When the National Air and Space Museum in Washington D.C. opened its new building for the US Bicentennial in 1976, it had a special surprise for fans and friends of Star Trek – Kirk’s iconic USS Enterprise had become a part of the collection and it was the first time that the underside of the engineering hull and its multiple markings could be seen by the general public (it’s different now as the original model has been tainted, painted, gridded, weathered and seam welded...
).
Very fortunately Phil Broad has kept a visual record of the (almost) untampered original condition of the VFX model: http://www.cloudster.com/Sets&Vehicles/STEnterprise/SmithsonianMiniature.htm
I had the privilege of seeing the Enterprise in 1976 and 1980 and expectedly wondered about the functionality of these markings and/or hatches. What really caught my eye was the (only exterior) small print near the saucer’s underside windows which I believe to be a good starting point trying an attempt to decipher the design philosophy behind these markings.
Saucer Hull underside: http://www.cloudster.com/Sets&Vehicles/STEnterprise/ent66.jpg
The text near the upper grey rectangles reads “Inspection Door Vent Systems Connections”. I believe this to be mean inspection door / maintenance hatch for the vent systems connections to which the Enterprise connects at a service station in space (air exchange). Thus grey could indicate “must do” maintenance hatches once at a service station in space.
On the starboard side just below the grey rectangle there is a yellow rectangle. In our current color warning applications (mixed with black) yellow indicates a hazard (moving part or an obstacle). As an airlock door it would be a moving part and the location for the airlock here seems in accordance with the deck level of the airlock in TMP where for some odd reason it’s on the port side: http://movies.trekcore.com/gallery/albums/tmp2/tmphd2072.jpg
The trapezodial panels (almost triangular in appearance) have invited a variety of interpretation proposals. I favor the extendable landing leg / stabilizing pole theory after the saucer has detached from the engineering hull and the neck section and needs to land on a planet, seen here http://www.cygnus-x1.net/links/lcar...l/USS-Enterprise-Officers-Manual_Page_038.jpg, but reject the obvious allusion to “Forbidden Planet” (i.e. extending sensor cylinder) and no longer believe the neck will stick like dead weight to the saucer in an emergency separation scenario:
The inevitable question is of course where the third landing pole would be. In my world it’s at the stern of the saucer but covered by the neck section attached (this would probably require a diagonal ‘throat’ turboshaft. Since I strongly advocate such a shaft – even in a diagonal shaft the turboshaft lights would seem to pass by vertically! - I have no doubt that this is where the third pole is hidden from our view). The landing leg or stabilizing pole theory is also supported by TMP Enterprise where there...are...four...- landing legs at the underside of the saucer (suggesting landing capability of the saucer).
As for the two red stripes near the stern of the saucer I have no ideas at this time.
Saucer Hull upper side: http://www.cloudster.com/Sets&Vehicles/STEnterprise/ent58.JPG
We see the parallel red lines that run from the back of the ‘teardrop’ all the way to the stern of the ship near the flight deck clamshell space doors. This is most likely the ‘spine’ or central nervous system of the ship with a large number of (indiscernible) external maintenance hatches between these red lines.
In need to address the protrusion at the stern of the upper side before I proceed. Of all the interpretations I’ve read here at Trek BBS, I like the one of the gentleman who suggested this to be an exterior connect and/or docking hardpoint the best. In fact, it appears to be the only surface feature of Kirk’s Enterprise that would allow a docking or payload connection (e.g. the third warp nacelle if you prefer
).
As a docking connect point it would equally hold the entire ship in place or just the saucer hull after an emergency separation and would roughly look like Andrew Probert’s original docking concept for the Enterprise-D: http://images.wikia.com/memoryalpha...prise-D_docking_concept_by_Andrew_Probert.jpg
The TMP spacedock had a docking tube attached to the port side of the ship, the TOS Enterprise does not have such a feature at the outer rim of the saucer but I believe the yellow rectangle on the ‘teardrop’ is the access for such a docking tube (there is a better and more practical way to have telescopic equipment ship-mounted which I’ll address shortly). In naval terms this could make the deck below the bridge the “quarterdeck” or – in TOS terms – the “Q deck”.
Based on his World War II aviation experience the Enterprise’s designer Matt Jefferies felt that the ship’s engines would suffer wear and tear and would need to be replaced in intervals (nice explanation for the warp nacelle differences between the Pike and the Kirk Enterprise). So what’s the story for the impulse engines (especially after “Tomorrow Is Yesterday”)? The large L-shaped panels appear to correspond to the location of the impulse engines and could suggest an hatch access to remove the entire engine block and replace it with a new one.
The yellow rectangles near these hatches are probably (again) airlocks which would enable engineering personnel to go EVA to supervise and assist engine block exchange or payload connection.
Warp Nacelles underside: http://www.cloudster.com/Sets&Vehicles/STEnterprise/ent67.jpg
Probably same story like with the aforementioned impulse engine maintenance hatches. You may need to take the cover panel off to unlock the warp nacelle from the support pylon in order to exchange it for a new one. While there’s no airlock there for engineering personnel, the upper windows in the pylons could indicate the work stations from which to monitor such an engine exchange.
Engineering Hull underside
The yellow circle is definitely an enigma but also the perfect ‘excuse’ to wrap a circular ship’s corridor around it (suggested by various scenes in TOS that take place in the engineering hull). Ironically, I had never thought of this until I noticed the circular computer core in the engineering hull on the three-dimensional cutaway poster of the Enterprise-D where the artist was possibly looking for and eventually creating this kind of ‘excuse’.
Back in the 80’s I did a cutaway blueprint of the TOS Enterprise and assumed that the yellow circle should be the bottom part of a probe (launching) cylinder. After all, there needed to be an egress possibility for the flight recorder in “The Corbomite Maneuver”, the satellites in “Operation Annihilate” and the probes in “The Immunity Syndrome”. A 360° rotating cylinder would provide optimal flexibility to launch flight recorders, satellites and probes in almost any direction you wanted without changing the ship’s trajectory.
And unless you needed to launch sensory equipment as a probe, that same sensory equipment could be used during normal astrophysical examinations. Just turn the cylinder with the corresponding probe towards the stellar object you want to examine. Done with optical astronomy? Turn the cylinder a few degrees to do infrared, next. After that an examination of radio, ray, and neutrino astronomy by the little rotation of the cylinder in the shortest amount of time. I presume Stellar Cartography (“The Changeling”) aka Auxilary Control Room to be located somewhere above this cylinder (while I naturally enjoyed my theory being supported location-wise by TOS-R I disliked its “bomb bay door” concept. But then again, I’m probably biased
).
Peter Chung did a schematic to visualize / emphasize the exterior locations of the engineering hull underside markings but accidentally also illustrated how such a probe cylinder could look like when extended: http://www.trekbbs.com/showthread.php?t=119751&page=12
The T- and the I-shaped grey rectangles are separate from one another but seem to share some kind of internal connection. If the color grey points to a hatch that will be routinely examined at a service station, those two are definitely among it. I presume these to be loading or fueling ports for matter (and/or fusion fuel) and antimatter. If spacedock is like a gas station in space it doesn’t seem too farfetched to have your air and fuel checked...
The red frame below the flight deck could be a blow-away hatch (should the flight deck doors be inoperative) to allow crew members to use the shuttlecraft on the H deck as a means to abandon ship. The red frame marking style, however, suggests this to be just a maintenance hatch for maintenance work if necessary, i.e. replenishment of air after an emergency decompression of the flight and hangar deck (unless, of course, we assume all red frame markings to be blow-away hatches in emergency situations).
The white square is most enigmatic. As Peter Chung’s schematic quickly reveals it’s to small for a shuttlecraft to fit through (unless one seriously suggests a shuttle to take a nose-dive inside). So what is it for?
One of the puzzling issues in Star Trek is the size of standardized cargo containers. I couldn’t help but feel that some walls on Space Station K-7 looked like these had the capacity to store small cargo containers TOS Trek style: http://tos.trekcore.com/hd/albums/2x15hd/thetroublewithtribbleshd0712.jpg
These look much better and bigger in TMP http://movies.trekcore.com/gallery/albums/tmp2/tmphd0385.jpg but are still to small to accomodate the interiors of the cargo containers the TOS Enterprise left to Khan and company on Ceti Alpha V http://movies.trekcore.com/gallery/albums/twokhd/twokhd0201.jpg
Anyway, we have never seen a TOS cargo transporter or a TOS cargo Bee. I think the white spuare could be a cargo container chute of some kind where a ceiling tractor beam functions like a rope to lower a cargo container onto a planetary surface like Ceti Alpha V. Since the “orbital cargo elevator” function wasn’t ready until Tuesday while the shuttlecraft replacement delivery wasn’t scheduled prior to Wednesday, it was of no use to Sulu and his landing party on Sunday in “The Enemy Within”.
(The German dubbing of this episode made Sulu ask Captain Kirk to attach a couple of coffee cups to a rope and lower them down to the planet. While undoubtedly a rope would add a nice maritime touch to the ship, that’s a different concept and story - for now).
Summary:
Grey hull markings – must-do maintenance at service station (fuel and air)
Red hull markings – maintenance at service station only if necessary
Yellow hull markings – structure can move or extend any time (stay clear!)
White hull markings – un/loading process may be about to begin (stay clear!)
Bob

Very fortunately Phil Broad has kept a visual record of the (almost) untampered original condition of the VFX model: http://www.cloudster.com/Sets&Vehicles/STEnterprise/SmithsonianMiniature.htm
I had the privilege of seeing the Enterprise in 1976 and 1980 and expectedly wondered about the functionality of these markings and/or hatches. What really caught my eye was the (only exterior) small print near the saucer’s underside windows which I believe to be a good starting point trying an attempt to decipher the design philosophy behind these markings.
Saucer Hull underside: http://www.cloudster.com/Sets&Vehicles/STEnterprise/ent66.jpg
The text near the upper grey rectangles reads “Inspection Door Vent Systems Connections”. I believe this to be mean inspection door / maintenance hatch for the vent systems connections to which the Enterprise connects at a service station in space (air exchange). Thus grey could indicate “must do” maintenance hatches once at a service station in space.
On the starboard side just below the grey rectangle there is a yellow rectangle. In our current color warning applications (mixed with black) yellow indicates a hazard (moving part or an obstacle). As an airlock door it would be a moving part and the location for the airlock here seems in accordance with the deck level of the airlock in TMP where for some odd reason it’s on the port side: http://movies.trekcore.com/gallery/albums/tmp2/tmphd2072.jpg
The trapezodial panels (almost triangular in appearance) have invited a variety of interpretation proposals. I favor the extendable landing leg / stabilizing pole theory after the saucer has detached from the engineering hull and the neck section and needs to land on a planet, seen here http://www.cygnus-x1.net/links/lcar...l/USS-Enterprise-Officers-Manual_Page_038.jpg, but reject the obvious allusion to “Forbidden Planet” (i.e. extending sensor cylinder) and no longer believe the neck will stick like dead weight to the saucer in an emergency separation scenario:
The inevitable question is of course where the third landing pole would be. In my world it’s at the stern of the saucer but covered by the neck section attached (this would probably require a diagonal ‘throat’ turboshaft. Since I strongly advocate such a shaft – even in a diagonal shaft the turboshaft lights would seem to pass by vertically! - I have no doubt that this is where the third pole is hidden from our view). The landing leg or stabilizing pole theory is also supported by TMP Enterprise where there...are...four...- landing legs at the underside of the saucer (suggesting landing capability of the saucer).
As for the two red stripes near the stern of the saucer I have no ideas at this time.

Saucer Hull upper side: http://www.cloudster.com/Sets&Vehicles/STEnterprise/ent58.JPG
We see the parallel red lines that run from the back of the ‘teardrop’ all the way to the stern of the ship near the flight deck clamshell space doors. This is most likely the ‘spine’ or central nervous system of the ship with a large number of (indiscernible) external maintenance hatches between these red lines.
In need to address the protrusion at the stern of the upper side before I proceed. Of all the interpretations I’ve read here at Trek BBS, I like the one of the gentleman who suggested this to be an exterior connect and/or docking hardpoint the best. In fact, it appears to be the only surface feature of Kirk’s Enterprise that would allow a docking or payload connection (e.g. the third warp nacelle if you prefer

As a docking connect point it would equally hold the entire ship in place or just the saucer hull after an emergency separation and would roughly look like Andrew Probert’s original docking concept for the Enterprise-D: http://images.wikia.com/memoryalpha...prise-D_docking_concept_by_Andrew_Probert.jpg
The TMP spacedock had a docking tube attached to the port side of the ship, the TOS Enterprise does not have such a feature at the outer rim of the saucer but I believe the yellow rectangle on the ‘teardrop’ is the access for such a docking tube (there is a better and more practical way to have telescopic equipment ship-mounted which I’ll address shortly). In naval terms this could make the deck below the bridge the “quarterdeck” or – in TOS terms – the “Q deck”.
Based on his World War II aviation experience the Enterprise’s designer Matt Jefferies felt that the ship’s engines would suffer wear and tear and would need to be replaced in intervals (nice explanation for the warp nacelle differences between the Pike and the Kirk Enterprise). So what’s the story for the impulse engines (especially after “Tomorrow Is Yesterday”)? The large L-shaped panels appear to correspond to the location of the impulse engines and could suggest an hatch access to remove the entire engine block and replace it with a new one.
The yellow rectangles near these hatches are probably (again) airlocks which would enable engineering personnel to go EVA to supervise and assist engine block exchange or payload connection.
Warp Nacelles underside: http://www.cloudster.com/Sets&Vehicles/STEnterprise/ent67.jpg
Probably same story like with the aforementioned impulse engine maintenance hatches. You may need to take the cover panel off to unlock the warp nacelle from the support pylon in order to exchange it for a new one. While there’s no airlock there for engineering personnel, the upper windows in the pylons could indicate the work stations from which to monitor such an engine exchange.
Engineering Hull underside
The yellow circle is definitely an enigma but also the perfect ‘excuse’ to wrap a circular ship’s corridor around it (suggested by various scenes in TOS that take place in the engineering hull). Ironically, I had never thought of this until I noticed the circular computer core in the engineering hull on the three-dimensional cutaway poster of the Enterprise-D where the artist was possibly looking for and eventually creating this kind of ‘excuse’.
Back in the 80’s I did a cutaway blueprint of the TOS Enterprise and assumed that the yellow circle should be the bottom part of a probe (launching) cylinder. After all, there needed to be an egress possibility for the flight recorder in “The Corbomite Maneuver”, the satellites in “Operation Annihilate” and the probes in “The Immunity Syndrome”. A 360° rotating cylinder would provide optimal flexibility to launch flight recorders, satellites and probes in almost any direction you wanted without changing the ship’s trajectory.
And unless you needed to launch sensory equipment as a probe, that same sensory equipment could be used during normal astrophysical examinations. Just turn the cylinder with the corresponding probe towards the stellar object you want to examine. Done with optical astronomy? Turn the cylinder a few degrees to do infrared, next. After that an examination of radio, ray, and neutrino astronomy by the little rotation of the cylinder in the shortest amount of time. I presume Stellar Cartography (“The Changeling”) aka Auxilary Control Room to be located somewhere above this cylinder (while I naturally enjoyed my theory being supported location-wise by TOS-R I disliked its “bomb bay door” concept. But then again, I’m probably biased

Peter Chung did a schematic to visualize / emphasize the exterior locations of the engineering hull underside markings but accidentally also illustrated how such a probe cylinder could look like when extended: http://www.trekbbs.com/showthread.php?t=119751&page=12
The T- and the I-shaped grey rectangles are separate from one another but seem to share some kind of internal connection. If the color grey points to a hatch that will be routinely examined at a service station, those two are definitely among it. I presume these to be loading or fueling ports for matter (and/or fusion fuel) and antimatter. If spacedock is like a gas station in space it doesn’t seem too farfetched to have your air and fuel checked...
The red frame below the flight deck could be a blow-away hatch (should the flight deck doors be inoperative) to allow crew members to use the shuttlecraft on the H deck as a means to abandon ship. The red frame marking style, however, suggests this to be just a maintenance hatch for maintenance work if necessary, i.e. replenishment of air after an emergency decompression of the flight and hangar deck (unless, of course, we assume all red frame markings to be blow-away hatches in emergency situations).
The white square is most enigmatic. As Peter Chung’s schematic quickly reveals it’s to small for a shuttlecraft to fit through (unless one seriously suggests a shuttle to take a nose-dive inside). So what is it for?
One of the puzzling issues in Star Trek is the size of standardized cargo containers. I couldn’t help but feel that some walls on Space Station K-7 looked like these had the capacity to store small cargo containers TOS Trek style: http://tos.trekcore.com/hd/albums/2x15hd/thetroublewithtribbleshd0712.jpg
These look much better and bigger in TMP http://movies.trekcore.com/gallery/albums/tmp2/tmphd0385.jpg but are still to small to accomodate the interiors of the cargo containers the TOS Enterprise left to Khan and company on Ceti Alpha V http://movies.trekcore.com/gallery/albums/twokhd/twokhd0201.jpg
Anyway, we have never seen a TOS cargo transporter or a TOS cargo Bee. I think the white spuare could be a cargo container chute of some kind where a ceiling tractor beam functions like a rope to lower a cargo container onto a planetary surface like Ceti Alpha V. Since the “orbital cargo elevator” function wasn’t ready until Tuesday while the shuttlecraft replacement delivery wasn’t scheduled prior to Wednesday, it was of no use to Sulu and his landing party on Sunday in “The Enemy Within”.

(The German dubbing of this episode made Sulu ask Captain Kirk to attach a couple of coffee cups to a rope and lower them down to the planet. While undoubtedly a rope would add a nice maritime touch to the ship, that’s a different concept and story - for now).
Summary:
Grey hull markings – must-do maintenance at service station (fuel and air)
Red hull markings – maintenance at service station only if necessary
Yellow hull markings – structure can move or extend any time (stay clear!)
White hull markings – un/loading process may be about to begin (stay clear!)
Bob