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[1]
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.
 |
| Figure 1. Stylized view of Gemini-B/MOL in low Earth orbit. Note the absence of any maneuvering thrusters, antennae or reconnaissance telescope aperture, but the gratuitous addition of a red nose on the Gemini-B. (Credit: McDonnell-Douglas, 1967) |
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[2]
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?
 |
Figure 3.
Retrograde rockets in NASA Gemini (4 rockets) and Air Force Gemini-B (6 rockets).
(Credit: McDonnell-Douglas.)
|
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 it[3]
under similar conditions as for the original Gemini heat shield.[4]
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.[5]
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.[6]
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.[7]
Lower altitude aborts would have used only the ejection seats because the Titan
rocket without the solid rocket boosters represented less explosive potential.
 |
Figure 4. Gemini-B/Titan-IIIM abort modes. Note “salvo fire” of all six
retrograde rockets near point B during the period when the solid rocket
boosters are in operation, compared with one-at-a-time “ripple fire” near point
C for an abort late in the launch phase. (Credit: McDonnell-Douglas.)
|
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.[8]
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.[9] 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).
 |
Figure 5. Possible re-entry trajectories.
|
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[10]
(due south of Hawaii), began re-entry over Mexico south of Texas and splashed
down in the western Atlantic Ocean.[11]
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.
Table 1. Approximate travel in orbit from de-orbit maneuver to atmospheric entry
for the Gemini 3 standard and fail safe cases compared with Gemini 10 (typical
de-orbit) and theoretical minimum de-orbit maneuver.
|
Case
|
Distance
travelled in orbital trajectory
|
|
Degrees
|
Miles (Kilometers)
|
|
Gemini 3
|
From
retrograde rockets firing to landing
|
40
|
3,000
(4,800)
|
|
From OAMS
firing to fail-safe landing
|
140
|
9,500
(15,300)
|
|
Gemini
10 (typical)
|
From retrofire
to 400K ft
|
75
|
5,200
(8,380)
|
|
From
400K ft to landing
|
36
|
2,500
(4,020)
|
|
From
retrofire to landing
|
111
|
7,700
(12,400)
|
|
Theoretical
Hohmann descent orbit to 400K ft
|
180
|
12,400
(20,000)
|
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.
Table 2. Maximum-altitude circular Gemini orbit consistent with de-orbit using
four retrograde rockets, compared with highest typical Gemini mission orbits.
|
Case
|
Orbital
altitude, miles (kilometers)
|
|
Average
|
Apogee x
perigee
|
|
Theoretical
maximum via Hohmann trajectory
|
286
(461)
|
286
(461) circular
|
|
Gemini 7
|
137
(221)
|
174 x
100 (280 x 161)
|
|
Gemini
11
|
183
(295)
|
189 x
178 (304 x 287)
|
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
onboard maneuvering engines in the MOL’s Attitude Control and Translation
System (ACTS). Mockups 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).
Table 3. Maximum circular orbit for Gemini-B consistent with de-orbit using six retrograde
rockets, compared with highest (initial) and lowest (final) MOL orbits (see
Reference 5).
|
Case
|
Orbital
altitude, miles (kilometers)
|
|
Average
|
Apogee x
perigee
|
|
Theoretical
maximum via Hohmann trajectory
|
396
(638)
|
396
(638) x 398 (638)
|
|
Assumed
initial MOL orbit
|
137
(221)
|
214 x 92
(345 x 148)
|
|
Assumed
final MOL orbit
|
183
(295)
|
159 x 89
(256 x 144)
|
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.
 |
Figure 7. Apollo
Service Module (SM) used its forward-firing maneuvering thrusters to insure
adequate separation from the command module (CM) during re-entry. MOL
apparently did not have a similar capability due to its lack of forward-firing
thrusters (see Figure 6). (Credit: NASA.)
|
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[12]
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.
 |
| Figure 8. Separation points of Gemini-B and MOL. [A] Pyrotechnically-driven cutter for two mechanical linkages and electrical connection of re-entry vehicle and retrograde adapter. [B] Shaped explosive charge to separate retrograde and equipment adapter modules. [C] Shaped explosive charge to separate equipment adapter module and MOL unpressurized module. [D] Interface mating lugs (bolts) connecting Gemini-B and MOL (26 places). (Credit: McDonnell-Douglas with annotations by author.] |
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.
Acknowledgements.
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.
[4]
Launch Evaluation Report MOL/HST Spacecraft, McDonnell Co., Dec. 3,
1966. Not available on-line; contact author.
[5]
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.
[6]
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).
[7]
“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.
[9]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)
[11]
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)
[12]
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.
