Flight Question DG to Moon Ejection Burn and Midcourse Correction

[dykesc]

Hi

New fairly new to Orbiter. Not using with any mods and don't wish to use mods at this time. I am repeating the DG to Moon trip I watched in the tutorials.

No problems up to the ejection burn where I have a couple of questions. Does it matter what the orbital relationship of earth and moon are when you plan the burn with HTO? What is TLe? Does it need to be near 11 degrees for the planned burn? I am never anywhere close to 11 degrees.

The mid-course correction appears to be performed when earths gravitational influence is .20. Is that a good way to time when the mid-course correction occurs? I can't seem to get my lunar orbit to change to intersect with Brighton. I am using normal+ attitude but my orbital plane on the map doesn't budge when I burn. I made sure my orbital plane was displaying on the map. In the tutorial it doesn't take much DV to change the orbit to intersect with Brighton. I can't seem to get my orbital plane to move much at all.

I noticed my perilune is very very high at mid-course, something on the order of 24M. Is my lunar intercept just to poor?

Thanks in advance for any advice.

 

[boogabooga]

Are you using Transfer MFD?

Probably most people here use at least TransX, which isn't really an add-on since it comes with Orbiter.

 

[Tommy]

It's been years since I've done a Moon trip with TransferMFD, so I could be wrong but...

It sounds like you are out of plane with your target. That would explain the high perilune and the difficulty aligning with the base.

Before you launch, use Map to target the Moon, and set the display to show the orbital plane. Launch when the Moon's path goes over the Cape (as close as it gets, anyway). Once in orbit use AlignPlanesMFD to eliminate the RInc (should only be a couple degrees if you did the launch and ascent correctly.) You MUST be in plane to use TransferMFD.

Be patient and persistent, using TransferMFD is "doing it the hard way", but you will learn more by doing it this way.

 

[dykesc]

Thanks to both of you.

Boogaboogaboo I didn't realize TransX was included as an Orbiter module. Found it an activated it in modules. Went through one transfer with it. Its gonna be a winner. A lot of fine tuning and forward looking lunar intercept view is very nice.

Tommy I launched at low lunar orbit inclination just like the tutorial and used align planes MFD to reduce from 5.86 degrees to .04 degrees once in earth orbit. I don't know. All I can say is, at mid-course, my orbital plane in the map view looked very flat. No slight sinusoidal depiction like I saw in the DG to Moon tutorial.

By the way does TranX have a way to align an orbit at mid-course to intercept a space port like I'm attempting to do?

Again thanks to both of you.

 

[boogabooga]

Hi
No problems up to the ejection burn where I have a couple of questions. Does it matter what the orbital relationship of earth and moon are when you plan the burn with HTO? What is TLe? Does it need to be near 11 degrees for the planned burn? I am never anywhere close to 11 degrees.

1) Some people like to wait until the moon is an an equatorial node of Earth, but so long as your Earth parking orbit is in the plane of the moon, you should be fine.

2)TLe = True longitude at ejection. (I think). Anyway, the 11 deg thing does NOT matter and was just particular to the tutorial. TLe changes when you set
EJ+ or EJ- in the HTO. What DOES matter is that you set EJ+ and - to align the white line (where you intersect the moon's orbit) and the yellow dashed line (where the moon will be when you reach the moon's orbit). You want the moon to be there when you get there.

You eject from low Earth orbit when the green solid line (your current angular position) gets to the green dashed line.

The mid-course correction appears to be performed when earths gravitational influence is .20. Is that a good way to time when the mid-course correction occurs? I can't seem to get my lunar orbit to change to intersect with Brighton. I am using normal+ attitude but my orbital plane on the map doesn't budge when I burn. I made sure my orbital plane was displaying on the map. In the tutorial it doesn't take much DV to change the orbit to intersect with Brighton. I can't seem to get my orbital plane to move much at all.

I noticed my perilune is very very high at mid-course, something on the order of 24M. Is my lunar intercept just to poor?

Thanks in advance for any advice.

I would use the inverse criteria, doing the MCC when the moon's gravitational influence is dominant (turns green). Perhaps in the range of 0.5 to 0.6. You can burn "inward" to lower the perilune to about 100 km, or whatever you wish.


Look at the map MFD and set Brighton Beach as the target base. Notice that the trajectory of the track your spacecraft will take over the ground is like a sine wave on the map.


I would line up with Brighton Beach in two steps. When you are over the moon's equator, do a plane change in one direction or the other such that the maximum latitude that you will traverse is at least as high as Brighton Beach's latitude. This is like changing the amplitude of the sine wave such that the "peak" is at least as high as Brighton Beach. You are actually changing the inclination of your orbit. Then, when you get to the peak of the sine wave, you will reach the maximum latitude that your orbit will traverse. Do another plane change, and it will be like changing the phase shift of the sine wave. You are actually changing the longitude of the ascending node of your orbit. Rotate your entire orbit around until the ground track passes over Brighton Beach.

The slower you are traveling, the easier the plane change will be. You might consider first entering a high orbit over the moon, doing the plane changes, then lowering your orbit.

 

[Delta glider]

If you do orbit normal + when you are not in the Moon's SOI then the auto-pilot will point you orbit normal + to the most dominant body (most likely earth).




---------- Post added at 06:29 AM ---------- Previous post was at 06:28 AM ----------

And to OF.

 

[Tommy]

If you can, a screenshot taken once in orbit and before the ejection will be helpful. Have OrbitMFD open, as well as MapMfd. Map should be set to show orbital plane and target Moon.

The fact that your orbital path is "flat", not sinusoidal, tells me that you are not in plane with the Moon (or there is something strange in your install.) In map, your path and the Moon's path should match. The Moon has an equatorial inclination of about 22 degrees IIRC, yours should be the same, and you should also have the same LAn. Inc and LAn are both shown in OrbitMFD (Ref Earth, target Moon, and the mode should be Equ, not Ecl ). LAn will be constantly changing and should match.

 

[boogabooga]

The fact that your orbital path is "flat", not sinusoidal, tells me that you are not in plane with the Moon (or there is something strange in your install.) In map, your path and the Moon's path should match. The Moon has an equatorial inclination of about 22 degrees IIRC, yours should be the same, and you should also have the same LAn. Inc and LAn are both shown in OrbitMFD (Ref Earth, target Moon, and the mode should be Equ, not Ecl ). LAn will be constantly changing and should match.

No, I think he means his lunar orbit, not his Earth parking orbit. He said that he was aligned to 0.04 degrees, which should be fine.

First, make sure that in map MFD, the reference is set to Moon, not Earth. Second, the Map MFD referenced to Moon will only be effective if you are in the Moon's SOI. You can tell if you are in the Moon's SOI from Orbit MFD. Set the reference to Moon in Orbit MFD as well. When the gravitational influence is displayed in green and not red or yellow, you are in the Moon's SOI.

I suspect that you may have seen weird results in Map MFD because you were not in the Moon's SOI. Once you are, it should work fine. Lunar orbits have relatively low velocities, so plane changes should be fairly easy.

 

[Tommy]

Didn't occur to me that he may have been talking about his lunar orbital plane. If so, then you are correct about Map being funky when still in the Earth's SOI. Map relies on a two-body calculation and wouldn't consider the Earth's influence on the trajectory. The same would hold true for OrbitMFD, and you won't get an accurate result from the stock MFDs.

If the planes are aligned for the TLI, then you are probably right and the error is in the TEj. Even a minute or two of error will result in a big miss. It's also important to center the burn correctly. TransferMFD assumes the dV will be applied instantaneously, not over time. You need to start the burn when TEj = 1/2 of the estimated burn time.

 

[dykesc]

I repeated the flight last night and made sure the moon's gravitational influence was dominant before I burned to align my orbit with Brighton Beach. I also insured the moon was referenced in the HUD and MFD. Everything worked fine, but I used a lot of fuel getting my orbital path to intersect with Brighton.

Thanks again for your help.

 

[boogabooga]

Everything worked fine, but I used a lot of fuel getting my orbital path to intersect with Brighton.

Yes. But that's why the stock delta glider has such an enormous fuel load. The stock navigation tools are very basic.

There are MFDs that will make your transfer much more efficient, should you ever decide to use add-ons.

 

[Tommy]

Congratulations! You have done it "the hard way", and certainly learned a lot. I can offer a tip that may help with making the base alignment burn more efficiently.

As you noticed, the attitude autopilots automatically use the dominant body as a reference so they only help once you enter the Moon's SOI. The HUD can be changed. Open OrbitMFD and set the reference to Moon (even if you are still in the Earth's SOI) and press the HUD button on the MFD. Now you will have the HUD in orbit mode, and referenced to the Moon. You can use the HUD to manually orient your vessel to Lunar Normal or Lunar Anti-normal and make the base alignment plane change much sooner, while still well within the Earth's SOI.

 

[dykesc]

Congratulations! You have done it "the hard way", and certainly learned a lot. I can offer a tip that may help with making the base alignment burn more efficiently.

As you noticed, the attitude autopilots automatically use the dominant body as a reference so they only help once you enter the Moon's SOI. The HUD can be changed. Open OrbitMFD and set the reference to Moon (even if you are still in the Earth's SOI) and press the HUD button on the MFD. Now you will have the HUD in orbit mode, and referenced to the Moon. You can use the HUD to manually orient your vessel to Lunar Normal or Lunar Anti-normal and make the base alignment plane change much sooner, while still well within the Earth's SOI.

Thanks Tommy. I'll give it a try insuring OrbitMFD and the HUD are referenced to the moon.

 

[RonDVouz]

Hi

New fairly new to Orbiter. Not using with any mods and don't wish to use mods at this time. I am repeating the DG to Moon trip I watched in the tutorials.

No problems up to the ejection burn where I have a couple of questions. Does it matter what the orbital relationship of earth and moon are when you plan the burn with HTO? What is TLe? Does it need to be near 11 degrees for the planned burn? I am never anywhere close to 11 degrees.

The mid-course correction appears to be performed when earths gravitational influence is .20. Is that a good way to time when the mid-course correction occurs? I can't seem to get my lunar orbit to change to intersect with Brighton. I am using normal+ attitude but my orbital plane on the map doesn't budge when I burn. I made sure my orbital plane was displaying on the map. In the tutorial it doesn't take much DV to change the orbit to intersect with Brighton. I can't seem to get my orbital plane to move much at all.

I noticed my perilune is very very high at mid-course, something on the order of 24M. Is my lunar intercept just to poor?

Thanks in advance for any advice.

I haven't used Transfer MFD in a while, it's kind of obsolete in my opinion. I've been using LunarTransfer MFD, it's far simpler than TransX and features an autoburn that orients the spacecraft for critical burns. Sure it took some of the guesswork out of the transfer but in terms of running a sim with limited fuel having these precise computer controlled burns makes all the difference in the world.

You can download it here: http://users.kymp.net/p501474a/Orbiter/Orbiter.html

Also the Interplanetary MFD is a more complex version of LunarTransfer MFD but a ton easier to figure out than TransX. When you get good at working with Lunar MFD the Interplanetary MFD becomes an easier way of getting to Mars.