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  Mercury - Gemini - Apollo
  Did Gemini have a retrograde orbit like Mercury?

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Author Topic:   Did Gemini have a retrograde orbit like Mercury?
DFBrunswick
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Posts: 40
From: California, USA
Registered: May 2015

posted 05-19-2015 01:10 AM     Click Here to See the Profile for DFBrunswick   Click Here to Email DFBrunswick     Edit/Delete Message   Reply w/Quote
Once in orbit, the Mercury spacecraft flew 'backwards' as I recall so that the earth would be moving forward and away from the astronaut's perspective.

What was the reason for this kind of 'retrograde' flight path?

Also did the Gemini and Apollo spacecraft fly 'forward'?

SpaceAholic
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Posts: 4494
From: Sierra Vista, Arizona
Registered: Nov 1999

posted 05-19-2015 03:10 AM     Click Here to See the Profile for SpaceAholic   Click Here to Email SpaceAholic     Edit/Delete Message   Reply w/Quote
Mercury's flight path was pro-grade. What you are referring to is spacecraft attitude/orientation vice path.

space1
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Posts: 861
From: Danville, Ohio
Registered: Dec 2002

posted 05-19-2015 05:08 AM     Click Here to See the Profile for space1   Click Here to Email space1     Edit/Delete Message   Reply w/Quote
The reason Mercury oriented itself that way was to be ready for re-entry at any time if needed, given the experimental nature of the spacecraft. Later flights (Gemini and Apollo) were oriented according to mission requirements.

Gemini had orientation modes selectable by the astronauts that included "SEF" and "BEF," for Small End Forward and Blunt End Forward.

Jim Behling
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Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010

posted 05-19-2015 05:20 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
The Mercury attitude "blunt end forward (BEF)" was so the vehicle was a near entry attitude and was able to perform entry as easily as possible, For Mercury, each orbit was conditional and a GO was given on each orbit to continue the mission.

For Gemini and Apollo, the attitude was determined by mission requirement. Nose end forward (NEF) was required by Gemini during rendezvous so that the radar was pointed at the target. Once docked with the Agena, it was flown BEF so that the Agena propulsion systems could be used.

On Apollo on the way to the moon, the vehicle would fly in attitudes determined by sun angles and mid course corrections. During LOI, it would have to be with the SM engine towards the direction of flight. Of course, rendezvous and docking with the LM dictated other attitudes.

Headshot
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Posts: 891
From: Vancouver, WA, USA
Registered: Feb 2012

posted 05-19-2015 07:04 AM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
Jim, where did you find the reference to NEF (nose-end forward)? I read one or two Gemini mission documents and recall the orientation being referred to as SEF (small end forward). Just curious.

Lou Chinal
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Posts: 1332
From: Staten Island, NY
Registered: Jun 2007

posted 05-20-2015 08:51 AM     Click Here to See the Profile for Lou Chinal   Click Here to Email Lou Chinal     Edit/Delete Message   Reply w/Quote
In Apollo you will see reference to the "Barbecue Mode." The engineers were trying to distribute the heat evenly by keeping the spacecraft in a slow roll. In Mercury, the mindset was "we have be ready to Retro-Fire forthwith." In Gemini they very often performed a rendezvous sideways (90') to the target.

All times are CT (US)

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