- 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.
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Thanks everybody, that's gonna do it for me.
I'm Tim Dodd, the everyday astronaut,
bringing space down to Earth, for everyday people.
(laidback music)