Interplanetary in Vanilla. Is orbiting Kerbol 1st the best way?

[Splargo]

Heres how I do interplanetary in Vanilla career.

Lets say I'm going to Eve.

1. Get into LKO

2. Just barely escape Kerbin

3. Circularize my orbit around Kerbol.

4. Match Eves inclination.

5. do a A Hohmann Transfer to Eve.

I do it this way because of KSPs interface. When I'm orbiting around Kerbol, and not Kerbin, I can setup a maneuver node with a burn that will give me a perfect Hohmann transfer just kissing Eves orbit, and then slide the node back and forth around my orbit around Kerbol in order to get the best phase.

I find that getting the phase right makes a huge difference in efficiency (I will have much less relative velocity to my target when I arrive) but because of the games interface I cant get the phase perfect from orbit around Kerbin.

If I was in orbit around Kerbin,I can only change the phase by waiting, so I cant go backwards, also the interface gets fiddly (ie. when you pass a node you've setup it goes dead) and the process is awkward. ie. wait - setup node, check if its ok, if not wait some more - repeat.

Is there a better way?

Edited September 30, 2017 by Splargo

[Deddly]

Welcome to the forum, @Splargo! I put your question in the Gameplay Questions section so you get the best answers.

Getting an encounter directly from Kerbin is more efficient because you can benefit more from the Oberth effect. I'll let someone else explain exactly how to do it well, because I'm sure they can explain it better than I.

[Helmetman]

I have always been able to fiddle with the manoeuvre nodes from kerbin orbit to get any encounter. Maybe your mouse sensitivity is off or you don't have cool hands to fine adjust using the maneuver icons? If your really unable to get a encounter from kerbin easily you can use the precise node mod...

This will let you fine adjust maneuver node parameters by 2 decimals places. 

I'm also not qualified to tell you much about the mathematics concerning oberth transfers. If you haven't looked it up yet, it basically tells you that burning as close to the periapsis is more efficient then burning anywhere higher then your pe. Doing your encounter burn outside Kerbins SOI will thus lose some Delta-V efficiency. In kerbins case this means that doing your transfer burn as close to 70km is the most efficient escape burn. 

Also know that getting anywhere close that is from 1-10 million meters from your target (i.e. Eve) is a very close encounter. Doing a correction burn outside the Kerbins SOI usually involves 1-5 m/s Delta-V. So it's not paramount to actually hit the target absolutely close, as long as you have a encounter the correction burn is more then forgivable.

Also note that if you've set a maneuver node in map mode you can use the "tab" key on your keyboard to switch between planets. If your target is Eve you can switch to Eve, pan your camera to Kerbin where the maneuver node is, then fine adjust and keep a close look at Eve and eyeball your vessels orbit line as your adjust the nodes. Using Precise node at this point is very recommended if your aiming for a atmospheric entry from Kerbin orbit. It's hard to get this exactly right in any case, so you'll usually need a correction burn later on.

 

[OHara]

1 hour ago, Splargo said:

When I'm orbiting around Kerbol [the Sun], and not Kerbin, I can setup a maneuver node with a burn that will give me a perfect Hohmann transfer just kissing Eves orbit, and then slide the node back and forth around my orbit around Kerbol in order to get the best phase..

What you are doing makes perfect sense for finding the best interplanetary leg of the trip.

1 hour ago, Splargo said:

If I was in orbit around Kerbin, I can only change the phase by waiting, so I cant go backwards, also the interface gets fiddly (ie. when you pass a node you've setup it goes dead) and the process is awkward. ie. wait - setup node, check if its ok, if not wait some more - repeat.

What you might want to try next, is to leave a probe or even debris in orbit around the Sun, near Kerbin in its orbit around the sun.  You can set a maneuver node on that probe to show the path to your next interplanetary destination (whether the probe has fuel to get there makes no difference for the planning node).

Then when the planet Kerbin reaches the maneuver node on your probe,  you launch your real ship into low-Kerbin orbit, and plan a burn from low orbit that joins up with the probe's planned interplanetary transfer.  Boosting into the transfer trajectory  by burning while moving at high-speed in low orbit raises the ship's kinetic energy to the needed amount with significantly less fuel spent -- the Oberth effect -- as Deddly pointed out.

That probe was marking the transfer path and showing where Kerbin is at the 'transfer window', so when maintaining a probe in solar orbit becomes inconvenient, you will probably want to use an out-of-game transfer-window planner to find the right time to launch.

[Splargo]

20 minutes ago, OHara said:

What you are doing makes perfect sense for finding the best interplanetary leg of the trip.

What you might want to try next, is to leave a probe or even debris in orbit around the Sun, near Kerbin in its orbit around the sun.  You can set a maneuver node on that probe to show the path to your next interplanetary destination (whether the probe has fuel to get there makes no difference for the planning node).

Then when the planet Kerbin reaches the maneuver node on your probe,  you launch your real ship into low-Kerbin orbit, and plan a burn from low orbit that joins up with the probe's planned interplanetary transfer.  Boosting into the transfer trajectory  by burning while moving at high-speed in low orbit raises the ship's kinetic energy to the needed amount with significantly less fuel spent -- the Oberth effect -- as Deddly pointed out.

That probe was marking the transfer path and showing where Kerbin is at the 'transfer window', so when maintaining a probe in solar orbit becomes inconvenient, you will probably want to use an out-of-game transfer-window planner to find the right time to launch.

Ingenious! Thanks!

[Palaceviking]

Google transfer windows... 

[OhioBob]

@Splargo, your method has some benefits as far as simplicity goes, but it is very inefficient energy-wise.  Others have talked about the Oberth effect and how it is better to perform the ejection burn from low Kerbin orbit.  I'll supplement that with a little bit of the math.

To do what you suggest required two burns, one to escape Kerbin's gravity, and the other to lower the periapsis of the resulting solar orbit to achieve an intercept with Eve.  Let's say we start out in a 75 km parking orbit, from here it takes a minimum of 935 m/s to escape Kerbin's sphere of influence.  Now that we're in solar orbit, we have to set up the burn to intercept Eve.  Let's say we're able to perform a perfect Hohmann transfer and intercept Eve right at the periapsis of our transfer orbit.  Lowering the orbit's periapsis just enough to perform this intercept takes about 755 m/s.  In practice it usually takes more because we're rarely able to use a perfect Hohmann transfer.  This means that the total Δv needed to intercept Eve is at least 935 + 755 = 1,690 m/s.

Another way to do it is to perform one burn from low Kerbin orbit.  This time, however, we don't stop burning when we achieved escape velocity.  Instead we keep burning until we've give our spacecraft just enough velocity that when it leaves Kerbin's SOI, it's already going -755 m/s relative to Kerbin.  There's now no need to perform the second burn because we've already taken care of it.  The velocity that is left over after escaping Kerbin's gravity is called hyperbolic excess velocity, which has the formula,

v_∞² = v_bo² - v_esc²

where V_∞ is hyperbolic excess velocity, V_bo is the burnout velocity, and V_esc is escape velocity.

We want V_∞ to equal 755 m/s, and V_esc at an orbital altitude of 75 km is 3,235 m/s.  Therefore we can solve for the burnout velocity,

V_bo = SQRT( 755² + 3235²) = 3,322 m/s.

The required Δv is simply the burnout velocity less the initial orbital velocity.  At an altitude of 75 km, the orbital velocity is 2,287 m/s, therefore

Δv = 3322 - 2287 = 1,035 m/s

As you can see, this results in a dramatic savings of over 600 m/s.

Performing your interplanetary transfer burns from low Kerbin orbit is always going the best way to do it as far as velocity change goes, and hence fuel efficiency.  It is well worth the effort to practice and gain proficiency in setting up your maneuver nodes and establishing planetary encounters while in Kerbin orbit.
 

[Spricigo]

1 hour ago, OHara said:

(whether the probe has fuel to get there makes no difference for the planning node).

Just notice that, If Comnet is on, the craft need connection or a pilot onboard.

Another way to find good transfer windows: https://alexmoon.github.io/ksp/

[ARS]

This is precisely what I did for my interplanetary mission. Circularizing an orbit around kerbol first is my method of transferring my vessel's orbit to another celestial body, especially when it's far enough that I need to use a gravity assist to help my maneuver node (Such as building base in far-off distance). It's not entirely the most efficient method, but it's easier. I always used it, even until now

[Laie]

4 hours ago, Splargo said:

I do it this way because of KSPs interface. When I'm orbiting around Kerbol, and not Kerbin, I can setup a maneuver node [...] then slide the node back and forth around my orbit around Kerbol [...]

Is there a better way?

Short answer: no.

Longer answer: You can save some delta-V if you depart directly from LKO at the right time (that is, when Kerbin is at the point where you would put the Maneuver Node when orbiting the Sun). There is no in-game method to determine that point (1), and trying to figure it out through trial and error is bound to drive you mad. You'd have to gather the target's (and Kerbin's) orbital parameters and do some calculations.

Thankfully, several people have done that before and you can benefit from their work. The Launch Window Planner has already been mentioned, but seeing where you're coming from, you might find a Phase Angle Diagram to be more suitable to your playstyle.

(1)Turns out there is a roundabout way of doing it. OHara's suggestion is just excellent.

Edited September 30, 2017 by Laie

[MrChumley]

Here is a useful chart :

There is also a calculator at http://ksp.olex.biz

[OhioBob]

Launch Window Planner has also been made into a mod, which can be easily use in the game.