IMFD Couple of IMFD questions

[IronRain]

Hi everyone!

I have a couple of question about IMFD. I know the basics but sometimes I'm doing things just because the tutorial said so and I don't even know why I do these things..

1.
When I create a flighplan to Mars, I setup the flightdata so oV is as low as possible. a way to decrease it more is to setup a 'source plane' in the flight plan. Although it decrease your total oV, you need to do a plane change maneuver (don't you?). Why is it better to use 'source plane' and to do a plane change instead of a 'Direct' flight plan?

2.
I want to get deeper into the solar system. Now, I (think to) know (how) to slingshot with IMFD but how can I calculate when to launch if I want to slingshot a couple of planets? I've already tried to search the answer a couple of times but I've never found an answer.

I hope someone can answer my question.
Thanks in advance!

IronRain

 

[dgatsoulis]

1. The value you need to look at, is the Total dv.
Usually when you set up a plan for a planet, if you choose your departure and arrival dates carefully (to get the lowest oV possible) then the "Off-plane" plan have about the same cost in dV, as the "Source plane" plan.
BUT, you have to ALWAYS remember to open another IMFD instance (linked) on the other panel, to see your Orbit-eject plan.
Here is an example:

With the off-plane plan of the upper part of the pic, you can see that it takes only 3576 m/s dv to reach Mars.
The same plan set up as a Source plane plan, will require a 4021 m/s burn. That's a big difference, eventhough both plans show about the same Total dV
~7640 m/s.

2. Unfortunately IMFD doesn't support multiple slingshot planning. (To my knowledge).
The tool of choice here (and a very powerful one), is TransX.

 

[IronRain]

So where you basically want to look at is the Orbit-Eject window?

 

[dgatsoulis]

So where you basically want to look at is the Orbit-Eject window?

Exactly. The reason that there is such a big difference with the two plans i gave you in the example above, is that my initial orbit is coplanar with the off-plane plan. (upper right MFD EIn value= 0.024).
When i chose the Source plane plan it changed to 5.815 (lower right panel).
This means a 5.8 degrees plane change during the burn, making it much more costly than the first one.

 

[Tommy]

As dgatsoulis pointed out, the biggest reason for the difference in the Tot dV was due to his orbital alignment being aligned with the ejection plane for the off-plane transfer.

It's best to plan the trip before you launch, and use the Total dV, not just the oV. It's actually best to have the second (linked) IMFD and check the dV in Orbit Eject, and add that to the estimated PlC shown in Map - this will often be more accurate than Target Intercept's prediction. That way you can launch into the plane of the most efficient transfer.

One of the reasons a Planar Transfer can be more efficient than an Off-Plane transfer is because a plane change is often needed. With the Off-Plane transfer the plane changes take place at the ejection (and can take advantage of existing orbital velocity - but this lowers the portion of your existing velocity that will be applied to the oV. IIRC, this is related to the cosine of the angle between the source plane and the transfer plane - with the higher angles being less efficient) where your velocity is high. A second plane change occurs during the Orbit Insert, which can slightly increase the dV required for the insert (this is usually very minor - the targets gravity is used to make the plane change.) A Planar Transfer often has the PlC in between the planets, where velocity is lower, making the PlC more efficient.

A Planar Transfer will not always be more efficient. Much depends on where the nodes between the source and target are relative to your transfer. If you eject at one node, and arrive at the other, the "cost" of the plane changes will be minimal and can be more efficient than a Planar transfer.

In short, (assuming you are launching into the correct plane for the type of transfer) an Off-Plane transfer will only be more efficient than a Planar transfer if: the transfer coincides with the nodes between the source and target plane, and/or the RInc is quite small.