Flight Question IMFD to the moon, a little help please?

[SystemsLock]

So, I've orbited Earth a few dozen thousand times and I'm in need of some new scenery. Currently, I'm looking to take my XR-2 on a nice free return past the moon (and maybe a landing, but lets not get ahead of ourselves). I know there is a lunar transfer MFD, but I'd like to use IMFD as it is more applicable to non lunar transits (for when I go to Mars and such).

There are some nice tutorials out there but it seems none of them are really working for me. Some are very specific (only applicable to AMSO or NASSP) which I've tried to replicate in my XR-2 but the numbers didn't really come out. Others are too general, they explain the basic concepts (of which I feel I have a firm grounding on) yet don't lay out the specific steps to do them.

I'd like to get to the point where I can do an orbital maneuver to and from the Earth and Moon from any orbital situation (specificaly in the XR-2) and actually know what the heck the button's I'm pressing do. I'd also like the trip to be as realistic as possible, by which I mean minimum fuel consumption, a realistic flight plan, and no fudging the details at the last minute.

So far, I've launched from Earth in my XR-2 in such a way that I have a minimum orbital inclination to the moon. I've set up my left and right MFDs with IMFD (left is the transfer program, right is the map) and set my target to the moon. From here I'll list out some of the specific problem's I've had and you can answer them one by one or give me a more general answer.

1. I always set Realtime to Off-Axis. I have no idea what this does (something with burn timing?) but almost every tutorial has told me to do this for long burns. It alters my projected transfer a bit so some details on that would be nice.

Also, on occasion when I'm messing with the settings (TIn and TEj) IMFD will crash for a second and reset Off-Axis back to Realtime. If I try to change it back to Off-Axis without changing the settings to something different it will crash and reset it again. Not sure why this happens.

2. I'm really not sure what settings (if anything) I should be altering after I target the moon and before I burn. Some tutorials say I should adjust TIn and TEj for minimum fuel consumption, others say it gives me an optimal transit automatically. When I do mess with the settings I get some pretty weird projected orbits. For some reason I never get the nice gentle free return the videos have.

3. Is there any reason to do an Apollo like figure 8? I've tried to set it up that way but it never works out. An AMSO tutorial told me to set the Offset to Vel. Frame and set Rad to 8M (which I assume offets the target a bit in front of the moon). But no matter what I set TIn, TEj or Rad to it will never quite do it. It will get really close to a figure 8 but the moon always catches it and snaps it back. If I set super bizarre settings, guaranteeing the moon won't catch it (and wasting crazy amounts of fuel) it predicts I'll just go into crazy loops.

I'll start with these questions but I'm sure more will come up. Feel free to throw in some tips and tricks you have for the other orbital maneuvers as well and/or point me to the tutorials you found useful. Any help is appreciated. Thanks.

 

[Tommy]

I'll start with trying to answer the questions.

1. Off Axis calculates the burn ahead of time - but stops updating at TEj = 3 seconds. "Realtime" keeps on calculating during the burn. Realtime is a bit more accurate - but constantly "corrects" the burn - and often has to correct that correction - so it's less efficient. I tend to use "off axis" during planning and ejection for interplanetary flights - and "realtime" for MCC's and even TLI's.

2. I'll come back to this.

3. The "figure 8", or Free Return, is used as a safety measure. With a standard Hohman transfer, if the engines won't start when you get to the moon you are screwed - you will miss the moon insertion and end up drifting off to who-knows-where. With a Free Return, if the engines fail you end up back at Earth. This paid off during Apollo 13 - those guys would have died if they had used a Hohman trajectory.

A good flight starts with planning, before you even launch. I find it's easier to use GET instead of MJD for lunar flights. Also, we want to wait until the Moon's Orbital path is close to our launch site. We'll plan for two orbits before ejection - this gives us plenty of time to get into a good parking orbit plan the burn, and align for the ejection. Since we want to launch into the ejection plane (and not have to make a big, expensive plane change in orbit, we use Surface Launch with the "mode" set to "Lunar Off-Plane". To do this, we need to know when we will get to the Moon.

A Free Return takes about three days, and we'll add a bit for the launch and parking orbits. I use a 78 hour Time Of Flight, so add about 78 hours to the current GET and input that into the TIn in Surface Launch. Now I'll check the time to launch, and if it's more than an hour I'll add it to the TIn and check again. Once I'm satisfied, I'll make note of that TIn for later. Then launch according to the time and heading indicated by Surface Launch and establish a parking orbit of just over 200km altitude.

Once in Orbit, open Target Intercept. Set the offset mode to Velocity frame, Target = Moon, Source = "x" (sets source to self). I'll set the TIn to the value I used in Surface Launch (this keeps the alignment correct and avoids an expensive plane change), then I'll adjust TEj for lowest dV (should be around 3k - give or take a hundred or so). The dV will rise and fall as you increase it, I pick a time at least 5k away so I'll have an orbit to prepare.

No, in Map (make sure MFD's are shared), I'll set TGT = Moon, CNT = "p-moon", and ensure that Int and Plan are enabled. Now I'll add an offset in Target Intercept. 8M (be sure you use a capitol M) is a good starting point. Adjust the Rad (radius of offset) to get a Moon PeA of about 150k. You could fly this plan - but it's probably not a good Free Return (won't end up back at Earth in usable PeA)

To adjust the Earth return PeA, you'll have to have it selected in Map, and then make SMALL adjustments - alternating between adjusting the TIn and then the Rad to get the Moon PeA back to 150k - and then see where your Earth PeA is. It's a long, difficult process (one of the hardest things to do in IMFD)- and I recommend LTMFD for free returns.

With an XR-2, and it's high dV budget, I wouldn't worry much about the Earth return PeA - just get the Moon PeA right and go. Make an MCC about half way there, and then use Base Approach (or Planet Approach) when you are close to the Moon's SOI.

You will find a lot of good info about IMFD in this:
[ame="http://www.orbithangar.com/searchid.php?ID=4142"]IMFD Full Manual/Playbacks[/ame]

 

[SystemsLock]

Thanks for the detailed response. I never tried setting the TIn before takeoff. I'll follow your plan step by step tomorrow and see how I do. I'll keep you posted.

 

[Tommy]

Setting the TIn (in Surface Launch) is needed so that IMFD can find the proper plane to launch into.

 

[SystemsLock]

Doesn't the moon always orbit on the same plane, regardless of TIn? And because the moon is at a lower orbital inclination than KSC you're always going to be launching to 90 degrees. Luckily though it's only a little bit lower so once you've made it to orbit the difference is negligible. At least that's what I've read.

 

[Loru]

You can also launch from a base closer equator like Habana, SRC Polygon, Wideawake (Ascension Island) or Jarvis Space Center

 

[SystemsLock]

Thanks for the help. It took some time, but I've now successfully plotted two figure 8 trajectories and I plan to fly one. In retrospect it appears I was being too flippant in setting my TIn, giving me some bizarre trajectories. After locking down TIn to 3 days I found figure 8 trajectories reasonably easy to attain. Thanks again.

 

[Tommy]

Doesn't the moon always orbit on the same plane, regardless of TIn?

The same plane relative to the Sun, yes. Not the same plane in the equatorial frame.

And because the moon is at a lower orbital inclination than KSC you're always going to be launching to 90 degrees. Luckily though it's only a little bit lower so once you've made it to orbit the difference is negligible. At least that's what I've read.

if you leave from KSC, your heading will always be 90 degrees. However, depending on when you launch, you can have up to about 50 degrees RInc. To see what I'm talking about, open Orbiter, open MapMFD, change "groundtrack" to "Orbital Plane", target the Moon, and then accelerate time and watch the Moon's plane for a month.

 

[SystemsLock]

The same plane relative to the Sun, yes. Not the same plane in the equatorial frame.

if you leave from KSC, your heading will always be 90 degrees. However, depending on when you launch, you can have up to about 50 degrees RInc. To see what I'm talking about, open Orbiter, open MapMFD, change "groundtrack" to "Orbital Plane", target the Moon, and then accelerate time and watch the Moon's plane for a month.

Okay, I get that. The best period to launch would always be when the moon's orbit intersects (or gets close to intersecting) the launch point. However, the launch time suggested by Surface Launch doesn't correspond to the time of intersection on the map. If I ignore the suggested launch time and launch at the time of intersection I'll end up with a very low relative inclination. If the suggested launch time isn't telling me the point of the moon's orbit's intersection, I really have no idea what it's telling me.

 

[Tommy]

IMFD doesn't require you to in the Moon's plane, it calculates an Off Plane intercept. Therefore, the ejection plane can vary significantly from the Moons plane. IMFD is telling you when the best time is to launch into the ejection plane.

For instance, I've used IMFD to go from the ISS to the Moon, without making any plane change.

 

[SystemsLock]

Sure, nothing says you have to be aligned with moon's plane in an off-plane intercept. But isn't it most efficient when you are? Almost every tutorial I've seen either has me aligning with the moon's plane or launching into it. If that doesn't matter than what efficiency is surface launch supposedly calculating? Or to put it simply, what is the benefit of launching when surface launch tells me to?

 

[Tommy]

It is a bit more efficient if you are "in plane" with the Moon - the encounter velocity will be lower resulting in a less costly orbit insert burn. However, the difference is fairly low - around 100 m/s for a RInc of 45 degrees if done right.

Surface launch is responsible for getting you into an orbit that is aligned with the ejection plane. The timing it gives will get you into that plane the most efficiently. The savings can be up to about 400 m/s (if you launch at 90 degrees rather than, say 0 degrees), but unless your launch heading is quite aways away from 90 degrees you won't notice much (a 60 degree launch heading will result in a difference well under 100 m/s dV .

In other words - to absolutely maximize efficiency you would plan the trip so that you could launch when the Moon's plane is closest to KSC, and the RInc between the transfer plane and the target's plane is minimal. You would also use a Hohman trajectory, not a free return.

 

[SystemsLock]

Alright, I think I've got it now. Thanks for your help.