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miki1234

Mun - Orbit and landing

Question

Hi guys,

 

I am trying to reach the mun and back. So far no problem getting there just getting back was a challange not enough fuel.

This is my design.. What do you think?

Plus, The rocket wobbles / shakes a lot whilst in atmosphere, How can deal with this? 

 

4uQat5s.png

 

What

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Craft wobble when they have lots of weak joints.  Autostrut can help but the simple option is - get bigger fuel tanks so you don't need to have lots of small ones.

Check the deltaV map (https://i.imgur.com/gBoLsSt.png) for how much energy you need to get to Mun and back.  Mun is at the bottom-left and requires 3,400m/s to Kerbin orbit, plus 860m/s to transfer to Mun, 310m/s to capture into Mun orbit and 580m/s to land.  Conveniently, that's calculated for you under where it says 'Mun' as 5,150m/s.  Your ship has 4,825m/s according to the KER display, so you can't even get there if you want to land; it's ok if you just want to get into Mun orbit though.

Coming back is more or less the same as getting there, so 5,150 x 2 = 10,300m/s but you can make a few shortcuts.  Most importantly and obviously you don't need to spend that 3,400m/s to launch from Kerbin, so no more than 6,900m/s is necessary.  In practice you don't need to re-establish Kerbin orbit either and can save a bit there but it would be good practice to stick with that total until you know what you're doing.  Ultimately then, you need a ship with another 2,075m/s for a return Mun landing.  If you just want to orbit the total needed is still 5,740m/s (because the biggest non-atmosphere part is the 860m/s Kerbin-Mun-Kerbin transfer), still leaving you nearly 1km/s to find.

Edit: Your basic design is fine, although I'm not sure exactly what you have apart from the science juniour.  If not landing, just make sure your top (space-faring) stage has 2,540m/s for the return orbit-orbit-orbit trip (if you're making Kerbin orbit on the way back this gives the 'safest' return, with 200m/s left for deorbit).  Then, obviously, your launch stage(s) need 3,400m/s to get the whole lot up there.  Just with the science juniour I made that an FL 400 fuel tank plus FL 200 (or 3 x 200) and a terrier engine on top, 2 x FL 800 (or 4 x 400, or 8 x 200), a reliant engine and the two thumper SRBs for launch.  Help yourself by removing all the monopropellant from the command pod and thrust-limiting the SRBs to ~75% (1.5 TWR at launch - you can comfortably go down to 1.3 if you prefer)

Edited by Pecan

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Couple of thoughts...

You don't have enough dV there. I can see just two stages plus the SRBs. Think of adding another.

You don't need a lot of dV to get back from the Mun so make that a fairly small lander. Dump the experiments before leaving the Mun to save mass. 

Don't bother with making orbit at Kerbin on the way back, go straight for re-entry. It'll work if all you re-enter is the capsule.  

What are all those parts below the materials experiment?

You can save some dV getting to Kerbin orbit by sticking all the bits and pieces in a service bay. 

A single small chute on the top instead of the nosecone will be enough back at Kerbin. 

If you have unlocked the small solar panels then stick 3-4 on the top because the Mun and back is a long way on batteries. 

If you can fly the craft with just a scientist then you will only need to take one of each experiment because the scientist can reset them. 

 

Edited by Foxster

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Also just wanted to say that those AV-8 fins at the bottom of your ship are each twice the mass of a basic fin plus elevon 4.  With 4 fins that's only a difference of 0.2t but it all adds up!

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Hi i tried to implement some of the changes. Still not enough dv.

The main issue is that the rocket becomes super long! Even if i want to add another stage, won't that hurt it's stability? 

 

Here is my latest design

CaAWsK2.png

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Drop a science jr and a goo. At this tech level it's too ambitious. Goos weigh as much as parachutes so offset your one chute with your one goo.

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1 hour ago, miki1234 said:

The main issue is that the rocket becomes super long! Even if i want to add another stage, won't that hurt it's stability?

Adding more SRBs to the sides won't make it longer or harm stability. Another thing you can do is put hammers underneath your thumpers, that stage first. You can get rid of the drogue chutes on your SRBs -- unfortunately in the current game they won't help you any. If you put your SRBs in 4-way symmetry, then you can attach the steering fins to your SRBs instead of your central core. That way, the fins will stage off once you are above the thick part of the atmosphere and reduce your total weight.

 

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Since your TWR is more than high enough to start with, you can easily add small fuel tanks to the top of those SRBs. Either use advanced tweakables or use fuel lines to feed them into the core stack. You might need to offset the SRBs down a bit on their decouplers so that they peel off cleanly without hitting the core stack.

If you do that, you can then either (a) reduce the SRBs to two, decrease their thrust a touch, and run the main engine from liftoff. That should already net you a significant dV gain; or (b) place the SRBs in two pairs. Set one pair to about 90% thrust and the other pair to about 60%. Drop the first pair first. Put small fuel tanks on both pairs, making sure the first pair drains first. That lets you run the main engine for far longer before you even touch the fuel in the core stack, and should still keep you at a TWR of at least 2 at all times.

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Here's a craft that you can build with your tech that has a good dV safety margin (~1000dV) built in: https://www.dropbox.com/s/lf24nc56wikbo5e/Mun 1.craft?dl=0

zdQgFCz.png

I'd get the first level electrical next so that you have solar panels and then get the rest of the experiments then the nuke engines. By that point you can go anywhere. 

Edited by Foxster

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1 hour ago, Foxster said:

Here's a craft that you can build with your tech that has a good dV safety margin

Heh. ^That's total overkill. :)

 

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1 hour ago, bewing said:

Heh. ^That's total overkill. :)

 

True. Gets the job done though and it's solid with a good dV margin ;)

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Dang OP has a decent amount of tech happening, the only thing I'd want to be safe are solar panels. You could get to the Mun easily with a decent lander that could hit 2-3 biomes with your current tech, I won't give a design but I'll say don't be afraid to use 2.5m parts. Again though, you may have to micromanage your batteries because of the lack of a solar panel.

Edited by Waxing_Kibbous

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This is my design so far - I am able to reach a decent mun orbit. Not to land.

Another problem - re entry is too hot, i get burned out with no fuel.

 

Any thoughts? 

 

9k3IqcC.png

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49 minutes ago, miki1234 said:

re entry is too hot

Then you're simply aiming too low in the atmosphere.  Aiming at a Kerbin Pe of 50km you should pass right through several-to-many times as your Ap is reduced from Mun to Kerbin by the minimal aerobraking.  The lower you aim the quicker you'll brake but the more you'll heat up.

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So this is my new design.. Way more steerable.. Now i even got to trying to land, while gauges showed me an alt of 4K I suddenly crash landed! It was on the dark side so a bit hard to see..

 

Tx2qAky.png

 

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The altimeter shows you the altitude above "sea level" on every celestial body, not "altitude above terrain". On CBs that have no ocean, the sea level altitude is just an arbitrary reference.

There are several ways in stock to approximately calculate the altitude of the terrain (using Kerbnet, IVA mode, or a maneuver node), or you can use a mod. But yes, in general it can be quite difficult to land in the dark. You can attach spotlights to your craft, or you can increase the brightness of your screen, or you can just always try to land when you have sunlight.

 

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Welcome to the club! You'll get it in no time.

As bewing said, the altitude indicator only makes an approximate altitude based on the world's averaged sea level. This is why new folks are encouraged to try landing on Mimmus first because the many flats there are calibrated to the altimeter and so what you see in the indicator is correct. But the Mun and many other worlds will completely crash you with differentiations in altitude in the 100s or even 1000s of meters.

Better to land in the daylight. That means you'll likely want the Mun starting its orbit behind Kerbin. The Mun is tidally locked so one side always faces it, but that means your spacecraft won't be in communication if on the far side with a simple HG antenna (assuming you use or need Commnet in the difficulty settings) if you land sun-side while on the far side of the Mun. When Kerbin is in front of the Mun, the dayside of the Mun faces the planet.

Daylight should give you a critical hint to altitude: Your shadow. Your final landing speed should be 6 m/s or less or parts may get damaged. Try to burn under 10,000 or less to save fuel. Odds are you're using the Terrier engine. On light vehicles, it'll slow you down very fast, so wait as long as possible (about 3,000 m) before kicking it in. Blow off 300-400 m/s (you'll likely be coming in around 500 m/s) before 5,000 m, then let yourself drop between 50-100 m/s to 3,000 ft before slowing to 20-30 to about 100 m before the final slowdown.

If you have a little time, take a look at Scott Manley's Career Mode Tutorial videos on YouTube, specifically this one. It'll fill in your gaps and give you more tips. Hell...watch this entire tutorial series. It's slightly dated but still very relevant.

 

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3 hours ago, miki1234 said:

So this is my new design.. Way more steerable.. Now i even got to trying to land, while gauges showed me an alt of 4K I suddenly crash landed! It was on the dark side so a bit hard to see..

Hehehe - to put it another way ... you're already achieving a lot more than you thought you ever could when you asked your first questions :-)

Corollary:- you've accidentally learnt and are using a lot more real space-flight physics & maths than you imagined.  Cool, isn't it?

Applause!

Edited by Pecan

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You don't have enough margin safety (kinda), try adding more tanks in the lander stage.

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I almost never bring any Science Jr. modules, they're so damn heavy, also definitely replace that nose cone with a top mounted parachute and then ditch the radially mounted parachutes. It's not really necessary to bring the science modules back down on Kerbin, you can retrieve all the data on EVA, unless your Kerbals can't do EVA yet. You should be able to more or less copy this design with your current tech tree:

Ok8b1rx.jpg

It has close to 8 km/s delta-v and will easily make it to the surface of the mun and back to Kerbin with a soft landing. You can add a Science Jr. to the design and still have decent delta-v margin. I agree with the others, you're clearly learning as you go, you'll fly huge nuclear powered beasts to Jool and back in no time. :wink:

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10 hours ago, GRS said:

You don't have enough margin safety (kinda), try adding more tanks in the lander stage.

I disagree, 2100 is LOTS for a Mun landing and return from orbit.  Figure 600 + generous margin down, 600 + margin back up (orbit's not nearly as hard to do safely), and 270 + margin back to Kerbin.  Leaving a 10% margin for the easier maneuvers, that's around 500 extra for landing.  The main problem I have with it is how tall it is...

edit: I took a good look at the other stages and see your point, the craft needs another stage or so to at least set up a Mun flyby before the lander can afford to do any work.

Edited by Kryxal

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