Rocket stability

[The Range]

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.

[H Peacock]

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.

[The Range]

Thanks, that was quick.

[Dash8driver1312]

Said the rocket scientist to the research student...

I'll get my coat...

[TURIN]

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

[Radix]

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[onetrack]

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

[ChickenHouse]

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!

[wiggy]

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

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.

[Radix]

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[Windy Militant]

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!

[rjtjrt]

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?

[Radix]

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[Bushfiva]

You can read about the S1C (first stage of the Saturn V) here (http://history.msfc.nasa.gov/saturn_apollo/documents/First_Stage.pdf) (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 (http://www.capcomespace.net/dossiers/espace_US/apollo/lanceurs/saturn_support_arm.htm). 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.

[wiggy]

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/Apollo-8-Mission-Operation-Report

Also some fairly expert discussion here:

Saturn V hold down posts: dies and pins - collectSPACE: Messages (http://www.collectspace.com/ubb/Forum29/HTML/001327.html)

[jimjim1]

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

[jimjim1]

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?

[Molemot]

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.

[WHBM]

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.

[wiggy]

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.

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

[WHBM]

I may have actually merged the two then, because I certainly recall the stories being told, but I have that book as well - possibly bought in the shop on the same visit.

[baker3950]

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!
The problem of the iced up propellant lines in the S-II/S-IVB and the S-IC pogo are two different (though related) problems.

Pogo was a result of harmonic resonance in the S-IC propellant tanks and supplies. In brief there was a undampened, unstable feedback loop were a change in thrust would change propellant pressure which would in turn change thrust. The problem was solved by detuning the propellant lines with dampers (basically pneumatic shock absorbers). The phenomenon had been experienced on many (if not most) earlier rocket designs, and was seen on the first test flight of the S-V (Apollo 4) but was much more severe on Apollo 5 where the oscillations were strong enough that the crew might have been injured.

The iced up propellant line problem showed up on Apollo 5. The fuel line feed line (LH2) would ice up in ground testing because the cryogenic liquid hydrogen flowing through the flexible connection to the J2 engine would freeze moisture in the air surrounding. In space there is no moisture surrounding the line so there was no ice build-up. The mass of the ice built up during ground tests of the J2 buffered the vibration of the line. The stronger vibration observed in space resulted in failure of the line. The fix was simply additional reinforcement of the flexible fuel line. During the second stage burn of Apollo 5 one of the J2 engines failed because of the fuel line problem but the 'crossed wiring' problem resulted in the shutdown of a different engine. Even so, the guidance computer compensated and achieve a sufficient, though imperfect orbit despite the fact that the two failed engines WERE ON THE SAME SIDE. This was far outside the expected performance of the guidance and control system.

Despite the anomalies observed on Apollo 5, the next flight of the Saturn V booster was on Apollo 8, the first manned mission to leave earth orbit. I can't see NASA taking that kind of gamble today.

[noflynomore]

To get a feeling for the immensity of the task of developing rocket transport there is a suitably immense book, or rather four immense books written by Boris Chertok, the electrical engineer who became second in command of the USSR's rocket/missile/space programme under the great Korolev. NASA, bless them, publish it in downloadable form for Kindle completely f.o.c!

It's hard work and a bit dry in places but an utterly fascinating and incredibly detailed account of the difficulties they encountered in their gargantuan task, and imo is essential reading for anyone with a special interest in space exploration and rockets. It is also a fascinating insight into the mindset of Russian governments and people from the '20s to the modern era.

Boris Chertok (author). Rockets and People, Volume 1, 2005. ISBN 0-16-073239-5. Published by NASA.
Boris Chertok (author). Rockets and People, Volume 2: Creating a Rocket Industry, 2006. ISBN 0-16-076672-9. Published by NASA.
Boris Chertok (author). Rockets and People, Volume 3: Hot Days of the Cold War, 2009. ISBN 978-0-16-081733-5. Published by NASA.
Boris Chertok (author). Rockets and People, Volume 4: The Moon Race, 2011. Published by NASA.

[westhawk]

Rockets can also have natural stability due to the relation between Center of mass and center of aerodynamic pressure. Like an arrow or an airplane. A big advantage of using nozzle gimbals is that stability characteristics can be imparted at low dynamic pressure AND control the rocket attitude and trajectory. All without the extra drag and weight associated with fins, or with stabilizing and steering thrusters. But of course fins are only effective for atmospheric flight, so space vehicles will require one or more of the other steering, stabilizing and attitude control systems available.

That is what makes the Estes rockets I built and launched as a kid seem rather simplistic in comparison. But still, I thought they flew pretty straight!

[Max Angle]

You can clearly see the Shuttle main engines adjusting themselves on their gimbals as the engines start.Its a good demonstration of the gimbals in action, a pre flight check of the system was carried out after the APUs has been started, you can see this at 6:15 on the video.

The movement of the two bottom engines after start up was due to the fact that the engine bells vibrated during the light up sequence and in the flight position they may have contacted each other. They were therefore held apart for start up and returned to the correct position once they had stabilised, clever stuff.

The solid rocket booster engine bells where also gimballed and each SRB had its own dual APU and hydraulic system to provide the power.

The steering sequence was loaded in before launch and included commands to help reduce the stress on the stack caused by climbing rapidly through changing wind velocities, wind shear strength was one of many launch constraints. These commands were updated using the latest data gathered from balloons and aircraft until shortly before launch but once airborne it was purely an "open loop" system, no feedback or sensing was used to modify the commands.

After SRB separation the guidance system became "closed loop" and actively steered the shuttle based on real time position and velocity to fine tune the required trajectory.

[Dont Hang Up]

The SpaceX rocket is of course capable of landing vertically.

All of my engineering instincts scream at me that that must be a far more challenging task for stability control than the take-off.

But when I think about it more, I cannot quite decide if or why that would actually be the case.

[WHBM]

It's notable that a £5 cardboard fireworks rocket is also pretty stable when launched, display sequences of multiple fireworks in formation at height do normally come off quite well. There must be some inherent stability in the basic concept. It's pretty unusual for one in a display to go off at 45 degrees - in fact I think, certainly in their early days, NASA had a greater proportion that did so than in a fireworks show.

[noflynomore]

A firework rocket is stabilised by the weight of the stick, same principle as a wire-walker - get the c of g low enough and it can't fall over but at a huge cost to it's load carrying performance.

To prove this take a firework rocket and support it with a finger at the bottom of the tube where the fuse is ie the point where thrust is applied. It will sit upright on your fingertip quite happily. Remove the stick and try again - different result.

Hanging 1000ft of 30 inch gas pipeline behind a Saturn 5 was never going to catch on so they had to think of something else!

[Dont Hang Up]

Hanging 1000ft of 30 inch gas pipeline behind a Saturn 5 was never going to catch on so they had to think of something else!

The size of the glass milk bottle alone was enough to discourage the idea! :O

[Max Angle]

But when I think about it more, I cannot quite decide if or why that would actually be the case. Agreed, the actual balancing task is the same but it calls for pretty accurate control of thrust in the last few seconds and some fancy guidance systems. They use aerodynamic steering in addition to rocket gimbals, four flaps that look a bit like large fly swats are deployed from the side of the rocket and are used to steer and perhaps also reduce the rate of descent, not sure.

[H Peacock]

Applying the rocket thrust ahead of the CofG will provide natural static stability, although at high speed in the atmosphere the aerodynamics could well upset that balance.

Apply the thrust below the CofG and you'll need to be able to gimbal it. But the size and weight become a significant factor. Now try balancing a pencil vertically on your hand - I doubt you can react quickly enough to do it. Replace the pencil with a a heavier and longer broomstick and it becomes somewhat easier.

[tdracer]

I'd always wondered about the (relatively) small fins on the Saturn V (they are fixed fins BTW, not actively controlled). However when I read a book on the development of the Saturn rockets (I, IB, and V), it talked about those small fins on the S-V. While obviously of little use at low speeds, the static aerodynamic stability they provided at speed dramatically reduced the amount of nozzle gimballing required to keep the rocket on-course.