CoM, CoL, FAR, and flipping

[DrPastah]

I am using the mod FAR, and I've been struggling a lot with having a stable rocket that doesn't flip easily. I always try to aim to have the Center of Lift below the Center of Mass, but I find I really have to work hard to get that to happen. It's either use wings as fins, use many fins, or put my fuel tanks as high as it can be while adding empty-tanks/structural-fuselages at the bottom and putting the rocket and fins on there. All of these either look absurd or just sound absurd. Here's an example where I had to do a contract by making a station with 4,000 liquid and that can habit 9 Kerbins. I had to use an empty structural-fuselage at the bottom.

How can I achieve stable rockets without needing to above and beyond to fix the CoL and CoM balance? I feel like I'm using dumb tricks as a crutch to get my rockets to work. A lot of the time the stages of the rockets get much lighter per a stage, thus the CoM becomes very low. I also noticed that sometimes the fairings actually raise the CoL, I use this: forum.kerbalspaceprogram.com/index.php?/topic/161733-13-1

Any advice for designing rockets that have a good CoM and CoL? It also seems like asparagus staging is a no-no with this aspect.

Edited December 7, 2017 by DrPastah

[Aegolius13]

I am not a FAR user, however: technically it is not the COL you care about.  It's the center of drag (or aerodynamic center).  COL is used as a proxy because drag and lift kinda, sorta, go hand in hand, but this is more often on things like planes.  I don't know how applicable it is to rockets, especially in FAR.

Per the screenshot, your rocket looks pretty streamlined to me: no form factor changes, fairing around the draggy stuff, etc.  However, it's very long.  This could lead to wobble, which can lead to all kinds of instability. Autostruts or rigid attachment might help if you're wobbling.  

Your COM is also going to be pretty low while those Kickbacks are still attached.  Maybe you could remove their nosecones to improve drag at the rear?  You could also look at building your rocket out more instead of up - using come radial LFO boosters and a smaller/shorter core stage.  

What engine is underneath the core stage?  It's possible you're not getting enough control authority from vectored thrust, though this seems unlikely.  Most gimballed engines supply enough, and you have all those fins too.

 

1 hour ago, DrPastah said:

A lot of the time the stages of the rockets get much lighter per a stage, thus the CoM becomes very low.

I'm not sure I follow this.  As you burn fuel within a stage, and as you drop stages, the COM should move up - because the stuff at the bottom of the rocket is going away, and the payload at the top is staying the same.  If you're not already, you may want to try setting fuel priority amongst your tanks, so the bottom ones burn out first. You could also take this a step further, and instead of, say, one orange tank, you could use a set of smaller tanks and set individual fuel priorities for each.  

 

EDIT: one other thought: according to KER, your thrust gets relatively high as your SRBS burn down their fuel.  2.49 TWR sounds fine in stock, but might be too much in FAR .  You might try staggering their thrust to smooth out your curve.  I.e., half of them start at 100% and drop first, the other half start at 70% and drop off later.  You can also throttle down your LFO engine a bit to counteract the TWR increase.  

Edited December 7, 2017 by Aegolius13

[Tyko]

You can also use fuel flow prioritization to keep the COM forward. Usually all tanks in a stage drain evenly, but by adjusting priority you can change that.

Use multiple fuel tanks in each stage (which it looks like you're doing). Then for each stage right click on the the tanks and adjust the fuel priority so the bottom tanks are the highest priority. This will cause that tank to drain first and then drain each tank progressively up the stack always keeping most of the weight forward

[Aegolius13]

Also:  can you describe your flight path?  Any rocket is going to have problems if you point it very far off of prograde.

[Snark]

On 12/6/2017 at 7:08 PM, DrPastah said:

Any advice for designing rockets that have a good CoM and CoL?

CoL is completely irrelevant.  What matters is where the "center of drag" (more properly, "center of dynamic pressure") is, relative to the CoM.  Unfortunately there's no visual indicator for that, you basically have to eyeball it.  However, roughly speaking, for a ship that's basically just a big cylinder, that will be approximately in the geometric center, generally.

(Don't feel bad.  Lots of people seem to think that CoL has something to do with stability.  It's true that for an airplane, you want to have the CoL behind the CoM, and if you don't, you'll have problems.  But while it may be necessary, it is not even vaguely sufficient, so you really really shouldn't rely on it.)

As long as you bear in mind that CoL is irrelevant, the problem with your rocket flipping becomes immediately and blatantly obvious from your screenshot.  Look at your CoM.

Your CoM is in the bottom of your rocket.

That just won't fly.  All craft-- airplanes, rockets-- want to have the CoM in the front.  Trying to fly a rocket that has the CoM way in the back is like trying to throw a badminton birdie with the feathered end forward:  it just doesn't work.

You need to move the CoM way up.

Here are some things you can do:

• Make sure your Kickback boosters are located as low down on your rocket as possible.  Mount the radial decouplers as low on the central stack as you possibly can, and mount the Kickbacks as low on the decouplers as you can.
• Then put steerable fins on the Kickbacks, mounted as low on them as you possibly can.  Why so low?  Because you want to get the fins as far behind the CoM as you can.  The control authority of the fins is directly proportional to their lever arm, i.e. how far behind the CoM are they.  If you can put them twice as far behind it, they become twice as effective.  AV-R8 winglets work pretty well for this purpose.  If, after following all the advice here, you're still having problems, you can move to something bigger-- e.g. large fixed wings with big ailerons on them, or something like that.
• As @Tyko suggests above:  make sure you've properly set the fuel prioritization on all your fuel tanks so that they drain from the bottom up.  This will cause your CoM to rise rapidly as you burn fuel, which will in turn help your stability.
• Those Hitchhiker containers are very lightweight (relative to their height) compared with the fuel tanks they're sitting on, assuming that those fuel tanks are full.  So, rearrange.  Put the lightweight Hitchhikers under the heavy fuel tanks, not on top of them.
• Looks like you're taking off using SRBs alone.  Consider activating your center-stack LFO engine right off the pad.  If it's gimbaled, the engine gimbal will really help with the steering and stability.  (You can compensate for the loss of LFO fuel by reducing your Kickbacks from 5 to 4, and putting some 1.25m LFO tanks sitting on top of the Kickbacks.  Make sure the radial decouplers have crossfeed enabled.  That way, your center-core engine will be draining those tanks first and not consuming any of your center stack's fuel.)

 

[Aegolius13]

8 hours ago, Snark said:

(You can compensate for the loss of LFO fuel by reducing your Kickbacks from 5 to 4,

Wait, FIVE Kickbacks?  Can you even do 5 way symmetry in this game?  If the thrust from these is not aligned,  that will cause torque - all the more so if there is no gimballed engine to compensate. 

(Insert obligatory Atlas V reference here)

[Snark]

14 minutes ago, Aegolius13 said:

Wait, FIVE Kickbacks?  Can you even do 5 way symmetry in this game?

Not in stock, no, but it's possible in a modded game.

It's worth noting that odd-numbered symmetry configurations can also have a tendency towards aerodynamic instability, depending on configuration.  But that's not the problem in the current case.  The current problem is simply that the craft has the CoM way in the back.

[DrPastah]

10 hours ago, Snark said:

• As @Tyko suggests above:  make sure you've properly set the fuel prioritization on all your fuel tanks so that they drain from the bottom up.  This will cause your CoM to rise rapidly as you burn fuel, which will in turn help your stability.

How do I set fuel prioritization for direction of tanks being emptied? I'm not seeing it in the VAB.

EDIT: Nevermind, I had to enable advance tweakables.

10 hours ago, Snark said:

That just won't fly.  All craft-- airplanes, rockets-- want to have the CoM in the front.  Trying to fly a rocket that has the CoM way in the back is like trying to throw a badminton birdie with the feathered end forward:  it just doesn't work.

How do you even have the CoM in the front most of the time? Isn't it almost always heavier at the bottom due to staging and needing bigger engines for TWR and more fuel for delta-v? To me, this seems generally impossible for all rockets.

For instance, making a general satellite rocket give me a low CoM.

Edited December 9, 2017 by DrPastah

[Plusck]

9 hours ago, DrPastah said:

How do you even have the CoM in the front most of the time? Isn't it almost always heavier at the bottom due to staging and needing bigger engines for TWR and more fuel for delta-v? To me, this seems generally impossible for all rockets.

For instance, making a general satellite rocket give me a low CoM.

 

It's always difficult, because most of what you want as payload (satellite dishes, crewed parts) are bulky but light.

However there is the inconvenience of having CoM "a bit low" like the satellite you showed (but once in space, there is no airflow and much less of a problem with being heavy bottomed)...
And then there is the massive issue of having CoM so far down like in your original pic.
To be honest, I don't even know how it's possible for CoM to be quite so far down on that first pic.

I see you're using mods, but I'm not familiar with them (for example, what is that grey 2.5m cylinder at the bottom?

With three orange tanks full of fuel, you should have a good deal more LF/Ox than that. Which suggests that at least one of the orange tanks (top one?) is empty or is in fact carrying something else.
Given the Oxidizer figures, the top orange tank and Rockoma x200-32 have been emptied of all oxidizer and maybe even some LF.
You can't hope to have a stable rocket if you make it extremely tall and then deliberately get rid of the mass at the top!

edit: In fact, looking at the numbers again:

- you are trying to fulfil a contract to get 4000 liquid fuel and 8 Kerbals in orbit.
- to do this, you emptied all of the Ox from the top orange and top x200-32 tank
- you also removed 1/10th of the LF from the top orange tank.

So the flipping of the rocket is therefore guaranteed: the top half of the rocket is about 70% air!

On the contrary, using LF tanks solves the problem:

No fins, no reaction wheels. Bottom tank set to empty first. 4000 liquid fuel up top and the standard contract requirements.
With 50% thrust off the lanchpad it has much the same TWR and the same dv as your craft, for 15k less funds and one less SRB.

In stock aero, it"s perfectly flyable without SAS. Needs a bit of care at around 8000m altitude to keep it roughly prograde, but shortly after that it becomes aerodynamically stable (mostly empty SRBs giving drag, half-empty bottom tank causing CoM to shift further up) meaning you can just let go of the controls for a while. It becomes unstable again when the SRBs drop but only a touch, and the gimbal is enough to keep you roughly within the prograde circle.

I don't expect FAR will make a huge difference to that.

Edited December 9, 2017 by Plusck

[Snark]

13 hours ago, DrPastah said:

How do you even have the CoM in the front most of the time?

It doesn't have to be in the front, per se.  Just don't put it way, way in the back, the way you did.

Think of it this way:

• Picture your CoM as being the center of your ship.  More than that:  think of it as the pivot point, as if there were an axle running through it that your ship can spin around.
• There's a part of the ship that sticks out in front of the CoM, and a part that sticks out behind the CoM.  I'll call these the "fore end" and "aft end".
• Drag on the fore end of your ship (call it "fore drag") wants to flip your rocket arond and make it fly backwards.
• Drag on the aft end of your ship (call it "aft drag") wants to keep your rocket flying with the pointy end forward, the way you want it.
• Therefore it's your job to make sure that the aft drag's torque is bigger than the fore drag's torque.
• You make either end (fore or aft) have a bigger drag effect in two ways:
  • Make it physically bigger and more "aerodynamically active".
  • Make it longer so that its drag happens farther away from the CoM, because that gives it a bigger lever arm to work with.

Ideally, yes, your CoM would be located in the front of the ship.  But it doesn't have to be.  It can be slightly behind the middle of the ship... as long as you have some nice robust fins that are located far far behind the CoM to compensate.

Your problem is that you've got the CoM way in the back of the ship.  This means you have a humongous draggy "front end" that comprises most of the ship, and very very little "aft end" to compensate.  And your fins aren't doing squat, because they're so close to the CoM that they have almost no lever arm to work with and therefore don't help you.

So.  You don't have to have the CoM in the front of the ship, it just has to be in front of the center of pressure.  (Sort of like the old joke about the two hikers who are surprised when a grizzly bear suddenly bursts out of the bushes and charges them.  One of them starts running.  The other says, "What are you doing?  You can't outrun a bear!"  The runner replies:  "I don't have to outrun the bear... I just have to outrun you.")

So... you do what you can to move the CoM as far forward as you can.  The way you do that is with all sorts of little incremental improvements, such as have been suggested on this thread.   It all boils down to:  Move the CoM as far forward as you can, and then put fins on it as far behind the CoM as possible.

13 hours ago, DrPastah said:

For instance, making a general satellite rocket give me a low CoM.

What you've shown there has a low CoM.  But it's not a complete spacecraft.  What you just designed there is not an SSTO.  There is no way that thing will have enough dV to get to orbit.

What you've just shown isn't a "general satellite rocket", it's a "general satellite upper stage".  In a real launch, that craft wouldn't launch off the pad just as you've shown it; it would be sitting on top of some lower stages.  And that makes a world of difference.

Why?  Because those lower stages can be designed in a way that the CoM of the overall craft is as far forward as possible, and also so that the craft has fins far behind the CoM that can save the day.

So, here's my attempt to recreate your example craft.  Looks like you're using a non-stock antenna, so I've used the RA-100 as a stand-in.  Looks like the overall situation is about the same.

I'll set aside for the moment the fact that the payload on the front end is a horrible, terrible, no good, very bad design from the standpoint of aerodynamics.  Big antenna: ouch.  Stack size that goes abruptly from >2.5m to 0.625m to 1.25m to 0.625m to 1.25m again?  Ouchity ouch owie ouch.  That's the poster child for a problem payload, horribly draggy, and normally we'd solve it by putting a fairing around that whole thing.  But just to prove a point (i.e. that this is launchable, even as it is), I'll leave it alone. 

 

Okay, so here it is perched on top of a launch vehicle, i.e. something that can actually get to orbit:

You'll note that the CoM is, indeed, down towards the bottom of the rocket, which ain't great.  But... note that I've placed the SRBs so that they stick down as low as possible below the CoM.  In particular:  Look how far below the CoM all those SRB fins are.  Pretty far down, no?  This gives them a nice lever arm to work with, so they can compensate for that huge draggy nose sticking out way in front.

I've set up the staging so that it lifts on all four SRBs plus the Swivel on the bottom.  The east and west SRBs are running with thrust limiters of 70%.  The north and south SRBs are at 100%.  It lifts on all five, then ditches each pair of SRBs as they burn out.

Result:  It flies just fine.  Rock-solid steady.  Just start a gravity turn and follow all the way to orbit.

So, why is it stable?

• It's designed to move the CoM forward, rapidly, as it burns.  Those SRBs get lighter fast.  And the central engine drains that bottom tank first.  So as soon as it has burned off a reasonable amount of fuel, that CoM moves way forward.
• This means that it's most unstable right at launch, before it has burned off the fuel, so the CoM is the farthest aft.  However... that's when instability doesn't matter much, because 1. it's going nearly vertical for that first little bit off the pad, and 2. it's going fairly slowly at first, meaning aero forces aren't quite so huge.
• As you can see, I've spammed steerable fins as far aft as possible.
• I also have the Swivel firing right off the pad.  Since it has gimbal, that also helps with stability, especially in the early bit of the flight before it's built up much speed so that aero effects aren't as strong.

That upper stage-- the one that includes the FL-T800 tank, copied from your screenshot-- is still horribly aerodynamically unstable, by itself.  But that's irrelevant.  Why?  Because it's not by itself, it's on top of a bigger launch vehicle.  And by the time that the lower part is staged away and it is all by itself... well, by that time it doesn't matter that it's aerodynamically unstable, because it's way high up in the sky where the atmosphere is nearly a vacuum and aero forces become negligible.

Anyway:  I launched the craft above and easily flew it to orbit.  Never needed to use the Ant-- the entire upper stage (with FL-T800 tank and Swivel) made it to orbit, with quite a lot of fuel left over.

If I were to tweak the design so that the really-horribly-draggy front end were encased in a fairing, that would probably help it considerably.  But it got to orbit even without a fairing.  You can put pretty much anything in orbit, if you brute-force it enough.