Help with FAR Rocket

[Ringkeeper]

Hi,

i need some help because i don't get it, what i'm doing wrong.

That is my rocket:

http://imgur.com/3dIe8nX

The payload is 3.2 tons , the upper stage 6 tons , so the rocket is not too heavy at the top.

Still it tips over at around 3 km . Going up totally straight. Speed is also around 30 m/s slower then the thermal velocity . More canard didn't help.

Any idea why this happens?

Edited February 9, 2014 by Ringkeeper

[SpaceGremlin]

You might actually want to move your center of mass higher. Rockets want to fly heavy end first. The further apart the CoM and center of pressure the more stable it will be.

[ddenis]

Seems ok, I may want to change the throttle of main engine to 1.7, also adding a reaction wheel can help. If nothing helps, provide me with your .craft file, so I would be able to try it myself.

[thescientist]

The payload is 3.2 tons , the upper stage 6 tons , so the rocket is not too heavy at the top.

The top should be heavy to keep the rocket stable. Also, try to use bigger wings (as Delta Wings) and an engine with a good gimbal. Extra toque force (reaction wheels) might help.

Just a qustion: are you sure that an aerodinamic stall doesn't occur? because control surface became uneffective if stalled or at trans-sonic speed.

[Guest]

Something else you might try is using a fat upper stage instead of that skinny rail so that the fairing blends smoothly. And, as has been said, a reaction wheel on the top of the first or second stages never hurts.

[Captain Sierra]

Swap the upper stage out for a shorter 2.5m tank and the twin bell 2.5m engine (that's the poodle equivalent). The skinniness is what's killing you.

[Taki117]

Are you starting your gravity turn sooner? Since FAR changes the aerodynamics of KSP you don't need to wait until the usual 10km to start your turn. I usually start gradually turning about 200-300m. Also, instead of using Canards (Which have a tendency to want to fly backwards anyway) use the Delta Deluxe Winglet, it will help at trans-sonic speeds. I don't know why they help, but they do. (The same goes for the AV-R8 winglet I've had it start rockets spinning uncontrollably) Also, instead of just having more fuel in your second stage, use less fuel, but use boosters on the side, and depending on how big the booster is you can not fire your center stack until they burn out. One Question, how much dV does it have?

[Hodo]

I have to agree 150% with Taki. In FAR you can do more realistic launch profiles and perform your gravity turn at a much lower altitude. That and canards are horrible for rockets.

[RSwordsman]

You might also try throttling down. FAR greatly reduces the density of the atmosphere, meaning that you can get away with 50% or lower thrust for certain lifter designs and still hit that gravity/air resistance butter zone. Your rocket will get much higher and into the less disruptive air before you're going really fast, which helps me with those disobedient rockets.

[Ringkeeper]

wow didn't thought of that many answers that fast

Well, to answer the questions:

- Gravity turn i didn't even start, as already at 2-3km the rocket flips

- Sonic speed not reached yet, still below 200 m/s

- if the top needs to be heavier, how do you send light satellites to space?

Here is the craft file how its at the moment:

https://www.dropbox.com/s/tgzo95q64b7hat2/MiniMusHopper.craft

*EDIT*:

hmm... ok.. even the "make you rocket heavier" makes logical no sense , it worked....

changed the upper stage to 2.5m with the little twin engine and the lower stage to 2.5m long tank with quadruple engine and the rocket lifted off and flew like it should. More tax money spend but oh well

Thanks for the help

Edited February 9, 2014 by Ringkeeper

[ferram4]

Get rid of the skinny upper stage. The gigantic drop in cross-sectional area right behind the fairing (and thus, ahead of almost all of the mass) is creating a stupid amount of drag and is making it unstable.

And drop your TWR down to 1.4 or so. If you're hitting 200 m/s before you flip at 2 km you're not going to be able to control the thing. Use a smaller core engine (like say, the Vesta) and use small boosters if you need to get the TWR > 1.

[maccollo]

- Sonic speed not reached yet, still below 200 m/s

200 m/s sounds like an awfully low terminal velocity for any rocket. The lowest I've had is 300.

wow didn't thought of that many answers that fast

- if the top needs to be heavier, how do you send light satellites to space?

It's not about the mass, but the density. You have a huge ass fairing on top of the rocket acting like a big sail.

Try to make the payload smaller so that you can make the fairing smaller. Or make the second stage more compact.

[Sovek]

Ok, I can't help you as much as I could since I cannot load the ship in the VAB as you have some mod I do not have, and I have alot of mods.

Some things though, that skinny upper stage is killing you, ditch it in favor of either A: 2.5m parts or B: Parallel staging with other 1.25m parts.

Edited February 9, 2014 by Sovek

[Ringkeeper]

Ok, I can't help you as I cannot load the ship in the VAB as you have some mod I do not have, and I have alot of mods.

Well, resolved the problem anyway thanks to the help in the posts.

It might be the LED flood light from the KOSMOS mod. Even i don't use the mod anymore but i keeped the light, as it is small and strong

[Bobnova]

It's a drag thing, the highest drag part of the rocket wants to be in the back. If you have a massive fairing on the top it not only creates a ton of drag, it also blocks the airflow from getting to anything below it that is of the same diameter or less. Control surfaces that don't stick outside that area of blocked air do almost nothing, as they're sitting in the big vacuum pocket left behind by the fairing.

I've had a lot of issues with this