Picking up where I left off 4 years ago...

2/23/2017

(I guess I work on this blog for a short while every 4 years. It's not that I've lost interest in the subject, it's that life keeps getting in the way. I will try to do better!)

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The missing "Ca" (?) brought up the need to develop a wing area in which the entire assembly would produce a good polar curve (?), that is, one that would achieve low air resistance. This would be a better areodynamic state in which the pressure point would be constant in the total unit as far as possible. In this way the requirements of the control mechanism could be simplified. Each rotationally symmetrical elongated body without built up surfaces has a pressure point hike between subsonic and supersonic speeds. Also, bodies that are equipped with a tail in the rear, like the A-4, show this pressure point hike (see diagram Abb. 6) which requires proportionately large marginal control forces.

There the aerodynamic moment controls itself by normal forces of the unit and the [air force lever arm] acting together, and as such the control mechanism must be designed to employ maximum leverage on the air rudder.

Getting to the details now...

Figure 1 shows the line of fire image using the measured aerodynamic values. The profile of the lift coefficients of the body and tail on the angle of attack was known and would have given intolerable resistance at the Mach numbers of the A-4.

  Fig. 1

Thus a very large angle of attack, with intolerable resistance and unfavorable lateral force distribution, would have been necessary for the Wasserfall rocket. Finally a requirement  was identified for a subsonic angle of attack of 15^o and a supersonic angle of attack of 8^o.

 

Starting fresh 3 years later

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And so here I am in January 2013 after a 3+ year delay.....How is that even POSSIBLE?

The Aerodynamic Development of the Antiaircraft Missile "Wasserfall"

Munich, 15 March 1945

Department Head and Author: Dr. Kurzweg

Director: Dr. Hermann

PART 1: The Nature of the Task

In the beginning of 1943, at the Aerodynamics Institute in Karlshagen, a demand was put forth for an aerodynamically perfect antiaircraft rocket. We were given this task  to solve in a new aerodynamic way using large-scale wind tunnel measurements. The tactical purpose required a device with a diameter of about 90 cm, 9 caliber[?] long, start with zero velocity and reach three times the speed of sound at an altitude of 20 km, ground controls up to 50 km, or perhaps equipped with a homing device, and it should describe small flight deviations in order to contact the enemy plane and counter evasive maneuvers. A device that meets these conditions must be part projectile, part airplane. But there are currently no airplanes that fly at supersonic speed. 

Therefore, in the aerodynamic development of an antiaircraft rocket, we had to rely solely on the experience gained in the development of the A-4 [V-2]. Because the A-4 is the first and only rocket/missile, flying through a wide range of speeds from initial velocity of 0 m/sec to final speed of 1500 m/sec, transitioning perfectly through the speed of sound. Also, control surfaces were prior experiences [with the A-4].

In order to lose no time, the Wasserfall antiaircraft rocket incorporated the basic shape of the A-4, the external shape of the tail, and its overall arrangement. Control devices in the form of air rudders had to be developed. Some experience with that - but only in a wind tunnel - was available on a glider model, which was part of the development of the A-4 device.

The demand for narrowly curved flight - even at very high speeds - required the attachment of wings in order to achieve the required lifting forces. The four control surfaces, which are arranged in two mutually perpendicular planes, has - unlike the usual plane shape - a symmetrical cross wing structure attached to the body to allow for a quick pivoting of the rocket in all directions. The last thought led temporarily to investigate a basic body which would have been surrounded  concentrically by a circular airfoil, that would have been able to slightly tilt the rocket. The very high drag closed this issue after the first use of the wind tunnel measurements.

[This particular portion of the German text was very difficult to translate, and some words were not even found in my German dictionary. This was my "best guess" - MFW] 

The antiaircraft rocket should be based on static tests loaded to a load factor of 12 at an empty weight of ~1.5 tons, which must not be even slightly higher than that of the aircraft to be attacked. The load factor of each type are shown in the following table:

Type:                                                            Load Factor:

Fighters - Dive Bomber                                     6 - 8

Destroyer                                                           5 - 7

Glide bomber                                                     3 - 5

Horizontal Bomber and transport aircraft          2 - 3

It was therefore for the aerodynamic shape design that a lift value of 18000 kg had to be achieved. [MFW note: 1.5 metric tons = 1500 kg. 1500 X 12 = 18000 kg]

From the calculated trajectories with initially adopted aerodynamic coefficients, a thrust of 8 tons, a burn time of 45 seconds and a flight time of 90 seconds revealed that maximum dynamic pressures are reached, on the flight altitude, of the type of the shot - or whether orthogonal diagonally - dependent [MFW - Needs more work.]

MFW note: Definition of Load factor:
In aeronautics, the load factor is defined as the ratio of the lift of an aircraft to its weight and represents a global measure of the stress ("load") to which the structure of the aircraft is subjected:

 

where: 
n = Load factor 
L = Lift 
W = Weight

 

Rolling up the sleeves...

Armed with a German-English dictionary and Google Translate on my computer screen, I begin to translate.

However, I should perhaps offer a little more background on how I got to this point. I was led to the Wasserfall through a route that I'll bet is very common to those interested in WWII German technological advances; specifically those related to rocketry. I'm referring of course to the A-4 ("V-2") rocket. In reading just about everything I could get my hands on concerning the A-4 (and from a very young age), there are many incidental references to the Wasserfall missile, but they tend to be tantalizing snippets. Given the impact this missile might have had on the conduct of the war if it had commanded the time and resources afforded the A-4, it is definitely a candidate for focused study - but where to learn more?

Some of the topics I wanted to learn more about include:
(1) The mid-body fins. I know they were placed there in order to give the missile the enhanced maneuverability that would be needed during a short flight into the wings or fuselage of a B-17, but did these fins also have control surfaces, and, if not, why not? By the way, this mid-body fin configuration coupled with conventional fins at the rear of the missile is also found on the Nike anti-aircraft missile and I believe was the result of the influence of postwar German rocket scientists working with the U.S. government.
(2) The hypergolic fuel mixture and how the chemistry was chosen for this missile.
(3) The state of the guidance technology. This would have been the key to a successful implementation.

O.K., so after doing some research I learned that the Smithsonian Air & Space Museum archives department has a large collection of captured German technical documents related to the Wasserfall. Photocopies of the microfilms can be purchased for a reasonable price.

I thoroughly reviewed the abstracts for the 65 or so Wasserfall-related documents they offer and decided to begin with one entitled ""Die Aerodrynamische Entwicklung der Flakrakete 'Wasserfall'" (The Aerodynamic Development of the Anti-Aircraft Missile "Waterfall").

From the title I expect to be able to get some answers to topic 1 above, but for topics 2 and 3 I'll probably need to order some more documents. But one step at a time.

Tonight I translated the cover page and table of contents. It took me about an hour and a half. I started out using the dictionary frequently, but after awhile I started using the online "Google Translate" program because it is very easy to use and my eyes get tired quickly trying to read the dictionary's fine print.

I will say this: My training in German years ago gave me a great advantage over someone just translating words from the dictionary. All of the complex German sentence structures, the changed meanings of verbs whose helping hand prefixes appear at the END of the sentence (too late!), the crazy compound nouns, the dative and genitive case confusion, etc. - I'm not saying I'm GOOD at these after 30 some odd years, but let's just say I didn't have to start from scratch.

I don't intend to translate everything in this blog, just some of the more important and interesting parts. The table of contents meets this criteria and so I will reproduce it below:

Contents:
1) The nature of the task
2) The arrangement of the body and the tail unit
3) The development of the structure
4) The mutual location of wing and tail fins
5) Development of the air rudders
6) Measuring method and test results:
---> a) 3-component measurements
---> b) vibration measurements
---> c) zero torque measurements
---> d) pressure distribution measurements
---> e) spin and hinge moment measurements
7) The influence of the exhaust jet on the aerodynamic properties
8) The temperature of the [missile] surface in flight at supersonic speed
9) Bibliography, definitions, and terms

Juicy stuff, huh? I'll be diving into this in the weeks and perhaps months to come. Not sure how much I'll be able to accomplish during the Christmas holidays. On one hand, this might be the perfect time to settle in and get a lot of it done. On the other hand, with family get-togethers and basic revelry in progress - coupled with an out of town holiday trip - this might be the very worst time to do this. We'll see. No hurry.

And so the project begins...

And so the project begins....

I just received my first set of captured German Wasserfall documents (58 pages) from the Smithsonian Air and Space Museum archives. There is something really neat about reading actual reports from the scientists themselves. The document I received is dated March 15th, 1945 - just two months before the German surrender. They must have known it was futile.

So I will start translating. I took four years of German at the University of Texas, including two Scientific German classes (I was a chemistry major, and German was required for that major at the time). But that was in the 70's, so I'm guessing it will be a struggle...but a stimulating one...

Even before beginning to translate, just looking through the charts, graphs, and drawings is fascinating. These were some heavy-duty guys.