Have strange issues with Mechjeb 2 regarding orbiting another planet

[galaxy366]

Hey KSP forum,

I have been using Mechjeb 2 trough some occasions and i have stumbled in a annoying issue. So the issue goes this:

I left Kerbin from 600k on a flight to EVE. When i finally entered it's gravity field my ship would head towards the ground. So my ship was at 300k and i wanted to circulate it at 100k.

What Mechjeb tells me is that isn't possible and gives me a yellow warning stating i can't go below blabla. Does someone have a idea why this happens?

I have to do place a node manually before i could magically go below it but even then it took 2000km to adjust my orbit and it cost me lots of fuel.

Edit:

Putting autopilot at 100k also doesn't work

Edited April 29, 2014 by galaxy366

[FREEFALL1984]

I'm not sure about this one, can you explain what you have done, I' assuming you havent aerobraked into the eve system and require 2km/s Dv in order to circularize, but the rest of your cool story is a little confusing. Also mechjeb would normally give a warning if you produce a maneuver which will require extreme Dv,

Please clarify.

[biohazard15]

You need to do it in two stages.

First, you need to set your Pe at 100 km. Choose "Change periapsis" in maneuver planner, set your node at 60 seconds ahead (you may wish to tweak it if you have a heavy ship), execute node

Second, choose "Circularise", choose "at the next periapsis", execute node.

Note that at Eve, you can (and should) use aerobraking to reduce dV needed for circularization.

[galaxy366]

I'm not sure about this one, can you explain what you have done, I' assuming you havent aerobraked into the eve system and require 2km/s Dv in order to circularize, but the rest of your cool story is a little confusing. Also mechjeb would normally give a warning if you produce a maneuver which will require extreme Dv,

Please clarify.

I'll go ahead and clarify it.

So I send my rocket with a Kerbal up at 600k and circulized him around the planet Kerbin. When that was done I went ahead and transfered to the planet Eve.

When I got underway and was in orbit around the Sun I used the maneuver planer to change my inclination so I would intercept the planet.

When I got into the pull of Eve my node showed a path through the core of Eve (meaning I would crash instead of escaping).

So I went ahead and used Mechjeb to put me in a circularized orbit around Eve, this is where the issue started. I entered at a orbit of 2000k and wanted to orbit the planet at 100k.

When I entered 100k Mechjeb say'd that this was impossible below 2000k. I went ahead and did it manually but it resulted in having my rocket burn 1500km/s 90% to the right.

Hopefully this clarifies it!

[EdFred]

When you enter the SOI, change the periapsis to 100km.

Then circularize at the periapsis.

Two separate maneuvers - just like biohazard said.

[galaxy366]

When you enter the SOI, change the periapsis to 100km.

Then circularize at the periapsis.

Two separate maneuvers - just like biohazard said.

Alright I'll follow your instructions! I'll report back how it goes

[Red Iron Crown]

If you're doing it from scratch, you're better off fuel-wise to use "Fine tune closest approach" once you're about two thirds of the way through the transfer.

[EdFred]

If you're doing it from scratch, you're better off fuel-wise to use "Fine tune closest approach" once you're about two thirds of the way through the transfer.

Even when I was using MechJeb, it still never gave me what I asked for on that - and I would always slow warp on SOI.

I would ask for 100km periapsis (it would show it after the burn on map view), but after entering the SOI, it would be anywhere from impacting the surface to 300-400km.

[Pecan]

Make sure you are not using the 'official' MJ download - use the latest dev build instead (it hasn't been happy since 0.23.5).

[Sirrobert]

I'll go ahead and clarify it.

So I send my rocket with a Kerbal up at 600k and circulized him around the planet Kerbin. When that was done I went ahead and transfered to the planet Eve.

When I got underway and was in orbit around the Sun I used the maneuver planer to change my inclination so I would intercept the planet.

When I got into the pull of Eve my node showed a path through the core of Eve (meaning I would crash instead of escaping).

So I went ahead and used Mechjeb to put me in a circularized orbit around Eve, this is where the issue started. I entered at a orbit of 2000k and wanted to orbit the planet at 100k.

When I entered 100k Mechjeb say'd that this was impossible below 2000k. I went ahead and did it manually but it resulted in having my rocket burn 1500km/s 90% to the right.

Hopefully this clarifies it!

That's probably because you are not in orbit.

Do a manual burn to raise your periaps. Use the blue manouver node points to raise your periaps, excecute that burn, and than tell Mechjeb to do it again

[galaxy366]

Make sure you are not using the 'official' MJ download - use the latest dev build instead (it hasn't been happy since 0.23.5).

What is so different with the Dev build exactly?

[sojourner]

Lots of fixes.

[galaxy366]

Make sure you are not using the 'official' MJ download - use the latest dev build instead (it hasn't been happy since 0.23.5).

I just came back from a friend. I'll go ahead and follow the tips i got in this topic.

Mind telling me where i can get the DEV build for MechJeb ? There is no link on Spaceport or the KSP thread.

[Pecan]

Yes, it's easy to miss -

In the MJ OP just under the BIG text saying "DOWNLOAD HERE"

the small print says "get the latest dev builds here"

which is -> http://jenkins.mumech.com/job/MechJeb2/

Edit: there may still be 'issues' and development is continuing but it seems to be working properly for me. The reason it's all like this is because the person looking after has changed.

[MisterFister]

Not only all of the above suggestions, which I endorse, you're starting your transit from HKO at 600km. You're losing a lot of the benefits of the Oberth Effect by beginning your Kerbin escape from such a high parking altitude (assuming you're not using a mod that alters the size of Kerbin.)

Also, according to your OP, you're entering the Eve SOI on an impact trajectory, not on a flyby. MechJeb doesn't work with impact trajectories very well, necessitating breaking up into two separate maneuvers. (Also, this is the first I've heard of the devbuild of MechJeb, but I can assure you that understanding what you're trying to accomoplish -- at least in theoretical terms -- can allow you to methodically work through these problems either manually, or using the official MechJeb release. You'd just need to break the request into smaller pieces, such as here.)

[BudgetHedgehog]

I think that's what happened - if on an impact course, circularising at 100km would need a huge amount of dV because MJ would do it on one manoeuvre i.e. as soon as your craft gets to 100km, it'd do a burn to cirularise, regardless of your current trajectory. As mentioned, doing a small radial burn to move the PE into a flyby height and then telling MJ to circularise at PE is needed. Or you could aerobrake, I guess.

[galaxy366]

Seems i kind of managed to solve it. I followed someone his instruction about doing it in 2 maneuvers and I did get into orbit around the Mun! I'll try it again and see if it works again. I'll post here what I did. For the moment i'll have this be unanswered till I report back.

[galaxy366]

Alright guys it again didn't really work. So this isn''t with EVE but it's the exact same line that I got when I entered it's orbit.

http://i.imgur.com/8XkJ3ma.png

What do I have to do now?

[BudgetHedgehog]

Alright guys it again didn't really work. So this isn''t with EVE but it's the exact same line that I got when I entered it's orbit.

http://i.imgur.com/8XkJ3ma.png

What do I have to do now?

Change PE to the desired orbit height (either use the Change Periapsis window in MJ or just do it yourself with a small radial burn (the blue symbols on the manoeuvre node.). When at that new PE, circularise.

If you just immediately say 'circularise at 10km' or whatever, MJ will do just that - make a node at 10km that circularises. Because you're travelling at pretty much right angles to it and are going straight to the core, you need to kill the vertical velocity (with respect to the Muns surface) and replace it with sideways orbital velocity. Probably in the region of around 1km/s burn I'm guessing? A bit less? That's how much speed to have to kill and regain once you reach 10km. If you just do a small radial burn, you push your vector outside the Muns surface so are travelling sideways in respect to the surface. The you just need to slow down to orbital velocity.

[galaxy366]

Change PE to the desired orbit height (either use the Change Periapsis window in MJ or just do it yourself with a small radial burn (the blue symbols on the manoeuvre node.). When at that new PE, circularise.

If you just immediately say 'circularise at 10km' or whatever, MJ will do just that - make a node at 10km that circularises. Because you're travelling at pretty much right angles to it and are going straight to the core, you need to kill the vertical velocity (with respect to the Muns surface) and replace it with sideways orbital velocity. Probably in the region of around 1km/s burn I'm guessing? A bit less? That's how much speed to have to kill and regain once you reach 10km. If you just do a small radial burn, you push your vector outside the Muns surface so are travelling sideways in respect to the surface. The you just need to slow down to orbital velocity.

But won't this action burn lots of fuel? Even with my current vessel i'm at 35% with a nuclear engine.

[BudgetHedgehog]

But won't this action burn lots of fuel? Even with my current vessel i'm at 35% with a nuclear engine.

What you should do is in the first paragraph. You'll use like 30m/s or so for the radial burn and a couple hundred for the circularising. With 35% of fuel and a nuclear engine, you'll be fine.

Yes, the second way consumes a lot more fuel, is the most inefficient way to do what you want and I only wrote that to explain what happens when you do it. You can safely igore the second paragraph - just do what I told you to do in the first paragraph (change PE, then circularise at PE).

[Baythan]

I don't use Mechjeb much myself, so I'll be giving you the steps to end up in a decent orbit from that basis, but Mechjeb can do these steps itself if you find the right command. Also, use the Dev builds unless they have an official 0.23.5 release. KSP 0.23.5 kinda broke a lot of the maneuver planning for Mechjeb. From what I read on their forum while I was deciding between MJ and KER, MJ was waiting till you reached nodes to do burns instead of doing half the burn before the node and half after.

Reaching orbit from impact trajectory:

Step 1: From as far away as possible (to reduce Delta-V needed), increase Pe to desired orbit OR to a good aerobraking altitude (depending on astral body). For a prograde orbit, point to 90°, for retrograde point to 270°.

Step 2: (Skip to step 3 if not aerobraking) If correct aerobraking altitude was selected, Ap should now be inside the SOI of target body. Create node at Ap to INCREASE Pe to desired orbit.

Step 3: Create node at Pe to reduce Ap to desired orbit, which should be a retrograde burn.

Step 4: Orbit should now be circularized.

Even though the aerobraking has more steps, it can save you a LOT of Delta-V on Step 3, but only works on bodies with an atmosphere.

[galaxy366]

Alright I see! Does someone also know if i can make Mechjeb 2 auto circularize Kerbin?

Example: Now i have to manually put in 80k and when it reaches that press the circularize button.

Is there a way to make this happen automatically? I did have some instances where it did but i have no clue how i did that.

[MisterFister]

Again, sir, the picture you posted is of an impact trajectory. You need to FIRST convert it to a flyby by nudging the periaps outward to [theoretically any] altitude BEFORE you circularize.

On a more general point, you're revealing what is essentially the heart of the MechJeb Debate. Now, full disclosure, I use MechJeb too. The real space program uses autopilots, the Space Shuttle was only flown manually twice (evidence suggests the third time was an attempt to do so during the Shuttle Columbia disaster, though if so, it was obviously inconsequential and futile) and both known attempts were planned to be limited. One was aborted fourteen seconds prematurely, and the second time almost resulted in the loss of a mission objective.

The problem with manual OR autopilot inputs, in KSP or on Earth, is the Oberth Effect and something that I think may be called "arc distances." Essentially, think of it this way: If you're standing at the front of, say, a bus. The two sides of the windshield of the bus are wider apart than your whole armspan. If you go to the rear of the bus, however, you can cover the width of the windshield with your hand with outstretched fingers. The further away you are, the smaller adjustments you have to make to effect a HUGE course correction at the other end.

In the game, the Mun, Duna, even Kerbin itself are huge when compared alongside the actual size of the craft you're piloting. But the distance of even a few centimeters in travel direction coming out of an escape burn can literally mean the difference between crashing into the center of your target body such as in your photograph, and conversely being flung outward on a gravity assist, albeit an unintentional gravity assist.

Some elementary explanations are in order here. Orbits, you might need to know, are calculated around the CENTER of a planet. That's why you actually have an "orbital velocity" when sitting still on the Launchpad. The surface of Kerbin is rotating around the center of the planet, so technically you are orbiting. You may already realize, however, that the orbit is slower than escape velocity, so gravity takes hold. This is a fair approximation of what is happening here on Earth, too -- as you sit playing KSP, you are literally "orbiting" the center of the earth once every 24 hours. The International Space Station also does so, but at an altitude (above the average-sea-level surface of the planet, not above the atmosphere, and certainly not above the center of the Earth) of ~277km. To get your altitude there in your chair, or the altitude of the space station, above the center of the Earth, you need to take your "indicated altitude" (close to zero for you, ~277,000 meters for the station) and add that number to, literally, the radius of the planet from the center of the core to the surface.

When you see impact trajectories like the one you display in that imgur link above, that "escape angle" from the Munar SOI is deceptive, since it "looks" like you're going somewhere afterward. Mathematically, you would, if the entire mass of the Mun were a dimensionless dot in orbit around Kerbin. You'd swing around that center of mass at ostensibly extraordinary speeds, and fling outward in nearly the opposite direction on a parabolic trajectory. So, as far as MechJeb is concerned, you already "have" a periaps. The problem is that the Mun takes up space. It has a radius at its equator of two hundred thousand meters. That means a periaps of 200km from the gravitational center would be a [very high speed and likely crash] landing. In terms of orbital mechanics however, we consider that center-radius of 200km to be "zero." A negative periaps is therefore an impact. An "orbit," as far as we're concerned, of 25km is actually an orbit of 225km above the Munar center.

I mentioned the MechJeb Debate. Some people see it as cheaty, and would mock you and me for using it. Others find the game frustrating without it and "need" it. I personally tend to fall into the middle: I use it to give me suggestions, but I modify my maneuver nodes the way I want them (using the PreciseNode plugin, I highly recommend it) and I consider myself capable of burning them myself. Now, since a smidge left or right will sometimes result in missing my intended capture angle, I am willing to let MJ perform the burn for me, as well, but I don't let it go off uncorked; I pay attention to it with my mouse cursor hovering over the "Abort Execute" button.

MechJeb, or the autopilot on the Space Shuttle, is a wonderful tool when it is used to augment your flying skill as opposed to replacing it. Think of the game of pool on a billiard table. Show someone a birds-eye-view picture of a set of billiard balls in random arrangements like they do on television and any person can intuitively guess on some of the easier straight-line shots. But ask the average novice to hold the pool cue and actually execute them, that's a different story. You have to work on your aim and your cue-control, and this is the same with golf, or baseball, or hockey, rugby, or either of the two major forms of football. The same here: I can plan my burns well enough, but sometimes, even with fine-controls enabled on my keyboard, and my admittedly huge-*** high resolution monitor, sometimes a single button push to the left or right will be three times what the burn actually needed, and I'm thrown off elsewhere. So, I use MJ to perform those very sensitive burns that are often reported by the maneuver node to be less than 2s in length, or in situations where using PreciseNode tells me that even .01m/s in antiradial burn vectoring will change my capture periaps by more than 20km in altitude. Can you use your mouse or your keyboard to precisely burn .01m/s along a specific antiradial vector? Or change an existing burn plot by exactly .01m/s antiradial? I can't, and that's when I choose to use MJ.

I am gathering (and not mocking you for this) that you need MechJeb for more reasons than I do. The question you pose here leads me, and others I'd wager, to believe that you don't quite grasp basic orbital mechanics. And that's fine -- even the people who designed the Apollo program in the 1960s didn't fully understand them at first, either (lots of history to be studied there, if you like learning cool things about what people did yesteryear, I highly recommend reading up on it.) We, as a community of Kerbal Space Program players, weren't born knowing orbital mechanics, and I'm fairly certain that unless a KSP player among us actually happened to go to school for something where orbital mechanics was a thing, or happened to work for some point at JPL (such as the author of http://www.xkcd.com/1356/) chances are we didn't even know much about orbital mechanics before playing this game at all.

So, I'm not knocking you for your obvious reliance on MechJeb to tell you what's possible and how to do basic things. That's the great thing about this game -- it appeals to so many even without the mods, but with the mods that audience is increased by what I think is an entire order of magnitude. But I can say that your problem is rather easily solved when it happens, and PREVENTING it is even easier than solving it. I'm not talking down to you, this is the actual answer to your question: the Munar "orbit" pictured in your imgur above is preventable by tweaking your Kerbin transfer burn, likely by less than 1m/s in VERRRRY sensitive and finicky increments along what could end up being divided among all three axes (grade, normal, radial). Alternately, you can perform a correction burn, likely less than 1m/s at a midcourse node as well. Absent that, then you change your periaps to a desired level IMMEDIATELY after transitioning to the Mun SOI, and THEN you circularize at the new periaps.

Sorry dude, but there exists no other answer to your question in any form at all. If this answer does not help, then you need assistance understanding the fundamental elements of the answer, but again, there exists no other available answer other than the ones you've been given here.

And I do not apologize, because that would be a cop-out and my saying, essentially, "You're on your own dude." I'd be happy to Skype with you and walk you through it, step by step, soup to nuts.

[MisterFister]

Lots happened while I was typing and editing that post above. Largely, my point remains unaffected.

To your follow-up question -- pay close attention to your MechJeb windows. You can circularize "in X seconds [default 0 = now]", "at periaps," and "at apoaps." Use those left / right scroll buttons, dude. They ALL have a VERY SPECIFIC function.