Eggers, Alien, and Neice found that the skip vehicle, which would return to Earth by performing an intricate series of progressively steeper dips into the atmosphere, would need an extremely powerful boost to circumnavigate the globe and also would encounter a prohibitively large amount of aerodynamic heating.36 By contrast, the glider, although heavy, would require less boost and would keep the g forces imposed on the pilot during reentry at a quite acceptable level. Like the skip craft, the glider would provide the advantage of pilot control during the landing phase. It would radiate heat well, but since its thermodynamic load still would be high, the glider might experience dangerous interior heating during a "global" satellite) mission. So the authors suggested a high-lift glider; like the high-lift-over-drag glider, it would have a delta-wing configuration but also would feature thick, rounded sides and bottom to minimize interior heating. It would enter the atmosphere at a high angle of attack, then level off at lower altitudes to increase the lift/drag ratio.
The ballistic vehicle, the simplest approach of the three, could not be controlled aerodynamically, but its blunt shape provided superior thermal protection, and its relatively light weight gave it a longer range. If it entered the atmosphere at a low angle, deceleration forces could be kept at or below 10 g, with 5 g lasting for 1 minute and 2 g for not over 3 minutes. Therefore the three NACA researchers concluded that "the ballistic vehicle appears to be a practical man-carrying machine, provided extreme care is exercised in supporting the man during atmospheric entry."37
As time passed, Eggers personally became convinced of the overall desirability of the manned satellite glider as opposed to the ballistic satellite. He revealed his preference in a modified version of the earlier paper done with Alien and Neice, which he read before the annual meeting of the American Rocket Society in San Francisco, in June 1957. Eggers was skeptical about the relatively high heating loads and the deceleration forces characteristic of ballistic reentry, even at a small entry angle. He warned that "the g's are sufficiently high to require that extreme care be given to the support of an occupant of a ballistic vehicle during atmospheric reentry," and pointed out that such an object, entering the atmosphere along a shallow trajectory so as to hold deceleration down to 7.5 g, would generate a surface temperature of at least 2,500 degrees F. Thus, in Eggers' judgment, "the glide vehicle is generally better suited than the ballistic vehicle for manned flight at hypersonic velocities."38
Eggers realized that his glider design, if actually built, would be too heavy for the military rockets then under development. At the same time he remained concerned about the deceleration loads imposed on the space pilot and the heating  loads on the spacecraft structure. He also saw the difficulty of recovering a ballistic satellite, which since it was noncontrollable in the atmosphere, would have to land somewhere in a target area of several thousand square miles. As a consequence of these apprehensions, during the last half of 1957 he sketched a semiballistic device for manned orbital flight, blunt but having a certain amount of aerodynamic lift, with a nearly flat top and a round, deep bottom for heat protection. This design, which Eggers called the "M-1," fell about halfway between the high-lift glider and the ballistic vehicle discussed in his 1954 NACA study with Allen and Neice. About 10 feet wide and nearly seven feet long, the M-1 from above looked like an isosceles triangle rounded at its apex39 A more graphic description was offered by Paul Purser, who called it a "¼ egg lifting shape." 4040 The M-1's limited amount of lift would give it about 200 miles of lateral maneuverability during its descent through the atmosphere and about 800 miles of longitudinal discretion over its landing point. Eggers' calculations indicated that skillful piloting could keep reentry deceleration at about 2 g. 4141
36 The critical problem of aerodynamic heating on the skip vehicle was not considered in the theoretical work on an antipodal bomber done by Eugen Sänger and Irene Bredt for the Luftwaffe in World War II. See Eugen Sänger, Rocket Flight Engineering, NASA TT F-223 (Washington, 1965).
37 Alfred J. Eggers, Jr., H. Julian Allen, and Stanford E. Neice, "A Comparative Analysis of the Performance of Long-Range Hypervelocity Vehicles," NACA Tech. Report 1382, Forty-Fourth Annual Report, 1141-1160. A modified version of this paper is Allen, "Hypersonic Flight and the Reentry Problem," Journal of the Aeronautical Sciences, XXV (April 1958), 217-230.
38 Alfred J. Eggers, "Performance of Long Range Hypervelocity Vehicles," Jet Propulsion, XXVII (Nov. 1957), 1147-1151. Actually the peak temperatures on the heatshield of the Mercury spacecraft during its reentry from an orbital mission reached approximately 3,000 degrees F.
39 Eggers' design is sometimes erroneously referred to as the "sled," after the nickname for a quite similar proposal first made in 1957 by Antonio Ferri and two others at the Gruen Applied Science Laboratories, of Hempstead, New York. See Antonio Ferri, Lewis Feldman, and Walter Daskin, "The Use of Lift for Re-entry from Satellite Trajectories," Jet Propulsion, XXVII (Nov. 1957), 1184-1191. Ferri, Feldman, and Daskin described a high-drag configuration which by flying at a proper angle of attack could produce a comparable amount of lift. Their vehicle would have an open top surface, and in this open area would be located a bubble-canopy sealed cabin. Reentry would be along a phugoid skip trajectory, with the lower surface of the vehicle acting as a heat sink.
40 Purser log.
41 Eggers letter; Syvertson interview.