Orbital Alltitude question

[Viper_607]

So, I've gotten two of the largest tanks up into orbit, with lots of fuel to spare in my "tug" stage. I'm curious, which altitude around kerbin is more or less the ideal for docking procedures? Too low, and docking is difficult because of how quickly the ships move around the planet but too far up means that getting the ship that requires refueling to the station limits the design of that craft. So great Rocketeers, what are your experiences with this issue, and the altitude you're most comfortable with?

[Gniuz]

I go for around the 100Km mark.

G.

[Temstar]

The lower the better because you generally fuel up if you want go to somewhere far afterwards, and going far is best done while starting out in very low orbits due to Oberth effect. When you dock the orbital velocity of the spacecraft makes no difference, it's only the relative velocity between the spacecrafts that matter.

I personally use a 75km orbit. 100km is another popular LKO orbit altitude.

[Scialytic]

It depends on your priorities. For your convenience, a major consideration is that you need to be above 120 km to enable 100x time warp (although that can be circumvented by switching to another vessel). For maximizing efficiency, lower is better. With practice, docking at any altitude is easy so that shouldn't be an issue long term. Back when I was first learning docking I put things over 200 km, but now I put any Kerbin-based refueling craft at 75 to 80 km. That said, I haven't done much LKO refueling since I installed the Kethane mod. I have a Minmus mining & refining operation going, which now serves as my primary orbital refueling point.

[Pursuedtank]

You should however be in a high enough orbit so that if you end up behind something you can go into a lower orbit and catch it up. I usually go for about 100 km

[TriggerAu]

100kmish for my refuling station, as pursuedtank, it gives some room to play with lower orbits to catch up.

[Tristavius]

I always use just above 120km so as to be able to use 100x time warp and allow the inbound ship to be able to drop it's periapsis as low as 75km and catch up at a good speed (I always prefer the inbound ship to be chasing target rather than the other way round).

In short, lower is technically better, but very inconvenient, so find a good balance which in my opinion is just above 120km.

[Francesco]

you're right, the higher you are the easier it gets since the ships move slower, but it's also less efficient.

I usually stay below 100 km.

what matters more to me, though, is the difference in altitude, and thus in speed, between the two ships:

while you can go straight to rendezvous from launchpad if you're accurate enough, I prefer putting ship #2 on a lower orbit (let's say, 10 to 20 km lower), so that I can perform the rendezvous burn when I'm 25 to 50 km behind ship #1.

[capi3101]

I've done docking maneuvers low and high; in my experience, there's not much difference in the particulars. That said, I like most people go for 100 km; it's a good standard orbit height, it gives you room to adjust your orbit both up and down and you don't have to plan for a large booster to get it where it needs to go.

[Nao]

Above 120km is nice for timewarp, but 75-85 is where i do mine.

Remember that if you set the ships in a way that their docking axis is parallel your orbits rotation axis, you will cancel out almost all of the problematic effects of low orbit operations.

[Viper_607]

Above 120km is nice for timewarp, but 75-85 is where i do mine.

Remember that if you set the ships in a way that their docking axis is parallel your orbits rotation axis, you will cancel out almost all of the problematic effects of low orbit operations.

That's kinda what I was hinting at.. given how the lower the orbit, the more "Spin" you get on the ships, whilst the orbits are very similiar, i've noticed that the two aligned craft will drift off to one side or the other because of their natural movement around the gravity well... so what you're basically suggesting is to avoid this.. you approach from the "top" or "bottom" instead of behind or infront of?

[RoboRay]

No, you want to approach from the "left" or "right" of your target.

Think about this:

Your craft are in orbit, not rotating, but following a curved orbital path around the planet. Your two ships are parked nose to nose, 50m apart. A quarter of an orbit later, the one out front is now pointed straight down at the planet, while the one in back is pointed straight up at the stars. Half an orbit from the starting point, the craft are now tail to tail, still 50m apart, simply because of the curved path they follow, and they are not rotating to match the curvature.

This apparent rotation can be eliminated by turning both craft so that they point along the Normal axis, which is 90 degrees off your prograde direction, and on the horizon line of your NavBall. So, if you're orbiting to the east (heading 090), turn your craft to face north and south. This causes the craft to "roll" as they orbit rather than "pitch", giving you a stationary target to dock with.

So, maneuver into a parking-orbit 90 degrees to the left or right of your target's direction of motion, point at it, and dock from that angle.

[Viper_607]

The Top or bottom I refered to is with the right side being the planet, and the left side being outwards towards space... is that the one you're using aswell..?

[DMagic]

Left/right and top/bottom probably aren't the best descriptors to use when talking about orbit. And north/south aren't great either, but that's the direction you want the docking ports pointing. That would be at 90 or 270 degrees on the navbal, and directly between the orange and blue halves. This isn't always an issue in LKO since you are always orbiting fairly high up, but on small moons, where you might be only 10km above the surface, this can really make a difference.

[Scialytic]

And north/south aren't great either, but that's the direction you want the docking ports pointing. That would be at 90 or 270 degrees on the navbal, and directly between the orange and blue halves.

This is true in the special case of equatorial orbits. More generally it is the normal or anti-normal vector you want to align with.

[Viper_607]

So I had the right idea.. I was talking about the normal and anti normal... then again, I don't know if i'm good enough yet to put something in that exact position over it without a lot of work.. though.. I'm currently bringing my first tank up to it... to help increase the capacity of it, or rather, be a removable fuel source... I've put the whole rig in 200k orbit for now, seeing as at that height, I have wriggle room between there and the atmosphere.. and I'm horrible at launching my ships at the right time for a minimal time rendezvous. (that and with the rocket design I have for the refueling crafts... I barely make orbit :3)

[RoboRay]

Yeah, you had it right... I only used left/right because I was trying to put it in the terms you were using.

Align your docking ports along the Normal axis for the easiest docking, as it gives you a stationary target.

Just for future reference, the other primary axis not shown on that wonderful diagram I'm about to steal is Radial, which is a line from the planet's center through your craft, perpendicular to both Prograde/Retrograde and Normal.

Edited May 15, 2013 by RoboRay

[Viper_607]

SUCCESS, and with about 15% of a large 1 meter tank to spare

[TriggerAu]

Congrats

[Merinsan]

I have my fuel depot at about 100km. I use the space from 70km-100km for building interplanetary vessels, which I then use a refueling ship to refuel (too much lag to go to the fuel depot).

The height is not so important though, it depends on your ship designs, and what you are planning to do. For interplanetary missions, the later you refuel the better, but you need more fuel to get there. I've actually got another depot around Minmus for more refueling if I think I need it, not to mention depots all over the solar system...

The primary benefit of a depot in a low orbit is that it reduces your fuel requirement, so you can get a bigger payload up.

[Merinsan]

SUCCESS, and with about 15% of a large 1 meter tank to spare

Well done!

Keep working on it, you'll eventually be able to launch huge amounts of fuel into orbit.