We repeat
our post on the Apollo 11 Moon landing, 56 years ago today, because our post
was so exceptionally well written.
The
fascinating part of the historic Apollo 11 landing on the moon (50 years ago
today) is the handling of the 1202 and 1201
alarms during the decent of the Eagle to the lunar surface.
Some
Apollo-nerd stuff that you only get here:
The Eagle's computer had three
programs to run from the time it undocked to landing. The first was P63: which
controlled Eagle from undocking while it was still in orbit to powered descent.
P63 controlled the attitude (pitch and yaw) of Eagle and ran the burn which
took Eagle out of orbit and into a controlled descent to the lunar surface. PDI
(powered-descent initiation) occurred about 500 kilometers east of the landing
site and 12 minutes to landing. The first go-no-go from Houston after the
go-no-go for undocking was for PDI. When the Eagle is three minutes from
landing and 7K/M from the site, the computer ran P64. The P64 program pitched
the Eagle forward and gave Armstrong a view of the lunar surface so he could
check for landmarks. At this point the computer is telling the astronauts and
Houston where it intends to land. If you listen to the raw landing tapes, the
guidance officer in Houston is indicating that they are headed for a spot
longer down-range than anticipated.
When the Eagle is 600 meters (2000 feet) from the landing site, Armstrong activates P66 in
which he and the computer share the throttle while Armstrong alone controls the
descent.
Somewhere along the way a switch was flipped powering on the rendezvous radar
that was supposed to be off. The rendezvous radar began feeding more
information to the computer than it was designed to handle at exactly the wrong
time- during the dangerous descent phase. How Armstrong and NASA handled this
critical error is the stuff legends are made of.
35,000 Feet Above The Lunar Surface And The First 1202 Alarm
As PDI begins, Houston immediately loses signal and data from Eagle, prompting
a call to Collins in Colombia to tell Aldrin to re-aim an antenna. Meanwhile
Neil Armstrong has several issues to contend with, including no communication
with the men who are supposed to guide him.
As the Eagle descended Armstrong began to realize that the guidance computer
was taking him farther down range then it was supposed to and was putting the
Eagle into a crater. The Eagle has two guidance systems: PGNS (pings) is the
main system, and AGS is the back-up. Both Houston and Aldrin are
monitoring both systems and comparing their data to see if it matches as the
Eagle descends towards history.
As P63 turns the Eagle around so that now it's Engine is facing the lunar
surface, the landing radar and other radar (that's supposed to be off) both
lock on to the lunar surface. The computer is overloaded with data, and
now come the words that almost ended the mission: "1202...1202
alarm." Eagle is 33,500 feet above the surface of the
moon.
In the simulator, Armstrong and Aldrin had practiced with several different
scenarios, including the loss of various functions of the Eagle, including
wrong indicators, loss of communication, and the like. But in the
thousands of hours of training, they had never encountered a 1202 alarm.
MEANWHILE ON THE GROUND IN HOUSTON
Here is what happened on the ground in Houston, and their success showed why
NASA was able to achieve the extraordinary moon landing:
Gene Kranz was the flight director, later responsible for the saying "failure
is not an option" in regard to Appolo 13. Kranz
was the one who made the final decision to allow the Eagle to land.
Jack Garman was an engineer and part of the team working on the computers and
the landing guidance system of the Eagle. At a meeting several weeks before the
landing, Kranz told Garman to write down every possible alarm and the
response to the alarm.
Steve Bales was the guidance officer who was one of the men responsible to
answer Kranz during various "Go-No-Go" calls when Houston had to
tell Armstrong if he could continue to land.
Astronaut Charlie Duke was "cap-com" the man responsible for speaking
directly to Aldrin and Armstrong.
Several
seconds after the 1202 alarm, Armstrong having heard nothing from Houston,
asked "give us a reading on that 1202 alarm." When
you listen to Armstrong, there is some uncertainty in his voice. He is about
30,000 feet above the surface of the moon and less than seven and half minutes
from landing. He is looking for a place to land, watching his fuel, working on
the high-gain antenna issue (the Eagle had been intermittently losing
communication with Mission Control), and now an alarm that may scrub the
landing is going off and Armstrong does not recognize it.
Meanwhile in Houston, when the alarm went off, Kranz was looking towards
his guidance officer Bales, or anyone else who knew what the alarm was. Nobody
knew. There were blank stares all around as Krantz's landing team
started scrambling though massive three-ring binders looking for what a
1202 alarm was. Eventually Krantz asked Bales, and Bales called over to a back
room where there were dozens of engineers One of them- Jack Garman - knew
what the alarm meant.
Apollo 11's computer's were rudimentary. The landing radars started giving the
computer more data than it could handle. When this occurred, the computer
had a line of programing to tell it to prioritize its work and to trigger a
1202 alarm to let Houston and the Eagle know what it was doing. Essentially the
computer was rebooting without shutting down. If the computer had shut down,
Kranz would have ordered an abort.
Garman reasoned that as long as the alarm didn't continually repeat, which
would mean the computer was in a non-recoverable loop, that they were
"go" on the alarm. Garman told Bales. Bales told Kranz. Kranz told
Duke and Duke told Armstrong.
There was another 1202 alarm and then a 1201 alarm at 27,000 feet above the
surface. Aldrin tells Houston about why he thinks the alarm is occurring.
Meanwhile Garman quickly told Bales that the 1201 alarm was the same type
of alarm as the 1202 and that they were "go" on that.
Armstrong never doubted what Duke was telling him. Kranz had faith in Bales,
and Bales knew Garman knew the landing computer software better than
anyone.
5200 FEET ABOVE THE LUNAR SURFACE
At about 9 minutes into the landing, and 5200 feet about the lunar surface,
the computer switches to P64 and the program pitches Eagle over so that
the attitude of the Eagle is more upright, and it begins to descend in the
same attitude that it will have upon landing. Armstrong is now looking at the
surface so he can find a place to land. Kranz quickly runs through a
"go-no-go" for landing, and Retro (the controller monitoring the
engines) FIDO (flight dynamics), ECOM (electrical, environmental and
consumables), Guidance, and the flight surgeon, all give Kranz an enthusiastic
"GO!" for landing, which Charlie Duke as Cap-Com relays to Aldrin and
Armstrong.
3000 FEET ABOVE THE LUNAR SURFACE
At 3,000
feet they get another 1201 alarm, but they are quickly told they are
"go on that alarm" and the descent continues. In another minute, at
1300 feet, they get another 1202 alarm, but they are still go to
land.
At
600 feet the P64 program is steering the Eagle toward a sea of boulders and
craters. Armstrong decides to switch to the P66 program and manually take
control of Eagle. Perhaps the greatest pilot in the world in 1969 is
piloting the Eagle in our quest to land on the Moon. Armstrong begins to
use thrusters to navigate the Eagle past a large crater while looking for a
flat area to set the Eagle down, all while monitoring an
ever-dwindling fuel supply and a host of other issues.
300 FEET ABOVE THE LUNAR SURFACE
The Eagle
is 300 feet from the surface when Aldrin tells Armstrong that he is "pegged
at horizontal velocity", meaning they are going forward at the
top speed on the indicator. At 250 feet Armstrong is now slowing the forward
velocity. He sees a landmark he recognizes: "Little West
Crater", and he pilots Eagle just past it where he sees a relatively
smooth surface to land. The forward velocity has slowed from 50 feet/second to
19 feet/second. At 175 feet they have 94 seconds to land, or they will get
a "fuel-bingo" call from Houston and will have to abort.
The Eagle
had one chance to land on the moon. If the landing was aborted, they would
activate the ascent engine, fly back to Michael Collins in Colombia, and headed
back to earth having failed in their attempt to land. There were no second
chances on this flight.
At 100
feet they have 75 seconds of fuel left. Armstrong is now demonstrating
why he was chosen for this mission- he is at the top of his game and pulling
off the greatest landing in the history of aviation- and one of the most
difficult and dangerous to boot. The man has met the moment. Thousands of
hours of training are paying off for the Apollo 11 landing team.
60 SECONDS
At 75 feet
the Eagle's velocity has slowed to six feet forward/per second. At 60 feet,
Charlie Duke in Houston calls out "sixty seconds": they
have one minute of fuel left to land.
At 20 feet
Charlie Duke calls out "thirty seconds". There is almost no fuel left
to land.
With
seventeen seconds of fuel remaining Aldrin calls out "contact
light": a 1.5-meter probe below Eagle has touched the surface and
humans have landed on the moon.
The team
worked. The system worked, and Armstrong landed the Eagle with 17 seconds of
fuel remaining.
ON THE MOON
The first words spoken by a human being on another celestial body belong to
Buzz Aldrin: "Contact light. Ok. Engine stop. ACA out of descent. Mode
control both auto descent engine command override off. Engine arm off. 413
is in."
Hardly memorable or historic words, but before Armstrong tells the world that
from Tranquility Base "The Eagle has landed", Aldrin had
a checklist he needed to run through to make sure the descent engines were shut
down and the abort-ascent engine couldn't be accidentally triggered.
Charlie Duke responds "we copy you down Eagle".
Armstrong
says "The Eagle has landed" and Duke responds that there
were a bunch of guys about to turn blue but were breathing again.
This is in response to the fact that with less than 30 seconds of fuel, the
Eagle had not landed and everyone in mission control was holding their
breath.
A half a million people worked on some part of the Apollo program. But on July
20, 1969, it was Kranz, Steve Bales, and Jack Garman who gave the go ahead to
Armstrong and Aldrin to continue the landing in the face of 1202 and 1201
alarms.
Brave and historic actions indeed. And it is, in our humble opinion,
humanity's finest hour. It reminds us that if we try together, we as a species
can do great things.
Happy
Apollo 11 Moon Landing Day. We are go on that 1202 Alarm.