Author
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Topic: Which Apollo astronaut initiated TLI?
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stsmithva Member Posts: 1940 From: Fairfax, VA, USA Registered: Feb 2007
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posted 04-18-2018 06:14 AM
On each lunar mission, a TLI engine burn caused the spacecraft to leave Earth orbit (usually over the Pacific). I've done some research but apparently not well, because I can't find which astronaut (CDR, CMP, or LMP) was responsible for "flipping the switch" to initiate that burn. Specifically, on Apollo 11, was it CMP Michael Collins? |
Robert Pearlman Editor Posts: 43576 From: Houston, TX Registered: Nov 1999
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posted 04-18-2018 07:53 AM
Going by how Robert Kurson describes TLI in "Apollo 8," the burn was triggered by the onboard computers, not by the crew. About ninety seconds before the engine was to fire, a light came on in the command module indicating the final countdown to ignition...Just sixty seconds remained, but the crew could still cancel the burn by throwing the Inhibit switch if things didn't look right. At eighteen seconds, they would have no choice but to allow the engine to light. On Apollo 8, it was Frank Borman who read out the countdown in the final seconds and Jim Lovell who announced "Ignition!" |
Headshot Member Posts: 891 From: Vancouver, WA, USA Registered: Feb 2012
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posted 04-18-2018 10:59 AM
From the description given on pages 128-129 of "How Apollo Flew To The Moon," TLI was pre-programmed into the CM's computer. It would have been up to the crew to stop the program, by activating a certain switch, if there was something amiss. Otherwise all the crew had to do was sit back, watch the velocity readout increase, and enjoy the ride. |
Space Cadet Carl Member Posts: 225 From: Lake Orion, Michigan Registered: Feb 2006
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posted 04-18-2018 11:22 AM
Thanks, Robert. I figured ignition was performed to the split-second by automatic sequence. But isn't it also true that a crew member always kept their finger on the engine shutdown switch, just in case the engine didn't shut down exactly on time? I remember reading that Lovell had his finger on the shutdown switch on the back side of the Moon, just in case the engine kept firing during LOI. |
stsmithva Member Posts: 1940 From: Fairfax, VA, USA Registered: Feb 2007
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posted 04-18-2018 02:52 PM
Interesting! Thank you for the answers. |
David C Member Posts: 1039 From: Lausanne Registered: Apr 2012
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posted 04-18-2018 03:10 PM
I think perhaps there's some confusion here between an SPS burn controlled by the CMC and a launch vehicle burn under IU control. |
Jim Behling Member Posts: 1488 From: Cape Canaveral, FL Registered: Mar 2010
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posted 04-19-2018 08:37 AM
quote: Originally posted by Headshot: TLI was pre-programmed into the CM's computer.
The S-IVB burns were controlled by the IU and not the CMC. |
Lou Chinal Member Posts: 1332 From: Staten Island, NY Registered: Jun 2007
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posted 04-22-2018 08:50 AM
Good question Steve. I always thought it was the CMP. |
Blackarrow Member Posts: 3160 From: Belfast, United Kingdom Registered: Feb 2002
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posted 04-26-2018 12:32 PM
quote: Originally posted by David C: ...a launch vehicle burn under IU control.
What happened when the launch was late? For instance on Apollo 14 when there was a delay of around 40 minutes? Or on Apollo 17, when there was a delay of around 2 hours, 40 minutes? Or even on Apollo 13, when the spacecraft was slightly late getting into orbit because of an early J2 cut-off? Did Houston send an update to the Instrument Unit to adjust the timing of the TLI burn? |
indy91 Member Posts: 15 From: Germany Registered: Feb 2016
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posted 04-26-2018 03:16 PM
The LVDC (the computer in the IU) had targeting data for the whole daily launch window. So if the launch was late like on Apollo 14 and 17, it would use a slightly different set of targets for TLI.And for the Apollo 13 issue with the late orbital insertion, the LVDC solved a geometrical problem to find the time of ignition for TLI. So it didn't have a preset time for TLI, but instead a set of targeting parameters and an earliest possible time for the TLI and from that it checked every second while in Earth orbit, if the conditions for TLI are satisfied. So the late insertion didn't make much of a difference. |
Jim Behling Member Posts: 1488 From: Cape Canaveral, FL Registered: Mar 2010
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posted 04-26-2018 08:52 PM
quote: Originally posted by Blackarrow: What happened when the launch was late?
No different than existing vehicles for interplanetary missions. The guidance system can account for it. |
Blackarrow Member Posts: 3160 From: Belfast, United Kingdom Registered: Feb 2002
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posted 04-28-2018 06:06 PM
Pretty clever for a 50-year-old computer system. |
Jim Behling Member Posts: 1488 From: Cape Canaveral, FL Registered: Mar 2010
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posted 04-29-2018 09:08 AM
Not really. It is just simple programming. Atlas Centaur did the same thing earlier. |
Blackarrow Member Posts: 3160 From: Belfast, United Kingdom Registered: Feb 2002
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posted 04-29-2018 05:16 PM
It is if you don't know how computers work, which probably accounts for at least 99% of the world's population, including me. |
Jim Behling Member Posts: 1488 From: Cape Canaveral, FL Registered: Mar 2010
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posted 04-29-2018 09:16 PM
I do know how they work and it was the programming that allowed for launch delays. Guidance computers didn't have to be very powerful since they were basically single task computers. |
Space Cadet Carl Member Posts: 225 From: Lake Orion, Michigan Registered: Feb 2006
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posted 05-01-2018 02:25 PM
Besides the computer programming, I'm equally fascinated by the star sightings by optical sextant that made those firings not only on time, but correctly aligned. Again, Jim Lovell supposedly was a standout at it. |
Blackarrow Member Posts: 3160 From: Belfast, United Kingdom Registered: Feb 2002
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posted 05-02-2018 05:05 PM
quote: Originally posted by Jim Behling: I do know how they work and it was the programming that allowed for launch delays. Guidance computers didn't have to be very powerful since they were basically single task computers.
And yet, whether 50 years ago or today, how computers do what they do is beyond almost everybody. That the Saturn V computers could do what they did 50 years ago was very impressive then, and is still very impressive to almost everybody. I get it that you are one of the very few people who know how computers actually work, but most of us don't. |