Rescue a stranded Kerbil who is moving clockwise?

[Mister Spock]

I have a rescue mission in which the stranded Kerbil is moving clockwise -- i.e., her orbital inclination is roughly 180 degrees. Would it be fruitless to try to "match" orbits with her in the opposite direction, to save me the fuel costs of flipping my own orbit? Or would we just end up whizzing by each other at impossibly high speeds? I assume I need to suck it up and match her inclination, but I was curious what you all think. Thanks in advance.

Edited May 11, 2015 by Mister Spock

[Bobnova]

It's the whizzing by one. You'll have a closing speed of ~4200m/s. Taking off to the west rather than the east will require around 400m/s more delta-V than normal. Swapping orbit directions like that will take that ~4200m/s.

You could do some trickery like boost to ~Minmus (900dV), reverse your direction there (~400dV), then burn to recircularize your orbit going the new direction (~900dV again, less if you aerobrake). That drops the required dV to maybe 1300, but taking off to the west is significantly easier.

[cybersol]

Well around Kerbin for instance, you would be doing 2300+ m/s in one direction and she would be doing 2300+ m/s in the opposite direction. If you were to meet at the same spot, I bet it would be quite the sight.. for a moment.

To save dV there are a few useful approaches though. One would be to launch directly into the reverse orbit, costing only around 350 m/s extra m/s. If you are already in orbit, inclination changes are cheaper the higher up you are in orbit. For example, if you raise you apoapsis out to near minmus, then you are only traveling at less than 30 m/s near apo. You can thus switch your direction 180* by just burning retrograde and using only 30 x 2 m/s. You still have to pay to raise and lower you apo before and after the maneuver (930 x 2 m/s), but for large inclination changes like 180* it will save fuel overall.

Edit: Ninja's while I was afk.

[Mister Spock]

Thanks for the swift and helpful answers! I'll try taking off to the west. And cybersol, I'd long wondered about where it's most efficient to make inclination changes. Now I know! Thanks again.

[Bobnova]

Generally speaking(I don't like to say always, though I can't think of a situation where this isn't true) changing inclination costs a percentage of your current orbital velocity. That percentage varies based on how much you need to change the inclination.

As such changing it at a very low velocity is very cheap, while changing it at high speed is significantly more expensive.

The cheapest way of changing inclination is to make something else spend the dV (gravity assist off a planet/moon), but that is often impractical.

[Mister Spock]

Well, I launched west, and it worked a treat. I'm now on the same inclination as the stranded Kerbil. (But it's a pretty high Kerbin orbit -- well past the Mun -- so it's going to take a while for my capsule to catch up to the Kerbil-in-distress.) Thanks for the help!

[Fishslap]

All orbital changes are magnified over distance. Changing the peri on Kerbal from 100k to 500 k takes a long burn at Kerbal but just a slight tap on the shift key if your apo is out by Minmus. This is why planetary travel is even possible. The longer out you bubble your apo from orbit the further out you go when you stop.

[MarvinKitFox]

Or would we just end up whizzing by each other at impossibly high speeds?

Oh, you just need to aim right, and press "B" in the 0.0000217 second while she is within grasping range of the capsule door. And before she turns into a white-hot cloud of expanding plasma from the impact.

[RW-1]

One would be to launch directly into the reverse orbit, costing only around 350 m/s extra m/s.

Yes, not to be slammed here but when I read the thread I thought "What's the big deal?" you launch and go west instead of east, a little more DV but other than that it's all the same ...

[Mister Spock]

Update: I did launch west, and it worked fine -- I'd just never tried it before. And I had an easy time matching orbits with the stranded Kerbil and bringing her home. I did have excitement on re-entry, though -- I didn't change my inclination, so I went into re-entry heading clockwise, east-to-west, rather than the usual west-to-east. It was by far the hottest re-entry I have had in 1.x! My heat shield lost 40% of its ablative protection! I assume this was because I was moving faster, and undergoing more friction, once I hit the atmosphere -- although my speed as I approached Kerbin didn't seem much higher than usual. Anyway, my Kerbil survived, which is good, as this is a no-revert game.

[cybersol]

Glad to hear launching west worked well. I had not thought about the re-entry yet, but it makes sense that you were going around 350 m/s faster than normal on entry. I can also see ending up with a steeper trajectory because trying to land at KSC in that direction would be unfamiliar.