Soyuz Rocket Tilts Just Before Liftoff?

[Randy'sRocketShop]

Hey everyone.
I was watching the latest Soyuz launch two nights ago and noticed that just after the engines startup, but before liftoff, that the rocket rocks or tilts to the side and then swings back, basically doing about 1.5 or 2.5 oscillations as far as I could see. The oscillations damped out until the moment of liftoff. I was curious, so I looked back at a few older Soyuz launches and noticed basically the same phenomenon. Does anyone know anything about this? I would assume this is due to a staged engine startup of the four axially-mounted boosters. I know Space Shuttle started each of its three main engines a few milliseconds apart so that the entire force of engine startup was a little less jarring to the vehicle. Could it be the same thing with Soyuz, that a staged engine startup causes an initial sideways thrust imbalance that causes the stack to tilt to the side?

[YOUTUBE][video=youtube;Q_8VF7knp8E]Launch Video[/video][/YOUTUBE]

Thanks for any insight anyone has!

 

[Bluegillbronco2]

I am not sure why they do this. But I do know the space shuttle swayed just before liftoff exactly the way you described about the Soyuz.

According to Wikipedia: " Between T-6.6 seconds and T-3 seconds, while the SSMEs were firing but the SRBs were still bolted to the pad, the offset thrust caused the entire launch stack (boosters, tank and orbiter) to pitch down 650 mm (25.5 in) measured at the tip of the external tank. The three second delay after confirmation of SSME operation was to allow the stack to return to nearly vertical. At T-0 seconds, the 8 frangible nuts holding the SRBs to the pad were detonated, the SSMEs were commanded to 100% throttle, and the SRBs were ignited"

 

[mikec]

I think the shuttle "twang" was much more pronounced than this. The Soyuz has 20 engines in the first stage and I imagine that some kind of minor imbalance on startup would be inevitable.

 

[shreadvector]

The shuttle twang was because the SSMEs were mounted on the orbiter and the orbiter was bolted to the side of the External Tank (ET) and the entire stack was supported by the aft skirts on the two SRBs bolted to the sides of the tank.

https://www.google.com/search?q=spa...microsoft:en-US:IE-Address&ie=&oe=&gws_rd=ssl

 

[James Duffy]

The Soyuz has 20 engines in the first stage...

A minor quibble: the R7 booster family only has 5 RD-107 engines in the first stage. While there are 20 nozzles, each set of four main nozzles is connected to a common set of turbo pumps and associated plumbing, and is considered to be a single engine. The version of the RD-107 used on the strap-on boosters even have two smaller vernier nozzles connected to the whole works, resulting in a total of six nozzles associated with each engine.

See: https://en.wikipedia.org/wiki/RD-107

 

[georgegassaway]

The Shuttle "Twang" was so pronounced (about 30" total movement) that the moment of ignition was back-timed from T = 0 to match the expected duration when after twanging pitch-forward due to off-axis thrust from the SSME's, then swaying back, it would become perfectly vertical again, that is when the SRB's were ignited and explosive bolts fired (give or take some milliseconds).

IIRC, SSME ignition was at T- 3.5 seconds, and it took 3.5 seconds to twang one way, then sway back to vertical. If not waiting for the "Twang", then SSME ignition would likely have been timed sooner such as T-3.0 seconds or T-2.5 seconds.

This Soyuz launch swaying is a new one to me, but obvious in that video. Since it is a symmetrical rocket, there are no design imbalances.

Now, IIRC, the "release arms" are designed to hold the rocket tight when the rocket is sitting on the bases of the arms. But when the rocket lifts off, counter weights pull the arms away from the rocket. So, it could be that either there is no mechanical latch system locking it all into place until a command is given, or such a system is commanded to unlock at ignition BUT that only means the arms are free to move in accordance to what the rocket does (unlocking NOT meaning let go at that moment, just now free to move and not locked), and the rocket is still "sitting" on it for some time between ignition and building up enough thrust to lift off. So, for awhile, as the rocket is building up thrust, the release arms (and the supports at the bottom of the arms that the rocket bottom is sitting on) are free to move. Not enough to move away but enough to allow the rocket to wobble a bit. I could easily see wind being an issue in this regard. Less likely but also possible, some minor differences in thrust levels until liftoff thrust level is reached.

Keep in mind that hold down and release system was designed over SIXTY years ago! So, it's super-simple. And if it ain't broke, don't fix it, so they've had pretty much the same hold down system ever since, while the first stage of the rocket is mostly the same design from the mid-1950's as well, though with some upgrades over the years (After stretching and increasing te ful volume of the core for Soyuz, no much physical changes to that first stage.... technically stage and a half.)

i'm sure this must have been discussed in other places, such as NASAspaceflight Forum.

 

[Exactimator]

See, this is why I can't be a real rocket scientist. My first thought was "That looks like rod whip. Maybe they should have used the 1/2"."

 

[James Duffy]

See, this is why I can't be a real rocket scientist. My first thought was "That looks like rod whip. Maybe they should have used the 1/2"."

Nope. They should have used the rail. :wink:

 

[K'Tesh]

I agree... A rail would be much more stable than a rod.

I wonder if the Russian Federation could order those from McMaster-Carr? I know that here in the PRC, I can't.