Introduction

The Apollo 11 mission, one of the most remarkable achievements in human history, culminated with the successful landing of the lunar module on the moon and safely returning the astronauts back to Earth. 

This blog post delves into the postflight evaluation of the Command Module Computer (CMC) and associated systems during the mission’s entry phase. 

We will focus on the operation of the CMC, the Entry Monitor System (EMS), and an evaluation of the entry monitoring procedures. 

The analysis will shed light on the complexities and challenges faced during the mission while providing insights into the technological marvels of the Apollo 11 spacecraft.

ENTRY CONDITIONS

Reconstruction of the entry trajectory flown by the Apollo 11 CM:

Inertial velocity	36,194.368 ft/sec
Inertial flight path angle	-6.5230 deg
Inertial azimuth	50.1761 deg
Longitude	171.1960 deg East
Geodetic Latitude	3.1933 deg South
Geodetic Altitude	400,000.72 ft

Command Module Computer (CMC)

The CMC was the brain of the Apollo 11 spacecraft, responsible for guiding and controlling the vehicle during its journey. 

It was an essential component of the Apollo Guidance Computer (AGC), which was responsible for the guidance, navigation, and control of both the Command Module (CM) and the Lunar Module (LM). 

The CMC, a 70-pound device, was a marvel of its time, featuring approximately 36,000 integrated circuits and being capable of executing 85,000 instructions per second.

CMC’s Role in the Entry Phase

The entry phase of the Apollo 11 mission was critical, as it involved the spacecraft reentering Earth’s atmosphere at high speeds while ensuring the safety of the astronauts on board. 

The CMC played a pivotal role in this phase, providing real-time guidance, navigation, and control information to the spacecraft. 

The CMC calculated the spacecraft’s position, velocity, and attitude and provided the necessary commands to the spacecraft’s control systems to execute precise maneuvers and maintain the desired trajectory.

Entry Monitor System (EMS)

The EMS was an independent backup system designed to monitor the spacecraft’s trajectory during the entry phase. 

It provided the astronauts with essential information, such as altitude, velocity, and range to the landing site, and acted as a backup in case of a CMC failure. 

The EMS worked in conjunction with the CMC, allowing the astronauts to cross-check the CMC’s performance and verify the accuracy of the onboard guidance and navigation systems.

Apollo 11 Reconstructed Data

	Actual Apollo 11	Reconstructed Apollo 11
First maximum load factor	6.73 g	6.74 g
Velocity at first maximum load factor	31,810 ft/sec	31,776 ft/sec
First minimum load factor	0.48 g	0.60 g
Velocity at first minimum load factor	20,500 ft/sec	20,257 ft/sec
Second maximum load factor	6.00 g	6.00 g
Velocity at second maximum load factor	12,390 ft/sec	13,070 ft/sec

EMS Operation During the Entry Phase

During the entry phase, the EMS continuously recorded the spacecraft’s trajectory on a scrolling plotter, referred to as the “scroll pattern.” 

This pattern allowed the astronauts to visually monitor the spacecraft’s progress and compare it to the pre-planned trajectory. 

The EMS also included a set of manual controls, enabling the astronauts to take over control of the spacecraft in the event of a CMC failure.

Data Limitations in the Postflight Analysis

Evaluating the performance of the CMC and EMS during the Apollo 11 mission was a complex task, primarily due to the lack of inflight telemetry (TM) data. 

The limited data available for analysis included:

• Downlink data from ARIA aircraft: ARIA (Apollo Range Instrumented Aircraft) were modified military aircraft equipped with tracking and telemetry equipment. These aircraft were strategically positioned to receive signals from the spacecraft during critical mission phases, such as entry and landing.
• Recovery operations data: Data obtained during the recovery of the spacecraft and astronauts after splashdown, which included information on the spacecraft’s systems, instruments, and physical condition.
• EMS scroll pattern: As previously mentioned, the scroll pattern provided a visual representation of the spacecraft’s trajectory during the entry phase.

Given the scarcity of data, the postflight analysis required a meticulous approach, piecing together the available information to form a coherent picture of the CMC and EMS performance during the entry phase.

CMC Performance Assessment

Despite the limitations imposed by the lack of data, the analysis confirmed that the **CMC performed properly throughout the entry phase**. 

The onboard CMC-computed position at touchdown was:

• 169.15 degrees west longitude
• 13.30 degrees north latitude

This position was approximately 1.7 nautical miles from the planned touchdown point. 

The simulated CMC position at touchdown was:

• 169.14 degrees west longitude
• 13.34 degrees north latitude

These results indicate that the CMC was able to accurately guide the spacecraft during the entry phase, ensuring the safe return of the Apollo 11 astronauts.

Factors Affecting CMC Performance

The CMC’s performance during the entry phase was influenced by various factors, such as the spacecraft’s mass properties, aerodynamic characteristics, and external forces acting on the vehicle. 

Additionally, the CMC had to account for uncertainties in the spacecraft’s initial state and the atmospheric conditions during entry. 

The CMC used a set of pre-programmed algorithms and onboard sensors to estimate these variables and continuously update the spacecraft’s guidance and control solutions.

Despite these challenges, the CMC demonstrated remarkable performance, successfully guiding the spacecraft through the hazardous entry phase and ensuring the safety of the crew.

Environment Reconstruction

Reconstruction of the actual environment during the entry phase was achieved using the EMS scroll pattern trace and the onboard CMC solution for onboard reference trajectories at the exit from the Huntest phase, as recorded by the ARIA aircraft.

This reconstruction revealed that the entry flight path angle from the 21-day best estimate trajectory (BET) differed from the entry flight path angle, which best fits all available data for the Apollo 11 entry phase. 

This discrepancy suggests that the actual entry conditions may have been slightly different from the pre-mission predictions.

Implications of Environment Reconstruction

The reconstructed environment provided valuable insights into the performance of the CMC and EMS during the entry phase. 

It also highlighted the importance of having accurate trajectory predictions and the ability to adapt to changing conditions during the mission. 

The findings from the environment reconstruction can be used to improve future spacecraft design, guidance algorithms, and mission planning.

Entry Monitoring Procedures Evaluation

An evaluation of the recommended entry monitoring procedures showed that the procedures were followed by the crew, and all critical tests performed by the crew prior to and during the entry phase were successful. 

These tests accurately indicated the status of the onboard entry systems, ensuring that the astronauts were well-informed of the spacecraft’s condition throughout the mission.

Crew Interaction with CMC and EMS

The Apollo 11 crew played a crucial role in monitoring and controlling the spacecraft during the entry phase. 

They were responsible for verifying the accuracy of the CMC and EMS, conducting system checks, and performing manual maneuvers if necessary. 

The crew’s ability to effectively interact with the spacecraft’s systems was vital to the mission’s success.

Lessons Learned from Apollo 11

The Apollo 11 mission demonstrated the importance of having robust and reliable onboard guidance and navigation systems. 

The CMC and EMS proved to be invaluable tools for the astronauts, ensuring their safety during the entry phase. 

The lessons learned from Apollo 11 have been instrumental in shaping the development of future spacecraft and mission planning.

Conclusion

In conclusion, despite the data limitations, the analysis of Apollo 11’s entry phase confirmed the successful operation of the CMC and the EMS. 

The crew followed the recommended procedures, and the tests performed during the mission provided accurate status updates for the onboard systems. 

The findings from this analysis have contributed to our understanding of the challenges faced during the Apollo 11 mission and have highlighted the importance of having accurate guidance, navigation, and control systems for space exploration. 

These lessons have continued to influence the design and operation of spacecraft, paving the way for future human missions to the moon, Mars, and beyond.

To learn more about the incredible innovation that powered the Apollo 11 mission, check out our article on the F-1 engine: A Triumph of Innovation in Space.