Efficient Landing

[ajburges]

Woah,

Ignoring human error, anything less than full throttle is typically wrong in a vacuum (except for burns that specifically rely on the integration). Less thrust correlates to longer burns. Longer landing burns leads to heavier gravity loss in landing and takeoff. The reason we control throttle in atmo is to balance gravity loss against atmospheric loss. If there's not atmo loss, there's nothing to balance.

Apollo model landings are good, but they are not the most efficient (still pretty good). As mentioned, they needed visual observation of the landing site for final approach.

There are a few effects I am aware of that are notable for landing fuel efficiency. Oberth and force of gravity. Theoretically, the most efficient landing considering those two would be to arrange a (near) surface level periapsis from initial (highest) orbit. Then suicide burn centered at periapsis (the greater velocity from the elliptic orbit should realize more kinetic energy via the Oberth effect than lowering the orbit before the maneuver). This approach is not practical for most though. The more extreme circumstances would surely lead to efficiency losses from execution errors that would eliminate any gains! Not to mention TWR has a bigger impact on efficiency in this maneuver.

Still, if we are going to debate efficiencies, why is controlled lithobraking not getting more attention! Depending on object mass and rotational speed, you can get several hundred dV without any fuel usage. More precise landings don't need any burns after the deorbit burn! Plus those Kerbals love the ride.

Edited May 10, 2015 by ajburges

[Beowolf]

The most efficient way is to come in horizontally, close to the ground. Kill off your horizontal velocity while adjusting pitch to control your vertical speed. It's very easy to do.

LOL, easy. Yeah, it is now. Not my first dozen times, it wasn't! I had enough trouble I ended up having MechJeb do the landing over and over until I learned how by watching.

[Snark]

There's a very simple and easy UI technique (no mods required) that's helpful when doing your burn. The following explanation makes two assumptions:

1. That you've already made your trajectory adjustments and are on a collision course with the ground, and it's just a matter of knowing when to start your retro-burn.

2. That you have already gotten some 100% thrust time with your engines during your current "flying session" on the ship, so that the game knows what your acceleration is.

Here's the technique:

1. Go to map mode

2. Hover the mouse over your trajectory (like you do when you're going to make a maneuver node)

3. Make a maneuver node that's exactly precisely at the point where your trajectory intersects the ground.

4. Grab the retrograde handle and drag it out. Your projected orbit will start shrinking towards the impact point.

5. Let it keep doing that until it shrinks to zero and the prograde/retrograde indicators start flipping back and forth. Now you're done, you can leave map mode.

At this point your maneuver on the nav ball will tell you how many seconds it thinks you need to burn.

The actual time you need to burn will be typically be less than that. How much less depends on your angle of descent and your TWR. Shallower angle (i.e. more horizontal) = larger fraction of estimated time. High TWR = smaller fraction of estimated time. I generally find that it ends up being about 70%, in practice.

Anyway: you now have your burndown time. Set your SAS to "hold surface retrograde", poise your finger over Z, and wait until your "time until maneuver" is the calculated amount of time.

If you've estimated it exactly right, you'll slow to zero right at the surface. Assuming you left a bit of safety margin, you'll slow down to reasonably-slow at a fairly low altitude, and then you can complete your landing manually as per usual.

[Zephram Kerman]

...Theoretically, the most efficient landing considering those two would be to arrange a (near) surface level periapsis from initial (highest) orbit. Then suicide burn centered at periapsis (the greater velocity from the elliptic orbit should realize more kinetic energy via the Oberth effect than lowering the orbit before the maneuver)...

But, that's not kerbal.

...controlled lithobraking... ...several hundred dV without any fuel usage... Plus those Kerbals love the ride.

That's kerbal!

[rkman]

Turns out there's a more efficient way than just suicide burns as said earlier.

It should be noted that "constant altitude" descent and suicide burn are not mutually exclusive.

The trick of a "constant altitude" descent is that you lose most of your horizontal velocity while moving mostly perpendicular to the direction of gravity (thus minimizing gravity loss). There is no reason why that can not be followed by a suicide burn to kill that last bit of (mostly vertical) velocity just before touch down.