Apollo Atmospheric Entry Phase

e [Music] [Music] before presenting the actual entry of the Apollo Command Module into the atmosphere of Earth after its lunar Expedition we will review quickly the overall mission to this point the Apollo space vehicle and its crew were placed in an earth parking orbit then injected on a translunar trajectory a burn of the service module propulsion system provided braking and a plane change that placed the spacecraft into the proper orbit for the lunar Landing The Landing to be performed in the lunar module after the landing and exploration of the lunar surface the lunar module was launched from the surface of the Moon and rendevu with the orbiting Command Module with the rendevu completed the space vehicle is now preparing for transar injection and the trip back [Music] the basic problem is to bring the spacecraft and crew from the Moon to a specific Landing site on Earth to see how this is accomplished we must first look at the Earth Moon geometry the Moon is revolving around the earth the Earth is rotating on its axis each of these motions has a specific effect on the spacecraft's ability to reach the landing site to examine these effects and how they act on the return and entry trajectory we must first Define the antipode a line is drawn from the center of the Moon through the center of the earth penetrating the Earth's surface which faces away from the Moon the point of penetration is called the antipode which means simply the point opposite the return trajectory will pass over the antipode the antipode being detered at the time the spacecraft leaves the moon's vicinity the distance from the point of entry into the atmosphere to the antipode is relatively fixed the entry range limits of the spacecraft are also fixed so that Landing occurs near the antipode therefore the landing area is defined and the target Point can be selected within this area the recovery forces are of course located on the surface of the Earth and rotating with it it while the trajectory is fixed by selecting the proper trajectory we can vary the transit time of the spacecraft up to 24 hours in order to bring the spacecraft to the longitude of the recovery forces it is only necessary to control the return time of the spacecraft to the Earth while the Earth rotates into the proper position this gives us complete Freedom of Choice as to selection of the landing longitude The Landing longitude then can be selected with operational constraints in mind such as a required landing on water location of land masses convenience of recovery Force placement and so forth the other geometric constraint is the latitude at which the spacecraft will land the primary factor influencing the latitude is the declination of the Moon to understand this we must again look at the Earth Moon geometry as the moon revolves around the earth the plane described by the lunar orbit is called the moon orbit plane the moon orbit plane is inclined to the Earth's equatorial plane as the moon moves around the earth it changes its position relative to the earth's equator the angle between the equatorial plane and the Earth Moon line is called the angle of declination this angle varies from a maximum when the moon is here to to 0° when the moon is here the return trajectory begins at the moon and must pass over the antipode this means that the plane of the return trajectory must contain the Earth Moon Line the injection burn to start the lunar spacecraft on its return trip will also fix the plane of the return trajectory This Plane can be rotated around the earth Moon Line the limits of this rotation are determined operationally this range of planes along with the entry range limits determines the range of latitudes that can be achieved because of the relatively short entry range and thus the small distance from the antipode to the Target point it can be seen that the latitude range is quite small The Landing latitude selected within this range is essentially fixed at time of transar and injection while latitude range is determined by the possible range of return trajectory planes and the entry range the location of the latitude range is determined by the declination of the Moon as the moon changes its declination throughout the month the latitude range shifts accordingly across the surface of the Earth in review we have seen that accurate control of the landing longitude can be achieved by varying the transit time of the returning spacecraft until the Earth and Recovery forces have rotated to the desired position the range of Landing latitudes is determined by the range of return trajectory planes and the entry range the position of this band of Landing latitudes on the other hand is relatively fixed by the declination of the moon at the time of transar injection thus control of Landing latitudes is extremely Limited to understand how the entry trajectory is shaped it is necessary to understand the conditions existing at entry the spacecraft will enter the upper limit of the atmosphere at an altitude of 400,000 ft this altitude is by definition where the perceptible atmosphere of Earth is first encountered it will enter at a speed of approximately 24,500 mph this is a speed considerably greater than that required to put the spacecraft into circular orbit around the Earth this means that if we take away the atmosphere the spacecraft would approach the Earth and arrive at a point called the vacuum Pare altitude this would be the point of closest approach of the transar trajectory if Earth had no atmosphere since the velocity of the spacecraft is greater than the velocity required to achieve circular orbit the momentum of the spacecraft would cause it to move away from the earth after passing the point of closest approach but Earth does have an atmosphere the vacuum parag altitude is located well below the limit of the perceptible atmosphere if the spacecraft could be controlled to precisely the correct flight path angle capture could occur as a result of atmospheric drag on the spacecraft however if the spacecraft relied only on drag the corridor would be too narrow to guarantee a safe return to see why this is let's take a closer look to achieve a safe capture and entry using only atmospheric drag the spacecraft trajectory cannot exceed very narrow limits that is it must arrive within a given distance of the nominal vacuum Pary altitude this can be related directly to the angle at which the spacecraft enters the atmosphere this range of flight paths is called the entry Corridor if the spacecraft enters at an angle shallower than that of the upper or overshoot limit of the corridor it will skip off the atmosphere like a flat stone off a pond of water and go into an orbit that will not allow it to re-enter until its oxygen and power are exhausted if the spacecraft should enter at an angle steeper than the lower or undershoot limit the high G forces involved would incapacitate the crew and could exceed the spacecraft's Heat and structural capacities and cause it to burn up like a meteor therefore to achieve safe entry the spacecraft must follow a trajectory within the entry Corridor however with the limitations of hardware and trajectory design over trans lunar distances and velocities it is impossible to control the trajectory to guarantee an approach closer than this to the nominal vacuum Pary altitude since this is directly related to the entry Corridor it can be seen that if atmospheric drag alone is used as a controlling Factor it is impossible to guarantee a safe entry therefore aerodynamic lift is used to assist the drag force and widen the corridor to acceptable limits the aerodynamic characteristics of the entry vehicle or Command Module play a decisive role in the Apollo atmospheric entry phase especially since even after the corridor is achieved future errors could result in disaster we will look at this later now let's see how this lift is generated the Apollo Command Module is basically conical in shape it will enter the atmosphere blunt end forward as it does so it will strike molecules of air according to Newton's first law a particle at rest or in motion tends to stay at rest or in uniform motion in a straight line until acted upon by an outside force the spacecraft provides this Force deflecting the particles of air according to Newton's third law to every action there is an equal and opposite reaction then just as the air is acted upon by a force from the spacecraft the air imparts a force on the spacecraft of equal magnitude but in the opposite direction this line represents the symmetrical Center Line Of The Command Module if the center of gravity were located on this axis of symmetry then all Air striking the blunt end would be deflected equally in all directions this means that the aerodynamic forces of the deflections perpendicular to the axis of symmetry would be balanced then the only effect of the interaction of the spacecraft and the particles of air would be along the axis of symmetry and opposite to the direction of the spacecraft motion therefore the only aerody Dynamic force acting on the spacecraft would be in a direction opposite the direction of spacecraft motion this force is called drag if however the center of gravity is shifted off the center line a new set of conditions occurs the spacecraft now realigns itself according to the shift in the center of gravity this new new alignment is a stable trim attitude that is if the spacecraft should deviate from this attitude aerodynamic forces will tend to restore it to this position now the air strikes the spacecraft more on one side of the center of gravity than the other again the spacecraft behaves according to Newton's Laws more air is deflected downward thus a lifting force is created on the spacecraft the center of gravity gra it is fixed relative to the spacecraft by Rolling the spacecraft about its center of gravity the direction of lift can be changed and aerodynamic control achieved we can now see how this affects the limits of the entry Corridor if the entry angle is steep a lift up attitude can be held to bring the spacecraft onto a safe entry path if the angle is shallow lift down can achieve a safe entry therefore the entry range or entry Corridor is expanded the spacecraft must be controlled to arrive within slightly less than plus orus 15 mil of the desired vacuum Pare altitude this after falling along the return trajectory of over a quar of a million miles we have seen what defines the entry Corridor with its overshoot and undershoot limits how aerodynamic control of the command Command Module is achieved by Rolling it around its offset center of gravity thus changing the direction of lift and how this increases the depth of the entry Corridor we are now ready to see what happens to the Command Module during the entry phase since the spacecraft left the moon's vicinity it has been falling to Earth for 3 and 1 A2 days over a distance of about a/4 million miles before actually entering the atmosphere the service module which contains the primary propulsion system and all but a very limited oxygen and power supply will be jettisoned this is roughly analogous to a scuba diver taking off his air tanks before starting up from the Bottom of the Sea and holding his breath all the way to the surface once the Command Module enters the atmosphere it has about 1,000 M and a bit over 2 and 1 12 minutes of control ability divided into two brief control periods most of this controlability will be used up during the first control period the trajectory The Command Module will follow is basically as follows it will enter on a given flight path and proceed downward to the pull-up point just before pullup it enters the atmosphere dense enough to allow aerodynamic control of its Landing point and begins its first control period The Command Module will then climb until it skips out of this denser atmosphere the skip is primarily to increase the maximum range of the spacecraft and to reduce the aerodynamic heating load on the heat shield as the skip is approached aerodynamic control is reduced to practically zero during the skip the crew will be experiencing weightlessness the skip will not take the Command Module beyond the 400,000 ft level it will then descend starting at second entry and go on to the second control period the entry phase is terminated at the 23,500 Ft level at which point the drogue parachutes are deployed and the spacecraft is committed to its Landing point we will now examine the entry trajectory in detail and see what occurs during each portion as the Command Module enters the Fringe of the atmosphere at 400,000 ft the there will be as yet no aerodynamic control before this point is reached the Command Module will be aligned to its stable trim attitude it should be noted that there are two stable trim attitudes the desired one is blunt end forward the other is with the small end forward since the heat shield is on the blunt end this entry alignment would be disastrous proper alignment of the spacecraft is determined by the onboard computer from data received from the tracking Network prior to entry after proper alignment there will be control only in the roll axis pitch and yaw being stabilized by aerodynamic forces at the 05g Mark the point at which deceleration can be sensed for the first time limited aerodynamic stabilization of the spacecraft will be evident the spacecraft will be in a lift up attitude which in the nominal case will be maintained until the first control period when the 2 to 2 and 1/2 G region is reached capture is so highly probable as to be considered achieved the spacecraft continues in a lift up attitude until it enters the dense portion of the atmosphere just before pullup maximum G loads will be placed on the spacecraft just before this the first control period begins during the first 7 seconds of this control period any error or failure to correct an error could result in what is called in great understatement a non-recoverable situation that is too much lift up could put the spacecraft and crew into an orbit that would last beyond the lifetime of the Command Module systems too much lift down could take the spacecraft and crew beyond the stress and heating rate limits and result in destruction of the Command Module as the spacecraft approaches the skip the aerodynamic forces become less and approach zero so aerodynamic control approaches zero it is during this first control period that the real determination of where the spacecraft will land is made to understand this let's take a look at a flat map of the Earth this point represents the Target that is the point at which the spacecraft should Land This Is The Landing footprint or area in which the spacecraft might land at the beginning of the first control period the usable portion of this footprint is designed to compensate for ACR errors and to provide alternate Landing sites in case of bad weather of this total footprint only 1,000 nautical miles or 1,150 statute miles are usable during the first 70 seconds of the first control period the spacecraft must be maneuvered so that this 20,700 Mi footprint shrinks around the target to this footprint about 270 Mi long 90% of the maneuverability is used up during this period this 70 seconds then is the critical period of the entry phase during the remainder of the first control period until the skip the landing footprint will be reduced to this area about 690 M long only Minor Adjustments of the trajectory can be made as the skip is approached if all goes well during the first control period the spacecraft and crew should be on a trajectory that can be safely controlled during the second entry we are now ready to look at the Skip and the final part of the entry phase during the skip the crew will ride along and monitor the trajectory but will have no control over it however the Command Module will be aligned to its trim position in preparation for the second entry as in the first entry aerodynamic stabilization begins at the 05g Mark the spacecraft must be properly aligned by this point the second control period beginning at 2 G's will last a minute and a half while a dangerous skip out cannot occur an error could take the spacecraft and crew beyond the stress limits it is during this second control period that the fine tuning of the entry trajectory takes place that is the spacecraft will be maneuvered to shrink the landing footprint as determined during the first control period so that the spacecraft will arrive at the Target the Apollo entry phase will terminate at about 23,500 ft altitude at this altitude the drog shoots will come out and the spacecraft descend to splash down by Parachute reviewing now we see that before entering the atmosphere the service module containing all but the entry oxygen and power supplies will be jettisoned and the Command Module positioned in its trim alignment it enters the atmosphere at the 400,000 ft level at 05g aerodynamic stabilization begins at the 2 to 2 and 1/2 G level capture is considered achieved The Command Module continues on the trajectory until pullup for the control skip maneuver this is where maximum G loads will be placed on the spacecraft and crew just before pullup the first control period will begin during the first critical 70 seconds of this control period the initial Landing foot footprint 20,700 mi in length will be shrunk around the target to this 270 M footprint in the remainder of this first control period the footprint will shrink further to this 690 Mi long area the first control period is followed by the controlled skip during the skip the Command Module will be aligned for the second entry the second control period begins at0 2 G's and last lasts about 1 and 1/2 minutes during this period the trajectory will be tuned to bring the Command Module down on the target point the Apollo atmospheric entry phase terminates at an altitude of 23,500 Ft with the opening of the drogue parachutes we have seen how the landing area on Earth is determined by the time of travel from the Moon the declination of the Moon relative to the Earth's equatorial plane and the plane of the transar trajectory we have seen how the entry Corridor is defined how aerodynamic control of the Command Module is achieved by Rolling it about its offset center of gravity and we have followed the spacecraft and crew through the entry trajectory just how important the atmospheric entry phase of the lunar mission is can be highlighted by the fact that while it is one of the briefest phases in time it requires one of the largest onboard computer programs once the spacecraft is in the atmosphere things happen fast and there is little time for corrections as compared with earlier portions of the mission for instance an error made during the critical 70 seconds of the first control period could cause a high-speed skip resulting in placing the Command Module and crew into a trajectory that would exhaust the oxygen and power supply fly before landing yet during this period Communications blackouts exist between the ground and the spacecraft caused by the ionization of the air from the heat of Entry therefore entry will be entirely controlled through onboard computations just before the entry phase the ground will transmit the necessary data to the spacecraft that data being where you are where you're going where you want to [Music] go we have presented the major factors affecting the atmospheric entry phase of the man lunar Mission it requires careful planning to assure the safe return to Earth of spacecraft and crew and this is the final part of an overall plan a plan designed to carry us safely to the Moon and back [Music] all