Best parking orbit for interplanetary voyages?

[Ketatrypt]

So I am doing a bit of experimentation on interplanetary orbits, and I am trying to figure out the most convenient starting location. LKO is pretty basic, and from there, with a nice moon slingshot I can get to interplanetary space with ~ 875 delta v.

 

And then I could also use a station in orbit of one of the moons, but, in that regard, I am wondering if it is worth the trouble of making a moon-station for interplanetary voyages?

 

and also, I have been wondering... how can I use kerbin as a slingshot assist starting from a moon? Its simple enough when going from kerbin to the mun, I can pick up several hundred deltaV from the muns gravity assist, but I don't quite understand how to do it in reverse with the planet.

 

thanks

Edited February 2, 2016 by Ketatrypt

[Evanitis]

3 minutes ago, Ketatrypt said:

And in that regard, how can I use kerbin as a slingshot assist starting from a moon?

As far as I know it can't be done, as you need a satellite (read: moon) in the SoI of the body you are orbiting. But you -can- exploit the Oberth-effect: lower your PE from the munar orbit to almost touch Kerbin's atmo, and burn there. People more knowledgable than myself says this way is the most dV efficient way to get an escape trajectory. Of course it comes with the added hassle of refueling around the Mun / Minmus, and timing for a precise launch window is harder. Also, Minmus is the more efficient option of the two, but than you also need to deal with the inclination.

[Plusck]

18 minutes ago, Evanitis said:

As far as I know it can't be done, as you need a satellite (read: moon) in the SoI of the body you are orbiting. But you -can- exploit the Oberth-effect: lower your PE from the munar orbit to almost touch Kerbin's atmo, and burn there. People more knowledgable than myself says this way is the most dV efficient way to get an escape trajectory. Of course it comes with the added hassle of refueling around the Mun / Minmus, and timing for a precise launch window is harder. Also, Minmus is the more efficient option of the two, but than you also need to deal with the inclination.

Agreed with almost all of that. Especially the "slingshot" bit: that requires you to enter at one angle (and altitude) into the SOI of a body in orbit, and leave at another angle (and altitude).

For exploiting the Oberth effect, however, that depends on where you're going. If you're going to Duna or Eve, you actually expend the least energy to get there if you're already in a very high (close to Mun) orbit. If you add a burn to return to a low Kerbin Pe, you're actually wasting that energy.

"Someone more knowledgable" has worked out the most efficient orbits for getting from one planet to another (and from one moon to another within the Kerbin and Jool systems):

http://forum.kerbalspaceprogram.com/index.php?/topic/33699-efficient-hohmann-transfer-altitudes/

So for Eve and Duna, the optimal orbits straddle the Mun's orbit.

That certainly does not mean that you should raise your orbit to the Mun before making an interplanetary burn, but it does mean that if you are at the Mun, you should just burn straight out of the Kerbin system.

If you're going further than Eve or Duna, then yes, the Oberth effect from Kerbin starts being more useful, so dropping your Pe to a lower Kerbin orbit will help. Exactly how much it will help, and how far into Kerbin's gravity well you should go, I do not know.

[Plusck]

(um forget this post I was being daft)
 

Edited February 2, 2016 by Plusck

[Warzouz]

Coming from ground, LKO parking orbit should be as close as 70km possible. Personnaly I set orbits to 75km and park space stations at 80km.

There are 2 reason you want to do otherwise

• You have to wait a lot and low orbits can't warp high. In that cas you may want to raise orbit to be able to warp more quickly. But there is an alternative : you can always warp in the tracking station. You have no limit there. This is very safe when you use KAC mod
• Your ship has a low TWR and you burn may take several minutes. You may loose quite a lot of fuel, event cross the atmosphere in worst cases. In that case you might want to start from a higher orbit.

Orbit	Periode (min	dV m/s	Max time for 30°	Min TWR 30°
80	31	1100	2.6	0.72
150	36	1100	3.0	0.62
200	40	1100	3.3	0.56
500	64	1100	5.3	0.35
1000	113	1100	9.4	0.20

This table show the burn you can make in a 30° angle

• Orbit : where you are
• Period : orbital period
• Max time for 30° (1/12 of a period) : the time you can burn in 30° ark (in reality it's smaller as your speed would increase)
• dV : my test value of 1100m/s
• TWR : Min TWR you must have to do the 1100 burn in the time allowed.

[Superfluous J]

It is worth noting that using Mun to "save dV" on tossing you out of Kerbin's SOI is not ideal. It's cheaper than burning up to escape Kerbin's SOI and then - later in Sun's SOI - burning to your target, but it's more expensive than doing a proper Hohmann transfer directly from LKO.

Of course, if you can nail a Mun assist on a Hohmann transfer you can stack the Hohmann dV savings along with the Mun gravity assist savings, but I'm not smart enough to do that one.

[Evanitis]

2 minutes ago, 5thHorseman said:

Of course, if you can nail a Mun assist on a Hohmann transfer you can stack the Hohmann dV savings along with the Mun gravity assist savings, but I'm not smart enough to do that one.

I don't believe you. Hell, I'm a guy who has problems adding up numbers with multiple digits, but even I can do that. After a reload or two. But that only shows I didn't practice it much.

[Geschosskopf]

6 hours ago, Ketatrypt said:

And then I could also use a station in orbit of one of the moons, but, in that regard, I am wondering if it is worth the trouble of making a moon-station for interplanetary voyages?

No.  It is not worth it in time, money, fuel, or hassle to build any sort of station within Kerbin's SOI for the purpose of assisting interplanetary ships.  This has been discussed many times, based on all the various mod and stock refueling systems that have come out over the years, and the answer is still the same.  Because the KSP universe is so small, the practical benefit from doing this is so small that the system will never pay for itself, nor be worth the trouble of using.  You just never need more fuel than you can lift.

And then, by the time you're doing interplanetary missions, lifter and transfer fuel is really a small fraction of the cost of a rocket---the payload is where you spend your money.  If you can afford the payload, you can afford to lift it and shoot it out from Kerbin.

Now, things might be different if you use RSS or some other rescaled solar system, for the same reason that such ideas have some traction in the real world.  But in stock KSP, don't bother.

-------------------------------

So, back to the thread's title of interplanetary parking orbits, the best ones are the lowest you can manage.  Here's why....

To go to another planet, you need to reach Kerbin's escape velocity plus whatever extra is required to raise your Ap (or lower your Pe) to the altitude of your target.  Therefore, the higher your velocity to start with, the less dV you need to reach the required transfer velocity, and you have a higher starting velocity the closer you are to Kerbin.  In addition to this, the higher your velocity, the less fuel is required to create a given amount of dV, thanks to the Oberth effect.  Thus, a ship in a higher orbit will require more fuel to create the same amount of dV than a ship in a lower orbit.  Therefore, the higher your parking orbit, the more dV you need to create, and the more fuel that dV costs, to get to the same place.  This means that the lowest possible parking orbit is the best---lower dV required and it's cheaper per unit of dV on fuel.

HOWEVER, transfer burns take a non-zero amount of time, which means that at the start of the burn, your ship will be pointing off prograde and losing efficiency.  In fact, it will be pointing at Kerbin somewhat.  Thus, if your burn is more than about 5 minutes (which is about 20% of an orbit around Kerbin in LKO), you lose a lot of efficiency to cosine loss.  And if your starting orbit is too low, you might even push yourself down into the atmosphere.

Therefore, if your burn time is 5 minutes or less, start in about an 80km orbit.  If it's 6-7 mintues, maybe start at 100km.  If it's any longer than that, either do the burn in multiple passes or start in an orbit big enough to minimize the cosine loss and just accept the higher dV required and the reduced Oberth efficiency.

Of course, if you've got a really long burn (10-20 minutes, or even more), then odds are you're using some really super-efficient but very low-thrust transfer engine.  In which case, you've probably got enough fuel in the tank to go anywhere you want so don't care about the loss of efficiency from starting in a high orbit.

 

 

 

 

[Foxster]

The only thing that matters for me is whether I can use max warp, which means my usual Kerbin orbit is 605km. 

It takes a little more dV to get to 605km than, say, 100km but it also then takes less dV to escape Kerbin's SOI. I just checked and it was a difference of 127dV on a mission to Duna. 127dV is nothing and for the real-time saving by being able to warp at the maximum it is definitely worth it for me. 

 

Edited February 2, 2016 by Foxster

[Warzouz]

7 hours ago, 5thHorseman said:

Of course, if you can nail a Mun assist on a Hohmann transfer you can stack the Hohmann dV savings along with the Mun gravity assist savings, but I'm not smart enough to do that one.

Neither do I. I understand how to do it but not managing to get a proper encounter then. On the other hand, It's easy to use it to slow down on arrival, especially in Jool system (and quite fun to do).

2 hours ago, Foxster said:

The only thing that matters for me is whether I can use max warp, which means my usual Kerbin orbit is 605km. 

It takes a little more dV to get to 605km than, say, 100km but it also then takes less dV to escape Kerbin's SOI. I just checked and it was a difference of 127dV on a mission to Duna. 127dV is nothing and for the real-time saving by being able to warp at the maximum it is definitely worth it for me. 

 

Yes true, but you can warp from within the Tracking Station whatever altitude you are.

[Foxster]

1 hour ago, Warzouz said:

Yes true, but you can warp from within the Tracking Station whatever altitude you are.

Possibly, but I'm not going to do that. 

[Evanitis]

1 hour ago, Warzouz said:

Neither do I. I understand how to do it but not managing to get a proper encounter then. On the other hand, It's easy to use it to slow down on arrival, especially in Jool system (and quite fun to do).

You are the second well established player to state that in this thread. What's so hard in a Mun assist for escape? I have much more trouble to do the same on the arrival. Well, not around Jool of course - it has a moon at every corner. But when coming back to Kerbin, I can't slow down and wait for Mun to be at the correct angle, so many times I can only get an inefficient assist. That's usually enough, and Minmus can help a bit too, so it's not a problem.

But lining up the escape is much easier, unless the window is really 'thin', like the Moho one. But for like Duna or Eve, you get weeks while the required dV difference is around ~100 m/s. Waiting for the Mun to get into the perfect position saves a lot more than that.

Edited February 2, 2016 by Evanitis

[Superfluous J]

15 minutes ago, Evanitis said:

You are the second well established player to state that in this thread. What's so hard in a Mun assist for escape? I have much more trouble to do the same on the arrival. Well, not around Jool of course - it has a moon at every corner. But when coming back to Kerbin, I can't slow down and wait for Mun to be at the correct angle, so many times I can only get an inefficient assist. That's usually enough, and Minmus can help a bit too, so it's not a problem.

But lining up the escape is much easier, unless the window is really 'thin', like the Moho one. But for like Duna or Eve, you get weeks while the required dV difference is around ~100 dV. Waiting for the Mun to get into the perfect position saves a lot more than that.

I suppose it's not that hard. Just plan your Mun encounter and note where it has to be to eject at roughly the proper angle. Then wait until the Mun is there at the closest time to your window, and eject then. You could probably even tweak at Mun periapsis so long as you're really close after the eject burn.

Now I want to try it

[Norcalplanner]

There was a fabulous thread on this a while back. It starts with where the crossover point is for dropping periapsis first vs burning straight from a higher orbit, but the discussion goes all over the place with regard to orbital height. http://forum.kerbalspaceprogram.com/index.php?/topic/122445-oberth-effect/&page=1

[Hannu2]

Absolutely best parking orbit is 70 km circular orbit. It should be inclined so that you have exact velocity vector after burn which keeps always prograde direction. Burn trajectory is part of the spiral. However, it is very difficult task to execute with KSP's maneuvering tools. Unfortunately there are also not suitable mods, as far as I know. At least MechJeb can not calculate such burn. I tried it because it was interesting engineering challenge to program software to calculate burn and try it couple of times but it is far too laborious to be a permanent practice for every mission for me.

Practical best depends on your personal preferences but I use 300 km parking orbit for interplanetary ships. I get typically accurate burn with TWR of 0.3 and can use 0.2 with typical correction of about 100 m/s (for outer planets). Cost difference between correctly inclined 300 km parking orbit and prograde burn on spiral trajectory from lower orbit is just few percents if I do not want to have extreme low TWR and burn times. At many times it is practically nothing because I can not save even single smallest fuel tank of SRB.

[Warzouz]

41 minutes ago, Hannu2 said:

Absolutely best parking orbit is 70 km circular orbit. It should be inclined so that you have exact velocity vector after burn which keeps always prograde direction. Burn trajectory is part of the spiral. However, it is very difficult task to execute with KSP's maneuvering tools. Unfortunately there are also not suitable mods, as far as I know. At least MechJeb can not calculate such burn. I tried it because it was interesting engineering challenge to program software to calculate burn and try it couple of times but it is far too laborious to be a permanent practice for every mission for me.

Inclined parking orbit... This is quite hard to calculate... It's not only a matter of inclination (even Sun relative inclination is not the same as Kerbin relative inclination), it's about being in the same plane as the target.

I tried to do that when I went to Eeloo in beta 0.9. I roughly calculated (and 3 tries) I had to be in a 30° inclination to be grossly in Eeloo plane. But with that, the launch was quite hard. I had to launch exactly on the opposite side of my calculated burn node (which can't be set until you launch...). Then doing half a revolution around Kerbin and when crossing the equator, do the escape burn.

There was a lot of eyeballing and guessing. I'm not even sure I did it correctly. In the end, I still needed to do a 450m/s correction burn to be co-planar to Eeloo.

That was a lot of stress. Now I have a more pragmatic vision :

• Launch 2 weeks before transfer window, 0° Kerbin inclination.
• Plan a prograde escape node AND plane change node when in parking orbit (75 to 80km)
• Wait for transfer window (I usually do other things...)
• Reset the nodes then do the real burn
• Reset the plane change/encounter after leaving Kerbin SOI

It's certainly not the most efficient way, but at least it's easy and I don't need other tools than a "Precise Node"-like mod (I don't even bother with ejection angle, my way of setting the node takes care of it, "de facto"). I just pack a bit more fuel.

I tried MJ calculator, but I had a lot of difficulties with it. I got nice flyby encounter which did very bad "orbital insertion"-friendly encounters. I use MJ, but not for that.

[Hannu2]

Look at my Dres mission: http://imgur.com/a/ATbax

That is not very good example because inclination is only 2° but procedure is explained. Launch Window Planner (or Alexmoon's calculator) gives angle to prograde of ejection burn and you use information given by MechJeb to time launch to get correct LAN. It saves about 300 m/s compared to equatorial parking orbit if I go to Dres and need 25° inclination. Transfers to Eeloo can save more. I use inclined parking orbit if inclination is 5° or more. When you used to use it it takes maybe minute or two per launch. Typical correction is less than 100 m/s (except Moho).