and discussed the landing operation in a “sequence of events” narrative.
This week we will review a few key control systems on the Lunar Module and the landing operations. Then we will discuss the maneuvers performed to land safely and softly on the lunar surface. This will help us understand how the astronauts controlled the LM. The LM has two main propulsion engines located on the Descent and Ascent stages. The LM also has four reaction control thruster assemblies located on the ascent stage to reorient (or point) the main engines in the proper direction. These thrusters are called attitude thrusters because they are used to point the craft in the proper direction or place it in the right ‘attitude'. Thruster firings to achieve the proper attitude are often called reorientation or attitude maneuvers. The proper attitude is very important to ensure that the large main engines apply their thrust vector (to change the orbital velocity) in the proper direction. This type of a burn is called a “delta velocity” (or delta-v) maneuver. In the Apollo missions the descent engine was pointed into the direction of travel (or the velocity vector) to perform a “retro” burn. The 30 second ‘Descent Orbit Insertion’ (DOI) burn reduces the craft’s velocity and lowers the LM's perilune (point closest to the Moon) to within about 50,000 feet (15 km) of the surface, about 260 nautical miles (480 km) up-range of the landing site.
At this point, the main engine was started again for ‘Powered Descent Initiation’ (PDI). During this time the crew flew on their backs, depending on the computer to slow the craft's forward and vertical velocity to near zero. Control was exercised with a combination of main engine throttling and attitude thrusters, guided by the computer with the aid of landing radar (LR). During the ‘braking phase’, the altitude decreased to approximately 10,000 feet (3.0 km), and the ‘final approach phase’ took the craft to approximately 700 feet (210 m). During final approach, the vehicle pitched over to a near-vertical position, allowing the crew to look forward and down to see the lunar surface for the first time.
The ‘landing phase’ began approximately 2,000 feet (0.61 km) up-range of the targeted landing site. At this point manual control was enabled for the Commander, and enough fuel reserve was allocated to allow approximately two minutes of hover time to survey where the computer was taking the craft and make any necessary corrections. (If necessary, landing could have been aborted at almost any time by jettisoning the descent stage and firing the ascent engine to climb back into orbit for an emergency return to the CSM.) Finally, three-foot-long probes extending from three footpads of the LM touched the surface, activating the ‘contact light’ that signals time for descent engine cutoff and allowing the LM to ‘touchdown’ softly onto the surface of the Moon.
Maneuvering the Lunar Lander
The maneuvers performed by the Lunar Module (LM) were DOI-2 (Descent Orbit Insertion #2), PDI (Powered Descent Initiation) and Landing maneuvers. DOI was a maneuver to insert the spacecraft in the correct orbit from which to initiate descent. DOI-1 was performed by the CSM with the LM still docked. The LM, using its RCS thrusters, performed DOI-2. PDI was the maneuver that brakes the LM out of lunar orbit and lands it softly on the surface of the Moon. This was the only maneuver to use the main engine of the Descent Propulsion System (DPS).
The DPS engine was ignited at 10% throttle and held there for 26 seconds to allow the DPS engine gimbal to be aligned through the spacecraft center of gravity before throttling up to maximum thrust. The braking phase was designed for efficient reduction of orbit velocity and, therefore, used maximum thrust for most of the phase; however, the DPS was throttled during the final two minutes of this phase. The DPS was able to be throttled only between 10% and 60%.
The approach phase provided visual monitoring of the approach to the lunar surface. At 'high gate' (from old aircraft-pilot parlance meaning the beginning of the approach to an airport in a landing path) the LM pitched forward to give the command pilot a view of the moon. 'Low gate' was the start of the landing phase, and was the point for making a visual assessment of the landing site to select either automatic or manual control. The entire sequence took about 12 minutes with only about two minutes for the astronauts to manually make any corrections to land the LM safely on the Moon.
Remember last week’s blog post when Neil Armstrong almost ran out of fuel landing the LM on the Moon!
The historic landing of Apollo 11 is a great human accomplishment that provided many pioneering experiences and ‘lessons learned’. After the great success of Apollo 11, NASA's next step was honing the Lunar Module's (LM) ability to make a pinpoint landing. Many of the future landing sites corresponded to areas with rough topography; the LM would have to come in steeply and set down within a few hundred meters of a designated point. The next Apollo mission used the lessons learned from the Apollo 11 lunar landing to demonstrate ‘pin-point’ landing capability, as well as the tremendous value of teamwork. Let’s listen to the great team effort of Commander Pete Conrad and Lunar Module Pilot Alan Bean as they land the Apollo 12 LM on the Moon with “loads of gas ... plenty of gas, plenty of gas” and “got it made” pinpoint accuracy. Listen for the call outs of key events and operational parameters such as the ‘pitch over’ maneuver and the constant monitoring of the altitude (feet), fuel remaining (%), LPD angle (degrees), and velocity (fps):
Apollo 12 - Approach and Landing - November 19, 1969
Why did the Apollo 12 lunar landing need to be so precise? (Hint: Rocks and soil were not the only things that the astronauts took back to the Earth to be analyzed – the answer will be published in the next blog post)
Practice is Everything!
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'An ounce of practice is worth more than tons of preaching.' - Mahatma Gandhi
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Simulators are often used to validate operational procedures and ‘practice’ maneuver execution skills, especially in a team environment. Control of the thrust vector is key to both the landing and the simulation. A series of throttle settings and pitch angles must be derived to guide the LM to a successful landing. The following is an example showing how the LM pitched forward and the engine was throttled back during the approach (visibility) and landing phases of a mission. This is just a generic diagram and is not specific to any particular mission. The pitch angles, thrust settings, velocities, altitudes and distances shown may be different for each mission.
The Commander (CDR) looks through a set of scribed marks on his window to assist him while the LM pilot gives him the Landing Point Designator (LPD) angle from the guidance computer. The LPD angle will tell him where to look along the vertical scale to find the place where the computer thinks they are going to land. If the CDR doesn't like the spot, he can move his hand-controller to tell the computer that he wants to change the landing spot up or back or to either side. A single movement of the hand-controller, which moves the landing point by a half degree or so, is usually referred to by the astronauts as a 'click'.
Here is a simulation of the “OUTSTANDING” teamwork of the Apollo 12 Lunar Landing crew:
Did you notice the set of LPD angle marks scribed on Commander Pete Conrad’s window to help him control the flight and the engine throttle while the LM Pilot kept the CDR informed of the systems' status and navigational information? Why is it important to have an initial visual landmark (like a crater)? Did you hear Alan Bean’s continuous communication of the key landing parameters that the Commander needed to control the landing – altitude (feet), fuel remaining (percentage), LPD angle (degrees), and velocity (fps)? Did you notice how the key parameters changed during the descent phases? Why is it important to watch for the dust? Did you hear the call for ‘contact light’, ‘engine off’ and other key operational events? What is a safe velocity (fps) to softly land the LM? We heard the three most important tools of teamwork ... Communication, Communication, and Communication!!!
Now for the FUN!
Curiosity is a sign of a healthy mind – as long as it is limited by good sense. But have you ever seen something and then try to do it yourself because you thought 'it can’t be that hard, in fact it looks so easy!' Well, as Murphy’s Law proves time and time again … nothing is as easy as it seems. There is always something that can come up as the 'surprise' element or the unexpected detail (like the 1202 alarms on Apollo 11). However, once you start practicing and doing something over and over again, you may encounter just about all of those unforeseen events that could eventually go wrong. Through practice we learn to understand the range of unanticipated (or anomalous) situations that can occur, thereby, acquiring valuable 'hands on' experience. This is the power of accurate simulators as a learning tool.
Next it’s your turn. Here is a simulation that you can use to practice your lunar landing skills. This classic video game accurately simulates the real motion of the lunar lander with the correct mass, thrust, fuel consumption rate, and lunar gravity. Its ‘dashboard’ display provides the critical operational parameters you need to monitor while executing landing phase maneuvers. The dashboard also provides the craft’s altitude (meters), range (meters), horizontal (v_x) and vertical (v_y) components of the velocity vector (mps), fuel gauge (kg) and thrust (N). During the simulation, the main engine thrust is graphically represented. At the top of the dashboard is an arrow that indicates the craft’s inertial pointing alignment through the spacecraft's center of gravity and thrust vector. You can enable/disable both the ‘Sound’ and ‘Vectors’ options in the simulation. The ‘Vectors’ option illustrate both the magnitude (size) and direction of the velocity and acceleration vectors. The simulation also provides the necessary keyboard manual control keys to fire ('click') the main descent engine and attitude thrusters (to ‘tilt’ the craft). Make sure to read the instructions including those in the ‘Help/Pause’ button. In addition, there is a ‘Reset’ button so you can practice, practice, practice...
Can you avoid the boulder field and land safely, just before your fuel runs out, as Neil Armstrong did in 1969? Can you ‘reorient’ (or tilt) the LM by using the attitude thrusters? What happens when you apply the thrust in different directions? Make sure to monitor both the x and y components of the velocity vector. Note the difference between the velocity and acceleration vectors as you fire the engine (apply thrust). At an altitude of 50 meters, can you ‘scoot across’ the surface to find a smooth surface to safely land like Neil Armstrong? Can you make a pinpoint landing like Apollo 12? How close can you come to the designated landing site (note the starting range of 45 meters)? How many points can you accumulate before you run out of fuel? How many soft landings can you perform? How about hard landings? How many ‘man-made’ craters did you create? The real lunar lander is very hard to control too. Good Luck and have Fun!
You can send us your best scores to magic@kelvin.net and we will publish it on the blog post or just leave a comment below to let us know what you learned and how well you did.
Hope you have lots of FUN!
Mystical Moon
'All one can really leave one's children is what's inside their heads. Education, in other words, and not earthly possessions, is the ultimate legacy, the only thing that cannot be taken away.' - Dr. Wernher von Braun
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