24 October 2016

Talking to the Farside: A 1963 Proposal to Use the Apollo Saturn V S-IVB Stage as a Radio Relay

The moon's Farside hemisphere: a mosaic of Lunar Reconnaissance Orbiter images. Image credit: NASA 
A Saturn S-IVB stage awaits shipment from the Douglas Aircraft plant in California. Various red protective covers would be removed before flight. Image credit: NASA
The S-IVB rocket stage played several important roles in NASA's 1960s and 1970s piloted space programs. The 58.4-foot-long, 21.7-foot-wide stage, which comprised a single restartable J-2 rocket engine, a forward liquid hydrogen tank, and an aft liquid oxygen tank, served as the second stage of the two-stage Apollo Saturn IB rocket and the third stage of the three-stage Apollo Saturn V.

The Saturn IB S-IVB's J-2 engine would ignite at an altitude of about 42 miles and burn until it placed a roughly 23-ton payload into low-Earth orbit. After that, it would shut down and the spent stage would separate. The Saturn V S-IVB's J-2, on the other hand, would ignite twice to accelerate the stage and its payload: once for 2.5 minutes at an altitude of about 109 miles and again for six minutes about two and a half hours later. The first burn would place the S-IVB and payload into a low parking orbit between 93 and 120 miles above the Earth; the second would place the S-IVB and payload onto a path that would intersect the moon, about 238,000 miles away, about three days after Earth launch. Departure for the moon was called Translunar Injection (TLI).

During Apollo moon landing missions, the payload was a three-man Command and Service Module (CSM) and a Lunar Module (LM) moon lander. The astronauts would separate the CSM from the four-segment shroud linking it to the S-IVB about 40 minutes after TLI. They would then maneuver it clear of the S-IVB and turn it end-for-end so that its nose pointed back at the top of the stage. The shroud segments, meanwhile, would hinge back and separate to reveal the LM spacecraft mounted atop the S-IVB. The crew would guide the CSM to a docking with the LM; then, about 50 minutes after docking, the joined CSM and LM would move away from the S-IVB. The stage would then vent residual propellants and ignite auxiliary rocket motors to place itself on a course away from the CSM-LM combination.

Schematic representation of Apollo 8 mission events (not to scale). Apollo 8 was the first mission to inject the Saturn V S-IVB stage into orbit around the Sun. Image credit: NASA 
Roughly 60 hours after launch from Earth, the docked CSM and LM would enter the moon's gravitational sphere of influence. About 12 hours later, they would pass behind the moon over the Farside, the lunar hemisphere that is turned always away from Earth. There, out of visual, radar, and radio contact with Earth, the astronauts would ignite the CSM's Service Propulsion System (SPS) main engine to slow the CSM and LM so that the moon's gravity could capture them into lunar orbit. This critical maneuver was called Lunar Orbit Insertion (LOI). Orbital mechanics dictated that LOI should occur more or less over the center of the Farside.

A few hours later, two astronauts would separate from the CSM in the LM. They would fire the moon lander's throttleable descent stage engine - again over Farside, as dictated by orbital mechanics - to begin their descent toward their pre-selected landing site on Nearside, the lunar hemisphere that is turned always toward Earth. Following a safe landing and a period of surface exploration (about one Earth day for the earliest Apollo landing missions), the LM ascent stage would lift off.

About two hours later - again over the moon’s hidden hemisphere – the CSM would rendezvous and dock with the LM. The lunar landing crew would rejoin the CSM pilot, the astronauts would cast off the LM ascent stage, and preparations would begin to ignite the SPS to depart lunar orbit for Earth. The critical lunar-orbit departure maneuver, also carried out over Farside, was called Trans-Earth Injection (TEI).

The S-IVB stage would, meanwhile, swing past the moon and enter orbit around the Sun. Although it would travel to the moon and beyond, as of early 1963 no one had identified any further role for the S-IVB after the CSM and LM separated from it.

The silhouette at left shows the position of the S-IVB third stage in the Saturn V stack. The cutaway illustration (right) shows the interstage faring that linked the S-IVB to the Saturn V S-II second stage and the relative sizes of the liquid oxygen and liquid hydrogen propellant tanks. Helium stored in spherical tanks push propellants into the J-2 engine. Image credit: NASA
For six months in 1963, engineers at The Bissett-Berman Corporation in Santa Monica, California, working on contract to NASA Headquarters, studied another use for the Apollo-Saturn V S-IVB stage. In a series of "Apollo Notes" prepared beginning in March of that year, they identified a need for a relay satellite to enable Earth-based radar tracking of the Apollo CSM and LM while they carried out crucial maneuvers over the Farside. They then proposed that the spent S-IVB be outfitted to serve as a relay.

The first note, authored by H. Epstein and based on a concept suggested by L. Lustick, proposed a radar relay satellite for tracking the Apollo CSM during LOI and CSM rendezvous and docking with the LM ascent stage. Epstein and Lustick's satellite would include an omnidirectional antenna for near-lunar operations and, for "deeper phase operation," a steerable four-foot parabolic dish antenna.

The relay satellite, Epstein wrote, would separate from S-IVB stage along with the Apollo LM and CSM after TEI, then separate from the CSM-LM combination before LOI. It would fly past the moon on a path that would keep both Earth and most of the Farside in view during LOI and CSM-LM rendezvous and docking. The omni antenna would relay radar from Earth until the satellite was 40,000 kilometers beyond the moon, then the dish would take over.

The second Bissett-Berman Apollo Note, dated 16 April 1963, raised the possibility of placing a "special purpose relay package" on the S-IVB stage. The package would either remain attached to the stage or would eject from it when activated. The Apollo Note's author, L. Lustick, attributed the S-IVB relay concept to one Dr. Yarymovych, whose organizational affiliation was not stated.

For his analysis, Lustick assumed that the S-IVB would retain enough propellants for its J-2 engine to restart a third time shortly after CSM-LM separation, raising its speed by 160 feet per second. He calculated that, at the time of CSM-LM LOI, the S-IVB or ejected relay package would have in view simultaneously both Earth and more than three-quarters of the Farside hemisphere. At the time of CSM docking with the LM ascent stage, about 100 hours after Earth launch, the relay would have in view Earth and a little more than two-thirds of the Farside. Throughout the approximately 28-hour period between LOI and CSM rendezvous with the LM ascent stage, the S-IVB or ejected relay package would remain within 143,000 miles of the moon.

The S-IVB would rely for attitude control guidance on the ring-shaped Instrument Unit (IU), the Saturn V's "electronic brain." The IU, located on top of the S-IVB during launch, encircled the LM descent stage and provided attachment points for the four separable shroud segments. It was not intended to operate for more than a few hours, so would need modifications to ensure that it could reliably stabilize the S-IVB throughout the relay period.

The ring-shaped Instrument Unit (IU) rode atop the S-IVB stage on both Saturn IB and Saturn V rockets. Image credit: NASA
The Instrument Unit assembly line at the IBM plant in Huntsville, Alabama. Image credit: NASA
In an addendum to Lustick's 16 April Apollo Note dated 18 April, engineer H. Epstein looked at simplifying the S-IVB Farside Relay concept by assuming that the stage would lack attitude control while it acted as a data relay. Replacing steerable dish antennas – one for Earth-S-IVB communication and one for S-IVB-Apollo CSM communication – with two passive omnidirectional antennas would permit data relay no matter how the S-IVB stage became oriented, he wrote.

The use of relatively low-power omni antennas would produce few problems as far as Earth-S-IVB communication was concerned, for NASA could call into play large antennas on Earth to ensure reception of the weak signal. Epstein proposed increasing from four feet to five feet the planned diameter of the dish antenna on the CSM to enable it to receive data from Earth relayed through the S-IVB-CSM omni antenna. He noted, however, that, even with a larger CSM dish antenna, radio interference from the Sun might stymie the omni antenna relay concept.

An undated Apollo Note by Lustick and C. Siska explored the S-IVB Farside Relay concept in yet greater detail, and included evidence of NASA interest in the scheme: for the first time, the authors cited NASA Headquarters-imposed study requirements. The space agency told Bissett-Berman to assume that the S-IVB could increase its speed by up to 1000 feet per second for up to about seven hours after TLI, and that the maximum range between the S-IVB Farside Relay and the CSM should not exceed 40,000 nautical miles throughout the relay period.

NASA, Lustick and Ciska explained, sought to learn whether relay of voice (not only data or radar) would be possible using an S-IVB Farside Relay during the roughly 30-hour period between LOI (a "particularly important" time to have voice relay capability, NASA asserted) and CSM-LM ascent stage rendezvous and docking. The authors found that boosting the S-IVB's speed by 1000 feet per second 7.6 hours after TLI would place it on a path to relay voice between Earth and Farside from 72 hours after Earth launch until 102 hours after Earth launch, at which time the S-IVB would reach NASA's 40,000-nautical-mile operational limit. In fact, they found that the S-IVB would have Farside in view as early as 60 hours after Earth launch (this was of purely academic interest, however, because the Apollo spacecraft would not be orbiting over the Farside at that time).

Lustick and Ciska also noted that the S-IVB would pass out of sight behind the moon (that is, become occulted by the moon) as viewed from Earth 102 hours after Earth launch. They added, however, that slight adjustments in S-IVB boost direction would postpone loss of Earth contact with the S-IVB Farside Relay for long enough to ensure that voice communication could continue during CSM rendezvous with the LM ascent stage.

In Bissett-Berman's penultimate examination of the S-IVB Farside Relay concept, author Ciska noted that a 1000-foot-per-second boost could be planned for as early as TLI. This would, however, leave no propellant margin for later correction of S-IVB boost aim errors. On the other hand, S-IVB attitude control was expected to "drift" over time, making accurate boost pointing later than TLI increasingly unlikely. Furthermore, boil-off of liquid hydrogen from the S-IVB stage would rapidly reduce the amount available to fuel a later boost. Both of these factors favored an "all-or-nothing” early boost.

Ciska noted also that, regardless of the S-IVB boost aim point selected, the stage would pass out of sight behind the moon as viewed from Earth for roughly half an hour at some point along its curved path during the voice relay period. For a 1000-foot-per-second boost applied 7.6 hours after TLI with an aim point slanted 100° relative to a line linking the Earth and moon, for example, the half-hour occultation would occur about 99 hours after Earth launch.

The last Bissett-Berman Apollo Note devoted to the S-IVB Farside Relay concept, also by Ciska and dated 20 August 1963, was an extension of his earlier note. In it, he examined an S-IVB boost 4.15 hours after TLI and considered further the impact of boost direction. Ciska did not attempt to plot S-IVB attitude drift or liquid hydrogen boiloff rates; nevertheless, he proposed as realistic a 700-foot-per-second boost 4.15 hours after TLI with an aim point slanted 100° relative to the Earth-moon line. Following this maneuver, the S-IVB Farside Relay would pass out of view of Earth for about 30 minutes a little more than 83 hours after Earth launch and would pass beyond NASA’s 40,000-nautical-mile limit about 103 hours after launch.

The S-IVB stage converted into the Skylab Orbital Workshop retained its IU and liquid oxygen tank - the latter launched dry and used as a dumpster - but lost its J-2 engine and saw its liquid hydrogen tank converted into a large habitable volume (note astronaut on lower deck for scale). Image credit: NASA
Though the Bissett-Berman S-IVB relay proposal was not taken up, S-IVB stages did play key non-propulsive roles in NASA's manned space program. NASA converted Saturn IB S-IVB 212 into the Skylab 1 Orbital Workshop. Skylab was launched into low-Earth orbit on the last Saturn V to fly and staffed by three three-man crews in 1973-1974. Saturn V S-IVB 515, originally intended to boost the Apollo 20 mission to the moon, was converted into the Skylab B workshop, but was not launched and became a walk-through display in the National Air and Space Museum in Washington, DC.

Of the 10 Apollo Saturn V S-IVBs that departed low-Earth orbit between 1968 and 1972, half reached orbit about the Sun and half were intentionally crashed into the moon. The Apollo 8, Apollo 9, Apollo 10, Apollo 11, and Apollo 12 S-IVBs departed the Earth-moon system, while those that boosted Apollo 13, 14, 15, 16, and 17 out of low-Earth orbit toward the moon were intentionally impacted on the moon's Nearside. The impacts were part of a science experiment: the seismic waves their impacts generated registered for hours on seismometers left behind on the lunar surface by earlier Apollo crews, helping to reveal to scientists the structure of the moon's deep interior.

The irregular ray crater blasted out by the impact of the Apollo 13 S-IVB stage in 1970 as imaged by NASA's Lunar Reconnaissance Orbiter in 2010. Image credit: NASA
The Apollo 12 S-IVB, launched on 14 November 1969, flew past the moon too fast to receive a gravity-assist boost into orbit about the Sun, so circled the Earth in a loosely bound distant orbit until 1971, when, through gravitational perturbations from Earth, Sun, and moon, it finally escaped into solar orbit. It orbited the Earth again for about a year in 2002-2003, during which time it was observed and mistakenly identified as a near-Earth asteroid. Spectral analysis revealed the presence of titanium-based paint, however, confirming the object's identity as Apollo 12's errant S-IVB.

Sources

Apollo Note No. 35, Lunar Far Side Relay Technique – Some Basic Radar Considerations, H. Epstein, The Bissett-Berman Corporation, 21 March 1963

Apollo Note No. 44, Back of Moon Relay Trajectories, L. Lustick, The Bissett-Berman Corporation, 16 April 1963

Addendum to Apollo Note No. 44, Communications Capability of Unstabilized S-4-B Satellite Relay System, H. Epstein, The Bissett-Berman Corporation, 18 April 1963

Apollo Note No. 87, Section 7, Far-Side Relay, L. Lustick and C. Ciska, The Bissett-Berman Corporation, no date

Apollo Note No. 90, Further Examination of Far-Side Relay Trajectories, C. Ciska, The Bissett-Berman Corporation, 6 August 1963

Apollo Note No. 97, Minimum Boost Velocity Requirement for Far-Side Relay, C. Ciska, The Bissett-Berman Corporation, 20 August 1963

More Information

Reviving & Reusing Skylab in the Shuttle Era: NASA Marshall's November 1977 Pitch to NASA Headquarters

What If Apollo Astronauts Could Not Ride the Saturn V Rocket? (1965)

Solar Flares and Moondust: The 1962 Proposal for an Interdisciplinary Science Satellite at Earth-moon L4

2 comments:

  1. There's a lovely little animation of the Apollo 12 S-IVB entering the Earth/Moon system, and then getting ejected: https://www.youtube.com/watch?v=EWRrbqxU_Wc

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