APOLLO LUNAR LANDING MISSION SYMPOSIUM
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APOLLO LUNAR LANDING MISSION SYMPOSIUM
JUNE 25-27, 1966
MANNED SPACECRAFT CENTER
APOLLO EARTH RETURN ABORT CAPABILITIES By Ronald L. Berry
5.0 ABORTS DURING TRANSLUNAR COAST PHASE
Since the nominal translunar coast trajectory is a free return to earth, the minimum delta-V to abort is essentially zero. Thus, as will be shown later, large delta-V capability margins exist which can be used in an abort situation to speed up the return to earth and/or control the point of landing.
Figure 19 presents a summary of the abort modes for the translunar coast phase. Note that redundant abort capability exists throughout the entire phase due to the presence of three independent propulsion systems.
The first abort mode listed is a direct abort available with either the SPS or LM DPS propulsion systems. The basic features of this abort mode are shown sketched in figure 20. This mode consists of a single unconstrained attitude abort burn which returns the spacecraft directly to reentry without circumnavigating the moon. Shown sketched, are the two extreme types of returns possible in this mode--time critical and fuel critical. A fuel critical return usually passes through apogee following the abort maneuver; whereas, a time critical or a fast return usually does not pass through apogee following the abort maneuver. All of the seven sub-modes described previously for a translunar injection phase [see '*' below] are available for this mode as well as for all of the other modes for this phase. If the SPS is available, this mode of abort produces the fastest possible returns to the earth for aborts performed prior to reaching approximately the lunar sphere of influence. This is the reason why this particular mode is considered prime for approximately the first three-fourths of the coast period from the earth to the moon.
[* - These seven submodes were described in the prior section on Aborts During TLI Phase:
- a. Returns to primary recovery sites
- b. Time critical returns to a contingency recovery area
- c. Fuel critical returns to contingency recovery areas
- d. Time critical returns to a water landing
- e. Fuel critical returns to a water landing
- f. Time critical returns to unspecified landing area
- g. Fuel critical returns to an unspecified landing area
A paragraph was provided to explain each sub-mode in more detail.]
The DPS direct abort mode, as shown in the summary chart, is available as a backup mode to the SPS direct mode for approximately the first two-thirds of the coast period from the earth to the moon. Use of this mode during the latter portion of the translunar coast phase would result in excessively long return times. Thus, the use of this mode as a backup during this latter period is not considered as a possibility. As will be shown in more detail later, the LM DPS direct abort mode, when used in the event of an SPS failure, will result in the fastest possible return time only if the abort maneuver is performed during approximately the first 25 hours of translunar coast.
The next abort mode listed in the summary chart consists of delaying the abort maneuver until the vicinity of pericynthion or perilune is reached, as shown sketched in figure 21. This mode of abort is available continuously throughout the entire translunar coast phase with either the SPS, DPS, or marginally with the Service Module RCS system. If the SPS is available, this mode of abort will generally result in the fastest possible return to the earth for abort decisions made near the sphere of influence or thereafter. For this reason, it is shown in the summary chart as the prime mode of abort for this portion of the translunar phase. In the event of an SPS failure, the LM DPS propulsion system used in this mode will result in the fastest possible return time for abort decisions made approximately 25 hours or later out along the translunar coast trajectory. In the event of an SPS failure and failure to obtain the LM DPS propulsion systems, the Service Module RCS would be marginally available for this mode of abort due to the fact that the nominal translunar coast is a free-return trajectory.
The third abort mode listed in the summary chart is a circumlunar abort available with the SPS, DPS, or marginally with the Service Module RCS continuously throughout this mission phase. Figure 22 shows the basic features of this abort mode to be a single unconstrained burn performed at some point along the translunar coast trajectory which returns the spacecraft to the earth after circumnavicating the moon. The altitude of pericynthion or perilune is allowed a certain amount of freedom in order to obtain landing area control upon return to earth. This mode of abort is not as yet thoroughly understood. Preliminary analysis indicates that this type of abort many times produces the absolute minimum delta-V required to return to a particular landing area, especially if the translunar coast profile is a non-free-return type. However, for just as many cases the minimum delta-V required to return to a particular recovery area has been found to be minimized by use of delay to pericynthion abort mode. A complete understanding of this effect is currently under investigation.
Figure 23 shows a special case of the circumlunar abort mode. This particular case consists of merely using mid-course corrections to correct back to the nominal free-return trajectory, assuming this type of return is the nominal profile. Use of the circumlunar abort mode in this fashion is essentially equivalent to the fuel critical unspecified area sub-mode described previously.
The last mode listed on the summary chart is a two-burn mode for the special case of when the translunar coast trajectory is on an impact course with the moon. As shown, this mode of abort is available continuously throughout the entire phase with either the SPS, DPS, or marginally with the Service Module RCS. Figure 24 shows the basic features of this abort mode. The first maneuver, usually a small one, is used to raise perilune or pericynthion altitude to an acceptable value. The second burn is then performed in the vicinity of perilune to return the spacecraft to earth in one of the seven sub-modes described previously. The most brobable need for this abort mode would be in the event of a badly executed second midcourse correction near the sphere of influence due to a G&N or an SPS failure.
Questions and Answers
EARTH RETURN ABORT CAPABILITIES
Speaker: Ronald L. Berry
1. Dr. Rees - Why not use the APS [Ascent Propulsion Stage, LM] for a backup propulsion system?
- ANSWER - Since the ascent engine is not gimballed, the possible c.g. offset effects cannot be controlled.
2. What is the RCS engine burn time limitation?
- ANSWER - Specifications limit is 1000 seconds.
4. Dr. Haeussermann - Have we looked into using a DPS-SPS combination?
- ANSWER - No. The main reason for using the DPS as backup is because of an SPS failure.
5. Mr. Richter - Can't we use ADS [sic, APS?] some of the time - isn't it load dependent?
- ANSWER - The control authority is marginal in most cases for the APD since the APS engine is not gimballed.
6. Mr. Richter - Can we use two SPS burns for fast earth return transfer?
- ANSWER - This is being looked into. This may not be a desirable way to achieve a gain in return time, since it could
- be dangerous. If the SPS did not fire the second time, you might exceed acceptable entry velocity or conditions.
7. Mr Green - When the DPS is used for backup propulsion, do we jettison the SPS propellants?
- ANSWER - There is no capability to jettison SPS propellants.