Reentry: "Apollo Atmospheric Entry Phase" 1968 NASA Misson Planning and Development Project Apollo [VIDEO]

[SSgt Baloo]

Possibly educational: 

 

[DerekL1963]

For all the talk about the 'skip' trajectory, it was never actually used.

[Bill Phil]

I think I saw this a while ago.

23 minutes ago, DerekL1963 said:

For all the talk about the 'skip' trajectory, it was never actually used.

Out of curiosity, do you have a source?

[Motokid600]

1 hour ago, Bill Phil said:

I think I saw this a while ago.

Out of curiosity, do you have a source?

I was under the impression that they skipped as well, but..

https://en.wikipedia.org/wiki/Skip_reentry

"Guidance of a skip trajectory can be tricky due to trajectory sensitivity. The Apollo Skip Guidance^[3] was engineered, but never utilized in a manned mission. More recent work relies on advances in computing technology to compute a trajectory on board the vehicle.^["

^{Not too sure. Id like to hear it from somewhere other the Wikipedia.}

[DerekL1963]

9 hours ago, Bill Phil said:

I think I saw this a while ago.

Out of curiosity, do you have a source?

Thirty years of studying space exploration, etc?  That's about the best I can do.

[KerbonautInTraining]

Apollo flight journal

Skip to 191:48:xx and read the explanation below it. A PAD is a massive set of condensed information relayed orally to the crew, which is then written down in a labeled grid sheet for future reference. That's 1960's tech for ya.

They most definitely did skip on the lunar missions. (Read discussion below)

Sorry if this looks like cherry picking, but I can guarantee the Apollo lunar surface/flight journals are the best damn sources for info on the Apollo missions. 

Edited February 28, 2016 by KerbonautInTraining

[DerekL1963]

Yes, it looks like a skip if you "cherry pick" - but if you read the whole exchange, Evans reads up and Armstrong repeats back "Additional comments: Use non-exit EMS pattern".  The EMS is the Entry Monitor System, which the CMP used to monitor the progress of the re-entry to ensure the vehicle remains on trajectory and within G-limits.   "No-exit pattern" specifically means no skip. [1]

Further if you scroll down to 293:20:53 in your link, when Collins discusses the entry procedure with Evans, there's no mention of a skip.  They're holding the lift vector up to stretch the range a bit (215 miles according to the PAO at 192:00:07), not skipping.

The AFJ/ALSJ is indeed a good source, but it's not entirely without errors and it does often require a bit of background to understand.

Doing some research on skips also leads me to the master's thesis of an individual who is studying skip re-entry, on page 13 he states that "However, the skipping capabilities of the guidance were never utilized during that particular mission, nor any other Apollo Mission."

[1]  Mission Planning for Apollo Entry, page 12

 

Edited February 23, 2016 by DerekL1963

[KerbonautInTraining]

6 hours ago, DerekL1963 said:

Yes, it looks like a skip if you "cherry pick" - but if you read the whole exchange, Evans reads up and Armstrong repeats back "Additional comments: Use non-exit EMS pattern".  The EMS is the Entry Monitor System, which the CMP used to monitor the progress of the re-entry to ensure the vehicle remains on trajectory and within G-limits.   "No-exit pattern" specifically means no skip. [1]

Further if you scroll down to 293:20:53 in your link, when Collins discusses the entry procedure with Evans, there's no mention of a skip.  They're holding the lift vector up to stretch the range a bit (215 miles according to the PAO at 192:00:07), not skipping.

The AFJ/ALSJ is indeed a good source, but it's not entirely without errors and it does often require a bit of background to understand.

Doing some research on skips also leads me to the master's thesis of an individual who is studying skip re-entry, on page 13 he states that "However, the skipping capabilities of the guidance were never utilized during that particular mission, nor any other Apollo Mission."

[1]  Mission Planning for Apollo Entry, page 12

 

In the PAD it mentions fluctuating G-forces (going as low as 0.84 before picking up again) so while they aren't out of the atmosphere they would have climbed considerably. That's assuming my 1-in-the-morning brain is interpreting the PAD correctly. Was the PAD for a hypothetical off-nominal re-entry?

[DerekL1963]

1 hour ago, KerbonautInTraining said:

In the PAD it mentions fluctuating G-forces (going as low as 0.84 before picking up again) so while they aren't out of the atmosphere they would have climbed considerably. That's assuming my 1-in-the-morning brain is interpreting the PAD correctly. Was the PAD for a hypothetical off-nominal re-entry?

Yes, they could climb without skipping.

The PAD is for a normal entry.  How I read the PAD is they've used the data slots for what appears at first glance to be a skip trajectory because lofting is the first step of a skip - but you can stop short (that is not apply enough lift to actually exit the atmosphere) and still move the landing point downrange (just not as far as a full skip).  Which is what they did - stopped short of skipping.  The comments appended by the journal editors to the interpretation of the PAD describe just that - using P65 (entry lift control) for the loft phase, then skipping P66 (ballistic guidance during the exoatmospheric portion of the skip) since they didn't actually skip, and going straight into P67 (final entry).

[magnemoe]

You can climb without skipping, this is common in KSP as the planet is so small, Pe at 35 km, the aerobrake reduse your speed to below 2.4 km/s so your get an ballistic trajectory with Ap at say 60 km before reenter the thick atmosphere. 
 

[benzman]

According to 'How Apollo flew to the Moon' by W. David Woods, the skip was only planned to be used to extend the re-entry flightpath and in the event, only Apollo 11 used it because of bad weather in their planned recovery area.  Apollo 11 'skipped' an extra 600 kilometres to a safer recovery area.    CSM pilot Michael Collins was not pleased by this as almost all their simulator work had been with the standard re-entry parameters.

[KerbonautInTraining]

On 2/23/2016 at 3:31 AM, DerekL1963 said:

Yes, they could climb without skipping.

The PAD is for a normal entry.  How I read the PAD is they've used the data slots for what appears at first glance to be a skip trajectory because lofting is the first step of a skip - but you can stop short (that is not apply enough lift to actually exit the atmosphere) and still move the landing point downrange (just not as far as a full skip).  Which is what they did - stopped short of skipping.  The comments appended by the journal editors to the interpretation of the PAD describe just that - using P65 (entry lift control) for the loft phase, then skipping P66 (ballistic guidance during the exoatmospheric portion of the skip) since they didn't actually skip, and going straight into P67 (final entry).

Whoa, yeah looking back at it they definitely didn't skip. I looked at some of the other entry transcripts and Ctrl + F "skip" returns no results.

How about that! Out of all the stuff I've seen on Apollo, I'm pretty sure all of it mentions a skip when talking about re-entry. Crazy how stuff like this goes unnoticed sometimes.

Edited February 28, 2016 by KerbonautInTraining

[John FX]

I`m loving the dragons and sea monsters on the globe they use to show orbital trajectories...