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Construction Help: Rocket tilts out of control?


McCoy
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I'm playing career, trying to get a satellite 'round the Mun.

The problem is that this particular rocket and other rockets I have built have a tendency to spin out of control -- this one in particular goes towards 90 degrees and then will just spin and spin and in the process lose lots of acceleration by virtue that the vector has changed.

I don't know if it is a center of gravity problem, the CoG ball in the builder is a bit vague when you're looking for precision.

Here is the contents of .craft file: http://pastebin.com/6E2ABxhB

You can copy that into a text file, rename it .craft and place it in a sandbox game save's VAB folder.

I ask the advice of the Kerbal Elite to see where my construction methods (if that is the case) have gone wrong. Please try my .craft and tell me what is going on!

Thank you in advance,

McCoy

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I look at your craft file. It has strange fuel lines( 4 to the center tank). The staging ejects the tanks that are full first. Only two rcs thrusters?

The flipping (I am assuming you are talk about in atmo) is Center of mass issue usually. Swivel engine for the central stack to add control and fins or other control surface. Should fix some of the problem. The ship is really overpowered. Meaning that drag will affect you ship more (ie flipping). Keep speed below 300m/s below 10,000 meters will make it more controllable.

Trying not to steal to much of you fun. I hope this guides you in the right direction.

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Made a picture of the craft, the second one is my suggestion: http://imgur.com/a/ZEVRj

You have an extremely traditional pre-1.0 pancake lifter. All your stages are arranged horizontally and keep your CoM low while giving you an insane amount of thrust. At the same time your payload is rather unaerodynamically shaped and has a lot of draggy bits sticking out in every direction which makes it easy for the aerodynamic forces to spin you out of control once you reached the speed of sound (which you will hit early and low with that TWR). And as AlanP said: the staging and the fuel lines don't make sense.

You need to keep you CoM up and your Center of Pressure low. That means adding upper stages with enough mass and shielding you payloads with fairings and/or adding fins to the bottom. The second picture in that album is my quick and dirty version of your craft. I ditched one of the booster sets, added an upper stage and fins (the basic fins are not enough to keep it stable). It is still absolutely overpowered but can get through the atmosphere.

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Pictures of the ship are usually enough to diagnose problems like this. Can you upload some screenshots to Imgur or one of the other image-sharing sites and include their links in your posts?

I apologize, I hadn't thought of that! :blush:

I look at your craft file. It has strange fuel lines( 4 to the center tank). The staging ejects the tanks that are full first. Only two rcs thrusters?

The flipping (I am assuming you are talk about in atmo) is Center of mass issue usually. Swivel engine for the central stack to add control and fins or other control surface. Should fix some of the problem. The ship is really overpowered. Meaning that drag will affect you ship more (ie flipping). Keep speed below 300m/s below 10,000 meters will make it more controllable.

Trying not to steal to much of you fun. I hope this guides you in the right direction.

Not at all! I was really frustrated because I've played the game a lot, mainly in sandbox mode, especially pre 1.0; Scott Manley's videos were (and still are) of great help.

Yeah, the staging was messed up ejecting the wrong tanks, usually I fixed at the launch pad, boo-boo on my part. I figure that perhaps that the large TWR ratio in ascent along with 4x radial fuel tanks caused the imbalance upon ascent. I did try adding controllable fins to the ejectable tanks before I came onto the forums but... didn't make much of a difference.

I always make it a point to not break the sound barrier if possible under most conditions (below 200m/s speed) because the atmospheric drag, although perhaps things are a bit different post v1.0.

Made a picture of the craft, the second one is my suggestion: http://imgur.com/a/ZEVRj

You have an extremely traditional pre-1.0 pancake lifter. All your stages are arranged horizontally and keep your CoM low while giving you an insane amount of thrust. At the same time your payload is rather unaerodynamically shaped and has a lot of draggy bits sticking out in every direction which makes it easy for the aerodynamic forces to spin you out of control once you reached the speed of sound (which you will hit early and low with that TWR). And as AlanP said: the staging and the fuel lines don't make sense.

You need to keep you CoM up and your Center of Pressure low. That means adding upper stages with enough mass and shielding you payloads with fairings and/or adding fins to the bottom. The second picture in that album is my quick and dirty version of your craft. I ditched one of the booster sets, added an upper stage and fins (the basic fins are not enough to keep it stable). It is still absolutely overpowered but can get through the atmosphere.

Rhodan, I tried a test lift vehicle based on your second photograph and I got into orbit. The only mod I use is Kerbal Engineer Redux and although it provides readout while in VAB, it can sometimes get buggy and lie about delta-v; I found with your design, delta-v was under-estimated once in orbit because of the low payload mass.

What do you mean by pre-1.0 pancake lifter? I'm playing career so researching fairings will be next, with the new aerodynamics in v1.0 it seems that things are a lot more sensitive, which is great -- but it sucks when you've played pre-1.0 for so long you get used to bad habits that need to be broken.

Given my original design, what engines would you use to keep the TWR above 1.0? I've always had the center engine the swiveling LV-T45 and the external engines the T30s. This low in research tree the LV303s have terrible atmospheric ISP and the weight of adding them on versus the thrust they'd provide seems overall a loss to total delta-v.

Also, what is center of pressure? I love this game, I just find that the default tools don't give you enough information or if it does, it doesn't give it accurately enough.

What mods do you guys use? I want to construct better rockets and spacecraft and as I mentioned previously, all I use is Kerbal Engineer Redux, in terms of mods, but what mods do you guys use to help build better spacecraft? Or what heuristics or rules of thumb do you guys use when building a rocket, in general?

I appreciate the help, honestly. If anyone wants to add me on STEAM, you can send me a private message on here.

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I always make it a point to not break the sound barrier if possible under most conditions (below 200m/s speed) because the atmospheric drag, although perhaps things are a bit different post v1.0.

Actually, you can go a bit faster than that-- 300 m/s is fine. Once you're over 10 km, can crank it up.

What do you mean by pre-1.0 pancake lifter?

"Pancake lifter" = a ship with a flat-pancake front end. Before 1.0, it didn't matter what the shape of your craft is, the drag stayed the same regardless. Therefore, very short, squat craft with "flat" front ends did just fine. However, 1.0 has more realistic aero, and you really increase your drag that way-- need to go for taller, slenderer craft, and make sure that the front end is pointy.

Given my original design, what engines would you use to keep the TWR above 1.0? I've always had the center engine the swiveling LV-T45 and the external engines the T30s.

The 45's and 30's make great early-game engine. Good TWR, reasonable Isp both in vacuum and atmosphere, nice and low in the tech tree.

Definitely save the vacuum engines (like the Terrier) for upper stages that don't fire until you're over 20km-- they're simply not worth it to use at sea level, just a waste of fuel.

What mods do you guys use? I want to construct better rockets and spacecraft and as I mentioned previously, all I use is Kerbal Engineer Redux, in terms of mods, but what mods do you guys use to help build better spacecraft? Or what heuristics or rules of thumb do you guys use when building a rocket, in general?

Personally, I don't use any mods at all at design time-- after a while you develop a pretty good seat-of-the-pants feel for what will do the job. When I want to know dV a bit more precisely, I just haul out the ol' calculator and plug in Tsiolkovsky's formula.

General rule of thumb is that each lower stage should be an approximately constant multiple of the stage above it, in terms of mass-- for example, double.

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I'd consider making the center stack have two of the 800s ... your TWR is too high for current conditions. This would likely allow you to drop to two radial tanks (and engines) instead of four. This would also help launch stability, though you really should add some fins too.

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If you have the spare delta v/fuel, go slower. The benefit to this solution is it requires no design changes. (as you go slower, there's less flip inducing drag on your nose) which buys you more altitude, which reduces drag, which gets you faster, which gets you higher, which gets you less drag...

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I'm playing career, trying to get a satellite 'round the Mun.

The problem is that this particular rocket and other rockets I have built have a tendency to spin out of control -- this one in particular goes towards 90 degrees and then will just spin and spin and in the process lose lots of acceleration by virtue that the vector has changed.

I don't know if it is a center of gravity problem, the CoG ball in the builder is a bit vague when you're looking for precision.

Here is the contents of .craft file: http://pastebin.com/6E2ABxhB

You can copy that into a text file, rename it .craft and place it in a sandbox game save's VAB folder.

I ask the advice of the Kerbal Elite to see where my construction methods (if that is the case) have gone wrong. Please try my .craft and tell me what is going on!

Thank you in advance,

McCoy

Well, I copied your data and flew it.

I don't know what parts you are limited to if you are in career mode, but I added some fins. This helped with the stability, but it still didn't like the shedding of the first two tanks. I then added the Probodobodyne OKTO unit. Very little weight.

Yes, as pointed out, your staging is wrong. You need to switch your first two staging decouplers sequence.

Anyway, once in space, I didn't even make a Kerbal orbit. Almost, but not quite. It won't go to the Mun.

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I don't understand why are you looking at delta v of 4.7 and thinking it will go to the mun. Especially when you are not looking ar athmospheric delta v.

You need 2500 ATHMO delta v + 1500, + 1300 to make Mun orbit. Then another 300 to return or 2000 to land and return.

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Rhodan, I tried a test lift vehicle based on your second photograph and I got into orbit. The only mod I use is Kerbal Engineer Redux and although it provides readout while in VAB, it can sometimes get buggy and lie about delta-v; I found with your design, delta-v was under-estimated once in orbit because of the low payload mass.

What do you mean by pre-1.0 pancake lifter? I'm playing career so researching fairings will be next, with the new aerodynamics in v1.0 it seems that things are a lot more sensitive, which is great -- but it sucks when you've played pre-1.0 for so long you get used to bad habits that need to be broken.

Given my original design, what engines would you use to keep the TWR above 1.0? I've always had the center engine the swiveling LV-T45 and the external engines the T30s. This low in research tree the LV303s have terrible atmospheric ISP and the weight of adding them on versus the thrust they'd provide seems overall a loss to total delta-v.

That wasn't a really good design. I just tried deviating as little as possible from of your design. You should actually build something that resembles a real rocket instead of a bunch of small boosters. Try aiming for long and sleek with radial boosters only as an option to provide additional thrust to push the rest of the rocket away from the launch pad. That high TWR is telling you that you could carry much more fuel per engine than you are actually carrying right now. A TWR around 1.5 - 2 is enough to launch and in the best case you shouldn't even have to throttle down during ascent.

Also, what is center of pressure? I love this game, I just find that the default tools don't give you enough information or if it does, it doesn't give it accurately enough.

What mods do you guys use? I want to construct better rockets and spacecraft and as I mentioned previously, all I use is Kerbal Engineer Redux, in terms of mods, but what mods do you guys use to help build better spacecraft? Or what heuristics or rules of thumb do you guys use when building a rocket, in general?

The CoP is more or less the median point of the applied aerodynamic forces on your launcher. If it rises above the CoM (the point your machine is basically rotating around) it might overpower your control authority. After breaking the sound barrier those forces multiply and a lot of draggy stuff at the top of your rocket will make the CoP jump upwards. That's why it is generally better to follow the golden rule and add draggy stuff at the bottom (fins) and heavy stuff at the top (well fueled upper stages). The problem with the CoP is that there's no tool telling you where it is because it mostly relies on the shape and direction of your rocket. And on your rocket the top parts of the boosters are above the CoM and therefore add drag on the wrong end, that design is just unsavable.

An explanation with a nice picture: http://forum.kerbalspaceprogram.com/threads/121353-Sooo-why-exactly-rockets-are-flipping?p=1939964&viewfull=1#post1939964

Another problem seems to be that the small rocket parts are really prone to flipping. Probably because of the higher surface area compared to their mass.

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I appreciate all the input and I'm sure I'll have this post bookmarked to go over it a few times, let it soak in. I think we can safely say solved/answered. You are right about the drag, pushing F12 and looking at the aerodynamic forces, the RCS radial tanks really are killer. The thing is, even with two of them (160 units of RCS fuel), not much in the way of delta-v to make deviations as I intended. I found a mod that helps with that and thrust in general, though.

windows_x_seven: I don't turn 90 degrees in a split second. Although it has me thinking of making a USB HID for finer-tuned control. I don't know much about FAR to use it rather than stock.

Thanks, anyone feel free to post your best try for a basic moon satellite and if you want to add me on STEAM, you can send me a PM.

Obligatory photographs: https://imgur.com/a/w4ApU

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I don't understand why are you looking at delta v of 4.7 and thinking it will go to the mun. Especially when you are not looking ar athmospheric delta v.

You need 2500 ATHMO delta v + 1500, + 1300 to make Mun orbit. Then another 300 to return or 2000 to land and return.

In this case there's more than enough monopropellant to make the transfer and circularize in any desired orbit. They don't really need to come back to Kerbin after that.

@McCoy -

Grats on getting it working!

Another approach to keeping your mass forward for the sake of aerodynamic stability is to lock the top fuel tank during launch and unlock it again later when the bottom tank is almost empty. The advantage of this approach over a two-stage launch when you don't have SAS is that the larger bulk doesn't rotate as badly as a smallish stage during the orbital insertion phase.

(In this example the main stage in an LV-45, a pair of Flea boosters get it moving fast enough off the pad for the fins to start being effective ASAP, and the launcher is dumped before final circulation. Monoprop is used from there. I also threw on a quad of linear RCS thrusters on the payload just to cut burn times.)

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My solution to this problem is incredibly cheesy, but it almost always works: flip the payload upside down and put it at the BOTTOM of the launch vehicle. :)

Among other engineering nonsense, you'll need to have 1st-stage engines to the sides of the payload to make this work, but I find it particularly satisfying to watch a formerly-unstable payload fly a perfectly stable launch trajectory, while I make rude gestures at the screen and go "screw you, physics!"

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