Recommendations about the orbit for my "high" station?

[davidpsummers]

I'm building a shiny new station. My old one was just my SkyLab with a bigger habitat module stuck on it and parked ships to hold fuel (thought I got 9 good games years of use out of it). Rather than just deorbit it, I'm planning to boost it to a high oribit (giving me a "high" and "low stations". I'm going to build a nice shuttle to carry fuel and people back and forth between them).

So recommendations on the orbit? I was thinking of orbiting Minmus (give the Kerbals something to look at), but the inclination will get old pretty quick? Mun is too low (I want to be able use it to fuel ships heading out into interplanetary missions). So I'm thinking of maybe putting just within the Kerbal SOI?

Edited August 24, 2014 by davidpsummers

[Tex_NL]

If your main objective is to refuel ships I'd say stay low. Thanks to the Oberth effect starting your interplanetary burn from a low orbit is much more efficient than doing it from a high orbit. (I am not going to explain Oberth here as there are already dozens of topics explaining it in detail.) Refuelling in a high orbit is just not worth the effort.

[NNukBomba]

For me, it's 1000 km max. Just too high.

[davidpsummers]

I'm aware that circularizing a high orbit, to meet the station, costs fuel. But refueling passes that inefficiency onto another ship and allows to my mission to clear the Kerbin SOI with almost 1,000 m/s more of fuel. Given how big my rockets are getting (I'm sending bigger and bigger playloads) it seems worth it.

I do also want to put it up there "just because" (and I don't need two low stations), but I guess then I would put it some place picturesque (like over Minmus).

[Tex_NL]

You indeed negate the fuel loss by refuelling but in return you will loose about the same amount through inefficiency. Add to that the cost of getting that fuel in a high orbit in the first place ....

A direct burn from LKO to a Jool intercept costs roughly 2000m/s dV: 1000m/s to leave Kerbins SOI and another 1000m/s to intercept Jool.

Split this in two by first going from LKO to a solar orbit, then burn for a Jool intercept you'll spend a total of roughly 3000m/s: 1000m/s to leave Kerbins SOI and about 2000m/s to intercept Jool.

Edited August 23, 2014 by Tex_NL

[cantab]

If you have a station in a very high orbit, then for departures from it on interplanetary trips you should burn retrograde so your periapsis is low over Kerbin, then burn for ejection there. Delta-V wise this does indeed take less than departing from LKO.

Here's how you can work out how much delta-V you'll save this way. Put a ship in LKO, then set up a transfer to a higher orbit. The delta-V saving for a ship departing a station in the higher orbit is the first burn minus the second burn.

The thing is you need to make that retrograde burn in the right place. With a station too far out it orbits so slowly that by the time it's come round to said right place you've missed the transfer window! Minmus is manageable (except for going to Moho) and a popular choice for kethane users.

[GoSlash27]

I'd recommend edge of the SOI; circular orbit of 70 million meters and follow cantab's suggestion.

You can save even more DV if you use the mun to slingshot you.

Best,

-Slashy

[davidpsummers]

<blockquote>

A direct burn from LKO to a Jool intercept costs roughly 2000m/s dV: 1000m/s to leave Kerbins SOI and another 1000m/s to intercept Jool.

Split this in two by first going from LKO to a solar orbit, then burn for a Jool intercept you'll spend a total of roughly 3000m/s: 1000m/s to leave Kerbins SOI and about 2000m/s to intercept Jool.

</blockquote>

This doesn't make sense to me. If you burn from LKO, with 2000 m/s, then you coast out of Kerbin's SOI at 1000 m/s. Once you are there, it doesn't matter how you go there. (Potential Energy is, as they say, a "state function".) So if you are in the same spot from HKO, then why do you need more than 1000 m/s to match the first example.

[GoSlash27]

<blockquote>

A direct burn from LKO to a Jool intercept costs roughly 2000m/s dV: 1000m/s to leave Kerbins SOI and another 1000m/s to intercept Jool.

Split this in two by first going from LKO to a solar orbit, then burn for a Jool intercept you'll spend a total of roughly 3000m/s: 1000m/s to leave Kerbins SOI and about 2000m/s to intercept Jool.

</blockquote>

This doesn't make sense to me. If you burn from LKO, with 2000 m/s, then you coast out of Kerbin's SOI at 1000 m/s. Once you are there, it doesn't matter how you go there. (Potential Energy is, as they say, a "state function".) So if you are in the same spot from HKO, then why do you need more than 1000 m/s to match the first example.

This does actually work out to be roughly correct, and it's due to the Oberth effect. But as cantab mentioned, once you're up at the edge of the SOI and refueled (which effectively resets the clock) slingshotting from Kerbin* makes it cheaper. A lot of the time, the total fuel expended in the mission is far less important than the DV required to complete a single leg of a mission.

[edit] It only really takes 200 m/sec or so to get from a circular SOI orbit to an LKO periapsis, so your trip to Jool would only cost about 1,200 m/sec from up there instead of the usual 2,000. Even less if you can catch the Mun on your way out and slingshot from it. [/edit]

*It's not technically a "slingshot", but rather gaining a whole lot of velocity for cheap in order to make use of the Oberth effect at periapsis...

Best,

-Slashy

Edited August 23, 2014 by GoSlash27

[mhoram]

A while back there was a thread about ideal altitudes for refuelling stations with some charts.

Might be interesting for you.

[Kasuha]

I definitely recommend station at 650 km or so.

It allows you highest time warp

Transfers to Duna/Eve come cheaper than from LKO

Transfers to Jool/Eeloo/Moho come about the same

Kerbin looks great from that distance

Orbital drift causing docking problems to some people is negligible

---

Regarding high (Minmus-level) parking orbits and using drop to low periapsis and burn from there, they are very effective but come with their own problems. You can only reach about 90° sector (in two 45° parts), the rest is getting ineffective fast and you need to wait till you coast to a suitable place which may cause you to miss the whole launch window. It can be fixed with very careful planning, but that can get very bothersome - you need to prepare three very well adjusted maneuvers and you need to do so up to 60 days in advance.

Edited August 23, 2014 by Kasuha

[Belphegor]

i'd say 420 km

[davidpsummers]

OK, I guess it all comes down to the Oberth effect, which I have to say I don't really understand. I mean, it all comes down to your velocity, but wrt what reference frame? I mean, the spot of highest orbit velocity around Kerbin can be very different than the highest velocity wrt to the sun's reference frame.

[davidpsummers]

I definitely recommend station at 650 km or so.

It allows you highest time warp

Transfers to Duna/Eve come cheaper than from LKO

Transfers to Jool/Eeloo/Moho come about the same

Kerbin looks great from that distance

Orbital drift causing docking problems to some people is negligible

Ironically, I hadn't thought about the drift and that can quite annoying! The time warp issue isn't bad. Maybe I'll move it there. I can always move it again.

[Eric S]

This doesn't make sense to me. If you burn from LKO, with 2000 m/s, then you coast out of Kerbin's SOI at 1000 m/s. Once you are there, it doesn't matter how you go there. (Potential Energy is, as they say, a "state function".) So if you are in the same spot from HKO, then why do you need more than 1000 m/s to match the first example.

Others have given numbers, I'll try to explain why the numbers are right at a conceptual level rather than the technically correct reason.

Basically, the faster you leave a gravity well, the less time that gravity well has to slow you down. Given that LKO orbital speed is over 2200 m/s, that's a lot of velocity that can be taken with you.

As an example, consider a uniform (as opposed to realistic) gravity well that has 1 m/s of acceleration towards the center of the gravity well and a 50 m sphere of influence. If you pass through the center of the gravity well at 10 m/s, in 10 seconds you will travel average velocity * time, or (10+(10-10))/2 m/s *10s = 50 m, so you arrive at the edge of the SoI with zero relative velocity. If on the other hand, you expend 2.5 m/s delta-v instantaneously at the center of the SoI, you wind up leaving the SoI in half the time: (12.5+(12.5-5))/2*5s = 50. Note in this case that your exit velocity is 7.5 m/s, despite the fact that you only added a third as much velocity, meaning that you got 5 m/s of velocity for "free." Not literally free, but do you understand why the delta-v burn resulted in an exit velocity greater than the burn?

This same process works on a realistic gravity well, I'm just not awake enough to do the math for that.

If you want a more correct answer, the thing to remember is that kinetic energy isn't linear, but squared, so the faster you're already moving, the more energy a fixed amount of acceleration adds to the system. This means that acceleration while you're whizzing around a planet at over 2 km/s means a lot more than when you're standing still relative to that planet. This is the heart of the Oberth effect.

OK, I guess it all comes down to the Oberth effect, which I have to say I don't really understand. I mean, it all comes down to your velocity, but wrt what reference frame? I mean, the spot of highest orbit velocity around Kerbin can be very different than the highest velocity wrt to the sun's reference frame.

I'm not exactly certain of that myself, not having gotten as far as orbital mechanics in physics during college. I think that the reason that your velocity relative to your current SoI is the important one is because despite people saying that the Oberth effect has nothing to do with gravity, in the absence of gravity (or other external acceleration cause), the Oberth effect is just navel gazing. So you'll see the strongest Oberth effect from the strongest gravity influence you're currently in, which is pretty much defined by your SoI.

Edited August 23, 2014 by Eric S

[GoSlash27]

OK, I guess it all comes down to the Oberth effect, which I have to say I don't really understand. I mean, it all comes down to your velocity, but wrt what reference frame? I mean, the spot of highest orbit velocity around Kerbin can be very different than the highest velocity wrt to the sun's reference frame.

It doesn't make any sense intuitively, so it's just not you. You *do* have a potential energy advantage up there, but unless you convert that potential energy to kinetic energy, you may as well not bother, because the potential energy itself is worthless in interplanetary transfers.

The Oberth effect is gained IRT Kerbin's inertial frame, while the benefit is reaped IRT Kerbol's inertial reference.

I look at it this way: The height advantage of near-SOI orbits is really nil in Kerbol system scale. What matters is velocity. Say you need 4,500 m/sec (on top of Kerbin's 9,300) to set up a Hohmann transfer to Jool. At LKO, you've already got 2,300 of it. Up at the edge of SOI, you've only got 220 (assuming you're in a circular orbit). It would be a huge disadvantage to burn from up there to make up the lost velocity... *but* if you spend 200 m/s to get you down to LKO in a highly elliptical orbit, then once you get there, you're booking along at about 3,500 m/sec.

Put another way... getting a payload to Jool is work. Work is thrust applied over a distance. If you nail a rocket to a tree and light it, it provides thrust but does no work, because all of it's chemical potential energy is expended in accelerating propellant out the back. None of the potential energy applies to the *real* work of accelerating the rocket itself. Likewise, if the rocket is travelling at a high velocity, pretty much all of it's potential energy is converted into kinetic energy, resulting in more work accomplished.

The faster the rocket is going, the more work it can do with it's conversion of potential chemical energy into kinetic energy.

Best,

-Slashy

Edited August 23, 2014 by GoSlash27

[O-Doc]

My personal preference is stationary orbit. Easy on the mission planning. You always get to and from Kerbin with identical burns. You can retro burn back to Kerbin for planetary transfers or burn up for Mun slingshot. Optimal transfer orbit is around 120km, you get fast(er) time speed up, low cost refueling and decent Oberth effect. Higher orbit position is up to personal preference. Minmus reduces transfer window opportunities and has an inclination problem. Mun is OK, with added ability to service bases. I still prefer stationary orbits for reasons mentioned.

[nilof]

To the people championing a low orbit depot because of the Oberth effect: you are making the faulty assumption that the interplanetary transfer burn has to happen at the depot. In practice, to go to another planet you drop down from your high orbit to a highly eliptical orbit with a low periapsis, and do the interplanetary burn at periapsis.

Putting a depot at a high orbit is preferable IMHO because you can make both your departure stage and the depot smaller for the same flights, leading to less lag and wooblyness during docking. Also, randezvous at high orbits is significantly less time consuming and less frustrating. I find that the small burn you need to make to circularize/decircularize at high orbits is similar to or less than what i waste doing a lazy randezvous with a small plane change in LKO. At high orbits you can approximate your trajectory as a straight line for adjusting final approach, and plane changes are really cheap.

A Minmus or high Mun orbit depot is a good option. You can use it to stage missions to unexplored lunar biomes in career mode, or to test the lander on a new craft, and the delta-v to decircularize is low enough. On the other hand SOI changes can be a bit annoying.

Also, avoid stationary orbit. That is a terrible staging orbit delta-v wise.