Making "Hard" Stop in Space (How Does RCS Work?) Q&A

• Making "Hard" Stop in Space (How Does RCS Work?) Q&A by ecosta on 03 Apr, 2020 13:20
• So in particular I guess this question pertains to Apollo missions, especially during transposition / docking, and pre-LM docking spacecraft checks. I understand basically how the RCS works, using thrusters to control attitude and translation of the spacecraft.
  
  But when the Apollo CSM translates to dock and extract the LM, they rotate, and then all of a sudden stop once they've flipped 180 degrees and are in line with the LM. How does this work? Is the RCS hooked up to the computers / gyros that let it know its position and when to fire thrusters to decelerate and stop at a specific point? Or is this done manually by the CSM pilot?
  
  This also occurs once the LM ascent stage rendezvous with the CSM, they both rotate to ensure there is no damage to the spacecraft and then make a hard stop once they're lined up appropriately. Again, is the RCS linked to the computers to allow them to stop in a specific orientation?
• #1 by Proponent on 03 Apr, 2020 14:15
• It was done manually in both cases by the CMP (Command Module pilot).  The CM had a Crewman Optical Alignment Sight on the left-hand window and the LM had a docking target, both of which made it easier to line up properly.
  
  Originally, the hatch that the astronauts used to exit and re-enter the LM on the lunar surface was to double as a docking port when the LM returned to the CSM.  In 1965, however, it was found that it was difficult for a fully suited astronaut to exit the LM through the round docking hatch as he made his way to the lunar surface (it was Roger Chaffee, later to die in the Apollo 1 accident, who discovered this while working with a mock-up at Grumman).  The round hatch was replaced by a square one, which could not be used for docking.  A window was built into the roof of the LM to allow the CDR (commander) to docking with the CSM on return to lunar orbit.  This turned out to be quite awkward, however, and it was decided that the CMP would perform that second docking maneuver too.
  
  You can try your hand at the docking maneuver -- and many other aspects of a space mission -- if you install the free Orbiter spaceflight simulator.
• #2 by ecosta on 03 Apr, 2020 16:28
• Right I understand the actual docking was done manually by the CMP. I more talking about the translation maneuver of the spacecraft. They rotate and then "Stop on a dime" so to speak, how is this achieved?
  
  Theoretically the pilot could fire the thruster to start moving, and slowly fire it in the opposite direction to slow himself down until he has flipped a full 180. But the CM would slow down as it got closer to the final position in that case. In the videos of the transposition move, the CM rotates at a constant speed and then just stops when its flipped 180, how is that done?
  
  Or for instance in the videos of  the LM coming to rendezvous with the CM, as it moves around it makes slight movements and stops abruptly. I know the footage is probably being played back at the wrong frames per second so it looks robotic, but they are still stopping quickly without slowing down toward the end of a maneuver. How is it stopping so abruptly? Is it the RCS doing its job through the computer? I know on aesent the computer took over but was allowed manual inputs through the RCS.
• #3 by Jorge on 03 Apr, 2020 18:06
• Quote from: ecosta on 03 Apr, 2020 16:28

  Right I understand the actual docking was done manually by the CMP. I more talking about the translation maneuver of the spacecraft. They rotate and then "Stop on a dime" so to speak, how is this achieved?
  
  Theoretically the pilot could fire the thruster to start moving, and slowly fire it in the opposite direction to slow himself down until he has flipped a full 180. But the CM would slow down as it got closer to the final position in that case. In the videos of the transposition move, the CM rotates at a constant speed and then just stops when its flipped 180, how is that done?
  
  Or for instance in the videos of  the LM coming to rendezvous with the CM, as it moves around it makes slight movements and stops abruptly. I know the footage is probably being played back at the wrong frames per second so it looks robotic, but they are still stopping quickly without slowing down toward the end of a maneuver. How is it stopping so abruptly? Is it the RCS doing its job through the computer? I know on aesent the computer took over but was allowed manual inputs through the RCS.
  

  
  Normally, the Digital Autopilot (DAP) software running in the Apollo Guidance Computer (AGC) interpreted the pilot's hand controller inputs and the gyro outputs from the Inertial Measurement Unit (IMU) to produce the proper RCS firings. Manual prox ops were normally performed in "attitude hold" mode, where the DAP would command RCS firings to hold attitude based on the IMU attitude and rates. When the pilot deflected the RHC, say to command a "pitch up", the DAP would command RCS firings to achieve a pre-selected rate and hold that rate until the pilot released the RHC, at which point the DAP would command RCS firings to null the rate.
  
  For translation, the CDR in the LM had use of the range and range rate from the Rendezvous Radar (RR) until about 100 ft. The THC was normally set to operate in a "pulse" mode where a single deflection produced a pre-selected delta-V. The CDR would pulse the THC a desired number of times to change the range rate by a desired amount. If the CSM was the active vehicle, the CMP had use of VHF ranging for the same purpose (the AGC estimated range rate in this case), but the minimum range for VHF ranging was larger than for the RR... how much I don't remember. Once the RR or VHF range were unavailable, the pilot would continue the approach until the target reached a certain size in the COAS, then pulse the THC to null the range rate (target stops growing in the COAS). The pilot knew what the range rate was when the RR/VHF lost lock so he generally had a pretty good idea already of how many pulses to input to null the rate.
  
  There were backup systems in both vehicles. The CSM had BMAGs to provide backup gyro capability in the event of IMU failure and the LM had the AGS.
• #4 by joema on 07 Jun, 2020 14:05
• Quote from: ecosta on 03 Apr, 2020 13:20

  ...when the Apollo CSM translates to dock and extract the LM, they rotate, and then all of a sudden stop once they've flipped 180 degrees and are in line with the LM. How does this work? Is the RCS hooked up to the computers / gyros that let it know its position and when to fire thrusters to decelerate and stop at a specific point?...

  
  I don't recollect much exterior footage of the CSM actively maneuvering during S-IVB or LM proximity operations. By "all of a sudden stop" the spacecraft was controlled by the Digital AutoPilot. The CSM pilot watches the 8-ball and maybe digital attitude indicators on the DSKY. The instant he releases the ACA hand controller the DAP will halt all motion.
  
  If you mean the visual appearance of the LM "snapping" to a stop after an attitude change (esp. as seen during the docking phase after lunar ascent on Apollo 11), this is a combination of the DAP (Digital AutoPilot) plus (a) low motion rate on the combined LM or (b) higher motion rate on the low-mass ascent stage in lunar orbit. It may also be visually exaggerated by a low frame rate on the film camera. The DAP was really a digital fly-by-wire system, the term "autopilot" is misleading.
  
  The ascent stage rotation or pitch motion does not really stop instantly, but it can look that way. The LM RCS were located on the ascent stage, yet had enough impulse to control a fully-fueled heavy combined LM vehicle. So even the heavy vehicle - if in a slow rotation rate - will appear to stop pretty fast and if the film camera was at 10 fps or slower, that visual jerk will be magnified. The RCS are fixed-thrust at approx 100-lb force, not throttleable.
  
  The RCS could be "blipped" for a few milliseconds so in theory using pulse-width modulation you could effect a more gradual acceleration in attitude. However the RCS were inefficient in that mode so I think the DAP would just do one big firing to achieve the rate commanded by the hand controller.
  
  For the "J" missions the total LM weight was about 36,700 lbs. By contrast the LM ascent stage at rendezvous was only about 5,700 lbs. In the lunar orbit rendezvous phase, the RCS (sized for good control over the heavy combined LM) will initiate and halt motion on the light LM ascent stage very fast.
  
  I think the normal RCS mode was "rate command/attitude hold". So more displacement on the ACA handgrip produced a faster rate. Then when the stick was released, the DAP system will automatically halt that motion.
  
  Based on the info in AIAA paper 69-892 ("Manual Attitude Control of the Lunar Module" by Robert F. Stengel, 1969), the depleted ascent stage had an inertia of 4400 kg-m^2. The pitch RCS jets produced torque of 746 Nm^2. If my calculation is correct that implies a pitch acceleration of about 10 degrees per sec^2, which is *really* fast: http://www.stengel.mycpanel.princeton.edu/AIAA69LM892.pdf
  
  I think under normal conditions the max angular attitude rate was 1 deg/sec, which could be achieved with a 1/10th sec RCS firing, which might fall between  two film frames at 10 fps.  If that is correct it would corroborate why the LM ascent stage attitude rates visually go from motionless to moving then "jerk" to an instant stop. It's the combination of digital control, low vehicle mass, high torque from the RCS and slow film frame rate.