Mun Landing Descent

[Ncog Nito]

Good Evening all, hope you're having a pleasant night.  I was wondering, in all of your opinions, what is the optimal altitude above the Mun that you should start your landing descent?

[Kryxal]

If you do it aiming for minimum delta-v, maybe 500m AGL to be safe?

If you're aiming for a precise landing, start higher and burn harder.

[Alshain]

As low as possible.  The height of the tallest peak on the Mun is a bit more than 7km, and that's near the poles.

If you are inexperienced however, it is easier to land from a higher altitude, you have more time for corrections.  This of course costs more fuel.

[bewing]

It also depends a bit on the TWR of your ship. Most ships will have plenty of TWR, so it's usually not an issue. But if you are using (for example) a Spark engine on a normal-sized ship, then you're going to need a lot of altitude while you slowly come to a stop.

 

[Ncog Nito]

Appreciate the input guys...thanks.

[Zeiss Ikon]

Important to note (from personal experience): if you start too low, you have to make a much larger deorbit burn to get an acceptably steep descent; otherwise, there's a hazard of flying into terrain (especially if your landing site is in a deep crater).

That is to say, if your final descent trajectory is too shallow, mountains and crater rims may rise above your trajectory before you get close enough to ground for your terminal landing burn, and taking that into account, there's actually little difference in delta-V between, say, a 25 km orbit and a 10 km orbit.

[Scarecrow]

I usually park my Command module in an orbit anywhere between 12 and 20km and don't find a great deal of difference in fuel use on my lander when landing and returning to those sort of altitudes. 

[Spricigo]

4 hours ago, Scarecrow said:

I usually park my Command module in an orbit anywhere between 12 and 20km and don't find a great deal of difference in fuel use on my lander when landing and returning to those sort of altitudes. 

Some players will pursue that last bir of optimisation, some will just carry the extra fuel/engines that makes the optimisation unnecessary. 

Most will do both,  according to the mood. 

[Kryxal]

Of course, before all that, many will go there, thinking they have enough, only to discover otherwise.  Often enough, that's due to landing difficulties ... after all, you can't near-hover forever!

[Spricigo]

2 hours ago, Kryxal said:

... after all, you can't near-hover forever!

Does "stranded in orbit"  counts as hovering ? 

[Alshain]

On 4/30/2017 at 6:50 AM, Zeiss Ikon said:

Important to note (from personal experience): if you start too low, you have to make a much larger deorbit burn to get an acceptably steep descent; otherwise, there's a hazard of flying into terrain (especially if your landing site is in a deep crater).

That is to say, if your final descent trajectory is too shallow, mountains and crater rims may rise above your trajectory before you get close enough to ground for your terminal landing burn, and taking that into account, there's actually little difference in delta-V between, say, a 25 km orbit and a 10 km orbit.

I know this has been marked as solved, but I just wanted to say, this shouldn't a problem if you are landing using a slow burn you can always adjust up and if you keep an eye on your map you can see if you would be intersecting the ground.  I'm not sure what you mean by an "acceptably steep descent".  A good descent would kill lateral movement right before touchdown, it's not steep at all.  A steep descent just means you are fighting gravity longer, and that costs fuel.

[Ncog Nito]

10 hours ago, Scarecrow said:

I usually park my Command module in an orbit anywhere between 12 and 20km and don't find a great deal of difference in fuel use on my lander when landing and returning to those sort of altitudes. 

 

4 hours ago, Kryxal said:

Of course, before all that, many will go there, thinking they have enough, only to discover otherwise.  Often enough, that's due to landing difficulties ... after all, you can't near-hover forever!

 

1 hour ago, Alshain said:

I know this has been marked as solved, but I just wanted to say, this shouldn't a problem if you are landing using a slow burn you can always adjust up and if you keep an eye on your map you can see if you would be intersecting the ground.  I'm not sure what you mean by an "acceptably steep descent".  A good descent would kill lateral movement right before touchdown, it's not steep at all.  A steep descent just means you are fighting gravity longer, and that costs fuel.

Thanks again guys.  I actually started at 50 KM and cover a distance of 1/4 of the Mun to land in the Northwest Crater.  Picture perfect, text book landing if I do say so myself, and, I had enough fuel to get back into Mun orbit, transfer to Kerbin orbit, and make a de-orbital burn that allowed me to land 2 KM from the KSC.  I didn't use the traditional command module/lander set-up that the Apollo missions used.  My lander was my command module.  Here is a picture during my descent:

[Alshain]

@Ncog Nito Congratulations!  The command module/LM setup isn't practical in KSP because the engines are typically the same.  In KSP we only have a few options for engines.  At their most basic they can be split into two groups, atmosphere and vacuum.  In the real world they are so much more complex.  Of course you can do them, but they aren't necessary.  Lander cans are still useful for lower mass when you have a craft that will be making trips between a station and the surface, but less useful for a one shot like Apollo because you end up carrying more mass in parts than you would in fuel. We used to have the O-10 engines which made for nice ascent engines if you dropped everything else because you could dock using normal RCS and only needed one tank, but they changed the model for the O-10 and now they are too big.  Sadly the devs have a bad habit of assuming nobody uses parts and replacing them rather than adding to them.

Edited May 2, 2017 by Alshain

[Starman4308]

On 4/30/2017 at 3:15 AM, bewing said:

It also depends a bit on the TWR of your ship. Most ships will have plenty of TWR, so it's usually not an issue. But if you are using (for example) a Spark engine on a normal-sized ship, then you're going to need a lot of altitude while you slowly come to a stop.

 

That isn't strictly speaking 100% true; I'm pretty sure the maximally efficient descent profile is a constant-altitude descent, where you start pitching up to keep your vertical velocity at 0.

Of course, this means you're skimming over the terrain at a scarily low altitude, and an ideal constant-altitude descent is outright impossible if there is any terrain higher than your landing site between said landing site and where you begin your burn, but the concept remains roughly the same: by keeping your vertical velocity under control, you can minimize gravity losses during descent, because you're not picking up large amounts of kinetic energy in the downwards direction*.

What I usually wind up doing is a half-eyeballed sorta-constant-velocity descent where I try not to let my downwards velocity get too large; I generally also keep an eye on the MechJeb suicide burn indicator. If the MJ suicide burn indicator is negative, that means you're probably doing it right. If it's too negative, however, you may want to consider pointing your vehicle straight up before you slam into terrain.

*Oberth, etc.

@Ncog Nito Again, congratulations. Not unexpected that direct ascent winds up better at the stock scale. The theoretical basis for lunar-orbit-rendezvous missions is that you get to leave the propellant to return home in orbit, at the cost of having a separate landing vehicle (entailing some duplication of parts). The problem in stock is that it's much tinier than the real world, so delta-V requirements are much smaller, so direct ascent costs you much less, because you need so much less return-home propellant.

If you ever get tired of that, try installing Sigma Dimensions and up-scaling the system a bit, possibly using either SMURFF or Real Fuels to bring fuel tanks and engines closer to reality as well*.

*Stock engines have poor TWR, specific impulse consistent with very good kerolox/methalox engines, but throttle and restart perfectly. Stock tanks are ridiculously heavy relative to the contained propellant, but the fuel is absurdly dense and doesn't have issues like boiling off or any of the other 5,439 complications that come with real fuels.

[Zeiss Ikon]

6 hours ago, Starman4308 said:

That isn't strictly speaking 100% true; I'm pretty sure the maximally efficient descent profile is a constant-altitude descent, where you start pitching up to keep your vertical velocity at 0.

@Ncog NitoOf course, this means you're skimming over the terrain at a scarily low altitude, and an ideal constant-altitude descent is outright impossible if there is any terrain higher than your landing site between said landing site and where you begin your burn, but the concept remains roughly the same: by keeping your vertical velocity under control, you can minimize gravity losses during descent, because you're not picking up large amounts of kinetic energy in the downwards direction*.

 

This is what I was referring to as "acceptably steep" above.  Unless you're landing on a mountain top, you need a certain descent angle to be sure you don't intersect terrain before you finish killing your velocity -- and if your descent stage has something like a single Terrier (as my early ones have had -- why bring a heavy engine with a lot of thrust to land on Mun, or moreso Minmus?) dodging terrain isn't always possible.  And in my opinion, dodging terrain winds up costing more fuel than a steeper descent, plus it really puts you off your planned landing location (you're certain to overshoot, possibly by as much as halfway around Mun, if you have to maneuver upward late in your suborbital trajectory).

I try to make my deorbit burn late enough and hard enough that my prograde is around 15 degrees below horizon if I'm landing on higher ground, and may go as high as 20 degrees for a lowlands landing site.  I've made few enough Mun landings that I might still modify this rule of thumb -- Mun has some steep terrain in spots.  Minmus has such a low orbital velocity that you can pretty much kill your orbit and drop straight down if need be, unless you're flying with near-zero dV margin.  And when you get to landing on Gilly (which I did for the first time a couple days ago), bring a good book -- you'll spend a lot of time waiting to get to the surface.  It's almost like docking with an asteroid, except there is enough gravity to hold your ship in place once you get all the legs planted (10 km orbit is something like 25 m/s).

[Wanderfound]

7 hours ago, Starman4308 said:

That isn't strictly speaking 100% true; I'm pretty sure the maximally efficient descent profile is a constant-altitude descent, where you start pitching up to keep your vertical velocity at 0.

Nope.

The maximally efficient landing profile is a pure suicide burn [1]; point retrograde the whole way, full throttle the whole way, vertical and horizontal velocity simultaneously zeroed just as you touch down [2]. Every degree of pitch above retrograde costs you dV in gravity losses. If you need to pitch up, then pitch up, but avoid it if you can.

However, a perfect suicide burn is tricky to get right, and by definition has no room for error (hence the name). So, instead, you can do an elevated suicide burn: same thing, but aiming to bring you to a half a short distance above ground level instead of right on it. This allows for a bit of error without instant death.

How high that distance is depends entirely upon piloting skill; you'll likely want to give it a kilometre or two when beginning, but once you get the hang of it you can shave it down to a few dozen metres or less.

 

[1] Difficult to get right without computer assistance.

[2] Unless your TWR is perfectly calibrated to your orbit, you'll need to lower the periapsis a fair way before beginning the final suicide burn phase. Deorbit burn, wait a bit, suicide burn.

[Starman4308]

2 hours ago, Wanderfound said:

Nope.

Nope.

I'm pretty sure the theory boils down to kinetic energy and the Oberth effect, that by allowing negative vertical velocity to build up, you're going to need to deal with it later by spending excess delta-V. That said, for high TWR and small bodies, I'm pretty sure constant-altitude and perfect suicide burns come pretty close to each other.

[Kryxal]

It's actually a fairly simple thing ... consider a takeoff where the engine GENERATES fuel, and plan the ascent where you make the least.  You'll want to build up horizontal speed as soon as possible.  Now run this in reverse and it's your landing, with the engine consuming fuel.

Oh, by the way, for Oberth, height is potential energy, and you want to do your burn when as much as possible is kinetic energy.  The faster you're going, the more energy you get out of your delta-v, and your start and end energy states are defined.

[Alshain]

3 hours ago, Kryxal said:

It's actually a fairly simple thing ... consider a takeoff where the engine GENERATES fuel, and plan the ascent where you make the least.  You'll want to build up horizontal speed as soon as possible.  Now run this in reverse and it's your landing, with the engine consuming fuel.

Oh, by the way, for Oberth, height is potential energy, and you want to do your burn when as much as possible is kinetic energy.  The faster you're going, the more energy you get out of your delta-v, and your start and end energy states are defined.

The engine generates fuel?  What?

 

Why do we keep bringing Oberth into this?  The Oberth maneuver is when the craft uses the speed of a falling into gravity well to accelerate.  The boost in efficiency from that maneuver is the Oberth effect.  None of that really applies to landing.  When you land, you are decelerating (I hope) while Oberth is explicitly acceleration.

Edited May 2, 2017 by Alshain

[Spricigo]

1 hour ago, Alshain said:

Why do we keep bringing Oberth into this?  The Oberth maneuver is when the craft uses the speed of a falling into gravity well to accelerate. 

incidentally:

Quote

Oberth effect

From Wikipedia, the free encyclopedia

Not to be confused with Slingshot maneuver....

....Rocket engines produce the same force regardless of their velocity. A rocket acting on a fixed object, as in a static firing, does no useful work at all; the rocket's stored energy is entirely expended on accelerating its propellant. But when the rocket moves, its thrust acts through the distance it moves. Force multiplied by distance is the definition of mechanical energy or work. So the farther the rocket and payload move during the burn, (i.e. the faster they move), the greater the kinetic energy imparted to the rocket and its payload and the less to its exhaust.

I know that wikipedia it's not exactly the best source out there, but...

 

Quote

When you land, you are decelerating (I hope) while Oberth is explicitly acceleration.

Deceleration its just negative acceleration. If nothing else Oberth is capable to do his trick because under the conservative forces the same equations will describe the motion if we invert the direction the time is flowing.

 

Now if you ask me which is more efficient, continuous descent or gravity turn. I have no idea, didn't check the maths or tested it. However I would like to point that there is some confusion among the various concepts:

Suicide Burn: maneuver where all deceleration is executed as close to the ground as possible.

Continuous Descend: approaching the landing site with a gentle and continuous angle.

Gravity Turn: maneuver that use only the gravity to steer a vehicle.

I think there is no question about the suicide burn, no matter what kind of approaching we are doing it will be better if we burn as close as possible to the landing(impact?) point.

Continuous Descent vs Gravity Turn depends on how much gravity and/or steering will affect the efficiency.

 

Edited May 2, 2017 by Spricigo

[Alshain]

55 minutes ago, Spricigo said:

incidentally:

I know that wikipedia it's not exactly the best source out there, but...

 

Deceleration its just negative acceleration. If nothing else Oberth is capable to do his trick because under the conservative forces the same equations will describe the motion if we invert the direction the time is flowing.

Wikipedia is especially not the best source when you clip out the important parts that directly defy your position.... like the first sentence which explicitly defines the Oberth maneuver as:

Quote

a maneuver in which a rocket falls into a gravitational well, and then accelerates when its fall reaches maximum speed.

I'm afraid it doesn't work in reverse because the acceleration of your rocket in retrograde is a counteracting force to it's orbital velocity.  It would be like saying that you can move forward more easily if I were holding you in place than you can if you were unencumbered, or even better if I were pushing you in the direction you were trying to move.  That is obviously not true, I am a force acting against your movement, in fact I may be strong enough to impede all movement entirely (but probably not, I've got no muscle, I'm a weakling).  Likewise, your velocity which is created by gravity on your descent from Ap, is pulling you one way while your rocket engine is pushing the other.  It's just not the Oberth effect.  In order to be the Oberth effect you have to be traveling in the same direction you're burning so you get that energy boost, which means only acceleration or only when raising your orbit, something you do not do while landing.

 

Edited May 2, 2017 by Alshain

[Spricigo]

I'm afraid we started a quoting war:

Quote

In astronautics, a powered flyby, or Oberth maneuver, is a maneuver...

So wikipedia starts the oberth effect article defining something related but not quite the same. I suggest we find a better source if we'll furthers argue about what is the oberth effect.

 

about working in reverse:

13 minutes ago, Alshain said:

I'm afraid it doesn't work in reverse because the acceleration of your rocket in retrograde is a counteracting force to it's orbital velocity.

If we reverse the direction the time is flowing the time dependent variables, like velocity, are also reversed, that is, our landing turn in a lift-off, the acceleration will be prograde and the velocity will increase.

[Alshain]

7 minutes ago, Spricigo said:

I'm afraid we started a quoting war:

So wikipedia starts the oberth effect article defining something related but not quite the same. I suggest we find a better source if we'll furthers argue about what is the oberth effect.

The Oberth maneuver is what I have been discussing the whole time though.  Read the first thing you quoted from me.  The Oberth effect is simply a result of the Oberth maneuver.

 

Quote

about working in reverse:

If we reverse the direction the time is flowing the time dependent variables, like velocity, are also reversed, that is, our landing turn in a lift-off, the acceleration will be prograde and the velocity will increase.

No, you can't reverse your velocity without expending energy, that's called magic.

Edited May 2, 2017 by Alshain

[Kryxal]

7 hours ago, Alshain said:

I'm afraid it doesn't work in reverse because the acceleration of your rocket in retrograde is a counteracting force to it's orbital velocity.  It would be like saying that you can move forward more easily if I were holding you in place than you can if you were unencumbered, or even better if I were pushing you in the direction you were trying to move.  That is obviously not true, I am a force acting against your movement, in fact I may be strong enough to impede all movement entirely (but probably not, I've got no muscle, I'm a weakling).  Likewise, your velocity which is created by gravity on your descent from Ap, is pulling you one way while your rocket engine is pushing the other.  It's just not the Oberth effect.  In order to be the Oberth effect you have to be traveling in the same direction you're burning so you get that energy boost, which means only acceleration or only when raising your orbit, something you do not do while landing.

 

As it happens, the Oberth effect doesn't require burning in the same direction as you're moving.  It's about how moving faster means using delta-v creates a larger energy change, no mention of sign of that change.

[Alshain]

Guys, believe what you want to believe.  I just don't care anymore.  Please stop tagging me in this thread.