Monday, December 18, 2017

The Radome Road to a 32-inch Radome



During the design phase, just as the mission of the F4H-1 changed several times, so did the radar that was proposed to help the Phantom see its prey.  Early on, when the proposed aircraft mission was ambiguous, many proposals emerged. When it looked like the aircraft would be primarily an attack aircraft the choices were:
  • Westinghouse AN/APQ-56 (modified) with a Teledyne AN/APN-79 Doppler set for navigation
  • North American Aviation Autonetics Division’s NASARR (North American Search and Ranging Radar) which was being developed at the same time. This radar was optimized for the attack role and thus fit in nicely with the anticipated role of the AH-1 and the early F3H/F4H designs. The early F4H-1 models showed this pedigree with their 24-inch radome which had been designed around the NASARR radar requirements.
But in 1956, when BuAer changed the mission and dropped the cannons in favor of an all missile armament, the North American Design was dropped.  So, the scramble was on for a replacement radar that would fulfill the new mission requirements but still fit in the space carved out in the design for the NASARR. There were two immediate contenders. A modified Westinghouse AN/APQ-50 X-band fighter interceptor radar (similar to that flown in the McDonnell F2H-3 Big Banjo and Douglas F4D Skyray), and the Hughes AN/APG-51B (similar to that flown in the McDonnell F3H-2N Demon and Douglas F3D Skyknight).

Both were proven, although by no means state of the art designs, that used discrete components that were connected by long wiring harnesses. In an aircraft where real estate was at a premium, this was not ideal. Also, their design posed other problems. Long wiring runs connecting the components weakened the radar signals passing through them as well as made the introduction of noise into the system more probable.  And each time the signal strength was boosted, the amount of noise introduced into the system increased as well.

Westinghouse was the first to come up with a solution. The team at Westinghouse decided to combine the AN/APQ-50 modules into a cylinder shape that would fit into the aircraft nose right behind the radar antenna so that the signals did not have to travel through long wiring harnesses. McDonnell need only supply cooling air and electricity to a central inlet and Westinghouse distributed it. The entire radar was supported on an I-beam on which it could be extended out for easy maintenance. (This design feature was one of the improvements that helped the F4H-1 win the fly-off with the Vought F8U-3.)

But, during testing Westinghouse could not satisfy the most basic requirement of the radar and that was the range at which the APQ-50 detected a target aircraft. Westinghouse blamed the radar antenna. McDonnell's engineers had designed the F4H nose to hold the 24-inch diameter antenna of the NASARR radar they had intended to use. Westinghouse claimed that 24 inches was simply too small, as their calculations showed that the APQ-50 needed at least a 32-inch antenna to meet the required range. In testing they had confirmed the size at an outdoor radar range where they took a prototype of the 32-inch antenna and shot it at aircraft landing at Baltimore airport. The Navy approved the bigger antenna renaming the system the APQ-72, and gave Westinghouse a production contract.

By this time the prototype F4H aircraft (first 18 airframes) were in various stages of completion on the assembly line, all designed for a 24-inch antenna. The 32-inch antenna design changes wouldn't be implemented until the next series of aircraft (Block 3).

The 32-inch antenna would pose several engineering challenges. First came the fuselage redesign to widen the nose cylinder to allow for a 32-inch antenna. From FS 77.0 forward the fuselage was redesigned to deepen and widen the nose. (All future nose changes also took place from FS 77.0 forward like the RF-4, and F-4E)
Fuselage structure modifications forward of FS 77.0

Another technical challenge that they faced was building a radome for a high-speed aircraft that was 32-inches wide.  This had never been done before. Rain hitting a radome that large at Mach 2 could have some very serious consequences. The radome had to be structurally strong while being light-weight and aerodynamically correct. But most of all it needed to be transparent to the radar waves both outgoing and incoming. Radar manufacturers like Westinghouse worked with an exposed antenna in the lab and in the field trials, and then expected the radome manufacturers to provide a transparent dome that did not absorb or reflect the radar waves. Any imperfections in the radome, either in thickness or density, could distort the radar signal and give false information.

After extensive research into the state of radome technology, McDonnell chose the Brunswick Company in Virginia (maker of bowling balls and fiberglass boats) to construct the new radome. The Brunswick engineers came up with a way to wind fiberglass filaments around a conical form provided by McDonnell in the exact shape of the radome. The filaments are wound in alternate layers which are 90 degrees from each other. One layer of filaments, called circs, are wound around the form circumference and the other layer of filaments, called longos, are wound around the radome lengthwise. The layers of filaments are then saturated with resin, to seal it from absorbing moisture, and baked in an oven until it is cured into a strong yet flexible cone. The engineers at Brunswick then constructed a grinder connected to a special meter that measured the phase shift of an electromagnetic wave as it passed through a section of the radome. The grinder would grind the thick or dense areas of the radome until the whole radome registered identical readings. The bonded fiberglass shape is then covered in a layer of neoprene to act as a rain erosion barrier and a small metal nose cap is affixed to the radome to prevent rain and airflow from peeling back the neoprene shell. (I have seen a pin hole on the neoprene peel half the radome like a banana when it came back from a flight.)

(Note: the lone exception to this process was the RF-4s which had a painted radome (epoxy enamel finish) with a fabric/neoprene boot only covering the first 12-inches of the radome.)


Please understand that this isn't a definitive list. I am having a hard time finding pictures to validate each aircraft. I have included BuNos. of the ones that I have been able to verify and will keep it updated as I find more information. 

VARIATION 1 : All Metal 24-inch Nose with Metal Rib Structure (opens left)
Verified on BuNos: 142259a, 142260a, 143388a, 

This was an all metal nose with internal metal rib structure not intended to be compatible with radar, so these aircraft were not equipped with radar while equipped this radome. Many of these aircraft had an instrumentation pallet in the nose where the radar would have been.

Internal Structure of the 24-inch metal nose.

BuNo. 142259a with metal nose. Notice TAT probe installed later in test program.
BuNo. 143388a Showing to good effect the screw pattern on the metal nose.
BuNo. 143388a Showing screw pattern on metal nose.
BuNo. 143388a With radome open. Notice instrumentation pallet swings out to the right (just beyond the open radome).
VARIATION 2: Glass Fiber 24" Nose with Metal Rib Structure (opens left)
Verified on BuNos: 145307b,  

Again, aircraft fit with this radome would not have had radar installed because of the metal rib structure that underlies it.
BuNo. 145307b - The screw pattern can easily be seen showing the underlying structure.
VARIATION 3: Glass Fiber 24" Nose (opens upward)

Verified on BuNos: 145308b, 145315b, 145317b

For some reason three aircraft had a modified radome which opened upward instead of to the left like the others.  The radome was a new 24 inch glass fiber type, without internal structure other than the mounting ring so it would be compatible with a radar set. Externally I am not sure you would see any differences, just that internally it was hinged differently.

Structural Repair manual shows a glass fiber radome which opens upward.
VARIATION 4: Glass Fiber 24" Nose (opens left)
Verified on BuNos:  143389a, 143390a,

Aircraft equipped with this radome could have had AN/APQ-50 radars installed because of the lack of internal metal structure. 

BuNo. 143389a with 24-inch fiberglass nose.
VARIATION 5: Hybrid 32-inch Long Nose with Metal Rib Structure
Verified on BuNos: 143392a, 145311b 

These aircraft had a much longer radome which actually was built over a 24-inch radome structure. The nose appears to be much straighter on top as well as being more symmetrical in shape than the eventual production noses. This nose was used to test out the aerodynamic qualities of the proposed 32-inch radome. Since it had two layers of metal ribs (one on each radome) and a test instrument boom installed, these aircraft did not have a radar installed.
Illustration showing the outer structure built around the normal inner 24-inch structure. 

BuNo. 143392a showing its 32-inch nose

BuNo. 145311b showing its 32" nose
VARIATION 6: 32" Glass Fiber Nose
Verified on BuNos: 145313b, 146817c and subsequent aircraft.

This was to be the production standard for all future F-4(B,C,D,K,M,N,S) Phantoms starting with Block 3. The airframe forward of FS 77.0 was modified to accommodate the antenna of the APQ-72 radar and the larger radome it required. (FS 77.0 was the point where all future nose modifications started - RF-4s, F-4E,F, etc.)
The new 32" radome was laminated glass filaments bonded together and then covered with a neoprene exterior which was to keep rain from eroding the glass fibers. It had no internal metal structure other than the mounting ring and a small metal nose cap is affixed to the radome to prevent rain and airflow from peeling back the neoprene shell. The new radome opened to the right unlike its predecessors. This made a lot more sense as it wouldn't impede access to the cockpit when completely open.

32-inch production radome 

Revision History:

  •  18 Dec 2017 - Original Post
  • Glenn E. Bugos, Engineering the F-4 Phantom II - Parts into Systems 
  • NAVWEPS 01-245FDA-3-1 - F-4A, F-4B & RF-4B-Structural Repair Manual 15 MAY 1965
  • Photos found on the Internet