LM Exhaust question

[AstroBrant]

If I was a hoax nut, this would be inappropriate for this section of the ApolloHoax forum. Ironically, I may have come up with a better hoax claim than most CTs have ever come up with. They would be very jealous of me, I think.

But no, I really am puzzled by this and hope someone will have an answer.

We are repeatedly told that the LM descent engines were shut down just before landing. I've read the relevant material on:
http://www.collectspace.com/ubb/Forum29/HTML/001189.html
But I am bewildered by the fact that we see loose regolith being blown out by the exhaust for several seconds after touchdown in all the Apollo missions. Particularly odd to me is Apollo 14. There we see thick dust blowout for 7 seconds after touchdown, and then faint dust expulsion for a full 13 seconds after that!

Can someone explain that? All I've been able to get from my friends is speculation. I'd really like a solid answer if that's possible.

Edit: I did see where someone said that A14 actually didn't shut the engine down until after it landed. But then there is still that extra 13 seconds of slight dust blowout.

[Allan F]

Off the top of my head, I remember A11's shutdown was 1.5 seconds after touchdown. There is an excellent site called "Apollo By The Numbers" where the engine on/off times is detailed.

[Zakalwe]

If I have read the Apollo by Numbers correctly:

Apollo 11:  Engine shutdown occurred 1.5 seconds after landing.
Apollo 12:  Engine shut down 1.1 seconds before landing
Apollo 14:  Engine shutdown 1.83 seconds after the first pad touched down. The remaining pads touched down 2 seconds after the first pad.
Apollo 15:  Engine shutdown 0.7 seconds before landing
Apollo 16:  Engine shutdown occurred 4 seconds after landing
Apollo 17:  Engine shutdown occurred exactly at touchdown

[Mag40]

But I am bewildered by the fact that we see loose regolith being blown out by the exhaust for several seconds after touchdown in all the Apollo missions. Particularly odd to me is Apollo 14. There we see thick dust blowout for 7 seconds after touchdown, and then faint dust expulsion for a full 13 seconds after that!

As I see it, the engine is still expelling exhaust gas even after it has shut down, and much less forcibly. The 13 seconds of haze is where dust is settling locally, rather than being fired hundreds of feet away by the active engine.

[JayUtah]

Apollo 11 definitely ran the engine to touchdown and beyond.  This is because the "Contact light!" callout was missed.

Several authors report that a few CMPs told them they had no intention of shutting the engine off until touchdown.  But in the film analysis I do agree that you're seeing a lot of residual entrainment even on the flights that did manage to drop the last meter or so unpowered.

[Allan F]

What about the attitude control thrusters - did they shut down automatically at touchdown, or did they try to correct the tilt from the not-level ground?

[Zakalwe]

What about the attitude control thrusters - did they shut down automatically at touchdown, or did they try to correct the tilt from the not-level ground?

From memory, no they weren't automatically shut-down. I'm off to find the command that was used to disable them.

[JayUtah]

What about the attitude control thrusters - did they shut down automatically at touchdown, or did they try to correct the tilt from the not-level ground?

They did not shut down automatically.  Indeed they tried to "correct" the LM's attitude.  However, the solution is simply to wiggle the joystick out of its default position, forcing the DAP to accept the LM's current orientation as the set-point until the RCS circuit breakers could be pulled.  You hear Armstrong report, "ACA out of detent," shortly after landing, in conjuction with Aldrin's "413 is in."  The latter operation poked a non-zero value into memory location 0413, which was consulted by programs whose operation depended on knowing whether or not the LM had landed.

In program P66 the ACA hand controller functioned almost identical to the cyclic stick in a helicopter.  If you pulled back, the LM would pitch nose upward at a rate proportional to how far you deflected the handle.  If you release it back to detent, the pitch rate is nulled and the DAP accepts that as the new ATT HOLD set point.  The computer receives a discrete signal when the ACA is out of detent in any direction, activating the "manual rate control" routine.  So in P66, resetting that signal tells the software, "whatever the attitude is at this instant, that's the attitude set-point."  Then the ordinary DAP loop handles nulling all the rates that the manual routine ordered.

[Zakalwe]

Jay beat me to it 

[Allan F]

So all the CDR had to do to null the attitude thrusters was to click the handcontroller in a random direction? Clever.

[JayUtah]

Yes it was. Only in P66 though. But it's characteristic of the practical ways the Apollo engineers solved problems.

The CM engineers worried about having the caution and warning system go off spuriously when the CM was first powered up and before its sensors had "settled."! One of the astronauts reminded them that cars do that all the time and we're used to it. So when you power on a CM you get 30 seconds or so of warning lights and buzzers. Then you push Master Reset and you're good to go.

[AstroBrant]

(snip...)
As I see it, the engine is still expelling exhaust gas even after it has shut down, and much less forcibly. The 13 seconds of haze is where dust is settling locally, rather than being fired hundreds of feet away by the active engine.

I guessed that there might be some residual exhaust after shutdown, but I couldn't figure out how long that would last. Zakalwe's "by the numbers" list as well as Jay's reference to comments by the CMPs, (I suppose he meant LMPs), confirm another of my suspicions: that the astronauts may have thought, "Simulations be damned -- I'm shutting this engine off when we hit the ground."

Still, this residual haze, (13 seconds after the main dust blowout had stopped), has me baffled. I can see how the dust took a couple of seconds to clear, but it is finished long before this haze stopped. All I can figure is that the shutoff valve didn't completely close. What I was hoping for was some documented information on why this "dust anomaly" happened, but it simply may not exist.

[AstroBrant]

If I have read the Apollo by Numbers correctly:

Apollo 11:  Engine shutdown occurred 1.5 seconds after landing.
Apollo 12:  Engine shut down 1.1 seconds before landing
Apollo 14:  Engine shutdown 1.83 seconds after the first pad touched down. The remaining pads touched down 2 seconds after the first pad.
Apollo 15:  Engine shutdown 0.7 seconds before landing
Apollo 16:  Engine shutdown occurred 4 seconds after landing
Apollo 17:  Engine shutdown occurred exactly at touchdown

Well, at least this explains Apollo 16.

[JayUtah]

I guessed that there might be some residual exhaust after shutdown, but I couldn't figure out how long that would last.

The graph of thrust over time shows a parabolic decay at cutoff.  For most motors the decay time is roughly two-thirds of the transient time.  IIRC the DPS has an ignition transient time of about 350 milliseconds, so call it 225 milliseconds for the decay.

...Jay's reference to comments by the CMPs, (I suppose he meant LMPs)

Actually I meant CDR.  I just starting typing one acronym with C and another one snuck out instead.  The CDR's line of sight was usually out the window, flying the ship.  For most purposes the LMP monitored the instruments and called out significant readings.  It was his job to call out the contact light, but it was the CDR's decision when to actually cut the engine.

All I can figure is that the shutoff valve didn't completely close.

I'll have to look closely at the films and see if I can see what you're seeing.  The remaining DPS propellants were vented after landing.  That may explain it.

[Allan F]

Wasn't the venting manually done - some time after landing?