New claim: lunar module ascent stage too nimble

[ToSeek]

Interdimensional Warrior is on another site claiming that the ascent stage of the lunar module maneuvers in an unrealistic way, viz., "Instantaneouas acceleration from one velocity to another is positively impossible."

I'm trying to calculate just how fast the ascent stage could start turning given the strength of the RCS thrusters and its moment of inertia, but my math is kind of rusty for that. If anyone knows how to do these calculations, feel free.

A couple of YouTube videos illustrating the behavior. Not sure if these are real-time, though, but I haven't found any authoritative sources for the film - even my Apollo 11 DVD set from Spacecraft Films doesn't include this sequence:

Apollo 11:
http://www.youtube.com/watch?v=uazzYgghQD4&NR=1

Apollo 9:
http://www.youtube.com/watch?v=AG6A1wcL2Lk

[matthewota]

On Apollo 10, they accidentally left the Abort Guidance System (AGS) active when they separated from their descent stage. The Reaction Control System (RCS) quads fired to get the ascent stage into position and made it tumble so fast that the astronauts shouted expletives in surprise.

The LM, with the descent stage gone, was quite a sporty spacecraft.

[Donnie B.]

After watching the first clip, I would say it's not real-time, but made with a DAC camera on one of the slower frame rates.

The evidence is the strobe light on the LM. In real-time it would be flashing regularly, but in this video we only see it flash occasionally and at wildly differing intervals. That suggests that the camera is not running continuously but is snapping a shot periodically with relatively long spaces in between, and therefore missing many of the strobe flashes.

[Dave J]

It's pretty obvious in watching the usual media films that the rendezvous footage is sped up. The real-time maneuvers were just too slow and methodical in the final stages to be "dramatic".
Such time accelleration would tend to make rotational inputs seem instantaneous, and therefore "impossible". Finding real time footage, or the equivilant thereof, would be valuable here.

Kind of like watching some footage of the Shuttle/ISS dockings, sped up from the snail's pace to fit the allotted show time...

[JayUtah]

The DAC nominally operates at 6 fps during docking.

The LM RCS was sized for the docked, fully-fueled vehicle.

[peteshimmon]

The dvd available with one of the Apollo
mission reports has the complete video of
the descent and ascent. I was fascinated by
the slow limit cycling of the ascent stage
while getting into lunar orbit from the
surface. They had to be so damn careful with
the mass distribution prior to lift off so
that the rocket was thrusting through the
centre of mass. The astronauts has lost
weight, the rocks had to be weighed, so many
items dumped overboard. And they did it OK
six times!

[PetersCreek]

Take the source for what it's worth...and my understanding is that it's a fairly accurate model...but I just popped into Eagle Lander 3D and noted that the Rate Scale switch allows a selection of 25 deg/sec (or 5 deg/sec) at full deflection of the controller. If the flight model is accurate, the LM will indeed move (and null rates) quite crisply.

I'll just add that my rates have gotten away from me more than a few times.

[JayUtah]

The moment arm of the APS is actually pretty short. The APS throat is well up inside the bulk of the LM, almost between the APS propellant tanks which form the overwhelming majority of the ascent stage launch mass. Hence off-axis thrust in the APS generates relatively small moments.

At ascent docking, the APS propellant is essentially depleted. The resulting vehicle is little more than an aluminum can. Recall that the RCS was sized to provide usable rates in a vehicle originally massing some 17 tons. While that still doesn't result in "instantaneous" acceleration, it will likely produce rates whose initial motion is not captured at 6 fps.

[PetersCreek]

Just to try it hands-on, so to speak, I started the sim with the complete LM docked with the CSM. After separation, with Rate Scale set to 25 deg/sec and PNGS in Attitude Hold, it did respond much more sluggishly. It took considerably more thrust to get the rate up and to null it, as one should expect from Jay's description.

[CJSF]

I mentioned this in the other thread started by ToSeek (regarding video of Earth from the Moon's surface):

Could someone clarify for me, what is BAUT's policy for posting and debating claims from another forum? I thought it was against the rules, but here we have 2 threads by an admin doing it. I don't see a major problem with it, but I have seen mods warn/ask members to desist from debating points brought up on other websites.

CJSF

[ToSeek]

I think the key issue is whether or not someone is advocating on behalf of a third party. I'm not, I'm advocating on my own behalf.

[Gillianren]

Also, no one's making fun of anyone and the argument itself isn't brought forward--there isn't a lot of "well, you said X on the other board, and I think you're Y." This seems as much for our edification as anything.

[Swift]

I mentioned this in the other thread started by ToSeek (regarding video of Earth from the Moon's surface):

Could someone clarify for me, what is BAUT's policy for posting and debating claims from another forum? I thought it was against the rules, but here we have 2 threads by an admin doing it. I don't see a major problem with it, but I have seen mods warn/ask members to desist from debating points brought up on other websites.

CJSF

The advice for CT supporters are suggestions that are not quite rules, but come pretty close. In that list, # 9 and 10 cover this case:

9. The CT forum is mostly designed for advocates of a Conspiracy Theory to defend their ideas, and not a general forum for CT discussion. However, we do allow limited posting of other threads related to CTs. If you have a specific question related to a CT (example: “Why can’t I see stars in photos from the moon?”), it is probably best to ask that in the Space/Astronomy Questions and Answers forum.

10. If you are not advocating a CT, but are looking, for example, for help in answering questions about one (“I got this friend at work who thinks….”), you can post in the CT forum when looking for help or advice. You must make it very clear that you are not advocating the CT. If you use these exceptions to advocate your own belief in a CT, you will be suspended or banned.

[KA9Q]

I ran into this exact claim on another forum (probably Youtube) a while ago. His video made a big deal of Isaac Newton and his laws of motion, as though that's all he needed to prove Apollo a hoax.

I analyzed it and found that the observed behavior was well within the capability of the ascent stage with empty tanks. I did not notice the irregular strobe light (good catch!) but I did look up the frame rate and also found it to be 6 fps.

I got the mass properties from the mission report, specifically the moment of inertia around the pitch axis, and computed the angular acceleration imparted by a continuous 4-thruster pitch RCS firing. It was *quick*. I don't have my numbers handy but again it was quite capable of producing the filmed behavior.

The guy kept trying to tell me that it was still impossible for the LM to stop instantly even after repeated explanations that it didn't have to stop instantly. It only had to stop within one or two film frames, and in fact it looked like it took two frames for all motion to cease. I think concepts like moment of inertia were well over his head, so eventually I gave up.

[JayUtah]

...

His video made a big deal of Isaac Newton and his laws of motion...
I think concepts like moment of inertia were well over his head...

I have found it to be most often the case that people who claim the "laws of physics" suggest Apollo film and video are fake really don't have much grasp of the laws of physics.

Those little Marquardt thrusters are deceptively powerful: 100 lbf thrust each. And the way the RCS quads on the LM are designed, you get four thrusters giving you pitch moments. Shepard let Mitchell fly the LM manually a bit during the ascent, and Mitchell is the one who described it as "sporty."

[tofu]

Did the various Apollo spacecraft have something like an auto pilot that would zero their roll/pitch/yaw rates? I'm wondering if they start a maneuver, like rolling 90 degrees, then have to stop the maneuver manually, or if they just hit a button and let the autopilot stop it.

[PetersCreek]

In the case of the LM, the Primary Navigation and Guidance System (PNGS) provided an Attitude Hold mode. When hand control input was nulled, the PNGS would automatically provided RCS thrust to null the rate(s).

[JayUtah]

That's the joy of fly-by-wire. The attitude-control joystick can be defined in software to behave however you want. There was some debate whether the stick should be considered to indicate a desired rate or a desired attitude. That is, if you push the stick forward should you have to pull it backward to null the pitch rate when you get to where you want? Or if you push the stick forward, should you just release it back to detent in order to null the rate?

Nevertheless the digital autopilot (DAP) software was the heart of the flight control systems. Engineers quickly realized that the "error" (deflection from desired direction) and "error rate" (angular velocity) in each axis were set-points that could be manipulated by software, and constituted a fairly rich vocabulary of flight-control actions.

The "ACA out of detent" call just after landing is a funny example. The LM lands in ATT HOLD mode. But because it may land catty-wompus to the existing attitude set-points, the RCS would go nuts trying to "correct" the landed LM to the proper attitude. The solution is simply to wiggle the joystick so that when it pops back to the detent the existing attitude angles are plugged into the DAP as the "desired" angles. And since the LM sitting on the ground isn't going to budge, the DAP never has to correct them. A simple, elegant solution to an amusing problem.

[KA9Q]

In the case of the LM, the Primary Navigation and Guidance System (PNGS) provided an Attitude Hold mode. When hand control input was nulled, the PNGS would automatically provided RCS thrust to null the rate(s).

Correct. That was the so-called "manual" mode (P66 I believe) used during the landings. But it was far from pure manual.

You can hear an "att hold" callout on the flight loop during the A11 landing.

[ToSeek]

But because it may land catty-wompus to the existing attitude set-points

Don't get all technical on us, Jay.

[Peter B]

...Shepard let Mitchell fly the LM manually a bit during the ascent...

Are you sure, Jay? According to Chaikin, Conrad let Bean fly the LM while above the far side of the Moon on Apollo 12, but I've never read anything suggesting Shephard let Mitchell fly the LM on Apollo 14.

...Mitchell is the one who described it as "sporty"...

I'm going for two from two here, but wasn't it Charlie Duke who described the LM as "...a sporty ride..." on the BA's Fox Apollo page?

[Glom]

So that call is saying that he is wiggling the stick to reset the attitude setpoint?

[Gillianren]

Don't get all technical on us, Jay.

Completely OT--this is a word I would essentially never correct, no matter how it's spelled. I spell it "cattawampus," when I spell it, but it is one of the words with the strongest regional differences, and it's even more complicated that British vs. American.

[JayUtah]

Are you sure, Jay?

I recall hearing it from Mitchell himself. I asked him about the LM handling characteristics. He may have borrowed "sporty" from someone else for all I know.

[LaurelHS]

On page 300 of W. David Woods' book How Apollo Flew To The Moon, Buzz Aldrin also describes the LM as "sporty."

[JayUtah]

--this is a word I would essentially never correct, no matter how it's spelled. I spell it "cattawampus," when I spell it, but it is one of the words with the strongest regional differences...

I've always considered it unequivocally vernacular and thus highly susceptible to regional variations in spelling and pronunciation; cf. "kitty-corner" and "catty-corner." Here in Utah kids taunt each other saying "neener neener!" whereas in the Midwest where I grew up it went "nanny nanny." Similarly in the Midwest the second syllable of our word in question is distinctly ee.

I've also heard "skee-wompus" as a variant.

[Anla'Shok]

The guy kept trying to tell me that it was still impossible for the LM to stop instantly even after repeated explanations that it didn't have to stop instantly. It only had to stop within one or two film frames, and in fact it looked like it took two frames for all motion to cease. I think concepts like moment of inertia were well over his head, so eventually I gave up.

Point out that the LM did not 'stop' at all, it just matched the velocity of the CM, or near enough to maneuver to dock. You know what they say ‘It’s all relative’.

I hope I did not make an imbecile out of myself here and address the wrong question, I can not access YouTube at work so did not see video.

[gwiz]

I recall hearing it from Mitchell himself. I asked him about the LM handling characteristics. He may have borrowed "sporty" from someone else for all I know.

According to this link, Conrad used the word in December 1967, presumably from his simulator experience.

http://www.astronautix.com/craft/lmweight.htm

[JayUtah]

Sounds like multiple lines of evidence suggest that "sporty" is a universal term among the Apollo crews for describing the performance of the undocked ascent module.

[AGN Fuel]

Sounds like multiple lines of evidence suggest that "sporty" is a universal term among the Apollo crews for describing the performance of the undocked ascent module.

...in contrast to the universal terms used by the Apollo 10 crew for describing the performance of the undocked ascent module with an incorrect switch configuration on descent stage sep...