Low TWR pinpoint landings

[Jackissimus]

Hi!

I am learning to land this way:

But I don't know when to start the burn when I want to do a precision landing. Does anybody have any tricks? Can I perhaps read it from the craft's TWR somehow? Or can I somehow use a maneuver node to give me an estimate?

EDIT: I think I need to clear up my question a bit. The whole burn to kill your horizontal speed can take up to 3 minutes. So you have to start burning maybe 50-100km before your target area. Problem is you don't really know when exactly. I don't want to do trial and error all the time to determine the right moment to start the burn. It's different for every moon, each craft design and sometimes even each craft (because it carries different amounts of fuel). I was hoping I could make some prediction before the actual landing. I tried two things:

- to guesstimate it somehow from the numbers: I know the thrust (240kN), the initial weight (80t) and the horizontal speed (500m/s), I thought I could get a reasonable number if just calculate - my TWR is then about 0.3, which means I can produce an acceleration of about 3m/s^2, and that means I will need about 160s, or 2m40s to kill my horizontal speed. However this isn't a good way to go at it and my number ended up about 50% off

- to make a maneuver node right above my target and hold the retrograde marker all the way to 0 final speed. It then gives me a time estimate, but it's way off too.

I don't know, I just thought there would be some way to guess about the right number.

Edited August 10, 2013 by Jackissimus

[Johnno]

You need a TWR above 1, that's all there is to it. The 'trick' as such is keeping an eye on your descent rate (top of your screen, right side of the altimeter, arrow pointing at a scale). The hard part is landing on locations with height differences, but if you know the area you can keep your altitude steady at a given range while you're still taking down your horizontal velocity.

When I was participating in a challenge for Mün landings (this was before the surface changes) I started around 2500 meters above what I knew to be the surface altitude, then just burned hard to kill horizontal velocity, trying to keep my vertical velocity at -10. If I noticed I was dropping too fast, I'd tilt up to decrease my descent rate.

I would advice that you put your craft into a specific orbit, say 20x20km on Mün, then start practicing killing your horizontal velocity while maintaining 20km altitude. Once you've got the hang of that, move to lower orbits and/or parabolic trajectories where you've anticipated the touchdown area (and know its altitude).

[lammatt]

it takes forever...makes me sleepy..

i'd rather just kill most of the horizontal speed up high and let it free fail till it almost crashes and then quickly burn toward the zenith to touchdown.

---

anyways

to answer your question

i've heard trial and error would be a nice way to figure out things.

[RocketPilot573]

That huge lander has a TwR of more than 1 with only a 909? Wow. Anyways, nice video.