Question IMFD terms

[jambooger]

I am so confused about some terms on the Imfd. It seems to change depending if Im within SOI or out. During a voyage I get very confused how to set these. Can someone please give me an easy to understand explanation.

Src, Ref, Tgt. Yes I know Source, Reference, and Target, but they are different depending what Im doing.

Thanks

 

[tblaxland]

Tried this? [ame="http://www.orbithangar.com/searchid.php?ID=4142"]IMFD Full Manual/Playbacks[/ame]

 

[Tommy]

One of the ways to describe this that makes it a bit easier to understand is to assign a "level" to objects, depending on what they orbit. You start with the Sun, which has a value of zero. Being in orbit around an object adds 1 to the value, so an object in solar orbit would have a value of 1 (0+1=1). Therefore, the Earth has a value of 1. If you are in orbit around Earth, add 1, so a vessel (or moon) would have a value of 2 (0+1+1). If you are orbiting the Moon, you have a value of 3, 0 (sun) + 1 (Earth) + 1 (Moon) + 1 (orbiting Moon).

So, for instance, if you are in Earth Orbit, and want to get to the Moon, you and your target have the same value (2). If you are in Earth orbit and want to go to Mars, you and your target have different values (you are a 2, and Mars is a 1).

In IMFD's Target Intercept, the Source and the Target must both have the same value, one level higher than the Reference, and both must be orbiting the same reference (you can't have a Source = Moon and Target = Phobos even though both have the same value - each is orbiting a different level 1 body).

Orbit Eject and Slingshot programs can be used to lower the value of your vessel. In both programs, the Reference will be the body you are orbiting (or slinging off of).

So, for our Moon trip, the Reference would be Earth (value 1), the the Source would be yourself (value of 2) and the Target would be Moon (value 2).

For the Mars trip, it's a bit more complex. We start out at a value of 2, and Mars has a value of 1. Since our Target's value is 1, we need a Reference of zero value, the Sun. We also need a source with the same value as the Target. In this case, our vessel has a value of 2, so it won't work. Earth, the planet we are orbiting, has a value of 1, so it is the Source. Once Target Intercept is set up, we use Orbit Eject (reference Earth, set to "plan") to lower the value of our ship by leaving the Earth's SOI, which leaves us in orbit around the Sun. Our vessel's value now matches our Target's value, so we change the Source (in Target Intercept) to "Self".

To get from orbiting the Moon (value of 3) to Mars, once again our Source will be Earth, since it has the same value as our Target, Mars. Reference is the Sun again, of course. Once our course is plotted in Target Intercept, we use Orbit Eject (reference Moon, set to "slingshot") to leave the Moon, lowering our value to 2. Then we can use Slingshot (reference Earth, set to "plan") to leave Earth's SOI and lower our value to 1. Then we can change the Source in Target Intercept to "Self".

Going from the Moon to Phobos you would use the same method as above, and once in a Mars orbit, Target Intercept would be set up with Reference = Mars (1), Source = Self (2), and Target = Phobos (2).

I recommend (shameless plug) IMFD Full Manual:

[ame="http://www.orbithangar.com/searchid.php?ID=4142"]IMFD Full Manual/Playbacks[/ame]

This contains a PDF manual with instructions on how to get from anywhere to anywhere, and includes charst's listing Reference, Source, and Target for most trips.

Mark and I have been working on an update to this tutorial. Unfortunately, neither of us has had much time to spend on it, so it's taking much longer than we would like. I'm trying very hard to finish the last section (KSC to Titan via Jupiter) this week. Even then, there will be some time to put everything together and check for errors, etc. I hope we can have it up at the Hanger sometime later next month.

Version 1 of this Manual is still usable and contains a wealth of information - including a simple process to find launch windows and set up Target Intercept, although I'd skip the sections on Target Offsetting and Slingshot - both contain errors. This will be corrected and expanded upon in version 2, and a new section on Delta Velocity program is included.

For my part of this update, four new flight recording/ manual sections will be included. One is a "fix" for the original (incorrect) slingshot tutorial. Second is setting up a lunar free return with Target Offsetting. Third is an Off-Plane transfer using Delta-Velocity program. This flight takes you from the ISS to the Moon - with no plane change and a direct insertion into a lunar orbit that passes over Brighton Beach. Last is the "Advanced Concepts" tutorial, which uses the techniques learned in the other sections to travel from KSC to a landing on Titan (via Jupiter Slingshot) in a stock DG, with over 1/3 fuel remaining at touchdown. Titan is targeted directly after the sling - no Saturn insertion is used.

 

[jambooger]

Excellent responses guys. I will download the IMFD full manuel right away. I had searched for such a thing but did not find it.

Thanks Tommy for the detailed summery. Ive printed it out to read over.

:tiphat:

 

[Wishbone]

Thanks Tommy for explanations yet let's just say the intuitiveness of IMFD rivals that of Colossus/Luminary. Quite unlike the brilliant interface of Lunar Transfer MFD.

 

[Moach]

well pretty much everything in spaceflight is of a hardly intuitive nature.... this IS, in fact, rocket science :lol:

and that's why it's s much FUN!

 

[Tommy]

let's just say the intuitiveness of IMFD rivals that of Colossus/Luminary. Quite unlike the brilliant interface of Lunar Transfer MFD.

I agree, but let's face it - it's much easier to design an interface for a MFD with a specific and limited function. LTMFD is only useful for trips from a planet to it's moon/moons and back. IMFD is more like the swiss army knife of nav tools. It's a bundled set of programs that are capable of interacting with each other.

I think the true beauty of IMFD is that it can be quite simple to use, or quite complex - depending on what you want. You can follow a fairly simple process to get from one place to another without actually understanding the mechanics involved. Yet, once you understand the mechanics, and get a feel for how IMFD works, it is extremely flexible and powerful, capable of complex, even unorthodox manuevers (I've used it to complete the Lua Challenge #4 - which requires some pretty non-standard navigation).

And IMFD's Map is unmatched by any other MFD available for accuracy and detail.


Also, while I am no expert when it comes to TransX, I believe the concept of "levels" applies to it as well - if in a slightly different way. For instance, TransX's "Escape Plan" serves the same purpose as IMFD's "Orbit Eject" to change a vessel's "level". For that matter , it even applies to TransferMFD if used for interplanetary flights (although it provides no tools to assist in changing levels.)

 

[Wishbone]

TransX is definitely worse than IMFD in that it IIRC uses conics (am I correct in assuming that IMFD does the hard work of RK integration?) Yet the interface, while a bit convoluted (so few buttons and so many details!) is way better than IMFD which requires me to set up two MFDs and do some non-trivial headology to make eject work.

 

[Tommy]

IIRC IMFD uses RK solvers in all programs. However, most IMFD programs use a fairly simple two body solution which isn't really significantly more accurate than conics. Map program, however, uses Encke-Fehlberg methods. Different types of integration are available, all are third or fourth order. Step size can also be adjusted, larger step sizes can sometimes increase overall accuracy (but will reduce the graphics quality of the trajectory displayed).

As for ease of use, I think it depends on what you are used to. I find TransX's variables difficult to navigate (one variable "hidden" under another, never sure which line I should be on to hit VAR ++ to get the one I need), but that's simply due to my lack of practice with that tool.

As for Orbit Eject, it's easiest if you don't open this program until you are in your final parking orbit. When you first open Orbit Eject and set it to "Plan", it places the ejection point at the optimal point - assuming nothing changes before ejection. ANY change to Target Intercept or change in the parking orbit will result in a change in the optimal ejection point, and you will need to adjust TEj in Orbit Eject (adjust to find lowest dV as shown by Orbit Eject).

IMHO, IMFD's biggest drawback is it's inability to plan sequential intercepts (ie, slinghots). There's no way to find a valid launch window, and the altitude of the swingby can't be calculated until you enter the SOI of the sling body (by which time any change in the approach altitude can be quite expensive.) For instance, in the EJS sling in the "Advanced Concept's" tutorial I used a known window (the 1976 Voyager window) and knew the correct swingby altitude only form prior experience.