Author Topic: Gemini 11 Retro Abilities  (Read 439 times)

Online bknight

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Gemini 11 Retro Abilities
« on: September 07, 2019, 10:58:40 AM »
Reading the newest JR thread and AtomicDog's post concerning Gemini didn't have a "air/vacuum" lock either, got me to reminiscing about Gemini.  I remember flight 11 where the Gemini/Agena combo increased the Apogee to +/- 750 miles. 
So a question developed and I'm sure those "dumb" NASA technicians thought about it prior to the mission.
After the increased apogee procedure, did the Gemini have sufficient orbital maneuvering fuel and/or reentry fuel to lower the orbit and reenter?  I search the press kit and mission report, but couldn't find any information concerning that contingency.
Does anyone have a link to answer?
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Offline ka9q

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Re: Gemini 11 Retro Abilities
« Reply #1 on: September 08, 2019, 04:05:14 AM »
They used the Agena only to raise the apogee; the perigee was still down at only 290 km. So the Gemini would still have easily re-entered using its own thrusters, if necessary.

I haven't worked it out, but it might have required even less delta-V to re-enter from the higher apogee than from a lower, more circular orbit. Not sure.

Online bknight

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Re: Gemini 11 Retro Abilities
« Reply #2 on: September 08, 2019, 09:47:34 AM »
They used the Agena only to raise the apogee; the perigee was still down at only 290 km. So the Gemini would still have easily re-entered using its own thrusters, if necessary.

I haven't worked it out, but it might have required even less delta-V to re-enter from the higher apogee than from a lower, more circular orbit. Not sure.

No, the Agena was used to raise and lower the apogee, but I'll accept your premise on the delta-v
https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-081A
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Offline ApolloEnthusiast

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Re: Gemini 11 Retro Abilities
« Reply #3 on: September 08, 2019, 12:27:22 PM »
They used the Agena only to raise the apogee; the perigee was still down at only 290 km. So the Gemini would still have easily re-entered using its own thrusters, if necessary.

I haven't worked it out, but it might have required even less delta-V to re-enter from the higher apogee than from a lower, more circular orbit. Not sure.

No, the Agena was used to raise and lower the apogee, but I'll accept your premise on the delta-v
https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1966-081A
While the Agena did raise and lower the apogee, I think ka9q was saying the perigee of the new orbit was still low enough to do a standard reentry retrofire in the event the Agena was unable to lower the apogee after boosting them to the farther orbit. 

Disclaimer: I'm not a scientist or engineer and have done exactly zero math; I'm just interpreting the statements as I see them pertaining to your initial question.

Offline gwiz

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Re: Gemini 11 Retro Abilities
« Reply #4 on: September 09, 2019, 06:28:02 AM »
That's how I interpreted ka9q's reply.  A retro burn in the higher orbit would take place at apogee and lower the perigee by the usual amount.  The re-entry trajectory would have a higher apogee than normal, hence rather higher velocity, so the question is not the retro-rockets' ability but the heat shield's.  Personally, I'd expect it to be still within the design margins, but don't have anything to back this up.
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Offline ka9q

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Re: Gemini 11 Retro Abilities
« Reply #5 on: September 12, 2019, 12:00:23 PM »
That's how I interpreted ka9q's reply.  A retro burn in the higher orbit would take place at apogee and lower the perigee by the usual amount.  The re-entry trajectory would have a higher apogee than normal, hence rather higher velocity, so the question is not the retro-rockets' ability but the heat shield's.  Personally, I'd expect it to be still within the design margins, but don't have anything to back this up.
Exactly right.

Offline rocketman

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Re: Gemini 11 Retro Abilities
« Reply #6 on: September 17, 2019, 04:14:03 PM »
I am getting that the amount of fuel required would actually increase for a while as we move from a circular orbit to an elliptical one, but then start to go down, and go down very dramatically as the orbit begins to approach a parabolic trajectory.

However, I am a long way from confident that I did this right.  I definitely want to double-check.

ETA - Oh bugger.  I treated a parameter as fixed, when it is a function of eccentricity.  Need to redo.  Ignore.
« Last Edit: September 17, 2019, 04:18:56 PM by rocketman »