Mun landing without mun orbit

[MajorTomtom]

Hey,

Just a quick question. Is it any more/less efficient to land on the mun with or without a prior munar orbit ? I would tend to think that it doesn't change anything because of energy conservation etc, but it would be nice to have confirmation

Thanks in advance

[Superfluous J]

48 minutes ago, MajorTomtom said:

Hey,

Just a quick question. Is it any more/less efficient to land on the mun with or without a prior munar orbit ? I would tend to think that it doesn't change anything because of energy conservation etc, but it would be nice to have confirmation

Thanks in advance

It's technically more efficient, but practically speaking it's not enough more to justify it.

More important to consider is what you lose by not orbiting first, most notably the ability to pick your landing site.

Energy conservation isn't directly involved. Imagine you're in a 100 km orbit, then change to a 200 km orbit. Changing back to the 100 km orbit doesn't get your energy back.

There is a "best" way to do things to minimize dV, and a perfect horizontal burn to a dead stop at ground level is that way in this case. However, a single periapsis burn at say 10km over Mun's surface, followed by a tiny burn to intersect your landing site, and the final landing burn is so much easier, less prone to failure, and costs such little more fuel, that other than to say you did it (a valid reason, as this is a game and we're here to have fun) there really is no reason to do the direct landing.

[Spricigo]

Technically you are not landing without an orbit.

You enter in Mun SoI in a hyperbolic orbit with a periapsis very close to the ground. At the periapsis you circularize the orbit and procede to landing without a pause between the maneuvers.

Notice that you periapsis is below the ground you will be less efficient because you need to raise the periapsis to ground level.

[Human Person]

It depends on your TWR.

You want to do burns nice and close to the surface to take advantage of the obearth effect If your TWR is low, you should enter orbit first because otherwise you would have to start your landing burn very high up. 

Then there's the problem of when to start your suicide burn. Doing it to early can cost a lot of extra fuel, doing it to late can cost the life of your Kerbals.

[Pand5461]

Depends very much on where your initial periapsis is.

If it's above the ground, no difference from the case when you go to parking orbit first.

Ideal case: Pe is just above or just below the ground. If you manage to stop right there, that's the most fuel-efficient way to do a landing.

Worst case: you're falling vertically on the Mun. To stop, you must constantly burn against the gravity vector. Probably, burning radial to move Pe at or above the ground level while you're still far away will be more efficient than braking at vertical descent.

Everything else falls somewhere in between, impact trajectories being usually worse because thrust vector must be more aligned with gravity vector.

In stock, there's no reason no to go for a parking orbit first because then you can choose where to land. IRL, first Moon landings were from an impact trajectory. My guess is that landing guidance was much simpler that way.

[wumpus]

19 hours ago, 5thHorseman said:

There is a "best" way to do things to minimize dV, and a perfect horizontal burn to a dead stop at ground level is that way in this case.

If you really want to do an ideal landing, I would recommend Minmus.  First it is far more forgiving (less gravity), second you can really do a perfect horizontal burn in the flats (add wheels perpendicular to your engines) and then land on the wheels.  Trying to land on Mun (especially without being able to pick your landing zone) with extreme horizontal velocity means crashing into a ridge or similar.

If landing a spaceplane that can land backwards, it might help to put an octo probe at the rear and press "control from here", this will allow all RCS input to be properly reversed.  As noted above, on all surfaces this is extremely dangerous for little savings (while it might be less dangerous on Minmus, the delta-v saved is tiny).

[Snark]

On 8/21/2017 at 0:34 PM, Spricigo said:

Technically you are not landing without an orbit.

Depends how you define "orbit".  In the strict sense of "are traveling ballistically within the SoI", then you're in an orbit all the time.    If you mean in the sense of "you have an elliptical orbit, with a Pe that may or may not be above ground level" (i.e. counting suborbital as an "orbit"), then you're in an orbit as soon as you've braked below escape velocity.  But if you mean "you have an elliptical orbit with a Pe above ground level", the answer is "not necessarily".  Someone could enter the Mun's SoI on an escape trajectory that's already on a collision course for the Mun, and in the course of braking, you could go straight from an escape trajectory to a suborbital one.

On 8/21/2017 at 0:34 PM, Spricigo said:

You enter in Mun SoI in a hyperbolic orbit with a periapsis very close to the ground.

Yes, in this case, there will be a non-suborbital elliptical orbit.  But if the initial hyperbolic periapsis is below ground level, then no, you won't get an "orbit" in that sense-- you'll be transitioning straight from hyperbolic to suborbital.

None of the above points are directly relevant to the OP's question, though.  Just a technical point. 

Going to the OP's question:

On 8/21/2017 at 10:56 AM, MajorTomtom said:

Just a quick question. Is it any more/less efficient to land on the mun with or without a prior munar orbit ? I would tend to think that it doesn't change anything because of energy conservation etc, but it would be nice to have confirmation

The answer to your question is "Yes." 

Of course, your very next question is going to be "well, is it more, or is it less?  And by how much?"

To which the short answer is "It depends."

The slightly longer (more practical) answer is:  For actual KSP ships, it's generally best to approach the surface at a very shallow angle (i.e. mostly horizontally), and then do a "suicide burn" as close to ground level as possible, such that you reach zero surface speed at the moment of touchdown.  It's better than descending straight down, for example.  However, how much better it is depends on a lot of factors, mainly TWR and your suicide-burn skill.

 

The long answer, with the "why is it that way", is below:

It's probably easiest to start by describing the hypothetical, physically-impossible ideal (i.e. least dV) case, and then work backwards from that to practicality.

The ideal (least dV) way to land is this:  you have infinite TWR.  You plunge to the surface, without burning engines at all.  Precisely at the moment of about-to-impact, right at the surface, you do an instantaneous burn with infinite acceleration, such that it precisely brings your velocity to zero right at ground level.

In the above scenario, it doesn't matter what angle you're approaching the surface at.  You could be dropping straight down.  You could be hurtling horizontally and only barely grazing the surface right where your periapsis is tangent to the surface.  Doesn't matter.  If you do your whole burn at ground level, then it doesn't matter what angle you're at.  It's all the same.

Now, in real life (did I really just describe KSP as "real life"?  sheesh, I've been playing too long), your angle of descent does matter, because your TWR isn't infinite.  Also, while it's true that in the abstract "the higher your TWR, the less dV you need to land", that doesn't mean you should try to make your ships have stupidly high TWR, for the following reasons:

• Getting that kind of high TWR means packing lots of engines (i.e. "more than you need"), which are dead weight, which means wasted fuel.
• Also, the really high TWR engines tend to have lower Isp, which means they generally have less dV to play with than if you were using a more efficient engine.  Actual ship design involves a tradeoff between "more efficient engine, which provides more dV for landing" and "higher-thrust engine, which requires less dV to land."  The ideal engine would be one that has both high thrust and high dV, but that would be an overpowered engine that breaks game balance, which is why there's no such engine in stock.
• Diminishing returns.  Yes, "the higher the TWR you have, the less dV you need"... but it's not linear.  TWR makes a big difference if your local TWR is less than, say, 2.  The needed dV approaches infinity as your TWR drops towards 1.  So, it's a lot more efficient to land with a TWR of, say, 1.8 than it is for 1.2.  But once you get much higher, there's diminishing returns.  A TWR of infinity is only slightly more efficient that a TWR of, say, 5.  And since the Mun's gravity is fairly low, it's pretty easy to get a Mun TWR above 5, even with fairly lightweight, high-Isp engines.  TWR of 10 is not uncommon.  So you generally have a fair amount of TWR to play with.

The reason why you want to be doing your landing burn at as low an altitude as possible is to get the maximum benefit from Oberth effect.  No need to go into the details about that here-- go read about it, if you're curious.    For purposes of current discussion, though, suffice to say that the most dV-efficient way to land is to do as much of your burn as possible at the lowest altitude possible.

That's why it's generally favorable to approach the surface at a shallow angle.  You don't have infinite TWR, which means you travel a fair distance while you're braking.  If you're approaching the surface at a really shallow angle (i.e. nearly horizontally), then that means most of your "braking path" is all right down next to the surface, thus optimum dV efficiency.  On the other hand, if you're dropping straight down, then you have to start braking while you're still high above the surface, which means doing a lot of your burn while you're still very high up, which is less efficient.

How much difference does it make, though?  That depends entirely on your TWR.  The answer is probably "not very much".  Essentially, the amount of dV you waste by going straight down towards the surface (as opposed to the optimal nearly-horizontal approach) will be, roughly, Mun gravity times the length of your deceleration burn.  And that's generally not a huge amount, meaning "it doesn't make much difference."

Let's check the numbers.  Math in spoiler section.

Spoiler

Let's say you're approaching the Mun such that your velocity at the surface would be, say, 900 m/s if you just left your engines off and let it crash.  Let's suppose you've got a somewhat typical small Mun lander-- basically just a command pod, couple tons of fuel, a Terrier, and a few lightweight accessories such as solar panels, landing legs, and science instruments.  Let's say 3.5 tons for the whole package.  A Terrier has 60 kN thrust, so it'll get about 17 m/s² acceleration.  That means the braking time to land will be about 53 seconds.  The Mun's surface gravity is 1.63 m/s², so multiply that by your 53 seconds and you find that you're wasting roughly 85 m/s of dV by dropping straight down instead of coming in shallow.  That's 9.5% of your 900 m/s ideal burn.

(Disclaimer:  To be clear, I'm cutting some mathematical corners here; this is an approximation for the sake of brevity.  But it's a pretty close approximation, good enough for purposes of current discussion.  I could do a much longer, much more complicated, mathematically-rigorous answer, that would give essentially the same results.)

So, for a typical Mun lander, the difference between "optimal" and "worst case" is under 10% of dV efficiency.  So yes, it matters, but no, it doesn't matter much.  It would matter a lot more if you had a really low Mun-local TWR, but that usually doesn't come up.

That said, though... there's another reason why it's generally a good idea to approach on a shallow angle, and that has to do with the practicalities of piloting.  Specifically, because properly executed suicide burns are hard unless you have automated aids.  You're likely to get it not-quite-right, i.e. there will be some "error" there (starting the burn a little too early or a little too late).  And a mostly horizontal approach to the surface is a lot more forgiving of such errors.

If you're dropping straight down towards the surface, suicide burns are really unforgiving.  If you start the burn even one second too late, you go splat and there's absolutely nothing you can do to prevent it.  Which means you have to start the burn extra-early for the sake of caution... but now you're going to be wasting a lot of fuel, and the inefficiency can add up to a lot more than the 9.5% number that I quote above.  On the other hand, if you're doing a mostly-horizontal approach, it's much better in several ways:

• If you realize that you started the burn a little too late, you can recover:  just tip your ship so it's pointed a bit above retrograde (i.e. the nose is pointed higher in the sky); this will buy you some time to slow down more.
• This means you can shave it closer and are unlikely to start the burn way too early.
• Even if you do start the burn too early, it doesn't hurt you as much, because you're moving mostly horizontally and the altitude difference won't be much.

All the above arguments become moot if you're doing an automated landing with something like MechJeb, because then it perfectly nails the ideal suicide burn all the time, and the only difference between vertical and horizontal landings is the roughly 10% figure I quote above.  But unless you're doing that-- if you're doing this entirely with your human eyes and hands and brain-- then unless you're an ace, you probably get considerably better efficiency with the horizontal approach.

[Spricigo]

1 hour ago, Snark said:

Depends how you define "orbit".  In the strict sense of "are traveling ballistically within the SoI", then you're in an orbit all the time.   

  did I really just describe KSP as "real life"?  sheesh, I've been playing too long),

And that's what I meant. So I'm technically correct,  the best kind of correct. 

But I failed to make it clear.  Thanks for the clarification. 

Quote

 Just a technical point. 

Maybe,  but relevant point associated to this is the fact that in both situation we need to consider the same factor for a efficient landing (TWR, approaching trajectory, timing)

 

[invision]

ive found it uses less fuel without a proper orbit set up but you have to make sure your craft has enough thrust the gravity pull wont overcome your craft.

minmus is super easy to do this on almost any engine, the mun needs a bit more power or you will smash into the ground and ive had this happen a few times.

[Johnny Wishbone]

9 hours ago, Pand5461 said:

IRL, first Moon landings were from an impact trajectory. My guess is that landing guidance was much simpler that way.

Thats not true, unless by "first moon landings" you are referring to probes that were intended to impact the surface.

All the manned Lunar landings (Apollo) were initiated from a Lunar orbit.

[Pand5461]

5 hours ago, Johnny Wishbone said:

Thats not true, unless by "first moon landings" you are referring to probes that were intended to impact the surface.

Why that protein chauvinism? Of course I am referring to Luna-9 and Syrveyors (that were never intended to impact the surface, btw).

[Johnny Wishbone]

7 hours ago, Pand5461 said:

Why that protein chauvinism? Of course I am referring to Luna-9 and Syrveyors (that were never intended to impact the surface, btw).

Sorry. Not following you. what is "protein chauvinism"?

My reply was more to the point that since most people have generally limited knowledge and understanding of Lunar exploration, they probably don't know diddly squat about the Luna, Ranger, or Surveyor programmes. To most people, the "first moon landings" are the Apollo landings, which were done from an orbit.

This is not to dismiss the achievements of the Luna, Ranger, and Surveyor programs.

[Dark Lion]

7 hours ago, Pand5461 said:

Why that protein chauvinism?

Uh oh! I think we've got a proud, flag-waving hyooman here in the forums... 

LOL! Pand must've missed your clarification about the landings from orbit being manned @Johnny Wishbone, LOL!

[OrbitalBuzzsaw]

Bar the occasional zero-margin insanity, usually it's not worth it. Sure, it's possible, but there's no real point.

[Pand5461]

3 hours ago, Johnny Wishbone said:

Sorry. Not following you. what is "protein chauvinism"?

That intended to be a joke meaning "unmanned space programs also deserve to be remembered". I apologize if it offended you.

I just quietly assumed that people on this forums usually know that there were pre-Apollo unmanned Moon landings, so I don't need to explicitly say it.

Edited August 23, 2017 by Pand5461

[Johnny Wishbone]

2 hours ago, Pand5461 said:

That intended to be a joke meaning "unmanned space programs also deserve to be remembered". I apologize if it offended you.

I just quietly assumed that people on this forums usually know that there were pre-Apollo unmanned Moon landings, so I don't need to explicitly say it.

no offense taken. i just had no idea what you meant or were referring to.

[MajorTomtom]

Alright thanks for the answers. The question popped because I saw that the early probes for the moon were indeed in direct vertical descent, and I imagined it had to do with savings, but it is as you mentioned probably just a question of reduced complexity (with limited ignitions, batteries etc..)

Thanks again

[DerekL1963]

5 hours ago, MajorTomtom said:

Alright thanks for the answers. The question popped because I saw that the early probes for the moon were indeed in direct vertical descent, and I imagined it had to do with savings, but it is as you mentioned probably just a question of reduced complexity (with limited ignitions, batteries etc..)

Thanks again

In terms of real life, a direct launch/direct landing also vastly simplifies guidance and navigation requirements.