Apollo 8 - Navigation musings

[TheGreenGoblin]

I was watching a documentary on Apollo 8, the first mission ever to obit the moon and began to consider the issues of navigation for such a mission and then realized I had many more questions then answers.

a) First musing was did any of the Apollo craft actually measure air speed, i.e. did they use or even have a pitot tube, at any stage of its flight in the earth's atmosphere? Given that initially the spacecraft's trajectory was almost vertical, its ground speed would have been virtually 0 knots (did the atmospheric navigation even use knots as a unit of measure?) and as it climbed and its trajectory changed it would have picked up angular momentum and it would have developed (from my limited earth bound pilot's mind) an appreciable ground speed and a true airspeed whose accuracy would have dwindled to the point of nothingness as the spacecraft left the earth's atmosphere. Clearly a spacecraft is not an aircraft for long and the usual concepts of aircraft speeds such as IAS, TAS, GS and CAS really wouldn't have been that relevant in achieving the 17,000 miles an hour velocity (or was that speed?) required to acquire earth orbit. All these questions before one even comes to considering which inertial and reference frame the spacecraft used for measuring velocity.

b) Once in orbit how did the spacecraft measure its speed or, most likely, velocity, accelerometers, its own radar, star sights etc) or was all of this done from the earth using radar?

c) Once the spacecraft was approaching the moon it would have had to have slowed down to a precise velocity by implication an angle to safely achieve orbit. if the velocity/angle was out, it would have either swung by in an orbit into deep space or curved in to crash on the other side of the moon. Another point to consider was that if the spacecraft was relying on radio communication from the earth for this kind of precise navigation data then once it was on the other side of the moon its reception of radio signals would have been interrupted (blackout). In the case of data from the earth, the time lag would have been appreciable (roughly 5 seconds for a round trip) during which time the spacecraft traveling at many times faster than speeding bullet would have moved a long way in a specific direction!

Any thoughts from ops-normal's resident astronaut corps, while I do a bit of searching for some answers to these questions.

[TheGreenGoblin]

As my first pass in this whimsical research, like the earth bound buffoon that I am, I thought I might start with an Apollo flight plan to see how it might differ from a normal pilot's PLOG and plan...

I couldn't find Apollo 8's plan but Apollo 11 will suffice as it extended Apollo 8's plan significantly.

Apollo 11 Flight Plan

[Alisoncc]

Obviously Pitot and Static pressure differentials require the presence of a medium. No air, no discernable diference. Given the speeds involved would think that doppler shift of light from a known source directly inline with direction of travel would give an accurate indication of velocity of observing body.

[TheGreenGoblin]

Obviously Pitot and Static pressure differentials require the presence of a medium. No air, no discernable diference. Given the speeds involved would think that doppler shift of light from a known source directly inline with direction of travel would give an accurate indication of velocity of observing body.

One of the reasons I wondered about airspeed related to the escape capsule immediately after launch and the use of the parachute in the case of an abort. Use of the system at too high an airspeed would have destroyed the chute. I believe the reentry chute was triggered by a barometric pressure sensor.

Your point about the use of Doppler shift and radar is well made. It would certainly have been the most accurate way to measure velocity.

[TheGreenGoblin]

It seems that the use of radar and ground stations was the primary basis for navigation.

Apollo ground tracking

Besides handling communications between the Control Center and the spacecraft, the network must provide the extremely vital function of space navigation. The decision to use the MSFN as a primary system for navigation, made some time ago, was based on analyses which showed that position and velocity (trajectory) can be determined more accurately in a very short time (in the order of minutes) by using ground-based tracking data-range, range rate, and two angles (azimuth and elevation or equivalent)-rather than onboard tracking information. Since both systems will be needed and will be used to capacity when appropriate to fulfill Apollo navigation and guidance requirements, the role of onboard systems should not be underestimated.

Interesting to note that as Von Braun was the primary architect behind the Saturn V, so it was a German who masterminded the MSFN.

[Boac]

First musings:
There will always be a 'TAS' even in space, since 'TAS' is simply movement through the medium. No 'Air', so 'TAS' becomes movement RELATIVE to some space-time frame - earth or universal. 'Orbital velocity' is not dependent on orbital direction.

Spacecraft use accelerometers to assess attitude and velocity changes while manouevring.

How 'airspeed' is determined on re-entry for the drogue chute I know not, if indeed it is. I wonder if the drogue chutes are simply 'stressed' for opening at the craft's terminal velocity?

[Boac]

Before the maths/physics purists jump on me, I need to correct the above

'Orbital SPEED' is not dependent on orbital direction.

[TheGreenGoblin]

First musings:
There will always be a 'TAS' even in space, since 'TAS' is simply movement through the medium. No 'Air', so 'TAS' becomes movement RELATIVE to some space-time frame - earth or universal. 'Orbital velocity' is not dependent on orbital direction.

Spacecraft use accelerometers to assess attitude and velocity changes while manouevring.

How 'airspeed' is determined on re-entry for the drogue chute I know not, if indeed it is. I wonder if the drogue chutes are simply 'stressed' for opening at the craft's terminal velocity?

I was about to question your comment about orbital velocity having no direction but you have corrected what was clearly a typo...

I agree that the spacecraft's speed in space, measured using the differential in angles to another point in space (as per trigonometry), would be the analogue of speed through a medium such as air. Whether the former would be called TAS in space is a moot point though. The only spacecraft that I can think of that would need to measure TAS, in the sense of that term, would be those that are not ballistic in low earth orbit or in the atmosphere and actually fly primarily using aerodynamic means though the air, although I am aware that even the overtly ballistic capsules used in reentry in the Apollo missions etc. used aerodynamic effects once in the atmosphere to stay aligned.

Craft such as the X15 or the space shuttle definitely used air data to fly as an aircraft would...

Space Shuttle Air Data Systems

The shuttle used inertial guidance systems to give AoA, sideslip and TAS data during the re-entry phase although a Doppler radar system was proposed as a replacement for the inertial guidance system.

https://arc.aiaa.org/doi/abs/10.2514/6.1984-2111

With respect to the parachutes I suspect you are right. The chute used in the Apollo escape pod would have been used at relatively low altitude and speed and the chutes used during reentry would be have been barometrically or radar altimeter triggered so the speed would not have been too much for the drogue and the complex parachute system to manage.

Apollo 15 encountered a parachute anomaly it seems.

Apollo 15 parachute anomaly.

I am utterly devastated to find that I cannot find a METAR for the day that incident occurred.

[CremeEgg]

Where are you Fox? - just the sort of questions I'd expect a beautifully crafted reply to from Fox. Now to have a read up.

[PHXPhlyer]

Currently reading "Carrying The Fire" by Michael Collins; the Apollo 11 command module pilot. Haven't got to the nuts and bolts of the navigation part yet.
So far, a good read.

PP

[k3k3]

There is a programme about Apollo 8 on the Smithsonian channel now, navigation was by sextant!

[TheGreenGoblin]

There is a programme about Apollo 8 on the Smithsonian channel now, navigation was by sextant!

The sextant or Apollo Guidance and Navigation System was used to augment the inertial navigation system and by 1968 the ground network was considered to be the primary navigation system.

The Apollo Guidance and Navigation System - The sextant fulfilled the need for a device to aid the alignment and bound the drift of the inertial system. The instrument consisted of two telescopes. The first was a one-power, wide-field scanning telescope, which was used to locate a star or constellation in space. The second was a 28-power sextant, which took the actual reading. Although it did not look like a traditional sextant, it operated in a similar manner. The astronaut sighted on two heavenly bodies: two stars, or a star and the horizon of the Earth or Moon, adjusted the optics until they were aligned over one another, and then pressed a button marking the instrument’s reading and the time. One of the axes of the telescopes was fixed, so that the process of finding the Earth or Moon typically consisted of orienting the entire spacecraft around until that body came into the field of view. Once a reading was taken, the on-board Apollo Guidance Computer (AGC) computed the spacecraft’s position, based on those readings and data stored in its memory.

The Apollo 8 mission, in December, 1968, provided the first real test of this device, although it had been tested in Earth orbit for Apollo 7. Apollo 8 took three astronauts from the Earth to an orbit around the Moon and back again, safely. In the chronicles of Apollo, that mission marked a number of famous milestones: the first human crew lofted by the Saturn V booster, the Christmas Eve reading from the book of Genesis, and the famous “Earthrise” photograph showing a blue Earth rising above a barren and forbidding lunar horizon. It was also the first mission in which a human crew actively navigated across the depths of space from one heavenly body to another.

By 1968, however, the on-board system had evolved from the primary to the back-up to a ground-based navigation system. In the initial planning stages for Apollo, this extensive use of ground facilities was not foreseen. One concern was that the Soviet Union, whom the Americans were racing to the Moon, might jam the signals from Earth. This was a real possibility, but by 1968 NASA felt that it was less likely. Another advance was the completion by 1961 of the Deep Space Network: a set of three 85-foot parabolic dish antennas placed approximately 120 degrees apart on the Earth. These were able to track the Apollo spacecraft using several techniques, including precise timing of S-Band radio signals sent to the spacecraft and returned back to Earth.

https://www.ion.org/museum/item_view.cf ... =5&iid=293

See Apollo Ground Tracking link in one of the posts above and for a full and detailed, very interesting take on the the Apollo 8 navigation systems see below.

https://repository.si.edu/bitstream/han ... 1085SH.pdf

[Boac]

I was about to question your comment about orbital velocity having no direction but you have corrected what was clearly a typo...

- phew! I think I got away with it. I was using PN's productive sex.
PS Don't fret about the METAR. I'm sure no-one will mind.

[TheGreenGoblin]

"All balls" was the Apollo astronauts way of saying that there was zero difference between the star sights they made, and sightings made using the between a star and the earth's horizon and a star, that is 00 on the on-board computer, and that these sightings matched what the ground stations were saying as well.

Such accuracy is really impressive and was achieved many times during Apollo 8's flight by Jim Lovell. The fact that the ground system was matching the on-board systems gave mission control the confidence to make the ground system the primary navigation system but also ensured confidence in the manual systems as well and this paid dividends when the main computer had to be powered down during the crisis encountered on Apollo 13. Jim Lovell was the right man in the right place againd on the latter ill fated flight.

See page 16 of this fascinating document...

https://repository.si.edu/bitstream/han ... 1085SH.pdf

[TheGreenGoblin]

"All balls" was the Apollo astronauts way of saying that there was zero difference between the star sights they made, and sightings made using the angle between another star and the earths circumference, that is 00 on the on-board computer, and that these sightings matched what the ground stations were saying as well!

There, hopefully that fixed that balls up!

[ExSp33db1rd]

.....navigation was by sextant!

Hence the question posed to me by a young Aero Club student pilot. i.e. " What was a sextant ?"

I subsequently gave a presentation entitled " Navigation before GPS " - and told them.

One could weep.

Moments of Clarity.
"As I sat, strapped in my seat waiting during the countdown, one thought kept crossing my mind .....
every part of this rocket was supplied by the lowest bidder. "
-John Glenn

[sidevalve]

I think you'd enjoy Mike Mullane's memoire "Riding Rockets" - sub-titled: The Outrageous Tales of a Space Shuttle Astronaut.

On February 1, 1978, the first group of space shuttle astronauts, twenty-nine men and six women, were introduced to the world. Among them would be history makers, including the first American woman and the first African American in space. This assembly of astronauts would carry NASA through the most tumultuous years of the space shuttle program. Four would die on Challenger.

USAF Colonel Mike Mullane was a member of this astronaut class, and Riding Rockets is his story -- told with a candor never before seen in an astronaut's memoir. Mullane strips the heroic veneer from the astronaut corps and paints them as they are -- human. His tales of arrested development among military flyboys working with feminist pioneers and post-doc scientists are sometimes bawdy, often hilarious, and always entertaining.

Mullane vividly portrays every aspect of the astronaut experience -- from telling a female technician which urine-collection condom size is a fit; to walking along a Florida beach in a last, tearful goodbye with a spouse; to a wild, intoxicating, terrifying ride into space; to hearing "Taps" played over a friend's grave. Mullane is brutally honest in his criticism of a NASA leadership whose bungling would precipitate the Challenger disaster.

Riding Rockets is a story of life in all its fateful uncertainty, of the impact of a family tragedy on a nine-year-old boy, of the revelatory effect of a machine called Sputnik, and of the life-steering powers of lust, love, and marriage. It is a story of the human experience that will resonate long after the call of "Wheel stop."

He's not the NASA vanilla personality type that you might expect - he also gives out his email address in the book and he's willing to enter into correspondence.

A great read!

[TheGreenGoblin]

Many thanks for that steer Sidevalve.

Kindle edition purchased.

Any book that starts at the arse end of the subject, as this one does, is already on the right track!

Expurgating the AME's report. I like this guy already and I am only on page 2!

[TheGreenGoblin]

A brief departure from track...



And swinging right back onto track, the urine dump film in that video verifies the issue of taking star sights when each droplet of frozen urine looks a star as the whole lot follows the space craft in its orbit, as noted by the Apollo 8 astronauts.

[sidevalve]

I identified with MM - he tells it like it is. Not PC at all..
I'm surprised he got as far as he did!