Landing System

Apollo Lunar Module
NASA Apollo Lunar Module at Kennedy Space Center Visitor Complex


NASA Apollo 11 and Surveyor 3 (with Apollo 12 in background) on the Moon


"Lunniy Korabl", LK


NASA Viking lander on Mars

https://terakuhn.neocities.org/5c3ad554a3106c65fff4fa23.jpeg
Photo taken by the Yutu-2 rover on Jan 11, 2019 shows the Chang'e-4 lander. The Yutu-2 rover and the Chang'e-4 lander took photos of each other. [Photo/Xinhua/China National Space Administration]

A landing system is a system that allows a spacecraft to reach the surface of a celestial body such as the Moon, Mars, an asteroid (such as Ceres) or the Earth. For the US Apollo program to the Moon the Lunar landing system was the Apollo Lunar Module. A total of six Apollo Lunar Modules landed on the Moon, and two others went to the Moon. For the USSR N1/L3 program to the Moon the Lunar landing system was to be the lunar lander or ("Lunniy Korabl", LK). The LK had a weight of about 5.5 tons and could carry only one cosmonaut to the surface of the Moon, unlike the Apollo Lunar Module which carried two astronauts. The LK was tested in Earth orbit, running through the various burns it would need to complete a Moon mission; however, it never went to the Moon.

The Apollo Lunar Module included both a descent stage for landing and an ascent stage for returning the astronauts to the Apollo Command Module in Lunar orbit. The descent stage and the ascent stage each had their own liquid fueled rocket engine. While the LK had landing gear that would remain on the Moon, it was designed to use the same engine for both descent to the Moon and ascent from the Moon.

In addition to Apollo, autonomous landers have also landed on the Moon and on Mars. These include US Surveyor, USSR Luna, US Viking, US Mars Pathfinder, China's Chang'e and many others. Earlier landers mostly relied on rocket engines to slow their descent to the surface. While the Apollo and many other landers used liquid propellant rocket engines, the Surveyor lander used both solid propellant and liquid propellant rocket engines, and the proposed OMOTENASHI CubeSat mission would use a solid propellant rocket engine. Landers on Mars have also incorporated parachutes since, unlike the Moon, Mars has a thin atmosphere. Spacecraft landing on Mars have incorporated air bags to cushion the landing. Before using air bags to land on Mars, a similar technique was used to land on the Moon by the Luna 9 spacecraft. The OMOTENASHI CubeSat mission is also proposing to use an air bag to land a small lander on the Moon. However, the latest approach to landing the heavier Curiosity Rover on Mars does not use air bags. Airbags aren't effective for landing more massive spacecraft on more massive planets.

Spacecraft that have successfully soft landed on other planets
Landing Spacecraft mission type country of development payload to surface (kg) techniques guidance control system technology
Luna 9, 13 Lunar Lander USSR 99 retro rockets, air bags
Surveyor 1, 3, 5, 6, 7 Lunar Lander USA 292 to 305.7 retro rockets
Apollo 11, 12, 14 Lunar Sample return USA 15,200- retro rockets Apollo Guidance Computer (AGC architecture)
Luna 16, 20, 23, 24 Lunar Sample return USSR 1,880 retro rockets
Luna 17, 21 / Lunokhod 1, 2 Lunar Rover USSR 1,514 retro rockets
Apollo 15, 16, 17 / with Lunar Rover Lunar Rover and Sample return USA 16,400- retro rockets Apollo Guidance Computer (AGC architecture)
Viking 1, 2 Mars Lander USA 572 retro rockets, parachute Guidance Control and Sequencing Computer (GCSC) consisting of two Honeywell HDC 402
Mars Pathfinder / Sojourner Mars Rover USA 264 retro rockets, parachute, air bags IBM RAD6000 (POWER architecture) on Pathfinder lander and an Intel 80C85 (8080 architecture) on Sojourner rover
MER-A, B / Spirit, Opportunity Mars Rover USA 533 retro rockets, parachute, air bags IBM RAD6000 (POWER architecture)
Phoenix Mars Lander USA 350 retro rockets, parachute IBM RAD6000 (POWER architecture)
MSL Curiosity Mars Rover USA 899 retro rockets, parachute RAD750 (POWER architecture)
Chang'e 3, 4 / Yutu-1, 2 Lunar Rover China 1,200 retro rockets
InSight Mars Lander USA 358 retro rockets, parachute two RAD750 (POWER architecture)

In addition to spacecraft that have landed on the Moon and Mars, spacecraft also land on Earth. Early space capsules returning to Earth also relied heavily on parachutes to decrease their velocity. While the Space Shuttle relied on wings to generate aerodynamic forces to slow its velocity, the most recent approach to landing large spacecraft on Earth rely on rocket engines. Both Blue Origin and SpaceX are developing rockets that can go to space and then land back on Earth. This is interesting since similar technology could be applied to landing on Mars and the Moon, while wings and parachutes don't work well or at all on Mars and the Moon.

In addition to the forms of navigation used by spacecraft, landers typically also use RADAR to detect the distance to the planets surface or a Doppler RADAR to detect distance and velocity relative to the surface. By knowing the distance to the surface and the velocity relative to the surface of the planet, a guidance control system can determine when to deploy mechanisms to slow the landers decent.

The Apollo Lunar Module was powered by batteries. It had an Apollo Guidance Computer (AGC) for guidance and control which the two astronauts could run programs on to conduct various maneuvers. Most landers have used electric batteries. The Apollo Lunar Module used silver oxide zinc batteries. The Viking Lander used nickel cadmium batteries along with a radioisotope thermoelectric generator.

Further Reading


Digital Apollo: Human and Machine in Spaceflight (MIT Press)
On Mars: Exploration of the Red Planet, 1958-1978--The NASA History (Dover Books on Astronomy)
Sojourner: An Insider's View of the Mars Pathfinder Mission by Andrew Mishkin (2003-12-02)
Roving Mars: Spirit, Opportunity, and the Exploration of the Red Planet

Experiments

For students who want to do experiments related to landing systems, they probably can't send something to the Moon to test out landings there.
However, they could do science experiments that demonstrates what happens when objects hit a surface by using plaster of paris in pie pans to record the result of the impact. For the experiment, the student could change the size of the object hitting the surface, the height that the object is being dropped from, or maybe the angle that the surface is being impacted at.
They could also do science experiments that try to prevent an egg, or other fragile item, from breaking when dropped from a high location, by using cushions or parachutes or other mechanisms. Here are some ideas for science experiments:

Science Questions (concerning landing system)
Question What is Tested? What Stays the Same? Data Collected
How does changing the height from which a ball is dropped impact the size of the crater it makes? Drop height (and thus impact speed) Size and mass (and weight) of ball Crater size made in plaster of paris in a pie pan
How does changing the size and mass of a ball dropped impact the size of the crater it makes? Size and mass (and weight) of ball Drop height (and thus impact speed) Crater size made in plaster of paris in a pie pan
How does the angle of impact affect patterns observed in falling water balloons? Angle (e.g. straight 90 degree vs. angled 45 degree) Size of water and procedures for making water balloon, force, height of drop Pattern observed at impact site
What is the best shock absorber for dropping an egg? Shock absorbers (e.g. cloth, foam, balloons, etc.) Size and mass (and weight) of egg, height of drop (and thus impact speed) Wholeness of the egg (was the egg intact, cracked, or broken open?)
What is the best shape for a parachute? Shape of the parachute Size and mass (and weight) of payload, drop height, wind, area of parachute Time for parachute to complete descent
What is the best material to use for a parachute? Different materials Size and mass (and weight) of payload, drop height, wind, area and shape of parachute Time for parachute to complete descent

Working Example

For students who want to experiment with a working example, in addition to the Moon or Mars, you can test landing systems right here on Earth. Before building anything it's a good idea to know exactly what you're trying to accomplish. For example, if you want to build a lunar lander, are you building it to land a camera or scientific instrument, lunar rover, a sample return craft, a terrarium or greenhouse, humans? That will tell you something about how much mass you want to land on the Moon. You don't need an Apollo Lunar Module if you're only trying to land a small rover. Also, are you landing from lunar orbit or coming from Earth. This will tell you something about how fast you will be going as you approach the Moon.

The job of a lander is to change what would be an high energy impact into a low energy landing. During an impact, an object's kinetic energy is equal to one half of the object's mass times the square of its relative velocity: KE = m / 2 * v^2. To decrease the impact force of a lander, you need to decrease its kinetic energy before it reaches the surface and its kinetic energy is released all at once.

If your goal is to develop something that simulates landing on the Moon, you shouldn't rely on parachutes or wings. Since the Moon has no atmosphere to use to decrease the landers velocity through heat shields or parachutes, you will likely have to rely on rocket engines to decrease the velocity. These could be liquid fueled rocket engines or solid propellant rocket engines like what was used on NASA Surveyor landers. The amount of force the rocket engine will have to generate will depend on the mass and velocity the lander starts at. You probably shouldn't be experimenting with LOX and certainly not with UDMH nor dinitrogen tetroxide. However, you could use a solid propellant model rocket engine as a retro rocket.

Further you will have to aim those rocket engines in the direction that will decrease the velocity relative to the Moon. Radar is quite often used to determine at what altitude a lander is at and when and how much thrust should be generated by the rocket engines. Doppler radar is sometimes used to determine relative velocity with the ground and how much additional thrust is needed to decrease the velocity. Also keep in mind that the Moons surface is also moving as the Moon rotates. Sometimes cameras are used to take multiple images during final decent to determine if the lander is drifting relative to the surface features. Additionally gyroscopes and acelerometers might be used to track the landers change in position.

All of these navigation sensors need to feed back to the control system for smaller rocket thrusters that can adjust the direction and speed of the lander. This control system could be built from discrete electronics, but typically today they’re built from small computer control systems.

Also, most landers don’t land perfectly on the surface with zero kinetic energy. They quite often zero out their kinetic energy slightly above the surface and then drop. Compressible shock absorbers in their lander gear or air bag mechanisms like those used by Luna 9 and 13 (as well as Pathfinder, Spirit, and Opportunity on Mars) are used to absorb any remaining energy. You could create an air bag from balloons or plastic bags.

Finally, put all those rockets, sensors, landing gear, and control mechanisms into a rugged structure where everything can activate and deploy appropriately and somehow get a ride to the neighborhood of the Moon, or drop test on Earth. Have a look a OMOTENASHI for simulated videos of how the small OMOTENASHI lander is proposed to work.

Finally, here are some more links to an interesting lunar landing project:

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