Jool delta V question

[miklkit]

I have established a base on Bop and sent a spaceplane there to transfer crew for the first round trip there.  The spaceplane made it there just fine with plenty of fuel left over, refueled and took off.  It then took so much fuel just getting out of the Jool system that when it got to Kerbin it had to aerobrake to slow down enough to then get an orbit, then more aerobraking was needed to slow down enough to finally land.  I would think that it would take less fuel to return than to get there.  Is this how it is or did I do it wrong?

Also, I just installed restock and it seems to be working well, but this fiery arc on the craft is new.  is this normal?

[king of nowhere]

1 hour ago, miklkit said:

I have established a base on Bop and sent a spaceplane there to transfer crew for the first round trip there.  The spaceplane made it there just fine with plenty of fuel left over, refueled and took off.  It then took so much fuel just getting out of the Jool system that when it got to Kerbin it had to aerobrake to slow down enough to then get an orbit, then more aerobraking was needed to slow down enough to finally land.  I would think that it would take less fuel to return than to get there.  Is this how it is or did I do it wrong?

Also, I just installed restock and it seems to be working well, but this fiery arc on the craft is new.  is this normal?

From a purely energy perspective, it takes the same energy to get from kerbin to jool than it takes to make the reverse trip. that's because of energy conservation.

however, there are many differences in the ways you can save energy.

one is gravity assist. to get to jool, you don't have to pay for an insertion burn because you can use a gravity assist from tylo or laythe. another is oberth effect; haing a massive body helps you in prograde burns. Jool is very massive, but at bop you are also pretty far from it. another is aerobraking; on kerbin, if you have a plane with enough heat resistance, you don't have to pay for insertion either.

 

that said, your experience is very surprising. because if you sent a spaceplane from kerbin to jool you had to spend 2 km/s to get to jool from kerbin orbit, plus whatever you had to spend to reach kerbin orbit in the first place, plus whatever you have to spend to orbit and land on bop from jool orbit - which is generally at least another km/s.

while from bop you can get a kerbin intercept for 1 km/s. plus 200 m/s to orbit the small moon. you can easily reach kerbin with less than half the deltaV it took you to get there. and without even counting what you spent to reach kerbin orbit. are you sure it took more fuel to return? in that case, you must surely have made some major mistake, but without knowing your trajectory i can't guess it.

actually, i can't even imagine what you could have done to spend more than 3 km/s getting a kerbin intercept from bop. so I'm more likely to put it down to you misreading or misremembering fuel cost.

Edited October 29, 2022 by king of nowhere

[Streetwind]

Did you wait for the correct Jool->Kerbin transfer window?

If you try to leave Jool just whenever you feel like it, you'll pay for an extremely expensive transfer.

[miklkit]

Full disclosure.  The spaceplane took off and went to Minmus, refueled, took off and got into a solar orbit, then went to Jool for a cost of 2800m/s.  It had plenty of fuel left as it didn't take much to get to Bop after arrival.  It then refueled on Bop.

It took off from Bop, circled around and burned for a solar orbit and just sat there.  Once it eventually did reach a solar orbit it then set course for Kerbin but didn't have much fuel left.  I hope this doesn't happen again as it was nerve wracking

[Streetwind]

The way back from Jool is typically more expensive, regardless of what conventional wisdom says (orbital mechanics doesn't always obey dV maps). But not that much more expensive if you find the right transfer. Finding it is not always easy though, and that may be what tripped you up.

Let me explain, using Alexmoon's handy calculator:

(The calculator assumes performing the burn in an equatorial orbit around the origin body, 100km by default, though I set 200km for Jool. I also checked the box for "no insertion burn", since we're only interested about departure burns here.)

The earliest available transfer window would have you leave Kerbin for Jool on year 2, day 256. expending slightly less than 2000 m/s (departure burn only), and arriving year 6, day 161. If you look at the porkchop plot (the colorful chart at the bottom), you'll see that there are clear vertical strips of color, red morphing into blue morphing into red, back and forth. The X axis of that chart is the departure date; the Y axis is the travel time. Mousing over the chart will show you a dV cost for that specific combination of departure date and travel time. You can also click the chart to get directions for executing that particular transfer, but the calculator will have the best one pre-selected.

The thick, even vertical bands, and the huge differences in dV costs between red and blue areas, show a strong dependance on transfer windows. If you pick a departure date (X axis) within a red area, then no matter where you move the mouse on the Y axis, it'll always be in a red area, and the dV costs are always going to be high. Leaving at that date is a bad idea. You have to wait for the right time.

Assuming you took that transfer and arrived on year 6, day 161, the most convenient window back would then be on year 6, day 365, expending some 2930 m/s. You can compute that transfer by swapping origin and destination, and adjusting the earliest possible departure time to the time of your arrival from the first transfer.

If you look at the plot of that calculation, however, you'll see a completely different picture: the colored bands are not nice and thick and vertical, they're extremely slanted and pretty thin. The difference in dV costs between red and blue areas still exists, but it's much smaller than it was with the Kerbin->Jool trip. This shows a weak dependance on transfer windows. Each point on the X axis is going to have at least one blue band somewhere, possibly even multiple ones. Meaning no matter when you want to leave, there's a cheap transfer you can find.

But the same applies to the red bands. Each point on the X axis is going to have at least one red band somewhere, possibly multiple ones. Meaning no matter when you want to leave, there's room for finding a terrible transfer that has you paying way more than you should. And though the differences between the best and worst transfers are not as bad as they could be, they might well be enough to make or break a mission.

When you're sitting ingame without any such handy tools available, just pulling the handles of a maneuver node until you get an encounter, then it's quite possible to accidentally find a pretty bad one. An experienced player might be able to tell that they're being overcharged, and will continue looking for a different route, but someone on their first trip to Jool wouldn't know any better.

[king of nowhere]

11 hours ago, miklkit said:

Full disclosure.  The spaceplane took off and went to Minmus, refueled, took off and got into a solar orbit, then went to Jool for a cost of 2800m/s.  It had plenty of fuel left as it didn't take much to get to Bop after arrival.  It then refueled on Bop.

It took off from Bop, circled around and burned for a solar orbit and just sat there.  Once it eventually did reach a solar orbit it then set course for Kerbin but didn't have much fuel left.  I hope this doesn't happen again as it was nerve wracking

ah, that's it. you are not making use of oberth effect.

Oberth effect means that making a prograde burn close to a planet is cheaper. making such a burn in solar orbit is a lot more expensive.

you spent 2800 m/s to go from kerbin orbit to jool in solar orbit. that's already a lot more than it should be. from low kerbin orbit, you can spend 2000 m/s and reach jool. it's much cheaper, even though you have to spend 900 m/s just to get out of kerbin's gravity. from minmus you are already almost out of it, and you can spend less than 1300 m/s; the best way is from minmus to fall down on kerbin and make the ejection burn close to it, as shown in this picture

160 m/s to go from minmus to low kerbin orbit, then it's only 1150 m/s to jool.

 

and the same applies to jool, except you are potentially losing eve more there. I already showed you a pic where you could have returned from pol with 1 km/s, because jool is very big and has a lot of oberth effect. making the burn around bop would also add a tiny bit of oberth from the moon. by getting out of jool, then lowering periapsis in solar orbit, you spent a fortune leaving the gas giant and you also lost your oberth effect.

frankly, i am surprised that you got to the point where you can make a spaceplane that can ssto from kerbin with enough fuel left to reach minmus (while carrying a lot of payload in the form of heavy passenger cabin and mining equipment) but you don't know about oberth effect and efficient interplanetary transfers. it's like finding a civilization that has discovered electricity but never invented the wheel.

[miklkit]

Ah, you both assume I know how to do orbital mechanics.  I do not.  I get into solar orbit and then let MechJeb do it.  Upon arrival I know enough to eventually get to where I want to go.  Jool is complicated and this was my first time exiting that system.

  As near as I can tell when I left Bop the game decided that I was in a retrograde orbit instead of a prograde orbit and it took forever to flip the orbit around before it would let the vessel actually leave the Jool system.  That is where most of the fuel went.  I need to learn more about that. 

I do spaceplanes because I don't like rockets.  There are 70+ rovers scattered around the Mun and Minmus from the days I was using rockets to get there.  They had to be left behind when returning to kerbin.  Not so with spaceplanes as they can reload the rover and bring it back.   Oh, they do not carry any mining equipment as that is what the gas trucks are for.  For Bop, first to land was a science lab, then a gas truck, both with rockets, then this spaceplane.   Round trips are now possible.

[king of nowhere]

16 hours ago, miklkit said:

Ah, you both assume I know how to do orbital mechanics.  I do not.  I get into solar orbit and then let MechJeb do it.  Upon arrival I know enough to eventually get to where I want to go.  Jool is complicated and this was my first time exiting that system.

  As near as I can tell when I left Bop the game decided that I was in a retrograde orbit instead of a prograde orbit and it took forever to flip the orbit around before it would let the vessel actually leave the Jool system.  That is where most of the fuel went.  I need to learn more about that. 

 

ok, I guess it makes sense.

So, I already told you about oberth effect and why you want to make those burns in the planetary orbit. As for how to make the maneuver, I suggest a simple way that's relatively accurate.

First, go to alexmoon.github.io/ksp. it's a very useful tool. So you want to go from kerbin to jool, tell the tool just that. In this case, it will tell you that the best time to start is in year 2:258, but you get a good transfer window also around 1:200. This will let you know the transfer windows.

Then you scroll down, there are a bunch of additional informations. most of them I myself don't know how to use, because they don't have an easy ksp reference. but one you can use, the ejection deltaV. In this case it says 1987 m/s, so you know you should spend some 2 km/s to get there. you'll likely spend a bit more because you're not perfect, but try to stay within 10% of that.

So now it's the transfer window, time to plan your maneuver. take your ship in kerbin orbit, and plot a prograde maneuver for 2 km/s. Any direction is good, you'll fix it later. So now your planned trajectory will exit kerbin and go in solar orbit, where it will produce an apoapsis (or periapsis if you're leaving on the wrong side. Highlight the apoapsis. You want to push apoapsis up as high as possible. So move your maneuver around kerbin's orbit, you'll see apoapsis increase, until you hit some point where you get maximum apoapsis and then it decreases. you found the optimal position for your ejection burn. Since this is the transfer window, chances are by now you'll also be seeing a close approach, because this is jool. now you can make small corrections to turn that into an encounter.

for other, smaller planets you're unlikely to yet be seeing a close approach. In this case I suggest you also plot a second maneuver, on the planar nodes- they are marked as AN or DN, ascending or descending node, meaning that your ship is passing through the plane of the target orbit going up or down. if you maneuver on the descending node, make a burn upward (purple triangle). if you maneuvr on the ascending node, make a burn downward (purple triangle with lines coming out of it). once you match the target inclination, you'll see a close approach. again, you'll need small corrections, but they will be small.

 

[Zhetaan]

On 10/29/2022 at 7:09 PM, miklkit said:

Full disclosure.  The spaceplane took off and went to Minmus, refueled, took off and got into a solar orbit, then went to Jool for a cost of 2800m/s.  It had plenty of fuel left as it didn't take much to get to Bop after arrival.  It then refueled on Bop.

It took off from Bop, circled around and burned for a solar orbit and just sat there.  Once it eventually did reach a solar orbit it then set course for Kerbin but didn't have much fuel left.  I hope this doesn't happen again as it was nerve wracking

That'll do it.  In both cases, being already out at a distant moon, it's tempting to think that since you're already close to solar orbit, you ought to go the rest of the way there and uncomplicate the manoeuvre.  Maybe that's not your reasoning, specifically, but either way, it doesn't work very well.

Here's the main reason why this wastes propellant and delta-V:  once you get to solar orbit, as in the instant you leave the local sphere of influence (whether Kerbin or Jool), your vessel still has an orbit that looks a lot like that of the planet that you just left.  Burning for solar orbit does change it just enough that you become separate entities, but only barely just enough.  In other words, with the way that you're doing it, most of the burn for solar orbit is wasted propellant because once you're done, you're effectively only marginally different from where you started.  One may imagine launching a rocket by burning until the engines are absolutely spent ... and then releasing the launch clamps.  That's not a perfect analogy, but it's illustrative of the idea:  you move, but the way that you're using the propellant isn't helping you much.

I don't know how you run your Minmus missions, so this is an honest enquiry:  when you do a mission to and from Minmus, do you return to Kerbin by burning to break Minmus orbit into Kerbin orbit, and only then burn to drop your periapsis and actually get to Kerbin?  Or do you arrange the Minmus return by burning such that your Minmus escape also happens to throw you back along Minmus's own retrograde so that you both break Minmus orbit and return to Kerbin with one burn?

If you do the latter, then that (or something similar) is what you need to do, albeit on a larger scale, for interplanetary transfers.  The reason is backed up with a lot of mathematics, but the short version is that there is a lot of energy bound up in a high orbit of a planet, whether or not you happen to also orbit a moon.  This is because high orbits have a lot of potential energy, which is actually a circular definition because these orbits get potential energy by virtue of being high:  in an important way, potential energy refers to how far something has to fall.

The problem is that when you burn to break from high orbit without first using that potential energy in some way, then it ends up literally being wasted potential.  The thing to do is to burn to dive for a low pass of Kerbin, and when you're at that periapsis, that is where you complete your burn for Jool.  The reverse applies for returning to Kerbin from Jool.  There's a lot that goes into the timing for a burn like that, and I won't get into it because @king of nowhere and @Streetwind already covered it, but what I want you to take from this is that high orbits have a lot of potential energy, and that you can save yourself a lot of propellant if you make the effort to realise that potential by first diving into the lower planetary system from these high-orbiting moons.  If potential energy is a measure of how far your rocket has to fall, then actually falling essentially converts that potential into energy of motion--and energy of motion is what gets your rocket to new and interesting places.

[miklkit]

I've been using that Oberth maneuver to get to Mun and Minmus.  Planets?  Nope, I kept missing.  Hence MJ.  I recently discovered "focus view" and that is a big help in getting a good intercept,  but still have a lot to learn about it.

How do I get from Minmus to Kerbin?  I wait until just past the prograde point and burn to Kerbin orbit.  It usually works well but sometimes it takes 3 burns.  Accuracy is not my strong suit. 

I have a spaceplane at Bop now and will try this Oberth thing.

[miklkit]

Jool is complicated.  The high point was when prograde became retrograde right out of Bop.  That made me not try to dive at Jool but just get away.  Maybe next time. 

[Zacspace]

17 hours ago, miklkit said:

Jool is complicated.  The high point was when prograde became retrograde right out of Bop.  That made me not try to dive at Jool but just get away.  Maybe next time.

Your navball shows your vectors relative to your current sphere of influence, so once you leave Bop's orbit and enter Jool's it'll suddenly switch to showing you Jool-relative vectors. if you ejected from Bop in the opposite direction that it orbits Jool then your navball would probably flip the the whole way around then you change SOI, the reason being that you're moving the opposite direction relative to Jool from what you are to Bop.

To put it another way, you're moving away from Bop while still orbiting Jool in the same direction as it. The same motion is just being shown to you from different perspectives throughout your flight based on which planet/moon's gravity is effecting your craft.

[miklkit]

Methinks I have found part of the problem.  The game is intermittently flipping the prograde and retrograde buttons.  Prograde shows as the position of the ship but it is pointed retrograde, and vice versa.  Not always but just once in a while.

I hired 4 new crew and am taking them around to level them up.   Loaded them up into a MK2 and went to Mun, flag, Minmus, flag with no issues.  Off to Ike and nothing but trouble.  When doing the retro burn into Ike orbit the ship was flipped and it ended up out in solar orbit again.  In the map view it looked like the earlier Jool burn with an orbit suddenly showing up and then flipping.  Now I have to visually check the ship before every burn.

[king of nowhere]

8 hours ago, miklkit said:

Methinks I have found part of the problem.  The game is intermittently flipping the prograde and retrograde buttons.  Prograde shows as the position of the ship but it is pointed retrograde, and vice versa.  Not always but just once in a while.

I hired 4 new crew and am taking them around to level them up.   Loaded them up into a MK2 and went to Mun, flag, Minmus, flag with no issues.  Off to Ike and nothing but trouble.  When doing the retro burn into Ike orbit the ship was flipped and it ended up out in solar orbit again.  In the map view it looked like the earlier Jool burn with an orbit suddenly showing up and then flipping.  Now I have to visually check the ship before every burn.

that is... there are ways to introduce this error accidentally.

do prograde/retrograde flip when changing sphere of influence? this is normal. do they change because you are doing something to your ship? maybe you're controlling it from another position? that can also be normal. there is a command to invert the direction of command in the menu of any probe core and crew pod, though it's not something you can press by accident. also if you do docking, that can flip the ship; the game can get persuaded that the front of the ship is now the docking port you just used, even though that docking port may be pointed backwards. you mentioned having "gas stations" around, so you dock stuff every time, so I guess that's what happened. make sure of where your ship is pointing. you can open the menu of the Mk2 cabin and select "control from here" to reset the control point.

if it's not one of those issues, then i have no idea

[miklkit]

I really don't know.  The buttons themselves change.  The prograde button flips the ship retrograde.  And it got worse earlier tonight. 

I was returning from Dres to Kerbin in that crewed MK2 and it was going ok.  It was about a 16 minute burn and with around 5 minute's left in the burn being controlled by MechJeb when the engines shut down, then I noticed the navball rolling.  Checking the map out showed MJ had changed the target to Jool and was swinging the ship around to start burning there!  Aborted everything and exited the game.  Restarted and the buttons were reversed.  Started over from the last save and this time it worked normally and I made it to kerbin and landed just fine.  Thinking about uninstalling and reinstalling KSP.

[Zhetaan]

6 hours ago, miklkit said:

I really don't know.  The buttons themselves change.  The prograde button flips the ship retrograde.

Normally, I'd call that a 'control from here' problem as @king of nowhere mentioned.  However:

6 hours ago, miklkit said:

Checking the map out showed MJ had changed the target to Jool and was swinging the ship around to start burning there!

... I'm honestly not sure of how to parse this one.  Even if you had previously set up a manoeuvre for your Dres-Kerbin return using MechJeb, that would just be a standard manoeuvre node in the game--you could shut down or even disable MechJeb and the node (and planned burn) would remain.  MechJeb works with KSP, not independently from it, which means that a lot of what it does is done merely using automatic control of features that are already accessible to the player.  What I mean is that MechJeb shouldn't be randomly changing your target, but even were that to happen, it definitely shouldn't be cancelling existing manoeuvre nodes and setting up new ones.

The only time I've ever seen similar behaviour from MechJeb is when someone deliberately created a new manoeuvre in MechJeb's planner and asked it to execute that plan immediately.

Obviously, you're not doing that--at least not deliberately.  The only way I can think that you might be doing it accidentally is if you keep MechJeb windows open under other menus while you do other things.  One thing that a lot of mods in KSP do not have is click-through opacity (assuming that that is even the correct term), meaning that if you have menus open and overlaid on one another, then clicking on one also registers as clicking on any menu underneath that one, because the mods only register the cursor's screen position, not the menu 'depth' under the 'surface' of the screen, to coin an analogy.  Is it possible that your MechJeb manoeuvre planner is under, say, the resource panel or a part action window that you're using on your rocket?  If so, then you may be accidentally telling MechJeb to plan a manoeuvre and burn for Jool.

[king of nowhere]

7 hours ago, miklkit said:

I really don't know.  The buttons themselves change.  The prograde button flips the ship retrograde.  And it got worse earlier tonight. 

I was returning from Dres to Kerbin in that crewed MK2 and it was going ok.  It was about a 16 minute burn and with around 5 minute's left in the burn being controlled by MechJeb when the engines shut down, then I noticed the navball rolling.  Checking the map out showed MJ had changed the target to Jool and was swinging the ship around to start burning there!  Aborted everything and exited the game.  Restarted and the buttons were reversed.  Started over from the last save and this time it worked normally and I made it to kerbin and landed just fine.  Thinking about uninstalling and reinstalling KSP.

ah, then it looks like a mechjeb problem of some sort. i'm not familiar with mechjeb, so others can help you better there.

as a purist of finding my own trajectories who takes great fun simulating my own ejections, insertions and gravity assists, I would say it's karma punishing you for trying to skip that part and relying on a mod to do your job . but I realize it's unfair and other people have fun in different ways than i do.

[miklkit]

My SOP is to get into a solar orbit and then turn on MJ and let it do it all from there.  And these are with crewed vessels and there is never any "control from here" stuff touched.  I only do that with rovers.  I normally do this in map mode too.  Oh, i am paranoid about leaving maeuver nodes open and delete them as soon as I am done with the manuevers.   I did find 2 screenies that show what I see.  It's a pretty clean screen.

[Zhetaan]

19 hours ago, miklkit said:

It's a pretty clean screen.

Wow.  I don't normally see screens that pristine even in stock.  Also, nice plane.

Anyway, were you executing this burn by hand, I see a potential problem in that you're set to prograde hold in your second screenshot.  A sphere-of-influence change would cause a problem there, since prograde is relative to the body you're orbiting.  But MechJeb uses a form of manoeuvre hold to execute its burns (actually, it just knows which way to point the rocket, and points it in that direction--which is a throwback to how everyone had to do it back before SAS manoeuvre hold was a thing), so the sphere change wouldn't affect that.

I'm starting to think that you've got a bug of some sort; this stopped making sense three posts ago.

[miklkit]

In that screenie the plane has just left Minmus and is in solar orbit with MJ all set up to go but still in manual mode.  After taking the screenie I activated MJ which turns off SAS, and then went to map mode.  It was then on its way to Bop.  That plane is a rover carrier that is carrying a science rover that is being delivered to Bop and is now bebopping around Bop collecting science.  It is also carrying 2 fuel cell arrays to add extra power to the gas truck.  Solar panels are useless that far out, I have found. 

That is basically a stock screen.  MechJeb in the upper left and some mod stuff in the lower right are the only non stock things there. 

Yeah there is a bug somewhere.  The plane had been flown to Ike, then Dres by MJ and it was only when going from Dres to Kerbin that it changed targets to Jool, so there should not have been any old things to trigger it.  I'm going to reinstall MJ and if this still happens, then the whole game will be reinstalled. 

[king of nowhere]

2 hours ago, miklkit said:

Yeah there is a bug somewhere.  The plane had been flown to Ike, then Dres by MJ and it was only when going from Dres to Kerbin that it changed targets to Jool, so there should not have been any old things to trigger it.  I'm going to reinstall MJ and if this still happens, then the whole game will be reinstalled. 

or, you could learn to perform those maneuvers manually. it will be hard at first, but you can be a lot more efficient than mechjeb is, and it will be a lot more fulfilling.

besides advertising my own agenda on how this game should be played, I can attest that this game has a mod to fix everything, but the more mods, the more it becomes instable. all those mods add their own issues. In the end I found that it's better to not rely on a mod unless you really have to.

Wait, does that still count as advertising my own agenda on how this game should be played? Damnit!

[miklkit]

Yes you are expressing your opinion. 

When I first started trying to go interplanetary I found that MJ is better than I am.  A lot better.  I can do it myself, but choose not to.  It saves me hours of frustration this way. 

As for mods making the game unstable, MechJeb is built into this game!  MJ2 is a mod.  Almost all the mods I do have are graphics, not functional.

Anyway, i uninstalled MJ last night and played a bit.  Scaled a 6600 meter mountain on Bop.  Now to reinstall and see what happens.  Really not sure just how to reinstall the entire game while keeping this save file.