Trans-Mars injection & orbital plane

[NFunky]

Hey guys,

So I've been playing with RSS for a while now, but only just begun trying to send probes interplanetary, namely to Mars.  I've got a good launcher, a decent probe design, and what I think is a good window, but I'm running into unexpected dV issues.

I've read a lot of material about trans-Mars injection burns, and a good amount on the Earth-Mars transfer window and the minimum energy window, but nowhere can I find any material on plane change maneuvers.  When I attempt to get an intercept from LEO, I simply can't.  Mars is about a 1.5 degrees inclined, and the minimum energy transfer window will always have me crossing Mars' orbit about as far from the AN/DN as you can get.  This means I end up having to do an 800+ dV correction burn during the transfer, and that's about 115 days from the injection burn.  That means that I have to carry extra hypergolic fuel, since hydrolox or anything using LOX will have long boiled off by that point.  Carrying an extra 800-900 dV's worth of hypergolics (which are heavy) means I now need a much bigger launcher and that, in turn, means it's going to cost a LOT more.

Anyway, does anyone know how real world rockets deal with this?  Is there anything in particular I should be typing into google to find info on this subject?  How do you all go about solving this problem?

Edited March 29, 2016 by NFunky

[Vanamonde]

@NFunky, it seems you accidentally posted in the wrong place. Your post has been moved to its own thread in Gameplay Questions, since that seemed to be the best place for it. Feel free to edit the temporary title I have given it. 

[Streetwind]

13 hours ago, NFunky said:

Anyway, does anyone know how real world rockets deal with this?

A real-world rocket would minimize the plane change by launching directly into a low Earth orbit of the matching inclination.

What you have here is a bit like the "Minmus problem" in stock KSP. If you are not intercepting Minmus near its AN/DN, and you are coming from an equatorial orbit, then you will struggle to get an encounter. Even a large normal component during your transfer burn will not help you - that just twists your orbit around the axis drawn through your vessel and your orbit's primary focus. In this case, while making a transfer burn, this equals the axis drawn through the apses. So you're twisting the orbit without moving the apses themselves. You would however need to move your apoapsis up or down relative to Kerbin's equatorial plane... and there simply does not exist a maneuver that can do this, apart from an in-flight course correction.

The good thing about Minmus is that there are no transfer windows to worry about. You can wait in orbit until it nears its AN/DN so you can plot a direct transfer. With your Mars window, you don't have that luxury: you need to transfer now.

As a result of that, to avoid the in-flight plane change, you have only one option: you must match inclination with Mars' orbit before you perform your transfer burn. You can do this either by launching directly into a properly inclined orbit, or by performing the plane change in low Earth orbit. The latter may be very expensive. So the former is really the way to go. I'm sure there are tutorials for launching into inclined orbits around the forums somewhere. If you have MechJeb (or any other tool that can show you your current deviation from a target orbit's inclination), it becomes much easier.

Edited March 29, 2016 by Streetwind

[OhioBob]

There are basically two common ways to get to Mars...

 

In Figure 5.5, what you first do is launch from Earth into a zero-inclination ecliptic orbit.  Then, when you are 90 degrees from Mars, you perform a plane change.  As you said, however, this can be a fairly large burn - 800 m/s sounds reasonable.  The other thing you can do is to launch directly into an inclined orbit as shown in figure 5.6.  Figuring this our gets messy.  If you don't mind using mods, you might try Transfer Window Planner, which is the in-game version of this web tool.  I've only used Transfer Window Planner in the stock game, but its my understanding that it pulls its information from the game files and will work with RSS.