Wet spaghetti rockets

[Scarecrow88]

I have to say that I thought the era of rockets behaving like wet spaghetti once they had left the launch pad was long behind us, but apparently not. I recently thought I'd put together a simple rocket to go to Mun; no separate CSM/LEM or rendezvous and docking here. Just a basic lander with enough dV to land on Mun and return to Kerbin, on top of a two stage rocket for getting to LKO and a munar insertion stage. Boy oh boy. On launch of the first iteration of my design I thought someone had snuck in to the VAB and replaced all the rocket parts with imitations made of rubber! This thing would have given a bendy bus a run for its money in a 'how close to your tail can you get your nose' competition.

After a few tweaks and design alterations, I now have a rocket that fulfils the original design brief, and pretty much behaves itself in flight, though I have still had to use rather more struts than I would have liked. But that first flight is indelibly printed in my memory, and all Kudos to Jeb and the rest of the crew for not hitting the abort button when prudence dictated that's exactly what they should have done.

[TrooperCooper]

Yeah I dislike that game mechanic as well. If we want to launch anything bigger than sattelites, we basically have to either build pyramids or use struts,... which always gives an odd look to the rockets imo. Just recently I found the joint reinforcement mod and tried it out for the first time. And voila... all of a sudden you can build real rockets that dont behave like underpressurized bananas thrown into the air...

Give it a try.

http://forum.kerbalspaceprogram.com/threads/55657-1-0-2-Kerbal-Joint-Reinforcement-v3-1-3-4-27-15

[WWEdeadman]

Most of what I've seen with wobbly rockets as of 1.0 is because of the gimbal of the engines when SAS is on. I set the gimbal down to like 30-40 percent now mostly, and viola, no wobbling.

[serolrom]

It also helps a lot to use a joystick. Steering with the keyboard is an all-or-nothing every time you press the key.

[Scarecrow88]

I did think about trying it with the SAS switched off, as most of the wobbling did seem to be centred around where decouplers were attached to rocket nozzles.

Here's a pic of the final ship that I finally got to behave itself after a lot of trial and strutting.

[SAI Peregrinus]

IIRC there's a bug with engine gimballs right now that makes them reach full deflection instantly. So the old "rapidly tap the keyboard key to get a partial deflection" doesn't work, it just vibrates the rocket into ripping apart.

[Jouni]

The reason why rockets wobble is usually because they look like this:

There's a heavy payload connected to a heavy lower stage through a narrow midsection, separated by a decoupler that's way too small. Both the payload and the lower stage have too much control authority. Because the payload and the lower stage have different periods of oscillation, SAS actively drives the oscillation instead of damping it. Aerodynamics may also contribute to the oscillation in one way or another.

[Green Baron]

Like TrooperCooper said: Kerbal Joint Reenforcements immediately makes a rocket out of a snake.

:-)

k

Edit: Oops, just saw the design ... probaly even KJR might be overstrained here. Pls let me suggest to streamline it a bit and maybe 3 economically placed struts ...

Edit again: ninja'd by sal_vager .... yes, that's a rocket :-)

Edited May 30, 2015 by kemde

[sal_vager]

If you're careful, you can get away with far fewer struts than you think, for example with this rocket.

There are a total of eleven struts, eight are under the fairings, two are on opposite sides of the narrow midsection, and one more lives between the upper fairing base and the payload, of these only the two exposed struts on the "neck" are contributing to drag.

Not only that, but the craft only wobbled along the east/west axis during launch, so all the struts are east/west, there was no need for any more.

The SRB's don't use struts at all, as they are attached at the tops (offset tool), when they burn they are forced out, to jam themselves in one place against the decouplers maximum joint flex.

As a bonus, the decoupler force is at the top so they separate much more cleanly than SRB's that are held by their center.

[Scarecrow88]

I've just downloaded KJR, and it does make a difference. I don't like relying on such mods to make my rockets flyable, and I've never had to in the past. Any wobbles I have had previously have been manageable, but what I experienced with this latest design was in another league. Admittedly, I did resolve the problem eventually, but I think it would be nice if a mod like KJR was standard, as I am now aware that if I build a rocket that I'd like to share because it flies perfectly, the first person to download it that doesn't have KJR installed could be in for a nightmare ride.

[marza]

What was the TWR of your rocket at launch?

I don't have KJR installed and have seen my share of spaghetti rockets. I've found that if struts are not effective, another thing to consider would be to adjust engine gimbal (as mentioned above) or to simply slow down. Every spaghetti rocket I've had was due to a heavy payload/configuration that has a large amount of drag at the top of the rocket and a powerful first stage with a TWR of 2.5 or more. When the rocket in question has a mass of a few hundred tonnes, the forces acting on it are mind boggling huge. Keeping your TWR at no more than 2 during launch will get most, if not all, stock spaghetti rockets out of the atmosphere.

[Murph]

Speed isn't the major problem. Stock 1.0.2 just doesn't allow tall and heavy 2.5m rockets properly, with quite low TWR (1.3-ish on pad, never rising above 2.0 while there's air to worry about, nowhere near going supersonic in the early stages of ascent). 3.75m, on the other hand, I find they are much better and are mostly ok with no struts on the 3.75s themselves, just to prevent boosters flexing and giving non-aligned thrust, and to prevent the payload wriggling like a caught worm. Squad need to stop buying Bend-o-Maticâ„¢ brand stack joints for 1.25m and 2.5m, and actually get something decent instead.

[Jouni]

Speed isn't the major problem. Stock 1.0.2 just doesn't allow tall and heavy 2.5m rockets properly, with quite low TWR (1.3-ish on pad, never rising above 2.0 while there's air to worry about, nowhere near going supersonic in the early stages of ascent). 3.75m, on the other hand, I find they are much better and are mostly ok with no struts on the 3.75s themselves, just to prevent boosters flexing and giving non-aligned thrust, and to prevent the payload wriggling like a caught worm. Squad need to stop buying Bend-o-Maticâ„¢ brand stack joints for 1.25m and 2.5m, and actually get something decent instead.

The problem is more nuanced than just too bendy joints. I'll demonstrate it with a tall 2.5 m rocket.

The rocket goes supersonic at a low altitude, but it doesn't wobble too much at that point. The problems start later in the ascent, when the center of mass rises above the decoupler. As long as the center of mass was within the same relatively rigid region of the rocket as the engines, everything went fine. Bad things happen, when the engines try to rotate the rocket around a point they aren't rigidly attached to.

Let's make a small change to the rocket and add a probe core to the lower stage.

Now the rocket starts bending as soon as I start the gravity turn. It seems that having many joints close to each other is a bad idea, if the lower part is pushing the upper part too hard. We can solve this by reducing the number of joints, the TWR, or the mass above the joints.

Let's fix the rocket.

Now it goes to space just fine.

[Bev7787]

I'm building a Saturn V. It bends immediately on launch even with engine gimbals turned off, except for the first stage gimbals. It is annoying.

[mythbusters844]

I'm building a Saturn V. It bends immediately on launch even with engine gimbals turned off, except for the first stage gimbals. It is annoying.

Got reminded of this thing I made when I tried making a Saturn V

It took two days of strutting before the thing stopped bending on launch, but it still wobbled in flight. It took a week of trial-and-error before it flied satisfactorily.

Edited May 30, 2015 by mythbusters844

[problemecium]

This thread is strange to me. I play 1.0.2 with no joint reinforcement mods and full engine gimbals, and my rockets barely flex at all, with the exception of floppy payloads that obviously should be strutted down.

[Motokid600]

Ladies and gentlemen. Change your root part to that closest to the center of mass after your finished construction.

No more wobbly rockets.

[gogozerg]

Most of what I've seen with wobbly rockets as of 1.0 is because of the gimbal of the engines when SAS is on.

Speaking of stock SAS, I always wondered why it was so bad (compared to Mechjeb's Smart A.S.S.). Wrong impulses, wobbling, stabilization... My conclusion is that Squad do not want efficient assists. But the problem now is we got assists that make the game more difficult with default configuration.

Ladies and gentlemen. Change your root part to that closest to the center of mass after your finished construction.

No more wobbly rockets.

Really? Why would the root part change the simulation? Which manual page please?

And when you split the ship?..

Edited May 31, 2015 by gogozerg

[Fel]

This thread is strange to me. I play 1.0.2 with no joint reinforcement mods and full engine gimbals, and my rockets barely flex at all, with the exception of floppy payloads that obviously should be strutted down.

I tend to take it as "I don't like mechanical engineering." Regardless of how people perceive real rocket design to be, this is a game and hence has game based limitations and game removed limitations; struts are just an answer to restricting people from making obscenely large rockets as if childsplay.

I also ask that people remember that momentum is in effect; trying to maneuver heavy rockets without RCS is very difficult.

- - - Updated - - -

Really? Why would the root part change the simulation? Which manual page please?

And when you split the ship?..

Well, each part has its own idea of the forces being applied to it; and orientation of the root part can cause really crazy things (I forget the exact problem but it is RCS associated), so the root part is likely used in the SAS calculations, being placed closer to the center is more accurately the vessel forces than on top.

[raidernick]

struts are just an answer to restricting people from making obscenely large rockets as if childsplay.

That is not true. Struts were introduced by Squad as a lazy fix to their wobbling joints problem. This "problem" was supposedly fixed in latest iterations of the game but like every other problem with the game, obviously not fixed at all. If anything struts are there as a crutch for people with ridiculous designs to somehow get them to fly, not the other way around.

Edited May 31, 2015 by raidernick

[Motokid600]

Speaking of stock SAS, I always wondered why it was so bad (compared to Mechjeb's Smart A.S.S.). Wrong impulses, wobbling, stabilization... My conclusion is that Squad do not want efficient assists. But the problem now is we got assists that make the game more difficult with default configuration.

Really? Why would the root part change the simulation? Which manual page please?

And when you split the ship?..

Crafts in ksp are built like trees. Everything is branched off of the root part of the craft. So your root part is the command pod up top for instance. Imagine holding a penicil at the tip and trying to control it. Now imagine holding the same pencil at the center and now control it. Much easier and more stable. No splitting of the ship and im not sure what you mean by manual page. With the root part gadget or whatever its called tool in the VAB. After your finished construction flip on the CoM indicator to see where its at. ( will most likely be in the first stage fuel tank so thats the one you want rooted ) Hit the root part button, click on a part next to the part you wish to be the new root then select the part. Now your craft is rooted from the center out.

Im not sure of the technicalities and forgive me if that explanation is a bit foggy. But it works really well. Combine that with KJR and its perfect. As for the people who dont like KJR because it makes things too strong, well then you havent tried FAR yet..

Edited May 31, 2015 by Motokid600

[StrandedonEarth]

That's not wobbly. This is wobbly.

[Warzouz]

Yeah I dislike that game mechanic as well. If we want to launch anything bigger than sattelites, we basically have to either build pyramids or use struts,... which always gives an odd look to the rockets imo. Just recently I found the joint reinforcement mod and tried it out for the first time. And voila... all of a sudden you can build real rockets that dont behave like underpressurized bananas thrown into the air...

Give it a try.

http://forum.kerbalspaceprogram.com/threads/55657-1-0-2-Kerbal-Joint-Reinforcement-v3-1-3-4-27-15

I use it also this should be the normal game mechanic. At least everything in fairing shouldn't wobbling at all.

[hellion13]

Kerbal joint reinforcement for the win. On the other hand, it makes for easy flight. Almost too easy. Struts don'teed to be used anymore if you follow a logical rocket design.

A full on 5 stage kerbalized one man Saturn V rocket with lander(and fairings) flies like a Saturn V, even has a minor(very minor) bit of full body flex against the atmosphere right before first stage burn out. Add some canards(2x symmetry x2on the big engine cowlings) and it will fly stable a full 20 degrees out side of the velocity vector.

[Scarecrow88]

I'm building a Saturn V. It bends immediately on launch even with engine gimbals turned off, except for the first stage gimbals. It is annoying.

Strangely enough I didn't have a problem with my Saturn V looky likey.