An astronaut's view on reentry
-Temporarily not more than a scratchpad to sort my memory; structure and "eyecandy" will follow later ;-)-
-Parts of entry: Pre-entry (P61,62,63), Initial steering (P64), Constant drag & upcontrol (P65) + Ballistic Phase (P66) (will only be entered when specific conditions are met; otherwise: P64 -> P67!); Final steering (P67 w. N66); End of steeering (P67 w. N67)
How everything starts:
V37E61E
P61
FL V06N61
+/- XXX.XX IMPACT LAT - Latitude of desired splashdown point. + is north, - is south
+/- XXX.XX IMPACT LONG - Longitude of desired splashdown point. + is east, - is west
+/- 00001 HEADS UP/DOWN - Switch for roll attitude when approaching 0.05g. + is heads up (lift DOWN), - is heads down (lift UP). Lunar entry was flown with lift up (-00001), earth entry with lift down (??)
V25E to change data. Note that P37 stores its predicted splashdown point here (handy, isn't it?;-)).
PRO to proceed
FL V06N60
XXX.XX G MAX - predicted peak G during entry
XXXXX VPRED - predicted velocity at 400.000 ft altitude in fps
XXX.XX GAMMA EI - flight path angle (= entry angle) at 400.000 ft altitude in degrees
PRO to proceed
FL V06N63
XXXX.X RTOGO - range to go from 0.05g in nautical miles
XXXXX VIO - velocity at 0.05 g
XXBXX TFE - time from now to 0.05 g. NOT UPDATED CONTINOUSLY!
RTOGO and VIO can be used to initialize the EMS.
PRO to P62
P62
FL V50N25
00041 - checklist code for: "please perform CM/SM separation"
BLANK
BLANK
Perform CM/SM separation; key PRO when done
-CSM DAP off; entry DAP on in attitude hold; further V37 inhibited from now on-
FL V06N61
+/- XXX.XX IMPACT LAT - Latitude of desired splashdown point. + is north, - is south
+/- XXX.XX IMPACT LONG - Longitude of desired splashdown point. + is east, - is west
+/- 00001 HEADS UP/DOWN - Switch for roll attitude when approaching 0.05g. + is heads up (lift DOWN), - is heads down (lift UP). Lunar entry was flown with lift up (-00001), earth entry with lift down (??)
V25E to change data. Last chance to change the splashdown point.
PRO to proceed
-entry DAP maneuvers to entry attitude; SC CONT to CMC for auto-maneuver or to SCS for manual control (CMC drives error needles on FDAI for desired attitude); manual is recomended (entry DAP has wide deadband and damps rates very slowly), when needles are centered SC CONT to CMC-
V06N22
XXX.XX OG ROLL
XXX.XX OG PITCH
XXX.XX OG YAW
Desired attitude in yaw, pitch, roll. Note that display is skipped and P63 is selected when alfa (angle of attack) is already > 45°.
P63 automatically selected 21 s after alfa > 45°.
P63
FL V06N64
XXX.XX G - actual acceleration in G.
XXXXX VI - actual inertial velocity in fps
XXXX.X RTOGO - range to go to splashdown point. Negative and decreasing when approaching
Display is updated continously. When 0.05 g (R1 reads 00005 or more) is attained, P64 is selected automatically.
P64
V06N74
XXX.XX BETA -commanded bank angle in degrees
XXXXX VI - actual inertial velocity in fps
XXX.XX G - actual acceleration in G.
Now (finally) the entry starts -and the CMC will do 'nothing' (obvious) at first. Till 0.2 g is attained, P64 keeps the attitude in pitch and yaw in the limits which are necessary for a controlled dive into the atmosphere. Roll attitude will be the one which was selected earlier. Attitude control is especially vital during earth entries, due to the shallow entry angle: the CM could just flip over earth's atmosphere like a flat stone over the pond. It will enter somewhere again, of course, as the velocity is to low for an orbit, but there might be no recovery forces where it will splash down then -not very desirable (you could miss your first warm meal since over a week, just for example ;-)).
During a lunar entry, this problem will not occur; the entry angle is (should be!) so steep, that the CM will enter the atmosphere, no matter what. But even here, attitude control has its purpose: with a constant 'lift up' roll attitude the CM will climb by itself when the air gets thicker (and the G's rise), so that you can't burn up.
At 0.2g, P64 has to make its first decision: if VI (the inertial velocity of the CM) was smaller then 27000 ft/s at 0.05 g, which is usually the case during earth entries, it calls P67 and ends.
If not, it's a lunar entry and the CM will dive (due to its trajectory) into the atmosphere and the G's ramp up -but not forever, at one point the aerodynamic forces will come into play and the CM will start to climb again (remember: it is still rolled heads down aka lift up!). If we don't do anything, it will fly out of the atmosphere again and we will face the same problem as above (no warm meal for today...). What now? Roll heads up (lift down), of course. But stop! We want to hit a specific splashdown point, so timing is everything. And maybe rolling up might even be wrong: maybe we have to fly a short time out of the atmosphere to come closer to our target. Complex thing, isn't it? Be assured: no astronaut had to take out it's slide ruler (calculators didn't exist those days) -they had the CMC for it (resp. the EMS -but that's another article). Just after 0.2 g it had started to calculate: where am I now? Where will I splashdown? Where should I splashdown? It does this for two alternative ways:
- leaving the atmosphere after the first entry and re-enter; this is called 'upcontrol solution', as the CMC will maintain the lift up roll until the CM has left the atmosphere again
- stay inside the atmosphere and fly the rest of the entry like an earth entry. This is called 'constant G solution' -we will see later why it's called this way
When HDOT (vertical velocity) of the CM becomes more positive then -700 ft/s, i.e. it is about to climb again, the CMC looks at both solutions and decides which one is better. If the upcontrol solution is better, it switches to P65. If the constant G solution is better, it switches to P67. On a nominal entry P65 wasn't entered, as the range-to-go from 0.05 g to splashdown point was so short that the upcontrol solution would overshoot the target and constant-G was the only way to fly. P37 also aims for an entry without P65. It's a case-of-emergency program and intends to bring you back to earth the shortest way.
P65
-Note that this section and P66 are written from the docs. I haven't seen P65 and P66 working yet.-
FL V16N69
XXX.XX BETA -commanded bank angle in degrees
XXX.XX DL - drag level at skipout
XXXXX VL - inertial velocity at skipout in fps
Now the CMC has decided to skip out of the atmosphere and reenter. Of course, just flying up, up and away will not do it. The skip out has to happen at a specific velocity, so that it might be necessary to roll up (lift down) for a moment to dissipate velocity. The CMC informs the astronauts when it will start to leave the atmosphere, i.e. it will 'pull up' when the velocity displayed is reached. Key PRO to signal the CMC that this data has been acknowledged. It switches back to the well known
V06N74
XXX.XX BETA -commanded bank angle in degrees
XXXXX VI - actual inertial velocity in fps
XXX.XX G - actual acceleration in G
The CMC 'burns' the unneeded velocity away now by rolling up and down if necessary. At VL, it pulls up and switches to P66 when the drag drops below 0.2 g, i.e. the CM has left the atmosphere. It is possible that P65 switches to P67 and skips P66. This will happen when velocity drops below ca. 27000 fps during P65 (the precise velocity is calculated by P65). This prevents an unnecessary skipout -P67 can deal with this velocity alone.
P66
V06N22
XXX.XX OG ROLL
XXX.XX OG PITCH
XXX.XX OG YAW
Now that we've left the atmosphere, we've got to get ready to enter it again. Attitude is everything, and the CMC calculates (and steers to) the proper one for this second entry. When 0.2 g is reached again, P67 is selected.
P67
V06N66
XXX.XX BETA -commanded bank angle in degrees
XXXX.X X RNG ERR - predicted crossrange error in nautical miles
XXXX.X DWN RNG ERR - predicted downrange error in nautical miles
We've now reached the final phase of the entry. The CMC now kills cross- (flying left or right form the target) and downrange (flying to short or to long) error by rolling up or down (downrange) and left or right (crossrange). Note that sometimes the CMC rolls 'over the top' or 'over the bottom' instead of 'taking the direct way' to desired attitude. That's okay, the CMC keeps the crossrange error small by doing so. When velocity drops below 1000 fps, aerodynamic forces are to small to control the CM anymore. P67 now switches to
FL V16N67
XXXX.X RTOGO - range to go to splashdown in nautical miles
+/- XXX.XX LAT - present latitude + is north, - is south
+/- XXX.XX LONG - present longitude + is east, - is west
It's over now. Flip up your ELS LOGIC and ELS AUTO switches and wait for the chutes.:-)