Efficient descent profile

[AlexinTokyo]

OK, so I see a lot of posts/threads here about efficient ascent profiles (speed, timing and extent of gravity turn, etc.).

But what does a fuel efficient descent profile look like?

Assume a body with no atmo, so a fully powered landing with no drag and, of course, no 'chutes.

I have a feeling that the extremes would be something like:

• Retrograde burn to 0 horizontal velocty, freefall, braking burn at minimum altitude required to slow to landing speed with a full-thrust burn.
  
• Retrograde burn until the predicted trajectory just intersects the ground, then burn the absolute minimum required to reach landing velocity over the entire distance remaining.
  

But that is only a gut feeling, so what in fact is an efficient way to land?

Edited March 11, 2014 by AlexinTokyo

[Kasuha]

Most efficient descent profile on non-atmospheric body I know is horizontal landing.

- bring your periapsis as low as possible (above terrain)

- set up a maneuver at periapsis to circularize

- start your burn the usual way (in the direction the maneuver is indicating and half the time estimated for the burn ahead)

- delete that maneuver, don't stop burning

- clear your horizontal velocity while keeping yourself above terrain and your vertical velocity low by burning slightly above prograde as needed

- when your speed is sufficiently low, let the ship fall near terrain and finish the landing

[MockKnizzle]

I can't recite the math for you and it's all very dependent on your TWR, but generally for a non-atmospheric body your best bet is to get into the lowest circular orbit you can, then burn at max thrust while using your pitch to keep your vertical velocity constant. You should be skimming sideways along the ground until the very last instant, where you pitch vertical right as your horizontal velocity goes to zero and settle ever-so-perfectly down on the ground with little to no hang-time.

[celem]

Your other example, kill horizontal and freefall is pretty much the opposite end of the spectrum. Its about the least efficient descent profile. Having said that its also probably the easiest to execute as it gives you forever to think about what you are doing and shuffle the landing point to find flat ground. A major drawback of this tactic is that if you freefall from too high the planet/moon will rotate beneath you and your landing spot will drift during vertical descent

The suicide burn is what you really want. You pick a low periaps (not quite ground-intercepting as you suggested) and follow the retrograde surface marker with the burn (rather than burning horizon until horizontal is 0 then canceling vertical as you would using the other method. Doing that with a suicide trajectory will probably mean a crash. You can plot a suicide burn to some degree by tweaking the retrograde node handle on the map until your trajectory after the burn appears to be straight down on the map toward the surface. Thats robbed both horizontal and vertical velocities, since you burn retrograde surface rather than horizon the vertical path shown after the burn is a freefall under local-g, its gonna be a dozen or so ms.

In practice you want a tiny mix of these 2, you do still want a short free-fall period to make any final adjustments (mainly to allow for surface slope, double-check gear deployment etc.), the true suicide all-or-nothing landing is best used when you know the ground you are coming down on, otherwise do the suicide with a view to ending 1000m or so up so that you have a chance to make those tweaks.

Edited March 11, 2014 by celem

[Kasuha]

For pinpoint landings (i.e. landing at certain selected spot) i recommend

. He uses calculator but these things can be guesstimated with a bit of practice, too.

[AlexinTokyo]

That makes a lot of sense, it's basically the direct opposite of the ascent profile - thrust sideways as soon as you're clear of terrain. Now I just have to practise doing it

I had already seen the rescue tutorial (and used it to rescue some stranded Kerbals), but thanks for the reminder.