Advice on space dock orbit

[Captain Sierra]

So, I wrote a blog yesterday that was some theorycrafting about a new station design. I'll be posting another later tonight as I work on preliminary design sketch builds and some proof of concept stuff.

Bottom line is I have yet to decide where to put it. The station itself is meant to be a space dock for large utility ships and interplanetary missions to Duna, Jool, and maybe Eve. For more info beyond that, read the blog post. I was tossing around some ideas and have roughly settled on 3-4 potential orbits. These currently are 350x350, 500x500, 1,000,000x1,000,000, and ~1,500,000x1,500,000. The station is an orbital construction space dock meant as a staging ground for missions. Missions are often sent up empty (or close to it, enough for rendezvous) and fueled at the docks. The dock's fuel supply comes from my Mun and Minmus kethane operations. I need an orbit that is good for receiving crafts inbound from Munar orbital altitude as well as from Kerbin. Bear in mind the Mun orbits somewhere around 5,000,000km up. So, suggestions? I'm looking to launch the first of the three gantry sections tomorrow. The gantry itself is fairly easily moved so if I can't get a solid answer in a day, I can always push it around some. That aside, there is no moving it once I start building the respective facilities on the ends of the gantry.

[TechnicalK3rbal]

I would suggest 700km, that way you can time warp fully. (I think.)

[Captain Sierra]

Well, I still have two very high orbits above that. I could use some more input by others.

[Sirine]

Make use of the Oberth Effect.

Launch from Kerbin KSC to LKO is the best. You will benefits from less DV needed to destination, hence less parts counts, and less times for mining, planing, rendezvous, docking...etc...

My test result...

http://forum.kerbalspaceprogram.com/threads/61387-Kerbin-SOI-versus-Sun-SOI?p=834037&viewfull=1#post834037

Full chart.

http://forum.kerbalspaceprogram.com/threads/61387-Kerbin-SOI-versus-Sun-SOI?p=834183&viewfull=1#post834183

unless, you enjoy fantasy of having a station that does nothing...

[Soda Popinski]

There is the question of, is will you have more delta-V to spare if you start from LKO, or if you start from a high orbit fully fueled from Minmus / Mun sources? I was contemplating starting my Duna mission from Minmus orbit, as I set up a kethane operation there. I ultimately decided I was too inept to calculate ejection angles while orbiting Minmus, and wasn't sure the timing would be right, as Minmus should be going prograde to the sun at the time that I'd want to burn out.

[Captain Sierra]

I'm fully aware that rational thinking disagrees with my plans. Hasn't stopped me yet! There are some other problems which I discuss in the blog I wrote as a theorycraft. There is now a link in my sig to a forum thread which has the links of all my blogs relating to the project in order. Feel free to give them a read and comment.

[capi3101]

Mün's at 12,000,000 m, isn't it?

I'll second the suggestion for an orbit closer to Kerbin; you could always transfer back your Mün and Minmus mining operations and aerobrake them into somewhere close to the station's orbit. The same cannot be said for a craft headed the other way.

[Captain Sierra]

There is the question of, is will you have more delta-V to spare if you start from LKO, or if you start from a high orbit fully fueled from Minmus / Mun sources? I was contemplating starting my Duna mission from Minmus orbit, as I set up a kethane operation there. I ultimately decided I was too inept to calculate ejection angles while orbiting Minmus, and wasn't sure the timing would be right, as Minmus should be going prograde to the sun at the time that I'd want to burn out.

Given that this is a space dock, any mission launched from it will be fully fueled. I am also trying to situate it low enough to get some benefit from Oberth while still being able to get the advantages of less dV to escape velocity and being in closer proximity to the Mun and Minmus. In my experience, the first 50% of your Munar transfer dV is in the first 25% of orbital altitude. Being up in the million km range shaves a huge amount off the fuel requirements of the tankers to get back to the Mun.

[Alistone]

I'd recommend the 350x350. The further out the orbit the more time you spend trying to wait for the launch window. A lower orbit makes the windows occur more frequently making docking easier (in my opinion). Similarly when you are getting ready to leave you don't want to be waiting two weeks for your orbit to circle around to the correct phase angle for your ejection burn.

I played around with this tool http://ksp.olex.biz/ by picking a target and moving the parking orbit (that you leave from) at different alititudes and discovered that the optimal parking orbit depends on how far you are going. For close objects (Duna) it is helpful to leave from a higher orbit. For distant targets it is better to leave from lower orbit to better utilize the oberth effect.

At less than 100km orbits I sometimes had issues with the daocking target "spinning" as it progressed around it's orbital path while I was trying to line up the clampotrons. Higher orbits reduce and would support a higher orbit.

Overall I think it doesn't matter much. By the time you get atomic engines you usually have dV to spare and as long as you do your maneuvers correctly you'll be fine. Consider though that the gain you get from leaving from a higher orbit could easily be lost by using a non-ideal launch window because you weren't in the right part of your orbit place when the planets aligned.

Edited January 9, 2014 by Alistone

[technion]

There is the question of, is will you have more delta-V to spare if you start from LKO, or if you start from a high orbit fully fueled from Minmus / Mun sources?

I'd be very interested in seeing this calculated. Getting maximum efficiency on the burn is one thing, but my last mission took six launches just to assemble - and each one of those involved either abandoning fuel or using it to extend to orbit. I ended up launching form 1.5M km, fully fuelled, with the hope that allows more total dv than spending reasonable a portion of my burn getting the ap from 70km up to that level.

[Captain Sierra]

I'd be very interested in seeing this calculated. Getting maximum efficiency on the burn is one thing, but my last mission took six launches just to assemble - and each one of those involved either abandoning fuel or using it to extend to orbit. I ended up launching form 1.5M km, fully fuelled, with the hope that allows more total dv than spending reasonable a portion of my burn getting the ap from 70km up to that level.

This is one of the theorys I'm basing this off of. Bear in mind, the station may be functional but 600 part station + 300 part interplanetary ship != rational. Rational thinking is being thrown out the window. Efficiency is a moot point here. I'm focused on ease of access from both Mun and Kerbin. Higher orbits tend to align much more frequently for docking purposes which makes rendezvous easier on me. It's going up high, the question now remains how high.

[BlackBicycle]

I just regularly go 610km circular. That is enough for a full time warp (600+). The extra 10 km helps when your stations orbit gets skewed a bit due to the craft docking/departing.

[thiosk]

Add a vote for 350x2. Mine are at 300. This is a great, useful orbit. IT doesn't take TOO much juice to get circular there, and the circle is big enough that its feasible for eject with nuclear engines. I'll explain this: last night I had my ship in 150x150 orbit. I was going to do a jool transfer. I had these two orange droptanks on board, 2 orange tank equivalent fuel, and only two engines. The orbit was way to fast to let me do a reasonable burn. I ended up doing two passes to get an escape, then had to burn outside kerbin SOI. This ended up adding in the neighborhood of 500+ dv to the trip (but I got to jool with 2 full orange tanks of fuel to facilitate my ongoing science operations there!!)

From the 300x2 orbit, it would not have been so bad.

For long timewarps, I have a package sitting on kerbin with a protractor mod on it. I switch to it and let the clock run when its time to await windows.

Edited January 9, 2014 by thiosk

[sjwt]

I recently read a thread somewhere about the optimal orbit to leave Kerbin's SOI from for a planetary transfer, apparently you can save in the realm of 100's DV if you leave from the right orbit, as apposed to a lower or higher orbit.

[SkyHook]

I would go for a low orbit. 180+ will get you far enough away from Kerbin to take advantage of the lower resolution textures, and it's not THAT much more delta-v than, say, a 100Km orbit. Any higher, and your maximum payload per launch will decrease and your delta-v (and engineering) requirements will just go up. My spacedock will be at about 250Km (for RP purposes).

[LethalDose]

I'm fully aware that rational thinking disagrees with my plans. Hasn't stopped me yet! There are some other problems which I discuss in the blog I wrote as a theorycraft.

Theorycrafting that eschews rationality?

Anyway, higher orbits maximize Oberth Effect, and if you're trying to optimize interplanetary craft, then you want to optimize Oberth. This is done by burning retro in a high circular orbit until Pe is just above Kerbin's atmo, then riding it down and burning at Pe to maximize your kinetic orbital energy.

You've stated that all craft leaving this station will be fully fueled, so fuel consumption from Kerbin to the station is pretty irrelevant.

I've found higher orbits increase the frequency of transfer windows, since there's less synchronicity between Kerbin's surface and high orbits than between Kerbin's surface and LKO.

For stations to prep interplanetary missions, I don't see any strong argument for LKO.