I am not trained in orbital mechanics, also called “astrodynamics,” as practiced by Rich Purnell in the movie The Martian. But I feel some kinship with him because, except for his youthful good looks, his grasp of extreme mathematics and his access to the “NASA Supercomputer,” he and I are a lot alike. He used orbital mechanics to solve a life-or-death problem on a Mars mission gone wrong twenty years in our future. I used orbital mechanics to decipher an obscure feature of a military space program cancelled almost fifty years ago.
If the U.S. Air Force’s secret Manned Orbiting Laboratory (MOL) had flown into low Earth orbit in the 1970s, its astrospy pilots would have ridden in the Gemini-B variant of NASA’s retired Gemini spacecraft during launch and landing (Figure 1). Gemini-B looked outwardly very similar to its predecessor (see Figure 2), but it was stripped down for its supporting role during month-long reconnaissance missions. It would have gotten its on-board electricity from batteries instead of hydrogen-oxygen fuel cells, giving it an independent lifetime of only 14 hours, shorter than all but two Gemini missions. Gemini-B would have been launched already bolted to the MOL, so it wouldn’t have needed rendezvous radar or a full set of maneuvering thrusters. Fuel cells and maneuvering thrusters would have been on the MOL, the central component of the mission.
One area in which Gemini-B was not stripped down was its retrograde rocket complement. It was to carry six of the same Star-13E rocket motors as Gemini (see Figure 3). But the MOL mission called for orbits as low as or lower than those of Gemini, which had only used four retrograde rockets: de-orbiting from a lower orbit should not have required more retrograde rockets. Why did Gemini-B need six?
Not being an engineer or astrodynamicist like Rich Purnell, I inquired among known experts. They didn’t know either, but they made some reasonable guesses.
Was it because Gemini-B was carrying more mass than Gemini at deorbit? I estimate that Gemini-B was to be only 10% heavier than Gemini,* certainly not requiring 50% more retrograde rocket thrust for de-orbit.
Was it some sort of military requirement to "get 'em down ASAP," or to simulate a lunar re-entry profile, or a need for a shorter orbital arc from retrofire to re-entry to minimize any guidance (“aiming”) errors during the de-orbit maneuver. The first two seem unlikely, but the shorter arc was mentioned by a few experts as being a factor in NASA Gemini re-entries. Using an even shorter arc on Gemini-B might have stressed its heat shield with more thermal loading than Gemini experienced. But a re-entry test validated the modified heat shield with a plug hatch cut into itunder similar conditions as for the original Gemini heat shield. Clearly the re-entry conditions for Gemini-B were planned to be the same as for Gemini.
Was it somehow driven by the geographical limitations of available equatorial ground stations tracking the re-entry trajectory of a polar orbiting spacecraft? This suggestion seems to assume that the entire de-orbit, re-entry and landing sequence could be accomplished within view of a single tracking station, which were scattered around the Earth within about 30 degrees of the equator. Such an extremely abrupt de-orbiting seems unlikely, unsafe and unnecessary; more likely, a tracking ship or aircraft could be stationed in the high northern or southern latitudes far outside the existing U.S. network, which sounds like a good idea in any case.
The only justification I have ever seen for carrying six retrograde rockets is that they were primarily for off-the-pad launch aborts of the Titan III-M launcher with its two highly-explosive side-mounted seven-segment solid boosters (see Figure 4). If an abort was required before liftoff or up to 31 seconds later, salvo-firing all six retrograde rockets simultaneously would rocket the Gemini-B to a safe distance from the exploding booster, allowing the pilots to eject and land under their personal parachutes. In any abort from 31 seconds to separation of the solid rocket boosters, the pilots would not eject but would stay in their Gemini-B capsule through re-entry and splashdown. The NASA Gemini also had a salvo-fire option of its four retrograde rockets, but only for launch aborts above 70 thousand feet. Lower altitude aborts would have used only the ejection seats because the Titan rocket without the solid rocket boosters represented less explosive potential.
In fact, I have concluded that Gemini did not even need its four retrograde rockets to de-orbit at all, and Gemini-B certainly did not need six. The first two piloted Gemini missions demonstrated a fail-safe de-orbit option in case their retrograde rockets failed to fire. On its final orbit, Gemini 3 fired its Orbital Attitude and Maneuvering System (OAMS) thrusters, already known to be functioning correctly from maneuvers on earlier orbits, for two minutes while passing near Hawaii, setting up an orbit with a low point of 54 miles, well below the 76-mile altitude used as the “top” of the atmosphere. Then the retrograde rockets were fired as planned near Los Angeles, bringing the spacecraft to its intended landing site about 70 miles east of Grand Turk Island in the Atlantic Ocean. If the rockets had not fired, the spacecraft would still have landed about 1,000 miles west of Ascension Island in the central Atlantic (see Figure 5).
Of course, the Gemini retrograde rockets worked on-time every time on every mission, and the fail-safe option was discarded after Gemini 4, permitting the full maneuvering fuel supply to be applied to rendezvous maneuvers. For example, Gemini 10 de-orbited near Canton Island in the Pacific Ocean (due south of Hawaii), began re-entry over Mexico south of Texas and splashed down in the western Atlantic Ocean.
The fail-safe maneuver provided only slightly more than the theoretical minimum velocity change required, which would have produced an arc of 180 degrees and 12,400 miles (20,000 km)—halfway around the Earth—in what is called a Hohmann orbit (see Table 1.) Thus, both Gemini 3 and Gemini 10 started their descents from approximately the same longitude, but Gemini 3 followed a shallower trajectory until it fired its four retrograde rockets to end up splashing down approximately where Gemini 10 did.
The highest circular orbit from which the four retrograde rockets could de-orbit a standard Gemini (using a Hohmann orbit with a perigee of 400,000 feet, which is 122 kilometers or 76 miles) was much higher than any Gemini ever flew unless it was docked to an Agena-D rocket stage (see Table 2). This demonstrates that the four retrograde rockets were overkill for de-orbiting purposes.
Gemini-B/MOL would have been in an even lower orbit than Gemini to improve its high-resolution Earth photography, and constant atmospheric drag would have been slowing the vehicle enough to de-orbit it in hours or days. This would surely have required frequent orbital boosts from the on-board maneuvering engines in the MOL’s Attitude Control and Translation System (ACTS). Mock-ups and images of MOL from late in its design phase show the largest ACTS thrusters were those pointed to the rear (“+x” in spacecraft parlance) (Figure 6) to speed up the MOL. There didn’t seem to be any thrusters at all pointed forward; maybe the designers didn’t foresee any need to slow MOL down more than atmospheric drag would already achieve.
Based on the same type of analysis as for the NASA Gemini orbits, the six retrograde rockets on Gemini-B would have permitted de-orbiting from a circular orbit over twice as high as the final orbit we assumed for the MOL missions and forty percent higher than the initial orbit we assumed (see Table 3).
If the MOL provided adequate propulsion capability and if the retrograde rockets were even more overkill on Gemini-B than on Gemini, why didn’t Gemini-B dispense with retrograde rockets entirely and utilize the MOL’s ACTS thrusters to de-orbit the entire vehicle? This would obviously have been immediately followed by separating the Gemini-B from the MOL so it could land safely while the single-use MOL burned up in the atmosphere as intended.
I have not seen an authoritative discussion of this topic, but maybe it is there, deep in some yet-to-be-declassified documents. So, I can only guess. Perhaps there was concern about ensuring adequate distance between Gemini-B and MOL to avoid re-contact and collision during the buffeting of re-entry. For comparison, the Apollo service module actively distanced itself from the command module during re-entry (see Figure 7) with no instances of recontact. It used a thruster configuration apparently not available on MOL, so perhaps that was one reason.
Air Force mission planners may also have been interested in targeting MOL to a different disposal site than the splashdown site of Gemini-B. Dan Adamo and I speculated that Gemini-B would be aimed to land near Hawaii but MOL would be targeted to the Marianas Trench several thousand miles to the west to prevent Soviet retrieval of any heavy elements that survived re-entry. This may have required MOL to remain in orbit several hours longer than Gemini-B.
There were also other risks. The ACTS thruster fuel could have been exhausted before the planned end of the mission, preventing a targeted de-orbit and leaving the military MOL pilots to an inevitable but uncertain landing in a large swath of the Earth—including in a country they may have been spying on from orbit. However, ACTS fuel status would certainly have been monitored regularly and the mission could have been shortened if necessary to protect re-entry capability.
There was also a small risk of failure to separate the Gemini-B equipment adapter module from the laboratory after the ACTS de-orbit maneuver, but the MOL design already envisioned a sequence of separations between the Gemini-B and the MOL: first a shaped charge would have split the connection to MOL at the bottom of the equipment adapter (C in Figure 8); then, prior to retrofire, another shaped charge would have split the retrograde adapter from the equipment adapter (D in Figure 8); finally, after retrofire, two pyrotechnic charges would have broken the structural and electrical connections between the re-entry vehicle and the retrograde adapter (A in Figure 8). Those same steps could have provided triply-redundant assurance of Gemini-B separation from MOL after de-orbiting using the ACTS.
These alternatives all have one thing in common: once safely in orbit, without having to salvo-fire the retrograde rockets during a launch abort, it was a better idea to fire them for de-orbiting and use them up than to have unexpended ordnance in proximity during re-entry heating, when they would certainly explode, spraying shrapnel in the vicinity and damaging the Gemini-B’s heat shield, or possibly fire and push the retrograde module into a collision with the re-entry vehicle.
Thus, Gemini-B would simply have continued the tried-and-true Gemini practice and used its available launch abort rockets to shorten the arc of its re-entry orbit. One may quibble over whether those rockets should have been named “launch abort rockets” instead of “retrograde rockets” but the former would have represented an improbable eventuality while the latter represented a certainty.
Still, questions remain. Wouldn’t applying fifty percent more retro-thrust have made the Gemini-B re-entry significantly steeper and hotter than it was qualified for? And if so, were there options that maintained those conditions while not leaving unfired ordinance in proximity to the re-entering Gemini-B?
Those are the topics of an upcoming post.
Thanks to Roger Balettie, Jorge Frank, Jonathan McDowell, Jim Oberg and Ryan Whitley, among others, for their patience in explaining aspects of orbital mechanics to me, and to Dr. Dwayne Day for documents and illustrations used in this analysis.
* The six rocket motors would weigh 58.7 kg (129.4 lb) more than just four on Gemini, until they were fired. The film canisters from the KH-10 DORIAN cameras, assuming four of the type flown by Corona reconnaissance satellites weighing 36 kg (80 lb) each, would add 145 kg (320 lb) more than Gemini. This was estimated based on data retrieved from searches on various websites, including the National Reconnaissance Office collection of declassified documents; see for example http://www.nro.gov/foia/CAL-Records/Cabinet2/DrawerC/2%20C%200052.pdf (accessed Sept. 27, 2015).
 “Astrospies,” NOVA, PBS, air date Feb. 12, 2008, http://www.pbs.org/wgbh/nova/military/astrospies.html (accessed Oct. 4, 2015).
 Also called the TE-M-385; see ATK Thiokol's solid fuel STAR motors (source: ATK catalog), updated July 26, 2012, http://www.b14643.de/Spacerockets_2/Diverse/ATK-Thiokol_STAR/index.htm (accessed Nov. 15, 2014).
 “A Jones for MOL #3: Down the hatches,” http://www.astrocryptotriviology.blogspot.com/2012_09_01_archive.html (accessed Oct. 4, 2015); “A Jones for MOL #7: Hatches? We Don’t Need No Stinking Hatches!” http://www.astrocryptotriviology.blogspot.com/2012_12_01_archive.html (accessed Oct. 4, 2015).
 Launch Evaluation Report MOL/HST Spacecraft, McDonnell Co., Dec. 3, 1966. Not available on-line; contact author.
 Charles, John B., and Daniel R. Adamo, “Thirty Days in a MOL: Biomedically Relevant Aspects of a Reconnaissance Mission Inferred from Orbital Parameters,” Quest, The History of Spaceflight Quarterly, vol. 22, no. 2, pp. 3-14, 2015.
 Shayler, David J., Space Rescue: Ensuring the Safety of Manned Spacecraft, Springer Praxis, Berlin-Heidelberg-New York, 2009, p. 204-6, “Launch escape, 2: Ejection seats. Gemini and Manned Orbiting Laboratory”, http://books.google.com/books?id=wEHL8MIhRa8C&pg=PA206&lpg=PA206&dq=gemini-B+retrorocket+abort&source=bl&ots=IW1teNACQ_&sig=wuZFuNxmbpZOBPGhjogDzpm8g4Y&hl=en&sa=X&ei=ftlfVPjsOomuyQT72IDgAw&ved=0CDsQ6AEwBA#v=onepage&q=gemini-B%20retrorocket%20abort&f=false (accessed Nov. 9, 2014).
 “Launch to insertion abort boundaries, launch heading = 72°,” Gemini Design Certification Report, Feb. 19, 1965, p. 2.1-11, Figure 2.1-2. Not available on-line; contact author.
Short news article quoting Dr. Christopher C. Kraft originally appeared in the Galveston News-Tribune, Feb. 16, 1965, reproduced in the NASA Astronautics and Aeronautics Report for 1965, p. 68, http://history.nasa.gov/AAchronologies/1965.pdf (accessed Sept. 2, 2013)
 “Canton Island Tracking Station (CTN),” http://wikimapia.org/19994259/Canton-Island-Tracking-Station-CTN (accessed Oct. 10, 2015).
 Gemini X Mission Report, NASA Manned Spacecraft Center, Houston, Texas, August 1966, p. 4-38, Figure 4-2c Re-entry, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750067644.pdf (accessed Nov. 22, 2014)
 Charles, John B., and Daniel R. Adamo, “Thirty Days in a MOL: Biomedically Relevant Aspects of a Reconnaissance Mission Inferred from Orbital Parameters,”Quest, The History of Spaceflight Quarterly, vol. 22, no. 2, pp. 3-14, 2015.