When / where to launch for Minmus straight-shot?

[Xyphos]

Every single of trips to Minmus have been from a stable orbit, after matching the planes. was wondering if there was a proper time/angle to just fly in a straight(ish) line and end up in Minmus' SOI

[Camaron]

Of course you can, but its such a small target. It's far, far easier to just use the maneuver nodes like a normal person. Personally I don't match the inclination of slanted orbits, I shoot my apoapsis up straight along the dotted line that marks the angle of incline. Then I expand the peri to get the intercept on the following orbit pass like a time-wasting master of inefficiency. 

[Guest]

Yes.  Launch when KSC is under (or slightly behind) the ascending or descending node for Minmus.  If you're unsure where that is, focus on Kerbin, zoom out until you can see Minmus' orbit, rotate your view until the orbit is a single line, then timewarp until KSC is under the orbit line.  Once in a parking orbit, plan your trip the same as any other.  Matching the plane during launch will reduce delta-V requirements for the trip.

You can also launch directly to Minmus but I am personally not sure when is a good time to do that, basically the same as above only you skip the parking orbit.

[TyrannoFan]

If you want to go to Minmus without doing a plane change, you need to intercept it at its ascending/descending node. This means you have to intersect Minmus' path at the point in its orbit where it is crossing the orbital plane of the Mun. From a 100km orbit, you can raise your apoapsis to Minmus' orbital height of 47,000km. It will take (approximately) 9 days, 1 hour and 10 minutes to reach this apoapsis from a 100km periapsis. Minmus' orbit takes 49 days, 5 hours and 15 minutes, so if you do this maneuver when Minmus is 66.366° away from its ascending/descending node, you will get an encounter with Minmus. This is because it will take Minmus 9 days, 1 hour and 10 minutes to get from that position to the position of your apoapsis. I made a graphic here to illustrate:

Edited March 18, 2016 by TyrannoFan
Fixed typo. 9 days, not 19.

[Dafni]

thank you @TyrannoFan, very nice detailed post, very helpful.

[Tfin]

28 minutes ago, Dafni said:

thank you @TyrannoFan, very nice detailed post, very helpful.

Except for the typo. (19 or 9?) Graphic says 9, text says 19... twice.

Edited March 18, 2016 by Tfin

[TyrannoFan]

Yep, it is meant to say 19. I'll see if I can make a quick edit.

EDIT: Actually, I think 19 is the typo. It doesn't seem to make sense if it's 19.

Edited March 18, 2016 by TyrannoFan

[Tfin]

6 minutes ago, TyrannoFan said:

Yep, it is meant to say 19. I'll see if I can make a quick edit.

Wait, it's supposed to say 19!? Maybe I'm just tired... but I was thinking that piece of orbit looked more like 9. That 19 was more than a third of 50, and you'd have an arc twice that.

[TyrannoFan]

Yeah the 19 figure is actually a typo :P, I just checked and there's no way 66 degrees of Minmus' 49 day orbit checks out to 19 days.

[bewing]

I always launch straight shots. You save 25% on your fuel that way. Going into LKO first is a waste of time (and fuel). You only need to wait until KSC is about 30 degrees ahead of Minmus, anywhere on Minmus' orbit. Turn on SAS Stability mode and launch dead straight up. Get your Ap to about 45 Mm. Rotate and zoom until you are looking back toward Kerbin from Minmus' orbital plane. You will clearly see whether the tip of your orbit is above or below the plane. Make a small burn (a few seconds) either north or south until the tip is exactly in line with the plane. Then set Minmus as your target. It'll show you the position of Minmus at closest approach. You need it to be below 2 Mm for an encounter. Generally, you almost always need to carefully burn a little more prograde to get an encounter. Then you need 200 m/s of deltaV to get a capture once you are there.

PS. One further note: tourist flyby missions only count if your ship was in orbit around Kerbin before your Minmus encounter. So when you are just about to get to your encounter, you may want to do a 60 or 70 m/s burn east, to establish an orbit. Minmus will capture you anyway, a few seconds later.

Edited March 18, 2016 by bewing

[Snark]

Gameplay question moved to Gameplay Questions.

One thing that's handy when you have a ship sitting on the launchpad and are trying to judge "when am I right under Minmus orbit?" is to go to the map view and double-click on Kerbin itself to set the focus to the planet rather than your ship.  Then just rotate the map view until you're looking at Minmus orbit perfectly edge-on.  Timewarp.  When your ship appears to be right on the line, it's time to launch.

The reason it's handy to set the focus to the planet rather than your ship is so that you don't have to keep fiddling with the camera angle to keep Minmus orbit edge-on in the view as your ship moves.

[Xyphos]

Thankyou everybody!

[ElWanderer]

If you want to launch into a Low Kerbin Orbit that matches Minmus's inclination, you need to wait until the orbit plane passes over the launchpad (happens every three hours), then launch towards a heading of 83.5° or 96.5°, depending on whether Minmus is ascending or descending. This is not so easy without some kind of aid. I've written a kOS script to automate launching at the right time and keep the right heading, which usually gets me about 0.1° relative inclination. Even then, I still match planes (at much less cost than it would be from an equatorial orbit) to make the transfer easier.

[Guest]

39 minutes ago, ElWanderer said:

This is not so easy without some kind of aid.

It's not nearly as hard as you're making it out to be, I launch into roughly equatorial solar system plane without aid all the time in Realism Overhaul; it's basically a requirement for any interplanetary launch.  No aids that I know of (KER/MJ) can tell me the relative inclination of something outside the SOI I'm in so it's pretty much looking at orbit lines and comparing your own orbit line the entire launch.  You can easily do the same in stock/vanilla without aids.  Yes, you may have to adjust inclination during your ejection burn but that's nothing big if you're within a degree or two of your target.

The bonus with Minmus is that you can set it as a target and watch the AN/DN readout on the map screen, even without aids installed.

Edited March 18, 2016 by regex

[jhousen]

On 3/18/2016 at 11:48 PM, regex said:

...

You can also launch directly to Minmus but I am personally not sure when is a good time to do that, basically the same as above only you skip the parking orbit.

That would be about 7 minutes before where you would put a maneuver node if you were in a stable orbit (takes about 6 minutes to get to orbit plus about 30 seconds for the first half of the tmi burn, you end up getting into the parking orbit just in time to burn for minmus 

[Guest]

1 minute ago, jhousen said:

That would be about 7 minutes before where you would put a maneuver node if you were in a stable orbit (takes about 6 minutes to get to orbit plus about 30 seconds for the first half of the tmi burn, you end up getting into the parking orbit just in time to burn for minmus 

Six minutes to orbit?  With every craft?  No matter the TWR of uppers and lowers?  Injection burn will always be 1 minute in length?

Better to test the craft before relying on those hard numbers.

[Xyphos]

9 hours ago, regex said:

Six minutes to orbit?  With every craft?  No matter the TWR of uppers and lowers?  Injection burn will always be 1 minute in length?

Better to test the craft before relying on those hard numbers.

Well, in my experience, it's 3 minutes for rockets and about 7 minutes for rapier-powered vessels, even longer if there's a lot of parts involved due to physics lag.

[Plusck]

On year 0 day 1 at 0:00 (i.e. when each new game starts) you can launch to LKO orbit, but not circularise and keep burning prograde to get a Mun encounter. At the time of launch, the Mun is almost exactly on the eastern horizon.

The same applies to Minmus (i.e. waiting until Minmus is on the eastern horizon) but because the target is smaller, your exact launch time will need to be more precise, and that depends on TWR, ascent angle and so on. Assuming a reasonable-to-high TWR (about 1.7 on launch, and about the same on starting the second stage) and an efficient ascent profile, you should leave when Minmus is about 5-7° under the eastern horizon.