Best type of landing approach

[flightmaster]

Hey,

So I was doing some thinking about the best type of landing approach/angle to do to land on a planet with no atmosphere. And by "best" I mean lowest Delta V expended.

Here are the two main approaches I can think of:

A) Small retrograde burn from the Apoapse so your Periapse hits the moon/Mun/planet at your rough landing point (on the opposite side of the body you're trying to land on). Then warp and wait until you're several kilometres away from your landing point and retrograde burn all the way to the ground, killing both vertical and horizontal velocities in one burn, OR

Burn retrograde as close as possible to being directly above your landing point, killing all your horizontal velocity and then doing a suicide burn to kill your vertical velocity once you have gotten closer to the ground.

I can make some pictures if my question doesn't make sense but if it does, which would use less Delta V? My thoughts are that you'd be using the same amount, no matter what approach you do, or it'd be very similar...

[ROFLCopter64bit]

I alternate between them everytime I load from quicksave because I'm horriffic at landing without cheating (AKA parachutes)

[Cesrate]

According to Oberth Effect, the faster the ship moves in the gravity field while it burns, the higher the efficiency of the fuel will be.

As for me, I'd like to do a full-manual landing without RCS, began to do the vertical descend when at several hundred meters above.

[metaphor]

Option A is more delta-v efficient. It's always better to combine burns if possible. A 1000 m/s burn up and then 1000 m/s horizontally has the same effect as a 1414 m/s burn at a 45 degree angle, but the second one uses less delta-v. Plus in option B you're fighting the gravity of the planet. In the time between when you kill your horizontal velocity and when you land on the ground, the planet's gravity is accelerating you downward and adding to the delta-v required to come to a stop. For example, if the gravity is 10 m/s^2 and you spend 100 seconds falling to the ground, that's 1000 m/s extra delta-v that you need to use to land.

[SalmonellaDingDong]

I always do A.

[Jason Patterson]

The most efficient landing mirrors the most efficient launch. As a practical matter, you want to set it up so that you're coming in very shallow, with your periapsis barely above the ground. Wait until periapsis and burn horizontally as long as possible. This combines the benefits of Oberth with the fact that you're killing your velocity by burning sideways and avoiding gravity drag. As your velocity begins turning vertical, follow it until you're burning vertically and landing at the same time.

Getting that to line up with actually landing at the spot you want to land at is more than just tricky, so what you can do, without losing any delta-v, is burning at your periapsis (which is at maybe 2-5km, depending on the body you're landing on) and circularizing your orbit. Then you're able to orbit until you're at whatever position you want and descend from there.

ETA: Stopping and then falling onto your target is very wasteful of delta-v. It's equivalent to launching by shooting straight up, then burning straight sideways at your apoapsis. It works, but you burn an awful lot of gas doing it.

[katateochi]

"Just follow the force of gravity, its easy" - Ford Prefect

(sorry totally not helpful, it's getting/got late, I'll just go).

[Mr Cord]

I had a lander sitting in orbit around the Mun so I thought I would give this a go.

Starting from a 100k Orbit

Option A. Deorbit at AP so PE is roughly around Landing Target

De-orbit Burn - 54 m/s

Deceleration Burn - 672 m/s

Landing - 267 m/s

Total - 993 m/s

Option B. Decelerate to 0 m/s above Landing Target and descend straight down.

De-orbit Burn - 0 m/s

Deceleration Burn - 484 m/s

Landing - 519 m/s

Total - 1003 m/s

Option C. Let Mech Jeb 2.0 do the landing.

De-orbit Burn - 125 m/s

Deceleration Burn - 622 m/s

Landing - 51 m/s

Total - 747 m/s

So it looks like there isn't much difference between the 2 options (at least in the game). Mech Jeb doing better was due to doing a better landing than I did manually.

It would be interesting to see what values others come up with.

Edited April 25, 2013 by Mr Cord

[flightmaster]

I had a lander sitting in orbit around the Mun so I thought I would give this a go.

Starting from a 100k Orbit

Option A. Deorbit at AP so PE is roughly around Landing Target

De-orbit Burn - 54 m/s

Deceleration Burn - 672 m/s

Landing - 267 m/s

Total - 993 m/s

Option B. Decelerate to 0 m/s above Landing Target and descend straight down.

De-orbit Burn - 0 m/s

Deceleration Burn - 484 m/s

Landing - 519 m/s

Total - 1003 m/s

Option C. Let Mech Jeb 2.0 do the landing.

De-orbit Burn - 125 m/s

Deceleration Burn - 622 m/s

Landing - 51 m/s

Total - 747 m/s

So it looks like there isn't much difference between the 2 options (at least in the game). Mech Jeb doing better was due to doing a better landing than I did manually.

It would be interesting to see what values others come up with.

Nice! Mechjeb 2.0s landing function won't work for me, it just keeps crashing but thanks for the experiments!

[Jono]

NASA's Moon landings were most similar to A. The main difference being that they didn't lower the periapsis to the surface from the parking orbit. The CSM would stay at an altitude of ~110 km and the Lander would lower periapsis to ~15 km. They then did a breaking burn at periapsis. Here's a diagram:

[reptile2001]

To me (not at all a math or physics whiz), it would make the most sense to let anything and everything that can help you do just that - if gravity/atmosphere is going to bring you where you want, why fight it? To me the most efficient way would be burning to close to your intended landing point and adjusting from there.

[CalculusWarrior]

I agree, the less time spent burning, the better.

[AmpsterMan]

I descend on a body in multiple stages.

First stage is landing insertion burn in which I burn retrograde to bring periapsis over target area. Next, I begin breaking burn to bring me into a landing course. Depending on how accurate I was in my breaking burn determines how strongly I burn or how much I deviate from the velocity vector. A perfect burn has me cancelling horizontal and vertical velocity on the same instant my legs touch. Final stage I get into a hover 50 meters above surface and veru slowly descend to touchdown