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How Will We Go to the Moon ?

 

November 14, 2024

Plans to send man to the Moon are soon coming upon us, although this type of mission is not considered new since it was already accomplished 55 years ago during the U.S. Apollo program.

What is considered new or different this time compared to the “old” Apollo days ?  Will it be anything like the old Apollo missions ?

There are some obvious improvements in energy storage, and the old fuel cells of yesteryear are no longer needed which were a primary blame for the near-catastrophe of Apollo 13.  Along with a myriad of equipment refinements - regarded as improvements – are practice procedures involving launch and mission control, and astronauts.

Terms such as “escape velocity” or “trans-lunar insertion” .. are some of the jargon used back in the Apollo days, but some of these old-school terms may now be outdated ..

Terms such as “escape velocity” or “trans-lunar insertion” or “trans-lunar injection” are some of the jargon used back in the Apollo days, but some of these old-school terms may now be outdated in view of later developments of Spaceflight.

The term “trans-lunar injection” implies there is something to the actions of leaving Earth-orbit and then heading toward the Moon.  But “trans-lunar injection” may be a misnomer because the word “injection” implies there is some critical balance of actions (like threading a needle) to leave Earth-orbit and begin a path to the Moon.  In fact, this is not true.  There’s not really a set of critical actions to perform for this part of the mission, which is actually simpler than the old school term implies.

While the spacecraft is in orbit about the Earth, it is balanced by centripetal and centrifugal forces.  When the time comes, aft thrust is fired and the spacecraft reacts by moving forward (action-reaction). And that’s really all there is to it.  It is a simple matter of an aft-thrust moving the spacecraft forward at a specific location in the Earth-orbit.  The amount of thrust can be any amount to overcome the remaining pull of Earth’s gravity, and only depends on how fast you want to travel to the Moon.  It is not a critical balance of any sort, and only requires a minimum of thrust to overcome the remaining Earth’s gravity, which is not so difficult if it is a high enough orbit.  The amount of thrust really depends on how fast you want to travel, and there is no upper ceiling.  Just remember that when you reach the Moon, you will need to slow down to begin orbital insertion.

What is really affecting the spacecraft in respect to gravity is its distance from the Earth, ..

What is really affecting the spacecraft in respect to gravity is its distance from the Earth, which then questions what is meant by “escape velocity”.  The only way to “escape” the Earth’s pull of gravity is by the action of another force, which is Thrust (F = ma).  Thrust counteracts the force of gravity and moves the spacecraft farther from Earth, which distance “escapes” the Earth’s gravitational pull.  This is accomplished by acceleration and distance – not by velocity.  Therefore, “escape velocity” is possibly another misnomer in spaceflight jargon because it doesn’t have a true practical basis nor even a true physical basis.  In fact, one never truly escapes the Earth’s gravitational pull, it only becomes negligible as you travel farther from it.  All the way to landing on the Moon, you will be under the influence of the Earth’s gravity.  The Earth’s gravity is what keeps the Moon in orbit, and if you have landed on the Moon, you are now “one-with-the-Moon” under the influence of Earth’s gravity.  In a strict sense, no gravity field is completely overcome (never zero) – it only becomes negligible the farther from the massive object.

Now that we have “annihilated” NASA’s “trans-lunar injection” and Konstantin Tsiolkovsky’s “escape velocity”, it would be a good idea to summarize the actual concepts taking place.  The following walks through these concepts in a straight-forward manner:

 

1)  When the spacecraft is in Earth-orbit, it is balanced by the centripetal and centrifugal forces, FCP and FCF.  The spacecraft (and everything inside) is weightless. 1

2)  When aft-thrust is applied, it begins to move the rocket forward in a straight path.  This “breaks” the circular orbit and the centripetal and centrifugal forces become zero.  The only force remaining is what is left of Earth’s pull of gravity, FG.  After leaving the orbital path, the spacecraft is no longer weightless.

3)  The remaining force of Earth’s gravity, FG, can be shown as a vector, broken into two orthogonal components, FGy which is directly counter to the spacecraft’s Thrust, and the other component FGx which is acting lateral on the spacecraft.

                   

While orbiting, the spacecraft applies thrust and now moves in a straight line.  The curved orbital path is “broken” and the centripetal and centrifugal forces automatically goto zero. The remaining force acting on the spacecraft is what is left of Earth’s gravity force.  This remaining gravity force FG can be broken into its components, FGx and FGy (inset on right).

 

4)  The spacecraft’s thrust needs to overcome the direct component of Earth’s gravity FGy, but this is not so difficult if the spacecraft is in a high orbit (distant from the Earth).  Therefore, the amount and duration of the thrust burn mostly dictates how fast you want to travel to the Moon, and is not critical in determining a trajectory.

5)  The small lateral component of Earth’s gravity FGx will tend to draw the spacecraft in a lateral direction, and this small amount of lateral force can either be compensated or utilized by the spacecraft by choosing which location in the Earth-orbit to start the burn.  From the diagram below and left, if you want to approach the Moon from the right side (orbit the Moon counterclockwise) you will begin the burn at the “X” so that the small lateral force places you on the preferred path, or lunar trajectory.  This method compensates for the small lateral gravity force.  If you want to utilize the small lateral gravity force instead, and approach the Moon on the left side, then the burn can begin at the “X” (diagram below and right).

 

Two trajectories to reach the Moon.  Since the Moon itself is in orbit, the position of the Moon is shown when the spacecraft arrives.  On the left, the lunar trajectory begins at “X” to compensate for the small lateral gravity force FGx  which pulls the spacecraft toward the trajectory shown by the straight dotted line.  On the right, the lunar trajectory begins at “X” and the lateral gravity force pulls it toward the trajectory shown by the straight dotted line.  Since the thrust burn takes place later in the flight for the diagram on the right, Mission Planners will often prefer this trajectory to save a little extra fuel (utilizing the lateral gravity force instead of opposing it).

 

6)  Whichever lunar trajectory you decide to take, right or left side of the Moon, you will be on your way, but still under the influence of the Earth’s gravity.  Because of this remaining amount of Earth’s gravitational pull, it will slightly slow down the spacecraft.  An optional second burn can be performed to makeup for this loss of speed, or the slightly slower speed is decided as satisfactory.  Also, adjustments to the lunar trajectory are made with attitude control thrusters (orientation).  Due to the available attitude thrusters, the “insertion” or “injection” point of the lunar trajectory is considered not so critical.

7)  When approaching the Moon, the gravity of the Moon will now affect the spacecraft and counterbalance the Earth’s gravity.  The spacecraft will be in equigravitation, and any unrestrained objects within the spacecraft will be free floating.

8)  As the spacecraft continues toward the Moon, the Moon’s gravity becomes dominant and reverse thrust is applied to slow the spacecraft to begin orbital insertion.  The term “orbital insertion” is considered correct because it requires a balance of parameters: mass, velocity, and altitude to balance the Moon’s gravity, and “insert” the spacecraft into a lunar orbit according to mv2/r (the equation for centrifugal force).

 

“Orbital insertion” is considered correct, but “trans-lunar injection” 2 has less understanding these days, and “escape velocity” - which has no universal meaning - may be impractical for spaceflight since velocity is not an action taken (velocity is a data measurement). 

Complexities in old jargon should be avoided if it is only causing extra complications. Simplification and clarification is considered advancing Spaceflight.  There will be plenty of other complexities ahead such that any unnecessary complexities should be disregarded.

As man continues his plight toward occupying the Moon, he can concentrate more on what he should be doing when he gets there.  This includes accessing the Moon by way of a lunar space station and building structures  on the Moon’s surface.  A lunar space station, with multiple docking ports, can act as a hub to accommodate incoming spacecraft of supplies or astronauts, and allow access to the Moon by using docked lunar landers.  Man can start occupying the Moon by building structures for shelter 3 and for processing fuel, water, and food.  Robots can help in constructing facilities on the Moon and it is possible to send robots ahead of humans to build the structures before they are humanly-occupied.  Robots don’t need to wear spacesuits and are considered to have more dexterity and mobility than a human wearing a spacesuit.

 

Simplification and clarification is considered advancing Spaceflight.

 

1.  Weightlessness in Space is achieved more often during orbit compared to other aspects of spaceflight. During orbit, the spacecraft (and everything in it) is balanced by centripetal and centrifugal forces, which results in everything becoming weightless (unrestrained objects float around). The term “orbit” itself implies a free-falling of the spacecraft with its falling-path matching the curvature of the Earth.  During other types of spaceflight, for example lunar trajectory, centripetal and centrifugal forces do not exist, and unrestrained objects within the spacecraft will “fall” to the rear of the spacecraft (pull of Earth’s gravity).  When approaching closer to the Moon, unrestrained objects will tend to “fall” to the front of the spacecraft.  After lunar orbit insertion, the spacecraft will become weightless again.

2.  The term “lunar trajectory” can be substituted for “trans-lunar injection”.  An example in practice is giving the command “begin lunar trajectory” instead of “begin TLI burn”.

3.  Since the Moon lacks a significant magnetic field, and is mostly outside Earth's protective magnetosphere, any duration of time spent on a lunar space station or on the Moon's surface will need protection against Cosmic Rays.  Artificial magnetic fields will need to be implemented for inhabited structures on or near the Moon.  The Apollo missions were of short duration and didn't require this protection.