Going Interplanetary - I've seen two methods. Pros and Cons?

[JoeSchmuckatelli]

1.  I don't have the link handy - but there's a nicely colored image that shows the best alignment of the planets compared to Kerbin for each destination.  The idea is you use the Tracking Station to warp until you get Kerbin in the approximate 'best' location with relation to your target planet and begin planning your transfer from there.  You set the Maneuver Node on the appropriate side of Kerbin and pull your prograde marker until you get AP to touch the orbital path of the destination planet, and then fiddle with it, then run the escape burn.  Once outside Kerbin's SOI, in Kerbolar orbit you do a plane change as needed and set up a mid course correction to get the intercept.

2. The other one I've seen is to just get an escape trajectory in the appropriate direction (in line with Kerbin's prograde for the outer planets, retrograde of Kerbin for the inner).  You're not going for a full burn to the destination planet's distance from Kerbol - just an escape from Kerbin's SOI and into a Kerbolar orbit.  Once you're in a Kerbolar orbit, you set target, get your plane aligned and then place a maneuver node wherever, pull till AP is at the destination orbit line & then pull the node around your current orbit (with appropriate fiddling) until you find an intercept - then run that.

 

I can see benefits to both; but what I don't know is what the ramifications are of choosing one or the other.  Anyone know?

[Scarecrow71]

First, I recommend the TransferWindow mod.  It will auto-warp you to the correct angle and such for whatever your destination is.  There is no thread here on the forums, but here is the link on Spacedock:

https://spacedock.info/mod/3297/Transfer Window

Anyhow, I always recommend doing the burn you need to get an intercept.  Unfortunately, there is a major bug in interplanetary transfers and maneuvers right now that makes it necessary to do an SOI change burn to get into Kerbolar orbit and then plot your intercept.

[FleshJeb]

I swear I'm not picking on you, I just click the most recent post on the side, AND you ask good questions.

Method 2 doesn't benefit as much from the Oberth Effect. https://en.wikipedia.org/wiki/Oberth_effect

The simplified version is that it's always more efficient to do your burns lower in a gravity well, because your orbit is faster down there, and the faster you're already moving, the more additional kinetic energy you get out of the same amount of delta-v expended. A bigger change in kinetic energy means a bigger change in the resultant orbit (Each orbit represents a constant sum of potential and kinetic energy at any given point.)

Kinetic energy equation: K = 0.5 * Mass * Velocity^2

If you're already moving 2.0 km/s you have 2.0*Mass km/s of kinetic energy. Add a 1.0km/s burn to that and you have 4.5*Mass km/s. The difference is 2.5*Mass km/s

If you start at 3.0 km/s, you have 4.5*Mass km/s. Add 1.0km/s again, and you have 8.0*Mass km/s. Difference of 3.5*Mass km/s for the same amount of delta-V expended.

There's a longer, more comprehensive explanation, but that's the short version. Also, the mass of the spacecraft is irrelevant here, but I left it in for completeness.

[JoeSchmuckatelli]

14 minutes ago, FleshJeb said:

I swear I'm not picking on you, I just click the most recent post on the side, AND you ask good questions.

Method 2 doesn't benefit as much from the Oberth Effect. https://en.wikipedia.org/wiki/Oberth_effect

The simplified version is that it's always more efficient to do your burns lower in a gravity well, because your orbit is faster down there, and the faster you're already moving, the more additional kinetic energy you get out of the same amount of delta-v expended. A bigger change in kinetic energy means a bigger change in the resultant orbit (Each orbit represents a constant sum of potential and kinetic energy at any given point.)

Kinetic energy equation: K = 0.5 * Mass * Velocity^2

If you're already moving 2.0 km/s you have 2.0*Mass km/s of kinetic energy. Add a 1.0km/s burn to that and you have 4.5*Mass km/s. The difference is 2.5*Mass km/s

If you start at 3.0 km/s, you have 4.5*Mass km/s. Add 1.0km/s again, and you have 8.0*Mass km/s. Difference of 3.5*Mass km/s for the same amount of delta-V expended.

There's a longer, more comprehensive explanation, but that's the short version. Also, the mass of the spacecraft is irrelevant here, but I left it in for completeness.

So - for a duffer like me; doing method 1 is better because it's more efficient and will let me retain more fuel to do work once I get there...  But 2 is okay if you're overbuilt and just goofing around?

 

16 minutes ago, FleshJeb said:

I just click the most recent post on the side, AND you ask a lot of questions  (FTFY!)

I really appreciate it. 

Full disclosure: I just finished a contract job and don't go back to work until after Spring Break, so I've been playing a lot yesterday and today.  I kind of feel like the Forum's ADD kid atm.  Just can't stop posting bugs and asking questions!

[Superfluous J]

If you JUST escape from Kerbin's SOI, you're essentially going the same speed around the Sun as Kerbin is. So, when you burn to the other planet you must do so from the velocity of Kerbin's orbit.

If you do that burn instead from LKO, you're traveling 2000 or so m/s relative to Kerbin, so you get a "boost" of about that (a bit less due to stupid Gravity pulling you back but let's not worry about that right now, it's tiny compared to the gains).

So if Kerbin's going X m/s around the sun, and to get to Duna you need to be going X+3000m/s, then if you do the burn from LKO you only need to burn about 1000m/s

If instead you do it the "easy" way and burn first to just the edge of Kerbin's SOI, first you must burn about 900m/s from LKO to get to that edge, and then when you get there you need to then burn 3000m/s to reach Duna.

So doing it "the harder but better" way saves you TONS of dV.

(note all numbers were pulled out of the Nether and are subject to being totally completely wrong. But they give the basic idea)

4 minutes ago, JoeSchmuckatelli said:

So - for a duffer like me; doing method 1 is better because it's more efficient and will let me retain more fuel to do work once I get there...  But 2 is okay if you're overbuilt and just goofing around?

Technically yes, but being a duffer has its drawbacks, mostly that there comes a point where you simply can't get your ship launched off the surface because it's too big or too clunky. While the ship that's 1/3 the size but can do 2x the stuff is far easier to launch.

[FleshJeb]

16 minutes ago, JoeSchmuckatelli said:

So - for a duffer like me; doing method 1 is better because it's more efficient and will let me retain more fuel to do work once I get there...  But 2 is okay if you're overbuilt and just goofing around?

It's KSP, you can try both and see which one you prefer! EDIT: Entirely-Unsuperfluous-J's answer is the more comprehensive one that my brain would not articulate.

16 minutes ago, JoeSchmuckatelli said:

Full disclosure: I just finished a contract job and don't go back to work until after Spring Break, so I've been playing a lot yesterday and today.  I kind of feel like the Forum's ADD kid atm.  Just can't stop posting bugs and asking questions!

I've been enjoying KSP vicariously through your videos. I quit my job of 15 years in January, and I'm HEAVILY caffeinated.

Edited March 22, 2023 by FleshJeb

[Jacke]

Method 1 close to the optimum phase angle means you depart Kerbin at about the right time for an optimum Hohmann transfer orbit (which requires the least delta-V for nearly all cases of interplanetary travel).  Using Method 2 means the phase angle could be any value, which means you might have to do a fast transfer which is a lot more delta-V.  While a fast transfer to Mun or Minmus isn't that much more delta-V, interplanetary it's a lot more.

[JoeSchmuckatelli]

Okay...  All this stuff is starting to seep in. 

And it's - this shiz is hard.  Now I'm seeing why NASA stuff takes years.  So there is really one window per year per planet for the most efficient transfers. 

... 

It's been so long since I have played KSP that I've clearly forgotten much of what I learned back then!  Thing is - I was wondering if the difference was a wash given that either way you have to burn 970 (or whatever) to get the escape and then refine.  But now I see that there is a magnitude to the refinement - and the second method was a gamification just short of using an orbit editor hack.  Yeah you can do it in the game, but you would not do it IRL. 

 

[The Aziz]

While other said correct things in general, I say this: visualize this as getting a gravitational slingshot from the planet. Or another moon since these are your first steps in interplanetary journeys. You ever noticed how the trajectory changes when you go past the Mun? That's the boost you're getting from doing the transfer maneuver as close to the planet as possible.

So, performing single burn is cheaper because you're already moving relative to the parent body in roughly correct direction. Just sending yourself outside the SOI would mean you're pretty much stationary relative to Kerbin.

[farelinho]

Method 1 is clearly the best if you are short on fuel and have a lot of time. Real life space travelling uses gravity assists and the Oberth Effect for maximal efficiency ( meaning a lot of weight reduction and fuel sparing in a industry where the $$$ for Kg is really astronomical ). As an example, you can check the BepiColombo probe flight plan to Mercury: it makes 9 fly-bies between Earth, Venus and Mercury but it will take 7 years to get there! 

 

Method 2 is faster and more Kerbalish : just add moar boosters!!! I found it useful when learning the basics of KSP: Hommann transfers, orbital inclinations / plane alignment and understanding how to get anywere without external aids such as MechJeb, maneuver planers and the like.

 

In KSP2, I'm using mostly method 1.5: trying the first one and correcting with the second because of the insufficiencies the game still presents in precise planning and execution of IP maneuvers. (KSP1 is much better at that at the moment).

[FleshJeb]

20 hours ago, JoeSchmuckatelli said:

So there is really one window per year per planet for the most efficient transfers. 

It's when the two planets arrive at the same phase angle again. This is dependent on the ratio of their orbital periods.

Let's say you want to go from Planet 1 to Planets 2 or 3. As it happens, the angle between 1 and 2 or 3 is optimal at Year 0, Day 0.

How long does it take until the transfer to each is optimal again? This is a bit incomplete, but it's when they've returned to the same relative positions, which is going to happen when they're at round integers in the chart below.

As you can see, the less difference between the orbital periods, the more rarely they both line up. In the real world, Mars has an orbital period of about 1.88, and we can get an optimal transfer about every 25 months. A Neptunian year is 165 of ours, so you can basically go every year and change (without gravity assist shenanigans).

EDIT: If I'd been smart, I'd have compared this all to the hands of an analog clock. It takes longer than one hour for the minute hand to cross over the hour hand again.

Edited March 23, 2023 by FleshJeb