Post by inquisitivemind on Nov 3, 2010 10:51:06 GMT -4
In this thread, I discuss about the flight of the lem from the LCM to the moon.
I am going to show that the normal orientation for the LCM and the LEM at the start of the flight is horizontal.
One member of this forum has a wrong misconception that makes him think that the LCM has a fixed orientation in an absolute system which makes that its attitude constantly changes relatively to the moon as it orbits it; this is the way he imagines the LCM orbits the moon:
So the LCM would alternatively be horizontal and vertical relatively to the moon with all the intermediary attitudes.
He thinks that keeping the LCM horizontal relatively to the moon would need an attitude control.
I have told him that the LCM remains naturally horizontal relatively to the moon as it orbits it, without the need of an attitude control; this is the way the LCM naturally behaves:
But this member doesn't believe me; he thinks he detains the knowledge, and that I'm just an "ignorant layman" who knows nothing about space navigation.
I have told him that, if he makes an object turns with a string, he can see that the attitude of the object naturally follows its orbits by the property of the centrifugal force.
But he thinks that it's because there is a string, and that, without a string, the object does not behave the same.
In fact, there is a string, but it's invisible, it's the lunar attraction which is a virtual string which is attached to the center of gravity of the object.
The orientation that the orbiting object takes relatively to the body it orbits around depends on its distribution of mass.
In the case of the LCM, this natural orientation is along its longitudinal axis.
This schema explains how the centrifugal force automatically regulates the attitude of the LCM:
When the LCM is oriented this way, the right part is in an inner orbit, and the left part is in an outer orbit; because the right part is in an inner shorter orbit, it turns a little faster than the left part, and thence it is submitted to a little stronger centrifugal force than the left part; consequently the LCM turns counterclockwise by a lever effect.
The same if the LCM is in the opposite orientation.
The LCM will naturally remain parallel to its orbit around the moon, and no artificial attitude control is needed for that.
Of course, it is possible to make the LCM perpendicular to the moon with the lateral reactors, but it will remain in this position for as long as an effort is sustained on the lateral reactors; if this effort is stopped, the centrifugal force will make the LCM go back to its horizontal position.
I have found an article for this reluctant member proving that a satellite has a natural orientation which is always the same relatively to the earth:
celestrak.com/columns/v04n09/
There is this schema in this article:
with this comment:
"The despun section rotates, keeping the antennas pointed at the earth and preventing the satellite from going into a flat spin (which is the natural tendency)."
This means that, if no attitude control was exerted on the satellite, then it would always show its flat side to the earth, but it won't do, because it's not on this side that the antenna is mounted, so the attitude must be changed, and that's why an attitude control is necessary in this case.
But the reluctant member denied that this was what the article was meaning, and persisted in his misconception, and went on snubbing me as being ignorant.
Well, if he is true, then, when an obus is fired by a cannon, it should keep the orientation which has initially been given to it, and fall this way:
But anybody who has seen an obus fly knows that the orientation of the obus follows its trajectory, and that it flies this way:
And, if the big satellites always have an attitude control, there also exists small satellites, called "nanosatellites", which don't always have an attitude control.
These satellites are balanced in mass so that they have the right orientation toward the earth, and they always show this orientation to the earth; of course they are not very precise, and have little variations because of perturbations (like solar interferences) which prevents from using them for some applications which require precision, but they still can be used for applications which don't require too much precision.
These satellites absolutely don't behave like this member imagines, that is that they would have a fixed orientation in an absolute system which would make that their orientation toward the earth would constantly change.
So, I must say to this member: Although you persist in your misconception, it's really a misconception you have, and you should get rid of it.
Now, if you continue to think that your conception is correct and that I'm only an ignorant idiot who thinks wrong, then show me a serious scientific article which confirms what you say.
Before leaving the LCM, the LEM is attached this way to the LCM:
In fact, the LEM already has the good attitude, because this is the way it must start its travel to the moon.
Indeed, the LEM initially has the same horizontal speed relatively to the moon as the LCM, that is around 6000 km/h.
This speed creates a centrifugal force which counters the lunar attraction.
The LEM must decrease this horizontal speed to allow the lunar attraction to attract it to the moon, and also because it must have nullified this speed before reaching the moon.
It starts countering the horizontal speed by taking a horizontal attitude and giving a horizontal thrust with its main reactor.
As the LEM loses its horizontal speed, the LEM must progressively change its attitude to make it more vertical, in order to also control the vertical speed, because the lem must also nullify the vertical speed when it reaches the moon, and furthermore it must arrive vertical on the moon.
The vertical lateral reactors allow to control the attitude of the lem; the attitude of the lem allows to distribute the thrust of the main reactor on the horizontal axis (control of horizontal speed) and the vertical axis (control of vertical speed).
At any moment, the AGC reads the current actual decelerations on the accelerometers and compares them with the computed decelerations the LEM should ideally have at this moment of the trajectory;from the difference between the readings and the computed values, the AGC computes corrections to apply to the main thrust and the attitude (lateral vertical reactors).
This process is continuously made, and the control is smooth enough not to need a fast computational period (which is 2 seconds, according to the NASA documentation).
Now you might ask: Why is it necessary to start countering the horizontal speed as soon as the LEM leaves the LCM (green trajectory) , why couldn't the LEM start to descend vertically, and start countering the horizontal speed when it is closer to the moon (red trajectory)?
Well, if the LEM starts descending vertically, and starts countering the horizontal speed when it is closer to the lunar ground, then the lem will have lost its reserve of height, and will not be allowed to lose as much height as he could if it starts earlier to counter the horizontal speed; as a result, the LEM will have to counter more the lunar attraction than with the first trajectory, not to lose too much height.
If the lem was compensating the lunar attraction the same way on the second trajectory as it does on the first one, this is what would happen:
The Lem would bump into the lunar ground before it has finished nullifying the horizontal speed, with the result you can imagine; it would end in a disaster.
So, the lem will have more to compensate the lunar attraction in the second trajectory, and thence give more vertical thrust.
So, the second trajectory is possible, but will consume more fuel than the first trajectory.
I could give an analogy with an helicopter.
An helicopter lands along a parabolic trajectory:
The helicopter could also land this way, that is first brutally descend, and then fly closer to the ground till its landing point:
But it will not usually do it, because that makes it consume more fuel.
So, in conclusion, the trajectory in which the lem starts horizontal is the most economical trajectory, even if other trajectories are possible.
So, if the most natural orientation for both the LCM and the LEM is horizontal, why would the lcm take a vertical orientation when the LEM leaves the LCM?
Now, you can say:
- a trajectory along which the lem first descends vertically before starting countering the horizontal speed is possible.
Yes, it is possible!
- And the LEM could start flying over the LCM, and make rolls, like on these sequences:
Yes, it is possible!
So, what's my point?
My point is that if would make the LEM consume more fuel than just doing the optimal trajectory (in which the lem starts in a horizontal position).
And why is it so important to save fuel in this lunar adventure?
Well, the moon is an unknown universe.
The astronauts don't know in advance what they will find when they get close to the moon.
The relief may be broken, dangerous; they may arrive at a spot which is unfit for landing (a rocky spur or a crater), and they may have to look for the right spot to land on, by maneuvering the LEM with the lateral reactors.
But, during this phase they look for the right spot to land on, it's not with the lateral reactors that they will burn the more fuel, it's with the main reactor instead!
Why?
Because they constantly have to counter the lunar attraction to maintain the LEM above the lunar ground; indeed, they no more have the horizontal speed to create a centrifugal force to counter this lunar attraction.
So, the longer they look for a spot to land on, and the more they will consume fuel.
And they don't know in advance how long this phase will take.
And on the moon, no rescuers, and no gas pumps!
And they also have to save enough fuel for the return to the LCM!
In these conditions, it is absolutely obvious that they have to put all the chances on their side by not wasting fuel before getting close to the moon and saving as much of it as they can!
Making the clowns before really starting for the moon could cost them their lives!
This is especially true when we know that Neil Armstrong was almost out of fuel when he landed on the moon.
This is what we can find in the relation of his flight:
"Words of warning came from Earth: just 60 seconds of fuel left before he would have to abort the landing".
So, he was almost out of fuel, fuel that he could not refill on the moon, and he would however have made the fantasy of flying over the LCM which would have taken the fantasy of being vertical?
DOES IT MAKE SENSE, SERIOUSLY, DOES IT MAKE SENSE???
I am going to show that the normal orientation for the LCM and the LEM at the start of the flight is horizontal.
One member of this forum has a wrong misconception that makes him think that the LCM has a fixed orientation in an absolute system which makes that its attitude constantly changes relatively to the moon as it orbits it; this is the way he imagines the LCM orbits the moon:
So the LCM would alternatively be horizontal and vertical relatively to the moon with all the intermediary attitudes.
He thinks that keeping the LCM horizontal relatively to the moon would need an attitude control.
I have told him that the LCM remains naturally horizontal relatively to the moon as it orbits it, without the need of an attitude control; this is the way the LCM naturally behaves:
But this member doesn't believe me; he thinks he detains the knowledge, and that I'm just an "ignorant layman" who knows nothing about space navigation.
I have told him that, if he makes an object turns with a string, he can see that the attitude of the object naturally follows its orbits by the property of the centrifugal force.
But he thinks that it's because there is a string, and that, without a string, the object does not behave the same.
In fact, there is a string, but it's invisible, it's the lunar attraction which is a virtual string which is attached to the center of gravity of the object.
The orientation that the orbiting object takes relatively to the body it orbits around depends on its distribution of mass.
In the case of the LCM, this natural orientation is along its longitudinal axis.
This schema explains how the centrifugal force automatically regulates the attitude of the LCM:
When the LCM is oriented this way, the right part is in an inner orbit, and the left part is in an outer orbit; because the right part is in an inner shorter orbit, it turns a little faster than the left part, and thence it is submitted to a little stronger centrifugal force than the left part; consequently the LCM turns counterclockwise by a lever effect.
The same if the LCM is in the opposite orientation.
The LCM will naturally remain parallel to its orbit around the moon, and no artificial attitude control is needed for that.
Of course, it is possible to make the LCM perpendicular to the moon with the lateral reactors, but it will remain in this position for as long as an effort is sustained on the lateral reactors; if this effort is stopped, the centrifugal force will make the LCM go back to its horizontal position.
I have found an article for this reluctant member proving that a satellite has a natural orientation which is always the same relatively to the earth:
celestrak.com/columns/v04n09/
There is this schema in this article:
with this comment:
"The despun section rotates, keeping the antennas pointed at the earth and preventing the satellite from going into a flat spin (which is the natural tendency)."
This means that, if no attitude control was exerted on the satellite, then it would always show its flat side to the earth, but it won't do, because it's not on this side that the antenna is mounted, so the attitude must be changed, and that's why an attitude control is necessary in this case.
But the reluctant member denied that this was what the article was meaning, and persisted in his misconception, and went on snubbing me as being ignorant.
Well, if he is true, then, when an obus is fired by a cannon, it should keep the orientation which has initially been given to it, and fall this way:
But anybody who has seen an obus fly knows that the orientation of the obus follows its trajectory, and that it flies this way:
And, if the big satellites always have an attitude control, there also exists small satellites, called "nanosatellites", which don't always have an attitude control.
These satellites are balanced in mass so that they have the right orientation toward the earth, and they always show this orientation to the earth; of course they are not very precise, and have little variations because of perturbations (like solar interferences) which prevents from using them for some applications which require precision, but they still can be used for applications which don't require too much precision.
These satellites absolutely don't behave like this member imagines, that is that they would have a fixed orientation in an absolute system which would make that their orientation toward the earth would constantly change.
So, I must say to this member: Although you persist in your misconception, it's really a misconception you have, and you should get rid of it.
Now, if you continue to think that your conception is correct and that I'm only an ignorant idiot who thinks wrong, then show me a serious scientific article which confirms what you say.
Before leaving the LCM, the LEM is attached this way to the LCM:
In fact, the LEM already has the good attitude, because this is the way it must start its travel to the moon.
Indeed, the LEM initially has the same horizontal speed relatively to the moon as the LCM, that is around 6000 km/h.
This speed creates a centrifugal force which counters the lunar attraction.
The LEM must decrease this horizontal speed to allow the lunar attraction to attract it to the moon, and also because it must have nullified this speed before reaching the moon.
It starts countering the horizontal speed by taking a horizontal attitude and giving a horizontal thrust with its main reactor.
As the LEM loses its horizontal speed, the LEM must progressively change its attitude to make it more vertical, in order to also control the vertical speed, because the lem must also nullify the vertical speed when it reaches the moon, and furthermore it must arrive vertical on the moon.
The vertical lateral reactors allow to control the attitude of the lem; the attitude of the lem allows to distribute the thrust of the main reactor on the horizontal axis (control of horizontal speed) and the vertical axis (control of vertical speed).
At any moment, the AGC reads the current actual decelerations on the accelerometers and compares them with the computed decelerations the LEM should ideally have at this moment of the trajectory;from the difference between the readings and the computed values, the AGC computes corrections to apply to the main thrust and the attitude (lateral vertical reactors).
This process is continuously made, and the control is smooth enough not to need a fast computational period (which is 2 seconds, according to the NASA documentation).
Now you might ask: Why is it necessary to start countering the horizontal speed as soon as the LEM leaves the LCM (green trajectory) , why couldn't the LEM start to descend vertically, and start countering the horizontal speed when it is closer to the moon (red trajectory)?
Well, if the LEM starts descending vertically, and starts countering the horizontal speed when it is closer to the lunar ground, then the lem will have lost its reserve of height, and will not be allowed to lose as much height as he could if it starts earlier to counter the horizontal speed; as a result, the LEM will have to counter more the lunar attraction than with the first trajectory, not to lose too much height.
If the lem was compensating the lunar attraction the same way on the second trajectory as it does on the first one, this is what would happen:
The Lem would bump into the lunar ground before it has finished nullifying the horizontal speed, with the result you can imagine; it would end in a disaster.
So, the lem will have more to compensate the lunar attraction in the second trajectory, and thence give more vertical thrust.
So, the second trajectory is possible, but will consume more fuel than the first trajectory.
I could give an analogy with an helicopter.
An helicopter lands along a parabolic trajectory:
The helicopter could also land this way, that is first brutally descend, and then fly closer to the ground till its landing point:
But it will not usually do it, because that makes it consume more fuel.
So, in conclusion, the trajectory in which the lem starts horizontal is the most economical trajectory, even if other trajectories are possible.
So, if the most natural orientation for both the LCM and the LEM is horizontal, why would the lcm take a vertical orientation when the LEM leaves the LCM?
Now, you can say:
- a trajectory along which the lem first descends vertically before starting countering the horizontal speed is possible.
Yes, it is possible!
- And the LEM could start flying over the LCM, and make rolls, like on these sequences:
Yes, it is possible!
So, what's my point?
My point is that if would make the LEM consume more fuel than just doing the optimal trajectory (in which the lem starts in a horizontal position).
And why is it so important to save fuel in this lunar adventure?
Well, the moon is an unknown universe.
The astronauts don't know in advance what they will find when they get close to the moon.
The relief may be broken, dangerous; they may arrive at a spot which is unfit for landing (a rocky spur or a crater), and they may have to look for the right spot to land on, by maneuvering the LEM with the lateral reactors.
But, during this phase they look for the right spot to land on, it's not with the lateral reactors that they will burn the more fuel, it's with the main reactor instead!
Why?
Because they constantly have to counter the lunar attraction to maintain the LEM above the lunar ground; indeed, they no more have the horizontal speed to create a centrifugal force to counter this lunar attraction.
So, the longer they look for a spot to land on, and the more they will consume fuel.
And they don't know in advance how long this phase will take.
And on the moon, no rescuers, and no gas pumps!
And they also have to save enough fuel for the return to the LCM!
In these conditions, it is absolutely obvious that they have to put all the chances on their side by not wasting fuel before getting close to the moon and saving as much of it as they can!
Making the clowns before really starting for the moon could cost them their lives!
This is especially true when we know that Neil Armstrong was almost out of fuel when he landed on the moon.
This is what we can find in the relation of his flight:
"Words of warning came from Earth: just 60 seconds of fuel left before he would have to abort the landing".
So, he was almost out of fuel, fuel that he could not refill on the moon, and he would however have made the fantasy of flying over the LCM which would have taken the fantasy of being vertical?
DOES IT MAKE SENSE, SERIOUSLY, DOES IT MAKE SENSE???
