DECA 12 grain KNER / KNDX – 2010

Introduction

Propellant mass: 1700gr

Total impulse: 1730Ns

Average thrust: 690N

Thrust time: 3,5s

ISP, delivered: 104s

Motor classification: K670

Propellant type: KNER (Erytritol) 65/35

Goals

12 bates grains in a 54mm motor

Design & Construction

Motor

The DECA 12 grain is a 54mm motor quite similar to the DECA 8 grain but instead carries 12 bates grains. Total impuls will be approximately 1700-2100Ns. The nozzle has a 16mm throat and is optimised for the 20mm core Bates grains (core-to-throat ratio of about 1,5:1 to avoid erosive burning). Ae/At ratio is 6,0:1. The nozzle is held in place with 2 rows of 6x – M3 countersunk hex socket bolts. Forward bulkhead is retained by the coupler which is secured the same way as the nozzle.

Ignition

Update: for the flight motor a revised igniter a 50/50 (8gram total) mix is felt to work better, e.g. lower initial peak and more heat transfer.

Igniter static test 04.09.2010: 8,0 grams of ignition mixture. Peak pressure 32 bar requires approximately 5gr BP igniter charge. Mixture used: 5,0 grams of blackpowder & 3,0 grams of magnesium powder. Additionaly all grains coated with BP isopropanol slurry. Thrust curve shows initial peak with extended delay in pressure built up.

Pc . At = ( F / g ) / CF

Pc • 2,02= ( 700 / 9,81 ) / 1,10

Pc = 32 Bar

CFmax = 1,10

Throat aera ( At = 2,02 )

(g = 9,81 m / s)

Propellant

For 4 grains, weigh out the right amount of KN & ER in a bowl: 468gr KN + 252gr ER = 720gr KNER and mix / blend thoroughly.

Per bates grain 143gr + 2 gr = 145gr.

Casting pan set at mid 165° C.

While casting KNER bates grains the freezing characteristic of Erythritol was observed and found to be annoying. The top closure froze the KNER to the inhibiter and when pressure was applied it crumbled the inhibiter rather then compressing the propellant. An cold closure did not work but a closure heated to 100º C. also did not do the trick. The procedure which gave a 50/50 succes rate was carefully poor the propellant in 2-3 times. Clear the in hibiter and to push down the closure untill molten KNER (mostly ER) was extruded between coring rod and closure. Then waited for the KNER on top of the closure to freeze after which the spring was tightened.