Summary.

In the design calculations which Henschel had undertaken for their missile, they had concluded that they needed a powerplant offering a specific weight of less than or equal to 12g /kg s which had to be regulated to give a constant speed close to the speed of sound. They had shown these requirements to both principal manufacturers of rocket motors in Germany, BMW and Walterwerke, in April 1943. BMW had agreed that they could produce such a powerplant and had been given the initial primary contract. But Walterwerke, having conducted some studies of their own concluded that they could acheive better than 13 g/kg s and were also awarded a contract as secondary supplier.

Left is a preserved 109-729 combustion chamber, held at the RAF Museum in Cosford, England. The fuel valves are shown on the left, the venturi to the right.

In common with a number of Walter's designs, the basic motor used an oxidant into which was pumped a hydro-carbon fuel, combustion being initiated by an accelerant. However, this was not a hydrogen peroxide motor, but one based on nitric acid.

The liquid fuels for the HWK 109-729 were, as an oxidant, SV-Stoff (88.5% Nitric Acid, 11% Sulphuric Acid and 0.5% water) with the fuel being standard German petrol - (or "gasoline" which in German is written "benzine", not to be confused with the organic chemical reagent benzene). As these two fuels are not spontaneously ignitable, combustion was initiated by the introduction of Fantol (Furfural Alcohol) into the combustion chamber, in similar fashion to the HWK 109-739 for "Enzian".

Schmetterling is a small missile, and to keep the all-up weight of the motor low, instead of Walter's usual steam powered fuel pump, compressed air was used to drive propellants to the combustion chamber.

Description.

The Walter 109-729 consisted of a number of motor parts mounted within the body of the Henschel missile.

There were four spherical tanks, one for the compressed air, one for the gasoline fuel and two tanks containing SV-Stoff, control valves, combustion chamber and associated pipework.

In addition, as the motor was designed for flight at a constant speed, there was also a regulator unit, connected to the fuel delivery valves at the forward end of the combustion chamber.

Operation.

Compressed air is stored in its welded sheet steel tank at 180 atmospheres, prevented from passing into the system by an electrically operated valve having a bursting disc.

There is one fuel tank and two SV-Stoff tanks. The rationale here was, acid driven from one tank to the second would cause a lesser moment around the centre of gravity than two separate tanks emptying sequentially. The propellant air was free to come into contact with the surface of the fluids. A delivery pipe was submerged beneath the liquid in the tanks, and it was both weighted and free to swing so that it would follow the fluids during manouvering and not become uncovered and capable of letting air into the fuel lines. (This swinging fuel pipe had been tested by Walter at the Me.163 test unit with some limited success. Maintaining a good weight balance between the fuel and the pipe itself was difficult, and with differing inertias, the pipe would not follow the fuel well, would risk becoming uncovered and allow air into the fuel lines).

When the Hs.117 is launched, the solid rocket boosters propel it away from the launch frame up to its operating speed. Then in the main motor, an electrical impulse detonates a cartridge which pierces the bursting disc, allowing air to escape through to a reducing valve then into the propellant tanks.

The liquids are retained in their tanks also by a bursting disc, and the pressure build-up causes these to rupture. When the propellant is driven forwards, downstream of the flow is a small tank which contains Fantol. The Gasoline/SV-Stoff mixture is not self igniting, and the increasing fuel pressure bursts the discs holding the Fantol, which is then forced in advance of the liquids into the combustion chamber, igniting when it comes into contact with the acid, initiating combustion of the main fuel flow. The lack of a hypergolic reaction between the fuel and the oxidant was viewed as an advantage for storage and transport, in that inadvertant mixing from leaks would not cause a fire or explosion.

Control of the valves admitting fuels to the combustion chamber is by means of a regulating unit which operates on a comparison of static and total air pressures (static plus dynamic). Propellant taken from the main flow is reduced in pressure by a valve and used as the hydraulic means of adjusting the fuel and oxidant valves. Thus, the total pressure is used as a measure of velocity, which adjusts the thrust of the rocket motor during flight.

The combustion chamber was a new departure for Walter, in that it was uncooled. The motor only needed to operate reliably for around twenty seconds, and the great saving in weight of having an uncooled combustion chamber led to Walter's being able to acheive good specific weight figures.

The combustion chamber (see the figure at the top of the page) is made from welded steel, with an inner surface coated with a triple layer of potassium silicate, asbestos and graphite. Despite the coating process taking up to three weeks to fully dry, the combustion chamber was good for 180 seconds of continuous operation. At the efflux end, the motor was fitted with a graphite insert venturi.

Three pressure balanced, cylindrical valves were mounted at the forward end of the combustion chamber. These are shown here in close-up on the preserved motor from RAF Museum, Cosford.

Each cylindrical valve was set with six orifices. When the valve was rotated these could be opened, and come into operation in the order 1-6, then 2-5 and 3-4; by looking at the valve spindle here, you can see the offset holes which are designed to come into operation sequentially. Fuel and oxidant were sprayed in fine conical jets onto a target plate to mix for combustion.

Discussion.

The initial aerodynamic designs of the 8-117 project underwent some modification, based on wind-tunnel testing, but were largely successful. Control and guidance for "Schmetterling" were always to be subject to ongoing development. The biggest problems affecting the project were surrounding the propulsion systems. Rheinmetall-Borsig RI-503 solid boost rockets were used initially for test launching; until Schmidding-WASAG boosts became available. Some of the early tests showed that the Rheinmetall-Borsig rockets had too great an uneveness in burning and unacceptable deflections of the thrust line due to high temperature deformations of the nozzle.

WASAG designed a new powder core, specifically for "Schmetterling", to be used in Schmidding boosts, with a better eveness of burning, and less sensitivity to temperature variation - WASAG also produced a compound that replaced the flame efflux which had been causing problems in night launches. However, this resulted in a black smoke trail, which was then found to obscure the missile and target for daylight launches. WASAG had told Henschel that the powder compound could be re-engineered to return the flame efflux with no loss of performance, but further tests were curtailed by the end of the war.

Sighting the round was initially thought to have been possible by following the main rocket motor's efflux, but practically, this proved insufficiently bright over the usual ten mile or so range that was deemed to be the average flight distance. Chemicals and dyes in the fuels did little to improve the situation, and Henschel reverted to designing a flare carrier into the tail of the missile, as they had done with the Hs.293.

It was also found that the boosts needed to be extinguished before complete combustion as the missile usually reached its operating velocity before the powder was fully consumed. When the Mach regulator detected operating velocity, ports in the boost casing were opened, dissipating part of the thrust. At this, sensors detecting the drop in thrust of the boosts automatically blew them free of the missile and initiated the starting of the on-board motor. The same blow-off system was used if the pressure in the boost throat fell naturally due to exhaustion of the powder fuel.

(Interestingly, Henschel considered employing an explosive mechanism to detonate the free boosts into fragments as a protection for battery crews against large, hot, expended rockets falling back into the emplacement. However, this was not pursued, owing to time constraints and the complexity of adding another mechanism, and crews were left to look out for themselves.)

Another problem found during flight testing was the difficulty of manufacturing and fixing the solid boosts accurately, so that they fired symmetrically through the missile's centre of gravity. Faults here sent the missile wildly and often irretrievably off course, spoiling the shot. To combat this, Schmidding designed a combustion venturi on a ball joint, and Henschel developed a wooden tool - launch crew could place the tool into the venturi of the boost and manually align the thrust line through the centre of gravity, simply, accurately and in a few moments.

In a post-war evaluation of the system, Henschel design engineer J.J. Henrici, made a number of observations about the BMW and Walter motors.

"The weight of this [BMW] combustion chamber was 13kg [and] a helical path from the nozzle up to the head piece was provided, along which the salpetric [nitric] acid was ducted for cooling. A considerable pressure drop was the consequence. "

"The first drawback of this combustion chamber was the friction of the sliding valves which required a high moment for the adjustment. The second drawback was the fuel consumption which met our requirements only when maximum thrust was produced, whereas already at the prescribed [medium] thrust, the required momentum was no longer obtained."

Walter's combustion chamber was simpler, comparatively easy and economical to manufacture, robust and 3kg lighter than the BMW model. The roller-type control valves were more easily moved, and the delivery pressure in the tanks could be reduced by about 15%. Not only that, it had a lesser fuel pressure drop, and gave better performance, particularly at reduced thrusts.

However, Henschel were seriously concerned about the Walter method of delivering the fuel - the swinging fuel hose was a simple item to manufacture, but the possibility of air entering the fuel lines during rapid or violent manouvering was too much of a concern. The stalling of the motor would render the missile useless, as there would be no means of restarting combustion - the fuels not being hypergolic (able to spontaneously re-ignite on contact).

Henrici, was obviously torn on the best design for the "Schmetterling". BMW's fuel tank with piston was a good design, but heavy, and the design tolerances (an internal clearance of only 0.012 inches) needed the manufacture to be of high quality. The lack of performance in the tests at Peenemunde had left Henschel searching for a better motor, and they were pleased to receive Schmidt's design from Walterwerke. Henrici states,

"The decisive delay of the completion of the Butterfly arose from the tolerance between piston and tank of the motor-rocket. We had been given the assurance by a firm which was experienced in pressing small gauge light-alloy pieces... whereby the tolerance of 0.2mm could be observed. Half a year after this assurance, however, the firm was [still] not able to meet these conditions."

"Later on we got a satisfactory design, which met our wants, by Dr. Schmidt of Walter... Schmidt had made a new regulation device and an uncooled combustion chamber."

"As far as manufacture was concerned the Walter propulsion unit was particularly favoured as the pressing of half-spheres [for the fuel tanks]... and subsequent welding is a simple problem. With the combustion chambers the matters were similar as with the tanks. The required tolerance of the narrowest throat diameter of the [BMW] nozzle was lower than the admissable tolerance of the sheet metal thickness. This problem disappeared, when we made use of the Walter-combustion chamber whose nozzle consisted of coal [sic]." [I suspect the translator of the report was searching for the word 'graphite']

"There was a more important problem to be solved, still whether the emptying of the tanks could be performed on any service conditions...   I was afraid that on account of the exposed surfaces and the changing accelerations an increasing part of the fuel would be mixed up with air and hereby overthrow the balance. Tests were being made to clear the doubts."

Based on information available at the time, Henrici summarises that, costs aside, the favoured production design for "Schmetterling" was likely to be a combination of Walter's combustion chamber, the BMW fuel tank system and Henschel's own electric Mach regulator.

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