You can't get there from here ???

[Tachyon]

Okay - need help ... and actually the kind of help that you all can give...  (i.e. not psychiatric)

I need to get my head wrapped around the physics of this navigation. I´m in a LO around Europa and want to get to
Io. In my head I first want to leave the gradational influence of Europa and the retro (with respect to Jupiter) to a
lower orbit around Jupiter to get to Io. AND time it right so that we end up in the same place at the same time.

I'm looking for a flip this switch and adjust this variable. No, that comes later. I want to know the "overview" if you
will.

Signed: Lost-in-Space !

[ar81]

This is some sort of "interplanetary travel".
I will explain for interplanetary.

Interplanetary travel has 3 stages.

1.Escape from source body gravitational sphere of influence SOI.
You can escape to a higher orbit (i.e. escaping from earth to mars or outer planets) or to a lower orbit (i.e. escaping
from earth towards mercury or venus).  Higher orbit will throw you outside, and lower orbit will throw you inside.

2.Then you fly around the central body (sun is the central body of Earth)
If you fly from earth to mars, you may need to make an orbit alignment (just like when you fly from ISS to MIR) as
soon as you reach the point where mars and earth orbit planes meet (node).  This is usually known as mid course
correction.

3.As you reach the node (between your trajectory and planet equator) you make the proper corrections to have the
proper orbit inclination.  This involves normal or antinormal burn, relative to target planet.

4.As you get close to target planet periapsis you start retro burn to circularize your orbit, which is called "orbit
insertion".

If you go from Earth to Mars you have:
Source planet Earth
Central body Sun
Target planet Mars

If you fly from one Jupiter moon to another, the central body is Jupiter.
The problem you may have is that moons may have a SOI smaller than the planet and the fact that multiple moons
and sun will be pulling your ship in different directions, making MFD calculations inaccurate and very changing in time.

Probably that could be causing you problems.

[Tachyon]

okay - I understand that and actually that is the part that I do get.

  Lets take the Jupiter system as an example: I'm on Europa and want to go Io. This will be a higher orbit transition to
a lower orbit (ref Jupiter) Leaving Europa - Do I leave Europa so that my trajectory is opposite of the orbital direction
of Europa so that I'm heading towards a "retro" trajectory (again ref Jupiter) ?  And if so - how does this work on
Transfer MFD. (Please don't bring up TransferX - baby steps here) Ref=Jupiter, Source=Europa, & Target=Io  - can this
even be planned while sitting on Europa or do I have to wait till I'm in orbit before the TransferMFD will properly
operate. And if that is so ... how do I know what direction to take off from Europa to set this whole thing up
correctly.  

..... Now... what was my name again??  

[Pirx]

OK, it sounds like you are trying to fly something similar (or identical) to 'Rescue mission on IO' scenario that comes
with DGIV. Yes, it is possible to do it using only standard Orbiter MFDs though it is exactly this mission that made me
start looking for more advanced navigational tools. It was not classified as 'hard' for nothing!

First you must decide whether you launch immediately or stay on Europa waiting for a launch window. I will focus on
the latter being IMHO more realistic.

Bring up Transfer MFD and select REF: Jupiter, SRC: Europa, TGT: Io. It is important to set the source to the
planet/moon you are currently landed on as you would be travelling with it for quite some time. Otherwise, a
trajectory around Jupiter based on your ship's current speed relative to it will be used by the MFD.

Next use HTO, DV+/-, EJ+/- to find a transfer trajectory to Io. For 'Rescue mission on Io' it turned out to be -1.666
km/s about 3/4 of Europa's orbit later. Wait for Europa to go near (~20ks) the time of the transfer manoeuvre and
launch into pro-grade equatorial orbit.

Start the ejection burn approximately when your ship (in pro-grade orientation) is facing the disc of Jupiter and cut off
your engines soon after you reach escape velocity. You should do this in order your trajectory to be close to parabolic.
Then the final direction you will be travelling will be perpendicular to your ship's orientation at the time of ejection.
More precisely, it will be in the direction from your ship at the time of ejection towards the centre of the body you
were orbiting. This means that you will loose some (small) delta-V relative to Jupiter.

Note: It is much more efficient to do the whole burn that will bring you to Io at the time of ejection. But
calculating it, especially the orbital plane and the moment of the burn is rather difficult. It is better and easier to learn
IMFD or TransX MFD for that purpose.

Now wait for the gravitational influence of Europa to wear off, which should not take too much time. Change in
Transfer MFD REF to self ('x') and plan the Io intercept using your current orbit, which will differ from Europa's. Perform
transfer and plane alignment burns at the appropriate times.

The approach to Io is also challenging. Normally, I used (in my pre-IMFD days) to plan approach waiting for the target
body to become close enough and changing the Orbit MFD to reference it. Then you try to change your PeA and Inc to
the desired ones and keep them there as your trajectory is influenced by other bodies. With Io this is not possible as
both the range at which its gravitational influence becomes predominant is too close to the surface and both your and
Io's trajectories have significant curvature under Jupiter's influence. This makes relative position and orientation
change constantly. My advise is to still use Orbit MFD and pay attention to the trends in PeA. Make corrections only if
it does not go in the desired direction and when you do, over-correct as you are going against the trend.

If you are playing 'Rescue mission on Io' you also want your inclination to be higher than 23 deg or lower than -23
deg and, if possible, close to either value. This will make your landing near GLMinning easier.

[Tachyon]

Pirx -

Yes - that mission is what sparked this but generally the astronavigation is what I'm hung up on. You gave me some
very good insight and raw information. I'm going to put this into practice and see what happens (where I end up )

You said "...and launch into pro-grade equatorial orbit"  pro-grade with respect to Jupiter or Europa?  As I understand
what I've just asked - if the answer is Jupiter then I can leave the Europa either with the rotation or against the
rotation of Europa. If the answer is Europa then it can only mean one thing .... in the direction of the rotation of
Europa. Is my understanding, correct ?

And you've confirmed another assumption of mine and that was the Orbit and transfer MFD's are dynamic in that the
answers they provide the pilot are always changing depending on the src and gravitational influences as well as
many other factors.

okay .. it's off to Europa I go ... and BTW ..the navigation I'm practicing here would be the same as if I were to say go
from the Earth's moon to the ISS.... correct

[ar81]

Going retro looks a bit inefficient and dangerous.
Inefficient because the braking would use lots of fuel.
Dangerous because hitting the ground would be dangerous.
It is better to reach it as if you were trying to reach ISS.

It is better to go prograde and stay below target planet orbit.
That would make you to reach that planet.

[Pirx]

Tachyon wrote:
You said "...and launch into pro-grade equatorial orbit"  pro-grade with respect to Jupiter or Europa?  As I understand
what I've just asked - if the answer is Jupiter then I can leave the Europa either with the rotation or against the
rotation of Europa. If the answer is Europa then it can only mean one thing .... in the direction of the rotation of
Europa. Is my understanding, correct ?

I meant relative to Europa. And the only purpose of this being to use the rotation of the moon to your advantage.
However, for Europa and Io this is mostly theoretical. Both moons of Jupiter are tidally locked, meaning that like our
Moon they always face Jupiter with the same hemisphere. This means they rotate very slow. Unless I am mistaken, by
launching pro-grade you gain a whopping 32 m/s for Europa and 74 m/s in case of Io

On the other hand you should eject retro-grade with respect to Jupiter if you are going to a moon with a lower orbit.
Which will be the case if you are in pro-grade orbit of Europa and perform ejection when facing Jupiter. Of course, this
only means that you will lose some speed, not that you  are going in a retro-Jupiter orbit.

Tachyon wrote:
okay .. it's off to Europa I go ... and BTW ..the navigation I'm practicing here would be the same as if I were to say go
from the Earth's moon to the ISS.... correct

In theory, yes. In practise there are some differences:
* The relative inclination of Moon's and ISS's orbits is much bigger. You would want to take care of this while far from
the Earth.
* With ISS you can afford to get to low Earth orbit and then synchronize with ISS in a couple of revolutions. With Io,
you would prefer to intercept it in less than one orbital period. Otherwise the crew stranded there may start getting
impatient
* Not to mention that with ISS you may use aerobreaking to save some fuel. Would not advise you to try the same
with Jupiter.