The Landing Point Designator (LPD)
The grid pattern in the Lunar Module’s left window is the Landing Point Designator (LPD). It was a graduated vertical and horizontal scale marked in degrees, etched onto both panes of the window in front of the Commander.
Purpose of the LPD
The LPD was used to help the Commander determine the spacecraft’s attitude relative to the lunar surface, especially during the landing. By observing how the Earth and Moon moved through the grid, astronauts could get a rough idea of how much the spacecraft was deviating from its intended orientation.
The LPD was particularly important because precise attitude control was crucial for successful lunar landings and rendezvous maneuvers. It served as a valuable backup to the LM’s computer system, particularly in case of computer malfunctions or unexpected situations.
How Astronauts Used the LPD
- Before Landing: Prior to the powered descent, the Commander used the LPD and a stopwatch to measure the speed at which landmarks on the surface moved along the scale. This helped them estimate the LM’s altitude.
- During Landing: During the landing, the computer provided the Commander with a number, which they used to locate the point on the LPD where the computer thought the LM would land. The Commander could then manually adjust the landing point by using the hand controller to move the target up range, downrange, left, or right.
Challenges and Limitations:
- The LPD was difficult to use, especially because the LM was constantly pitching during the descent, and propellant sloshing caused the spacecraft to move.
- The actual computer target was often different from where the Commander thought the LPD was pointing.
- On Apollo 11, a propellant slosh uncovered a fuel gauge, causing the crew to lose flight time and making the LPD unreliable.
- On Apollo 12, Pete Conrad used the LPD to change the landing site, but he actually moved away from his target because propellant slosh affected his view through the grid.
Enhancements in Later Missions
Later, Apollo missions benefited from a redesigned baffle system in the propellant tanks, which reduced sloshing and improved the reliability of the LPD. Furthermore, later versions of the LM guidance software made it easier to switch between manual and computer control during the descent, which eased the workload on the Commander.
Functionality of the LPD
The Landing Point Designator (LPD) served as a manual visual aid for astronauts to adjust the Lunar Module’s (LM) landing point during the final descent phase. It functioned as follows:
- Grid Reference: The LPD consisted of a grid with lines intersecting at specific angles, etched onto the LM’s window. This grid provided a reference frame for the astronaut to observe the positions of the Earth and Moon as the LM descended.
- Deviation Detection: As the LM descended, the astronaut monitored the Earth and Moon’s positions through the grid. Any drift from their intended positions within the grid indicated a deviation from the planned landing trajectory.
- Manual Adjustments: If a deviation was observed, the astronaut could make small adjustments to the LM’s trajectory using a hand controller. These adjustments aimed to realign the Earth and Moon with their intended positions on the LPD grid.
Precision and Backup System
The LPD offered a relatively simple yet effective method for fine-tuning the LM’s landing point. It also acted as a valuable backup system in case of computer malfunctions or unforeseen situations, supplementing the LM’s primary computer system, which handled most of the landing calculations.
Astronaut Skill and Significance
The LPD required the astronaut to possess significant skill and training to interpret the grid accurately and make timely adjustments. By utilizing the LPD, astronauts could ensure a safe and precise landing at the designated site on the Moon’s surface.
Challenges and Limitations
However, there were challenges associated with the LPD:
- Propellant Slosh: Propellant sloshing within the LM tanks during descent could cause spacecraft motions, rendering the LPD unreliable. This issue was particularly noticeable around the halfway point of the descent when the propellant level was around 50%, leading to significant spacecraft wobbling and rendering the LPD virtually unusable.
- Data Lag: The LPD readings provided to the Commander from the computer could be as much as 2 seconds old due to the constant pitching of the LM after initiating the Approach Phase (P64). This lag, combined with the propellant slosh, added to the difficulty of using the LPD effectively.
- Discrepancy with Computer Target: The actual computer target for landing was often different from the point where the Commander perceived the LPD to be indicating. This discrepancy, exceeding 500 feet in some cases, further highlighted the limitations of the LPD in achieving pinpoint landings.
Addressing Propellant Slosh in Later Missions
To mitigate the propellant slosh issue, later Apollo missions (starting with Apollo 14) implemented a redesigned baffle system in the propellant tanks. This improvement reduced sloshing to a manageable level, enhancing the reliability of the LPD and contributing to smoother descents.
Advancements in Guidance Software
In addition to the hardware improvements, later Apollo missions also benefited from enhanced guidance software, which simplified the process of switching between manual and computer control during descent. These advancements eased the workload on the Commander and contributed to more controlled landings.
Apollo 11 Landing and LPD Usage
Despite the challenges, Neil Armstrong successfully used the LPD during the Apollo 11 landing, demonstrating its value as a backup system. However, he acknowledged that he did not rely solely on the LPD, particularly in the final stages of descent. Instead, he primarily focused on clearing computer alarms and maintaining control of the spacecraft.
Limitations of Simulations
The Apollo 11 crew also highlighted the limitations of simulations in replicating the uncertainty of intermittent communication and the distractions caused by unexpected computer alarms. These real-world challenges underscored the importance of astronaut adaptability and problem-solving skills during missions.
Use of the LPD during the Apollo 11 Landing
The sources offer a detailed account of the Apollo 11 landing, including how Neil Armstrong and Buzz Aldrin used the Landing Point Designator (LPD).
Initial Use for Altitude Estimation:
Before the powered descent, Armstrong used the LPD, along with a stopwatch, to track landmarks and estimate the LM’s altitude.
Limited Use During P63 and Early P64:
During the initial phases of the powered descent (P63) and early into the approach phase (P64), the crew’s attention was focused on managing unexpected program alarms (1202 and 1201), ensuring the spacecraft systems were functioning correctly, and confirming their position relative to landmarks. Due to these issues, they didn’t use the LPD to re-target the landing site.
First LPD Reading:
It wasn’t until they were at an altitude of approximately 2,000 feet that Armstrong requested an LPD reading from Aldrin.
Aldrin, reading from the Primary Guidance and Navigation System (PGNS) display, reported an LPD angle of 47 degrees.
However, Armstrong, possibly distracted by the challenging terrain ahead, requested the reading again. This exchange highlights the high-stress environment and divided attention during the landing.
Manual Control and LPD Observation:
As they descended further, Armstrong took manual control of the LM at approximately 1,500 feet, transitioning into P66.
He continued to monitor the LPD, which indicated they were approaching a large, rocky crater (later identified as West Crater).
Decision to Overfly West Crater:
Based on his observations through the LPD and the window, Armstrong determined that landing short of the crater wouldn’t be safe due to the boulder field.
He decided to fly the LM manually further downrange, effectively abandoning the computer’s intended landing target.
Focus on Manual Control in Final Stages:
From this point onward, Armstrong concentrated on manually flying the LM to a safe landing spot, prioritizing control over precise targeting.
While Aldrin continued to provide LPD readings, Armstrong primarily relied on his visual assessment of the terrain and the LM’s shadow to guide the final descent.
Primary and Backup Navigation Systems in the Apollo Lunar Module
The sources describe the Primary Guidance and Navigation System (PGNS) and the Abort Guidance System (AGS) used in the Apollo Lunar Module (LM).
Primary Guidance and Navigation System (PGNS)
The PGNS was the main navigation system in the LM, and it was responsible for guiding the spacecraft during its descent to the lunar surface. It used data from an inertial platform to estimate the spacecraft’s velocity. The PGNS provided more information than the AGS and was the primary system used during the landing.
Astronauts interacted with the PGNS through the Display and Keypad (DSKY), which allowed them to input commands and view data. The PGNS could track the signal received via the high-gain antenna and adjust the antenna’s pointing to maintain optimal signal strength. The PGNS also calculated and displayed the Landing Point Designator (LPD) angle on the DSKY, aiding the Commander in visually identifying the intended landing site.
Abort Guidance System (AGS)
The AGS served as the backup navigation system in the LM and was primarily used for emergency return-to-orbit scenarios. It used data from less accurate, body-mounted accelerometers to estimate spacecraft velocities.3 Before the descent, the PGNS and AGS were cross-checked to ensure accuracy and build confidence in both systems.
During the Apollo 11 landing, the AGS played a crucial role when the PGNS experienced program alarms (1202 and 1201) due to data overflow. The AGS data reassured the crew and Mission Control that the landing could proceed safely.
Conclusion
Although the LPD was intended as a tool for precise landing site selection, the challenges encountered during the Apollo 11 descent, including program alarms and difficult terrain, led Armstrong to prioritize manual control and visual judgment. He used the LPD for initial assessment and to confirm his position but ultimately relied on his flying skills and situational awareness to land safely.
