Landing. Manual vs. Mechjeb for fuel efficiency

[iEvermore]

Hello, I trying to find the most fuel efficient way to land. I've landed on Duna twice manually and a third time with Mechjeb but all three times where different ships. I'm trying to figure out which is the most fuel efficient way to descend to land. Mechjeb slows the ship down and then uses the engines almost the whole way down to keep speed at about 400+. When I do it manually, I would get to about 15k (Duna atmosphere) and pound on the accelerator to bring me down from 800+ to a vertical decent as fast as possible. I'm wondering if the full burn I do is more fuel efficient than the way Mechjeb does it?

As a second part to my question ...The only reason I wanted to try Mechjeb was because on Duna it is much harder to pin point a specific spot to land (i.e. save stranded Kerbals) than it is to do manually. (For my novice skill set that is) But not only do I think Mechjeb is less fuel efficient, it still ends up being at least 60-80 km away from the target which I can do manually anyways. I start with "Land on target" from a 100km orbit. Do I need to have a higher or lower orbit to help Mechjeb hit the spot?

[Eric S]

I'm pretty sure that the landing code got a total rewrite between 1.9.X and 2.0.X. In 1.9.X, Mechjeb would do something very close to a suicide burn, and I could never come close to matching it. I haven't used 2.0 much for landing, but it seems to have backed off a bit and gone for something a bit more gradual, though still probably a bit better than I do (I'm just not good at manual landings). Suicide burns are definitely more efficient, but are also more dangerous if you don't get them just right.

[tavert]

I don't remember if this was exactly the case in MechJeb 1.9.x, but in MJ2 there's a substantial difference between "Land at target" and "Land anywhere." The former maintains a controlled gradually decreasing speed as it goes down, and tends not to be full-throttle so will use a bit more fuel than a suicide burn would. Land anywhere is more aggressive, at least 80-90% throttle for most of its landing burn. I'm guessing it's leaving a bit of a safety factor, but I bet that factor could be made adjustable. Time to head on over to their Github and file another feature request...

Not sure about accuracy with Land at target, it will occasionally go nuts and be way off for me. If you find a case where MJ2 goes crazy, it's good to back up the quicksave and submit to via their Github so they can look into what's going wrong.

Edited April 24, 2013 by tavert

[Panichio]

Yeah, as the others have said. The newest MechJeb does suicide burns when landing, which is much more fuel efficient than a long burn. I don't think there are many people that could match it's accuracy in timing and location when it comes to landing, so I like to use it. Some rockets though, where the center of thrust might be very low because it's a tall rocket, MechJeb can easily flip it and loose control.

[Kosmo-not]

I have not tried MJ2 for landing yet. From past experience, it has always crashed the ship when I set it up for a fuel-efficient landing.

[TooManyErrors]

MechJeb is probably more efficient, but its definitely not as fun. (Well, for me at least.)

Suicide burns are probably the most efficient way to land, but you take the risk of lithobraking if you don't time it properly.

[ThatBum]

Or as Scott Manley would call it, "hardware-assisted braking."

[iEvermore]

Ok thanks guy. I'll give Mechjeb 2 a shot. Did a few more tests yesterday. I found that a mix of manual and Mechjeb worked out best. But ultimately I only used Mechjeb for the last 3k since I had to land on an angle to save the Kerbals. Unfortunately the rover I built and tested on Kerbin didn't fair too well on Duna. But it's all about trial and error right !

Edited April 24, 2013 by iEvermore

[maltesh]

One of the interesting features of Mechjeb 2 (requiring that you set up a custom window for it) is a Suicide burn Timer, giving you a countdown to the last point where you can save yourself with a full throttle-burn.