I built a interplantaery spacecraft, but I'm worried I made a design mistake.

[DalisClock]

So, I've been playing around with KSP(off and on since roughly .24) for a while and I'm fairly comfortable with all the Kerbin Orbit stuff, I've built a decent sized space station in LKO just to prove that I can, landed on the Mun and sent an an unmanned Lander/Orbiter mission to Duna, which has both mapped the planet with SCANSAT and successfully put my robot lander on the surface of Duna(I undershot my intended landing zone due to drag but at least I didn't break the lander).

So my next challenge to myself was to design an interplanetary mother-ship to reach other planets, which could go there and back without needing refueling(and can be refueled via one of it's many docking ports), take a crew of 9 and attach various landers and modules depending on mission requirements. My resulting craft is fairly long, with the front half being the crew/control areas and the back half being a huge fuel tank and a 2.5 m NERV engine, not to mention some large solar panels to provide plenty of power. It's about 2.5 m pretty uniformly up and down the ship(except the Mk3 liquid fuel tank that bulges out somewhat near the end), and looks like a large tower in orbit.

I built it, somehow managed to get each big piece into orbit(the aft and forward sections had to be launched separately and mated in orbit), then designed all of the landers for a Duna landing and docked them to the main craft. I finally selected my crew, got them up there and was ready to go.

When I got up to the window for the best Kerbin-Duna transfer, I started notcing something. One, the burns take a very long time. I kind of expected that using such a large craft and NERV engine(but it's annoying) The problem I hadn't anticipated was that it works fine at low thrust, but once I got above 1/3 power I start drifting off target(up on the navball), even with SAS set to hold on target. If I use RCS, this isn't a problem at higher thrusts but the act of holding on target burns through monoprop like crazy and I know I'm gonna need it.

I purposefully built the craft with reaction wheels that are the same diameter as the main ship and it gives me a decent ability to rotate in orbit without using RCS, so I didn't think this would be a problem. I've been able to get up to about 90% thrust but only if I raise thrust very slowly and incrementally after the 33% mark, and it's annoying. Too much and I start drifting off course(and always up)

My craft is roughly 90K tons(including lander weight) with a dV of about 6500 m/s(most of the landers are not coming back on the return trip, which should make it a little easier). It's currently in a 100 km parking orbit above Kerbin and I have a save before I start burning out of that orbit.

The only thing I can think of is that my landers are not balanced around the axis of my craft, though the idea that I need a second NERV engine has also come to mind. I can provide a screenshot if that helps.

Any suggestions would be appreciated.

Edited September 2, 2015 by DalisClock
Question Ansewered

[Supernovy]

A screenshot would really help, here. Especially one with the centre of mass and centre of thrust displayed.

[LaytheDragon]

It is probably a unbalance of landers on each side, resulting in the center of mass being off and the craft spinning out when there is too much thrust offset to the side from the center of mass, as a decent number of reaction wheels can't fight against so much torque from the thrust. A screenshot would be helpful though, as stated above.

Edited November 6, 2015 by LaytheDragon

[Snark]

One thing to watch out for: if you have landers docked radially to the main craft, then when you turn on your mothership engine, it could be draining fuel out of the landers... and might not be doing so symmetrically (e.g. it might be draining it out of one of the radial landers until it's empty, etc.) That would cause an uneven mass distribution that could be the issue.

One way to check this, take a look at the contents of your various fuel tanks (after you do your burn and notice the drift becoming a problem); check whether there's a lopsided fuel distribution there.

If that does turn out to be the problem, there are a couple of ways to solve it. One would be to just drain all the landers dry before you start (if you have spare tank space available in the central core ship). Another approach would be to click on the radial docking ports where the landers are attached and choose "Disable Crossfeed"-- that will stop the mothership's engine from draining fuel from the lander tanks.

No idea if this is actually your problem, just something worth checking.

[The_Rocketeer]

A screenshot would help immensely.

In general, the longer a craft is the more likely it is to bend under acceleration. This is more true when you have radial payloads, especially when they are mounted near the opposite end of the craft to the source of thrust. If your craft is being piloted from a pod at one end and propelled by an engine at the other, it's likely that even a small amount of bending will cause you to drift steadily off course. The heavier this end of the rocket is and the higher the rate of acceleration (read thrust/throttle setting), the worse it will be.

Another issue that arises is that any reaction wheels in your radial-mounted landers/payloads will cause a positive feedback issue by trying to steer the craft to correct the bend. This puts strange torsions through the craft at various places which are often counter-productive and actually increase the bending. However, having totally inert payloads can be worse, as they begin to sway back and forward causing a torsion-oscillation that is not corrected by the reaction wheels at all.

To help with this, design your ship so that any detachable crafts or payloads dock/mount to in-line/parallel docking ports and not radial/perpendicular ports. There are obviously many ways to do this, and strange torsion forces can still occur, but this arrangement helps reduce the unwanted effects acceleration.

EDIT: Also, adding another NERV will not help with your problem at all, if anything it will probably make the problem worse because the acceleration force will be greater. What you probably need to do is take the craft back to the drawing board and design it with a more rigid structure and better mass distribution.

Edited August 30, 2015 by The_Rocketeer

[Pecan]

Welcome to the forums. Where have you been since February?!

Sounds like you've worked it all out well and carefully but, yes, I'd agree that something is assymmetric on your fully-assembled ship, putting the thrust-axis below the CoM and forcing the vehicle to pitch up.

Your only solution is going to be adding something beneath the ship or removing something on top of it to bring the CoM inline with the thrust.

Handy tip for the future - build it ALL in the VAB, as one piece so you can check this. Then separate and save the various parts you're actually going to launch.

[problemecium]

On the topic of preventing fuel drain out of landers, I find a more reliable method is to just right-click each tank in the lander(s) and click the little green triangle to the right of the fuel bar, so that it turns into an "x" - this "disables" the tank, preventing its fuel from being used by anything. When the time comes to land, just right-click and press the button again and it'll re-enable, saving you the trouble of redistributing fuel.

This "disabled" state reliably saves in the persistence file, unlike disabling crossfeed on ports, which in my experience seems to always revert to "enabled" on loading the ship.

[ForScience6686]

You can use fuel to balance the Landers until its time to deploy. I think that would be your best option. Unless the imbalance is to great, in that case re evaluate the landers to see where adjustments can be made. A less efficient way would be to thrust limit and use the engine on the inside of the turn to balance.

[cantab]

If your centre of mass and centre of thrust aren't aligned then you'll veer off course when thrusting. Engines can generate quite a lot of torque, enough to overcome reaction wheels.

If you have Kerbal Engineer installed it can tell you "Thrust Torque". Try and get that to zero. If you don't have KER just watch the way the craft veers.

In future if you use a cluster of engines you can individually thrust limit them to compensate for small mass offsets. Note that while it's called a "limiter" it's actually a multiplier - for example with throttle at 60% an engine with 40% thrust limit will deliver 24% of its normal maximum thrust.

To save this ship you could simply go everywhere at low thrust. If you don't want to do that the first thing I'd try is to move fuel to get the centre of mass in line with the main engines. The next thing is to reposition the landers if possible. The next thing I might try is using the lander engines, appropriately thrust limited, to help keep things in line

[DalisClock]

Here are pictures from the VAB

Here are orbital pictures of how the landers are laid out.

[The_Rocketeer]

Wow, ok. Yes the payload mass is just very unbalanced.

If you add mass to one side of a craft, you need to add EQUAL mass to the other side, otherwise you pull the CoM out of alignment with the CoT. One way to do this is to double up on all your payloads, so take 2 of each lander, each docked directly opposite it's counterpart. Another is to use a fuel tank with just the right amount of fuel, which you can then use to top up the main craft once the lander has detached.

Although I can see you've arranged your payloads in a (sort of) even distribution around the core, they'll all weigh different amounts, so u still end up with and off-centre CoM.

So right now, either take it straight back to the drawing board, or get some extra payloads up there to balance it out before u go and pray it doesn't mess up your dV allowances!

[cantab]

You've made life harder for yourself by not standardising on one docking port. Attached to the hab section you have three-way symmetry on the docking ports and only two landers to use it.

Maybe send up a drop tank to attach to a third docking port on the hab section. It'll act as ballast on the way out, then you could transfer the fuel to the main tanks and use it for the return trip.

[mrclucks]

Besides being very in balanced, your vessel is super long and skinny. You probably have multiple sas modules and they have been known to fight against each other, occasionally being so bad that it rips itself apart

[OhioBob]

I've never tried this before in KSP, but you could attempt to gyroscopically stabilize your craft during the burn. Get the vehicle carefully aligned in the right direction for the burn, and then put it into a roll around its long axis. Once spun up, turn off the SAS so that it continues to spin on its own. The spin should counteract the asymmetry. Once you've made it through the burn, turn SAS back on an de-spin. Be sure to do a save beforehand in case it doesn't work.

[Claw]

I've never tried this before in KSP, but you could attempt to gyroscopically stabilize your craft during the burn.

I'm interested to hear how this turns out for a vessel this size. Although I susped with a craft that big, it might wobble and snap while trying to do the T-handle oscillation.

Interesting design.

Cheers,

-Claw

[cantab]

Spin-stabilisation works reasonably well for balanced craft and can be handy if you haven't got SAS for whatever reason. But I'm not sure how well it will work for an unbalanced craft, I think you might just wobble.

[DalisClock]

Appreciate the feedback. I knew that everything had to be balanced for atmospheric ascent or I just wasn't going to orbit that day, but didn't think it really mattered once I got to orbit. Once again, KSP makes me feel like a dumbass. I did the 120 degree seperation because it seemed a nice way to maximize the amount of landers I could attach, but I guess that was a bad idea.

I'm gonna try balancing the lands and adding a drop tank on the third side, not to mention cutting crossfeeds and disabling all the SAS mods and seeing if that helps. I also have KAS/KIS installed so there's the possibility I can re-arraign some of the docking ports on the forward end for a 180 degree separation instead.

If not, I've got plenty I want to do while waiting for the next Duna window to open up.

- - - Updated - - -

Welcome to the forums. Where have you been since February?!

Lurking, reading various threads here and there to see if my questions had already been answered, as well as getting a feel for the community.

That and I've been on a break from KSP for a while. After putting my Duna ship in orbit and running into this problem, I got discouraged and stopped playing for a couple months so I could focus on some other games(and projects).

I recently discovered the FASA mod and that pulled me back in.

[Claw]

Once again, KSP makes me feel like a dumbass.

No need to feel that way. KSP has a steep learning curve. To me, that learning curve was most of the fun.

I still remember slamming into the Mun over and over, and at the time I didn't even know you could save. So I would fly the whole thing over just to practice that 15 seconds of terror, watching my kerbals get vaporized. Heh...good times.

Keep at it!

[OhioBob]

But I'm not sure how well it will work for an unbalanced craft, I think you might just wobble.

That is my concern as well. No harm in trying, though.

DalisClock, if you do try spin stabilization, please report back and let us know how it worked. It might be a good experiment that we can all learn from.

Edited August 31, 2015 by OhioBob

[Starwhip]

That is my concern as well. No harm in trying, though.

DalisClock, if you do try spin stabilization, please report back and let us know how it worked. It might be a good experiment that we can all learn from.

Indeed, you'll be teaching us something.

[DalisClock]

Besides being very in balanced, your vessel is super long and skinny. You probably have multiple sas modules and they have been known to fight against each other, occasionally being so bad that it rips itself apart

I had issues like this on ascent but I've tried the trans-duna burn several times and the craft has held together without incident. Even the landers, which I was worried would be the weak point and shift/break off(especially the ones attached by mini-docking ports on the aft section) have held steady at 90% burn.

Edit: Assuming I manage to fix the overall balancing issue and don't have to go back to the drawing board on this, am I correct in assuming I'm gonna need to perform a series of trans-duna burns due to the low thrust of the NERV engine? It would seem that even under the best thrust conditions it takes an incredibly long burn to put my on a duna intercept from LKO and that tends to mess up the orbital shape.

Edited September 1, 2015 by DalisClock
Addendum to orginal message.

[DalisClock]

Update:

I tried a couple of different things, including cutting crossfeeds, playing around with the reaction wheels and the spin method. None of them did any good trying to get to full power, and turning off some of my ship reaction wheels started causing some serious wobbling in the main ship.

So part imbalance is very much the problem. It turns out there's an easy solution for now. I did a mass check of each lander and while most of my landers were between 10-20K tons, that big lander on the bottom of the forward half is a whopping 40. I decoupled it, moved it away a little and then tried a full power burn. There's still some drift but not so much the reaction wheels couldn't compensate for it and hold on target.

So I'm gonna go ahead and leave the big lander in orbit(I still want to use it later and it was a massive pain to get up there), and just balance out the forward half with a couple more lander of the same type.

[Mikki]

Hi there

I solved the problem by making a wider construction, this interplanetary tug has only little to no thrust offset, me wonders too:

It has no overpowered SAS nor needs much RCS (Vernor in this case), just depleted all the equipment on the centerboom from nearly all liquids... then its possible to shift around the weights, or rebalance it with the fueltanks at the enginebooms (close them afterwards).

Its mass is very compact and shaped more like a sphere than a long rocketthing, it turns great and precise for manouvers.

Its in itself like a big gyroscope somehow.

Its quiet stockish, Atomic Age mod and Space Shuttle Parts mod, Stock Fuel Switch, not much more, KJR for some more stiffness.

Edited September 1, 2015 by Mikki

[lukethecoder64]

Woah, way to many radiators! Unless those engines get pretty hot, you'll never need that many.

Looks good otherwise!

Luke

[Warzouz]

Yep nice ship. That would have been a pain to launch, no ?