Minimizing Delta-V in landing burn

[Kasuha]

Make a lander that has a TWR of 1.5 - 2.0 (relative to Mun) and try both methods. Record your results (video would be awesome).

I have a different proposal. Make a quicksave with the ship at the apoapsis of a sub-orbital trajectory of your choice and let's check who can land it with more fuel left.

[numerobis]

I watched that video and I think that if there was a hill in your path before you killed your horizontal velocity you'd be toast.

There was a hill in the video, which cost some extra delta-v to avoid. But indeed, it's a risky method on mountainous terrain. Best to map your approach path ahead of time.

And I also think your method is not the most efficient as you're burning a lot in non-retrograde direction, moving your orbit along the surface instead of braking.

The least delta-v is used when you freefall on sub-orbital trajectory and then stop right on the surface after just one full throttle retrograde burn. That applies to any craft. You can use maneuver node to estimate it.

The video basically shows what you say you should do, no? With low thrust the single burn is going to be long.

[Kosmo-not]

I have a different proposal. Make a quicksave with the ship at the apoapsis of a sub-orbital trajectory of your choice and let's check who can land it with more fuel left.

I would be happy to. Too bad I have to put this off until tomorrow evening.

[Specialist290]

I'll be looking forward to seeing the results of this little contest, myself.

[Kasuha]

There was a hill in the video, which cost some extra delta-v to avoid.

No. He changes pitch but his retrograde vector is glued to the horizon all the time meaning he was flying perfectly horizontally the whole time. He started at the right height to avoid any obstacles that might interfere.

The video basically shows what you say you should do, no? With low thrust the single burn is going to be long.

No. It is supposed to be pure retrograde burn.

Of course we are talking about which landing maneuver is most efficient here, not which is easiest to execute.

[tavert]

No. It is supposed to be pure retrograde burn.

Of course we are talking about which landing maneuver is most efficient here, not which is easiest to execute.

Run the numbers, do the tests, we all have or we wouldn't be so confident in our recommendations. You do suffer some steering losses with the constant-altitude method, but the steering losses are lower than the gravity losses you incur if you do a retrograde suicide burn. Falling in altitude speeds you up, costing extra fuel.

[Kasuha]

Run the numbers, do the tests, we all have or we wouldn't be so confident in our recommendations. You do suffer some steering losses with the constant-altitude method, but the steering losses are lower than the gravity losses you incur if you do a retrograde suicide burn. Falling in altitude speeds you up, costing extra fuel.

All the time you spend covering the distance from your position to the landing place gravity is acting on you giving you certain acceleration, i.e. increasing the budget of dv you need to compensate to land successfully. The longer time you spend fighting it, the more fuel you spend on it.

But to be honest I am also curious how will it come out. I find Kosmo-not's method hard to execute anyway - particularly I have problem orienting the ship exactly against my velocity to be able to use "just pitch", I always end up with some sideways speed which makes things much less manageable. Maybe it's because he's using mods, I don't know. I can admit that if starting at the position shown in the video there may not be any better way to land because the initial position is too low already. I believe it's not the apoapsis though.

[Kosmo-not]

Here's my result from a quick thing I did.

http://imgur.com/bPCwCIi,MF7ZjYo#0

Persistence file can be downloaded here if you would like to have a go at it.

I used mechjeb only for delta-v stats and vertical speed readout. My first and only attempt so far landed using 926 m/s.

Have fun and let me know how it goes for you.

[Vector]

From his persistence file, first try using Kosmo-not's method:

http://i.imgur.com/cR8iAAs.png

For some reason MechJeb didn't load, but I had 32.13 liquid fuel remaining at touchdown. I made an extra trip around the Mun because I hate landing in the dark.

Also pitched up for a bit to coast past an inconveniently located crater, probably didn't cost much fuel. Final low-speed low-altitude descent was essentially retrograde burn for a very small minority of the descent. Definitely not suicide burn because for me those frequently end up suicidal.

Normally when I land on the Mun I have high TWR so I don't need the constant-altitude approach, but I definitely like it. It's also much easier and less stressful than my typical landing. Nice to know that it's also efficient.

[Kosmo-not]

From his persistence file, first try using Kosmo-not's method:

http://i.imgur.com/cR8iAAs.png

For some reason MechJeb didn't load, but I had 32.13 liquid fuel remaining at touchdown. I made an extra trip around the Mun because I hate landing in the dark.

Also pitched up for a bit to coast past an inconveniently located crater, probably didn't cost much fuel. Final low-speed low-altitude descent was essentially retrograde burn for a very small minority of the descent. Definitely not suicide burn because for me those frequently end up suicidal.

Normally when I land on the Mun I have high TWR so I don't need the constant-altitude approach, but I definitely like it. It's also much easier and less stressful than my typical landing. Nice to know that it's also efficient.

You landed using 989 m/s.

I'm glad you like the new landing method. It's how they landed on the moon in real life (more or less).

[togfox]

Im surprised no one has mentioned the mechjeb suicide burn read out. It counts down in seconds the exact point you need full thrust to slow to 0 m/s at altitude 0 (ground). Its called suicide because if you miss this point by even a second you'll never slow enough for a landing. Of course i go full thrust about 10 seconds before te suicide point so ive got a buffer and to allow for flosting point inaccuracies.

[Kosmo-not]

Im surprised no one has mentioned the mechjeb suicide burn read out. It counts down in seconds the exact point you need full thrust to slow to 0 m/s at altitude 0 (ground). Its called suicide because if you miss this point by even a second you'll never slow enough for a landing. Of course i go full thrust about 10 seconds before te suicide point so ive got a buffer and to allow for flosting point inaccuracies.

I did that going straight down before making my attempt. I ended up using 1048 m/s.

[tavert]

All the time you spend covering the distance from your position to the landing place gravity is acting on you giving you certain acceleration, i.e. increasing the budget of dv you need to compensate to land successfully. The longer time you spend fighting it, the more fuel you spend on it.

Sure. If you assume perfect piloting in the ideal case, in either method you want to coast until the start of the braking burn, then burn at full throttle until you hit the ground at the exact moment you hit zero velocity. So in this case minimizing fuel use is equivalent to minimizing the time length of the landing burn. As I described, in a retrograde burn when you have a nonzero vertical speed, gravity is not perpendicular to your velocity vector so it causes you to speed up as you fall. This extra speed requires extra time to counteract in the landing burn. In a constant-altitude burn gravity does not speed you up at all since your velocity vector is always perpendicular to the direction of gravity, but not all of your thrust is in the horizontal direction. When you integrate this out, as I said, gravity losses in retrograde burn end up increasing the length of the braking burn more than the steering losses in the constant-altitude method.

[rkman]

TWR ~1.7 (2.1 @ touchdown)

starting in 30km orbit

~40 m/s used to get intersect with Mun surface

suicide burn guided by Mechjeb's numbers (started about 90 seconds before touchdown)

719m/s total delta-v expended

edit:

Same with higher TWR (4.6 @ touchdown) (LV-909 instead of one 48-7S)

suicide burn started about 30 seconds before touchdown

676m/s total delta-v expended

Edited October 7, 2013 by rkman

[Kasuha]

Persistence file can be downloaded here if you would like to have a go at it.

Just to be sure - does it start high above Mun with zero speed, i.e. when I do nothing I crash at the area between day and night, or was something lost in the translation? It does not look very much like a normal sub-orbital trajectory to me. The first time I loaded it I couldn't drive the ship at all, had to switch to Space Center and back.

Here's my result from a quick thing I did.

http://imgur.com/bPCwCIi,MF7ZjYo#0

I don't have MechJeb installed so I'd prefer if you just told me your remaining fuel after landing.

[Kasuha]

As I described, in a retrograde burn when you have a nonzero vertical speed, gravity is not perpendicular to your velocity vector so it causes you to speed up as you fall.

Wrong. Gravity causes you to speed up all the time regardless of your vertical speed. You either counteract it or you don't. Counteracting it to achieve constant altitude counts as hovering, regardless of your horizontal speed.

You need to kill your horizontal velocity before you need to start hovering, otherwise you're wasting fuel on hovering while killing your horizontal velocity.

This constant altitude approach is just the other extreme of killing your horizontal speed at altitude and then braking vertical freefall. Neither is optimal. Both are easier to execute than the optimum approach but both are also less effective.

Edited October 7, 2013 by Kasuha

[tavert]

You're only considering the radial influence of gravity. That's not what I'm talking about, as that only depends on altitude. Gravity also affects your angular velocity, when you have nonzero vertical speed.

Edit: You could think of this as being a coordinate acceleration from the rotating reference frame, or gravitational acceleration changing the magnitude of velocity as well as its direction, when you are at any velocity other than a circular orbit.

Your horizontal speed absolutely matters, due to centrifugal acceleration. When you're in a circular orbit, you require zero vertical thrust to counteract gravity. Below orbital speed, the amount of vertical thrust required to counteract gravity is a quadratic function of horizontal speed. Refer to equations (14) and (20) here https://dl.dropboxusercontent.com/u/8244638/Constant%20Altitude%20Landing%20and%20Takeoff%20Derivation.pdf, both of which were before I imposed any constant-altitude conditions.

Edited October 7, 2013 by tavert

[Kosmo-not]

Just to be sure - does it start high above Mun with zero speed, i.e. when I do nothing I crash at the area between day and night, or was something lost in the translation? It does not look very much like a normal sub-orbital trajectory to me. The first time I loaded it I couldn't drive the ship at all, had to switch to Space Center and back.

I don't have MechJeb installed so I'd prefer if you just told me your remaining fuel after landing.

You told me to make it at the apoapsis of a suborbital trajectory.

Fuel remaining after landing was 35.24

[Kasuha]

You told me to make it at the apoapsis of a suborbital trajectory.

Yes, obviously. But as long as you use mods which I don't have and I wasn't able to control the ship after I loaded it I am trying to make sure we're both solving the same problem.

So I ask again. Is the trajectory supposed to look like this after I load the file?

Edit: okay, now that I loaded it the second time it doesn't have landing legs. There's definitely something fishy.

Edited October 7, 2013 by Kasuha

[Kosmo-not]

So I ask again. Is the trajectory supposed to look like this after I load the file?

Yes, the trajectory is supposed to look like that.

[Kasuha]

Yes, the trajectory is supposed to look like that.

Okay. In that case I admit defeat because even though my best attempt zeroed all speeds at literally few meters above ground I still ended up with only 33.58 fuel (and 41.05 oxidizer).

[blizzy78]

But is that a realistic trajectory you'd normally use to land?

[Kosmo-not]

I could set it up so that you start with the lander just after the TMI burn. That way, we can tweak the trajectory to our liking using very little dV. That would make for a more realistic (and fair) scenario.

[rkman]

I could set it up so that you start with the lander just after the TMI burn.

I'm not sure what gives, but the scenario in the video you posted seems quite a bit different than the scenario in the persistence file and different than the alternative scenario that you suggest.

In the video you start out in low orbit (15km), and put periapsis a few km above the surface.

In the persistence file the craft is at very high altitude and periapsis is far below the surface.

In the alternative scenario the craft would be coming in from transfer trajectory, first requiring a lot of delta-v to establish low orbit.

[Kosmo-not]

I'm not sure what gives, but the scenario in the video you posted seems quite a bit different than the scenario in the persistence file and different than the alternative scenario that you suggest.

In the video you start out in low orbit (15km), and put periapsis a few km above the surface.

In the persistence file the craft is at very high altitude and periapsis is far below the surface.

In the alternative scenario the craft would be coming in from transfer trajectory, first requiring a lot of delta-v to establish low orbit.

With the situation in the persistence file, I burned about 65 m/s to the side and then went into low orbit until I could bring my periapsis down for landing on the light side.

1700 m/s is plenty enough to land from a transfer trajectory.