Duna landing with mech jeb oh the crashes.....

[OgreMagi]

Hello I am a new player to the Kerbal space program, I downloaded the free version and decided that I would try the advanced version.

So far doing great, I played with the stock version and only down loaded mechjeb to help with the tedious parts. I made several trips to the mun, minius, and eve. all landing with one a few fatalities ( my manned rovers needed some redesign). but the problem is DUNA......

Mech jeb is making it hard to make a landing, I have tried my similar approach I used on eve, Parachutes to slow to final desent and enough rockets to get back up but landing with mechjeb assistance is impossible.

Whats the issue with mech jeb? I tried deaccelerating and then deploying the chutes but it never slows the lander to a safe landing speed, even when I burn my ship to only 100m/s in the atmosphere.

any ideas on what I may be doing wrong?

[Streetwind]

Addon support questions should technically go into this forum: http://forum.kerbalspaceprogram.com/forums/13-Add-on-Requests-and-Support

In the meantime, have you managed a successful manual landing (without mechjeb) using this vehicle? Maybe it is struggling to offer enough deceleration. Eve has a very, very thick atmosphere (5 times as thick as Kerbin's at sea level), so parachutes work great there. Duna on the other hand has a very thin atmosphere only. Parachutes there slow you down a little, but you must still make a powered landing in nearly all cases. If you don't have any engines to land with, then mechjeb can't land it (and neither can you).

Edited February 18, 2014 by Streetwind

[helaeon]

Duna's atmosphere is really thin. You need lots of chutes or ones designed for use in that thin atmosphere deployed from a high altitude. Real Chutes is good for this, but even there don't expect a lot from your parachutes if you're using a heavy manned lander, or even a light one.

MechJeb is a great assistant but usually having it do stuff for you is not the best and can be frustrating. I'd use the landing assist to target where you'd like to end up and help keep you on course but you fly. Keep your engines pointed at your retrograde vector as you come in and use the suicide burn read-out to give you an idea when you should turn those engines on full blast (I usually do it at about 1s then back off as needed).

Other trick is come in really really shallow. You want to spend as much time cutting through that thin atmosphere as possible. If you're trying to use parachutes to slow you down so your engines do less on the final burn this will help even more (I think the stock drogue chutes will deploy super high). It's kind of like the opposite of landing on Kerbin in this way.

The burn should happen mostly at the end, not the beginning. You want to get to 6-8m/s generally before you hit, 2 m/s is even better (usually by ramping up your engines quickly right before touchdown and cutting them) and that works everywhere. That's a practice thing though.

Maybe that will give you a start to figuring it out.

[Skorpychan]

With two drogues and four of the blue chutes, my 15-ton Duna lander made it down on just chutes most of the way, with the engines burning to slow it down for the last few meters before impact.

The atmosphere is barely there, but it's usable. Drop the lander from the cruise stage at a high apoapsis, and burn until you get a periapsis in the atmosphere. Then you can take multiple passes through it, or just a single low shallow approach that'll activate your chutes.

[SSSPutnik]

Do you use engines during landing? You probably don't have enough chutes.

[KerikBalm]

An approach that works on Eve, or even Kerbin, will not neccessarily work on Duna.

You can basically come in perpendicular to the surface of Kerbin for a safe landing (given current game mechanics allow you to enter as steep as you want without burning up) - all it requires is deployment of chutes at the right time (set the full- deployment altitude low, so you don't rip your craft apart - give it more time to decelerate to terminal velocity).

With Duna, you need to come in shallow, if you come in steep, you'll cut right through the atmosphere as if it wasn't even there (FYI, the real life Mars has an atmosphere 1/20th that of Duna's and ~25% higher gravity, hence the airbags and sky cranes and such).

For your capture set your perapsis at about 13 km, I did another flight to duna last night, 13 km perapsis on the first go around, and then a 17km perapsis the second time, then ejected the lander, raised perapsis to 43km, and lowered the lander's perapsis to 7 km.

You want to be coming in relatively slow (ie orbital velocity, not some high speed flyby trajectory launched outside a transfer window that relies on heavy aerobraking for capture), and shallow - you should still have a perapsis visible when you enter the atmosphere (your trajectory shouldn't intersect the surface prior to entering the atmosphere).

My lander used 6 radial parachutes, and 2 drogue chutes (carrying basically 2 fl-800 tanks worth of fuel, and a 2 person lander cabin, 2 aerospikes, and a manned rover).

The drogue chutes will semi deploy at 10km, I set them for a full deploy at 5km (beware opening shock - I used them because their fully deployed drag is smaller than any other chute), then I staged my remaining 6 to fully deploy at 2.5 2, and 1.5 km.

I aimed for a low spot, atmospheric pressure at the landing site was .107 atmospheres, so if I had aimed better, I could have slowed the craft even more (its .2 at the lowest spots I believe).

6 chutes was a bit overkill, and the landing was survivable with no engine thrusting - the landing legs were all damaged, but I was able to repair them and right the craft for 0 fuel usage, I think I came in at 14 m/s? 16? eliminating some chutes might be better - spending 20 extra delta v for a soft touch down may be regained by reduced weight during the ascent (not to mention reduced weight getting the lander to Duna, but the chutes are pretty light, so I'm not sure, and I leave the lander in orbit - after refueling it - to be reused by the next craft coming to duna).

So... shallow trajectories, low flat landing spot, staggered , high altitude full deployments - the first deployment is the most important, as that will be the most G load, so that chute/pair of chutes should be "well strutted", I again recommend drogues, as they slow you the most partially deployed, and generate the least force when fully deployed

[goldenpeach]

What I usually do with my manned Duna lander is that I use mechjeb for the initial burn(with the landing autopilot). Then,I disable it and let the lander fall to the ground,deploying the chutes as soon as I can.

I use the engine to pass from an horrible crash(i.e everything break) to a survivable crash(i.e nothing break but the landing is not soft at all).

Of course, with that you can't do a very precise landing(with mechjeb and by using that method) which is why I do a "real" powered landing with my Duna rover: the landings are precise hut it use more fuel(a lot more); however, while my Manned lander is meant to go in orbit after landing, the rover is meant to stay on the ground.

[OgreMagi]

thanks for the answers.

I have a 4 nuke engine lander ( standard nuke engines no mods) 88.98 tons total mass with 12 tons of detachable ground rovers, with 4 Mk 2 radial parachutes.

This design landed easily on kerbin and eve, using mech jeb, but has a problem landing on Duna using mech jeb. I have the kethan mod and want to land in a kethan rich area to reuse the ship but mech jeb will not work properly.

this is what I happened and what I have tried to do to correct it.

first try I select the site using the landind guidance feature of mech jeb, then warp to high orbit, let mech jeb correct for the landing site and wait for the braking burn.

then its stops working, I watched it deorbit without firing the engines and have the vehicle ripped apart from the chutes deployin usually just as the main engins come on around 10k meters

So I try again, set the point let it do the de orbit burn then try to keep the orbital speed low enough to deploy the chutes but not so much to move my landing site out of the kethane field. I get the chutes to deploy then active mech jeb to land anywhere when it is low enough, then I have trouble.

But mech jeb does very odd things, the engines stutter, comming on and off in rapid succession and the landing speed never gets low enough to make a safe landing .

I have tried the burn high up but my landing points moves to far from the kethane field. even near the ground mech jeb on land anywhere simply does not function very well with parachutes.

I will try manual landing but it is such a pain to do.

Ogre

[SSSPutnik]

I would say MechJeb is having trouble finding a solution.

4 radial chutes is not enough and the nukes probably don't have enough thrust. What is your TWR in Duna orbit?

Remember Eve has a soup atmosphere and Kerbin is MUCH thicker than Dunas.

Edit: Suggest Google Kerbals chute calculator.

Edited February 19, 2014 by SSSPutnik

[helaeon]

Nukes probably aren't the right answer for this application. Remember nukes are also very heavy so sometimes you get better results from using regular rocket engines with ISPs around 360-400, because the ship will be a lot lighter. It's why so many use the LV-909s on landers. Maybe won't have as much deltaV but that's not everything. Use as many as you need to get the TWR you need. 1.2-1.6 works well for the lander when it's full.

Also with a lighter ship you'll need less SAS to have enough torque to give control authority, which will make your lander less prone to tipping, so that can make your ship lighter as you can remove SAS modules (or keep them and have a ship with a ton of control authority). Same goes for RCS, you'll have more RCS delta V and control authority so you can possibly pack even less of that. Which increases all of these values even more and maybe when you're done you end up with a better flying and more capable ship with about the same amount of delta V as with the nukes.

[Geschosskopf]

Bottom line is, MJ's landing autopilot doesn't work very well on Duna, even if it works just fine elsewhere. For some reason, it just can't come to grips with the thin atmosphere. So what you have to do is perform most of the landing yourself. That is, you do the de-orbit burn, you burn during the descent to keep your speed from getting too high, and you pop the chutes when you want to. Then, when you get fairly close to the ground, and only then, you can tell MJ to "land somewhere" and it will get you down safely the rest of the way. But by then you might as well finish the job yourself.

The good news here is that kethane deposits are big targets so you don't have to land on a specific spot, just somewhere within state lines. Do your de-orbit burn so you'll come down just inside the far edge and you'll be fine as the rotation of Duna and the (limited) drag of the atmosphere will move you more to the middle of the deposit, by maybe 1 or 2 kethane map hexes.

[SSSPutnik]

In cases like this I actually use MJ in two stages.

On for the initial deorbit and a little longer to ensure I'm headed to target.

Then off and do manual deceleration.

Then do land anywhere as per above once you get low over target.

Nice thing about this, I find I'm doing more and more manual and relying less on MJ as time goes by.

[KerikBalm]

So simple answer, your craft is way too heavy, with not enough chutes, and no drogue chute to slow you down so the main chutes don't rip the craft apart during landing.

The spacecraft design is the main problem

[OgreMagi]

thanks for the replys,

I managed to get mech jeb to get me to a crash landing, after multiple attempts. the lander crashed but my rovers survived intact,.. only lost a light or 2 on them. ( I use a space staton cupola as the base the seem to resist damage better.)

I saved a picture of the crahs, but I do not know how to load pic's to the page, but I used the rovers solar panels to right them and well it was a unmanned lander with unmanned rovers,

( the rovers were being staged for when the kerbals returned for a manned landing, similar to the mars direct concept I read in science fiction analog years ago.)

Well I will try to redesign the lander, less fuel ( or half the fuel in the tanks to reduce weight since I don't really need it) and more parachutes, still it is kerbal space program nothing more dangerous than a atomic power rocket....... just cant cannot seem to find it in my heart to put a less dangerous rocket on it..

thanks again for all the advice.

Edited February 20, 2014 by OgreMagi

[Taki117]

thanks for the replys,

...

I saved a picture of the crahs, but I do not know how to load pic's to the page, but I used the rovers solar panels to right them and well it was a unmanned lander with unmanned rovers,

( the rovers were being staged for when the kerbals returned for a manned landing, similar to the mars direct concept I read in science fiction analog years ago.)

...

You need to use a web hosting site like Imgur or Photobucket However, Imgur has the added functionality of being able to add albums with the following:

[*imgur]Mdm0i (URL of Album not including imgur.com/a/)[/imgur]

(Without the asterisk or stuff in parenthesis

Which will give you:

Javascript is disabled. View full album

(This is my latest Duna Mission)

[SSSPutnik]

Nice ship!

[Skorpychan]

thanks for the replys,

I managed to get mech jeb to get me to a crash landing, after multiple attempts. the lander crashed but my rovers survived intact,.. only lost a light or 2 on them. ( I use a space staton cupola as the base the seem to resist damage better.)

I saved a picture of the crahs, but I do not know how to load pic's to the page, but I used the rovers solar panels to right them and well it was a unmanned lander with unmanned rovers,

( the rovers were being staged for when the kerbals returned for a manned landing, similar to the mars direct concept I read in science fiction analog years ago.)

Well I will try to redesign the lander, less fuel ( or half the fuel in the tanks to reduce weight since I don't really need it) and more parachutes, still it is kerbal space program nothing more dangerous than a atomic power rocket....... just cant cannot seem to find it in my heart to put a less dangerous rocket on it..

thanks again for all the advice.

The nuclear rocket is probably half the problem.

Note: http://imgur.com/a/uc1J8#0

Four LV-909s. Small, light engines with a decent TWR. A rockomax x16 tank's worth of fuel, a good amount of chutes, and a weight-efficient lander can. The docking port is so it doesn't have to haul the interplanetary tug around with it everywhere.

[flyboy67109]

What I did was build the lander and take into Kerbal orbit. Then, I tell MJ to land at KSP. If it crashed, I add more rockets. I also found that I need to watch the relationship between the center of thrust and the center of mass. If they are too close together, MJ does this stupid "dance" right before touchdown and flips the lander and crashes. If I put the engines above the center of mass like a skycrane or low, it lands fine. I was able to land a Orange fuel tank with 8 radial engines and a poodle engine just fine using MJ on Kerbal.