[AlphaKilo]

Sorry about the delay friends. Caught up with a presentation work, will update today evening(3.5hrs behind Indian evening) for sure.
Please bear with me, as this is one important post and will be very detailed and long. I don't want to post at random and spoil the quality. So, in the meanwhile keep your questions, doubts and comments flowing.


Happy Stargazing!!

[AlphaKilo]

So here goes the post as promised:

Satellites - Design, how, why and more!

Hello, I know what a satellite means, but how do I design one?

ya, hmm, not so tough job. First, basic idea for a satellite is necessary that is, what will our satellite do? Why do we need it? Can there be a cheaper alternative on-ground instead of satellite? If one answers these questions and comes out with a solution of "yes, I still need a satellite" then starts the design phase. Believe it or not, before every satellite being built/sponsored we engineers must answer a group of "experts" from the Govt. and only if they are satisfied, we get to build the satellite!

A satellite is subdivided into several systems and sub-systems. There are four major systems which need individual experts and team leads.

1. Power and Thermal system:

Power and Thermal systems are responsible for the on-board electricity supply, and thermal(heat, temperature, radiation) control inside the satellite. So, basically, without this system, no satellite will work.

It is subdivided into:

-Power accumulation, storage and distribution
-Thermal control

have experts from the individual fields like batteries, thermal control guys and electronics and electrical guys working on solar arrays and power bus lining and so on.

Instruments:
Batteries, Solar panels - Power generation and storage

Power bus - electricity supply to all instruments and payload on-board

Thermal control - heaters, radiators - to maintain the temperature inside the satellite within operable limits.
Thermal control could be both active (using heaters, radiators) or passive - by radiation by conduction of heat into the colder side of the deep space, Multi-Layer insulation foils(the gold wrap around foils that we see!), or kapton foils (silver coloured ones).


2. Attitude and Orbital control system

Another very important system of the satellite design. They control the movement of the satellite while in-orbit and make sure that necessary maneouvres are made when payload cameras or the mission aim needs some change in spacecraft orientation. They have several on-board devices which is used to perform the maneouvres.

Subsystems: Attitude control, Orbital control

Instruments: Thrusters - small rockets (this by itself is a chapter - for sure I will talk about this in detail!), Magnetic coils(uses earth's magnetic field to produce current and change the orientation of the satellite), momentum wheels (spin at high RPM's to create a torque inside the satellite at the Centre of Gravity by changing the Moment of inertia of the satellite - makes the satellite to revolve/rotate).

3. Telemetry and Telecommand system
This is the communication link between the satellite and the ground station. Telemetry is the data that is received from the satellite about its health status and payload condition. And when data take(data take = taking pictures, weather data or any other type of work done to fulfill the mission goal!) those data's are sent down through telemetry. Telecommand is the information or the command sent by the operators from control room to the satellite - commanding it to do something or perform software updates/do some changes in its automated routine and so on.

Subsystem: Telemetry, Telecommand(uplink/downlink), ground station

Instrument used:
On board computer (OBC), PCDU(Payload control data unit), Antennas, radars, lasers and all other communication equipments.

4. Payload systems, Launch, data operations
So far, only engineers and people from satellite operational group/designers were involved. But when designing payload, the scientists, end users, launch providers(people who build rockets) are also involved. They are needed because payload scientists define their requirement to which the instrument is built/bought/developed.

Launch guys are important because we cannot build a satellite that no rocket can launch or will not fit into any rocket's payload fairing.

Data operations guys are involved to define the communication channels necessary to develope the storage facility like how much amount of data per day is expected and how big should the storage be, what sort of backup/archive/storage facility is necessary and so on is defined at the phase a1 or phase 1 stage itself.

Once the design is complete, we cross the phase 0 into Phase 1 after a PDR (preliminary design review) - here feasibility study is conducted and a possible design layout of the satellite is developed. If everything goes well and the end user is satisfied we go into the next stage of design and development and finalizing of designs which goes into CDR (Critical design review) phase 1 ends and phase 2 starts where the actual components are being built and once thats is over goes into assembly, testing followed by FDR(Final design review) before the satellite is put to full tests or pre-launch tests. Once prelaunch test is complete, the satellite design which gets frozen at the CDR stage, is no more called a prototype rather, it is called as "Flight hardware" that is it has qualified the mandatory "Acceptance tests". Hence, will be subjected to final series of tests before being prepared for launch.

I am sorry if I am too boring and monotonous. I want to go into detail as much as possible but, these being intro posts, I am trying to stick to basics. So, anybody feeling bored, please feel free to comment and I will do the best to keep you guys interested. No pictures for this post, as more will come in the coming days.

Comments welcome!

[Guite]

Interesting thread. I believe there is a typo in "attitude control". A lot of people can do with attitude control though, if such a thing exist.

[AlphaKilo]

Originally Posted by Guite

Interesting thread. I believe there is a typo in "attitude control". A lot of people can do with attitude control though, if such a thing exist.

No, it is indeed called as "Attitude Control system" - it controls the attitude of the satellite. Wish we had such thing for humans too!

Next chapter: Will be about different satellites, missions, lunar/mars rovers, manned space flights.

[Guite]

Okay, I thought it is supposed to be aLtitude control.

[AlphaKilo]

Hello everybody, the eagerly awaited next post - well will have to wait untill this weekend! But untill then I shall use this thread to post some space related information(current research achievements) in the field of space science:
First, to start with, I will post the link which shows that Space science is not a waste of money and how the derivates from this field could be put to use in day-to-day applications:

http://www.dlr.de/dlr/en/desktopdefa...151_read-5318/

From space to life on Earth
VibroTac is a novel device in the form of a wristband capable of transmitting information, whose origins lie in the telepresence concept employed by space robotics. The original aim of this technology is to use human-system interfaces to equip an operator with the best possible access to remote worlds. In space travel, telerobotics is a key technology used for the construction, maintenance and reparation of satellites and space stations. Building upon this experience, the development process of VibroTac takes due account of ergonomic product design as well as human perception.

Followed by that some good news on Ozone layer rebuilding:
(means we can drive more cars )

http://www.dlr.de/dlr/en/desktopdefa...5393/year-all/

Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have been instrumental in the preparation of a report by the World Meteorological Organization (WMO) regarding the development of the ozone layer in the stratosphere. Based on estimates, by about the mid 21st century, the ozone layer will have the same thickness as it had in the early eighties. The latest evaluations of space-based measurements acquired by the DLR Remote Sensing Technology Institute, combined with model computations from the DLR Institute of Atmospheric Physics support the statement that ‘the regeneration of the ozone layer continues’.

Enjoy driving!

Edit: i am not doing any PR for any organisation. This applies to the links posted here, even though its the organization who pay for my bread and butter. Its just that I want to share with you all the latest research achievements from the field of space technology!

[dhawcash]

Instruments: Thrusters - small rockets....., Magnetic coils....., momentum wheels......

So these are the naughty boys responsible for correcting orbital courses without the need for on board fuel! waiting for more details on these.

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.

Another deep question i have, indirectly related to your profession.

Us earthlings are constantly burning up solids/liquids (fossil fuels, cigarettes , waste, industrial processes, etc..) and converting most of it into gas. So i have a theory- as the gas volume is ever increasing due to this, would it increase the diameter of the earth's atmosphere? I suppose That would affect a rocket's performance/trajectory if unaccounted for? (correct me if my worries are baseless)

PS: why is the thanks button missing in this thread?

[AlphaKilo]

Originally Posted by dhawcash

So these are the naughty boys responsible for correcting orbital courses without the need for on board fuel! waiting for more details on these.

Roger. Detailed info Coming up soon!

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.

Mate, you touched the " " -nerve in me! Expect a longg post on this topic, as this happens to be my area of work.

Us earthlings are constantly burning up solids/liquids (fossil fuels, cigarettes , waste, industrial processes, etc..) and converting most of it into gas. So i have a theory- as the gas volume is ever increasing due to this, would it increase the diameter of the earth's atmosphere? I suppose That would affect a rocket's performance/trajectory if unaccounted for? (correct me if my worries are baseless)

negative! They just react with atmosphere in the sameway like every other gas we expel So, there is no expansion of atmosphere happening anywhere! Actually, there is a wrong(if I may use that word) rather I would say a sort of misconception, earth is round, reality is there is no fixed outer boundary where the earth ends and outerspace starts(I said this in my very first post too!). So any expansion(if at all any!) just escapes into atmosphere! Remember, gases travel upwards not downwards and as long as the core below doesn't expand, we are safe and if that happens, well, please book the next possible space flight to the nearest safe planet!!

Please bear with me as I am undergoing a training this week and also have to shift house, so for sure, will fulfill my promise in the next couple of days, but in the meanwhile, questions are absolutely welcome!

[AlphaKilo]

Hello Everybody,
First of all sorry about the longer delay. There is a slight change of plan. Initially I said I would be writing about various missions so far, but lets push it little back in terms of priority.

This post will be on "Attitude and Orbital Control subsystem"

1. What is attitude and orbital control subsystem?

Basically, please refer to my post detailing about various subsystems of satellites. Well, it is as necessary in a satellite, as much as a steering wheel in an automobile. More or less, both do the same job, but the basic difference being, being inside Earth, we control the steering manually but since satellites are up in space and there are certain restrictions on real-time control, the attitude control is mostly autonomous.(read correctly: Its Autonomous and not automatic)

- We need this system to
a. Maintain the orientation of the satellite, so that the payload (eg. Cameras), solar panels and other instruments are pointing towards the correct direction.
b. Necessity to maintain a pre-defined orbit (track) for various reasons such as like for eg. keeping lanes on a highway, up in the space satellites need to maintain the orbit to avoid colliding with one another, or in-order to be able to make payload operations(remote sensing/imagery/experimental studies etc. etc.)
c. To compensate the atmospheric(rest) drag and to counter act the sink caused by the gravity pull and drag.

2. All said and done, but then How do they work? How is attitude control achieved?

Now that's an easy question to answer. The following image shows the three axes that is used in a spacecraft.
Source



Basically, the axes definition varies a little from satellite to satellite. Well, but just go ahead with this example. Whenever a satellite is launched in the earth orbit, it is necessary for the satellite to point its solar panels towards the sun(power generation for the systems and to charge the batteries). In this case it is the Z axis that needs to turn by 90° (assuming sun is on the left side of the satellite). Lets see the sequence of operations:

After launch and after seperation from the rocket, the satellite starts to tumble uncontrollably. The angular velocity (omega) is quite high during this time. The on-board computers remain in safe-mode(sleep mode), that means, that neither the satellite nor the controller in the ground know anything about the satellite's actual orientation. But with time, the spin/tumble starts to loose energy and satellites starts to stabilize over its centre of mass (i'm purposefully not using centre of gravity - can someone tell me the reason for it? ). Once this happens,(the slowing down process is known as detumbling and the controllers on the ground know to a rough extent how long this would take!) hence, the on-board computers are pre-programmed to wake up once this time is elapsed.

Once woken up, the computers do a quick health check, to see if all the systems are normal and within nominal limits. Once all green, it slowly starts to switch its sensors (sun-sensors, star sensors, earth sensors, Inertial Measurement Unit (Gyroscopes) etc. etc.). Usually, big, heavy satellites have lots of sensors, to be able to achieve the desired orbit with very precise accuracy whereas, smaller, cheaper ones have usually only one or two of the sensors on board. Some small cube-satellites(built by students) even use Earth's gravity to stabilize itself(passive stabilization!).

So, first to start working will be the sun-sensor. Once that is on, it starts to scan the space for the sun(sun-sensor is of many types - cheapest one being a CCD chip searching for a light source powerful enough to be a sun!). Usually, sun sensor is mounted all over the body(one for each side of the satellite) and the on-board computer knows exactly which sensor has found the sun. Once, found it commands the Attitude and Orbital control system(AOCS) to turn the satellite/solar panels into that particular direction.
Sun sensors: Source


This turning is achieved by again, various means such as reaction(momentum) wheels, thrusters(generally used in bigger satellites), magnetic torquers.
Micro-Thrusters: Source


Micro-thrusters are nothing but small(mini) rockets. There are many types. Most common types produce thrust in the range of 10-24 N and are used in large number aboard a single spacecraft(more than 12 at times! precision defines the number!). They use cold-gas fuels such as MMH/N2O4 (mono-methylhydrazine and diNitrotetraoxide) and are fired not continuously but in short pulses. These pulses generate thrust, there by performing push/turn/any required maneouvre and effectively re-orienting the satellite. These thrusters are usually used during initial stages where large corrections are needed and during rest of the life, they remain as back-up systems for other Orbit control system(OCS). The following is the one used in Apollo Lunar Capsule:Source





Reaction Wheels:source


More or less, very commonly used system to achieve the Attitude control. This is nothing but a mass mounted on a electric motor, made to spin at high rpms. This induces a momentum inside the satellite(upon its Centre of Mass). These reaction wheels are strategically placed(usually two for each axis -cheaper way one for each axis-much cheaper way use totally three set of wheels places exactly at the centre of mass and just spin on of the three to create an imbalance in the moment of inertia). The reaction wheel simply induces a gyro effect, thus forcing the satellite to spin in the opposite direction. This is used mostly in combination with devices like magnetic torquers.

I have video of reaction wheels in action (taken by me while testing a satellite), I will post it later.

Magnetic Torquers: Source


Working principle of Magnetic torquer: source


so, now the last of the ACS tool. This is nothing but a coiled electro magnet placed inside the satellite. When current passes through it, and when placed in the earth's magnetic field lines, generates a resultant force which is used to turn or reorient the satellite in the necessary direction. (mostly useful only for earth observation(LEO) orbits). Here, I am not going into the physics behind it as, more or less everybody who has done physics in 12th standard know it!

So, bascially by activating one or more or a combination of these devices, either only attitude control or only orbital control or both attitude and orbital control is achieved in a satellite.

Time for your questions and doubts!!

[dhawcash]

After launch and after seperation from the rocket, the satellite starts to tumble uncontrollably..... But with time, the spin/tumble starts to loose energy and satellites starts to stabilize over its centre of mass (i'm purposefully not using center of gravity - can someone tell me the reason for it? ).....

The reason for using center of mass?

F= Mass x acceleration ( and gravity is just the force exerted on an object due to its mass and earth's gravitational pull ( 9.8 m/s^2)

But as the satellite is in equilibrium with gravity and centrifugal forces cancelling each other out, its mass that matters. it is essentially a weightless thing floating out there. ( zero acceleration if i may say so?)

Another question i have. If the satellite is released into the vaccum of space, how would it lose its energy? i mean there would be no air resistance to dampen its tumbling?

[AlphaKilo]

Originally Posted by dhawcash

The reason for using center of mass?

F= Mass x acceleration ( and gravity is just the force exerted on an object due to its mass and earth's gravitational pull ( 9.8 m/s^2)

But as the satellite is in equilibrium with gravity and centrifugal forces cancelling each other out, its mass that matters. it is essentially a weightless thing floating out there. ( zero acceleration if i may say so?)

The reason I love TBHP!! Full of geeks by the way, answer is correct, even I wouldn't have done any better!

Another question i have. If the satellite is released into the vaccum of space, how would it lose its energy? i mean there would be no air resistance to dampen its tumbling?

Yes and no. Outer space is not completely vaccum. Its more or less, micro-gravity(if I may say so!) lower orbits do experience atmospheric drag, remember I told in my first post that there is no specific boundary where the atmosphere ends and empty space starts. Moreover, even if you go higher, its not technically empty, we constantly have cosmic radiation, solar particles flying around(photons and plasmas). They have energy, although negligible when compared to the mass of the body we send up, yet, they cause significant changes over time.

1. Heard of solar radiation pressure? Can some one tell me whether is it due to the solar wind or something else?

2. See the image below!source


Now say whether outer space is completely vaccum or not!

[AlphaKilo]

I apologize for a grave mistake in quoting the original sources:

The first two images are from
Sun sensors: http://spot4.cnes.fr/spot4_gb/attitude.htm

Apollo Lunar Module: http://en.wikipedia.org/wiki/File:LM_RCS.jpg

Reaction wheels: http://lunar.gsfc.nasa.gov/gallery-hardware.html

Magnetic torquer is from: http://bass.gmu.edu/~pceperle/WebPro...1/mag_torq.htm

Solar winds and Earth's magnetosphere: http://upload.wikimedia.org/wikipedi..._rendition.jpg

Sorry about the mistake!

[CLIX]

Very good material AK! Thanks !

I’ve a question on something I came across some time ago (umm very long ago ! ) : “Ion Motors”
If I understood right these can [if “done” right !] be used to propel spacecraft. And I also remember this being part of some hush hush research going on. Would you be able to throw some light on this? Where are we with this technology? Before that – What exactly is this?

Thanks in advance. And do keep the interesting posts ticking!

[AlphaKilo]

Originally Posted by CLIX

I’ve a question on something I came across some time ago (umm very long ago ! ) : “Ion Motors”
If I understood right these can [if “done” right !] be used to propel spacecraft. And I also remember this being part of some hush hush research going on.

They do exist and as you rightly pointed out, there is always a school of thought among the "Propulsion experts" who are working to find out something new and to some, this is a "hush-hush" research and even believe that future lies here! well, every one is free to have their own opinions

What exactly is this?

Before we go to see what this actually is, we will have to know the ranges of various propulsion mechanisms: (as explained by the graph below)
Source: http://dawn.jpl.nasa.gov/mission/ion_prop.asp


So, as we see here, chemical propulsion(conventional rockets) run out of breath after a max 400-500 secs (I_sp), whereas the electric propulsion can go on and on, albeit their thrust levels will seem like a joke (unbelievably low) when compared with that of the conventional chemical propulsion. Hence, in general, electrical propulsion is gaining fame and is being favoured by the Propulsion engineers these days over chemical ones, not only due to I_sp but also due to other issues like fuel on board and so on.

1. Basically uses Ion's as the fuel to generate thrust! Means, simply speaking, emission of ion's at high speeds (remember action-reaction, Newton's third law). In free space(more or less free), this small thrust when exerted for long (really long) durations, creates enough force to propel (take it with a pinch of salt!) or to provide orbital/attitude corrections(more or less possible - tested in some missions already!).

2. The high velocity is achieved by accelerating the ions upto very high speeds at the cost of lots of power. Means, in order to be able to accelerate these ion particles to speeds which can produce thrust, we would need a power grid!(a huge one! eg. See below!)

3. Just quoting the description from Wiki:

Ion thrusters use beams of ions (electrically charged atoms or molecules) to create thrust in accordance with momentum conservation. The method of accelerating the ions varies, but all designs take advantage of the charge/mass ratio of the ions. This ratio means that relatively small potential differences can create very high exhaust velocities. This reduces the amount of reaction mass or fuel required, but increases the amount of specific power required compared to chemical rockets. Ion thrusters are therefore able to achieve extremely high specific impulses. The drawback of the low thrust is low spacecraft acceleration, because the mass of current electric power units is directly correlated with the amount of power given. This low thrust makes ion thrusters unsuited for launching spacecraft into orbit, but they are ideal for in-space propulsion applications.
Various ion thrusters have been designed and they all generally fit under two categories. The thrusters are categorized as either electrostatic or electromagnetic. The main difference is how the ions are accelerated.

• Electrostatic ion thrusters use the Coulomb force and are categorized as accelerating the ions in the direction of the electric field.
• Electromagnetic ion thrusters use the Lorentz force to accelerate the ions.

Power supplies for ion thrusters are usually solar panels, but at sufficiently large distances from the Sun, nuclear power is used. In each case the power supply mass is essentially proportional to the peak power that can be supplied, and they both essentially give, for this application, no limit to the energy.
Electric thrusters tend to produce low thrust, which results in low acceleration. Using 1 g is 9.81 m/s/s; F = m a or a = F/m
An NSTAR thruster producing a thrust (=force) of 92 mN[7] will accelerate a satellite with a mass of 1000 kg by 0.092 / 1000 = 0.000092 m/s/s (or 9.38E-6 g).

Do you see the acceleration achieved by this over a mass of 1000 kg? But being electric, the maximum (if not all) of the input energy is converted as kinetic energy! Hence, scores high on efficiency (good for "Kitna deta hain?" type junta!)

Small example: Boeing Thruster (http://www.boeing.com/defense-space/...ion_medium.gif)

Would you be able to throw some light on this? Where are we with this technology?

Today, lots and lots of research work and money is going into this(similarly into many electric propulsion methods), as chemical propulsions seems to be loosing out the edge for the want of high delta V and longer I_sp (specific impulse - see my initial post to know what I_sp means in case you had missed reading this thread from the beginning).

Many private companies like Boeing, Lockheed Martin, EADS Astrium, and many companies in Australia and Russia are actively working on it.

Some test satellites flown so far:
NASA: DAWN
http://dawn.jpl.nasa.gov/mission/ion_prop.asp

The Dawn spacecraft uses ion propulsion to get the additional velocity needed to reach Vesta once it leaves the Delta rocket. It also uses ion propulsion to spiral to lower altitudes on Vesta, to leave Vesta and cruise to Ceres and to spiral to a low altitude orbit at Ceres. Ion propulsion makes efficient use of the onboard fuel by accelerating it to a velocity ten times that of chemical rockets. This efficiency is measured in terms of the specific impulse of the fuel (Isp).

Dawn's engines have a specific impulse of 3100 s and a thrust of 90mN. While a chemical rocket on a spacecraft might have a thrust of up to 500 Newtons, Dawn's much smaller engine achieves an equivalent trajectory change by firing over a much longer period of time. The figure below shows the specific impulse and thrust of different thruster types.

PAS-5 (commercial satellite to carry Xenon propulsion system)
http://www.boeing.com/defense-space/...xips/xips.html

[MaNishi]

Great Stuff, AK!

You have made Rocket science real easy for layman like us to understand.

Keep them coming
and
Thanks for sharing .