ISS > Moon Possible?

[bradfirj92]

Been getting used to the XR5 Vanguard recently, and I love it, and I just managed to take off from KSC and dock at the ISS fairly efficiently.

"Next stop, the moon!" I thought, until I realised that with limited fuel I couldn't align the orbital planes, because the ISS is so off-plane. (RInc of ~70.89)

What sort of fancy maneuver could I do with TransX to get from the ISS to the Moon, is it even possible?

 

[ar81]

You should attempt an Off plane transfer.
Or attach a plasma engine to ISS and align it...

 

[agentgonzo]

You don't need to align the planes in order to do a transfer. The moon takes ~28 days to orbit the earth, so every ~14 days you'll have a launch window. Create a menouvre and put in the DeltaV to transfer to the moon (~3.1km/s) from your ISS orbit. Then advance the menouvre date forward until the closest approach lines up with the orbital intersect line (grey). It's a bit tricky and you'll need to advance the date in fine mode to get a rough idea of the 'closest approach' minimum and how far away from the grey node line this is. Then use coarse or medium to advance a few days and try again with the fine tuning. This allows you to get the closest approach where the yellow and grey lines line up and you'll encounter the moon.

 

[bradfirj92]

Bear with me, I'm still getting the hang of TransX.

I switch Maneuver to 1, and add Prograde Velocity to 3.1k as you say, but I see no Grey line, do I need to designate a target somewhere?

 

[orwellkid]

First, refuel at the ISS, and then do an off-plane transfer with IMFD. I can reliably do this... it ain't fuel efficient, but it works.

 

[bradfirj92]

I'd rather learn to use TransX at the moment, take things one step at a time.

 

[agentgonzo]

This time with diagrams

Set up the menouvre to be around 3.1km/s DeltaV with the target as the moon. You should see something like the below. There are 3 main lines.
The grey line is the intersection of the moon's orbital plane and your orbital plane. The two yellow lines show the position of the moon (bottom yellow line) and your craft (top yellow line hidden behind the 6 in this case) at the point of closest approach between your two craft.



Obviously, this is rubbish. Now, switch to menouvre date on Ultra or Hyper and advance it until the two yellow lines are aligned on top of eachother. This will be when your craft and the moon are in the same orbital position with respect to the earth at the time of closest approach.



However, if you flew this plan, you would get to this position many thousands of km below or above the moon because of the difference in orbital inclinations. What you need to do is to align the two yellow lines and the grey line on top of each other so that when you get the closest approach you are where the two orbital planes intersect and you will be close to the moon. Switch the adjustment mode on the Menouvre date to be ultra or fine and advance until this happens



At this point, open up TransX in the second MFD and advance to the second (moon centred) stage. Use this as your targeting view. Hit VW to get the encounter view that will give you your altitude above the moon.

In the first MFD, adjust the date in Hyper mode and tweak prograde DeltaV to get a close approach to the moon in the second stage. You may need to adjust the menouvre date by a few orbits in order to get the best trajectory (otherwise you may still be passing over/below the moon although you are very close).

 

[bradfirj92]

Thanks Agentgonzo, thats exactly what I was looking for, I'll try it later today.

PS. What's the maths, or the technique used to calculate rough dV?
When travelling from planet to planet, one can use the hypothetical orbit, adjusting Prograde so it just brushes the target and fine tuning from there.

This doesn't seem to apply here?

 

[agentgonzo]

The technique for getting to ~3.1km/s is just from having done lunar missions far too many times!

The adjusting prograde technique is fine. However, because the projection drawn to the screen is normally ecliptic, and the ISS's orbit is highly inclined to this orbit, it will seem as if the orbit is not as eccentric as it really is.

Simplified explanation follows.

If you do your TLI burn when you are on the ecliptic, the apogee will be similarly on the ecliptic - which happens to be in the orbital plane of the moon*. This is where you can use the 'adjust-prograde-until-it-brushes-the-orbit-of-the-moon' technique. However, if you do the TLI burn at a high latitude, then your apogee will also be at a low latitude. Since the MFD is drawn from above the ecliptic, even though the apogee will be the same distance from the earth as the moon, the orbit will seem as though it its apogee is well inside the orbit of the moon.

You can see this change as you advance the menouvre date in hyper. The apogee of the orbit seems to grow and shrink wildly as you move the time of the burn around the orbit, reaching its maximum when the point of burn is on one of the nodes (points where two orbital planes intersect - denoted by the grey line).

The technique would be similar to how you've done it before with increasing the prograde until it grazes (or is slightly bigger than) the orbit of the moon - except in this case, set some DeltaV so that you get a noticably elliptic orbit. Then adjust the Menouvre time so that the Apogee/Perigee of your hypothetical orbit are on the node (grey line). Then adjust the Prograde DeltaV so that it's grazing the orbit of the moon.



*Actually, the moon's orbit is inclined at 5° to the ecliptic, but ignore that here.

 

[bradfirj92]

Hey again.

My problem now is that I can't seem to advance the date in such a way that both of the yellow lines are parallel to the grey one. If I make one line match up, the other one is way off. Am I doing something wrong?

EDIT: Disregard, turns out I hadn't gone far enough forward, current launch window looks to be something like Sim + 3 MJD

 

[TeddyJohn]

Been getting used to the XR5 Vanguard recently, and I love it, and I just managed to take off from KSC and dock at the ISS fairly efficiently.

"Next stop, the moon!" I thought, until I realised that with limited fuel I couldn't align the orbital planes, because the ISS is so off-plane. (RInc of ~70.89)

What sort of fancy maneuver could I do with TransX to get from the ISS to the Moon, is it even possible?

I use IMFD with the XR5 and have launched from the ISS to the moon using both methods. The Off Plane transfer only saved me 45 seconds of burn time. That's what I like about the XR5, I can always carry extra fuel and oxygen in the cargo bay to re-supply.


-----Posted Added-----


You can also swing by the Mir to refuel. It's only about a 25 second alignment burn from there.

 

[bradfirj92]

Made it with Gonzo's excellent guide diagrams.

Now to try and land it, Lola MFD just engages 1/2 hover engines and blasts me into space.