How are rockets (real ones) stable when they lift off?

[Anizer]

So MAVEN was launched yesterday on an Atlas V rocket and from the pictures i've seen of it, there are no "tail wings" or anything of that sort to stabilize the rocket as it accelerates. If so how does do rockets lift off so steadily?? I've seen pictures of the Soyuz rocket and the fuselage sorta bulges for the three engines which can act like an tail wings I guess, i'm not too sure about that one either.

(I haven't taken fluid dynamics yet.)

Lets say there's SAS in real life, how would that even work??

[Guest Brody_Peffley]

Actually they have no real sas. Its controlled by vectors under the rocket engine, Which move the thrust from the engine which moves the aircraft.

[OtherDalfite]

As for rotation, I'm not quite sure. But for pitch and yaw they use gimballing with engines. You can view this in KSP when you launch as your rocket engine will look like it's angling itself to the opposite direction of the key you are pressing. The thrust being angled provides a force that is below the center of gravity and rotates the rocket.

[SuperBigD60]

Yeah, I think an Atlas V gimbals its engines to maintain its proper attitude

[numerobis]

Real rocket engines gimbal, just like many of those in KSP (but with smarter control algorithms, and with a wider range of motion). But I'm not up on exactly what the Atlas V does.

[edit: I'm a slow typer]

[FenrirWolf]

They do have SAS in the sense that a computer program controls the way the gimbals turn to keep the rocket on course. But they don't have super powered reaction wheels like we do of course.

[Decho]

As others have said, it's thrust vectoring. This is typically either moving the nozzle or entire motor on a mechanical pivot (the former being what happens in KSP), but there are a few other methods - a good example being a Japanese system which introduces water into part of the exhaust, which cools it and reduces the thrust in that area, effectively altering direction of net thrust for the motor. As for rotation control, if you have more than one engine then this is easily achieved with thrust vectoring again - simply gimbal the nozzles/motors on opposing sides in opposite directions, if only one engine though then you'd probably be wanting fins, or RCS for upper stages.

[numerobis]

Or you design the rocket so that it doesn't tend to roll, and you accept what roll occurs anyway. Roll helps keep you going straight.

[sgt_flyer]

Well, at first, they try to make aerodynamic rockets basically, if the rocket has an high enough acceleration, if your nose veers away from the prograde marker, one side of the rocket will encounter more resistance to air than the other - if you don't apply control inputs, the rocket will tend to get back prograde on it's own.

Then, for actively controlling / correcting the rocket course (due to the winds, etc) the rocket will either have gimballed main engines nozzles (like on saturn V where the outer nozzles were gimballed on the first and second stage)

Or using verniers pods (which are small nozzles getting their thrust from the main engines combustion chamber) with greater gimbal range, like on the soyuz rocket boosters, core stage and upper stage. (Although a new soyuz upperstage is undergoing tests with more a efficient engine and gimballed main nozzles instead of verniers)

[Specialist290]

I think this fits the Science Labs a little better Moved.

[Smidge204]

Sometimes they aren't!

No Kerbals (or Humans) were harmed.

=Smidge=

[LtHeckard]

The Rockets are Aerodynamic

[jjwb22101]

Well, at first, they try to make aerodynamic rockets basically, if the rocket has an high enough acceleration, if your nose veers away from the prograde marker, one side of the rocket will encounter more resistance to air than the other - if you don't apply control inputs, the rocket will tend to get back prograde on it's own.

It dosen't quite work this way. There are two important points on a rocket with regards to (static, and without gimballing engines) stability. One is the center of mass, which we should all be quite familiar with as the point where the rocket balances. The second is called the center of pressure or lift, depending on weather you are dealing with rockets or aircraft. This one is a bit more difficult to explain, and pretty darn hard to find, but it's essentially the point where all the aerodynamic forces balance. You want this point to be behind the center of mass, because effectively this is where any aerodynamic force will be applied, and the aerodynamic forces you discussed in your post will only make the rocket point the correct direction if the center of pressure is far enough back. This point is manipulated by changing the shape of the nose of your rocket, or much more so by manipulating the shape and locations of fins. This is why model rockets always have large fins at the back.

Most modern rockets designed to go into space will use thrust vectoring, which completely ignores all of the above, though the less stable a rocket is, the harder it is for the vectoring to compensate.

[Anizer]

So how did they accomplish this before computers?

[Kryten]

They didn't-the earliest space launches used electromechnical analogue computers. Some Soyuz variants use them even now.

[jjwb22101]

So how did they accomplish this before computers?

As i said earlier, with fins. And to be perfectly honest, rockets large enough that fins aren't enough have been around for less time than computers.

[zekes]

As for rotation, I'm not quite sure. But for pitch and yaw they use gimballing with engines. You can view this in KSP when you launch as your rocket engine will look like it's angling itself to the opposite direction of the key you are pressing. The thrust being angled provides a force that is below the center of gravity and rotates the rocket.

They have little tiny fins at the bottom. or you can "twist" all the engines in one direction to spin the ship...

[magnemoe]

As i said earlier, with fins. And to be perfectly honest, rockets large enough that fins aren't enough have been around for less time than computers.

For artillery rockets fins are used to introduce spin, who works great for increasing stability. You can also use fins in the exhaust to spin the rocket up, benefit is that you don't have stupid fins sticking out and the rotation force dependent on trust not airspeed giving you more stability during launch.

V2 used fins in the exhaust controlled by an analog regulator. Not sure if you could call it an analog computer, it was also the first liquid fueled rocket who was more than an experiment.

It also had massive external fins, if you look at rocket designs for larger rockets, you see that external fins get smaller and smaller until they disappear, saturn 5 small fins was part of the escape system, keep the rocket stable if engine shutdown until the command module was ejected, had they made an heavy unmanned lifter version they would have dropped them.

[AeroEngy]

As others have said, initially they maintain their balance via thrust vectoring. This is necessary since initially there is not enough velocity for the center of pressure/natural stability to matter at all.

Regarding roll control. Some rockets I have worked on have very little roll control initially. No fins and RCS is only enable minutes into the flight. Generally this not a problem as the roll rates aren't that high and don't matter regarding steering the rocket. The RCS would enable later to eliminate roll for payload sep or prevent over the roll rates being high enough to effect the liquid propellant (throw it to the outside of the tank). I have a high speed video of the GIF below I am uploading to Vimeo I can share later but for now here is a cool GIF.

[Sternface]

Interesting. I always assumed they just stuck an SAS on and hit T.

Seriously though, awesome image ^

[AeroEngy]

Interesting. I always assumed they just stuck an SAS on and hit T.

Seriously though, awesome image ^

I put the video in the Space Lounge here. You will also notice in the GIF/Video that the rocket angles away from the tower. That is to prevent any accidental contact from wind induced lateral drift.

[thereaverofdarkness]

If you correctly place your rocket's center of lift between its center of mass and center of thrust, and have the center of thrust at the back, the rocket will stabilize itself. The further back from the center of mass youo have the center of lift, the more stable the rocket will be, but the harder it will be to maneuver. The real reason cheap rockets don't fly straight is because of imperfections in their design. A rocket that is shaped and weighted precisely enough needs no maneuvering tools to keep it flying straight, though air turbulence can still throw it off.