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Learning how to land on Tylo and do it well.


Klapaucius
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I've spent a lot of time with planes, but I am pretty poor at efficient rocketry, both in building and in flying.  I'm trying to do a bit of upskilling in that department by breaking things down into specific areas. Right now, I am using Mission Builder to put myself in orbit around Tylo.  Just to make it interesting, I am trying to land a Mk1-3 Command pod attached to a mobile processing unit and eventually put it back into orbit. Once I manage that, I can work on setting up a docking scenario to get them home.

I'm close on the landing bit. The 3 in the capsule managed to survive the landing, but everything and everyone else was destroyed.

My engineering needs a lot of tweaking, but I think I can work that out if I have a better idea of what I am actually striving for in my flight profile. I've got over 6000 m/s delta V, which if I read correctly, should be more than enough.  It would be helpful to know what is the ideal landing profile from orbit on an airless moon.  I figure if I can nail Tylo, I can then be a lot better at all the other, easier bodies, such as the Mun and Minmus, where my landings are successful but really inefficient. 

 

So:

1. Is there an ideal orbit height to start at? Obviously, if I am at 8,000 meters I am never going to get my orbital velocity down before smashing into the surface. 

2. Am I right in assuming a nice continuous arc describing 90 degrees that somehow gets me exactly vertical just a few tens of meters above the surface is the best profile?  (as opposed to losing all my orbital speed at altitude and then just fighting gravity on a vertical descent?

3. Is it better to descend in multiple stages or just go with one ?  I'm planning on leaving the final descent stage behind either way.

4. Favorite engines for this kind of thing?  With the weight on the large capsule and processing lab and of course fuel, I need something fairly powerful. I tried a mainsail and then tried multiple smaller engines. Both times I ran out of fuel before landing, and I could not shed the rest of the speed.  I have a sneaking suspicion that I am being quite wasteful.

 

Edited by Klapaucius
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1) I like to start "low", from 15k to 20k meters.

2-3-4) My fav is the vector, it is easier to obtain a good twr with a good form factor for a lander; I usually try to sap all the horizontal speed as fast as possible - since I am starting low then I am left with an affordable amount of altitude to go.

*) My Tylo landers are usually made with a lot of staging and asparagus tanks - it does not seem to me this is your case

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The most efficient way to take off from an airless body is to give it just enough vertical velocity to clear the terrain, and then burn horizontal all the way.  Therefore the most efficient way to land is to do the opposite, but it can be tricky to get right.

The Apollo landers went for a constant rate of descent, pitching the lander up more a you slow, this works great for low TWR landers where your burn time is pretty long.

Sadly it looks like @GoSlash27 post on his alternative Reverse Gravity Turn has sadly been borked by Photobucket

In general though set a low orbit, but then drop the Pe above your target area, rather than trying to drop from a higher circular orbit.  If you get the Pe right you can burn retrograde all the way to the ground.  I personally prefer a constant rate of descent deceleration and come to a horizontal stop a decent distance above the target, not as efficient but it does prevent lithobraking incidents.

 

11 minutes ago, Klapaucius said:

Can you explain what you mean by the vector?

The Vector engine, very high TWR

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Veteran Tylonaut here. I even did a fully recoverable crewed mission there once.

I've tried lots of different approaches, most of which have ended up in a crater. Ultimately it's pretty simple: build your craft with the stats of a Kerbin SSTO, except forget aerodynamics (obviously) and make it rugged enough to survive about 12 m/s worth of lithobraking, and practice.

The ideal trajectory is exactly the same as an ideal ascent trajectory, except in the other direction. I've ended up starting my descent at about 20-25 km. Much higher than that means you're wasting fuel; much lower means that you'll need unnecessarily heavy engines. 

The ideal is one continuous burn at maximum thrust until you're within a few metres of the surface, then lower yourself onto it. The reality is always a bit different and depends on your craft design. Practice with simulated runs to find the trajectory that works for you: if you're switching off your engines a lot, you started too high; if you ended up in a crater, you started too low. It's easy to screw up at the very end by trying to gently kiss the ground like on a Mun landing; build your lander to survive a 10-12 m/s landing and you're much less likely to run out of fuel at the end.

My recommendation for a first-time Tylonaut is to start with a light payload, one way only. For example, try dropping a rover there on a one-way mission. I did it with this one in my latest career; it's powered by four Sparks:

rDeMVkG.png

I almost screwed up the final landing BTW -- ran out of fuel a few metres above the surface. The Sparks took the impact, the fuel tanks were destroyed, but the lander itself was intact. And I actually meant it that way -- that's why I used those wedge-shaped tanks, just for this eventuality, although wheels are really good at taking bumps too.

Once you've got that down, you know how it works and can scale it up however much you like. 

So, basically, give it the stats of a Kerbin SSTO except rugged enough to survive hitting the surface at 12 m/s, and practice. Good luck!

 

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56 minutes ago, Klapaucius said:

I am trying to land a Mk1-3 Command pod attached to a mobile processing unit and eventually put it back into orbit.

IME the key for landing on Tylo intact is very high TWR, like 60m/s^2, and very light dry mass, so that the legs can absorb the impact. So use command seats instead, and land everything else separately.

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1 hour ago, Signo said:

2-3-4) My fav is the vector, it is easier to obtain a good twr with a good form factor for a lander; I usually try to sap all the horizontal speed as fast as possible - since I am starting low then I am left with an affordable amount of altitude to go.

Can you explain what you mean by the vector?

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2 hours ago, Signo said:

2-3-4) My fav is the vector, it is easier to obtain a good twr with a good form factor for a lander; I usually try to sap all the horizontal speed as fast as possible - since I am starting low then I am left with an affordable amount of altitude to go.

 

 

48 minutes ago, RizzoTheRat said:

 

The Vector engine, very high TWR

Sorry, was thinking Vectors as in geometry of descent, not engine names :confused:

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On 1/30/2020 at 6:05 AM, Klapaucius said:

Sorry, was thinking Vectors as in geometry of descent, not engine names :confused:

Indeed.  I'll have some very strong words for Squad if they introduce an engine called the 'Delta-V'.

To reply to your original question, Slashy's method has a lot to be said for it, though I don't think anyone else has the pictures.  He did do a good job of explaining what he was doing, so you ought to be able to duplicate the technique from only the written description.

If you're feeling adventurous, then you can take screenshots as you go and upload a new iteration!

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On 1/30/2020 at 9:26 AM, Klapaucius said:

2. Am I right in assuming a nice continuous arc describing 90 degrees that somehow gets me exactly vertical just a few tens of meters above the surface is the best profile?  (as opposed to losing all my orbital speed at altitude and then just fighting gravity on a vertical descent?

Yes.

The problem with efficient landings is that you will have a lot of sideways velocity until nearly the last moment. Then it suddenly cancels out, you're motionless a little above the surface, and need to do a few meters worth of vertical descent, carefully.

It now becomes a game of

  1. how maneuverable is your lander -- at the end you may find that you'd want to pitch from near-horizontal to near-vertical in an instant. It will matter if it takes you two seconds to do so, or ten.
  2. how quick are your reactions, and how finely can you control the throttle. It's easier if you have a relatively low TWR at that point, though you want a high TWR until just one moment earlier.

Being a bit slow to react (either your reactions, or because the lander takes time) will require more fuel. A lot more fuel, because this is Tylo. There's no helping it.

Staying within 100m/s of the theoretical limits will require god-like skill (a.k.a. autopilot or several rounds of saveloading). Cutting it even thinner than that is bound to fail.

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There's a sick video on youtube I'd watch if I were you presented by the youtuber "Kosmo-not" titled "How to land". (I'd link it to you but I can't because of the fact I'm at work rn.) Anyway if you employ that landing method and have good enough TWR and fuel to lift off once you land then you should be fine. :retrograde: Good Luck!

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@James M

 

Thanks for that. I've been intuiting that a bit, but that was just so skillfully done. I always end up trying to bleed of the speed just before landing, overdoing it and then I start ascending again.  Usually I do this a few times before I finally land, often too hard and I bounce and fall over.  That was just perfect.

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You can look at my Tylo lander in my Jool-5 mission (link in signature). It's a staged lander both on descent and ascent, with only a small fraction of the original lander returning to the mothership. Think of it as sort of a small, weird-looking Saturn V.

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