Rocket stability
 

Hi all,
Hope someone can answer my question. I was wandering the other day what makes a rocket go straight up at lift off without getting destabilized before it picks
up enough speed.

Big rockets are steered by changing the thrust vector. The main engine(s) are mounted on a gimbal allowing small changes in the thrust vector to be made. An inertial platform coupled with pitch and yaw rate sensors tell the guidance system where to steer the engine.

A bit like you balancing a broomstick on your hand, providing you can react quickly enough.

Thanks, that was quick.

Rocket stability
 

Said the rocket scientist to the research student...

I'll get my coat...

I've often WONDERED what stopped it WANDERING. :)

Rocket stability
 

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I'm old enough to recall that it wasn't always amazing technology. There were numerous early rocket failures that were spectacular (and incredibly costly, no doubt), to say the least. :(

I often wonder how the scientists and engineers managed to find the cause or rectify the design/assembly/electrical/fuel/name-your-problem errors, after such destructive explosions and losses.

https://www.youtube.com/watch?v=Z5Zy4cVrAME

Rocket science was quite tough to develop in the early days, as there are huge forces to balance with high accuracy. I just had a smile thinking of a 2.934,8 tons broomstick (Saturn V start weight) balancing ... nice metaphor!

Indeed, and a few of those who rode the Saturn V commented that in the few seconds after launch the balancing/steering process wasn't at all progressive or gentle. Mike Collins commented that it felt like you were in a car being steered very jerkily, and I think it might have been Bill Anders who commented that it was so bumpy just after liftoff that he thought they must hitting the launch tower.

Rocket stability
 

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A lot of it was trial and error, I've read that the Pogoing on the fortunately unmanned test flight was caused by resonance in the flexible fuel lines used. This hadn't occurred during ground testing. Eventually they figured out that the fuel lines were becoming coated with ice during the ground test but as the air was less moist at altitude the pipes were free to flap about! They fitted solid lines after that was discovered.
I think they also had an engine cut out problem where sensors shut down a good engine because they'd cross wired the the looms. After this they made the looms to each engine a different length so it couldn't happen again!

Another question - when a rocket is stood up on its end on launch platform, where and how is it's weight supported. It can't sit on its main engine.
3,000 ton Saturn 5 would need a lot of bracing, and thwe arms that swing back into launch tower at liftoff can't be the things supporting it, can they?

Rocket stability
 

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You can read about the S1C (first stage of the Saturn V) here (dated December 1968). The thrust structure holds the engines and distributes the thrust evenly. There are 4 hold-down points on the structure, seen here. They support the weight of the entire stack, and hold it down until full thrust is reached. Although I can't find a pic, it's actually a soft launch with the rocket pulling tapered pins through holes for half a second or so.

Saturn V Pins and Dies
 

There's a bit more detail and a schematic in this report (page 61 of the paper doc, page 161 according to the PDF count).

http://www.scribd.com/doc/44107815/A...eration-Report

Also some fairly expert discussion here:

Saturn V hold down posts: dies and pins - collectSPACE: Messages

Rocket steering
 

You can clearly see the Shuttle main engines adjusting themselves on their gimbals as the engines start.
9m 53s in - link starts at correct place.


https://www.youtube.com/watch?v=ShRa2RG2KDI&t=593

V2 steering
 

The V2 used carbon steering vanes in the exhaust stream that were controlled by gyroscopes.

Google [v2 steering vanes] - without brackets. Many hits and images.

The steering "program" must have been pre-loaded before launch to achieve targeting. Maybe just direction and burn time. Fuel load?

The V2 autopilot guidance system had two free gyroscopes and simply kept it going in one direction...the targetting was set by rotating the launch platform so that this direction pointed towards the target. That sorted out the azimuth...the range was set by an accelerometer which cut off the engine at a predetermined velocity. Some later models had radio beam guidance.

Went around Kennedy some years ago on the all-day visitor excursion. The elderly tour guide had worked there all his career and had fascinating personal experiences going back to these early days, which got a couple of us asking him for more and more reminiscences.

He said that Werner von Braun, and his German colleagues, would be in the concrete bunker watching it all. When there was a malfunction they would start gabbling in high speed German, with one another, gesturing, scribbling designs, etc, which the launch director and others could not understand until they were persuaded to calm down and start the debriefing in English.

Von Braun had of course done all this before getting the V2 launches which attacked Britain in WW2 going, and had been spirited off to the US in 1945 and given a prominent position on the early rocket team. At the end of our tour I asked our guide how the US would have felt if the designers of the 9/11 attack had been taken to Britain and given significant positions in the UK government. He said it wasn't the first time such a question had been asked.

I think Gene Kranz covers that story in his book "Failure is not an Option" ...the launch Director in question was Chris Kraft.