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Schwartzkopff Bonn |
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In the postwar period every material - reports, microfilms and publications - concerning water entry were collected by order
of the "Amt Blank" (predecessor of the German Ministry of Defence) by the company:
Berliner Maschinenbau Here a test facility was build similar to the plant of H.G.Snay. It consisted of a water basin, air pressure gun and a Fastax-highfrequency camera. The next figure shows the faciliy in a later stadium after a move from Brühl to Bonn
Germany wanted to buy torpedos from the USA for mission in the Baltic Sea where the average depth is 52 m. The american torpedos were developed for mission in the ocean. After dropping from aircraft they dived as far as 50 m in the sea before steering brought them in the normal running depth. Therefore in extended ranges of the Baltic Sea the torpedo would not be applicable. From the investigation of Snay it was known that water entry of torpedos produces large cavities which prevent the controllability on long distance. Therefore it was tried to minimize the entry cavity. Two posibilities were experimentally tested: One technical solution was a central rod on the head which diminishes the cavity and guarantees an early controllability what was approved in tests. A second solution was suggested by W.Trinks (Ministry of Defence). Therefore a beam of water was ejected from the head of the torpedo immediately before water entry. This was also tested and proved as the better solution. Both solutions were not realized because substantial modifications were required in the head zone of the torpedo. The selected solution consisted of an attachment on the rear of the torpedo with the shape of a cone. It inhibited that the torpedo could touch the upper side of the cavity. So deep diving was avoided.
In this period the reports of Reichardt and Münzner became available to Schwartzkopff. Within the precision of measurements the experiments could be explained with their theories. In 1974 the results of experiments below water were very sucessful so the employer of the Ministry of Defence requested a onesided summary to inform other interested departments:
Translation:
R.F.Steffen
March 4, 1974
The cavitation bubble contour body (cbcb)
a body running in water with low resistance at high speed.
Prinziple:
The body generates with its head a cavitation bubble which mainly depends on its velocity and the shape and dimension of its
head. The cavitation bubble moves with the body through water (dynamic cavitation bubble).
The shape of the cbcb is something smaller than the cavitation bubble so that only the head is in contact with water.
In the rear of the body steering equipments can be fixed that hold distance to the cavitation wall and guarantie a
straight-line trajectory.
With growing velocity the head area can be reduced without changing the maximum cross section of the cavitation bubble.
The drag in water therefore only grows straight proportional with the velocity and not quadratically as usual.
Stage of experiments
At present a body of caliber 20 mm with a front head of 3.8 mm² is tested at velocity 200 m/s. The body has a drag
coefficient cw of ca. 0.01 related to to the largest cross section area of 314 mm². The Ministry of Defence showed no reaction at all.
In 1975 Dr. Walter Trinks (Ministry of Defence) retired. In honor of him a commemorative volume was published. A contribution
treated a computation method to determine roughly the bubbles generated at the immersion of solids of revolution on the basis
of Reichardt`s rules of cavitation bubbles (Lit.4). In 1975 a report (Lit.5) was delivered to the Ministry of Defence concerning the topic "drag reduced running of bodies of revolution within cavitation bubbles". It contained calculations and applications of Reichardt`s rules of cavitation bubbles. Summary of report: By application of cavitation phenomena the frictional drag of bodies of revolution with approximate ellipsoidal respectively half ellipsoidal shape is largely avoided. Under certain circumstances the so called full cavitating running can be achieved. It is characterised by an enforced separation of flow at the head of the body generating a streched cavitation bubble which closes behind the body. In this case the predominant part of the body is only surrounded by the gas within the bubble contributing only a little to surface friction. The reduction of drag characterized by the drag coefficient cw is the larger the higher the velocity and the length-diameter-ratio of the body. Usually the value of cw for wetted bodies lies between 0.3 and 0.1. Fully cavitating bodies can achieve drag coefficient smaller by magnitudes. The principle of drag reduced fully cavitating running is applicable with gain on range or velocity on all water running bodies. Some examples are described like projectiles with velocities of 800 m/s with range 1000 m, torpedos with velocities above 50 m/s, projectiles against helicopter and flat diving rockets against vessels.
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With the original plant - build about 1960 - small-scale test were performed to test the water entry of torpedos dropped
from aircraft.
In the following years Schwartzkopff improved the experimental technics, enhanced the velocity of the acceleration to 200 m/s
and rebuilt the basin for horizontal shots.