Wikipedia:Reference desk/Archives/Science/2015 November 11

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November 11

Space Question - "Stiff" or "rigid" motions in zero/low-gravity

A question that comes to mind when watching this video made during one of the Apollo missions to the moon. If you watch certain parts, for example 4:30 to 5:00, a number of the motions can be described a "stiff" or "discrete." https://www.youtube.com/watch?v=uazzYgghQD4 Skeptics might even assert that the motions in that portion of the video look "fake." It appears the lunar module as it prepares to rendezvous and dock with the command module in orbit is using thrusters to adjust course/position. After each correction, the motions seem, for lack of a better term "discrete" or "rigid." This would almost seem counterintuitive....one might hypothesize that in a vacuum with low gravity, each motion would cause continual motion in the opposite direction of the thruster, or, taken to an extreme, could cause endless spinning. The result appears to be the case here. Rather than spinning endlessly, each motion seems very short and defined. Does anyone have an explanation? — Preceding unsigned comment added by 67.244.30.139 (talk) 01:37, 11 November 2015 (UTC)

Spacecraft use feedback control and are able to command and control movements very precisely, whether these movements are changes in velocity, attitude, or position. Apollo's manned spacecrafts could control their attitude using the onboard reaction control system, which allowed true 6-DOF maneuvering. Each craft is a sophisticated machine executing precise maneuvering, sometimes entirely under the control of a computer. The spacecraft is not (usually) allowed to drift inertially - even when idle, the spacecraft is constantly station-keeping, so your intuitions about where it is going, and when it should stop moving, are pretty meaningless.

Here are a couple places to start reading:

• Apollo Guidance, Navigation, and Control: Hardware Overview, including lots of additional references
• Apollo Lunar Module: Propulsion Systems Overview, including lots of additional references

Both documents are hosted on the NASA Technical Reports Server, a resource that you will find is far more informative than Youtube if you wish to study spacecraft design.

Nimur (talk) 02:01, 11 November 2015 (UTC)

Thank you! --67.244.30.139 (talk) 02:15, 11 November 2015 (UTC)

• For rigid body motion, the zero gravity, zero friction conditions of orbit are a physicist's dream. Design your thrusters well, with minimal turbulence from the nozzles and nice precise valves and pumps, and you can very precisely move your spacecraft. Effectively all you have to worry about is conservation of momentum and conservation of energy (and conservation of angular momentum, but good thruster placement and gyroscopic reaction wheels keep spinning in check), and these are so easy to calculate that they teach it in secondary school. Shoot X grams of propellant out of one thruster at Y m/s, and then shoot another X grams out of the opposite thruster at -Y m/s. Your net momentum is zero, and your net speed becomes zero. Smurrayinchester 09:55, 11 November 2015 (UTC)

Several factors explain what you see. First, the computer on board was able to make fixed rotation and translation. You ask for a 9 degree rotation and the computer make both the relevant acceleration ÃND deceleration. That explain why it stop on a dime so often. Second, the amount of thrusters and gyro and their position affect how you can accomplish a turn. The computer may add 3 thrusters in different directions to achieve on movement in another direction. So it affect widely the acceleration per millisecond one should expect. Next point worth noting, the pilot use semi-manually a docking maneuver that he can understand and adjust in case of problems instead of giving full control to that 1969 flight computer. I don't blame him. The maneuver is a bit more complex and require lot more steps then you'd think because both ships are orbiting, they are far from flying straight as you might be lured to believe. Finaly, add to the mix a camera that probably can't deal with more then 24 frames per second and therefore is unable to capture the shortest acceleration. Iluvalar (talk) 17:04, 11 November 2015 (UTC)

Controlling a spacecraft's position by using a thrust to start motion, wait an appropriate time for the motion to occur, then using a counter-thrust to stop it, seems like it would work, but doing a lot of steps like that also seems wasteful of fuel. I wonder if they have a more sophisticated system these days where you can specify the overall target position and velocity vector, and it can figure out the minimal thrusts needed to get the job done. By no means a trivial problem, but the fuel savings might be worth it, especially if the docking and undocking operations must be repeated frequently. (Think about picking an object up with your hand, do you do several discrete movements of each joint or do they all flow together into one efficient movement, involving many joints moving at once ?) StuRat (talk) 18:48, 11 November 2015 (UTC)

To some extent, yes. For example, a lot of satellites are put into frozen orbits, which are ones that minimize the perturbation that pulls the satellite around. This is very difficult to calculate, because you need to take into account lots of small effects like air drag, solar radiation pressure, the lumpiness of the planet you're orbiting etc. For the general case, this is probably a chaotic system, which is why docking is such a slow, piecemeal process. You get a bit closer, correct for things like thruster imperfections, sensor inaccuracy, unexpected (or at least uncalculatable) motion, then repeat. Smurrayinchester 10:17, 12 November 2015 (UTC)

Yeah, an orbiter mission to Pluto] is probably gonna be a bitch. :) Wnt (talk) 15:14, 12 November 2015 (UTC)

I'm not sure it's relevant to the question, but video compression artifacts also make the video look odd at times. For example the jerkiness of the rotation at around 4:20 is due to bad motion estimation. -- BenRG (talk) 20:06, 11 November 2015 (UTC)

What is the biggest and smallest skull bones?

I know that there are 22 skull bones, but I don't know what of them is considered as the biggest or smallest. I thins that the frontalle is the biggest but I'm not sure. 78.111.186.232 (talk) 07:01, 11 November 2015 (UTC)

If you include the auditory ossicles, the stapes is the smallest and lightest bone in the human body. --NorwegianBlue ^talk 09:19, 11 November 2015 (UTC)

(e/c)See Human skull. The largest is either the Frontal bone or the Parietal bone. The smallest is the Lacrimal bone.--Shantavira|^{feed me} 09:22, 11 November 2015 (UTC)

I'm fairly certain the mandible is the most massive. Our article says it is "...the largest, strongest and lowest bone in the face." (though note the emphasis added). 99.235.223.170 (talk) 02:25, 14 November 2015 (UTC)

Venlafaxine & blood pressure

Why does Venlafaxine increase blood pressure, while most of other antidepressant decrease it? What is the molecular mechanism, if there is one? Are there published scientific studies about it? Thanks! --93.66.238.130 (talk) 13:30, 11 November 2015 (UTC)

Venlafaxine is a different structural class of drugs compared to most antidepressants, so it's not strange that it causes a different spectrum of side effects. Although there are a number of case reports and analyses out there suggesting it does indeed increase blood pressure, I haven't been able to find a mechanistic study. However, the difference between venlafaxine and most other drugs is that it is a noradrenaline reuptake inhibitor, as well as a serotonin reuptake inhibitor (ie it will increase noradrenaline concentrations as well as serotonin concentrations). Now, noradrenaline raises blood pressure, and this is consistent with the fact that both the noradrenaline uptake effects and blood pressure effects are more pronounced at higher doses. This is the mechanism that most publications speculate about, but as far as I know, it has not been proven. (So no doubt someone will tag this as OR and hat it soon). Fgf10 (talk) 15:25, 11 November 2015 (UTC)

Antenna theory

I wasnt getting any useful responses to my Q on radiation resistance so I have decided to rephrase my question. Whet is the theory that determines how much power is radiated from an arbitrary antenna (ie an antenna of arbitrary design)?31.55.69.147 (talk) 23:05, 11 November 2015 (UTC)

This recent book, Antenna Theory and Design looks good [1]. There are of course well-known classes but WP:OR I think for a truly arbitrary given design you have to work a lot out from first principles on a case-by-case basis. SemanticMantis (talk) 23:54, 11 November 2015 (UTC)

Exactly. I assume the OP has read Antenna (radio) and a reasonable selection of the articles in Category:Antennas. To answer the question as posed, the general theory is classical electromagnetism as expressed in Maxwell's equations. The specific part of the theory is impedance matching, the impedances in question being the impedance of free space and the impedance of the antenna. The difficult bit is calculating the impedance of an arbitrary antenna. The only general solution is to solve Maxwell's equations (probably by numerical rather than analytical methods) for the precise geometry and materials of the antenna. Tevildo (talk) 00:25, 12 November 2015 (UTC).

Ok I have had a look at the near field far field page and it appears that the Z of free space only applies in the far field. Nearer to the antenna it can be capacitive (hi X) or inductive (low X). So Tevildo is correct that it depends on frequency and the exact geometry of the so called antenna. However, I dont have an antenna as such, just a piece of coax chopped off square at the end. Im surprised that there isnt a simple formula to determine what impedance the end sees as it peeks into space. Would it be capacitive, inductive, or resistive, that is the big Q. In Tdr experiments, it looks just like an open circuit ie: very high impedance cf 50R. But why?.?31.55.64.102 (talk) 15:45, 12 November 2015 (UTC)

I can't find an intuitive explanation. All I can find is the maths, based on Maxwell's equations, at Radiation from the Open End of a Coaxial Cable. Just below equation (15) it estimates the power radiated from a typical coax to be about −90 dB of the power input. I think it follows that the radiation resistance is about 10⁹ * Z₀. That's why your TDR result looks very much like infinite impedance, and certainly not 377 Ω.--Heron (talk) 18:44, 12 November 2015 (UTC)

You should have taken the square root of the power fraction. But that does not affect the impedance situation much at all. It looks like there is a very high resistance attached to the end by this calculation. Graeme Bartlett (talk) 21:04, 12 November 2015 (UTC)

Well Herons find is indeed atreasure and just the sort of thing I was looking for.Brilliant piece of searching Heron. WELL. DONE31.55.64.91 (talk) 23:26, 12 November 2015 (UTC)