the SLA, it deployed at a right angle to the SM. Four omnidirectional S-band antennas on the CM were used when the attitude of the CSM kept the high-gain antenna from being pointed at Earth. These antennas were also used between SM jettison and landing. Specifications Length: Diameter: Mass: Structure mass: Electrical equipment mass: Service Propulsion (SPS) engine mass: SPS engine propellants: RCS thrust: 2 or 4 × RCS propellants: MMH/ SPS engine thrust: SPS engine propellants: (UDMH/)/ SPS Isp: 314 s (3,100 N·s/kg) Spacecraft delta-v: Electrical system: three 1.4 kW 30 V DC fuel cells Modifications for Saturn IB missions The payload capability of the Saturn IB launch vehicle used to launch the Low Earth Orbit missions (Apollo 1 (planned), Apollo 7, Skylab 2, Skylab 3, Skylab 4, and Apollo–Soyuz) could not handle the mass of the fully fueled CSM. This was not a problem, because the spacecraft delta-v requirement of these missions was much smaller than that of the lunar mission; therefore they could be launched with less than half of the full SPS propellant load, by filling only the SPS sump tanks and leaving the storage tanks empty. The CSMs launched in orbit on Saturn IB ranged from (Apollo–Soyuz), to (Skylab 4). The omnidirectional antennas sufficed for ground communications during the Earth orbital missions, so the high-gain S-band antenna on the SM was omitted from Apollo 1, Apollo 7, and the three Skylab flights. It was restored for the Apollo–Soyuz mission to communicate through the ATS-6 satellite in geostationary orbit, an experimental precursor to the current TDRSS system. On the Skylab and Apollo–Soyuz missions, some additional dry weight was saved by removing the otherwise empty fuel and oxidizer storage tanks (leaving the partially filled sump tanks), along with one of the two helium pressurant tanks. This permitted the addition of some extra RCS propellant to allow for use as a backup for the deorbit burn in case of possible SPS failure. Since the spacecraft for the Skylab missions would not be occupied for most of the mission, there was lower demand on the power system, so one of the three fuel cells was deleted from these SMs. The command module was also partially painted white, to provide passive thermal control for the extended time it would remain in orbit. The command module could be modified to carry extra astronauts as passengers by adding jump seat couches in