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1 hour ago, Nivee~ said:

Thanks, i will try it... but why in the name of Kraken do imperial units even exist???? Measurement is the most basic branches of mathematics, something every human does, almost every day, and we still don't have a common standard for this?
Oh and please nobody tell me that 'There are 2 types of countries: those who follow metric units, and those who went to the Moon' garbage.. :mad:

The short version is tradition but why those particular units were chosen in the first place is sometimes interesting as well. Fahrenheit was a very skilled tool maker and it's likely that his scale for temperature was chosen because he made more consistent thermometers at the time. Fast forward a few hundred years and it becomes very difficult to break that tradition.

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That's not to say that the US is staunchly opposed to the metric system (or rather SI). In fact, the US customary units are actually based on the metric system: https://www.nist.gov/sites/default/files/documents/pml/wmd/metric/1136a.pdf 

The funny thing is, many major industries in the US use strictly the metric system. Speaking from experience, the auto industry is almost exclusively metric with a handful of exceptions like specialty manufacturers (like smaller bus manufacturers). In fact, the American auto industry had started doing new design in the metric system in the 80s and customary units were phased out with old models. Even things like government regulations have been converted to metric when it comes to cars: distances in meters or millimeters, temperatures in C, speeds in km/h (although I would prefer m/s), etc.

So the US isn't necessarily a curmudgeon, progress has been made, it's just much slower than the science and engineering community would like. Also, public perception is very different from what's happening in the industries.

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1 minute ago, ARS said:

Why airship is extremely difficult to shoot down? Holes on it's rigid body should've vented out the lifting gas inside, right?

Great question!

Zeppelins were gigantic warships of the air, with lifting gas being stored in numerous separated cells. Even if you puncture one cell, the others remain full, and with a fabric exterior a bullet only leaves a tiny hole. The hydrogen inside was not under pressure, so there was nothing to make it "vent" or otherwise leak out...it could take days before a bullet puncture hole would leak out enough hydrogen to cause significant lifting problems.

They were also effective because they could fly much higher than the planes of their time, meaning that fighter pilots were extremely exposed to the zeppelin's massive firepower. 

Incendiary rounds, which we know as the tracers that show up during machine gun fire in wars, were first invented as a way of igniting the hydrogen gas in zeppelins and thus taking them down with fewer bullets. 

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10 minutes ago, ARS said:

Why airship is extremely difficult to shoot down? Holes on it's rigid body should've vented out the lifting gas inside, right?

The rigid body was not a single air chamber, but usually filled with a series of smaller bags (still big though).   That way, if there was a leak in one of them, the airship would not lose all it's buoyancy.  Plus these bags were big, and little bullet holes take a long time for the lifting gas to leak out.   Also, unless the holes go through the top of the bags, some gas would remain inside damage bags.  

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2 hours ago, sevenperforce said:

Great question!

Zeppelins were gigantic warships of the air, with lifting gas being stored in numerous separated cells. Even if you puncture one cell, the others remain full, and with a fabric exterior a bullet only leaves a tiny hole. The hydrogen inside was not under pressure, so there was nothing to make it "vent" or otherwise leak out...it could take days before a bullet puncture hole would leak out enough hydrogen to cause significant lifting problems.

They were also effective because they could fly much higher than the planes of their time, meaning that fighter pilots were extremely exposed to the zeppelin's massive firepower. 

Incendiary rounds, which we know as the tracers that show up during machine gun fire in wars, were first invented as a way of igniting the hydrogen gas in zeppelins and thus taking them down with fewer bullets. 

This, the US used tethered balloons in Iraq as an 24/7 drone above many bases and critical places. They are pressurized but pressure is tiny, in the middle east it's common to fire guns in the air to celebrate stuff and the balloons was nice to aim at doing this, in short they took way more fire than enemy action. Over the weeks the balloons started loosing attitude and they found up to 50 bullet holes in them, they just had one chamber and was much smaller than Zeppelins

In short large volume and low to none over pressure and you don't get much leak. 
The real Zeppelin killer was bad weather, as they was huge and pretty fragile shear forces from wind, that it wind speed is much stronger in the front than 100 meter back could damage structure and rupture them. 

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16 hours ago, Nivee~ said:

Thanks, i will try it... but why in the name of Kraken do imperial units even exist???? Measurement is the most basic branches of mathematics, something every human does, almost every day, and we still don't have a common standard for this?
Oh and please nobody tell me that 'There are 2 types of countries: those who follow metric units, and those who went to the Moon' garbage.. :mad:

It cost to change as you has to learn new stuff, this generate resistance. 
You also has standards, in Norway boards are typically classified by dimensions in thumbs like an 2"x4" or 1"x6" simply as you has to use the same dimension as previous in perhaps an 100 year old house. 
Its sold in decimeters, probably just to give round numbers, with meters you has to use decimals and cm will give higher numbers.

Regarding standards, the most convoluted standard ever is probably the bore of shotguns. most common today is 12, followed by 16, 10 is not used anymore as I know. 
Now 12 bore is larger than 16 who is pretty counter intuitive. 

The reason, it the number of round bullets you can make of one pound of lead, the same ways cannons tended to be classified by the weight of a round bullet they could shoot. 
This has luckily been dropped as nobody uses round cannon balls also : An 12 pound cannon yes I can easy carry that one. 
Leaving shotguns yes you have slugs but round ones are rare. So its just an standard who also specify the length of the cartridge. 

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On 3/15/2019 at 8:32 PM, cubinator said:

When was Jupiter named, and why was the name chosen? It seems weirdly coincidental that the planet was named after the king of the gods before we had any idea it was actually the largest planet. And Venus is brighter most of the time.

Well, the Romans obviously named Jupiter Jupiter and the name was traded through the Middle Ages. But it has different Names in different cultures.

In modern times it was chosen to name planets after Roman deities, so Jupiter just kept its designation. One could as well have chosen Arabic names, as in high Middle Ages their astronomy was more advanced. But now it is as it is, and i too think that by coincidence the boss of the Roman pantheon became the boss of planets :-)

 

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2 hours ago, Green Baron said:
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@cubinatorasked:

When was Jupiter named, and why was the name chosen? It seems weirdly coincidental that the planet was named after the king of the gods before we had any idea it was actually the largest planet. And Venus is brighter most of the time.

Well, the Romans obviously named Jupiter Jupiter and the name was traded through the Middle Ages. But it has different Names in different cultures.

In modern times it was chosen to name planets after Roman deities, so Jupiter just kept its designation. One could as well have chosen Arabic names, as in high Middle Ages their astronomy was more advanced. But now it is as it is, and i too think that by coincidence the boss of the Roman pantheon became the boss of planets :-)

Originally, Venus was not regarded as a planet, but as a star. This is because it orbits nearer the sun than we do. Mars, Jupiter, and Saturn all orbit farther from Earth, meaning that their positions across the sky are readily predictable using reasonably predictable epicycles, deferents, and so forth. We can observe Jupiter at many times of the year. In contrast, planets nearer the sun like Mercury and Venus are only visible close to sunset or sunrise, so it is more challenging to track their movements and recognize that they are, indeed, planets. In fact, Venus was originally imagined to be two different stars (the morning star and evening star) which appeared near to the sun. 

Jupiter was the brightest of the three distant planets known to the ancients and so it was regarded as the greatest of all. 

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2 hours ago, Green Baron said:

But it has different Names in different cultures.

Yup! We call it Brihaspati, the teacher of Gods. According to our mythology, he counsels Gods on stuff and is good friends with the easily irritable Shani (Saturn, God of Justice). :)
He has a professional rivalry with Shukra(Venus), since Shukra is the teacher of Demons. Demons regularly challenge and defeat Gods despite being mortal and born several degrees weaker than Gods. This implies Shukra(Venus) is the better teacher, and of course, it irritates Brihaspati(Jupiter). :D

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Yeah, justice is frequently tossed around. Ooops ... :-)

Mesopotamian (Babylonian) astronomers tracked Jupiter i recall having read somewhere. I think it held the name of their respective deity-in-chief Maruk (?) Martok (?) no that's Star Trek :-) as well, just as it did in antique Greece (Zeus), from whom he was taken over by the Romans and rebranded Jupiter.

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The thing which I don't understand is how could one associate a bright point in the sky with a deity who were usually portrayed in very... fleshy manner.

It's clear why the constellations were associated with agricultural events and seasons: just because they are visible in corresponing periods.
(And this was the original sense of astrology.)

But how could one think that this migrating sky pixel is the antropomorphic deity  What common?

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9 minutes ago, kerbiloid said:

The thing which I don't understand is how could one associate a bright point in the sky with a deity who were usually portrayed in very... fleshy manner.

Totally depends on the culture and their perception of their deities/gods/spiritual concepts. The star could simply be a symbol for the god, or signal the deities wakeup/bedtime, or be a vehicle for him/her, or just a signal to do/leave something irl, whatever. Just because they were portrayed as representing something in the shape of something from the real world (a humanoid, an animal, a hybrid, a contraption like a cart or so) doesn't mean they were restricted to that behaviour. Some of them could fly, or travel between the real world and a spiritual level of any sort (typical for shamanism and derived beliefs), live under the sea (in an octopus' garden), play roles as messengers, show benign or malign behaviour of sorts, beat each other up, change shape and form, etc.

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On 3/24/2019 at 10:40 PM, Nivee~ said:

Say, if I want to convert a value in btu/(hr*ft2*oF) to kJ/(s*m2*K), how do I do it? I mean I get it for all other quantities, but I am a lil unsure about oF.....

Oh hey! I actually use those, they're contact (or thin film) thermal conductivities. Let's do the standard thermal conductivities first, and then convert them to the contact equivalent. The imperial one was the most common thermal conductivity unit in the states up until recently. Now though, we almost always use W/m.K (for bulk materials) or kW/m2K (for contact).

Here's the conversion:

6.933 Btu.in/(hr . ft2 . F) = 1 W/m.K = .001 kW/m.K   (Thermal conductivity)

0.1761 Btu/(hr . ft2 . F) = 1 W/m2.K = .001 kW/m2.K   (Contact/film thermal conductance)

Or in other words, divide your imperial value by 176.1 . Depending on what you're working on, typical contact thermal conductances are often on the order of 2-20 kW/m2.K

Best of luck with your project! Let me know if I can help any more with the translation :)

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On the Lunar surface, with it's lower gravity than earth, is it practical for astronaut to deploy light microsatellite using shoulder mounted rocket launcher? Only instead of warhead it carries satellite payload and orbit circularization thrusters, so the rocket will be fired horizontally on Lunar surface on parabolic suborbital trajectory before using the onboard thrusters to circularize the orbit

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37 minutes ago, ARS said:

On the Lunar surface, with it's lower gravity than earth, is it practical for astronaut to deploy light microsatellite using shoulder mounted rocket launcher? Only instead of warhead it carries satellite payload and orbit circularization thrusters, so the rocket will be fired horizontally on Lunar surface on parabolic suborbital trajectory before using the onboard thrusters to circularize the orbit

Probably, one mars sample return idea uses an rocket to reach orbit, rocket looks more like an anti tank missile. 
Moon gravity is lower and its no atmosphere. 

Something like an stinger can reach 4000 km/h on earth so should be able to do it, you would however need two stages since the rocket burn out fast. One interesting option here is an rover with an orbital stage for sample return. 
You might even be able to get 2.7 m/s and return to Earth 

 

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1 hour ago, ARS said:

On the Lunar surface, with it's lower gravity than earth, is it practical for astronaut to deploy light microsatellite using shoulder mounted rocket launcher? Only instead of warhead it carries satellite payload and orbit circularization thrusters, so the rocket will be fired horizontally on Lunar surface on parabolic suborbital trajectory before using the onboard thrusters to circularize the orbit

An off the shelf missile system? Maybe. Being a weapon, the Stinger missile doesn't have a ton of technical information publicly available but enough to get you close. https://en.wikipedia.org/wiki/FIM-92_Stinger Solid propellant specific impulse and the square-cube law really kill you on this but my guess is that a Stinger missile could just barely get something into lunar orbit.

The warhead is 3 kg and total mass of the missile is 10.1 kg so assuming a dry/wet ratio of 0.65 of the remaining mass and an ISP of 266 s (from another small SRM), you get 1591 m/s of dV. A 2U cubesat has a max weight of 2.66 kg and you wouldn't need the tracking system of the Stinger (which I don't know the mass of) so I'd say you're right on the cusp of making a Stinger work. The whole Stinger system (launcher and all) weighs ~15kgm which would be around 2.5kgm under lunar gravity, light enough to handle 1 handed. A purpose built launcher would be a piece of cake for a 3U cubesat.

More importantly, the moon's gravity field has been described as "lumpy" so putting something into orbit is one thing but getting it to stay there would be a serious challenge. 

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31 minutes ago, ARS said:

On the Lunar surface, with it's lower gravity than earth, is it practical for astronaut to deploy light microsatellite using shoulder mounted rocket launcher? Only instead of warhead it carries satellite payload and orbit circularization thrusters, so the rocket will be fired horizontally on Lunar surface on parabolic suborbital trajectory before using the onboard thrusters to circularize the orbit

I love this thread!

To answer this (very excellent) question, we need to look at two factors: energy and practicality. I'll start with the latter. Except for a very small class of frozen orbits, most low lunar orbits pass over mass concentrations (leftover from the Late Heavy Bombardment) buried just below the regolith; these mascons will aggressively perturb any low orbits and eventually cause surface impact. For this reason, microsatellite orbits originating from the surface are impractical. It's also impractical because you have to take extra mass down to the lunar surface; if you are already going to be in orbit at some point, and so you're much better off pulling an Apollo and just dropping the satellites while still in orbit. Granted, such maneuvers are limited to the plane of your manned vehicle's orbit, but if you have an active enough manned lunar landing program that you can afford the mass budget for a shoulder-fired orbital launcher, you can probably afford to come in for a landing (from LOP-G or whatever) in the desired plane. 

Even setting aside those considerations, it is hard to imagine a situation in which the risks of firing a rocket in close proximity to an astronaut on EVA would be outweighed by benefits. Would be much better to go tripod.

Now, to the question of energy. As @magnemoe pointed out, the Stinger pushes 750 m/s; it deploys a 3-kg warhead. The very different (but also man-portable and nearly shoulder-fireable) Javelin launches an 8.4-kg warhead at up to 600 m/s. The shoulder-fired PZR Grom, from Poland, has a 1.3-kg warhead that reaches 650 m/s and has a ridiculous range of 5.5 kilometers. It is similar to the Russian 9K333 Verba, which launches a 1.5-kg warhead at similar speeds. The fastest shoulder-launched short-range SAM in the world is the two-stage British Starstreak, which launches a 2.7-kg cluster warhead to around 1400 m/s.

Firing on the moon would obviously improve performance. Gravity drag would not be any different due to the high TWR of all these missiles, but aerodynamic control surfaces could be discarded and there is no drag. It would not be implausible to imagine a man-portable, shoulder-fired two-stage missile launcher that would get a 1-2 kg payload into a nearly orbital trajectory which could then self-circularize with ease. You only need about 1.7 km/s to get into low lunar orbit.

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I can imagine a scenario when a Lunar exploration grows to a level where a permanent research facility is erected on the moon's surface, a shoulder-fired rocket launcher could be useful in regularly sending research samples from moon's surface to earth where recovery teams can pick it up instead of using manned spacecraft. Probably a small reentry capsule like those used in Hayabusa probe? And if the rocket fired could reach a speed of 1400m/s or more, the lack of atmosphere and low gravity should improve it's performance a lot. Perhaps a slower-burning propellant could be used for better control and maneuvering?

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I'd imagine the main problem with using existing missiles to launch payloads on the moon is that being designed to work in atmosphere, the lack of air for control surfaces means that they'd be unable to stay on course. Unless the missile chosen already has some sort of thrust vectoring or such.

Edited by V7 Aerospace
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5 hours ago, ARS said:

On the Lunar surface, with it's lower gravity than earth, is it practical for astronaut to deploy light microsatellite using shoulder mounted rocket launcher? Only instead of warhead it carries satellite payload and orbit circularization thrusters, so the rocket will be fired horizontally on Lunar surface on parabolic suborbital trajectory before using the onboard thrusters to circularize the orbit

Lunar OneWeb.
 

Spoiler

article2.jpg

 

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Well, there's an abstract. Which doesn't deliver much real world information.

tbh, judging from a short overview there is nothing in there we do not already know. Only difference is that the information was reviewed with a strong "i see what i want to see" bias. Which makes the Journal of Astrobiology the right place for it :-)

Was i too quick to judge ? I don't think so, for example the "stromatolites" thing, which is presented as if but actually has no proper publication and is only a rumour, derived from a blurry image, if i am not mistaken. To me they don't even remotely resemble earth-like stromatolites. But i know for sure and out of experience than one should never judge geological formations from afar. 1st semester geoscience ;-)

The next rovers, Mars 2020 and ExoMars will tell us more !

Edit: a short search questions the ALH84001 argument as well:

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/3755/1/Morphological-behavior-of-inorganic-precipitation-systems/10.1117/12.375088.short?SSO=1

tl, dr: You see what you want to see and as long as there are other explanations the shape of an object alone is not an argument for extraterrestrial life. And may i add, not even for intrat. life as a review of some very old indirect traces of life has shortly revealed ;-)

Edited by Green Baron
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