Thursday, December 27, 2012

A Jones for MOL #7: Hatches? We Don’t Need No Stinking Hatches!

I addressed the topic of hatches in MOL back in #3 of this series. But there is more to be said on the topic. Plus, I just recently thought of the title for this blog post, which is much better than the title of #3, and could not resist using it. If I compile these individual posts into a longer article or a book chapter, I will use the title from this blog post. For a brief history of the entertaining catch phrase behind this title, see “Stinking Badges” at Wikipedia (ref. 1).

In #6 I explained the evidence for my assertion that the Gemini-B capsule was to be sealed and depressurized for most of the 30-day manned MOL mission. The isolation of the capsule seems straight-forward. One obvious approach would be for the last pilot leaving the Gemini-B to close a hatch at the lab end of the transfer tunnel, permitting the subsequent depressurization of the transfer tunnel and the adjoining open capsule while maintaining the air pressure in the MOL. But as noted in #3, the evidence for a hatch at this location is ambiguous at best. I concluded that there was no independent pressure-seal capability for the transfer tunnel: the tunnel would have been open from the Gemini-B heat shield all the way to the interior of the MOL habitable volume.

This is not unique in spaceflight: the Spacelab modules carried in the payload bay on many Space Shuttle flights were connected by a long transfer tunnel to the Shuttle’s crew compartment, and there was no hatch at the Spacelab module opening (ref. 2). But the crew module of the Shuttle was never intended to separate from the Spacelab while in-flight, while the Gemini-B was required to separate from the MOL for return to Earth. This will be the topic of a future blog post in this series.

If the Gemini-B/MOL tunnel could not be closed off at the lab end, then maybe it would have been possible to seal off just the capsule by reversing the large pressure bulkhead (LPB) hatch, passing it through the hatch opening and then re-installing it from the tunnel side to seal the Gemini-B’s large pressure bulkhead  (see Fig. 1). In this scenario, the separate heat shield (HS) hatch would have stayed stowed in its alcove in the wall of the transfer tunnel. 
Figure 1. McDonnell Douglas illustration of the crew transfer hatches in the Gemini-B: the large pressure bulkhead (LPB) hatch and the heat shield (HS) hatch. Note the common features of both hatches, including the crank handle and the six dog arms. Note also the stowage location for the LPB hatch in the small alcove between the seats.

The following analysis is based on my reading of the collected Gemini-B/MOL documentation, much of it provided by Dr. Dwayne Day from his visit to the U.S. Air Force archives at Maxwell AFB in Alabama in late 1999. The drawings from those archives are detailed and thorough, and bear corporate file numbers but not dates. However, a date may be inferred from the fact that the drawing numbers all contain the abbreviation “MDAC” clearly indicating the McDonnell Douglas Astronautics Corporation. McDonnell Douglas was formed from the April 28, 1967, merger of the McDonnell Aircraft Company, builders of the Mercury and Gemini spacecraft and prime contractor for the Gemini-B, and the Douglas Aircraft Company, prime contractor for the MOL itself (ref. 3). Thus, I have assumed that the drawings represent mature designs for their subject components, since MOL was heading into manufacture by 1967 and was cancelled just over two years later.

There are some problems with the approach of reversing the LPB hatch. First, the hatch was too big to be passed through the hatch opening. The diameter of the hatch was 25.1 in. (63.8 cm.) (per Fig. 1), but the hatch opening diameter was only 24 in. (61.0 cm.) (per Fig. 2). This does not mean that such an approach is impossible under any circumstances: I have watched in amazed disbelief as a large airliner door is swung inward and then snugged up to the door frame from the inside to make an airtight seal, but that apparently requires some rotation of the rectangular doors to pass through the frame and then de-rotate to fit snuggly. No amount of angling or rotation would have allowed the circular hatch to pass through its circular opening that was over an inch smaller.
Figure 2. McDonnell Douglas illustration of the edges and seals of the LPB and HS hatches. (In this illustration, the LPB hatch is labelled as the crew transfer hatch.) I have highlighted the LPB knife edge and the corresponding gasket, neither of which were labelled in the original illustration. I have also indicated the area of interest in the small diagram of the Gemini-B, to the left of the hatch detail.

Second, the cross-section of the LPB hatch shows it to have had a type of “knife-edge” seal (ref. 4) which would work only if the hatch were oriented correctly (see Fig. 2). An extended ridge called the knife edge near the rim of the hatch would be compressed a recessed gasket in the rim of the hatch opening to make the seal airtight. The gasket was only on the Gemini-B side of the opening, so if the hatch had been flipped around so that the crank handle was accessible from inside the transfer tunnel, then the knife-edge seal would have been facing away from the gasket.

Third, the crank handle for securing the hatch was only on the Gemini-B side of the hatch (see Fig. 2). This handle was to be turned to drive six dog arms on the hatch into corresponding fittings on the hatch opening rim, mechanically wedging the knife edge of the hatch onto the gasket of the hatch opening, making the seal airtight. Conceivably the hatch could have been left in its usual place on the Gemini-B side of the opening, but the handle moved to the tunnel side, or a second handle could have been inserted on the tunnel side of the hatch. But the available diagrams give no evidence that a handle could be removed and reinserted on the MOL side. In addition, leaving the hatch on the Gemini-B side of the opening meant that it would have been on the vacuum side of the bulkhead when the capsule was depressurized for the 30-day duration of the mission. The hatch was clearly designed to be on the high-pressure side of the bulkhead, using the air pressure of the manned capsule to reinforce the seal against the gasket. Whether it could have provided an adequate seal for 30 days if the air pressure from the MOL was trying to push it into the depressurized Gemini-B capsule is unclear to me, but I suspect not.

Fourth, I just realized that there would have been no need for the permanent structure between the seats to retain the LPB hatch (see Fig. 1), if the hatch were to have been used to seal the tunnel for up to 30 days. A temporary stowage location, perhaps just velcroed to a wall, would have sufficed.

Thus, it appears that the LPB hatch could not have been reversed, installed and sealed from the tunnel side.

Summarizing the key points of several blog posts:
  1. The Gemini-B capsule was to be evacuated, sealed and decompressed for the duration of the MOL missions (discussed in #6 in this series).
  2. There was no hatch or other fixture to seal the transfer tunnel from the Gemini-B to the MOL at the lab end of the tunnel (discussed in #3 in this series).
  3. The LPB hatch of the Gemini-B capsule could not be made to seal nor could it be operated from the MOL side of the hatch (as discussed in this article).
I can only reconcile any two of these three points. First, there would be no need for either a transfer tunnel hatch or a reversible LPB hatch if the Gemini-B were not to be depressurized. (This is the only case for which the title of this blog post is literally correct.) Second, there would be no need for a tunnel hatch if the LPB hatch were operable from the tunnel side of the capsule’s heat shield. Third, there would be no need for a reversible LPB hatch if there were an operable hatch at the MOL end of the tunnel.

But at this juncture, I cannot say how all three of those conditions were to be fulfilled simultaneously on manned MOL missions. It is a mystery, and I hope that someone reading this blog post knows the answer and will share it with me. If that happens, I will pass it on.

Why this was important to the end of the crew-occupied phase of each MOL mission, when the Gemini-B was to separate from the MOL and begin its return to Earth, will be the topic of a future blog post, but first I want to address other biomedical and human factors aspects of the MOL missions.

  1. “Stinking Badges,” (accessed 17 Dec. 2012).
  2. Spacelab News Reference, NASA Marshall Space Flight Center, document number 14M983 (undated, ca. 1980), Sec. 3.2.2. Tunnel Systems, p. 3-19, (accessed 29 Aug. 2012).
  3. “McDonnell Douglas,” (accessed 27 Dec. 2012). There is surprising ambiguity on the Internet whether the “A” in MDAC” stands for astronautics, aerospace or aircraft. I have opted for astronautics.
  4. For a discussion of seal types in nautical hatches, see Freeman Marine Equipment, (accessed 6 Aug. 2012), and Standard Equipment Co., (accessed 27 Dec. 2012).

Monday, December 17, 2012

A Jones for MOL #6: Not Quite a Vacuum.

If you have read any of my preceding “A Jones for MOL” blog posts, you might have been wondering where all of this hypertrivial analysis of a defunct spaceflight project was leading. Your patience will now be rewarded, and if your response is, “That was the blockbuster?” I will also tell you why it is so interesting that it motivated these six blog posts and a few more yet to come.

The Revelation

Here it is: during their 30-day mission, while the two MOL pilots were to have been evaluating space reconnaissance techniques as well as being guinea pigs for space biomedical research, their Gemini-B capsule would have been sealed off and depressurized to 0.1 psi (5 mm. Hg, 0.7 kPa, 0.007 atm)—the same as atmospheric pressure at 42 km (26 miles) above Earth’s surface (ref. 1). Humans need supplemental oxygen above about 3 km (10,000 feet) altitude, and space suits above 19 km (12 miles), so the cabin pressure would have been just a smidge better than a vacuum, uninhabitable for the duration of the mission.

I stumbled upon this surprising and unexpected aspect of the MOL’s human factors plan while re-reviewing some declassified MOL documents at NASA’s Technical Reports Server (  As far as I know, this has not been reported anywhere since 1968. I do not remember ever having seen it reported, or even mentioned, anywhere else, certainly not among the rote, repetitive assertions that accompany any new discussion of MOL on the Internet.

Powered Down

It has long been known that the 90 ft3 (2.6 m3) Gemini-B capsule (ref. 2) was to be powered down, that is, many of its electrical systems were to be turned off, during the 30-day operational mission of the MOL reconnaissance station. Such a power-down has been, and continues to be, common practice in space station missions. It was not even unprecedented during the MOL planning period, although the single time it had happened before was unplanned. In August 1965, Gemini 5, the third manned capsule in the series, was testing electricity-producing fuel cells in flight for the first time. These were required for future long-duration missions, including the Apollo moon flights. According to NASA (ref. 3), “About [2 ½ hours after launch], the crew [command pilot Gordon Cooper and co-pilot Charles Conrad] noticed the pressure in the oxygen supply tank of the fuel cell system was dropping. At some point earlier in the flight the oxygen supply heater element had failed, and the pressure dropped from nominal pressure of 850 psia to a low of 65 psia 4 hours and 22 minutes into the flight. This was still above the 22.2 psia minimum but it was decided to […] power the spacecraft down. An analysis was carried out on the ground and a powering up procedure was started on the seventh revolution. Over the rest of the mission the pressure slowly rose in the fuel cells and sufficient power was available at all times.”

After entering the lab module, an early task of the Gemini-B co-pilot would have been to activate the lab’s systems so the command pilot could then shut down most of the Gemini-B’s systems for long-term quiescence. All guidance and navigation and communications systems would be off or on stand-by. Spacecraft power from the laboratory would preserve the capsule’s non-rechargeable batteries for re-entry. During the month-long mission, the Gemini-B would remain quiescent, and the pilots in the lab would monitor 10 parameters of its systems, including the pressure in two gaseous oxygen and two nitrogen tanks, in two fuel and two oxidizer tanks for the maneuvering engines, and the status of two coolant pumps. Meanwhile, 73 Gemini-B parameters would be monitored from Earth via the lab’s communication system whenever its orbit passed over a tracking station (ref. 4).

The phrase “powering down a spacecraft” brings to mind the images of the inert command module in Apollo 13 (both the movie and the actual spaceflight). The cabin would not be continually heated by its electronics, and would get cool. Then it would get damp, too, as the pilots’ warm, moist breath diffused throughout the open tunnel from the laboratory module into the capsule. This had been a problem on Gordon Cooper’s 34-hour Mercury flight in 1963: the short-circuiting of critical re-entry control systems led to improved packaging for electronics on the Gemini spacecraft, as well as relocating much of those systems outside the crew cabin, in the vacuum of space. Ironically, Cooper faced the challenge of a power-down on his second spaceflight on Gemini 5.

Closed and Depressurized

After its power-down, the Gemini-B was to be sealed off for its long dormancy and then depressurized. The MOL pilots would have lived in the 400 ft3 (11.3 m3) pressurized laboratory module (ref. 5) for all but the first and last few hours of the flight. This plan for the isolation and prolonged decompression of a portion of a manned spacecraft during flight was and is unprecedented in the annals of human spaceflight, but reflects the knowledge, experience and concerns of the manned spaceflight community of the day, 1964.

Of course, the Gemini capsule was designed to be depressurized because space walking, or extravehicular activity, was a goal of the Gemini program. The capsule needed to be opened to the vacuum of space so an astronaut could exit it and perform useful tasks outside. But these were episodic, rare and brief events, not continuous for weeks at a time.

In other spaceflight experience, during Skylab missions, the unpowered Apollo command module was accessible continuously and was used for privacy by the astronauts because temporary ducts circulated fresh, dry air through the capsule. The Russian Soyuz was also accessible during Salyut and Mir missions—and remains so during International Space Station (ISS) missions. Ditto for the Chinese Shenzhou-9 while docked with Tiangong-1. Of course, the Space Shuttle was open to the Mir and ISS on all of its docking missions, and was fully powered the whole time, even drawing current from the ISS’s solar panels to extend its own orbital lifetime.

However, the depressurization is mentioned specifically in only one document (ref. 4), a report produced by Bellcom, Inc., a Washington-based contractor which analyzed Apollo program technical decisions for NASA Headquarters. Early in 1968, Bellcom’s R.K. McFarland met with representatives of McDonnell–Douglas to learn how the Gemini-B capsule was being modified for long-duration quiescence, in a mode comparable to that planned for the Apollo command-service modules during upcoming NASA space station missions. Among other things, McFarland was told that the Gemini-B would be depressurized to 0.1 psia, with the crew not re-entering the capsule until the end of the 30-day mission.

Independent, albeit indirect, evidence for the long-term isolation of the Gemini-B is found in a fire safety report (ref. 6) by The Aerospace Corporation, the systems engineering contractor for MOL, in mid 1967, before McFarland’s visit. This report documented the responses of the MOL Program to the recommendations of the review boards for the Apollo 1 fire and the Brooks AFB chamber fire (ref. 7), both of which occurred within a period of four days at the end of January 1967.

The report noted that, “[u]nder the original baseline, the Gemini B was [to be] repressurized with 100 percent oxygen prior to crew transfer from the Laboratory. In case of a fire emergency in the Laboratory requiring the crew to abort to the Gemini, this pure oxygen atmosphere would have made a hazardous situation worse. A capability is being added to permit repressurization of the Gemini B with a two-gas atmosphere from the laboratory atmosphere supply source to minimize this hazard. In addition Gemini B emergency repressurization time is being sharply reduced.”

This is a telling comment: a repressurization of the Gemini-B capsule would have been necessary only if it had been depressurized earlier in the mission.


As originally designed, the Gemini-B’s life support system would have required remote control—either from Earth or from the MOL—to repressurize its cabin by releasing pure oxygen from on-board storage tanks. The added capability mentioned in the Aerospace report might have been as simple as a valve in the hatch between the MOL and the Gemini-B which would allow MOL atmosphere to flow into the Gemini-B.

There is another indication that the Gemini-B would have been left unattended, if not actually sealed and depressurized, for the 30-day MOL mission. In a memo reporting the results of a visit by MOL engineers to the GE space simulation facility in Valley Forge, Pa., astronaut Scott Carpenter is quoted as saying he wouldn’t leave the Gemini unattended for 30 days without several checks on its condition (ref. 8). The fact that he was saying anything of the sort is evidence that someone, either GE or the Air Force, must have asked his opinion about leaving the Gemini unattended for 30 days. In a vehicle with only 400 cubic feet of volume, if the secluded the Gemini cabin plus the attraction of its two windows were available, it would undoubtedly have prompted frequent visits from off-duty MOL pilots, who could then almost casually have checked on its systems—unless they were physically blocked from such visits.

In the absence of any explicit contemporary reports to the contrary, the two contractor reports from Bellcom and The Aerospace Corporation establish that the Gemini-B was indeed to be depressurized while in quiescent in-flight storage. All of the other primary and secondary sources are silent on the pressurized state of the Gemini-B during the MOL mission. Judging from discussions on some authoritative listservs, this plan to isolate and decompress the Gemini-B capsule during the MOL missions is literally unknown by space history buffs today.

Why the Depressurization?

The available documents don’t mention the reason or reasons for the prolonged depressurization, but I can suggest a couple. First, as previously mentioned, the pilots would have been the source of substantial humidity. The unpowered Gemini-B capsule would have been a natural cold-trap for that humidity, and would probably have become quite damp quite quickly. This subsequently happened on Apollo 13, whose astronauts actually experience a bit of a rain shower as their damp command module entered the Earth’s atmosphere and the condensation dripped onto them from the control panel and other overhead structures.

Sealing off the Gemini-B would also have reduced the air leakage that was an inevitable feature of any manned spacecraft, especially one with two large crewmember hatches.

As I said, I have not seen any documentation of the reasons for the decompression, but these may have been contributing factors.

How the Depressurization?

But how would the Gemini-B capsule have been sealed off? There were hatches between the Gemini-B cabin and the MOL’s pressurized laboratory—surely they were used for that purpose. In my next post I will describe how it might have been done, or not.

  1. Calculated using the equations at (accessed 16 Dec. 2012).
  2. Cohen, M., “Testing the Celentano Curve: An Empirical Survey of Predictions for Human Spacecraft Pressurized Volume,” 38th International Conference on Environmental Systems, San Francisco, California, June 29-July 2, 2008, SAE TECHNICAL PAPER SERIES 2008-01-2027, (accessed 28 Aug. 2012).
  3.  __, “Gemini 5,” National Space Science Data Center, (accessed 16 Dec. 2012).
  4. McFarland, R.K., , Bellcom, Inc. “Subsystem Modification to Develop Quiescent Operation for Gemini B, Case 620”, February 28, 1968, (accessed 6 Aug. 2012). McFarland briefed C.W. Matthews, Director, Apollo Applications Program, on Feb. 15.
  5. Wade, M., “MOL,” Encyclopedia Astronautica, (accessed 28 Aug. 2012).
  6. __, “MOL safety evaluation based on Apollo 204 Review Board findings and recommendations and Brooks Air Force Base Accident Investigation Board Conclusions,” Aerospace Corporation, AD-856687L, 58p., September 1967.
  7. __, “Apollo-1 (204),” (accessed 17 Dec. 2012).
  8. Anderson, J.J., Col., AFRMO, “Memorandum for the Record, Trip Report to Valley Forge, GE, Simulation Tests,” 20 Aug. 1964, courtesy of Dr. Dwayne Day.

Sunday, December 9, 2012

A Jones for MOL #5: A MOL, not a chipmunk.

This is the only image I could find of something close to a
chipmunk in space,even though it would probably have
been breathing pure oxygen in the A7L NASA-style spacesuit.
Credit: Avanti Press, 2011,

The presence of helium in the gas mixture of the Manned Orbiting Laboratory’s internal atmosphere has enabled four decades of amused speculation that the two MOL pilots would have sounded like Alvin, Simon and Theodore when they spoke to each other or with their ground controllers.

Substituting helium for nitrogen in breathing gas mixtures for deep-sea diving (to help prevent oxygen toxicity and nitrogen narcosis) is well-known to cause speech distortion, especially under high atmospheric pressures (ref. 1), so it has been assumed the same would be true onboard MOL. The famous example of astronaut-turned-aquanaut Scott Carpenter’s telephone conversation with President Lyndon Johnson, in which the president betrayed no hint of any struggle to understand the explorer’s squeaky voice, was the result of Carpenter’s presence in a decompression chamber at 89 psi of heliox at 20% oxygen and 80% helium, after his 30-day sojourn at a depth of 200 feet (61 m) in Sealab II (refs. 2,3) (I was surprised to find out that the high pressure alone would not have increased the frequency; ref. 4).

The U.S. Air Force investigated voice quality and other aspects of candidate MOL atmospheres before settling on the final gas mixture of 70% oxygen and 30% helium at 5 pounds per square inch (psi). A complex of connected altitude chambers at Brooks AFB in San Antonio, Texas, for prolonged habitation was laid out like the MOL with an adjacent area to accommodate “ground control” (ref. 5). A smaller “space cabin” at Wright-Patterson AFB in Dayton, Ohio, was used for brief studies (ref. 6). Subjects for these studies, at least at Brooks, were “informed volunteers” drawn from the available population of young, fit basic trainees at nearby Lackland AFB (ref. 7).

My last blog described the Air Force’s process of selecting helium as the diluent for MOL’s atmosphere. Helium’s relative inertness, unusual heat coefficient and reduced solubility in body water and fat all recommended it as a substitute for atmospheric nitrogen even at reduced cabin pressures (ref. 1). Voice quality characteristics of interest included distortion, attenuation and error rate between airmen within the cabin and by external monitors using headsets (ref. 1). An Air Force study (ref. 1) of a 50% oxygen, 50% helium mix at 7 psi (slightly less than ½ atm) for up to 12 hours found no greater incidence of communication problems than in pure oxygen at 3.5 psi or 5 psi (as would be used in US EVA suits and US spacecraft, resp., through the end of Apollo). Fatigue of the test subjects, who were awakened early and spent a long day in the chamber, was judged to cause the same degree of problems. Another study (ref. 8) evaluated almost the same atmosphere, 44% oxygen and 56% helium at 7.8 psi (½ atm), in 4 subjects but for 14 days this time. On average, the fundamental and second formant frequencies of the voice, which would largely determine voice pitch, were higher in helium than in air, but the fundamental decreased toward its pre-exposure value over the two-week confinement. In fact, the fundamental frequency in one subject dropped and remained at or below baseline for most of the study. A third study (ref. 6) of bioacoustics during 5-8 hours at 30%, 50% and 70% helium atmospheres at 258, 360 and 760 mm. Hg (0.34, 0.5 and 1 atm) found over 50% increase in voice formant frequency at both 30% and 50% helium, and 90% increase at 70% helium, consistent with other studies.

Those studies dealt mostly with higher helium fractions and shorter exposures than a MOL mission would entail. A more realistic Air Force study (ref. 9) confined 4 subjects for 56 days at the selected MOL atmosphere. Only 1 subject showed a marked rise in the fundamental frequency of his voice over the 2-month confinement, although all 4 exhibited increases in their second formant frequencies.

Soviet investigators had also investigated an unspecified helium atmosphere, even before the Air Force tests, in which the voice pitch of the test subjects was reported to be increased “seven tenths of an octave, but the distortion was tolerable” (ref. 10). I spent a little time Googling articles about voice quality to understand what “seven-tenths of an octave” corresponded to in terms of “cycles per second” but just came away confused.

The Soviet study aside, at least the Air Force studies were consistent in their conclusions that, even if the voice frequency were increased, intelligibility was more influenced by background noise, and crewmembers learned to modify their speaking styles to minimize the effects of the helium, the noise and the lower cabin pressure.

What’s more, the frequencies in the MOL pilots’ voices would be increased only 9% compared to air, much less than that observed in the divers (see table below) (ref. 11). The MOL pilots would have been breathing almost exactly the opposite ratio of the gases used by deep sea divers whose voice quality derives from the higher proportion of helium with its low gas density and high velocity of sound.

So, no chipmunks or Donald Duck on MOL.

The pitch of the MOL pilots' voices would have been determined by the
relative frequency of sound in the cabin's 70% oxygen, 30% helium atmosphere,
about 9% higher than in normal air. This is less of an increase than
under cabin atmospheres that were considered, and much less than
deep-sea divers breathing heliox.

  1. Cooke, J.P., and S.E. Beard, “Verbal Communication Intelligibility in Oxygen-Helium, and Other Breathing Gas Mixtures, at Low Atmospheric Pressures,” Aerospace Medicine, 36(12): 1167-1172, Dec. 1965.
  2. “LBJ & the Helium Filled Astronaut,” Produced by Larry Massett, (accessed 22 Mar 2006). The NPR website incorrectly listed the date as 1964, but it was September 27, 1965. See also Carpenter, S., and K. Stoever, For Spacious Skies (Orlando: Harcourt, Inc., 2002), p. 321.
  3. Speed of sound formulas, (accessed 14 Aug. 2012).
  4. “Speed of sound - temperature matters, not air pressure”, (accessed 21 Aug. 2012)
  5. Nunneley, S.A., and J.T. Webb, “Aerospace Medicine at Brooks AFB, TX: Hail and Farewell,” Aviation, Space and Environmental Medicine 82(5, sec. 1): 567-570, May 2011.
  6. Nixon, C.W., and H.C. Sommer, “Subjective Analysis of Speech in Helium Environments,” Aerospace Medicine 39(2):  139-144, Feb. 1968.
  7. Barry, S.J., and J.E. Endicott, “Comparison of Speech Materials Recorded in Room Air at Ground Level and in a Helium-Oxygen Mixture at a Simulated Altitude of 18,000 Feet,” Aerospace Medicine 40(4): 368-371, Apr. 1969.
  8. Nixon, C.W., W.E. Mabson, F. Trimboli, J.E. Endicott and B.E. Welch, “Observations on Man in an Oxygen-helium Environment at 380 mm. Hg Total Pressure: IV. Communications,” Aerospace Medicine, 39(1): 1-9, Jan. 1968.  
  9. Nixon, C.W., W.E. Mabson, F. Trimboli and B.E. Welch, “Study of Man During a 56-Day Exposure to an Oxygen-helium Atmosphere at 258 mm. Hg Total Pressure: XIV: Communications, Aerospace Medicine 40(2): 113-123, Feb. 1969
  10.  __, “Helium Test,” Aviation Week & Space Technology, Nov. 23, 1964, p. 25.
  11. This conclusion is drawn from calculations based on information in two sources: Hess, D.R., J.B. Fink, S.T. Venkataraman, I.K. Kim, T.R. Myers and B.D. Tano, “The History and Physics of Heliox”, Respiratory Care, 51(6): 608-612, 2006, (accessed 7 Aug. 2012); and Speed of Sound in a Gas (accessed 7 Aug. 2012).

Sunday, November 4, 2012

A Jones for MOL #4: He for two, and two for He.

This was supposed to be an easy blog entry to write. A few months ago, I was wondering whether the two pilots really would have sounded like chipmunks in the MOL’s atmosphere of 70% oxygen and 30% helium at 5 pounds per square inch (psi) (see * endnote below). I Googled for some basic information on inspired gas composition and voice quality, and thought I had enough for a short essay. But a few questions presented themselves, and I didn't know the answers: why was a diluent gas added to what started out as a 100% oxygen spacecraft environment? Why was helium (the “He” in the title) chosen? And why was 5 psi even in the discussion?

Last question first: MOL was initially planned to use pure oxygen at 5 psi because Gemini was designed to use it—and eventually did use it without problems in ten manned flights in 1965-1966; Gemini used it because Mercury had used it without problems in six flights in 1961-1963. The Mercury spacecraft was designed by NASA in 1958 and built by McDonnell Aircraft Company in 1959 using pure oxygen at 5 psi because the required life support system was relatively lightweight. Gemini had started out as a two-man “Mercury Mark 2” to be built by McDonnell, so many design decisions were just rolled over from Mercury to Gemini to minimize the costs. Ground studies had shown no pulmonary problems in up to 30 days of 5 psi of pure oxygen (ref. 11). Thus, there was previous flight and ground experience in terms both of health and of knowledge of structure and systems design behind the selection of this low pressure.

But why was 5 psi even chosen for Mercury? If the goal was to maintain oxygen partial pressure (designated “pPO2“ in the short-hand of physiologists that I learned in grad school) in the lungs at sea-level values, why not use the value adopted by Mother Nature? Oxygen makes up 21% of our 14.7-psi atmosphere for a pPO2 of 3.1 psi. Why did Mercury adopt a cabin pressure 1.9 psi (61%) higher than found at sea level?

Part of the answer seems to come in a NASA report written by John Kimzey at NASA’s Manned Spacecraft Center in early 1966 (ref. 9). First, pPO2 had to be greater than 2.8 psi to prevent hypoxia (too little oxygen for respiration) but less than 7.3 psi to prevent oxygen toxicity. Oxygen toxicity included such problems as pulmonary atelectasis (a type of lung collapse), decreased hematopoiesis (production of red blood cells), visual effects, seizures, nausea, tinnitus (ringing in the ears), dizziness, lightheadedness, retching, parasthesias (uncomfortable skin sensations) and poor concentration (ref. 10).

Second, the total pressure must be such that during a rapid decompression from cabin pressure to emergency spacesuit pressure, a maximum pressure-change ratio should be less than 2 to 1. Since 1960s-era space suits restricted their wearers’ mobility unacceptably when they were pressurized to more than 3.5 psi (ref. 8) above the external environment—namely, a vacuum, in the case of a major spacecraft leak in orbit—a cabin pressure of no more than 7 psi was required. 

Kimzey didn't say why that led to a pressure of 5 psi rather than, for example, 6 psi but I think it may have been because it was uncomfortably close to the oxygen toxicity value of 7.3 psi, allowing only 5% possibility for error in the sensors and gas supply system. A cabin pressure of 5 psi almost exactly split the difference between the upper limit of 7.3 psi and the lower limit of 2.8 psi necessary for life. This argument also worked in connection with planned—not accidental—decompressions, to permit extravehicular activity (EVA), or space walks.

In addition to health concerns, there were considerations of fire safety of materials used inside the cabin which apparently motivated the Air Force to evaluate use of mixed gases a instead of pure oxygen (ref. 8). Man-rating a pure oxygen atmosphere even at 5 psi would drastically limit material choices when considering toxic contamination and flammability (ref. 9). 

As noted by an Aerospace Corporation report (ref. 3), MOL’s mixed gas atmosphere would also have reduced the risk of an in-flight fire. This was a fortuitous benefit in the post-Apollo fire time frame  The loss of three NASA astronauts in 1967 in a pre-launch ground test of an Apollo capsule (ref. 1) pressurized with 100% oxygen to 16 psi had literally re-ignited the debate over NASA’s choice of a 5 psi pure oxygen environment for its Mercury, Gemini and Apollo series vehicles. Draconian steps were taken to fireproof the Apollo command module to allow NASA to retain the relatively simple and lightweight hardware of the 100% oxygen environment. Gemini-B had already demonstrated its compatibility with pure oxygen and thus could accommodate the MOL’s less fire-supportive oxygen-helium mixture, but MOL was not planned to be fire-proofed for a 100% oxygen environment.

By November 1964, the Air Force was debating the relative merits of a one-gas system, 100% oxygen at 5 psi, and a two gas system, 40% oxygen and 60% nitrogen at 7 psi (ref. 4). That same month, it also conducted its first manned tests of a third variant: half an atmosphere (7.8 psi) at 50% oxygen and 50% helium (ref. 2). Another study compared the decompression sickness risks of mixed oxygen-nitrogen and oxygen-helium atmospheres at 7 and 5 psi, with oxygen maintained at 3.5 psi (ref. 11).  

In early January 1965, the Air Force had not yet decided whether to fly MOL with an environment of pure oxygen or mixed gas (ref. 5). But during 1965, a major study of the physiological and psychological effects of 56 days in a chamber built to simulate the MOL on four aircrewmen tested only the 5 psi 70% oxygen 30% helium atmosphere—not pure oxygen and not oxygen-nitrogen (ref. 8). Thus, it appears that the decision on the oxygen-helium cabin atmosphere for MOL had been made by that time. 

But why helium as the diluent gas? Helium had never been used in a spacecraft cabin before MOL adopted it, and it has never been used since MOL. The Soviet Union evaluated a helium-rich atmosphere for future space vehicles (ref. 2) but every Soviet and Russian spacecraft type from Vostok to Mir successfully used a cabin environment very close to Earth’s atmosphere at sea level, as did the U.S. Space Shuttle. Both the Russian and U.S. segments of the International Space Station (ISS) are baselined for a sea-level atmosphere. While Mercury, Gemini and Apollo used pure oxygen at 5 psi, after 1967 Apollo was pressurized with 60% oxygen, 40% nitrogen on the launch pad to minimize the chance of another fire (ref. 10). Skylab later used an atmosphere of 70% oxygen and 30% nitrogen at 5 psi to provide the same benefit (ref. 6).

Helium was selected apparently because it was relatively inert chemically, was insoluble in blood or retained in the blood like oxygen to prevent bubbles when environmental pressure is reduced, and suppressed combustion, and in spite of the facts that helium was not trouble free from the standpoint of vocal communication and did not have a small diffusive leak rate through seals and gaskets (thus satisfying only three of the five criteria established by Kimzey for inert gases as atmospheric diluents in spacecraft) (ref. 9). However, a study of 50-50 gas mixtures found no greater protective effect of oxygen-helium than for oxygen-nitrogen during decompressions from likely cabin atmospheres (ref. 7). 

Somewhere out there is a study that supports 70% oxygen-30% helium at 5 psi, but I have not seen it.

EVA was an early requirement for the MOL research program, to evaluate the pilots’ capabilities for military tasks outside the spacecraft (ref. 12). EVA was also an early option for transfer between the Gemini-B and the MOL pressurized module, before the decision was made to implement the heat shield hatch. It is not clear that the program maintained major EVA requirements after 1965, when MOL was refocused from a testbed for military astronaut capabilities to a reconnaissance test platform. But the weightless transfer tests on the KC-135 in 1966 described here included parabolas with the pilots wearing what appear to have been Gemini G4C EVA suits made by The David Clark Co. The new MOL-specific space suits ordered from Hamilton-Standard in 1967 and delivered in 1968—just a year before the program was cancelled—also had EVA capability (ref. 12). Therefore, EVA requirements influenced MOL planning and design throughout the program’s lifetime.

As an aside, if EVA were not even a contingency requirement, it might have made more sense to outfit the MOL pilots with the light-weight G5C space suits worn on Gemini 7, which I blogged about previously. They could have been doffed and donned even in the confines of the Gemini-B cabin, and would have made it much easier to pass through MOL’s narrow hatchways and long tunnel while still providing the necessary backup to the cabin pressurization system during launch and early orbital flight.

Anyway, for a variety of reasons, after mid-to-late 1964, helium was in consideration as the diluent gas for the MOL pressurized laboratory, and after mid-1965, it appears that the decision had been made to use helium.

My wife told me not to make these articles so long—after all, they’re just blogs—so I will leave the discussion of voice quality in MOL’s oxygen-helium atmosphere to the next one.


1. __, Apollo 1, (accessed 1 Oct. 2012).

2. __, “Helium Test,” Aviation Week & Space Technology, Nov. 23, 1964, p. 25.

3. __, “MOL Safety Evaluation Based on Apollo 204 Review Board Findings and Recommendations and Brooks Air Force Base Accident Investigation Board Conclusions”, September 1967, prepared by MOL Systems Engineering Office, Aerospace Corporation, for Deputy Director, Manned Orbiting Laboratory Program, MOL System Program Office, OSAF Headquarters, Space Systems Division, Los Angeles, California 90045.

4. __, “Six Astronauts Picked for MOL; Assignment to Specialties Planned,” Aviation Week & Space Technology, Nov. 23, 1964, p. 25.

5. __, Weekly Activities Report (For Week Ending 15 January 1965), Assistant for Manned Space Flight, MOL Program, courtesy of Dr.  Dwayne Day.

6. Bacal, K., G. Beck and M.R. Barratt, “Chapter 22: Hypoxia, Hypercapnia, and Atmospheric Control,” in Barratt, M.R., and S.L. Pool (eds), Principles of Clinical Medicine for Space Flight (New York: Springer, 2008).

7. Beard, S.E., T.H. Allen, R.G. McIver and R.W. Bancroft, “Comparison of Helium and Nitrogen in the Production of Bends in Simulated Orbital Flights,” Aerospace Medicine 38(4): 331-337, April 1967.

8. Hargreaves, J.J., W.G. Robertson, F. Ulvedal, H.J. Zeft and B.E. Welch, “Study of Man During a 56-day Exposure to an Oxygen-Helium Atmosphere at 258 mm. Hg Total Pressure I. Introduction and General Experimental Design,” Aerospace Medicine, 37(6): 552-555, June 1966.

9. Kimzey, J.H., Flammable and Toxic Materials in the Oxygen Atmosphere of Manned Spacecraft, NASA TN D-3415, NASA, May 1968, (accessed 30 Sep 2012).

10. Michel, E.L., J.M. Waligora, D.J. Horrigan and W.H. Shumate, “Chapter 5, Environmental Factors,” in Johnston, R.S., L.F. Dietlein and C.A. Berry (eds.), Biomedical Results of Apollo (Washington, D.C.: NASA, 1975), (accessed 4 Nov. 2012).

11. Robertson, W.G., J.J. Hargreaves, J.E. Herlocher and B.E. Welch, “Physiologic Response to Increased Oxygen Partial Pressure II. Respiratory Studies,” Aerospace Medicine, 35(7): 618-622, July 1964.

12. Thomas, K.S., and H.J. McMann, US Spacesuits (Chichester, UK: Springer-Praxis, 2006), “Chapter 8: U.S. Air Force spacesuits,” pp. 179-193; see also the discussion of the Integrated EVA (IEVA) suit from Hamilton Standard contracted by the U.S. Air Force in 1967, in Harris, G.L., The Origins and Technology of the Advanced Extravehicular Space Suit (San Diego: Univelt, Inc., 2001), “Chapter 4: The Apollo Application Program Era,” pp. 201-2.

Sunday, September 16, 2012

A Jones for MOL #3: Down the Hatches!

Any launch from a standing start into low Earth orbit has to be an exhilarating experience by itself. It can also prompt physiological responses. During a MOL launch, the Titan-IIIM booster would have leapt off the pad with the ignition of the paired solid rocket boosters flanking the first stage, the acceleration load increasing as the solid fuel was burned and the mass of the stack decreased. When the exhausted solids were jettisoned and the first stage continued to burn its fuel, acceleration would have built up steadily again, then dropped to zero at staging, and almost immediately resumed and built up again as the second stage accelerated the vehicle into orbit, only to drop immediately to zero again at burn-out about 8½ minutes after launch (ref. 1). But for the now-weightless MOL pilots that would have been only the start.

One of the pilots, probably the co-pilot on the right side of the cockpit, would have unstrapped from his ejection seat and turned around in a volume literally smaller than a 1960’s telephone booth (ref. 2). Facing the aft bulkhead of the capsule, he would have unsealed and stowed the 24-inch (61-cm) large hatch in the capsule’s pressure bulkhead, and after that, the similarly-sized heat shield hatch (see Figure 1).

Figure 1. View of the two crew transfer hatches in Gemini-B, the heat shield hatch installed and the large pressure bulkhead (LPB) hatch stowed. Credit: McDonnell Douglas Aircraft Corporation, courtesy of Dr. Dwayne Day.
With the two hatches stowed, the co-pilot, still wearing his bulky MH-8 pressure suit (ref. 3), would have squeezed through the slightly wider tunnel, 31 inches (79 cm) in diameter and 11 feet (3.3 m) long, passing through the Gemini-B adapter module and then the MOL’s unpressurized equipment compartment finally to arrive inside the pressurized laboratory module, the pilots’ home for the next 30 days.

This procedure for moving from the Gemini-B capsule to the MOL was evaluated in brief weightlessness during parabolic flight aboard an Air Force KC-135 in March 1966. Project engineers went through all the motions while wearing Gemini-style space suits (ref. 4), both unpressurized and pressurized, and then repeated them again in light-weight aviators’ flight suits (ref. 5). At least three of the Aerospace Research Pilots assigned to MOL repeated the process that same month (ref. 6) also wearing Gemini-style suits. Each test run occurred in discrete 25-sec. steps, because that was the duration of the parabolas that the KC-135 could fly. As might be guessed, two men could make the transfer easily in a single parabola while in flight suits, but it took several parabolas for just one man to struggle through it with his suit pressurized and stiff.

In those early days of spaceflight, the problem of space motion sickness was suspected but not yet demonstrated.  But if anything would have provoked it, the MOL gyrations would surely have done so.

Provocative or not, such gyrations were not unprecedented. Wally Schirra, on his Gemini 6 flight, reported getting completely out of his ejection seat on the left side of the cockpit and turning around to close the balky door on the storage compartment between the two seats’ headrests (ref. 7). That storage compartment was to be replaced by the third hatch on Gemini-B, so Schirra unintentionally evaluated the first transfer step in flight. Other Gemini astronauts must also have moved around in their small cabins—probably like shifting in your economy-class airline seat—maybe just to experiment with weightlessness, but probably to unzip their space suits around the crotch and a little way up the back to use the plastic bags provided for calls of nature (ref. 8).

That there were two separate hatches on the Gemini-B is not widely understood. People have seen the circular cut through the heat shield on the reused Gemini 2 capsule tested in re-entry in 1966 and the circular hatch on the back wall of the unflown Gemini test capsule in the National Museum of the U.S. Air Force (see Figure 2), and have assumed, logically enough, that those were front and back views of the same hatch. However, MOL documentation acquired from the Maxwell Air Force Base archives in 1999 by Dr. Dwayne Day shows that there were two separate hatches (see Figure 1 again). Even watching the films from those KC-135 flights (ref. 5) doesn’t make the distinction obvious: opening and closing the heat shield hatch are plain enough when viewed from inside the transfer tunnel, but manipulation of the large pressure bulkhead hatch is almost completely obscured by the faux pilot in the cramped Gemini cabin space.

Figure 2. Heat shield hatch in Gemini 2 capsule on display at NASA Kennedy Space Center visitor center (top left) and in unflown Gemini-B test capsule at National Museum of the U.S. Air Force (top right). Large pressure bulkhead hatch of the Gemini-B test capsule (bottom right) in the same location as the storage compartment in the standard Gemini (here shown in the Gemini Mission Simulator). Photo credits: NASA (top and bottom left), John Charles (top and bottom right).
But the evidence is ambiguous as to whether there was another sealable hatch between the Gemini-B heat shield and the interior of the lab module. A set of watercolor illustrations of typical MOL scenes, probably commissioned by the McDonnell-Douglas Aircraft Company, MOL's manufacturer, in late 1967, shows a hinged hatch at the laboratory end of the tunnel in one view but not in another (see figure 3). However, there is no such hatch in the full-scale mockup flown on the KC-135 or in a photo of the full-scale high-fidelity mockup of the MOL’s interior made by McDonnell-Douglas (see figure 4) at about the same time (ref. 9). Richard Truly, one of the MOL pilots who trained on the KC-135 mockup in March 1966, clearly remembers the two heat shield hatches he struggled with during those parabolas, but is not sure about another hatch at the other end of the tunnel.
Figure 3. Two watercolor images circa late 1967 illustrating the MOL pilots’ initial entry into the laboratory module. Notice the presence of a hinged hatch at the lab end of the transfer tunnel in the panel on the right, but the absence of any such hatch in the panel on the left. The circular feature behind the pilot in the left panel is not in the same position as the hinged hatch in the right panel, and is thus probably not meant to represent it. Credit: McDonnell Douglas Aircraft Co.
At least, the mockups don’t show signs of a permanent, hinged hatch, unlike the watercolors. But that does not mean there wasn’t a removable hatch there. The two Gemini-B hatches described earlier were both removable. In addition, the Apollo command module had a removable plug hatch at the top of its docking tunnel, so the concept was neither unprecedented nor unknown to the MOL designers.
Figure 4. Views of two MOL mockups showing no evidence of a tunnel hatch. Top left shows cut-away mockup used on KC-135 parabolic flight testing of transfer from Gemini-B (left end) through tunnel to where MOL would have been (right end). Bottom left view is 90-degree rotation of top left view. Right panel shows view up from MOL floor toward top of module, with tunnel entry near left center. (Mark-ups on original.) Credits: McDonnell-Douglas Aircraft Co., courtesy of Dr. Dwayne Day (top left), Spacecraft Films (bottom left) and Andrew Cochrane (right). 
Apollo’s forward docking hatch is similar to what might have been a removable MOL tunnel hatch. It was 30 inches (76 cm) in diameter and weighed 80 pounds (36 kg). It was latched in six places and operated by a pump handle. At the center was a pressure equalization valve, to match the pressures in the tunnel and lunar module before the hatch was removed (ref. 10).

Something like the Apollo plug hatch sounds ideal to seal off the tunnel, but there is no evidence for its existence in any of the available source documents. The KC-135 tests, which analyzed all the steps in the transfer from Gemini-B to MOL and back, did not include such a hatch, either hinged or removable. Clearly such a hatch was not part of the flight design, at least not in March 1966.

In the absence of any evidence to the contrary, I conclude that there was no independent pressure-seal capability for the transfer tunnel: the tunnel would have been open from the Gemini-B heat shield all the way to the interior of the MOL habitable volume.

This is not unique in spaceflight: the Spacelab modules carried in the payload bay on 15 Space Shuttle flights (ref. 11) were connected by a long transfer tunnel to the Shuttle’s crew compartment, and there was no hatch at the Spacelab module opening (ref. 12).  Gemini-B/MOL and Shuttle/Spacelab were admittedly different vehicles, but seeing the same unique feature in different spacecraft reassures me that it was real and not an oversight on my part.

By now you might be thinking that all of this is akin to calculating how many angels may dance upon the head of a pin. But the absence of a hatch at the lab end of the transfer tunnel is important because it leads directly to the the very topics that originally inspired this blog series, and which will be covered in upcoming posts.


1. Estimated from “Titan III Typical Flight Sequence” (p. 278), in Isakowitz, S.J., International Reference Guide to Space Launch Systems (Washington, D.C.: AIAA, 1991), pp. 263-80.
2. “How Many in a Phone Booth?” no writer attributed, The Harvard Crimson, July 23, 1959, (accessed 30 Aug. 2012). The article gives the volume of a phone booth as 63 ft3 (1.8 m3); Gemini cabin volume is widely reported as 95 ft3 (2.5 m3), for both pilots.
3. Thomas, K.S., and H.J. McMann, US Spacesuits (Chichester, UK: Praxis Publishing, 2006), Chapter 8: U.S. Air Force spacesuits.
4. Astrospies, PBS Nova, excerpt starting approximately 8:30 elapsed, (accessed 26 Aug. 2012).
5. “MOL zero gravity testing, McDonnell photography, Wright-Patterson AFB, March 1966,” on CD: Man in Space, U.S. Air Force manned space projects, Spacecraft Films, 2007,
6. Truly, Richard, personal communication (email), to John Charles, June 29, 2012, recalled flying on the KC-135 with his friends Michael Adams and Jack Finley on March 24-25, 1966, at Wright-Patterson AFB.
7. Schirra, W.M., and T.P. Stafford, “Gemini VI Technical Debriefing, Dec. 20, 1965,” reproduced in Godwin, R., Gemini 6, The Mission Reports (Burlington, Ont., Can.: Apogee Books, 2000), p. 113, 117.
8. Kemmerer, W.W., and J.W. Morar, A Review of Spacecraft Waste-management Systems, NASA TM X-1851 (Washington, D.C.: NASA, 1969). See figure 10; (accessed 16 Sep. 2012). Poor quality document reproduction observed.
9. Cochrane, Andrew, MOL Update #4, July 6, 2012, (accessed 20 Aug. 2012).
10. Apollo Command/Service Module, (accessed 30 Aug. 2012)
11. Spacelab Payloads on Shuttle Flights, (accessed 16 Sep. 2012).
12. Spacelab News Reference, NASA Marshall Space Flight Center, document number 14M983   (undated, ca. 1980), Sec. 3.2.2. Tunnel Systems, p. 3-19, (accessed 29 Aug. 2012).

Monday, September 3, 2012

A Jones for MOL #2: 2 Cool 2 B 4 Gotten.

What is it about MOL, the U.S. Air Force’s 1960s “Manned Orbiting Laboratory” program (fig. 1), which keeps reeling me back in? Announced by Defense Secretary McNamara in 1963, endorsed by President Johnson in 1965 and cancelled by President Nixon in 1969, MOL was the unflown but extensively-planned rudimentary Earth-orbiting space station and reconnaissance test bed. By 1965, the man-in-space research on MOL was already being downplayed in favor of detailed Earth observations using the secret KH-10 “Dorian” telescope to be attached to the MOL (1).
Figure 1. USAF Manned Orbiting Laboratory configuration in 1967.

I have recently blogged here about nomenclature, specifically, why the Air Force gave its highly-specialized piloted reconnaissance test bed such a non-specific title. In doing so, the Air Force simultaneously co-opted previously developed concepts that had used that same bland title into its military program and pre-empted future actual space laboratories from using it. (I have retroactively designated that post as #1 in this series.)

MOL would have used modified Gemini capsules for crew transportation in an era when NASA had moved on from Gemini to Apollo to carry astronauts to the Moon. These so-called “Gemini-B” capsules were to have several important but easily overlooked differences—simplifications, mostly—but one very dramatic complication: they would have had big holes in their heat shields.

I have also blogged here about holes in the heat shields of space vehicles, and why they have been more common and less worrisome than you might imagine. The Gemini-B crew capsule would have had such a hole in the heat shield for its third hatch. Hatches 1 and 2 were the two large hinged doors, sized to permit ejection on rocket-propelled seats if necessary, and through which the pilots entered the capsule on the launch pad (fig. 2) and exited after splashdown (fig. 3).
Figure 2. Large ingress-egress hatches on Gemini 4, after astronaut ingress before launch (credit: NASA).
Figure 3. Large ingress-egress hatches on Gemini 9, during astronaut egress on the aircraft carrier deck after splashdown (Credit: NASA).

This third hatch is probably the most well-known modification of the basic Gemini design for the MOL mission. It was really in two parts: a removable “crew transfer hatch” (fig. 4) which sealed the 24-inch (61 cm) diameter pass-through in the large pressure bulkhead—to maintain air pressure inside the capsule—and a separate “heat shield hatch” (fig. 5), the same size and also removable, which plugged the corresponding hole in the actual heat shield—to keep the 3,000-degree F (1650 degree C) plasma (2) safely outside the capsule during re-entry into Earth’s atmosphere at the end of the mission. Many people assume these two hatches were one and the same, but they weren't.
Figure 4. “Third hatch” on Gemini-B. Bottom diagram shows the crew transfer hatch (labelled “Heatshield Hatch”--a common mistake) inside the crew cabin, where the storage compartment was located on legacy Gemini capsules (credit: John Fongheiser). 
Figure 5. “Third hatch” on Gemini-B. Actual heat shield hatch on exterior of re-entry vehicle of the unflown Gemini 3A capsule in the National Museum of the U.S. Air Force (credit: Steven Jones).

As an Air Force spy satellite project, MOL was heavily classified, but as a manned program in the early “heroic” days of space exploration, there was inevitably much publicity in which some information was released, and much speculation in the absence of more information. This led to a tension between public curiosity and operational secrecy, both then and now. In recent years, many aspects of MOL have been declassified, but others remain secret. The men selected as MOL’s “Aerospace Research Pilots”(3) seem eager to discuss what they knew way back then, after long and successful aerospace careers following its cancellation in June 1969. They tread carefully lest they reveal still-secret topics, but several of them spoke on camera and on the record in “Astrospies”, an episode of the PBS series NOVA which aired in February 2008. Astrospies was an hour-long documentary on MOL and its Soviet counterpart based on work by investigative journalist James Bamford (4).

Shortly thereafter, Dr. Dwayne Day published some clarifications of the Astrospies script as well as a good listing of previous MOL-related publications already in the public domain (5). He has researched and published extensively on spy satellites, and had collected much of the available documentation on the project during a visit to the Air Force’s archives at Maxwell AFB in Alabama in late 1999.

Dr. Day (6) and Scott Lowther (7) have posted sales presentations given by the MOL’s manufacturer, the Douglas Aircraft Co., to NASA in April 1967 and May 1968. (Douglas was merged with McDonnell Aircraft Company, builders of the Mercury and Gemini spacecraft, in April 1967, but retained responsibility for MOL.) Douglas tried to interest NASA in some or all of MOL’s components for the agency’s pre-Skylab version of its first space station. Presumably these declassified charts and diagrams accurately reflect the habitable portions of MOL: Douglas would probably have tried to market the same mature design to a different customer, because redesigning for a possible (but unlikely) NASA sale would not have been cost-effective. Also, the secret aspects of the program had more to do with the reconnaissance payload at the aft end of the MOL stack than with the cramped living quarters for the pilots.

In addition to his other contributions, I owe Dr. Day thanks for the title of this blog series. In addressing some on-line comments about the difference in the progress of records declassification between MOL and unmanned spy satellites, he wrote: “Despite the fact that lots of people have a jones for MOL, it never actually flew, meaning that it had little impact upon the Cold War” (8).

During the 43 years since the project was cancelled, enough has remained secret that those of us with a “jones” for MOL must supplement documentation with deduction or even imagination.

One long-standing question has been the orientation of the Gemini-B/MOL stack in orbit, while photographing Russian military secrets on Earth: was the 70-inch primary mirror of the powerful telescope aimed out the back end of the MOL, or through an aperture in the bottom side of the cylindrical mission module? From a human factors perspective, the question was whether the Gemini-B capsule was pointed up into space away from the Earth, so that its two forward-facing cockpit windows would have viewed only deep space while the telescope was in use, and its attitude was stabilized by the gravity-gradient effect. Or was it pointed forward along the line of flight, so that it flew nose-forward like a bullet or an airplane, requiring regular thruster firings to maintain its attitude parallel to Earth’s surface? Artists’ concepts and models have supported both attitudes, but a recently declassified document (9) makes it clear that images were to be obtained through a large opening in the side, meaning that the Gemini-B/MOL stack flew along its orbital path like an airplane (fig. 6).
Figure 6. Planned operational attitude of MOL during photo-reconnaissance activities (credit: Giuseppe De Chiara; available online in “MOL Unofficial Presentation,”, 15 Aug. 2012,;all, accessed 24 Aug. 2012).

In other topic areas, we MOL-jonesers have made some arbitrary and literally unsupported assumptions about MOL and its capabilities.

One assumption was that there would have been an on-going program of MOL flights, including clusters of modules linked for specific reconnaissance purposes. This is based on artists’ concepts showing such configurations (fig. 7). In fact, the approved program was based on independent flights of 7 modules, one after another: 2 unmanned test flights followed by only 5 piloted missions (10). These would have tested various capabilities for future manned and unmanned reconnaissance systems, but MOL itself was only a test bed, not an extensible architecture (11). Every American and Russian manned spacecraft series has inspired plans for such growth concepts, but—with the notable exceptions of Apollo begetting Skylab and the Apollo-Soyuz mission, and a few one-off Soyuz missions—these concepts have usually gone unrealized.
Figure 7. Conceptual growth application of MOL: deployable antenna for signals intelligence or radar imaging (Credit: U.S. Air Force).

Another assumption was that the MOL stations could have been revisited and reused (fig. 8). The MOL itself was a limited life item, not reusable (12). After its 30-day useful lifetime, it was to be expended. No future Gemini-B vehicles would have been launched to dock with an orbiting MOL and reoccupy it. This was part of the reason that the NRO was not enthusiastic about MOL: its sophisticated telescope system was only to be available for a short time.
Figure 8. Conceptual growth application of MOL: clustered MOL modules, with one module apparently ready to receive a replacement Gemini-B shown being boosted from Earth (Credit: U.S. Air Force).

This limited life was reflected in the means of attaching the Gemini-B to the MOL. The Gemini-B was to be launched physically bolted to MOL, and the pressurized transfer tunnel was to be sealed by a spring-loaded compression gasket pressed hard against the Gemini-B heat shield at one end (fig. 9), and apparently attached to a flange on the MOL at the other end. There was no reusable docking system for future visits: once the bolts were fractured and the airtight seal permanently broken, the MOL could not have been reoccupied and would have been left to whatever limited capabilities it had without men aboard, perhaps only able to deorbit itself under ground control to avoid somehow falling into the Soviets’ hands.
Fig. 9. Detail of crew transfer tunnel seal to Gemini-B heat shield using a spring-loaded flexible seal (credit: McDonnell-Douglas Aircraft Co.).

As I have learned more about MOL, I have come to understand how ambitious it was for its day, and how restricted it would have been in practice. When it was conceived and planned in 1963, astronauts were flying missions of only a few hours, and then a few days, duration. Re-entry at orbital velocities was a still a challenge, recently but imperfectly demonstrated by Mercury and Vostok capsules, and not without incident (13). Proposing to depend on astronauts performing highly sophisticated tasks adequately for a whole month in weightlessness, and then to re-enter using a heat shield with a hole in it, took organizational courage and self-confidence.

MOL was an Air Force project, but orbital reconnaissance was a responsibility of the National Reconnaissance Office (NRO), and by 1966 it was on a path to consolidating its control over all such efforts (9). NRO was already using unmanned satellites and was developing plans to fly MOL unmanned, literally taking the man out of the MOL (fig. 6). That agency would have preferred not to involve MOL pilots selecting ground targets for targets of opportunity for high-resolution photography, when spy satellites were already taking large volumes of targeted photographs (1). In addition, when not actively spying, the off-duty pilots’ movements in MOL would likely jiggle the telescope, blurring the photos.

Unlike other “MOL-jonesers,” my primary interest is in other end of the MOL stack, where the human factors would have taken place. They will be the topic of future blog posts.

1. Day, D.A., “All along the watchtower,” The Space Review, Feb. 11, 2008, (accessed 3 Sep. 2012).
2. Gemini Program Mission Report, GT-3, Gemini 3, MSC-G-R-65-2, NASA Manned Spacecraft Center, April 1965, p. 5-4.
3. “U.S. Air Force Manned Orbiting Laboratory Program,” Fact Sheet, United States Air Force, March 1968.
4. “Astrospies”, PBS NOVA, first U.S. broadcast Feb. 12, 2008, (accessed 4 Aug. 2012).
5. Day, D.A., “Astrospies, corrected”, The Space Review, April 14, 2008, (accessed 4 Aug. 2012).
6. Day, D.A., Gemini for MOL and Circumlunar,, (accessed 20 Aug. 2012).
7. Gordon, T.J., McDonnell Douglas Aircraft Cp., PSAC Briefing, “MOL for NASA”, July 20, 1968, offered by Scott Lowther at (accessed 3 Jan. 2011), password required.
8. Day, D., Re: MOL Unofficial Presentation, 15 Aug. 2012,;all (accessed 24 Aug. 2012).
9. Semi-Annual Report to the Presidents [sic] Foreign Intelligence Advisory Board on the Activities of the National Reconnaissance Program, 1 November 1965 – 30 April 1966, (accessed 15 July 2012).
10. Normyle, W.J., “Five Orbital Shots Planned in MOL Tests,” Aviation Week & Space Technology, Sep. 12, 1966, pp. 33-4.
11. “U.S. Air Force Manned Orbiting Laboratory Program,” Fact Sheet, United States Air Force, March 1968 (c/o Don Pealor).
12. “30 Day Orbit of MOL Key to AF Space Effort,” Air Force Times, June 21, 1967.
13. Glenn’s Mercury capsule re-entry was marked by concern about a detached heat shield; Carpenter’s, by an alignment error that sent him 250 miles past his targeted splashdown site. Gagarin’s landing capsule failed to separate cleanly from its instrument compartment, causing unplanned and terrifying gyrations during re-entry.