Construction

[Esoteric106]

I could really use some help building rockets that don't oscillateto the point of uncontrollability. I have the math down as well as orbital mechanics rendezvous and docking I can do manually landing I'm somewhat accurate but for the life of me I can't get a command pod and lander in orbit with out it shaking

[Snark]

6 minutes ago, Esoteric106 said:

I could really use some help building rockets that don't oscillateto the point of uncontrollability. I have the math down as well as orbital mechanics rendezvous and docking I can do manually landing I'm somewhat accurate but for the life of me I can't get a command pod and lander in orbit with out it shaking

Could you post a screenshot?  Hard to offer advice about what's wrong with your ship without seeing your ship.   

Also, just to verify, which version of KSP are you running?  (Relevant question because SAS got a whole lot better and less jitter-prone in 1.2, so if you're running an older version of KSP than 1.2, that could be part of the reason right there.)

[Esoteric106]

1.2.2 and I will in like 5 min

[Esoteric106]

so how do i up load a pic?

 

[Esoteric106]

It doesnt have a option to attach a pic? I'm confused

 

[HvP]

18 minutes ago, Esoteric106 said:

so how do i up load a pic?

 

Take a screenshot in the game using F1, then go into your KSP folder on your computer and find the folder called screenshots. You'll then need to upload your picture to a file sharing site because this forum doesn't host uploads itself. Imgur.com is the easiest and most popular site to use. Go to imgur.com and you can either create an account to upload to or just click the "New post" button at the top of the page and then drag your file into the box that pops up. You don't have to have an account there to do this.

After your image has uploaded then you can just click on it and copy the image link then paste that URL directly into your text box here in your forum post. It will show up automatically.

Edited May 9, 2017 by HvP

[HvP]

In the meantime, there's a few things that could cause excessive shaking during your launch.

- Having powerful engines with a high degree of gimbaling. The Vector engine is especially prone to this. You can right-click the engines and lower the degrees of gimbal and see if this helps.

- Uneven weight distribution. For simplicity try to keep your craft balanced around the center of thrust, with equal weight on both sides.

- Fins attached with tri-symmetry sometimes cause wobble. Four-times symmetry is more stable when the fins are used to control the pitch, yaw, etc. on a vertical rocket.

- Using large reaction wheels on light craft, or having too many reaction wheels. Right-clicking the reaction wheels will give you the option to turn down their power, and you can also change how they react to SAS control, or turn them off completely.

- Having a long rocket with a lot of flex in the joints. This may be self explanatory, but when your rocket flexes the point you are controlling from is no longer in line with your direction of travel and the SAS over compensates to bring it back in line. Solutions include but are not limited to, adding more struts, using wider parts, designing so that the stack doesn't connect heavy parts with small joints, using the autostrut function when you have advanced tweakables enabled, launching at a steep upward angle with very little turning movement, using a point of control closer to the center-of-mass of the rocket (you can right-click a docking port, for example, closer to the center of your stack that will be close to the pivot point of your rocket).

- Part clipping. Sometimes the physics of some parts can interact with other parts if they come into close contact with each other and create unpredictable jittering. Service bays and landing gear are the most prone to this. Make sure that they don't push on other parts of the ship.

- Aerodynamically unstable designs. Keep your drag at the bottom of the rocket and your mass towards the front. Otherwise it's like trying to throw a dart backwards, it will try to flip around.

Edited May 9, 2017 by HvP

[Esoteric106]

Thank you for the patience 

it oscillates in a 15-20degree somewhat circle  

[HvP]

1 hour ago, Esoteric106 said:

it oscillates in a 15-20degree somewhat circle

Ok, so you've got quite a few struts to stiffen the joints, but you've also got a lot of changes in diameter from wide to narrow and back again, plus a fairing 2/3s of the way up. Every time you change diameter it adds drag and every strut adds drag too. Fairings can be quite light for their size and so they are likely to act like a sail when you start turning. All of those add up to massive drag and/or aerodynamic torque in one way or another. I can't tell if you're using LVT-30 Reliant, or LVT-45 Swivel engines at the bottom, but I doubt you have enough steering gimbal to direct such a massive rocket.

Here's my suggestion just based on what I can see. I would replace the 3.75.m to 2.5m fuel tank adapter with another short size 3 tank. That way you can can use the large size 3 fairing base and put your reaction wheel inside of it or move it up to the 2.5 meter portion of your stack. Use a Poodle engine, Skipper Rhino there in the middle stage, or if you prefer you can use the medium engine but build a size 3 fairing around it. The goal here is to use a consistent diameter all the way up the rocket stack so that drag is reduced and the joints are solid.

For the engines on your bottom stage you will either need engines with plenty of gimbal control (such as the Vector or Mammoth), or add fins for stabilization and steering.

If you don't want to make too many changes to start with, then the easiest thing to do right now is just put big fins at the bottom. That will give you steering control and stabilize your center-of-lift.

Edited May 9, 2017 by HvP

[Esoteric106]

ok thank you. From what I gather I just don't have all the required tech to make this design work as of yet

 

[HvP]

53 minutes ago, Esoteric106 said:

ok thank you. From what I gather I just don't have all the required tech to make this design work as of yet

 

Not necessarily. You don't have to do everything I mentioned. Any improvement in the streamlining will help. As a rule of thumb it's not a great idea to have a hard transition between parts of different diameters. So if you don't have the largest fairing yet, then use your size adapters to create a more gentle transition between your 3.75 meter and 2.5 meter parts. You can also take that upper fairing and turn it upside down, attach the base to the decoupler at the top, and close the fairing around a 3.75 tank below it.

If those are Reliant engines at the bottom, then replace them with Swivels since they have gimbal steering. You may have to compensate with a larger engine in the middle though, or more of them.

And if you can't do anything else just put some big fins on the bottom and try not to turn very hard.

Edited May 9, 2017 by HvP

[Spricigo]

Just say what tech is missing and we can try to figure out the alternatives. 

While what @HvP said is solid advice and may help to solve your problem there one point that  would be better if you don't need to adopt. 

That is: changing the Reliants for Swivels. 

Let me explain, yes it may help you to control the rocket.  But is always better if you remove the problem instead of countering it. 

Gimbal in particular solve the problem by causing another,  which is your thrust not steadily at direction you want but moving around to counter the wobbling. (to make things worse the cause of wobbling may be excessive gimbal) 

Well,  we don't know what is the intended purpose of the ship,  so we can't say if reducing some stage is possible. ( Or if spreading into lateral stack,  for the matter.) But making the rocket shorter may also help.  (on a side note,  I'm curious about the destination) 

[Cpt Kerbalkrunch]

All good points. As @HvP said, different size stages (large to small and back to large), are definitely not going to help your stability. If you're set on keeping this rocket as is, you can make a couple of small changes that I think might help. I've experienced the oscillating, wobbling rocket dance myself, and it usually is a two-fold problem. The instability at the top gets your rocket wobbling, then your engine gimbals to try to compensate, but it's actually an overcompensation because your rocket is already wobbling back the other way, and now it's been made worse by trying to stop it. Your only source of gimbal is at the bottom of your rocket, right in the center. Not the best place to stop it from wobbling. The mammoth has a pretty good gimbal, and in your case it might actually be excessive, as stated by @Spricigo. I would limit the gimbal in the VAB (maybe to 50%, maybe less; you'll have to experiment a bit), and place Vernors on your top sections to keep the rocket stable. Other than that, the strut connector is one of my best friends, I use them liberally. A good launch profile certainly helps as well. As soon as you can, get your gravity turn and point that thing prograde. I've gotten much flimsier rockets into orbit. You'll just have to feel your way a bit, I think.

[HvP]

46 minutes ago, Spricigo said:

Just say what tech is missing and we can try to figure out the alternatives. 

While what @HvP said is solid advice and may help to solve your problem there one point that  would be better if you don't need to adopt. 

That is: changing the Reliants for Swivels. 

Let me explain, yes it may help you to control the rocket.  But is always better if you remove the problem instead of countering it. 

Gimbal in particular solve the problem by causing another,  which is your thrust not steadily at direction you want but moving around to counter the wobbling. (to make things worse the cause of wobbling may be excessive gimbal) 

Well,  we don't know what is the intended purpose of the ship,  so we can't say if reducing some stage is possible. ( Or if spreading into lateral stack,  for the matter.) But making the rocket shorter may also help.  (on a side note,  I'm curious about the destination) 

I agree it's a gamble. So I guess it's a gimble gamble.

Normally, oscillation would usually seem to indicate excessive oversteering. If it was just drag I would expect it to flip over pretty quickly instead of precessing as @Esoteric106 seems to be describing. Knowing what engines are on that first booster stage would help a lot. If none of them gimbal then there's almost certainly not enough steering control to turn such a big craft with just that reaction wheel unless you can do a perfect gravity turn. I also agree with @Cpt Kerbalkrunch, if there's a Mammoth down there then it probably needs to have its gimbaling reduced. If those are Swivels around it then replace them with Reliants, etc.

Other considerations include possibly having an offset weight distribution in the payload or components with high drag placed asymmetrically, but that doesn't appear to be the case from what can be seen so far.

And I want to ask you Esoteric106 something. Despite all of the struts, is the stack bending and flexing much? I'm willing to bet that payload inside that fairing is attached at the top with a 1.25m docking port which would be a particularly flimsy connection during launch. At least make sure that you have struts running inside the fairing from the inside of the base to the underside of the top.

Another more challenging alternative is to try flying without SAS on. Nailing a smooth gravity turn is one of the most elegant and underappreciated skills to master in KSP.

Edited May 10, 2017 by HvP

[Esoteric106]

so i discovered that i did not have advanced tweakables enabled, i turned that on clicked on rigid attachment  and was able to get this to minimus and had enough for a landing on the mun same trip it still is flexing more than i like but is in the realm of adjustability thank you again for your time and the bottom stage is a mainsail in the middle and reliants  (6) around it it is very satisfying to do it all manually with no mods thank you again

 

I seem to pretty good at the gravity turn( i think i suck but i consistently get craft to orbit with about 3000-3200dv and that seems to be pretty good

 

On Minimus!!!

 

of course I feel stupid right now... contract was to return the landed craft back to kerbin.... 

[HvP]

29 minutes ago, Esoteric106 said:

so i discovered that i did not have advanced tweakables enabled, i turned that on clicked on rigid attachment  and was able to get this to minimus and had enough for a landing on the mun

Yay! And rigid attachment... the one thing that none of us mentioned, haha. Glad you got it.

As for the contract, well that just gives you the opportunity to design a reentry capable lander.

[Spricigo]

Nailing a smooth gravity turn is the number one design goal of any of my launch vehicles. But since I'm a mediocre pilot further limited by keyboard controls the number two goal it's to allow it to happen with minimal player imput.  

A much underpreciated skill mindeed.  Maybe because a screwed gravity turn is much more noticeable. 

About the contract,  any part of a craft in the requested situation will suffice.

A small probe,  launched on top of a hammer (trust limited)  can do it quick and cheap. Something along these lines:https://kerbalx.com/Spricigo/SL51D 

Or in a truly kerbal way you can try to make a debris survive after crashing at Kerbin with the vessel you used.  (supposing it cannot perform a proper landing) 

 

[Esoteric106]

Spricigo, it was to land on minimus. I didn't have advanced tweakables on. It was successful with some wobble. I did a second trip with a remodeled lander that could reenter on return to kerbin docked to command module separate and return. I managed 750 plus science on the trip

Also I figured out the oscillation it was happening after booster separation and throttle up from 3/4 I didn't throttle up and no shaking. Tomorrow I'm going to redesign with the added tech because I was happy with the lifting power of the rocket (named it the beast) it will be good for raising staton parts when I get there . That and work on accurate landings. I did manage to change landing spots beause initial point was too sloped any tips for landing accurate on non atmospheric bodies?

Sorry rambled a bit there

Edited May 10, 2017 by Esoteric106

[bewing]

On minmus it's easy. Go into "Surface" mode. Burn retrograde until your velocity is 0. Look straight down and that's the spot you will land on.

 

[swjr-swis]

6 hours ago, HvP said:

So I guess it's a gimble gamble.

Points for the gimble gamble.  Thanks for the chuckle.

[Spricigo]

If you need to keep throttle at 75% to avoid wobbling you should consider to reduce/change the engines to cut the extra,  unused power. This will, in turn,  reduce weight  and  may allow  reduce fuel amount.  Keep goling until  there is nothing left to reduce 

[Esoteric106]

so after the minimus return with that rocket and the science return I was able to make this.... wayyy more stable thank you all for your help

 

 

[HvP]

2 hours ago, Esoteric106 said:

so after the minimus return with that rocket and the science return I was able to make this.... wayyy more stable thank you all for your help

Now that's a rocket that means business. I like it. I'm glad we could help.

As for pinpoint landings, it's really going to take practice. If you are doing orbital rendezvous then I'm guessing you are familiar with how to maneuver towards a target in space. You may have noticed that burning near your prograde will pull the prograde marker closer to the direction you are pointing. And the opposite is true if you are burning retrograde. Burning close to the retrograde marker will push it away from your line of travel. Aiming at the azimuth (straight up) will let you "hover" to extend your glide. You can use that technique to adjust your landing site on the way down.

I was going to give a lengthy explanation, but I realized that I simply cannot do better than Scott Manley's excellent tutorial on that topic (you can ignore the math in the middle.)

 

Edited May 10, 2017 by HvP

[Spricigo]

11 hours ago, bewing said:

On minmus it's easy. Go into "Surface" mode. Burn retrograde until your velocity is 0. Look straight down and that's the spot you will land on.

 

which is not the most (fuel)efficient way, but since we are talking about Minmus(is near and has low gravity) that is hardly a serious concern.

Later in the game 'proper' landing procedures may serve you better (inverse gravity turn, constant descent trajectory, suicide burn are some concepts to look for). Also you may adopt some mods to help you with this or any part of the game you find too difficult or simple uninteresting.

 

 

 

 

 

Or simple enjoy doing it in 'unproper' 'inefficient' way because what really matter is only how much fun you have.

 

[Spricigo]

37 minutes ago, HvP said:

I was going to give a lengthy explanation, but I realized that I simply cannot do better than Scott Manley's excellent tutorial on that topic (you can ignore the math in the middle.)

His tutorials are quite nice indeed.

Something to consider when looking for KSP tutorials: the game changed a lot through the versions, something that used to work in early iteration may not work anymore so be cautious with old content. But there is some awesome stuff like this  constant descent landing:

Spoiler

 

No, its not easy like he make look to be.