North American B-25 Mitchell

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The Falcon fire control unit

Not long ago the fixed 75mm cannon, in the nose of the B-25H, was regarded as a handy anti-shipping weapon that ought to be handier. With a range of over 5000 yards, the B-25H and it’s 763 lb. cannon represented a unit of highly mobile artillery that could stay clear of the light flak thrown up by Jap shipping while hammering away at the target.

But the cannon had no way of getting accuracy at long range; instead it had to be fired on the optical judgment of the pilots, which is a particularly tough ASV assignment. As a result, B-25H pilots worked mostly at short distances where optical range errors were small but the danger of getting shot down high. What was needed was something to give range data to the gunsight so that the necessary superelevation correction, allowing for the gravity drop of the 15-lb. projectiles, could be accurately applied.

The answer was AN/APG-13 (Falcon), radar range finder introduced early in 1944.* Falcon eliminates guesswork all the way from 5100 down to 300 yards, keeps the gunsight continuously corrected for range.

It turns out that pilots take readily to Falcon, and ring up good scores with little or no training in 75mm firing. Their performance isn't surprising, since their job,--providing the gunsight is fed correct data by the radar operator sitting alongside--is stripped down to flying so that the hairlines in the sight window are properly positioned on the target and then pushing the firing button as fast as the cannon is loaded.

*AN/APG-13 was the crash-built, pre-production Falcon; test data mentioned in this report refer to that. The production model, incorporating several refinements, but essentially the same equipment, is designated ANIAPG-13A.

Falcon now has been theater tested by the 5th, 13th and 14th Air Forces. Only the 14th was able to give it a real workout. Its performance in China (mainly along the Yangtze) resulted in a heavy requirement, fulfillment of which has been affected by the critical China base problem, though Jap river traffic still offers targets from existing westward bases. The 5th and 13th just didn't have the shipping to pit it against. But a Marine squadron fitted with Falcon now is in the Pacific and 6 more will follow.

The fact that the Japs have taken to using small, flimsy cargo vessels in substantial numbers (partly for reasons of cargo dispersal, partly because of the increasing dearth of larger ships) gives the B-25H 75mm Falcon team added value. Against light shipping the 1.5 lbs. of TNT in the 15 lb. projectile is powerful enough to kill. It isn't enough to hurt a heavily constructed vessel or a warship. And in comparison, the 500-lb. bomb dropped by an LAB B-24 carries 250 lbs. of explosive. That may be why the 5th Air Force sees the Falcon-fitted B-25H as best suited for armed reconnaissance.

Another possibility for Falcon is its adaptation for 105mm cannon in the A-26, which would require an increase in the radar sweep to 12,000 yards or more.

The theater tests have proved Falcon inept against most land targets because the rough terrain often encountered in Jap warfare doesn't allow good target discrimination at ranges above 1500 yards. But a "range-over-land" development (Vulture) is under way which may see the equipment modified to give range data against tanks, motor vehicles, trains, ammunition dumps and other non-isolated land targets.

Falcon is just one of 10 or more airborne fire control radars on their way into operation. It is the first scheduled for extensive use, with both Army and Navy participating. Others, however, are fast following. AN/APG-15, a combined ARO and AGS type (see page 18) for the B-29 tail turret, has been delivered to the theater in small quantities and should be in use about the time this issue is being read. AN/APG-5, similar to Falcon but needing no radar operator, will soon be used in bomber turrets to feed range to computing sights. Additional systems are being designed and built for tail defense of various aircraft, for use with fighter computing sights, for blind fire control in the Black Widow. Most of these are more complex and weigh more than Falcon (a 100 baby). In return they handle in reported complex assignments.

 
 
 
 

Without Falcon the fixed 75mm cannon in the nose of the B-25H is airborne artillery useful at pointblank range. With Falcon it can knock out a Jap supply ship at 3 miles. In operation, the procedures unfold as follows:

WITHOUT FALCON: The pilot presets the gunsight for a certain range by rotating the adjusting knob. As he flies, his line of sight is on the target as seen through the gunsight. When he estimates his range to the target to be equal to the prescribed distance he pushes the firing button. He then quickly resets the sight to a shorter range and repeats.

With this technique, about 3 shells can be fired per run. The big trouble is that estimating ranges is tricky practice, so the pilot flies in close where his judgment is better but the flak thicker.

WITH FALCON: The radar operator and the pilot, sitting side by side, work as a team. The pilot homes on a target. He motions the operator. When the target becomes visible the operator is ready. Watching it on his 6000 yard range sweep he begins tracking, keeping the range step lined up with the left hand side of the target echo by turning a crank on the side of the indicator unit. This automatically turns a cam that varies the elevation an angle of the optical reflector sight to provide the required superelevation.

 
 
 
 

Coincidentally, the data is fed into the range dial mounted near the sight, enabling the pilot to open and break off fire on the basis of his range readings.

The pilot, for his part, keeps the target positioned in the reflector sight, computes windage as usual, and fires as often as the loader can ram the projectiles (20 in the rack and maybe more on the floor) into the breach. With Falcon constantly providing range data for the gunsight, 10 or more rounds can be fired during a single run.

In its young lifetime Falcon has undergone a number of tests at home and abroad, all of which add up to pretty much the same thing. The picture is one of accurate 75mm firing as far as 3 miles from the target and the fuller realization of B-25H as an offensive weapon. It's also a picture of increased understanding of Falcon's applications and limitations, particularly based on its tests.

AAF BOARD TESTS.Official trials at ranges from 5,000 down to 2,000 yards resulted in 56% hits on a destroyer silhouette. Between 5,000 and 4,000 yards the average was 35%.

BOCA RATON (FLA.) TESTS. Three pilots handling the 75mm for the first time fired 117 times, making 4 to 5 shots on each pass on ranges between 4800 and 2000 yards. They made 1.04 hits, 13 misses.

Here is what they reported:

Lt. Col. John J. Mullen: "Any pilot with a minimum of training can be expected to fire an almost perfect score. The operation of the radar is quite simple and should require little training. The test results were amazing. Each time the cannon was fired while the target was in the sights a direct hit was registered."

Capt. R. S. Bowditch: "After firing 46 rounds, my first aerial gunnery, I consider my score remarkable. Use of the sight involved no skill or experience, and after firing a few rounds even the most inexperienced pilot should have no difficulty getting a high percentage."

5TH AF PROJECT. Sixteen B-25Hs, commanded by Lt. Col. Neil A. Newman, tested Falcon in June 1944 for its tactical value in the 5th AF Theater. Most missions, due to the scarcity of enemy shipping, developed into training and practice flights. Good scores were counted against small rocks and islands in most of the 19 such operations. Three missions failed because of radar electrical and mechanical failures, two because of radar operator inexperience. Two additional missions that started as anti-shipping developed into land-strafing which found Falcon useless. In the lone operation that found an enemy transport, the pilot failed to fly at the airspeed required for the gunsight computation and all shots fell short. As a result of this error, the production Falcon now allows the pilot to set his own speed so long as he maintains it throughout the firing run.

14TH AF PROJECT. Three B-25Hs got operational tests in China during June 1944 under Major P. J. Riemar. Two were fitted and flown from Eglin Field, another was fitted with spare equipment in the field. Major General Claire L. Chennault, who saw Falcon as a way to get at Jap river traffic supplying the central China invasion, briefed the crews first. He acquainted them with the Jap strategy, moving shipping along the Yangtze after dark, anchoring during daytime with large gunboats hovering nearby. Long range firing seemed a likely answer. Against such opposition 3 test missions were flown, the following results noted:

  1. Against gunboats on the Yangtze, with fighter cover by 13 P-38s, firing began at 6,000 yards and broke off at 2,000, with most firing around 4,700. Forty-two rounds were fired by the 3 aircraft and a considerable number of hits recorded. The gunboats, about the size of small destroyers, were heavily armed with AA guns up to 40mm. During 35 minutes over the target no sinkings were noted.
  2. Twelve hits seen, many probables, out of 61 rounds against Changsha shipping. One cargo ship, I fuel boat and several small motorpowered cargo boats sunk.
  3. Two B-25Hs, the pilot of one firing the 75mm. for the first time, made a one hour sweep along the Yangtze. One large steamer and 4 riverboats destroyed. Total damage to all B-25Hs on all missions was I flak hole.

Falcon's tests *are finished now. The present job is to figure ways---the base problem being what it is---to get planes into combat. The 14th AF, striking against Jap river traffic deep in China's provinces, has the best chance. Other AF's are primed for its use as soon as takeoffs become possible. With Falcon by day and LAB by night, they have a double-barreled menace against Jap shipping that is slated for plenty of action as the forces come to closer grips in the months to come.

 
 
 
 
 
 
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Endfire antenna is on axis of 75mm cannon, gives 28 of azimuth, elevation.
 
Reflector Gun Sight
 
Reflector sight tilts as range data feeds in. Pilot keeps target in aiming circle, reads range dial to determine breakoff point.

 
 
 
 
"M" Scope is what radar operator, alongside the pilot, see. He tracks manually, traning a crank at the side of the scope to adjust sight.