Douglas A/B-26 Invader

SHORAN














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SHORAN

High Tech Bombing in Korea

In so far as B-26 crews were concerned 'high technology' and 'Korean War' were an oxymoron. The one exception was SHORAN (SHOrt RAnge Navigation), a beacon navigation and bombing system introduced during the closing days of WWII in the ETO. SHORAN was initially installed in B-26's in January 1951, but it was not until 2/17/51 that the B-26's flew the first SHORAN missions in Korea. It was something less that satisfactory due to:

  • (a) ground stations located too far from the targets,
  • (b) inadequate maintenance of both ground and airborne equipment,
  • (c) insufficient training of operators and maintainers
  • (d) inadequate geographical knowledge of Korea, among other things.

Until the MIG's drove the B-29's from the daylight skies, the SHORAN system languished in neglect. In June 1951 when the B-29's switched from daylight to night operations, SAC became interested, and when SAC became interested in any project, improvements were generally forth coming. As a result of SAC intervention, the ground stations were moved to islands off the west coast and to mountain tops just behind the battle lines in eastern Korea (closer to the targets), maintenance and maintainer training were improved, better geographical information was obtained and operator training was improved. In addition the computation of critical bombing parameters was concentrated in the Tokyo area were better facilities were available. By November 1952 these changes had developed SHORAN into a reliable accurate blind bombing system which was used by B-29's and B-26's for the remainder of the war.

 

By January of 1953 the Korean War had settled into a WWI type of fixed battlefront and the primary objective of allied forces was to reach a peace settlement which retained the integrity of South Korea. In a stagnant war, well-defended strong points are the norm, and the main UN objective was to prevent the enemy from building up to the point he could launch an all out victorious offensive. The CCF and NK forces took advantage of the "peace talks" to re-supply and re-stock their war material. One of the methods used by the CCF was to locate the main supply points close to POW camps to discourage bombing attacks. Another objective of the CCF and NK forces was to re-establish airfields in North Korea, so that just prior to the armistice an overpowering AF could be stationed in North Korea. By mid April 1953, photo recon showed that there were 200 IL-28 twin engine jet bombers stationed in Antung just across the Yalu from Sinuiju. Countering these objectives, the USAF initiated a sustained airfield campaign and a sustained interdiction campaign. Precision results were obtained by relying on the SHORAN system. The B-26 force was used to supplement the B-29 force because the number of B-29s (with its 12 sorties per night capability) did not provide for a sufficient number of sorties to keep all of the airfields neutralized. In addition, the B-26 SHORAN attacks could be used to lure the MIGs away from the more vulnerable B-29s. The CCF was developing an effective night-fighter capability against the B-29s, which was ineffective against the B-26s, because of their small size and lower operational altitude.

In March 1953, Fifth Air Force concentrated the B-26 SHORAN capability in the 17th Bomb Wing, and initiated extensive training of the SHORAN Operators for a proposed maximum effort beginning in May, in support of precision interdiction and airfield destruction. Being an engineer I was fascinated by the" high tech" SHORAN system and believed that its precision capability made trips up "North" more worth while; therefore I volunteered to become SHORAN qualified. Because SHORAN targets were considered more difficult and escape from the SHORAN compartment of the B-26 was considered improbable if not impossible in case of emergency, Shoran operator was an all-volunteer endeavor. Even when the May/June offensive came and emphasis of the B-26s shifted from night interdiction to close air support, the level of the SHORAN effort in the 17th Bomb Wing stayed at a 10 to 12 % of all missions.

Note: The WWII 17th Bomb Group flew some of the earliest SHORAN missions during WWII.

  • SHORAN bombing from a B-26 in Korea

     

    C.O. Smith ex-NAV-BOMB

    37TH Bomb Squadron, 17TH Bomb Wing (L/NI)

    During the late spring and early summer of 1953, the ChiComs made every effort to take advantage of the unusually heavy monsoon season to re-build airfields and to build up supplies for one final offensive. Visual night or day interdiction missions were severely handicapped by the bad weather. SHORAN, a blind bombing system used by both the B-26s and the B-29s proved to be a very valuable asset to the Fifth AF and FEAF

     

    SHORAN is an acronym for SHOrt RAnge Navigation and is the name give to the precision radar beacon type electronic navigation/bombing system used by both the B-26 and the B-29 for precision bombing in the Korean War. Its origin lies in WWII, when the Army Air Force was striving to develop an accurate navigation system for flying in the often less than ideal European weather. In 1943, an early system was demonstrated to the 8th Air Force in the UK and was well received. Unfortunately all experienced engineers and technicians involved in the design and development of the system were killed a plane crash in Newfoundland on the return flight from the UK. The System had to be reconstructed and redesigned from the hand written notes and sketches left by the original design crew -a long-term tedious operation. Do to its short inherent range (~300 miles) it was not used extensively in the European theater; though it did receive use in the Italian theater by the 17th Bomb Group (M) where it proved to be very accurate and successful and in addition permitted the bombing of targets not achievable with visual bombing.

    The system requires an airborne AN/APN-3 set and two AN/CPN -2 or -2A ground stations. The airborne equipment consists essentially of a transmitter, a receiver, an Operator's Console FIG 1 and a K-1A bombing computer FIG 2.

     

    Figure 1: Operator's Console

     

    Figure 2: K1-A Bomb Computer

     

    The transmitter alternately transmits pulses to one of the two ground stations and, the system by measuring the elapsed time between transmitter pulse and the returned signal, computes the range (in Statute miles) to the interrogated station. While the system was designed primarily as a navigation system, it was soon recognized that its inherent accuracy could be used to perform "blind" bombing with a degree of accuracy previously unattainable. Integrating the K-1A bombing computer with the previously designed navigation system produced the SHORAN as we knew it in Korea. shows the basic principle of SHORAN and some of the required information.

     

    Figure 3:Basic SHORAN

     

    By design, when facing the target and standing on a line joining the two stations the Low Frequency Station always lies on the left, and the High Frequency Station always lies on the right. Computing the SHORAN distances from the LF Station and the HF station required and exact knowledge of the geographical position of the stations and the target. The technical details of computing the bombing parameters are beyond the scope of this article, the purpose of which is to describe its use on SHORAN bombing missions in Korea. When installed in the B-26, the gunner's station and the upper and lower turrets are removed. 

     


    Figure 4

     

    The upper turret is replaced by a dome, which can be distinguished from the upper turret by its larger, higher domed shape (in which the transmitter is installed) and the lack of gun barrels. The Indicator, receiver and K-1A Computer are installed on the left side of the gunners compartment facing the right side of the A/C. The bomb bay opening is sealed with a metal plate in which there is a 4'x4' opening so that the SHORAN operator, could observe the fall of the bombs from the bay and ensure that all bombs had fallen clear during a bomb run. The operator faces forward and operates the system over his left shoulder, for there is not enough room for him to turn and face the equipment. The only means of entrance or egress is through the gunner's right side access door. Entering or exiting the compartment is difficult. To enter, the operator stands on a ladder, opens the right side door, removes the seat, puts his parachute in, climbs into the compartment, puts on his chute, then receives and installs the seat. Egress is the reverse. Under emergency conditions the operator is to jettison the door, jettison the seat and exit via the door.

    All of these features can be seen in FIG 4 (above).  The large dome replaces the upper rear turret. In the picture the tech is removing the K1A Computer

    LIMITATIONS

    There are certain inherent limitations that must be understood so that the effects of the limitations can be minimized to the greatest extent possible:

    1. The Maximum range is ~= 300 STATUTE miles (radio path must be clear)
    2. No more than 20 A/C may interrogate a pair of stations
    3. Complex parameter calculations made prior to flight must be adhered to during the bomb run. (The bombing parameter calculations are based upon the aircraft maintaining the predicted TAS {true air speed} and bombing altitude.)
    4. Station Angle must be between 30 degrees and 150 degrees. FIG 5 (below)
    5. Exact geographical position of each of the two ground stations and the target must me known.
    6. The 100 mile ambiguity must be recognized and taken into account
    7. There are only four possible approaches to any one target: all predefined by the geometry of the system. (FIG 4)
    8. Because the system is line-of-sight limited, the A/C must fly at altitudes above 14k feet and sometimes as high as 16K feet. An altitude not easily attainable by a fully loaded B-26 and one at which the engines must be operated at a fuel consuming "High Blower".
    9. Only stationary targets can be attacked
    10. An imagined shortcoming is that in keeping with USAAF practices in effect when the system was developed, all ranges are in statute miles in stead of nautical miles



    Figure 5

     

    The following is the description of a mission I flew on 14 May 1953.

    My pilot and I reported to the 1430 briefing to find that we had been separated for this mission. He was to be an IP on a 3rd $ ride for a new crew and I had been assigned to fly as a SHORAN operator with C.I. Smith (pilot) Beryl Baker (navigator) and a 1st $ ride pilot whose name escapes me. I thought this to be unusual, for we rarely had $ riders along on a SHORAN mission. I was comfortable with this crew for C.I was an experienced WWII pilot, Beryl was a great navigator and I was a competent SHORAN operator having flown more than a few SHORAN missions. All of us were up in the 30 mission range except the $ ride pilot who was up for his first ride. We were assigned A/C #748, (See A/C Picture) a SHORAN equipped, dual control B-26C. Beryl groaned for he knew that he would have to endure the takeoff and landing strapped into the tunnel-no one was allowed in the nose on take off and there was no access to the nose from the cockpit in a dual controlled plane.

    We attended the regular briefing where we heard the usual: weather briefing, intelligence briefing, aircraft and target assignment, take off times etc.

    We then attended the special briefing where peculiar SHORAN information was briefed. Flimsies containing the pre-calculated SHORAN data, for the particular A/C and target were distributed. Latest target information and FLAK info was passed out. The primary and secondary routes were briefed. All sorties were individual, i.e. there were no formation drops, and each A/C made its own drop, generally in bomber stream style. We would be the last A/C in a five plane SHORAN bomber stream taking off at two minute intervals and would be the only BDA A/C .The lead and final A/C were usually both tagged with BDA requirements. Our target for the night was an airfield at Sondokchong-Ni, which lay on the East Coast half way between Wonson and Hamhung. (See Mission Map) There had been definite signs that the ChiComs were activating this airfield. The Navy had attacked the airfield but the FLAK was too heavy for daylight, low -level fighter-bomber attacks. The FLAK was primarily automatic fire so should not reach our altitude - or so we were told. Our mission was to destroy as much as possible of the airfield, and to drop five hundred pound, long -delay fused bombs. We were to use approach No. 3 (See Mission Map), which would bring us in from the northeast in an attempt to surprise the ChiComs. In Addition the #3 approach brought us down the strip where a string of bombs would do the most damage. Our bomb load would be 4 - 500# GP wing bombs and 6 - 500GP # bombs in the bomb bay. (See A/C Picture) The bomb bay bombs would be fused with the M124A1 long delay fuses set to explode form 4 to 12 hours after impact. We were to release the wing bombs first, so that the Instantaneous-Non-delay fused bombs did not detonate the long delay fused bombs. In addition we would carry two photoflash bombs one on each wing for BDA purposes. The photoflash bombs automatically triggered the K-14 BDA camera.


    SHORAN Mission Map

     


    A SHORAN A/C


     

    An hour and a half prior to take off, we, the four man crew (pilot, copilot navigator, SHORAN operator), reported to the equipment shack, picked up parachutes, survival kits etc. and went in for a last minute intelligence briefing to see if the situation had change since the afternoon brief. At that time we discovered that out right seat pilot was to be none other than Col. Wasem, the wing Commander. I do not know why he chose to fly this mission- but 2nd Lts don't ask a Colonel "Why?' We then proceed to the plane for the preflight. All four participated in the preflight - four pair of eyes was better than one or two. We inspected the bomb load to determine that we had the load that was briefed. Just before entering the aircraft, the navigator and I removed the pins from the bombs. The arming propeller on the nose fuses for the bomb bay bombs were very carefully safety wired so that the nose fuses would not arm and the bombs would appear to be duds. (Early on we just left the plugs in the nose, but if the maintenance crews dug the bombs out and saw plugs, they recognized the rear fuses as the LD type.) The long-delay fuses were equipped with an anti-withdrawal feature so if an attempt were made to disarm the bomb, it would explode immediately. Externally the M124A1 looked like the M101A2 non-delay tail fuse. A discouragement for the airfield maintenance crew. Diabolic? -"War is hell".

    Except for the thirty minutes of the bomb run and the thirty minutes of SHORAN checkout, the SHORAN operator is more or less a very interested passenger. Upon entering the SHORAN compartment, I perform a very limited pre-flight while the pilot, copilot and the navigator completed the cockpit pre-flight. Until take off is completed and the first power reduction the SHORAN could not be turned on. There was very little room for movement in the compartment and even less visibility, so the SHORAN operator had plenty of time to sit back and contemplate," What burst of insanity had caused him to be here?"

     

    The Mission A/C


    After takeoff and climb out to the first power reduction point, I energized the SHORAN and began the check out. Upon completion of the check out, I broke out my flimsy and inserted the necessary constants into the bombing computer. I then started looking for the appearance of the range and drift pips on the 100-mile scale so that I could tune the receiver and maximize the gain on each channel. I confirmed with the pilot that the set was O.K and then began the double check of the parameters inserted into the SHORAN and the K-1A computer. The navigator identified the parameter; I read back the value. Upon completion, I folded my flimsy so that the secondary approach data was visible in order to be prepared to make rapid changes if an alternate approach became necessary. The flimsy generally carried data for two approaches and bombing data (ATF and Trail) for the programmed altitude and values plus or minus 500 feet. Because SHORAN signal was by line-of-sight radio transmission, all flights were generally at altitudes greater than 14-k feet. Sometimes when making a turn at those high altitudes with a fully loaded B-26, altitude would be lost which could not be regained down the run. The operator must be prepared to reset ATF and Trail data to compensate for the lost altitude. On other occasions is would be necessary to take the alternate approach (If the ChiComs had plotted the primary approach, it might be advisable to use the alternate.) The exception was, of course, when running against airfields, the path had to parallel the runway. When making an approach on the East Coast, I always preferred Approach No 4, since it usually took the A/C over the water and after acquiring the arc the operator could generally measure the true bombing altitude using the SHORAN set (plus the fact there were no AA guns over water). Approach No. 3 required a 120-degree left turn to acquire the arc. On the West Coast, the situation was reversed. The Approach No. 1 was best with No 2 being second best. The overland approaches were best avoided because by that time (1953) the ChiComs had become rather adept at plotting the path of bombers flying the arc and they knew as well as we where the valuable targets were - so they knew where to place the FLAK.

    At 1930 we began the take off roll. After leaving K-9 on a course of 000 degrees, we coasted out over Sam'chock- one of the few times we avoided Mike Roger (the LF splasher, and the 17th Bomb Wing inbound checkpoint.) I was busily checking out the SHORAN, tuning it, while watching the drift pulse. The navigator indicated that coast-out point has been reached, the pilot checks in with TDC, and turns the A/C to a course of 355, and initiates the climb to the bombing altitude of 14K feet. The pilot and the navigator are busy with their front-end duties while I switch the range knob to the 100-mile range and locate the drift marker. I perform the final receiver tuning and gain adjustment. When the range to the drift arc is less than 10 miles, I take over and have the pilot continue present course until drift marker is within one mile.

     


    Figure 6: The SHORAN Picture

     

    At that time I direct the pilot to turn left to a course of 210 while monitoring the position of the drift marker on the J scope. When within range, I go to the one-mile range and talk the pilot onto the arc. Once the A/C is on the arc, I switch to the 10-mile range and confirm the altitude by observing the ghost return from the transmitter. By placing this return on the reference pip, I can make a good altitude measurement. (Altitude is measured in statute miles and converted to feet). It has been a good turn and we have lost no altitude but we required a 200 foot correction. I then return to the 1-mile range and again directed the pilot back to the no-wind drift arc. Returning to the 100 mile range I place the range pip on the reference pip using the Range Position crank and start my stopwatch. I returned to the 1- mile range and again correct the course of the plane. Alternating between the 100 and the 1-mile range, I guide the pilot with GCA like commands to keep the A/C on the no-wind drift arc. When the stopwatch indicates that 1/3 of the ATF has passed, I place the range pip back on the reference marker using the Range- Rate knob. This should establish the range rate fairly closely. When the range arc gets within 10 miles, I switch the K-1A computer switch to the "BOMB" position. Alternating between the 1 and the 10-mile range I refine the range rate and talk the pilot down the arc. At 7 miles the "Seven Mile" light comes on and I call" 7 miles to target". From then on I call the range every mile. At five miles from the target the bomb bay doors are opened with the accompanying buffeting and the armament switches are set for release. We open the doors early so that the pilot can settle the A/C down and get the proper airspeed. At two miles from the target I note the A/C heading and set the K-1 Airplane Azimuth control dial to this true heading value. This establishes the crosstrail correction, caused by wind, and moves the drift pip from the no-wind circle to the final arc. The pilot makes the required correction, to account for the drift. When the A/C is settled back on the arc, the "Bombs Away" light illuminates, the first photoflash and the bombs are released. I call "Bombs Away" to the pilot. With my flashlight I ensure that the bomb bay is clear and notify the pilot, "Bomb bay clear". The navigator and pilot clear the wing bombs. The pilot holds the course for thirty seconds while the second photoflash is released and the second BDA photo is taken. I now secure the SHORAN set, for we do not know how many other A/C will require the beacons that night, and sit back for what I hope will be an uneventful ride home. The pilot turns left to 090 and heads for the water. At some point, Beryl has C.I. turn South for the run home.

    All in all it has been a successful run. During the run, while I was busy, I had followed the talk from the front. Evidently our plan had been successful. FLAK was moderate but inaccurate [unlike later when I revisited this area as the front end Nav/Bomb and watched a heavy {probably and 86mm) track us along the arc in a complete overcast. Evidently the fire control system could not accurately project the future position of an A/C flying a continually changing flight path]. The weather was clear, the A/C performed perfectly, the SHORAN operated as designed and we had a very smooth bomb run. The BDA photos showed good results. Landing was at 2325 - a relatively easy 3:55 mission. Not all SHORAN missions were this easy. I was pleased that every thing had gone so well with Col. Wasem on board.

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