Message 2967, Jun 17, 2001
Hi Bones, Well, the clam-shell was not really pressure-gear in the conventional sense. It was not used as part of a partial or full pressure suit assembly, but rather as an integrated oxygen breathing and crash protective helmet (as Figmo already referenced, the original Navy concept was termed the Aviator Oxygen Helmet model 1, or AOH-1). It simply integrated the 02 breathing system into the area around the front of the face, rather than into an oronasal mask, as in a conventional system. Only the facial area, which was sealed off from the rest of the helmet by an air-damming seal, was exposed to the incoming oxygen. The aft 2/3rds of the helmet was open to ambient (cockpit) air (and pressure).
The story of the clamshell and its entire, fascinating story of development, makes for one very interesting reading. The ideas behind it were sound, even if the technology wasn't yet quite capable of meeting the design objectives; it died a premature death for a number of practical physiological and human-factor reasons, having much to do with concerns involving pilot comfort, High-G spinal loading, peripheral visual fields restriction, conflict with other items of flightgear (snagging on chute harness Capewells and Koch fittings) and just plain old pilot bias for unrestrictive, lighter weight headgear.
The clamshell was originally flight tested by the Air Force in F-105s and F-104s, and later in the Navy by A-7, A-4, F-8, and F-4 drivers. The reports written about the operational flight testing experiences by the Air Force pilots participating in these tests make very engaging reading, also. It is significant that the casual observer could readily mistake the clam-shell for a pressure helmet, given the similarities in both design and appearance. If you want to see a couple of REALLY interesting photos, ask Figmo about what he believes are the "clamshell precursor" helmets the Navy tested (an example of one of which was apparently auctioned off on eBay about a year ago).
Cheers, DocBoink

Message 2965, Jun 17, 2001
Greetings Neat Photo!! Why would a guy be wearing pressure gear in an A-7? I thought that A-7s were for low level. That nose reminds me, I had a pilot grouping from a Crusader pilot, APH-5 helmet, with a MS22001 set up for liquid oxygen, the guy's name was Rice. I also had his flight suit (brown, not orange or khaki), and g-suit.
Bones

Message 2962, Jun 16, 2001
Hi John, Definitely post some photos of your clamshell. If you show me yours, I'll show you mine, heh-heh. The clamshell is a special interest helmet for several of us (including meself and Figmo). Love to see yours and $500 is a give-away, considering what these rarities bring now. Consider yourself LUCKY! There is an old expression of American slang that you may recall: "Happy as a clam in a high tide at Puget Sound". Well, anyone should know that real happiness is "Another clamshell safe in a museum".
Cheers, DocBoink

Message 2961, Jun 16, 2001
Hi Steve, Thank's for the info on the HGU-20/P, Do you have any info on how many 20/P's the Navy bought? I heard 750 when I picked up mine ( paid 550.00 for it about 10 years ago when I first started collecting gear, had no idea what it was but knew it was rare) Mine was issued to a USMC Capt. F4 backseater (It's the helmet in JAFC with the Phantom guy sitting on a Sidewinder) If anyone is interested, I could post some Jpegs, let me know.
John

Message 2957, Jun 16, 2001
Thanks Figmo, My world picture is back in sync. This helmet ought to get its owner a pretty penny.
Cheers, Bluelight

Message 2955, Jun 16, 2001
Nicely summarised, Figmo! It should also be pointed out that the information that Trey Turner has on the clamshell (HGU-20/P) on his site (in the "X-files" section detailing helmets of particular interest) is essentially incorrect and misleading. I was so concerned by his inaccurate spin on the clam-shell, a few years ago, that I went to the trouble of preparing a small paper on it for him to use and possibly to include directly in his X-Files description. Although Trey acknowledged receipt of it, he apparently never undertook modifying his original remarks and one unhappy result is that with his large audience of site visitors, a great many people end up reading his remarks and laboring under misapprehensions about this helmet's actual history. Although Trey has helped our area of interest greatly by spreading the word about flightgear throughout the world, this particular entry has had the reverse effect in terms of separating fact from fiction: the spread and furtherance of bad and just plain inaccurate information does no one a favor. Keep this in mind if any of you ever visit his site and look up the helmet in his "X-files" menu.
Cheers, Doc(History-Police)Boink

Message 2953, Jun 16, 2001
HGU-15/P vs HGU-20/P Clamshell. The JAFH stated that the HGU-15/P was a modified version of the HGU-20/P. IT IS NOT POSSIBLE for an earlier type designation to be based on a later designation. Simply stated: type designations are assigned, and never re-used. It defeats the purpose of the designation system. The history of the clamshell is fairly straight forward AND WELL DOCUMENTED. The US Navy had a program called the Aviator's Oxygen Helmet aka AOH-1. Based on the Robertshaw Mini oxygen regulator and a helmet designed by Protection Inc. The program was lagging and Robertshaw tried to get the USAF interested. First, the helmet had to be officially assigned a number.
HGU-15/P was assigned 10 Jan 1963. During 1963-64 a field test at George AFB was conducted using F-104 and F-105 pilots (Report 909-026-025 issued 5 Jan 1964). At least 2 of the actual HGU-15/Ps still exist. The results were not favorable and the type designation was cancelled 1/16/64. There are literally several hundred pages of information on this helmet: type designation sheet, reports, Field manuals for maintenance and fitting of the helmets.
The HGU-20/P type designation was assigned 1-30-68 to Helmet, Aviator's, Intergrated Oxygen. Requested by Naval Air Engineering Center. Fully five years after the HGU-15/P. The HGU-20/P was not well received and didn't last long although there are photos showing the helmet being worn into combat by A-7 pilots. Again, NAVAIR documents running into several hundred pages exist for this helmet.
In 1971, a study was undertaken to test the clamshell design for CB use. The helmet on E-Bay is one of the 16 study examples from this experiment. The report of this study clearly states that the helmet is NOT the HGU-15/P but CB helmet modified FROM the HGU-15/P. An honest mistake was made and then reported in the JAFH book.
For the record: Gentex purchased Protection Inc. in the mid 1960s. It appears that Gentex Western Operations and Flight Suits Ltd are the two organizations that emerged from this merger. The clamshell design was used by NASA for early shuttle missions. Contrary to earlier information, the NASA version is identical to the earlier clam shells except for the addition of a second microphone. Clint Eastwood wore a modified clam shell for the Foxfire 3 movies provided from Gentex Western Operations. The clamshell design has been used in non aviation applications most notably for chemical cleaning suits.
Cheers all, FIGMO
PS: Much of this information was provided in a telephone interview with a gentelmen at Gentex Western operations who in all actuality, designed the clam shell helmet for Protection Inc.

Message 2948, Jun 16, 2001
Heads up pressure helmet buffs, There is an HGU-15/P pressure helmet avaliable on eBay. Why would a helmet modified from an HGU-20/P be designated HGU-15/P? It defies my logic that it gets a lower number.
Cheers, Bluelight