FAR ruining the rocket physics?

[9911MU51C]

Hello, after downloading the FAR mod, none of my rockets can even take off. Even the incredibly simple rockets just start violently turning end over end after about 5km, even with 5 SAS kits and massive wings. Can somebody tell me what's wrong with my rockets?

[Pecan]

Not just from that picture. Will it go straight up? The big thing with proper aerodynamics is you must keep manoeuvres subtle, so the facing indicator doesn't move far from the prograde marker, otherwise you'll stall and flip.

[Taki117]

I'm curious what that is on top of your rocket. If it's a fairing then there is another issue present, if it's not a fairing, then aw have issues in that FAR is going to calculate all that drag, plus whatever is inside that thing, and that could result in a very bad day.

[Jimbimbibble]

Take only the small winglets and move them to the very bottom of the rocket. Remove the big wings. When you fly it, don't move outside the prograde marker by more than a few degrees until you get to about 40km. When you design your rocket, keep the center of lift behind the center of mass or it will be unstable.

[FenrirWolf]

I can't tell what those parts are on the rocket either. All I know is that using a ton of wings and SAS modules to try fighting the air isn't going to do you favors like in stock.

What is your ascent profile like? Are you veering sharply away from prograde at any point? What is your TWR? You should be aiming somewhere around 1.2 to 1.8

[Amaroq]

I guarantee this is what is happening:

You have too much thrust, and you're turning too sharply.

Think of your rocket's current behavior as "A high-horsepower race car accelerating constantly from a stop down the main straightaway and into a tight first corner." The car spins out, of course -- that's basically what's happening to your rocket.

You need to re-learn the game with real aerodynamics, which means a very very different ascent profile than the "Up 10km, 45 degrees over" version.

1. Start your gravity turn almost as soon as you're clear of your launch clamps.

2. Try to keep your nose at the bottom edge of the circle created by the "prograde" reticule on the navball throughout the flight (while also making sure you're pointing above the horizon until your apoapsis is about 80km)

3. Throttle down as you ascend, so that you are sticking with a current TWR around 1.6 even as you bleed off fuel. (You can use Kerbal Engineer Redux to see the TWR created by your current engine, fuel load, etc.)

Stick with it -- once you have it working, its way more fun with FAR!

[Hodo]

I guarantee this is what is happening:

You have too much thrust, and you're turning too sharply.

Think of your rocket's current behavior as "A high-horsepower race car accelerating constantly from a stop down the main straightaway and into a tight first corner." The car spins out, of course -- that's basically what's happening to your rocket.

You need to re-learn the game with real aerodynamics, which means a very very different ascent profile than the "Up 10km, 45 degrees over" version.

1. Start your gravity turn almost as soon as you're clear of your launch clamps.

2. Try to keep your nose at the bottom edge of the circle created by the "prograde" reticule on the navball throughout the flight (while also making sure you're pointing above the horizon until your apoapsis is about 80km)

3. Throttle down as you ascend, so that you are sticking with a current TWR around 1.6 even as you bleed off fuel. (You can use Kerbal Engineer Redux to see the TWR created by your current engine, fuel load, etc.)

Stick with it -- once you have it working, its way more fun with FAR!

I can not agree more with this statement right here.

Right now the OP's rocket is just a bad design all around. It has TOO much power, to little fuel, and probably using a nose or parts that are not FAR compitable.

[Luckfish]

A while back I did a few dozen test ascents with FAR trying to learn what works best, and what affects my stability. In addition to everything that's been mentioned, I found that having NO wings or tailfins at all gave me better stability and control. Smaller = better, but the best I found (with the particular rocket I was using) was no control surfaces, only gimballing.

Edited July 28, 2014 by Luckfish
typo

[maccollo]

To much power? That's just a skipper, which would give a TWR of just under 1.5. That's perfectly reasonable.

What appears to be making this thing aerodynamically unstable is all those SAS units. Pressumably they are very low density like the stock SAS so they create a ton of drag above the CoM. The wings aren't exactly helping the situation. The small ones are above the center of mass and the big delta wings are right on top of it.

To make it work just remove all the wings and all the low density stuff on top. As long as you don't do any excessive turns it will fly just fine.

Even the KS-25x4 with it's crappy gimbal range doesn't need any controls surfaces to fly well.

http://youtu.be/qMojczhdDUw

Edited April 21, 2014 by maccollo

[DaMichel]

Tbh. this rocket looks okay. The only obvious design flaw is that the wings should be as far back as possible. The delta wings are so big, they have a huge stabilizing effect. I manage to launch pretty crazy things with the help of these ...

I'm not familiar with the mod parts. For the sake of getting better help, could you post a rocket with stock parts only please?

Check the position of the CoL in relation to the CoM with full and empty fuel tank. CoM has to be always in front of CoL. Another thing to check is if those mod parts make excessive drag. Right click on them in flight to see their drag force.

Edited April 22, 2014 by DaMichel

[Starstrider42]

Since nobody else has brought this up...

The flaps on the bottom set of fins are almost certainly too powerful. When combined with SAS, you get a rocket that's unstable in flight simply because it keeps overcompensating for tiny course changes. Either remove the flaps entirely (if you have five SAS modules, plus gimbals, you don't need aerodynamic control surfaces), or right click on them in the VAB and turn the "control authority" setting way down.

Had a lot of trouble launching rockets until I learned that trick, though most of them spun instead of flipping.

[9911MU51C]

The rocket in my picture was a result of me trying to get it flying straight after having just winglets and 1 SAS, and the top is http://kerbalspaceprogram.com/anvil-rockets/ fairings, with just a simple one man rocket in it. I'll list off what I've heard and my response

(The inside of the rocket and stats)

Maneuvering too rough/steep? - My problem was it'd start tipping at only 2km high and I was just trying to keep it upright, not that it'd start flipping as I turned

Too much thrust? - I did indeed have a 1.4 TWR, But I only had the engine turned up about 75%. I use the full thrust once I clear the thick atmosphere (But that isn't the problem here)

Aerodynamics problems? - Honestly I think this may be the problem, as I thought FAR just changed how the game in general calculated drag. If the anvil parts aren't compatible, that is probably the issue here. I'll test it out

tl;dr - I'm guessing it's incompatible ANVIL fairings that are causing drag on the top and making it turn end-over-end

Edited April 22, 2014 by 9911MU51C
Added image

[Hodo]

The rocket in my picture was a result of me trying to get it flying straight after having just winglets and 1 SAS, and the top is http://kerbalspaceprogram.com/anvil-rockets/ fairings, with just a simple one man rocket in it. I'll list off what I've heard and my response

(The inside of the rocket and stats)

http://i.imgur.com/oRaDYVX.png

Maneuvering too rough/steep? - My problem was it'd start tipping at only 2km high and I was just trying to keep it upright, not that it'd start flipping as I turned

Too much thrust? - I did indeed have a 1.4 TWR, But I only had the engine turned up about 75%. I use the full thrust once I clear the thick atmosphere (But that isn't the problem here)

Aerodynamics problems? - Honestly I think this may be the problem, as I thought FAR just changed how the game in general calculated drag. If the anvil parts aren't compatible, that is probably the issue here. I'll test it out

tl;dr - I'm guessing it's incompatible ANVIL fairings that are causing drag on the top and making it turn end-over-end

I don't think ANVIL fairings are compatible try using Procedural Fairings or KW parts. Those are two parts packs that I know are FAR compatible.

[Amaroq]

Yeah, mod incompatability would do it.

If you're moderately competent at config files, you can figure out how to update your local ANVIL fairings yourself; at a very minimum, please inform the mod developer of the problem, if that turns out to be it!

[federally]

Also, lift generating wings on the rear of Rockets cause stability issues.

You have a rocket that typically carries a lot of weight in the nose, your payload, which does not decrease as you travel. This is initially counter balanced by the fuel load in the bottom, however you burn through this as you go. Then you add wings that create lift at the rear of the rocket.

Eventually the weight in the bottom will decrease enough that the lift generated by wings will overpower the rockets SAS and send the rear end up over the nose.

Actual rockets don't use lift generating wings. They use flat surfaces that cause aerodynamic forces to push the tail down or up if it begins to move out of line with the direction of travel.

Stop putting wings on rockets, save them for planes.

[Pecan]

Oohhhh - nice looking ship, now you explain what it is. Good luck.

[Tortoise]

Well for a start, I have no idea what all those wings are on there for, but they increase drag as-well.

Edited April 22, 2014 by Tortoise

[9911MU51C]

Well for a start, I have no idea what all those wings are on there for, but they increase drag as-well.

As I stated above, they were there in a vain attempt to stop my rocket flying out of control, before it was just 4 winglets at the bottom of the rockomax tank.

- - - Updated - - -

I don't think ANVIL fairings are compatible try using Procedural Fairings or KW parts. Those are two parts packs that I know are FAR compatible.

Thank you! I'll go download those fairings instead

[thereaverofdarkness]

I guarantee this is what is happening:

You have too much thrust, and you're turning too sharply.

I guarantee you that is NOT what is happening. That rocket has its center of lift too far behind the center of mass. He wouldn't be able to turn it if he tried, not while going fast anyway.

Sounds like what happened to me the first time I tried FAR. Everything I flew would just go crazy shortly after I left the launch pad. Incredibly simple designs, perfctly stable designs, nothing worked. I did some testing and discovered that there were some automatic controls that were intentionally pushing my rocket off-course. I made several posts about it on the forums and also one on the FAR page in the Spaceport, but I never got a response from anyone who actually understood what was happening.

I never figured out how or why it was happening, but the second time I tried FAR it mysteriously didn't happen and everything was fine. I'd suggest you try uninstalling it completely and reinstalling FAR from scratch. This time, make sure you have the most up to date version. Also try keeping the FAR flight manager closed. You really don't need it. Just put the big fins on the back and your rockets will be prevented from going unstable.