[CLIX]

Very interesting post ! And explained very well too.

Reminds me of the days my friends & I used to rush to the little library in the street corner to get the Pop Science magazines before anyone else - in those days our only source of interesting info.

Thanks AK.

[AlphaKilo]

Originally Posted by MaNishi

Keep them coming

Originally Posted by CLIX

Very interesting post ! And explained very well too.

Reminds me of the days my friends & I used to rush to the little library in the street corner to get the Pop Science magazines before anyone else - in those days our only source of interesting info.

More info coming up. Soon, I will posting more info on Solar panels, batteries and other forms of power sources used in the space craft. Keep your questions pouring

Wear seat belts while in a 4wheeler and helmets when on 2
Happy Stargazing untill my next post (which will take some time, I excuse in advance for the delay!).

[CLIX]

ALERT!

I am not sure if you would like to answer this as part of this thread - but what is your take on Objects sighted while in space from the spacecrafts, that are many times discounted as space junk, dead pixels or lens flare among other things, but have been known to move in strange trajectories and varying velocities
The moment I use the three letter acronym its rubbished away as fantacy - I shall let you take it from here.

[AlphaKilo]

Originally Posted by CLIX

ALERT!

I am not sure if you would like to answer this as part of this thread - but what is your take on Objects sighted while in space from the spacecrafts, that are many times discounted as space junk, dead pixels or lens flare among other things, but have been known to move in strange trajectories and varying velocities
The moment I use the three letter acronym its rubbished away as fantacy - I shall let you take it from here.

hahaha, Mate! My question to you is, do you really believe such news?

Secondly, from my experience (I am in Space craft operations sitting in the control room monitoring spacecrafts and analysing the telemetry!). So far, I have never ever seen any such thing like you are saying. I have seen ISS(international space station) operations live, and never even once heard from them on the likes of what you are quoting.

Hence, I cannot personally answer to your question since I have no experience with it and hence I don't believe in any such existence, except for the fact that outer space is full of some meteorites, asteroids and debris/cosmic particles.

Always remember to use some common sense and logical questioning before believing in anything said to you. (I know that, that's the very reason you had put this question up in the first place, yet its my duty to remind you of the same as a fellow BHPian). Most of the time, its more or less hoax or some attention diverting mechanism or some badly drunk scientist trying to gain some cheap publicity.

[CLIX]

A perfectly "buyable" answer! Agree fully.

I too am a skeptic in these sighting theories, however my deep interest in this field and crazy curiosity seems to leave open this small door to such possibilities.

My take has been - Given the probability of intelligence existing elsewhere, I am open to lending an ear if the credibility is high / conclusive. But then that has not happened yet.

[Ricci]

Theoretical question - AlphaKilo (envious job you have ! )
Since acceleration is caused by application of force, and in space speed will remain constant due to absence of air/friction .On earth, air drag limits the speed achievable for a given thrust, but since there is no air in space, is it correct that a rocket/spacecraft will carry on accelerating as long as rocket motor is providing impulse ? I know the speed of light limitation, but is there some other physical limit on how fast a rocket/spacecraft can go , and is it limited by thrust ?

Unrelated - but what would happen to a human or living animal who got ejected into space without a spacesuit ? The air would be sucked out of the lungs, the blood might boil over and cause death , will the absence of external pressure cause the body to explode , or will the body freeze instantly (or both freeze and explode ? ). Scary even asking !

EDIT : just read page 2 - ok, so there's not pure vacuum in space and there's some particles and/or energy to resist motion, however small. But can we assume somewhere in deep space so far away, that there's no gas cloud or radiation from the stars and galaxies to make our theoretical all-vacuum space for the purpose of my question ?

[AlphaKilo]

Originally Posted by Ricci

Theoretical question - AlphaKilo (envious job you have ! )
Since acceleration is caused by application of force, and in space speed will remain constant due to absence of air/friction .On earth, air drag limits the speed achievable for a given thrust, but since there is no air in space, is it correct that a rocket/spacecraft will carry on accelerating as long as rocket motor is providing impulse ? I know the speed of light limitation, but is there some other physical limit on how fast a rocket/spacecraft can go , and is it limited by thrust ?

The basic limitation - if its a chemical propulsion system = Maximum energy(after discounting thermal losses) produced by the chemical reaction will determine the max. acceleration. Its constant, does not increase. -Electrical/any alternate propulsion - can only have maximum fixed acceleration value.
-Structural limits prescribe the maximum do not exceed acceleration.

Unrelated - but what would happen to a human or living animal who got ejected into space without a spacesuit ? The air would be sucked out of the lungs, the blood might boil over and cause death , will the absence of external pressure cause the body to explode , or will the body freeze instantly (or both freeze and explode ? ). Scary even asking !

See this:

http://www.youtube.com/watch?v=SmQmP4QlrSs

http://www.youtube.com/watch?v=RPlCO...eature=related

EDIT : just read page 2 - ok, so there's not pure vacuum in space and there's some particles and/or energy to resist motion, however small. But can we assume somewhere in deep space so far away, that there's no gas cloud or radiation from the stars and galaxies to make our theoretical all-vacuum space for the purpose of my question ?

For that first we should know the boundaries of the universe itself we are even yet to reach the boundary of the Milky way, so I would safely say, may be or may not be is the answer for your question.

[Ricci]

Thanks AlphaKilo , I get the limit on acceleration part F=ma , but my query was more about terminal speed - assuming enough fuel , can a rocket/shuttle with the same thrust continue toward the speed of light ? I guess I may have to work it out mathematically using Newton's laws , esp given as the fuel burns, the mass of the rocket decreases giving more acceleration. This is true of aircraft, as the fuel weight decreases, the aircraft accelerates more given the same peak thrust, however the aerodynamic drag (mainly , at low altitude structural limits rather than lack of thrust ) limits the top speed for a given thrust value.

That shaving foam looks like a chef's hat !

[AlphaKilo]

So, after a long time, Its time for a update:
What's it all about this time then? Its all about Power! No don't worry, I am not going to ask you all to contest an election or ask you to vote for me.

The following paragraphs will contain information about the Power system of the satellite and rockets:

First of all, basics:

Any machinery to work or to perform the desired task, it needs some source of power. On the Earth we use several sources of power such fuelled, electrical, mechanical and manual(sometimes). So, in the same way, even satellites and rockets need power source in order to be able to perform various operations which they are tasked with.

But, then in outerspace we do not have the liberty of putting up petrol stations or send some humans every time to refuel the satellites when ever they run out fuel. Hence, certain amount(mass, if chemical/solid fuel) of capacity is pre-calculated, during the design phase itself. But on the other hand, in order to be able to operate the electronic/electrical equipment there is a free natural source of power available at a much higher efficiency rate than at the surface of the earth. Hence, all of the satellites around the earth utilize sun as a part of their power system or at times even as the only source of power generation.

What are the basic ways of power generation on board a satellite/rocket?

1. Solar panels - satellites only

2. Batteries - primary batteries (both satellites and rockets use it!)

3. Nuclear Thermal generators - A mini nuclear plant

4. fuel cells

So, how is the power generated here utilized?

well, in order to understand this we need to go to some basics. First of all, the power system on board is divided in three stages, generation, distribution and utilization, where generation is the phase where the actual energy conversion happens, distribution includes, on-board computer based control, complex power lines (cabling) and charge control (will talk on the later), and finally utilization is when the power is actually used by the devices.

So, here lets go into details of the power generation system. First of all lets start with the easy ones, primary batteries. I am sure every one knows what primary batteries mean? correct? It is nothing but a normal AA/AAA/other sized non-rechargeable cells that we use in our day to day life.

Source: http://images.tutorvista.com/content...dry-cells.jpeg

More or less dry cells. These are best during the initial stages of launch, where either the satellites are inside rocket or the rocket itself is on its way to space and hence there is no possibility of utilizing the solar panels or high power is required only for a short duration. Mostly rockets use these kinds of batteries (pack of cells) as re-useability is not a requirement. At times, some small university satellites or rather "Pico" satellites too use this technology, which are designed for not more than a month(at the max) to be in space. So with the help of such primary batteries mostly the flight computer in the rockets are powered and in "pico" satellites, all systems are powered by such batteries.

So far to the primary batteries, lets see about the other power generation source, our beloved, Surya bhagwan. Sun's light inputs/increases the energy of the surface it impacts on by a value of roughly 1367 kW/m² of area! So, this is partially because of the atmosphere blocking some of the rays inbound, filtering some wavelengths out! But in outerspace, at earth distance or 1 AU (Angstrom unit) the value remains more or less the same irrespective of whether one is inside or outside of the atmosphere. Hence, even solar panels for satellites are also designed to operate at this level of power at their surface. Its nothing but a photovolatic cell made of silica which converts the incident heat into electrical energy!
The following is a solar cell in test/assembly phase:
source:http://www.astrium.eads.net/media/im...ar-array01.jpg

[AlphaKilo]

contd..

So these solar panels are then connected to rechargeable batteries placed inside the satellite, or the power is directly fed to the devices live. But this then limits the operability of the satellite as any satellite involving this technology cannot to afford to be shadowed by any celestial object or lose power if any shadowing happens! One more problem with connecting the solar panels directly to the devices is that, if the power required is too high, then the wet area of solar panel must also be large enough to be able to generate so much power, hence causing cost and manufacturing issues. The efficiency of solar panels (recent developments) is between 30-40%!!

Weight is not of a concern here. why? Any guesses?

Hence, logical solution is to connect the solar panels to a rechargeable battery and simply connect the devices to that battery. The most preferred battery type is Ni-Cd/Ni-H2(for non-chemistry guys -> nickel-cadmium or nickel-hydrogen) battery system. But slowly, Lithium-Ion batteries are catching up, given their weight and power/weight ratio advantages, slowly space field is opening up to adopt lithium ion batteries.

Some may be wondering the meaning behind the last sentence above, well whether you know it or not, space technology is very very conservative, and we do not believe in changing anything that has worked so far! Hence, although space engineering sounds like a hi-tech field, its actually very very low tech if seen in real technology parameters or tech-specs.

Thirdly, the RTG's (Radio-Isotope Thermal Generators): Nothing but mini-nuclear plants, use mostly Pu-238(Plutonium-238) for its safety and low decay rate. But the efficiency of such a system is around 8%. But naturally, the advantages provided by this independent power system is well known from the defence technology. Satellites using RTG's can remain longer in eclipse/shadows or fly deeper in our galaxy, say beyond jupiter (>5 AU). Moreover, such independent power sources, help in reducing overall costs by eliminating the need for solar panels. Mostly used in landers or missions designed for deep space(beyond earth's orbit into dark space away from the sun).

But an RTG' on board brings in following complications:

1. safety/Shielding the space craft from radiation
2. excess heat radiation
3. Very expensive!
4. environment unfriendly (if landing on other planets, might cause mutation, and we might end up having to fight star wars or to see some hulks invading earth.)

Source: http://upload.wikimedia.org/wikipedi...n_GPHS-RTG.jpg


P.S: European Space Agency (ESA) has a policy not to use RTG's at all even for missions beyond jupiter!

It is very safe to use system and has been space proven several times over and over again.

Further, lets go into the details of the Power distribution system. Like what we have even in our electricity supply, power is generated at one point and served to various places through a grid system. The satellite too utilizes a similar system but this being smaller in size and more or less controlled by on-board computers. There will be mostly a dedicated computer called "Power Control and Distribution Unit-PCDU" available on board satellites (cheapers one's - hmmm! its like buying a lx model in satellite which lacks abs, and airbags). So this PCDU, monitors the power generated at the solar panel and makes sure that battery is charged and always remains fully charged or charges the battery to max. possible level whenever the energy levels drop and simultaneously the solar generator is producing power! This also ensures that the batteries are not either over-charged or over discharged. In order to protect over charging, several shunting and resistors are used and to prevent over-discharging, certain safety margins are pre-set while programming the PCDU. This triggers a system shut-down or pushes the satellite into a so-called safe-mode where only the very essential devices such as the ones required to keep the satellite in orbit are kept on and remaining ones are put to sleep.

Image of a PCDU box.

Source: http://www.astrium.eads.net/media/im...wer-pcdu01.jpg




Finally, the device level usage will be quite clear to everyone. Isn't it? Its like, you have power at home, all we need to do is to turn the switch to on position and bingo, light glows! (well unless your bulbs have fused out and still you call your local EB office to file a complaint!! in such a case, please consult a doctor immediately. )

So a power control logic looks like as below:




Can somebody, explain me the above image? Electrical/electronics guys..anybody?










If you believed me on that!! Just kidding!!! Wanted to wake you guys up after reading a boring post!

The actual overall setup looks something like this below:
https://directory.eoportal.org/image...=1339683956438



The overall(full satellite) architechture looks as below:

https://directory.eoportal.org/image...=1344353440706



And finally, an interesting image: Exploded view of a full satellite:

http://ars.els-cdn.com/content/image...000636-gr4.jpg



questions??

Originally Posted by CLIX

Reminds me of the days my friends & I used to rush to the little library in the street corner to get the Pop Science magazines before anyone else - in those days our only source of interesting info.

Originally Posted by dhawcash

Also, What kind of surface areas do satellite solar panels have? their wattage?
What kind of batteries do they use (AFAIK batteries have limited life spans) ?(Regarding the dark periods when they are in the shadow of earth receiving no sunlight) I guess these are the lifeline of the satellites as it would be dead if any of these systems fail.

Hope I have answered your doubts in the above posts! Feel free to ask if you need more info.

[CLIX]

Once again - very interesting write-up AK! Thanks.
Some sections with some serious OHT [over head transmission ] you know what I mean; like that satellite Arch. diagram! But I am not giving up! Do keep them coming!

[AlphaKilo]

Originally Posted by CLIX

Once again - very interesting write-up AK! Thanks.
Some sections with some serious OHT [over head transmission ] you know what I mean; like that satellite Arch. diagram! But I am not giving up! Do keep them coming!

Mate, can you tell me which of the parts where OHT? I will try to filter it down further.

[CLIX]

Originally Posted by AlphaKilo

Mate, can you tell me which of the parts where OHT? I will try to filter it down further.

Thanks AK - sure will.

[PGA]

Thanks a lot for starting this tread, AlphaKilo. Pretty Good exposition on satellite, engines, electrical systems et al. I read about 'Ion Engines for Space Travel' in one of the editions of Popular Science in 1984/85 and gave a talk in the college about them which was quite a hit. In that regard your post has enabled me recollect some of my lost knowledge.

It will be great if you could talk about the navigation in space. As to how are these satellites put into the orbit around the earth, what I mean is how do you chart the route from here to the moon, mars or jupiter, what I mean is how do you decide on which road to take. Sorry TBHP fixation, Can't put this question in any better way.

Secondly I was wondering why the GEO orbit has to be at 35k, it should be dependent on relationship of speed between the earth and the satellite. Can't you accelerate the satellites in lower orbits to catch up on earth's rotation.
Cheers

[AlphaKilo]

Originally Posted by PGA

It will be great if you could talk about the navigation in space. As to how are these satellites put into the orbit around the earth, what I mean is how do you chart the route from here to the moon, mars or jupiter, what I mean is how do you decide on which road to take. Sorry TBHP fixation, Can't put this question in any better way.

Seperate post coming up soon!

Secondly I was wondering why the GEO orbit has to be at 35k, it should be dependent on relationship of speed between the earth and the satellite. Can't you accelerate the satellites in lower orbits to catch up on earth's rotation.
Cheers

I will answer your second question first.

1. Well, at LEO, the satellites are travelling(revolving) at more or less 7-8 km/s, what does this mean? You just need 90 mins to complete one revolution around the earth! How much more faster do you want to go when you are already faster than earth's revolution? To give you some numbers, the International Space station revolves per earth day around 14 times. Means, the Astronauts on board see day/night cycle 14 times (based on the normal earth times - 1 orbit ~90-100 minutes, for 1440 minutes (24hrs) you have ~14 orbits). Hence, the need for GEO, where it is "Slower" ( relatively slower! remember in GEO we are still rotating at the same speed as that of the earth's revolution around its axis) farther you go, the slower you tend to be, relatively! Thats why we see aircrafts flying at very very high altitude to be relatively stationary, although their true air speed will be anywhere close to 800-900kmph. So if its the case for a 3.6 kms (10000feet) altitude, imagine 36,000.00 kms!

2. The GEO satellites are mostly communication/Television satellites which have a very omni-directional antenna. Would you prefer to watch the HD channels only in your bedroom or all over India? See the image below to understand what I am talking about. Its called "Antenna footprint". The farther you go, the larger area you cover and longer you stay above the same place. That is also another reason why the GPS satellites are put in a MEO (medium earth orbit ~24000kms) rather than in LEO. Try this (or probably you already understood what I said, yet I will write this example, may help others), switch on the headlights of your car and drive straight onto a wall, what do you see? (Well, if you keep looking for something and fail to brake in time, don't blame me )when you are at a distance, the headlight spread is wide and as you go closer to the wall the spread narrows down.
Its the same principle with the antenna signals being focussed towards the earth. Imagine LEO to be very close to the wall and GEO to some distance away. Of course, then you do have further techniques called high-gain narrow beam antenna and low gain wide beam antenna and bla bla bla! I am not going to talk about it, but well then if you insist I will surely put a seperate post on that! Let me know if you need to know in such details.

Below is an image showing various radius of concentrations for different communication satellites over Europe. The ellipses indicate the antenna diameter required for reception.
http://upload.wikimedia.org/wikipedi..._Footprint.png




Below image shows the foot print of a directional antenna over Canada (Northern hemisphere)
http://www.freewebs.com/vo1one/siriu...%20antenna.JPG