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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The development of bubble sextants
From: George Huxtable
Date: 2009 Aug 18, 21:12 +0100
From: George Huxtable
Date: 2009 Aug 18, 21:12 +0100
In [9565], Hanno asked- "Who was Vannevar Bush?" He became heavily involved in the politics of US science, particularly military science, during and after World War 2, after his participation in the atomic bomb project. Look him up on Google. I think he had respectable credentials. That doesn't mean that all his notions were worthwhile ones. Just because it was never actually made (or so we presume) doesn't mean that this stabilised mirror wouldn't work. Indeed, it might, but it would be a lot more convincing if there was an actual instrument for the inventor to point to, and say "There it is, it works, and its performance is so-and-so". Without that, it's really no more than a notion. Hanno has been convinced, but I am less credulous. The spherical geometry is needed to deal with all the accelerations and tilts that occur in three dimensions. That makes the construction of the instrument very complex. However, to investigate the principles involved, let me simplify the problem to just one dimension. Let;s imagine we have a smooth straight railway track, but with undulations. Just to pre-empt diversions about Coriolis force, put it on the equator, aligned East-West (though that's really unimportant). On this track we put a flatbed railway truck, and a navigator who wishes to take observations of the Sun, to his East, using a horizontal mirror as an artificial horizon. The rail track tilts as it goes over hills and valleys. Attached to the truck is a shunting engine. How do we mount the mirror, to make it insensitive to such motion? We could put a pair of knife edge bearings athwart the truck, and on those mount a platform carrying the mirror. That would have a ballast weight attached below it to supply a vertical reference. Then, whatever the tilt of the track, as long as the car was stationary, and had been so for a long time so that all oscillation had died out, we would have a good horizontal mirror. But now, with the shunting engine, start moving the flatcar backwards and forwards. The pivoted mirror will respond to the acceleration forces, and it will start swinging. To reduce that effect, you could add a sort of dumbell arrangement, like a see-saw, with its centre of gravity aligned with the pivot. tThat would act like a trapeze artist's balance pole, to increase the moment of inertia. Now you have a system with a long period. Its response to short-period perturbations will be reduced by the enhanced inertia. You can make the period as long as you like by increasing the dumbell weights, or decreasing the weight and spacing of the ballast. Even so, it will respond to those horizontal accelerations by tilting, to some extent, especially if those accelerations are long-lasting. And that will give rise to a slow swing, which will continue for a long time, as we haven't introduced any damping. So let's do so, by attaching a light vane to the platform, that dips into a bucket of water (or some more treacly fluid), that sits on the bed of the truck. Now the energy of those swings can be dissipated. We can arrange the damping to ensure that the swinging dies away fastest, to achieve so-called critical damping. But what have we done? By that damping vane, we have introduce a connection between the tilt of the railcar and the tilt of the platform, that had been absent before. Now, any sudden tilt of the car will produce an immediate tilt of the platform, which will due away with the long time-constant of the system, so it will continue for some time. What I've tried to show is the complex interplay, in such a system, between tilts and accelerations, in their combined effect on the mirror, and the unwanted side-effects of the necessary damping. It's by no means obvious that a system can be designed to eliminate such effects. Bush says nothing about the proper choice of such oscillation periods and damping constants for optimum effect. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Hanno Ix"To: Sent: Tuesday, August 18, 2009 6:42 PM Subject: [NavList 9565] Re: The development of bubble sextants George: I simply assumed you would look up the rest of the patent yourself - which you did. So why attach it? I also assume you read it. His goal, you will agree, was to create a mechanical structure with a very low proper frequency and a well chosen damping. Simply, he anticipated results of modern signal theory. A practical example of a similar concept is the seismometer which of course has been implemented many times. It, too, is a lowpass filter just used differently! I claim, the idea, the concept, the line of thought of this patent is correct. Look, Charles Babbage' s invention was much more complicated than this and he could not implement it in his times. We found better means. And now, we both use the descendant of his idea at this very moment - because the idea was right. I am not an ME, but I agree with you about the complexity of the inventor's proposed implementation. But that alone means nothing in the long run. BTW: Who was Vannevar Bush? Regards --- On Tue, 8/18/09, George Huxtable wrote: From: George Huxtable Subject: [NavList 9561] Re: The development of bubble sextants To: navlist@fer3.com Date: Tuesday, August 18, 2009, 3:44 AM Hanno included 3 attachments with his last message, but without accompanying text. First, as a general request, can I ask that when contributors provide pointers or links for us to look at, a few words are added to the email text to say what's likely to be found there, so only those with an interest will be bothered to look. In my case, Hanno's attachments were rather rewarding. I found three diagrams from patent no 1912358 of 1929, by Vannevar Bush, a name that Americans of a certain age may well recognise. It can be readily located, from that number, within Google Patents. The patent referred to the use of a mirror as an artificial horizon, and a proposal for stabilising it against accelerations. It included several pages of text, omitted by Hanno, without which those diagrams would make little sense. An amazing proposal, which I greatly doubt could ever have been constructed. It shows a horizontal mirror suspended on springs within a liquid-filled transparent sphere, that floated on a bath of mercury within another transparent sphere. Clearly, the author relished complication. It put me in mind of those Eastern cosmologies in which the Earth was held by a comely maiden, who stood on the back of an elephant, which was then supported by the shell of a giant turtle, which swam in the ocean, and so on, combined with the concentric crystal spheres that carry the planets in the Ptolemaic universe. All with the idea of decoupling the mirror from the accelerations that it was being subjected to. Not for the faint-hearted to attempt constructing, then. It seems to have got no further, which is unsurprising. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Hanno Ix" To: Sent: Tuesday, August 18, 2009 4:57 AM Subject: [NavList 9559] Re: The development of bubble sextants Gentlemen: Please refer to: US Patent 1,912,358 V. Bush: Apparatus for establishing an artificial datum Filed April 8, 1928 The inventor means an artificial horizon. Please refer to the attachments for the drawings. He has implemented a mechanical lowpass filter. We have discussed LP's before. Better yet: This filter simultaneously works in 2 coordinates, roll and pitch. He also points out that the natural oscillating frequency of the mirror should be as "low as possible" but he doesn't give any data. Since the roll frequency of ships is rather low to begin with (judging from you-tube: about. 0.1 Hz = 1 period per 10 sec for an oil-tanker ) the filter has to have a cut-off frequency of about 0.01Hz which amounts to 1 period per 100 sec. The filter also needs to be sufficiently damped. The reason is simply avoiding resonant oscillations of the filter/mirror in response to the ship's movements. To build an LP of this kind is the challenge! The inventor has made every effort to decouple the housing of the mirror from the body of the sextant. So to speak, he created something like a bubble level floating within bubble level. With a lowpass of this kind, influences from pitch/roll would be reduced by a factor of 100, possibly more. So, a 10 degree roll would create 0.1 degree (6 arc-min) deflection of the mirror. Smaller ship will have higher roll/pitch frequencies than oil-tankers, maybe 0.5 Hz. Accordingly, on smaller ships 1 arc-min oscillation of the mirror might be possible. Is that error sufficiently low given the circumstances? It is certainly much less than I saw once in a bubble sextant. Perhaps most importantly, this patent points out how to separate accelerations of the sextant which are instantaneous from gravitation which is constant in time. When I made my proposal with the tubular ring the other day I had similar ideas in mind, however I had not fully understood the interaction between bubble and spirit. Regards --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---