Standard orbits for Interplanetary shuttles & Fuel stations?

[Cloakedwand72]

What's the best orbit for ships leaving the home world would I want to put the ship at the lowest possible orbit if it the TWR is around 1 or so? And would 115,000 miles orbit be the minimal for Stations & resupply outpost?

[Ultimate Steve]

It really doesn't matter as long as the ship has sufficient TWR to do the departure burn in one orbit without hitting the atmosphere, or as long as you can do multiple kick burns.

However, above 120km it's good because you get more time warp, and above 250km is good because it unloads the surface is unloaded and you get less lag.

But if you want the lowest possible orbit with a TWR of 1, you don't need to go super high. 80-90km should be good for that, although you won't be able to time warp much.

[qzgy]

5 minutes ago, Ultimate Steve said:

although you won't be able to time warp much.

There are mods to get around that. And you can always hop back to the tracking station, so its not a huge issue

[Cavscout74]

I typically use ~120km for LFO-powered interplanetary missions, primarily for the higher time-warp.  For NERV's & Ions, I usually use 160-180km to ensure I'm clear of the atmosphere during the burn.

[Gargamel]

I like to put my interplanetary missions in a much higher orbit, 500k-1500km.  I've placed big asteroids there too for fueling stations.    But I'll get the ship fueled up, and then park it in those high orbits, so they are already a ways out of the gravity well.   The only fuel 'penalty' comes when I launch the small shuttle vehicle to rendezvous with the mother ship.  But I really don't care about a few extra drops of fuel for getting the 10 ton shuttle to a high orbit vs bringing that 850t mother down into LKO. 

Right now, due to a contract, my BFFR (Big flying fuel rock) is around the Mun, so I fuel up there, and then drop back down, but with my mother ships carrying > 15k dv, I can stand to lose a little. 

But with a TWR of 1, I'd be happy at 80km and a full tank of gas.  It's when the TWR's get low (my mother ships are around .25-.4 depending on cargo and fuel load) that I want some elbow room to complete the burns. 

I have also done my refueling stop around minmus, and then boosted to an orbit above that to await the window.  While you may lose a bit of perfect window Dv waiting for the ship to come around it's orbit to it's node, you save about 1k dv in just getting that far out of the gravity well. 

Edited June 12, 2018 by Gargamel

[Kesa]

Distances are measured in kilometer (km), not sure what you mean by "miles".

Due to Oberth effect, you want to do you ejection burn as close to Kerbin as possible.

TWR is irrelevant. If you don't have enough acceleration, you can split the burn, performing what's called apsis kicking. Even if you absolutely want to make your burn all at once, doing a suboptimal insertion from LKO is more efficient than going into a high orbit and then make an "optimal" burn there. TWR of 1 is more than comfortable enough to make interplanetary ejection burns at 80km in LKO.

 

For a spacce station or something meant to be docked with, I would go a bit higher than the bare minimum, say a circular 120 km one. It makes it easier to rendez vous from Kerbin, as well as from your resupply outpost on the Mun or on Minmus after aero breaking (preferably the latter).

[StrandedonEarth]

“Standard orbit, Mr Kulu. “

”Aye, Kap’n. Circular orbit at 245km”

I’ve started setting my departure orbit above 240 km for more accurate burns and more time warp. Sure, I may lose a little dV from Oberth, and there’s workarounds for warp, but it’s just quicker and simpler than jumping somewhere else and back, or doing a series of kick burns. I never seem to have enough time to play KSP, so I tend to want to skip any unnecessary flatulating around. Launching the refueling tanker takes enough time as it is, and setting up the rendezvous is easier with the target up higher, IMO.

On 6/11/2018 at 2:36 PM, Ultimate Steve said:

Above 250km is good because it unloads the surface is unloaded and you get less lag.

 Oh, well, I guess I’ll have to go those extra kilometres for that bonus unloading.

 

[MythicalHeFF]

When I’m just launching an interplanetary ship, I usually won’t put much thought into what orbit I’ll put it in. Usually around 100km is the orbit I’ll go with.

For space stations however, it isn’t as simple as just having one ideal altitude, although I wouldn’t bother putting one below about 120km due to lag and lack of time warp, like others have said already. If you’re going for a basic scientific/crew station, I wouldn’t bother going much above that. However, I like putting fuel stations in much higher orbits, usually 500-1000 km in order to make them easier to get to when coming down from a Minmus base to refill their tanks. 

Edited June 13, 2018 by Jack Joseph Kerman

[Gargamel]

13 hours ago, Kesa said:

Due to Oberth effect, you want to do you ejection burn as close to Kerbin as possible. 

 

12 hours ago, StrandedonEarth said:

Sure, I may lose a little dV from Oberth,

Edjumacate me a little here then.

How much of an Oberth penalty could there be for say a fully fueled ship at 2.5k km vs 100 km?  I would have to imagine the Oberth losses are really offset by the lack of needing to climb out of the gravity well.  If I can reduce the burn by 300-600 dV by starting higher (everything else being equal), why wouldn't I want to do that?

Of course, there are dV penalties and logistical issues involved in starting higher, but once you have worked out the logistics of setting up a high altitude refueling depot, it's really not that big of a deal.  Especially when you are considering the amount of fuel consumed getting a small shuttle up to your mothership, vs the amount of fuel the mother ship would have to burn to climb back up out of the hole.  Sure, they're both burning 300 dV or so, but the shuttle only uses one little tank. 

[blakemw]

I like 600km. There is the concept of a gate orbit, that is the cheapest orbit to perform an ejection burn from to a certain destination. There are two factors: first is being further out of the gravity well which makes it cheaper, secondly is the reduced Oberth effect which makes it more expensive. Where the cost of the ejection burn is minimized is the gate orbit. 600km makes a pretty good orbit for a range of destinations (in general, "climbing out of the gravity well" is more valuable for near destinations like Duna and Eve resulting in a relatively high gate orbit, Oberth effect is more valuable for far destinations like Moho and Jool resulting in a relatively low gate orbit). But note: getting up to the gate orbit requires more fuel than ejecting directly from a lower orbit, but higher orbits can still be good places for fuel depots if that fuel is coming down from Minmus rather than up from Kerbin, or if you're lifting so much fuel at once you don't care.

[Wolf Baginski]

This is one of those things where Hyper-Edit and MechJeb can be use without shouts of cheating.

1: Go into sandbox mode

2: use Hyper-edit to put the craft into a parking orbit

3: Use Mechjeb to find the minimum delta-V for the transfer to the target planet

4 Change orbit and repeat.

Plot a graph of parking orbit against delta-V, find the minimum, that's the  gate orbit.

The delta-V for that orbit is what your booster needs. The delta-V for the transfer could be on the probe, or on a small booster. An option would be to have a small fuel margin and stage the booster on arrival at the target, you will be making a burn there.

This sort of simulation is part of how RL rocket  scientists plan missions.

Personally, I have Hyper-edit installed, but I don't use it very much. I think it's OK for routine stuff like filling a fuel-station, after I have done it with a real mission. 

 

[Superfluous J]

3 hours ago, Gargamel said:

Edjumacate me a little here then.

How much of an Oberth penalty could there be for say a fully fueled ship at 2.5k km vs 100 km?  I would have to imagine the Oberth losses are really offset by the lack of needing to climb out of the gravity well.  If I can reduce the burn by 300-600 dV by starting higher (everything else being equal), why wouldn't I want to do that?

If it took you 1000 dV to get there?

I don't know the numbers, but basically it's always cheaper to launch to LKO and then go interplanetary. There is a specific orbit though that is the cheapest to launch from to go to each planet, called the "gateway orbit" for that planet. Below it or above it, it costs extra dV to make the burn. However, it costs more dV to get to it from LKO plus launch interplanetary from there than it does to just go to LKO. So, for launch-and-go missions, it's always best to just go to the planet.

However, if you are going back and forth or refueling, then a gateway station becomes more attractive. I've never personally done it because it's a LOT of extra fiddling for not all that much benefit IMO, but it's there and it IS better in these circumstances.

The big problem is that generally there's no real reason to send a ship back and forth to a planet like the Hermes did in The Martian. Except to do it. Which of course is a great reason

But as to "how much cheaper" you can find out pretty simply in game or on the Alexmoon calculator, by plugging in higher and higher values for your starting orbit around Kerbin. Once the numbers start going back up, you've found your Gateway Orbit.

[Lisias]

3 hours ago, Gargamel said:

Of course, there are dV penalties and logistical issues involved in starting higher, but once you have worked out the logistics of setting up a high altitude refueling depot, it's really not that big of a deal.

We must free ourselves from the assumption that the fuel will come from Earth. Space will be only viable when we manage to get our fuel from near bodies, as Moon and perhaps asteroids.

It's far more cheaper to ferry 100 tons of something from Moon to LEO than from the Earth's surface.

[Superfluous J]

12 minutes ago, Lisias said:

We must free ourselves from the assumption that the fuel will come from Earth. Space will be only viable when we manage to get our fuel from near bodies, as Moon and perhaps asteroids.

It's far more cheaper to ferry 100 tons of something from Moon to LEO than from the Earth's surface.

And likely cheaper to send the ship to the Moon than the fuel to LEO, especially if aerobraking is out of the question.

[Lisias]

15 minutes ago, 5thHorseman said:

And likely cheaper to send the ship to the Moon than the fuel to LEO, especially if aerobraking is out of the question.

Not sure if it applies to every situation... I need to bleed a lot more delta-V to orbit Moon than to orbit Earth if I'm coming from the outer bodies, so if my fuel tanks are under budget, it's probably a better idea to go for a LEO's fuel station.

Even by aiming the Moon, I could save some fuel launching the vessel without the fuel needed to the Moon's voyage, getting it from the fuel station. This would save me some mass to be kicked out of this world and, so, some fuel to reach the needed delta-V.

Edited June 14, 2018 by Lisias

[Superfluous J]

2 minutes ago, Lisias said:

Not sure if it applies to every situation... I need to bleed a lot more delta-V to orbit Moon than to orbit Earth if I'm coming from the outer bodies, so if my fuel tanks are under budget, it's probably a better idea to go for a LEO's fuel station.

But that's only looking at the dV. Likely, that ship you're sending interplanetary is a lot lighter than massive fuel tanks, if for no other reason than it would (or at least should) be built and fueled to arrive at the Moon station on fumes.

[Lisias]

1 minute ago, 5thHorseman said:

But that's only looking at the dV. Likely, that ship you're sending interplanetary is a lot lighter than massive fuel tanks, if for no other reason than it would (or at least should) be built and fueled to arrive at the Moon station on fumes.

Looking only at the dV is missing the real problem: fuel mass.

Kicking the butt of a 100t payload needs the same dV than kicking a 10t one's. But the last one would use a lot less fuel due less mass and less drag due the 1st stage smaller tanks.

So, if you have a Fuel Station strategically positioned, it's better to launch your vessel without the fuel for the injection burn, getting it from the Station instead - that by its turn, would get its fuel from the Moon (for example), making it cheaper.

The savings for a single launch perhaps would not be that bigger, but if you plan to implement and support a regular flight, the savings pile up.

Essentially, it's what BFR is going to do (except by the Station's fuel source).

[Kesa]

6 hours ago, Lisias said:

We must free ourselves from the assumption that the fuel will come from Earth. Space will be only viable when we manage to get our fuel from near bodies, as Moon and perhaps asteroids.

It's far more cheaper to ferry 100 tons of something from Moon to LEO than from the Earth's surface.

A LKO/ LEO station still is cheaper to refuel from the Moon or an asteroid than to a gateway orbit, so there are logistical losses anyway. Apparently at least, since as Lisias and 5-th Horseman point, you gain from carrying less stuff around, especially smaller interplanetary stages. And not to a point it matters, for refueling from space (in addition to it being cheaper, logistical Dv losses of going to the gateway orbit rather than LKO are less than half coming from the Mun than from Kerbin).

 

 

Gargamel, you make a very important point that I missed, and kudos for Blackmw for bringing gateway orbit, and providing altitude for a good median one.

 

10 hours ago, Gargamel said:

Edjumacate me a little here then.

How much of an Oberth penalty could there be for say a fully fueled ship at 2.5k km vs 100 km?

I'll crunch the numbers probably tonight. I could have sworm somebody talked about a 15,000 km orbit, and I remember that long ago I thought that the border of Kerbin SOI (either side) was the best place for a station midway through interplanetary travels. That was what I had in mind when I said as close as possible to Kerbin.

2.5k km, and even 1.5k km sounds high to me, but I could not have predicted gateway orbit were at 600km, so I'll likely be surprised.

[Black-Two-]

On 6/11/2018 at 11:36 PM, Ultimate Steve said:

above 250km is good because it unloads the surface is unloaded and you get less lag.

Is it 250km? I thought it was 160.

[Gargamel]

20 hours ago, 5thHorseman said:

on the Alexmoon calculator

LInky? You mean KSP Launch Window Planner by alexmoon ?  I've had this bookmarked for years, but I think I've only looked at it once. 

Good discussion guys, keep it rolling.

I may have been playing this game for a long time, but I'm always learning new stuff. 

Edited June 15, 2018 by Gargamel

[Superfluous J]

1 hour ago, Gargamel said:

LInky? You mean KSP Launch Window Planner by alexmoon ?

Yup. It's the same code that the Transfer Window Planner mod uses.

 

[suicidejunkie]

22 hours ago, Kesa said:

I'll crunch the numbers probably tonight. I could have sworm somebody talked about a 15,000 km orbit, and I remember that long ago I thought that the border of Kerbin SOI (either side) was the best place for a station midway through interplanetary travels. That was what I had in mind when I said as close as possible to Kerbin.

2.5k km, and even 1.5k km sounds high to me, but I could not have predicted gateway orbit were at 600km, so I'll likely be surprised.

If your station is at the very edge of the SoI, then a ~20m/s burn to drop down to the atmosphere I expect should save you something on the order of 1000m/s from Oberth.

And vice versa on the way to the station.  Couple hundred m/s (aero)braking burn, and an RCS scale rendezvous burn rather than a 2km/s rendezvous burn.  Timing arrivals and departures will be a huge pain however.

[LordFerret]

On 6/13/2018 at 1:32 PM, Kesa said:

Distances are measured in kilometer (km), not sure what you mean by "miles".

If we gave distances in Kellicams, would you have balked at that as well?

For future reference: 1 mile = 1.60934 kilometers

In the OP's case, he's talking about an 185074.56km orbit, in-between Mun and Minmus. I have three huge ships which I've assembled in a similar orbit, two of which are now on their way to Jool, and the third on its way to Eeloo. All three being Ion powered, using such an orbit as a start point made a lot of sense to me, and it worked out quite well.

[Kesa]

9 minutes ago, LordFerret said:

If we gave distances in Kellicams, would you have balked at that as well?

Yes, even more so, although I would assume you knew what you are talking about.

 

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For future reference: 1 mile = 1.60934 kilometers

I know that and I don't see it being useful for future reference since distances are given in meters and multiple in KSP.

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In the OP's case, he's talking about an 185074.56km orbit, in-between Mun and Minmus.

No. He is most likely assuming m denotes miles and is talking about a 115 000 meters orbit. He is talking about "the lowest possible orbit". And 185074.56km is well beyond Kerbin's SOI, not lower than Minmus.

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I have three huge ships which I've assembled in a similar orbit, two of which are now on their way to Jool, and the third on its way to Eeloo. All three being Ion powered, using such an orbit as a start point made a lot of sense to me, and it worked out quite well.

Good for you. The OP is asking about vessel with TWR around 1 though. For ion probes, high altitude make sense. A gateway orbit (600km) should give 10-20 min to perform a burn, and a 3 000 km one (about keostationary) should give one hour. Even disregarding the Dv gain, a 3 000 km orbit offer a lot more flexibility in departure time than transmunian one. If I were dedicated enough to make crafts requiring burns longer than one hour (and not easily wrapable), I would probably bother kicking periapsis too, and make two or more burns from a 3000km orbit rather than one from a higher one.

[Kesa]

On 6/14/2018 at 9:19 AM, Gargamel said:

Edjumacate me a little here then.

How much of an Oberth penalty could there be for say a fully fueled ship at 2.5k km vs 100 km?

I would have to imagine the Oberth losses are really offset by the lack of needing to climb out of the gravity well.  If I can reduce the burn by 300-600 dV by starting higher (everything else being equal), why wouldn't I want to do that?

Here you go : https://docs.google.com/spreadsheets/d/1aJqOQ6EknlOIW76JqZrH9661G7KHZv9bQQ_YICAa31E/edit?usp=sharing

 

The answer depends a lot on the destination. For Moho and Dress, the gateway orbit (the one minimizing the Dv of the interplanetary transfer) seems to be around 600 km, and there is no noticeable difference between 100 km and 2.5k km, which both are less than 5% worse than the gateway orbit. For Jool and Eloo, LKO is 100 and 150m/s better than 2.5k km, respectively.

For Eve and Duna, the difference is massive, nearly 400 m/s in favor of the 2.5k km one, and the gateway orbit seems around the Mun altitude.

 

That's assuming you perform a single ejection burn. For planet other than Duna and Eve, from a 2.5k km, you should first drop your Pe to do your ejection burn there. Going the dive route, the higher the better basically. 2.5k km becomes 200 m/s better than LKO for planet other than Eve and Duna.

Pushing that route to the extreme, you should actually refuel on the edge of Kerbin SOI, dive down to 70 km and fire your engines there. A Minmus like orbit further saves 500 m/s across all bodies compared to the 2.5k km one.

 

To make justice to LKO though, if you count Dv from the launch, LKO beats 2.5k km by a large 660 m/s (duna) up to a 820 m/s (Mhoh Eloo), meaning that without refuel, or refueling with fuel from Kerbin, it is largely beneficial to refuel low (for ejection burn 120km is still near optimal, at 240km, losses start to be noticeable).

Even when refueling, it is more relevant than it seems : this hundreds of m/s of Dv are not just "logistic losses". For the most part, you lose them when climbing from Kerbin to your station, meaning you'll need a higher launch mass.

I tried to make a model simple enough for single refuel that encompasses the fact that lower transfer burn let you build a lighter ship, that could exhibit which orbit minimizes launch mass. Even plugin numbers that seems pretty unfair to LKO, it got a clear lead. (the model direguard cost, in particular of more engines vs more fuel, and it assumes single refuel, no complexe highly modular stuff).

A nice way to put it : LKO is the best place to refuel if you intend to minimize the use of Kerbin's ressources, and conversely maximize the use of free/cheap deep space ressources.

 

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Of course, there are dV penalties and logistical issues involved in starting higher, but once you have worked out the logistics of setting up a high altitude refueling depot, it's really not that big of a deal.  Especially when you are considering the amount of fuel consumed getting a small shuttle up to your mothership, vs the amount of fuel the mother ship would have to burn to climb back up out of the hole.  Sure, they're both burning 300 dV or so, but the shuttle only uses one little tank. 

I'm not sure I follow you here. If you need to make round trips, or reutilize the interplanetary stage/mother ship, yeah, you should park it in a rather high orbit.

But when launching stuff from Kerbin, the only things that matter from the upper stage to your orbit are the energy of the target orbit and the amout of mass you bring there. Bringing it bits by bits may make it easier for rendez vous and assembly, but it does not save fuel (it may even cost a bit more if your shuttle don't have the efficient engines you mothership has).

 

Summary :

Pros of refueling low (< 250 km) :

- Lowest launch mass (you can launch empty vessels)

- Most efficient trajectory without refueling means it's also the best strategy if the tanks you refuel from are from Kerbin as well.

- Aerobraking helps refueling the station

 

Cons :

- Slightly bigger transfer stage mass

- short burn windows

 

Pros of refueling medium (below Mun) :

- Convenient burn window, which come often and are long

- Gateway orbit : lowest transfer single burn for distant bodies

con :

- don't have the big pros of others

 

Pros of refueling High (Mun and Minmus) :

- lowest transfer single burn for Duna and Eve (for the Mun, and Mimus is decent)

- lowest transfer burn if you're readdy for a dive (lowest transfer mass)

- proximity with the mining site

 

Con :

- impractical timing

- Requires interplanetary abilities and fuel to even get to the refueling post in the first place

 

It's possible to get the best of both world by refueling twice, though likely overkill.