This picture shows the flight path of the SA-206 (Skylab 2) during ascent. The S-IB performed its task in only the first 144 seconds of the flight and was then disposed of. After a 9½ minutes ballistic flight, the S-IB stage plunged into the Atlantic Ocean about 500 km off the east coast of Florida.

This picture shows the flight characteristics of the SA-206 (Skylab 2) from lift-off to orbit insertion. The mass of the whole Saturn IB stack is expressed in payload mass. The payload mass is the combined mass of the command module and the service module which is around 14 metric tons (around 2.4 % of the launch mass). The red curve shows a dramatically large rate of propellant consumption in the first 141 seconds of flight. At the moment of S-IB stage burn-out, the Saturn V has already lost around 69% of its launch mass. The expression "MR shift" stands for "Mixture Ratio shift". It is referring to an automated procedure to change the ratio between the amount of fuel and amount of oxidizer which is supplied to the thrust engines of the S-IVB stage. The objective of this procedure is to optimize the performance of the S-IVB stage by depleting the propellants during flight as much as possible, to keep the mass of the launch vehicle at stage burn out as low as possible. The blue curve representing the acceleration during flight helps us to imagine what the crew must have felt during ascent. In the first minute of the flight the acceleration was slowly building up. But the sudden drop of acceleration at S-IB stage burn-out (from over 4 G's to weightlessness within a second) must have been quite an experience. Two seconds later the astronauts were thrown back in their couches when the S-IVB stage engines ignited and the acceleration went from zero to 0.8 G almost instantly.

This picture shows a set of other flight characteristics of the SA-206 (Skylab 2) from lift-off to orbit insertion. Also is shown which abort procedures have to be used in time during ascent in case of emergencies. Aerodynamic pressure (Q) is depending on the velocity and the air density. Q is zero when the velocity is zero at the moment of lift-off and is also zero when the air density is zero. This implies that during ascent there is a moment when Q has a maximum value. This Max. Q is attained at around 95 seconds into the flight at an altitude of 14 kilometers. For the ascent phase from lift-off to orbit insertion, abort procedures have been devised to return the flight crew safely to Earth in case of emergency. The requirements for an abort procedure however, are much related to the velocity of the launch vehicle and its altitude. Therefore three flight regimes have been distinguished: - atmospheric flight; - transitional or sub-orbital flight; - and space flight. There is an abort procedure for each flight regime. The objective of all these three abort procedures is to attain a quick return of the flight crew to Earth. Each procedure also requires a different mode of operation with regard to the spacecraft, to onboard software for event sequencing and guidance, to ground based stations and to Mission Control. There is however a fourth abort mode which is meant to attain a contigency orbit. This mode provides more options in selecting landing area's and, if possible, to meet some of the mission objectives. Short description of the four abort modes: Mode IA: From launch pad until 61 sec. elspased time: LES propels CM to a safe distance from the exploding launch vehicle. The pitch control motor in the top of the LES is used to push the CM downrange, off the coast, into the Atlantic Ocean. Mode IB: 61 sec. up to 30 km altitude: Same as Mode IA. The launch vehicle has cleared the beach after 42 seconds. After sepration from the launch vehicle a canard system in the top of the LES is used to induce a pitch tumble to put the CM in the proper attitude for parachute deployment. Mode IC: From 30 km altitude up to 3 min. after Lift-Off: The LES is used to seprate the CM. After jettison of the LES, the CM is put in a proper reentry attitude with its RCS. Mode II: Service Module RCS engines or SM main engine propel CM away from the launch vehicle. When the CSM is at safe distance, the CM is separated from the SM and manoeuvred into a reentry attitude. Mode IIIA: This submode has been introduced for high lattitude ascent flight paths. Service Module Propulsuion (SPS) engine is not only used to propel the CM away from the launch vehicle, but also to fly over the cold water water in the North Atlantic and land at a predetermined point. After CM-SM separation normal entry procedures will be followed. Mode IIIB: The SPS engine is used to slow the CSM down to land at a predetermined in the Atlantic Ocean. Upon completion of thois retrograde maneuver anormal entry procedure will follow.. Mode IV: Service Module main engine is used to separate the CSM from the launch vehicle and insert the CSM into a contingency orbit.

Skylab 2 ground track during the Boost Phase (Ascent)

This picture shows the ground track of the SA-206 (Skylab 2) during ascent. The S-IB performed its task in only the first 2½ minutes of the flight and was then disposed of. The S-IB stage plunged into the Atlantic Ocean about 500 km off the east coast of Florida. The S-IVB provided thrust for about 7½ minutes and was then separated about 6 minutes after engine cut-off. The deorbit command was given to the S-IVB at 4.5 hours GET (Ground Elapsed Time). At 6 hours GET the S-IVB stage plunged into the Pacific Ocean, some 1045 km north east from Hawaii.

Skylab 3 ground track during Ascent and Orbital Maneuvering

This picture shows the ground track of the SA-207 (Skylab 3) mission during the boost phase and the first six revolutions. The performance of the Apollo Service Propulsion System In this picture the milestones are mentioned of the socalled coelliptic rendez-vous approach to allow the Skylab 2 spacecraft to dock with the Skylab Orbital Workshop (OWS). This rendez-vous approach has been selected by NASA for the Gemini and Apollo missions because of its relatively ease of use by the astronauts in the terminal phase of this procedure. The required maneuvers were executed with the Apollo Service Propulsion System. The S-IB and the S-IVB stages were just only used to attain the first milestone: to insert the spacecraft into an elliptical orbit (INS). Milestones: INS Insertion into Earth Orbit SEP CSM - S-IVB Separation NC1 Normal Catchup Phasing Maneuver 1 NC2 Normal Catchup Phasing Maneuver 2 NCC Corrective Combination Maneuver NSR Normal Slow Rate Coelliptic Maneuver TPI Terminal Phase Initiation TPF Terminal Phase Finalization DCK Docking (Strictly speaking this discussion about the coelliptic rendez-vous approach actually exceeds the scope of this page. But the maneuvers are an integral part of the groundtrack, so they fit in somewhat naturally in this discussion.)

Skylab 3 Rendez-vous Sequence

This picture provides a schematic overview of the coelliptic rendez-vous approach. The orbital plane of the Skylab OWS and the Skylab 3 spacecraft is depicted. This orbital plane has an inclination of 50.0°. The red colored elliptical orbits are overly drawn for illustration puposes, their real shape were more circular (apogees/perigees: 222km/150km and 385km/222km. The apogee/perigee of the target orbit: 441km/426km. In this picture the milestones are mentioned of the socalled coelliptic rendez-vous approach to allow the Skylab 2 spacecraft to dock with the Skylab Orbital Workshop (OWS). This rendez-vous approach has been selected by NASA for the Gemini and Apollo missions because of its relatively ease of use by the astronauts in the terminal phase of this procedure. INS Insertion into Earth Orbit SEP CSM - S-IVB Separation NC1 Normal Catchup Phasing Maneuver 1 NC2 Normal Catchup Phasing Maneuver 2 NCC Corrective Combination Maneuver NSR Normal Slow Rate Coelliptic Maneuver TPI Terminal Phase Initiation TPF Terminal Phase Finalization DCK Docking The angular/lap positions of the milestones are expressed in fractions of revolutions with respect to the point of orbital insertion (INS), whose angular/lap position is 0.00 revolutions by definition. So the docking (DCK) occurs during the 6th orbital revolution of the spacecraft. The Skylab 3 spacecraft went, roughly speaking, through two types of orbits: The two elliptical orbits. The spacecraft has been inserted in an elliptical orbit (150 km perigee, 222 km apogee) With a socalled apogee kick at point NC1 the spacecraft transferred from the inner elliptical orbit to the outer one; At point NC2 it transferred into a spiral shaped orbit towards the 440 km high circular orbit of the OWS.

References Skylab Saturn IB Flight Manual MSFC-MAN-206, September 1972 Saturn IB Launch Vehicle Flight Evaluation Report SA-206 Skylab 2 Prepared by Saturn Flight Evaluation Working Group George C. Marshall Space Flight Center MPR-SAT-FE-73, July 23, 1973 Press Kit Skylab 3 NASA, Washington DC July 23, 1973

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Copyright 2022 by   Sander Panhuyzen Comments and questions are welcome. All pictures and drawings contained on and through these pages are the author's, unless otherwise noted. No unauthorized reproduction without permission.