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Lunar lander

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A Lunar lander or Moon lander is a kind of lander (spacecraft) designed to conduct a Moon landing. The design requirements for these landers depend on factors imposed by the payload characteristics and purpose, flight rate, propulsive requirements, and configuration constraints.[1] Other important design factors include overall energy requirements, mission duration, the type of mission operations on the lunar surface, and life support system if crewed. The relatively high gravity and lack of lunar atmosphere negates the use of aerobraking, so a lander must use propulsion to decelerate and achieve a soft landing.

Several studies indicate the potential for both scientific and technological benefits from sustained lunar surface exploration that would culminate in the utilization of lunar resources, or in the development of the necessary technology to land payloads on other planets in the Solar System.[2]

Challenges unique to lunar landing[edit]

As of 2019, landings have been achieved on several solar system bodies. These can be broadly broken into two categories - landings on bodies large enough for gravity to be a significant factor, and landings on asteroids.

Landing on any solar system body comes with challenges unique to that body. The moon has relatively high gravity compared to that of asteroids, and no atmosphere. Practically, this means that the only method of descent and landing that can provide sufficient thrust with current technology is chemical rocket-based.[3] In addition, the moon has a long solar day. Landers will be in direct sunlight for more than two weeks at a time, and then in complete darkness for another two weeks. This causes significant problems for thermal control.[4]

Lack of atmosphere[edit]

To date, space probes have landed on all three bodies other than Earth that have solid surfaces and thick enough atmospheres to make aerobraking possible - Mars, Venus, and Titan (moon). These probes were able to leverage the atmospheres of the bodies on which they landed, and could descend using parachutes and much less fuel. The upshot is that larger payload could be landed on these bodies for a given amount of fuel. For example, the Mars Science Laboratory landed the Curiosity rover, which weighed approximately 900Kg, and had a mass (at the time of Mars atmospheric entry) of 2400Kg,[5] of which only 390Kg was fuel. In comparison, the much lighter (292Kg) Surveyor 3 landed on the moon using nearly 700Kg of fuel.[6] The lack of an atmosphere, however, also removes the need for a moon lander to have a heat shield and aerodynamics are not a factor in its design.

High gravity[edit]

Although it has much less gravity than Earth, the moon has sufficiently high gravity that descent must be slowed considerably. This is in contrast to an asteroid, in which "landing" is more often called "docking" and is a matter of rendezvous and matching velocity more than slowing a rapid descent.

Since rocketry is used for descent and landing, the moon's gravity necessitates the use of more fuel than is needed for asteroid landing. Indeed, one of the central design constraints for the Apollo program's moon landing was mass (as more mass requires more fuel to land) required to land and take off from the moon.[7]

Thermal Environment[edit]

The Lunar thermal environment is influenced by the length of the Lunar day. Temperatures can swing between 25K (during the Lunar night) to 390K (during the Lunar day). These extremes occur for fourteen Earth days each, so thermal control systems must be designed to handle long periods of extreme cold or heat.[8] In contrast, most spacecraft instruments must be kept within a much stricter range of between 233K and 323K.[9] This means that the lander must cool and heat its instruments.

The length of the Lunar night makes it difficult to use solar electric power to heat the instruments, and nuclear heaters are often used.[4]

Landing stages[edit]

Achieving a soft-landing is the overarching goal of any lunar lander, and distinguishes landers from impactors, which were the first type of spacecraft to reach the surface of the moon.

All lunar landers require rocket engines for descent. Orbital speed around the moon can, depending on altitude, exceed 1500m/s. Spacecraft on impact trajectories can have speeds well in excess of that.[10] In the vacuum the only way to slow down from that speed is to use a rocket engine.

The stages of landing can include[11][12]:

  1. Descent orbit insertion - the spacecraft enters an orbit favorable for final descent. This stage was not present in the early landing efforts, which did not begin with Lunar orbit. Such missions began on a Lunar impact trajectory instead.[13]
  2. Descent and braking - the spacecraft fires its engines until it is no longer in orbit. If the engines were to stop firing entirely at this stage the spacecraft would eventually impact the surface. During this stage, the spacecraft uses its rocket engine to reduce overall speed
  3. Final approach - The spacecraft is nearly at the landing site, and final adjustments for the exact location of touchdown can be made
  4. Touchdown - the spacecraft achieves soft landing on the moon

Touchdown[edit]

Lunar landings typically end with the engine shutting down when the lander is several feet above the lunar surface. The idea is that engine exhaust and Lunar regolith can cause problems if they were to be kicked back from the surface to the spacecraft, and thus the engines cut off just before touchdown.[14] Engineers must ensure that the vehicle is protected enough to ensure that the fall without thrust does not cause damage.

The first soft lunar landing, performed by the Soviet Luna 9 probe, was achieved by first slowing the spacecraft to a suitable speed and altitude, then ejecting a payload containing the scientific experiments. The payload was stopped on the Lunar surface using airbags, which provided cushioning as it fell.[15] Luna 13 used a similar method.[16]

Airbag methods are not typical. For example, NASA's Surveyor 1 probe, launched around the same time as Luna 9, did not use an airbag for final touchdown. Instead, after it arrested its velocity at an altitude of 3.4m it simply fell to the Lunar surface. To accommodate the fall the spacecraft was equipped with crushable components that would soften the blow and keep the payload safe.[17] More recently, the Chinese Chang'e 3 lander used a similar technique, falling 4m after its engine shut down.[18] Perhaps the most famous lunar landers, those of the Apollo Program, were robust enough to handle the drop once their contact probes detected that landing was imminent. Apollo 11's lunar lander, for example, contacted the surface with its probe at 1.6m above the Lunar surface, at which point the engine was shut down and the spacecraft fell the remaining distance.[19]

Examples[edit]

Examples of lunar landers or programs to design lunar landers include:

See also[edit]

References[edit]

  1. ^ [Lunar Lander Stage Requirements Based on the Civil Needs Data Base]. (PDF). John A. Mulqueen. NASA Marshall Space Flight Center. 1993.
  2. ^ Lunar Lander Configurations Incorporating Accessibility, Mobility, and Centaur Cryogenic Propulsion Experience. (PDF) Bonnie M. Birckenstaedt, Josh Hopkins, Bernard F. Kutter, Frank Zegler, Todd Mosher. Lockheed Martin Space Systems Company. 20006.
  3. ^ Wertz, James; Larson, Wiley (2003). Space Mission Analysis and Design (3rd ed.). California: Microcosm Press. ISBN 1-881883-10-8.
  4. ^ a b Okishio, Shogo; Nagano, Hosei; Ogawa, Hiroyuki (December 2015). "A proposal and verification of the lunar overnight method by promoting the heat exchange with regolith". Applied Thermal Engineering. 91 (5): 1176–1186. doi:10.1016/j.applthermaleng.2015.08.071.
  5. ^ http://spaceflight101.com/msl/msl-landing-special/. Missing or empty |title= (help)
  6. ^ https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1967-035A. Missing or empty |title= (help)
  7. ^ Cole, E.G. (November 1965). "Design and Development of the Apollo Three‐Man Spacecraft With Two‐Man Lunar Excursion Module (LEM)". Annals of the New York Academy of Sciences. 134 (1): 39–57. doi:10.1111/j.1749-6632.1965.tb56141.x.
  8. ^ Hager, P; Klaus, D; Walter, U (March 2014). "Characterizing transient thermal interactions between lunar regolith and surface spacecraft". Planetary and Space Science. 92: 101–116. doi:10.1016/j.pss.2014.01.011.
  9. ^ Gilmore, D. G. (2003). Spacecraft Thermal Control Handbook (2nd ed.). Segundo, California: Aerospace Press. ISBN 1-884989-11-X.
  10. ^ https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-045A. Missing or empty |title= (help)
  11. ^ https://www.nasa.gov/mission_pages/apollo/missions/apollo11.html. Missing or empty |title= (help)
  12. ^ http://spaceflight101.com/change/change-3/. Missing or empty |title= (help)
  13. ^ https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-045A. Missing or empty |title= (help)
  14. ^ https://www.quora.com/What-are-those-long-metal-rods-below-the-Lunar-Modules-landing-gear-for-and-how-do-they-work. Missing or empty |title= (help)
  15. ^ "Nasa: Luna 9".
  16. ^ https://www.drewexmachina.com/2016/12/24/the-mission-of-luna-13-christmas-1966-on-the-moon/. Missing or empty |title= (help)
  17. ^ https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-045A. Missing or empty |title= (help)
  18. ^ https://www.bbc.com/news/science-environment-25356603. Missing or empty |title= (help)
  19. ^ http://heroicrelics.org/info/lm/lunar-surface-probe.html. Missing or empty |title= (help)
  20. ^ Robotic Lunar Lander, NASA, 2010, accessed 2011-01-10.