|
Author
|
Topic: Riding through translunar injection (TLI)
|
Hogboy13
New Member Posts: 7
From: Bowling Green Ohio USA
Registered: Oct 2012
|
posted 12-10-2012 04:16 PM
  
Translunar injection (TLI) was one of the more "routine" parts of the moon flights. But it never ceases to amaze me the speed they traveled during this phase of a flight (nearly 7 miles per second). I always wonder if it was a smooth "elevator" ride or did that have a lot of rattle and rolling? Would be interested in the board's comments or observations about the TLI segments of the flights. |
Headshot
Member Posts: 894
From: Vancouver, WA, USA
Registered: Feb 2012
|
posted 12-10-2012 05:00 PM
During Apollo 10's TLI, the S-IVB vibrated so violently that the crew could not read any gauges or dials. Concerned that the vibrations would shake the entire vehicle apart, mission commander Tom Stafford was ready to abort, but decided to ride it out. |
Explorer1
Member Posts: 182
From: Los Angeles, CA, USA
Registered: Apr 2019
|
posted 04-12-2020 09:57 AM
If escape velocity is 25,000 mph, why does it take 2.5 days to travel the quarter million mile distance to the Moon? I suppose the better question to ask is after the command module breaks away from the Saturn IV-B stage and docks with and extracts the lunar module, what is the velocity of the docked spacecraft combination in heading to the moon after the main engine burn? Similarly, on the return trajectory where there is no interruption such as spacecraft docking, I believe the command module hit the atmosphere at 25,000 mph on its first dip into the atmosphere, then slowed down before it made its full descent into the atmosphere. So the command module I assume was traveling on the return trajectory at 25,000 mph but still took 2.5 days to return to Earth. Can someone explain the time lag? Editor's note: Threads merged. |
One Big Monkey
Member Posts: 171
From: West Yorkshire, UK
Registered: Jul 2012
|
posted 04-12-2020 10:24 AM
25,000 mph was the maximum speed, it didn't travel at that rate for the whole journey.No sooner had the fire gone out than they started to slow down as a result of Earth's gravitational pull. Speed dropped to roughly 10 percent of escape velocity by they time they got within the moon's sphere of influence, after which it picked up again. Likewise for the return trip the speed at re-entry was mostly from Earth's gravitational pull. |
Colin Anderton
Member Posts: 160
From: Great Britain
Registered: Jan 2005
|
posted 04-12-2020 10:28 AM
Speed drops off rapidly as the spacecraft tries to tear itself away from Earth's gravitational pull. By the time it crosses the point where the moon's gravity becomes the dominant influence on the trajectory, it's travelling only about one mile per second; it then begins to speed up again.Reverse this procedure for the journey home. The service module engine doesn't have to reach anywhere near the speed required to leave Earth orbit. |
Space Cadet Carl
Member Posts: 226
From: Lake Orion, Michigan
Registered: Feb 2006
|
posted 04-12-2020 10:40 AM
Using rough numbers, when an Apollo flight got within 39,000 miles of the Moon, it entered the Moon's sphere of gravitational influence. The spacecraft had slowed down to only 2,000 miles per hour at that point. Then, it began picking up speed again as it "fell" toward the Moon. |
AstroCasey
Member Posts: 49
From:
Registered: Feb 2019
|
posted 04-12-2020 10:48 AM
Technically, the Apollo missions never actually reached escape velocity. The fastest velocity ever reached was 24,791 mph by Apollo 10 during re-entry. Since escape velocity wasn't achieved, the spacecraft lost speed during the entire trip to the moon. By the time the astronauts reached the moon's sphere of influence (region where the moon had the dominant gravity force), the spacecraft slowed enough to a reasonable speed that it could go into lunar orbit.I am currently watching the website Apollo 13 in Real Time. I recommend anybody with an interest in Apollo to check this out (Apollo 11 and 17 have similar websites). Notice how velocity drops immediately after the S-IVB third stage shuts down. Likewise, notice how the spacecraft speeds up as it gets closer to Earth. |
space1
Member Posts: 863
From: Danville, Ohio
Registered: Dec 2002
|
posted 04-12-2020 11:52 AM
quote:
Originally posted by Headshot:
During Apollo 10's TLI, the S-IVB vibrated so violently that the crew could not read any gauges or dials.
If I may comment on that 2012 entry, I recall reading the reason the Apollo 10 crew experienced such vibrations (I think in Cernan's "Last Man on the Moon"). The crew couch assembly was supported by shock absorbing struts within the spacecraft. These struts would allow the couch assembly to sway slightly as the crew would ingress the vehicle. To minimize this a small brace was attached. This brace was inadvertently left in place for the Apollo 10 launch and TLI, causing the excessive vibration for the crew. |
Explorer1
Member Posts: 182
From: Los Angeles, CA, USA
Registered: Apr 2019
|
posted 04-12-2020 12:15 PM
Thank you so much for all of the great answers. I suspected it was something like that. What is confusing is the return trajectory is how close the speed of Apollo 10 was at its maximum speed to the speed of escape velocity. Although not a fair comparison, I have heard that the New Horizons probe that went to Pluto reached the orbit of the Moon in just 9 hours with a peak speed of 36,400 mph. Different than going to the actual Moon. |
Jim Behling
Member Posts: 1494
From: Cape Canaveral, FL
Registered: Mar 2010
|
posted 04-13-2020 07:24 AM
Escape velocity is not a constant. It varies by altitude. quote:
Originally posted by Explorer1:
...what is the velocity of the docked spacecraft combination in heading to the moon after the main engine burn?
There is no engine burn after LM extraction. Watch this, especially case 2.The velocity varies at different points in elliptical orbits. Fastest at perigee and slowest at apogee. That is why the spacecraft is slowing down as soon as TLI and is speeding up after TEI. |
Colin Anderton
Member Posts: 160
From: Great Britain
Registered: Jan 2005
|
posted 04-16-2020 05:41 AM
Jim, I hesitate to challenge your facts here, but surely the spacecraft begins slowing after TEI, until it reaches the point where Earth's gravitational pull becomes the dominant feature?In other words, isn't the moon's gravity acting — to a lesser degree, of course — the same way Earth's gravity acts on a spacecraft after TLI? |
oly
Member Posts: 980
From: Perth, Western Australia
Registered: Apr 2015
|
posted 04-16-2020 06:27 AM
Listening to the Apollo 13 voice communication as part of the BBC series 13 Minutes to the Moon, Part 2 (great series) you can hear the Capcom tell the Apollo 13 crew during their post TEI coast that the spacecraft has entered the sphere whereby Earth's gravity has become the dominant force, meaning that the vehicle will begin to accelerate, speeding their voyage homeward. |
Jim Behling
Member Posts: 1494
From: Cape Canaveral, FL
Registered: Mar 2010
|
posted 04-16-2020 12:43 PM
quote:
Originally posted by Colin Anderton:
...until it reaches the point where Earth's gravitational pull becomes the dominant feature?
The moon's SOI only has a radii of 41,000 miles.
|
oly
Member Posts: 980
From: Perth, Western Australia
Registered: Apr 2015
|
posted 04-16-2020 08:57 PM
From the Apollo 11 Flight Journal: Leaving the Lunar Sphere of Influence: PAO: This is Apollo Control at 148 hours, 7 minutes. In about 24 seconds from now, the spacecraft will pass the imaginary line into the Earth's sphere of influence.148:07:14 McCandless: Apollo 11, this is Houston. Stand by for a Mark leaving the lunar sphere of influence. 148:07:22 McCandless: Mark. 148:07:23 McCandless: You're leaving the lunar sphere of influence. Over. 148:07:31 Collins: Roger. Is Phil Shaffer down there? 148:07:34 McCandless: Negative. But we've got a highly qualified team on in his stead. 148:07:42 Collins: Rog. I wanted to hear him explain it again at the press conference. 148:07:50 McCandless: Okay. 148:07:54 Collins: That's old Apollo 8 joke but tell him the spacecraft gave a little jump as it went through the sphere. 148:07:58 McCandless: Okay. I'll pass it on to him. Thanks a lot, and Dave Reed is sort of burying his head in his arms right now. Perhaps this description make sense. Less than four hours ago over the far side of the Moon, the crew carried out a burn of the SPS engine to leave lunar orbit. From the point of view of the Moon, this burn accelerated them onto a hyperbolic trajectory that will exit the Moon's sphere of influence. But from the point of view of Earth, what the burn did was to counter the Moon's orbital velocity of one kilometre per second, thereby bringing the spacecraft almost to a halt. The Moon then continues on its path around Earth leaving the spacecraft behind to begin its long fall towards splashdown in three days time. |
Colin Anderton
Member Posts: 160
From: Great Britain
Registered: Jan 2005
|
posted 04-17-2020 09:11 AM
But surely the moon's gravity would still begin pulling down the spacecraft speed as soon as the TEI burn was complete. But it depends on which body you relate the speed to.Perhaps this is why you often heard a PAO comment on the speed "with respect to the Earth." |
oly
Member Posts: 980
From: Perth, Western Australia
Registered: Apr 2015
|
posted 04-17-2020 09:20 AM
From the Apollo Flight Journal: In his autobiography, Mike [Collins] tells how [Phil] Shaffer struggled to explain to non-technical journalists what the concept of crossing the equigravisphere actually meant. At this point, the trajectory experts would change the reference for their calculations from Moon-centered to Earth-centered and, due to the relative speeds of these two objects, there would be a jump in stated velocities. Apparently some journalists were convinced there must be a physical jump felt by the crew, a nonsense that crews played with throughout the Apollo programme. |
ejectr
Member Posts: 1760
From: Killingly, CT
Registered: Mar 2002
|
posted 04-17-2020 01:14 PM
Correct, a jump in only the numbers but not in the mass which they were riding.Why is this so difficult to understand? |
advertisement