altitude and an inclination that minimizes dV. Then link Surface Launch and Orbit-Eject with
this Course. You will need around 20km/s delta-V from a LEO. "Venus Weather" DGIV scenario is suitable for such trip.
But it beats me why you would want to do this. You are not going to see any details on the surface of the Sun nor are
you going to be evaporated so it is hardly a realistic experience ;)


Offline GXE3

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Reply #20 - 24 February 2008, 19:23:21
Thankyou
:wor:

I'm printing out this whole topic right now. Then I'll compile it into a tutorial. I could see it now:

"Getting Started with the IMFD"
originally by Pirx
edited by GXE3

;)


-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #21 - 24 February 2008, 23:49:49
Oh well. It seems that when I was introducing Course / Planet Approach and BaseApproach programs for one reason
or another I did not mention that besides fly-bies they also work with established elliptical and circular orbits. In other
words, as long as you are under the predominant influence of the reference body, you can use these programs to
adjust your orbit:
* Planet Approach - the altitude of your periapsis and the orbital inclination
* BaseApproach - to reentry or pass over a selected base

Using BaseApproach for return from the Moon and Planet Approach for visiting the Sun are two examples when the
initial trajectory is close to circular (low eccentricity).


Offline GXE3

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Reply #22 - 25 February 2008, 00:02:46
You also didn't clarify if you should Autoburn after you set up the Base Approach and Planet Approach programs. I got really
confused at that part. You talk about setting up the Planet Approach program and then setting up the Base Approach program,
but then you just continue on to talk about orbit insertion. Do you Autoburn? Please clarify.

thanks again ;)

PS: I made my first trip to mars today and it's thanks to you. :)
      I didn't get to actually form a good orbit around mars though because of what I talked about in the above paragraph.



Post Edited ( 02-25-08 00:07 )

-GXE3
A 15 year old.
Long Live Java


Offline Pirx

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Reply #23 - 25 February 2008, 07:38:19
First you do AutoBurn (OK, several corrections) using the approach program (which one depends on whether you are
targeting a base or just a specific altitude/inclination). This adjusts your periapsis. Then near periapsis you perform
orbital insertion. How near? Just activate Orbit Insert AB when you are finished with approach course corrections. The
auto-pilot will select the right time.

So, you do AB both in the approach program and after that in Orbit Insert. They solve different problems:
* Approach - tune your periapsis. If you only use this program you will start moving away from your target once you
pass the periapsis
* Orbit Insert - changes your orbit to circular (or elliptical if you choose so; default is circular)


Offline GXE3

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Reply #24 - 12 March 2008, 03:32:46
This is the last thing I will bug you with. How do you use the "slingshot" program in the imfd. Also, what is the point of
the "Orbital" program.


-GXE3
A 15 year old.
Long Live Java