Maximise Landing efficiency.

[Tw1]

While definitely can land, sometimes, it can get a bit hairy. And if it's very hairy, sometimes I need to let mechjeb do it for me, which feels a bit like defeat. I'd like some advice on how to maximise the efficiency when landing.

I tend to use a manoeuvre node at the end to give an idea of how long it's going to take to kill my velocity, but this often leaves me almost still in mid air (vacuum?) Or I end up violently lithobraking. Not desirable.

Any tips, links, or other advice you're willing to give, all greatly appreciated.

Edited July 12, 2013 by Tw1

[Kosmo-not]

To maximize efficiency, you need to come in horizontally (set a periapsis close to the surface and burn at periapsis). This will give you the least amount of gravity loss.

[Tw1]

Gravity loss is the main issue, ok.

More than the fact that you'll have greater horizontal velocity to kill?

I expect there must be a happy medium, last time I tried landing from an extremely low orbit:

[Skorpychan]

Generally, I kill off my velocity a few times partway down. The less speed I come screaming in with, the less I have to kill off at the last minute in a panic. I also cover the last few thousand meters at an absolute maximum of 50m/s in munar gravity, to ensure a nice landing.

Pick your descent engines carefully! I use a set of four of the tiny radial engines rather than a single -909 engine, for more thrust with lower weight, and less fuel consumption.

Set your speed indicator to surface once you're properly sub-orbital, and burn retrograde. Eventually, the marker will point straight down. Aim to keep it that way to avoid sliding sideways.

[Kosmo-not]

Gravity loss is the main issue, ok.

More than the fact that you'll have greater horizontal velocity to kill?

I expect there must be a happy medium, last time I tried landing from an extremely low orbit:

Killing horizontal velocity is a non-issue. Managing your descent rate is the only thing you have to worry about (and that's easily done in most cases due to high TWR landers).

Generally, I kill off my velocity a few times partway down. The less speed I come screaming in with, the less I have to kill off at the last minute in a panic. I also cover the last few thousand meters at an absolute maximum of 50m/s in munar gravity, to ensure a nice landing.

Pick your descent engines carefully! I use a set of four of the tiny radial engines rather than a single -909 engine, for more thrust with lower weight, and less fuel consumption.

Set your speed indicator to surface once you're properly sub-orbital, and burn retrograde. Eventually, the marker will point straight down. Aim to keep it that way to avoid sliding sideways.

We're talking about efficient landing here.

Here is a video showing an efficient Mun landing with a low TWR lander:

Edited July 12, 2013 by Kosmo-not

[Mr Shifty]

There was a very involved discussion a couple days ago about this very question in this thread:

http://forum.kerbalspaceprogram.com/showthread.php/39390-How-to-calculate-DeltaV-to-get-to-orbit-or-to-get-from-orbit-to-land

tl;dr: Kosmo-not is correct. The most efficient landing method, particularly for low-TWR vehicles, is to do a Hohmann transfer to the surface, and burn off your horizontal velocity near periapsis (i.e. at low altitude) while maintaining a vertical velocity of near zero.

I've watched Kosmo-not's landing video 4 times now start-to-finish since it first appeared in Mulbin's thread. Beautiful. And easily the best vacuum landing tutorial out there.

[Tw1]

A lot make sense now. Thanks guys.

It's a bit late at night for me to much maths here though, I'll just have to take your word for it, and read that thread some other time.

I now consider this question answered. Next time I play, I'll have to practice precision landings with this technique.

Edited July 12, 2013 by Tw1

[skiingrules88]

I don't think the landing in that video is very efficient at all. For much of it the burn is at least 45 degrees to the horizontal, so you're putting at least as much thrust into fighting gravity as into killing the horizontal velocity. It might be the best that can be done with a lander with such a low TWR, but I definitely feel like you'll use less fuel overall if you use a bigger engine and reduce total burn time and thus reduce gravity loss.

[tavert]

I don't think the landing in that video is very efficient at all. For much of it the burn is at least 45 degrees to the horizontal, so you're putting at least as much thrust into fighting gravity as into killing the horizontal velocity. It might be the best that can be done with a lander with such a low TWR, but I definitely feel like you'll use less fuel overall if you use a bigger engine and reduce total burn time and thus reduce gravity loss.

Yes, but not by as much as you think. See http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse for numbers. You quickly get into diminishing returns past local TWR of around 2. And engines are heavy, so you actually start getting worse in payload fraction for higher TWR as more of your craft mass is engines.

The gravity losses in the video are basically negligible. If your velocity vector is perpendicular to gravity (moving horizontally), gravity losses are zero. The losses in the constant-altitude landing method are in the form of steering losses, since you have to thrust higher than retrograde to maintain altitude. Still, for the same initial TWR, it is more efficient than performing a retrograde suicide burn.

Edited December 13, 2013 by tavert

[astropapi1]

low-periapsis-above-landing-site method + the new Mun = http://i.imgur.com/czIvV5H.png"]This.

[Kasuha]

I don't think the landing in that video is very efficient at all. For much of it the burn is at least 45 degrees to the horizontal, so you're putting at least as much thrust into fighting gravity as into killing the horizontal velocity. It might be the best that can be done with a lander with such a low TWR, but I definitely feel like you'll use less fuel overall if you use a bigger engine and reduce total burn time and thus reduce gravity loss.

I did not think it is efficient, either. I had a contest with Kosmo-not who will land with more fuel left, he with his method or I with suicide burn. On new Mun. He won.

There are two points about this way of landing:

- Like it or not, it is very efficient

- It is actually quite easy to do once you learn a few basics

Lunar landers used exactly the same descent profile, too.

low-periapsis-above-landing-site method + the new Mun = http://i.imgur.com/czIvV5H.png"]This.

Of course you cannot pull your periapsis to zero with the new Mun. That's all it takes.