I tried 3 different methods to land on Mun.

[mobylettespaceindustries]

Disclaimer: this is not a tutorial, it's a detailed experiment! Don't rush me!

The last one, the reverse gravity turn by @GoSlash27 gave me the best result but i don't know when to switch to final visual approach, so, perhaps it was poorly executed, i don't know.  What do you think? 

 

Edited January 28, 2017 by mobylettespaceindustries
user mention

[GoSlash27]

@mobylettespaceindustries,

 When executing the "reverse gravity turn", I don't reduce the periapsis first. I just shoot it straight from orbit (for the reason you encountered). It costs a little more DV that way, but I think the safety, precision, and ease of use makes it worth it. *edit* Incidentally, I tend to stick with the map view until I have trouble differentiating where I am on the line between apoapsis and touchdown.

The "constant altitude" approach *should* yield better results AFA total DV expenditure, but I find it to be a lot more hazardous and a lot less precise.

Cool video, thumbs up!

-Slashy

Edited January 28, 2017 by GoSlash27

[mobylettespaceindustries]

  On 1/28/2017 at 11:45 PM, GoSlash27 said:

@mobylettespaceindustries,

 When executing the "reverse gravity turn", I don't reduce the periapsis first. I just shoot it straight from orbit (for the reason you encountered). It costs a little more DV that way, but I think the safety, precision, and ease of use makes it worth it.

The "constant altitude" approach *should* yield better results AFA total DV expenditure, but I find it to be a lot more hazardous and a lot less precise.

Cool video, thumbs up!

-Slashy

Expand  

Thanks for your answer, i'm just learning. I will do a similar video next, but this time with a precise landing spot, something i have deliberated omitted here,  and starting the rgt from parking orbit.

I started the three approach from clearly a too high altitude, for a mun landing, usually, i start from about 13-15km, not 25, but my first try at 13km leads me to almost similar dv results, so i started from higher to 'force' the difference. 

are there others methods to land than this 3 ones? 

 

Edited January 28, 2017 by mobylettespaceindustries

[Gaarst]

The method in which you drop the altitude to 10m and the orbital velocity to 0 right above the ground is theoretically the most efficient method. The problem, besides its riskiness, is its efficiency is directly related to your TWR: you need very high TWR to achieve good efficiency because otherwise you will end up burning radial-in to avoid crashing into the ground.

The more time you spend burning out of prograde, the more fuel you waste. So for low TWR landers, or unknown terrains, the reverse gravity turn is more efficient because it's easier to do the manoeuvre correctly.

Edit: try this method on Minmus where the orbital speed is low (high TWR) and you have great flats, it should be the most efficient, costing barely more than your orbital speed.

Edited January 29, 2017 by Gaarst

[GoSlash27]

@mobylettespaceindustries,

 Those are the 3 main techniques in use, but I'd recommend getting some pointers from someone who's more adept at the constant altitude method. You really don't want to do a free-fall suicide burn at the end of it. That skews the results.

Best,
-Slashy

[mobylettespaceindustries]

  On 1/28/2017 at 11:58 PM, Gaarst said:

The method in which you drop the altitude to 10m and the orbital velocity to 0 right above the ground is theoretically the most efficient method. The problem, besides its riskiness, is its efficiency is directly related to your TWR: you need very high TWR to achieve good efficiency because otherwise you will end up burning radial-in to avoid crashing into the ground.

The more time you spend burning out of prograde, the more fuel you waste. So for low TWR landers, or unknown terrains, the reverse gravity turn is more efficient because it's easier to do the manoeuvre correctly.

Edit: try this method on Minmus where the orbital speed is low (high TWR) and you have great flats, it should be the most efficient, costing barely more than your orbital speed.

Expand  

@Gaarst

I decided to give it a try on Mun instead of Minmus because it's so ridiculously easy to land on Minmus i suppose the difference in dv costs should be too small to be noticeable. Don't you think? 

  On 1/29/2017 at 12:00 AM, GoSlash27 said:

@mobylettespaceindustries,

 Those are the 3 main techniques in use, but I'd recommend getting some pointers from someone who's more adept at the constant altitude method. You really don't want to do a free-fall suicide burn at the end of it. That skews the results.

Best,
-Slashy

Expand  

@GoSlash27

well, it's not really a suicide burn, more a final descent. i think. 

Edited January 29, 2017 by mobylettespaceindustries
users mention

[Gaarst]

  On 1/29/2017 at 12:12 AM, mobylettespaceindustries said:

I decided to give it a try on Mun instead of Minmus because it's so ridiculously easy to land on Minmus i suppose the difference in dv costs should be too small to be noticeable. Don't you think? 

Expand  

If you do several runs and average the dV required, you'll see a difference, but in an actual mission you don't care about such a small difference, especially on Minmus.

Trying on x64 or x10 Minmus would probably give good results.

[GoSlash27]

  On 1/29/2017 at 12:12 AM, mobylettespaceindustries said:

well, it's not really a suicide burn, more a final descent. i think. 

Expand  

The point of the constant altitude method is that there is no final descent at the end if done properly. You should be very low to the ground when horizontal velocity is cancelled. Think of it as an efficient launch to orbit, just in reverse.
 Unfortunately, I'm not the guy to talk to about that, since I almost never use the method.

If you really want a good indication of the relative efficiencies, I recommend doing it on Tylo. Nice flat terrain and the high gravity will accentuate the differences.

Best,
-Slashy

Edited January 29, 2017 by GoSlash27

[mobylettespaceindustries]

  On 1/29/2017 at 12:15 AM, Gaarst said:

If you do several runs and average the dV required, you'll see a difference, but in an actual mission you don't care about such a small difference, especially on Minmus.

Trying on x64 or x10 Minmus would probably give good results.

Expand  

well, and what about my social life ? hein?

  On 1/29/2017 at 12:17 AM, GoSlash27 said:

The point of the constant altitude method is that there is no final descent at the end if done properly. You should be very low to the ground when horizontal velocity is cancelled. Think of it as an efficient launch to orbit, just in reverse.
 Unfortunately, I'm not the guy to talk to about that, since I almost never use the method.

Best,
-Slashy

Expand  

noted. I think everybody should use the three methods, the suicide burn, when you have a rush and a high twr, the constant alt method for best efficiency and the reverse gravity turn to land as near as possible to a target, to assemble a base for example. 

Thanks for your advices.

[Gaarst]

  On 1/29/2017 at 12:20 AM, mobylettespaceindustries said:

well, and what about my social life ? hein?

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What about your social life? This is SCIENCE!