Help me figure out what the hell is wrong with my vehicle

[Analogy]

I'm trying to design a craft that can accomplish a grand tour - visit the SOI of every object in the Kerbol system. So, nuke engines, lots of fuel, and a booster to put all that fuel into orbit.

So, the problems:

* Near the end of the third stage, it has a tendency to start uncontrollably spinning out of control. Usually between 6 and 8 tanks empty.

* However, the last few iterations, it's started randomly exploding around the time of second stage separation. No collisions, and no real changes to the part that's exploding. It's the same one every time, and around the same time in the flight, but I can't identify what's causing it.

I've gotten this craft into orbit. Now with no fundamental design changes, it's failing to do so. Help me KSP forum, you're my only hope.

http://imgur.com/a/lZnet

http://analogy.cupofnoodles.com/ksp/Grand%20Tourist.craft

[Analogy]

Quick possible theory on the random exploding booster: Because it's near the end of the stage, the thrust to weight ratio at that moment is fairly high, could it be G-forces making the connection fail?

[cardgame]

Way too much thrust and too little mass to push along results in instability, which is why I never use multiple 2m tanks and mainsail engines. You'll need to reduce thrust if it's shaking to pieces.

[Analogy]

My standing theory about the cause of vehicles losing control is that as fuel is used, the center of mass moves downward because you have full tanks on bottom and empty tanks on top. Moving the center of mass toward the center of thrust means that thrust vectoring has less ability to exert control authority over the craft. Usually, figuring out how to move mass upward or thrust downward at that point in the launch helps me out. The other theory is that as boosters drop off, so are all the bracing connections between them, so the vehicle is a bit more wobbly.

[Analogy]

Way too much thrust and too little mass to push along results in instability, which is why I never use multiple 2m tanks and mainsail engines. You'll need to reduce thrust if it's shaking to pieces.

Thrust to weight ratio is about 1.5 at the beginning of each stage, maybe 2.5 max at the end. That's well within decent tolerances.

[Nibb31]

The reason it's spinning, is that as you drain your fuel, the back end of your rocket gets lighter and your center of mass moves forward with all the thrust at the back. Also, it is super top-heavy. Those Nerva engines wheigh several tons each. Why do you need so many of them on your top stage? You don't need lots of thrust in space. You should use that mass for fuel, not engines. If it's for a lander, 2 should be more than enough.

For your boosters, it's the opposite. You have way too many fuel tanks and not enought engines. I'm pretty sure that each engine isn't capable of launching the fuel it's carrying. You are spending a whole lof of fuel just lifting the rest of the fuel off the ground, but you're not going anywhere. You shouldn't have more than 2 or 2.5 large tanks on each engine.

Try cutting down on the fuel tanks. If you really need that much fuel (which I doubt), add more stages so that you get rid of the empty tanks and keep your center of mass as close as possible to the center of thrust.

[Analogy]

The reason it's spinning, is that as you drain your fuel, the back end of your rocket gets lighter and your center of mass moves forward with all the thrust at the back.

Uh... No. Your center of mass moves backwards as you burn fuel. You drain from the top tanks first, so the top end of the rocket gets lighter. This keeps moving the center of mass back until the last tanks drain, then it moves forward again.

Also, it is super top-heavy.

Top-heavy rockets are more controllable by thrust-vectored engines for the same reason that it's easier to balance a broom on your finger from the handle end than the bristle end. You have a longer lever on which to act.

Those Nerva engines wheigh several tons each. Why do you need so many of them on your top stage?

Look at the staging. Those engines are hauling 5 3200L tanks, which are necessary to provide enough delta-v to visit every SOI in the system. One or two nervas will do about... 1 m/s^2 of acceleration with that. Would you want to try and calculate a 2000 m/s burn with that using only KSP's (lack of) orbital planning tools, or would you rather take a mass penalty and put enough thrust on your vehicle to make those burns a little easier? All of the transfer calculators assume a burn which is short enough that it can be considered instantaneous for the purposes of the calculations. You need at least .3 or .4 TWR to do that. One nerva pushing 5 large tanks is a TWR of about .06. I would love to see anyone demonstrate a successful interplanetary transfer burn using one or two nervas pushing 16000L of fuel without resorting to extensive trial and error.

If it's for a lander, 2 should be more than enough.

Did you not pay *any* attention to my post? I mention that this is a *grand tour* mission in the *very first sentence*. Nothing is landing on anything. There aren't even any landing legs on the craft, anywhere.

I'm pretty sure that each engine isn't capable of launching the fuel it's carrying.

I assure you they are. The thrust to weight ratio of the vehicle never drops below about 1.5, perfectly reasonable for a super heavy lifter. Yes, a bit on the low side, but not wastefully so. When I don't run into tumbling or random explosions, the vehicle hits orbit with fuel to spare before separating the interplanetary stage.

Is it seriously too much effort to throw the .craft into your saves directory and watch mechjeb try to fly it?

[Nibb31]

No need to be snarky when people are just trying to help.

[Commander Zoom]

My advice:

1. You probably don't need that many NERVAs. For testing purposes, at least, try deleting half of them and see what happens to your ascent.

2. Are you deliberately tilting, or is that a result of your rocket being only semi-under-control? Because I would not be trying to turn such a beast at that point in the ascent, and definitely not in the midst of staging. You're practically asking for something to crash into something else.

[togfox]

I'm currently experimenting with six large tanks with the rockstar engine (whatever it's called) and I'm also finding the fuel tanks will disengage midflight and destroy my craft. I've tried strutting the frek out of it. I get about one good launch to every three launches. Once I clear about 10km altitude and the SRB's drop off, I found the craft less prone to seperation. I suspect there is too much weight hanging off my fuel stacks.

Try putting 3 tanks beside 3 tanks (with decoupler and fuel line) and see if that makes a difference. That is a departure from your current design but eh, if you want a tour, you gotta get off the ground first.

Good luck!

[Wayfare]

Hi Analgoy,

I fixed your boat

[ATTACH]34219[/ATTACH]

The problem I believe was in the Asparagus staging. The violent nature of stage separation combined with the sort-of weak structural integrity of your ascent stage (pylons on pylons does not make for a very strong connection), and the not-quite-symmetrical fuel line arrangement causes the rocket to wobble. This gets progressively worse as more stages are dropped. I don't think there was any one magic moment during ascent that caused critical failure: it was the build-up of structural stress throughout the launch that eventually caused stuff to go boom.

So I took out the asparagus staging, re-arranged the fuel lines to suit, and presto!

That said, I only fixed the primary ascent: MechJeb doesn't like your craft very much and has trouble controlling the very long rocket it is left with after jettisoning the boosters, to the point where it can't make orbit. You'll have to fly it manually after the gravity turn and make sure you start burning prograde well early to circularize your orbit. After that it should be ok...

...Kind of.

I say "kind of" because I tried to stay as close to your original design as possible. Frankly, though, I don't think it will work very well. According to Kerbal Engineer you get a bit over 7000 m/s of delta-v out of your interplanetary stage. That's a good number but it's an awfully big stage for what it does. I've taken the liberty of also modifying your craft more thoroughly to create an ascent stage that will get you to a 100km orbit (assuming you take manual control after the gravity turn) with some fuel to spare for your exit burn out of Kerbin's SOI. The remaining interplanetary stage has more than 8000 delta-v for you to play with:

[ATTACH]34220[/ATTACH]

[ATTACH=CONFIG]34221[/ATTACH][ATTACH=CONFIG]34222[/ATTACH]

Yes, I stripped off half the nuclear engines. If timing your exit burns a few degrees early is problematic, I recommend you use Protractor to give you the data you need. Note that the centre stack of the ascent stage is a bit wobbly once the booster stages have come off: you may want to strut that up a bit if you find it causes you problems.

Edited October 4, 2012 by Wayfare
Added pictures

[bsalis]

Ok, first up i'd like to say I admire your ambition. However, I do not think a Grand Tour is possible. At the least it is a huge navigation challenge rather than a huge rocket one. I recommend dropping Bop and Gilly as a minimum. A practice run of just the planets would be useful I think.

Anyhow, as for your rocket. My 2c follows...

1. Strut-o-rama! I think there is more than needed. Less is sometimes more. Good placement is key.

Thrust to weight ratio is about 1.5 at the beginning of each stage

2. I think this is too low. A read elsewhere that 2.2 is a good guide as a minimum. A guide that has served me well. With the mainsail thrust buff in 0.17, I am finding stacks of three big tanks a good match for a single engine. A good working example being this.

I've gotten this craft into orbit. Now with no fundamental design changes, it's failing to do so.

3. I advise that once your reach a stable version during development, you save it under a new name to continue (just increment a number at the end is easy to do). That way you can go back if you end up down the wrong path.

Edited October 4, 2012 by bsalis

[NannerManCan]

Also I only saw a few struts on those first stages. Add more struts and see if that helps.

[Wayfare]

I think I also figured what MechJeb's problem is - the long rocket takes far too long to turn towards prograde after it's coasted to apoapsis. You'll want to do that manually ahead of time, disable the ascent autopilot, and then just hit "Circularize" once you're close to your desired orbit height.

[r_rolo1]

@ Wayfare

Asparagus is inherently unstable because of the strut forces of the fuel lines and the non-radial display of them in there, but that normally can be circumvented by strutting and more control ( even putting some extra SAS can help ). I've done some rockets of that size with asparagus and it can definitely be done. IMHO the real issue in most cases ( not here though ) is the lack of strutting in the bottom: pylons and decouplers are inherently more flexible than what you want in here and they will make the engines to wobble slightly out of the vertical. This will get worse as the rocket loses mass, but it can be solved by extra strutting in the lower parts to the center part and even between tanks in the same stage ... dirty but effective.

@ Analogy:

I agree with the poster in the last page that says that putting 5 tanks in top of all the big LV is wasteful. I will not say that you don't need that fuel, but if you want to take it you would be better doing drop tanks in the side ( say 2 drop tanks in the side and 3 in the LV proper ). Empty big tanks still weight 2t ( thus less mass for you to accelerate if you drop them soon ) and, more important, makes your rocket shorter and far less susceptible to bending stress failures.

BTW that is what you have here: a bending stress failure on the lower side rocket ( the more susceptible to that in your ship when it starts get horizontal ) as soon as the angle of the ship with the horizon reaches a certain level and the center of mass gets low enough while still having that huge mass in the top. The only solution for that are assuring that the center of mass is higher relative to the rocket length ( by reducing the top load and/or making the rocket shorter ), increase the structural strength of the faulty areas ( more strutting and/or reducing the length of the side rockets ) or simply start turning later, when the Kerbin g-force is lower ( I know, it is inefficient fuel wise ... )

P.S Other thing you can do is to put some thrusting in the top part of the rocket at launch, thus increasing the center of thrust and reducing the bending and compression stress due to gravity in the gravity turn phase. You have a lot of open space in between the side rockets and the central stage, so you could in theory fire your nuke engines at launch while draining the outer tanks ( you already have the fuel line work done and all ). I say in theory because nuke engines can gimbal and that might end making that the exhaust hits something, but I've done it a lot with aerospikes in the .16 times due to the weak decoupler issues. In your particular case you would also need to rotate the engines in top to ensure that you don't hit the side rockets

Edited October 4, 2012 by r_rolo1
Added cooment on increasing center of thrust

[antbin]

Top-heavy rockets are more controllable by thrust-vectored engines

This makes sense to me, but atmospheric drag might counteract that effect.

My experience is that very top-heavy rockets are uncontrollable during the gravity turn phase because of drag forces (which KSP calculates proportional to mass). 2 degrees of thrust vectoring isn't enough to control 4+ degrees of pitch error drag from a very top heavy rocket.

I would very much like to launch my capsule with a 3200L tank strapped to the top (to fuel an interplanetary tug stage), but I've failed every time, with uncontrollable pitch and/or structural failure.

Having a super long main booster should help by increasing the leverage of the Mainsail's thrust vectoring, and pulling the center of drag a bit further aft. However long (4) tank stacks seem to snap like twigs, despite strutting.

Edited October 4, 2012 by antbin

[EndlessWaves]

While a 900 ton rocket with 300 parts isn't a problem itself, it's definitely not the solution to launching a robust 70 ton vehicle. To put it bluntly, you're wasting masses of fuel.

A simple launcher would be something like this:

[ATTACH]34239[/ATTACH]

It has sufficient fuel to just make it into orbit (well, mine was actually 500m short of clearing the atmosphere at perihelion but mechjeb made a bit of a hash of it's turn timing on the test run and I wasn't paying attention). It is single stage to orbit so not as fuel efficient as it could be but it's simple and does the job in under 100 parts total (excluding launch clamps) and at less than half the launch weight of the original.

[Commander Zoom]

Amazing weight-reduction job there, EndlessWaves.

[EndlessWaves]

Amazing weight-reduction job there, EndlessWaves.

The key is not so much the weight launched but the inherent wobble of the payload. The aerospikes have no thrust vectoring and you need a stable upper section if the command pod gyros are going to manhandle the entire rocket on their own. In this case the payload seems very stable so you can get away with chucking out all the other steering and going for maximum efficiency.

[Wayfare]

You went a bit more radical than me it seems

I still think the biggest problem with this ship is the payload. The interplanetary stage is just far too inefficient in terms of delta-v. It can easily be trimmed down to 60 tons - half the weight of the current payload - and will actually gain delta-v in the process. The trimmed version still has a TWR of 0.62 in low Kerbal orbit to make for realistic burn times.

[cardgame]

Yeah, definitely doesn't need all those nuclear engines – and oh god, why jet fuel?

[Analogy]

Awesome, guys, thanks. I have to go run an errand right now but I have downloaded your modified .crafts and will be checking them out when I get home.

[Wayfare]

and oh god, why jet fuel?

Those are structural fuselages, no fuel in them - they're actually a pretty good way to build out your craft if you need to place some tricky modules (like those long nuclear engines) without sacrificing too much mass.

Awesome, guys, thanks. I have to go run an errand right now but I have downloaded your modified .crafts and will be checking them out when I get home.

Looking forward to hear what you think of them

[Qumefox]

Those are structural fuselages, no fuel in them - they're actually a pretty good way to build out your craft if you need to place some tricky modules (like those long nuclear engines) without sacrificing too much mass.

Aren't the engine pods a bit lighter, mass wise, than the structural fuselage?

[Wayfare]

Aren't the engine pods a bit lighter, mass wise, than the structural fuselage?

Ah, true - and they look sexier too