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Rocket stability

Old 12th Mar 2015, 14:07
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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.
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Old 21st Jan 2017, 16:29
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Originally Posted by 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!
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.
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Old 25th Jan 2017, 15:22
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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.
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Old 26th Jan 2017, 08:58
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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!
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Old 27th Jan 2017, 09:19
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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.

Last edited by Max Angle; 27th Jan 2017 at 09:47.
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Old 2nd Feb 2017, 10:42
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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.
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Old 2nd Feb 2017, 13:23
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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.
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Old 2nd Feb 2017, 13:50
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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!
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Old 2nd Feb 2017, 14:06
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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!
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Old 3rd Feb 2017, 09:31
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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.
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Old 5th Feb 2017, 09:27
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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.
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Old 4th Mar 2017, 18:32
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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.
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