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Mars-Bound Hokie

Timing my Minmus Launch

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When I use the MJ Maneuver Planner to fly to Minmus, I've noticed that the maneuver required varying amounts of delta-V. Sometimes it got a little past ~1300 m/s, others I've seen it go as low as ~900 m/s. My goal is to minimize the amount of fuel it takes to make the burn from LKO to Minmus so that I would have enough to safely land my SSTO.

 

Rng5IlI.png

  • Image of the Mk. III Duna SSTO.

 

I believe that, to decrease the amount of fuel I use for the Minmus burn, I need to not only launch at the right inclination, but the right time at well. Since Minmus has an orbital inclination of 6 degrees, I need to launch my plane at an 84-degree azimuth (that's also a 6-degree inclination). However, what I DON'T know is how I'm going to time my launch right so that my orbital inclination matches Minmus' AND requires the least amount of fuel to reach it.

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900-ish m/s is the correct number for a transfer that does not include a plane change. Since Minmus is in a zero eccentricity orbit around the same body you are starting from, this cost never changes, no matter where Minmus currently is. There are no transfer windows, and there are no times when the transfer is cheaper or more expensive.

If you do spend more than that, it is spent on adjusting inclination.

To avoid having to adjust your inclination, there are two valid tactics, each with its own advantages and drawbacks.

The first is to launch into a standard, equatorial Kerbin orbit. Then, select Minmus as your target. The game will now show you the ascending and descending nodes, which are the points where changing your orbital inclination to match the target is the cheapest. But, even more importantly, these are the points where the orbital planes (yours and Minmus') intersect. You can abuse that fact to get a free inclination change. To do this, you put your maneuver node exactly on the ascending or descending node, and then add your 900-ish m/s dV for the transfer to Minmus. Now, you will notice that (unless you are incredibly lucky) you won't actually get a Minmus encounter this way, because Minmus will simply not be there when you arrive. So you must push the maneuver node forward in time, orbit by orbit, until you do get an encounter.

Advantages: Minmus' sphere of influence will capture you without the need to perform an inclination adjustment anywhere on your route. You will most likely arrive in an equatorial orbit around Minmus, or close to it.
Disadvantages: you may have to wait many, many days after launching until you can execute your maneuver node.

The second tactic is to launch directly into Minmus' inclination, which is six degrees - either north or south, depending on whether Minmus is ascending or descending relative to you at the time. To do so, you must launch at the ascending or descending node, mentioned above. It doesn't matter which one you pick, as your low Kerbin orbit will allow you to quickly loop around to your correct transfer burn position later. Unfortunately, you cannot actually see the ascending and descending node while you're not in flight... but, there's a trick you can use. In map mode, zoom out so that you can see Minmus' orbit lines on both sides of the planet. Now, orient yourself so that both the orbit of the Mun and the orbit of Minmus appear only as a single line - i.e. that you are looking at them directly from the side. You will notice that, when you do this, these two lines intersect on top of Kerbin. Ideally, dead center on Kerbin's own center. You have now found the ascending or descending node of Minmus relative to the Mun. And since the Mun is in a zero inclination orbit, and your default launch site (KSC) is also at zero inclination, you can use those nodes for your own launch. Leave your map view oriented as it is, and zoom right up to Kerbin so you can see it clearly. Timewarp until KSC is in the middle of your view, on the side of the planet facing you. This is the correct time to launch. (If you want to be perfect, launch ever so slightly before KSC is dead center, because you'll need a few minutes to get to orbital velocity.)

Advantages: you'll not need an inclination change (beyond fixing minor inaccuracies), and you can execute your transfer to Minmus at any time you want.
Disadvantages: launching precisely into a specific inclined orbit is more difficult, steering-wise, than launching into an equatorial orbit. The launch will consume a small bit of extra dV. You will most likely arrive in an inclined orbit around Minmus.

 

Edited by Streetwind

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13 hours ago, Streetwind said:

900-ish m/s is the correct number for a transfer that does not include a plane change. Since Minmus is in a zero eccentricity orbit around the same body you are starting from, this cost never changes, no matter where Minmus currently is. There are no transfer windows, and there are no times when the transfer is cheaper or more expensive.

If you do spend more than that, it is spent on adjusting inclination.

To avoid having to adjust your inclination, there are two valid tactics, each with its own advantages and drawbacks.

The first is to launch into a standard, equatorial Kerbin orbit. Then, select Minmus as your target. The game will now show you the ascending and descending nodes, which are the points where changing your orbital inclination to match the target is the cheapest. But, even more importantly, these are the points where the orbital planes (yours and Minmus') intersect. You can abuse that fact to get a free inclination change. To do this, you put your maneuver node exactly on the ascending or descending node, and then add your 900-ish m/s dV for the transfer to Minmus. Now, you will notice that (unless you are incredibly lucky) you won't actually get a Minmus encounter this way, because Minmus will simply not be there when you arrive. So you must push the maneuver node forward in time, orbit by orbit, until you do get an encounter.

Advantages: Minmus' sphere of influence will capture you without the need to perform an inclination adjustment anywhere on your route. You will most likely arrive in an equatorial orbit around Minmus, or close to it.
Disadvantages: you may have to wait many, many days after launching until you can execute your maneuver node.

The second tactic is to launch directly into Minmus' inclination, which is six degrees - either north or south, depending on whether Minmus is ascending or descending relative to you at the time. To do so, you must launch at the ascending or descending node, mentioned above. It doesn't matter which one you pick, as your low Kerbin orbit will allow you to quickly loop around to your correct transfer burn position later. Unfortunately, you cannot actually see the ascending and descending node while you're not in flight... but, there's a trick you can use. In map mode, zoom out so that you can see Minmus' orbit lines on both sides of the planet. Now, orient yourself so that both the orbit of the Mun and the orbit of Minmus appear only as a single line - i.e. that you are looking at them directly from the side. You will notice that, when you do this, these two lines intersect on top of Kerbin. Ideally, dead center on Kerbin's own center. You have now found the ascending or descending node of Minmus relative to the Mun. And since the Mun is in a zero inclination orbit, and your default launch site (KSC) is also at zero inclination, you can use those nodes for your own launch. Leave your map view oriented as it is, and zoom right up to Kerbin so you can see it clearly. Timewarp until KSC is in the middle of your view, on the side of the planet facing you. This is the correct time to launch. (If you want to be perfect, launch ever so slightly before KSC is dead center, because you'll need a few minutes to get to orbital velocity.)

Advantages: you'll not need an inclination change (beyond fixing minor inaccuracies), and you can execute your transfer to Minmus at any time you want.
Disadvantages: launching precisely into a specific inclined orbit is more difficult, steering-wise, than launching into an equatorial orbit. The launch will consume a small bit of extra dV. You will most likely arrive in an inclined orbit around Minmus.

 

It should be noted that, if you have it, the Dessert Launch site is 6 degrees below the equator, so due east launch will automatically put you into the same inclination as minmus without needing a plane change or a dogleg.

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15 hours ago, Mars-Bound Hokie said:

However, what I DON'T know is how I'm going to time my launch right so that my orbital inclination matches Minmus' AND requires the least amount of fuel to reach it.

Just because your inclination is 6°, it doesn't necessarily match Minmus' inclination. It also needs to be coplanar with Minmus.

These days, perhaps the easiest and most obvious method of coming to grips with the concept is to use the "Set Orbit" cheat: put a vessel into a high inclination (something that's rather obvious, 30-40°), and then enter different values in the "LAN" field. LAN is "Longitude of Ascending Node" in degrees (0-360). See what happens when you try 40, or 90, or 130?

Now do that again, with your vessel set to a 6° inclined orbit, while Minmus is selected as a target, and click one of the ascending/descending nodes in order to see the relative inclination. Watch how that figure changes as you try different LANs.

The whole exercise takes about two minutes, and, (I hope), will make it abundantly clear what you need to do.

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On 10/16/2019 at 10:33 PM, Mars-Bound Hokie said:

When I use the MJ Maneuver Planner to fly to Minmus, I've noticed that the maneuver required varying amounts of delta-V. Sometimes it got a little past ~1300 m/s, others I've seen it go as low as ~900 m/s. My goal is to minimize the amount of fuel it takes to make the burn from LKO to Minmus so that I would have enough to safely land my SSTO.

 

Rng5IlI.png

  • Image of the Mk. III Duna SSTO.

 

I believe that, to decrease the amount of fuel I use for the Minmus burn, I need to not only launch at the right inclination, but the right time at well. Since Minmus has an orbital inclination of 6 degrees, I need to launch my plane at an 84-degree azimuth (that's also a 6-degree inclination). However, what I DON'T know is how I'm going to time my launch right so that my orbital inclination matches Minmus' AND requires the least amount of fuel to reach it.

Wow really nice Starship. Have you got too, or landed on Minmus yet? Love the air-brakes and use of solar panels!

BTW, just watched your video about KSP Career Mode Pt. 1! Being I just started career mode myself, so enjoying your example. Great tips as well! I will look at Pt 2 soon.

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5 hours ago, Saturn5tony said:

Have you got too, or landed on Minmus yet?

Yes; I asked this question so I know how to save fuel.

  • On one of my test runs, the tanks went dry before touchdown.

 

5 hours ago, Saturn5tony said:

just watched your video about KSP Career Mode Pt. 1!

When I saw your profile picture, I knew you were the one who commented. I haven't posted on my channel for months due to my life being busy (college, family, girlfriend), but I got some pretty nifty creations to share.

  • If I were you, I'd install the following mods:
    • MechJeb (whichever version works for you)
    • Kerbal Alarm Clock
  • I think you'll really love my fuel truck.

 

If you want more career mode advice, have any more questions, or just want to chat, feel free to DM me. I've already completed the tech tree, and have sent spacecraft EVERYWHERE. But don't take my word for it; read some of my Mission Reports from my current career save.

  • What started out as a simple exploration mission turned into two kerbalnauts being stranded on Eeloo. After they were rescued, there was a full colonization effort on that snowball.

 

  • My rover has discovered a Jool moon with breathable air (but really low temperatures) and liquid water, so I sent several kerbalnauts to check out Laythe.
    • Neptune III is still in progress, and I have no idea what to do for Neptune IV.

 

  • The name says it all.
  • Anyone who's not involved in any ongoing missions (surveys, exploration missions, tourist runs) is coming back home.

 

Glad you like the Mk. III, by the way. SSTOs are not as easy as @Matt Lowne makes them seem, but if you build them right it's worth the effort.

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