Targeting the Mun - Delay

[Philae & Rosetta]

Hi all,

 

Since a few days I have a problem.

I need to go to the Mun with some tourists but when I try to target the Mun, the NavBall indicates the current position of the Mun,  and doesn't take into account the delay to go there (something like two hours).

So when I arrive the mun is not anymore at this position...

Few weeks ago, I already targeted the Mun (with another ship, and without tourists), and it was taking into account the delay. So I could manage to approach it easily.

 

Do you know how it can be explained? I don't understand it...

Thanks,

 

 

Edited November 4, 2017 by Philae & Rosetta

[luizopiloto]

There are two methods you can use to get captured by the Mun:
The most simple one is to start you transfer burn when the Mun rises from Kerbin's horizon, but only works fine if your ship have more than 0,5 TWR and orbit altitude is lower than 100Km.
The most precise one is to open the Map View (M) key, click on your orbit line to create a maneuver node, drag the Prograde indicator until the maneuver Δv indicator reach 830~840m/s
and drag the maneuver node alongside your orbit line, clicking on the middle, until it shows a Mun capture. Then you start your transfer burn at the indicated point, pointing towards the node vector indicator on your navball.

[Geschosskopf]

When you're in a prograde low orbit at Kerbin, just burn about 950m/s prograde as soon as you see the full disk of Mun rise above Kerbin's horizon.  This will get you a Mun encounter.  It will probably need some tweaking along the way or once you get there, but you and Mun will end up in the same place at the same time.

This trick works whether you have maneuver nodes or not.  You can do it totally by eyeball.  Thus, it's not dependent on upgrading KSC.  When you do have the upgrades to use maneuver nodes, however, just move the camera so you're sighting tangent to Kerbin's surface and can see Mun's disk just above it.  Then tweak the node as desired.

[Philae & Rosetta]

Thanks,

So you are both assuming (and the game training too) that in order to go to the Mun, we need first to orbit (low) Kerbin. Isn't more economic, and less propellant waste to go directly from Launch, without orbiting Kerbin?

For me it's a non-sense...

Edited November 4, 2017 by Philae & Rosetta

[Rocket In My Pocket]

26 minutes ago, Philae & Rosetta said:

Thanks,

So you are both assuming (and the game training too) that in order to go to the Mun, we need first to orbit (low) Kerbin. Isn't more economic, and less propellant waste to go directly from Launch, without orbiting Kerbin?

For me it's a non-sense...

No, not really. (Also, welcome to the forums!)

It's slightly more efficient to go direct, by like...a hand full of Dv. Nothing worth worrying over.

Also, no one is assuming you have to orbit Kerbin, if you are quick you could certainly setup your maneuver node while on a direct ascent. It's just kind of pointless to make yourself hurry like that. You need to target the Mun and look for the "closest approach markers" the same as when you rendezvous with a craft, these tell you where you and it will be when you meet. (The Mun is typically about a 1/4 of an orbit ahead of Kerbin if that makes sense to you, so start off by aiming there.)

Edited November 4, 2017 by Rocket In My Pocket

[Geschosskopf]

18 minutes ago, Philae & Rosetta said:

Thanks,

So you are both assuming (and the game training too) that in order to go to the Mun, we need first to orbit (low) Kerbin. Isn't more economic, and less propellant waste to go directly from Launch, without orbiting Kerbin?

For me it's a non-sense...

Well, the eyeball method works both from the ground and from LKO.  Only if you're starting on the ground, you should launch a bit sooner, like when less than the full disk is above the horizon.

As to the fuel requirements, if your ship is going to enter Munar orbit, there's no difference in dV required whether you enter LKO or not.  Getting into Munar orbit requires doing 2 things that consume fuel.  When viewed from Kerbin's frame of reference, these 2 things are:

1. Raising your Ap from Kerbin's surface to the desired altitude above Mun. This is the transfer burn.
2. Once you get to Mun, you then pull your Pe up after you.  This is the capture burn.

Whether you pause in LKO or not, you're still moving your Ap and Pe the same total distance away from Kerbin's surface.  Thus, the amount of dV required is the same whether you pause in LKO en route or not  All you're doing is altering the timing of when you pay for changing your Pe.  You do a little of it up front to get into LKO, then do the rest once you get to Mun.

Note, however, that the above is in terms of dV.  The more massive the ship, the more fuel required to create a given amount of dV, due to inertia and all that.  Because ships get lighter over time by burning fuel and dropping stages, the later in the mission you do burns, the less fuel they consume, although you still need to make the same amount of dV regardless.  So by going straight from the ground to Munar orbit, you don't need QUITE as much fuel for the Pe change, because you're doing it all after the big expense of the transfer burn.

Still, this is only an insignificant difference.  The 100km or so you move your Kerbin Pe to achieve LKO is a tiny fraction of the total distance between Kerbin and Mun.  Thus, you're still doing the vast bulk of the Pe change during the capture burn at Mun, after you've done the transfer burn and the ship is light.  So IMHO, you don't save anywhere near enough fuel to justify going straight from the ground for that reason alone.  If you just want to do things that way, then go for it.  But you really won't get any real difference in mission pricetag.

Besides, there's a good reason for pausing in LKO en route to Mun.  It gives you a chance to inspect and test the ship and its functions before committing to a long trip away from help.  If you forgot the solar panels or whatever, you can still land immediately before you run out of juice  

[Philae & Rosetta]

Woah tank you for this very detailed answer.

 

I indeed tried (for the first time) from Kerbin Orbit and it's indeed easier to manage the way to enter the Mun Influence area.

Maybe it's a bit less fuel consommative from launch, but it's really more difficult to manage as you said, so it's not really brining value to perform like this

 

I have another issue I want to talk about but I will open a new topic for this.

[bewing]

If you want to do a direct launch to the Mun, the important angle to know is 45 degrees. That's how much you need to lead the Mun by, in order to hit it. IOW, that's how far it will move in the time it takes for you to get there.

 

[Vanamonde]

My guide is a couple of years old, but should still work.

[Superfluous J]

11 hours ago, Philae & Rosetta said:

So you are both assuming (and the game training too) that in order to go to the Mun, we need first to orbit (low) Kerbin. Isn't more economic, and less propellant waste to go directly from Launch, without orbiting Kerbin?

I just want to confirm, are you talking about burning STRAIGHT UP to Mun, or doing a gravity turn to go sideways, but not stop burning until your Ap is at Mun's orbit?

If you are going STRAIGHT UP, then no. It's cheaper (sometimes a LOT cheaper) to get into orbit first and then burn to Mun. Why? Because every SECOND you burn straight up, you WASTE almost 10 m/s of dV in fuel, fighting gravity. If you are burning sideways, you waste NONE of that fuel. The ONLY reason you start burning straight up is because you need to get out of the thick atmosphere before turning sideways.

If you are burning sideways, and nail your Mun encounter perfectly from launch (possible, but not likely, at least not on the first try) then yes, it is a teensy tiny bit cheaper to just keep burning. However, you can EASILY waste more fuel fixing the inevitable incorrect encounter, than you would have by getting a nice LKO first and benefiting from the extra time you spent creating a maneuver node.