landing questions

[JtPB]

1. How do I calculate the optimal orbit for an orbiter to wait for a lander around a moon or planet?

2. How do I determine the exact reentry spot where atmosphere is peresent?

are there tutorials for those problems(all are without mods)?

Edited July 30, 2014 by JtPB

[impyre]

1. optimal orbit for orbiter is generally as low as you can safely get and still land without smacking into the terrain with a bunch of horizontal velocity. That said, it will depend on lander design. My typical lander has a kerbin TWR of about 1.6, so when it fires the engines at mun or minmus it really slams on brakes. It can safely descend from 12-14km at the mun, and from 8-9km at minmus (depending on terrain elevation). If you use something with less thrust, just know that it will take longer to kill your horizontal velocity... and the whole time you're fighting gravity. The best way to learn is to simply experiment.

2. If you select a planet in mapmode or from the tracking center, a small box appear on the right side of the screen with an "i" in it. This contains useful information like ASL gravity and atmosphere height.

[Boomerang]

For your second question, if you're looking to land on bodies with atmospheres without mod assistance, then a whole lot of trial and error is the ticket. You're always going to need to lead your target by making sure that your trajectory is beyond your landing point, because the atmosphere will rob of you of at least some of your velocity.

The trouble is that depending on your initial velocity and your reentry angle, *how much* the atmosphere slows you down can vary a great deal. Though a reasonable rule of thumb if you're returning from Low Kerbin Orbit is to aim for a periapsis of around 30-35km right above KSC. That should drop you around the space center. Ish. Practice

[KingPhantom]

the 2 posts are good answers. i can only add the fact, that's easier to meet againg with the orbiter when this one has a inclanation of 0° or 180°... otherwise you have use a lot of fuel to meet again (because of the body's own rotation)

[impyre]

the 2 posts are good answers. i can only add the fact, that's easier to meet againg with the orbiter when this one has a inclanation of 0° or 180°... otherwise you have use a lot of fuel to meet again (because of the body's own rotation)

Actually, as long as the target orbit isn't 100% polar, you can meet it from launch with no inclination change if your timing is good.

If your timing is poor, or the target is 100% polar then the best solution may be to raise your apoapsis well above the target in such a fashion as to bring your apoapsis very close to an ascending or descending node, then correct inclination at apoapsis. At smaller bodies with relatively low gravity, it doesn't take a great deal of delta-v to raise your apoapsis. And since prograde burns are most effective at peri, and inclination changes most effective wherever velocity is least, this type of maneuver can sometimes save fuel. You could fiddle with maneuver nodes to do a cost comparison just to be sure though.

[Spyritdragon]

Speaking of proper orbit&burn for landing.. its best to cancel out as much velocity as you can pointing at an angle of 45 degrees, right? Cause diagonally you'd only need sqrt(2) m/s for every 1 m/s horizontally and vertically you want to reduce, as opposed to 2 if you did it independently.

Or am i calculating something wrong there? My head is incredibly derpy right now.

Edited July 29, 2014 by Spyritdragon

[AlexinTokyo]

For waiting in orbit for a lander, I assume you mean Apollo style landings. In that case, I'd recommend the orbiter wait in a higher-than-minimum orbit. The reason being that it gives more margin for error doing the rendezvous. If you launch the (ascent stage of the) lander as the orbiter's orbit passes overhead, in the same direction as the orbiter, you should be able to get an intercept fairly simply.

For landing, the most efficient is the so called 'suicide burn', where you burn directly along your retrograde vector at exactly the right point to reach 0 velocity exactly at the ground. As you can probably guess from the name, it's not actually a very useful technique.

The most efficient practical method is a 'constant altitude' landing. I learned about it from kasuha's posts on the forums; you should be able to find descriptions of it. There is also a good YouTube tutorial, but I'm afraid I can't remember the author.

As for precision landings in atmo; trial and error and practice, as has been said.

[monophonic]

Speaking of proper orbit&burn for landing.. its best to cancel out as much velocity as you can pointing at an angle of 45 degrees, right? Cause diagonally you'd only need sqrt(2) m/s for every 1 m/s horizontally and vertically you want to reduce, as opposed to 2 if you did it independently.

Or am i calculating something wrong there? My head is incredibly derpy right now.

Yes, to your last question. You are forgetting that when pointing diagonally, you are not getting the full thrust in either horizontal or vertical direction. In the (bit simplified) end you need (approximately) the same amount of dV regardless of whether you do one burn diagonally or two cardinally. The effects of gravity will cause some difference on the two routes, and the point of touchdown wont be exactly the same either. But the actual difference comes in at an advantage to the diagonal burn.

[LaytheAerospace]

For landing, the most efficient is the so called 'suicide burn', where you burn directly along your retrograde vector at exactly the right point to reach 0 velocity exactly at the ground. As you can probably guess from the name, it's not actually a very useful technique.

Pfft, real Kerbals lithobrake!

[JtPB]

1. optimal orbit for orbiter is generally as low as you can safely get and still land without smacking into the terrain with a bunch of horizontal velocity. That said, it will depend on lander design. My typical lander has a kerbin TWR of about 1.6, so when it fires the engines at mun or minmus it really slams on brakes. It can safely descend from 12-14km at the mun, and from 8-9km at minmus (depending on terrain elevation). If you use something with less thrust, just know that it will take longer to kill your horizontal velocity... and the whole time you're fighting gravity. The best way to learn is to simply experiment.

2. If you select a planet in mapmode or from the tracking center, a small box appear on the right side of the screen with an "i" in it. This contains useful information like ASL gravity and atmosphere height.

1. yep there are many orbits you could performe good landing from them, but i simply meant the one which reqiure the least dV for descending & ascending to, of course.

2. and how exactly I do the calculate the spot from these parameters? you didnt answer that.

For your second question, if you're looking to land on bodies with atmospheres without mod assistance, then a whole lot of trial and error is the ticket. You're always going to need to lead your target by making sure that your trajectory is beyond your landing point, because the atmosphere will rob of you of at least some of your velocity.

The trouble is that depending on your initial velocity and your reentry angle, *how much* the atmosphere slows you down can vary a great deal. Though a reasonable rule of thumb if you're returning from Low Kerbin Orbit is to aim for a periapsis of around 30-35km right above KSC. That should drop you around the space center. Ish. Practice

and for FKO or from the moons?

For waiting in orbit for a lander, I assume you mean Apollo style landings. In that case, I'd recommend the orbiter wait in a higher-than-minimum orbit. The reason being that it gives more margin for error doing the rendezvous. If you launch the (ascent stage of the) lander as the orbiter's orbit passes overhead, in the same direction as the orbiter, you should be able to get an intercept fairly simply.

For landing, the most efficient is the so called 'suicide burn', where you burn directly along your retrograde vector at exactly the right point to reach 0 velocity exactly at the ground. As you can probably guess from the name, it's not actually a very useful technique.

The most efficient practical method is a 'constant altitude' landing. I learned about it from kasuha's posts on the forums; you should be able to find descriptions of it. There is also a good YouTube tutorial, but I'm afraid I can't remember the author.

As for precision landings in atmo; trial and error and practice, as has been said.

yep, i mean apollo style.

constant altitude is when i keep the thrust vector on the retrograde marker until touchdown?

[AlexinTokyo]

constant altitude is when i keep the thrust vector on the retrograde marker until touchdown?

Nope, that's suicide burn.

Here is kasuha's description of the constant altitude (also called horizontal) landing:

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

And here is

that I was thinking of.

[Laie]

Speaking of proper orbit&burn for landing.. its best to cancel out as much velocity as you can pointing at an angle of 45 degrees, right? Cause diagonally you'd only need sqrt(2) m/s for every 1 m/s horizontally and vertically you want to reduce, as opposed to 2 if you did it independently.

Or am i calculating something wrong there? My head is incredibly derpy right now.

The general notion that a single diagonal burn will be better than two straight ones is correct (does than sentence even make sense?). Don't get too hung up on the 45°, though; you will waste more fuel setting up a 45° burn than you save by performing it.

And in the context of a landing, the best practical approach is to burn as horizontal as possible, for as long as possible. Hmmm. Let me get at this from the other end: as they say, a landing is like a launch in reverse. The best possible launch doesn't go up. Going up means "fighting gravity": when you turn off your engines, your speed decreases and becomes negative and you fall back down. It's much better to go sideways, pushing your trajecory over the edge of the world, so that the combination of your momentum and gravity turns it into an orbit. When you shut down your engine *then*, you'll neither slow down nor hit the ground.

TLDR: vertical momentum will eventually be eaten by gravity, any horizontal momentum is yours forever.

Now, for practical purposes you can't just lay your rocket flat on the ground and start the engines. First you have to get high enough as to not run into the next hill; but from then on, you should point as horizontal as possible, pitching only just enough as to maintain altitude.

The most fuel-efficient landing is doing just the same, but in reverse: kill your horizontal speed while pitching just enough to maintain your altitude. That pitch will be slight until you're almost there; due to the way vectors add up, fighting gravity along the way will actually cost you very little fuel.

[JtPB]

Nope, that's suicide burn.

Here is kasuha's description of the constant altitude (also called horizontal) landing:

And here is

that I was thinking of.

ok, knows them both. so whats the more efficient way?

The general notion that a single diagonal burn will be better than two straight ones is correct (does than sentence even make sense?). Don't get too hung up on the 45°, though; you will waste more fuel setting up a 45° burn than you save by performing it.

And in the context of a landing, the best practical approach is to burn as horizontal as possible, for as long as possible. Hmmm. Let me get at this from the other end: as they say, a landing is like a launch in reverse. The best possible launch doesn't go up. Going up means "fighting gravity": when you turn off your engines, your speed decreases and becomes negative and you fall back down. It's much better to go sideways, pushing your trajecory over the edge of the world, so that the combination of your momentum and gravity turns it into an orbit. When you shut down your engine *then*, you'll neither slow down nor hit the ground.

TLDR: vertical momentum will eventually be eaten by gravity, any horizontal momentum is yours forever.

Now, for practical purposes you can't just lay your rocket flat on the ground and start the engines. First you have to get high enough as to not run into the next hill; but from then on, you should point as horizontal as possible, pitching only just enough as to maintain altitude.

The most fuel-efficient landing is doing just the same, but in reverse: kill your horizontal speed while pitching just enough to maintain your altitude. That pitch will be slight until you're almost there; due to the way vectors add up, fighting gravity along the way will actually cost you very little fuel.

well you can use the hill as a boost to the orbit in minmus, doesnt it?

and you all didnt answer what's the formula for the optimal waiting orbit. is it the lowest orbit possible?

it's seems wrong because as close you are to the body you have more horizontal speed to kill & gain back...

[Laie]

and you all didnt answer what's the formula for the optimal waiting orbit. is it the lowest orbit possible?

it's seems wrong because as close you are to the body you have more horizontal speed to kill & gain back...

There is no single ideal waiting orbit.

If you're optimizing for delta-v and nothing else, stay in the first orbit you get -- just expend enough delta-v to stay in the SOI and be done with it. The most delta-v effective way of doing /that/ is to aim for the lowest possible periapsis in the encounter, then organize your capture from there. The resulting, highly eccentric orbit makes the eventual rendezvous more tedious to set up and execute, however.

If you're looking for comfort (ease of rendezvous), get a circular orbit that is somewhat higher than minimum, though not by too much. The lander ascents to the lowest possible orbit, and the greater the difference between their orbital periods, the shorter is your wait until the next opportunity. Though you can shorten the waiting time in orbit if you make your ascent at the right time (basically, when the target just passes overhead or a little later).

The smaller the difference between orbits, the shorter your burn needs to be. When setting up a rendezvous, burn times should be rather short for higher precision. Depends on your skills, really.

So, no formula. If you have delta-V to spare, use it to make your life easier. If you don't, try to make the best of it (and bring a little more fuel next time).

[Kosmo-not]

Coming in as horizontal and as low to the ground as possible is the most efficient way to land. Sure, you're fighting gravity, but much less so than doing a traditional suicide burn (and without the danger of burning too late).