Human	Lunar	Landing	Experience	On	Project	Apollo	

How	We	Did	It	

Why	We	Did	It	That	Way	

Conclusions	and	Implications	for	Human	Landing	System	Design	

31	December	2019	

Dean	Eppler	

The	Aerospace	Corporation	

dean.b.eppler@aero.org	

dean.b.eppler@SanAntonioMountainConsulting.com	
Presentation	Sections	 • Introduction	

• Basic	terminology	and	acronyms	

• Apollo	approach	to	lunar	landing	–	Basic	Approach	

• Primary	Lunar	Module	Systems	

o Descent	Propulsion	System	

o LM	Reaction	Control	System	

o LM	Landing	Radar	

o Primary	and	Abort	Guidance	System	(“Pings	and	Ags”)	

o Lunar	Module	Guidance	Computer	

o Cockpit	controls	

o LM	Commander’s	Window	

o CDR	and	LMP	

• Apollo	approach	to	lunar	landing	–	the	actual	implementation	

o Spacecraft	elements	–	which	vehicle	is	doing	which	task	and	why?	

o Aborts	and	Abort	Modes	

o Site	selection	constraints	

o How	we	flew	the	approach	

• Apollo	landing	crew	experience	–	how	did	it	come	off	for	real?	

o Apollo	11	–	Getting	it	on	the	ground	in	one	piece	and	coming	home	

o Apollo	12	–	Precision	landing	in	“relatively”	benign	terrain		

o Apollo	14	–	Precision	landing	in	more	challenging	terrain		

o Apollo	15,	16	and	17	–	Landing	in	the	mountains	on	the	Moon		

• Conclusions	and	Implications	for	future	Human	Landing	System	(HLS)	design	

• Source	Listing	

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Introduction
Study	Background	and	Purpose	

• This	study	was	undertaken	largely	because	in	the	4-5	decades	since	the	end	of	

the	Apollo	Program,	the	detailed	methodology	and	history	of	how	we	landed	on	

the	Moon	has	been	largely	lost	from	NASA’s	institutional	memory	

• This	has	led	to	many	misconceptions	and	confusion,	including:	

o The	flight	crew	did	all	the	flying	

o The	flight	crew	did	none	of	the	flying	

o The	Lunar	Module	Pilot	was	flying	the	LM	

o The	Lunar	Module	Pilot	wasn’t	doing	anything	at	all*	

o The	Commander	was	flying	the	LM	

o The	ground	was	flying	the	LM	

o The	computer	was	flying	the	LM	

• This	history	has	significant	relevance	for	the	preliminary	development	of	the	

Artemis	human	lander	

• The	teams	selected	by	NASA	for	the	Artemis	lander	are	in	the	process	of	making	

critical,	up-front	engineering	decisions	that	cannot	be	easily,	or	cheaply,	“undone”	

later	if	they	prove	to	be	incorrect	

• If	we’re	going	to	go	back	to	the	Moon,	we	need	to	understand	how	we	got	there	

in	the	first	place,	including	what	worked,	what	didn’t,	how	we	did	things,	and	

who	was	doing	them	

*Note	the	following	comment	from	Donald	Wilhelms	1993	memoir	titled,	“To	A	Rocky	Moon”:	“....[on	Apollo	8]	no	LM	was	ready	for	Anders	to	fly	(not	that	the	

LMP	actually	piloted	the	LM	anyway)..”	
A	Disclaimer	and	Acknowledgements	

• Understanding	how	Apollo	landed	on	the	Moon	is	like	peeling	an	increasingly	

complex	onion	

o There	are	(still)	several	levels	of	complexity	below	what	I	was	able	to	ferret	out	or	

report	on	for	this	presentation	and	this	report	is	a	significant	simplification	of	an	

extremely	complex	process	

§ “The	lunar	landing	was	a	very	complex	series	of	maneuvers,	including	subtleties	and	

2nd	order	effects	that	have	not	really	been	published	(as	far	as	I	know).”	[Dave	Scott,	email	to	Eppler,	2	January	2020]

o However,	I	believe	the	more	detailed	layers	are	about	implementation,	not	about	the	

high-level	approaches	to	landing	on	the	Moon;	consequently,	I	think	these	

conclusions	will	be	valid	for	initial	Artemis	definition	

• The	number	of	people	we	can	ask	directly,	“How	did	you	land	on	the	Moon?”	is	

diminishing	all	the	time,	and	like	all	history,	it’s	getting	harder	to	nail	down	

questions	that	should	have	been	asked	years	ago...

o Several	folks	have	expressed	dismay	we	didn’t	capture	as	much	knowledge	from	

Apollo	as	we	should	have,	but	the	key	information	that	is	often	missing	is	“assumed	

knowledge”...the	everyday	information	that	people	shared	and	didn’t	think	to	define	

in	memos,	during	space-to-ground	conversations	and	in	debriefs	

§ Think	of	it	this	way	–	if	you	got	hit	by	a	bus	on	the	way	to	work	tomorrow,	would	

someone	that	didn’t	have	a	background	in	your	part	of	the	space	business	understand	

everything	you	did	just	by	reading	your	e-mails,	files	and	reports?	

o The	fact	is	that	NASA	did	an	excellent	job	of	capturing	the	experience	of	Apollo,	

largely	in	the	form	of	130	individual	documents	called	“Apollo	Experience	Reports”,	

covering	everything	from	LM	landing	techniques	to	space	suit	engineering	and	

procurement	to	food	

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A	Disclaimer	and	Acknowledgements	

• This	work	benefitted	greatly	from	discussions	and	critiques	by	many	folks,	

including	Anthony	Ceccacci,	Gerry	Griffin,	Jeff	Hanley,	Frank	O’Brien,	Harrison	

Schmitt,	David	R.	Scott	and	David	Woods	

• HOWEVER...any	flaws,	misconceptions,	incorrectly	filled-in	blanks	and	

wrong	conclusions	should	be	pointed	out	to	me	with	no	reservations	–

it’s	the	only	way	we’ll	get	this	story	straight	and	be	able	to	influence	the	

next	generation	of	lunar	landers	and	the	crewmembers,	engineers	and	

operators	who	will	develop	them	and	fly	them	

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Critical	Sources	

• I	will	include	a	more	complete	reference	list	at	the	end,	but	a	few	sources	

need	to	be	called	out	up	front,	as	they	were	critical	to	understanding	what	

we	did	50+	years	ago	

• First,	the	Apollo	Lunar	Surface	Journal	and	its	companion	source,	the	Apollo	

Flight	Journal,	continue	to	be	the	go-to	place	for	any	information	about	

detailed,	minute-by-minute	transcripts	and	for	interpretations	by	

crewmembers	and	very	talented	auxiliary	contributors	to	the	journal	

o Throughout	this	report,	the	use	of	italics	denotes	a	direct	quote,	either	from	the	

ALSJ	or	other	sources	

• Second,	David	Mindell’s	“Digital	Apollo”	is	the	best	source	of	both	how	we	

did	the	Apollo	human-machine	interface,	as	well	as	a	great	discussion	of	

how	flying	in	the	atmosphere	and,	by	evolution,	flying	in	space	changed	

from	the	pre-World	War	II	era	all	the	way	forward	through	Apollo	into	the	

Space	Shuttle	era	–	everyone	in	this	business	should	read	this	book!	

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Sources	(cont.)	

• Third,	Don	Eyles’	“Sunburst	and	Luminary:	An	Apollo	Memoir”	captures	

how	the	computer	code	for	Apollo	was	both	conceived,	written	(and	in	

some	cases,	rewritten	during	flight)	and	implemented	(hint	–	if	you	read	

Mindell’s	and	Eyles	book	in	parallel,	you	get	a	great	view	of	two	sides	of	the	

coin	on	software	development)	

• Fourth,	Frank	O’Brien’s	“The	Apollo	Guidance	Computer:	Execution	and	

Operation”	is	the	most	complete	compilation	of	data	on	the	LM	guidance	

computer	and	its	software	out	there.	

• Lastly,	the	post-flight	crew	debriefs	are	critical	in	understanding	how	the	

crew	remembered,	immediately	post	flight,	their	experience	landing	the	LM	

and,	when	combined	with	the	transcripts	from	the	ALSJ	and	other	sources,	

allows	another	dimension	to	be	added	to	this	activity	

o These	can	be	found,	along	with	a	host	of	other	sources,	on	the	ALSJ	

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Sources	

• I	will	include	a	more	complete	reference	list	at	the	end,	but	a	few	sources	need	

to	be	called	out	up	front,	as	they	were	critical	to	understanding	what	we	did	

50+	years	ago	

• First,	the	Apollo	Lunar	Surface	Journal	and	its	companion	source,	the	Apollo	

Flight	Journal,	have	continued	to	be	the	go-to	place	for	any	information	about	

detailed,	minute-by-minute	transcripts	and	interpretations	by	crewmembers	

and	very	talented	auxiliary	contributors	to	the	journal	

• Second,	David	Mindell’s	“Digital	Apollo”	is	the	best	source	of	both	how	we	did	

the	Apollo	human-machine	interface,	as	well	as	a	great	discussion	of	how	flying	

in	the	atmosphere	and,	by	evolution,	flying	in	space	changed	from	the	preWorld	War	II	era	all	the	way	forward	through	Apollo	into	the	Space	Shuttle	era	

–	everyone	in	this	business	should	read	this	book!	

• Second,	Don	Eyles’	“Sunburst	and	Luminary:	An	Apollo	Memoir”	captures	how	

the	computer	code	for	Apollo	was	both	conceived,	written	(and	in	some	cases,	

rewritten	during	flight)and	implemented	(hint	–	if	you	read	Mindell’s	and	Eyles	

book	in	parallel,	you	get	a	great	view	of	two	sides	of	the	coin	on	software	

development)	

• Third,	the	post-flight	crew	debriefs	are	critical	in	understanding	how	the	crew	

remembered,	immediately	post	flight,	their	experience	landing	the	LM	and,	

when	combined	with	the	transcripts	from	the	ALSJ	and	other	sources,	allows	

another	dimension	to	be	added	to	this	activity

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PDI	to	Touchdown	Videos	

• The	Apollo	Flight	Journal	has	put	together	a	set	of	videos	that	combine	the	

crew	air-to-ground	with	the	CAPCOMM	and	PAO	loop	from	just	prior	to	the	

start	of	Powered	Descent	through	to	touchdown	

o Apollo	11,	PDI	to	touchdown	(note:	updated	version	from	David	Woods	which	

includes	the	Flight	Director	loop	as	well	as	the	crew	voice	loop)

https://history.nasa.gov/afj/ap11fj/10day3-flight-plan-update.html#0581404

o Apollo	12,	PDI	to	touchdown:	https://www.youtube.com/watch?v=kFSa6vUix70

o Apollo	14,	PDI	to	touchdown:	https://www.youtube.com/watch?v=oZZe-xXx9_o

o Apollo	15,	PDI	to	touchdown:	

https://www.youtube.com/watch?v=AxqKlDsgMzc&t=7s

o Apollo	16,	PDI	to	touchdown:	

https://www.youtube.com/watch?v=JSXhb3J05ps&t=3s

o Apollo	17,	PDI	to	touchdown:	

https://www.youtube.com/watch?v=A7y5feeMvEo&t=869s

• Watching	each	of	these	is	time	well	spent,	if	only	to	hear	the	interaction	in	the	

LM	cockpit,	as	well	as	between	the	LM	and	the	ground	

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A	Brief	Note	on	Units	of	Measurement	

• Apollo	did	its	planning	at	time	when	NASA	was	initially	using	a	variety	of	

units,	including	aviation	units	(knots,	nautical	miles),	English	units	and	

metric	units	

o However,	early	in	the	1970s,	the	Agency	decided	to	standardize	to	all	metric	

units	

• Remarkably,	this	never	caused	a	technical	hiccup	in	any	operation,	but	it	

did	result	in	a	plethora	of	units	being	used	for	different	phases	of	flight	

o For	instance,	orbital	velocities	were	often	called	out	in	feet	per	second	

(abbreviated	here	ft	s-1),	but	orbital	altitudes	were	often	designated	in	

nautical	miles	and	EVA	distances	on	the	surface	were	called	out	in	kilometers	

• For	the	purposes	of	consistencies	with	historical	documents,	and	because	

this	report	largely	covers	orbital	operations	down	to	a	surface	landing,	I’m	

going	to	stick	with	the	units	used	in	those	documents	rather	than	convert	

units	or	list	joint	units	(e.g.,	feet	(km))	

• I	hope	this	does	not	confuse	anyone	unduly...

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Basic	Terminology	and	Acronyms	

• State	Vector	–	In	simplest	terms,	it	gives	the	position	of	the	spacecraft	in	3	

numbers	and	the	velocity	in	3	numbers,	with	the	added	7th	number	the	

time	at	which	the	state	vector	was	determined;	in	short,	it’s	where	the	

spacecraft	is	and	how	fast	it’s	going	in	which	direction	at	a	discrete	time	

• Descent	Propulsion	System	(DPS,	or	“Dips”)	–	the	rocket	engine	in	the	

descent	stage	that	allowed	the	LM	to	descend	from	orbit	to	the	Moon	

• Ascent	Propulsion	System	(APS)	–	the	rocket	engine	that	either	took	the	

crew	off	the	Moon	under	a	nominal	mission,	or	allowed	the	LM	to	abort	

from	various	points	of	during	descent	and	get	back	up	to	orbit	for	an	

eventual	rendezvous	with	the	Command-Service	Module	

• Landing	Radar	(LR)	–	the	continuous-wave	radar	system	that	provided	

the	Lunar	Module	Guidance	Computer	(LGC)	with	radar	altitude	and	

altitude	rate	information,	which	was	used	by	the	guidance	computer	to	

calculate	range	and	rate	to	the	target	point	

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Basic	Terminology	and	Acronyms	

• Primary	Guidance	and	Navigation	Control	System	(PGNCS,	or	“Pings”)	

–	The	Inertial	Measuring	Units	(IMUs,	which	included	gyroscopes	and	

accelerometers),	computers,	I/O	devices	and	software	that	allowed	the	

crew	to	operate	both	the	LM	and	the	CSM;	the	heart	of	the	system	–IMU	–	is	

often	referred	to	simply	as	“the	platform”	

• Abort	Guidance	System	(AGS,	or	“Ags”)	–	The	secondary	system	of	

gyroscopes,	computers,	I/O	devices	and	software	that	backed	up	the	PGNCS	

and	would	allow	the	crew	to	fly	in	the	event	of	a	total	PGNCS	failure		

• LM	Guidance	Computer	(LGC)	–The	computer	on	board	the	LM	that	took	

inputs	from	the	on-board	IMU,	the	crew	and	the	ground	and	executed	

specific	programs	that	controlled	the	LM	main	propulsion	system	and	

Reaction	Control	System	to	execute	a	particular	phase	of	a	landing,	abort	or	

ascent	

• Display-Keyboard	(DSKY,	pronounced	as	“Diskee”)	–	the	computer	

interface	in	both	the	LM	and	CSM	that	allowed	the	crew	to	input	data,	

receive	data,	or	initiate	a	particular	flight	phase	

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Basic	Terminology	and	Acronyms	

• LGC	Programs	(also	referred	to	with	by	their	“P”	numbers,	such	as	

“P63”)	–	Specific	loaded	programs	within	the	LGC	that	would	execute	a	

defined	set	of	maneuvers	to	complete	a	particular	flight	phase;	the	first	

number	of	a	particular	program	was	generic	for	a	given	flight	phase	(e.g.,	

in	the	LGC,	all	P6X	numbers	dealt	with	lunar	landing)	

• LGC	Verbs	(identified	with	specific	numbers,	such	as	Verb	37)	–

Specific	actions	that	told	the	computer	what	to	do;	e.g.,	Verb	37	told	the	

LGC	to	standby	for	new	program	initiation	instructions	

• LGC	Nouns	(identified	with	specific	numbers,	such	as	Noun	33) –

Instructions	to	the	LGC	to	display	specific	data	on	the	DSKY,	such	as	time	

to	engine	ignition	for	a	given	flight	phase,	or	input	data,	such	as	the	

angular	position	of	navigational	fixes	

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Basic	Terminology	and	Acronyms	(cont.)	

• Landing	Phase	–	the	particular	segment	of	a	lunar	landing	that	involved	a	

specific	set	of	vehicle	actions	designed	to	place	the	LM	on	the	lunar	surface	

• TIG –	Time	of	ignition	for	a	particular	burn;	often	called	out	as	“Tig”	or	“Delta									

Tig”	in	burn	residual	reports	

• TGO –	Time	to	Go	until	the	start	of	a	particular	landing	phase;	sometimes	called	

out	as	TTG

• H		-	Symbol	for	altitude	above	the	lunar	surface	

• H-dot	–	Symbol	for	altitude	change	rate	

• LOS/AOS	–	Loss/acquisition	of	LM	data	when	the	LM	either	went	behind	the	

Moon	(LOS)	or	came	in	sight	of	the	Earth	after	a	far	side	pass	(AOS)	

• Ullage	burn	– A	brief	burn	of	the	LM	Reaction	Control	System	(RCS)	prior	to	

the	initiation	of	powered	descent	insertion,	performed	to	position	the	LM	fuel	

and	oxidizer	to	the	bottom	of	the	prop/oxidizer	tanks;	often	referred	to	by	the	

crew	as	just,	“Ullage.”	

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How	To	Land	on	the	Moon	–	The	Basics
Apollo	Lunar	Approach	and	Landing	–	Basic	Approach	

• There	are	two	approaches	to	landing	on	any	planet	–	a	direct	descent	from	the	

inbound	trajectory	without	going	into	an	initial	parking	orbit,	or	a	descent	

from	a	parking	orbit	

o The	Surveyor	robotic	landers	used	the	direct	descent	approach,	which	works	

well	as	long	as	the	descent	procedure	was	nominal	

o However,	if	there	was	a	major	propulsion	anomaly,	there	was	very	little	chance	

for	recovery	from	a	catastrophic	surface	impact	

§ In	fact,	Surveyors	II	and	IV	were	likely	lost	in	this	manner	

• Apollo	chose	the	latter	approach,	starting	with	the	Command-Service	Module	

(CSM)/Lunar	Module	(LM)	stack	going	into	an	≈60	nautical	mile	(nm)	

circular	parking	orbit	

• The	problem	of	getting	safely	to	the	lunar	surface	was	then	broken	into	four	

segments	by	one	of	the	Space	Task	Group	members	named	Donald	Cheatham	

o Phase	1:	Get	the	LM	into	an	new	orbit	to	set	up	the	descent	to	the	surface	

o Phase	2:	Put	the	LM	in	a	continuous	braking	trajectory	to	get	it	near	the	surface	

o Phase	3:	Put	the	LM	in	a	final	approach	trajectory	to	the	planned	landing	site	

o Phase	4:	Land	
Apollo	Lunar	Approach	and	Landing	–	Basic	Approach	

• Phase	1:	Put	the	LM	into	an	elliptical	orbit	(called	the	descent	orbit)	using	a	

minimum	energy	Hohmann	transfer	orbit	burn	(called	the	Descent	Orbit	

Insertion	burn	or	DOI	burn)	with	an	apoapsis	of	≈60	nm	and	a	periapsis	of	

≈50,000	ft	that	was	≈250	nm	up	range	from	the	landing	site,		

• Phase	2:	The	Braking	Phase	

o At	the	periapsis	of	the	Descent	Orbit,	the	Descent	Propulsion	System	(DPS)	

would	be	ignited	to	put	the	spacecraft	on	a	trajectory	to	both	reduce	altitude	and	

velocity,	bring	it	to	an	altitude	and	location	from	which	the	final	landing	

approach	and	touchdown	would	be	made	

o This	maneuver	was	called	“Powered	Descent”	

§ It	lasted	8-9	minutes	and	took	the	vehicle	from	≈50,000	ft	and	≈5,500	ft	s-1	horizontal	

velocity	at	Powered	Descent	Insertion	(PDI)	to	a	point	≈7,000	ft	above	the	lunar	

surface	and		≈700	ft	s-1	vertical	velocity	(called	High	Gate)	

§ During	most	of	the	Braking	Phase,	the	LM	would	be	flying	engine	bell/crewmember	

feet	forward,	“eyeballs	up”	with	a	largely	horizontal	thrust	vector,	giving	the	crew	

limited	view	of	the	lunar	surface	

§ The	only	change	to	the	procedure	was	on	Apollo	11,	which	flew	the	early	portion	of	

the	powered	descent	“eyeballs	down”	so	the	crew	could	get	time	hacks	on	when	

particular	points	on	the	lunar	surface	flew	by	

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Apollo	Lunar	Approach	and	Landing	–	Basic	Approach	

• Phase	3:	The	Approach/Visibility	Phase	(starting	at	High	Gate)	

o Although	the	altitude	at	High	Gate	varied,	it	was	usually	between	5,000	and	8,000	ft	

AGL	

o At	this	point,	the	LM	rotates	about	the	pitch	axis	(called	the	pitchover	maneuver),	

giving	the	DPS	thrust	vector	a	significant	vertical	component	and	allowing	the	crew	

to	see	the	surface	on	which	they	would	be	landing,	and	enabling	them	to	manage	the	

touch	down	point	of	the	LM	

o Phase	4:	The	Landing	Phase	(starting	at	Low	Gate)	

o This	phase	began	at	<1,000	ft	AGL	

o At	this	point,	the	LM	is	in	a	largely	vertical	attitude,	giving	the	DPS	a	primarily	vertical	

thrust	vector	

o Changing	the	LM	attitude	in	pitch	and	roll	would	allow	the	crew	to	change	the	DPS	

thrust	vector,	thereby	changing	their	predicted	touchdown	point	while	still	

continuing	the	descent	

o In	addition,	the	engine	could	be	throttled	up	or	down,	allowing	the	crew	to	manage	

descent	rate	

• This	basic	approach	was	used	for	each	of	the	lunar	landings	

o The	details	of	these	phases	were	worked	out	by	a	group	in	the	Mission	Planning	and	

Analysis	Division	(MPAD)	under	Floyd	V.	Bennett	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	19	of	154	
Apollo	Lunar	Approach	and	Landing	–	Basic	Approach*	

Figure	8.3	scanned	from	David	Mindell,	Digital	Apollo.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	20	of	154	
Apollo	Lunar	Approach	and	Landing	–	Basic	Approach*	

*Figure	created	by	Steve	Tellier,	retired	LPI	librarian

Apollo	15	ground	track	from	powered	descent	insertion	to	landing	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	21	of	154	
Primary	LM	Systems
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• The	LM	was	a	classical	system-of-systems,	each	system	having	a	different	

purpose,	but	each	dependent	on	interaction	with	other	systems	to	carry	out	

the	aggregate	task	of	landing	on	the	lunar	surface	

• The	key	LM	systems	discussed	here	will	be	as	follows:	

o The	LM	Descent	Propulsion	System	(DPS)	

o The	LM	Reaction	Control	System	(RCS)	

o The	Landing	Radar	(LR)	

o The	Primary	and	Abort	Guidance	Systems	(PNGCS	(“Pings”)	and	AGS)		

o The	Lunar	Module	Guidance	Computer	(LGC),	including	the	input	hardware	and	

software	

o The	cockpit	controls	

o The	LM	Commander’s	window	

o The	Commander	(CDR)	and	the	Lunar	Module	Pilot	(LMP)	

• These	elements	formed	a	tightly	knit	interactive	system	that	allowed	us	to	

safely	land	on	the	lunar	surface	six	times,	and	bring	the	crew	home	safely	

each	time	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	23	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems*	

*Note:	Figure	scanned	from	Don	Eyles,	Sunburst	and	Luminary,	pg.	71.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	24	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• LM	Descent	Propulsion	System	(DPS)	

o The	DPS	was	a	throttleable	hypergolic	engine	with	an	in-space	restart	capability	

§ Isp –	311	seconds	

§ Thrust	in	vacuum	–	10,125	lbf	

§ Engine	dry	weight	–	394	pounds	

§ Fuel/Oxidizer	–	Aerozine	50/Nitrogen	tetroxide	

§ Throttle	capability	–	10%	-	60%,	and	≈90%	-	100%	

- Engine	nozzle	erosion	issues	limited	continuous	operation	between		≈60%	and	

≈90%	

§ Pressure	fed	with	supercritical	He	

o The	DPS	was	the	first	throttleable	rocket	engine	developed,	and	it	was	an	

absolute	necessity	for	landing	on	the	lunar	surface	

o Depending	on	the	abort	mode,	DPS	could	be	also	be	used	for	the	initial	stages	of	a	

aborted		landing	to	get	back	to	orbit	

o Engine	bell	extension	modification	on	the	Block	II	LM	allowed	an	increase	in	

engine	performance	and	increased	down	mass	for	the	Apollo	J-missions	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	25	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• LM	Reaction	Control	System	(RCS)	

o The	LM	RCS	was	a	series	of	16	thrusters	mounted	at	four	corners	of	the	Ascent	Stage	that	

allowed	rotation	of	the	vehicle,	initiated	the	pre-Powered	Descent	ullage	burn	and	were	part	of	

the	contingency	Ascent	Stage	impulsive	thrust	in	the	event	of	an	under-performing	ascent	engine	

§ This	was	a	hot-gas	system,	so	LM	structure	had	to	be	protected	from	RCS	burn-through	by	

providing	plume	deflectors	and	limiting	firing	durations	

o The	RCS	was	a	hypergolic	system	capable	of	multiple	firings	throughout	the	descent,	landing,	

ascent	and	CSM	rendezvous	

§ Fuel/Oxidizer	–	Aerozine	50/Nitrogen	tetroxide	

§ Pressure	fed	with	gaseous	He	

§ The	RCS	had	two	independent	control	and	plumbing	systems	to	provide	redundancy	

§ Typical	performance	during	a	mission	was	>1,000	seconds	operating	time	and	>10,000	firings	

o The	RCS	had	three	firing	modes:		

§ Direct	crew	firing	using	the	translational	(TTCA)	and	rotational	(ACA)	hand	controllers,	with	no	

guidance	computer	input	–	essentially	“hand	flying”	

§ Guidance	computer-modulated	firing	in	response	to	crew	a	single	“pulse”	of	the	rotation	hand	

controller	during	the	Approach	and	Landing	Phases	

§ Independent	guidance	computer	firing	to	manage	dynamic	disturbances	to	the	LM	(primarily	fuel	

sloshing)	during	the	Approach	and	Landing	Phases	

o In	addition	to	the	firing	modes,	the	crew	could	select	two	dead	band	modes	(±5°,	±20°)	that	

would	define	how	far	out	of	a	selected	attitude	the	LM	could	rotate	before	the	system	would	

automatically	fire	to	adjust	back	to	the	original	attitude	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	26	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• LM	Landing	Radar	

o The	LM	Landing	Radar	was	a	multi-beam	radar	system	that	provided	slant	range	distance	to	the	

landing	site	using	a	3-beam	Doppler	radar	system,	and	altitude	using	a	single	beam	vertical	radar	

o Radar	data	was	provided	to	the	LM	guidance	computer	to	provide	near-real	time	altitude	(H),	

altitude	rate	(H-dot),	and	range	to	touch	down	

§ The	derived	H	and	H-dot	was	critical	throughout	the	landing	approach	

- LGC	used	a	“spherical	Moon	model”	that	couldn’t	take	into	account	detailed	topography,	

meaning	errors	in	calculated	H	and	H-dot	could	be	significant	if	not	corrected	by	Landing	

Radar	data	

- In	addition,	the	lunar	surface	did	not	provide	good	visual	cues	that	allow	pilots	to	judge	the	

height	above	the	touchdown	point	and	the	range/range	rate	to	the	touchdown	point,	so	radar	

data	was	essential	to	judging	the	landing	

• A	cratered	and	ridged	surface	looks	much	the	same	from	10’000	ft	as	it	does	from	100	

ft,	making	the	normal	altitude	cues	pilots	use	during	terrestrial	flight	ops	invalid	

- However,	if	H	was	<1,000	ft		and	a	radar	failure	occurred,	and	flight	rules	did	allow	the	crew	

to	choose	to	land	without	radar	data	if	there	was	good	PGNCS	and	AGS	data	

o Consequently,	lack	of	Landing	Radar	data,	or	a	significant	lack	of	confidence	in	that	data,	triggered	

a	mandatory	abort	for	all	the	Apollo	landings	

§ Before	the	radar	acquired	the	surface,	two	lights	on	the	the	computer	interface	(the	DSKY)	–	the	

Altitude	and	Velocity	Lights	–	remained	illuminated,	meaning	radar	data	was	not	useable	

§ Once	good	data	was	coming	in,	these	lights	went	out,	which	was	a	key	call	during	Powered	Descent	

§ Once	the	Altitude/Velocity	lights	were	out,	the	crew		and	the	ground	had	to	assess	the	data	using	a	

routine	within	landing	program	P63	called	“Delta-H”,	and	manually	tell	the	LGC,	“This	data	is	good,	

start	using	it.”	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	27	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• LM	Landing	Radar	

o The	radar	was	located	on	the	aft	port	underside	of	the	descent	stage,	allowing	

the	antennas	to	see	the	surface	throughout	the	landing	approach	

o The	attitude	of	the	LM	during	Powered	Descent	and	some	of	the	yaw	maneuvers	

during	that	phase	were	designed	to	give	the	radar	a	good	view	of	the	surface	

o System	weight	–	42	pounds	

o System	power	consumption	-	≈130	W	

o Altitude	precision	

§ >2,500’	-	±20’	

§ <2,500’	-	±4’	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	28	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• Primary	Guidance	and	Navigation	Control	System	(PGNCS,	or	“Pings”)	

o To	quote	Frank	O’Brien,	“Alone	by	itself,	the	computer	is	blind	to	the	world	around	it,	

lacking	the	most	basic	information	on	orientation	and	the	accelerations	the	

spacecraft	undergoes	(from,	“The	Apollo	Guidance	Computer”)”	

o The	PNGCS	consisted	of:	

§ 	An	inertial	measuring	unit	(IMU)	(AKA,	the	platform)	with	a	3-axis	gyroscope	to	establish	a	

stable	platform,	and	accelerometers	in	each	axis	to	measure	vehicle	acceleration	during	

dynamic	maneuvers	

§ An	optical	observation	system,	connected	to	the	LGC,	that	allowed	the	crew	to	determine	

the	LM	attitude	with	reference	to	a	number	of	guide	stars	or	lunar	landmarks	

- Although	the	optical	interfaces	on	the	LM	were	different	the	CSM,	the	PNGCS	in	both	

vehicles	were	identical	

§ A	mechanical	interface	that	connected	the	PGNCS	rigidly	to	the	LM	structure	

§ Electronic	interfaces	that	allowed	Landing	Radar	data	to	be	fed	directly	into	the	PGNCS	

o These	elements	allowed	the	LGC	to	know	where	it	was	in	inertial	space,	how	it	was	

moving,	and	allowed	calculation	of	the	appropriate	DPS/RCS	burns	for	landing	on	the	

lunar	surface,	initiating	a	nominal	return	to	the	CSM,	or	aborting	back	to	the	CSM	

o Several	of	the	key,	mandatory	abort	flight	rules	during	LM	descent	and	landing	

covered	failures	of	the	PNGCS,	or	failure	of	the	PGNCS	to	agree	with	input	data	from	

the	Landing	Radar	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	29	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• Abort	Guidance	System,	or	AGS	

o The	AGS	was	designed	to	be	able	to	provide	backup	navigational	data	to	the	LGC	in	

the	event	of	a	total	PGNCS	failure	during	descent	and	landing		

§ The	name	is	a	little	confusing	in	that	the	“abort”	referred	to	is	NOT	a	landing	abort,	it’s	a	

failure	of	the	PNGCS	

- An	abort	of	a	landing	because	of	a	DPS	failure,	with	no	concomitant	failure	of	the	

PGNCS,	would	still	be	flown	with	PGNCS	data	

- However,	an	abort	initiated	because	of	a	total	PGNCS	failure	would	be	flown	on	the	

AGS	

o The	AGS	used	a	strapdown	IMU,	as	opposed	to	the	gimbaled	system	in	PGNCS,	and	

Landing	Radar	data	was	not	fed	into	the	AGS	

§ It	had	a	separate	computer	(the	abort	electronics	assembly	(AEA)	and	display	unit/

keyboard	(the	Data	Entry	and	Display	Assembly,	or	DEDA))	mounted	on	the	LMP	side	of	

the	cockpit	

§ It	also	had	a	separate	system	of	software	commands	that	had	to	be	entered	by	the	LMP	at	a	

number	of	times	during	the	LM	descent	and	landing	

o Although	the	AGS	had	a	lower	degree	of	precision,	it	was	continually	used	to	check	

the	health	and	quality	of	PNGCS	data	

§ The	call	by	the	LMP,	“Pings	and	Ags	agree”	was	often	heard	during	lunar	descent	

o The	crews	came	to	trust	the	AGS	as	much	as	the	PGNCS,	particularly	for	it’s	ability	to	

keep	up	with	the	LM	as	it	flew	the	approach	

§ In	various	debriefs,	Gene	Cernan	commented	that	if	they’d	lost	the	PNGCS	after	High	Gate,	

he	felt	the	AGS	provided	good	enough	data	to	be	able	to	land	with	only	AGS	data	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	30	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• The	Lunar	Module	Guidance	Computer	was	a	early	digital	computer	that	

provided	computational	capability	and	guidance,	navigation	and	control	

interfaces	to	the	LM	systems	during	descent,	landing	and	ascent,	acting	as	a	

digital	autopilot	

o The	LGC	was	designed	at	the	MIT	Instrumentation	Laboratory	under	Charles	Stark	

Draper,	with	hardware	design	led	by	Eldon	C.	Hall		

o Early	architectural	work	came	from	J.	Hal	Laning	Jr.,	Albert	Hopkins,	Richard	Battin,	

Ramon	Alonso,	Hugh	Blair-Smith	and	Herb	Taylor	

o Programming	for	the	LM	landing	was	largely	done	by	Don	Eyles	of	Draper	Labs	

o The	flight	hardware	was	fabricated	by	Raytheon,	weighed	≈70	lbs.	and	used	only	55	

W	of	power	

• Most	of	the	LM	software	was	stored	in	Read-Only-Memory	created	by	weaving	

copper	wire	around	or	through	magnetic	cores	(called	a	“core-rope	memory”)	

o This		software	provided	all	of	the	instructions	for	the	LM	to	execute	the	Braking,	

Approach,	Landing	and	Ascent	Phases	of	the	LM	mission	

o Although	there	was	a	limited	amount	of	what	we	think	of	today	as	RAM,	the	LM	

largely	operated	on	preset,	ROM-loaded	routines	that	were	specific	to	a	particular	

landing	mission	–	in	effect,	Apollo	14	could	only	go	to	the	planned	Apollo	14	landing	

site	

• Interface	with	the	LGC	by	the	crew	was	through	the	Display/Keyboard	(DSKY)

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	31	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• The	DSKY	provided	basic	interface	between	

the	crew	and	the	computer,	allowing	the	

crew	to	initiate	programs,	display	critical	

vehicle	navigational	data	and	get	information	

about	computer	issues,	such	as	error	codes	

• The	interface	consisted	of	two	lines	that	

displayed	the	main	program	running	at	the	

time	(there	were	often	several	programs	

running	simultaneously,	although	only	one	

was	displayed	in	the	DSKY)	and	relevant	

verb/noun	combinations,	three	lines	of	data	

display	(Register	1,	2	and	3)	

• In	addition	to	the	navigation	data	displayed	

in	the	upper	right,	there	was	a	computer	

status	display	on	the	upper	left	and	an	input	

keyboard	below	with	a	“hand-calculatortype”	interface	and	specific	action	buttons	for	

uploading	and	initiating	programs,	and	

entering	appropriate	noun-verb	

combinations	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	32	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	33	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• LGC	programs	

o The	nominal	and	contingency	actions	of	the	LM	during	flight	were	controlled	by	a	

series	of	programs	that	executed	specific	actions	for	the	LM	to	either	land	on	the	

Moon,	conduct	a	nominal	ascent	back	to	the	CSM	or	abort	back	to	the	CSM	

o The	specific	programs	that	executed	landing	phases	were	the	P60	programs:	

§ P63	–	Braking	Phase	

§ P64	–	Approach	Phase,	High	Gate	to	Low	Gate	

§ P65,	P66	and	P67	–	Landing	Phase,	Low	Gate	to	Touchdown	

§ P68	–	Landing	Confirmation	

o For	Apollos	9	through	12,	there	were	separate	programs	for	different	landing	

methods,	including:		

§ P65	which	was	a	completely	automatic	landing		

§ P66,	which	put	some	of	the	key	landing	elements	under	crew	control	

§ P67	which	was	a	full	manual	landing,	specifically,	disconnecting	the	LGC	from	the	

“action	chain”,	with	the	CDR	subsequently	directly	flying	the	LM,	controlling	DPS	

thrust	and	RCS	firings	using	the	translational	(TTCA)	and	rotational	(ACA)	hand	

controllers	

§ However,	it	was	discovered	that	entry	into	P65	or	P67	would	not	allow	the	crew	to	

switch	back	to	intermediate	landing	modes	between	full	auto	and	full	manual	

§ After	Apollo	12,	different	modes	of	crew	control	were	compiled	into	P66	as	separate	

“execution	activities”	that	could	be	initiated	during	the	Landing	Phase	

• One	of	the	puzzles	in	doing	this	study	was	interpreting	what	a	CDR	actually	

meant	when	he	said,	“I	took	it	out	of	AUTO	and	landed	it	manually...”	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	34	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• LGC	verbs	and	nouns	

o Verbs	were	specific	actions	that	could	tell	the	computer	to	display	and/or	

update	data	or	carry	out	a	specific	action,	expressed	as	a	2-digit	code,	e.g.,	

VERB	57,	which	permitted	landing	radar	updates;	VERB	16	was	to	display	

data;	VERB	25	was	to	update	data

o Nouns	specified	the	data	that	would	be	used	by	the	computer	to	complete	a	

specific	action,	such	as	Noun	95,	which	displayed	data	on	a	completed	burn	on	

the	DKSY	to	compare	to	the	pre-burn	planning	numbers

o A	DSKY	Sequence	of	events	could	look	like	this	(e.g.,	initiating	Braking	Phase):	

§ VERB	37	ENTER	(V37	means	“change	program”)	

§ 63	ENTER	(means	“execute	Braking	Phase	Program	63”)		

§ Once	the	computer	was	ready	(and	there	were	numerous	steps	in	P63	before	this	

happened),	it	displayed	a	flashing	VERB	99	in	the	Verb-Noun	register	–	this	was	

saying	“Enable	engine	firing”	and	it	was	flashed	at	Tig	=	–	5	seconds	

§ At	Tig	=	0,	the	crew	would	hit	the	PROCEED	(PRO)	button,	telling	the	LGC	to	initiate	

the	burn	

§ On	a	checklist	this	would	be	shown	as:	

V37	ENT	63	ENT	V99	N61	PRO	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	35	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

o The	DSKY	display	on	the	right	shows:	

§ Program	P63	entered	

§ VERB	06,	which	prompts	the	LGC	to	display	

specific	P63	data	in	digital	form	on	the	

DSKY;	

§ NOUN	61,	which	specifies	which	data	to	

display	on	the	lower	3	data	displays	(from	

top	to	bottom,	Registers	1,	2	and	3),	

specifically	

- TGO	in	P63,	9	mins	49	secs	to	completion	

(this	would	remain	at	-09	49	until	TIG

reached	0	00)	

- TIG	for	P63,	Ignition	in	22	mins	45	secs	

- Crossrange	to	the	landing	site	in	tenths	

of	km	

Simulated	DSKY	Data	scanned	

from	Don	Eyles,	Sunburst	and	

Luminary,	pg.	143.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	36	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• LM	Cockpit	Controls	

o The	LM	cockpit	had	two	primary	controls	that	were	provided	for	both	the	CDR	and	

the	LMP	

§ To	the	left	of	each	crewmember	was	a	T-handle	called	the	Thrust/Translation	

Controller	Assembly	(TTCA)	

- During	on-orbit	maneuvering,	the	TTCA	was	configured	to	allow	the	LM	to	translate	

using	the	RCS	jets		

- During	the	lunar	landing,	the	TTCA	was	configured	to	serve	as	the	throttle	control	for	

DPS,	with	hard	stops	at	10%	and	92.5%	thrust	and	a	soft	stop	at	62.5%	thrust	

- In	addition	on	the	CDR	side,	there	was	a	toggle	switch	just	to	the	right	of	the	TTCA,	

called	the	Rate	of	Descent	(ROD)	switch,	that	could	incrementally	control	the	rate	of	

descent	of	the	LM	during	the	Landing	Phase	

§ To	the	right	of	each	crewmember	was	a	stick-type	control	called	the	Attitude	

Controller	Assembly	(ACA)	

- The	ACA	had	two	critical	roles	–	first,	control	the	LM	attitude	during	flight,	and	

second,	provide	direction	to	the	LGC	for	landing	site	redesignation	during	landing	

- During	RCS	firing,	the	ACA	worked	on	in	two	modes	–	a	quick	pulse	told	the	LGC	to	

fire	the	RCS	in	a	single	pulse,	while	a	rotating	the	ACA	to	its	hard	stop	disconnected	

the	LGC	inputs	and	fired	the	RCS	as	long	as	the	ACA	was	held	in	position	

- During	the	landing,	rotating	the	LM	attitude	pointed	the	DPS	engine	directly,	giving	it	

a	lateral	thrust	vector	and	allowing	the	LM	to	move	both	forward	and	backward	or	

left	and	right	

o Both	the	ACA	and	ROD	switch	could	be	used	to	provide	the	LGC	with	commands	to	

translate	the	LM	to	an	appropriate	landing	spot	and/or	modify	the	rate	of	descent	

based	on	a	real-time	assessment	of	the	approach	

Eppler/Apollo_Lunar_Landing_Experience_Report_20021_R3.pptx/Slide	37	of	153	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• The	Commander’s	window	–	Landing	Point	

Designator	(LPD)	

o The	left	hand	window	of	the	LM	was	scribed	on	

both	the	primary	and	redundant	pressure	panes	

with	a	vertical	and	horizontal	scale	that	defined	

degrees	down	and	degrees	left	and	right	

§ With	the	angle	data	scribed	on	both	panes,	it	

allowed	the	window	to	act	as	a	nomograph	–	when	

the	CDR	lined	up	a	particular	angle	on	both	panes,	

the	point	he	was	looking	at	on	the	surface	would	

be	the	spot	the	LM	was	heading	toward	

§ During	the	Approach	Phase,	the	LGC	would	

provide	an	numerical	value,	called	the	LPD	angle,	

based	on	the	LM	instantaneous	state	vector,	pitch	

attitude	and	subsequent	propagated	trajectory,	

which	the	CDR	lined	up	on	to	see	the	surface	

location	that	the	LM	was	headed	for	

§ In	this	way,	the	window	would	act	like	Precision	

Approach	Path	Indicator	(PAPI)	lights	on	a	runway	

o The	LMP	was	responsible	for	calling	those	“look	

down	numbers”	out	to	the	CDR	so	he	could	decide	

on	the	suitability	of	the	targeted	landing	spot,	and	

make	real-time	decisions	on	whether	to	retargeted	the	LM	to	a	more	suitable	landing	spot	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	38	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• The	Crew	

o OK,	the	guys	on	board	are	obviously	not	hardware,	but	the	Commander	

(CDR)	and	Lunar	Module	Pilot	(LMP)	were	a	critical	part	of	the	LM	

system	and	performed	roles	that,	in	concert	with	the	hardware,	effected	

6	successful	landings	on	the	Moon	

o The	Commander	

§ The	CDR	was	Pilot-in-Command	in	the	purest	14	CFR	Part	91.3	sense:	the	

primary	decision	maker	for	flight	safety,	for	selection	of	the	specific	landing	

spot	and	for	decisions	related	to	aborting	or	continuing	a	particular	lunar	

descent	

§ There	were	obvious	flight	rules	that	governed	specific	aborts	based	on	

hardware	issues	or	failures,	and	clearly	the	flight	controllers,	represented	

by	the	Flight	Director,	had	significant	input	to	flight	decision-making,	but...

- During	the	Approach	and	Landing	Phases	(<4	mins	total	duration),	the	data	the	

ground	was	receiving	was	11⁄4		seconds	late,	and	subsequent	ground	calls	would	

take	another	11⁄4		seconds	to	get	back	to	the	crew,	making	real-time	decision	

making	by	the	CDR	a	critical	part	of	landing	

§ Accordingly,	throughout	the	final	approach	and	landing	phase,	a	successful	

landing	or	abort	was	entirely	the	responsibility	of	the	CDR	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	39	of	154	
Apollo	Lunar	Approach	and	Landing	–	LM	Systems	

• The	Crew	(continued...)	

o The	Lunar	Module	Pilot	

§ The	LMP	was	a	true	co-pilot,	and	acted	in	the	same	role	that	co-pilots	act	on	airline	and	

military	flights:		

- Provide	data	to	the	pilot-in-command	on	the	projected	landing	spot,	including	LPD	angles,	

rates	of	descent,	lateral	velocity	and	fuel	remaining	

- Monitor	spacecraft	systems	and	generally	provide	the	data	that	the	CDR	would	need	for	

landing	decisions	so	he	could	keep	his	eyes	outside	the	cockpit	on	the	landing	site	

§ During	the	Powered	Descent	phase,	the	LMP	was	generally	responsible	for	inputs	to	the	

LGC	through	the	Display/Keyboard	(DSKY)	

- Some	CDRs	preferred	to	be	the	guy	that	did	the	inputs	to	the	DSKY		

- In	this	case,	the	LMP	was	the	person	that	did	acted	as	safety	backup,	confirming	the	

inputs	before	executing	a	particular	guidance	program	

§ In	any	event,	the	LMP	was	usually	the	crew	expert	on	the	LGC	and	the	Landing	Radar,	and	

understood	how	to	make	the	systems	work	together		

• In	summary,	the	LM	was	a	vehicle	that	required	two	pilots,	and	while	in	theory	it	

could	be	landed	automatically,	every	approach	and	touch	down	had	significant	

crew	input	during	the	Approach	and	Landing	Phases	

o In	particular,	on	every	mission	(not	just	Apollo	11),	the	initial	landing	point	targeted	by	the	

automatic	system	was,	in	the	CDRs	assessment,	unsatisfactory		

o While	the	CDR	was	never	flying	the	LM	in	a	“direct	mode”	(more	on	that	later),	every	CDR	

needed	to	select	a	better,	alternate	landing	spot	and	drive	the	P66	activity	through	to	

landing	and	engine	stop	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	40	of	154	
How	We	Did	It	–		

Aborts	and	Abort	Modes
Aborts	and	Abort	Modes	–	How,	When,	Why	

• Any	discussion	of	landing	aborts	and	abort	modes	should	be	informed	by	the	

crew’s	view	of	aborts:	

o Neil	Armstrong:	“Well,	I	think	--	in	simulations	we	have	a	large	number	of	failures	and	

we	are	usually	spring-loaded	to	the	abort	position	and	in	this	case,	in	the	real	flight,	we	

are	spring-loaded	to	the	land	position.	We	were	certainly	going	to	continue	with	the	

descent	as	long	as	we	could	safely	do	so...our	procedure	throughout	the	preparation	

phase	was	to	always	try	to	keep	going	as	long	as	we	could	...	(Apollo	11	Post-flight	

Press	Conference	at	MSC,	12	August	1969)”	

o Gene	Cernan	:	"We	used	to	play	around	in	the	simulator	to	see	if	the	AGS	gave	us	

enough	information	to	land;	and,	quite	honestly,	if	we	were	close	I	think	we	could	have	

landed.	There	was	a	mission	rule,	as	I	remember,	that	we	wouldn't	land	if	we	had	a	

primary	guidance	failure.	But	one	thing	that	people	never	paid	much	attention	to	is	

that,	for	most	of	the	flight	-	and	this	is	one	very	visible	point	-	the	ground	has	no	control	

over	what	you're	doing...you're	in	command,	you're	flying	the	machine,	and	you're	

making	the	decisions.	So	who	is	to	know	what	decision	would	have	been	made	if	you	

were	down	around	three	or	four	hundred	feet	or	six	hundred	feet	or	a	thousand	feet	and	

you	had	a	question	about	your	primary	navigation	system	but	your	abort	navigation	

system	looked	good...we	didn't	want	to	go	that	far	and	not	land	because	of	a	computer	

problem.”	(ALSJ,	Apollo	17,	Debrief	comments	at	MET	112:50:39)	

• In	short,	while	landing	aborts	are	often	defined	by	hard	rules,	the	actual	in	flight	

application	of	those	rules	would	have	been	informed	by	the	crew’s	interpretation	

of	what	was	reasonable	at	a	particular	moment	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	42	of	154	
Aborts–	General	Conditions,	Responsibilities	and	Modes	

• Abort	Conditions/Responsibilities	–	General	

o MSC	Houston	(MCC-H)	was	responsible	for	detecting	“slow,	insidious	drift	

malfunctions”	that	might	contribute	to	aborts	

§ This	required	as	much	high	rate	data	between	the	LM	and	the	ground	as	possible,	as	

downlinked	telemetry	gave	the	flight	controllers	the	information	to	detect	this	

malfunctions	

o The	crew	was	responsible	for	detecting	errors	that	require	immediate	action,	largely	

because	communications	cycle	time	for	an	immediate	abort	would	be	as	long	as	20	

seconds	

o The	abort	modes	discussed	below	cover	general	conditions	–	there	are	pages	of	

detailed	abort	conditions	and	actions	that	are	summarized	in	the	backup	slides	

• Abort	modes	

o The	LM	had	two	abort	modes	Abort	and	Abort/Stage,	based	on	the	conditions	causing	

the	abort	and	the	time	during	the	landing	when	abort	was	initiated	

o An	abort	that	used	the	ascent	and	descent	stages	–	i.e.,	aborts	initiated	when	the	

failure	did	not	involve	significant	degradation	of	the	DPS	–	was	referred	to	as	an	

Abort	

§ In	this	case,	the	DPS	was	used	to	start	the	LM	stack	(both	Ascent	and	Descent	stages)	back	

uphill	toward	into	an	orbit	where	the	LM	could	rendezvous	with	the	CSM	

§ The	DPS	continued	firing	until	fuel	was	exhausted,	at	which	point	the	Descent	Stage	was	

jettisoned	and	the	rendezvous	was	continued	using	the	Ascent	Propulsion	System	

§ This	abort	required	pitching	the	LM	to	reorient	the	thrust	vector	of	the	DPS	such	that	the	

climbout	began	as	soon	as	possible	after	the	abort	condition	was	detected	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	43	of	154	
Aborts–	General	Conditions,	Responsibilities	and	Modes	

• Abort	modes	(cont.)	

o An	“Abort/Stage”	abort	was	initiated	when	a	failure	of	the	DPS	occurred	at	any	time	during	the	

landing	sequence	

o This	involved	jettisoning	the	Descent	Stage	and	initiating	an	immediate	burn	on	the	Ascent	

Propulsion	System	

§ Abort/Stage	meant	that	explosive	devices	severed	the	electrical	and	data	connections	to	the	Descent	

Stage	and	conducted	an	immediate	“fire	in	the	hole”	burn	with	the	Ascent	Propulsion	System	

o It	was	assumed	that	the	“decision	cycle”	to	initiate	an	Abort/Stage	was	≥4	sec	

• Abort/Stage	abort	boundaries	

o During	the	Approach	and	Landing	Phases	of	the	flight,	there	were	critical	abort	boundary	

conditions	that	drove	whether	the	LM	could,	in	fact,	successfully	do	an	Abort/Stage	

o These	boundary	curves	were	similar	to	the	altitude-airspeed	curve	that	defines	whether	a	

helicopter	can	successfully	auto-rotate	in	the	event	of	a	complete	engine	failure	

o A	Bellcomm	memo	(ref	2.)	defined	the	following	formula:	

Minimum	safe	altitude	=	|80	ft	+	10*H-dot|	

o What	this	meant	is	that	if	the	[80+10*H-dot]	value	was	less	than	the	altitude	of	the	LM	at	that	time,	

there	was	insufficient	time	and	height	above	the	lunar	surface	to	execute	an	Abort/Stage	

o While	the	CDRs	did	a	good	job	of	staying	above	that	altitude	boundary,	you	can	do	the	math	and	

see	that	below	≈100	ft	with	an	H-dot	>2	ft	s-1,	the	LM	was	committed	to	a	landing	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	44	of	154	
Specific	Abort	Conditions	

• Abort	or	Abort/Stage	decision	would	be	made	when	malfunctions	precluded	a	

safe	descent	and	landing,	or	inability	to	abort	later	in	the	landing	sequence	

o Abort	(not	Abort/Stage)	procedures	were	designed	to	place	the	LM	in	an	orbit	

with	a	periapsis	>30,000’	and	in	a	position	to	rendezvous	with	the	CSM	on	the	

Ascent	Propulsion	System	

• Overall	abort	decision	points	

o General	

§ Loss	of	S-band	high	rate	data	or	voice	communications	

§ CSM	malfunctions	that	may	affect	rescue	capability	

o Propulsion	systems	

§ Zero	or	low	DPS	thrust	

- Failure	of	the	DPS	to	start	automatically	after	P63	PRO	and	after	an	

attempted	manual	start	by	the	CDR	

- DPS	entry	into	a	degraded	condition,	abort	to	be	initiated	after	attempted	

CDR	manual	start	using	the	Descent	Engine	Start	switch	

§ Failure	of	the	DPS	to	throttle	

§ Impending	DPS	propellant	depletion	

§ Loss	of	APS	redundant	start	capability	

§ Uncontrollable	DPS	gimbal	failure	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	45	of	154	
Specific	Abort	Conditions	(cont.)	

• Overall	abort	decision	points	(cont.)	

o Guidance	and	Navigation	

§ PGNCS	malfunction	

§ AGS	malfunction	prior	to	High	Gate	(P63-P64	transition	at	Pitchover)	

§ Lack	of	Landing	Radar	(LR)	data	

§ Incompatible	difference	between	PGNCS	and	LR	data	

o Trajectory	constraints	

§ Violation	of	specific	abort	limits	seen	on	MCC-H	displays,	data	not	available	to	

the	LM	crew;	in	this	case,	the	Flight	Director	would	call	the	abort	

o Landing	Site	Condition	

§ Unsatisfactory	for	safe	landing	

§ Redesignation	unreasonable	given	LM	trajectory	and	fuel	state	

• If	you	look	at	the	history	of	various	landings,	clearly	some	of	these	abort	rules	

were	subject	to	some	interpretation	

o For	instance,	the	Apollo	11	landing	had	very	ratty	comms	throughout	the	Braking	

Phase,	requiring	several	yaw	maneuvers	to	put	the	LM	high	gain	antenna	in	a	more	

favorable	position	

§ If	the	abort	rule	on	communications	had	been	strictly	enforced,	Apollo	11	might	have	had	

to	abort	very	early	in	the	Powered	Descent	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	46	of	154	
How	We	Did	It	–		

How	Vehicle	Roles	Changed	as	Apollo	Evolved
Lunar	Descent	and	Landing	–	Changing	Vehicle	Roles	for	

Descent	Orbit	Insertion	

• Remember	back	to	our	four	phases	of	a	landing,	Phase	1	is	Descent	Orbit	

Insertion,	which	put	the	LM	in	an	orbit	from	which	to	start	down	to	the	lunar	

surface	

• Descent	Orbit	Insertion	(DOI)	Burn		

o On	Apollos	11	and	12,	the	DPS	initiated	the	burn	after	separation	from	the	CSM	

on	the	lunar	far	side,	out	of	contact	with	the	ground		

§ This	burn	was	done	after	P52	guidance	system	alignment	by	the	LM,	and	getting	a	

state	vector	uplink	to	the	LM	prior	to	Loss	of	Signal	(LOS)	as	the	LM	went	behind	the	

Moon	

§ This	was	clearly	a	very	busy	evolution	

o The	DOI	burn	was	≈30	seconds	long,	with	a	∆V	of	<100	ft	s-1	performed	by	the	

DPS	

§ The	use	of	the	DPS	meant	that	≈30	seconds	of	fuel	was	not	available	for	the	actual	

descent	and	landing	

o 30	seconds	doesn’t	sound	like	much,	but	literally	every	second	of	DPS	fuel	counted	

o A	good	example	of	the	importance	of	seconds	of	burn	time	is	the	removal	of	the	≈200kg	

Apollo	Lunar	Scientific	Experiments	Package	from	the	Apollo	11	manifest,	in	order	to	give	

the	LM	three	additional	seconds	of	fuel	burn	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	48	of	154	
• After	Apollo	12,	it	was	decided	that	on	future	missions,	the	DOI	burn	would	be	handled	by	the	

CSM,	leaving	the	total	DPS	∆V	budget	available	for	descent	and	landing	

o The	SPS	DOI	burn	was	≈23	seconds	long,	with	a	∆V	of	≈200	ft	s-1	

§ This	shift	of	DOI	responsibility	was	done	to	increase	hover	time	for	the	LM	over	the	increasingly	

challenging	landing	areas,	and	to	increase	the	landed	downmass	

§ By	the	time	detailed	planning	for	Apollo	13	was	undertaken,	Apollo	Program	management	had	

developed	sufficient	confidence	in	the	Service	Propulsion	System	to	transfer	the	DOI	burn	

responsibility	to	the	CSM	

• However,	there	were	a	number	of	key	post-DOI	burn	events	that	had	to	occur	before	the	LM	

could	initiate	PDI	

o The	CSM-LM	stack	had	to	be	in	a	safe	orbit	prior	to	undocking		

§ An	SPS	overburn	during	the	DOI	burn	of	only	1	second	would	leave	the	stack	in	an	orbit	that	would	

impact	the	Moon	in	a	successive	orbit	

- A	“bail	out	burn”	was	scheduled	≈35	minutes	after	DOI	to	get	the	stack	back	into	a	safe	orbit	if	

there	was	a	significant	overburn	

o The	LM	had	to	safely	separate	from	the	CSM	and	both	vehicles	had	to	have	a	clean	systems	check	

o The	CSM	had	to	successfully	initiate	a	burn	back	up	to	the	60	nm	circular	orbit	

§ This	meant	that	the	SPS	had	to	check	out	prior	to	the	burn	

§ If	the	SPS	did	not	fire	properly,	the	landing	would	have	to	be	aborted	and	the	DPS	would	take	the	redocked	stack	back	up	to	the	60	nm	orbit	

- On	Apollo	16,	the	LM	landing	was	delayed	because	of	an	issue	with	SPS	gimballing	that	took	

several	orbits	beyond	the	nominal	CSM	circularization	burn	to	be	cleared	and	allow	PDI	

Lunar	Descent	and	Landing	–	Changing	Vehicle	Roles	for	

Descent	Orbit	Insertion	(cont.)	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	49	of	154	
How	We	Did	It	–		

Landing	Site	Constraints	
Lunar	Descent	and	Landing	

Constraints	for	Landing	Site	Selection	

• During	the	later	Apollo	missions	(specifically	Apollo	15,	16	and	17),	a	great	

deal	of	appropriate	emphasis	was	made	on	selecting	sites	that	the	science	

community	felt	would	answer	lunar	science	questions	

• However,	for	all	landing	sites	under	consideration,	there	were	key	

operational	constraints	that	had	to	be	satisfied,	regardless	of	how	important	

the	science	was	

o These	constraints	ultimately	“rattled	back”	onto	launch	window	dates,	launch	

window	durations,	the	launch	inclination	the	Saturn	V	stack	and	the	duration	of	

various	mission	phases	leading	up	to	landing	(for	a	great	discussion,	see	

https://history.nasa.gov/afj/launchwindow/lw1.html)	

o In	particular,	this	meant	that	Apollo	launch	windows	were	roughly	a	month	

apart,	and	it’s	why	Apollo	8	had	to	be	orbiting	the	Moon	during	Christmas,	1968	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	51	of	154	
• Landing	Site	Lighting	

o In	order	to	safely	land,	the	crew	had	to	be	able	to	assess	the	“LM-scale”	surface	

roughness	of	the	landing	spot	prior	to	touching	down,	which	required	lighting	that	

would	enhance	surface	topography	

o It’s	really	important	to	emphasize	that	during	Apollo,	our	understanding	of	“footpadscale”	topography	was	very	poor	for	each	site	except	the	Apollo	17	site	

o This	meant	that	the	landing	site	had	to	be	lit	from	behind	the	LM	during	it’s	approach,	

with	a	sun	angle	between	5°	and	14°	above	the	horizon,	so	that	at	pitchover	the	crew	

could	see	the	topographic	features	of	the	landing	site	

o This	meant	that	the	final	approach	ground	track	azimuth	had	to	be	oriented	so	the	

Sun	was	behind	the	LM	at	pitchover	

• State	Vector	Update	and	Trajectory	Confirmation	

o The	other	critical	operational	constraint	was	that	the	LM	state	vector	had	to	updated	

prior	to	PDI	based	on	uplink	data	from	the	ground	and	position	checks	in	orbit	

o The	former,	in	particular,	could	not	be	accomplished	until	the	ground	reacquired	the	

LM	when	it	emerged	from	the	lunar	far	side	

o Once	in	contact,	it	was	necessary	for	the	ground	determine	and	uplink	the	LM	state	

vector	prior	to	PDI	–	this	took	time,	and	consequently	eliminated	landing	sites	that	

were	on	the	far	eastern	limb	of	the	Moon	

• Note	that	we	will	almost	certainly	have	the	same	constraints	for	any	future	lunar	

landing,	regardless	of	the	targeted	landing	site	

Lunar	Descent	and	Landing	

Constraints	for	Landing	Site	Selection	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	52	of	154	
How	We	Did	It	–		

	The	Nuts	and	Bolts	of	Flying	an	Actual	Landing
Lunar	Descent	and	Landing	–	How	We	Actually	Did	It	

• The	Braking	Phase	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	54	of	154	
Lunar	Descent	and	Landing	–	Braking	Phase	

• The	Braking	Phase	was	initiated	by	the	Powered	Descent	Insertion	(PDI)	burn,	

performed	by	the	DPS	≈250	n.m.	uprange	of	the	landing	site	

o PDI	was	controlled	by	the	LGC	running	the	P63	program,	which	was	called	up	by	the	crew	

≈10-15	minutes	before	DPS	ignition	

o P63	went	through	a	pre-burn	targeting	routine	which	set	the	LM	up	for	the	burn		

o This	would	be	a	V37	ENTER	63	ENTER,	followed	by	a	flashing	VERB	06	NOUN	61	

confirming	the	key	data	for	the	braking	phase:	TGO,	TIG	and	cross	range	

• PDI	was	preceded	by	a	number	of	key	navigation	checks	and	updates	that	were	

done	by	either	the	ground	over	the	Manned	Space	Flight	Network	(MSFN)	or	the	

crew	in	the	LM	using	the	optical	sighting	system	and	doing	a	Platform	Alignment	

using	the	LGC	Program	P52	

o The	first	check	was	transferring	the	CSM	platform	alignment	to	the	LM	about	an	hour	

before	undocking,	which	provided	a	coarse	platform	alignment,	and	a	subsequent	P52	

platform	optical	fine	alignment	by	the	LM	crew	using	the	LM	optical	system	after	

undocking	

§ At	this	point,	the	LMP	would	update	the	AGS	as	well,	so	both	guidance	systems	had	the	same	

state	vector	data	prior	to	initiating	the	burn	to	the	surface	

o The	second	check	was	an	LM	state	vector	update,	which	was	uploaded	into	the	LGC	by	the	

ground	through	the	MSFN	after	LM	AOS,	with	crew	authorization,	at	about	PDI	-30	mins	

§ This	ensured	that	the	LGC	had	the	latest	understanding	of	where	it	was	in	space,	and	how	fast	it	

was	going	in	which	direction	

§ The	LGC	then	updated	the	LM	state	vector	every	2	seconds,	based	on	input	from	the	PNGCS,	

which	used	the	inertial	platform	to	sense	orientation	and	acceleration	during	Powered	Descent	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	55	of	154	
Lunar	Descent	and	Landing	–	Braking	Phase	

• Pre-PDI	Checks	(cont.)	

o The	third	check	was	done	by	the	crew	using	the	LM	optical	tracking	system	to	

check	at	what	time	they	passed	a	number	of	geographic	points	on	the	lunar	

surface	

§ These	points	were	selected	prior	to	the	mission,	and	the	crew	had	a	table	of	nominal	

overflight	times	

§ If	they	overflew	these	points	early	or	late	it	would	tell	them	that	they	were	targeting	

long	or	short	on	the	landing	site	

§ For	instance,	on	Apollo	11,	at	MET	102:36:30	Armstrong	reported	that	they	had	

overflown	a	ground	target	≈3	seconds	early,	indicating	that	they	would	be	≈3	mi	

downrange	of	the	intended	landing	point	

o The	fourth	check	prior	to	engine	ignition	involved	the	LGC	running	a	

subroutine	called	“Average	g”	

§ Average	g	was	a	complicated	integration	of	a	platform	data	and	assessments	of	the	

value	of	the	lunar	gravity	field	that	was	designed	to	correct	the	initial	state	vector	

for	the	effects	of	lunar	gravity	

- When	the	LGC	began	Average	g,	the	DSKY	would	go	momentarily	blank	and	

then	reset	

§ Given	the	known	“lumpiness”	of	the	lunar	gravity	field	(blame	geology	and	isostasic	

disequilibrium),	Average	g	was	critical	to	correct	the	state	vector	prior	to	the	start	

of	powered	descent	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	56	of	154	
Lunar	Descent	and	Landing	–	Braking	Phase	

• PDI	was	initiated	by	preceded	by	a	brief	ullage	burn	(≈7.5	seconds)	done	

with	the	RCS	to	bring	the	DPS	fuel	and	oxidizer	to	the	bottom	of	the	tanks	

where	it	could	flow	into	the	engine	once	the	burn	was	initiated	

o Once	the	ullage	burn	initiated,	at	PDI	TIG=-5	sec,	the	LGC	would	flash	VERB	99	

NOUN	62	on	the	DSKY,	essentially	saying,	“I’m	ready	to	go	with	P63	when	you	

are	(V99),	and	here’s	the	data	you’ll	need	to	monitor	(N62))”	

o The	crew	would	hit	PRO	on	the	DSKY,	which	would	tell	the	computer	to	proceed	

with	P63	and	start	PDI	

• At	PDI,	the	DPS	went	through	two	actions	

o An	≈26	second	burn	at	10%	thrust,	which	allowed	the	DPS	gimbals	to	align	the	

engine	thrust	vector	through	the	LM	center-of-gravity	(Cg)	

o At	Tig	≈+26	seconds,	the	DPS	throttled	up	to	>92.5%	thrust,	reducing	the	LM	

forward	velocity	and	starting	down	to	the	lunar	surface	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	57	of	154	
Lunar	Descent	and	Landing	–	Braking	Phase	

• At	this	point	(except	for	Apollo	11),	the	LM	is	pointed	face	up	and	engine-bellforward,	making	it	impossible	for	the	crew	to	pick	up	subsequent	check	points	

o During	Apollo	11,	the	LM	was	face	down	for	post-PDI	geographic	position	checks	

o At	around	3	minutes	into	Powered	Descent,	Eagle	was	yawed	to	face	up,	engine	bell	

forward	

o After	that	yaw	maneuver,	Armstrong	and	Aldrin	were	in	the	same	attitude	as	

subsequent	landings	-	flying	blind,	face	up	–	and	they	had	to	rely	on	the	LGC	and	

ground	assessments	of	their	trajectory	to	know	if	the	landing	evolution	was	

proceeding	in	a	nominal	fashion	

o The	LM	would	start	Powered	Descent	in	an	attitude	that	was	pitched	up	≈100°,	and	as	

the	Braking	Phase	continued,	the	pitch	would	gradually	decrease	to	<60°	just	prior	to	

transition	to	the	Approach	Phase	

§ During	Powered	Descent,	the	LMP	often	gives	the	CDR	cues	on	“the	Ball	number”,	which	is	

a	reference	to	the	pitch	angle	he	can	see	on	the	Flight	Director	Attitude	Indicator	(FDAI)	or	

simply,	“the	Ball”

• Throughout	Powered	Descent,	the	LGC	is	flying	the	LM	by	automatically	

commanding	two	systems:	the	DPS	and	the	RCS	

o The	RCS	did	a	variety	of	short	firings	commanded	by	the	LGC	to	compensate	external	

disturbances	like	fuel	sloshing	in	the	prop	tanks	

o The	LM	simulators	at	KSC	and	JSC	did	not	simulate	this	and	on	Apollos	11	and	12,	

there	are	crew	comments	about	how	the	“RCS	is	banging	away”	during	P63	

o This	is	a	very	dynamic	phase	–	the	LM	weight	and	Cg	is	changing,	the	trajectory	is	

varying	constantly,	and	the	LGC	is	furiously	calculating	state	vectors,	altitudes	and	

range	to	the	landing	site	every	2	seconds	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	58	of	154	
Lunar	Descent	and	Landing	–	Braking	Phase	

• The	key	crew	task	here	is	to	monitor	the	progress	of	the	burn	against	the	

“theoretical”	perfect	powered	descent	(displayed	on	a	“cheat	sheet”	velcroed	to	

the	instrument	panel)	using	the	1)	H,	H-dot	and	Range-to-Landing	data	shown	on	

the	DSKY,	2)	fuel	and	oxidizer	quantities	and	3)	a	PGNCS	to	AGS	comparison,	and	

to	ensure	that	key	data	sources	like	the	Landing	Radar	provide	data	to	the	LGC	at	

the	appropriate	times	

• The	ground	would	also	monitor	the	LM	position	with	MSFN-acquired	tracking	

data	and	project	a	landing	point,	and	could	determine	potential	over-	or	undershoots	of	the	landing	site	using	a	program	called	DLAND	that	was	implemented	

after	Apollo	11	

o As	appropriate,	the	crew	could	update	the	LGC	using	a	VERB	21	NOUN	69	on	the	

DSKY	to	enter	an	uprange	or	downrange	correction	

o However,	an	incorrectly	entered	NOUN	69	entry	could	cause	significant	problems,	

so	the	the	N69	ENTER	was	only	done	when	the	crew	and	the	ground	agreed	on	the	

parameters	to	be	entered	and	confirmed	that	they	were	typed	correctly	into	the	

DSKY	prior	to	proceeding	

• In	addition,	the	LMP	would	periodically	manually	update	the	AGS	(which	did	not	

get	Landing	Radar	data)	on	a	separate	keyboard	(the	DEDA)	with	the	altitude	

readout	from	the	DSKY		

o For	example,	the	LMP	could	key	in	18,000’	into	the	DEDA	and	when	the	DSKY	showed	

an	H	value	of	18000,	he	would	hit	ENTER	on	the	AGS	keyboard,	updating	the	AGS	

data		

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	59	of	154	
Lunar	Descent	and	Landing	–	Braking	Phase	

• At	about	3	minutes	into	Powered	Descent,	the	next	critical	event	was	the	Landing	

Radar	getting	good	returns	from	the	the	surface	(“radar	lock”)	at	≈40,000	ft	

o Based	on	both	a	crew	and	ground	check	of	the	quality	of	the	radar	data,	the	crew	

would	tell	the	LGC	to	accept	that	data	and	use	it	in	the	recurring	state	vector	updates	

§ The	radar	data	“quality”	was	checked	using	a	LGC	function	called	“Delta-H”	which	

compared	the	radar	H	with	the	LGC	calculated	H	

- For	the	first	part	of	the	Braking	Phase,	H	and	H-dot	were	based	on	state	vector	data	

and	often	had	pretty	big	error	bars	

- Very	often	during	the	first	part	of	powered	descent,	you	can	often	hear	the	LMP	

calling	“H	and	H-dot	are	a	little	high...”	

- Being	a	little	high	was	OK	–	for	“Kentucky	windage”,	the	approaches	were	often	

started	high	and	H	and	H-dot	converged	as	they	got	closer	to	the	landing	site	

- A	good	Delta-H	calculation	was	essential	to	confirm	radar	data	quality	

o Good	radar	lock	was	a	big	deal	–	without	Landing	Radar	data,	the	crew	would	have	to	

abort,	as	pure	state	vector	data	alone	was	not	considered	sufficiently	precise	to	be	the	

sole	navigational	source	for	landing	

§ Gene	Cernan,	on	Apollo	17,	obliquely	referred	to	the	nominal	time	for	acquisition	of	good	

radar	data	as,	“The	Day	of	Reckoning”...

• Also	at	≈3	minutes	into	the	burn,	the	crew	do	the	“ED	Batts	Check”,	to	confirm	

the	batteries	that	fired	the	explosive	devices	in	the	event	of	an	“Abort/Stage”	

abort	were	working	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	60	of	154	
Lunar	Descent	and	Landing	–	Braking	Phase	

• In	addition,	the	ground	would	monitor	the	fuel	gauges	and	choose	the	most	

conservative	for	the	crew	to	monitor	

o There	were	two	independent	systems	for	measuring	fuel	and	oxidizer	remaining,	so	

early	on	in	powered	descent,	the	ground	would	make	a	decision	on	which	set	of	

gauges	to	watch	

§ A	switch	on	LM	Panel	1	allowed	the	crew	to	select	which	gauge	they	would	monitor	

during	approach	

o This	was	done	to	ensure	that	the	crew	was	conservative	on	fuel	during	the	approach	

and	landing	phases	

o A	typical	call	would	be	for	the	crew	to	“Monitor	Descent	Fuel	2”	

• Lastly,	the	CDR	would	briefly	change	the	PGNCS	Mode	Control	switch	from	

AUTO	to	ATT	HOLD,	which	would	allow	him	to	check	the	handling	of	the	LM	

before	going	into	the	final	landing	phase	

• At	around	7	mins,	30	seconds,	the	DPS	would	go	through	“Throttle	Recovery”	-	

throttle	down	to	≈57%,	from	the	full	power	setting	initially	used	during	

Powered	Descent,	to	avoid	the	power	settings	where	significant	DPS	throttle	

assembly	erosion	occurred	

o The	“throttle	down”	call	was	a	critical	one,	as	it	was	impossible	to	land	the	LM	at	full	

power,	or	if	the	throttle	went	too	far	down	and	wouldn’t	recover	

§ This	call	took	place	about	2	minutes	before	the	transition	from	the	Braking	Phase	to	the	

Approach	Phase	(P63-P64	transition)		

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	61	of	154	
Lunar	Descent	and	Landing	–	How	We	Actually	Did	It	

• The	Approach	Phase	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	62	of	154	
Lunar	Descent	and	Landing	–	Approach	Phase	

• As	the	crew	flew	the	Braking	Phase,	they	were	gradually	losing	altitude	and	

velocity,	headed	toward	“High	Gate”	and	Approach	Phase	transition	

o Throughout	the	last	≈8	minutes	of	Powered	Descent,	the	crew	was	face	up	

allowing	the	Landing	Radar	to	see	the	lunar	surface	and	provide	the	

appropriate	H,	H-dot	and	range	to	the	landing	site	data	to	the	LGC	

• Approach	Phase	Initiation	

o The	Approach	Phase	started	at	about	7,000’	AGL	when	LGC	program	P64	

initiated	the	pitchover	maneuver	

o This	pitched	the	LM	around	its	Y-axis	from	the	feet-forward,	face-to-the-sky	

attitude	to	one	where	the	crew	could	now	see	the	landing	site	for	the	first	time	

o Pitchover	was	a	BIG	DEAL!		It	meant	you	could	tell	how	accurately	the	LGC	had	

navigated	you	to	the	landing	site,	and	if	you	were	going	to	be	able	to	land	

o Note	this	dialogue	from	Apollo	12	at	P63-P64	transition:	

110:29:17	CAPCOM	Carr:	 Roger.	Copy	P64.	

110:29:18	Bean: Okay,	there's	6000	update.	

110:29:20	Conrad: (Very	excited)	Hey,	there	it	is!	There	it	is!	Son-of-a-Gun!	

Right	down	the	middle	of	the	road!!!	

110:29:25	Bean:	 Outstanding!	42	degrees,	Pete.	

110:29:27	Conrad: Hey,	it's	targeted	right...	

110:29:28	Conrad: 	...for	the	center	of	the	crater!	

110:29:29	Bean: (Garbled)	look	out	there.	

110:29:30	Conrad: I	can't	believe	it!	

110:29:32	Bean:	 Amazing!!	Fantastic!	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	63	of	154	
Lunar	Descent	and	Landing	–	Approach	Phase	

• Approach	Phase	Initiation	(cont.)	

o Approach	Phase	initiation	was	controlled	by	an	

automatic	switchover	keyed	to	TGO	in	P63	–	that	is,	when	

the	TGO	value	in	P63	reached	zero,	the	LGC	would	

immediately	shift	to	executing	P64	

o Once	the	LGC	was	in	P64,	it	would	show	a	DSKY	display	

similar	to	the	one	shown	on	the	right	(scan	from	Don	

Eyles,	Sunburst	and	Luminary)	

- In	the	upper	right,	“64”	indicates	that	the	LGC	is	in	P64	

- VERB	06	and	NOUN	64	indicate,	respectively,	the	displays	

are	in	decimal	data	in	the	lower	three	data	displays,	and	

that	they	are	displaying:	

- In	Register	1:	Left,	the	time	available	to	redesignate	the	

landing	point,	and	right,	the	LPD	angle	that	the	CDR	

needs	to	look	to	see	the	targeted	site	

- In	Register	2:	The	computed	H-dot	in	ft	s-1	(there	is	an	

“assumed”	decimal	point	between	the	two	digits	to	the	

right;	“1249”	is	actually	124.9	ft	s-1)

- In	Register	3:	H	from	Landing	Radar	data	

• When	the	LGC	went	into	P64,	the	Verb/Noun	line	

flashed,	meaning	the	LGC	was	asking,	“Is	this	OK”?	

o Either	the	LMP	or	CDR	would	then	hit	PRO	to	accept	the	

data	

Apollo	11	DSKY	at	the	start	of	

P64,	captured	while	the	LM	is	in	

the	process	of	pitching	over.	

Time	to	completion	of	P64:	99	s	

LPD	angle:	70°	

H-dot:	124.9	ft	s-1	

H:	7,129	ft	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	64	of	154	
Lunar	Descent	and	Landing	–	Approach	Phase	

• During	the	landing	approach,	the	CDRs	was	able	to	redesignate	the	landing	spot	using	the	

ACA	

o One	pulse	forward	or	back	moved	the	landing	spot	0.5°	

o One	pulse	left	or	right	would	move	the	landing	spot	2°	

• Dave	Scott,	for	instance,	found	that,	“We	were	south,	and	I	redesignated	immediately	four	

clicks	to	the	right	(north)	and	then	shortly	thereafter...I	redesignated	two	more	to	the	right	

and	three	uprange.	(Dave	Scott,	quoted	from	ALSJ,	crew	debrief	comments	after	MET	

104:40:28)”	

o Dave	Scott	would	ultimately	make	18	redesignations,	the	most	of	any	Apollo	landing	(quoted	in	

Mindell,	Digital	Apollo,	pg.	254,	para.	1)	

• The	teamwork	between	the	CDR,	the	LMP	and	the	LM	during	the	Approach	Phase	was	the

key	to	flying	the	LM	to	the	landing	spot	

o Unlike	the	Braking	Phase,	in	P64	the	crew	was	directly	interacting	with	the	LM	through	the	LGC	to	

decide	where	they	would	land	

o The	LGC	was	still	doing	the	nuts-and-bolts	of	controlling	the	vehicle	–	changing	thrust,	changing	LM	

attitude	–	but	the	CDR	was	deciding	if	the	spot	the	LGC	was	taking	them	to	was	good,	and	telling	the	

LM	where	to	go	if	it	was	not	

o The	data	that	allowed	the	CDR	to	make	that	decision	was	coming	out	of	the	LGC	in	the	form	of	LPD	

angles,	H	and	H-dot	values,	read	out	by	the	LMP	so	the	CDR	could	keep	his	head	out	of	the	cockpit	to	

control	the	landing	spot		

o The	LMP	had	a	cue	card	for	a	“theoretical	perfect	approach”	that	showed	fuel	burn	rates,	H	and	H-dot	

at	30	second	intervals,	enabling	him	to	keep	the	CDR	apprised	on	how	close	they	are	to	a	“perfect”	

approach	and	how	close	they	are	to	fuel	limits	and	prohibitively	large	H-dots	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	65	of	154	
Lunar	Descent	and	Landing	–	How	We	Actually	Did	It	

• The	Landing	Phase	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	66	of	154	
Lunar	Descent	and	Landing	–	Landing	Phase	

• Entry	into	the	Landing	Phase	is	(obviously)	critical,	and	it’s	probably	the	least	

“scripted”	of	any	of	the	landing	phases	

o First,	during	Apollo,	our	actual	knowledge	of	the	meter-scale	topography	of	the	landing	

site	prior	to	pitchover	was	often	very	poor	

§ On	Apollo	16,	in	particular,	the	best	images	we	had	of	the	landing	site	prior	to	landing	didn’t	

allow	us	to	see	anything	smaller	than	about	a	20	meter	object	

§ This	would	be	equivalent	to	being	on	an	actual	ILS	35L	approach	to	Ellington	and	when	you	

break	out,	you	could	find	that	Building	45	is	sitting	on	the	approach	end	of	35L	

o Consequently,	the	transition	from	the	Approach	Phase	to	Landing	Phase	(P64-P66	

transition)	was	dependent	on	the	CDR	designating	to	an	acceptable	landing	site	in	P64,	

and	getting	low	enough	to	begin	to	actively	manage	LM	attitude,	approach	path	and	

descent	rate	

• There	were	three	options	in	P66	

o Full	Auto	–	essentially	stand-back	and	let	the	LM	land	itself,	with	the	Landing	Radar	

and	the	LGC	making	all	the	decisions	about	descent	rates	and	vehicle	attitudes	

(originally	P65)	

o Attitude	Hold	–	in	this	case,	the	LGC	keeps	the	roll,	pitch	and	yaw	attitude	of	the	vehicle	

steady	and	the	CDR	manages	the	descent	rate,	although	he	can	command	a	change	in	

attitude	with	the	ACA	(originally	the	only	option	in	P66)	

o Full	Manual	landing	–	in	this	case,	the	LGC	is	completely	out	of	the	loop	and	the	CDR	is	

flying	the	vehicle	directly,	using	the	TTCA	to	control	DPS	thrust	and	the	ACA	to	control	

vehicle	attitude	(originally	P67)	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	67	of	154	
Lunar	Descent	and	Landing	–	Landing	Phase	

• Entry	into	P66	was	accomplished	by	the	CDR	moving	the	PGNCS	switch	on	

the	MODE	CONTROL	SWITCH	from	AUTO	to	ATT	HOLD	

o The	Mode	Control	Switch	was	on	the	LM	STAB/CONT	Panel	just	above	the	

DSKY

• Once	ATT	HOLD	was	selected,	the	CDR	

would	engage	the	ROD	switch	just	

inboard	if	the	TTCA	

o The	ROD	Switch	controlled	the	thrust	

in	1	ft	s-1	increments	–	one	“up”	pulse	

of	the	switch	reduced	the	rate	of	

descent	by	1	ft	s-1	;	one	“down”	pulse	

increased	the	rate	of	descent	by	1	ft	s-1		

o By	pulsing	the	ROD	switch	with	the	

ATT	HOLD	engaged,	the	LGC	

automatically	rolls	over	into	P66,	and	

the	landing	proceeds	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	68	of	154	
Lunar	Descent	and	Landing	–	Landing	Phase	

• At	this	point,	the	CDR	is	flying	the	LM	through	the	LGC,	directing	the	vehicle	

to	the	chosen	landing	point	by	managing	the	vertical	velocity	with	the	ROD	

switch	while	LGC	sends	appropriate	commands	to	the	RCS	to	hold	a	chosen	

attitude	

o A	short	pulse	on	the	ACA	could	pitch	or	roll	the	LM,	which	would	change	in	the	

DPS	thrust	vector	and	cause	the	LM	to	move	laterally	

§ When	you	see	the	films	of	Eagle	flying	downrange	above	the	lunar	surface,	this	is	

how	Armstrong	is	executing	the	approach	

o In	effect,	at	this	point,	the	CDRs	job	is	to	manage	the	LMs	horizontal	position	

using	ACA	inputs	to	the	LGC,	and	manage	the	descent	rate	using	the	ROD	switch	

until	touchdown	

§ Nominally,	the	plan	was	to	null	out	any	horizontal	velocity	(0°	roll/0°	pitch)	and	use	

the	ROD	switch	to	set	the	LM	down	

o During	P66,	the	LMP	is	acting	as	the	flight	engineer,	feeding	information	on	H,	

H-dot,	lateral	velocities	and	fuel	state	using	the	DSKY	and	other	onboard	

instruments	to	ensure	the	CDR	has	the	data	he	has	to	manage	the	landing	

o When	you	listen	to	the	transcripts	of	Apollo	landings,	about	90%	of	the	time,	

you’re	listening	to	the	LMP	updating	the	CDR	with	the	pertinent	approach	data	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	69	of	154	
Lunar	Descent	and	Landing	–	Landing	Phase	

• This	activity	continued	until	one	of	the	≈6’	long	landing	probes	made	

contact	with	the	lunar	surface	and	was	torqued	out	of	position,	which	sent	a	

signal	to	illuminate	the	“Lunar	Contact”	lights	on	the	control	panel	

• Once	the	Contact	Light	illuminated,	the	CDR	hit	the	“Engine	Stop”	switch,	

terminating	DPS	ops,	and	the	LMP	entered	413+1(0000)	into	the	AGS	and	

keyed	PRO	on	the	DSKY,	which	rolled	the	LGC	into	P68	(touchdown)	and	

entered	the	landing	coordinates	into	LGC	memory	for	use	on	return	to	orbit	

• In	addition,	the	CDR	pulsed	the	ACA	to	tell	the	LGC	to	not	fire	the	RCS	

thrusters	in	response	to	any	perceived	non-level	orientation	precipitated	by	

the	slope	of	the	landing	surface	

• Once	that	was	completed,	the	crew	ran	through	the	post-landing	checklist,	

safing	the	DPS,	configuring	the	cockpit	to	either	stay	or	immediately	depart	

if	there	was	an	unsafe	condition	

• Note	these	callouts	from	Buzz	Aldrin	from	Apollo	11:	

o “Contact	Light?...OK,	engine	stop...ACA	out	of	detent...Mode	Controls,	Both	AUTO...

Descent	Engine	Command	Overide	OFF...Engine	Arm	OFF...413	is	in.”	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	70	of	154	
Lunar	Descent	and	Landing	–	Landing	Phase	

• Lunar	surface	directly	

below	Apollo	11	DPS	

engine	bell	

• Notice	the	groove	

starting	directly	

below	the	engine	bell	

and	going	to	the	upper	

left	

o This	is	the	spot	where	

we	first	touched	the	

Moon,	caused	by	the	

initial	contact	and	

subsequent	dragging	

of	the	lunar	contact	

probe	forward	and	to	

the	left	as	the	LM	

touched	down	with	

some	residual	lateral	

velocity	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	71	of	154	
Lunar	Descent	and	Landing	–	Summary	

Flight	Phase	 LGC	Actions	 Crew	Actions	 Why	Were	We	Doing	It	

This	Way?	

Braking	(PDI	to	High	

Gate,	P63)	

• Navigate	to	High	Gate	

using	state	vector	data	

derived	from	the	platform	

to	send	appropriate	

commands	to	the	DPS	

• Manage	engine	thrust	to	

reach	High	Gate	on	the	

basis	of	engine	limitations	

and	platform	and	radar	

data		

• Manage	LM	Cg	and	

moment	of	inertia	

changes	with	DPS	

gimballing	and	RCS	

firings	

• Provide	appropriate	H,	Hdot,	slant	range	and	TGO

data	to	the	crew	

• Monitor	the	progress	

of	the	flight	phase	

against	pre-planned	

trajectory	data	

• Act	as	“quality	gate”	

for	data	coming	into	

the	LGC,	such	as	

Landing	Radar	data	

and	NOUN	69	

corrections	

• Monitor	the	vehicle	

for	indications	of	

degraded	conditions	

that	might	lead	to	an	

abort	

• The	complexity	of	the	

phase	was	better	

managed	by	the	LGC	

and	monitored	by	the	

crew	

• The	vehicle	attitude	

through	most	of	the	

phase	precluded	crew	

visibility	of	the	lunar	

surface	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	72	of	154	
Lunar	Descent	and	Landing	–	Summary	

Flight	Phase	 LGC	Actions	 Crew	Actions	 Why	Were	We	Doing	It	

This	Way?	

Approach	(High	Gate	to	

Low	Gate,	P64)	

• Provide	LPD	angles	to	

show	the	targeted	

landing	spot		

• Provide	H	and	H-dot	

information	

• Accept	redesignation	

data	from	the	CDR’s	

ACA	

• Manage	engine	thrust	

on	the	basis	of	both	

CDR	inputs	and	P64	

instructions	

• LMP	provides	LPD	

angles,	H,	H-dot	and	

lateral	velocity	data	

to	the	CDR	

• CDR	uses	LPD	angles	

to	manage	the	

location	of	the	LGC	

targeted	landing	spot	

• The	CDR’s	judgment	of	

the	quality	of	the	LGC	

targeted	landing	spot	

is	better	than	the	LGC	

knowledge	of	the	

lunar	surface	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	73	of	154	
Lunar	Descent	and	Landing	–	Summary	

Flight	Phase	 LGC	Actions	 Crew	Actions	 Why	Were	We	Doing	It	

This	Way?	

Landing	(Low	Gate	to	

Touchdown,	P66)	

• Based	on	the	CDRs	

input,	flying	the	LM	as	

per	the	appropriate	

actions	in	P66	

• Manage	engine	thrust	

on	the	basis	of	both	CDR	

inputs	and	P66	

instructions	

• For	each	Apollo	flight,	

this	meant	using	PNGCS	

Mode	Control	in	ATT	

HOLD,	with	the	ROD	

managing	DPS	engine	

thrust	and	the	ACA	

controlling	LM	roll	and	

pitch	attitude	

• CDR	lands	the	vehicle	

using	the	ATT	HOLD	

capability,	the	ROD	to	

manage	the	vertical	

thrust,	and	the	ACA	to	

manage	lateral	

velocity	

• LMP	provides	data	on	

H,	H-dot,	and	fuel	

remaining	

• Each	LGC-targeted	

landing	spot	was	

judged	by	the	CDR	to	

be	too	rough	at	an	

“LM-Footpad”	scale,	

precluding	allowing	

an	automatic	landing	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	74	of	154	
Apollo	Crew	Experience	–		

	How	Did	Each	Landing	Proceed?
Fly	Me	To	The	Moon...For	Real	

• There	are	really	three	histories	of	landing	on	the	Moon	–	1)	Apollo	11,	2)	Apollos	

12	and	14,	and	3)	Apollos	15	through	17	

• The	Apollo	11	landing	was	really	in	a	class	by	itself,	largely	because	there	were	so	

many	significant	problems	that	were	overcome	by	the	aggregate	team	–	the	crew	

in	the	LM	and	the	Flight	Controllers	on	the	ground	–	and	which	never	happened	

again,	or	to	the	same	critical	extent	they	happened	on	Apollo	11	

• Apollo	12	and	14	were,	like	Apollo	11,	engineering	flights,	driven	by	either	

executing	precision	landing	at	a	specific	point	(Apollo	12)	or	by	landing	in	

increasingly	challenging	terrain	(Apollo	14)	

• Apollo	15,	16	and	17	built	on	the	earlier	missions	by	precisely	landing	in	

challenging	terrains	(the	lunar	highlands,	into	narrow	valleys,	approaching	right	

over	high	mountains)	at	sites	that	were	critical	for	understanding	lunar	geology/

geophysics	

• Each	landing	followed	the	approach	described	above,	but	each	had	elements	that	

deviated	from	the	“pure”	approach	procedure	described	above

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	76	of	154	
Fly	Me	to	the	Moon	

Apollo	11
Fly	Me	To	The	Moon	–	Apollo	11	

• The	Apollo	11	approach	began	with	the	LM	executing	the	DOI	burn	on	the	

lunar	far	side	after	executing	an	undocking	from	the	CSM	and	an	“inspection”	

maneuver	where	the	CMP	confirmed	that	the	LM	looked	intact	and	ready	to	

land	

o What	we	did	not	appreciate	at	the	time	is	that	various	pre-PDI	maneuvers	and	

seemingly	minute	additions	to	thrust,	such	as	residual	tunnel	atmosphere	release	

and	LM	cooling	water	venting,	were	adding	error	to	the	approach	path,	leading	to	

significant	errors	in	the	targeted	landing	spot	

• The	start	of	powered	descent	was	nominal,	and	the	Braking	Phase	proceeded	

as	planned,	although	at	MET	102:36:11	(TIG	+3	min	8sec)	the	LM	passed	a	

landmark	target	≈3	seconds	early,	indicating	it	was	going	to	land	≈	3	mi	long	

o Despite	the	navigational	error,	everything	was	still	proceeding	nominally	

• At	MET	102:38:04,	the	LM	got	good	Landing	Radar	lock	on	the	surface,	which	

would	provide	H	and	H-dot	data	to	the	LGC	after	the	crew	and	the	ground	

agreed	it	was	good	data	

o At	this	point,	Aldrin	keyed	in	a	VERB	16	(monitor	digital	data)	and	NOUN	68	

(show	slant	range,	Tgo	in	P63	and	the	delta	between	the	calculated	state	vector	

altitude	and	the	Landing	Radar	data,	called	Delta-H)...and	things	started	to	get	

interesting...

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	78	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	

• At	MET	102:38:21,	the	delta-H	data	showed	a	2900’	delta-H	difference	

between	PGNCS	calculated	altitude	and	radar	altitude,	acceptable	to	continue	

powered	descent	

• At	MET	102:38:26,	the	first	program	alarm	happened,	requiring	Aldrin	to	do	a	VERB	5	

NOUN	9	ENTER	to	display	a	1202	Alarm	Code	on	the	DSKY	

o This	alarm	arose	from	the	background	EXECUTIVE	program,	which	was	running	

a	myriad	of	jobs	with	different	priorities	related	to	the	landing	

o EXECUTIVE	was	programmed	to	call	a	1202	when	there	was	no	space	left	in	the	

queue	of	jobs	waiting	to	be	calculated	

o This	was	particularly	critical	at	this	point,	because	the	LGC	is	responsible	for	

taking	the	raw	radar	data,	converting	it	to	digital	data,	integrating	that	into	the	

state	vector	and	showing	corrected	H	and	H-dot	data	on	the	DSKY	

o Without	that	calculation,	the	crew	could	not	tell	if	the	radar	data	was	good,	and	

subsequently	could	not	tell	the	computer	to	accept	the	data	into	its	calculations	

(a	VERB	57	ENTER)	

o Armstrong	chose	to	accept	the	data	anyway,	before	the	1202	alarm	evaluation	

was	complete	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	79	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	

• In	what	was	a	remarkable	display	of	rapid	thought	and	gutsy	decision-making,	

only	16	seconds	later,	the	Control	Team	decided	that	a	1202	Alarm	would	not	

jeopardize	the	landing,	and	CAPCOM	Charlie	Duke	reported,	“...we’re	go	on	that	

alarm!”		

o Prior	to	Apollo	11	launch,	Kranz	had	given	the	team	the	assignment	to	understand	

every	computer	alarm	that	could	come	up,	and	to	understand	how	to	respond	to	

them	in	flight	(this	was	probably	the	most	important	pre-mission	exercise	that	made	

the	AS-11	landing	possible)	

o Also,	Aldrin	recognized	that	the	1202	happened	when	the	V16	N68	was	executed,	

suggesting	to	him	that	it	was	somehow	related	to	the	additional	computations	

required	by	those	actions	

o The	actions	by	the	Flight	Control	Team	would	later	earn	them	a	Presidential	Medal	of	

Freedom	

• At	MET	102:39:34,	DPS	went	though	the	planned	throttle	down	to	<60%,	exactly	

on	time	and	to	the	correct	throttle	setting	

• At	MET	102:41:35,	the	LGC	switched	over	to	P64,	and	the	LM	pitched	over	so	the	

crew	could	assess	the	landing	site	using	LPD	angles	

• At	this	point,	H	was	≈5,000	ft	and	H-dot	was	100	ft	s-1,	which	placed	the	minimum	

Abort/Stage	altitude	at	≈1,100	ft	

o At	MET	102:42:05,	Armstrong	took	the	PGNCS	Mode	Control	switch	from	AUTO	to	

ATT	HOLD	to	check	how	the	LM	responded,	and	then	went	back	to	AUTO...this	was	

going	to	be	critical	over	the	next	4	minutes	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	80	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	

• Then,	at	MET	102:42:17	with	the	LM	at	3,000	ft,	the	first	1201	alarm	

sounded	

o CAPCOMM	Duke	immediately	called,	“Roger...1201	Alarm...we’re	go,	same	type,	

we’re	go”,	and	the	crew	continued	descending	

o As	it	turns	out,	the	1201	was	only	one	of	the	critical	things	going	on	

• At	MET	102:42:32,	Armstrong	saw	West	Crater	looming	large	in	his	window	

and	called	for	an	LPD	from	Aldrin	

o Initially,	the	targeted	landing	spot	looked	good,	but	at	MET	102:43:10,	about	40	

seconds	later,	Armstrong	realized	that	the	LGC-targeted	landing	spot	was	

unacceptable	due	to	large	boulders	on	the	West	Crater	ejecta	blanket	

• At	MET	102:43:15,	Armstrong	took	over	control	of	the	vehicle,	selecting	

PGNCS	ATT	HOLD	on	the	Stability	and	Control	panel	

o Armstrong	then	“bumped”	the	system	into	P66	by	pulsing	the	Rate	of	Descent	

(ROD)	switch,	and	started	pulsing	the	ACA	(rotational	hand	controller)	forward		

o This	started	the	LM	flying	down	range,	beyond	West	Crater	to	a	more	suitable	

landing	site	

o At	this	point	H	was	600	feet,	H-dot	was	19	ft	s-1

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	81	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	

• At	MET	102:43:26,	the	LM	was	down	to	500	ft	with	a	forward	velocity	at	58	

ft	s-1	forward	and	an	H-dot	was	15	ft	s-1	down	

o The	forward	velocity	was	almost	40	mph,	and	16	seconds	later,	Aldrin	reported	

that	the	forward	velocity	on	the	velocity	cross	pointer	display	was	“pegged”	

o Although	it’s	not	like	a	fighter	buzzing	the	tower	(despite	what	is	shown	on	

various	Discovery	Channel	specials),	Neil	has	placed	the	LM	in	an	attitude	where	

the	descent	angle	has	decreased	considerably	and	the	LM	has	a	larger	horizontal	

vector	component	to	allow	them	to	move	across	the	surface	

o Note	that	at	this	point,	the	Abort/Stage	boundary	is	≈230	ft	

• At	this	point,	Armstrong	has	gone	off	script	and	is	aggressively	flying	the	LM	

downrange	in	P66	-	PGNCS-ATT	HOLD,	using	the	ACA	to	pitch	the	LM	over	

and	give	it	more	forward	velocity	

o Although	the	flight	controllers	had	no	idea	what	was	going	on,	to	the	Flight	

Director’s	and	control	team’s	credit,	they	stood	by	to	assist	and	let	the	crew	fly	

the	LM	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	82	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	

• At	MET	102:43:46,	Armstrong	reduced	his	pitch	angle	to	slow	the	LM	up	in	

preparation	for	landing	

o Throughout	this	evolution,	Aldrin	has	been	giving	him	constant	updates	on	H,	H-dot	

and	horizontal	velocity	so	Armstrong	could	stay	out	the	window,	finding	an	

acceptable	landing	spot	

o Without	Aldrin’s	input,	Armstrong	would	have	a	limited	idea	of	the	LM	H,	H-dot	and	

lateral	velocity,	and	it	is	unlikely	he	would	have	been	unable	to	continue	the	approach	

without	looking	in	the	cockpit	and	losing	lock	on	where	the	LM	was	with	respect	to	

the	landing	spot	

• At	MET	102:44:04,	Aldrin	reported	seeing	the	shadow	of	the	LM		

• At	MET	102:44:45,	H	was	100	ft,	H-dot	was	3	1⁄2	ft	sec-1	down,	forward	velocity	

was	9	ft	sec-1	and	fuel	was	down	to	about	8%	-	they	were	getting	very	close	to	a	

“Bingo	fuel	call”	

o This	meant	they	were	committed	to	landing,	as	they	were	below	the	safe	Abort/Stage	

boundary	

• At	MET	102:45:31,	CAPCOMM	Duke	called	30	seconds	of	fuel	remaining	

o H	is	20	ft,	H-dot	is	≈	1⁄2	ft	sec-1,	and	forward	velocity	is	4	ft	sec-1	

• At	MET	102:45:40,	Aldrin	called	“Contact	Light”,	Armstrong	hit	the	Engine	Stop	

pushbutton,	and	the	LM	dropped	the	last	few	feet	to	the	surface	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	83	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	Assessment	

• There	has	been	a	lot	of	ink	spilled	over	what	happened	on	Apollo	11,	particularly	

in	terms	of	the	significance	of	the	1202/1201	alarms,	and	I’m	most	likely	not	

going	to	illuminate	that	discussion	

• The	cause	of	the	alarms	has	been	debated	a	lot,	but	based	on	Don	Eyles	and	the	

Draper	Lab	experience,	it	appears	that	the	likely	“first	order”	cause	was	an	issue	

with	the	Rendezvous	Radar,	particularly	getting	incompatible	data	from	

Rendezvous	Radar	when	the	LM	panel	switch	was	in	either	SLEW	or	AUTOTRACK	

o This	led	the	LGC	to	what	I	would	(irreverently)	call	Whiskey	Tango	Foxtrot	response	

as	it	tried	to	handle	the	data,	could	make	no	sense	of	it,	and	still	tried	to	work	with	it,	

stealing	the	limited	RAM	that	was	left	in	the	LGC	after	calculating	the	state	vector	

every	2	seconds	

o Even	then,	in	other	phases,	the	spurious	activity	would	not	have	been	a	problem...

except	when	you	were	doing	the	most	difficult	task	of	the	mission,	landing	on	the	

Moon	

o Ironically,	this	had	been	seen	in	testing,	but	only	2%	of	the	time,	and	it	was	considered	

low	enough	probability	that	it	shouldn’t	occur	in	flight...except	that	it	did	

• The	good	news	is	that	it	was	a	simple	fix,	ensuring	that	the	incompatible	data	

issue	never	happened	again	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	84	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	Assessment	(cont.)	

• I	think	an	equally	important	part	of	the	discussion	is	the	response	of	the	crew	and	

the	MOCR	team	

o First	and	foremost,	the	crew	kept	flying	in	spite	of	hub-bub,	although	responding	to	the	

problem	dominated	what	would	have	been	time	devoted	to	flying	a	nominal	approach	

§ "Normally	...from	P64	onward,	we'd	be	evaluating	the	landing	site	and	checking	our	position	and	

starting	LPD	activity.	However,	the	concern	here	was	not	with	the	landing	area	we	were	going	

into	but,	rather,	whether	we	could	continue	at	all	(because	of	the	program	alarms).	

Consequently,	our	attention	was	directed	toward	clearing	the	program	alarms,	keeping	the	

machine	flying,	and	assuring	ourselves	that	control	was	adequate	to	continue	without	requiring	

an	abort.	Most	of	the	attention	was	directed	inside	the	cockpit	during	this	time	period	and,	in	my	

view,	this	would	account	for	our	inability	to	study	the	landing	site	and	final	landing	location	

during	the	final	descent.	It	wasn't	until	we	got	below	2000	feet	that	we	were	actually	able	to	look	

out	and	view	the	landing	area.”	[Neil	Armstrong,	from	the	Apollo	11	Post-Mission	Technical	

Debrief,	quoted	after	MET	102:42:35	on	the	Apollo	11	ALSJ	landing	page]		

§ The	bottom	line	is	they	kept	their	cool,	kept	flying	the	vehicle	and	kept	their	faith	in	the	

machine,	the	systems	and	the	folks	on	the	ground	

o In	the	MOCR,	they	were	able	to	decide	in	16	seconds	that	the	landing	could	continue,	

particularly	with	the	help	of	Jack	Garmin,	Steve	Bales,	Draper	Lab’s	Russ	Larson	and	

various	back	room	controllers	

§ This	was	largely	due	to	Kranz’s	assignment	to	find	out	and	understand	every	possible	alarm	

code	that	could	come	up	on	the	LGC	during	landing	

o This	speaks	to	the	value	of	the	extensive	training	the	whole	team	had	prior	to	Apollo	11,	

and	the	robustness	of	the	LM,	the	LGC,	and	the	humans	in	the	loop	to	deal	with	a	

problem	that	might	have	otherwise	called	an	abort	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	85	of	154	
Fly	Me	To	The	Moon	–	Apollo	11	Assessment	(cont.)	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	86	of	154	

• The	other	critical	event	on	Apollo	11	is	Armstrong’s	response	to	the	autotargeting	taking	him	into	a	spot	where	safe	landing	of	the	LM	was	in	question	

• This	is	(again,	at	least	to	me)	the	best	example	of	“Pilot-in-Command”	actions	in	

the	Federal	Aviation	Regulations	Part	91.3	sense	

o Armstrong	saw,	using	the	LPD	angles,	that	the	LM	was	headed	for	an	unsafe	spot,	with	

only	limited	options	

§ To	land	short	meant	they	would	have	to	increase	their	rate-of-descent,	which	might	have	

been	possible,	but	would	have	been	very	sporty	

- “As	we	approached	the	1500-foot	point,	the	program	alarm	seemed	to	be	settling	down	and	we	committed	

ourselves	to	continue.	We	could	see	the	landing	area	and	the	point	at	which	the	LPD	was	pointing,	which	

was	indicating	we	were	landing	just	short	(and	slightly	north)	of	a	large	rocky	crater	surrounded	with	the	

large	boulder	field	with	very	large	rocks	covering	a	high	percentage	of	the	surface.	I	initially	felt	that	that	

might	be	a	good	landing	area	if	we	could	stop	short	of	that	crater,	because	it	would	have	more	scientific	

value	to	be	close	to	a	large	crater.	(However),	continuing	to	monitor	the	LPD,	it	became	obvious	that	I	

could	not	stop	short	enough	to	find	a	safe	landing	area.”	[Neil	Armstrong,	ALSJ	entry,	MET	102:42:35]

§ West	Crater	was	to	the	left	(south)	of	the	LM’s	path,	so	that	was	no	good,	and	to	the	right	

(north),	the	LM	would	still	be	flying	into	the	West	Crater	ejecta	blanket	

o The	only	real	option	was	to	fly	downrange,	which	he	did		

• Having	a	pilot	aboard,	able	to	take	over	and	fly	to	a	safe	spot,	was	the	key	to	

success	on	Apollo	11,	and	without	Armstrong	having	the	option	to	do	that,	Apollo	

11	would	have	either	ended	in	a	very	bad	day	(LM	crash,	LoC,	LoM)	or	a	very	

unsatisfying	failure	(LM	aborts	home,	no	landing	on	Apollo	11...maybe	no	

landing	before	the	decade	was	out...)	
Fly	Me	to	the	Moon	

Apollo	12
Fly	Me	To	The	Moon	–	Apollo	12	

• If	everything	that	could	go	wrong	with	the	LGC	and	the	navigation	went	wrong	

on	Apollo	11,	then	everything	that	could	go	right	with	the	same	systems	went	

right	on	Apollo	12	

• Apollo	12’s	main	objective	was	to	demonstrate	not	only	that	the	LM	could	

land	safely	on	the	Moon,	it	could	land	with	pinpoint	accuracy	at	a	location	that	

was	important	for	both	science	and	spacecraft	engineering	

o The	key	science	objective	was	to	visit	another	mare	site	to	compare	the	returned	

samples	with	the	Apollo	11	samples	

§ Another	key	science	objective	was	to	emplace	the	first	geophysical	station,	the	Apollo	

Lunar	Scientific	Experiment	Package	(ALSEP),	which	had	been	left	off	Apollo	11	due	to	

weight	and	crew	EVA	time	considerations	

- The	ALSEP	array	on	Apollo	12	weighed	in	at	>175	pounds	(≈85	kg),	which	

represented	seconds	in	hover	time	

- In	addition,	the	crew	time	necessary	to	deploy	the	package	(3-4	hours)	would	

exceed	the	planned	Apollo	11	EVA	time	

§ Both	of	these	objectives	could	have	been	accomplished	without	precision	landing	

o The	key	engineering	objective	was	to	return	samples	of	the	Surveyor	III	

spacecraft,	which	had	been	on	the	Moon	for	≈2	1⁄2	years	

§ Given	the	limitations	of	walking	EVAs,	this	dictated	that	the	LM	had	to	land	<1	km	

from	the	Surveyor		

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	88	of	154	
Fly	Me	To	The	Moon	–	Apollo	12	

• As	with	Apollo	11,	the	DPS	performed	the	DOI	burn	while	on	the	lunar	farside	at	

MET	109:23:29	

o Unlike	Apollo	11,	the	Apollo	12	crew	went	to	great	pains	to	minimize	any	spurious	

delta-V	inputs	(one	MPAD	memo	indicated	even	urine	dumps	would	be	enough	to	

screw	up	the	trajectory...)	during	the	runup	to	DOI	

o The	DOI	burn	put	the	LM	into	a	60.5	nm	by	8.9	nm	orbit,	with	minimal	residuals	and	

good	agreement	between	AGS	and	PGNCS,	which	was	the	first	step	to	a	precision	

landing	

• At	MET	109:50:52,	the	ground	updated	the	LM	state	vector	and	then,	based	on	

these	data,	updated	the	AGS	in	preparation	for	executing	the	Powered	Descent	

burn	

• At	MET	110:13:38,	Conrad	enters	a	VERB	50	NOUN	18,	which	puts	the	LM	in	the	

proper	attitude	for	P63	PDI	

• At	MET	110:20:06,	the	LGC	signals	it	is	running	the	Average	g	program	by	a	

momentarily	blank	DSKY	

• At	MET	110:20:29,	the	LM	started	PDI	on	time,	with	throttle	up	26	seconds	later	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	89	of	154	
Fly	Me	To	The	Moon	–	Apollo	12	

• At	MET	110:21:57	,	≈1	min	30	sec	into	Powered	Descent,	the	crew	and	the	

ground	implemented	the	DLAND	trajectory	correction	on	the	basis	of	MSFN	

tracking	data,	which	gave	them	an	opportunity	early	in	Powered	Descent	to	

correct	any	trajectory	issues	with	a	“fudge	factor”	that	would	bring	the	LM	to	

its	planned	landing	spot	

o As	noted	earlier,	DLAND	was	not	available	on	Apollo	11	

o Entering	the	appropriate	data	in	DLAND,	a	NOUN	69,	was	critical	to	a	good	

approach	–	entering	the	incorrect	data	was	a	serious	problem,	so	ENTER	was	

only	pressed	after	both	the	crew	and	the	ground	agreed	on	the	value	to	be	

entered	

§ Unlike	most	air-to-ground	exchanges,	which	were	pretty	terse,	the	NOUN	69	

confirmation	and	execute	discussion	took	≈30	seconds,	indicating	the	interaction	

necessary	to	confirm	it	was	entered	correctly	

o In	this	case,	the	NOUN	69	value	entered	was	+4200	ft	in	the	downrange	direction,	

with	no	correction	required	for	left	or	right	retargeting	

o The	entry	string	was	VERB	21	NOUN	69	ENTER	–	the	VERB	21	told	the	LGC	that	it	

was	only	a	downrange	correction	(Register	1	on	the	DSKY;	Register	2	and	3	are	

crossrange	and	altitude,	which	were	not	changed)	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	90	of	154	
Fly	Me	To	The	Moon	–	Apollo	12	

• At	MET	110:23:58,	the	Altitude	and	Velocity	lights	on	the	DSKY	went	out,	

indicating	that	the	Landing	Radar	had	good	lock	on	the	surface	

o The	Delta-H	comparison	(≈900-1,000	ft)	was	considered	good,	so	at	MET	

110:24:26,	Landing	Radar	was	incorporated	into	the	LGC	calculations	

• Pretty	much	all	through	the	initial	part	of	Powered	Descent,	everything	went	

nominally	(one	might	say,	“nominal	in	the	extreme”...)	

o PGNCS	and	AGS	agreed,	sometimes	with	zero	residuals,	engine	performance	was	

right	to	spec,	and	about	the	only	crew	comments	were	a	significant	number	of	

RCS	firings,	something	that	did	not	happen	in	the	simulator,	something	seen	on	

Apollo	11	as	well	

o The	RCS	activity	would	later	be	found	to	be	caused	by	fuel	and	oxidizer	sloshing	

in	the	prop	tanks,	which	did	not	have	anti-slosh	baffles	

§ After	Apollo	12,	Grumman	would	undertake	a	very	delicate	“ship-in-a-bottle”	

modification	to	the	prop	tanks	(which	had	already	been	delivered	for	each	LM)	that	

welded	anti-slosh	baffles	through	the	tank	mouth	

§ This	solved	the	problem	on	future	landings	

• Throttle	down	(throttle	recovery)	occurred	at	MET	110:27:07,	at	Tig	+6	min	

26	secs,	which	was	immediately	followed	by	another	AGS	update	

o At	this	point,	everything	was	completely	nominal,	in	comparison	to	Apollo	11	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	91	of	154	
Fly	Me	To	The	Moon	–	Apollo	12	

• At	MET	110:29:10,	the	LGC	picks	up	running	P64	and	the	LM	pitches	over	to	give	

the	crew	the	first	view	of	the	landing	site	

o I	printed	the	transcript	earlier,	so	I	won’t	repeat	it	here	except	for	one	callout	that	

seems	to	sum	up	the	Apollo	12	descent	to	landing:	

§ MET	110:31:06	Bean:	Hey!	Look	at	that	crater;	right	where	it's	supposed	to	be!		

§ During	pre-flight	planning,	Conrad	decided	to	use	as	the	LM	aim	point	the	center	of	

Surveyor	crater,	assuming	that	the	guidance	would	never	be	that	precise	–	to	his	chagrin,	

the	rest	of	the	Approach	Phase	and	the	Landing	Phase	were	spent	moving	the	touchdown	

point	so	they	didn’t	land	right	in	the	middle	of	Surveyor	Crater	

• From	this	point	until	the	start	of	the	Landing	Phase,	Conrad	uses	numerous	LPD	

angle	callouts	from	Bean	to	redesignate	the	landing	point	away	from	the	center	

of	Surveyor	Crater	

o During	P64,	Conrad	will	make	seven	redesignations	of	the	landing	point	before	

switching	into	P66	

§ Conrad,	from	the	1969	Technical	Debrief	-	"I	gave	her	one	click	forward,	let	her	go	for	a	while,	

and	decided	that	we	were	high	and	fast.	I	didn't	like	the	size	of	the	area	short,	where	we	had	

normally	been	trying	to	land,	and	I	looked	for	a	more	suitable	place.	('That	much	I	remember,'	

Pete	said	[during	Eric	Jones’	interview	for	the	ALSJ]	in	1991.)”	

o During	the	same	time,	Bean	will	make	continuous	callouts	on	LPD	angle,	H,	H-dot	and	

fuel	state	to	keep	Conrad	apprised	of	the	condition	of	the	vehicle	and	where	it’s	going	

§ MET	110:30:08	Bean:	“38	(degrees	on	the	window).	38	degrees.	36	degrees;	you're	1200	feet,	

Pete.	A	thousand	feet	coming	down	at	30	(feet	per	second).	You're	looking	good.	Got	14	

percent	fuel.	(Glancing	out	the	window,	again)	Looks	good	out	there,	babe.	Looks	good!	

(Pause)	32	degrees.	You're	at	800	feet.	33	degrees.	You're	at	680	feet.	33	degrees,	600	feet.”	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	92	of	154	
Fly	Me	To	The	Moon	–	Apollo	12	 • At	MET	110:30:52,	at	an	altitude	of	≈500	ft,	Conrad	switched	the	LM	into	P66	and	

proceeded	to	fly	the	landing	in	a	“manual	mode”	

o I	use	the	quotes	because	at	this	point	in	the	evolution	of	LGC	programs,	there	were	3	modes	

the	crew	could	land	in:	

§ P65	–	“Pure”	automatic	mode	–	the	LGC	would	null	out	lateral	rates,	and	then	gradually	reduce	 thrust	until	lunar	contact	

§ P66	–	A	manual	mode	where	the	vehicle	is	PGNCS	Mode	–	AUTO	HOLD,	and	the	CDR	uses	the	ROD	

switch	to	manage	the	rate-of-descent	while	the	LGC	holds	the	vehicle	attitude	steady	automatically	

§ P67	–	“Pure	manual	mode”,	where	the	LGC	is	taken	out	of	the	loop	and	the	CDR	is	commanding	

DPS	thrust	with	the	TTCA	and	the	attitude,	and	the	DPS	thrust	vector,	with	the	ACA	

o Other	than	a	“P66”	call,	there	was	never	an	“ATT	HOLD”	call	or	an	indication	Conrad	is	

using	the	ROD	switch,	but	in	the	absence	of	the	call	to	P65	or	P67,	I’m	reasonably	sure	

they’re	in	the	mode	that	all	other	landings	used	

• From	here	until	touchdown,	Conrad	is	flying	the	LM	around	Surveyor	Crater	to	an	

acceptable	spot	on	the	NW	side	of	the	crater,	while	Bean	continues	to	give	him	cues	

on	H,	H-dot	and	fuel	

o MET	110:31:06 Bean:	Hey!	Look	at	that	crater;	right	where	it's	supposed	to	be!	Hey,	you're	

beautiful.	Ten	percent	(fuel	remaining).	257	feet,	coming	down	at	5;	240	coming	down	at	5.	

Hey,	you're	really	maneuvering	around.	

o MET	110:31:21 Bean:	Come	on	down,	Pete.	

o MET	110:31:23 Bean:	Ten	percent	fuel.	200	feet;	coming	down	at	3...’need	to	come	on	

down.	

o 110:31:31	Bean:	190	feet.	Come	on	down.	180	feet;	9	percent	(fuel	remaining).	You're	

looking	good.	Going	to	get	some	dust	before	long.	130	feet;	124	feet,	Pete.	120	feet,	coming	

down	at	6.	You	got	9	percent,	8	percent.	You're	looking	okay.	96	feet,	coming	down	at	6.	

Slow	down	the	descent	rate!	80	feet.	80	feet,	coming	down	at	4.	You're	looking	good.	70	feet;	

looking	real	good.	63	feet.	60	feet,	coming	down	at	3.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	93	of	154	
Fly	Me	To	The	Moon	–	Apollo	12	

• By	MET	110:32:06,	Conrad	nulled	out	all	the	lateral	motion,	and	was	probably	

managing	H	and	H-dot	down	to	the	landing	spot	using	the	ROD	switch	–	in	

other	words,	he	was	flying	as	per	the	P66	landing	program,	not	P65	or	P67	

o Conrad,	from	the	1969	Technical	Debrief	-	"At	that	point,	the	dust	was	bad	enough	

and	I	could	obtain	absolutely	no	attitude	reference	by	looking	at	the	horizon	and	

the	LM.	I	had	to	use	the	8-ball	[DE-Note:	the	Flight	Director	Attitude	Indicator		an	

“artificial	horizon”)].	I	had	attitude	excursions	in	pitch	of	plus	10	(degrees)	and	

minus	10,	which	happened	while	I	was	looking	out	the	window	making	sure	that	

the	lateral	and	horizontal	velocities	were	still	nulled.		I	would	allow	the	attitude	of	

the	vehicle	to	change	by	plus	or	minus	10	degree	in	pitch	and	not	be	aware	of	it,	

and	I	had	to	go	back	in	the	cockpit	and	keep	re-leveling	the	attitude	of	the	vehicle	

on	the	8-ball.	I	was	on	the	gauges	in	the	cockpit	doing	that	at	the	time	the	Lunar	

Contact	light	came	on.	I	had	that	much	confidence	in	the	gauges.	I	was	sure	we	

were	in	a	relatively	smooth	area.”	

• At	MET	110:32:35,	Bean	called	“Contact	Light”,	and	Conrad	hit	the	Engine	

Stop	button,	dropping	from	≈3	ft	to	the	lunar	surface

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	94	of	154	
Fly	Me	To	The	Moon	–	Apollo	12	Assessment	

• Apollo	12	performed	an	essentially	flawless	landing	on	the	lunar	surface,	

demonstrating	that	all	of	the	issues	that	were	raised	during	the	Apollo	11	

landing	had	been	fixed,	and	that	while	landing	would	never	be	“routine”,	the	

LM	systems	and	the	crew	performance	should	allow	further,	more	challenging	

landings	to	proceed	

o Conrad	showed	that	while	blowing	lunar	regolith	can	obscure	the	landing	site	in	

the	final	stages	of	touchdown,	using	the	instruments	and	settling	in	with	minimal	

lateral	movement	would	still	give	you	a	safe	touchdown	evolution.	

• In	a	way,	Apollo	11	and	12	were	“bookends”	–	one	flight	showing	that	you	

could	land	on	the	Moon,	but	all	kinds	of	systems	needed	to	work	flawlessly,	

and	one	flight	showing	that,	if	the	systems	worked	flawlessly,	you	could	go	

wherever	you	wanted	to		

• The	crew	would	go	on	to	do	two	EVAs,	deploy	the	first	ALSEP,	bring	back	

samples	that	were	significantly	different	from	Apollo	11,	and	bring	back	

pieces	of	the	Surveyor	spacecraft	to	show	how	the	lunar	environment	treats	

man	made	structures	

o The	Surveyor	parts	are	still	the	only	man-made	structures	brought	back	from	the	

lunar	surface,	and	their	condition	and	history	should	still	inform	lunar	surface	

system	design	work	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	95	of	154	
Fly	Me	to	the	Moon	

Apollo	14
Fly	Me	To	The	Moon	–	Apollo	14	

• Given	the	success	of	Apollo	12,	Apollo	13	was	set	to	land	in	the	first	

“topographically”	challenging	area	near	the	crater	Fra	Mauro,	which	was	

characterized	by	low,	rolling	hills,	in	contrast	to	the	flat	maria	sites	on	Apollo	

11	and	12	

• After	Apollo	13’s	abort,	it	was	decided	that	the	Fra	Mauro	site	was	still	critical	

for	lunar	science,	so	Apollo	14	was	targeted	for	the	same	landing	site	

• Apollo	14	was	also	the	first	mission	where	the	shift	in	DOI	“responsibility”	to	

the	CSM	was	implemented		

• The	CSM-LM	stack	successfully	entered	orbit	at	MET	81:56:42	into	a	58.4	nm	

by	169	nm	orbit,	which	was	the	orbit	planned	to	stage	the	stack	for	the	DOI	

burn	

• The	DOI	burn	came	≈4	hours	8	minutes	later	at	MET	86:10:53,	which	put	the	

stack	in	a	9.1	nm	by	58.4	nm	orbit	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	97	of	154	
Fly	Me	To	The	Moon	–	Apollo	14	

• LM	undocking	came	at	MET	103:47:52	–	undocking	was	nominal,	but	

then	things	started	to	get	interesting...

o As	the	LM	was	going	through	the	pre-PDI	checks,	the	ground	noticed	that	it	

was	getting	data	that	suggested	that	the	LM	abort	discrete	was	being	set,	

automatically	and	incorrectly,	without	crew	input	

§ A	“discrete”	was	a	signal	in	the	LGC	that	,	while	not	an	“automatic	abort	

trigger”,	was	an	indication	that	an	inadvertent	abort	during	Powered	

Descent	was	a	significant	possibility	

- Basically,	every	0.25	seconds	the	LGC	Abort	Monitor	Routine	would	

verify	that	an	abort	signal	(the	LETABORTBIT	Flag)	was	set	in	the	

LGC	

- Positive	verification	would	lead	to	a	check	of	the	discrete,	and	if	

both	bits	were	positive,	the	LGC	would	automatically	command	

either	an	Abort	(P70)	or	Abort/Stage	(P71)	

o The	crew	tapped	on	the	abort	button,	which	caused	the	discrete	to	go	on	and	

off,	suggesting	that	a	piece	of	FOD	in	the	switch	was	causing	a	short	and	

triggering	a	false	discrete	signal	

§ The	concern	was	that	during	PDI,	the	acceleration	would	move	the	FOD	

into	the	switch	and	trigger	an	automatic	abort	without	crew	input	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	98	of	154	
Fly	Me	To	The	Moon	–	Apollo	14	

• At	first,	Don	Eyles	(who	wrote	the	original	code)	thought	that	the	LETABORTBIT	

flag	could	be	programmed	off,	which	would	disallow	automatic	aborts,	but	would	

still	allow	the	crew	to	manually	abort	if	necessary	

o This	would	have	used	a	NOUN	5	to	spoof	the	LGC	into	shutting	off	the	LETABORTBIT	flag	

o However,	a	closer	inspection	of	the	code	showed	Eyles	that	while	the	LETABORTBIT	was	

normally	off	preceding	DPS	ignition,	it	would	automatically	(spoofing	notwithstanding...)	

reset	at	PDI/Tig	=	0,	regardless	of	the	pre-existing	NOUN	5,	making	it	likely	an	abort	would	

be	triggered	before	the	crew	could	intervene	and	re-execute	the	NOUN	5	

• At	this	point,	it	was	realized	that	a	whole	new	program	sequence	needed	to	be	

written	by	Eyles,	tested	in	the	simulator,	validated,	verbally	sent	up	to	the	crew	

(there	was	no	printer	on	board	during	Apollo),	entered	by	Mitchell,	all	in	<4	

hours	so	they	didn’t	miss	a	second	shot	at	the	Fra	Mauro	landing	site	

• Essentially,	Eyles	made	the	computer	believe,	during	P63,	that	a	P71	abort	was	

already	in	progress	(even	though	it	wasn’t...)	

o This	used	a	register	called	MODREG,	which	kept	track	of	which	program	was	actually	

running	

o To	quote	Eyles	directly,	“Setting	MODREG	to	71	during	63	seemed	radical,	but	perhaps	it	was	

not	as	dangerous	as	it	sounded.		It	would	not	actually	initiate	the	abort	programs... BURNBABY	–	the	master	ignition	routine...was	calling	the	shots,	and	the	paths	taken	by	

BURNBABY	were	controlled	by	a	parameter	independent	of	MODREG.		Mostly.”	[Don	Eyles,	

“Sunburst	and	Luminary,	pg.	257]	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	99	of	154	
Fly	Me	To	The	Moon	–	Apollo	14	 • BTW,	if	about	now,	you’re	thinking,	“Holy	[insert	your	favorite	expletive	here]!”,	you’re	not	

alone	–	I	think	this	was	the	most	critical	moment	in	any	lunar	landing	other	than	Apollo	11	

• In	the	end,	the	crew	had	to	execute	the	following	sequence:	

o To	set	up	the	cockpit,	the	Mode	Control	Switch	was	set	to	ATT	HOLD,	the	Throttle	Control	Switch	

on	the	Thrust	Control	Panel	was	set	MANUAL	

o After	the	DSKY	started	counting	down	to	P63,	but	before	ignition,	the	LMP	keyed	in	VERB	21	

NOUN	1	ENTER	1010	ENTER	107	ENTER	

§ 	This	would	set	the	mode	register	to	71	(abort	in	progress)	with	the	address	of	MODREG	=	1010,	

with	107	being	the	octal	code	for	P71	

o At	exactly	Tig	=	+26	seconds,	the	CDR	manually	pushed	the	TTCA	to	throttle	to	100%	

o At	completion	of	throttle	up,	the	LMP	keyed	in	VERB	25	ENTER	NOUN	7	ENTER	101	ENTER	200	

ENTER	1	ENTER	

o This	reset	ZOOMFLAG	and	enabled	landing	guidance	equations	supporting	LGC	ops	

o Next,	the	LMP	keyed	in	VERB	25	NOUN	7	ENTER	105	ENTER	400	ENTER	0	ENTER,	which	would	

immediately	disable	the	LETABORTBIT	abort	flag	

o Finally,	the	LMP	keyed	in	VERB	21	NOUN	1	ENTER	1010	ENTER	77	ENTER,	which	set	the	

program	register	back	to	P63	

o Finally,	the	CDR	reset	the	Throttle	Control	and	Mode	Control	switches	to	AUTO,	returning	the	

throttle	to	MIN,	leaving	the	LGC	in	charge	of	throttling	DPS	automatically	during	P63	(which	was	

back	to	happily	managing	Powered	Descent)	

• A	downstream	effect	of	this	“spoofing”	was	that	any	aborts	would	have	to	be	manually	

initiated	and	run	on	the	AGS,	including	an	immediate	abort	from	the	surface	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	100	of	154	
Fly	Me	To	The	Moon	–	Apollo	14	

• At	MET	108:02:26,	the	crew	initiated	Powered	Descent	

o Manual	throttle	up	was	executed	by	Shepard	at	Tig	+27.25	seconds	(only	11⁄4	

seconds	late...),	and	Mitchell	proceeded	to	upload	the	remaining	command	

strings	into	the	LGC	to	spoof	the	LM	into	continuing	the	descent		

§ The	various	DSKY	strings	were	uploaded	by	MET	108:03:57,	and	the	Braking	Phase	

proceeded	nominally	from	there	on...well,	almost...(remember	that	a	critical,	

mandatory	abort	happens	if	the	crew	does	not	get	Landing	Radar	returns,	confirm	a	

good	Delta-H	comparison,	and	tell	the	LGC	to	accept	radar	data	by	30,000	ft...)	

• At	MET	108:04:50,	the	crew	executed	the	DLAND	targeting	correction	by	

entering	a	NOUN	69,	with	a	downrange	correction	of	+2800	feet	

o This	would	ultimately	enable	the	surface	science	ops	after	the	landing	by	

ensuring	the	crew	landed	at	the	spot	from	which	the	planned	EVAs	originated	

• At	MET	108:08:16,	6	mins	40	secs	into	Powered	Descent	and	down	to	32,000	

ft,	the	Altitude/Velocity	lights	were	still	lit,	indicating	that	the	Landing	Radar	

was	not	locked	onto	the	surface	

o By	the	time	they	were	below	25,000	ft,	they	still	did	not	have	radar	data,	and	

things	were	looking	dicey	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	101	of	154	
Fly	Me	To	The	Moon	–	Apollo	14	

• At	MET	108:08:42,	CAPCOMM	Haise	directed	the	crew	to	recycle	the	Landing	

Radar	circuit	breaker,	and	it	worked	

o At	MET	108:09:14,	Shepard	reported	that	the	Altitude/Velocity	Lights	were	out,	and	

the	approach	proceeded	

§ MET	108:09:39	Mitchell:	“Great.	Whew;	that	was	close.”

o It	was	shown	later	that	either	an	unexpectedly	strong	return	from	the	surface	or	a	

side	lobe	bouncing	off	of	LM	structure	had	switched	the	radar	into	short	range	mode,	

which	prevented	lock	up	until	that	circuit	breaker	was	cycled	

• The	rest	of	the	approach	(PGNCS-AGS	agreement,	DPS	throttle	down)	was	

completely	nominal,	in	contrast	to	the	other	excitement	during	the	early	parts	

of	Powered	Descent	

• At	MET	108:11:13,	the	LM	went	into	P64	and	pitchover	occurred	right	on	

time,	with	Shepard	immediately	sighting	Cone	Crater,	the	major	science	target	

of	EVA	2		

• The	LMP	immediately	began	giving	LPD	angles;	the	agreement	between	the	

planned	landing	site	and	the	guidance	was	very	good,	and	Shepard	only	

redesignated	once	during	P64	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	102	of	154	 *...when	in	doubt,	reboot...
Fly	Me	To	The	Moon	–	Apollo	14	

• At	MET	108:13:05,	Shepard	pushed	the	LGC	into	P66	by	switching	the	PGNCS	

Mode	Control	Switch	to	ATT	HOLD	and	pulsing	the	ROD	several	times,	and	

proceeded	to	drive	the	LM	to	his	planned	landing	site	

• At	MET	108:13:24,	Shepard	leveled	the	LM,	nulled	out	his	lateral	velocity,	and	

began	letting	the	LM	down	to	his	selected	landing	spot	

• At	MET	108:14:52,	CAPCOMM	Haise	made	the	60	seconds	to	Bingo	Fuel	call	

• At	MET	108:15:11,	Mitchell	called	the	Contact	Light,	and	the	LM	landed	at	

MET	108:15:12	after	Shepard	hit	the	Engine	STOP	switch	

o At	landing,	the	LM	was	moving	≈3	ft	s-1	down,	forward	at	about	2	ft	s-1,	and	left	

about	1	ft	s-1	

o In	addition,	the	LM	was	<60	ft	from	the	planned	landing	spot,	which	was	critical,	

as	this	crew	had	the	longest	walking	EVAs	on	Apollo	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	103	of	154	
Fly	Me	To	The	Moon	–	Apollo	14	Assessment	

• Apollo	14	would	probably	have	to	abort	the	landing	(and	potentially	doom	

the	subsequent	3	flights	of	Apollo)	if	it	wasn’t	for	three	key	personnel	

o The	first	is,	obviously,	Don	Eyles,	whose	knowledge	of	the	software	made	it	

possible	to	quickly	understand	the	problem,	understand	the	work-around	

quickly	enough	to	write	a	new	code	sequence,	test	it	and	find	lurking	bugs,	and	

then	complete	the	procedural	change	sequence	and	get	it	radioed	up	to	the	crew	

in	time	to	do	a	successful	landing	

o The	second	is	Ed	Mitchell,	who	from	the	start	of	his	participation	in	Apollo	had	

specialized	in	the	LM	and	its	subsystems,	particularly	the	LGC,	making	him	one	of	

the	most	knowledgeable	astronauts	about	the	vehicle	

o The	third	is	Fred	Haise,	who	had	a	similar	level	of	understanding	of	the	LM	and	in	

his	role	as	CAPCOMM,	was	able	to	understand	and	call	up	the	procedure	for	

spoofing	the	LGC	so	Mitchell	could	implement	it	

§ "I	was	on	the	backup	crew	for	[Apollo]	10.	Also,	I'd	been	on	the	support	crew	for	[Apollo]	

9.	Fred	Haise	and	I	probably	knew	more	about	the	lunar	module	than	any	two	guys	alive	

at	that	point,	since	we	helped	build	it	at	Grumman/Bethpage	(the	facility	at	Bethpage,	

New	York,	where	the	LMs	were	designed	and	built).	And	had	been	through	all	the	cycles	

with	all	the	spacecraft	-	with	all	of	the	lunar	modules.”	Ed	Mitchell,	ALSJ	debrief	

comments	after	MET	106:38:13.

o Without	all	three	of	these	guys,	it	is	unlikely	Apollo	14	would	have	been	

successful	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	104	of	154	
Fly	Me	To	The	Moon	–	Apollo	14	Assessment	

• Apollo	14	also	points	out	the	need	to	get	into	the	code	and	be	able	to	rewrite	it	in	

real	time	to	meet	a	short-timeline	contingency	

o Regardless	of	the	talents	of	the	crew,	the	MOCR	team	and	the	software	folks,	it	would	

have	been	for	naught	if	they	were	unable	to	figure	out	a	solution,	and	unable	to	

implement	it	in	real	time	

• The	Landing	Radar	issue	shows	that	even	with	simple	fixes	(cycling	a	switch)	can	

work	in	a	pinch	

o Ed	Mitchell’s,	“...whew,	that	was	close”	comment	when	they	had	good	Landing	Radar	

data	was	spot	on!	

o Lack	of	Landing	Radar	data	would	have	meant	a	mandatory	abort,	which	would	have	

been	doubly	frustrating	after	all	the	work	to	get	them	to	PDI	

o The	Apollo	14	crew	was	very	lucky,	particularly	that	the	Landing	Radar	data	fix	

(cycling	the	circuit	breaker)	was	simple	compared	with	the	code	strings	that	Mitchell	

had	to	upload	while	the	Braking	Phase	was	underway	

• Lastly,	the	ability	to	land	as	close	to	the	planned	touchdown	spot	was	critical	to	

completing	the	science	objectives,	given	that	the	Lunar	Roving	Vehicle	(LRV)	was	

not	available	until	Apollo	15	

o As	it	was,	Apollo	14	EVA	2	was	the	most	physically	challenging	of	all	the	Apollo	EVAs,	

with	the	largest	metabolic	rates	during	Apollo,	due	to	the	difficulty	of	walking	uphill	

in	the	A7L	pressure	garment	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	105	of	154	
Fly	Me	to	the	Moon	

Apollo	15
Fly	Me	To	The	Moon	–	Apollo	15	

• Apollo	15,	16	and	17	jointly	comprised	the	“J-missions”	–	missions	that	included	

an	upgraded	LM	(often	referred	to	as	a	“Block	II	LM”)	with	more	delta-v,	an	

extended	DPS	engine	bell	and	the	capability	for	increased	downmass	

• These	upgrades	allowed	the	LMs	to	carry	sufficient	consumables	for	three	days	on	

the	lunar	surface,	and	the	ability	to	carry	a	Lunar	Roving	Vehicle	(LRV),	which	

increased	the	crew’s	range	of	travel	from	no	more	than	≈2	km	to	as	far	as	10	km	

away	from	the	LM	

• Making	good	use	of	these	capabilities	for	exploration	dictated	the	need	to	land	in	

more	challenging	terrains	–	approaching	over	some	of	the	steepest	mountains	on	

the	Moon,	landing	in	the	highlands,	and	landing	into	relatively	narrow	alpine	

valleys	

• The	Apollo	15	mission	was	the	first	to	test	these	capabilities,	landing	at	a	maria	

site	in	western	Mare	Imbrium	that	would	require	an	approach	over	the	Apennine	

Front,	which	had	a	maximum	altitude	of	≈15,000	ft	and	a	12,000	ft	ridgeline	that	

the	LM	approach	path	crossed	over	at	22,000	ft	

o Because	of	the	terrain	clearance	considerations,	Apollo	15	approach	path	from	

pitchover	to	touchdown	would	be	twice	as	steep	as	the	earlier	landings	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	107	of	154	
Fly	Me	To	The	Moon	–	Apollo	15	

• LM	undocking	came	at	MET	100:39:37,	about	20	minutes	late	due	to	a	minor	issue	

with	the	CSM	probe	umbilicals	which	CMP	Worden	had	to	take	care	of	prior	to	

undocking	

o Otherwise,	undocking	and	separation	was	nominal,	and	the	CDR	(Scott)	and	the	LMP	

(Irwin)	went	through	normal	pre-PDI	checks,	including	helping	CMP	with	prep	and	

execution	of	the	CSM	circularization	burn	

• At	MET	104:09:14,	about	20	minutes	prior	to	PDI,	the	crew	put	a	3.3	nm	“south	

to	north	(left	to	right)”	correction	into	the	LGC	using	a	NOUN	61,	based	on	MSFN	

tracking	data;	this	meant	that	sometime	during	PDI,	the	LM	would	yaw	to	change	

the	DPS	thrust	vector	to	put	in	that	correction	

o “This	was	important,	but	an	MCC	input	which	did	not	affect	the	final	descent	from	our	

perspective	in	the	LM.			If	MCC	made	the	call	at	this	point,	we	complied	without	

hesitation,	MCC	“flew”	the	vehicle	(had	the	stick)	until	pitchover	(and	then	we	had	the	

stick).”	[Dave	Scott,	e-mail	to	Eppler,	2	January	2020]	

• PDI	occurred	at	MET	104:30:12,	with	nominal	engine	sequencing	and	

performance	

o The	Braking	Phase	proceeded	nominally	throughout,	with	a	DLAND/NOUN	69	

downrange	correction	to	the	trajectory	of		2800	ft	about	1	min	30	sec	into	the	burn	

o At	about	3	minutes	into	the	burn,	the	LM	yawed	right	to	“pure	face	up”	to	correct	an	

earlier	left	yaw	to	get	better	comm	coverage	earlier	in	Braking	Phase	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	108	of	154	
Fly	Me	To	The	Moon	–	Apollo	15	

• At	MET	104:33:25,	the	Altitude/Velocity	lights	went	out,	and	the	Delta-H	calculation	(3400	

ft	difference)	was	acceptable,	allowing	the	crew	to	accept	Landing	Radar	data	into	the	LGC	

• At	MET	104:38:38,	about	10	secs	prior	to	pitchover,	Scott	began	to	see	the	mountains	in	the	

lower	corner	of	his	window	at	an	H	of	≈9,000	ft	

• At	about	MET	104:39:00,	the	MOCR	determined	that	the	LM	was	targeted	south	of	the	

planned	landing	spot	by	≈3,000	ft;	

o Although	the	Flight	Director	originally	was	against	relaying	that	information	to	the	crew,	Ed	Mitchel,	the	

CAPCOMM,	felt	it	was	important	to	keep	Scott	and	Irwin	apprised	and	shortly	relayed	that	data	

o 104:39:17	Mitchell:	“Falcon,	Houston.	We	expect	you	may	be	a	little	south	of	the	site,	

maybe...”	

o 104:39:18	Irwin:	“Okay.	Coming	up	on	8000	(feet	altitude).”	

o 104:39:19	Mitchell:	“...3000	feet.”	

o 104:39:24	Scott:	“(Responding	to	Mitchell)	Okay.”	

§ “As	noted	from	this	exchange,	during	final	descent,	MCC	told	us	we	were	about	3,000	feet	south	of	the	

planned	touchdown	point.	So,	immediately	after	pitchover,	I	began	moving	us	north	with	the	LPD.		

However,	when	I	could	finally	identify	some	landmarks,	I	realized	we	were	going	too	far	north,	and	

began	to	move	us	back	south	with	the	LPD.		Eventually	we	got	pretty	close	to	the	original	target	

point.		But	lo	and	behold,	when	we	received	the	3,000	ft	south	call,	the	state	vector	trajectory	had	

already	been	corrected	for	the	3,000	feet;		therefore,	we	were	actually	on	the	correct	trajectory	when	

we	got	the	3,000	feet	north	call	-	that	is,	MCC	made	a	double	correction.		See	page	253	and	Note	58,	

page	300,	in	Digital	Apollo.		This	double	correction	plus	the	simulator	confusion	(also	in	Digital	

Apollo),	made	the	selection	of	a	touchdown	point	quite	a	challenge....!!	[Dave	Scott,	e-mail	to	Eppler,	

2	January	2020]

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	109	of	154	
Fly	Me	To	The	Moon	–	Apollo	15	

• At	MET	104:39:32,	the	LGC	hit	the	P63	Tgo	of	zero	and	rolled	over	into	P64,	

initiating	pitchover	at	High	Gate	

o The	Braking	Phase	took	almost	a	minute	longer	than	on	previous	flights	due	to	the	

increased	weight	of	the	upgraded	LM	

o Scott,	from	the	1971	Technical	Debrief	-	"When	we	pitched	over,	we	got	P64	right	on	

time...I	couldn't	convince	myself	that	I	saw	Index	Crater	[a	key	landmark	–	DE]	

anywhere.	I	saw,	as	I	remember,	a	couple	of	shadowed	craters,	but	not	nearly	as	many	as	

we	were	accustomed	to	seeing	(in	the	LM	simulator)...We	were	south,	and	I	redesignated	

immediately	(with	the	LPD)	four	clicks	to	the	right	(north)	and	then,	very	shortly	

thereafter,	after	[Irwin]	called	me	again	on	the	LPD	numbers,	I	redesignated	two	more	

right	and	three	uprange...		

o Scott,	from	the	1971	Technical	Debrief	–	“I	got	busy,	at	that	time,	attempting	to	select	a	

point	for	the	actual	landing.	I	guess	our	pre-flight	philosophy	had	been	that,	if	we	were	

on	target,	we	would	try	to	land	exactly	on	target.	If	we	had	a	dispersion,	we	would	select	

some	point	within	the	1-kilometer	circle	which	looked	like	a	good	place	to	land	and	

(then)	would	land	as	soon	as	possible	so	as	not	to	get	behind	in	the	propellant	curve.	

Once	I	realized	that	we	were	not	heading	for	the	exact	landing	site,	and	that	I	didn't	

have	a	good	location	relative	to	Index	Crater,	I	picked	what	I	thought	was	a	reasonably	

smooth	area	and	headed	directly	for	that.”	[ALSJ,	multiple	integrated	comments,	quoted	

after	MET	104:40:28]	

o As	noted	earlier,	Scott	would	go	on	to	make	18	total	redesignations,	more	than	any	

other	landing

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	110	of	154	
Fly	Me	To	The	Moon	–	Apollo	15	

• At	MET	104:41:08	and	at	H=400	ft,	Scott	switched	the	LGC	into	P66	and	began	to	set	up	for	

touchdown	

• At	MET	104:41:51	(H=60	ft),	blowing	dust	began	to	obscure	the	landing	spot	

o At	this	point,	Scott	largely	nulled	his	lateral	velocity	and	brought	the	LM	down	to	the	surface	with	

ROD	switch	pulses	

o Scott’s	situational	awareness	was	largely	coming	from	Irwin’s	crisp	annunciation	of	LM	attitude	

and	velocity	data,	based	on	LGC	and	cockpit	instruments	

§ ALSJ	Comment	by	Scott	after	MET	104:41:54:	“I	was	still	[looking]	outside.	We	had	trained	enough	

that	Jim	would	call	anything	to	me	that	didn't	look	exactly	right.	And	he	would	call	the	things	that	I	

wanted	that	looked	right.	So	what	he	was	telling	me	is	something	I	would	have	normally	looked	at,	at	

the	last	moment,	to	check	myself.	In	the	dust,	I	can't	see	whether	we're	going	this	way	or	that	way.	I	

could	have	looked	at	them	[the	Cross	Pointer	Instrument	showing	lateral	velocities-	DE].	But	I	didn't	

have	to	look	at	them	because	he	did	it	for	me,	'cause	he	knew	I	wanted	to	look	at	them,	and	I	knew	he	

knew	what	he	was	looking	at.”	

• At	MET	104:42:29,	Irwin	called	the	Contact	Light	on,	and	Scott	immediately	hit	the	Engine	

Stop	Switch	

o MET	104:42:29	Irwin:	“Contact.	(Pause)	Bam!”	[ALSJ	callout]	

o Irwin	-	"We	did	hit	harder	than	any	of	the	other	flights!	And	I	was	startled,	obviously,	when	I	said,	

'Bam!'	“	[ALSJ	callout	after	MET	104:42:29]	

o “As	noted	[in	the	ALSJ	–	DE],	we	landed	in	on	a	crater	rim.		“Contact”	by	the	+Z	(forward)and	+Y	

(right)	footpads	and	by	Jim’s	call	was	at	1	fps	(target	rate),	but	was	actually	was	at	0.5	fps;	this	was	

followed	by	the	other	footpads		(-Y	(left)	and	–Z	(aft))	into	the	crater	(as	the	LM	rotated	about	4	feet	

into	the	crater	to	final	“touchdown”	at	which	the	descent	rate	was	6.8	fps	(thus	the	firm	impact	of	

the	final	legs).		This	resulted	in	a	final	tilt	of	about	11	degrees.		The	crater	was	about	5	feet	deep	and	

15	to	25	feet	across.		The	descent	engine	was	shutdown	immediately	upon	contact	due	to	the	

anticipated	pressure	buildup	inside	the	lengthened	engine	bell	and	potential	backpressure	into	the	

descent	stage	(unlike	e.g.,	A-12	when	Pete	counted	two	seconds	until	he	shut	the	engine	down).”	

Dave	Scott	e-mail	to	Eppler,	2	January	2020.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	111	of	154	
Fly	Me	To	The	Moon	–	Apollo	15	Assessment	

• The	approach	to	landing	on	Apollo	15	was	about	as	nominal	as	you	could	get	–	in	

fact,	the	discussion	between	the	crew	and	the	ground,	and	between	Scott	and	

Irwin,	was	so	casual	that	you	would	think	they	were	shooting	a	VFR	straight-in	

approach	to	Ellington	Field	

• First,	I	think	this	is	an	outcome	of	Scott	spending	a	lot	of	time	talking	to	the	

previous	crews	and		thinking	about	how	he	would	fly	the	approach	

o Scott	ALSJ	comment	after	MET	104:42:48:	“Another	objective	we	had,	based	on	

the	previous	flights,	was	to	stay	on	a	constant	flight	path	-	a	constant	rate	of	

descent	-	and	get	it	down.	The	previous	flights...had	all	leveled	out	high	and	then	

had	come	back	down.	And	we	looked	at	their	trajectories,	and	it	seemed	to	be	a	

trend,	that	the	guys	would	start	stopping	too	soon	and	use	up	a	lot	of	propellant,	

doing	a	stairstep	thing.	So	one	of	the	things	that	we	trained	on	and	thought	about,	

was	to	keep	it	going	and	keep	it	coming	down	a	constant	flight	path	so	that	we	

could	save	gas	for	the	hover,	if	we	needed	it.	The	stairstep	appeared	to	be	a	trend	

that	people	got	into	because	there's	no	definition	on	the	ground.	There's	no	

runway...There's	nothing	there	to	tell	you	how	high	you	are;	and	I	think	the	trend	

had	been	for	people	to	start	slowing	up	their	rate	of	descent	too	soon	-	because,	of	

course,	you	don't	want	to	get	too	close,	too	fast,	'cause	then	you	can't	stop.”	

o Watching	the	cockpit	view	from	High	Gate	to	landing,	the	approach	to	landing	is	

marked	by	a	continuous	descent	right	to	the	surface	(it’s	actually	a	little	scary	to	

watch),	with	little	hesitation	in	either	the	approach	angle	or	descent	rate,	right	

down	to	the	P64-P66	transition	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	112	of	154	
Fly	Me	To	The	Moon	–	Apollo	15	Assessment	

• Second,	I	think	Scott	and	Irwin	spent	a	lot	of	time	working	out	their	“cockpit	protocol”:	who	

would	do	what,	the	data	they	would	exchange,	how	they	would	communicate,	and	then	

trained	the	approach	exactly	as	they	would	fly	it

o Scott	ALSJ	comment	after	MET	104:42:48:	"As	I	recall,	I	went	out	the	window	as	soon	as	we	

got	down	there	(that	is,	at	pitchover).	Everything	inside	is	for	Jim.	For	me	to	come	in	and	go	

back	out,	really	takes	too	much	time...I	wanted	as	much	from	Jim	as	I	could	get.	I	mean,	I	

was	outside	the	window.	Everything	from	inside	the	cockpit	was	from	Jim.	So	I	had	a	lot	of	

sources	of	information...So	when	Jim	and	I	worked	on	this,	I	remember	we	worked	on	him	

giving	as	much	as	he	could,	because	I	wasn't	going	to	do	any	talking.	I	was	going	to	do	the	

flying.	I	was	going	to	do	outside	the	window,	and	he	was	going	to	tell	me	what	was	going	on	

inside.	We	were	comfortable	doing	it	that	way.”	

o The	result	was	concise	communication	between	the	crewmembers	during	the	entire	

landing	

• In	addition,	the	LM	worked	flawlessly,	and	the	coordination	between	the	crew	and	the	

ground	was	also	spot	on	

• In	short,	Apollo	15	represented	a	kind	of	[DE	words	here...]	“plateau	of	maturity”	that	

integrated	all	the	lessons	learned	from	the	first	three	landings	and	applied	them	to	making	

lunar	landing,	in	effect,	routine	

• The	Apollo	15	would	end	up	doing	3	EVAs	with	the	LRV,	and	returned	77	kg	of	samples,	

including	the	famed	“Genesis	Rock”,	which	remains	one	of	the	most	iconic	samples	returned	

on	Apollo	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	113	of	154	
Fly	Me	to	the	Moon	

Apollo	16
Fly	Me	To	The	Moon	–	Apollo	16	

• LM	undocking	came	at	MET	96:13:33	after	a	largely	uneventful	pre-undocking	prep	

o During	the	LM	prep	for	undocking,	it	was	discovered	that	the	LM	S-band	antenna	was	not	rotating	

around	the	yaw	axis,	although	it	appeared	the	antenna	itself	was	working	OK	

§ It	was	decided	that	the	output	bit	rate	through	the	Omni	antennas,	if	captured	by	the	Goldstone	

210	ft	antenna,	would	be	more	than	adequate	to	support	LM	descent	and	landing	

- However,	updates	to	the	LM	state	vector	could	not	be	uploaded	automatically	as	in	previous	

flights,	and	now	had	to	be	sent	up	verbally	and	loaded	by	the	LM	crew	(CDR	John	Young	and	

LPM	Charlie	Duke)	by	hand	

o In	addition,	there	were	regulator	issues	in	the	LM	RCS	tanks	that	indicated	there	was	a	leak	in	the	

system,	requiring	fuel	transfers	to	keep	tank	pressures	within	limits	

§ In	the	end,	RCS	System	A	was	deemed	sufficiently	healthy	to	conduct	the	landing	

• At	MET	≈97:30:00,	a	significant	issue	arose	during	CSM	prep	for	initiating	the	CSM	

circularization	burn		

o During	CSM	pre-burn	checkout,	the	SPS	Yaw	Gimbal	Number	showed	significant	oscillation	every	

time	it	was	actuated,	vibrating	the	CSM	

o If	not	corrected,	this	would	require	the	LM	to	redock	and	bring	the	CSM-LM	stack	up	to	the	

nominal	60	nm	circular	orbit,	aborting	the	landing	and	triggering	TEI	burn	in	several	hours	with	

the	DPS	

§ MET	97:37:35	Mattingly	(CM	comm):	”Okay.	Brother,	what	a	way	to	start	the	day,	huh?”	

§ MET	97:40:30	Mattingly	(CM	comm):	”...my	burn	rules	say	I	got	to	have	two	sets	of	servo	loops	-	

two	on	each;	got	to	have	all	four	servo	loops	to	go.”	

§ MET	97:40:39	Young	(LM	comm):	”I	think	that's	right,	Ken.”	

§ MET	97:40:40	Mattingly	(CM	comm):	”I'm	sorry,	gang.	I	don't	know	what	to	do	with	the	darn	

thing.	It	does	it	both	when	the	CMC	drives	it	and	when	we	drive	it.	I've	started	it,	restarted	it,	and	it's	

just	apparently	really	in	the	servo	loop.”	

o As	per	mission	rules	(see	Slide	48),	this	caused	a	temporary	postponement	of	Powered	Descent	

Insertion	on	the	first	available	opportunity	while	the	problem	was	worked	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	115	of	154	
Fly	Me	To	The	Moon	–	Apollo	16	

• At	MET		98:11:15,	CMP	Mattingly	began	working	the	problem	with	the	MOCR	

while	both	spacecraft	initially	co-orbited	at	a	separation	range	of	≈1	nm	and	

then	ultimately	re-rendezvoused		

• After	several	hours	of	analysis	and	simulations,	it	was	decided	that	the	

problem	did	not	constitute	a	structural	hazard	to	the	CSM	and	operation	of	

the	SPS	was	not	compromised	

o MET	102:01:36	CAPCOM	Irwin:	”Okay,	Orion	can	always	tell	Casper	what	his	

problem	is,	but	it	looks	like	an	open	circuit	in	the	rate	feedback	and	your	servo	loop.	

We've	run	exhaustive	tests	down	here	on	the	West	Coast	and	East	Coast	on	

controllability	aspects	and	structural	aspects,	and	everything	looks	satisfactory...So	

we're	convinced	down	here	that	we	have	a	satisfactory	control	mode	if	we	have	to	

revert	to	that	one.”	

• Subsequently,	the	LM	was	given	and	Go	for	PDI,	which	commenced	at	MET	

104:17:26,	three	orbits	and	≈6	hours	late	

o The	delay	cost	the	crew	some	hours	on	the	surface,	and	postponed	the	first	EVA	

until	after	a	6-hour	sleep	period	

o It	was	realized	that,	if	the	original	flight	plan	was	followed,	the	LM	crew	would	

have	done	a	6-8	hr	EVA	directly	after	landing,	leading	to	an	≈27	1⁄2	hour	long	day	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	116	of	154	
Fly	Me	To	The	Moon	–	Apollo	16	

• PDI	occurred	at	MET	104:17:28,	with	nominal	engine	sequencing	and	

performance	

o The	Braking	Phase	proceeded	nominally	throughout,	with	a	DLAND/NOUN	69	

downrange	correction	to	the	trajectory	of	only	800	ft	about	1	min	44	sec	into	the	

burn	

o This	would	be	the	smallest	NOUN	69	correction	of	any	Apollo	landing	

o If	nothing	else,	it	shows	that	the	extra	three	orbits	managing	the	CSM	problem	did	not	

cause	significant	trajectory	dispersion	(another	problem	from	Apollo	11	solved...)	

• In	addition	to	the	low	N69	correction,	the	Altitude/Velocity	Lights	went	out	a	

50,000	ft,	earlier	than	any	other	landing,	indicating	very	good	performance	by	

the	Landing	Radar	

o 104:21:02	Duke:	“I	say,	there's	no	way	to	get	the	Altitude	light	at	this	height.	(Pause)	

(Four)	minutes...	(Pause)”

o 104:21:28	Duke:	“We're	50,000	(feet).”

o 104:21:30	Young:	“Look	at	that!	Altitude	and	Velocity	lights	are	out	at	50k!”

o 104:21:34	Duke:	“Isn't	that	amazing?	Copy	that,	Houston?”

o 104:21:38	Irwin:	“We	copy.”	

• Throttle-down	occurred	at	MET	104:24:54,	exactly	on	time	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	117	of	154	
Fly	Me	To	The	Moon	–	Apollo	16	

• P63	–	P64	transition	occurred	at	MET	104:26:55,	with	pitchover	showing	that	the	LM	was	

targeted	for	the	planned	landing	site	

• During	the	Approach	Phase,	the	CDR	retargeted	the	LM	a	number	of	times	to	take	out	

dispersions	in	the	spot	targeted	for	landing	

o Young,	from	the	Apollo	16	Mission	Report	-	"After	pitchover,	a	comparison	of	the	Landing	

Point	Designator	with	the	computer	and	the	movement	of	the	vehicle	showed	that,	if	no	

further	trajectory	corrections	were	made,	the	Lunar	Module	would	land	approximately	600	

meters	north	and	400	meters	west	of	the	center	of	the	landing	ellipse.	Therefore,	between	

altitudes	of	3000	and	4000	feet,	an	estimated	total	of	five	redesignations	to	the	south	were	

made.	The	vehicle	responded	properly.”	[ALSJ	after	MET	104:27:02]	

o Young,	from	the	1972	Technical	Debrief	–	“I	think	the	LPD	was	perfect.	I	don't	have	any	

gripes	there	whatsoever.	When	we	pitched	over,	we	were	north	and	long	and	you	could	see	

that.	I	was	just	letting	the	LM	float	in	there	until	I	could	see	where	it	was	going.	I	took	out	

the	north	because,	according	to	our	pre-flight	maps,	the	north	country	was	a	little	rougher.	

There	were	more	(map)	contour	lines	up	north	and	down	south;	so	we	took	those	out	and,	

when	we	got	in	close,	we	backed	up	a	little	and	put	in	some	rear	updates."	[ALSJ	after	MET	

104:27:02]	

• The	CDR	would	ultimately	make	12	redesignations	in	P64,	although	he	did	not	try	to	put	

the	LM	at	the	exact,	pre-planned	landing	spot	

o Young,	from	the	Apollo	16	Mission	Report	-	"It	was	clear	that	the	vehicle	was	going	to	be	

north	and	west	of	the	pre-mission	designated	landing	spot	-	75	meters	north	of	Double	Spot	

Craters.	However,	there	was	no	major	attempt	to	land	at	the	pre-mission	designated	spot,	

nor	had	there	been	any	intent	to	do	so	prior	to	the	flight	because	the	surface	traverse	

capabilities	of	the	Lunar	Rover	negated	the	requirement	to	land	precisely	at	the	designated	

landing	spot."	[ALSJ	after	MET	104:27:20]

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	118	of	154	
Fly	Me	To	The	Moon	–	Apollo	16	

• The	LM	continued	in	P64	down	to	<300	ft	before	the	CDR	transitioned	into	P66	with	ROD	clicks	

at	≈250	ft	

o Unlike	the	other	Apollo	landings,	the	LMP	spent	more	time	looking	out	the	window	and	giving	the	CDR	

specific	flight	control	inputs,	specifically	information	on	ROD	clicks		

§ MET	104:28:45	Duke:	“Okay,	200	feet,	11	(fps)	down.	Give	me	a	couple	of	clicks	up.”			

- Duke	-	"He's	going	down	too	fast.	Okay?	And	our	profile	calls	for	5	foot	per	second,	H-dot;	and	

he's	11.	And	12	was	our	maximum.	And,	so,	I	wanted	him	to	slow	down	a	little	bit.	We	were	

sinking	too	fast.	I	wanted	to	get	him	closer	back	to	profile.”	[ALSJ	debrief	comment	after	MET	

104:28:25]	

• At	MET	104:28:29,	a	distinct,	recognizable	LM	shadow	appears	in	the	direct	down-Sun	spot,	

which	gave	the	CDR	a	superb	reference	for	descent	rate	and	landing	spot	

o Young,	from	the	1972	Technical	Debrief	-	"At	that	Sun	angle,	we	could	see	the	rocks	(through	the	dust)	all	

the	way	to	the	ground	and	I	think	that	was	a	great	help.	From	200	feet	down,	I	never	looked	in	the	cockpit.	

It	was	just	like	flying	the	LLTV;	your	reference	is	to	the	ground	outside.	You	had	another	thing	that	nobody	

has	ever	remarked	about	before,	and	that	was	the	shadow.	I	really	didn't	have	any	doubt	in	my	mind	how	

far	above	the	ground	we	were	with	that	shadow	coming	down	(that	is,	getting	closer	to	them	as	they	

approach	the	surface).	I	had	no	scale	of	reference	to	the	holes;	but,	with	the	shadow	out	there	in	front	of	

you	and	coming	down,	it	really	takes	all	of	the	guesswork	out	of	it.	For	that	kind	of	Sun	angle,	if	the	radar	

had	crumped,	I	don't	think	you'd	have	had	a	bit	of	trouble	in	just	going	right	in	and	landing	just	like	a	

helicopter.	First,	we	could	see	the	thing	(rocks	and	other	surface	features)	all	the	way	to	the	ground;	

second,	the	shadow	was	right	there	to	help	you	with	the	rate	of	descent.”	

• At	MET	104:29:22,	the	CDR	came	to	a	hover	at	≈40	ft,	and	with	lateral	rates	nulled,	settled	the	

LM	onto	the	surface	

• At	MET	104:29:36,	the	Contact	Light	illuminated,	the	CDR	hit	the	Engine	Stop	switch	and	the	LM	

dropped	onto	the	surface	at	a	vertical	speed	of	≈1.8	ft	s-1	

o 104:29:36	Duke:	“Contact!	Stop.	(Pause	while	they	drop	to	the	surface)	Boom.”		

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	119	of	154	
Fly	Me	To	The	Moon	–	Apollo	16	Assessment	 • Once	the	CSM	SPS	gimballing	issues	got	straightened	out,	the	Apollo	16	approach	and	landing	was	another	wellexecuted	evolution	

• Although	the	crew	dynamic	was	different	than	on	Apollo	15,	Young	and	Duke	clearly	worked	well	together,	

including	LMP	Duke	giving	the	CDR	specifically	flight	directions	so	Young	could	continue	to	look	out	the	

window	and	assess	the	landing	site	

o Duke,	from	the	1972	Technical	Debrief	-	"We'd	agreed	(pre-flight)	that	I	was	going	to	look	out	since	I	had	two	good	craters	

on	my	side	(Palmetto	and	Dot).	[ALSJ	Comment	after	MET	104:27:08]”

• As	with	Apollo	15,	Young	took	the	LM	down	close	to	the	surface	(≈250	ft)	before	he	took	over	and	moved	the	

LGC	into	P66	with	ROD	clicks	

o According	to	the	debrief	comments,	he	briefly	considered	letting	the	LM	land	itself	before	having	concerns	as	to	the	

surface	roughness	at	the	auto-designated	landing	spot	

o Young,	from	the	1972	Technical	Debrief	-	"It	was	working	so	well	I	was	tempted	to	let	it	(the	PGNS)	do	the	thing	all	by	itself;	

but	the	trouble	is,	we	got	down	low	and	I	could	see	that	we	were	going	to	land	in	that	pothole	down	there.”	[ALSJ	Debrief	

Comment:	The	'pothole'	in	question	is	a	shallow,	15-meter	crater.	John	will	overfly	it	and	land	with	his	rear	foot	pad	about	

3	meters	beyond	the	west	rim	of	the	crater.]	[ALSJ	Comment	after	MET	104:28:33]	

o Note	the	following	ALSJ	exchange	as	Young	got	out	on	EVA-1:	

§ MET	119:04:43	Young:	...Well,	we	broke	the	probes	off,	going	straight	down.	The	probes	are	all	standing	straight	

up.	

- There	were	probes	hanging	down	from	three	of	the	four	footpads	to	trigger	the	Contact	light.	Each	probe	was	

68	inches	(1.73	meters)	long...During	the	landing,	the	probes	bent	as	the	spacecraft	fell	the	last	few	feet	to	the	

surface;	and	the	probe	orientations	were	a	good	indication	of	spacecraft	motions	during	the	last	seconds	of	the	

landing.

• Apollo	16	was	the	only	“pure	highland”	site	visited	on	Apollo,	and	the	samples	returned	were	not	only	unique,	

they	were	different	from	the	results	expected	from	pre-mission	photogeologic	planning	

o The	crew’s	EVA	performance,	in	the	face	of	site	geology	that	was	different	from	the	mission	plans	and	shortened	EVAs,	

was	superb,	and	it	put	their	extensive	geologic	training	to	work,	validating	the	benefit	of	detailed	geology	training	

o One	complex	breccia	(“Big	Muehly”,	the	largest	rock	returned	on	Apollo	at	11.6	kg)	is	still	being	carefully	dissected	and	

studied

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	120	of	154	
Fly	Me	to	the	Moon	

Apollo	17
Fly	Me	To	The	Moon	–	Apollo	17	

• Apollo	17	was	the	last	mission,	but	it	was	one	that	broke	new	ground	

• First	and	foremost	was	the	assignment	of	field	geologist	Harrison	Schmitt	as	

LMP,	the	first	scientist-astronaut	to	be	assigned	to	a	mission	

o Schmitt’s	assignment	caused	angst	in	some	circles	because	he	was	not	a	test	

pilot,	although	he	had	in	excess	of	1,000	hrs	flight	time	in	high	performance	

jets	

§ Although	some	of	the	test	pilots	felt	that	his	experience	was	not	adequate	to	fly	as	an	LMP,	

the	belief	was	not	universal	

o Schmitt	and	CDR	Cernan	would	go	on	to	build	a	remarkable	partnership	

where	Schmitt	functioned	as	an	extremely	capable	LMP	to	Cernan,	while	

Cernan	learned	to	be	a	highly	capable	field	assistant	for	Schmitt	while	they	

were	executing	their	EVAs	

o Cernan	-	"In	thinking	about	why	we	worked	so	well	together,	the	most	important	point	was	

that	we	had	confidence	in	each	other.	Jack	is	a	very	astute	and	dedicated	individual,	very	

talented,	and	I	felt	very	comfortable	with	him.	He	was	not	an	aviator	and	he'll	be	the	first	to	

tell	you	that.	But	he	took	his	responsibilities	very	seriously,	learned	quickly,	and,	in	terms	of	

technical	understanding,	was	as	good	as	anybody.	He	wasn't	a	passenger;	he	had	learned	how	

to	be	a	good	co-pilot.	In	much	the	same	way,	I	wasn't	a	geologist,	I	wasn't	a	scientist;	but	I	had	

learned	to	be	a	good	observer.	As	soon	as	we	hit	the	lunar	surface,	although	I	was	still	

commander	of	the	crew,	Jack	was	the	expert	and	I	was	his	assistant.	We	had	very	different	

backgrounds....but	we	both	knew	enough	about	all	aspects	of	the	missions	that	we	could	be	

confident	about	working	together.”	[ALSJ	Comment	after	AS-17	MET	113:02:30]

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	122	of	154	
Fly	Me	To	The	Moon	–	Apollo	17	

• An	operational	issue	that	was	initially	raised	was	that	the	site	chosen	for	-17	

was	the	Taurus-Littrow	valley,	a	narrow,	fault-bounded	valley	in	the	

mountains	on	the	southeastern	edge	of	Mare	Serenitatis	

o The	site	was	chosen	because	it	had	a	variety	of	significant	geologic	features,	

including	potentially	young	volcanic	vents	and	steep,	fault-bounded	massifs	that	

shed	boulders	on	to	their	lower	slopes	from	identifiable	rock	units	on	the	upper	

slopes	

§ The	juxtaposition	of	these	different	rock	units,	combined	with	the	ability	to	range	

over	the	valley	with	the	LRV,	had	the	potential	to	allow	a	complete	field	study	of	a	

single	locality,	as	well	as	returning	some	of	the	oldest	and	they	youngest	lunar	rock	

units	

o Once	the	site	was	chosen,	MPAD	began	working	the	approach	path	and	the	

expected	landing	ellipse	and	discovered	there	was	an	issue	with	fitting	the	

ellipse	into	the	valley	

§ Simply	put,	the	3-σ	cross	range	component	initially	the	included	steep	slopes	of	the	

North	Massif,	indicating	that	there	was	a	low	but	finite	probability	that	the	LM	could	

end	up	on	an	approach	trajectory	into	a	landing	spot	that	was	too	rugged,	or	had	

slopes	that	were	too	steep	to	land	safely	

o However,	MPAD	determined	that	scheduling	a	late	state	vector	update	to	the	LM	

prior	to	PDI,	which	was	in	turn	based	on	precise	landmark	tracking	by	the	CSM,	

allowed	landing	ellipse	to	be	reduced	sufficiently	to	fit	safely	into	the	TaurusLittrow	Valley

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	123	of	154	
Fly	Me	To	The	Moon	–	Apollo	17	

• At	MET	112:49:56,	after	a	nominal	DOI	burn,	entry	into	the	LM	and	CSM-LM	

separation,	the	crew	began	the	Braking	Phase	

o Prior	to	Apollo	17,	CDR	Cernan	had	served	as	Stafford’s	LMP	on	Apollo	10,	during	

which	there	was	an	uncontrolled	roll	and	pitch	excursion	of	the	LM	at	staging	

because	of	miscommunication	in	the	cockpit	on	switch	settings	

o As	a	result	of	this,	Cernan	decided	that,	with	very	few	exceptions,	the	CDR	was	going	

to	do	all	the	key	entries	into	the	LGC	on	the	DSKY	and	the	LMP	would	do	the	entries	

into	the	AGS	on	the	DEDA	

• At	MET	112:50:59,	the	crew	was	given,	and	entered	a	NOUN	69	correction	to	the	

descent	trajectory	of	3,400	ft	

• In	addition,	at	about	the	same	time	as	PDI,	the	descent	fuel	quantity	light	came	

on,	indicating	a	low	propellant	quantity	

o Although	not	an	urgent	issue	at	that	point,	it	was	a	nagging	issue	for	the	first	two	

minutes	of	the	Braking	Phase	

o At	MET	112:51:51,	CAPCOM	Fullerton	suggested	the	crew	cycle	the	Propellant	

Quantity	Gauging	System	(PQGS)	switch,	which	fixed	the	problem	(amazing	how	that	

approach	even	works	on	the	LM...)	

• At	MET	112:52:31,	PNGCS	and	AGS	converged	and	remained	in	good	agreement	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	124	of	154	
Fly	Me	To	The	Moon	–	Apollo	17	

• From	MET	112:54:15,	the	Altitude/Velocity	lights	went	out,	indicating	a	

good	Landing	Radar	lock	on	the	lunar	surface	

o The	Delta-H	comparison	in	the	LGC	looked	good,	and	at	MET	112:54:28	the	crew	

did	a	VERB	57	and	accepted	radar	data	into	the	LGC	

• At	MET	112:56:07,	the	LM	flew	over	the	rough	terrain	that	was	uprange	of	

the	landing	site,	causing	the	Delta-H	calculation	to	respond	to	the	difference	

between	the	calculated	“assume	a	spherical	Moon”	approach	of	the	LGC	with	

the	real	terrain	data	provided	by	the	Landing	Radar	

o MET	112:56:07	Cernan:	“Okay.	(Pause)	Okay,	Houston.	As	we	went	over	the	

hump,	Delta-H	just	jumped.”	

o MET	112:56:16	Fullerton:	“Roger.”	

§ Cernan	-	"The	PGNS	was	keeping	track	of	our	altitude	relative	to	the	landing	site	while	

the	radar,	of	course,	was	getting	raw	data	from	the	terrain	below	us.	Knowing	ahead	of	

time	that	we	were	going	to	cross	the	mountains	east	of	the	landing	site,	we	were	looking	

for	the	altitude	difference	to	change,	and	it	was	a	comfortable	feeling	to	see	it	

happen.”	[ALSJ	Debrief	Comment]

o MET	112:56:18	Cernan:	“And	looks	like	it's	back	down.”	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	125	of	154	
Fly	Me	To	The	Moon	–	Apollo	17	

• At	MET	112:59:18,	the	LM	pitched	over	and	the	LMP	hit	the	PRO	key	to	

transition	the	computer	into	the	P64	Approach	Phase	program	

o MET	12:59:18	Cernan:	“Pitchover.”

o MET	112:59:19	Schmitt:	“There	it	is!	Proceeded.”

§ Cernan	-	"We	still	wanted	to	slow	ourselves	down	by	firing	forward	but	now	we	also	had	

to	keep	ourselves	from	falling	in	too	fast	toward	the	surface.	I	don't	know	how	much	we	

pitched	over,	maybe	from	about	sixty	degrees	to	about	twenty	or	thirty	degrees	

(actually	from	60	degrees	to	20);	but,	when	we	did,	all	of	a	sudden,	bam,	the	lunar	

surface	filled	up	almost	the	entire	window."

o MET	112:59:21	Cernan:	“And	there	it	is,	Houston.	There's	Camelot	(Crater)!”

o MET	112:59:22	Schmitt:	“Wow!”

o MET	112:59:23	Cernan:	“Right	on	target.”

o MET	112:59:24	Schmitt:”	I	see	it.”

o MET	112:59:25	Cernan:	“We	got	them	all.”

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	126	of	154	
Fly	Me	To	The	Moon	–	Apollo	17	

• From	MET	112:59:26	through	MET	113:00:51,	the	Approach	Phase	proceeded	

nominally,	with	the	CDR	assessing	the	approach	trajectory	and	landing	spot	and	

the	LMP	providing	the	LPD	and	H/H-dot	data	

o MET	112:59:26	Schmitt:	“42	degrees,	37	degrees	through	5500.	38	

degrees...”

o MET	112:59:32	Fullerton:	“Challenger,	you're	Go	for	landing.”

o MET	112:59:34	Schmitt:	“...5000	feet;	42	degrees	(LPD	angle)	through	4000;	

47	now;	47	degrees	through	3500;	49	degrees.	(Pause)	3000	feet;	53	degrees.”

o MET	112:59:54	Cernan:	“Okay,	I've	got	Barjea;	I've	got	Poppie;	I've	got	the	

Triangle.”

o MET	112:59:59	Schmitt:	“At	2500	feet,	52	degrees.	H-dot	is	good.	At	2000,	

H-dot	is	good.	Fuel	is	good.	1500	feet,	54	degrees,	Gene.	Approaching	a	

thousand	(feet).	Approaching	a	thousand	feet;	57	degrees.	Okay,	you're	

through	a	thousand,	and	I'm	taking...Radar	altitude	and	PGNS	altitudes	

agree.	You're	through	800	feet.	H-dot's	a	little	high.”

o MET	113:00:28	Cernan:	“Okay;	I	don't	need	the	(LPD	angle)	numbers	any	

more.”	

• Although	I	can’t	find	any	specific	citation	on	the	number	of	ACA	redesignations	

done	by	the	CDR,	based	on	definite	roll	or	pitch	changes	during	the	approach,	it	

looks	like	Cernan	redesignated	the	LM	<10	times	during	the	Approach	Phase	

o This	is	also	a	testimony	to	the	approach	trajectory	accuracy	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	127	of	154	
Fly	Me	To	The	Moon	–	Apollo	17	

• At	MET	113:00:51,	the	CDR	switched	the	LGC	into	P66	and	proceeded	into	the	

Landing	Phase	at	≈300	ft	

• From	P66	transition	to	touchdown,	the	LM	did	only	a	bit	of	maneuvering	as	the	

CDR	picked	the	landing	spot,	and	the		

o MET	113:00:55	Schmitt:	“Okay;	9	feet	per	second	down	at	200.	Going	down	at	5.	

Going	down	at	5.	Going	down	at	10	(fps);	cut	the	H-dot.	The	fuel's	good.	110	feet.	Stand	

by	for	some	dust.	Little	forward,	Gene.”

o MET	113:01:15	Schmitt:	“Moving	forward	a	little.	90	feet.	Little	forward	velocity.	80	

feet;	going	down	at	3.	Getting	a	little	dust.	We're	at	60	feet;	going	down	about	2.	Very	

little	dust.	Very	little	dust,	40	feet,	going	down	at	3.”	

§ ALSJ	Comment:	Gene	wants	about	5	feet	per	second	down	for	the	final	phase;	2	feet	per	

second	would	burn	too	much	fuel	and	oxidizer.	They	will	land	with	about	1225	pounds	of	

fuel	remaining,	enough	for	about	117	seconds	of	hovering.	For	comparison,	Neil	Armstrong	

landed	with	only	50	seconds	of	fuel/oxidizer	remaining...	Apollo	11	was	the	only	flight	on	

which	less	than	100	seconds	of	fuel/oxidizer	remained	at	touchdown.

§ ALSJ	Comment:	In	the	landing	film,	dust	becomes	easily	visible	at	about	113:01:38;	but,	by	

this	time,	Gene	knows	exactly	where	he	is	going	to	land.	Even	in	the	film,	rocks	and	small	

craters	are	visible	until	the	last	few	seconds.	The	16-mm	camera	is	mounted	in	Jack's	

window.

o MET	113:01:42	Cernan:	“Stand	by	for	touchdown.”	

o MET	113:01:43	Schmitt:	“Stand	by.	25	feet,	down	at	2.	Fuel's	good.	20	feet.	Going	

down	at	2.	10	feet.	10	feet.”	

o MET	113:01:58	Schmitt:	“Contact.”	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	128	of	154	
Fly	Me	To	The	Moon	–	Apollo	17	Assessment	

• Apollo	17	proved	that	a	well-trained	LMP	did	not	have	to	be	a	test	pilot,	and	

that	putting	a	trained	professional	geologist	on	a	lunar	crew	brought	a	

significant	delta	in	the	scientific	return	from	the	mission	

o Subsequent	use	of	scientist-astronauts	during	Skylab,	and	Mission	Specialist	in	

numerous	flight	roles	during	Shuttle	has	proved	this	point	many	times	over	

• In	all	aspects,	the	Apollo	17	mission	showed	the	Apollo	Program	at	its	best	–

with	the	exception	of	an	errant	fender	on	the	LRV	(fixed	by	duct	tape	and	

landing	site	approach	photos),	every	system	worked	flawlessly	

o The	LRV,	in	particular,	traversed	slopes	of	20°+,	and	drove	over	9	km	away	

from	the	LM	to	allow	the	crew	to	complete	all	the	planned	traverses	throughout	

the	Taurus-Littrow	Valley	

o In	addition,	the	ability	of	MPAD	to	reassess	standing	mission	rules	to	allow	the	

landing	ellipse	to	fit	into	the	valley	showed	that	the	Program	had	reached	a	

stage	where	previously	very	conservative	boundaries	could	now	be	challenged	

safely,	due	to	the	maturity	of	the	hardware,	the	mission	planning	and	training,	

and	combined	astronaut-mission	operations	team	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	129	of	154	
Conclusions	and	Implications		

for	Human	Lunar	Lander	Systems	(HLS)	Design
Conclusions	and	Recommendations	for	HLS	–	Who’s	Flying?	

• There	has	been	a	lot	of	words	written	about	whether	the	Apollo	crews	actually	“flew”	the	LM,	but	

that	discussion	seems	to	be	largely	written	by	people	who	are	not	pilots	or	don’t	understand	

what	flying	an	aircraft	means...

• If	by	“flying”,	one	means	that	there	is	a	direct	mechanical	linkage	between	the	pilot	and	the	flight	

system	being	actuated,	the	fact	is	that	with	the	exception	of	General	Aviation	aircraft,	nobody	flies	

anything	today	

o Every	military	jet,	from	fighter	to	bomber	to	helicopter	to	transport,	and	every	major	commercial	

airliner,	has	a	fly-by-wire	system	that	takes	the	pilots’	inputs	on	the	stick/yoke/cyclic/collective/throttle	

and	passes	them	through	a	computer,	which	then	turns	those	inputs	into	movements	of	control	surfaces	

and	engine	parts	

• The	fact	is	that	direct	control	of	airplane	gizmos	(control	surfaces,	engines,	rotors,	etc.)	is	only	a	

small	part	of	the	science	and	art	of	flying...the	largest	part	is	the	pilot’s	moment-to-moment	

assessment	of	what’s	going	on	with	their	vehicle,	and	knowing	what	to	do	to	complete	any	flight	

successfully	

o By	that	standard,	the	Apollo	crews	were	flying	the	LM,	even	if	they	never	operated	the	vehicle	without	

the	LGC	helping	out	

§ During	Powered	Descent,	their	job	was	to	monitor	the	vehicle	and	be	ready	to	abort	as	necessary	

§ During	the	Approach	Phase,	the	LMP	was	giving	guidance	data	to	the	CDR,	who	was	looking	out	of	

the	cockpit	and	redesignating,	as	needed,	to	put	the	LM	in	the	best	spot	

§ During	the	Landing	Phase,	the	CDR	was	using	P66	to	maneuver	the	LM	and	set	it	down,	with	inputs	

on	fuel	state,	H	and	H-dot	right	to	touch	down	

o Based	on	my	1,700+	flight	hours,	I	would	say	this	is	flying,	in	no	uncertain	terms	

§ Vehicles	landing	on	any	surface	need	the	capability	to	be	flown	by	pilots,	and	we	should	

acknowledge	that	up	front	and	plan	for	piloted	operations	with	future	lunar	landers	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	131	of	154	
Conclusions	and	Recommendations	for	HLS	–	System	of	Systems	

Design	Considerations	

• Landers	are	intrinsically	“system-of-system”	vehicles,	due	to	the	complexity	

of	a	landing	on	an	airless	body,	and	the	concomitant	complexity	of	the	vehicle	

needed	to	execute	the	landing	

o While	all	the	systems	in	the	LM,	including	the	crew,	were	required	to	both	

operate	separately	and	in	concert	with	the	other	systems,	it	was	the	crew	

overseeing	and	managing	any	given	landing	phase	that	ultimately	led	to	each	

successful	landing	

o ALSJ	Comment	from	Dave	Scott:	“I	think	we	-	as	a	group	flying	the	machine,	the	

PGNS,	AGS,	Irwin,	and	me,	we're	all	flying	that	thing	-	I	think	we	as	a	collective	

entity	are	safer	and	more	efficient	if	there's	a	focal	point.	And	I	was	the	focal	point.	

Jim	fed	things	into	my	ears.	The	Moon	fed	things	into	my	eyes,	and	I	could	feel	the	

machine	operating.”	[ALSJ,	Apollo	15,	Comment	after	MET	104:42:48]	

o Discussions	with	Harrison	Schmitt	have	pointed	out	that	human	crewmembers	

represent	the	best	risk	reduction	system	you	can	put	on	any	future	lunar	landing	

vehicle,	provided	they	are	properly	trained,	that	they	are	part	of	the	whole	

landing	vehicle	system	design,	and	that	the	system	is	designed	in	such	a	way	that	

the	crew	can	take	over,	as	they	did	at	some	point	on	every	Apollo	landing,	and	fly	

the	vehicle	to	a	safe	conclusion	of	any	given	landing

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	132	of	154	
• We	will	need	both	sensor-aided	guidance	computers	to	fly	the	entire	landing	sequence	and

pilots	that	can	take	over	at	critical	points	in	the	approach	

o The	vehicle	will	have	to	be	flown	in	some	phases	by	a	guidance	system	that	is	at	least	as	good	at	the	

LGC	

§ The	moment-to-moment	flying	of	the	LM,	particularly	during	the	Braking	Phase,	was	extremely	complex–

hence,	the	need	for	a	highly	capable	autopilot	to	manage	thrust,	vehicle	Cg,	and	state	vector	calculations	

o However,	you	also	need	trained	pilots	with	a	good	sense	of	the	“moment-to-moment”	random	

perturbations	that	are	present	in	landing	any	vehicle	on	any	surface,	who	are	qualified	to	take	over	

and	fly	the	vehicle	in	the	event	that	the	guidance	system	fails	

§ Every	Apollo	landing	involved	the	crew	taking	over	and	diverting	to	a	different	touch	down	point	than	was	

being	targeted	by	the	LGC,	due	to	issues	with	“LM-footpad	scale”	surface	roughness	and	slope	at	the	original	

touchdown	point	

§ “Given	the	proper	controls	and	displays,	the	LM	could	be	“hand	flown”	throughout	the	descent,	given	an	effective	

flight/descent	profile.” Dave	Scott	e-mail	to	Eppler,	18	January	2020

o With	respect	to	the	use	of	Heads-Up	Displays	(a	technology	not	available	to	Apollo),	Dave	Scott	made	

this	pertinent	comment:		

§ "I	would	much	prefer	to	have	Jim	read	the	numbers	rather	than	have	anything	else	(other	than	the	LPD)	in	the	

window	to	scan	and	interpret	--	my	job	was	to	find	a	landing	point,	total	focus	on	the	surface,	with	no	distractions	

in	focus	or	observation.	Heads	Up	displays	are	great	for	some	things,	but	not	for	landing	on	the	Moon....!!!”	(ALSJ,	

Comment	after	MET	102:42:48)	

§ “Good	idea	if	limited	to	an	image	that	identifies	the	LGC	target	point,	such	as	a	transparent	circle.			

Otherwise,	bad	idea.			The	CDR	must	be	able	to	focus	exclusively	on	finding	a	suitable	touchdown	point	–	

scanning	back	inside	to	a	screen	with	numbers	and	then	interpreting	the	numbers	would	increase	the	

difficulty	and	risk	significantly.		One	doesn’t	close	one’s	eyes	when	aiming	at	a	target.	The	transmission	of	

LPD	numbers	from	the	LMP	to	the	CDR	was	very	convenient,	and	it	optimized	the	LMP’s	inside	view	and	

scanning	of	both	the	LGC	and	the	instrument	panels.	This	also	utilized	one	of	the	CDR’s	important	input	

channels;	i.e.	ears.”	Dave	Scott	e-mail	to	Eppler,	18	January	2020

Conclusions	and	Recommendations	for	HLS	–	System	of	Systems	

Design	Considerations	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	133	of	154	
Conclusions	and	Recommendations	for	HLS	–		

Astronaut	Aviation	Experience	

• The	LM	was	a	two-pilot	vehicle,	and	both	the	CDR	and	LMP	had	critical	roles	to	play	and	could	not	

have	been	replaced	by	the	LGC	

o Although	some	of	the	LMP	function	can	be	replaced	with	HUDs,	etc.,	the	co-pilot	function	is	still	very	much	

required	(this	is	why,	even	with	the	variety	of	pilot	aids	we	have	at	present,	there	are	still	2	pilots	required	

in	every	airliner)	

o Each	future-CDR	will	need	to	have	a	lot	of	experience	flying	all	kinds	of	vehicles,	both	conventional	and	

Short/Vertical	Take-Off	(S/VTOL),	in	all	kinds	of	situations	–	that	usually	means	test	pilots	

§ One	thing	that	comes	out	loud	and	clear	in	this	research	is	how	much	that	last	1,000’	to	the	lunar	

surface	was	about	assessing	the	landing	surface	and	balancing	lateral	motion,	vertical	motion	and	fuel	

to	fly	the	LM	to	the	best	landing	spot		

o Future	LMPs	do	not	need	to	be	test	pilots,	but	they	do	need	to	have	a	good	background	in	flying,	fully	

experienced	with	in-flight	operations	and	landing	flight	hardware	

§ Harrison	Schmitt	did	an	excellent	job	as	the	AS-17	LMP	without	test	pilot	experience,	but	any	co-pilot	

has	to	be	qualified	to	provide	critical	flight	data	(vertical	and	horizontal	velocities,	altitudes,	fuel	

state,	vehicle	status,	abort	system	status)	to	the	Commander,	as	well	as	be	able	to	trouble-shoot	

systems	failures	while	the	CDR	is	flying	

- “As	you	know,	the	LM	is	a	very	complex	and	highly	integrated	system.		For	a	lunar	mission,	of	particular	

importance	is	the	communications	system,	again	very	complex.		The	primary	function	of	the	LMP	is	the	flight	

engineer.				If	e.g.	one	of	the	three	Earth	tracking	sites	fails,	the	crew	is	on	their	own	and	any	failures	will	depend	on	

the	experience	and	intuitive	skills	of	the	flight	engineer	(and	the	pilot).		After	eight	successful	Apollo	flights,	

exchanging	a	geologist	for	the	flight	engineer	position,	especially	a	highly	trained	very	smart	geologist,	was	a	

reasonable	approach	for	the	final	and	very	mature	Apollo	mission.		As	usual,	this	was	risk	vs.	gain.		The	issue	is:		if	

the	flight	engineer	trained	as	a	geologist	fails,	lose	some	science.			If	the	engineer/geologist	fails,	lose	the	mission	

(or	die).		Therefore,	for	the	first	several	flights	in	a	vehicle	such	as	the	LM,	a	flight	test	engineer	(or	test	pilot)	

maximizes	the	risk	reduction.		As	to	Apollo	science,	I	doubt	that	A-17	returned	more	science	than	A-15	or	A-16.		

Therefore,	as	a	general	statement	of	qualification,	I	would	not	conclude	that	for	early	flights	the	LMP	functions	

could	be	satisfied	by	a	scientist.” Dave	Scott	e-mail	to	Eppler,	18	January	2020	

o The	only	situation	that	provides	the	experience	needed	by	both	crewmembers	is	flying	real	vehicles,	in	all	

the	random	situations	that	flight	vehicles	are	subject	to	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	134	of	154	
Conclusions	and	Recommendations	for	HLS	–	IT	Design	

• The	LM	IT	assets	–	the	hardware	and	the	software	that	ran	the	vehicle	during	the	landing	 –	worked	very	well	together,	and	without	them,	we	would	not	have	been	able	to	land	on	

the	Moon	

o There	was	an	elegance	to	system	that	was	created	because	(1)	there	were	very	smart	folks	at	

Draper,	and	(2)	the	same	individuals	worked	on	the	system	from	start	to	finish,	so	they	really	

cared	about	what	the	product,	and	had	an	intuitive	feel	for	the	hardware	and	software	

o I	think	the	other	part	of	the	reason	there	was	such	elegance	was	because	the	LGC	had	no	room	

for	junk	code	that	might	have	unintended	consequences	in	real	flight	

o This	suggests	we	need	to	build	this	system	from	scratch	

§ I	don’t	think	adapting	an	operating	system	based	on	a	consumer	product	is	the	answer	to	our	

software	development	(i.e.,	the	HLS	should	not	be	a	Windows	or	iOS	driven	vehicle!)	

• The	real	crux	of	this	issue	is	anticipating,	preventing	and	recovering	from	failures	in	

electronics	and	IT	systems,	where	failure	probabilities	are	low	but	consequences	are	very	

high	

o All	systems	fail	sometime	–	it	doesn't	matter	if	the	system	is	human	or	machine	

§ The	key	is	to	provide	paths	to	graceful	recovery,	regardless	of	the	hardware	fails	or	the	human	

crew	are	unable	to	perform	their	respective	flight	duties	as	Pilot	in	Command	or	Co-Pilot	

o Having	astronauts	involved	in	the	design	phase	of	any	future	lunar	lander	will	be	critical	for	

incorporating	flight	crew	input	into	both	nominal	and	contingency	system	design	

§ Air	France	Flight	447	and	the	737	Max	debacle	illustrates	what	happens	when	the	hardware	

systems	unexpectedly	fail	and	the	human	operators	are	unable	to	take	over	and	recover	from	that	

failure	

o The	Beresheet	and	Chandrayan	2	landing	failures	and	the	Boeing	CST-100	OFT	launch	failure	

point	out	that	system	errors	during	dynamic	flight	phases	happen,	and	may	not	be	recoverable	

with	automated	flight	systems	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	135	of	154	
Conclusions	and	Recommendations	for	HLS	-	Training	Vehicles		

(and	nobody	is	going	to	like	this	one...)	

• We	will	need	some	kind	of	flight	training	vehicle	allow	CDRs	to	learn	the	dynamic,	

random	variations	in	approach	path	that	will	occur	for	all	lunar	landings	

o The	Lunar	Landing	Training	Vehicle	(and	its	Research	Vehicle	predecessor)	were	challenging	

vehicles	to	fly	(3	of	the	5	built	crashed,	although	the	pilots	successfully	ejected	in	each	case),	

but	every	CDR	that	came	back	from	the	Moon	said	that	the	LLTV	trained	them	better	than	any	

other	device	

§ Dave	Scott	-	"And,	as	long	as	something	will	keep	that	vehicle	stable...The	LLTV	landings	were	

manual	landings,	and	the	LLTV	was	a	great	trainer.	I	mean,	boy,	am	I	glad	we	had	that,	because	it	

gave	me	confidence	that	I	knew	what	I	was	doing	on	the	Moon,	and	I	didn't	have	to	think	about	

things.	I	didn't	have	to	consciously	program	myself	to	do	things.	I	was	automatic.”	[ALSJ	Comment	

after	MET	112:59:25]		

§ Gene	Cernan:	“I	think	the	most	significant	part	of	the	final	phases	from	500	feet	down,	as	far	as	the	

CDR	was	concerned,	was	that	it	was	extremely	comfortable	flying	the	bird,	either	LPDing	in	P64,	

and/or	flying	manually	in	P66.	I	contribute	that	primarily	to	the	LLTV	flying	operations.	That's	why	

the	rates	of	descent	and	what	have	you	were	just	very	comfortable.”	[Comment	in	Apollo	17	Technical	

Debrief]		

§ Gene	Cernan:	"Although	there	is	nothing	quite	like	the	real	thing,	flying	the	Lunar	Landing	Training	

Vehicle	(LLTV)	had	been	a	step	toward	realism	from	"flying"	the	stationary	simulators.	In	the	LLTV	

you	had	your	butt	strapped	to	a	machine	that	you	had	to	land	safely	or	you	didn't	make	it.	It	still	

wasn't	landing	on	the	lunar	surface,	but	it	gave	you	a	feel	for	what	the	actual	landing	would	be	

like.”	[ALSJ	Comment	after	MET	112:59:25]

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	136	of	154	
Conclusions	and	Recommendations	for	HLS	–	Training	Approach	

• Each	Apollo	crew	had	a	different	personal	dynamic,	but	each	LM	crew	clearly	

worked	extremely	well	together	as	a	team,	and	were	able	to	manage	the	giveand-take	associated	with	flying	an	extremely	complicated	machine	

o That	speaks	to	a	lot	of	time	training	together	in	the	simulators	and	the	classroom	as	

well	as	time	in	terrestrial	flying	machines	

§ This	was	not	derived	from	occasional	computer-based	training	exercises	–	it	was	lots	

of	hours	in	the	simulator,	in	the	LLTV	and	in	T38s	working	out	the	partnership	for	

getting	to	the	Moon	

§ Test	pilot	schools	(at	least	the	US	Air	Force,	the	US	Navy	and	the	British	Empire	Flight	

Test	School)	have	a	mantra,	“Train	to	Test,	Test	to	Train”,	which	means	you	train	

exactly	how	you	would	do	the	same	activity	for	real	–	Apollo	clearly	applied	that	

mantra,	and	it’s	one	we	should	continue	to	follow		

o This,	in	turn,	suggests	that	we	need	to	budget	lots	of	training	hours	for	each	

integrated	Artemis	crew	

§ The	15	July	1969	Apollo	11	Crew	Training	Summary	indicates	that	out	of	959	hours	of	

training,	Neil	spent	285	hours,	or	30%	in	the	various	LM	simulators	and	Buzz	did	1017	

hours	of	training,	of	which	332	hours,	33	%,	spent	in	the	LM	simulators	

§ These	figures	do	not	include	Neil's	LLTV	flights	nor	the	56	hours	each	of	them	spent	in	

briefings	about	LM	systems	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	137	of	154	
Conclusions	and	Recommendations	for	HLS	–	Lighting	Constraints	

• Lighting	constraints	to	landing	were	a	key	feature	on	Apollo,	and	they	ensured	that	when	

the	LM	went	into	the	Approach	Phase,	the	crew	could	see	the	terrain	they	were	landing	

on,	and	that	any	obstacles	to	landing	could	be	seen	clearly	

o “The	nature	of	the	lunar	surface	was	the	most	difficult	part	of	the	final	descent	–	there	is	no	

scaling	to	provide	a	basis	of	estimating	altitude	(height).		This	was	the	reason	the	early	flights	

“stair	stepped”	the	final	descent.		If	the	size	of	specific	craters	could	be	memorized	and	interpreted	

quickly,	then	they	might	provide	a	relative	scale	for	height	estimation...”	Dave	Scott	e-mail	to	

Eppler,	18	January	2020	

• These	constraints,	in	turn,	drove	launch	windows,	which	is	why	it	was	possible	to	launch	

only	once	a	month	

• These	kinds	of	visual	constraints	have	to	be	factored	into	site	selection	and	launch	

window	determination	for	Artemis,	particularly	on	the	first	landing	to	any	site	

o Regardless	of	any	artificial	aids	to	landing	(LIDAR,	high-precision	landing	radar,	etc.),	the	crew	

has	to	be	able	to	see	what	the	ground	they	are	landing	on	looks	like,	so	they	can	back	up	the	

other	technology	

§ Also	note	the	observation	made	previously	that	all	systems	fail	sometime...

o This	will	be	particularly	true	for	very	low	Sun	angle,	very	high	contrast	landing	spots	on	the	

lunar	poles	

o Without	this	ability,	you’re	just	shooting	for	a	spot	on	the	Moon	blindly	and	hoping	when	the	

dust	settles,	your	lander	is	right-side	up	and	the	crew	can	leave	again	safely	–	I	don’t	think	this	

is	the	risk	management	approach	we	should	apply	to	Artemis	

o Among	other	things,	even	Apollo	management	wasn’t	that	gutsy,	and	management	today	is	a	lot	

more	risk	averse	than	management	was	during	Apollo	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	138	of	154	
Conclusions	and	Recommendations	for	HLS	–	Landing	Site	Data	

• The	lunar	surface	provides	few	cues	to	estimate	altitude,	descent	rate	and	

lateral	velocity	

o Craters,	at	the	“final	approach	scale”,	are	all	circular,	regardless	of	size,	so	the	

view	out	the	window	does	not	provide	good	clues	to	estimating	descent	rates	

and	time	before	landing	in	the	last	1,000	ft	to	the	touchdown	

§ Alan	Shepard,	from	the	1971	Technical	Debrief	-	"	The	elevation	and	distance	

estimation	of	landmarks	is	always	a	problem	as	far	as	I	am	concerned.	And	the	only	

thing	I	can	recommend	is	that	the	CDR	(Commander)	carry	in	his	head	the	geometry	of	

the	landing	site,	the	size	of	the	craters,	and	the	difference	between	the	crater	

landmarks	that	are	used.	He	should	know	exactly	what	those	distances	and	dimensions	

are	ahead	of	time.	That's	one	thing	you've	got	to	memorize	because,	as	far	as	I'm	

concerned,	at	least,	the	L&A	doesn't	give	you	the	feeling	of...looking	at	a	crater	which	is	

unfamiliar	to	you	and	saying	that	I'm	5000	feet	above	the	ground	or	2000	feet	above	

the	ground.	It's	just	something	that	you	can't	do.	You	can't	relate	it	to	your	Earthbound	experience.”	

o Curiously,	we	grump	about	lunar	regolith	(correctly,	from	a	surface	ops	status),	

but	the	horizontal	blowing	of	dust	caused	by	DPS	plume	impingement	was	a	

critical	cue	that	the	LM	was	getting	close	enough	to	the	surface	to	land,	and	the	

upper	sheet	of	the	blowing	dust	often	provided	a	surface	on	which	to	view	the	

LM	shadow	in	the	final	seconds	before	landing	

o The	more	data	we	have	about	each	site,	the	better,	but	until	we	have	prepared	

landing	pads,	each	landing	will	be	challenge	that	cannot	be	met	by	prepackaged	landing	instructions	in	a	guidance	computer	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	139	of	154	
Conclusions	and	Recommendations	for	HLS	-	Navigation	

• I	will	paraphrase	a	line	from	“The	Right	Stuff”	–	the	issue	isn’t	landing,	the	issue	is	

navigation	

• The	biggest	challenge	for	Artemis	will	not	be	the	actual	act	of	landing	the	spacecraft,	it	

will	be	navigating	to	the	spot	where	we	can	be	reasonably	sure	that	the	local	topography	

will	not	tip	the	lander	over	on	touchdown	

o LRO	data	is,	at	its	best	(and	don’t	get	me	wrong,	LRO	data	is	fantastic),	0.5	m	per	pixel,	but	1	

pixel	doesn’t	tell	you	much	–	you	need	≈5	pixels	to	get	a	reasonable	idea	of	what	the	

topography	is,	which	means	your	actual	knowledge	of	blocks	and	holes	is	around	1-2	meters	in	

size	

• This	means	getting	the	3-σ	landing	ellipse	down	to	where	there	are	NO	large,	

permanently	shadowed	areas	in	the	ellipse	–	NONE	

o The	Apollo	17	mission	almost	didn’t	go	to	the	Taurus-Littrow	site	because	an	initial	calculation	

of	the	3-σ	landing	ellipse	did	not	fit	into	the	“landable”	portion	of	the	valley	

o It	was	only	through	diligent	interaction	between	MPAD,	CB	and	MOD	that	they	were	able	to	

work	out	the	procedures	that	would	reduce	the	ellipse	to	fit	within	acceptable	topography	

o Again,	this	was	not	a	photographic	issue	–	by	AS-17,	we	had	good	panoramic	and	metric	

mapping	camera	data	from	AS-15	–	it	was	a	function	of	navigation	precision	during	PDI	to	

ensure	when	the	crew	pitched	over,	they	would	see	their	nominal	landing	site	

• This	means	we	really	have	to	start	working	the	entire	approach	methodology	now,	

understanding	how	a	variety	of	approach	ground	tracks	and	landing	ellipses	map	out	

onto	the	presumptive	south	polar	landing	area	

o As	a	first	step,	we	can	place	the	Apollo	11	landing	ellipse	(18.5	km	x	4.8	km)	over	the	terrain	

around	Shackleton	in	real	(not	averaged)	lighting	conditions	to	see	how	ugly	things	are	

o We	need	to	make	sure	that	the	first	Artemis	crew	will	see	visible,	well-lit,	landable	terrain	in	

front	of	them	when	they	pitch	over,	NOT	a	black	hole!	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	140	of	154	
Sources
Sources	

1. Bennet,	Floyd	V.,	Mission	Planning	for	Lunar	Module	Descent	and	Ascent,	NASA	

Technical	Note	(Apollo	Experience	Report)	TN	D-6846,	March	1972.	

2. Ennis,	W.	W.	Apollo	APS	Abort	Boundary	in	LM	Descent	in	Apollo	Lunar	Landing	

Missions,	Bellcomm	Letter	Report,	B72	02004,	14	February	1972.	

3. Eyles,	D.,	Sunburst	and	Luminary:	An	Apollo	Memoir,	Fort	Point	Press,	Boston,	2018,	

357	pp.	

4. Mindell,	D.,	Digital	Apollo:	Human	and	Machine	in	Spaceflight,	MIT	Press,	

Cambridge,	MA,	2008,	359	pp.	

5. NASA	Mission	Operations	Directorate,	Apollo	11	Technical	Crew	Debrief,	31	July	

1969,	accessed	through	the	Apollo	Lunar	Surface	Journal,	

https://www.hq.nasa.gov/alsj/,	accessed	numerous	times	between	1	August	and	1	

December	2019.	

6. NASA	Mission	Operations	Directorate,	Apollo	12	Technical	Crew	Debrief,	1	

December	1969,	accessed	through	the	Apollo	Lunar	Surface	Journal,	

https://www.hq.nasa.gov/alsj/,	accessed	numerous	times	between	1	August	and	1	

December	2019.	

7. NASA	Mission	Operations	Directorate,	Apollo	14	Technical	Crew	Debrief,	17	

February	1970,	accessed	through	the	Apollo	Lunar	Surface	Journal,	

https://www.hq.nasa.gov/alsj/,	accessed	numerous	times	between	1	August	and	1	

December	2019.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	142	of	154	
Sources	

8. NASA	Mission	Operations	Directorate,	Apollo	15	Technical	Crew	Debrief,	14	August	

1971,	accessed	through	the	Apollo	Lunar	Surface	Journal,	

https://www.hq.nasa.gov/alsj/,	accessed	numerous	times	between	1	August	and	1	

December	2019.	

9. NASA	Mission	Operations	Directorate,	Apollo	16	Technical	Crew	Debrief,	5	May	

1972,	accessed	through	the	Apollo	Lunar	Surface	Journal,	

https://www.hq.nasa.gov/alsj/,	accessed	numerous	times	between	1	August	and	1	

December	2019.	

10. NASA	Mission	Operations	Directorate,	Apollo	17	Technical	Crew	Debrief,	4	January	

1973,	accessed	through	the	Apollo	Lunar	Surface	Journal,	

https://www.hq.nasa.gov/alsj/,	accessed	numerous	times	between	1	August	and	1	

December	2019.	

11. NASA	Manned	Spacecraft	Center	Internal	Note	01296,	Apollo	Mission	Techniques,	

Mission	H-2	and	Subsequent:	Abort	from	Lunar	Powered	Descent	and	Subsequent	

Rendezvous,	7	January	1970.	

12. O’Brien,	F.,	The	Apollo	Guidance	Computer:	Architecture	and	Operation,	SpringerPraxis,	Chichester,	UK,	2010,	439	pp.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	143	of	154	
Backup	Data
LM	Descent	and	Landing	Abort	Decisions	

• Abort	Conditions/Responsibilities	–	General	

o MSC	Houston	(MCC-H)	is	responsible	for	detecting	“slow,	insidious	drift	malfunctions”	that	might	

contribute	to	aborts,	requiring	as	much	high	rate	data	between	the	LM	and	the	ground	as	possible	

o The	crew	is	responsible	for	detecting	errors	that	require	immediate	action,	largely	because	comm	cycle	

time	for	an	immediate	abort	may	be	as	long	as	20	seconds	

• Overall	Abort	Decision,	Regardless	of	Abort	Mode	and	Stage	of	the	Lunar	Landing	

o Either	an	Abort	or	Abort/Stage	decision	will	be	made	when	a	malfunction	has	been	detected	that	precludes	

a	safe	descent	and	landing,	or	inability	to	abort	later	in	the	landing	sequence	

o Abort	procedures	are	designed	to	finish	with	the	LM	in	an	orbit	with	a	periapsis	>30,000’	and	in	a	position	

to	rendezvous	with	the	CSM	

• Overall	abort	decision	points	

o Propulsion	systems	

§ Zero	or	low	DPS	thrust	

- Includes	failure	of	the	DPS	to	start	either	automatically	after	P63	PRO	or,	if	the	DPS	went	into	a	

degraded	condition,	after	attempted	CDR	manual	start	using	the	descent	engine	command	override	

switch	

§ Failure	of	the	DPS	to	throttle	

§ Impending	DPS	propellant	depletion	

§ Loss	of	APS	redundant	start	capability	

§ Uncontrollable	DPS	gimbal	failure		

o Guidance	and	Navigation	

§ PGNCS	malfunction	

§ AGS	malfunction	prior	to	High	Gate	(P63-P64	transition	at	Pitchover)	

§ Lack	of	Landing	Radar	(LR)	data	

§ Incompatible	difference	between	PGNCS	and	LR	data	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	145	of	154	
LM	Descent	and	Landing	Abort	Decisions	

• Overall	abort	decision	points	(cont.)	

o Trajectory	constraints	

§ Violation	of	specific	abort	limits	seen	on	MCC-H	displays	(probably	data	not	available	in	LM)	

o Landing	Site	Condition	

§ Unsatisfactory	for	safe	landing	

§ Redesignation	unreasonable	given	LM	trajectory	and	fuel	state	

o General	

§ Loss	of	S-band	high	rate	data	or	voice	communications	

§ CSM	malfunctions	that	may	affect	rescue	capability	

• Go/No	Go	for	Powered	Descent	Initiation	(PDI)	

o PDI	is	the	first	key	event	in	a	lunar	landing	

o If	the	LM	isn’t	in	a	condition	to	safely	land,	it	is	(obviously)	better	to	remain	in	orbit	and	either	work	the	

condition	to	a	satisfactory	conclusion	or	recover	to	the	CSM	

o For	a	“Go”	decision	on	PDI,	the	LM	must	be	able	to:	

§ Safely	land	

§ Ascend	

§ Rendezvous	

§ Re-dock	with	the	CSM	

o A	“Go”	decision	means	proceed	with	the	landing	sequence	start	

§ PDI	was	just	the	first	event	in	the	landing		

§ There	were	more	than	a	few	potential	events	that	would	trigger	an	abort	between	PDI	and	landing	

o A	“No	Go”	decision	means	an	abort	to	an	orbit	that	will	allow	docking	with	the	CSM,	or	a	delay	for	1	rev	to	

deal	with	the	problem	

o In	addition	to	appropriate	LM	system	health	status	(DPS,	Ascent	Propulsion	System	(APS),	LM	ECLSS,	

appropriate	systems	for	landing),	a	“Go”	for	PDI	requires	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	146	of	154	
LM	Descent	and	Landing	Abort	Decisions	

• Go/No	Go	for	Powered	Descent	Initiation	(PDI)	(cont.)	

o In	addition	to	appropriate	LM	system	health	status	(DPS,	Ascent	Propulsion	System	(APS),	LM	ECLSS,	

appropriate	systems	for	landing),	a	“Go”	for	PDI	requires	

§ Agreement	between	PGNCS	and	MCC	computed	target	data	(uplinked	to	the	crew	on	Preliminary	

Advisory	Data,	or	PADs)	

- After	AOS	on	the	planned	landing	orbit	(≈PDI	-33	mins),	MCC-H	used	tracking	data	to	compute	

a	new	LM	state	vector	and	to	assess	orbital	perturbations	induced	during	the	previous	orbit	

§ Acceptable	alignment	of	PGNCS	

§ Acceptable	PGNCS	and	AGS	agreement	(AKA	PGNCS/AGS	residuals)	

- AGS	was	required	through	completion	of	P63	at	High	Gate;	however,	depending	on	the	realtime	situation	and	the	particular	failure	mode,	a	decision	would	be	made	to	either	continue	or	

abort	the	descent)	

§ Adequate	high	bit-rate	and	voice	comms	

§ Proper	LM	pitch	attitude	for	DPS	ignition	

§ LGC	functioning	correctly	

§ A	verified	ullage	burn	at	TIG	-7.5	seconds	

o A	“No-Go”	decision	either	required	an	abort	burn	back	up	to	CSM	altitude	(≈60	nm)	at	the	nominal	

PDI	time	+	12	minutes	or	a	1-rev	delay	for	assessment	and	working	

§ Decision	made	between	33	minutes	and	6	minutes	remaining	to	ignition	of	the	DPS	at	PDI	

(TIG).	

o Ullage	maneuver	was	performed	with	the	RCS	just	prior	to	DPS	PDI	initiation	in	order	to	ensure	that	

DPS	fuel	and	oxidizer	was	properly	positioned	at	the	fuel	inlet	valves.	

o This	would	have	been,	in	effect,	a	decision	to	not	land	at	any	time	during	that	mission.	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	147	of	154	
LM	Descent	and	Landing	Abort	Decisions	

• Abort	vs.	Abort/Stage	Decision	

o When	the	choice	is	available,	DPS-to-orbit	(Abort)	is	used	for	aborts	up	to	PDI	+	5	1⁄2	minutes	

o After	PDI	+	5	1⁄2	minutes,	Abort/Stage	is	required	following	DPS	fuel	depletion	or	non-recoverable	

malfunction	

o Abort/Stage	decision	implies	one	of	the	following:	

§ DPS	fuel	depletion	following	an	Abort	decision	

§ DPS	performance	limits	are	violated	

§ Excessive	RCS	plume	impingement	

§ LM	tipover	after	lunar	surface	contact	

		

• PGNCS	to	AGS	Switchover	Decision	

o PGNCS	performance	is	more	critical	for	the	LM	when	flying	the	powered	descent	trajectory	than	when	

flying	the	actual	landing	

§ This	imposed	more	constraints	to	continuing	the	lunar	landing	in	the	event	of	PGNCS	degradation	during	

the	Powered	Descent	to	Pitchover	at	High	Gate	(P63-P64	transition)	

o The	abort	boundary	for	poor	PGNCS	performance	was	at	30,000’	periapsis,	requiring	a	switch	to	AGS	for	

the	subsequent	abort	(this	abort	sequence	was	practiced	on	Apollo	10)	

o Conditions	for	the	PGNCS	abort	were:	

§ Unacceptable	residuals	between	the	ground-based	Powered	Flight	Processor	data	(PFP)	and	the	lunar	

orbit-based	PGNCS	

§ Unacceptable	residuals	between	PGNCS	and	AGS		

§ Unacceptable	residuals	between	the	trajectory	as	established	by	tracking	data	from	the	Manned	

Spaceflight	Network	(MSFN)	and	PGNCS		

§ Unacceptable	residuals	between	the	trajectory	as	established	by	tracking	data	from	the	Manned	

Spaceflight	Network	(MSFN)	and	AGS		

o These	aborts	were	all	of	the	“slow	insidious	drift”	type	and	were	called	from	the	ground	based	on	data	that	

was	not	available	in	the	LM	

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LM	Descent	and	Landing	Abort	Decisions	

• Post-PDI	Abort	Decisions	for	Powered	Descent	

o Failure	of	the	DPS	to	ignite	after	entry	into	P63	Braking	Phase	program	and	after	CDR	

attempt	at	manual	ignition	using	the	descent	engine	command	override	switch	

o PDI	initiation	starts	with	a	10%	thrust	trim	phase	for	26	seconds,	after	which	the	engine	

goes	into	a	Fixed	Throttle	Phase	(FTP)	at	92.5%	thrust	

§ If	DPS	fails	to	go	into	the	FTP	phase	after	the	trim	phase,	the	crew	will	stop	thrusting		

§ To	safely	accomplish	the	landing,	PNGCS	must	be	able	to	control	DPS	throttle,	so	

manual	throttling	as	the	only	capability	is	not	acceptable	and	an	abort	is	called	for	

o DPS	throttle	performance	

§ If	DPS	thrust	degrades	to	<80%,	the	Thrust/Translation	Controller	Assembly	(TTCA)	

control	switch	is	cycled	from	CDR	to	LMP	

§ If	this	does	not	recover	thrust	to	≥92.5%	within	100	seconds,	an	abort/stage	decision	

must	be	made	due	to	unacceptable	DPS	nozzle	degradation	between	≈62.5%	and	

92.5%,	which	would	cause	a	catastrophic	failure	

• LM	Attitude	and	Attitude	Rate	Monitoring	

o LMP	will	verify	that	the	control	system	and	Flight	Director	Attitude	Indicator	(FDAI)	is	

working	by	comparing	steady	state	readings	of	the	FDAI	error	needles	on	the	CDR	and	

LMP	FDAIs	

§ Unexpected	or	prolonged	errors	of	5°	sec-1	indicates	a	PGNCS	failure	

§ Abort	on	the	AGS	is	subsequently	performed	

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LM	Descent	and	Landing	Abort	Decisions	

• LR	Acquisition	

o If	the	LR	good	data	signal	is	not	received	by	the	PGNCS	at	H=10,000’,	landing	will	be	aborted	

o If	ground	data	indicates	that	the	PGNCS	performance	has	exceeded	3σ	performance	limits,	the	

abort	altitude	is	raised	to	H=18,000’	

• PS	Gimbal	Drive	Actuator	(GDA)	

o DPS	Gimbal	Drive	degradation,	and	subsequent	disabling,	can	be	compensated	for	with	

increased	RCS	activity	for	a	wide	range	of	offset	errors	without	violating	RCS	plume	

impingement	limitations	

o RCS	compensation	capability	increases	as	a	function	of	the	duration	of	powered	descent	

o GDA	shutdown	will	occur	any	time:	

• DPS	engine	gimbal	trim	light	is	on		

• Attitude	errors	show	offset	>±3°	

• Excessive	RCS	firings	

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LM	Descent	and	Landing	Abort	Decisions	

• DPS	Throttle	Performance	Limits	during	Powered	Descent	(Note:	DE	Summary	of	a	number	of	

potential	abort	situations)	

o During	the	Powered	Descent	Phase,	the	LGC	was	driving	the	thrust	profile,	with	the	

requirement	of	getting	the	LM	to	High	Gate,	within	range	of	the	targeted	landing	site,	at	an	

altitude	and	speed	that	allowed	a	safe	approach	to	the	landing	site	

• Within	the	DPS	limits	discussed	above,	the	LGC	would	command	the	DPS	to	provide	

sufficient	delta-v	to	achieve	the	landing	

• If	(and	this	is	Eppler	surmising	here...)	the	DPS	performance	began	to	be	erratic,	the	

LGC	would	command	the	engine	appropriately	to	reach	the	projected	delta-v,	

horizontal	velocity	and	descent	rate	constraints	for	landing	

• However,	a	serious	DPS	failure	(low	or	unstable	thrust,	cyclic	variation	in	commanded	

thrust)	would	cause	the	LM	to	undergo	“large	attitude	excursions”	

o This	resulted	in	a	number	of	complicated	abort	scenarios	that	considered	throttle	

excursions	

• If	the	DPS	throttle	performance	became	erratic	prior	to	≈80	secs	left	in	P63	(TGO>80),	

an	abort	would	be	recommended	

• If,	however,	the	throttle	performance	became	erratic	with	TGO<80	secs,	the	LM	could	

continue	to	P64	initiation	at	High	Gate	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	151	of	154	
LM	Descent	and	Landing	Abort	Decisions	

• DPS	Throttle-Down	

o As	noted	above,	the	DPS	had	issues	with	continuous	operation	between	62.5%	and	92.5%	

power;	consequently,	DPS	flew	the	landing	with	an	initial	high,	fixed	throttle	setting	until	

throttle	down	≈7	minutes	into	Powered	Descent,	at	which	point	it	would	throttle	back	to	

<62.5%	for	the	rest	of	the	landing	

§ Throttle	down	was	a	critical	point	in	the	landing,	as	inability	to	throttle	down,	or	

throttling	down	too	much,	could	jeopardize	the	safety	of	the	crew	and	successful	

landing	

o During	Powered	Descent,	MCC-H	monitored	DPS	thrust	and	made	predictions	on	when	

throttle	down	would	occur	during	Powered	Descent	

§ If	throttle	down	did	not	occur	and	DPS	would	not	throttle	(indicating	a	DPS	hardware	

failure),	the	crew	would	abort	(probably	an	Abort/Stage)	

§ If	throttle	logic	has	failed	(this	is	an	LGC	or	engine	controller	issue),	thrust	will	go	to	

≈0%,	prompting	a	large	LM	pitch	change	at	≈40	secs	after	throttle	down	

§ If	throttle	down	has	not	occurred	by	the	time	of	P63-P64	program	change	time	(end	of	

Powered	Descent	and	entry	into	the	Approach	Phase)	+≈15	seconds,	the	LM	will	

develop	radial	velocity	that	prevents	landing	and	the	crew	would	abort	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	152	of	154	
LM	Descent	and	Landing	Abort	Decisions	

• Approach/Visibility	Phase	(P64	initiation	to	Landing	Phase)	

o If	H-dot	exceeded	the	safe	limit	and	didn’t	return	to	a	safe	amount,	the	crew	would	abort	

o If	the	no	landing	site	if	available	during	the	limits	of	H-dot,	H	and	propellant	exhaustion	

limits,	the	crew	will	abort		

o After	entering	P64,	MCC-H	will	monitor	which	tanks	(fuel	or	oxidizer)	have	the	lowest	

remaining	quantity	

§ Based	on	this	assessment,	the	crew	will	switch	the	propellant	quantity	measurement	

display	to	the	lower	tank	for	subsequent	monitoring	

§ However,	MCC-H	will	assume	prime	responsibility	for	advising	the	crew	on	propellant	

remaining	and	will	advise	the	crew	at	60	seconds	and	30	seconds	fuel	remaining	

§ A	“Fuel	Depletion	“call	from	MCC-H	means	that	no	more	than	20	seconds	of	hover	

capability	or	6	seconds	of	full	thrust	remains	

• Landing	Phase	(P66	initiation)	

o Propulsion	systems	

§ Zero	or	low	DPS	thrust	

§ Failure	of	the	DPS	to	maintain	acceptable	rate	of	descent	through	throttle	capability	

§ Impending	DPS	propellant	depletion	

§ Uncontrollable	DPS	gimbal	failure		

o Landing	Site	Condition	

§ Unsatisfactory	for	safe	landing	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	153	of	154	
LM	Descent	and	Landing	Abort	Decisions	

• Post-Landing	Phase	(P68	program)	

o Immediately	after	contact,	DPS	is	shut	down	and	the	crew	monitors	LM	pitch	and	roll	rates	

to	determine	if	a	tipover	is	imminent	

§ If	pitch	and	roll	rates	indicate	tipover,	an	automatic	Abort/Stage	abort	is	initiated	

o If	tipover	is	not	imminent,	the	crew	configures	the	LM	for	the	T1	and	T2	stay	decision	by	

finalizing	DPS	Shutdown	configuration	and	storing	the	landing	point	in	the	AGS	in	case	an	

abort	after	T1	or	T2	is	indicated	

o After	a	“Go”	call	for	lunar	surface	stay	is	made,	the	crew	manually	enters	Landing	

Confirmation	Program	(P68)	into	the	LGC	(VERB	37,	ENTER,	68,	ENTER,	PROCEED)	

Eppler/Apollo_Lunar_Landing_Experience_Report_20070_R4.pptx/Slide	154	of	154