Oberth effect

[Red Iron Crown]

That's a neat technique, there! I'm kinda surprised you aren't having the tanker act more as a Von Braun tug, though -- gassing up the mission craft, completing all or as much of the transfer burn as it can get away with, then undocking and slowing itself back down to head back to Minmus.

I....did not think of that. May have to do some further testing (I am concerned that the tanker won't have the dV to get back to Minmus).

[magnemoe]

That's a neat technique, there! I'm kinda surprised you aren't having the tanker act more as a Von Braun tug, though -- gassing up the mission craft, completing all or as much of the transfer burn as it can get away with, then undocking and slowing itself back down to head back to Minmus.

I have done this, tug works as first stage and have say 4-9 LV-N in the back, ship has LV-N radially so both can burn at once, I pump fuel to ship.

Then fuel in tug get low I disconnect it, if I have an light load I might push into escape speed and then brake.

I used to return to LKO, aerobrake to save fuel an round trip to Minmus would usually also delay things and I have dedicated tankers with far more capacity than the tug who is also not designed to land at base.

[Musil]

Great post.

[FleshJeb]

Bob, just found this: https://en.wikipedia.org/wiki/Gate_orbit

[Freshmeat]

Reading the Gate Orbit paper. At a glance it ignores the dV requirements to reach the given orbits, but I need some coffee and paper before I dismiss it completely. But since it just might be relevant for a high school student of mine, I can do it on company time ;-)

[OhioBob]

Bob, just found this: https://en.wikipedia.org/wiki/Gate_orbit

That's interesting. At least we now know what to call it. The equation is the same one that I derived back in post #40 on page 4.

[LostOblivion]

That's interesting. At least we now know what to call it. The equation is the same one that I derived back in post #40 on page 4.

Indeed. What you came to discover as this optimal orbit seems to be called a Gate Orbit.

[Hannu]

Gate orbit is interesting thing, but it seems to be only limitedly practical. It does not take dv needed between surface and gate orbit into account. Maybe it would be a good idea to put space station for interplanetary ships on the gate orbit, if there is much traffic to some certain planet. But if there is also lot of surface activity, it is probably most economic to optimize orbit for landers than for interplanetary tugs (typically nuclear powered with huge dv reserves).

[NecroBones]

Bob, just found this: https://en.wikipedia.org/wiki/Gate_orbit

Great find! It's good to know what it's called.

[OhioBob]

 

Freshmeat said:

Reading the Gate Orbit paper. At a glance it ignores the dV requirements to reach the given orbits…

Hannu said:

Gate orbit is interesting thing, but it seems to be only limitedly practical. It does not take dv needed between surface and gate orbit into account.

That issue has been discussed in this thread. From my investigations, the total Δv when measured from launch pad to orbital ejection will always be least when the minimum altitude parking orbit is used. The Δv expended to reach the higher gate orbit is more than the savings that comes from the smaller ejection burn.
 

Edited April 9, 2016 by OhioBob

[Superfluous J]

That issue has been discussed in this thread. From my investigations, the total ÃŽâ€v when measured from launch pad to orbital ejection will always be least when the minimum altitude parking orbit is used. The ÃŽâ€v expended to reach the higher gate orbit is more than the savings that comes from the smaller ejection burn.

Which means, of course, that the best place to put your ships before burning out to other planets is "as low as you can."

[EladDv]

Which means, of course, that the best place to put your ships before burning out to other planets is "as low as you can."

IF you can get the kraken to cooperate you can try and put your ship at the heart of kerbin

[Superfluous J]

IF you can get the kraken to cooperate you can try and put your ship at the heart of kerbin

Actually that wouldn't work IRL, though I bet it would in KSP. IRL, inside a planet gravity is less.

I define "as low as you can" by the highest mountain on an airless world, and the atmosphere on an aired one. With a margin to account for the fact that your ejection burn will take you a bit lower.

[LostOblivion]

Which means, of course, that the best place to put your ships before burning out to other planets is "as low as you can."

Not on it's own, no, only if you came from a lower orbit, such as the ground.

[Superfluous J]

Not on it's own, no, only if you came from a lower orbit, such as the ground.

Ah. Yes. All those people coming from places other than the surface of a planet, specifically into an ideal orbit of that planet to make ejections to other planets most efficient. I forgot about them.

[KerikBalm]

Ummm that sounds like sarcasm... but...

The KSP system has more planets than just Kerbin... and assuming you want to visit one, you will choose which orbit to insert into.

It may be worth it to make your small lander use a bit more dV to get to and from the surface, than to make your main vessel that will depart and got back to kerbin, go to a lower orbit (particularly with life support mods and such)

[oglommi]

If im playing a game where i've setup up some refining/mining infrastructure on Mun/Minimus. I park the Interplanetary vessels in a 2000-5000km orbit after take off. Because 900DV of orbiting manuver to get to a low orbit is very fuel costly for a fully tanked fueltanker from minimus/mun.

[Superfluous J]

Ummm that sounds like sarcasm... but...

The KSP system has more planets than just Kerbin... and assuming you want to visit one, you will choose which orbit to insert into.

It may be worth it to make your small lander use a bit more dV to get to and from the surface, than to make your main vessel that will depart and got back to kerbin, go to a lower orbit (particularly with life support mods and such)

It was sarcasm, and I did not take other worlds or heavy transfer ships (as with life support) into account. I doubt I'll do it in any of my missions, but I accept there are some cases where it would show some sort of benefit.

[Red Iron Crown]

The other scenario in which this is useful is for fueling in orbit before departure. Minimizing the ejection dV is relevant then because the ascent dV is less important, the ship will be fully fueled before ejecting no matter which orbit it starts from.

[Hesp]

Sorry for digging up an old topic, but I did some calculations for my possible Mun Fuel station and something interesting came out:

Mun Altitude [km]		Dv Jool	Dv Duna
25		1958.8	456.3
50		1974.8	454.4
75		1989.0	453.3
100		2001.7	452.7
150		2023.7	452.9
200		2042.1	454.0
300		2071.3	457.8
500		2111.8	466.9
Reference
Kerbin 75km		2002.8	1070.3
Kerbin-Jool GO		1992.6	965.6
Kerbin-Duna GO		2321.8	636.4

GO stands for Gate Orbit

The main assumption is that the ejection is performed prograde from Mun orbit, but it results in a smaller burn even compared to Kerbin gate orbits.

[Hesp]

Probably I need to review the calculations, I got an ejection burn Mun-Duna of 413 m/s. Better than expected! 

http://imgur.com/a/Yb5Us

[FleshJeb]

On 2/5/2017 at 12:17 PM, Hesp said:

Probably I need to review the calculations, I got an ejection burn Mun-Duna of 413 m/s. Better than expected! 

http://imgur.com/a/Yb5Us

I'm necroing this, because it's one of the most comprehensive threads on efficient orbital mechanics, and I refer to it often (like today).

Hesp, I'm wondering if the difference you observed might be due to the SOI vs Infinity discrepancies noted upthread. I imagine the Mun has an even more artificially-truncated SOI, thus causing a bigger difference. Did you ever figure it out?

[RizzoTheRat]

Good topic to revive, there's some great stuff on here.  I've learned loads but one of the key things is I've forgotten a lot of maths I've learned over the years

I was planning to refuel in high Mun orbit, but I might be revisiting that plan once I've reread and understood all this.  I like RedIronCrowns approach of a tanker/tug to lift stuff out to near the SOI though,

[Hesp]

4 hours ago, FleshJeb said:

I'm necroing this, because it's one of the most comprehensive threads on efficient orbital mechanics, and I refer to it often (like today).

Hesp, I'm wondering if the difference you observed might be due to the SOI vs Infinity discrepancies noted upthread. I imagine the Mun has an even more artificially-truncated SOI, thus causing a bigger difference. Did you ever figure it out?

Honestly I did not perform further calculations, but my gut feeling is that could be a plausible explanation.  I have to remark that @HebaruSan wrote a great tool to plan this kind of manouvers with his Astrogator.

 

After completing that Duna mission i restarted a career in a 2.5x scale "real" system, and I should perform soon some calculations on where to place the Lunar fuel depot.

The current Lunar station for surface operations is in a 100x100km orbit at 28.5° inclination from Earth equator (no plane changes needed if the launch is correctly timed). 

For the fuel depot I'm planning to keep an higher orbit (tentative 300x300) and aligned to the Ecliptic plane for interplanetary missions.

 

[FleshJeb]

10 hours ago, Hesp said:

Honestly I did not perform further calculations, but my gut feeling is that could be a plausible explanation.  I have to remark that @HebaruSan wrote a great tool to plan this kind of manouvers with his Astrogator.

After completing that Duna mission i restarted a career in a 2.5x scale "real" system, and I should perform soon some calculations on where to place the Lunar fuel depot.

The current Lunar station for surface operations is in a 100x100km orbit at 28.5° inclination from Earth equator (no plane changes needed if the launch is correctly timed). 

For the fuel depot I'm planning to keep an higher orbit (tentative 300x300) and aligned to the Ecliptic plane for interplanetary missions.

 

Thanks for following up. I should try doing the calcs myself. Do you work backward from V_excess from parent SOI and chain it down through SOIs? Do you have to add the orbital velocity of the Mun about Kerbin to your craft velocity about Mun? 

Great mod! I'll be using it.

I've tried spaceplaning in 3.2x, it was loads of fun.

I thought that meant you could only transfer to the the Luna station twice per month, then I realized you can match inclinations by adjusting the time of day of launch.

10 hours ago, RizzoTheRat said:

Good topic to revive, there's some great stuff on here.  I've learned loads but one of the key things is I've forgotten a lot of maths I've learned over the years

I was planning to refuel in high Mun orbit, but I might be revisiting that plan once I've reread and understood all this.  I like RedIronCrowns approach of a tanker/tug to lift stuff out to near the SOI though,

I think that might even be better/easier than the Von Braun tugs I wanted to try out. At least you don't have to retrobrake them before they reach the SOI.