For Questions That Don't Merit Their Own Thread

[KG3]

22 hours ago, ARS said:

What caused bombs dropped from aircraft to "whistle"? I know not all bombs dropped have a whistling effect, but what caused it in the first place?

Probably just a sound effect added when they edited the movie that you heard the bomb being dropped in.  Luckily nobody has ever dropped a bomb on me so I'm probably no expert though.

[kerbiloid]

7 hours ago, Spaceception said:

So, I'm trying to find the geostationary height for a planet 0.37x the mass, 0.66x the radii or Earth (~0.85 g), and I got the value of around 21,308 km. Which seems right. BUT, I calculated my finished value as follows: 295948.832 km x 0.072, why did I multiply it by 0.072? FIrst, I tried out the equation with Earth values (Several times), and multiplying it by 0.072 was the only way I could figure out how to get Earth's geostationary height. So when I did the equation for my planet, I did the same thing after converting the initial value from m to km (If you haven't figured it out by now, this was the first time I've ever used the equation )

I used a solid 24 hr day in seconds (86400 s), multiplied by 0.798 for the planet (19.152 hr day), Earth's mass in kg (expanded out to get rid of scientific notation), then times 0.37 for the planet, and Earth's radius in meters, multiplied by 0.66 for the planet. I used parentheses on all values with an exponent. And I used the calculation on the KSP wiki, which used cube root instead of square root on another website I found, which gave me even worse answers. (a = cube-root(G x M1 x t^2/4pi^2) - R) was the one of the KSP wiki.

R_synch = (G * (M_planet + M_sat) * (T/(2*pi))² )^1/3 ~= (G * M_planet * (T/(2*pi))² )^1/3  

***

Earth:
GM = 3.986*10¹⁴, (SI units)
T = 86400 s

R_synch ~= (3.986*10¹⁴ * (86400/(2*pi))² )^1/3  = (3.986*10¹⁴ * (86400/6.283...)² )^1/3 =  (3.986*10¹⁴ * 13750.98708314² )^1/3 =  (3.986*10¹⁴ * 1.891*10⁸ )^1/3 = (7.503488*10^22) 1/3 ~= 42178170 m ~= 42178 km

***

not Earth

R_synch = (G * M_planet * (T/(2*pi))² )^1/3  = (G /(2*pi))² )^1/3  * (M_planet * T² )^1/3 = const * (M_planet * T² )^1/3 = R_{synch Earth} *  ( M_planet/M_Earth * (T_planet / T_Earth)² )^1/3 

R_synch = 42178 * (0.37 * 0.798²) ^1/3 = 26051 km.

H_synch = R_synch - R_planet = 26051 - 6371 * 0.66 = 21846 km.

Edited September 27, 2018 by kerbiloid

[Spaceception]

3 hours ago, kerbiloid said:

R_synch = (G * (M_planet + M_sat) * (T/(2*pi))² )^1/3 ~= (G * M_planet * (T/(2*pi))² )^1/3  

***

Earth:
GM = 3.986*10¹⁴, (SI units)
T = 86400 s

R_synch ~= (3.986*10¹⁴ * (86400/(2*pi))² )^1/3  = (3.986*10¹⁴ * (86400/6.283...)² )^1/3 =  (3.986*10¹⁴ * 13750.98708314² )^1/3 =  (3.986*10¹⁴ * 1.891*10⁸ )^1/3 = (7.503488*10^22) 1/3 ~= 42178170 m ~= 42178 km

***

not Earth

R_synch = (G * M_planet * (T/(2*pi))² )^1/3  = (G /(2*pi))² )^1/3  * (M_planet * T² )^1/3 = const * (M_planet * T² )^1/3 = R_{synch Earth} *  ( M_planet/M_Earth * (T_planet / T_Earth)² )^1/3 

R_synch = 42178 * (0.37 * 0.798²) ^1/3 = 26051 km.

H_synch = R_synch - R_planet = 26051 - 6371 * 0.66 = 21846 km.

Thanks a ton! Screenshotting this so I can look at it in the future for other stuff  

[kerbiloid]

A large set of astronomical formulas

http://robert-ibatullin.narod.ru/utilities/astro_formulas.pdf

(Descriptions in Russian, formulas in Latin)

Upd.
That's funny, but looks like google translates.
https://translate.google.com.tr/translate?sl=ru&tl=en&js=y&prev=_t&hl=ru&ie=UTF-8&u=http://robert-ibatullin.narod.ru/utilities/astro_formulas.pdf&edit-text=

Edited September 27, 2018 by kerbiloid

[magnemoe]

13 hours ago, KG3 said:

Probably just a sound effect added when they edited the movie that you heard the bomb being dropped in.  Luckily nobody has ever dropped a bomb on me so I'm probably no expert though.

No, once heard the  whistle then some jerks threw snowballs from an high building, the sound was very noticeable. The sound and the long drop time was nice as they made them easy to evade.
So the sound effect is real, think mortar shells make them to, an artillery shell is supersonic and would not make any sound. 
 

[ARS]

I've heard a tale that since the V2 rocket flies in supersonic speed, when it hits the target, there's no audible warning since it flies faster than the sound. So the target suddenly just blown up, then after that, the V2's noise catches up, looming over the smoldering remains

[StrandedonEarth]

1 hour ago, ARS said:

I've heard a tale that since the V2 rocket flies in supersonic speed, when it hits the target, there's no audible warning since it flies faster than the sound. So the target suddenly just blown up, then after that, the V2's noise catches up, looming over the smoldering remains

I believe that’s true. I also read that the V2 could have been more destructive, but more powerful explosives were too sensitive and would be set off by the reentry heat, so it lobbed a ton of weaker explosives instead.  

[Gargamel]

23 hours ago, Tonka Crash said:

Timing from bomb release was unlikely because precise height above ground is difficult to know about a couple thousand feet. It's easier to use a barometric or radar altimeter to detonate at a specific height above the target

I know they have used props for timers, and that was the only thing I could think of.  The detonator answer is the correct one, I could only remember the timer one. 

[Gargamel]

2 hours ago, ARS said:

I've heard a tale that since the V2 rocket flies in supersonic speed, when it hits the target, there's no audible warning since it flies faster than the sound. So the target suddenly just blown up, then after that, the V2's noise catches up, looming over the smoldering remains

On the flip side, the V-1's pulse jet engine was so loud, it could be heard coming from 10 miles away.    It was only when it reached it's target did early models cut the engine (unintentionally) after starting their dive.   The sudden silence indicated a buzz bomb was about to explode. 

[kerbiloid]

If you are staying aside of an archer, you can hear how the arrows shuffle. At least in an indoor shooting range.

Also you can hear the blades of a propeller when the engine is off.

And you can attach a ruler to a rope and rotate it above head.

[Gargamel]

12 hours ago, kerbiloid said:

And you can attach a ruler to a rope and rotate it above head.

We just recently had an in depth thread about bullroarers.  

[kerbiloid]

Do THEY suppose to live inside or outside of a Dyson sphere?

[p1t1o]

2 minutes ago, kerbiloid said:

Do THEY suppose to live inside or outside of a Dyson sphere?

A Dyson Sphere is not a surrogate planetary surface, its point is that the vast majority of a star's energy output is utilised. You can make the living arrangements in any way you please.

It would take a vast civilisation indeed to require the surface area of a Dyson Sphere for accommodation, given that a mere Ringworld (@1AU radius) has an area of roughly 3million Earths.

But most likely the inner surface would be used for whatever mechanism is used to capture the star's energy, or things related to that.

 

On 9/28/2018 at 6:52 AM, kerbiloid said:

If you are staying aside of an archer, you can hear how the arrows shuffle. At least in an indoor shooting range.

Also you can hear the blades of a propeller when the engine is off.

And you can attach a ruler to a rope and rotate it above head.

You know what else is weird? You can hear skydivers in freefall make a noise audible from the ground, sounds like slowly ripping fabric.

Also, the noise an airliner makes is mostly the noise it makes moving through the air, the engines only contribute a minor amount of the total.

[wumpus]

31 minutes ago, p1t1o said:

Also, the noise an airliner makes is mostly the noise it makes moving through the air, the engines only contribute a minor amount of the total.

Inside, I assume that most of the noise is from the engines.  Does it travel through the airframe and resonate in the tube, or is that outside noise resonating at a frequency associated with turbines?

[p1t1o]

1 hour ago, wumpus said:

Inside, I assume that most of the noise is from the engines.  Does it travel through the airframe and resonate in the tube, or is that outside noise resonating at a frequency associated with turbines?

Unknown, although the wind outside the window is going more than 500mph relative, at cruise, I'd expect that to be part of the sound.

Any type of resonance, especially related to engine vibration, is avoided at all costs.

[wumpus]

1 hour ago, p1t1o said:

Any type of resonance, especially related to engine vibration, is avoided at all costs.

An airplane is basically a more or less hollow tube.  The air will resonate, if nothing else.  Although you can presumably dampen most of the audio frequencies, and I'm sure "the drone of flying" can't be due to the length of the tube, but might be a side-to-side resonance.

[ARS]

Is it possible to jam sonar (Passive and Active) by setting off high frequency noise (For example by using torpedo or naval mine purposely built for high frequency noise generation) underwater that masks other noise signature to make it difficult for sonar operator/ sonar receiver on enemy submarine to pinpoint the location of the target?

[p1t1o]

3 hours ago, ARS said:

Is it possible to jam sonar (Passive and Active) by setting off high frequency noise (For example by using torpedo or naval mine purposely built for high frequency noise generation) underwater that masks other noise signature to make it difficult for sonar operator/ sonar receiver on enemy submarine to pinpoint the location of the target?

Yup - https://en.wikipedia.org/wiki/Sonar_decoy - jamming sonar is essentially how they work, and there are torpedo-like versions which last longer, can follow a preset course etc.

I have also heard that submarines can use, for example, a civilian surface ship to mask their own signature if it is between them and a possible sensor.

However, submarines can be detected by listening to other sounds bounce off them, making local deception jamming less likely -  Your jamming signal may actually "illuminate" the sub you are trying to hide. The "sonar decoys" above are last-ditch torpedo defence for when you have already been found.

 

You might also find this interesting: https://en.wikipedia.org/wiki/Prairie-Masker

[Gargamel]

On 10/3/2018 at 4:43 AM, p1t1o said:

Yup - https://en.wikipedia.org/wiki/Sonar_decoy - jamming sonar is essentially how they work, and there are torpedo-like versions which last longer, can follow a preset course etc.

I would argue that is not jamming, as per the question, but a decoy, as in the name.  It creates false targets for the attacker, confusing the sonar operators or the torpedo itself into believing it is an actual submarine.   For it to be jammer, it would have to overwhelm or confuse the sensors so that any info they provide would be useless.  

Jamming sonar really wouldn't work to hide anything.  The modern sonar system can track direction and frequency.  By analyzing the frequency of a sound, and any 'rhythms' to it, a sonar system can identify a source from a library of noises they have on record.    You could mask what a target is by overwhelming all the frequencies, but it would be very obvious as to what direction it is, as the jammer would be a point source on their display.   Sort of like when a car is approaching you at night with it's high beams on, you're not sure what make and model it is coming at you, but you're dang sure where it is.   

As mentioned above, the PM system is designed obfuscate the sonar signature of a vessel.  The problem is, it sounds like something else, namely rain.   Which, for a sub, is something they would normally ignore.  But if they know a target they are hunting may have the PM system, they can either get a weather report for their locale, or pop up and take a peak with the periscope.  If they don't see any rain clouds, or even hear any other rain around them, then they can be pretty sure that very localized rain shower is probably their target.  

On 10/3/2018 at 4:43 AM, p1t1o said:

The "sonar decoys" above are last-ditch torpedo defence for when you have already been found.

There are a couple types of decoys in use.   Some are just large cans of compressed air that create a wall of bubbles that will reflect sonar, causing false positives.    Yes, these are the sonar equivalent of popping chaff/flares in an aircraft to lure away a torpedo.  

There is also the MOSS (Mobile Submarine Simulator), which you allude to, it is basically an unarmed torpedo, fitted with speakers and other noise makers to emulate the sounds of  a submarine.  It doesn't really work on a torpedo that is homing under active detection, but more of a tool of misdirection.  Subs mainly work on passive sonar, just listening to the world around them, not emitting any noise themselves.   The MOSS can be steered so that an enemy sub would believe there is a sub over there, while you are attacking from over here. 

[p1t1o]

 

5 hours ago, Gargamel said:

I would argue that is not jamming, as per the question, but a decoy, as in the name.  It creates false targets for the attacker, confusing the sonar operators or the torpedo itself into believing it is an actual submarine.   For it to be jammer, it would have to overwhelm or confuse the sensors so that any info they provide would be useless.  

IMO, "confusing sensor operators with false targets" is exactly "jamming", and decoys can "overwhelm or confuse" targetting sensors, so whilst I know what you mean (ie: its not as sophisticated as deception or camoflage jamming), it was enough for me to pick it up as a relevant example.

 

Or we're both wrong

 

jam¹

dʒam/

verb

gerund or present participle: jamming

improvise with other musicians, especially in jazz or blues.

"he had the opportunity to jam with Atlanta blues musicians"

 

[kerbiloid]

Spoiler

 

[wumpus]

16 hours ago, Gargamel said:

Jamming sonar really wouldn't work to hide anything.  The modern sonar system can track direction and frequency.  By analyzing the frequency of a sound, and any 'rhythms' to it, a sonar system can identify a source from a library of noises they have on record.    You could mask what a target is by overwhelming all the frequencies, but it would be very obvious as to what direction it is, as the jammer would be a point source on their display.   Sort of like when a car is approaching you at night with it's high beams on, you're not sure what make and model it is coming at you, but you're dang sure where it is.  

If you want to jam something, you basically need multiple white noise generator (because submarines tend to drag long lines with plenty of microphones in an array).  To be honest, I'd strongly recommend the "white noise generators" run as close to a randomly generated FFT as possible, and change the FFTs relatively slowly.

On the other hand, if you know the location and "sample" of what the jammers are producing, you now have active sonar, something typically only used by destroyers and never submarines.  This might be more valuable than the effects of the jamming.

But that "original signal" is still buried underneath all  that noise.  Too constant, and it becomes easier to subtract out the jamming noise as the jamming noise and target sound change relative position.  Too (pseudo) random a jamming noise, and you can average the signal out over time and hear the target signal.

[Gargamel]

13 hours ago, p1t1o said:

IMO, "confusing sensor operators with false targets" is exactly "jamming

No, that's a decoy.   A decoy gives a false positive return on a sensor.   The operator believes the sensor is operating properly, and may be misled to believe there are more targets than there actually are.   Jamming constitutes making a sensor useless.    The operator is fully aware that they are being actively jammed, and cannot trust most of the info they are receiving. 

Imagine you are standing in the middle of an arena full of silent people.   One singer is singing loudly, and is located somewhere in the crowd.   Your job is to locate that singer.   Now a couple of the people are holding up speakers that are playing a recording of the singer.  You now have a couple false targets to investigate.  Those are decoys.  You can fully use your ears for other uses, such as talking to and listening to other people, and using triangulation to home in on the multiple sources of the singing. 

Now the other 40,000 people in the stadium start screaming as loud as they can.   You can not even hear your self think.  This is jamming.   Not only can you not hear the singer, but you can't even hear somebody standing right next to you. 

 

The problem with sonar jamming is that creates a point source.  You have no idea whats over there, but you know something noisy as heck is in that direction.    The only way to really jam a passive sonar system is to have multiple jamming sources surrounding your target.   This is very impractical, as in peace time, it would be merely annoying and wasteful, but in wartime, you already know where your target is, so just drop a few torpedoes on them. 

 

EDIT: Dangit... can't quote in an edit.... see below:

 

 

Edited October 5, 2018 by Gargamel

[Gargamel]

2 hours ago, wumpus said:

On the other hand, if you know the location and "sample" of what the jammers are producing, you now have active sonar, something typically only used by destroyers and never submarines.  This might be more valuable than the effects of the jamming.

I wouldn't say never, but the concept is true enough.   Subs hunt by passive sonar, and if one sub surprises another sub, the targeted sub can fire off some active pings to level the playing field.  It will quickly expose which passive sonar contacts are real and which are decoys, allowing them to fire off a quick snap shot in the correct direction. 

2 hours ago, wumpus said:

But that "original signal" is still buried underneath all  that noise.  Too constant, and it becomes easier to subtract out the jamming noise as the jamming noise and target sound change relative position.  Too (pseudo) random a jamming noise, and you can average the signal out over time and hear the target signal.

And yeah, that takes a lot of time, but that's what subs are really good at, slowly tracking targets over time.   Given various environmental conditions, they might be able to track a target hundreds of miles away, and over the time it takes to get close enough to shoot, they can eventually filter out the noise.  

[K^2]

3 hours ago, Gargamel said:

The problem with sonar jamming is that creates a point source.  You have no idea whats over there, but you know something noisy as heck is in that direction.    The only way to really jam a passive sonar system is to have multiple jamming sources surrounding your target.   This is very impractical, as in peace time, it would be merely annoying and wasteful, but in wartime, you already know where your target is, so just drop a few torpedoes on them.

If you synchronize jammers, such as via radio, you can do something similar to a phased array to create completely false apparent location of the source. If you have a carrier group moving, for example, and you really don't want the subs targeting the carrier, but without painting a giant bullseye on any other ship either, you can sync up jammers on your support fleet to create an apparent false source of interference. It will probably be pretty obviously false, too diffuse and spread to be a single jammer on a single ship, but it won't indicate actual location of any particular ship in the network, either, until the sub gets really close.

The beauty of the approach is that a single sub won't have enough information to do a reverse mapping to resolve it into true sources from safe distance, and establishing communication between multiple subs or even with any one sub is notoriously difficult. (Otherwise, subs would just use satellite feed to target surface ships instead of sonar. It's not like carrier groups are hard to find.)

This is pretty computationally expensive, so hard to picture it on more than 30 year old hardware, and 20 year old is probably more realistic for when this would become completely viable. At the same time, the principles are solid and not exactly new, so if it's not a feature of modern support ships by now, somebody in the navy isn't doing their job. I haven't heard anyone talking about it, though, so grain of salt, but I'm inclined to suspect that anything like this would be heavily classified.