Code Name Mistletoe
MOST MAJOR WARS OF THE TWENTIETH CENTURY have contained deadly previews of the next conflict on the timeline. Aviation technology and aerial warfare provide excellent examples of this pattern. In World War I the airplane was hardly decisive, yet the opposing powers discovered its usefulness in reconnaissance, bombardment, and air-to-air combat, a lesson applied during the 1930s in the Spanish Civil War, which today is seen by many as a rehearsal studio for the German and Soviet fighter and bomber tactics of World War II. In the latter conflict, for the first time, air warfare was a key to victory. By the Gulf War it had become the chief means to the end of containing Saddam Hussein. In Bosnia in 1995, NATO warplanes achieved an 87 percent success rate with their precision weapons dropped from B-2 bombers, stopping a four-year-old war in its tracks.
The maturing of air warfare is a product of three factors: evolving doctrine (the methods of applying airpower in war), improvements in aircraft, and improvements in weapons. Rarely does one device incorporate advances in all three areas. But during World War II an aerial weapon was developed in Germany that anticipated today’s precisionguided short-range attack missiles, penetrating warheads, and fly-by-wire technology—all in one package. (In flyby-wire, the plane is flown by an electrical or electronic device that takes the pilot’s inputs, processes them, and sends commands to the plane’s control surfaces—the rudder, elevators, ailerons, and so forth. It differs from an automatic pilot in that the automatic pilot actually flies the plane by itself, while a fly-by-wire system acts as an intermediary between pilot and plane.)
The device was code-named Mistel, which translates as Mistletoe. It embodied a new doctrine of using self-guided “standoff” weapons to reduce the exposure of pilots and delivery aircraft to enemy defenses. World War I had seen an early step toward this goal with the Bug, designed for the U.S. Air Force by an all-star team that included Orville Wright, Elmer Sperry, Robert Millikan, and Charles Kettering. The Bug was a pilotless plane filled with 180 pounds of explosives that was launched from a kind of railcar. After it had flown a preset distance in a straight line, which was measured by counting engine rotations, its engine would shut off and it would glide to a crash landing and explode—somewhere near the target, it was hoped. The project reached the test stage in October 1918, but the war ended before it could be used in combat.
The Bug idea was revived by U.S. forces in World War II, first with planes like those of 1918 and later with worn-out B-17s and B-24s equipped with autopilots. Neither scheme worked very well; the tiny Bugs had too short a range, while the larger planes suffered a variety of mechanical failures and were vulnerable to anti-aircraft fire. The Americans also used winged glide bombs that were released from B-17s to glide to their targets. Some of these could be radio-controlled by the bomber’s pilot, though that required him to remain in the vicinity of the target.
For its part, the Luftwaffe developed a pair of radiocontrolled, air-dropped guided missiles. The Henschel Hs.293-A and the Ruhrstahl Fritz-X, both fielded in August 1943, were used against shipping, primarily in the Mediterranean. The former was rocket-powered and the latter was an armor-piercing gravity bomb. They sank several ships and damaged many more. The Italian battleship Roma was sunk by two Fritzes in their first engagement (after Italy had surrendered to the Allies). The use of these weapons encouraged the Allies to develop radio-jamming destroyers.
In the closing days of the war, the Germans demonstrated what would become the ultimate form of standoff weapon, by launching several thousand long-range bombs against Britain. These were of two types: the V-I, essentially a pilotless aircraft with a primitive pulse-jet engine running on air and aviation fuel, and the V-2 (or A-4), which was rocket-powered. The V-I was the forerunner of today’s cruise missiles, and the V-2 was the forerunner of today’s ballistic missiles.
All these weapons allowed destruction at a distance, and some of them allowed a modest amount of control over where that destruction would take place. But with the shortrange weapons, the aiming was crude at best, while with the long-range weapons, all the user could hope for was to hit a general area. Standoff weapons that could hit a specific target with precision would not come along until later wars—except in the case of the Mistel.
Its first use in combat, in June 1944, made it a contemporary of the V-I and V-2 and the Henschel Hs.293-A, as well as of the Messerschmitt Me.262 fighter, the first jet-powered combat plane. All of these, fortunately, came along too late and were too few to turn the tide of the war, which was by then running strongly against the Axis. After the war, however—thanks in large part to the contributions of engineers from Nazi Germany—all these concepts were brought to maturity. Today they account for the exceptional accuracy and survivability of modern air forces and their tools of destruction. But weapons are not the Mistel’s only legacy. Its electronic control system and design as a composite—that is, piggyback—aircraft found later application in commercial planes like the Boeing 777 and research vehicles like NASA’s space shuttle orbiter, with its earthbased B-747 carrier vehicle.
The Mistel story begins in 1941. Siegfried Holzbaur, chief test pilot for the Junkers aircraft company, was cruising the skies over Dessau in a Ju.88, looking for practice targets. The Ju. 8 8 was one of the most versatile aircraft in World War II, a twin-engined level or dive bomber that showed up on every front from the Battle of Britain to the African desert and frozen Russia. There were versions that carried torpedoes for antishipping raids, cannon for low-level attacks on tanks, and cameras for reconnaissance.
Holzbaur’s Ju.88 was equipped with a three-axis autopilot not found on the production models used in combat. Autopilots had been around almost as long as airplanes themselves and had improved steadily over the course of several decades. Holzbaur’s was augmented by a gyroscopic bombsight that neutralized target speed and wind effects. This made it useful for attacking moving targets, such as trains and truck convoys.
Holzbaur lined up the aircraft for a simulated dive attack on the chimney of a house and engaged the autopilot to see what would happen. The result was an unerring powered flight directly at the target. On his way back to the Junkers factory in Nordhausen, Holzbaur considered the idea of using the entire Ju.88 as a bomb. If the massive plane were filled with explosives, and if some way could be found to take it to the vicinity of the target and aim it, it might serve as a powerful weapon—much more powerful and much more accurate than conventional bombs dropped from the air.
Other countries were experimenting with this concept, but their flying bombs all required a pilot. The Italians and Americans loaded planes with explosives and had the pilot bail out short of the target. Radio control from a second plane would guide the bomb on its final run. Pilot mortality was the obvious problem with this scheme. Joseph Kennedy, the older brother of the future President, was killed on such a mission in August 1944 when his plane exploded before he ejected. The Japanese ensured accuracy by having the pilot stay with the airplane all the way to the target—the infamous Kamikaze.
The German solution was to use a smaller aircraft to fly the Ju. 8 8 to a spot near the target and an autopilot to guide it after separation. (Since mistletoe is a parasitic plant that attaches itself to trees, the idea of a small plane sitting atop a larger one may have suggested the nickname Mistel.) Mistel’s piggyback arrangement, with the smaller plane guiding the larger, was the reverse of that used in modern air-to-ground missiles, which are dwarfed by their carrier aircraft. It had to use the engines of both planes throughout, since that of the smaller plane alone would have been nowhere near powerful enough, whereas today’s missiles don’t engage their propulsion systems until they are launched. This presented a problem. Since the Ju. 88 would be unpiloted, some way had to be found to control it from the cockpit of a plane sitting on its back.
DFS (Deutsche Forschungsanstalt fÜr Segelflug), the German aeronautical research center, had done some preliminary work on composite aircraft. Its engineers had envisioned having a bomber tow a fighter for protection over enemy territory, an idea that found postwar fruition when gargantuan B-36 Peacemaker bombers carried both defensive fighters and high-speed attack and reconnaissance aircraft in tests. DFS had also mounted fighters on top of their gliders, instead of in the normal towing arrangement. At first the engineers were uninterested in Holzbaur’s suggestion for a piggyback fighter and suicide-bomber pairing, but they eventually came around and ordered tests using a Ju.88A-4 and a Messerschmitt Bf.l09F in early 1943.
Junkers put Fritz Haber in charge of the project. The 31-year-old Haber was a graduate of the Technical University in Darmstadt, a distinguished school of science and engineering with excellent professors of applied mathematics, whose graduates were good at finding engineering solutions with strong underlying science. After receiving his doctorate and working for several years at the Darmstadt wind tunnel, Haber joined Junkers in April 1939.
Haber’s wind-tunnel experience and work on control systems at Junkers made him a logical choice to solve the two major problems standing in the way of a composite aircraft: the safe separation of the two planes and the host’s remote control of the flying bomb. The solution of the first problem contributed to the development of other aircraft composites, such as the B-747/space shuttle orbiter; resolving the second led to the technology for fly-by-wire flight control.
Germany had a different approach to airplane controlsystem design from its opponents. Large American aircraft had their control surfaces moved by mechanical cables connected to hydraulic actuators that acted like power steering and power brakes in automobiles, essentially as force augmenters. But German airplanes had no hydraulics. Instead, each control surface on a Luftwaffe aircraft was carefully balanced and had small trim tabs attached. As the control surface began to move in response to the pilot’s inputs, the slipstream would push against the trim tabs and add force to the pilot’s efforts. Solid push-rods were used instead of cables.
The modified Ju. 88 added an autopilot to this arrangement. The autopilot put the pilot’s commands into effect while sending electrical signals from potentiometers in the gyro pack to electric servomotors, which moved the pushrods. The gyro pack sensed if the airplane was deviating from the set attitude, and the potentiometers governed the speed of the resulting control commands. Thus the pilot’s stick would be “heavy” or “light” depending on what was needed to maintain stable flight. The fact that this system was already electric (similar to Sperry autopilots on American aircraft) simplified the solution to the problem of controlling the composite airplane, which would clearly require some sort of electrical link between its two components.
To a certain extent, this system resembled modern fly-bywire: Instead of having the pilot move the control surfaces directly through mechanical connections, an electrical system processed the pilot’s commands and transmitted them. Still, it differed from today’s fly-by-wire systems in several respects. First of all, it had no computer, just an autopilot. Second, it worked only for straight and level flight; when making turns and other maneuvers, the pilot was on his own. And third, the Ju.88 was an inherently stable plane, whereas the goal of modern fly-by-wire is to control aircraft that are designed to be unstable.
The composite airplane, which came to be called by the official code name Vater und Sohn (Father and Son) or, for some reason, Beethoven Gerät (Beethoven Device) could be flown in several modes. The one that pointed the way to the fly-by-wire future was the direct coupling of the Messerschmitt fighter’s control devices to the Ju. 88’s autopilot. In this mode the composite became essentially a large tri-engined biplane. As with modern fly-by-wire, the electrical control system allowed a pilot to fly an aerodynamically complicated aircraft that he could never have controlled through direct mechanical means.
When the fighter pilot moved his control stick, a set of potentiometers sensed the displacement and sent proportional signals over an electrical cable to the Mistel (the Ju.88). These electrical signals, rather than the signals from the Ju.88’s gyro pack, activated the servos. Adjusting the gains (the strength of the electrical signals) gave the pilot more desirable flying qualities, such as making the controls more or less sensitive—just as in modern active flight-control systems. When the pilot got near the target, he would aim the Ju. 8 8 at it in a 15-degree dive, switch the controls to the Ju. 8 8’s autopilot, and release it. This system was an early form of fly-by-wire in that the Mistel was always under electrical control, but whereas sensors enable active control in a modern flyby-wire airplane, the brain of the fighter pilot did that job before separation and the gyros of the autopilot after.
This design was tested by combinations or Messerschmitt fighters and Ju.88s during 1943. The Mistel candidates were the luckiest planes in the Luftwaffe, for only worn-out airframes that had survived thousands of hours of hard use were chosen to be converted into flying bombs. Haber directed wind-tunnel work and flight tests to solve the problems related to separation. The departing fighter took its lift and airflow field with it, so most of the problems centered on avoiding propeller strikes or tail-to-tail collisions.
The final attachment structure consisted of twin tripods connected to the main spar of the fighter’s wings and a single long strut up to the tail. Explosive bolts caused the separation at the pilot’s command. (The research done in this phase of the program found application in the 1970s when NASA was designing the system for the space shuttle orbiter’s approach and landing tests. In these runs the orbiter would be detached from its B-747 carrier aircraft at high altitude and glide to the ground. Haber was able to advise NASA on the correct angle of attack for the orbiter to obtain a clean break.)
In early 1944 the Mistel became a complete weapons system. The Ju.88s were loaded with 6-foot, 3,500kilogram hollow-charge warheads in place of the cockpit. This type of warhead penetrated more than 60 feet of concrete in tests, making it a formidable weapon. Operational squadrons were formed with either Bf.109 or Focke-Wulf Fw.190 fighters. The few Fw.l90s were problematic despite their superior power, because they used different fuel from the Ju.88s.
The original plan was to make large group attacks on reinforced targets, such as fleets of warships or plants with concrete structures. A raid on the British fleet at its Scapa Flow mooring in Scotland was one objective. The Allied advance disrupted these plans, causing small flights of the Beethoven Gerät to be committed to combat prematurely. The attack on Scapa Flow and later mass attacks on Soviet power-generating plants were postponed and then reduced or canceled as the Germans were forced to fly from airfields increasingly far from the targets. The final uses of the Mistel were in destroying bridges to slow the Allied ground forces rolling into German territory. Instead of mounting one big offensive blow, more than 100 Mistel bombs were frittered away in the last months of the war.
Even considering its limited use, the Mistel was one of the most effective flying bombs in World War II. It helped prove the practicality of standoff weapons, despite its awkward size and vulnerability to anti-aircraft fire during its dive to the target. The Nazis tried to solve these problems and increase accuracy by including a television system in the nose that would transmit a picture to the fighter pilot, enabling him to guide the missile all the way. This technology (which the Americans also tried unsuccessfully with their glide bombs) was finally worked out during the Vietnam War.
After World War II, by a lucky twist of fate, Fritz Haber ended up in the United States. Since Dessau was in eastern Germany, the Soviets had occupied it and had captured most of the scientists there. But Haber happened to be in western Germany for the birth of a son at the time, so the Americans got him. His brother Heinz, a pilot who later specialized in space medicine, was included with Wernher von Braun, Helmut Hoelzer, and others as part of Operation Paperclip, the American effort to bring as many German aviation and rocket scientists as possible to the United States. Heinz recommended that Fritz be “paper-clipped” as well. The two brothers were assigned to Randolph Air Force Base in Texas, working on aeromedical problems.
They suggested the use of large cargo aircraft flying parabolic arcs to create periods of weightlessness for spacemedicine research. This technique—the so-called Vomit Comet—has been used in thousands of flights over the last four decades to train astronauts and even to shoot the weightless scenes in the film Apollo 13 . Fritz Haber left government employment in 1954 for a successful career at Avco Lycoming, where he developed some of the first gas-turbine engines for aviation, including the T-53, which went into the famed UH-I Huey helicopter. Haber and Holzbaur, the test pilot who had suggested Mistel, remained the best of friends, keeping in regular contact even though Holzbaur had stayed in Germany. Remarkably, they died on the same day, August 21, 1988.
The singular irony of modern air forces is that once the shooting starts, their aircraft and weapons technology actually save lives. In World War II fleets of B-17 bombers had to drop thousands of tons of bombs—most falling on civilian areas—to be certain of destroying one military or industrial target. Hundreds of fliers and thousands of noncombatants were put at risk. Today a single stealth airplane launching a precision-guided standoff weapon has roughly a 90 percent chance of hitting its target. The biggest surprise to everyone in Baghdad the morning after the first Coalition air strike in 1991 was the extensive destruction to command and control facilities with practically no “collateral damage” in the neighborhoods in which they were located. This is the legacy of the doctrine of air warfare and the underlying technology pioneered by the Mistel flying bomb.