Since the historic Apollo missions of the 1960s and 70s that first landed humans on the Moon, NASA has continued to study and develop concepts for advanced lunar landers to return astronauts to the lunar surface. These post-Apollo lander designs have grown progressively larger and more capable to support longer-duration missions with more crew.
This article provides an in-depth overview of how NASA’s lunar lander concepts have evolved from the basic Apollo Lunar Modules to the expansive Altair lander designed for the Constellation program. It highlights the key features and technologies considered over the decades as NASA has refined its designs for an affordable, reliable, and capable vehicle to enable sustainable human exploration of the Moon.
The Apollo Lunar Modules
The Apollo program’s Lunar Modules (LMs) were the first vehicles to land humans on the Moon, enabling one of the greatest achievements in human history. A total of six LMs successfully carried two-person crews to and from the lunar surface on missions from 1969 to 1972.
Lunar Module Design
The LMs that supported these bold missions were purpose-built spacecraft optimized for the specific requirements of landing on the Moon. The design of the LM was driven by the physics of descending to the lunar surface from orbit and ascending back to lunar orbit to rendezvous with the Command/Service Module.
To perform this mission while minimizing mass, the LM utilized a two-stage design:
- A descent stage with landing gear, fuel tanks, a descent engine, and storage space for equipment to be used on the lunar surface
- An ascent stage crew cabin with its own fuel tanks, ascent engine, life support, and flight controls for launching from the lunar surface and returning to lunar orbit
This staging approach allowed the ascent stage to be as small and light as possible since it did not have to carry the descent propulsion system and landing gear back to lunar orbit.
Lunar Module Characteristics
The LM had several distinctive characteristics driven by its unique mission:
- A lightweight, angular shape to maximize internal volume while minimizing mass
- Four landing legs with round footpads and crushable honeycomb material to absorb impact forces
- Triangular windows oriented to provide the best view for the commander to pilot the vehicle to a landing
- Fuel tanks, engines, and equipment bays mounted on the outside to maximize space in the cramped crew cabin
- Drogue and probe docking mechanism to mate with the Command/Service Module in lunar orbit
The LM design was optimized to support two astronauts for a relatively short stay of a few days on the lunar surface. It provided a habitable volume of just 6.7 cubic meters in the ascent stage crew cabin.
Propulsion
The LM used hypergolic propellants that ignited on contact for both the descent and ascent engines. This eliminated the need for an ignition system and allowed the engines to be easily throttled and restarted.
The descent stage had a single gimbaled descent engine that could throttle from 10% to 60% of full thrust. This allowed the LM to hover and translate in the final approach to the landing site.
The ascent stage used a fixed, constant-thrust ascent engine with a thrust-to-weight ratio greater than one to lift off from the Moon. It also had sixteen small reaction control thrusters for attitude control.
Guidance, Navigation, and Control
The LM had a digital computer and inertial guidance system that autonomously controlled the spacecraft through all phases of the mission. The astronauts could manually control the spacecraft using two three-axis hand controllers – one for pitch, roll, and yaw, and one for translation in three axes.
During the landing phase, the commander looked through a wide-angle window and used an optical alignment sight to provide visual cues to help guide the LM to the desired landing point. Vertical and forward velocities were displayed on gauges in the cabin.
Power and Life Support
The LM used silver-zinc batteries to provide electricity and had a water-glycol cooling system to reject heat. Oxygen was stored in cryogenic tanks in the descent stage and water was produced as a byproduct of the fuel cells.
The ascent stage had an environmental control system that provided a pure oxygen atmosphere at 4.8 psi. Lithium hydroxide canisters removed carbon dioxide and odors and activated charcoal removed trace contaminants.
Post-Apollo Lander Studies
After the success of the Apollo missions, NASA continued to study advanced lunar lander concepts to support future human exploration of the Moon. These studies aimed to build upon the Apollo LM design while increasing the lander’s size, capability, and mission duration.
Space Exploration Initiative
In 1989, on the 20th anniversary of the Apollo 11 landing, President George H.W. Bush announced the Space Exploration Initiative, a long-range plan that called for completing Space Station Freedom, returning to the Moon, and eventually sending humans to Mars. NASA conducted extensive studies of lunar lander concepts to support this ambitious initiative.
Lunar Excursion Vehicle
One of the primary lander concepts developed during this period was the Lunar Excursion Vehicle (LEV). The LEV was a two-stage lander that was essentially a scaled-up version of the Apollo LM.
The LEV descent stage had four landing legs and a cluster of four throttleable engines. The larger ascent stage could carry a crew of four for missions of up to 14 days on the lunar surface.
Like the Apollo LM, the LEV would be unloaded from a separate spacecraft in lunar orbit and descend to the surface. After the surface mission, the ascent stage would return to lunar orbit to rendezvous with the orbiting spacecraft.
First Lunar Outpost Lander
Another notable lander concept from this period was the First Lunar Outpost (FLO) lander. The FLO lander was designed to deliver a crew of four and a large cargo to the lunar surface to establish a small, temporary lunar base.
The FLO lander was a massive, single-stage vehicle with eight landing legs. It had a cluster of five engines – four for descent and landing and one for ascent. The engines used liquid oxygen and liquid hydrogen propellants.
The FLO lander included an airlock for crew egress and a pressurized logistics module with all the supplies and equipment needed for a 45-day lunar surface mission. This large, capable lander represented a significant departure from the minimalist Apollo LM design.
Faster, Better, Cheaper
In the mid-1990s, NASA adopted a “faster, better, cheaper” design philosophy that emphasized innovative, low-cost approaches to space exploration. During this period, NASA explored some radically different lunar lander concepts.
Phoenix/LUNOX Lander
One of the most interesting concepts was the Phoenix lander, also known as the LUNOX (lunar liquid oxygen) lander. The Phoenix was a single-stage lander that would use electricity to extract oxygen from the lunar regolith. This in-situ resource utilization (ISRU) would allow the lander to produce its own oxidizer for the return trip to lunar orbit, greatly reducing the amount of propellant that would need to be brought from Earth.
The Phoenix lander had a unique tripod landing gear arrangement with electric motors in each leg to allow the lander to “walk” on the surface to visit multiple sites. The crew cabin was an inflatable structure that would be expanded to provide a larger habitable volume.
The Phoenix was an innovative concept that aimed to reduce costs by utilizing local resources and minimizing the mass that needed to be landed on the Moon. However, the technology for oxygen extraction was still very immature and the concept was not pursued.
Human Lunar Return Lander
Another “faster, better, cheaper” concept was the Human Lunar Return (HLR) lander. The HLR was a minimalist, two-stage lander designed to support short-duration human missions to the Moon.
The HLR descent stage had four landing legs and used storable hypergolic propellants to minimize the lander’s size and complexity. The ascent stage was a simple crew taxi with a pressurized volume just large enough for a crew of two to stand.
The HLR aimed to reduce costs by minimizing the lander’s size and capability. It represented a return to the basic principles of the Apollo LM design, but with some additional risk.
Decadal Planning Team
In the early 2000s, NASA established the Decadal Planning Team (DPT) to create a series of strategic roadmaps for human exploration of the solar system. The DPT developed a number of lunar lander concepts to support human missions launched from the Earth-Moon L1 Lagrange point.
The DPT lander concepts were generally larger and more capable than the Apollo LM to support crews of four on longer-duration lunar surface missions. The landers used liquid oxygen and liquid hydrogen propellants and had clusters of throttleable engines.
Some of the DPT lander concepts included in-situ resource utilization to produce oxygen from lunar regolith. This would reduce the amount of oxidizer that would need to be brought from Earth for the lander’s return to lunar orbit.
The DPT lander concepts represented an evolutionary step in NASA’s lunar lander designs, building upon the Apollo LM design but with increased size and capability to support longer, more ambitious surface missions.
Exploration Systems Architecture Study
In 2005, NASA conducted the Exploration Systems Architecture Study (ESAS) to define a new architecture for human exploration of the Moon and Mars. The ESAS recommended a lunar lander design to support human missions to the Moon by 2020.
The conceptual ESAS lander was a two-stage vehicle designed to carry a crew of four to the lunar surface for seven-day missions. It would be launched on a heavy-lift rocket and would rendezvous with the Orion crew vehicle in low lunar orbit.
Several variants of the ESAS lander were studied, including:
- A lander with liquid oxygen/liquid hydrogen propulsion on both stages
- A lander with liquid oxygen/liquid hydrogen on the descent stage and pressure-fed hypergolic propulsion on the ascent stage
- A cargo version of the lander with an unpressurized payload bay
The ESAS lander concepts represented a significant increase in capability over the Apollo LM, with a larger crew, longer mission duration, and the ability to deliver cargo to the lunar surface.
Lunar Surface Access Module Pre-Project
Following the ESAS, NASA initiated the Lunar Surface Access Module (LSAM) Pre-Project in 2005 to further refine the lunar lander design. During this phase, NASA centers studied a wide range of lander configurations and subsystems.
Over 50 different lander concepts were developed during the LSAM Pre-Project, including:
- Two-stage landers with various propulsion system and structural arrangements
- Landers with airbag and crushable block impact attenuation systems
- Horizontal landers that could be used as habitats on the lunar surface
- Dedicated cargo landers for delivering rovers, power systems, and other infrastructure
- Minimal ascent vehicles for crew transport only
Some of the key trades that were studied included:
- Number of stages (one vs. two)
- Propulsion type (cryogenic vs. hypergolic vs. methane)
- Crew cabin shape (cylindrical vs. conical vs. hemispherical)
- Landing gear arrangement (cantilever vs. tripod vs. articulated)
- Cargo accommodation (internal vs. external)
The LSAM Pre-Project significantly expanded the design space for NASA’s lunar lander concepts and helped the agency understand the key requirements and constraints for a human-rated lander. The study results directly informed the development of the Altair lunar lander.
Lunar Architecture Team
In 2006, NASA established the Lunar Architecture Team (LAT) to develop a baseline architecture for human lunar exploration. The LAT conducted a series of analysis cycles to refine the design of the lunar lander, building upon the results of the ESAS and LSAM Pre-Project.
The LAT lander design converged towards a two-stage configuration with liquid oxygen and liquid methane propulsion. Methane was selected over hydrogen to reduce the lander’s size and improve commonality with other exploration elements.
The LAT lander was designed to support a crew of four for missions of up to seven days on the lunar surface. It included an airlock for surface egress and dedicated cargo storage capacity.
The LAT studies further matured NASA’s lunar lander concepts and helped establish the technical foundation for the Altair lander that would be developed for the Constellation program.
Altair Lunar Lander
In 2007, NASA formally initiated the Altair lunar lander project as part of the Constellation program. Altair was designed to be a highly capable vehicle for human exploration of the Moon, building upon the lessons learned from Apollo and the extensive lander studies conducted in the decades since.
Descent Stage
The Altair descent stage was responsible for delivering the lander to the lunar surface from low lunar orbit. Its key features included:
- Propulsion: Four liquid oxygen/liquid methane descent engines, each capable of throttling from 10% to 100% thrust. The engines could gimbal for thrust vector control.
- Landing gear: Four articulated landing legs with honeycomb crush cores in the footpads to absorb impact loads.
- Cargo capacity: An unpressurized cargo bay with a volume of 10 cubic meters for delivering payloads such as a lunar rover.
- Power: Fuel cells for generating electrical power and cryogenic tanks for storing reactants.
The descent stage also served as a platform for the ascent stage, with umbilical connections to transfer power, data, and consumables between the two stages.
Ascent Stage
The Altair ascent stage was designed to support a crew of four for up to seven days on the lunar surface and return them to lunar orbit to rendezvous with Orion. Its key features included:
- Propulsion: A single liquid oxygen/liquid methane ascent engine and sixteen pressure-fed hypergolic reaction control thrusters for attitude control.
- Crew cabin: A pressurized crew module with 31 cubic meters of habitable volume, an airlock, and docking hatch.
- Life support: A closed-loop life support system to minimize consumables and an active thermal control system.
- Avionics: A distributed, fault-tolerant avionics architecture with extensive automation to reduce crew workload.
The ascent stage crew cabin was designed to operate independently from the descent stage as a self-sufficient spacecraft once it separated for the return to lunar orbit.
Mission Profile
The Altair lunar lander was designed to support a nominal seven-day mission to the lunar surface. The lander would be launched on an Ares V rocket and would autonomously dock with the Orion crew vehicle in low lunar orbit.
The crew would transfer to Altair and descend to the lunar surface, with the commander taking manual control in the final approach to the landing site. Once on the surface, the crew would perform a series of moonwalks and deploy scientific payloads and experiments.
At the end of the surface mission, the crew would enter the ascent stage crew cabin and launch from the descent stage to return to lunar orbit. The ascent stage would rendezvous and dock with Orion, and the crew would transfer back to Orion for the return to Earth.
Summary
NASA’s lunar lander concepts have evolved significantly in the decades since the Apollo missions, reflecting changing mission requirements, technological advancements, and exploration goals. The landers have grown progressively larger and more capable, with the ability to support more crew for longer durations on the lunar surface.
However, the fundamental physics of landing on and ascending from the Moon have not changed. All of NASA’s human-rated lunar lander designs have used separate descent and ascent stages to optimize the mass that must be carried to the lunar surface and back to orbit. Large propellant tanks, a robust impact attenuation system, and a pressurized crew cabin remain the dominant design features.
NASA’s extensive studies have produced a rich set of lunar lander concepts that can inform future vehicle designs. By leveraging the lessons learned from the Apollo LM and the decades of analysis that followed, NASA can develop an affordable, reliable, and capable lander to enable sustainable human exploration of the Moon.
The Altair lander represents the culmination of NASA’s post-Apollo lunar lander development efforts. Although it was not ultimately built, Altair provides a valuable reference design for future landers. Its key technologies and subsystems, such as liquid oxygen/methane propulsion and advanced avionics, can be applied to new lander designs.
As NASA prepares to return humans to the Moon with the Artemis program, it is poised to take the next giant leap in lunar lander capabilities. By building upon the legacy of the Apollo LM and the wealth of knowledge gained in the decades since, NASA can develop a truly remarkable vehicle to carry humanity back to the lunar surface and enable a sustainable presence on the Moon.
