8,6 cm R.Spgr. L/4,5Pappmine
Chapter 3
21-cm R-Lg ROCKET

Caliber: 21 cm.
Bourrelet diameter: 21.4 cm.
Total weight: 60.0 kg.
Length: 5.1 cal.
Length (fuzed): 109.0 cm.
Maximum velocity: 560 m/sec.
Maximum ordivate: 5,400 m.
Spin Velocity: 11,900 rev/sec.
Moment of Inertia:
Longitudinal: 0.0334 mkg/sec².
Transverse: 0.363 mkg/sec².
Stability factor: 1.5.
Launching Velocity: 19.1 m/sec.
Launching Time: 0.118 sec.

Motor performance:

Nozzle K: 500 cm²/cm².
Burning Time: 3.3 sec.
Impulse: 3,040 kg sec.
Velocity: 560 m/sec.
Motor Volume: 12,600 cm³.
Number of nozzles: 6.
Throat Diameter: 1.48 cm.
Nozzle Area: 10.3 cm².
Nozzle Length: 7.5 cm.
Cant angle: 9°.
Expansion cone angle: 12°.

DESCRIPTION. The rocket projectile is a high-altitude, rearward-ejecting parachute-suspended flare. (See fig. 223.) The rocket consists of two concentric cylinders, the in-ner tube containing the flare and the outer containing the rocket propellant. Six nozzles, arranged symmetrically around the base of the motor, are canted to provide the spin necessary to stabilize the projectile. The launcher used for the tests at Unterlüss was a single-barreled launcher designated "21 cm R Ag M 42", which had a barrel 1.12 meters in length.

Rocket Motor. The 21-cm R Lg rocket motor has a unique design, considering the usual German construction. The motor tube has a groove cut into the inner wall, one near each end of the tube. The purpose of the two grooves is to provide a seat for a ring-type retainer to secure the closure plates. The closure plates have thin obturator disks to prevent the escape of the hot propellant gases around the closures. The upper closu-res uses one obturator disk and the lower closure two. The second obturator disk is un-necessary at the top of the motor because of the threaded fit of the central tube con-taining the flare unit and the upper closure.

The assembly procedure is rather complicated due to the closure plate system. First, the nozzle plate is assembled with the grid and obturators. The assembled nozzle plate is fit-ted into the tube and the locking ring retainer seated in the retainer groove. An annular locking plate, which has the skirt crimped onto it, is bolted to the nozzle plate. The lock-ing plate seats on the base of the motor tube and stresses the nozzle plate against the ring retainer. Three bolt positions, 180° apart, are used to attach the nozzle plate and the locking plate.

The propellant grain consists of a single tube of R 6 m diglycol propellant 19.2 by 10.8 cm in diameter and 51.0 cm in length, weighing 15.2 kg. This grain is placed upon the grid in the motor tube. An igniter of 70 grams of black powder, contained in a ring-shaped plas-tic case; is placed on the top of the propellant grain. The front motor closure is then se-cured to the motor by means of a retainer ring and locking plate. A chamber for the elec-tric igniter is permanently attached to the top closure and provided with a screw-on cap to enable insertion of the igni-ter after the motor has been located. After insertion of the electric igniter, the flare unit, with the wind-shield attached, is fitted into the motor and threaded into the upper motor closure. The manner in which the electric igniter makes an external contact cannot be determined from the information available.

Two construction designs are given for the nozzle assembly. Both types have insert noz-zles, i.e., the nozzles are individually assembled to the nozzle plate. In both cases, the nozzle plate is drilled and reamed 6 places, 60° apart, at the desired cant angle of 9°. The hole is then drilled out to a larger diameter for a short distance on the exit side of the plate forming a seat for the flange on the nozzle. The inside of the nozzle plate hole is made slightly conical in the first design. After the nozzle has been inserted, the en-trance cone of the nozzle is swaged outward into this cone, thus securing the nozzle to the nozzle plate. The newer desgin shows the nozzle pressed into a cylindrical hole in the nozzle plate omitting the swaging operation. This is sufficient, since the effect of the high-pressure gases flowing out of nozzle is to have a net forward thrust on the nozzle, thus forcing it more firmly against the shoulder of the nozzle plate.

Flare Unit. The flare unit is contained in a cylinder which is equal to the full length of the rocket. The front of the cylinder forms the nose of the rocket and carries the time-fuze and the ejecting charge. The ejecting charge consists of 35 grams of black powder. A thin concial wind-shield streamlines the forward end of the flare chamber into the body of the motor. The flare unit contains 7.5 kilo-grams of illuminant and has a burning time of 120 seconds. The flare unit fits in the forward part of the tube and the parachute in the rearward part of the tube. The parachute is contained in a split metal container which transmits the thrust of the ejecting charge to the base closure plug. The base closure plug is attached to the central cylinder by 3-mm diameter wires which are shear-ed by the thrust from the ejecting charge. The entire flare unit fits into the rocket and threads onto the forward closure. The rearward end of the flare unit projects through the nozzle plate and is obturated by a thin collar.

Figure 223 – 21-cm R-Lg Rocket

8,6 cm R.Spgr. L/4,5Pappmine