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# Suggestion on space station altitude

## Question

I'm planning to put to orbit a space station/orbital habitat/refuelling station. I want to ask, what would be the most optimal altitude to put my station in...

I also use MechJeb if that helps...

Many thanks.....

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Interesting question.  There are a few different ways to answer it in my opinion and I'm sure someone else will come along later with even better ideas.

In terms of computing power (this is from the KSP Wiki Community-Developed Campaigns - "Kerbals in Space") the altitude affects timewarp capability as well as graphics.  Directly from that link:  A station at 120km has a 50x timewarp available, at 160km the graphics for Kerbin downgrades helping to improve resolution and video processing, and at 240km the 100x timewarp is available.

Let's think physics now in terms of subsequent rendezvous and docking.  Unless you are spot on with your launch, you will need to optimize your closest approach by orbiting faster (lower) than the station if behind or slower (higher) than the station if ahead.  If the station is right at 70 km, you don't really have room to go lower so you have to go higher.  This could lead to longer times required for intercept and more deltaV expended (since you push you Ap above the station and then have to bring it back down).  At the same time, you don't want to go too high since it takes more deltaV to get there in the first place.

As for what is optimal, I hope someone else will be able to chime in with more details.  In my current save, I have a refuelling station at 80 km so my ships use less deltaV achieving orbit and a "research" station at 120 km for the timewarp.

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A lot depends on what you want.  I've outlined my preferred 'traffic separation' rules before.  Briefly, they are:

• 70km - (de)orbit = ships waiting for the right burn-point to deorbit.
• 75km - launch orbit/low phasing-orbit = target launch altitude and phasing-orbit for ships going higher
• 150km - low parking/rendezvous orbit = shipts waiting for something to come up to them
• 250km - Station orbit
• 400km - high phasing orbit = usually for ships coming back to Kerbin station, sometimes meeting high-parked ships
• 600km - high parking orbit = waiting for interplanetary windows

The idea is simply, as others have pointed out for their own preferences, that this gives you room for all your orbital-work below the station without being too hard for launching vehicles to reach.  Anything that can barely make it to orbit can be met by one of the station tugs to bring it up to dock.  I tend to have nine or ten ships 'working' in Kerbin orbit at any one time, apart from the stations themselves, with a few more waiting for transfer windows so I need to keep things organised!  Plus, I just find it helps with mission-planning to know at what altitudes everything will be when I need to meet them.

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41 minutes ago, Temstar said:

Yeah in that situation don't follow the node marker, instead just follow prograde + any normal/anti-normal angle you need. If you thrust with an offset from prograde you're losing delta-V to gravity drag.

Yeah, but the Δv loss is minor (20-25 m/s).  If you split your burn 50/50 around the maneuver node, then following the maneuver node marker results in an ejection trajectory that is much closer to the planned trajectory (i.e. the maneuver node trajectory).  Following the prograde marker results in greater deviation from the planned trajectory, therefore necessitating a larger course correction.  There was a thread discussing the pro and cons of this about a month ago.  Below is a graph that I produced from a simulation that illustrates the path of the two trajectories in comparison to the theoretical maneuver node trajectory.

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11 hours ago, Geschosskopf said:

There is no practical benefit in owning a space station in the Kerbin system for your own use so I recommend you not build one on your own initiative.

I can't believe no one has challenged this statement. That's ridiculous.

The theme among all of the reasons this statement is wrong is planning. If you have every single aspect of your space program planned from here to the end of time, then maybe your statement can be correct. If you can perfectly calculate your delta-V for every mission with little fuel wasted (whether by hand or by mod), then you may not have a use for a place to stash surplus fuel left over after launch or at the end of a mission. If your ships never have an emergency that requires you to quickly deliver them fuel before they crash into the Mun or get ejected from the system, then sure, you won't need that fuel stash around the Mun. If every new craft design you come up with works perfectly on the first go, then you wouldn't see any benefit from having a single design fine-tuned for efficiently launching to a station at a known and practiced orbit, at which point it can be refueled for the rest of its journeys elsewhere.

If all of those conditions describe you, then you won't benefit from having a space station.

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It depends a little bit on what you want from the station.

I also like 120, for these reasons:

• The dV difference between this and lower heights isn't much, in my opinion.
• You get a faster timewarp.
• You reduce the risk of debris collision (I have a lot of junk floating at 70-80km).
• Docking craft can fly at 70km if I need to "fly faster" to catch the station. At 120km, this gives you a pretty decent margin.
• Oberth differences are marginal for most flights.
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Just to hammer it home, if you plan on using an MPL you better be at least be above 240km or you'll be waiting forever for your science.

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Another reason to be somewhat above 70km is, if you're going to do an nuke transfer burn from station orbit (like a 4+minute burn) your periopsis might lose 10-20km (meaning your trajectory would take you well into the atmosphere if you started your burn at 70, but if you were at 120+, you'd be completely safe)

3 hours ago, Temstar said:

Then it's a matter of raising your orbit slightly so you meet up in next orbit. hard burning straight at it, damn the torpedoes.

Fixed this for you.

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7 minutes ago, Venusgate said:

Another reason to be somewhat above 70km is, if you're going to do an nuke transfer burn from station orbit (like a 4+minute burn) your periopsis might lose 10-20km (meaning your trajectory would take you well into the atmosphere if you started your burn at 70, but if you were at 120+, you'd be completely safe)

Wait what? That's not suppose to happen, you must be executing your burn incorrectly to be dropping your PE. For a long burn you're suppose to burn prograde all the way regardless and start your burn earlier than the node, so that when the burn is completed your ejection angle is exactly the same as if you had infinite TRW and executed your burn on the dot at the node.

As for burning straight at the target then cancelling out the relative speed once you are beside it, if you think about it that's basically the same as raising your orbit to meet it the next time around, only instead of meeting it after a whole orbit, you're meeting the target after only an section of one orbit (in others words an arc).

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3 minutes ago, Venusgate said:

if you plan a maneuver on a circular orbit, your blue maneuver node marker is below 0 degree pitch when you start your burn, and should be equally above 0 degrees at the end of your burn. in the middle (when you reach the Pe), it should be 0 degrees pitch, but you spent the first half of your burn shooting slightly "down" which shrinks your Pe.

Yeah in that situation don't follow the node marker, instead just follow prograde + any normal/anti-normal angle you need. If you thrust with an offset from prograde you're losing delta-V to gravity drag.

How early you should start that burn before the node depends on your TWR (and so your burn time) as well as the amount of TWR change during the burn. There's unfortunately no general solution to this problem.

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2 hours ago, Geschosskopf said:

The 1st question to ask is, why build a station anywhere within the Kerbin system?  The only good answer is "because an irresistibly lucrative contract asked me to."  So if you don't have such a contract, don't build a station at Kerbin, Mun, or Minmus.  There is no practical benefit in owning a space station in the Kerbin system for your own use so I recommend you not build one on your own initiative.

Now, I know some will say that they're required for SSTOs to refuel at.  To which I ask, where does the fuel for SSTOs at the station come from?  If the answer is "from Kerbin", then there's no point in having an SSTO to begin with.  If the answer is "from mining at Minmus", then it's even less cost-effective to have SSTOs.

Same goes with using the station to farm science.  The Mobile Processing Lab takes FOREVER to grow science, and it doesn't grow anywhere near as much as you can find lying on the ground while biome-hopping Minmus in WAY less time and at far less cost.

Stations, like refueling operations, are only of practical value at other planets.

Now, if you don't care about any of the above and are just building a station for the experience, or for role-playing, or whatever, then knock yourself out.  But in this situation, you obviously don't care about practicality or efficiency to begin with, so the orbit you put it in doesn't matter.  Put it wherever you think it looks the coolest.

I'm playing sandbox and for the sake of achievement, I want to put my station there. Might I ask, in terms of "coolness", what do you think is the "coolest" altitude? I'm eyeing for a 150-250 km orbit....

Thanks for all who replied too. Your answers are very much appreciated. You guys are the best!

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I prefer 75km orbit, yes delta-V to higher orbit isn't much, but being that the rocket equation is logarithmic it does matter when that extra delta-V filter downwards to the launch vehicle, Particularly if the launch vehicle IS the spacecraft (ie SSTO).

I don't find rendezvous much harder. Yes if you find yourself behind your target it will take longer to catch up. However instead of targetting the station exactly at your AP you could just launch slightly ahead of it so when you get into circular orbit you are ahead of the station. Then it's a matter of raising your orbit slightly so you meet up in next orbit.

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For Kerbin, I prefer 267.19km (45 minutes), although 250km is fine as well for a station.  My preferred parking orbit for inter-planet craft is 400km (450.52km is a 60min orbit and full of comm satellites in my games).

Getting from 75km to 200km circular orbit costs about ~165 dV.  Getting from 75km to 400km is around 410 dV and 450km is about 465 dV.  So you lose a bit of Oberth effect at 450km, but not enough to matter.  Especially if you do your refueling at 400-450km altitude.

During rescues, I'll regularly jump up  to a 200km orbit and circualize in order to speed up the availability of a transfer window back down to the kerbal waiting at 80-85km orbit.

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5 hours ago, Atlas2342 said:

I'm planning to put to orbit a space station/orbital habitat/refuelling station. I want to ask, what would be the most optimal altitude to put my station in...

I also use MechJeb if that helps...

Many thanks.....

The 1st question to ask is, why build a station anywhere within the Kerbin system?  The only good answer is "because an irresistibly lucrative contract asked me to."  So if you don't have such a contract, don't build a station at Kerbin, Mun, or Minmus.  There is no practical benefit in owning a space station in the Kerbin system for your own use so I recommend you not build one on your own initiative.

Now, I know some will say that they're required for SSTOs to refuel at.  To which I ask, where does the fuel for SSTOs at the station come from?  If the answer is "from Kerbin", then there's no point in having an SSTO to begin with.  If the answer is "from mining at Minmus", then it's even less cost-effective to have SSTOs.

Same goes with using the station to farm science.  The Mobile Processing Lab takes FOREVER to grow science, and it doesn't grow anywhere near as much as you can find lying on the ground while biome-hopping Minmus in WAY less time and at far less cost.

Stations, like refueling operations, are only of practical value at other planets.

Now, if you don't care about any of the above and are just building a station for the experience, or for role-playing, or whatever, then knock yourself out.  But in this situation, you obviously don't care about practicality or efficiency to begin with, so the orbit you put it in doesn't matter.  Put it wherever you think it looks the coolest.

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2 hours ago, Temstar said:

Wait what? That's not suppose to happen, you must be executing your burn incorrectly to be dropping your PE. For a long burn you're suppose to burn prograde all the way regardless and start your burn earlier than the node, so that when the burn is completed your ejection angle is exactly the same as if you had infinite TRW and executed your burn on the dot at the node

if you plan a maneuver on a circular orbit, your blue maneuver node marker is below 0 degree pitch when you start your burn, and should be equally above 0 degrees at the end of your burn. in the middle (when you reach the Pe), it should be 0 degrees pitch, but you spent the first half of your burn shooting slightly "down" which shrinks your Pe.

2 hours ago, Temstar said:

As for burning straight at the target then cancelling out the relative speed once you are beside it, if you think about it that's basically the same as raising your orbit to meet it the next time around, only instead of meeting it after a whole orbit, you're meeting the target after only an section of one orbit (in others words an arc).

I was half joking. If I'm within 50km of my target and im lazy and have fuel to burn, I wont bother with a whole orbit to snug up the gap, I just point straight there and burn up to 100m/s relative

47 minutes ago, Geschosskopf said:

There is no practical benefit in owning a space station in the Kerbin system for your own use so I recommend you not build one on your own initiative.

Kerbin SOI stations are easier to build in parts over time. And does "because it's cool" count as a benefit?

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9 minutes ago, Venusgate said:

Kerbin SOI stations are easier to build in parts over time. And does "because it's cool" count as a benefit?

As I said in the part of my post you didn't quote, if you're building a station anywhere within Kerbin's SO on your own initiativeI, you have already thrown practicality and efficiency out the window to begin with, so the ONLY reason you can possibly have is "because it's cool".  If that's what makes you happy, knock yourself out.  But once you've taken that step, there's no point at all in quibbling over where exactly you put the station because if you cared about the bottom line of the thing, you wouldn't have built it in the 1st place.

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1 hour ago, Geschosskopf said:

you have already thrown practicality and efficiency out the window to begin with

I'm having trouble wrapping my head around doing anything in KSP for a reason other than "because it's cool"

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It's not 50/50, you should start the burn later than the 50/50 time. The reason is the during the burn your orbit steadily shifts towards the ideal ejection path and your TWR increases. At some point starting a burn earlier than node time results in an ejection angle that EXACTLY match the ideal ejection angle and that's what you want. Of course under this condition although your ejection angle is exactly correct you will be displaced position-wise from the ideal instant velocity change orbit. But we're talking about a change in position on the order of kilometers over a trip that will take months so it's trivial to correct.

The hard part is to work out the correct time split. The burn definitely start later than the 50/50 burn time but the exact timing is different for each ship and probably different again even for the same ship but different burn lengths.

Think about it this way: if you want to go to another planet and transfer window is here, you ideally want an instantaneous velocity change relative to Kerbin's orbit around Kerbol in prograde or retrograde direction. But even if you have a magic engine that could give you this instant change in velocity you still do not do the burn exactly at Kerbin midnight or Kerbin midday because as you coast your way out of Kerbin SOI Kerbin's gravity will curve your path so that it starts to deviate from the ideal Kerbin-Kerbol prograde or retrograde direction. So instead you do your burn earlier, somewhere around the Kerbin terminator to compensate for Kerbin's gravity bending your flight path.

Split burn around the node is the same idea. you start your burn earlier than the ideal ejection angle suggests to compensate for Kerbin bending your path while your engines fire.

The most extreme example of this idea is the spiral orbit:

Here you have an engine that has a TWR so low it needs MONTHS to complete the interplanetary ejection burn. So what you do is point your ship prograde several month before the transfer window and fire up the engine. As you engine fire over the month you slowly build up velocity and spiral out in an ever wider orbit until you finally build up enough velocity to escape the SOI right as the transfer window arrive.

It's not hard to work out how long you need to fire your engine for to get escape velocity. The hard part is to figure out where about in LEO/LKO should you start your burn so that just as you reach escape velocity you also eject out of the SOI in the correct direction.

Edited by Temstar
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41 minutes ago, OhioBob said:

Following the prograde marker results in greater deviation from the planned trajectory

Thanks for putting this more definitively than I could have! XD

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1 hour ago, Temstar said:

The hard part is to work out the correct time split.

I definitely am picking up what you're putting down, and appreciate the perseverance.
And if I am in a purist mood (as I sometimes am) I can see where this comes in handy - to be both minimal dV expenditure and precise maneuver final trajectory (I know an exact 50/50 maneuver vector burn is not exact and takes some post-burn tweaks). The calculation for when to start one's pure prograde burn, however, is not as handy as the ingame burntime values, so I'll probably continue using those, and for those who also do, my point of having a higher than 70km station still stands

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1 hour ago, Temstar said:

It's not 50/50, you should start the burn later than the 50/50 time.

But the 50/50 split is the common practice that almost everybody does.  It's even built into KER, with the time to burn being the time to maneuver node less 1/2 the burn time.

Quote

The hard part is to work out the correct time split.

Correct, which is why almost everybody doesn't bother to work it out and just does the 50/50 split.  If you do what almost the entire KSP playing community does, than there is less trajectory error by following the maneuver node marker than there is by following the prograde marker.

Quote

But we're talking about a change in position on the order of kilometers over a trip that will take months so it's trivial to correct.

And the Δv that one saves by following the prograde marker versus the maneuver node is likewise trivial.  For a measly 20 m/s or so, I'd rather split the burn 50/50 and burn to the maneuver node marker.  It's quick and easy to figure out and the ejection error is reasonably small and easy to correct.  Burning prograde either needs a more complex method to determine the burn start time or, if the 50/50 split is used, there will be greater error.

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It's not that hard to experimentally work out the compensated ejection angle though. I recall back in the days there was this mod called Protractor that made this very easy:

What protractor did was to show the phase angle, current ejection angle, delta-V for the transfer burn and closet approach all on a single line. With a very low TWR craft such as this one here what you could do was save the game. Then perform the burn at 0 degree ideal ejection angle and then when you complete the burn note how many degrees off course you are from the ideal ejection angle. Then you load your saved game and just do your real burn that many degrees earlier.

Here for example I'm going to Eve and I need to start my burn nearly 17 degrees before the ideal ejection angle to compensate for TWR.

And here's the result:

Eve SOI entry without ever having to go into map mode.

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59 minutes ago, StarManta said:

I can't believe no one has challenged this statement. That's ridiculous.

The theme among all of the reasons this statement is wrong is planning. If you have every single aspect of your space program planned from here to the end of time, then maybe your statement can be correct. If you can perfectly calculate your delta-V for every mission with little fuel wasted (whether by hand or by mod), then you may not have a use for a place to stash surplus fuel left over after launch or at the end of a mission. If your ships never have an emergency that requires you to quickly deliver them fuel before they crash into the Mun or get ejected from the system, then sure, you won't need that fuel stash around the Mun. If every new craft design you come up with works perfectly on the first go, then you wouldn't see any benefit from having a single design fine-tuned for efficiently launching to a station at a known and practiced orbit, at which point it can be refueled for the rest of its journeys elsewhere.

If all of those conditions describe you, then you won't benefit from having a space station.

Well said. Another reason that was just plain wrong is the "efficiency" argument:

11 hours ago, Geschosskopf said:

As I said in the part of my post you didn't quote, if you're building a station anywhere within Kerbin's SO on your own initiativeI, you have already thrown practicality and efficiency out the window to begin with, so the ONLY reason you can possibly have is "because it's cool".  If that's what makes you happy, knock yourself out.  But once you've taken that step, there's no point at all in quibbling over where exactly you put the station because if you cared about the bottom line of the thing, you wouldn't have built it in the 1st place.

If you assume the only function of a space station is research, and to a far lesser extent, refueling, then this is almost ok. However, I look at space stations as an opportunity to consolidate many launches into one. It's obviously far more efficient (and dramatically less expensive) to combine several vehicles in space, then send them as an exploration package, rather than flying each piece to the destination individually. For instance, my Minmus mission included 3 landers, 2 fuel modules, the lab/core hub, and a power boom with 4 popcorn-style satellites. I moved 11 vehicles, stripped Minmus of science, and completed several satellite contracts in a single trip. From an efficiency standpoint, you'll never in your life come close to that without a space station.

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14 hours ago, Geschosskopf said:

The 1st question to ask is, why build a station anywhere within the Kerbin system?  The only good answer is "because an irresistibly lucrative contract asked me to."

You may be right if you play the stock game and your only objective is to get all science with minimal effort. There is too few use of stations in stock game. But second good answer is that I have mods (Station science) which give science and need long periods of presence of scientists. But in my opinion best answer is that it is fun to build stations.

I put my stations typically at 300 km. I use them mainly as a spaceports for interplanetary crew transports after beginning. I bring fuel from Minmus. It is less efficient than to have station on Minmus orbit, but Minmus's orbital period is so long that I would lost an advantage because unoptimal transfer windows or burning against Minmus's orbital velocity.

My interplanetary ships have typically acceleration of 2.5-3 m/s^2. I can make burns enough accurately from 300 km orbit. Lower altitude would give Oberth advantage but I would need larger and unpredictabe corrections from lower altitude. I have not make exact calculations but practically 300 km work well and I do not see reason to change it.

You will get only opinions but not a unique answer. I suggest that you try different altitudes and see what works best for your preferences and game style.

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11 hours ago, StarManta said:

I can't believe no one has challenged this statement. That's ridiculous.

The theme among all of the reasons this statement is wrong is planning. If you have every single aspect of your space program planned from here to the end of time, then maybe your statement can be correct. If you can perfectly calculate your delta-V for every mission with little fuel wasted (whether by hand or by mod), then you may not have a use for a place to stash surplus fuel left over after launch or at the end of a mission. If your ships never have an emergency that requires you to quickly deliver them fuel before they crash into the Mun or get ejected from the system, then sure, you won't need that fuel stash around the Mun. If every new craft design you come up with works perfectly on the first go, then you wouldn't see any benefit from having a single design fine-tuned for efficiently launching to a station at a known and practiced orbit, at which point it can be refueled for the rest of its journeys elsewhere.

If all of those conditions describe you, then you won't benefit from having a space station.

Well, actually, my ships always do work the 1st time and never run low on fuel.  It's not hard to do this.  If you're having trouble with it, you should work on your basic design and navigation skills instead of using a station as a crutch for the underlying problems.

Fuel is actually pretty cheap to buy compared to the income you make from contracts, to the point that by about 1/2way through the tech tree, your money worries should be permanently over.  You can then start doing things that don't even earn any contract income and still have plenty in the bank, plus more income coming in from contracts.  There is thus zero financial incentive to set up a refueling system in the Kerbin system, especially because the ore-related parts are fairly far up the tech tree, after you've reached this state of being independently wealthy.  And certainly if you're not solvent, you'll never save enough money on fuel to even pay off the set-up cost of a refueling system, let alone save enough on fuel to get youself solvent.

So, there you are.  No financial benefit to a fuel station (and in fact it becomes a permanent drain on income if you use life support, due to the necessity of resupply missions).  And as already discussed, mobile labs have been rendered pretty much useless since their single useful feature (resetting Goo and Materials) has been taken over by scientists and their ability to farm science is pathetic.  Thus, in practical game terms, there is no reason to build a station in Kerbin's SOI on your own initiative.  Now, if a well-paying contract wants one, go for it, and maybe even make some use of it yourself.  But if you don't have a "build a station" contract, then the only reason to build one yourself is because you think it's cool, accepting that it's a waste of your time and money.

19 hours ago, Atlas2342 said:

I'm playing sandbox and for the sake of achievement, I want to put my station there. Might I ask, in terms of "coolness", what do you think is the "coolest" altitude? I'm eyeing for a 150-250 km orbit....

Thanks for all who replied too. Your answers are very much appreciated. You guys are the best!

The coolest altitude for a station is the altitude at which you can take the best screenshot of it with just the right mix of Kerbin and space in the background.  The bigger the station, the more solar panels and docked SSTOs it has, the higher it needs to be for the screenshot to look good.  A lot of people make the mistaike of building colossal stations in low orbits, so that the background is almost totally Kerbin, little or no space.  This really makes the station look a lot less impressive.

You need to be high enough to see a significant curve of Kerbin's horizon, maybe even the entire planet as a circle.  Now THAT's a killer screenshot.  The picture says in no uncertain terms that not only did you put in the time and effort to build this huge thing out many, many separately launched modules, and can make SSTOs aplenty, but all this was done halfway to Mun.,  It also shows that you know the entire enterprise was a vast exercise in potlatch and make no apologies for that

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I tend to use a 100km orbit for my transit stations. That said, I DO need to declutter that orbital and defuel and deorbit some discarded stages...

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