What's more efficient - a quick burn to decelarate or a long slow steady burn

[alex the killa]

Out of curiosity

[alex the killa]

For landing in an atmosphere-less planet, for simplicity

[iamaphazael]

Assuming you're talking about coming in for a landing, the most efficient method is the "suicide burn": a max-thrust burn at the last possible second that lasts just long enough to bring the ship to safe landing speed just as it encounters the ground.

Most efficient, and also probably the most difficult to do right, for obvious reasons

[SkyRender]

A good compromise, incidentally, is an Apollo-style burn. It's very simple.

1. Get your periapsis down over your target landing site, and at a decent height. How high depends on your TWR (thrust-to-weight ratio) and the size of the body, but a general safe bet is 5,000m for anything Mun-sized or smaller, and 10,000m for most everything bigger. Tylo is best tackled from at least 40,000m up or more; not even kidding.

2. Once you've reached periapsis, start firing at full power towards retrograde, and switch the indicator over to Surface instead of Orbit mode. Watch the retrograde marker, and when it gets mostly vertical, then start chasing it.

3. Your actual descent to the surface will be completely vertical from here on out, and will likely take less than 2KM to reach the surface (unless you're trying to land on Gilly, Bop, Pol, or Minmus; they're all so low-gravity that you may have a bit of a wait ahead of you, but fortunately you won't suffer much in the way of losses to gravity in spite of that).

[CozmikR5]

I'd say "the burn you can control"

[Caelib]

Suicide burn!

[Carthaginian]

I'd say "the burn you can control"

THIS is a statement founded in wisdom.

If you don't survive the burn, then it don't matter how 'efficient' it was.

The Apollo 11 landing was hardly optimal or efficient; Armstrong said that it was typical to land their test lander on 15 seconds worth of fuel- the equivalent of 'landing on fumes.' Armstrong himself landed with about 25 seconds of fuel remaining. Don't be afraid to run your designated landing tanks absolutely dry if that is what it takes to get yourself to the deck safely and in one piece.

[rodion_herrera]

A good compromise, incidentally, is an Apollo-style burn. It's very simple.

1. Get your periapsis down over your target landing site, and at a decent height. How high depends on your TWR (thrust-to-weight ratio) and the size of the body, but a general safe bet is 5,000m for anything Mun-sized or smaller, and 10,000m for most everything bigger. Tylo is best tackled from at least 40,000m up or more; not even kidding.

2. Once you've reached periapsis, start firing at full power towards retrograde, and switch the indicator over to Surface instead of Orbit mode. Watch the retrograde marker, and when it gets mostly vertical, then start chasing it.

3. Your actual descent to the surface will be completely vertical from here on out, and will likely take less than 2KM to reach the surface (unless you're trying to land on Gilly, Bop, Pol, or Minmus; they're all so low-gravity that you may have a bit of a wait ahead of you, but fortunately you won't suffer much in the way of losses to gravity in spite of that).

Except for the last 200 meters or so, the real Apollo approach wasn't exactly vertical, so I don't know why you draw the comparison from Apollo, save perhaps, for how the throttle would be set. Anyways here's a good ref for the real thing: http://www.braeunig.us/apollo/LM-descent.htm

[tuguley]

I don't know about everyone else, but the following statement is what helped me out the most:

The perfect landing is the perfect liftoff in reverse

[Wabur]

Anyways here's a good ref for the real thing: http://www.braeunig.us/apollo/LM-descent.htm

Oh wow, thanks for that link!

[Pbhead]

I don't know about everyone else, but the following statement is what helped me out the most:

The perfect landing is the perfect liftoff in reverse

which... is a suicide burn.

but a fantastic way to describe a suicide burn.

[Tophir]

I think the most efficient way to land is to get yourself in as low of an orbit as you can about 2 or 3 km above the surface and above where you land kill your orbital velocity and descend as this will does not fight gravity as a suicide burn does I'm not sure how much more efficient this is but most likely only slightly.

[Anglave]

In interplanetary space, it doesn't matter. You can burn at 10% for 10 minutes, or 100% for 1 minute. So long as your ship is stable and can take the forces involved. If you're doing this with a maneuver node, make sure to center your burn time around zero seconds.

There are a few cases where it's more efficient to make a short hard burn than a long burn. For instance, in a gravity well (in other words, landing). Because you're fighting the planet's gravity for every second you're not on the ground, it is most efficient to minimize that time. Meaning you burn at full throttle for just long enough to kill your motion relative to the surface. Ideally, you reach 0m/s just as you touch down.

You may also wish to use shorter burns for orbital plane change maneuvers (the thrust will be most efficient when closest to the crossing node). And I use a series of short (4 minute) periapsis kicks to push my interplanetary missions out of Kerbin orbit. Due to the Oberth Effect, your rocket is slightly more efficient the faster you're moving. So doing these burns at periapsis is better. Also, starting a burn too far before your ejection will push your path into Kerbin, which is sub-optimal if your intent is an interplanetary voyage.

[Anglave]

I don't know about everyone else, but the following statement is what helped me out the most:

The perfect landing is the perfect liftoff in reverse

For an airless world, this is a great image. An ideal landing kills your vertical and horizontal speed relative to the surface in the minimum time, such that your speed is 0 m/s (or at least low enough for your landing legs) when you reach the surface. This means almost all the thrust should be applied at the very end of your landing (or the very beginning of your launch). Landing this way is going to be scary.

Atmosphere makes things substantially different.

Launching from a body with an atmosphere, we attempt to balance the thrust lost to atmospheric drag with the thrust lost to gravity drag. We use the throttle to keep our speed low while we're low, while trying to spend as little time close to the planet as possible. But we end up thrusting for a couple minutes, to fight atmospheric and gravity drag and get our speed up to orbital.

Landing on a body with an atmosphere, you can rely on atmospheric drag to slow your rocket down substantially. Even without parachutes, on any body in the Kerbin system, you'll probably only have to kill a couple hundred m/s with the engines. And with parachutes you can make an entirely unpowered landing.