Project Gemini

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Project Gemini
GeminiPatch.png
Duration 1962-1966
Goals Long-duration spaceflight; rendezvous and docking; extra-vehicular activity; targeted re-entry and Earth landing
Achieved Eight-day flight necessary for Apollo; 14-day endurance flight; first American spacewalk; first rendezvous; first docking; demonstrated ability to work in EVA without tiring
Crew 2
Vehicles Launch: Titan II GLV
Other: Agena, docking target
Organizer NASA
Related programs Mercury and Apollo

Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. Project Gemini was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966.

Its objective was to develop space travel techniques in support of Apollo, which had the goal of landing men on the Moon. Gemini achieved missions long enough for a trip to the Moon and back, perfected extra-vehicular activity (working outside a spacecraft), and orbital maneuvers necessary to achieve rendezvous and docking. All Gemini flights were launched from Cape Canaveral, Florida using the Titan II Gemini launch vehicle ("GLV").1

Program objectives

After the existing Apollo program was chartered by President John F. Kennedy on May 25, 1961 to land men on the Moon, it became evident to NASA officials that a follow-on to the Mercury program was required to develop certain spaceflight capabilities in support of Apollo. Originally introduced on December 7 as Mercury Mark II, it was re-christened Project Gemini on January 3, 1962, from the fact that the spacecraft would hold two crewmen, seated abreast, as gemini in Latin means "twins" or "double". Gemini is also the name of the third constellation of the Zodiac and its twin stars, Castor and Pollux.

The major objectives were:2

  • To demonstrate endurance of humans and equipment to spaceflight for extended periods, at least eight days required for a Moon landing, to a maximum of two weeks
  • To effect rendezvous and docking with another vehicle, and to maneuver the combined spacecraft using the propulsion system of the target vehicle
  • To demonstrate Extra-Vehicular Activity (EVA), or space-"walks" outside the protection of the spacecraft, and to evaluate the astronauts' ability to perform tasks there
  • To perfect techniques of atmospheric reentry and landing at a pre-selected location
  • To provide the astronauts with zero-gravity, rendezvous, and docking experience required for Apollo

Team

The two-man Gemini capsule was designed by a Canadian, Jim Chamberlin, formerly the chief aerodynamicist on the Avro Arrow fighter interceptor program with Avro Canada. Chamberlin joined NASA along with 25 senior Avro engineers after cancellation of the Arrow program, and became head of the U.S. Space Task Group’s engineering division in charge of Gemini. The prime contractor was McDonnell Aircraft, which had also been the prime contractor for the Project Mercury capsule.

In addition, astronaut Gus Grissom was heavily involved in the development and design of the Gemini spacecraft (the other Mercury astronauts dubbed the Gemini spacecraft the "Gusmobile").3 Grissom writes in his posthumous 1968 book Gemini! that the realization of Project Mercury's end and the unlikelihood of his having another flight in that program prompted him to focus all of his efforts on the upcoming Gemini program.

The Gemini program was managed by the Manned Spacecraft Center, Houston, Texas, under direction of the Office of Manned Space Flight, NASA Headquarters, Washington, D.C. Dr. George E. Mueller, Associate Administrator of NASA for Manned Space Flight, served as acting director of the Gemini program. William C. Schneider, Deputy Director of Manned Space Flight for Mission Operations, served as mission director on all Gemini flights beginning with Gemini 6A.

Guenter Wendt was a McDonnell engineer who supervised launch preparations for both the Mercury and Gemini programs and would go on to do the same for the manned section of the Apollo program. His team was responsible for completion of the complex pad close-out procedures just prior to spacecraft launch, and he was the last person the astronauts would see prior to closing the hatch. The astronauts appreciated his taking absolute authority over, and responsibility for, the condition of the spacecraft and developed a good-humored rapport with him.4

Spacecraft

A cutaway illustration of the Gemini spacecraft

NASA selected McDonnell Aircraft, which had been the prime contractor for the Project Mercury capsule, to build the Gemini capsule in 1961 and the first capsule was delivered in 1963. The spacecraft was 19 feet (5.8 m) long and 10 feet (3.0 m) wide with a launch weight of 8,490 pounds (3,850 kg). The Gemini capsule first flew with a crew on March 23, 1965.5

Unlike Mercury, which was equipped only with an attitude control system to change its pitch, yaw, and roll orientation in space, the Gemini spacecraft was equipped with an Orbit Attitude and Maneuvering System (OAMS) which also performed translation in all three perpendicular axes (forward/backward, left/right, up/down) in addition to attitude control. This was used to alter its orbital inclination and altitude, and to perform rendezvous and docking with the Agena Target Vehicle (ATV). The ATV had its own rocket engine which could be used to perform larger altitude changes.

Also unlike Mercury, and the later Apollo spacecraft, Gemini's emergency launch escape system did not use an escape tower powered by a solid-fuel rocket, but instead used aircraft-style ejection seats. The tower was heavy and complicated, and NASA engineers reasoned that they could do away with it as the Titan II's hypergolic propellants would burn immediately on contact, thus a booster explosion would be much smaller than on the cryogenically fueled Atlas and Saturn,citation needed and ejector seats were sufficient to separate the astronauts from a malfunctioning launch vehicle. At higher altitudes where the ejection seats could not be used, the astronauts would return inside the spacecraft, which would separate from the launch vehicle.

A major difference between the Gemini and Mercury spacecraft design was that all Mercury systems other than the re-entry rockets were situated within the capsule, most of which were accessed through the astronaut's hatchway. In contrast, Gemini housed power, propulsion, and life support systems in a detachable Equipment Module located behind the Reentry Module, which made it similar to the Apollo Command/Service Module design. Many components in the capsule itself were reachable through their own small access doors. Unlike Mercury, Gemini used completely solid-state electronics and its modular design made it very easy to repair.citation needed

Gemini was the first manned spacecraft to include an onboard computer, the Gemini Guidance Computer,6 to facilitate management and control of mission maneuvers. Unlike Mercury, the Gemini used in-flight radar and an artificial horizon—devices similar to those used in the aviation industry.

The original intention for Gemini was to land on solid ground instead of at sea, using a Rogallo wing rather than a parachute, with the crew seated upright controlling the forward motion of the craft. To facilitate this, the airfoil did not attach just to the nose of the craft, but to an additional attachment point for balance near the heat shield. This cord was covered by a strip of metal which ran between the twin hatches. This design was ultimately dropped, and parachutes were used to make a sea landing as in Mercury. The capsule was suspended at an angle closer to horizontal, so that a side of the heat shield contacted the water first. This eliminated the need for the landing bag cushion used in the Mercury capsule.

Early short-duration missions had their electrical power supplied by batteries; later endurance missions used the first fuel cells in manned spacecraft.

Gemini was in some regards more advanced than Apollo because the latter program began almost a year earlier. It became known as a "pilot's spacecraft" due to its assortment of jet fighter-like features, in no small part due to Gus Grissom's influence over the design, and it was at this point where the American manned space program clearly began showing its superiority over that of the Soviet Union with long duration flight, rendezvous, and extravehicular capability.7 The Soviet Union during this period was developing the Soyuz spacecraft intended to take cosmonauts to the Moon, but political and technical problems began to get in the way, leading to the ultimate end of their manned lunar program.

Launch vehicle

Main article: Titan II GLV

The Titan II had debuted in 1962 as the Air Force's second-generation ICBM to replace the Atlas. By using hypergolic fuels, it could be stored for long periods of time and be easily readied for launch in addition to being a simpler design with fewer components, the only caveat being that the propellant mix (nitrogen tetroxide and hydrazine) was extremely toxic compared to the Atlas's liquid oxygen/RP-1. However, the Titan had considerable difficulty being man-rated due to early problems with pogo oscillation.

The Titan II rocket that carried the Gemini spacecraft was guided by its own (separate) ASC-15 guidance computer. The Gemini Guidance Computer, sometimes called the Gemini Spacecraft On-Board Computer (OBC), was very similar to the Saturn Launch Vehicle Digital Computer. The Gemini Guidance Computer weighed 58.98 pounds (26.75 kg). The core memory had 4096 addresses, each containing a 39-bit word, composed of three 13-bit "syllables". All numeric data was 26-bit 2's complement integers (sometimes used as fixed-point numbers), either stored in the first two syllables of a word or in the accumulator. Instructions (always 4 bit opcode and 9 bits of operand) could go in any syllable.891011

Astronauts

Sixteen astronauts flew on 10 manned Gemini missions:

Group Astronaut Service Mission, crew position
Astronaut Group 1
(Project Mercury veterans)
L. Gordon Cooper USAF Gemini 5 Command Pilot
Virgil "Gus" Grissom Gemini 3 Command Pilot
Walter M. Schirra USN Gemini 6A Command Pilot
Astronaut Group 2 Neil A. Armstrong Civilian Gemini 8 Command Pilot
Frank Borman USAF Gemini 7 Command Pilot
Charles "Pete" Conrad USN Gemini 5 Pilot
Gemini 11 Command Pilot
James A. Lovell USN Gemini 7 Pilot
Gemini 12 Command Pilot
James A. McDivitt USAF Gemini 4 Command Pilot
Thomas P. Stafford Gemini 6A Pilot
Gemini 9 Command Pilot
Edward H. White II Gemini 4 Pilot
John W. Young USN Gemini 3 Pilot
Gemini 10 Command Pilot
Astronaut Group 3 Edwin "Buzz" Aldrin USAF Gemini 12 Pilot
Eugene A. Cernan USN Gemini 9 Pilot
Michael Collins USAF Gemini 10 Pilot
Richard F. Gordon USN Gemini 11 Pilot
David R. Scott USAF Gemini 8 Pilot

Crew selection

Edward White during spacewalk, Gemini 4, June 1965

Deke Slayton, as director of flight crew operations, had the main role in the choice of crews for the Gemini program. With Gemini it became a procedure that each flight had a primary crew and backup crew, and that the backup crew would rotate to primary crew status three flights later. Slayton also intended for first choice of mission commands to be given to the four remaining active astronauts of the Mercury Seven: Alan Shepard, Grissom, Cooper, and Schirra. (John Glenn had retired from NASA in January 1964 and Scott Carpenter, who was blamed by some in NASA management for the problematic reentry of Aurora 7, was on leave to participate in the Navy's SEALAB project and was grounded from flight in July 1964 due to an arm injury sustained in a motorbike accident. Slayton himself continued to be grounded due to a heart problem.)

In late 1963, Slayton selected Shepard and Stafford for Gemini 3, McDivitt and White for Gemini 4, and Schirra and Young for Gemini 5 (which was to be the first Agena rendezvous mission). The backup crew for Gemini 3 was Grissom and Borman, who were also slated for Gemini 6, to be the first long-duration mission. Finally Conrad and Lovell were assigned as the backup crew for Gemini 4.

Rendezvous of Gemini 6 and 7, December 1965

Delays in the production of the Agena Target Vehicle caused the first rearrangement of the crew rotation. The Schirra and Young mission was bumped to Gemini 6 and they now were the backup crew for Shepard and Stafford. Grissom and Borman now had their long-duration mission assigned to Gemini 5.

The second rearrangement occurred when Shepard developed Ménière's disease, an inner ear problem. Grissom was then moved to command Gemini 3. Slayton felt that Young was a better personality match with Grissom and switched Stafford and Young. Finally, Slayton tapped Cooper to command the long-duration Gemini 5. Again for reasons of compatibility, he moved Conrad from backup commander of Gemini 4 to pilot of Gemini 5, and Borman to backup command of Gemini 4. Finally he assigned Armstrong and Elliot See to be the backup crew for Gemini 5.

First docking; Agena target is seen from Gemini 8, March 1966

The third rearrangement of crew assignment occurred when Slayton felt that See wasn't up to the physical demands of EVA on Gemini 8. He reassigned See to be the prime commander of Gemini 9 and put Scott as pilot of Gemini 8 and Charles Bassett as the pilot of Gemini 9.

The fourth and final rearrangement of the Gemini crew assignment occurred after the deaths of See and Bassett when their trainer jet crashed, coincidently into a McDonnell building which held their Gemini 9 capsule in St. Louis. The backup crew of Stafford and Cernan was then moved up to the new prime crew of the re-designated Gemini 9A. Lovell and Aldrin were moved from being the backup crew of Gemini 10 to be the backup crew of Gemini 9. This cleared the way through the crew rotation for Lovell and Aldrin to become the prime crew of Gemini 12.

Along with the deaths of Grissom, White, and Roger Chaffee in the fire of Apollo 1, this final arrangement helped determine the makeup of the first seven Apollo crews, and who would be in position for a chance to be the first to walk on the Moon.

Missions

Gemini Mission Control in Houston during Gemini 5

There were two unmanned Gemini flights in 1964 and 1965, followed by ten manned flights in 1965 and 1966. All were launched by Titan II launch vehicles. Highlights of the Gemini program achieved its objectives in support of the Apollo program:

  • Edward H. White became the first American to make an extravehicular activity (EVA, or "space walk"), on June 3, 1965 during Gemini 4.
  • Gemini 5 demonstrated the 8-day endurance necessary for an Apollo lunar mission with the first use of fuel cells to generate its electrical power.
  • Gemini 6A and 7 accomplished the first space rendezvous in December 1965, and Gemini 7 set a 14-day endurance record.
  • Gemini 8 achieved the first space docking with an unmanned Agena Target Vehicle.
  • Gemini 11 set a manned Earth orbital altitude record of 739.2 nautical miles (1,369.0 km) in September 1966, using the propulsion system of its Agena target vehicle. This record still stands As of 2014.12
  • Edwin "Buzz" Aldrin on Gemini 12 became the first space traveller to prove that useful work could be done outside a spacecraft without life-threatening exhaustion.

Rendezvous in orbit is not a straightforward maneuver. Should a spacecraft increase its speed to catch up with another, the result would be that it is going to a higher and slower orbit and the distance thereby increases. The right procedure would actually be to slow down and go to a lower orbit first and then later to increase speed and go to the same orbit as the other.13 To practice these maneuvers special rendezvous and docking simulators were built for the astronauts.14

List of missions

Unmanned
Mission LV Serial No Mission Dates Launch Time Duration Remarks
Gemini 1 GLV-1 12556 8–12 April 1964 16:01 UTC 03d 23h First test flight of Gemini; spacecraft was intentionally destroyed during re-entry
Gemini 2 GLV-2 12557 19 January 1965 14:03 UTC 00d 00h 18m 16s Suborbital flight to test heat shield
Manned
Mission LV Serial No Command Pilot Pilot Mission Dates Launch Time Duration
Gemini 3
Gemini3.png
GLV-3 12558 Grissom Young 23 March 1965 14:24 UTC 00d 04h 52m 31s
First manned Gemini flight, three orbits.
Gemini IV
Gemini Four patch.jpg
GLV-4 12559 McDivitt White 3–7 June 1965 15:15 UTC 04d 01h 56m 12s
Included first extravehicular activity (EVA) by an American; White's "space walk" was a 22 minute EVA exercise.
Gemini V
Gemini5insignia.png
GLV-5 12560 Cooper Conrad 21–29 August 1965 13:59 UTC 07d 22h 55m 14s
First week-long flight; first use of fuel cells for electrical power; evaluated guidance and navigation system for future rendezvous missions. Completed 120 orbits.
Gemini VII
Gemini VII patch.png
GLV-7 12562 Borman Lovell 4–18 December 1965 19:30 UTC 13d 18h 35m 01s
When the original Gemini VI mission was scrubbed because its Agena target for rendezvous and docking failed, Gemini VII was used for the rendezvous instead. Primary objective was to determine whether humans could live in space for 14 days.
Gemini VI-A
Gemini 6A patch.png
GLV-6 12561 Schirra Stafford 15–16 December 1965 13:37 UTC 01d 01h 51m 24s
First space rendezvous accomplished with Gemini VII, station-keeping for over five hours at distances from 0.3 to 90 m (1 to 300 ft).
Gemini VIII
Ge08Patch orig.png
GLV-8 12563 Armstrong Scott 16–17 March 1966 16:41 UTC 00d 10h 41m 26s
Accomplished first docking with another space vehicle, an unmanned Agena Target Vehicle. While docked, a Gemini spacecraft thruster malfunction caused near-fatal tumbling of the craft, which, after undocking, Armstrong was able to overcome; the crew effected the first emergency landing of a manned U.S. space mission.
Gemini IX-A
Ge09Patch orig.png
GLV-9 12564 Stafford Cernan 3–6 June 1966 13:39 UTC 03d 00h 20m 50s
Rescheduled from May to rendezvous and dock with an Augmented Target Docking Adapter (ATDA) after the original Agena Target Vehicle failed to orbit. The ATDA shroud did not completely separate, making docking impossible. Three different types of rendezvous, two hours of EVA, and 44 orbits were completed.
Gemini X
Ge10Patch orig.png
GLV-10 12565 Young Collins 18–21 July 1966 22:20 UTC 02d 22h 46m 39s
First use of the Agena Target Vehicle's propulsion systems. The spacecraft also rendezvoused with the Agena Target Vehicle from Gemini VIII. Collins had 49 minutes of EVA standing in the hatch and 39 minutes of EVA to retrieve experiments from the Agena. 43 orbits completed.
Gemini XI
Gemini 11 patch.png
GLV-11 12566 Conrad Gordon 12–15 September 1966 14:42 UTC 02d 23h 17m 09s
Gemini record altitude with apogee of 739.2 nautical miles (1,369.0 km)12 reached using the Agena Target Vehicle propulsion system after first orbit rendezvous and docking. Gordon made a 33-minute EVA and two-hour standup EVA. 44 orbits.
Gemini XII
Gemini 12 insignia.png
GLV-12 12567 Lovell Aldrin 11–15 November 1966 20:46 UTC 03d 22h 34m 31s
Final Gemini flight. Rendezvoused and docked manually with its target Agena and kept station with it during EVA. Aldrin set an EVA record of 5 hours and 30 minutes for one space walk and two stand-up exercises, and demonstrated solutions to previous EVA problems.

Gemini-Titan launches and serial numbers

Main article: Titan II GLV
All Gemini Launches from GT-1 through GT-12
All Gemini launches from GT-1 through GT-12
USAF serial number location on Titan II
USAF serial number location on Titan II

The Gemini-Titan II launch vehicle was adapted by NASA from the U.S. Air Force Titan II ICBM. (Similarly, the Mercury-Atlas launch vehicle had been adapted from the USAF Atlas missile.) The Gemini-Titan II rockets were assigned Air Force serial numbers, which were painted in four places on each Titan II (on opposite sides on each of the first and second stages). USAF crews maintained Launch Complex 19 and prepared and launched all of the Gemini-Titan II launch vehicles. Data and experience operating the Titans was of value to both the U.S. Air Force and NASA.

The USAF serial numbers assigned to the Gemini-Titan launch vehicles are given in the tables above. Fifteen Titan IIs were ordered in 1962 so the serial is "62-12XXX", but only "12XXX" is painted on the Titan II. The order for the last three of the 15 launch vehicles was cancelled on July 30, 1964, and they were never built. Serial numbers were, however, assigned to them prospectively: 12568 - GLV-13; 12569 - GLV-14; and 12570 - GLV-15.

Program cost

In January 1969, NASA prepared for the US Congress an estimate of the costs for projects Mercury, Gemini, and Apollo (to the first manned Moon landing). This estimate gave the cost of Project Gemini as $1.2834 billion, broken down to $797.4 million for spacecraft, $409.8 million for launch vehicles, and $76.2 million for support.15 The Space Review estimated in 2010 the cost of Gemini from 1962 to 1967 as $1.3 billion in 1967 inflation-adjusted dollars, or $7.3 billion in 2010 dollars. Each manned flight cost $723 million in 2010 dollars.16

Current location of hardware

Spacecraft

Trainers

Proposed extensions and applications

Advanced Gemini

Main article: Advanced Gemini

McDonnell Aircraft, the main contractor for Mercury and Gemini, was also one of the original bidders on the prime contract for Apollo, but lost out to North American Aviation. McDonnell later sought to extend the Gemini program by proposing a derivative which could be used to fly a cislunar mission and even achieve a manned lunar landing earlier and at less cost than Apollo, but these proposals were rejected by NASA.

A range of applications were considered for Advanced Gemini missions, including military flights, space station crew and logistics delivery, and lunar flights. The Lunar proposals ranged from reusing the docking systems developed for the Agena Target Vehicle on more powerful upper stages such as the Centaur, which could propel the spacecraft to the Moon, to complete modifications of the Gemini to enable it to land on the lunar surface. Its applications would have ranged from manned lunar flybys before Apollo was ready, to providing emergency shelters or rescue for stranded Apollo crews, or even replacing the Apollo program.

Some of the Advanced Gemini proposals used "off-the-shelf" Gemini spacecraft, unmodified from the original program, whilst others featured modifications to allow the spacecraft to carry more crew, dock with space stations, visit the Moon, and perform other mission objectives. Other modifications considered included the addition of wings or a parasail to the spacecraft, in order to enable it to make a horizontal landing.

Big Gemini

Main article: Big Gemini

Big Gemini (or "Big G") was another proposal by McDonnell Douglas made in August 1969. It was intended to provide large-capacity, all-purpose access to space, including missions that ultimately used Apollo or the Space Shuttle.

The study was performed to generate a preliminary definition of a logistic spacecraft derived from Gemini that would be used to resupply an orbiting space station. Land-landing at a preselected site and refurbishment and reuse were design requirements. Two baseline spacecraft were defined: a nine-man minimum modification version of the Gemini B called Min-Mod Big G and a 12-man advanced concept, having the same exterior geometry but with new, state-of-the-art subsystems, called Advanced Big G.citation needed Three launch vehicles-Saturn IB, Titan IIIM, and Saturn INT-20 (S-IC/S-IVB) were investigated for use with the spacecraft.

Military applications

The Air Force had an interest in the Gemini system, and decided to use its own modification of the spacecraft as the crew vehicle for the Manned Orbital Laboratory. To this end, the Gemini 2 spacecraft was refurbished and flown again atop a mockup of the MOL, sent into space by a Titan IIIC. This was the first time a spacecraft went into space twice.

The USAF also had the notion of adapting the Gemini spacecraft for military applications, such as crude observation of the ground (no specialized reconnaissance camera could be carried) and practicing making rendezvous with suspicious satellites. This project was called Blue Gemini. The USAF did not like the fact that Gemini would have to be recovered by the US Navy, so they intended for Blue Gemini eventually to use the airfoil and land on three skids, carried over from the original design of Gemini.

At first some within NASA welcomed sharing of the cost with the USAF, but it was later agreed that NASA was better off operating Gemini by itself. Blue Gemini was cancelled in 1963 by Secretary of Defense Robert McNamara, who decided that the NASA Gemini flights could conduct necessary military experiments. MOL was cancelled by Secretary of Defense Melvin Laird in 1969, when it was determined that unmanned spy satellites could perform the same functions much more cost-effectively.

See also

References

 This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.

  1. ^ The only Gemini spacecraft not on a Titan II was the re-flight of Gemini 2 for a Manned Orbital Laboratory test in 1966, which used a Titan IIIC
  2. ^ Gemini Program. Venice, CA: Revell, Inc. 1965. p. 1. pamphlet included with 1/24 scale model of Gemini spacecraft; based on official records of NASA. 
  3. ^ "The Unsinkable Gusmobile". amyshirateitel.com. Retrieved 6 July 2012. 
  4. ^ Farmer, Gene; Dora Jane Hamblin (1970). First On the Moon: A Voyage With Neil Armstrong, Michael Collins and Edwin E. Aldrin, Jr. Boston: Little, Brown and Co. pp. 51–54. Library of Congress 76-103950. 
  5. ^ "McDonnell Gemini Spacecraft". Retrieved 30 March 2011. 
  6. ^ "Computer, Guidance, Gemini 8". Retrieved 23 December 2005. 
  7. ^ During the two years of the Gemini program, the Soviets made no manned flights, and despite achieving the first EVA, did no more EVAs until January 1969.
  8. ^ Ronald Burkey, "Gemini Spacecraft On-Board Computer (OBC)"
  9. ^ "Computers in Spaceflight: The NASA Experience", chapter one
  10. ^ "IBM and the Gemini Program"
  11. ^ C. A. Leist and J. C. Condell, "Gemini Programming Manual", 1966
  12. ^ a b Dumoulin, Jim (August 25, 2000), NASA Project Gemini-XI, retrieved April 12, 2010 
  13. ^ "Orbital Rendezvous". Buzz Aldrin. Retrieved 2011-10-09. 
  14. ^ "NASA, Project Gemini". NASA. Archived from the original on 2004-11-07. Retrieved 2011-10-14. 
  15. ^ Wilford, John Noble (July 1969). We Reach the Moon. New York: Bantam Books. p. 67. 
  16. ^ Lafleur, Claude (2010-03-08). "Costs of US piloted programs". The Space Review. Retrieved February 18, 2012. 

Further reading

External links