[Help] Problems transporting cargo

[Cmaj6]

Hi,

I'm having a lot of trouble transporting my cargo to space. The problems arise while still in atmosphere, at 7000-1000m altitude (the point where i try to do the gravity turn). The problem is that the rocket either keeps turning to the right, and/or the rocket bends at a point just below the lower most service bay, and gets uncontrollable from that point on.

Cargo weight: 1.6t

Goal: geostationary orbit (2863km orbit) (for continuous LKO scansat coverage/satellite linkage)

I've used this rocket before for smaller payloads without a problem. I tried everything, other types of wings, change thrust limit and fuel amount of the SRB's, strutting everything, etc., but unfortunately  to no avail. The previous payloads where in the range of 0.5-1 tons, this payload is only slightly larger, so i don't think it's because of the weight of the payload?

My restrictions: i play science mode and have not unlocked everything yet (see picture of what i have unlocked). Also, i try to build rockets with efficiency and economy in mind. Also, i try to not load launch parts of the payload with multiple rockets, because of 'economy and efficiency' (i do this because i play career mode also and wouldn't want to develop 'bad habits'). I tried with SAS both on and off, but no success. The biggest problem is the rocket bending at the previously mentioned point. Also, i tried deploying the fairing while in flight and i saw the payload bending too, just at the point where it is attached to the main rocket. I tried strutting the payload to the inner side of the fairings, but it didn't make my rocket and flight stable. I basically tried everything in my current possibilities (even the bigger sized fuel containers and bigger sized liqued fuel engines (see the tech tree for an overview of what i have available), but still no success :(.

My mods: KER, KIS, KAS, SCANsat, TACLS

I do not know what to do anymore and it has halted my game progress and enjoyment. Thank you in advance.

The payload and science tree: https://imgur.com/a/z3apL

The rocket (with payload on top, which i deleted the fairings off of for a better view): http://i.imgur.com/NFi9gKO.jpg

Edited March 1, 2016 by Cmaj6

[Vim Razz]

How fast are you generally moving when you flip?  Your rocket has quite a lot of engine, and the problems you're describing sound a lot like aerodynamic troubles arising from trying to go too fast while still too low in the atmosphere.

[Nich]

For some reason fairings create a lot of drag especially when transitioning through the super sonic region.  You will need struts connecting to the base of the fairing.  In addition you have fuel above batteries.  The joins between all those batteries are VERY flexible so you need to strut the top of the payload to the fairing like so.

 

You need to create a super structure around the fairing like this and run a strut from the payload to the beam.  

The fairing will be the final attachment point

Then remove the super structure

 

I am not entirely convinced fairings remove drag from what they encase from what I have seen anything with a fairing is more draggy with a fairing then without.  In 1.0.4 I was told only reason to use fairings are RP reasons or extreme aerobraking on another planet.

Edited March 2, 2016 by Nich

[Plusck]

4 hours ago, Cmaj6 said:

...

The problems arise while still in atmosphere, at 7000-1000m altitude (the point where i try to do the gravity turn). The problem is that the rocket either keeps turning to the right, and/or the rocket bends at a point just below the lower most service bay, and gets uncontrollable from that point on.

...

I think your problem is quite simply here - your gravity turn should start long, long before that. Basically, you should initiate the gravity turn right off the launchpad. A couple of degrees, and then let gravity (and your aerodynamics) do the rest.

It takes a bit of practice but with a normal, nose-heavy rocket with the fins you have on that rocket there, you would be able to start the gravity turn, keep SAS off and do nothing other than press the space bar a couple of times, turn SAS on at about 30km, and find yourself in orbit. Obviously, having the HEX core helps since it can follow prograde and lets you get away with a less aerodynamically stable ship.

However, the fairings complicate that, not because they produce drag but because they produce lift. Therefore, if you start turning at 7000m you have a massive lift force pushing the nose of the rocket perpendicular to the airflow. Impossible to survive.

So with fairings you really do have to keep exactly on prograde to avoid this problem. You want prograde to drop gently down, and once you are going fast in the atmosphere you have zero options for altering course: you must get the turn right before you're going more than about 180 m/s. And you certainly want the HEX or better probe core because you really need to let SAS follow prograde and do nothing but that until you're over about 35km.

One good rule of thumb is to turn about 5° at about 100 m/s, but the actual angle will completely depend on your TWR and aerodynamics. More TWR and less draggy ship = much slower drop to the horizon, so you have to turn much more aggressively off the launchpad.

[Snark]

So, a few things:

Lots of verbiage below, but first and foremost:  How is it possible that that ship is only 80.4 tons?  It's got four Kickbacks that ought to be 24 tons each.  Are you running the Kickbacks without a full load of fuel?  Is that on purpose?  Is it possible you accidentally reduced the fuel content when you were trying to set the thrust limiter?

Anyway, on to the advice:

First of all, that payload is the poster child for Thing That Wants to Bend In Half. 

• Smallest stack radius?  Check.
• Lots of short flat things stacked in a row, so there are lots of bendy floppy joints?  Check.
• Massive draggy thing on the top, to make it all sway around like a bobblehead?  Check.
• Icing on the cake:  The whole shebang mounted atop a 0.625m docking port, i.e. the single bendiest joint in KSP?  Check.

Those are far, far more batteries than you need, anyway.  The radially-attached 400EC batteries are plenty.  Just get rid of all those stack-mounted 200EC batteries, you don't need them.  Also, flip the science payload around so the engines & fuel tanks are on the bottom, since that's the most massive part and makes wobbling more of an issue.  It would also get rid of the joint on the docking port, which is just killing you; docking port connections are extraordinarily floppy.

Do you really need to have those big mono tanks and the lateral RCS thrusters?  Dropping those would save lots of mass.  Yes, they're kinda handy for docking, but for a little thing like this, you can dock without it if you're careful.  Or just maneuver it next to the target, switch ships, and let the target do the docking.

As for the main rocket, a few things.

First, the ratio between your first and second liquid-fueled stages seems out of kilter.  It looks like you've got 12 tons of LFO in the 1st liquid-fueled stage, and only 1 ton in the second.  That 12-1 ratio is quite inefficient; it means you're lugging around 1.5 tons of empty fuel tank and another more-than-a-ton of engine for a long part of your ascent.  A better ratio is something more along the lines of 2-to-1 or 3-to-1.  So, for example, consider making that 1-ton tank into a 4-tonner, then get rid of one of the 4-ton tanks in the stack below it.

Note:  For aero stability, vertical stacks of tanks are your worst enemy, since they drain from the top tank down.  That moves your CoM rapidly downward, and your rocket can become unstable.  So it's best to avoid vertical stacks when you can.  If you must have one, there's one trick you can do to help it a little:  disable the top tank in the VAB, which forces the bottom tank to drain first.  It means one more piece of micromanagement during the launch sequence, since you have to remember to enable the tank when the one below it is nearly empty.

Next, the SRBs.  That rocket seems far over-supplied with SRBs.  They're great off the pad, but they have crappy Isp and you shouldn't rely on them to take you to orbit.  Those Kickbacks are far too much SRB for that rocket; they're making up the large majority of your rocket mass.  Suggestion:  Replace the Kickbacks with Thumpers.  That would bring the launchpad weight down to around 46 tons (if I'm correctly visually estimating), which means that a quartet of Thumpers set to around 70% thrust or a smidgeon lower would give you a reasonable launchpad TWR.

I notice that you're using a combination of your liquid-fueled engine and your SRBs off the pad.  Don't do that.  To get the most dV, you always want to burn your lowest-Isp fuel first.  Since the SRBs have a much lower Isp than your liquid fuel engine, you should lift off using SRB power only, and don't activate your liquid-fuel engine until the SRBs burn out.

[FullMetalMachinist]

Just to add to Snark's excellent advise above, the fairings have a pretty horrendous bug where they apply their lift/drag forces waaaaay out in front of the rocket. This gives those aero forces a really big lever arm to wrench your rocket around backwards.  I highly recommend using Claw's Stock Bug Fix Modules.

 

[Plusck]

3 minutes ago, Snark said:

I notice that you're using a combination of your liquid-fueled engine and your SRBs off the pad.  Don't do that.  To get the most dV, you always want to burn your lowest-Isp fuel first.  Since the SRBs have a much lower Isp than your liquid fuel engine, you should lift off using SRB power only, and don't activate your liquid-fuel engine until the SRBs burn out.

While everything else you write is spot-on, I simply can't agree with that part.

Obviously you should limit your use of LF engines at the start if you have a large amount of SRB thrust, but for all but the lightest rockets (or ones with a huge amount of reaction wheel torque authority, which further increases weight, or RCS which adds cost and complexity, or Vernors which need a higher tech iirc) you need LF engines for control right at the start of your ascent.

So you should perhaps throttle down (or off) your LF engines as soon as you have started to get up to a decent speed, and use alternative control options if your ship needs RCS otherwise, but I can't agree with a general rule which says not to ignite LF off the launchpad: you risk spending your SRBs simply treading water (or treading atmosphere) and you won't be able to start your gravity turn soon enough unless you have other control options built in.

[Snark]

43 minutes ago, Plusck said:

While everything else you write is spot-on, I simply can't agree with that part.

Obviously you should limit your use of LF engines at the start if you have a large amount of SRB thrust, but for all but the lightest rockets (or ones with a huge amount of reaction wheel torque authority, which further increases weight, or RCS which adds cost and complexity, or Vernors which need a higher tech iirc) you need LF engines for control right at the start of your ascent.

So you should perhaps throttle down (or off) your LF engines as soon as you have started to get up to a decent speed, and use alternative control options if your ship needs RCS otherwise, but I can't agree with a general rule which says not to ignite LF off the launchpad: you risk spending your SRBs simply treading water (or treading atmosphere) and you won't be able to start your gravity turn soon enough unless you have other control options built in.

Well, it's a matter of taste, I suppose. 

On my own ships, I almost always lift off using SRBs alone, and I have no problems with control.  A very modest amount of torque authority from reaction wheels is plenty-- e.g. a single 2.5m wheel for a 2.5m stack, or a single 1.25m wheel for a 1.25m stack (or none at all, if it's a small vehicle and has a command pod on it).  If the rocket is very big and unwieldy (i.e. using Kickbacks on a ship that's over 100 tons on the pad), I may put a couple of AV-R8 winglets on the bottom of the longest-burning SRBs, on the north and south sides of the rocket, can help with pitch control.  But for anything up to mid-size rockets, a few Basic Fins for stability are all that's necessary.

The key is to start the first bit of the gravity turn very early, like practically right off the pad; I generally give the first eastward nudge when it's going no more than 20 m/s.  It takes a bit of practice to gauge the right amount of nudge, because at that early stage just a small difference in the early nudge can make a huge difference in the later part of the gravity curve.  One thing that helps is that I keep a religiously constant launchpad TWR of 1.5, which means that all my rockets, big and small, follow a fairly consistent launch profile, making it easier to judge the gravity turn with practice.  The key point, though, is that the first nudge is so small that a very modest amount of reaction torque is plenty.

Just to put some perspective on what I mean by "modest amount":

My typical rescue-a-kerbal-from-LKO single-passenger rocket is a three-stager consisting of:  #1 Mk1 pod, 2-ton tank, Terrier; #2, 4-ton tank, Swivel, four Basic Fins in an "X"; #3, two radial Hammers.  It lifts off on the Hammers alone and doesn't touch the Swivel until the Hammers burn out.  No reaction wheels at all, and no control surfaces; the torque on the command pod is plenty to start & tweak the gravity curve.

Another example:  a typical heavy-lift vehicle has a bottom booster stage that's a Big Orange Tank with Mainsail, surrounded by eight Kickbacks in two symmetry groups of 4.  One of those 4-groups of Kickbacks has AV-R8 winglets.  No reaction wheels on the booster stage.  Lifts off on the Kickbacks alone, all it needs is the four AV-R8s plus the small amount of torque from the upper stage to steer it.

The moral of the story:  do a very tiny (and just-right) amount of steering right at the start, and the gravity curve takes care of itself and needs practically no steering at all.

(That said, I suppose it's fine to start the LF engine together with the SRBs, if the throttle mostly stays at 0% and it's only used in brief blips to help make minor steering corrections.)

Edited March 2, 2016 by Snark

[InfiniteAtom]

Ok. Don't do the gravity turn until you hit 36000 meters. At that point, there is no atmosphere (or very little) and thus you wont de-stabilize. (As long as you have RCS and SAS).

Edited March 5, 2016 by InfiniteAtom

[FancyMouse]

I think the advantage of firing both LFO and SRB at launch is controlling the TWR at the end of the stage. I often just make the throttle less than 30% at the end of stage - especially helpful for "creative" payload designs.

[Snark]

3 hours ago, InfiniteAtom said:

Ok. Don't do the gravity turn until you hit 36000 meters. At that point, there is no atmosphere (or very little) and thus you wont de-stabilize. (As long as you have RCS and SAS).

Note that doing so would waste a lot of fuel due to following an inefficient trajectory.

Gravity turns are going to be with you as long as you're playing KSP-- they're a vital skill to develop.  I'd recommend practicing them and developing that skill, rather than just throwing fuel at the problem and ascending too far straight up.

(Occasionally you may find yourself needing to loft some hideously unstable monstrosity and there's no way around it because you have some very awkward payload that you have to get to orbit, and for such cases, you may need to bite the bullet and fly inefficiently, e.g. stay slow until you're above 15 km, wait until late to do your gravity turn, though 36km would still be way too high.  However, such occasions should be few and far between.  The normal launch should be:  design something that's stable, then follow a good gravity curve on the way up.)

[FancyMouse]

On 2016/3/2 at 6:15 PM, Nich said:

I am not entirely convinced fairings remove drag from what they encase from what I have seen anything with a fairing is more draggy with a fairing then without.  In 1.0.4 I was told only reason to use fairings are RP reasons or extreme aerobraking on another planet.

It depends on the payload design. Especially if the payload does not exhibit almost equal length and width, because fairing has to occupy a whole cross section area of a circle, it may increase the area by a lot. An example: if payload is 2.5m-sized with additional 2.5m-sized parts placed 2x symmetry, cross section area is 3*pi*1.25^2=14.7m^2. However if a fairing is used, then it has to be at least 3.75m in radius to cover the thing. That makes it pi*3.75^2=44.2m^2. This means unless fairing can bring down the (average) drag coefficient by 70% or more, you'll see increased drag.

On the other hand, if cross section area doesn't change too much, then I do see a decrease in drag, especially with 1.25m-sized rockets.

[Snark]

1 hour ago, FancyMouse said:

I think the advantage of firing both LFO and SRB at launch is controlling the TWR at the end of the stage. I often just make the throttle less than 30% at the end of stage - especially helpful for "creative" payload designs.

Yah, SRBs do tend to have issues with runaway TWR near the end, and LFO co-firing can help with that, as you point out.

My own solution to that problem is a technique I think of as "poor man's asparagus":  fire SRBs alone to take off, but in two symmetry groups rather than one, and at different thrust limiter levels.

Thus, let's say I'm building a rocket and decide it'll have 8 radial SRBs.  Instead of adding a single symmetry group of 8, I add two symmetry groups of 4 each (at 45 degrees from each other, so it looks like an 8-way symmetry).  Suppose that when I do the math, I conclude that to have my desired launchpad TWR of 1.5, I would need to set the thrust limiters on the SRBs to, say, 84%.  So instead of setting all the SRBs to 84%, I set group A to 100%, and group B to 68%.  Rocket lifts off on all 8 SRBs, but group A burns out first and is jettisoned, while the rocket continues on group B alone.

By that point group B generally has enough thrust to carry the load by itself, because, 1. the ship is a lot lighter (all of group A and most of group B are gone), 2. the SRBs are getting around 20% more thrust (the ship's high enough to get close to vacuum Isp), and 3. not as much TWR is needed anyway because it's already tipped over well into the gravity turn and isn't headed straight up.

Doing it that way smooths out the ride considerably.  Occasionally the math will work out in such a way that group B can't quite carry the load by itself, and in such cases I'll stage the LFO engine to come online when group A is jettisoned, so I can run it with enough thrust to keep things going (generally not full thrust).  But such cases tend to be more the exception than the rule-- it generally works out pretty well and I don't have to start the LFO stage until the last SRB stage is jettisoned.