IMFD P30 LVLH

[jedidia]

I think this is a more apropriate place to further elaborate my questions about the P30 LVLH burn mode than the thread of the manual.

There is something very weird about programming a course with this mode in the Delta-V program. At first, immediately after Orbit eject before setting up the long low-thrust burn, it seems to behave perfectly normal: Vf is prograde velocity, Vp is planar, Vo is outwards. No problem. Set it up, did the burn, and it was head on allthough I burned for a few weeks.

Now I just crossed the Orbit of Jupiter and am planning for the retro burn. But suddenly, the three Delta-V parameters seem to mean something different.

here's what the natural course looks like:



But now, when I diminish Vf, my course suddenly points to Nirvana, as if Vf wouldn't mean prograde velocity anymore.



Similiar results happen with the other 2 Prameters. None of them seems to align in a meaningful way to my ship anymore, so that planing the course is more or less just guesswork, which is a bit annoying. Can anyone give me some deeper insight to understand that burnmode better?

 

[Tommy]

I think I see two problems, but neither really has to do with the burn mode.

First, I think you forgot to account for the extended deccelleration when you planned the transfer. The original course prediction assumes you'll keep coasting right up to a normal, short insertion burn. By slowing down so soon, you've added weeks to the travel time. When you got to Saturn's orbit, it had already been there and gone. Since you were behind Saturn by 1.964 AU, next time plot a course for 1.967 or so head of Saturn to allow for this.

I suspect the second problem is that you're a bit confused by Map. It seems to be a simple program, no real variables to set, just an accessory to the other programs. Actually, it's the most complex of all the IMFD programs, and not easy to get the hang of. Map has only two dimensions to display information, but spaceflight occurs in more than two. Different projections help it display the first three dimensions, you wouldn't have gotten this far if you hadn't figured that out already. But spaceflight also takes place in the fourth dimension (time) which is even trickier to display on a 2D screen.

We tend to think of a map of any kind as a "top down" view, a "God's Eye View", yet we don't usualy think about where that eye is in space and time - it's just "up there" in a vague upward direction. Map program uses the location of the "eye", or Center (ie, CNT) as a way to reference the fourth dimension. It helps to think of your displayed course (actual or hypothetical) not as a line, but as a series of individual points, each representing the position of your ship at a particular point in time. In your case, the Center is Saturn, so eye is moving. The dots of your course represent your position relative to the moving viewpoint. In other words, you aren't really making that right turn, it just looks that way from Saturn.

If you set the CNT to "p-saturn" (will show up on display as Saturn - Pe), hit PG, and hit INT, you'll get intercept mode. It will show your course from a fixed point of view, and show two Saturns. One shows Saturn's current position, and the other shows Saturn's position at closest approach - assuming you don't change your velocity.

Anyway you look at it this is a tough manuever, and I wish you the best of luck. A tutorial would certainly be in order if you do!

 

[jedidia]

Thanks for your elaborations. They were not quite neccessary, but I see how the impression could arise.

The above screenshots were only to show that the parameters in P30 mode seem to act weird. The actual burn happend about three Ms later, and it succeeded achieving Orbit around Saturn, allthough a bit of a high one. Gave me the opportunity to notice that for a low-thrust vessel, aiming for a moon from a high orbit is easier than aiming for it from a low orbit, so that's ok. (I tried to reach titan, but my computer shut down because of overheating before reaching it, and I called it a day).

All in all, I figured the essentials of the Map MFD pretty well, I think. But all I need for a manouver like this is PeT, PeA and PeV. With the help of these the course can be set pretty comfortably with the Delta-V program.

But, and here comes my problem, the velocity frame mode seems to be vastly inprecise when your burn takes several weeks, and sometimes sends me of in the wrong direction alltogether. P30 mode on the other hands had a deviation of only about 2Gm after completing a 2.8Ms burn, which is good enough.
However, as stated, the parameters act weird. They don't allways seem to mean the same thing. In my initial burn after leaving the earths SOI, the parameters seemed to work correctly, allthough inverted, which isn't really a problem.
However, when I planned my insertion burn, suddenly Vf was... well, you see in the above screenshots what happened when I manipulated it. None of the three parameters refered to actual prograde direction of my ship, at least not relative to the sun. Prograde seemed to have been close to Vi, since that's where I ended up spending most delta-v, but it wasn't quite on the same vector. Somewhere betweenVi, Vp and Vf, but tweaking the parameters so I could actually adjust my course was simple try and error, just changing them and see what changed in the periapsis, which is a bit tedious.

So, if anyone can explain me how exactly the 3 vectors in P30 mode operate, I might even be able to know what I'm doing the next time, and not just fiddling three numbers until my PeA and PeV are satisfactory.

Oh yeah, since I'm allready asking questions here... There's not really a way to center the Map MFD on the target, is there? (i.e., on the panet you're aiming at, but in the position it will be when you get there. It is displayed of course, but centering on it would be kind of nice...)

 

[tblaxland]

None of the three parameters refered to actual prograde direction of my ship, at least not relative to the sun. Prograde seemed to have been close to Vi, since that's where I ended up spending most delta-v, but it wasn't quite on the same vector.

I haven't used P30 LVLH mode much but I'll give you my understanding of it anyway. The "LVLH" in the name reveals a lot of clues actually. What that means is that the vectors are in a local horizon/local vertical coordinate frame with respect to a reference body. The reference body can be set with the REF button. In your example above, that would be the Sun. IIRC, the alignment of the vectors is as follows:

1. Vp - aligned with the orbit normal. This direction can be found my taking the cross product of your relative position vector with the relative velocity vector. For a normal interplanetary Hohmann-ish type transfer this should be roughly perpendicular to the plane of the ecliptic.

2. Vi - aligned with the local vertical vector, ie, aligned with a line from your vessel to the reference body (also known as the relative position vector). This explains why a prograde/retrograde burn seemed to be mostly in this direction. At the time of your burn your velocity vector was pointing mostly away from the Sun.

3. Vf - lies on the intersection of the plane of your orbit with a plane perpendicular to the relative position vector (otherwise known as the local horizontal plane).

Vp, Vi, and Vf are thus aligned with the LVLH. IIRC, the LVLH coordinate frame used is the predicted LVLH at the time of the burn. Changing TEj will change the alignment of the coordinate system, depending on the trajectory of your vessel.

I hope that helps. Do some experimenting and see if it is consistent with my description above.

 

[jedidia]

Thanks a lot!

I understood Vi and Vp, and they make sense and seem to match my observations. However, I could not get my head around your description of Vf. Partly because my 3-dimensional thinking isn't the best, but partly also because I don't know exactly what you mean by "relative position vector". could you try explaining that one a bit more?

 

[tblaxland]

Sure. The relative position vector is a line from the reference body to the vessel (or vice versa - only the sign is different). The local horizontal plane is always perpendicular to that, ie, it is always perpendicular to vertical.

Another way to visualise it is to put the Map program projection (PRJ) into Self mode. That way Vp will be perpendicular to the screen, Vi will point toward the reference body and Vf will be perpendicular to Vi. Does that help?

 

[jedidia]

Sure. The relative position vector is a line from the reference body to the vessel

ah, silly me... you actually wrote that above, but I didn't get it. So, in explicit Vf is perpendicular to Vi, right?

 

[tblaxland]

So, in explicit Vf is perpendicular to Vi, right?

Yes. Vf, Vi, and Vp are all perpendicular to each other.

 

[jedidia]

great. Now that I know how things work, it will get easier. thank you.

 

[Tommy]

The above screenshots were only to show that the parameters in P30 mode seem to act weird

Map shows exactly the trajectory I expected given the burn plan in Delta V. That's what confused me, you seemed to expect something different.

Tblaxland's explanation is much better than my admittedly (in retrospect) cryptic explanation on the other thread. Note that in P30 mode a positive Vp is in an Anti-Normal direction, while in Velocity Frame a positive Vp is in the normal direction.

There's not really a way to center the Map MFD on the target, is there? (i.e., on the panet you're aiming at, but in the position it will be when you get there. It is displayed of course, but centering on it would be kind of nice...)

You can center map using the CNT button, then type in the name of your target. It's best to add a "p-" to the front, such as "p-titan". This centers on the periapsis and allows the use of Intercept mode, as I explained above.

 

[jedidia]

You can center map using the CNT button, then type in the name of your target. It's best to add a "p-" to the front, such as "p-titan". This centers on the periapsis and allows the use of Intercept mode, as I explained above.

ah, great. thanks a lot for your help.