Moon/Planet Base - Stuck

[JordanP60]

I am trying to put a base on the Mun. I downloaded a mods that gives you parts to build a base. I built the stage with the base but no matter what I do the rocker will always spin out of control about 10-15 seconds into the launch, the first 15 seconds are usually 100% fine. Adding wings, fins, RCS boosters, and modules to help stabilise the rocket all make literally 0 difference to the level of tilting that starts to happen. The base sticks out a bit but any base I would launch would have to be, and the things I've tried should counteract the tilting but it doesn't work. Is there any way to make the ship stable? 

[AHHans]

Hi @JordanP60, Welcome to the Forums.

You probably have too much drag at the top of your rocket. Do you have the base inside a fairing? A somewhat streamlined fairing has a lot less drag than a base with lots of bits and pieces poking into the airstream.

It would also be useful if you could post a picture of the rocket.

[JordanP60]

The Rocket it a bit oddly shaped atm, I've been testing the staging etc on Kerbin first. It's probably a badly designed moon base, and the rocket below is just what I made to test it on Kerbin. I'm not doing it for any other reason than to have a structure on the moon. Am I supposed to have the entire base with rockets lowering it down? And if so how can I fit enough fuel and engines below it so the landing gear will still reach.

My Ship: https://prnt.sc/r0awug

[AHHans]

47 minutes ago, JordanP60 said:

The Rocket it a bit oddly shaped atm, [...]

Well, I'm not surprised that a rocket like that is going to flip over. All these fiddly bits on the Mun-base and the open (not covered) tops of the rocket-stacks will create a lot of drag. Put the base inside a fairing, and any up facing attachment node outside the fairing should have something that closes it up - so another piece of the same diameter, an adapter piece to a different size, or a nose-cone. (Is there something with a smaller diameter between the decoupler and the uppermost large fuel tank?)

With a structure like this I would also add a few struts from the fairing-base to the arms of the Mun-base to keep it from wobbling around.

55 minutes ago, JordanP60 said:

Am I supposed to have the entire base with rockets lowering it down?

Depends on in how many pieces you want to base to arrive on the Mun.

O.K. seriously: yes, if you want to safely land the base on the Mun, then you need to slow it down with rockets. But you don't necessarily need to put the rockets and fuel tanks below the base, you can also attach them at the side with, e.g., radial decouplers. (And if you have some fuel left after landing, then you can start the engines at low thrust (not enough to lift the base) before decoupling and have the rockets fly away.)

[JordanP60]

I added a fairing to the top, it looks a bit silly with it on and only makes it slightly less of a problem. It also adds a new problem of it rattling so hard the rockets rip apart (even at low acceleration and m/s) . I've put cones at the top and at the first stage fuel tanks but it makes barely any difference. I don't understand how a few bits sticking out would cause a problem that can't be solved with the SAS modules, fins and RCS rockets. I don't know what else I can actually fit on the base that would make any difference, it just seems to turn slowly without any amount of counterforce making any difference, which it should. It seems like that only way to get a base off Kerbin is if it was laughably small.

Edited February 10, 2020 by JordanP60

[AHHans]

1 hour ago, JordanP60 said:

It seems like that only way to get a base off Kerbin is if it was laughably small.

Well, considering craft like this(*) got into orbit, that's not actually true.

1 hour ago, JordanP60 said:

I added a fairing to the top, it looks a bit silly with it on and only makes it slightly less of a problem.

Well, if it looks silly, then it probably doesn't work well. While even a badly designed fairing usually reduces drag, a somewhat streamlined fairing works much better.

But if you want to "brute-force" the problem with fins, reaction wheels and RCS thrusters, the you should consider: fins only help if they are behind the CoM and work better the farther they are away from the CoM, RCS thruster have rather little force at sea level and also create more torque farther away from the CoM.

(*) I didn't put it into a fairing, because no fairing is big enough...

[JordanP60]

I'm aware I'm probably doing it wrong, I just couldn't figure out a way to counteract it with what I know is in the game. Most of my ship is 100% symmetrical now, there are fins at many areas, as well as cones with a fairing I have changed the size, shape and model of several times and only minor differences are happening and the rocket tilts slowly without anything stopping it even a bit. With the fairing on it there's nothing not covered with it, the only bit that isn't symmetrical is the details of the station. It feels like the small imperfections in the ship should be easier to counteract so I don't know what else to add.

Update: https://prnt.sc/r0gura

This is the ship rn, it doesn't matter how fast I go, how much I try to manually steer its always fine for about 15 seconds, then around 25/30k it begins to slowly tilt like clockwork.

Edited February 11, 2020 by JordanP60

[AHHans]

8 hours ago, JordanP60 said:

Update: https://prnt.sc/r0gura

Well, the single biggest issue is that the payload (your Mun base) is supposed to go inside the fairing, or in other words: the fairing is supposed to go around the payload not on top of the payload. Place the fairing base-plate under the decoupler that decouples your payload from the rocket (so that the fairing base doesn't stick to the payload after decoupling) and then build up the fairing around your payload.

A smaller issue is that I think that many of your fins may be above the center-of-mass (CoM) and thus increase the rocket's tendency to flip. If you want to use fins to keep the rocket straight, then put them at the bottom of the rocket. Use the display of the CoM in the VAB (or SPH) to check where the CoM is. Also change the fuel levels in the tanks, to see how the CoM moves when fuel is burned. (The the CoM indicator in the VAB/SPH is good, but the center-of-lift indicator (or "Aerodynamic Overlay") is a bit misleading: it only shows the center of "lift" but there is also "drag" working on your craft which can mess up the aerodynamic stability of your craft.)

I don't know how you launch, but changing that may also help: a real gravity turn means that you tilt the rocket rather early while you are still slow, and then follow the prograde vector (in surface mode) for most of the launch. That way the "wind" is coming from straight on just along the axis of the rocket, thus minimizing the torque it applies to your rocket. Thus you can usually keep it steady with the engine gimbaling alone. I usually first go straight up, at 30 m/s I tilt over ca. 5 - 10 degrees, at ca. 150 m/s I lock SAS to prograde, and keep that until I'm in orbit. How much I tilt over exactly and when exactly I lock to prograde depends on the rocket. With at high drag / low TWR rocket I tilt over less and/or lock to prograde later (e.g. 5 deg. and 200 m/s), and with a low drag / high TWR rocket I tilt over more and lock to prograde earlier (e.g. 10 deg. and 120 m/s).

Edited February 11, 2020 by AHHans
fixed typo

[RizzoTheRat]

You've got a lot of drag at the front even with that faring, and quite a few draggy parts behind it too.  Where is the centre of mass compared to the fins?

As you burn the fuel in the side boosters your centre of mass will move forwards, which is probably why it's fine for the first 15 seconds or so.  The idea with fins is to get the side area behind the centre of mass bigger than in front, and the more drag you have up front the worse it is as well.

I would try and enclose more of the front end in faring, everything in front of that central fuel tank if you can, and get some fins, or even wings right at the back to get as much side area back there as you can.

 

 

ETA:  Ninja'd by Hans, agree with what he said, although I'd also add you can also reduce the drag, at the cost of fuel efficiency, by launching at a lower TWR, or throttling back a bit, until you're above 10-15km and the air starts to thin a bit,

Edited February 11, 2020 by RizzoTheRat

[Zhetaan]

23 hours ago, JordanP60 said:

I've put cones at the top and at the first stage fuel tanks but it makes barely any difference. I don't understand how a few bits sticking out would cause a problem that can't be solved with the SAS modules, fins and RCS rockets.

Good work on the nose cones.  They will help you a lot in the end.

However, part of the issue with your fins is in your choice of fin.  You appear to have two types:  AV-R8 winglets on your Twin Boar boosters, and Delta-Deluxe winglets high up on the main body of the rocket.  I strongly recommend getting rid of the Delta-Deluxe winglets.  Don't replace them with anything; you don't need fin control that high up on the rocket, and it won't be effective anyway, because they're too close to the centre of mass to have much leverage and be effective.  Also, I suggest moving the AV-R8 fins farther down the Twin Boar boosters to be closer to the engine.  You may need to include fins on the main stack; put them on at the base, between the Twin Boars.

Another possibility for you to try is to add the fairing base under the upper tank (the one with the most Delta-Deluxe winglets, which you'd definitely need to remove at that point) rather than the decoupler immediately underneath the payload.  You'd need to ensure that you use a base that is the same size as the tank (I assume that those are 5-metre tanks), but the idea is that a long, tapered fairing is better aerodynamically than a short one that tries to be flat on the bottom.

Good luck!

[NadenH]

I'm not sure how KSP figures drag. But I would try and do the following.

1) I think it is the fin placement people are mentioning. Although I mention it first it would be the last thing to do in the VAB. Turn on the CoM and CoL indicators. What I typically find is there is a center of mass only until I place a fin somewhere. The goal is to get the CoL (blue dot) below the CoM (yellow dot). Once that's done put your stages in proper order. Save. Now remove the parts that will be deattached from first stage. Is the blue dot still below the yellow dot? Fix. Repeat that for the next stages until you think you've reach a spot with minimal air resistance. Put your pieces back on or load you saved craft and place fins back on where needed.

2) I'm not sure what you have available in the tech tree but you might consider running over to Minmus and grab a few nodes by doing some science farming. The goal would be to unlock nodes that would allow you to two up the base in two separate pieces so it would better fit in the fairing. Some RCS, docking port and some probe control units will do.... or you can make a really cheap "worker bee" with mono and RCS that you can burn up in kerbin atmo so the base won't have the extra weight... although it would be kinda small.