[Tutorial] Non-Hohmann Interplanetary Transfers

[Mr Shifty]

EDIT: Note that this tutorial actually does not work. Specifically, the non-Hohmann transfers do not necessarily require you to exit the origin planet's SOI parallel to its sun-centric trajectory. I'll update and remove this notice at some point.

I'm loving alexmun's amazing Launch Window Planning web-app. It gives fantastic results, but it's not immediately obvious how to use them; this guide provides a step-by-step.

To provide an example, I thought I'd plan a trip (my first) to Jool from Kerbin. Here are the constraints for my example problem:

- My persistent game was about at 1yr:200d.

- I have a launch vehicle plus transfer vehicle with a total delta-V of about 11km/s. With at least 4.5km/s to get into LKO, I have about 6.5km/s for the transfer. The ultimate goal of the trip is to put a survey satellite in orbit around Laythe, so wanting to reserve some m/s for that, I wanted to limit the total transfer delta-V to less than 5km/s.

- My Kerbals have an election coming up in 2 Kerbin years (~106 Earth days) and need Jool intercept as a campaign booster. So, we need to make it there in less than 212 days.

To reiterate: Start after day 200 and with a transfer delta-V of less than 5km/s achieve orbit prior to day 412.

Step 1: Use alexmun's calculator to discover a transfer characteristic with your desired parameters.

Here's what I got when I entered the parameters above. (I use a 500km parking orbit so I can use 10,000x warp.)

Note my total delta-V is just under 5,000m/s. I've boxed the four important parameters in red.

Step 2: Establish your orbit and warp to about 2 hours prior to your launch window.

- Kerbal Alarm Clock is very handy for this.

Step 3: Rotate your display in map view so that you're looking directly down (south) onto your orbit.

Step 4: Zoom out until you can see Kerbin's procession around the sun, then orient your view so that Kerbin's prograde direction is pointing directly up to the top of the screen.

- Make sure you do this near in time to your launch window, since Kerbin's prograde direction changes as it rotates around the sun.

- Note that Kerbin, like all the planets, rotates counter-clockwise around the sun, so prograde means the sun is 90 degrees to the left of Kerbin when looking directly south.

- You can zoom in and Kerbin's orbit should look like a vertical straight line from top to bottom of the screen. Unfortunately it fades after a second or so, so you'll probably have to zoom in and out a few times to make sure it's right.

Step 5: Determine approximately where the ejection angle intercepts your orbit. This is the ejection point.

- If you're moving to a higher orbit (e.g. from Kerbin to Jool), the ejection angle will be to prograde. If you're moving to a lower orbit, the ejection angle will be to retrograde.

Here's the approximate position of the 101 degree ejection angle for my Jool transfer. (If you're clever, you'll notice that prograde is not actually at the top of this image. Please just pretend it is.)

Step 6: Advance your orbit until you're just past the ejection point, with one rotation to go until your launch window.

- Most LKO orbits run from 30-60 minutes. Add the times to and from apoapsis and periapsis to determine your orbital period, or just use MechJeb or Engineer Redux.

- You probably won't be able to hit your launch window exactly. You should be able to get within 1/2 of an orbital period.

Step 7: Add a maneuvering node at the ejection point, then add prograde delta-V until it equals delta-V_ejection*cosine(inclination_ejection).

- The cosine function lets you determine how much of the escape trajectory is in the prograde direction.

Step 8: Drag and rotate the maneuvering node around your orbit until the escape vector is parallel to the prograde or retrograde direction.

- The point of the ejection burn is to put your ship onto an escape trajectory that parallels Kerbin's orbit around the sun. That way you make maximum use of your ship's orbital velocity around Kerbin to either speed you up or slow you down for your transfer.

Here is my initial escape trajectory.

Now nicely paralleled.

Step 9: Adjust your maneuvering node for the ejection inclination.

- Add north (for positive inclinations) or south (for negative inclinations) until the total delta-V equals the total ejection delta-V.

Step 10: Zoom out and see how close you've gotten.

- You may need to tweak the maneuvering node slightly to get an intercept. The launch window and angle imprecision could cause minor adjustments to be necessary. MechJab's maneuvering node editor is very useful for this.

Note that my final trajectory passes much further from Jool than planned, is about 30 days shorter, and uses an extra 100 m/s or so of m/s.

Step 11: Perform the burn.

- These are typically long burns, 1-2km/s, so as with any burn, try to center the burn around the maneuvering node time.

- Watch the orbit in map view as you get to the end of the burn to ensure you make intercept.

- If you don't make intercept, then a very small (<100m/s) burn should get you there. Play with maneuvering nodes to find it.

During my Jool transfer, I made a 67 m/s burn 6 days out that pulled my Jool periapsis all the way into its atomosphere.

Now just coast to intercept and burn to achieve orbit. You're done!

Here's my hyperbolic approach to Jool.

Achieving orbit took less than 500m/s because I was able to aerobrake that hyperbola into a fairly small ellipse (semi-major axis of ~400km). I estimate my total delta-V for the transfer was well under 3500m/s. Plenty of energy left for a transfer from Jool to Laythe.

Happy trails!

Edited August 16, 2014 by Mr Shifty
Removed the note about the KAC index bug, which was resolved over a year ago.

[kahlzun]

I'm a bit confused.. how is this non-hohmann?

You're launching in a parabolic trajectory using a single, short burn, and allowing your momentum to carry you into the SOI of another body.

Textbook Hohmann.

Edited June 9, 2013 by kahlzun

[Mr Shifty]

I'm a bit confused.. how is this non-hohmann?

You're launching in a parabolic trajectory using a single, short burn, and allowing your momentum to carry you into the SOI of another body.

Textbook Hohmann.

Hohmann transfers are single plane transfers from one circular orbit to another, where the transfer ellipse is tangential to both orbits. A Hohmann intercept can only be accomplished if the launch burn occurs when the two orbiting bodies have a particular phase angle, so launch windows are infrequent. This tool (and tutorial) show how to calculate transfers with a wide range of launch windows, transit times, and delta-V requirements and between eccentric and inclined orbits.

[Geschosskopf]

Thanks muchos! I'm trying my 1st interplanetary flights now and have been having a horrible time because the tutorials I've found have all be for older versions of KSP and things are rather different now, especially how maneuver nodes appear in the game. Thus, I haven't been able to translate the old info into the current build, which is all I've ever played.

[Aldmehr]

Can you further explain step 8?

When you "Drag and rotate the maneuvering node around your orbit until the escape vector is parallel to the prograde or retrograde direction", doesn't this modify the 101 degrees to prograde ejection angle set up in step 7 to some other ejection angle?

What am I missing?

[Mr Shifty]

Can you further explain step 8?

When you "Drag and rotate the maneuvering node around your orbit until the escape vector is parallel to the prograde or retrograde direction", doesn't this modify the 101 degrees to prograde ejection angle set up in step 7 to some other ejection angle?

What am I missing?

The 101 degree angle is the angle of your impulse burn. That plus your ejection velocity should produce a final trajectory that is parallel to Kerbin's prograde or retrograde. So step 7 is to get the maneuvering node close; step 8 is to dial it in.

[SDIR]

Can't you, instead of zooming in and out, just use the side of the planet facing the sun, and have the bright side on the left amd the dark side on the right?