The landing stack has separated from the Lunar Transfer Vehicle. The first Artemis Project lunar landing crew set the LTV to take care of itself as it orbits about 60 nautical miles above the cratered landscape.
From inside the lunar habitat, the crewmembers control their descent to the lunar surface. The pilot's control station is mounted on the window side of the habitat, oriented so that he has his head up when the spacecraft is oriented vertically. One of his crewmates monitors their trajectory, calling out height above ground, velocity, range to target. The other crewmember is controlling cameras: cameras inside the habitat, outside the habitat, and on the lunar surface below.
The vertical orientation during landing is driven by the way the whole stack is packaged for launch and the flight to the moon. To land horizontally, we would have to launch the descent stage as a separate package and assemble it to the habitat in space.
Landing the stack vertically also makes it easier to control the craft during this phase of the flight. To understand this, try balancing a stick on your finger. If you place the stick horizontally with your finger at its center, you'll find it difficult to keep it from falling. However, if you stand the stick up vertically on your finger, you can chase it around the room and, with a bit of practice, keep it from falling over.
It works the same way with spaceships. The easier it is to control the spacecraft, the less maneuvering fuel we use, and less chance we have of losing control.
During flight, we use small maneuvering rockets control the attitude of the spacecraft. These little rockets turn on occasionally, whenever the vehicle starts to lean over. These thrusters work in reponse to control laws written by engineers who are experts in designing dynamic systems. The control laws tell each thruster how long to burn in response to an error in the vehicle's attitude.
There are other ways to design the control system. Instead of using maneuvering thrusters, we can mount the main engines in the descent stage on gimbals, and move them back and forth in respose to errors in the spacecraft's attitude. If we have precise control over the amount of thrust the engines put out, we also have the option of using small differences in the amount of force from each engine.
Each of those alternative design approaches has its merits. However, since the descent stage is used only once during the flight, we've chosen what appears to be the simplest, least expensive system as a starting point for design. If we rigidly mount the engines to the descent stage frame and get them aligned before we launch the spacecraft from Earth, we eliminate the cost and weight of the gimbal system. Further study is needed to determine which is really the best approach to controlling the spacecraft.
After the descent stage touches down on the surface, it drops a foot to provide a stable platform for leveling the habitat. After the foot is deployed, the crew does their first EVA on the lunar surface.
In this scenario, the three crewmembers use the habitat's supporting trusswork as a ladder. They gather at the level of the descent stage's thrust platform. Here we get a bit creative. To avoid the question of who was first set foot on the moon after forty years, they hold hands (well, gloves) and jump the last few feet together.
A robotic rover stands nearby, welcoming them to the moon as it records their arrival. This robot was one of several landed earlier, used to record for the first time a manned landing on the surface of the moon.
After a brief ceremony with flags, speeches, and leaping about, the crew gets to work. One crewmember moves to the foot of the support truss while the others take up positions on either side of the spacecraft. From these positions they carefully level the support truss. One at a time, the crew use hand wrenches to make small adjustments to jackscrews which control the length of each of the supporting legs. While one cremember is cranking a jackscrew, the others are sighting on bubbles of mercury which were meticulously aligned before the spacecraft was launched from Earth. (And which undoubtedly will get knocked askew before they get to the moon. We hope the error won't be enough to worry about.) When the platform is level, they lock the jackscrws in place.
They want to work quickly at this point. Ever since some time during the descent to the moon they've been inside the "dead man's curve," left without an emergency escape back to lunar orbit and the Lunar Transfer Vehicle that waits to take them home again. As soon as they get the habitat module rotated out of the way and the plumbing reconfigured for the ascent vehicle, they'll be able to leave the lunar surface on a moment's notice.
The crew monitors the descent of the habit as it rotates from the vertical to horizontal positions. There are a lot of design considerations in this activity, discussed below under "Design Issues." When the module is completely lowered and the ascent vehicle buttoned up, they're ready to go back into the habitat and set up for several days of housekeeping on the moon.
The horizontal position gives us more working space and a less claustrophic interior arrangement. Its additional stability is obvious; though there aren't winds or significant amounts of seismic activity (moonquakes) to worry about, we can generate quite a list of events that might tend to tip the module over.
It also has a show business advantage of providing greater camera relief; that is, a longer distance from the camera lens to the subject. This gives us more flexibility in setting the scene for interior shots during the first lunar visit. The lower profile of the horizontal orientation also will make it easier to use lunar regolith to shield the module for long-term operations. Finally, rotating the module moves it out of the way of the ascent stage; and from a show-biz standpoint, it looks neat.
On the other hand, leaving the module in the vertical orientation eliminates the cost of developing the mechanism for lowering the support truss and the module; and it eliminates the weight of that extra foot, mechanisms, and supporting trusswork. That extra weight could be invested in additional equipment and supplies delivered to the moon, increasing our assurance of mission success on later flights.
The current baseline reference mission calls for landing vertically and rotating the module to a horizontal position. Unless further analysis comes up with a very compelling reason to change this scenario, this is how we'll do it.
It's easy to imagine a computer-controlled electric motor lowering the support truss and the habitat to the horizontal position. After all, that's what spacecraft do. However, motors and batteries are heavy, and the electronics and software to control this one-time operation are very expensive.
Because of the weight and expense of using an electric motor, it's more likely that we'll build in a hand-operated winch or jack to lower the module. Or, since we're working in only one-sixth gravity, we might just let it crash and eliminate the extra trusswork altogether.
Further analysis is required to determine the best way to do this. For now, the reference mission calls for the crew to use a hand-operated winch to lower the module.
The images on this page are from an 890K Quicktime movie which shows the major events during this phase of the mission.