Even the simplest rockets flip over!

[m4inbrain]

Sorry, slightly off topic: Nathan Kell as in Kell Hounds?

[Bill Phil]

I think I know your problem...

The CoM (Center of Mass) is getting torqued into the airstream's direction. Essentially, make it longer. Add more tanks. Get the center of mass higher up.

[moogoob]

The key if using un-controlled fins is to not use too many - just enough to bring drag backwards. The new atmo is thin enough to do a proper gravity turn if you start it early enough, even with static fins.

[giltirn]

I think I know your problem...

The CoM (Center of Mass) is getting torqued into the airstream's direction. Essentially, make it longer. Add more tanks. Get the center of mass higher up.

This causes my rocket to flop around like a wet noodle. Until struts become available it looks like it is virtually impossible to make something that will reach orbit with more than a pod for payload.

[Bill Phil]

This causes my rocket to flop around like a wet noodle. Until struts become available it looks like it is virtually impossible to make something that will reach orbit with more than a pod for payload.

That's due to the sheer number of connection points. Not avoidable, since you get the smallest tank first in the tech tree...

Are you using SAS? That can actually make it hard to control.

[m4inbrain]

Struts won't help you there. You can kinda "circumvent" the problem by using SAS only occasionally. It's the SAS, turning your rocket into

[GoSlash27]

GoSlash27: Ah, gotcha. Well, TV will be close to what it is in FAR, I suspect, so it'll be pretty high, and darn hard to catch even at 2Gs. For the aeroFX, unless things have changed that's based on density * velocity^3 which has an extra factor of velocity compared to drag (drag is 1/2 density * velocity^2) so it probably won't be that reliable. However, it should be very reliable for heating intensity, if heating tracks v^3 (as it probably does).

NathanKell,

I know you're the go-to source on FAR aero, so glad to have you aboard!

While 2G is unlikely to catch Vt vertically, it should still be a hard limit for any angle short of vertical. The aero FX are rough for the job, but IIRC they kick out early enough to be useful.

It's sort of a "feel" thing.

Best and thanks!

-Slashy

[giltirn]

I am trying to launch this rocket:

It's about the simplest thing I could come up with to launch a 5T payload. Despite having the center of lift below the center of mass, it still tips over if I turn it even slightly towards the east. Any ideas what is going wrong?

[GoSlash27]

giltirn,

I recommend ditching all the stuff about "moar struts and SAS".

Your ship is adequate to hold it's heading as it is if you fly it correctly.

You just need to be gentle with it. If you can keep an eye on the acceleration, "feel" when it's starting to approach it's limits, and throttle accordingly, you'll be fine.

1.0 is placing a higher demand on your piloting skill and you probably won't be able to crutch it through design.

Best,

-Slashy

[smjjames]

Using the controllable winglets would probably help.

[Bill Phil]

I am trying to launch this rocket: http://i107.photobucket.com/albums/m309/giltirn/screenshot8.png

It's about the simplest thing I could come up with to launch a 5T payload. Despite having the center of lift below the center of mass, it still tips over if I turn it even slightly towards the east. Any ideas what is going wrong?

You then have the "pushing up on a broom" problem. It's better to be exactly on the dot.

Also, check it when fuel is gone.

- - - Updated - - -

giltirn,

I recommend ditching all the stuff about "moar struts and SAS".

Your ship is adequate to hold it's heading as it is if you fly it correctly.

You just need to be gentle with it. If you can keep an eye on the acceleration, "feel" when it's starting to approach it's limits, and throttle accordingly, you'll be fine.

1.0 is placing a higher demand on your piloting skill and you probably won't be able to crutch it through design.

Best,

-Slashy

I...

Actually flipped a few rockets over head over heels and managed to orbit them...

I actually enjoy it.

[Murph]

I...

Actually flipped a few rockets over head over heels and managed to orbit them...

I actually enjoy it.

It wasn't a loss of control, it was a … planned gyroscope calibration manoeuvre. *cough* ;-)

[roosterr]

I started using FAR about a year ago, and went through this same thing. The tanks drain from top to bottom, which lowers your center of mass. And of course you want to keep your COM high, so staging helps with this because you will then drain the bottom tanks first. You can also put a decoupler (and disable it in the VAB) directly above your booster engines, cutting them off from the fuels tanks, then run a fuel line from the engines to the high tank. I know this sounds counter-intuitive but that will pull the fuel up and drain the lower tank first, keeping your COM in check and even raise it. Then MaxQ (max aerodynamic pressure) comes into play. You gotta keep it slower in the thicker atmosphere, then hammer the throttle as you get to the higher altitudes. Sometimes an extra SAS module will help a lot too.

Funny thing, after a year of FAR, haven't flipped a rocket in a long long time, and dont even think about it. it is second nature to build and fly them stable now. You will get there.

- - - Updated - - -

I am trying to launch this rocket: http://i107.photobucket.com/albums/m309/giltirn/screenshot8.png

It's about the simplest thing I could come up with to launch a 5T payload. Despite having the center of lift below the center of mass, it still tips over if I turn it even slightly towards the east. Any ideas what is going wrong?

You need to bring the center of pressure lower than that.

[SRV Ron]

Testing of both my orbiter and three Kerbal orbiter found the following;

1. Single orbiter cannot be turned until the SRBs are staged, about 10,000 meters. Then it has to be turned gradually or it can swap ends.

2. The SRBs staged cleanly, no bumping

3. Reentry heat last much longer as slowing down takes longer. Aim for a 30,000 ,meters for a gentle slowdown to land.

4. With the heat shield, used about 160 units, the pod wants to swap ends. Still, it is easy to use SAS to hold in place. That needs to be fixed.

The three man design. (Without the monoprop)

1. Much more stable flight. Still need to turn gradually but easily went too high by burnout of the first state.

2. I needed to throttle back, then back to full throttle to control acceleration and stress in the denser atmosphere. Real Shuttle launches need to do the same for the same reason.

3. Aimed for the same 30,000 meter window. Same instability with heat shield as the one man pod. Only used one unit on the heat shield due to the ideal reentry window. Still easy to hold in place but needs to be fixed to be stable with SAS off.

Evaluation;

1. Had no problem getting into orbit with either design.

2. You do need to do the orbital turn gradually following the Navball due to the enhanced aerodynamics. Effects lessen with altitude.

3. Do shallow reentry. The new physics results in a longer slowdown. It is much easier on the craft and heat shields.

4. Bringing back Material Bay and other items, other then Goo Pods mounted high on the pods, is going to cause stability issues on reentry. Recover the science and ditch the equipment or drop them from orbit with their own heat shield and remote control pod.

Edited April 28, 2015 by SRV Ron

[Scotius]

I've read through this thread, and am slightly confused. I reached the space literally in my second flight yesterday - not orbital mind you, but AP was at 147 km. I had only small tank, LV-45 engine, first decoupler and small heatshield unlocked from tier 2 by then. No problems all the way - my tall and thin rocket was stable without any fins or struts. No flipping attempts, no wobbling. I started a gentle turn East couple of hundred meters above launchpad, throttled down a bit to not exceed 300 m\s while in dense atmosphere, and fully accelerated around 15 klicks up.

[giltirn]

I've read through this thread, and am slightly confused. I reached the space literally in my second flight yesterday - not orbital mind you, but AP was at 147 km. I had only small tank, LV-45 engine, first decoupler and small heatshield unlocked from tier 2 by then. No problems all the way - my tall and thin rocket was stable without any fins or struts. No flipping attempts, no wobbling. I started a gentle turn East couple of hundred meters above launchpad, throttled down a bit to not exceed 300 m\s while in dense atmosphere, and fully accelerated around 15 klicks up.

Can you provide a craft file? Mine flip end over end constantly, and are real pigs to turn; a gentle turn always ends up with an out-of-control drop towards the horizon then the ground. I just managed to reach orbit again for the second time in 6 hours. This is going to take some getting used to if there isn't a bug!

[moogoob]

Not necessarily - any fins that move the centre of mass far enough back will do.

Sorry to quote myself - went back and tested this (new career save) and yes the rigid fins make turning a real chore. Only solution is to wait until the LV-t45 and start turning right away or keep your speed low and pray. That sometimes works.

Man, 1.0 is exciting. It's a crapshoot even ascending, and they didn't have to model random part failure or anything to do it.

[tg626]

Now turn on the Com and col and move the fins till the col is just behind the com...

[NathanKell]

GoSlash27: thanks!

As it happens I have a plugin I wrote that helped during experimentals called AeroGUI. I meant to release it with 1.0 but I've been busy answering stuff on the forums. I'll add terminal velocity to it, though it will only be approximate--since vessel Cd shifts with mach, your instantaneous terminal velocity (i.e. based on your _current_ Cd) will not ever quite be your actual terminal velocity. If nothing else, the ratio of weight (in kN) to drag force will tell you what ratio of terminal velocity you're at.

[NathanKell]

aaaand AeroGUI is up.