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Author
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Topic: Apollo 8's original high-altitude mission
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Explorer1
Member Posts: 221
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 05-27-2020 04:02 AM
  
Originally, the Apollo 8 mission was planned to be a high altitude flight of the command and lunar modules. The apogee of their orbit would have been 4000 miles. Would that altitude have been sufficient for a high speed double-dip reentry as if from cislunar space? |
oly
Member Posts: 1040
From: Perth, Western Australia
Registered: Apr 2015
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posted 05-27-2020 05:17 AM
From Apollo by the Numbers (NASA SP-4029): The mission was originally designated SA-503, an unpiloted Earth orbital mission to be launched in May 1968 with boilerplate payload BP-30 instead of an operational spacecraft. The success of Apollo 6 (AS-502), however, led to the decision on 27 April that AS-503 would be a piloted mission with a CSM and LM instead of BP-30.The change to a piloted flight required that the S-II stage be returned to the Mississippi Test Facility for "man-rating." Additional tests for a piloted flight continued at KSC. The Mississippi tests were successfully completed on 30 May 1968 and the stage returned to the Kennedy Space Center on 27 June. After two months of testing, which started 11 June 1968, it was determined that the LM would not be ready for the projected early December launch. Therefore, the decision was made on 19 August that a 19,900-pound LM test article would be installed in the spacecraft/launch vehicle adapter for mass loading purposes, replacing the LM. It was also on this date that the crew was instructed to train for a mission to the Moon, officially designated "Apollo 8." The spacecraft would never reach the velocity equal to a lunar return mission from an apogee of 4000 miles because it would lack 234,855 miles of acceleration force of gravity. The Orion EFT-1 reached an apogee of 3604 miles and achieved of high-energy reentry at around 20,000 miles per hour (32,000 km/h; 8,900 m/s). |
Jim Behling
Member Posts: 1516
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 05-27-2020 07:49 AM
quote:
Originally posted by Explorer1:
Would that altitude have been sufficient for a high speed double-dip reentry as if from cislunar space?
It doesn't really matter about the altitude, they could fire the SPS to increase the velocity.Also, the double dip is for a reentry that is higher than orbital velocity and not just lunar return. |
Explorer1
Member Posts: 221
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 05-28-2020 03:28 AM
Jim, I thought the return velocity was about the same as escape velocity (25,000 mph) and this was the speed at which the double-dip was needed. So when you say lunar return velocity, when is there ever a situation in which lunar return velocity remains slow enough to allow for a simple ballistic reentry? The pull of Earth I believe speeds up the returning spacecraft. So unless there is some breaking done with thrusters, the lunar return speed is always going to be sufficiently high to require a double-dip. Is there anything I am overlooking here? And with regard to the altitude not being necessary for the returning spacecraft to reach lunar return velocity I agree that technically, altitude was not the factor for speed because the SPS engine would have to be used to bring the spacecraft up to that speed. But the altitude was needed for another reason — to give the cushion of time needed for separation of the Command Module and to bring it into proper alignment for reentry. LEO would have been very problematic in that regard time-wise. Correct me if I am wrong about this. |
Jim Behling
Member Posts: 1516
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 05-28-2020 08:59 AM
What I am saying is that a double dip entry can be used for other high speed entries (slower than lunar return) and not just lunar return. Any entry from a high orbit that has energy more than a low earth orbit can use it.But a double dip is not required, it can be a ballistic entry with no lifting. See AS-201 and 202. They were suborbital and performed multiple SPS burns. | |
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