IMFD dV Oscilation and target gravity sources

[ivan_w]

All,

I have a couple of questions here..

I'm redoing the cassini mission (just for fun) and noted that :

- While en route between Jupiter and Saturn, the IMFD/Intercept dV (source=self) seems to oscilate on a ~14 day basis with a magnitude of +/- 2m/s with the baseline eventually increasing by a fraction of a m/s at every oscilation.. (even when I'm halfway between Jupiter and Saturn). I thought it could have been a result of being in High Warp - but it also occurs at low Warp.. I am wondering : Is this an IMFD artifact or some mechanics I am unaware of ?

- I could be wrong, but it seems to me that IMFD/Intercept only goes into n-body solving inside the SOI of the target (and 2 body - sun/ship outside) - disregarding any effect of the grav source(s) outside the SOI sphere ? Meaning I have to kill any spurious dV when entering the SOI of the target if I want to keep my schedule - which is fuel waste ! (only affects me when going between sources with large difference in mass).

- Finally : Is there a way to target with IMFD/Intercept something else than dead center of the target ? That way I could intentionally try to hit a little bit outside - for example if I already know where my slingshot keyhole is for my next leg of the trip.. Hmmm.. if only planet approach would allow me to tweak that part while still outside the influence of the body itself.. Or maybe play with Delta Velocity.. Any ideas ?

Thanks !

--Ivan

 

[Izack]

- Finally : Is there a way to target with IMFD/Intercept something else than dead center of the target ?

That is called an offset and IMFD can indeed do it; a quick search can help you from here on.

 

[ivan_w]

That's it !

That'll teach me not to look into the "mod" options for the IMFD option of the day !

Exactly what I was looking for !

Thanks a lot !

--Ivan

PS : Mind it, only been using IMFD for 3 days - so I may come up with more stupid questions !

 

[jarmonik]

While en route between Jupiter and Saturn, the IMFD/Intercept dV (source=self) seems to oscilate on a ~14 day basis with a magnitude of +/- 2m/s with the baseline eventually increasing by a fraction of a m/s at every oscilation.. (even when I'm halfway between Jupiter and Saturn). I thought it could have been a result of being in High Warp - but it also occurs at low Warp.. I am wondering : Is this an IMFD artifact or some mechanics I am unaware of ?

The Target Intercept program is using 2-body approximation that will cause it to drift a little. The oscillation is cause by Titan. It will change the velocity of the Saturn in 15.9day periods. The IMFD should be targeting the barycenter but for some reason it's not.

I could be wrong, but it seems to me that IMFD/Intercept only goes into n-body solving inside the SOI of the target (and 2 body - sun/ship outside) - disregarding any effect of the grav source(s) outside the SOI sphere ? Meaning I have to kill any spurious dV when entering the SOI of the target if I want to keep my schedule - which is fuel waste ! (only affects me when going between sources with large difference in mass).

First of all the Map program is the only program that is capable of doing n-body calculations.

a) When the ship is orbiting the Sun, every moon like Titan, Io, Europa... are approximated using 2-body calculations, however, all planets are using n-body calculations. Therefore, plotting a near intercept of Titan, while the ship is outside the SOI of the Saturn, won't work very well.

b) When the ship is with-in the SOI of a planet then everything is computed using n-body calculations.

 

[ivan_w]

I'm surprised a function to ask for a solution for an intercept is less sophisticated than a real time display !

Personally, I wouldn't mind waiting 10 secs.. 30 secs, .. Or whatever it takes - to get the right vectors to properly intercept a target (and then to AutoBurn it !)

Not that I mind the math - but handling quaternions are not something I do on a daily basis (nor is integrating field equations or whatnot - although all these might be the related). (PS : I am no mathematician) !

....

And I'm really surprised a small gravity source such as Titan (compared to the humongous saturn sitting next to it) can afflect me from such a distance ! (but then again - I have no idea how much saturn is influencing my path overall in a saturn to jupiter travel - especially when I'm sitting in an almost equilateral triangle between those 2 giants). - But then again - the periodicity seems coherent with a Titan interaction ! (~15 Days + Increasing as I approach Saturn).

Anyway - Thanks for the answer ! I'm going to ponder over this - and try to wrap this all around my head - and this might take me some time - possibly a lifetime !

--Ivan

 

[Tommy]

And I'm really surprised a small gravity source such as Titan (compared to the humongous saturn sitting next to it) can afflect me from such a distance !

The problem isn't that Titan is affecting YOUR orbit as much as it is that Titan is affecting Saturn's orbit. Since Target Intercept uses a two-body solution, it doesn't properly account for the variation in Saturn's orbital velocity caused by Titan.

Also, Titan is a bigger gravity source than you might expect. Titan as a very thick (over 1000 kilometer) and dense atmosphere - which adds a lot of mass (and therefore gravity) to Titan. This isn't as obious when on the surface, since a significant portion of the atmosphere's mass is above you.

Personally, I wouldn't mind waiting 10 secs.. 30 secs, .. Or whatever it takes - to get the right vectors to properly intercept a target (and then to AutoBurn it !)

Not that I mind the math - but handling quaternions are not something I do on a daily basis (nor is integrating field equations or whatnot - although all these might be the related).

Doing that kind of advanced math isn't actually required - just use Map programs "Plan" feature to show the predicted path (will show trajectory in Blue, not Green).

I still don't know when I'll find time to finish my part of it, but there is an update to the "IMFD FUll Manual/Playbacks" add-on. One of the updates covers using offsetting to perform MCC's, how to use Delta - Velocity, and how to target a moon directly (flight is KSC to Titan via Jupiter sling - landing a stock DG on Titan with over 1/3 fuel remaining) Hopefully it will help - if I can find enough time soon!

BTW, as for the Jupiter sling, fly-by altitude will likely be close to 90M.

 

[ivan_w]

The problem isn't that Titan is affecting YOUR orbit as much as it is that Titan is affecting Saturn's orbit. Since Target Intercept uses a two-body solution, it doesn't properly account for the variation in Saturn's orbital velocity caused by Titan.

Ah ! Believe it or not - it does make sense now ! Let me see if I got that right : I'm not influenced by Titan - Saturn is ! and since I'm looking at Saturn - and Saturn is wobbling a bit (because the barycenter of the Saturnian system isn't the geometric center of saturn) the dV fluctuates in IMFD because it assumes the saturn system barycenter to be at the center of Saturn.. (feel free to correct me if I got it wrong !)

(did a quick calculation.. Seems like saturn's geometric center is wobbling around a ~300 km elipse - in a pure titan/saturn system (1.2GM*(1.3E23/(1.3E23+5.9E26))) - and that would look consistent - magnitude wise - with what I am seeing as far as dV is concerned)

Now - another question for you guys !

Played a bit with the 'offset'.. But can't make heads or tails out of it. Read the documentation - but I'm a bit confused with the explanation.

Manual says : Use Vel Frame - but doesn't explain what it is or what ECL Sphere is.. Also, indicates RAD is the length of the vector... But it doesn't say it what unit ! (Although I am assuming Radius of the target). Finally, when I set an offset, I have a 'blinking' offset indicator - but no course correction indication to attain that offset.. So I'm a bit confused !

 

[Tommy]

Manual says : Use Vel Frame - but doesn't explain what it is or what ECL Sphere is..

This is talking about the reference frame for the offset. In Vel Frame, the "reference axis" is the target body's prograde vector - if Lat and Lon are both zero the offset will be aligned with the prograde vector and the Lat and Lon are at 90 degrees to that.

ECL-Sphere is a LVLH (local vertical level horizon) reverence frame. In this case, the "reference axis" is the axis between the target's center and the Sun's center. The "zero (both Lat and Lon) axis" is at 90 degrees to that. In other words, If Lon and Lat are both zero, the offset will be in the "forward" direction and tangent to the circumference of the target's current distance from the Sun.

ECL-Sphere is only useful for interplanetary trips - and NOT for lunar trips. Vel Frame will work any time. I personally like ECL-Sphere for interplanetary MCC's, but you may prefer Vel Frame.

Rad is in meters, using standard Orbiter notation (k, M, G, etc). Lon = 0 means prograde (target body's prograde, not the vessel's - may mean "forward" if using ECL-Sphere) and Lon = 90 means "outward", Lon = 180 means retrograde, Lon = -90 means "inward". Positive Lat is "normal" ("north").

Lon can be used to adjust the TIn - for best efficiency in interplanetary trips (MCC's) do the following:

1; Check your current PeT in Map (ensure you are using actual (green), not Plan (blue). Set the TIn in Target Intercept to match the actual PeT.

2; Adjust the Rad to attain desired PeA.

3; Now it begins to get tricky. Set Map to Plan (trajectory shown in blue), center "p-targetname", and set Int (intercept mode). You will want to adjust the Lon to make the PeT shown in Map match the TIn you set in Target Intercept (this means you won't be changing the vessels speed (which is inefficient during a MCC), only the direction (no dVf, just dVi and dVp). Make small adjustments - adjusting the Lon will affect the PeA.

EDIT: Forgot to mention important point. Before making any adjustments, set TEj (in Target Intercept) to allow time to make the adjustments. TEj = 600 will give you 10 minutes, and you can reset TEj if you find you need more time (a few minutes won't affect the results to any extent). Just make sure that TEj is greater than zero until you have made all adjustments.

You will find that adjusting the Lon in one direction will make the PeT and TIn closer, and raise or lower the PeA. IIRC, for outward trips, lowering the Lon (into negative numbers) will lower the PeT. You will have to adjust BOTH Rad and Lon to get the desired PeA AND make the PeT match the TIn. You'll just have to experiment - the actual changes will depend on whether you are going inward or outward, and whether your current PeA is too high, or too low (or even on the wrong side of the planet). Make small changes to the Lon (a few degrees at most), then correct the PeA with Rad adjustments. Alternate between adjusting Lon and Rad repeatedly.

4; Now you may need to adjust the Lat. Look for the node marker in Map (will be a blue box on the trajectory - may also be a blue dotted radial (this disappears when the node is within the target's Soi). Using small adjustments (start with one degree) bring the blue box to match the Pe. Adjusting the Lat may affect the PeA and PeT, so alternate adjusting all values in small increments - correcting PeA and Pet as you correct the Node placement.

In the end, you should be able to attain the desired PeA, have PeT (in Map) match the TIn (in Target Intercept), and have the Node on (or close to) the Pe. It will take patience - keep all adjustments small or you will likely "lose" the solution and have to start over.

Finally, when I set an offset, I have a 'blinking' offset indicator - but no course correction indication to attain that offset.

Not quite sure what you are expecting for a course correction indication. Target Intercept is now aiming directly at the offset point - just use AB like normal. Target Intercept will show the location (IIRC a dotted gray line and marker) of the offset - but only Map (in Plan mode) will accurately show you where you will actually end up. Remember - Target Intercept has limited accuracy - offsets may be much larger or smaller (or in different places) than you would expect. Trust Map.