Apollo Investigation

Re-Entry Matters

by Mary Bennett

Detailed Investigation into Apollo Command Module Returns


When David Orbell wrote of his eviction from Autographica 2014, (Orbell, 2014) as a result of asking Apollo astronauts Stafford & Worden which method of entry into the Earth‘s atmosphere they had used during their Apollo missions, I realised that the process of returning a craft to Earth from space might be misunderstood by those not directly involved with the space program or otherwise familiar with the subject.

This is mainly due to the vocabulary used by NASA and its associates, which doesn’t sufficiently clarify the issues involved and, in this instance, it would seem that even individuals on the front line of Apollo – astronauts and flight director – are in a muddle. Hard to believe, and the reason why David Orbell had sought a final clarification from the astronauts relative to Apollo flight director Chris Kraft’s statement on re-entry:

"Because the velocity is so high, if you tried to come in directly, the heat-shield requirements would be too great. So what we did was get them into the atmosphere, skip it out to kill off some of the velocity, and then bring it back in again. That made the total heat pulse on the heat shield of the spacecraft considerably lower." (P. Mech, 2009)

Chistopher Kraft
Christopher C Kraft Jr

Al Worden
Alfred M Worden

Neither astronaut supported Chris Kraft’s statement but Al Worden’s response was vitriolic: "Chris Kraft is a bad guy. If we could feed him to a bomb, we would."

Reading about this spat, I thought that from any viewpoint, this response seemed a little harsh! However, from the point of view of Apollo re-entry procedures it raises some serious questions. Not least because on looking a little closer, I found that this was not a simple misunderstanding between people with fading memories of a single event, it turned out to be far worse than that.

Illusory Apollo again

Remarkably, rather than referring to a designated method for the Apollo return procedure, there are different accounts from those closely associated with this project regarding how the Apollo command module (CM) achieved a safe return to Earth.

Apollo flight director Chris Kraft has stated that Apollo used a skip re-entry; the astronauts and Johnson Space Center in 2011 describe it as a direct entry; and some experts qualify the Apollo direct entry as a 'double dip' (Kaya, 2008). And coming full circle, recent research from those studying the CEV Orion – the scaled-up version of the Apollo CM – has led to the same conclusion as Chris Kraft – a skip out of the atmosphere is the only way to neutralise the velocity of a lunar return safely and effectively with regard to the g-loading on a human crew, the heatshield capabilities, and the precision landings sought after by NASA.

For those who wish to go further into the material, the comparison between skip entry and direct entry is comprehensively dealt with in the referenced technical documents. Here the issue is why there are different accounts of the entry procedure emanating from within the Apollo record and those closest to the events, the astronauts and flight directors. After all, orbital parameters are non-negotiable, and even if rocket technology has its limitations, everyone involved with the same event should be relating the same narrative and importantly the same technical data. As this is not the case, and bearing in mind the so-called ‘cold war space race’, it occurred to me that poorly constructed sentences and misapplied jargon could usefully disguise the reality of an event.

Therefore, in order to ascertain what exactly might be Al Worden’s problem with Apollo flight director Chis Kraft, given that he is not about to enlighten us, perhaps we need to understand the entry procedures for the Apollo CM. Here then, is the nitty-gritty of the Apollo returns as described by NASA. If the discrepancies and anomalies within the data are confusing and unhelpful, it is still important to bear in mind that, whatever one’s personal opinion concerning the authenticity of the Apollo record, the overall NASA narrative concerning the lunar missions is what the agency expects us all to believe to be true.

Down to basics

Returning from the Moon, depending on the initial selected trajectory, the velocity reached just prior to entry into the atmosphere will be near to 36,725ft/sec. In order to neutralise this super orbital lunar return velocity and make a safe landing, the descent of the CM had to be carefully managed, especially from the atmosphere’s Entry Interface (EI) through to splashdown. For reasons both practical (and no doubt symbolic, given the relationships of Earth, Sun, and Moon) the EI is defined by NASA as being 400,000ft from Earth. And the agency certainly likes its symbolic numbers. Writing in Coming Home: Reentry and Recovery From Space, published in 2011, NASA chief historian Roger Launius’ description of the Apollo heatshield includes this:

“For Apollo, a brazed PH 14-8 stainless steel honeycomb sheet was attached to the structural shell, and a fiberglass-phenolic honeycomb with 400,000 individual cells was bonded to it…” (Launius & Jenkins, 2011)

Managing entry into the atmosphere is of major importance because a manned spacecraft cannot be permitted to burn up in the atmosphere as an umanned satellite might do. So, unsurprisingly, the figures used by engineers to develop craft that can fulfill the required mission criteria and flight trajectory must be very precise. But rather more surprising is the fact that NASA isn’t particularly fussy when it comes to presenting its data to the public. Take this extract from a table concerning the Earth entry data for Apollo:

Fig 1
Fig 1. Earth entry data extracted from NASA Entry, Splashdown, and Recovery Chart.

Here, there are differences of data: the flights up to Apollo 11 display two sets of figures for the EI, those of Apollo 12 onwards apparently managing with only one figure. Of those five flights with two velocities listed, two are on record as having entered from LEO. If these two lines of entry velocity data are based on theoretical calculations and actual values – the Apollo 13 saga eliminates the notion that all these missions performed as perfectly as any theoretical projection. Furthermore it is the heading angle which is relative to degrees east of north and not the flight-path angle. Since aerospace professionals have not thought to alert NASA to the ‘mistakes’ in this table, the Apollo project must be considered an anomaly within the industry.

However, NASA has form for losing Apollo 11 data as well as for storing Mars Cydonian data in the wrong place. As this table continues that tradition, one might also conclude that the intention is to confuse any layman examining Apollo. Then again, the lack of L/D ratios for Apollo CMs 7 & 9 is so ridiculous as to qualify as a whistle-blow, alerting us to the anomalies and inconsistencies within this document and across the entire Apollo record.

Entry ways

A lunar mission can return through the atmosphere and descend to Earth directly. It can also enter the atmosphere and then skip out of it again in order to kill its speed. The Russian Zonds were programmed for skip entry, Zond 6 virtually suceeded in 1968 – prior to Apollo 8. Zond 7 was a total success in August 1969, but no manned missions have made a skip entry to this day, nor was there an interim stop over in LEO. Thereby confirming the assertions made to David Orbell by Apollo astronauts Worden and Stafford that they had come in on a direct entry flight.

The six and a half degree ‘straight-in’ approach referred to by Al Worden in his exchange with David Orbell was the flight-path angle of -6.51deg ascribed to Apollo 15. The graphic below illustrates a target descent of 6.4 degrees illustrated from the 0.05g marker, some 150,000ft into the atmosphere from the EI. Note that the entry velocity was expected to be 36,500ft/sec.

Fig 2
Fig 2. ‘The straight-in corridor’. This NASA 1960s Apollo graphic illustrates the corridor from the 0.05g marker, which occurs around 150,000ft into the atmosphere from the 400,000ft EI. For a direct entry flight a nominal flight-path angle of -6.65deg ensures capture of the vehicle, and protects against skip out of the entry vehicle. (Arch. Study, 2005 Section

Authors Launius & Jenkins state that:

"Practical entry angles have an upper and lower limit. The lower limit, also called the overshoot boundary, is the angle at which the vehicle will skip back out of the atmosphere. The upper limit, or undershoot boundary, is the load-factor limit established by vehicle-structural, human-tolerance, or operational considerations." (Launius & Jenkins, 2011)

Back in November 1963, L.F. Crabtree and D.H. Peckham of the the Royal Aircraft Establishment at Farnborough had explained re-entry corridor matters rather more explicitly:

Up to now recovery from orbit has been along a ballistic trajectory. For future systems, however, the characteristics of ballistic re-entry appear altogether too restrictive, and some degree of manoeuvrability will be needed during re-entry to give control over range, deceleration and heating rates. This means the ability to generate lift. In this way a relatively deep corridor (compared with the ballistic case) is obtained within which a safe re-entry can be accomplished. The boundaries of this re-entry corridor have been termed "undershoot" and "overshoot," and correspond to steeper and shallower initial flight path angles respectively.

The overshoot boundary corresponds to the smallest angle at which re-entry can be made without the vehicle passing through the atmosphere and returning to space. For a ballistic vehicle this limiting angle must be steep enough to decelerate the vehicle to orbital speed before the opposite boundary of the atmosphere is reached, but a lifting vehicle can use negative lift to hold itself in the atmosphere at greater than orbital speeds while decelerating; it can therefore re-enter at a shallower angle. The undershoot boundary corresponds to the steepest re-entry angle for which the deceleration or heating are kept within prescribed limits (Crabtree & Peckham, 1963).

Fig 2a

Fig 2a. Typical super-orbital re-entry trajectory (altitude and range plotted to same scale) Image T.G. Sanial.

The fact that both the 1963 and 2011 commentaries refer to the drama of a vehicle unintentionally returning to space is more for public consumption and dramatic effect than it is a reality. The braking that occurs during the first entry into the atmosphere would preclude this eventuality, and in any event the craft would not be lost forever in the wilds of space as inferred. In the 1960s the Russians provided their cosmonauts with a week's life support in case their entry navigation failed – it never did. However, this concern is also a reflection of the state of the art of navigation processes both then and now, and it's interesting to note that skip entries have only recently become viable thanks to GPS as well as advanced computing.

Mercury astronauts returning from LEO with zero lift experienced the ‘ballistic case’ referred to above. The re-entry was short but very uncomfortable with a maximum of 11g – or just under 10g, depending on source (Iliff & Peebles, 2004). Interestingly today, no matter how briefly a time, NASA places the undershoot tolerance at 10g. Project Mercury ended in May 1963, by which time author David Portree writes:

"NASA engineers had understood the complexities of an Earth orbiting spacecraft entering from LEO at some 25,000ft/sec… However they were not fully confident that they could extrapolate the effects of re-entry at lunar return speed from the LEO re-entry data…until 1965 by which time they felt confident that they understood the atmosphere re-entry effects the Apollo CM would experience when it returned from the moon.." (Portree, 2012)

The Apollo CM, the craft they were trying to return safely through the atmosphere with its human cargo, was described by NASA as:

"A blunted cone with a spherical-segment base…the blunt design produces the drag necessary to efficiently dissipate the kinetic energies associated with the velocities of the lunar return mission. In addition the configuration has an offset centre of gravity (c.g.) to obtain the lift necessary to ensure a sufficiently wide entry corridor and to exercise control of the landing point. The centre of gravity for each flight is determined by preflight weight and balance procedures." (Apollo 4, 1969)

In testing the CMs we find more discrepancies of data and method. According to the record, AS 202, the CM re-entry tested on August 25, 1966, was described as a skip by author David Baker (who worked at NASA on the Apollo program). He writes that after separating from the service module, the CM flew ‘a typical lunar return trajectory: dipping on a shallow approach path to a height of 218,000ft, the CM skipped back to a height of 264,000ft to begin the final phase…’ However, the data supplied to Baker doesn't correspond with the data in the October 1967 NASA Technical Note relating to this test. Here, the maximum altitude of the so-called skip is recorded as under 260,000ft. And an entry velocity of 28,000ft/sec is well below that of a lunar return (Baker, 1996).


Fig 2b. Even at this early stage there are discrepancies in the technical data. NASA flight report of AS 202 showing the maximum altitude of some 257,000ft.

Then came the test flights of the unmanned Apollo 4 in November 1967 and Apollo 6 in April 1968. These two CMs were to test the worst case scenarios with lunar return conditions.

Launius and Jenkins state that:

“Apollo 4 tested the shallow re-entry, long and drawn out with a maximum total heat load, while Apollo 6 tested the steep re entry, short, sharp and with maximum total heat load.” (Launius & Jenkins, 2011)

Apollo 6 failed to achieve sufficient velocity to properly simulate a lunar return, leaving the earlier Apollo 4 as the only unmanned CM on record successfully simulating lunar return velocities. Even if the data in the Apollo 4 flight report might be considered suspect by some scientists, especially compared with modern papers on the problem of re-entry, this flight report is relevant for precisely that reason. It is what NASA officially adopted for Apollo returns – on paper (Apollo 4, 1969).

Lucky dip

Back in 1968 NASA was still describing the Apollo entry as a skip within the atmosphere. Writing in 2011 Launius and Jenkins called the Apollo entry ‘a semilifting entry’, which fits with the AS 202 test, (Launius & Jenkins, 2011) while Kaya, writing in 2008, called the Apollo entry a 'double dip' – which fits with the Apollo 4 test (Kaya, 2008).

In 2008, as a part of his Master’s degree, First Lieutenant Emre Kaya of the Turkish Air Force, then a graduate student at the US Air Force Institute of Technology (AFIT) Wright Patterson AFB, Ohio, wrote a thesis on the skip entry trajectory for the CEV (Orion). Comparing this future method of lunar returns for manned missions to the Apollo program he writes:

"In Apollo type missions, the return phase had to be initiated in a restricted time window so that the crew module could enter the atmosphere at the preplanned time and be able to land at the planned landing site. Using skip entry procedures, landing location and time will be more accurate in addition to having the time flexibility for reentry."

And that:

"Although total skip entry guidance has been done before for the trajectories of unmanned vehicles, application of this concept is quite new for the manned space missions. …Apollo used a 'double dip' entry."

Kaya emphasises:

"The views expressed in this thesis are those of the author and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government." (Kaya, 2008)

One wonders why the AFIT authorities thought it necessary to release a part of his degree work into the public domain. Surely there were enough NASA studies on the matter already. What this study does do however, is refute Chris Kraft’s assertion that the skip entry was the technique used for Apollo. And it is indeed odd that an Apollo flight director should appear to be so blatantly wrong. Taking Kaya’s words and Kraft’s statement together, could lead one to the inevitable conclusion that the Apollo missions were unmanned. Or that unmanned CMs attempted skip re-entries, but were not recorded as such. Something is surely not right with such disregard for definition and such a lack of clarity regarding this Apollo re-entry data.

No wonder the record reeks of confusion and, wittingly or otherwise, obfuscation.

Sleight of hand

Even Apollo 4 test flight details vary according to source and within NASA documentation. The NASA Apollo Program Flight Summary Report sets out the criteria for each flight and notes which were accomplished. Yet, while specifically stating that the Apollo 4 was the first CM to test entry at lunar return conditions, it says nothing about whether this was actually achieved, merely that the capsule was recovered safely. Nor does this supposedly comprehensive summary of all Apollo flights mention the re-entry procedures for any other Apollo mission (Apollo, 1972).

Consulting the post flight Apollo 4 Technical Note we learn that having been launched on a Saturn V (its first time out) Apollo 4 used the service propulsion system (SPS) to boost the CM to a highly elliptical orbit with an apogee of 11,242 miles, and then fired it again to boost the CM to a lunar return velocity of 36,545ft/sec. This second firing did not occur at the apogee but much, much closer to home (Apollo 4, 1969).

David Baker gives the orbit apogee of 11,242 miles, but then cites an entry velocity of 36,000ft/sec. (Baker, 1996) Fig 3 below shows the positions and timing of the CM at the various stages of this pre-entry flight. The boosting to 36,545ft/sec to EI is of the lunar return speed (No.13) therefore occuring approximately 3963mi (1 radian) from the Earth’s surface).

Fig 3

Fig 3. Positions and timing of the CM at various stages of this pre-entry flight.

From the EI, Apollo 4 carried out an entry procedure which has since been cited as the one used for all Apollo missions. Apollo 4 entry velocity and other parameters gave the unmanned CM a maximum skip altitude (as NASA puts it) of 241,602ft, after which the final entry phase was entered – also called the second entry. Again, the use of language leads to a great deal of confusion between the skip out of the atmosphere technique and this 'double dip' within the atmosphere.

Fig 4

Fig 4. Apollo 4 entry to splashdown at lunar return velocity (NASA), with added annotations juxtaposed to trajectory event locations.

Fig 5
Fig 5. NASA flightplan for Apollo 4 entry at lunar return test.

The Apollo 4 so-called 'double dip' entry has two peak points, each producing maximum g-force on the crew. It involves firstly plunging into the thinnest layer of the atmosphere at the 400,000ft EI using a pre-determined angle of approach and trajectory. The craft then moves downwards into the denser layers of the atmosphere.

Hitting the first loading of g-forces, (for Apollo 4 this was 7.3g) the trajectory was then manipulated through rolling the craft about its axis to raise it into a higher altitude – but all the while staying within the atmosphere – before descending once again through the denser atmospheric layers passing the second g loading, (for Apollo 4 this was 4.25g) and only then finally achieving the opening of the drogues and the descent to the ocean at a safe rate.

A 1968 NASA the mission planning and development project Apollo film Reentry explains the whole process (Reentry, 1968) and at: 9mins 37secs this film states:

If atmospheric drag alone is used as a controlling factor it is impossible to guarantee a safe entry. Therefore aerodynamic lift is used to assist the drag force and widen the corridor to acceptable limits.

This aerodynamic lift process starts at the 0.05g deceleration marker, and was computer controlled from the ground. As noted by David Orbell:

Al Worden had dismissed NASA's involvement in calculating his Earth re-entry trajectory, claiming outright credit for the calculation and execution of this manoeuvre, which contradicts the Apollo 15 mission flight journal (Orbell, 2014).

If the data from the Apollo 15 record demonstrates that the setting up of the entry procedure could have some navigational input from the Apollo astronauts, and all the Apollo mission records state that the CMs were equipped with an entry module system (EMS) enabling them to know where they were during entry, the unmanned tests for Apollo 4 and Apollo 6 demonstrate that the whole process could perfectly well be run from Houston (Apollo 4, 1969).

The relationship between lift and drag is called the lift-to-drag ratio, and is expressed as L/D. This is a complex subject and to fully address it is beyond the remit of this article – referenced sources below explain these matters fully.

Fig 6

Fig 6. Illustration of lift and drag relative to CM entry. NASA, with additional entry annotation.

The 1968 NASA film Reenty has this to say about the entry procedure from the 0.05g marker:

"The command module will then climb until it SKIPS out of the denser atmosphere. The SKIP is primarily to increase the maximum range of the spacecraft and reduce the aerodynamic heating load on the heat shield. As the SKIP is approached aerodynamic control is reduced to practically zero. During the SKIP the crew will be experiencing weightlessness. The SKIP will not take the command module beyond the 400,000 foot level, it will then descend starting its second entry and go on to the second control period. The entry phase is terminated at the 23,500 feet level at which point the drogue parachutes are deployed and the spacecraft is committed to its landing point." (emphasis added)

NASA might have intended to use the 'double dip' entry, but then again it originally intended to land on dry land with pinpoint precision. Computing not being up to the task, this option wasn’t pursued, and as it turns out, according to the record, the 'double dip' entry procedure attributed to the unmanned tests went through some modifications. Either that, or the data has been removed, because although termed ‘double dip’ the Apollo manned mission data lacks a second set of peak gs. Fig 7 below is the Apollo 11 flightplan data, which is typical of all missions except Apollo 10 and 17.

Fig 7Fig 7. Flightplan for Apollo 11 – only one peak g.

And the graph for the Apollo 8 flightplan is typical of a single peak entry – as is the timing of 13.5 minutes:

Fig 8

Fig 8. Apollo 8 flightplan – typical of a single peak entry.

The Apollo entry data gives the maximum g-force for each mission, and these average at a deceleration loading of around 6.56g for the manned Apollo entry and descent. Yet we have seen that the 'double dip' of Apollo 4 has two deceleration loadings, and higher gs. Here is the flight graphic of the Apollo 4-g loading:

Fig 9

Fig 9. Apollo 4 peak g – first peak 7.3g, second peak 4.25g (NASA TN D-5399 p37). The deceleration black line starts from the EI at 29,968.54sec and ends after drogue parachute deployment, at 30,678.6sec.

Note that the figure of 20,680 is incorrect, it should read 30,680. Also, the ground elapsed time on the graph doesn’t conform to the written statements in this document. The graph itself ends at the 30,760sec mark, and this is the maximum timing featured on all the graphs in this NASA Technical Note. Some of the graphs are a mixture of ground based simulations and flight data – and where this is the case the differences are pointed out on the graph itself. The deceleration graph does not state this fact, so it is presumably based on the actual flight test data.

The end timing on the graph is perhaps indicative of the assumed splashdown timing. However, it corresponds to 13mins 20sec which is the time taken for a single peak g event. The actual splashdown was recorded as 31,029.2sec (see fig 5 above). This gives the Apollo 4 CM re-entry under ‘double dip' conditions a total time from EI to splashdown of 17mins 40.7sec. The difference of some four and a half minutes between the hypothetical data and the actual splashdown data might also indicate problems encountered with the Apollo computers during this test. Then note that Apollo 17 has a flightplan with two peak g events, but the timing corresponds to the Apollo flightplan featuring only one peak g event.

Fig 10

Fig 10. Apollo 17 flightplan displaying two peak gs – but with no data.

The Apollo 17 mission report doesn’t mention the method of entry, nor does it mention the g-forces experienced, while the NASA entry data chart mentions only one figure of 6.49g. This lack of data makes it doubtful that a 'double dip' entry was actually performed on Apollo 17. Even the double dip within the atmosphere has to be managed carefully and perhaps the Apollo 4 test had revealed that the computational ability of the onboard computers was not reliable enough to risk human lives.

Apollo Lunar Surface Journal contributor and computer expert Frank O’Brien asserts that the Apollo Guidance Computer (AGC) had capabilities that are advanced even by today’s standards (O’Brien, 2010 ). This is a viewpoint not entirely shared by others. Jack Garman, who supervised the design and testing of the Apollo Guidance Computer said at the time of Apollo 11:

"Nobody knew much about computers in those days – and nobody knew much about flying spacecraft, either." (P. Mech, 2009)

Moreover, computer engineer Xavier Pascal has conclusively demonstrated that there are serious problems with the assumption that the AGC could manage the tasks attributed to it. (Pascal 2012/13) Given the NASA flightplan data and the limited and questionable abilities of the onboard computers in the Apollo craft, then the following comment from the 1968 NASA film can hardly be taken at face value: (Reentry)

"Just how important the atmosphere entry phase of the lunar mission is, can be highlighted by the fact that while it is one of the briefest phases in time it requires one of the largest onboard computer programmes." (emphasis added)

It would seem that the computer’s capabilites were understood, and that the simpler direct entry with a touch of lift and drag control was within the agency’s means – just. It’s instructive to note that modern documents (Arch. Study, 2005) state that the Apollo Guidance & Navigation system will only be used on the portions of the Orion CEV flight well into the atmosphere, the entry into and the skip out of the atmosphere will be managed by more advanced software and hardware.

Meaning every word

Which takes us back to Chris Kraft, because the problem here is that Kraft stated that due to the super-orbital velocities of a lunar return, Apollo needed to skip out of the atmosphere. Current research for the future Orion capsule has reiterated this fact, and elaborates on the choices for departure and landing that this skip then produces for the manned capsule. This then makes it look as if the Orion return is an entirely different case to Apollo. The only difference is that Apollo did not have the computing availability to achieve the skip in the first place. And had it been able to do so, then the heatshield requirements were of another order, hence the ceramic tiles proposed for Orion. So what might Chris Kraft mean by his statement:

"...So what we did was get them into the atmosphere, skip it out to kill off some of the velocity, and then bring it back in again." Chris Kraft (P. Mech, 2009)

If Chris Kraft was in fact inferring either ‘skip it out within the atmosphere’ or even ‘bring it back in again to the denser atmosphere’ the astronauts would have understood NASA jargon (or at least they ought to have done so) and therefore could have informed David Orbell as to what Chris Kraft had really meant. To react as Worden did rather indicates that he is aware that Kraft had meant every word.

Fig 11

Fig 11. Illustration of the SKIP re-entry (red line) stated by Chris Kraft for Apollo and now considered absolutely essential for the Orion lunar return.

Even more confusing is that irrespective of the techniques applied to Apollo, astronauts Worden and Stafford seemingly do not know enough about the overall mechanics of entry to be able to set David Orbell right as to the exact definition of a direct entry, and how that can translate into a choice of skip manoeuvres. Which is very hard to believe. But since neither man chose to set the intrepid Orbell right – Houston, there is an elephant in the room.

Unfortunately, NASA flightplans contradict the skip entry described by Chris Kraft. As does the fact that although the Apollo skip guidance was engineered, it was never used on a manned mission precisely due to navigation and control concerns during the skip manouevre out of the atmosphere.

Authors Launius and Jenkins might help with this entry conundrum:

"In order to provide adequate margins for guidance errors during super-orbital re-entries (i.e., direct return from the Moon) the original specification for Apollo required a ratio of 0.4 L/D …the Apollo CM was subsequently engineered with a lift ratio of 0.6 to 0.8." (Launius & Jenkins, 2011)

Over engineering, surely, when the means to control the skip were lacking. Scientists have noticed that NASA continually referred to the dangers of ‘skipping out of the atmosphere’ and the ‘straight narrow corridor', so this over engineering might well be an attempt to reassure their passengers that all would be well if a miscalculated entry did occur. Nevertheless, the NASA entry data across Apollo does not give the minimum 0.4 L/D ratio for a lunar return, but instead ranges between 0.28 and 0.30, which is also close to the L/D ratio of Gemini returning from Leo with a L/D of 0.25. Launius & Jenkins explain this by writing:

"However, the guidance techniques actually used during Apollo were sufficient. Therefore, most of the L/D was not required, and the flown missions used an L/D between 0.29 and 0.31, subjecting the crew to a deceleration of about 6g. (emphasis added)

From the above statement it can be seen that the numbers in the NASA entry data for maximum g (see the chart in Fig 1) have been massaged downwards by NASA, in another NASA publication. Furthermore, the Orion capsule studies have recommended the L/D of 0.5 for a lunar return; restricted to the ISS return from LEO, this has become L/D 0.30.

Fig 12

Fig 12. First Lt. Kaya provides a chart depicting the principles of g-loading relative to altitude for both skip and direct entries. Although not specifically attributed to Apollo testing, the parameters of the non-skip entry do correspond to data from the unmanned CM testing of AS 201 and 202. Confusingly, the Apollo manned CM g-loading on this chart would be seen to match the highest peak of a skip entry.

If the computer guidance system was only ‘sufficient’ then from the flightplans it could only manage a direct entry with a single lifting across 13-16mins to a pre-designated target, which then explains the lack of the double dip, that took over 17 minutes from EI to splashdown for Apollo 4. Also note that the next Apollo flight test, Apollo 7, the first manned CM return didn’t give the astronauts the benefit of this double dip experience. It actually entered the atmosphere from LEO at the much lower rate of 25,955ft/sec, and at 3.3g. As did Apollo 9.

This is about the same as the deceleration g-force from a Space Shuttle descent. Yet a returning long-bodied glider through the atmosphere with an L/D ratio of 1.1 is an entirely different process to that of the cone-shaped CM, as discussed by Phil Kouts in detail (Kouts, 2015).

Note that L/D ratios below 1.0 are suited to ballistic and semiballistic entry of such cone shapes as the CM. (Inges, 2012) Actually flying with L/D 0.28-0.30, instead of the 0.5 required for a skip entry, would infer that in fact the Apollo missions entered the atmosphere just like Gemini. Even more remarkable is that on the NASA entry chart for Apollo 7 and Apollo 9 flights – there is no L/D ratio data whatsoever.

Perhaps this would have drawn attention to the problem of entry and reminded us that it is not necessary to be travelling all the way from the Moon itself to simulate a lunar return, as was demonstrated by Apollo 4. And from the Apollo 6 technical report we learn how that was managed:

"Unmanned flights carry an Apollo Mission Programmer (MCP) with special equipment necessary to operate the spacecraft subsystem in the absence of crew. The VHF and S-Band omni-directional antennas will be modified accordingly. The MCP receives information from the updata link ground command, the Saturn instrument unit, ground support equipment, the guidance and navigation, Earth landing (from the automatic sequencing of drogue and main parachute systems onwards) and environmental control systems." (Apollo 6, 1969)

Measure for measure

Confusion over the actual method of re-entry has contaminated the record of other aspects of Apollo return procedures, and mixing up different measuring systems has helped to hide the truth. Despite the fact that present-day websites are replete with measurements expressed in kilometers, historically NASA used meters or statute feet per second (mtrs/sec or ft/sec) to express velocity; kilometers or nautical miles (km or nm) to express range distance, and meters or statute feet (mtrs/ft) to express altitude above sea level.

Amazingly, this system has apparently even confused some Apollo astronauts. Certainly, the Apollo 10 flight (remember it also had a double dip programmed into its flightplan) caused Eugene Cernan some number muddles. In his biography The Last Man on the Moon, he writes:

"The fifty five hour return was totally without problems and required only one tiny midcourse correction. We came hammering home on Monday, packing a velocity that carried the risk of the spacecraft skipping off the atmosphere rather than plunging through it. We had extra fuel when we left the Moon, and burning it off accelerated us to well over 25,000 miles per hour as we approached Earth, making us the three fastest humans in history…

"Our re entry corridor was tiny, plus or minus half a degree… We had to be confident that the slide-rule guys would make the shot, because, unlike in the simulators where you get a second chance, we couldn't start this part of the trip over again. It was all or nothing. After eight days of weightlessness, the force of gravity welcomed us back to our world, and the G-forces climbed as we punched into the heavy air. Half a G...two Gs...four...six...seven..." (Cernan, 2000)

There’s a lot wrong with the above statements. And not all of it down to hyperbole. Hammering home at 36,666ft/sec would be within the parameters of normal lunar return velocities. And Cernan knows perfectly well that in order to enter the atmosphere safely retro-rockets must be fired to bring the speed down. He knows that the actual EI velocity as recorded by NASA is the relatively more sedate 36,314ft/sec. If the ‘slide rule boys’ had worked the original maths he also knows that computers governed the entry process, and he also knows that the flightpath angle at entry has been calculated expressly to ensure capture of the CM and protect against skip out of the atmosphere. Finally for what it’s worth, the Apollo 10 narrative records that the maximum g-loading for this flight was under 7g.

However, this dramatic statement indicates that Apollo 10 ended up with an entry of only one peak g, otherwise Cernan would surely have described the experience. Despite the double dip shown in the pre-flightplan and press release, there is no indication that this ever occurred. And the post flight mission report has a distance from EI to splashdown commensurate with all the other manned Apollo flights of some 13.5 minutes.

Below is a graph showing the difference over time for a skip entry and an Apollo direct entry. But it contains three anomalies:

1) It shows the non-skip entry terminating at some 13.5min/810sec from EI to splashdown, which is the timeline for Apollo missions recording a single peak g according to NASA.
2) Yet this blue line displays a trajectory associated with two peak gs and if truly representing the ‘double dip’ entry as exemplified by Apollo 4, should terminate at around the 17.7mins/1060second mark.
3) While the green line correctly represents the EI at 121km (400,000ft), the non-skip entry line is highly exaggerated. It implies a climb after entry to an altitude of some 328,000ft – over 86,000ft higher than the lifting credited to Apollo 4’s ‘double dip'. So this apparently helpful graph again perpetuates the confusion around the issue of Apollo re-entry.

Fig 12a

Fig 12a. Between a direct entry and a skip entry, the dramatic difference in range flown from EI is most distinct between the trajectories. (Arch. Study, 2005 Section graphic Kaya, 2008.

Cernan might have reason to wish to point out a discrepancy with this flight, and whether this is a case of being confused by the numbers, or a form of whistle-blowing about that double dip non-event, remains to be seen. And Cernan is not alone. Back at the beginning of the manned missions, Tom Stafford (Apollo 10 again) asserted in his own 2002 biography We Have Capture that his entry speed was 28,547mph, which works out at 41,869ft/sec – faster than a return from Mars! (Stafford, 2002).

Unfortunately, finding discrepancies in the documentation across various NASA records for Apollo entry velocities is par for the course. NASA’s Apollo 10 web history page gives the figure of 36,314ft/sec as the velocity at the 400,000ft EI. And then adds a footnote concerning the inclusion of the Apollo 10 flight into the Guniness Book of World Records. Considered superfluous information by serious scientists, this bit of trivia is indicative of two things: firstly the need to establish a ‘world first’ for the so-called ‘space race’ record, thus anchoring Apollo into the collective consciousness as ‘the winner’ and secondly it demonstrates once again, the casual attitude of NASA to the public when publishing facts about Apollo.

The Guinness Book of World Records footnote reads:

The Guinness Book of World Records states that Apollo 10 holds the record for the fastest a human has ever traveled: 24,791 st mi per hour at 400,000 feet altitude (entry) on 26 May 1969. However, the Apollo 10 mission report states the maximum speed at entry was 36,397 feet per second, or 24,816 st mi per hour (Apollo 10 TestLM) (Apollo 10 MR, 1969).

This same statement then appears, virtually intact, as a footnote on page 265 of Apollo: The Definitive Sourcebook (published 2006 by Richard W Orloff & David M Harland). Although on the previous page the authors had cited the velocity of 36,314ft/sec as being the correct entry velocity.

So there seem to be three different velocities for the same point in the journey of Apollo 10.

A month after the Apollo 10 flight NASA issued an interim mission report, dated June 1969 (Apollo 10 TestLM) stating that the velocity at the EI was 36.314ft/sec. In August 1969 the full mission report (Apollo 10 MR, 1969) was published (at the time not intended for general distribution or referencing but now available on the web) and there is no mention of the velocity of the spacecraft at the EI that I could find, and certainly not in section 9.13, "Entry and Landing". Nor is it featured in the Apollo Program Flight Summary Report of June 1972.

However, that undated NASA table of Apollo mission data Entry, Splashdown and Recovery complied from various sources, (see again Fig 1) conveniently adds the maximum velocity figure of 36,397ft/sec to the already present 36,314ft/sec, but ignores the 36,360ft/sec linked to its world record.

All of which begs the question as to why there are three different entry interface speeds – or even two. It should be a matter of record that at the time of entering the atmosphere at the designated EI of 400,000ft only one entry velocity would be extant. If the other figures are related to moments either prior to, or post the EI, or even hypothetical values prior to any acutal flight test, then this should be stated as such since they do not qualify as being that of the EI itself.

Even if NASA likes to set the preflight numbers against those programmed into the computer (the real time numbers) and against those of the post flight record, (the actual numbers) surely Cernan and Stafford, supposedly in the same place at the same time, should be talking the same 'actual' numbers (Apollo 9, 1971).

Some have considered Stafford’s remark to be commensurate with stating the entry velocity in statute miles from a recorded nautical velocity. This assumption ignores NASA’s measuring conventions of the 1960s and the fact that nautical velocity is expressed in knots. It also creates another problem because if misunderstood, the calculation produces yet another figure for Apollo 10’s entry of 36,384ft/sec, and that does not feature in any NASA documentation.

These same reviewers of the Apollo record and attendant biographies have thought it a pity that the editors of Stafford’s book didn’t correct this ‘mistake’. Well, it also lies within Stafford’s power to correct any misprints and misstatements and as yet, he has not done so. One wonders if he has noticed that 28,547mph is 7.92 miles per second, which looks an awful lot like the 7.92 km/sec required to reach LEO and return therefrom.

Many happy returns

Maybe Chris Kraft’s use of the words ‘skip it out’ actually provides the clue to these anomalies in the Apollo record, because technically, the skip portion of the re-entry procedure spent outside the atmosphere is also called the Keplerian/ ballistic coast. (See Fig 13 below and Fig 4, Apollo 4) And one might say that Worden certainly went ballistic.

But perhaps it was all theatre.

If the intrusive audience member who had not liked David Orbell’s question on re-entry (why not?) did in fact have connections to the backroom boys at Autographica, Al Worden might have known what was coming. Especially since Tom Stafford had already been asked the same question. So might this dramatic response by Worden have been a set-up? David Orbell’s question providing exactly the right moment for Worden to make his point. Getting Orbell evicted shortly afterwards would certainly make sure his innocent questioner didn’t forget the incident. Speculation perhaps, and certainly if this was theatre, it took in David Orbell – as he later wrote:

"In retrospect one can see the dilemma that the Apollo veterans now face. Today they are locked between two realities. On the one hand they have to acknowledge NASA's explicit historic game plan. However we now know that no technical expertise existed with which to effect the skip re-entry manoeuvre suggested by Chris Kraft. The solution? Bluff your way through this dilemma and hope to God no one picks up on these impossible contradictions. Unfortunately I did, at Autographica. Solution? Remove me quickly from the scene." (Orbell, 2015)

Was Worden trying to bluff his way through the question? It’s quite possible, but the Apollo astronauts were selected for their ‘right stuff’ – the ability to remain calm in the face of extreme adversity, so I’m not so sure. Worden’s tantrum got Orbell’s attention and it got him chucked out, leaving him outraged and determined to follow up on the matter.


Even if the Apollo missions are recorded as returning at the ‘correct’ rate for a lunar return, they do not have to be coming back from the Moon to do so. The extreme confusion and reactions amongst those closest to the Apollo program indicates that all is not well with the official record.

Fig 13
Fig 13. Unmanned Apollo 6 flightplan. This failed mission is illustrated by a confusing NASA graphic in which a portion of the lunar trajectory is labeled ellipse, while the actual elipse used to simulate a lunar return is left unlabelled. No.9 is the 400,000ft EI. No. 11 is the target splashdown site. No 10 is the splashdown should the No.6 SPS burn not go to plan.

Had the Apollo astronauts actually experienced the entry manoeuvres as described in the Apollo record, it should have been easy enough to correct misunderstandings from members of the public and briefly explain how they had worked.

Maybe the astronauts did not themselves know the differences, because they didn't have the experience, or:

  • They think they did one thing, while their flight director thinks they did another, which would indicate a communication level so lacking in credibility as to be unbelievable;
  • It is the consequence of the level of need-to-know operating between management and astronauts within the Apollo project, even to this day;
  • They can’t remember/have distanced themselves from their Apollo mission to such a degree that it as if their minds are now totally blank. In which case why continue to trawl events such as Autographica?
  • There is a further option which is equally hard to believe, but no less worthy of consideration: Col. Al Worden deliberately chose to create a mega-fuss at a public meeting.

Perhaps engineering David Orbell’s departure from Autographica 2014 was an essential component of this bit of theatre. We might never know for sure, but the fact is that there is a very serious problem with the discrepancies across the record concerning the Apollo CM re-entry data. And this issue would have gone completely unnoticed – if it had not been for Chris Kraft’s statement, Stafford’s number crunching, Cernan’s hyperbole, Worden’s outburst and Orbell’s outrage.

It is possible that the enigmatic speech made by Neil Armstrong on July 20, 1994, particularly resonated with this problem of re-entry through the dense layers of the atmosphere. Read in the context of the whole speech his reference to the removal of one of truth’s protective layers is most revealing:

Wilbur Wright once noted that the only bird that could talk was the parrot, and he didn't fly very well. So I'll be brief. This week America has been recalling the Apollo program and reliving the memories of those times in which so many of us here, colleagues here in the first rows, were immersed. Our old astrogeology mentor, Gene Shoemaker, even called in one of his comets to mark the occasion with spectacular Jovian fireworks. And reminding us once again of the power and consequence of celestial extracurricular activities.

Many Americans were part of Apollo, about one or two in each thousand citizens, all across the country. They were asked by their country to do the impossible – to envisage the design and to build a method of breaking the bonds of earth's gravity and then sally forth to visit another heavenly body. The principal elements – leaving earth, navigating in space and descending to a planet unencumbered with runways and traffic control – would include major requirements necessary for a space-faring people.

Today a space shuttle flies overhead with an international crew. A number of countries have international space programs. During the space age we have increased our knowledge of our universe a thousand-fold.

Today we have with us a group of students, among America's best. To you we say we have only completed a beginning. We leave you much that is undone. There are great ideas undiscovered, breakthroughs available to those who can remove one of the truth's protective layers. There are many places to go beyond belief. Those challenges are yours – in many fields, not the least of which is space, because there lies human destiny. (emphasis added)

Speech by Neil Armstrong at the White House, July 20, 1994

Recent research carried out by NASA has demonstrated that the safest way to return from the Moon is by using a skip entry. Associating the Apollo direct entry with a skip entry through careful use of language infers that this fact was known at the time of Apollo – even if considered unachievable or in a worse case scenario, highly risky.

The fact that the Russians were experimenting with, and ultimately mastering the skip entry technique during the Zond program, not only implied superior guidance and navigation skills between ground and spacecraft, but also that the Russians held the key to a safe re-entry for bio-organisms at lunar return velocities. The skip entry would then have to be incorporated into the Apollo flight program – at the very least verbally. Consequently, it is hardly surprising that there are divergent views between those who planned the Apollo missions and those who flew – somewhere.

The responses of the astronauts at Autographica 2014 to David Orbell’s enquiry have highlighted yet another major problem with the Apollo record.

Mary Bennett

Aulis Online, May 2015


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