Why is Mechjeb botching this landing?

[DangerDave]

I just downloaded Kethane last night, and to test the concepts of it I'm simply trying to mine some of the Kethane that occurs on Kerbal itself. So I built a small drilling lander and put it into orbit, but for some reason whenever I tell Mechjeb to Land At Target (Specifically, the Kethane deposit) he crashes it. Admittedly, he crashes it dead center of the landing zone, totally sticks that part, but once he starts to enter the atmosphere he goes all wonky and tries to land it upside down. He drops the landing gear and does the braking maneuver he's supposed to right before he lands, but since he has the lander upside down, it just smashes into the ground.

Now, I know I can land it manually, and I don't even mind doing so, but I'm worried the problem is a design flaw and I'd like to know where I borked it up. For the record, I'm using the Impossible Innovations parts for this particular lander, mostly so that I could test the Kethane drilling without worrying about fuel and control, and the pure black piece is just a Deuterium tank that doesn't load the texture for some reason. ::shrug:: I've even tried loading four of the S.S.A.S. from the I.I. mod, but it didn't seem to help and was probably counter productive anyway.

Thanks in advance!

[stupid_chris]

MechJeb is not fail proof, sometimes it doesn't work. Why?No particular reason, but sometime MechJeb just hate your craft. There's nothing much you can do.

I had some success with starting it with land at target so it would get the landing properly aligned and then, deactivating it and just using the land somewhere feature when starting the breaking burn.

Else, Impossible Innovations is overpowered and MechJeb doesn't always like that. And the SSAS is just a strong SAS, it does nothing to help you control better just as normal SAS units don't.

[Cashen]

Even when I used MechJeb for a lot of things, I never quite trusted it to do landings.

Even when trying to land on a kethane deposit, I still do manual landings. Granted, I haven't tried to land from orbit on a kethane deposit on a body with an atmosphere yet. But when it's on an airless body I'd just bring my orbit as low as possible over a deposit, and as soon as the kethane scanner started beeping to indicate a deposit, engines to full power to kill lateral velocity. Most deposits are large enough that you can land on them with this method.

[Flixxbeatz]

Not to discourage you, but try landing it manually.

[DangerDave]

MechJeb is not fail proof, sometimes it doesn't work. Why?No particular reason, but sometime MechJeb just hate your craft. There's nothing much you can do.

I had some success with starting it with land at target so it would get the landing properly aligned and then, deactivating it and just using the land somewhere feature when starting the breaking burn.

Else, Impossible Innovations is overpowered and MechJeb doesn't always like that. And the SSAS is just a strong SAS, it does nothing to help you control better just as normal SAS units don't.

With some experimentation (Mainly, trying to land it manually) I've determined that the ship simply won't keeps its nose up. No amount of input (with or without Mechjeb) will allow me to bring the nose above the horizon and now I'm totally stumped.

[IllicitMedic]

It might be the cockpit, MechJeb functions based on what "part" the ship is being controlled from and its orientation relative to the ground...just a thought though idk.

I know I had that problem with a rocket on launch it kept tipping and pointing down and it was because it was being controlled from a part that faced the ground so it was trying to turn it up...simply selecting my capsule and setting control from here fixed it...so idk...

Try controlling it from different parts by right clicking and selecting "Control From Here" its worth a shot anyway.

[Coneshot]

I had the "YAY PINWHEEL TIME!!!" issue with landings as well, then I got the latest dev version and it stopped.. lands like a dream now:

http://jenkins.mumech.com/job/MechJeb2/

http://forum.kerbalspaceprogram.com/showthread.php/12384-PART-0-20-Anatid-Robotics-MuMech-MechJeb-Autopilot-v2-0-8

[ecat]

Some causes of MechJeb landing in unanticipated orientations (eg upside down)

Insufficient engine power.

Controlling from an upside down control pod/cockpit.

Having too much drag near the bottom of the vessel and too little drag near the top. IIRC, in game drag = 'mass of part' * 'drag of the part'.. I'm guessing this is your problem.

It's easy to test for the drag problem, just add some RCS near the top of the ship, 3 standard ports will do. You don't need too much RCS fuel as you don't need to enable RCS until the final part of the flight, during the last 10km or so should be enough to at least prove/disprove the drag problem.

The last minute of this video is a nice example of what we are trying to achieve...

[DangerDave]

Some causes of MechJeb landing in unanticipated orientations (eg upside down)

Insufficient engine power.

Controlling from an upside down control pod/cockpit.

Having too much drag near the bottom of the vessel and too little drag near the top. IIRC, in game drag = 'mass of part' * 'drag of the part'.. I'm guessing this is your problem.

It's easy to test for the drag problem, just add some RCS near the top of the ship, 3 standard ports will do. You don't need too much RCS fuel as you don't need to enable RCS until the final part of the flight, during the last 10km or so should be enough to at least prove/disprove the drag problem.

The last minute of this video is a nice example of what we are trying to achieve...

I gave that a try and still nothing. It did give the nose quite a bit more control, but still not enough to get above the horizon. I dunno, it's like the nose of the ship weighs 20 tons. I'm just gonna start from scratch with a different design.

[Gargamel]

My MJ does everything perfectly until about 10m off the ground, and then it loses it's mind, starts screaming "I CAN'T DO THAT JEBEDIAH!" and pinwheels out of control. Even with chutes attached.... lots of them (14).

Rest of the OP is identical to my experience aside from that and the design of my ship.

[ecat]

I gave that a try and still nothing. It did give the nose quite a bit more control, but still not enough to get above the horizon. I dunno, it's like the nose of the ship weighs 20 tons. I'm just gonna start from scratch with a different design.

So three are not quite enough. Can you post the craft file somewhere? And/or a video of the landing?

You can try 4 RCS ports in the usual pattern. You can try 2 sets of 4, one set just above the other. You can try extending the nose of the ship, placing the RCS ports as far from the bottom of the ship as possible. You can also try adding an additional set of RCS ports near the bottom of the ship to provide some counter force - I think this would be the next thing I would try. Just remember, more RCS ports = more RCS fuel use, especially with MechJeb.

Here I'm using one set of 4 near the bottom and two sets of 4 near the top to give MechJeb control of this 280t monster...

from: http://forum.kerbalspaceprogram.com/showthread.php/33503-Show-off-your-B9-Designs%21?p=508910&viewfull=1#post508910

While my ship is much heavier it has two advantages over yours. The RCS ports are most effective when placed as far from the centre of mass as possible, think long leavers vs short leavers. My ship has both length and width.

My MJ does everything perfectly until about 10m off the ground, and then it loses it's mind, starts screaming "I CAN'T DO THAT JEBEDIAH!" and pinwheels out of control. Even with chutes attached.... lots of them (14).

Rest of the OP is identical to my experience aside from that and the design of my ship.

Can you post the craft file or at least a picture?

Edited July 16, 2013 by ecat

[DerekL1963]

Just remember, more RCS ports = more RCS fuel use, especially with MechJeb.

A trick is to use the 'R' key - enabling RCS briefly when needed and disabling it when not.

[magnemoe]

I had much of this issue then trying to land on Pol, used the same landers who worked well on Minmus, on Pol everything went well until final approach. It then started rotating the craft with engines burning.

Was both normal rocket designs and pancake landers, suspected to high twr, but landed an huge kethane lander on nuclear engine only with the same issue.

Its not an huge issue main reason to use mechjeb landing is to get close to the target and it don't happens for me on higher gravity worlds.

[tavert]

One issue with your craft is that you're using an aircraft cockpit. Those have lower drag coefficients than most other parts in KSP, so under the stock aerodynamic model, your center of mass is ahead of your center of drag. This is great if you want the nose pointed forward, bad for landing when you want the nose pointed backwards. Try the same basic craft with a 0.2-drag capsule and see if that makes a difference.

[Starwaster]

Don't see anyone talking about balance. And that drill on the side is throwing your balance off in the worst way at the wrong time.

Since you're using parachutes, cut MJ off and apply just enough thrust manually (or limit MJ's use of thrust to maybe 10%-25%?)

JUST enough to keep your descent speed to the maximum your landing gear can take. Check its impact rating in the VAB. That number is the fastest you can be on descent when it lands.

Another tip that might let MJ finish the job in the final stages is to set its landing speed higher. Default is 0.5m/s. Increase that to no higher than the aforementioned impact rating for your gear. Maybe a bit less for safety margin. (but remember that Kerbals don't like safety margins)

One final MJ related tip, if it's having trouble with the landing, it might be trying to course correct at the last second, assuming that you're trying to 'land at target'. So one thing I find myself doing (A LOT) is on final descent just before the expected suicide burn, is that I abort the auto landing and click 'land somewhere' or is it called 'land anywhere'? I forget. That lets MJ just concentrate on getting down safely. Basically you're saying this location is good enough, just LAND ME ALREADY.

[Technical Ben]

Check your COB...

[Faark]

Is it possible that MechJeb (i am using 2.0.8) has some problems with vectoring engines? I was trying mechjeb's autoland on my landers and it always crashed. Only one with aerospikes worked fine. Anyways, so i tried another lander that uses radial mounted Mark55's. It lands fine if in case I lock those engines gimbal. Only in that case...

[Kosmo-not]

One issue with your craft is that you're using an aircraft cockpit. Those have lower drag coefficients than most other parts in KSP, so under the stock aerodynamic model, your center of mass is ahead of your center of drag. This is great if you want the nose pointed forward, bad for landing when you want the nose pointed backwards. Try the same basic craft with a 0.2-drag capsule and see if that makes a difference.

^^^ This is the real issue.

[ZothTek]

I would do land at target its best if you use Translatron and ounce you get close enough to the ground just hover over by setting your speed to 0 then change it back to -1 to land. Also i've tried land at target on Kerbin,Moho,Mun, and Minmus it just causes your ship to slam into the ground unless you get your Attitude correct which i do not understand how to do.

[DangerDave]

^^^ This is the real issue.

I thought that might be the problem at first so I changed it several times to use different pods (Both Lander cans and Command Pods), it didn't seem to make any difference so I went back to the cockpit just cuz I liked how it looked. I'll try several of these suggestions out tonight and let you know.

[ecat]

I've just read this again

I gave that a try and still nothing. It did give the nose quite a bit more control, but still not enough to get above the horizon. I dunno, it's like the nose of the ship weighs 20 tons. I'm just gonna start from scratch with a different design.

get above the horizon?

How and when did the pointy end of your ship get below the horizon? Ouch!

Once you allow the pointy end to point down you're in trouble.

[DangerDave]

I've just read this again

get above the horizon?

How and when did the pointy end of your ship get below the horizon? Ouch!

Once you allow the pointy end to point down you're in trouble.

That's just it. everything goes smoothly until after reentry and then the nose inexorably begins to swing below the horizon and I can't get it to come back up. =P

[ecat]

And you use RCS before or after this happens?

Hint:

The correct answer is before.