Flight Question Moon to Mars with slingshot around Earth

[Estelyen]

Hi there, I'm new to the forums although I've been using Orbiter for several years now. I have so far always been able to work out how to do things myself, but now I'm totally stuck with a problem, that's why I'm posting here.

I did use the search function and I already read Marks IMFD full manual (which gave me the idea to attempt this flight in the first place), none came up with an answer to my problem:

I'm attempting a flight from a custom built base on moon to a custom built base on Mars using IMFD. I checked that there's a suitable launch window to Mars; when I attempt that same flight starting from Earth, everything works out fine.

Before lift-off, I use the Target Intercept program to set a course to Mars with a dV minimum. Then I use the slingshot program to calculate a suitable burn at an Earth PeA of about 200k using data from the course program. Yes, I made sure that SRC was set to Earth. Next, I readjust the target intercept program to the TEj from the Slingshot program, repeating those steps until both are reasonably well matched. I then time warp forward until TEj minus 250k (so I have time to reach Earth from the moon), readjust the slightly changed values in both course and slingshot program, then use the Surface launch program (with data from the slingshot program) to lift-off into a lunar orbit with EIn close to zero. Then comes my actual problem: Once I have a stable lunar orbit, I open up the Orbit Eject program and try to calculate my trans-earth ejection burn using data from the slingshot program. However, instead of directing me towards Earth, the orbit eject program always attempts to send me into a completely bogus solar orbit (using a dV of over 4000 :huh: ) that gets somewhat close to the orbit I would expect after the slingshot around earth, but getting nowhere near Mars either.

I'm completely baffled as to why the orbit eject program acts that way... Did I miss a step somewhere or do something wrong?

 

[Enjo]

Hello!

It's very interesting. I haven't tried it myself, but here's Flytandem's "Surrogate ship" method, utilizing TransX, also as video playbacks
(homepage: http://flytandem.com/orbiter/ )
Dgatsoulis made it possible to setup such transfers without using the surrogate ship, just with second TransX instance (but I don't really know how). You need the latest TransX version for it and activate TransX2 module:
[ame="http://orbithangar.com/searchid.php?ID=6393"]TransX 2013.12.13 Auto-Min[/ame]

 

[dgatsoulis]

I'm completely baffled as to why the orbit eject program acts that way... Did I miss a step somewhere or do something wrong?

Hi Estelyen, welcome to the forums. :hello:

Then I use the slingshot program to calculate a suitable burn at an Earth PeA of about 200k using data from the course program. Yes, I made sure that SRC was set to Earth.

Actually you need Reference: Earth and Source: Moon.

I then time warp forward until TEj minus 250k (so I have time to reach Earth from the moon)

I can see the reason why you think that needs to be done, but that's not what's shown on Mark&Tommy's tutorial nor is it the way you are supposed to do it.
You advance the time until you are ~ 1 hour before the Time to Eject, simply to give yourself time to get the spacecraft into a parking orbit.
Keep in mind that this is a single burn solution for a Moon→Mars journey.

I advise you to watch Mark&Tommy's tutorial one more time and try again. For more info read the rest of this post. Again, welcome to the forums.

--------------------------------

It's very interesting. I haven't tried it... {snip} ...possible to setup such transfers without using the surrogate ship, just with second TransX instance...{snip}

There are lots of ways to get from the Moon to Mars, that we can basically separate them into 2 major categories. Single burn solutions and Multi-Burn solutions. (The burns are considered to be major burns to eject from a body, not the minor MCCs and/or plane changes that are required).

In terms of ΔV efficiency, a two-burn solution is the best for a Moon to Mars journey, with an average total cost of ~1.3 km/s to go from lunar orbit to Mars.
(0.85 km/s to eject the Moon and drop to low Earth altitude, + 0.45 km/s to eject Earth and go to Mars).

This kind of solution can be setup only with TransX, as IMFD cannot handle multi-burn plans. Flytandem's tutorials are excellent but if you are like me and absolutely hate using surrogate ships, you can have a look at BrianJ's post.
An excellent step by step guide from back in '08, that does not require a surrogate ship.

The only problem with that guide is that it doesn't provide you with a way to setup your lunar launch into orbit.

For simplicity I started from a roughly equatorial Lunar orbit. The inclination is not crucial, however for large inclinations the LAN should be close to 90 deg away from the Earth-Moon line at the time of departure from the Moon, in order to make an efficient Moon-Earth transfer. Launch from the Moon surface or adjust your
orbit with this in mind.

The LAN 90° away from the Earth-Moon line does help, but it is not very clear how to select which direction to launch when you are landed on a base on the Moon, especially if you are unlucky enough to be near the poles.

That's where I came up with using a second instance of TransX.
You simply setup a Moon-Earth plan there, and you can get a Launch Heading on stage1. Also, you can use the "Edge On" view on Stage2, to see if you need to apply any plane change in addition to the negative prograde to get back to Earth.
Once you are in lunar orbit, you can forget about the second TransX and proceed with the rest of the plan according to the guide.

----------------

The next best thing is a direct Moon→Mars flight aka single burn solution. And I guess this is the method the OP is interested in.
As the name implies, it uses a single burn from lunar orbit to get to Mars.
Average ΔV cost ~1.65 km/s (ranges from 1.5-1.75 km/s depending on the position of the Moon).

You can setup a TransX plan in a similar manner as BrianJ's example above, (this time you setup the PeD = Moon's distance) but it is a lot easier to use IMFD for this type of flight.

Here I have to note that the way this type of journey is flown in

Me and blixel are looking into making a better video explaining much more in detail what's going on and why it is set up in such a way. Also we will cover the two burn solution with TransX and finally show the flight using TransX and IMFD combined for best results.

 

[Estelyen]

[...]utilizing TransX

Thank you for the info, but the reason I posted this is because I wanted to do it with IMFD instead of TransX, the reason simply being that I am much more comfortable with using IMFD

Actually you need Reference: Earth and Source: Moon.

Yes, this is what I did Sorry, mixed it up while writing it in the forums *blush*

The next best thing is a direct Moon→Mars flight aka single burn solution. And I guess this is the method the OP is interested in.

Nope, it's the other way around: I wanted to do a Multi-burn flight with a slingshot burn close to earth

This kind of solution can be setup only with TransX, as IMFD cannot handle multi-burn plans.

OK, I guess I'll have to live with that... Anyway, thank you very much for your detailed post xD

I actually managed to do the flight with IMFD in the meantime, although I guess it was nowhere near optimal dV... I set my Deltaglider to unlimited fuel just to try my idea, which I got from the sugestion to use two instances of TransX: I tried using two instances of IMFD instead, using one course program in planet approach mode (Ref Earth), which I then used as data for the Lunar orbit ejection; then switching over to the other course program while on the way from the moon to earth to calculate a target intercept route to mars, using my earth Pe as the moment for the ejection burn. Using this method, my inclination turned out to be off by roughly 20° and the ejection burn at earth Pe was for 4500 total dV, but it DID sent me to Mars and I guess I could've done a lot worse :lol:

 

[Tommy]

What you are attempting isn't possible. There will be only one burn - when you eject from lunar orbit (and some MCC's as well, of course).

The reason for this is simple - the velocity you carry when you leave the Moon is more than adequate to get you to Mars. This is fairly unique to the Moon-Earth-Mars trip (and perhaps Earth-Moon-Venus as well).

In short, there will be no burn at Earth because you already have the energy required to get to Mars. If you wanted to perform a Phobos-Mars-Earth flight, you could use a two burn solution - but doing that in IMFD is quite complicated and I can't really cover that in a forum post.

Edit.....

You could use a two burn solution IF you plan an accellerated (non-hohman) transfer to Mars - but IMFD isn't really good at that. Getting into the proper plane around Earth requires some pretty advanced tricks using Delta-V program and Map Program - as well as some math (done by hand) to perform the launch and lunar ejection.

 

[sorindafabico]

How do I monitor the inclination I'll have in relation to the Martian equator on the Map screen?

E.g., yesterday I was going to Mars and was monitoring a value close to zero ... But I forgot that the value was relative to the plane of the ecliptic.

 

[dgatsoulis]

How do I monitor the inclination I'll have in relation to the Martian equator on the Map screen?

E.g., yesterday I was going to Mars and was monitoring a value close to zero ... But I forgot that the value was relative to the plane of the ecliptic.

If you had the Sun as the reference and Mars as the target, then the R.Inc you were seeing was the relative inclination of your heliocentric trajectory relative to Mars' heliocentric trajectory.

To switch between different reference points relative to Mars, you need to set Mars as your "REF" and then hit "TGT".
You will notice the title on the popup window:

"Set Target: (spec. 'g' 'r' 'e' 'l')"

g= geostationary (or in this case areostationary orbit)
This will find if an equatorial synchronous orbit exists around the planet/moon you have as reference and show it to you on screen. The plane is (ofcourse) always equatorial.

r= "Target Mars"
This will show you the relative inclination of your heliocentric trajectory relative to Mars' heliocentric trajectory.

e= "ecliptic"
This will show your inclination relative to the ecliptic plane.

l = "equatorial" (!?)
This will show the inclination relative to the reference body's equator. (same as 'g' but without the target orbit).

These show you just the data on screen. To actually change the projection of the map you need to press the PRJ button on the bottom left and look at the text under the reference body to know what projection you are looking at.
It is possible to look at one projection on screen (the drawing of the trajectory around a planet/moon) and the data from a different reference point.

In the pic below the map projection is set to the ecliptic but the data on the left of the screen is relative to the equator.

 

[sorindafabico]

Thanks! Did not realize I had to put Mars as the reference.


A little different question (but inside the topic, IMO): how do I plan a Phobos-rendezvous trajectory long before Martian SOI? Ok, I can get the EQ inclination as you explained, but, in this case, I need the same LAN, too.

E.g., I'm going with a DGIV from the Moon to Phobos and I want to do an aerocapture on Mars to save fuel. How do I align with Phobos using IMFD before entering martian SOI?

 

[dgatsoulis]

how do I plan a Phobos-rendezvous trajectory long before Martian SOI? Ok, I can get the EQ inclination as you explained, but, in this case, I need the same LAN, too.

Phobos is only ~1° inclined relative to the equator, so as long as you arrive with a low enough equatorial inclination, you should be ok.
The important thing is the node. You can get away with a high relative inclination, if the node is at periapsis, because then you can include a plane change in your aerocapture.
If the node is not at the periapsis, you will find it very difficult to get aligned with Phobos.


Some examples with more details:

Case1
Here you arrive at periapsis with ~11.6° degrees inclination relative to the equator. The problem is that the node (the intersection of your trajectory relative to he equator) is not aligned with the periapsis.
It will be impossible to align with Phobos during the aerocapture maneuver.

Case2
Here you arrive at periapsis with ~9° R.Inc. The node is exactly at periapsis
so you will be able to align with Phobos if you include some plane change in your aerocapture maneuver.

Case3
Here you arrive with a very low R.Inc, similar to what Phobos has relative to the equator and the node exactly at periapsis. This is what you want to aim for when you are still far away from Mars' SOI.

Once you are inside the Martian strong SOI (G>0.5 in OrbitMFD), you can open another map, reference Mars and target Phobos this time.
(Doing this earlier will not give you a very accurate prediction of your trajectory, so it is better to wait until inside the SOI).

Now you can watch your periapsis altitude on the left and the node relative to Phobos on the right. With a little bit of linear RCS you should be able to get the node exactly aligned with the periapsis.

You want to get the R.Inc. on the right (relative to Phobos) as close to 0 as possible during your aerocapture maneuver.

Again, going this last part while you are still very far away from the Martian SOI will result in a not so accurate trajectory prediction from IMFD, so it is better to target Phobos only after you have entered the SOI.

 

[sorindafabico]

Great explanation! Thanks!

 

[Tommy]

Dgatsoulis is correct for this flight - primarily because the Martian moons have so little mass. With a larger moon, it's not important to be in plane with the moon (relative to it's parent) because the moon itself will change your plane during the orbit insert burn.

If , for instance, you were performing a flight from Mars to Luna, there would be no real reason to worry about aligning with the Moon - you can intercept it directly. While this can be done at any point in the Moon's orbit, it is more efficient if you intercept it when it is at the point of it's orbit that is tangent to your transfer orbit and moving in the same direction. If you have a high inclination w.r.t the Moon, it can add a bit to your insertion burn, but a RInc of less than 30 degrees will have minimal effect on efficiency.

For an example, let's look at that Mars - Luna flight. I would start the flight like a regular Mars - Earth flight using Target Intercept. It would only be AFTER I LEAVE Mar's SOI
(and the "src" is set to "x") that things change. At that time, I would set Map to Ref=Earth, TGT=Moon, and CNT="p-moon". Insure that Int and Dsp are both enabled, as well as "Plan". At this point you can use Target Offsetting in Target Intercept, or use Delta-V program. I prefer the latter - because the spherical coordinate system used in target offsetting can be confusing to use. Map is a bit inaccurate - especially when it comes to your expected arrival time, so during my first MCC I will adjust my course so that it intersects the Moon's orbital path where I want - but not worry as much about the Moons position in it's orbit. (This is even more the case with faster moving moons like Titan, no point in trying to time the arrival until the last month of the flight but you should still be corrected to the target's orbital path).

So, I've adjusted my course so that it just touches the Moon's path on the "leading side" of Earth (so the Moon will be going in the same direction - if going outward like Earth-Titan you would target the "trailing side" of Saturn). I"ll want the Moon on the same side of the Earth, but don't need to be very accurate yet. I'll also make sure that the node displayed in Map is as close as possible to the intercept point. As I get closer, I'll be more picky about the Moon's position in it's orbit, that my path intercepts the Moon's path, and ensure that the node remains as close to the intercept as possible.

About a week from arrival, I'll switch Map to Ref-Moon, use the MOD button to enter approach mode, and begin fine tuning the approach using Delta-V program.

These are some advanced tricks, but using them I've flown Earth-Jupiter-Titan flights with less than 30 seconds of correction burns (after the initial Earth ejection burn), and been able to aerocapture and land (deadstick) at a base on Titan. And consider that most of that was to adjust for the effect of Jupiter's moons during the swing-by (less than 2 seconds of MCC's enroute to Jupiter). I could probably do Earth-Calisto with about 6 seconds of MCC's.