Mun Landing, acceleration and deltaV

[Ikare]

Hi everybody.

I'm pretty new in KSP, and while I've read a lot of tutorials and wiki pages, there is something that I don't understand in my current mission.

I've placed an orbiter around the Mun (on a very elliptical orbit, pe = 50km, ap = 2200km), and detached the lander.

Once I've modified the trajectory of the lander, its remaining deltaV is 1000m/s.

If I don't use my engines, I hit the mun at about 800m/s.

The way I understand things, I should be able to land, but in reality, I'm always short in fuel and hit the mun at around 230m/s.

How is that possible, given that spending its fuel, my lander should be less affected by gravity?

Thanks to anyone that has an idea on the problem.

Edited August 20, 2015 by Ikare

[Champ]

Hi everybody.

I'm pretty new in KSP, and while I've read a lot of tutorials and wiki pages, there is something that I don't understand in my current mission.

I've placed an orbiter around the Mun (on a very elliptical orbit, pe = 50km, ap = 2200km), and detached the lander.

Once I've modified the trajectory of the lander, its remaining deltaV is 1000m/s.

If I don't use my engines, I hit the mun at about 800m/s.

The way I understand things, I should be able to land, but in reality, I'm always short in fuel and hit the mun at around 230m/s.

How is that possible, given that spending its fuel, my lander should be less affected by gravity?

Thanks to anyone that has an idea on the problem.

If i understand, before burning, you are going 800m/s, and still have 1000 m/s in fuel in your vessel, and you dont understand why it's not enough to land ?

The answer is simple : gravity will make you accelerate.Save, and then dont burn, just let your lander crash and note the speed just before the crash : it is greater than 1000m/s.

Not-a-pro tip (but still a good advice) : when planning your mission and building your rockets, use DV maps (google KSP DV MAP) to know how much dV you will need

[Starhawk]

You are better off to start your deorbit burn from the lowest possible circular orbit. This will give you the lowest initial velocity and thus need a shorter burn to land.

You also need sufficient thrust-to-weight ratio. If your TWR is less than 1 relative to the Mun's surface gravity you will crash. Realistically, a TWR of 2 to 2.5 relative to the body you are landing on is probably needed for a comfortable landing.

How did you modify your lander's trajectory?

If your margins are very tight, then you will need to perform an efficient deorbit burn as well as a well-timed burn when you are just high enough to bring your velocity to zero as you reach the surface. This may take many tries to get right.

If the numbers you give are accurate, all you need is not to lose 200 m/s to gravity in the time it takes you to make your landing burn. As long as your TWR is high enough, that should be doable.

Good luck!

Happy landings!

[Ikare]

If i understand, before burning, you are going 800m/s, and still have 1000 m/s in fuel in your vessel, and you dont understand why it's not enough to land ?

I'm reaching 800m/s at the moment of the crash if I don't use my engines, my initial velocity is below this.

Maybe, as I slow down, I spend more time in mun attraction and that's why I actually need more.

I've watched http://forum.kerbalspaceprogram.com/threads/96985-1-0-4-WAC-s-Delta-V-Map-continued-2-1-%28Aug-13th%29-Swash-transition

If I understand it correctly, I need 910dV to go from an "elliptical orbit to SOI edge" to the surface. I have exactly 985dV, so I guess there is a way to land.

You are better off to start your deorbit burn from the lowest possible circular orbit. This will give you the lowest initial velocity and thus need a shorter burn to land.

The burn to modify my orbit will be smaller than the gain it offers? Anyway, I'll try that, thanks.

[Godot]

You are better off to start your deorbit burn from the lowest possible circular orbit. This will give you the lowest initial velocity and thus need a shorter burn to land.

...

This one.

The lower your orbit, the less chance gravity has, to accelerate your lander during descent.

Also you will ideally want to brake your landers free fall as late as possible.

If you already try to brake its vertical speed at higher altitudes (like 20km+) you will just prollong the time that Muns gravity has to accelerate your lander again ...

meaning that your brake will me worth less than if you brake low (below 10km) ... or it may even be worth nothing (if gravitical acceleration eats up the whole speed diff of your brake).

I for my part have noted 1000 m/s as well, for landing from an altitude of 20km.

So, it is doable with this amount of dV (if you are low enough, your landers TWR is high enough and you brake your vertical speed late enough )

[Starhawk]

The burn to modify my orbit will be smaller than the gain it offers? Anyway, I'll try that, thanks.

Actually, you want to get into that orbit with the transfer stage, before detaching the lander.

Adjusting your orbit with the lander's engine may help to shorten the final burn before landing (impact?) and make it easier that way, but I wouldn't expect any delta-v savings.

There's a lot of practice involved, and if your don't have much safety margin it can take quite a bit of repetition to get just right.

Upon achieving my very first Mun landing, I was so excited that I jumped out my chair, cheered, and danced around. It seemed like the most amazing achievement. Keep working on it.

Happy landings!

[Vegetal]

Think of it this way: The closer to the surface you orbit is, the least energy you will need to get to the surface. An elliptical orbit means you still haven't brought your orbital energy to a minimum before landing, so you will need more energy (fuel, delta-V) in your lander to land safely.

That's easy to understand on Kerbin. When you launch, you usually choose to circularize on a low orbit before other maneuvers, because a higher orbit will require a longer burn and more fuel (energy). If you are on a lower orbit, you will also heat up less when re-entering, because you will have less energy than on an eccentric orbit.

[Empiro]

It sounds like you ought to have enough fuel (1000 m/s in your lander, versus 800 m/s when you hit the surface if you don't fire your engines). You have a decent amount of leeway (200 m/s), but if you're not experienced with landing, then it might not be enough.

There are lots of inefficiencies that could be the cause of you running out of fuel. What is your TWR at the surface of the Mun? The Mun has low gravity, so it's not too hard to make it relatively high (4.0+ surface TWR on the Mun). You can land efficiently with less TWR, but it's harder to do (there are some guides that teach you to perform a constant altitude landing). Second, are you firing your engines as late as possible at full throttle? Any time you reduce the throttle or you stop firing, you've wasted some fuel. Third, make sure you're using the navball as much as possible to land -- burn against the surface velocity retrograde as much as possible. Any time you burn away from that, you've also wasted some fuel.

[SmashBrown]

Myself, i always have an orbit around the mun of at most 10-15km, usually under, then i pick a landing spot, burn retro so my orbit takes me into the landing zone i want, then do a slow controlled burn once i get low enough, usually hitting the surface at around 4 m/s. It's just practice. You'll get it.

Keep you orbit super low, then pick a good spot to arrest your horizontal speed, but don't allow your lander to speed up to much on the way down, as this is just wasting delta v IMO. Slow controlled burn. Try to spot your shadow on the surface, that will give you a good way to judge how long you have till you hit the ground.

[Admac]

Use a suicide burn counter

I'm sure there are lots of mods that provide suicide burn countdowns, mechjeb is the one I use. The theory being the less time between you starting your insertion and landing the less time gravity has to act on you. The practical way to optimize this is to burn as absolutely late as possible, so you velocity hits zero at the same time as your altitude hits zero. Like earlier posters explained, this is where higher TWR ratios help. the difference between 1.1- and 2 is huge, likely a few hundred m/s of DV on Mun, while the benefits drop off as it gets higher. I like about 3-5 TWR for airless body landing, especially if landing very accurately is the goal, like when building colonies or surface refuelling. 5 TWR on the Mun that is still only 8 m/s max acceleration, which is pretty easy.

With clever piloting, great TWR and a suicide burn countdown, your elliptical landing will be marginally more fuel efficient. Just a lot less safe!

[Ikare]

Thanks everyone. Here is the final story of my lander:

I had around 15 TWR, so it was more than enough (4x 48-7S Spark engine).

I've previously seen than reducing my apoapsis did not change my orbital velocity at periapsis (not significantly anyway),

so I just burned retrograde (prograde actually, I didn't get why but my rover was wrong-sided) at apoapsis to have a -1km periapsis.

At some point, I then burned at around half thrust, for it seems I have a small asymetry in my engines : my lander just make incontrolable spins if I go full thrust.

After a few tries to find the altitude of this "some point", I succeeded! But I was a little short on fuel and I forgot to lower the lander legs, so my rover tires exploded...

I'm really happy . I think I'll just accept this fate for my rover, instead of trying again.

And next time, if my lowtech Mechjeb has it, I'll use a suicide burn countown, this seems awesome!

Edited August 20, 2015 by Ikare

[SanderB]

Hi Ikare, you might enjoy a horizontal landing. It can be done fairly efficiently at an initial TWR (when you start the burn) of 1.0 and up. I prefer to start out doing it from the map view, pitching up and down to keep the Periapsis near my vessel until I need to pitch up to keep my vertical velocity above minus 10-30m/s. I hope I've adequately got it demonstrated here:

Horizontal landings are about being efficient with your fuel and funds, and its more of an advanced thing but it's not that hard to learn and I found it enjoyable. Slow horizontal landings give more time to precisely target your landings too.

[tsotha]

You are better off to start your deorbit burn from the lowest possible circular orbit. This will give you the lowest initial velocity and thus need a shorter burn to land.

Eh... is this true? I mean, the part about needing the shortest burn to land is true because your total energy is lower, but relative to the surface you should actually be going faster than you would at higher altitudes.

[Ripper2900]

You'd be going slower horizontally at the altitude where you do your deorbit burn, yes. But when you hit the surface, you will have picked up that speed vertically again.

[Starhawk]

Eh... is this true? I mean, the part about needing the shortest burn to land is true because your total energy is lower, but relative to the surface you should actually be going faster than you would at higher altitudes.

Yes.

At higher altitudes, the gravity of the target body has more time to accelerate your craft. The idea is that if your lander is very tight on fuel, your best approach is to get the transfer stage into as low an orbit as possible before detaching. This leaves the lander still full of fuel and needing less fuel to land from there than it would from a higher orbit, or one with a higher apoapsis.

Happy landings!

[GoSlash27]

Mathematically, I'd expect you to hit the Munar surface at 775m/sec from that orbit without a burn. The absolute minimum DV to arrest to a safe landing that way would also be 775 m/sec, assuming infinite thrust and no weight penalty for generating the thrust.

This does *not* mean that more thrust is the way to go about it, though.

As others have said, you really need to operate your lander from a low circular orbit.

If you're in a 10km circular orbit, you're orbiting at 557m/sec. Doing a zero descent rate approach from there would require 2 burns:

Burn #1 to set Pe just above the surface (6.8 m/sec)

Burn #2 to arrest lateral velocity (578 m/sec)

Total DV 584 m/sec. Again, this is a theoretical absolute minimum, but it's clearly preferable to 775 m/sec.

Now... this figure will go up with the approach method you're using. If you're using the "stop 'n' drop" method, it'll cost 578 m/sec to arrest your lateral velocity and then another 181 m/sec to arrest your freefall velocity at the surface for a total of 759 m/sec. Using an intermediate method will yield somewhere in between, but even this is preferable to bombing in from a 2 million meter apoapsis.

And here's where it gets weird: Raising the t/w very quickly yields diminishing returns, and soon adds no measurable benefit. Running a lower t/w will generate higher DV losses from gravity, *but* these losses may be offset by the reduced mass, structure, and fuel consumption attendant with carrying less engines.

Each engine has an ideal t/w for a given body fir high mass efficiency, and it's usually in the range of 1.2-1.6 for launches and *less than* 1 for landing.

We devoted way more time and energy to studying this than is probably healthy

Best,

-Slashy

Edited August 21, 2015 by GoSlash27