Delta-V Calculations - Kerbol Rescue Mission

[ChrisD]

I have had Sanrys Kerman patiently orbiting Kerbol (Sun) at about 2.4million KM since early on in the game. I reckon I now have a technology level that I can effect a rescue, however I don't know how to calculate how much delta-v I will need to recover him back to Kerbin. It's going to be a long mission so I want to have all by Kerbals in a row before I launch.

MechJeb has calculated that it will take about 3km/s to reach him from a 1000km orbit of Kerbin but how can I figure out how much will be needed to get him back home. Any tips?

Chris

[DeMatt]

As with most orbital mechanics, the delta-V necessary to travel one direction is the same as the delta-V necessary to travel the opposite direction.

So if MechJeb thinks it needs 3 km/s to get down to Sanrys, it'll need 3 km/s to get back up to Kerbin.

But... I think MechJeb is lying to you.

To reach low Moho orbit from LKO requires more than 4 km/s of delta-V, and you're trying to reach an orbit even deeper in Kerbol's gravity well. Double-check that MechJeb is also accounting for the circularization burn you'll need to match orbits (not just altitudes) with Sanrys.

[FancyMouse]

Not exactly equal when you want to return right after rescue. Interplanetary transfer dV is not a constant. It depends on the current location, how long you want to travel, etc. Most likely when you do an optimal transfer to somewhere else, it might not be the optimal launch window for going back.

[ChrisD]

DeMatt - you are correct - the delta-v indicated is just for the transfer to the orbit. I guess I'll need to factor in a decent margin to match velocities when I get to the correct orbit. It's looking like an 8km/s craft.

He might have to hang out where he is until I have fuel stations set up around Gilly, Duna and Moho! I hope he is well provisioned!

[tomf]

The way that I would approach this is to look in my persistance file and get the orbital elements for Sanrys. They are the block containing values such as SMA, LPE, LAN etc

I would then add Sanrys as a new planetary body using those values in the launch window planner http://alexmoon.github.io/ksp/ - tha add new vessel option doesn't seem to work for me

I would then use the tool to find the best time and delta v in order to rendevous with him and then use it again to find out how much delta-v I am going to need to get back

[MarvinKitFox]

I have had Sanrys Kerman patiently orbiting Kerbol (Sun) at about 2.4million KM since early on in the game. I reckon I now have a technology level that I can effect a rescue, however I don't know how to calculate how much delta-v I will need to recover him back to Kerbin. It's going to be a long mission so I want to have all by Kerbals in a row before I launch.

MechJeb has calculated that it will take about 3km/s to reach him from a 1000km orbit of Kerbin but how can I figure out how much will be needed to get him back home. Any tips?

Chris

2.4 million KM Kerbol orbit?

Yes, from 1000 km Kerbin orbit, you need 3018m/s to get there.

But that is the least of your worries.

Matching orbits, so you pick up a Kerbalnaut, not a bugsplat on your windshield: 6160.9m/s

Getting back out to Kerbin: another 6160.9m/s.

Total from 1000km orbit start: 15400m/s

This assumes zero losses to plane changes, bad navigation, and instantaneous burns.

Also assumes you have a ship that can withstand a 6.5km/s re-entry brake in Kerbin atmosphere. Otherwise add that, too.

Edited June 4, 2015 by MarvinKitFox