Mro On Approach, TCM-3 not required Options

[Redstone]

MRO has shifted from the cruise phase to Approach phase. Apparently, the trajectory is so good that TCM-3 was cancelled. This is good news for the prospects for a long life for MRO supporting future missions. TCM-4 is on Feb 28, and MOI on March 10. Only 5 weeks away!

[RNeuhaus]

After reading more details about MRO. I found it many interesting and surprises things:

As Mars Reconnaissance Orbiter approaches Mars traveling about 10,400 km per hour, it will need to fire six main engines for a long 25-minute burn, slowing the spacecraft down by about 3,584 km per hour! That reduction of speed relative to the planet will place the spacecraft into a long looping orbit around Mars.

The same as Doug has told us.

All subsystems and instruments on board (the so-called "dry mass") must weigh less than 1,031 kilograms (2,273 pounds) to allow room for 1,149 kilograms (2,533 pounds) of propellant for trajectory correction maneuvers that keep the spacecraft on target during the cruise to Mars and for burns that help capture the spacecraft into orbit around Mars.

The monopropellant hydrazine tank is big enough to hold 1187 kilograms (2617 pounds) of usable propellant. This amount of propellant, when used, will change the spacecraft's velocity by about 1.4 kilometers per second (3,100 miles per hour). Over 70% of the total propellant will be used during just one maneuver - Mars orbit insertion.

The propulsion combustible takes up more than 50% of spacecraft weigth.

Some way is needed to push the propellant to the thrusters. Mars Reconnaissance Orbiter feeds pressurized helium gas from a separate high- pressure tank, through a regulator, into the propellant tank where it puts the hydrazine propellant under pressure. Then, when any thruster is opened, the propellant will flow rapidly out, much like paint does from a can of spray paint.

Interesting, MRO uses HELIUM GAS to push Hydrazine away from the tank.
Pressurant Tank


About thrusters, a total of 20 rocket engine thrusters are onboard:

* Six large thrusters, each producing 170 Newtons* (38 pounds force) of thrust for performing the Mars orbit insertion burn. Together, all six produce 1,020 Newtons (104.5 kg 230 pounds force) of thrust. That's about the force you would feel if an NFL linebacker decided to sit on you.

* Six medium thrusters, each producing 22 Newtons* (5 pounds force) of thrust for performing trajectory correction maneuvers, and for helping to keep the spacecraft pointing in the right direction during the Mars orbit insertion burn.

* Eight small thrusters, each producing 0.9 Newtons* (0.2 pounds force) of thrust for controlling where the orbiter is pointed during normal operations as well as during Mars orbit insertion and trajectory correction maneuvers.

(*A Newton is a unit of force required to accelerate a mass of one kilogram one meter per second?every second!). One Newton is equivalent to 1.626 kg of push force.

So many thrusters! 20. I tought it had 7!!!

Spacecraft Configurations:

what the spacecraft will look like during various phases of the mission

During aerobraking, the solar panels have a special role to play. As the spacecraft skims through the upper layers of the martian atmosphere, the large, flat panels act a little like parachutes to slow the spacecraft down and reduce the size of its orbit.

The friction from the atmosphere passing over the spacecraft during aerobraking will heat it up, with the solar arrays heating up most of all. The solar arrays have to be designed to withstand temperatures of almost 200 Celsius (almost 400 degrees Fahrenheit!).

MRO will get very hot, up to 200 degree of centigrades at the perigee passing.

At Mars, the two panels together produce 1,000 Watts of power.

Nickel-hydrogen batteries

During each two-hour orbit around Mars the spacecraft will experience a "day" and a "night." During the "night," there is no sunlight because the planet is between the orbiter and the sun, and therefore blocks the sun's light from reaching the spacecraft. Astronauts on the shuttle experience this kind of pattern as well when they orbit the Earth.

During the nighttime periods, batteries are used to provide the necessary electrical power. The batteries charge during each "day" (using part of the electricity produced by the solar cells) and discharge during each "night' to keep the spacecraft supplied with electricity.

Mars Reconnaissance Orbiter uses two Nickel-Hydrogen rechargeable batteries, each with an energy storage capacity of 50 Ampere-hours - at 32 Volts that's 1,600 Watts for one hour. The spacecraft can't use this total capacity, because as a battery discharges its voltage drops. If the voltage ever drops below about 20 Volts then the computer will stop functioning - a very bad thing! So, to be safe, only about 40% of the battery capacity is ever planned to be used.
Comparing to MER, they have 1.2 M² of solar panel and its produces about 1000 wats when they had the best atmospheric conditions and about during the summer. The MRO has big solar panels, 10 M² and also produces about the same to MER? MRO will orbit Mars every two hours?, somewhat slower than ISS around Earth with 91 minutes.


There are three main mechanisms on board Mars Reconnaissance Orbiter:

* one that allows the high-gain antenna to move in order to point at earth
* two that allow the solar arrays to move to point at the sun

Each of these mechanisms, called gimbals, can move about two axes in much the same way that your wrist allows your hand to move in two axes: left/right and up/down. By contrast, your knee only has one axis of motion, but your neck has three.

As the spacecraft travels around Mars each orbit, these gimbals allow both solar arrays to be always pointed toward the sun, while the high-gain antenna can simultaneously always be pointed at earth.

What advanced is the space orientation software that the information from star tracker and sun sensors are feeding to the software in order to adjust constantly the solar panels and antenna pointing to Sun and Earth respectively.


The solar panels must be strong enough to survive launch, when the forces can exceed 5 g's. This means that the structure must be designed as if the spacecraft weighed five times what it does on earth! Extremely lightweight but strong materials are used to achieve this strength, including titanium, carbon composites, and aluminum honeycomb.

Any commercial airplane can withstand greater than 3 gravity but the military fighters can do up to 8 g's but the airman will be inconcient for that rather time.

Powerfull telecomunications
With its large-dish antenna, powerful amplifier, and fast computer, Mars Reconnaissance Orbiter can transmit data to earth at rates as high as 6 megabits per second, a rate ten times higher than previous Mars orbiters. This rate is quite high considering that Mars Reconnaissance Orbiter will achieve it while 100 million kilometers (62 million miles) from Earth.


Ten times faster than the previous Mars orbiters, which ones? I suppose it is refering to Odyssey?


High-gain Antenna
The high-gain antenna is a 3-meter diameter (10-foot) dish antenna for sending and receiving data at high rates.

The high-gain antenna will be deployed shortly after launch (see launch configuration), and will remain deployed for the remainder of the mission. It will serve as the primary means of communication to and from the orbiter.

The high-gain antenna must be pointed accurately and is therefore steered using the gimbal mechanism.

Low-gain Antennas

Two smaller antennas are present for lower-rate communication during emergencies and special events, such as launch and Mars Orbit Insertion. The data rate of these antennas is lower because they focus the radio beam much more broadly than the high gain antenna, so less of the signal reaches earth. But the Deep Space Network station on the earth can "see" the signal even when the spacecraft is not pointed at earth, and this is why these antennas are useful for emergencies. Think of how a flashlight works: with a tightly focused beam of light you can see farther directly ahead but not at all to the side. And with a wide beam you can see all around you but not very far. The low-gain antennas have the capability to transmit and receive.

The two low-gain antennas are mounted on the high-gain antenna dish - one on the front side and one on the back -- and are moved with it, although as just mentioned they do not require accurate pointing. Two are needed in that placement so that communication is possible at all times, no matter what the position of the spacecraft might be at a given time.

MRO has two LGA (front and back) and it has very good redundancy comparing to one of Hayabusa.


Mars Reconnaissance Orbiter uses a monopropellant propulsion system: there is fuel (hydrazine), but no oxidizer. Thrust is produced by passing the fuel over beds of catalyst material just before it enters the thruster, which causes the hydrazine to combust . (Other types of systems use bipropellant propulsion, where combustion is achieved by mixing a fuel with an oxidizer. The Space Shuttle, for example, uses liquid hydrogen as fuel and liquid oxygen as oxidizer, which spontaneously combust (explode) when they are mixed.)

Propellent: It is monopropellent. ONLY one : HYDRAZINE. It is a fuel. It does not need oxygen as an oxidizer to ignite. What is the catalyst material that causes the hydrazine to be fueled. Electrical ignition?



Reaction wheels:

While the reaction control system thrusters allow the spacecraft to turn quickly, they're not good at slow and steady turns. Slow and steady turns, however, are just what is required to take high-resolution images of Mars from orbit. Mars Reconnaissance Orbiter therefore has devices called reaction wheels. These are literally spinning wheels - four in total: one for each rotational axis plus a spare in case one of the three isn't working.



Good ones: 4 reactions wheels: one for three dimension axis plus one as a spare. Even better than Hayabusa.


Spacecraft Parts: Command and Data-Handling Systems

The Command and Data Handling subsystem is essentially the "brains" of the orbiter and controls all spacecraft functions. This system:

* manages all forms of data on the spacecraft;
* carries out commands sent from earth;
* prepares data for transmission to the earth;
* manages collection of solar power and charging of the batteries;
* collects and processes information about all subsystems and payloads;
* keeps and distribute the spacecraft time;
* calculates its position in orbit around Mars;
* carries out commanded maneuvers; and,
* autonomously monitors and responds to a wide range of onboard problems that might occur.

Space Flight Computer:
At the heart of the space flight computer, Mars Reconnaissance Orbiter employs the next generation of space-qualified processors, based on the 133 MHz PowerPC processor. While this speed may seem slow compared to the Gigahertz speed of the computer you're using to read this web site, it is fast by space standards. Commercial chips must be significantly enhanced and undergo long duration testing to prove they will survive the unforgiving radiation environment of space.

Flight Software:
The Flight Software is an integral part of the Space Flight Computer, and includes many applications running on top of an operating system, similar to the way your home computer has applications running on top of Microsoft Windows or MacOS. Mars Reconnaissance Orbiter's operating system is called VxWorks.
An example of an application is Fault Protection. Fault Protection continuously monitors hundreds of parts of the spacecraft for a wide range of problems, takes action to fix the problem if it can, and if it can't, keeps the spacecraft safe while it waits for instructions from Earth.


Smart enough to handle by itself during if there is a fault.

Solid State Recorder:

The Solid State Recorder is the primary storage for science instrument data onboard the spacecraft, with a total capacity of 160 Gigabits. That may seem like a lot, but not when you realize that a single image from HiRISE can be as big as 28 Gigabits!

The science data is stored on this recorder until it is ready for transmission to Earth, and then is overwritten with new science data.
The recorder is called a Solid State Recorder because it has no moving parts. Neither a tape recorder nor a hard disk drive, this device uses an array of over 700 memory chips, each with 256 Megabit capacity, to store Mars Reconnaissance Orbiter's data.

Whopping storage capacity in solid state record (RAM). The big computers has that amount storage. There very few computers have it.


Sensors

Sensors determine where the spacecraft is pointed, how fast it is turning, and how its speed is changing. They include:
sun sensors:

Sixteen sensors (eight are backups) deployed around the spacecraft body provide knowledge of where the sun is located. These sensors are pretty simple, and only give two answers: "I see the sun" or "I don't see the sun." The computer and flight software listen to all of the sensors and can deduce where the sun is from that information. Since the spacecraft relies on sunlight to create electrical power, this function is very important.

The sun sensors normally are used only when first waking up the spacecraft (for example, after launch) and during spacecraft emergencies - in both cases the spacecraft may not know where it is pointed.
The sun sensors give enough information so the spacecraft can continue to get power from the sun, but they don't give enough information for other things, like finding the earth, or a spot on Mars. For that, more sophisticated sensors coupled with computer software are required (see below).

Two star trackers are used to provide full knowledge of the spacecraft orientation, allowing the spacecraft to know not only where the sun is, but also where Earth and Mars are and how to point to any direction in the sky (which is necessary when doing a maneuver). As is the case with many components aboard Mars Reconnaissance Orbiter, the second star tracker is there as a back-up in case the first one fails.
The star tracker is a very smart camera. The star tracker takes a digital picture of the stars. Then, using its own catalog of thousands of stars, it compares the image with the catalog until it can identify the stars in the image. Once it does that, it knows exactly where it was pointing when it took the picture, and it sends a message to the computer with that information. And it does that ten times every second!

So many Sun sensors!!! up to 16??? (8 for backups) MRO has two star trackers which help to spacecraft ot only to know where the sun is located but also of Earth and Mars.


Sorry of that long post. It will permit us to be much better prepared to understand better of any MRO directions, movements, timing, so that we can discuss it better.

Rodolfo

P.D. More details, visit click here

[djellison]

[quote name='RNeuhaus' post='42269' date='Feb 19 2006, 06:14 AM']

Interesting, MRO uses HELIUM GAS to push Hydrazine away from the tank.
Pressurant Tank
[/quote]

I think just about every spacecraft I've ever seen does this.
[quote]
So many thrusters! 20. I tought it had 7!!!
[/quote]
Well - 7 main ones for the engine burn for MOI - but then lots of smaller ones for smaller manouvers and rotating the spacecraft to unload gyros.

[quote]
MRO will get very hot, up to 200 degree of centigrades at the perigee passing.
[/quote]

Ah - carefull - they say it's designed to withstand 200 degrees, not that they will nearly reach 200 degrees. They're get warm - but nothing like 200 deg. I think one of the tech papers I mentioned elsewhere shows CFD of how warm it might get.

[quote]
Comparing to MER, they have 1.2 M² of solar panel and its produces about 1000 wats when they had the best atmospheric conditions and about during the summer. The MRO has big solar panels, 10 M² and also produces about the same to MER? MRO will orbit Mars every two hours?, somewhat slower than ISS around Earth with 91 minutes.

[/quote]

Ahh - again, no - MER can produce up to about 1000 Watt-HOURS per sol, i.e. the equiv of 100 watts for 10 hours. Typically, the MER solar arrays would rarely produce more than 100 Watts at any one time - I'm not sure of the exact figures.

[quote]
Ten times faster than the previous Mars orbiters, which ones? I suppose it is refering to Odyssey?
[/quote]
and then some....

The very highest data rate I've seen quoted for Odyssey is 124425 bits per second - i.e. roughly 0.125 Mbits/sec

MRO's still a bit fluffy on quoted numbers - but the highest it can do is certainly around 5 Mbits/sec.

[quote]
Good ones: 4 reactions wheels: one for three dimension axis plus one as a spare. Even better than Hayabusa.

[/quote]
And the same as Cassini - which is already using its spare.
[quote]
Smart enough to handle by itself during if there is a fault.
[i]
[/quote]

Same as most spacecraft really.
[quote]
Whopping storage capacity in solid state record (RAM). The big computers has that amount storage. There very few computers have it.

[/quote]

Well - 160 Gbits is about 20 Gigabytes - and onboard that is equiv to the 'hard drive' for MRO - and I think we'd all be moaning if we only had that

Doug

[helvick]

Ahh - again, no - MER can produce up to about 1000 Watt-HOURS per sol, i.e. the equiv of 100 watts for 10 hours. Typically, the MER solar arrays would rarely produce more than 100 Watts at any one time - I'm not sure of the exact figures.
and then some....

The instantaneous peak output of the MER Panels at mars would vary between ~146 and 214watts if they were in orbit rather than on the surface. Taking the atmosphere and seasonal Tau into account that changes to ~140 to 164 watts.
I thought MRO had 2x10m^2 panels which could peak at between 2.2 and 3.3kw but I presume that they are rarely optimally oriented so an average of 1000 watts over each orbit is probably right.