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 shield to maintain a satisfactory thermal control surface during the rest of the mission back to entry. The actual jettison opera­tion of the LES, however, produces another thermal problem. The exhaust products of the solid jettison motors will impinge on the clean surfaces of the CM and SM and degrade the thermal performance to same extent. The complete nature of this problem is currently being investigated in ground tests and will be investigated in the early development flights. A later paper in the Symposium will discuss this in detail.

The second stage burn continues for a total duration of about 375 seconds, a little more than 6 minutes. At this point, orbital altitude of 100 nautical miles has essentially been reached but the spacecraft is still short of orbital velocity by about 3000 ft/sec. Inertial velocity is about 22,650 ft/sec. The vehicle is almost nautical miles down range from Cape Kennedy. The tracking stations at the Cape are out of range by this time but other stations at Grand Turk or Bermuda have picked up the space vehicle, so that communications have been uninterrupted.

Following shutdown of the five J-2 engines on the S-II stage, S-II/S-IVB separation occurs, and third stage operations being (Figure 18). The separation sequence is similar to the one described for S-IC S-II stage separation—the S-II retro-rockets and S-IVB ullage rockets fire and pyrotechnic devices separate the stage. The problem mentioned previously of thermal cotaingcoating [sic] degradation from the LES jettison motors is similar to one which occurs during S-II/S-IVB separation in that the S-II retro-rocket gases impinge on the Service Module thereby degrading its thermal coating. As mentioned previously, this problem is currently under investigation.

The S-IVB burn during the boost phase lasts for about 2½ minutes and imparts some 3000 ft/sec to the spacecraft velocity—boosting it up to the orbital velocity of about 25,600 ft/sec. Thrust of the S-IVB stage is about 200,000 pounds—produced by the single J-2 engine. At the conclusion of the S-IVB burn, the spacecraft is at 100 nautical miles altitude and has traveled another 600 nautical miles downrange for a total distance during the boost phase of almost 1500 nautical miles. The total ascent has taken about 11½ minutes (Figure 19).

During this third stage burn, communication with the land-based stations has been lost, but the spacecraft has been acquired by the insertion ship which has been specifically located to fill in this gap and to provide a period of tracking immediately after insertion for the purposes of confirming a safe orbit, and issuing the decision to continue the mission. The insertion ship coverage is shown in Figure 20.