Munar Lander Rocket no longer working in 0.23, but worked in 21.1, why ?

[NikkyD]

Its driving me crazy.

In version 21.1 i built a very minimalistic munar lander, removed part by part, tweaked with mechjeb and saved the rocket.

Now im loading it in 0.23 and its no longer working.

Rocket

http://imgur.com/xp71coG

The only major changes are the monoprop in the capsule and the 48-7S had a thrust increase from 20 to 30 kN. The monoprop i removed, the thrust should be massively in my favor.

The idea behind this rocket is, that the biggest starter stage is dropped while still in the atmosphere so that it would return to the surface to be recovered or at least not trash my orbit.

The second stage is the orbital + travel stage with the 909 engine with max efficiency and slim/short design.

The third stage, the lander, lands on mun, gets back up and back to kerbin and lands there without the need for a parachute.

It worked in 21.1 ! But now, the best i can manage is to get to a 100 km munar orbit with about 1300 m/s left. The latest mechjeb version that i have and use does some really weird things for munar landing, constantly burning with 1/3 thrust and stopping in a height of about 30m then drops and brakes again... wasting 800 m/s. And even if i start manually, with 400 m/s or so i cant make it back into space + munar escape! It once worked and there the lander had less thrust! The 20 to 30 increase in thrust should make the escape a cakewalk.

Does anyone have any ideas what could be the problem ? Did any part change that i missed ?!

[Armchair Rocket Scientist]

Hmm... I'm guessing that the picture you have is of the rocket in 0.23. Mechjeb says you have a dV of 7073 m/s. The following dV maps:

http://i.imgur.com/NKZhU57.png

http://wiki.kerbalspaceprogram.com/w/images/7/73/KerbinDeltaVMap.png

give requirements of 7110 and 7140 m/s for a Mun landing and return. Now, most dV maps include a safety factor, so that ship MIGHT pull off the mission if flown absolutely perfectly. However, I don't think the maps account for the time when your engines are running in atmosphere and have lower ISP. Also, your ship has no parachute, so you'd have to do a powered landing on Kerbin, which is at least another 100 m/s (atmospheric) with a suicide burn. Long story short, I don't think that ship can land on the Mun and return safely.

What is the calculated dV from .21? Is it different?

[NikkyD]

I dont really remember the .21 stats :/

I searching for my papers, i wrote down all the calculations to that point but... big pile of papers

What im wondering is that i am in lunar orbit with 1200 m/s left, and i remember that this was sufficient to land a craft that small and light and bring it back. I did let mechjeb do the landing and it thrusts always in the last second(s) and stop perfectly. But with .23 mechjeb keeps the engines running at 1/3 during the fall... its messing up somehow

I know its little safety in this rocket, but its the end result of a lot of tweaking etc. It did the job with almost nothing left when it landed on kerbin. This includes a very tricky atmospheric braking so maybe thats why the those "rule of thumb" maps make it look closer than it actually is

edit:

I didnt bother for a lunar orbit on the way back, it was a straight shoot up into the sky, escape and be on an almost crash course with kerbin. Return never costs much, i just need to touch the atmosphere so much that it brakes me down into a kOrbit. Second or third braking will do the final job.

Edited December 21, 2013 by NikkyD

[Kasuha]

I still have 0.21 on my disk so I re-created your ship in it.

0.21 stats:

Launchpad weight (including clamps): 26.52 t

Part count: 18 parts

Liquid fuel: 1665

Oxidizer: 2035

0.23 stats (with monopropellant tweaked out) are exactly the same.

Engine ISPs match too, so total dv must be the same as well.

So my guess is the difference is in handling. Perhaps you're landing from different orbit?

Edited December 21, 2013 by Kasuha

[tavert]

But with .23 mechjeb keeps the engines running at 1/3 during the fall... its messing up somehow

You might have left an acceleration limit enabled in MechJeb in your 0.23 install?

[Sirrobert]

Sounds like you let Mechjeb handle the landing. So that's where your problem is then.

Land manually

[NikkyD]

I still have 0.21 on my disk so I re-created your ship in it.

...

So my guess is the difference is in handling. Perhaps you're landing from different orbit?

Thx mate, that rules that out

You might have left an acceleration limit enabled in MechJeb in your 0.23 install?

Will check tomorrow, mechjeb is indeed fresh install and the .22 version (no .23 out yet ?! )

Sounds like you let Mechjeb handle the landing. So that's where your problem is then.

Land manually

Lets face it, if you wanna go all "real men do..." u win, but as a programmer i would write myself a plugin that lands as perfectly as possible, but someone already did that for me, why not use it ?

[sojourner]

Thx mate, that rules that out

Will check tomorrow, mechjeb is indeed fresh install and the .22 version (no .23 out yet ?! )

Get the latest dev release. There's a link to them in the first post of the mechjeb thread.

[entaran]

Actually mechjeb doesn't land perfectly, it lands safely. You can easily beat mechjeb's landing for efficiency doing it manually.

When you get to munar orbit, f5 quicksave, adjust your orbit so you're ~6km periapsis on the bright side of the moon somewhere (we're learning to land, not trying to hit a specific point). It's slightly better to do this from like 11/6 circular instead of like 90/6 but only marginal in terms of dV (like 10.)

As you approach what looks like a flattish spot (craters are good, or the high point just outside the edge of a crater), burn retrograde fullspeed with surface indicator on until you kill horiz vel. Then just fall. You can suicide burn at about 1km and just edge it in after that.

Mechjeb loves to just burn fuel to maintain a safe approach speed. It's also way too obsessed with exact co-ordinate targetting and not having any horiz vel at all. you can safely land with upto 5ms of horiz velocity if you have at least 4 legs. Faster if you have 8 or a better leg design.

Try it and then see how you go, I'm almost certain you can beat mechjeb to the surface by a considerable margin.

[John FX]

Actually mechjeb doesn't land perfectly, it lands safely. You can easily beat mechjeb's landing for efficiency doing it manually.

When you get to munar orbit, f5 quicksave, adjust your orbit so you're ~6km periapsis on the bright side of the moon somewhere (we're learning to land, not trying to hit a specific point). It's slightly better to do this from like 11/6 circular instead of like 90/6 but only marginal in terms of dV (like 10.)

As you approach what looks like a flattish spot (craters are good, or the high point just outside the edge of a crater), burn retrograde fullspeed with surface indicator on until you kill horiz vel. Then just fall. You can suicide burn at about 1km and just edge it in after that.

Mechjeb loves to just burn fuel to maintain a safe approach speed. It's also way too obsessed with exact co-ordinate targetting and not having any horiz vel at all. you can safely land with upto 5ms of horiz velocity if you have at least 4 legs. Faster if you have 8 or a better leg design.

Try it and then see how you go, I'm almost certain you can beat mechjeb to the surface by a considerable margin.

Also, if you want to do this sort of landing, This is a good way to do it using not too much fuel. Video is pre-0.22 so make your Pe 6-7km instead of 2.4 (Highest mountain on Mun is 7km.)

[Smidge204]

Lets face it, if you wanna go all "real men do..." u win, but as a programmer i would write myself a plugin that lands as perfectly as possible, but someone already did that for me, why not use it ?

Because maybe it's not as perfect as you think, and it's actually screwing you over?

I'm not about to look down on anyone who uses it, but at the very least you should develop the skills required to play without it. There's a difference between using MechJeb to take care of tedious business and using it to play the game for you, you know?

=Smidge=

[NikkyD]

So i played around a bit and the budget seems to be way off... i dont get it

The lander stage with about 1500 m/s on its way too the mun, landing, getting back up and "crashing" into kerbin was somehow possible but now, i really need the 800 m/s to land AND again to go back up... how did i pull this off in .21 and before ?!

I do remember that the last change was switching a 909 out for a 48-7s, as it was new at that point. And im not sure if the rocket was still operational with the 48-7s. But if i switch back to a 909, even if i double the fuel on top... im off by far.

Maybe it wasnt working in .21 but in .20, i stopped playing for quite some time so i cant be sure. But was there any major change from .20 to .21 that would explain this ?

[Eric S]

Is it possible you had FAR installed when you previously played? That tends to reduce the delta-v required to reach LKO by about 1 km/s.

[NikkyD]

No, the budget to the mun is pretty much what i remember. I had a tweaked ascent so reaching mun with 2nd stage still having 100 m/s or more left was always the case.

It's the mun descent and ascent that throws me off. I could swear it was less than 800 m/s for both... or somehow i didnt need that much or... i dunno

[Kasuha]

Mun's escape velocity is 807 m/s. There's no way you could squeeze approach, landing, liftoff and ejection to 800 m/s.

[Cashen]

I don't trust MechJeb to land me, ever. What I do like to use however is Smart ASS to point me either 'Orbit Horizontal Backward' or especially 'Surface Velocity Backward' and hold attitude while I control the throttle myself. Mun takes 840-860 m/s to land, and the same to take off (landing always takes more because it's never perfect). So for a single stage Mun lander I would normally aim for at least 1800 m/s for a safety margin, or even 2000.