Questions on Lander Designs

[Spengler]

Hi.  Hello there.  I'm new here (as if you couldn't tell).  I've been playing Kerbal Space Program for a couple of months now, and I'm at the point where I want to start branching out and doing better things...but I'm running into some stumbling blocks.  Particularly when it comes to designing landers.  Now, to be clear, I've gone through the tutorials, and I've watched a few videos and such.

In "How to Land on the Mun", Rocketscience gives a video with some decent pointers.  Unfortunately, he doesn't really go into how to design a lander, but rather focuses on how to get the lander to touchdown.  Great information, but not what I'm looking for.

In "Sporkboy's Guide to Mun Lander Design", we get a bunch of images with some tips on how to design landers.  Lot of good information there too, and I particularly like the Animal Farm quasi-reference.  But the tips are (at least to me) common sense, especially the part about building wide not tall.

Finally, in "Early Career Mun Landing", I start to see the type of information I'm looking for.  Clear pictures with descriptions of the parts being used.  This particular thread has become the go-to design for all of my craft at this point.

So what's my issue?  I can build a rocket and get into orbit, and I can take the design from Early Career Mun Landing and actually land on Mun and Minmus.  I'm bad at fuel consumption, but that's something I gotta get better at on my own.  But my issue is that all of my landers are the tall, skinny kind, which are hard as nails to land, even with 6 or 8 legs.  Something about the center of mass being too far up the column or some such.    Anyhow, I've tried designing low, wide landers, and I'm just not getting it.  Here's what I've tried so far:

1. I've tried designing a rocket similar to the ECML tutorial/thread, but the upper stage is a lander can and fuel wrapped in fairing.  I'm running out of fuel far too soon, and I've tried stripping away as much weight as I can to get as much dV as I can get.  I know that without images you can't really see what I'm doing, but it's essentially a lander can sitting on a fuel tank with a Spark engine and 4 legs.  And I just can't seem to get more than 3,000 dV out of it, which is not enough to transfer, orbit, land, and return.  I told you I'm not very good at fuel consumption; I have found that my sweet spot for fuel is around 3500 dV to do the entirety of the landing as I just put forth.  I know - I gotta get better.  Which is why I'm here asking about lander design.
2. I've tried using the docking ports to sit a lander on top of a command module, with having just enough fuel (about 1500 dV) to land and then return to the command module, but I've got multiple issues trying to do this.  I know I need docking ports on both the can and the module, but when I'm building the thing do I have to have them connected at the start?  Or could I go:  Command Module - Docking Port - Lander Can - Docking Port?  I'm confused about this because if I detach in space and then come back, will the 2 docking ports then be able to connect?  Oh, and let's not forget that docking is a serious pain in my behind, and I'm barely able to finish the tutorial in-game on docking with the help there.

So I guess what I'm asking is for some in-depth help on how to actually build a lander.  Which way is the right way?  Should I go down the path of rendezvous and docking, or use the can as the command module?  How do I build one that has enough dV to do everything I need it to do to get to [insert celestial body here] and back?  Assume we start with Minmus (as that's easier to get to than the Mun, for some odd reason); how do I design a lander that can get there?  Again, I'm using the ECML thread as the basis of my rockets, and that works.  But I want to get better, and I'm just not grasping something here.  I know I'm missing something, and I'm sure it's something rather basic that will trigger me to go "Aha!" as soon as I see it.  But it's elusive for now.

One last, semi-related question on landing please.  In Sporkboy's guide, he mentions at one point having a landing speed of less than 10 m/s.  What is the optimal speed to be landing at?  I've been shooting for less than 1 m/s, but can I get away with a higher speed to help save on some fuel?  If so, what's that sweet spot?

Any help anyone can give me would be most appreciated.  And until I get a response, I shall be off to try building...well, more rockets!

 

[Signo]

Try to get it easier. Go for a "direct ascent" design instead of doing it "Apollo style". 

You will notice it will be easier to get a good "form factor" for your lander too.

You do not really need legs if you land around 6m/s.

When landing reduce your speed to 90/100 m/s when you are at 10000, then sap all your relative horizontal speed when you are at 500. Just go down straight managing to keep it "brakeable".

About the "general design", maybe you need just a little bit more "oomp" in you transfer stage. 

Edited May 16, 2021 by Signo
oomp

[Snark]

6 hours ago, Spengler said:

But my issue is that all of my landers are the tall, skinny kind, which are hard as nails to land, even with 6 or 8 legs.  Something about the center of mass being too far up the column or some such.    Anyhow, I've tried designing low, wide landers, and I'm just not getting it.

(I'll assume that you're already avoiding steep landing places like crater walls and going for reasonably level surfaces).

There are a variety of lander designs that can work.  I find that "command pod on top of fuel tank on top of engine, with four landing legs around it" does okay, generally speaking.  It helps to make sure you're coming down perfectly vertically, and (if it's available to you) use SAS to help you, e.g. land while SAS is set to hold .

If you want a wider stance (and, incidentally, a bit of extra fuel storage) ... the Baguette fuel tank is really handy.  Put four of those around the central fuel tank in radial symmetry, and then attach your lander legs to those.  This moves your lander legs farther out from the center by an extra half a meter or so, and adds quite a bit of stability.

Could you post a picture of a typical lander you've tried, that you had issues with?  (It's easier to critique a concrete example than to offer general advice in a game with so many variables.  If we can see what you're doing, it'll be easier to offer specific suggestions for improvement.)

One last bit of advice:  if you find yourself having lots of problems with tipping, another thing that can be helpful is to add a reaction wheel.  Those will help maintain stability, and (depending on the mass and shape of the ship, and the power of the reaction wheel) might be strong enough so that even if you do fall over, they can right you again.

[Guest]

For a basic design, I use either a lander can or one of the pods. Under that I put one of the short/wide tanks (ie Rockamax or 1.75 tanks) then a lower profile engine. Then I’ll grab another tank similar to the first, change the symmetry to x4 and add them to the outside of the first tank.

I then add the LT2 legs to the outside of the outside tanks, then tweak their height so the feet are the same height when deployed as the end of the engine (so the engine acts as a 5th leg).

Usually the above gives ample fuel for Mun but you can also add another stage to do the deorbit burn if you don’t already.

[Spengler]

So, I took the advice of everyone in this thread, and I was able to come up with an odd lander that worked.  Pic of the craft is below so you can see what I eventually did.  I had to use the FL-T100/400 fuel tanks because the fairing I had access to at this point in my career save wasn't wide enough to get around some of the larger tanks.  You'll also notice that the craft is missing legs.  That's right - the fairing I have access to also isn't wide enough to get around legs on the tanks, so I had to land this bad boy right on the Spark engines.  But I got this thing in the Farside Crater on the Mun, did a whole bunch of science stuff, and came back home to a whopping (well, it's whopping to me!) 392.8 science gathered.  In one trip.  For me, that's a major development.

Now to speak directly to some in the thread to answer your questions/comments!

@Signo Thank you so much for the help here.  This idea actually did the trick.  Now I just gotta get better at designing rockets so I can get farther out in the Solar System.

@FruitGoose You get some credit here too because it's the same design.

@Snark Yeah, I generally aim for mostly level-ish landing spots; I've spent far too much time reverting flights and loading pre-landing saves thanks to just trying to stick anywhere, which usually results in my ending up on the side of a rather steep hill that you shouldn't land on.  As far as the Baguette tanks go, I actually tried using those before the FL-T400s, and they don't seem to generate enough dV for use.  At least, not for me they don't.  Which is probably a design issue on my end which, as I mentioned earlier, I need to get much better at.

And now for that tasty image I promised you!

 

[Snark]

1 hour ago, Spengler said:

As far as the Baguette tanks go, I actually tried using those before the FL-T400s, and they don't seem to generate enough dV for use.

Yep, it's not much.  I was mainly suggesting them because they provide a convenient way to widen the attachment point of the legs.  Having the legs be an extra Baguette-width further out from the center can really make a difference in stability.

Furthermore, since you can mount them really low down, you can set them to drain last, so they help to lower your CoM, too.

I just slapped this together and didn't add on other low-mass radial extras like antennas, other science instruments, solar panels, etc., but you get the idea.

Note the location of the CoM, and then look how far out from the center the legs are placed.  It's rock-solid stable and you could land on a 30-degree slope with no trouble.

It's set to burn the fuel in the Baguettes last, so that helps keep the CoM low as fuel is burned.  The pictured CoM is with a full load of fuel. Since the full-load CoM is slightly above the CoM of that 1-ton fuel tank, then this means that yes, the CoM will rise slightly as you start to burn fuel.  But only slightly, and even with a good 1000 m/s of its dV burnt, it's stil a low CoM overall and should land with no problem.

It provides 2400 m/s of dV, which isn't quite enough to go from LKO to the Mun's surface and back, but it's close enough that as long as you give it a reasonable boost from LKO, it can finish the job on its own.

And it's reasonably aerodynamic, so you don't need to put it in a fairing.  Just stick a decoupler under that Terrier and you can put it atop a 1.25m stack.

[Fraktal]

8 hours ago, Spengler said:

And I just can't seem to get more than 3,000 dV out of it, which is not enough to transfer, orbit, land, and return.

Whoa buddy, you are overengineering that lander a tiny bit. If I may, I'd recommend that you use your rocket's final pre-lander stage for the transfer, munar circularization and deorbiting and have the lander's own engine take over only during the actual descent towards the Mun's surface.

Examples with and without Making History parts:

All of these are capable of landing near-equatorially on the Mun and returning to Kerbin with about 200 m/s to spare. On the stage readout, Stage 1 is the lander, Stage 2 is the transfer stage immediately below the lander.

[Spengler]

@Fraktal

Yeah, one of my major design issues is that my first stage tends to barely get me into orbit around Kerbin, which then means I'm relying upon the lander itself to get me to the Mun and back (with a landing thereupon).  So yeah, I'm over-engineering the lander to handle far more than it really should.  To be honest, the lander should really only need to land and get back.  I need to get better with my stages; I have to design something that has 3 distinct stages:

1. Take-off and orbit Kerbin
2. Transfer to Mun (or another body)
3. Land and return to Kerbin

I'll figure it out at some point.  Your images are a great start, and I'll see if I can replicate that (although without knowing the parts you used, it might take me a while).

[Fraktal]

First design uses:

• One FL-T400 fuel tank and a single Terrier on the lander. There are three landing legs, one on each materials bay radially mounted onto the lander.
• The service bay under the command pod contains one experiment storage unit, one thermometer, one barometer, three goo containers and four Z-100 batteries.
• Two FL-T400 tanks and a Terrier on the transfer stage.
• Four FL-T400 tanks and a Swivel on the launcher's core stack.
• Four FL-T400 tanks and a Reliant on each of the launcher's two side boosters.
• Two Thumper SRBs.

Second design uses:

• Command pod is a Mk1 Lander Can. There are two radial parachutes on the nosecone, along with OX-STAT solar panels.
• The service bay above the command pod contains two experiment storage units, one thermometer, one barometer, one goo container, one Z-200 battery and an OKTO probe core so that you can bring a scientist instead of a pilot to repeat your single-use experiments for maximum science gain.
• Two Sparks with three Oscar-B tanks each on the lander. There are four landing legs, two on each Oscar-B stack.
• Same transfer stage as the first design.
• Same launcher core stack as the first design.
• Three FL-T400 tanks and a Reliant on each of the launcher's four side boosters.

Third design uses:

• Command pod is a KV-2 Pea. This part and the one used in the design below has a built-in heat shield that WILL survive reentry from the Mun, which saves some weight.
• The service bay above the command pod contains the same as the first design, plus a Small Inline Reaction Wheel clipped under the nosecone (the command pod doesn't have a built-in one).
• Two radial FL-T200 tanks and a single Terrier on the lander. There are four landing legs, two on each fuel tank.
• Three FL-TX220 and two FL-A151S tanks and a single Terrier on the transfer stage.
• Fourteen FL-TX220 and two FL-A151S tanks, one Reliant and two Thuds on the launcher's core stack. Feel free to use bigger tanks, I used these so that this particular rocket is available earlier in the tech tree.
• Same side boosters as the second design.

Fourth design uses:

• Command pod is a KV-3 Pomegranate. Same nosecone kit as the second design.
• The service bay above the command pod contains the same as the second design, plus a Small Inline Reaction Wheel.
• Two Sparks with four Oscar-B tanks each, plus one FL-T100 tank on the lander. There are four landing legs, two on each Oscar-B stack. The extra fuel compared to the second design is needed because the Pomegranate is very heavy (nearly four times the weight of a Mk1 Lander Can).
• Four FL-TX220 and two FL-A151S tanks and a single Terrier on the transfer stage.
• Twenty FL-TX220 and one FL-A151S tanks plus one Bobcat on the launcher's core stack.
• Two Kickback SRBs.

All of these except for the Pomegranate, Kickback and Bobcat are early-game parts. Those three are on the same tech level as the Spark, which you mentioned you're already familiar with.

Edited May 17, 2021 by Fraktal