Low altitude stability: gimballed or aerodynamic?

[Seret]

Forgive my noobness, but is there a clear difference between the two options for stability control in the lower atmosphere? I've been tending towards gimballed engines and ASAS for hassle-free launchers, but the non-gimballed engines do have more smash. How accurate is the modeling of drag for winglets? Do winglets making lots of little (ASAS?) adjustments for stability cost more in drag than the extra thrust is worth?

Or is this another one of the "how long is a piece of string" questions?

I take it for second and higher stages the choice leans more heavily towards gimballing for stability, as the effectiveness of aerodynamic control will drop off?

[Cykyrios]

With the current drag model, adding winglets will probably reduce your speed, but would be compensated by the higher thrust. If the stage your winglets are in gets above 10 km, they will lose their ability to stabilize your rocket.

I personally prefer using gimballed engines only, and if high thrust is needed, I put a gimballed engine on the center, and several non-gimballed ones around (for size 1 engines only, size 2 engines are all gimballed).

[Endeavour]

I prefer to use gimballed engines because winglets lose their usefulness very early on. But if it is a very large rocket, I use fewer gimballed engines and some winglets to keep the ASAS from overcompensating.

[Zephram Kerman]

For me, the decision to use winglets really depends on how I plan to fly while in the atmosphere. Usually, my gravity turn is almost exactly ballistic. (The nose is always pointed close to the direction of travel.) In this case, winglets are almost dead weight. But if the gravity turn needs to be more aggressive for some reason, winglets offer resistance to tumbling as well as a little bit of lift in the off-axis direction.

ASAS doing hundreds of little adjustments using winglets is not as troubling as it might seem. If you have MechJeb, you can see the "steering losses" are very small compared to gravity drag and atmospheric drag.

[preen]

If my craft is unstable or asymmetric, and tips uncontrollably on takeoff, I'll add some control surfaces to the boosters/takeoff stage. Then you can dump them < 10,000 meters, and no need for lots of vectoring engines.

[Vanamonde]

But you don't lose anything by using vectored engines, do you? They have a slightly lower thrust, but proportionally lower fuel consumption, yielding the same delta-v. (At least they did in .15, though I haven't thought to check that on the new engines.) So don't you get the same bang for your fuel buck either way?

[Taverius]

Winglets are useless unless you're running on pure solid boosters.

[olex]

Winglets are useless unless you're running on pure solid boosters.

Not true, six winglets have given my 7-engined (1200 thrust gimbaled engines) heavy launcher the final bit of stability it lacked for the Ascent Autopilot to put it up there.

[EndlessWaves]

They're potentially heavier depending on which ones you're comparing and then there are the situations where X non-vectored engines are enough but X+1 vectored engines are required which can quite considerably add to the weight.

There's also the gyro option (SAS modules) in addition to vectored engines and winglets but they can be fairly heavy.

Does anyone use jet engines as stability controls? They have a good range of vectoring.

[Taverius]

Not true, six winglets have given my 7-engined (1200 thrust gimbaled engines) heavy launcher the final bit of stability it lacked for the Ascent Autopilot to put it up there.

That's very strange. My large-tank asparagus rockets are all fine, and my small-tank asparagus are fine with just the central being a gimballing engine. Might be because of this bug - the more joints you have between the central and outer stacks, the more that will impact you.

[AmpsterMan]

Generaly, I use winglets on my small SRB's in the first stage. I find after 10km-12km they really become useless and should be discarded.

[UmbralRaptor]

Vectored thrust has better pitch and yaw control (especially at higher altitudes), but no roll control last I checked. Winglets do offer roll control, so may be of use below 30 km, depending on rocket stability.

[Seret]

no roll control last I checked.

Should be possible, providing you've got at least two of them and they're off the vertical axis. Or is it just not implemented?

[Vostok]

I use winglets on lower stages that get jettisoned whilst in atmosphere, since I find that gimballing engines do not control the ship's roll, meaning it can't be pointed on a heading reliably. Once it's in space, I ditch the winglets and the lower stages, and use SAS for roll control.

[Taverius]

Should be possible, providing you've got at least two of them and they're off the vertical axis. Or is it just not implemented?

The gimballed nozzles just rotate, instead of vectoring appropriately for the offset.

Its never given me an issue with mechjeb though, pod sas seems to be enough for roll on my designs.

[Vanamonde]

Or is it just not implemented?

If you watch what the engines are doing, they all move in unison. You never see some pointing in one direction and others in another direction. So they can pitch and yaw, but not roll the vehicle or stop its roll. But I don't consider that much of a problem anyway. If it gets out of hand it can overwhelm your guidance system, it's true, but as long as your axis of thrust is pointing in the right direction, it doesn't matter if you're rolling around that axis. If you can't stop a roll when you're out of atmo and it's time to make course changes, then you're boned, but if that's the case then it's a vehicle layout problem rather than a steering issue.