A look at the Van Allen radiation belts was the second objective. The first flight was to be an unmanned test to make sure that spacecraft and booster would be compatible for manned missions, but it would also carry biological experiments. Titan II would boost the spacecraft into a highly elliptical orbit, 160 kilometers above Earth at its lowest point but 1,400 kilometers out at its highest and through the Van Allen belts to acquire data on radiation.
Controlled landing was the third goal, to be pursued on all seven manned flights. This meant that the pilot had to have some means of flying the spacecraft toward a relatively limited landing area. The most direct method was to offset the spacecraft's center of gravity to yield some degree of aerodynamic lift, using the attitude control system to roll the spacecraft during its flight through the air and thus control the amount and direction of lift to correct any errors in the predicted landing point. Controlled land landing also demanded some way to cushion touchdown impact. This was a harder problem, but one to which the paraglider seemed to promise an answer.
Rendezvous and docking stood fourth in the list of objectives. The fifth, seventh, ninth, and tenth flights in the program each required two launches, so the Titan II-launched Mark II could meet and dock with the Atlas-launched Agena B in orbit. The planners foresaw the major problem in the first rendezvous missions to be the size of the "launch window," the length of time during which a spacecraft could be launched to rendezvous with its target. The larger the launch window, the greater the difference in speed between spacecraft and target that had to be made good. That was beyond the powers of the spacecraft alone, but the difference might be made up, in part, by the target. Later, with more experience, the engineers expected to reduce the size of the launch window. Then the extra power provided by the target might find other uses, perhaps in "deep space and lunar missions with the target vehicle being used as a booster following rendezvous." The fifth objective was astronaut training, mainly a useful byproduct of the program.18
The plan stressed extensive use of vehicles and equipment on hand, altered as little as possible. The Mark II spacecraft retained what the Mercury capsule had proved, its aerodynamic shape, thermal protection, and systems components. Some changes were demanded by new goals. In the longer flights, crew members needed improved pressure suits, fuel cells to replace batteries, and more stable propellants than hydrogen peroxide in the attitude control system. Although Mercury carried none of the gear required for rendezvous missions,  planners expected to meet these needs with little or no modification of existing inertial platforms, radar, and computers; of the major requirements, only a rendezvous propulsion system was not on hand.
Other major changes were limited to ejection seats instead of Mercury's escape tower and the environmental control system, in essence two Mercury systems hooked together. Since everything else in Mark II differed little if any from the equipment flight-tested in Mercury, the engineers looked forward to only a modest testing effort in the new project. They guessed that it would cost only $177 million to develop, procure, and test eight Mark II spacecraft, two of which were to be reused.19
The Mark II planners were just as sanguine when it came to launch vehicles. Atlas-Agena B could be used almost as it came off the assembly line, at a cost to the program of only $38 million for the four required. Titan II demanded more in the way of changes, but the Air Force would bear most of the cost. The chief exception was lengthened second-stage propellant tanks to increase the payload by 300 kilograms. As a manned booster, Titan II promised to be so simple and reliable that only one extra feature was needed to leave all decisions to abort a mission in the hands of the pilots. That was a redundant guidance and control system. Titan II's most dangerous potential failing, and the only one that demanded an automatic abort system, was first-stage engine hardover. A malfunction in the guidance and control system could drive the gimbaled engines to their extreme positions - hard-over - their thrust vector then being directed at the farthest possible angle from the proper flight path, accelerating the booster away from the correct course in the region where it would be subjected to the greatest dynamic pressure. The danger lay in the possibility of the booster's breaking up before the pilots could react. By adding a second first-stage guidance and control system, the hazards of this failing were all but erased. Since the booster demanded little in the way of new parts, testing could be quite limited. The best estimate of the price of the boosters was $86 million.
The cost of the entire program from drawing board through the last flight came to $347.8 million. It would be managed by a project office that would also take charge of the rest of the Mercury program, the three-orbit flights already planned and the proposed 18-orbit mission using the minimum-change capsule. Forming the core of the new project office would be the 76 members of STG's Engineering Division, at the time chiefly engaged in Project Mercury and largely outside the mainstream of Apollo. The planners were careful to stress that the new office could be fully staffed to a total of 175 and the new program could be carried on without threat to other programs. Mercury would not be hindered, Apollo would not be interrupted.20
Should the proposed project meet with complete success,  the stage would then be set for the sixth objective, which might supplant much of Project Apollo. If the Mark II spacecraft showed itself able to support a crew for seven days or more and if rendezvous proved to be practical, then the advanced program based on the Mark II might "accomplish most of the Apollo mission at an earlier date than with the Apollo program as it is presently conceived." By taking full advantage of the new spacecraft and rendezvous technique, "it is a distinct possibility that lunar orbits may be accomplished by the interim spacecraft after rendezvous with an orbiting Centaur." This prospect was the subject of an appendix to the development plan.
Centaur was a second-stage vehicle then under development that would use high-energy liquid hydrogen as its fuel. If Centaur were inserted into orbit by Titan II, it would have enough power after docking to boost the spacecraft to escape velocity. The deep-space version of Mark II differed from the rendezvous type only in having backup navigation gear and extra heat and radiation protection, 270 kilograms more on a 2,900-kilogram spacecraft. The appendix explored two possible mission sequences. One simply added four flights to the ten in the Mark II program. The first two extra flights were deep-space missions, with Centaur boosting the spacecraft into an elliptical orbit with an apogee of some 80,000 kilometers to study navigation and reentry problems. The last two flights, scheduled for March and May 1965, were circumlunar, and the whole package added only $60 million to the cost of the basic Mark II program.
The alternative was an accelerated program, nine flights in all. The first three flights were the same in both programs - an 18-orbit unmanned qualification and radiation test, an 18-orbit manned qualification test, and a manned long-duration test. In the speeded up program, the fourth and fifth flights developed the techniques of rendezvous and docking with Agena B as the target. Centaur launched by Titan II then replaced the Agena for the rest of the program - two deep-space missions and two flights around the Moon. This faster program put the first Mark II in lunar orbit in May 1964 for a cost not much greater than the basic 10-flight program: $356.3 million versus $347.8 million.21
During the week after its release, the Mark II plan had STG buzzing.22 A second version of the plan came out just a week later, on 21 August. It differed from the first in only one notable respect. All mention of a lunar mission for Mark II had vanished, leaving behind only a circumspect suggestion that, "if a vehicle such as the Centaur were used as the rendezvous target, the spacecraft would then have a large velocity potential for more extensive investigations."23 Even this hint dropped out of later versions of the plan.
The appeal of going to the Moon with Mark II, however, was not so easily quashed.  After cutting circumlunar flight from the Mark II plan, Chamberlin revived the even more daring idea of using the spacecraft in a lunar landing program.24 The booster was Saturn C-3 and the key technique was lunar orbit rendezvous. The scheme involved a lunar landing vehicle that was little more than a 680-kilogram skeleton, to which a propulsion system and propellants were attached. Fully fueled, it weighed either 3,284 kilograms or 4,372 kilograms, depending on choice of propellants. The lighter version used liquid hydrogen, the heavier used hypergolic propellants. Total Earth-launched payload in this mission fell between 11,000 and 13,000 kilograms, one-sixth to one-fifth of the 68,000-kilogram payload then in prospect for the direct ascent lunar mission. The cost was low, $584.3 million plus the expense of two Saturn C-3 boosters, but the risk was high.
The flight plan for this lunar landing program derived from the speeded up circumlunar proposal appended to the Mark II plan of 14 August. The first two flights, in March and May of 1964, were to be unmanned and manned qualification tests of the spacecraft and Titan II. The next two flights put the spacecraft in orbit for extended periods of time. Three flights then developed and demonstrated rendezvous and docking techniques with Agena as target. The eighth and ninth missions had Centaur boosting the spacecraft into an 80,000-kilometer deep-space orbit. Next came three flights to test rendezvous between the manned spacecraft and the unmanned lunar landing craft in Earth orbit, culminating with the crew transferring from one to the other. Flights 13 and 14 had Centaur boosting the spacecraft to escape velocity for an early demonstration of circumlunar capability. Saturn was to launch the 15th flight, a Moon orbital mission. Men would land on the Moon in the final flight, slated for January 1966.25
When Chamberlin proposed this scheme to Gilruth's senior staff at the start of September 1961, he was the first in STG to offer a concrete plan for manned lunar landing that depended on the technique of rendezvous in lunar orbit.26 STG so far had seen little merit in any form of rendezvous for lunar missions, but it reserved its greatest disdain for the lunar orbit version. The Langley partisans of lunar orbit rendezvous had first put their scheme before STG on 10 December 1960, when they rehearsed what they planned to say to Associate Administrator Robert Seamans and his staff a week later.27 On 10 January, John Houbolt and some of his colleagues met with three STG engineers and tried to convince them that lunar orbit rendezvous belonged in the Apollo program. The response was reserved, the scheme dismissed as too optimistic.* 28
 Three months later, Houbolt was back for another briefing, this time supported by a printed circular on "Manned Lunar Landing via Rendezvous." It included one project called MORAD (for Manned Orbital Rendezvous And Docking), a modest two-flight effort to be completed by mid-1963, intended as a quick proof of the feasibility of rendezvous. A small unmanned payload would propel itself to a linkup with a Mercury capsule, its maneuvers under the control of the Mercury Pilot. The key project, however, was MALLIR (Manned Lunar Landing Involving Rendezvous). Chamberlin, who attended this briefing, had known about Langley's rendezvous work, but had not before heard about the lunar orbit version. He asked Houbolt for a copy of the circular and for anything else he had on rendezvous.29
Others in STG had yet to be convinced. Gilruth saw rendezvous as a distant prospect, not something for the near future. Mercury was proving so troublesome that rendezvous, however simple in theory, seemed very far away. He strongly insisted on the need for large boosters:
Rendezvous schemes are and have been of interest to the Space Task Group and are being studied. However, the rendezvous approach itself will, to some extent, degrade mission reliability and flight safety. I am concerned that rendezvous schemes may be used as a crutch to achieve early planned dates for launch vehicle availability, and to avoid the difficulty of developing a reliable NOVA class launch vehicle.30This viewpoint was widespread in NASA, leading some to resist rendezvous, not because they believed it a poor idea but because it threatened to subvert another goal seen to be more important.
The efforts of Houbolt and his Langley colleagues to sell rendezvous in general, and lunar orbit rendezvous in particular, may have been frustrated less because their concept was faulty than because, as Chamberlin has suggested, "they were considered to be pure theorists with no practical experience." The major trouble with the lunar orbit rendezvous scheme may well have been that it simply looked too good to be true. Paper-and-pencil calculations did yield striking figures, but what looked good in theory might not stand up so well in practice. Chamberlin and his co-workers, although fully alive to the weight-saving features of rendezvous, stressed another aspect - it made a lunar spacecraft easier to design. Direct ascent posed a particularly thorny design problem because the spacecraft had both to land on the Moon and to reenter Earth's atmosphere. A rendezvous mission, however, allowed one design for a lunar lander, a second for a reentry capsule - a distinct spacecraft to meet the special demands of each of these  two most critical phases of the lunar flight. Chamberlin's group had, in fact, centered its work on a lunar-lander design, since reentry problems were already well in hand. Stressed as an answer to design constraints rather than a weight-saving expedient and sponsored by men with plenty of practical experience in Mercury, lunar orbit rendezvous in Chamberlin's plan for a Mark II lunar landing mission received its first serious hearing from STG.31
Toward the end of September 1961, Chamberlin's plan showed up as part of an "Integrated Apollo Program" STG presented to Silverstein and his staff at NASA Headquarters.  What "integrated" meant was adding a Mark II orbital rendezvous project to the Apollo program. Much of the presentation was drawn from the Mark II preliminary plan, but part of it was based on Chamberlin's lunar landing scheme of 30 August. Some of the figures were new: the lunar landing system, complete with propulsion unit and fuel, weighed little more than 1,800 kilograms, roughly half what the first version had. The cost now included the Saturn boosters for a total of $706.4 million, but the flight development plan had not changed.32
Silverstein proved to be no more excited by a Mark II lunar mission in September than he had been by an improved Mercury lunar mission in March. But he was willing to go along with the idea of a rendezvous development project. On 6 October, Silverstein asked for, and got, Associate Administrator Seamans' formal approval for the "preparation of a preliminary development plan for the proposed orbital flight development program." Seamans now granted STG sanction to begin talks with McDonnell on buying the Mark II spacecraft, with the Department of Defense on Titan II boosters and launch-stand alterations, and with the NASA Office of Launch Vehicle Programs on the Atlas-Agena.33
The Mark II project itself, however, had yet to be approved, even though Seamans remarked that "our present plans call for a Mercury Mark for test of orbital operations during 1963 and 1964."34 Still lacking was an approved project development plan. Such a plan, in fact, had yet to be submitted, although copies of Chamberlin's preliminary plan had been making the rounds of NASA Headquarters in search of comments. With his Mark II lunar landing scheme rejected, Chamberlin now set out to revise the Mark II plan and put it in shape for Seamans to sign.
* Houbolt, Clinton Brown, Manuel J. Queijo, and Ralph W. Stone, Jr., described the lunar orbit rendezvous idea to Kurt Straas, Owen E. Maynard, and Robert L. O'Neal. O'Neal's report to Associate Director Charles Donlan was distinctly skeptical of Langley's claims on weight saving.
16 "Preliminary Project Development Plan for an Advanced Manned Space Program Utilizing the Mark II Two Man Spacecraft," STG, 14 Aug. 1961.
17 Ibid., pp. 2, 20; U.S. Congress, House, Committee on Science and Astronautics, Aeronautical and Astronautical Events of 1961: Report, 87th Cong., 2nd sess., 7 June 1962, pp. 37-38, 53; James P. Henry, Biomedical Aspects of Space Flight (New York, 1966), p. 82.
18 "Preliminary Plan," 14 Aug. 1961, pp. 2-4, 8-13, 19-20.
19 Ibid., pp. 4-5, 6-8, 11, 19, Tables 3.1, 3.2, 3.4, 5.1, Fig. 8.1.
20 Ibid., pp. 17-18, 19, 22-23, 25-27, Fig. 8.1; Bastian Hello, interview, Baltimore, 23 May 1966.
21 "Preliminary Plan," 14 Aug.1961, pp. 29-33, Table A-2.2, Fig. A-5.1; cf. Figs. 5.4 with A-7.1, 8.1 with A-7.2, Table 5.1 with A7.1.
22 Memo, Purser to Gilruth, "Log for week of Aug.14, 1961," n.d.; Purser notes on meeting with Gilruth, Williams, Chamberlin, Charles W. Mathews, George M. Low, and Warren J. North, 15 Aug. 1961.
23 "Preliminary Project Development Plan for an Advanced Manned Space Program Utilizing the Mark II Two Man Spacecraft," STG, 21 Aug. 1961, p. 5.
24 "A Lunar Landing Proposal Using Rendezvous," STG, 30 Aug. 1961.
25 Ibid., memo, Jack Funk for Assoc. Dir., "Trip to Marshall Space Flight Center, August I, 1961, to discuss circumlunar payload," 14 Aug. 1961.
26 Williams interview.
27 John C. Houbolt et al., "Manned Lunar Landing through use of Lunar-Orbit Rendezvous" 1 (31 Oct. 1961): 5; Houbolt, interview, Princeton, N.J., 5 Dec. 1966.
28 Memo, Robert L. O'Neal to Assoc. Dir., "Discussion with Dr. Houbolt, LRC, concerning the possible incorporation of a lunar orbital rendezvous phase as a prelude to manned lunar landing," 30 Jan. 1961.
29 "Manned Lunar Landing Via Rendezvous," Langley Research Center, 19 April 1961; Houbolt, telephone interview, 30 Dec. 1966; letter, Chamberlin to James M. Grimwood, 26 March 1974.
30 Letter, Gilruth to Nicholas E. Golovin, 12 Sept. 1961; Gilruth, interview, Houston, 21 March 1968.
31 Purser, comments on Gemini draft history, 14 Jan. 1969; Chamberlin comments, 26 March 1974.
32 Memo, Purser to Gilruth, "Log for week of September 25, 1961," 5 Oct. 1961; "Project Apollo Slides: Integrated Apollo Program," STG, 26 Sept. 1961; Rose interview.
33 Memo, Abe Silverstein to Assoc, Adm., "Approval of Orbital Flight Development Program," 6 Oct. 1961, with Robert C. Seamans, Jr.s signed approval, same date.
34 Memo, Seamans to Silverstein, "Apollo Crew Selection," 13 Oct. 1961; memo, Low to Aleck C. Bond, "Possible Use of Titan II Booster in the Manned Space Flight Program," 22 Sept. 1961.