FAR and Gravity Turns

[Invader Myk]

or Why does my rocket keep falling over at 18km?

Here's my problem. While using FAR (and KIDS might contribute) when I start my g-turn I either need to perform the initial 40^o of elevation change from 10km to 18km ASL or my rocket continues to fall over until it is pointing at the ground.

Prior to installing FAR I would perform my g-turn with the following elevations and altitudes (assuming near terminal velocity) and end up with an apoapsis of 100km and periapsis of ~30km.

[TABLE=width: 500]

[TR]

[TD=align: center]Elevation in degrees[/TD]

[TD=align: center]Apoapsis Altitude in kilometers[/TD]

[/TR]

[TR]

[TD=align: center]45[/TD]

[TD=align: center]10

[/TD]

[/TR]

[TR]

[TD=align: center]40[/TD]

[TD=align: center]20[/TD]

[/TR]

[TR]

[TD=align: center]35[/TD]

[TD=align: center]30[/TD]

[/TR]

[TR]

[TD=align: center]30[/TD]

[TD=align: center]35[/TD]

[/TR]

[TR]

[TD=align: center]25[/TD]

[TD=align: center]40[/TD]

[/TR]

[TR]

[TD=align: center]20[/TD]

[TD=align: center]45[/TD]

[/TR]

[TR]

[TD=align: center]15[/TD]

[TD=align: center]50[/TD]

[/TR]

[TR]

[TD=align: center]10[/TD]

[TD=align: center]55[/TD]

[/TR]

[TR]

[TD=align: center]5[/TD]

[TD=align: center]60[/TD]

[/TR]

[TR]

[TD=align: center]0[/TD]

[TD=align: center]65[/TD]

[/TR]

[/TABLE]

With FAR I have to start turning at 9km ASL just to get an elevation of 60^o at 20km ASL. By the time I can fall over to 0^o with out losing control I'm over 30km ASL and my apoapsis is almost 70km. When I stop burning with an apoapsis of 100km, my periapsis is usually -200km.

What can I do to maintain control of my rocket while making the fairly quick turn from 90^o to an optimal initial elevation for a g-turn, and what is that elevation with FAR?

[smjjames]

Showing a screenshot of the ship would probably help. There are ways of helping to stabilize that, like adding fins/winglets and putting a fairing on the top of your rocket, or even add SRBs to ram your way out of the atmosphere.

As I understand it, it's happening because the front end of your rocket is too light, though having an engine with a somewhat low TWR can also contribute to the problem.

Of course though, it's possible that you're doing the gravity turn wrong or too fast.

[IRnifty]

FAR requires that your turns only be of a certain angle at any given time. You cannot turn very suddenly using FAR because that causes more drag and flight instability on your craft than it does proper change in direction of travel. I can't explain why this is because that's getting into real world aerodynamics, which I am not good at, but trust my when I say you cannot point your rocket at too great an angle from your direction of travel or it will make your craft unstable.

Now, in order to properly gravity turn, it actually depends on what speed your craft is when. Some crafts are so heavy that they need to start them late. Some are light and fast and need to start early. So as a rule of thumb, you should start your turn very early, around 5km or so, and turn very slowly. And I mean early and slowly. You should indeed be at 45 degrees at 10km, but you need to start early enough to get there with a slower turn.

[Crzyrndm]

Ferram quote

tl;dr

Have starting TWR of ~1.1-1.3 (NOT 2.0)

Turn about 5 degrees from vertial once speed reaches 100m/s

Let rocket fall over under it's own weight, regulate throttle as required to hit apoapsis at correct time/orientation. Craft heading should never leave the prograde vector

Numbers are guidelines, tweak as neccesary for your craft. Some control input or experimentation may be required

Edited February 11, 2015 by Crzyrndm

[Invader Myk]

As I understand it, it's happening because the front end of your rocket is too light.

Aerodynamics is counter-intuitive enough to me that this could be right, but I just want to make sure you meant "light" and not "heavy"

I think this might be a pic of the ship. (I'm not use to posting pics from websites).

The asparagus staging drops the first pair at ~1km, the second pair at ~8km, and the last pair at ~20km. The last pair are on the north and south side of the vessel.

[NathanKell]

Yes, 'light' is the issue. Think of a dart--will it fly stably if the front of it is light or heavy?

1.1 is a bit low--I'd say safe numbers are 1.2 to 1.5, depending on staging. If you're doing vegetables, then maybe a constant 1.8 is fine--the low starting TWR is predicated on it increasing quickly from a traditionally-staged LV.

To check stability, turn on CoM and CoL, and verify that at *all* times the CoM is in front of the CoL. That means testing both full and empty, and each stage separately. Or just add fins to the bottom to be safe.

[Invader Myk]

Yes, 'light' is the issue. Think of a dart--will it fly stably if the front of it is light or heavy?

1.1 is a bit low--I'd say safe numbers are 1.2 to 1.5, depending on staging. If you're doing vegetables, then maybe a constant 1.8 is fine--the low starting TWR is predicated on it increasing quickly from a traditionally-staged LV.

To check stability, turn on CoM and CoL, and verify that at *all* times the CoM is in front of the CoL. That means testing both full and empty, and each stage separately. Or just add fins to the bottom to be safe.

The dart analog helps.

The CoM-CoL locations appear to be my problem. The CoM stays around 1/3 the way up the vessel. Without winglets the CoL never gets behind the LV-909.

Any suggestions on placing winglets on the center stage of an asparagus design?

[NathanKell]

Between the boosters? :] Though by that point you probably don't have enough air to really need a statically stable rocket, your gimbal will be enough authority to counteract instability.

(Do get TheClaw's stock bug fixes mod, stickied in Addon Releases, as otherwise your decouplers won't work right)