The LM maneuvres pre docking

[Mag40]

On the Apollo 11 footage post landing docking, we see the LM doing a series of maneuvres. One claim I've seen(can't recall where) states that these moves.....followed by immediate stops are impossible. Now I think I know how it's done, but would love this clarified. I've been reading the Apollo experience report and couldn't see where the astronaut control tied in. Is this a matter of each joystick 'blip' left or right etc..... administering a measured amount, or was this simply down to the astronauts being extremely accurate? What I mean is, say a rotation is initiated of 60 degrees, then the LM stops rotating immediately, how is this performed dead stop.

This is a youtube video for reference, showing some Apollo 11 highlights....at the 5 minute mark

What would also be cool is if someone explained how they managed to move it sideways without rotating, is it a combination of joysticks and other controls?

[ka9q]

I think I've seen the same claims (on Youtube) that these motions were somehow impossible. Obviously, they weren't!

Two important points. First, the 16mm movie camera that took these sequences ran at only 6 fps (I think), so at the nominal 24 fps playback speed the movie plays back 4x faster than real time. (Don't quote me on these numbers before checking them.)

Second, the LM was extremely light at this point. At docking it has lost the entire descent stage and nearly all of its ascent propellants so its moments and products of inertia are all very low. Since the RCS thrusters are fixed at 100 lbf of thrust, the crews said the LM handled like a sports car during the docking. A "pure" pitch, roll or yaw maneuver required 2 or 4 thrusters, i.e., without introducing translation. A pure X axis translation required 4 thrusters, while a pure Y or Z axis translation required either 2 or 4 depending on the center of mass.

A while ago I looked up the LM's mass properties from a table in the mission report, computed the torques produced by the RCS thrusters, and worked out the maximum linear and angular accelerations that they could provide at docking. Taking the movie frame rate into account I found that the maneuvers shown were well within the LM's capability.

The LM was manually controlled with two separate joysticks, one in each hand, each with three degrees of motion. The conventional-looking stick on the right was for attitude control: pitch, roll and yaw. The T-shaped handle on the left was for translation: X (up/down), Y (left/right) and Z (forward/backward). The CSM pilot used the same two types of sticks.

With very few exceptions the LM and CSM operated in a "fly by wire" mode. The sticks sent signals to the computer which in turn decided which thrusters to fire. This was the case even in the so-called "manual mode" used just before landing by every Apollo commander. The "manual control mode" that Neil Armstrong famously used to fly over the boulder field was more accurately a semi-automatic mode. He actually put the computer into an "attitude hold" mode so that when he let go of the stick, the computer automatically fired whatever thrusters were needed to maintain the attitude he had selected.

Even in this so-called manual mode the computer controlled the descent engine, adjusting its gimbals to keep the thrust vector through the center of mass and adjusting the throttle to maintain a fixed descent rate while compensating for the LM's rapidly decreasing weight as it burned descent propellants. The commander adjusted the descent rate with a momentary toggle switch that worked very much like the cruise control in a car: flicking it in one direction slowed the descent rate by a foot per second and the other direction increased it by the same amount.

Except for the final phase of landing, LM powered flight was almost entirely automatic.

[Mag40]

Thanks Ka9Q! The signals to fire the thrusters....do they involve fixed amount of fuel, or is it astronaut skill?

[Glom]

Don't know about the specific claim you've heard, but on similar issues, it's worth noting that due to the low frame rate of recording that ka9q mentions, many of the RCS bursts didn't get caught so the LM looks like it's just randomly changing motion without cause, which looks strange.

[ka9q]

Thanks Ka9Q! The signals to fire the thrusters....do they involve fixed amount of fuel, or is it astronaut skill?

I know this is an old thread, but I just noticed you asked a question that I never answered.

The RCS thrusters had constant thrust -- 100 lbf each. Impulse was controlled by how long you held the propellant valves open. You could also effectively vary thrust by rapidly cycling them with a particular duty cycle. But because the valves are electromechanical, and because it takes finite time for the propellants to mix, ignite and build thrust, there is a practical "minimum impulse" for each firing. The computer took this into account when computing which thrusters to fire, when and for how long. This is one of the main reasons for "dead bands", an allowable margin of attitude error within which the computer would not try to correct the error further.

The computer could be instructed to maintain a given attitude within the dead band, which it would do as efficiently as possible, or an astronaut could fire them manually in which case fuel consumption depended on his skill. Transposition and docking was performed manually shortly after TLI, and the CMPs seemed to compete with each other for bragging rights as to who could do it with the least amount of gas.

[JayUtah]

LM RCS could be operated in "pulse mode," which is a minimum-impulse mode.  The servovalves were simply cycled as fast as possible such that a deflection of the hand controller would result (in some settings) in the fastest possible open-close cycle, or (in other settings) a rate-controlled series of pulses over time.  This was the answer to an RCS that had to maneuver the fully-fueled LM at rates that were fast enough to facilitate a safe landing, while later maneuvering the ascent stage only at controllable rates.

The basic autopilot mode was ATT HOLD, which will pulse or fire the RCS jets to maintain space-fixed orientation within a deadband.  The desired orientation is set in most modes with the rotational hand controller.  The size of the deadband was selectable.  There is no equivalent "position hold," so translations using the other hand controller had to be manually nulled.  However, ATT HOLD will null the rotational residuals.

If you have Orbiter, there is an Apollo add-on that will let you practice CSM transposition and docking, however the CSM dynamics feel markedly different than the LM.  For transposition and docking the CSM was the active vehicle.  For LOR, the LM was the active vehicle -- hence the jinking around you see on the film.

Regarding film speed, the Maurer on the CSM was indeed set to 6 fps during ascent and rendezvous.  Otherwise the CMP would have needed to change magazines right about the time the rendezvous got interesting.  As such the maneuvers seem abrupt.  Given the oversized RCS at this point, the starts and stops would be abrupt anyway.  Ed Mitchell told me that when Shepard let him fly the LM ascent module, he described its performance as "sporty."  The LM RCS is unquestionably in pulse mode during the ascent, and the pulses are short enough (something like 6-10 milliseconds) that you rarely catch one on a frame, nor would you even at full 24 fps.

[Sus_pilot]

Speaking of catching a pulse of the thrusters on a frame of film...

In Voyage to the Moon the CGI depicted the thrusters as leaving ice crystals (I think - could be other white dots) behind after firing, with no other exhaust artifact, such as flame.  I understand from reading here and elsewhere that the hypergolics used are virtually invisible when burning - IIRC, the visible flames from the Titan series (other than SRB's) were mostly from contaminants in the exhaust stream, from the engine bells, environment, etc. 

Not being an organic chemist by any stretch of the imagination, what would have been the residue from the thrusters? Would water have been a component of the exhaust (I'm guessing it would be)?  Would enough have been present to make visible crystals? Or was that an artifact added by the visual team in the series to show that the thrusters were firing?

Just as an aside, it does boggle the mind the sheer number of rocket engines used in the entire Saturn V/Apollo stack if you take into account thrusters, ullage motors,   etc., etc....

[Bob B.]

Not being an organic chemist by any stretch of the imagination, what would have been the residue from the thrusters? Would water have been a component of the exhaust (I'm guessing it would be)?

Water is definitely a large constituent of the exhaust.  The main gases produce by the combustion of nitrogen tetroxide and Aerozine 50 are water vapor, nitrogen, hydrogen, and carbon monoxide.   

Would enough have been present to make visible crystals? Or was that an artifact added by the visual team in the series to show that the thrusters were firing?

Since the water is expelled as a gas, I doubt ice crystals would form.  If the water were expelled as a liquid then, yes, some of it would freeze and form ice crystals.  However, I have a hard time seeing how it would go from gas phase to solid phase (though I could be wrong).

Just as an aside, it does boggle the mind the sheer number of rocket engines used in the entire Saturn V/Apollo stack if you take into account thrusters, ullage motors,   etc., etc....

Yes it is surprising how many engines/motors there are.  I added them all up once and I think it is over 80.

[JayUtah]

Those particular hypergols tend to throw an incandescent plume during in the ignition transient, but then burn without incandescence at steady state.  In pulse mode it's all about the transient, so if you were watching the thrusters live you could expect to see a very short visible burst.

[Allan F]

Now, we're talking hypergolic engines - is it possible to calculate the temperature of the exhaust of the LM's descent engine? This relates to the hoaxer idea of scorching on the surface. I know the exhaust wasn't capable of burning anything, but a solid calculation would be nice. I've tried myself, but got an improbable result, due to my lack of the correct formulae.

[Bob B.]

Now, we're talking hypergolic engines - is it possible to calculate the temperature of the exhaust of the LM's descent engine? This relates to the hoaxer idea of scorching on the surface. I know the exhaust wasn't capable of burning anything, but a solid calculation would be nice. I've tried myself, but got an improbable result, due to my lack of the correct formulae.

Yes, it's possible.  The gas expansion is isentropic, thus the effect of pressure on temperature is described by the following equation,

T₂ / T₁ = (P₂ / P₁)^{1 – 1 / k}

where k is the specific heat ratio.

The descent engine combustion chamber pressure was 713 kPa (103.4 psia).  According to my own calculations, I estimate the combustion chamber temperature at 3,060 K and the specific heat ratio at 1.23.  I further estimate the pressure at the nozzle exit to be about 1 kPa.  Therefore we can calculate the temperature at the nozzle exit,

T₂ = 3060 x (1 / 713)^{1 – 1 / 1.23}  = 896 K

Of course the gas continues to expand after leaving the nozzle, thus the pressure and temperature drops further, and quite rapidly.

[Allan F]

Yes, that was what I thought - I was way off. That equation you use, is that a derivative of the PV=nRT?

[Bob B.]

Yes, that was what I thought - I was way off. That equation you use, is that a derivative of the PV=nRT?

I think there's a derivation of it here,

http://www.grc.nasa.gov/WWW/K-12/airplane/compexp.html

[Allan F]

Yup - same equation. It was just me using the wrong chamber pressure, temperature, expansion ratio - basically all the important parts.

[ka9q]

Wouldn't you want to use the stagnation temperature and pressure if you're trying to find the temperature and pressure at the surface?

The plume expands after leaving the nozzle extension so it wouldn't reach the temperature in the combustion chamber, but it should be at least a little hotter than at the nozzle exit.