- Hi, it's me, Tim Dodd,
the Everyday Astronaut. It's been 50 years since Neil Armstrong famously stepped off the footpad
of an Apollo Lunar Lander and onto a new celestial body for the first time in human history. And once he was safely standing
on the surface of the moon he spoke perhaps some of the
most famous words of all time. - [Neil Armstrong] It's
one small step for man. One giant leap for mankind. - But something about
this always confused me. I mean, sure, the step off of the lunar lander's
91 centimeter footpad was a nice easy and small step, but the step just before
that was actually enormous. I'm talking about this. The huge gap between the last rung of the ladder and the ground. I mean why on the moon
is that gap so huge? Isn't it dangerous to require an astronaut to jump down onto the surface and then have to jump back up? The lunar lander's ladder had nine rungs, all spaced out at 22.8
centimeters apart from each other. But the gap between the
last rung and the ground was about three times that,
at a whopping 76 centimeters. So really the ladder
wasn't missing one rung, it was missing two. I mean, sure, the gravity's
only one sixth that of Earth's, but wouldn't it have been so much safer if there was even just one more rung that closed that gap up just a little. So today we're gonna look into why NASA and the lunar lander's
manufacturer Grumman chose a ladder of this length. We'll then talk about all
the design considerations of the hardware, the unknown conditions of
the lunar surface itself, and the astronauts who were basically just too smooth of pilots to
get the ladder's last rung any closer to the surface of the moon. Lets get started. - [Announcer] Three, two, one, liftoff! (energetic instrumental music) - [Neil] It's one small step for man. - On May 25th, 1961, President John F. Kennedy
announced his goal of putting humans on the moon by the end of the decade to Congress. At the time of this historic
and ambitious announcement, the U.S. had exactly 15
minutes and 22 seconds of human space flight experience with just one suborbital
launch under their belt. By the way, I should probably mention that Kennedy's speech in front of Congress is not the famous we choose
to go to the moon speech. That happened over a year later
at Rice Stadium in Houston. But the point is this
insanely ambitious goal almost immediately put a
lot of things in motion. A tsunami of engineering ensued. I mean so much so, sometimes it feels like
the left hand didn't know what the right hand was doing. One item that almost
immediately began development was the lunar lander itself. In July 1962, 11 companies
were asked to submit proposals for a lunar lander. Grumman won the contract
and began to design mock ups as early as 1963. Wait, what's all this
have to do with a ladder? Well, hold up, I'm setting up a story here and to me, these stories are half the fun, these little fun nuggets of history. But this particular bit of
history is one of the key reasons why the ladder is the wrong length. Grumman was building a lander
intended to land on a surface that we knew literally nothing about. This is understandable when the
only data we had at the time was from the orbiters taking footage from the ranger missions. And before that, we just
had ground based telescopes here on Earth. Well, I guess actually Ranger 6 through 9 actually recorded footage
all the way up until impact, resulting in some footage with an impressive
resolution of .3 meters. But, we still had almost no idea of what the surface of the
moon was actually like. It wouldn't be until June 2nd, 1966, before the U.S. would soft
land surveyor one on the moon, some four months after the
Soviet Union landed Luna 9. This would be the first time there'd be any data available to the U.S. about the composition
of the moon's surface. And even so, it was quite limited. This means during the first three years of development of the lunar lander, there were countless question marks. Many scientists thought the surface might be so soft and powdery, it might swallow a spacecraft or people entirely on the surface, sinking them so deep they
wouldn't be able to get out. While NASA was busy sending
spacecraft after spacecraft to observe the moon, Grumman was deep in development
of the lunar lander. The first couple years
saw a lot of changes, including things like going
from five legs to four legs, from a round cockpit to
more of a polyhedral shape, and even going from a round
hatch to a square hatch. Oh by the way, that round
hatch led to astronauts really struggling to get
back into the spacecraft, so Grumman switched to a square version to help make egress and
ingress that much easier. This is also around the same time that Grumman was playing
around with using a rope instead of a ladder, which astronauts found to
actually be impossible, and despite them using a
peter pan pulley system which would simulate one sixth gravity. So Grumman opted for a ladder down the front leg of the lunar lander. But even with something
as simple as a ladder, weight was an enormous factor. I mean, after all, for each kilogram landed on the surface of the moon, it takes about 400 kilograms of rocket to launch from Earth. This means Grumman went to
great lengths to cut weight including ditching seats and using lightweight golden
kapton foil for insulation, but it also means the
ladder, like all things, needed to be extremely lightweight. For example, I think my
favorite thing to think about is how if the lunar lander were standing on its own legs here on Earth, it would've likely collapsed
under its own weight because those legs weren't
meant for Earth's gravity. They were only designed to
handle the moon's weaker gravity. And the same thing goes for the ladder. If we were to try to ascend
the ladder, the actual ladder, here on Earth, it would've likely broken because it's not meant to
handle Earth's gravity. So they made it as
lightweight as possible, and only capable of
handling one sixth gravity. But weight likely isn't the main reason why the ladder stopped
short of the surface. For this, we need to
look at the landing gear the ladder is affixed to. The landing gear were designed to absorb the energy of impact and keep the lem from toppling over. Grumman engineers were
incredibly concerned with the lem tipping over, so a significant amount of work went into the design of the landing gear. The landing gear was so vital
that after the command module on the lunar excursion module undocked, the command module pilot was instructed to visually inspect all four legs to make sure they were properly deployed from their stowed position. A leg that wasn't properly locked in place would've been an immediate abort. But fortunately, none of
them failed to deploy. One interesting design consideration was the invention of a
single use shock absorber. NASA and Grumman developed a crushable honeycomb aluminum cartridge, otherwise known as a crush core. There was a great concern about using hydraulics in the landing gear and fear that they could leak and leave a landing leg unusable. So, for simplicity's sake,
they went with the crush core which is basically just
a set of cartridges made of aluminum that
either compress or stretch to provide smooth and reliable resistance. This technology is still used today for certain applications. Actually, SpaceX has crush
core in the landing struts of their Falcon 9 rockets. If the hydraulic shock
absorbers bottom out, they have additional travel
in the single use crush core. This was very obvious on Thaicom 8, which launched and
landed on May 27th, 2016. The booster had a little
bit of a rough landing, resulting in one of the landing legs eating into the crush core which made the booster stand lopsided. By some miracle, the booster
made it all the way back home, despite some really, really close calls. (laidback music) But back to the lunar lander. Its legs could absorb a large
amount of impact velocity. And due to their crush core absorbers, they did not rebound like a
traditional hydraulic absorber. They just compressed and then
they stayed in that position. They were designed to handle
up to three meters per second of vertical velocity with
zero horizontal velocity, or up to 2.1 meters per second with 1.2 meters per second
of horizontal velocity, which they could handle very reliably. Now since NASA didn't know
how much the shock absorbers would stroke or compress, and they were unsure of how
deep into the lunar surface the lem might sink, they didn't want the ladder attached to the movable part of the strut and potentially hinder any movement. As a matter of fact, NASA assumed the struts would travel a lot more than they did. Come to find out, two
factors prevented them from actually traveling that much, the pilots' skills and the
surface of the moon's ability to absorb energy. That's right. The pilots were in a sense
maybe too gentle with the lem and they all landed so stinking softly, there was hardly any
perceivable travel on the legs. But then again, there were so many unknown variables. NASA was playing this
whole thing extremely safe. The astronauts were
instructed to cut the engine when one of three 1.5 meter long lunar surface sensing probes made contact with the surface. You've probably heard them
make that contact call out. This was intended to not
only help prevent the engine from kicking up too much debris and dust for visibility and hole puncture concerns, but it's also because the
engine had engine thrust decay. This is where even after
successful shutdown of the engine, there's still some residual pressure that can last anywhere from half a second to a few seconds. All rocket engines have this. Fun side note, this little bit of extra thrust decay went unnoticed on the test stand when SpaceX upgraded their Merlin engine between flights two and three
of their Falcon 1 rocket. After safe separation, additional thrust decay led to a collision of the first and second stage, causing a loss of the vehicle. The apollo comadors were
intended to cut the engine at 1.5 meters in altitude, which would've been the
right height to touchdown at a safe velocity, and compress the legs enough
based on what NASA thought would be an adequate amount. But despite all this, all six landings had very different actual impact velocities. Neil Armstrong gave Apollo
11 the softest touchdown with a vertical velocity
of .54 meters per second. Apollo 17 was the next softest
at .91 meters per second. Apollo 14 was next, at
.94 meters per second. Then Apollo 12, at 1 meter per second. Apollo 16 at 1.7 meters per second. And lastly, Apollo 15's
commander David Scott finally gave her the beans
at 2.07 meters per second. But that still well within
the operational range to the legs. And now a big concern was
having the descent engine coming in contact with a lunar surface. And despite Apollo 15
carrying the additional mass of the lunar rover vehicle, having a 25 centimeter nozzle extension on the descent engine, landing on the steepest
lunar slope of 11 degrees. And despite the nozzle actually buckling, the nozzle still didn't
actually come in contact with the surface at all. The buckling of the nozzle was
due to a buildup of pressure from firing the nozzle
so close to the surface, and not from coming in contact
with the surface itself. But another factor that
was later accounted for on why the stress didn't
compress that much, was the fact that due to the
moon's somewhat powdery surface the surface itself would actually absorb around 80% of the impact velocity. And this is perhaps why the landing legs compressed so much less
than they had planned, leaving the final rung
higher than nominal. Okay, so basically the moral of the story is landing on the moon was
a big fat question mark with the Apollo program. NASA took the most conservative approach to as many things as possible, including fail proof shock absorbers, additional thermal protection
on the landing legs, big fat landing pads, and contact probes to name a few. Suffice to say they didn't
wanna make the ladder any longer than necessary and potentially hinder
movement of the landing gear. So assuming things went as planned and the shock absorbers went deep enough, the ladder would've been
a nice and easy step. But all this said, the final step still
worried some astronauts. The commanders were actually trained to immediately attempt to jump
back up to the bottom rung, after initial descent down the ladder. This way the pilot who
is still inside the lem could assist if there is any struggle to make that large gap. Neil actually commented on this before he stepped off the lander's pad. Different astronauts had
different experiences with the ladder. Such as Jim Irwin saying the
ladder was a real struggle. And 170 centimeter tall
Pete Conrad saying. So to summarize, the final rung height was a topic for much debate. But with all the unknown variables, a bounty out there for
every single kilogram to be shaved from the vehicle, and figuring jumping up and
down in one sixth gravity would make ascent and descent much easier. They also didn't know how
much the lunar surface would absorb energy, so NASA left the bottom
rung at it's final gap of 76 centimeters, from the bottom of the ladder to the top of the landing pad. Or maybe NASA could've just done what the Soviet Union planned to do with their LK Lunar Lander, and just used a hinge ladder. So what do you think? Did I help answer all your questions about why the lunar lander's ladder was the height that it was? Let me know if you have
any other questions about this topic or
about the Apollo program or just rocket science in general. And stay tuned, cause I have a lot of really awesome videos coming up. I owe a huge thank you
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Earth, for everyday people. (laidback music)
I always thought they left off the bottom section to avoid mechanical complexity.
Having a moving ladder was just one more failure point and that they thought the astronauts could leap that
Can’t watch rn but I thought it was just more weight saving
It's too long. I'm just going to come back here later and see if someone has posted the answer.