Apollo 16: The wave off

The Apollo 16 command ship, Caspar. Fifty years ago, April 20, with the words “The Command Module did not circ.,” the landing by the Lunar Module, Orion, seemed lost.


Coming around the edge of the moon at the start of our 13th revolution, We’re John Young and Charlie Duke in our lunar lander Orion flying free of the command ship Caspar, flown by T.K. “Ken” Mattingly, who will maintain watch from orbit.  You and I are were due to start powered descent to the surface, to the highlands of the moon in just minutes.  Past tense.  Something happened just minutes ago.  

We establish communications first.  And report, “The Command Module did not circ.”

That’s right, Houston — soak in in.  The Command Module which, on the backside, was suppose to to fire its big Service Propulsion System engine raising itself into a circular orbit, did not perform the circ. burn.   

And that’s all that’s needed to tell Houston that something is mightily wrong with the command ship.  Without hesitation, Houston calls, “Anticipate a wave-off for this orbit.”   

Minutes later, there it is, so close and so far away, our landing site at Descartes/Cayley passing 11 mi. below us.

It’s April 20, 1972 — and a bit ago, just after 3 p.m. EST back on Earth when everything went wrong.  The flight had been going pretty darn smoothly.   Go back a day, April 19, as we Approach the moon.   At 74 hrs. 28 min. 28 sec. into the flight, behind the moon, T.K., our Command Module pilot, fires our big Service Propulsion System engine to put us in in initial elliptical orbit.  The engine works like a champ — and I guess we’re lucky the problem didn’t occur then.    What a sight.  We radio, “Everybody is looking out their window . . .  We’re just staring at the ground.  Boy, this has got to be the neatest way to make a living anybody’s ever invented!”

Two orbits lager when T,.K. fires the SPS again to put us in a lower orbit, our descent orbit — think of it as the staging area for landing.  We fly as low as 11 mi. above the surface.  “It feels like we’re clipping the tops of the trees.”  Looking down at the landing site, we observe, “The general terrain appears frothy, vesicular-type territory.  Real craggy looking.”

Yep it looks rough as a cob down there at Descartes/Cayley.  We don’t have detailed photos of the area, as did prior missions.  Our photos were taken by Apollo 14 — and it’s high-resolution camera failed, so we only have lower-quality photos from hand-held cameras, which only show details more than 40-ft. across.

We take an 8 hr. rest period before beginning preparations for landing.  Sure, we encounter a couple small problems, corrected.  Then, power up our LM named Orion a bigger one rears up ugly.  The lander’s steerable  S-band antenna, used for receiving and transmitting at high data rates, vital to update our computer, is stuck — won’t steer in yaw.   And we can’t shake it free.

We’ve got two smaller omni (omni directional, meaning a wider beam) that we can use to communicate — at a lower data rate.  But they can’t be used to update the computer unless aligned perfectly with Earth.  Which means we have to find the time to key in the critical numbers ourselves, splitting tasks between the two of us, would you believe we get it done without falling behind the timeline?  And we undock, 96 hrs. 14 min. into the flight, and move away, and prepare for descent on the next orbit.  First, behind the moon, an hour before we land, T.K. will fire the SPS to raise the command ship Caspar, into a circular orbit of 60 nau. mi., his station for rendezvous should we have to abort the landing.  We’re about a half mile apart, coming up on 97 hrs. 40 min. into the flight, back there out of communications with Earth.  Prior to the burn, he checks all his systems.  And when he checks the Thrust Vector Control that steers the big engine during burns, keeping it thrusting through Caspar’s shifting center-of-gravity.  The primary TVC checks OK.  

Like everything, it has a backup loop.   When he hits the secondary system, he calls to us, “Hey, Orion?

“Go ahead, Ken.”

“I have an unstable yaw gimbal #2.   . . .When I turn it on, it feels as though it is shaking the spacecraft to pieces.”   To move the engine plus and minus 2 degrees, he usesa thumbwheel, much like the volume control on a radio.  And on the backup, he can feel the thing shake when he touches that thumbwheel.

“Oh, boy,” we can’t help but exclaim.  We need that engine to get home.  Or we could use the LM’s engines — that of course would mean no landing.  Damn. 

Don’t make that burn, we tell T.K., as if he doesn’t already know.  He’s heartbroken, wondering if he did something wrong.  “Me be a sorry bird.”

So once back in communications, we have to call the bad news, “No circ.  The Command Module did not circ.” 

While we have an hour or so on the front side of the Moon, T.K. works with Houston to explore the problem, engaging the TVC in several modes so controllers can gather data on the problem.

Communications go back and forth, like:

Capcom [Apollo 14’s Ed Mitchell]:  Ken, what we’d like you to do now is crank up the yaw 2 gimbal gain to that stable condition and then see what the MTVC [Main Thrust Vector Control] does to it.  See if that will excite the oscillation. 

T.K:  It did last time.  I now have the gimbal on again.  And I’m going to give it a little yaw.  And there it goes [oscillating].  Coming off — mark.  Would you like to take a look at it at accel. command?

Capcom:  Stand by . . .  OK, Ken — let’s try it in accel. command.

T.K.:  Ok, it’s diverging all on its own in accel. command.  I didn’t put any inputs into it.

Capcom:  Roger, copy.

And then an ominous silence from Houston for what seemed a long time.  We’re just 14 mins. from losing communications as we go around the back of the  Moon.  If all had gone well, we’d be on the lunar surface now, readying to make our first moonwalk about four hours later.

Houston calls, “Orion and Caspar, this is Houston.  It looks like we’re not going to have a decision on this rev.  We do have the capability of spending about 5 revs. in this configuration before making that decision.  We’d like you all to move in to station-keeping position.   . . . We recommend CSM active [the command ship, using it’s small RCS thruster] to move into position for station keeping.  And we’re going to run some simulations on this TVC, and we’ll get back to you.”

Five revolutions around the Moon equates to 10 hrs.  After that we’ll be too far out of position from the landing site.  

T.K. radios Houston before we lose contact, “Is there anything else you can think of us that we can do?  Otherwise, we’ll just be station keeping at about 100 ft. or so.

“We can’t think of anything else, Ken.”

And we lose contact, approaching the end of our 13th rev.  And our voices are stone flat.  The recently updated flight rules are clear — both SPS control systems must be working for landing.  And it sure looks grim on that.  In all likelihood, we’ll join up and in a few revs., come on home.  No landing, no explorations of the volcanic highlands of Descartes/Cayley. 

When we come back around and regain communications, Houston tells us, “We still do not have an answer.  But people are working very feverishly.”

“Orion, roger.  And thank you.  It’ll probably be awhile before we get to station keeping.”  Yep — our voices remain stone flat.  This isn’t looking good at all.  


We’re about to come back into communications range to start our 15th orbit.  Houston calls, “You do have a go, PDI on rev. 16.  And I have some words on that problems with the TVC when every you all are ready to copy.

“I’m all ears!”   PDI means Powered Descent Initiation and that means landing on the moon.

Here’s what they determined:  That a connection had gone ratty in the secondary’s feedback loop that sends commands and received data back on the engine’s movement.  Maybe a wire was loose or broken.  However commands were still getting through to the Thrust Vector Controller.  Tests way back on Apollo 9 had shown that the engine could operate successfully even with such oscillations.  Furthermore, say the primary controller failed; you could route commands to the secondary one using the primary’s control loop.  

Still the failure is a violation of flight rules — and all the wiring ran through the same bundle.  Who could say if other connections might fail.  It’s a risk to continue.  Maybe a small one, yet it shows how determined, shall we say, Mission Control is that we carry out the landing.  After all, there is only one more mission after us.

We’re ecstatic.  We’re going to land this baby on the moon and do some exploring!  We’re an orbit away — after 2.5 hrs. of station keeping, we move back away.   And once again on the farside of the moon, T.K. does his thing.  The big engine, on the primary controller, ignites, 103 hrs. 21 min. 43 sec,into the flight.  That bad boy burns for 4.66 sec.  As planned.  That’s all it takes to boost Caspar and T.K. into a 69 mi. circular orbit.  Can’t say we hat to see you go, friend — it means we’re on our own and just 56 min. from PDI — Powered Descent Initiation, the beginning of our 12-min. descent to the plains of Descartes.  We’ll be three orbits and 5 hrs. 40 min. late, but by golly we’ll get there.

We’re pressed for time now, and as we come around to the front of the moon again, try to lock up our omni antenna solid so that we can update our guidance.  We can’t get a lock — and damn if it doesn’t look like another wave off.  

Try yawing the spacecraft 20 degrees.

Good idea, Charlie, a cool move.  It works.  Whew — it’s taken us 12 min. to get our state vector updated.

It’s now 104 hrs. 13 min. into the flight, 4 min. to the start of our descent.  Our capcom, Apollo 15’s Jim Irwin, calls, “Orion, you’re go for PDI.”  

“Roger, go for PDI.”  Now aren’t those sweet words!

“Starting at about 10 mi.”  Charlie means we’ll be starting our descent from 66,000 ft. instead of the nominal 50,000 ft., but we can take out the extra on descent.   At the moment were flying face down, engine forward, which means we’re seeing back along the track we’ve come.    

Here go  “Pro.”  Meaning proceed — punching the button that tells the computer to proceed with the burn.  

“OK, Engine start.”  Mark — 104 hrs. 17 min. 25 sec. into the flight.  We’re engaged in a kind of technological call and response.

“Engine start.  Descent Engine command Override is on.”  That allows us to manually override the computer.  At PDI, we’re 320 mi. from the landing site.

“Stick your throttle at minimum.”  We start at minimum thrust, 10 percent, until the engine is locked firing through our center of gravity.  We’ll hold minimum thrust for 28 sec.

Our call and response continues.

“Stand by for throttle up; thrust to weight is OK.

” . . . Throttle up!  On time.”

“Feel that beauty come on!”  We can feel the G-force on our feet, as we stand, strapped in harnesses.  We also feel the high-frequency vibrations from the engine.

And it’s a numbers game.  We watch the numbers dip, dive, and rise, depending on their measure, herding them toward convergence with on the sweet curve of landing profile that ends on the plains of Descartes.  “We’re looking good at a minute.”  Even though way high on altitude.  

We take out the excess altitude as we build up descent rate.

“Systems look good.”  That’s it, Charlie — keep an eye on those systems — and key in the corrections Houston wants in the computer targeting.  “OK, that’s entered.”

Capcom Jim Irwin calls, “And you’re go at two” minutes.

We say, “Really looking good.”

Yep — “AGS and PGNS are tracking right on, Jim.”   Those are our two independent guidance systems — the Abort Guidance System, pronounced “Ags,” and the Primary Guidance and Navigation System, pronounced “Pings.”  When they agree, all is well.  Ags and Pings, brother.  “Within a tenth of a foot a second.”

“Orion, you’re go at three.”  

“Roger, go at three.”

By now, we’ve rolled the LM so that our windows are pointing up into the blackness of space.  This allows the landing radar at our base to sound out the surface.   

“Velocity light’s out, Charlie”  There are two lights on the panel between us, the Altitude/Velocity lights.  When the radar locks on each measure, the light goes out.  But the radar never achieves lock before about 40,000 ft.  It’s far, far too early for that.  “There’s no way to get an altitude light at this height height.”  Four minutes, approaching 50,000 ft.

“Look at that!  Altitude and velocity lights are out at 50k!”

“Isn’t that amazing?  Copy that, Houston?”

And the radar data is good.  We tell the computer to accept it.

“At 5 min.  Coming in like gangbusters.  Down to 39,000 ft. 

At 6 min., we should be at 32,000 ft.  We’re still 3,000 ft. high.  “Not quite back on profile, but almost.”

“Orion, you’re go at six.”  We’re 39 mi. from the landing site.

Fuel remaining, 45 per cent.  Right on the money.

We can see the sun getting lower as we approach it — the low post-dawn sun angle (but higher than it would have been three orbits earlier when we should have landed) will give long shadows to help us see surface features.  Still on our backs, but the vehicle is slowly tilting up.  Soon, our simulations show, the lunar horizon will slide into view.

It’s time to back off the power of the descent engine.  Jim in Houston calls when to expect it.  “Throttle down 7 [min.] plus 23 [sec.]”

“And you’re go at seven.” 

From the commander’s side, on the left, we call, “Throttle down.  Right on time.”

From the right side, Charlie comments, “It was on time, wasn’t it?”


We’re at 21,000 ft, coming up on 8 min. into the burn, two-thirds of the way through the landing.  And there it is — leaning forward at the window.  “I can see the landing site from here, Charlie.” 


Nine minutes, 12,000 ft. altitude.  About to pitch over to a vertical position.

“Pitchover.”  Into the approach phase of 

And now from the right window, Charlie’s window, you can see the site and identify craters, exclaims, “Pitchover.  Hey, there it is.  Gator, Lone Star.  Right on.”  Calling out the names we’ve given craters at the site.  Yes, can see the big craters, shadowed at the bottom in the low sun angle, Palmetto and Gator, and the smaller, sharp rimmed Lone Star on Gator’s flank.  A bit later, sees North Ray Crater, which we intend to explore, says, “Looks like we’re going to be able to make it, John.  There’s not too many blocks up there.”  Only time for a five-second glance — as gotta, as our trusty commander reminds us, keep calling out the numbers — keep our eyes on the instruments.

We’re correcting our course, which had us coming down about 2,000 ft. north and 1,300 west of the landing spots.  We do that by “clicks” of a controller that tell the computer to adjust the course a set amount.  The more clicks, the greater the adjustment.

Jim Irwin calls, “Orion, you’re go for landing.”  

We’re at 4,000 ft.  Still adjusting our landing point.  “A little more to the left.”  That’s to the south.

We’re at 2,000 ft. altitude and “on profile.”  We adjust by a couple more clicks.   “OK, Houston, we’re going to be just a little bit long . . .  We’re just now abeam of Double Spot.”   Those are the two small craters we wanted to land near.

We’re getting close.  Man, look at that — this place sure isn’t flat.  “Some big blocks over here to the left, John.”

We take over manual control.  Call out those numbers, help me on down.  

“OK, fuel is good 10 percent.  There comes the shadow.”  The shadow of the LM in front of us — as if  riding in to greet us.  When it’s shadow legs touch our, we’ll be on the surface.

And that shadow makes a great scale to measure rocks and craters on the surface.  We’re at 130 ft., sliding forward at 2 ft. per second.  

“Perfect place over here, John, a couple of big boulders.  Not too bad.”

Bringing her on down.  We’re 80 above the surface, and the dust begins flowing radially from us.  Have passed over a small but steep crater.   We’re backing up slightly.  Move forward and a bit to the right.  We’re  40 ft. altitude, hovering like a helicopter.  Looks flat — can see the rocks on the surface despite the dust.   But can’t judge slopes at all. 

“Stand by for contact.  Come on, let her down.  You leveled off.   Let her on down.”  Plenty of fuel left — 6 percent.  “Plenty fat.”  

Bringing her straight down.

“Contact.”  The 5-ft.-long probes below the footpads have touched ground.

Pause a moment and switch off the engine, drop the final 3 ft.  “Boom.”– we’re down.

Disarm the engine, switch to the program to light the Ascent Propulsion System engine should we have to make a quick escape.  And relax a moment.  “Well, we don’t have to walk far to pick up rocks, Houston.  We’re among them!”

And we are — rocks and small boulders everywhere, and craters.  Craters within craters within craters.  

“Old Orion is finally here, Houston.  Fantastic!”

Where exactly are we?  “We’re forward and to the north of Double Spot.  I would guess  about 200 meters to the north and maybe 150 meters to the west.”

There’s a ridge ahead of us and one to the side.  Looks like we could be in an very eroded subdued old crater shot with smaller craters.  “It’s not flatlands, though, Houston.”  Even though we’ve managed to land on a flat spot.  By luck.

What a place — look at all those rocks strewn everywhere, averaging, oh, 3 ft. across.  “All we got to do is jump out the hatch and we got plenty of rocks.”

“What a neat place!”

We’re so excited we almost forget, we need the word for Houston that we are OK to stay.  “Wait a minute, Charlie.  We gonna stay, Houston?”

“Orion, you’re stay for T -1.”

“Hey, Jim, hats off and a case of beer to FIDO.  I’ll tell you, that targeting was just perfect.”  FIDO is the Flight Dynamics Officer — the head guy for the team that did a beautiful job amid all the changes forced by our three-orbit delay.

We can’t stop exclaiming about what’s out there.  It’s unlike any previous site.  Man, we’re in the middle of a block field.  Already where looking along the routes the Rover will take — should have no problem going up the hills we want to explore.  “It’s going to be neat.”

Ah, but all that will have to wait for a new day (a new day for us — iy will be the same lunar day).   We’d originally planned to go right out, as quickly as we could after landing, which would take about four hours.  With the delay, that would mean a 29 hr. day for us.  As excited as we are, it’s tempting.  But best to get a good sleep and take on the surface tomorrow.  We power down, get out of our suits, and and string our hammocks crisscross in the tiny cabin.  Pleasant dreams.

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