EMS Interpretation

[Thespacer]

1. Having tried a couple of Apollo 8 re-entries, one automatic and the other manual, I find the AGC’s roll commands easy enough to interpret, and accurate enough to get close to the calculated splashdown with manual control.

The EMS, however, is less easy to interpret. Having saved the bitmap after splashdown and compared it to the actual Apollo 8 EMS scroll, it looks close enough to real life. But I still cannot figure out how the pilot is meant to use it during reentry. Again, I managed just fine with the AGC roll commands, so what else does the pilot need from the EMS?

I have read the relevant AOH manual on the SCS, particularly ch 2.3.7 on the EMS, but still can’t figure it out. Any able to explain further?

2. On a related point, my Apollo 8 reentry, which was within 2 mins of the actual splashdown, landed me in pre-dawn darkness (around 165 d west and 5 d north). Did the real flight land in darkness?

Tony

 

[Urwumpe]

Well, the scroll is supposed to tell you if the guidance commands and the navigation system are really working properly. Or if you are flying manually, the scroll helps you predict the future and change your roll angle before it is too late.

I'll try to explain it like that:

The scroll plots velocity vs deceleration.
Deceleration is caused by velocity and air density.
If deceleration increases, air density increases more rapidly than velocity decreases.
Thus, the deceleration is also a measure of decent rate.

For a successful gliding reentry, you need the right average decent rate. Since you have a fixed AOA, you need to wobble around this target trajectory.

Now, the lines on that scroll have different meanings, but you can say in most cases: If your past trajectory plot starts to follow or cross those lines in the wrong direction, you are in trouble.

The thick lines that curve from top left to bottom right describe what happens at full negative lift. If you drop below one of those, you have two seconds to react before reaching 10G (= you are dead). You can interpret those as maximum g-limits.

The lines that go from bottom left to top right are minimum G limits. Those only show up at higher speeds, when you could still skip out of the atmosphere. If you pass one of those lines going up, your vertical velocity is too high positive (climbing) and you need to turn the capsule around.

The vertical line is the orbital velocity line: Once you crossed if, you are suborbital and will land. Now the problem is just how you will land: Rare, medium or well-done. But you have survived the most difficult part of reentry already.

The dashed lines are range guidelines - they tell you which deceleration you need for landing at a certain range from now. They are not exactly precise, but at least bring you close to the carrier. The number at those lines is the range multiplied by 100, the final line applies to 50 NM distance to the landing site. The first line you will see is the 1200 NM line.

The top limit of the plot represents the maximum range deceleration: If you fly at this line you can maximize your flight distance. If you go above it, your capsule will "stall" and you will lose range.

If you fly manually, you would check your deceleration against the current range guideline and correct it for the following line - you are above the line for your current range to the landing site: Overshoot, reduce lift (=increase deceleration). Below the line: Undershoot, Increase lift (=reduce deceleration).

 

[indy91]

I have read the relevant AOH manual on the SCS, particularly ch 2.3.7 on the EMS, but still can’t figure it out. Any able to explain further?

Yeah, the AOH doesn't do a great job explaining how to use the EMS scroll. In addition to Urwumpe's explanation, here a picture of the EMS with some labels:

Don't go steeper than the on-set guide lines (1), then stay below/with the off-set guide lines (2) to not accidentally skip out. Follow the range potential lines (3, compare to EMS range counter) and you will get there.

2. On a related point, my Apollo 8 reentry, which was within 2 mins of the actual splashdown, landed me in pre-dawn darkness (around 165 d west and 5 d north). Did the real flight land in darkness?

Quoting from the Apollo 8 Mission Report:

Landing occured at 1552 G.m.t. ... the recovery forces held their positions around the spacecraft until first light at 1635 G.m.t., when swimmers from the helicopter were deployed to install the flotation collar. The hatch was opened and at 1714 G.m.t., the crew was hoised into the helicopter and brought about ship 1 hour 28 minutes after landing.

 

[Thespacer]

Thank you both very much. Much clearer!
Tony