During the following month, while trying to reduce that risk to a minimum, the von Braun team represented by Slattery, the Space Task Group represented by John A. Powers, and NASA Headquarters represented by Paul P. Haney, agreed to plan the public information for MR-BD to avoid "over-emphasis or overly optimistic assumptions relating to future manned flights."74
Redstone engineers meanwhile quickly fixed the MR-BD launch vehicle, making their seven technical changes during the first two weeks in March 1961. The foremost cause of previous Redstone booster overaccelerations was a small servo control valve that had failed to regulate properly the flow of hydrogen peroxide to the steam generator, which in turn powered, and in the case of MR-1A and MR-2 overpowered, the fuel pumps. Modifications were made to the thrust regulator and velocity integrator, in hopes that MR-BD would be physically incapable of exceeding the speed limit again. Another technical difficulty had been some harmonic vibrations induced by aerodynamic stress in the topmost section of the elongated Redstone. Four stiffeners were added to the ballast section and 210 pounds of insulation was applied to the inner skin of the upper part of the instrument compartment. Although oscillations at the second bending mode frequencies were less on MR-2 than on MR-1A, several other electronic changes were made to reduce the dangers from noise and vibration.  High winds aloft probably had added some extra stress in the former case, but in any event the next trajectory would smooth out the tilting maneuver in the region of high dynamic pressure, and 65 telemetry sensors were placed where the rocket's bending moments needed to be monitored. Finally, after a great deal of diagnostic study, five resettings were made to ensure that the booster engine cutoff time would not precede oxidizer depletion and hence cause another premature abort signal, as had happened with MR-2. All these changes proceeded smoothly while the boilerplate capsule was ballasted and corrugated to approximate the production model, McDonnell spacecraft No. 7, and fitted with an inert escape rocket. The capsule did not have a posi-retrorocket package.75
On the morning of March 24, 1961, the second half of the split countdown for MR-BD was in progress, and so far everything had proceeded without a hitch or a "glitch." To test procedures for the launch pad rescue crews, a manned M-113 armored personnel carrier was parked only 1,000 feet southwest of the unmanned Redstone. The firemen in this vehicle were going to endure bone-jangling noise and vibration during the launch to see how much emergency rescue crews could stand. Closer still, an unmanned asbestos-covered truck was  parked 65 feet from the MR-BD blast deflector to simulate the position that the "cherry picker," or mobile egress tower, would occupy during the launch of a manned missile.
Liquid oxygen loading for MR-BD began only two hours before the scheduled launch time. During the automatically controlled loading process, winds of about 20 knots swayed the Redstone and produced sloshing during the "topoff" operation. The fuel temperature began to rise toward the boiling point, and soon an overflow bled out the booster standpipe and boil-off valve. This potentially dangerous situation was governed by a computer, which, when its electronic bias in the topping circuit was lowered, continued the "lox- topping" normally. No holds were called, and the countdown proceeded to launch without further incident.
At 12:30 p.m., MR-BD lifted off straight and smooth from Cape Canaveral on its programmed trajectory. The people in the armored vehicle on the ground watched it all without discomfort, and a truck driver later moved the simulated "cherry picker" away undamaged. Although the actual exit velocity was 89 feet per second higher than planned, there was in general, said Hammack in his report to STG, "hardly a plotting difference between the actual trajectory data computed . . . and the nominal trajectory published in NASA working paper 178."76 The whole configuration impacted in the Atlantic 307 miles downrange (five miles short of the plan) and sank to the bottom, exploding a sofar bomb en route. MR-BD was highly successful; as George Low reported to Administrator Webb, it "demonstrated that all major booster problems have been eliminated."77 Telemetry revealed that the Redstone still wriggled a bit with high vibrations in the instrument compartment, but all the "quick-fixes" had worked properly. MR-BD satisfied von Braun's team, Debus' crew, and all of NASA that the Redstone was now trustworthy enough to be called "man-rated." Enough experience was at hand to tackle the next step in Project Mercury, manned suborbital flight.78
But the very next day, March 25, the Soviets announced the successful launch and recovery of their fifth Korabl Sputnik, containing a dog named Zvesdochka, or Little Star. Three out of five was their record now for successful recovery of "cosmic ships" and dogs from orbit. Three days later, at a Soviet Academy of Science press conference in Moscow, six of Strelka's pups, as well as four other space dogs, were on exhibit as evidence and harbingers of the imminent flight of man into space. MR-BD might have been that first flight had it been "MR-3," as originally scheduled, but the decision of a month before froze the Mercury-Redstone schedule for at least two months afterward. And the Mercury team, aware of but not dominated by the space race, could only hope that the "Sputnik Spacecraft Team" was having comparable final checkout difficulties.
At the beginning of April 1961, Mercury-Redstone launch vehicle No. 7 was erected on its launch pedestal at pad No. 5 and made ready for the first mating of the man-rated capsule No. 7. Feverish activity pervaded Hangar S and the  service structure, where another "white room" was being hastily rigged on the third level of the gantry at MR-3's capsule height. Rework on the capsule's reaction control system was completed during the first week in April, while the three chosen astronauts went through final procedures training and acceleration conditioning in centrifuge runs at Johnsville. The Space Task Group now believed that the development phase of the project was practically over. Symbolizing this shift, the Associate Director responsible for development, Charles Donlan, left STG formally on the first of April to return to Langley Research Center, leaving Walter Williams, the operations chief, as Gilruth's sole Associate Director.79
The Space Task Group nevertheless could not afford to become too preoccupied with the preparations for MR-3 because MA-3 and Little Joe 5B were scheduled first, and within two weeks, as prerequisites for the orbital objective. On April 10, foreign correspondents in Moscow reported rampant rumors sweeping the city that the U.S.S.R. had placed a man into space. That same day at Langley Field, Virginia, another rumor reached the attention of STG to the effect that the 10 members and four consultants of the President's Hornig panel were recommending at least 50 more chimpanzee runs before putting man in space. Gilruth remarked facetiously that if this were true, the Mercury program ought to move to Africa.80
This hearsay recommendation did not become a part of the "Report of the Ad Hoc Mercury Panel" or of the Hornig Committee, as it was more widely known, which was submitted on April 12, 1961. Having been delegated by President Kennedy and his scientific adviser, Wiesner, the panel visited the McDonnell plant, Cape Canaveral, and Langley Field and talked with representatives of supporting services and contractors. In its 18-page report it reviewed the accomplishments and failures of the Mercury program, assessed the risks and probability of success, and commented upon medical aspects of Project Mercury as a whole and medical readiness for manned suborbital flight in particular. It concluded with some reasonable medical reservations that a Redstone flight now would be "a high risk undertaking but not higher than we are accustomed to taking in other ventures," such as in the initial flights of the Wright Brothers, Lindbergh, and the X-series of research aircraft.81
In its reliability assessments, the Hornig panel graded the Mercury subsystems or components according to three classes of reliability percentages: Class 1, 95-100 percent; Class 2, 85-95 percent; Class 3, 70-85 percent. Eleven items were rated as Class 1: Capsule structure and reentry properties; separation mechanism and posigrade rocket; tower and abort rockets; voice communications; abort sensing instrumentation system; manual control system; retrorocket system; parachute landing system; ground environment system; recovery operation; and pilot training. Three items were rated in Class 2: Landing bag; environmental control system; and automatic stabilization and control system. The two items in the Class 3 category, booster (Redstone or Atlas) and telemetry, were explained as "not per se a cause for alarm" for pilot safety but only for mission success.82
73 Memo, signed, "Publicity Releases on Mercury," Marshall Space Flight Center, Feb. 8, 1961.
74 "Information Plan: Redstone Development Test: MR-BD,"NASA, March 21, 1961, 2. For an example of this policy, see NASA News Release 61-57, "Mercury Redstone Booster Development Test," March 22, 1961. One of the most famous of publicity fact sheets, issued by STG from Langley Field on April 10, 1961, was entitled simply "'IF': A Study of Contingency Planning for the Project Mercury Mission."
75 "The Mercury-Redstone Project," 8-16;"Final Report: Mercury Redstone Project Launch Operations," Appendix O, 2; memo, Geissler, "Project Mercury-Redstone: Trajectory Data for MR-BD," March 20, 1961; "Master Operational Schedule for MR-BD," Marshall Space Flight Center, March 6, 1961; memo, S. Snyder to NASA Technical Personnel, "Mercury-Redstone (MR-BD) Launch," March 23, 1961.
76 Hammack memo. Mercury working paper 178 was J. W. Maynard, T. J. Slopinski, and P. S. Leatherman, "Calculated Preflight Trajectory Data for Redstone Booster Test (MR-BD)," March 17, 1961.
77 Memo, Low to Administrator, "Mercury Redstone Booster Development Test," March 27, 1961. See also note, Eugene E. Horton to Powers on MR-BD publicity and point of test in "wriggling" Redstone; message, Powers to Paul Haney, date missing.
78 Of a total of 71 Redstone booster flights (including 4 Mercury-Redstone) through March 24, 1961, only 10, or 14.1 percent, were classed as failures by the latest revision of a composite document prepared under W. A. Mrazek, Director, Structures and Mechanics Division: "Redstone Vehicle Malfunction Study (Mercury-Redstone Program)", MSFC report No. DSD-TM-12-60, Rev. B, May 1, 1960, 8.
79 See "Final Report: Mercury-Redstone Project Launch Operations," Appendix M; Ms., George F. Killmer, Jr., et al., "Mercury Technical History—Preflight Operations," Dec. 30, 1963,85-90, Fig. 11; James M. Grimwood, Project Mercury: A Chronology, NASA SP—4001 (Washington, 1963), 131, 207, 218.
80 Purser, log for Gilruth, April 10, 1961; Purser, interview, Houston, Feb. 12, 1964; Gilruth interview; Silverstein, interview, Cleveland, May 1, 1964; Aeronautical and Astronautical Events of 1961, 15. See also "Rumors Fly as Moscow Alerts Press," Washington Post Apr. 11, 1961.
81 Donald F. Hornig, chairman, "Report of the Ad Hoc Mercury Panel," Apr. 12, 1961, 18, passim. The membership of this panel included, in addition to Hornig, Paul Beeson, W. John Beil, Miton V. Clauser, Edward H. Heinemann, Lawrence S. Hyland, Donald P. Ling, Robert B. Livingston, Harrison A. Storms, and Cornelius Tobias. The two technical assistants were Douglas R. Lord and James B. Hartgering, and two special consultants were Alfred P Fishman and Paul Wickham.
82 Ibid. See also "Debate Reported Over Space Shot: Kerr Asserts Kennedy Aides Disputed Flight's Wisdom," New York Times, May 10, 1961; Lord interview. Mae Mills Link, Space Medicine in Project Mercury, NASA SP-4003 (Washington, 1965), treats at greater length some of these problems in her chapter VIII, entitled "The Season of Crisis: 1961," 112-125.