Couple questions about flying over Mun's suface

[Khazar]

I have landed on the surface and now I want to go to another location, let's say about 5 km away.

What would be most efficient trajectory/ flight profile to do so ?

There are two velocities, vertical and horizontal. Reaching let's say 400 m/s of horizontal velocity means that before landing I would have to reduce it to around 0, so the smaller horizontal velocity the better. But at the other hand, each second in the flight requires me to fight against the gravity, so the shorter the flight the better. What is the relation ? Does it depend on TWR or something else ?

Going slowly I would't waste much fuel for braking but I would have to pull up constantly. Going fast would require a lot of braking but not much pulling up. Is one way superior to another or optimum is somewhere between ? Or maybe that doesn't matter at all ?

And similar concerns about landing from orbit. Are there any advantages/disadvantages of landing from very low orbit ? If I'm at low altitude then I fly fast, so I have a lot of horizontal breaking. If I am high then I go slow and don't have to brake a lot, but I have to reduce vertical speed a lot.

Also as there is no atmosphere to slow me down, Is there any difference whether i let my craft to fall all the way and use engines latest possible moment, or reduce speed gently in many burns not allowing craft to accelerate too much ?

[el_coyoto]

About the most efficient "hopping" strategy on the Mun, I'm not sure (it's a trade off between speed and gravity losses), but landing from low orbit is always better : by the time you have killed your horizontal velocity, you'll be at a lower altitude than if you had started from a high orbit.

By being at a lower altitude at the end of your retro burn, you will accumulate less vertical speed due to the gravity, and thus will have less vertical speed to cancel before touchdown, letting you save some fuel.

Also, using you engines at the last moment is called "suicide burn" even on an airless body around here, and has been shown to be efficient (and dangerous). I think that's partly due to the fact that you're cancelling vertical speed at the last moment, making sure you won't accelerated too much by gravity before touchdown. The Oberth effect may also have something to do (you're burning when you're deep inside the gravity well), but I may be wrong...

Please note that, a "perfect" low orbit landing wouldn't require a huge suicide burn before touchdown by definition. You can check the Scott Manley video link to see why (you're supposed to end up low and slow at the end of the maneuver).

This thread also deals with this problem, and might provide better answers than mine.

You can also check one of

on the subject.

Cheers!

Edited April 13, 2015 by el_coyoto

[TheXRuler]

Basically the flatter the trajetory the better, as going high means fighting gravity, wich means gravity losses. By going low and fast you will get a sub orbital trajectory which means your speed will fight gravity for you (very roughly explained).

Landing from the lowest possible orbit is best, for multiple reasons, including the oberth effect and gravity losses. Basically the lower you are when you brake, the smaller the time gravity has to accellerate you wich means less gravity losses. The best way to land is using the method detailed in this video, it is more efficient than putting your Pe in the body and then suicide burning. If you do not like this method, then one long suicide burn is better.

I have a Munar science vehicle wich uses kethane to refuel and then hops around the Mun on dual LV-N's, it has ~5600 m/s dV so I don't have to worry to much about efficiency, but I pride myself in being as efficient as I am capable of, so I have some experience with sub orbital hops.

[KerikBalm]

For short distances, you can use the approximations you might learn in high school physics (like treating trajectories as parabolas instead of elipses), and a 45 degree angle is best.

Pitch at 45 degrees, burn until your trajectory is just past your target, reverse on landing.

As your target gets farther and farther away, you want a flatter and flatter trajectory, as you'll be going closer and closer to orbit... for example, to reach the other side of the body, you'll nearly go to orbit.

[Warzouz]

Well On my Eeloo mission I had a lander which had to visit 4 spots before refueling (in polar orbit) and redo it again. I found a general spot where 4 biomes where quite near. I made 3 very flat and short hops. Science full great.

On my second trip the hop was much longer (nearly the other side of Eeloo), so I prefered to refuel before and drop directly on it (the small "Crater" biome).

I take off low, check the direction then continue prograde low until I reach the targeted biome. I overshoot a little bit. I wait until I reach the biome, then kill horizontal first then vertical. If not easy, I activate MJ where horizontal speed is very low.

Flat trajectories seems to be cheaper, quicker and easier to correct. Quicksave before you take off to reload if you managed to go in the opposite direction...

[Streetwind]

Landing from the lowest possible orbit is best, for multiple reasons, including the oberth effect and gravity losses. Basically the lower you are when you brake, the smaller the time gravity has to accellerate you wich means less gravity losses. The best way to land is using the method detailed in this video, it is more efficient than putting your Pe in the body and then suicide burning. If you do not like this method, then one long suicide burn is better.

You can't generalize it this way, at least not without a few footnotes.

The landing burn shown in that video is actually fairly inefficient, because it involves hovering. The maneuver is designed such that it keeps the vehicle's velocity vector pointing in the same direction (towards the horizon) for the entirety of the landing burn, regardless of horizontal velocity, right until the last moment where the vector disappears entirely and you transition into touchdown. However, hovering is really bad idea. The engine is not burning fully retrograde. It needs to be diverted down at an increasingly large angle in order to fight gravity, so that the vector can be kept from dropping. It is a very long burn and the craft is losing efficiency from fighting gravity every step of the way, just trying to maintain the status quo while not actually helping the slowdown process.

In fact, in terms of rocket fuel usage, hovering/preserving the status quo is the worst possible thing you can do for fuel efficiency. You can easily confirm this in an experiment where you drop a lander straight down from 10km while firing the engines on a low throttle setting to maintain a constant speed, compared to freefalling and just doing one quick, hard burn at the end.

So why is this generally regarded as "the best way of landing"? Because it was determined to be the best way to land an Apollo mission, specifically. The Lunar Module had a relatively small, puny engine to save weight and bulk, and was built to rely on manual, visual guidance for terminal descent. Technically the computer could handle it, but as Neil Armstrong himself famously demonstrated, the astronauts were very wise indeed to visually check out the projected landing site and decide themselves whether or not it was actually safe to let the computer touch down there. This slow, ponderous and somewhat wasteful descent was the most "efficient" one they could fly because it maximized overall mission success probability, not because it actually saved the LM fuel.

The most efficient landing is theoretically achieved by assuming the lowest possible safe orbit, then lowering the periapsis to exactly touch the surface at your intended landing site, and then finally performing an instantaneous suicide burn at periapsis that removes all lateral velocity in the moment of touchdown. That is, of course, physically impossible to pull off in practical application, but it it highlights one important factor: for efficiency you want to have the shortest possible burn to null your total velocity. To get that, you need to 1.) run your engine at full throttle, when you decide to run it - running it at less is always wrong; and 2.) burn fully retrograde* as long as possible.

This minimizes the time spent decelerating horizontally, while also minimizing the amount of vertical speed the lander gains from gravity during that time. As the burn progresses and your horizontal velocity gets lower and lower, you slowly turn the engine towards the surface in a smooth, gradual motion that keeps following the retrograde marker on your navball and touch down the exact moment when vertical and horizontal speed both hit 0 m/s together. This method is called a "reverse gravity turn" due to the similarity to a rocket launch played in reverse. It is the most efficient practical trajectory you can fly while still allowing for the fact that engine burns are not instantaneous and there might be obstacles to clear that steepen your path of approach.

* Due to sines and cosines, burning 10 seconds at a 45 degree angle imparts (or removes) both more horizontal and vertical speed than burning 5 seconds vertically and then 5 seconds horizontally... even in the absence of gravity. Always follow the retrograde marker.

Edited April 13, 2015 by Streetwind

[TheXRuler]

@streetwind

Oh, colour me surprised. Well I guess we never stop learning do we? Thanks for clearing that up.

It makes sense I guess, I often did wonder wether there should not be a more efficient way of landing by removing the need to thrust upward to keep from getting smeared all over the surface. Could it be agreed then that doing it that way is simply the safest way to land?

[Kerbart]

The most efficient way is simply burn retrograde until your orbit intersects the surface. The higher the altitude the better, as it requires less fuel. After that nothing is required; impact will follow sooner or later.

When you're adding extra criteria as "walking away from the landing site" or even "being able to return to orbit after landing" (I know! But believe it, there are Type-A perfectionists who insist on these ridiculous criteria!) things get more complicated and I revert to the above.

When you use Engineer you can see "suicide burn altitude" under "vessel". When it reaches exactly zero just hit full throttle, lean back and enjoy the landing (I yet have to do it that way).

[Streetwind]

@streetwind

Oh, colour me surprised. Well I guess we never stop learning do we? Thanks for clearing that up.

It makes sense I guess, I often did wonder wether there should not be a more efficient way of landing by removing the need to thrust upward to keep from getting smeared all over the surface. Could it be agreed then that doing it that way is simply the safest way to land?

Well, it IS rocket science, after all! We've come a long way towards making it accessible to the layman, but the fabled complexity of the topic should not be underestimated. I can't count the number of times I've had to revise my own preconceived notions here on the forums, or ingame.

(The particular insight about hovering being bad came after a long series of landing attempts on the Mun with permanently running engines. I just couldn't figure out why I was running out of fuel, then decided to look it up.)

But yeah, this hover trajectory is quite safe. Your vertical speed is near zero. The only thing that can endanger you in that sort of approach is encountering a cliff face that you cannot dodge because your engine is too puny. Thankfully in a real space program like Apollo, they knew there were no such cliffs anywhere near the landing site. Us KSP players, we often fly by eye, with sometimes

.