# QuestionIMFD and TransX differences?

#### cr1

##### Promoting vegetables in space
Now that I've circled the Earth round and round for a few thousand times already, launched, orbited, aligned planes, sync-ed orbits, docked, undocked, deorbited, reentered, etc, I decided to move to "the next step", which (for me) is to go to the Moon.

To do this, I heard that there are two MFDs available: IMFD and TransX. What are the differences between the two? What are the advantages and disadvantages for each of them?

Thanks

#### Jarvitä

##### New member
You can go to the moon using orbit MFD and sync MFD alone.

However, if you're going further out (or just want better precision), learning either IMFD or TransX, or both, is of course a logical course of action. The IMFD has a bit simpler user interface for simple transfers, but TransX is a lot more flexible, and also allows for setting up gravitational slingshots, while being somewhat more complex to learn.

#### flytandem

##### Tutorial Publisher
Tutorial Publisher
Using both is IMHO the best way to go.

But I'll give a shot at the differences. Since I decided early on to work with TransX my description of IMFD could be off a bit. I use IMFD mostly for the Map function.

Perhaps a good way to explain would to pretend we are out on a large open lake cruising along in a boat and we have an onboard mortar that we would fire shells in a trajectory to hit some other boat crossing the lake at some other angle and speed. To do this we in effect have to set the gun up to provide a sudden throwing velocity on some inclined angle and direction so that the shell lands exactly on the target boat at some future point in time and place.

Now with IMFD you would simply have to define which boat you wish to target, when you want the impact on the target to happen, and when you wish to fire the shot. It then automatically will set up the angles, and actually fire the shot for you. Very convenient. Not much unlike using Expedia or Orbitz to find and book an airline flight for you.

With TransX you have to manually set up by trial and error a future shot. You choose a hypothetical future firing time and what speed and direction you will make the shot. Then it shows what would happen and how much you will miss the target by and when the "miss" happens. From this you then keep changing either the time of the shot or the speed or direction (or any combination of these) and see what changes end up happening regarding the miss of the target.

I personally feel I get a better 3 dimensional feel of what is happening using TransX which is why I originally chose to learn TransX. In time however I realized that TransX most of the time isn't as precise as IMFD so I usually will use the Map function in IMFD to know where I am actually going. A nice marriage of the two works well. I start with a TransX setup of a burn and as I am approaching the end of the actual burn I use the Map in IMFD to see what my current trajectory is so as I end the burn I can end it a bit early or late as needed so that IMFD's Map info is showing the best result.

#### eveningsky339

##### Resident Orbiter Slave
Donator
Flytandem gave a wonderful description of the differences, but if you want something a little more brutally compressed, here it is.

IMFD is better for beginners because it calculates trajectories for you and comes with an autoburn function.

Transx is better for more intermediate orbiternauts because it requires all the info to be put in manually, but it is much more flexible.

I started with transx and then moved on to IMFD, but I find them both to be excellent navigation tools.

#### Tommy

##### Well-known member
TransX is better for multi-sling trajectories, but last time I checked didn't handle sunward slingshots - only outward.

IMFD also has tools for adjusting your arrival to have an orbit that passes over a specific base or enter the atmosphere at a specific place, a little better help for selecting launch times and headings, and it's Map program is extremely accurate and can account for multiple gravity sources - not just two.

Both definitely have their uses.

#### Andy44

##### owner: Oil Creek Astronautix
TransX is better for multi-sling trajectories, but last time I checked didn't handle sunward slingshots - only outward.

TransX handles slings from any direction just fine.

#### Tommy

##### Well-known member
TransX handles slings from any direction just fine.

O.K. Been a while since I used it. The reason I started using IMFD was because I wanted to get from Titan to Mars, by slinging around Saturn, and TransX couldn't do it. That was a while ago though, I know TransX has been updated a few times since then.

#### Andy44

##### owner: Oil Creek Astronautix
Well, since Titan orbits Saturn, you can't "sling" around Saturn, since it's the primary body of Titan. But I think I know what you mean, and TransX should be able to do it. 1st stage Titan, 2nd stage Saturn, 3rd stage Sun, 4th stage Mars.

#### silent_protagonist

##### Member
I like to mix transX and IMFD too. I use transX the plan my trip, then copy the burn data into IMFD's deltaV program, and then while watching the map program in the other MFD tweak the numbers to take advantage of map's higher accuracy. If your careful you can do single sling transfers, straight from ejection burn to insertion burn, without any course corrections at all (if you're doing multiple slings it gets a bit messier).

As for which one to tackle first, I'd recommend transX. IMFD is more convenient, but I think transX does a better job of teaching the fundamentals of interplanetary flight using patched conic approximations.

BTW, the version of transX included with Orbiter is a bit out-of-date. The latest version (available here) gives you an extra level of precision, as well as a few other bells and whistles.

#### Tommy

##### Well-known member
Well, since Titan orbits Saturn, you can't "sling" around Saturn, since it's the primary body of Titan.

Not precisely true. You won't gain velocity relative to a solar orbit, but you can change your velocity vector. It's not a traditional gravity assist type of slingshot, but it is still technically a slingshot.

In this specific instance, I was flying the "Mars Bound Veep" scenario, which I believe came with the DGIII (this was in '05 or thereabouts). When the scenario starts, Titan has entered the anti-sunward bound portion of its orbit. A direct flight from Titan to Mars would have necessitated eliminating the outward velocity imparted by Titans orbit. Using Saturn to re-direct that velocity inward saved substantial Delta-V. TransX didn't seem to consider that a slingshot, and while I could come close to the course I wanted it was not very efficient - only saving me about 20% of the fuel used compared to a direct transfer (fuel savings mostly due to Oberth effect)

IMFD was able to set up a slingshot course, but it was even further from what I wanted. It assumed that the only burn would be the Titan ejection burn, and only used Saturn for re-direction. It also planned a TEj months in the future. I simply used two OrbitMFD's (one referenced to Saturn, one Titan) to plan an ejection along a Saturn Retrograde direction, then once clear of Titan, used a Hohman transfer to place myself at the Saturn Pe called for in IMFD. When I got close I performed the TMI burn and saved fuel due to the Oberth effect. End result was a Mars arrival 2 months sooner than TransX could get me there, and an additional 25% fuel savings.

Granted, someone more knowelegeable may have been able to get better results from TransX. The version that I had at that time lacked much support for a retrograde slingshot, even the 3.0 manual says only that they are "problematic", and that future versions would hopefully have that functionality. That may have improved since Agentgonzo took over, but if so it doesn't appear to be documented.

TransX's main advantage is it's abilitiy to plan dozens of stages, allowing a much more complex flightplan than IMFD. A "Grand Tour", for instance, would be difficult if not impossible with IMFD. IMFD can't do everything TransX does, but what it can do it does easier and more accurately.

#### flytandem

##### Tutorial Publisher
Tutorial Publisher
Thanks Tommy. I learned a new word today. Oberth. Although I did know the existence of the effect, just not what it was called.

The last in my series of TransX tutorials covers accomplishing what you are asking about. In the case of the tutorial it starts at the moon, ejects, dropping into the gravity well of Earth and does a burn at Earth Pe to then go to Mars.
http://www.flytandem.com/orbiter/tutorials/Surrogate/index.htm

I'll admit that TransX isn't intentionally designed to be able to do this. But the tutorial shows that with just a little bit of visual eyeballing it actually works, and is fairly accurate at planning the two burns, namely ejecting from the moon and burning at the planet Pe.

#### Tommy

##### Well-known member
Hermann Oberth was one of the founding fathers of modern rocketry. He was a contemporary of Robert Goddard, and Werner Von Braun's teacher at the Technical University of Berlin. In the Star Trek universe, one class of star ship is the "Oberth" class.

The Oberth effect carries his name as a tribute, as far as I can tell he didn't "discover" it himself.

From Wikipedia:
As a vehicle falls towards periapsis in any orbit (closed or escape orbits) the velocity relative to the central body increases. Burning the engine prograde at periapsis increases the velocity by the same increment as at any other time, determined by the delta-v. However, since the vehicle's kinetic energy is related to the square of its velocity, this increase in velocity has a disproportionate effect on the vehicle's kinetic energy; leaving it with higher energy than if the burn were achieved at any other time.

One of the things TransX does better than IMFD is that it assumes burns will be made where they are most efficient, even if that means more than one burn at different times to aquire all the needed Delta-V. IMFD seems to assume the initial burn will provide all the needed Delta-V, and only course correction burns will be used after that.

#### Andy44

##### owner: Oil Creek Astronautix
I never knew it was called the Oberth Effect, either, but I did know that burning at periapsis takes adavantage fo the vis-viva equation.

#### T.Neo

##### SA 2010 Soccermaniac
Why do you need TransX/IMFD to go to the Moon? I've done it dozens of times with the stock Transfer MFD that comes with Orbiter. It needs very low, or no RInc between the source and target bodies, so can't allow off plane transfers.
I find it very easy to use, and there is a superb tutorial on Orbiterwiki.

#### Andy44

##### owner: Oil Creek Astronautix
Actually, you can do off-plane lunar transfers with the Transfer MFD, but it's not very precise. As for why use TransX to go to the Moon, Transfer MFD won't let you plan free-returns or precise encounters into a specific lunar orbit plane. Transfer is fine for simplestuff and joyriding in a DG, but if you want to fly more realistic missions you need better navigation tools.

#### T.Neo

##### SA 2010 Soccermaniac
Actually, you can do off-plane lunar transfers with the Transfer MFD, but it's not very precise. As for why use TransX to go to the Moon, Transfer MFD won't let you plan free-returns or precise encounters into a specific lunar orbit plane. Transfer is fine for simplestuff and joyriding in a DG, but if you want to fly more realistic missions you need better navigation tools.

I still need to get the hang of TransX.

I suggest using Transfer for your first lunar transfer, though.

#### Agra Barecasco

##### Skyswimmer
3D

It'll be great if the transX is upgraded with a 3D function..
Good for the intuition of the flight..
TransX gives us the freedom feeling in spaceflight,

But for me IMFD is more "complete" than transX...

#### Kveldulf

##### New member
But for me IMFD is more "complete" than transX...
I dont know about complete, but the reason I like IMFD more is that it feels smoother then TransX. For example, when I fish around for ejection and encounter dates with IMFD, the dV is updated smoothly, and if I overshoot, I can easily back up to a date. With my experience using TransX, if I overshoot a date, it doesnt back up to the date correctly.

It goes something like this (purely a hypothetical example mind you):
Encouter Date: Feb 10th
*single click to increase*

New Encounter Date: Feb 11th
Me: "Too much. Feb 10th it is"

*single click to decrease*
New Encouter Date: Feb 8th.:compbash2:

Dont get me wrong, I like TransX, and would probably use it over IMFD in planning, but its just too frustrating when my numbers aren't being modified by a set factor as far as I can see.

#### Quick_Nick

##### Passed the Turing Test
Donator
I dont know about complete, but the reason I like IMFD more is that it feels smoother then TransX. For example, when I fish around for ejection and encounter dates with IMFD, the dV is updated smoothly, and if I overshoot, I can easily back up to a date. With my experience using TransX, if I overshoot a date, it doesnt back up to the date correctly.

It goes something like this (purely a hypothetical example mind you):
Encouter Date: Feb 10th
*single click to increase*

New Encounter Date: Feb 11th
Me: "Too much. Feb 10th it is"

*single click to decrease*
New Encouter Date: Feb 8th.:compbash2:

Dont get me wrong, I like TransX, and would probably use it over IMFD in planning, but its just too frustrating when my numbers aren't being modified by a set factor as far as I can see.
TransX is usually accurate enough, especially if you download the 'new and improved' TransX here which allows even more accuracy. (I think that's the right version)
Also note that lower end computers cause a little bit more 'lag' and maybe even less accuracy than those with better specs.

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