Delta-V for Returns

[Chads]

Hi there,

I really like the subway-style Delta-V map on the wiki page for Kerbin (http://wiki.kerbalspaceprogram.com/wiki/File:KerbinDeltaVMap.png).

I'm curious, is there anything out there that talks about required Delta-V for returning to Kerbin?  I was able to safely return to Kerbin from Low Munar Orbit on 575m/s of Delta-V.  Along with the 810m/s escape velocity of the Mun, that'd be nearly 1400 Delta-V necessary from the surface of the Mun, correct?

Any pointers, tricks, lists, etc. to go on here?  Or, do I just need to experiment? (Which I'm happy to do. )

[HebaruSan]

Orbital velocity for Mun is usually around 550 m/s. Starting from that, you can return from Mun to Kerbin for less than 300 m/s:

And less than 200 m/s from Minmus:

Calculation of those burns was added to Astrogator in early February:

 

[Spricigo]

this webpage

 

[Chads]

I'll check those out!  Thanks!

 

[Spricigo]

32 minutes ago, Chads said:

Hi there,

I really like the subway-style Delta-V map on the wiki page for Kerbin (http://wiki.kerbalspaceprogram.com/wiki/File:KerbinDeltaVMap.png).

I'm curious, is there anything out there that talks about required Delta-V for returning to Kerbin?  I was able to safely return to Kerbin from Low Munar Orbit on 575m/s of Delta-V.  Along with the 810m/s escape velocity of the Mun, that'd be nearly 1400 Delta-V necessary from the surface of the Mun, correct?

According to the map you linked:

Form the surface to 14km orbit: 580m/s

From 14km orbit to scape velocity: 310m/s

subtotal thus far 890m/s.

Now, your obit around Kerbin will depend on the ejection angle. You may be either in collision course of Kerbin or headed to interplanetary space depending where you did the burn. If you are in collision course just a small burn to adjust the periapsis is required (assuming your vessel can resist the reentry), otherwise...who knows?!

[Chads]

1 hour ago, Spricigo said:

According to the map you linked:

Form the surface to 14km orbit: 580m/s

From 14km orbit to scape velocity: 310m/s

subtotal thus far 890m/s.

Now, your obit around Kerbin will depend on the ejection angle. You may be either in collision course of Kerbin or headed to interplanetary space depending where you did the burn. If you are in collision course just a small burn to adjust the periapsis is required (assuming your vessel can resist the reentry), otherwise...who knows?!

So, it looks like those numbers are bi-directional, then?  I assumed those numbers were from the perspective of "Kerbin-to-..."  So, 3400 to escape Kerbin, 860 for transfer to Mun, 300 to circularize orbit, and 580 to get to surface.

I looked at Astrogator--I'll give it a try.  

I believe it took me 280 to get into Kerbin Orbit from the Mun, but then another 290 to get the Periapsis into the atmosphere (at 20km).  Now, I was not waiting around to optimize that--it's quite possible I could have done it all in one burn, if I'd worked on it more.

[Spricigo]

Just now, Chads said:

So, it looks like those numbers are bi-directional, then?

Exactly.

Notice that the numbers in the map are approximations, because its hard to predict how much will be lost to gravity, drag and piloting. (Let alone the variation in engine performance for planets with atmosphere.)  Also, in some circumstances the mods the provide deltaV readout will get confused (a notorious case is when your craft has drop tanks)

 

[HebaruSan]

In general, going from Orbit A to Orbit B has the same cost as going from Orbit B to Orbit A, just in the opposite direction. This is because the magnitude of the burn is just the difference in your speed at that point in Orbit A versus Orbit B.

In the case of a return, you're essentially undoing the capture burn, which the map says is around 310 m/s, and which is not too far off from the 265 m/s from my screenshot. The tricky part is the timing; you need to burn at just the right part of your Mun orbit to do it that efficiently. If you then wanted to circularize at LKO after that with no aerobraking, it would cost the same as the initial burn to get an intercept, again because the two pairs of orbits are the same.

[Chads]

4 hours ago, HebaruSan said:

In general, going from Orbit A to Orbit B has the same cost as going from Orbit B to Orbit A, just in the opposite direction. This is because the magnitude of the burn is just the difference in your speed at that point in Orbit A versus Orbit B.

In the case of a return, you're essentially undoing the capture burn, which the map says is around 310 m/s, and which is not too far off from the 265 m/s from my screenshot. The tricky part is the timing; you need to burn at just the right part of your Mun orbit to do it that efficiently. If you then wanted to circularize at LKO after that with no aerobraking, it would cost the same as the initial burn to get an intercept, again because the two pairs of orbits are the same.

OK, good.  I think I'm getting it, then.  Many thanks!

[Zhetaan]

14 hours ago, HebaruSan said:

The tricky part is the timing [snip]

@Chads:

The numbers are bi-directional, as others have said, but you were correct, too.  The other tricky part about these is understanding that most of the maps are Kerbin-centric, which is to say that they are easiest to understand from the perspective of going from Kerbin to another planet.  With a little work (mostly craning your neck), you can grasp the necessary return delta-V (the trick here is knowing that you're not necessarily starting from the surface of the visited body; you may be starting from orbit), but assists and moon-to-moon transfers can't be read from the basic map.  Keep that in mind if you ever decide to try for a grand tour or multi-planet flyby.

For example, let's say that you want to go from the Mun to Minmus (or vice versa), perhaps to train a crew or for a couple of tourist contracts.  The delta-V map you have doesn't have good answers for that transfer:  you don't want the LKO-to-flyby transfers because you're not starting from LKO, but at the same time, the flyby-to-orbit numbers are based on an orbit that begins at LKO.  Either way, it appears on the map as though the transfer should take up to 500+ m/s, but in reality, you can get a Mun-Minmus transfer every eight days or so for between 210 and 220 m/s.

Does that sort of issue arise very often?  No.  But that's how it trips you.

[Tyko]

15 hours ago, Chads said:

So, it looks like those numbers are bi-directional, then?  I assumed those numbers were from the perspective of "Kerbin-to-..."  So, 3400 to escape Kerbin, 860 for transfer to Mun, 300 to circularize orbit, and 580 to get to surface.

I looked at Astrogator--I'll give it a try.  

I believe it took me 280 to get into Kerbin Orbit from the Mun, but then another 290 to get the Periapsis into the atmosphere (at 20km).  Now, I was not waiting around to optimize that--it's quite possible I could have done it all in one burn, if I'd worked on it more.

You're correct. With a perfectly timed burn you can return from the mun back to aerobraking on Kerbin for around 300. The trick to leaving the Mun and returning to Kerbin is placing your maneuver node correctly. Try this:

orient your view so the Mun is at the bottom of your screen and Kerbin is directly above at 12 O'clock. If you're in a prograde orbit ( counter clockwise) your maneuver node should be placed on your Mun orbital line at about 1-2 o'clock position.  This will cause you're to leave the Mun's SOI more or less directly backwards relative to the Mun's orbit. 

Orbits are defined by speed. To lower your orbit you have to slow down. By leaving the Mun heading backwards from the Mun's orbit you'll leave the Mun's SOI going slower than the Mun.

drop a maneuver node at that 1-2 o'clock position then play with it. Increase or decrease your burn and also try sliding the maneuver node back and forth around the 1-2 o'clock position and you can see how you projected orbit will be affected. 

Hope this helps. 

Edited July 6, 2017 by Tyko