Elliptical Intercepts

[Minimalist]

Rendezvous

As (I hope) you can see from the image I can get a rendezvous with a target in a highly elliptical orbit. In this case I'm sending fuel to a captured asteroid that ran out. My intercept is at, or very close to, periapsis.

My question is this: Is it easier to make the rendezvous closer to Ap or Pe?

Both craft would be moving more slowly near Ap, but I can't convince myself this represents an advantage.

As an aside, is there somewhere I can see the class of an asteroid after I've grabbed it?

--Min

[Spricigo]

 

I suppose that marginally better at AP where change in direction will be cheaper...if the relative inclination is high enough to matter at all.

[bewing]

It shouldn't matter much where you make your intercept, really. There might be a tiny deltaV difference depending on where the intercept is, but as long as it's near the Ap/Pe, it should be fine. If your two ellipses were perpendicular to each other, that would be a different matter.

And no, once you've grabbed a rock it becomes part of your ship. The only ways to see what "class" it is at that point are to look at the ship's mass and know what classes refer to what mass ranges -- or to temporarily let go of the rock and see what class it gets assinged to.

 

 

[Kryxal]

It looks like orbits are already pretty closely matched, you might be a bit better off higher up but it's not going to be a huge deal.

If you were still in a not-so-close orbit I'd probably aim nearer to apoapsis, for the sake of an easy burn to match speed and less gravity differential (the closer you are to the planet, the more difference in acceleration, though it's not a huge problem).

The only reason I can see for going near periapsis would be if you're not well-practiced in rendezvousing and trying to catch something still hyperbolic.

[Jaeleth]

Well... My opinion:

For an optimal intercept you should first match orbital planes of your target and your interceptor, this should be done right at launch (into the planet of target), if you use mechjeb there's an option that will do it automatically for you. So, assuming the planes are now the same you are left with the present case, two co-planar elliptical orbits. Assuming you are starting from a near circular LKO orbit with your interceptor you are interested in NOT getting into an higher energy orbit than your target, because that would be a waste of energy, this means you will match your Pe with the target and then push your Ap as necessary so that you and the target rendezvous at Pe. You will be trailing the target, as you should. If you are intercepting in any other point than Pe it means that your your Pe is higher than target's (waste if energy) and / or the angle of interception will be not zero (as it will be at Pe if Pe's match perfectly), meaning there is an additional DeltaV to spend. 

Bear in mind that, in the real world, it is very difficult to do this "one shot" Pe matching interception so what the Shuttle, Cygnus and so on do is to get into a circular slightly lower orbit, trailing the ISS circular orbit and approaching it orbit after orbit, pushing the Ap to match circular orbital radius of ISS when close enough (last orbit). But this is not the case you presented here so yes, in your case, match Pe's and intercept at Pe, it is the optimal way, immo.

Also bear in mind that there are no perfect circles, and the perfect circle is just a particular case of an ellipse

When intercepting high velocity asteroids in hyperbolic orbits forget perfection, you may not be able to do it at the sweet spot due to high speed of target versus your interceptor orbital period so, just get as close as you can to a low angle of interception and least possible velocity difference from target (sometimes this means intercepting well off Pe).

 

Edited June 8, 2017 by Jaeleth

[Kerbart]

You'll have to match orbits for a rendez vous. Unless there's a time constraint with doing it at Ap there's no gain energywise. What you save by not raising your apo for an intercept, you lose by matching higher velocity. Whatever is most convenient.

[Kryxal]

I suggested AP-ish because the rendezvous step will likely be easier up there.

[HebaruSan]

In the specific scenario from your screenshot, I would rendezvous near periapsis, because the two orbits are already very close there, whereas the apoapses have a significant gap between them. To make these orbits identical, you will need to burn at or near periapsis to raise the apoapsis. You might as well make that burn the close-approach burn as well.

[Draalo]

You can rename your vessel in flight from "shipname" to "shipname Class X carrier" right after catching an asteroid as a workaround.