DECA 4 grain KNDX – 2009 – VERTICAL LIMITS

Introduction

This is a description of my efforts searching for the maximum specifications within a fixed motor diameter using sugar fuels. This search will use some new techniques and are tested on the small 54mm size which can later be implemented on lager diameter motors. In the end these motors might be staged together reaching extreme altitudes on sugar fuels.

General

Nozzle – plain carbon steel with a Ø 13,5mm throat and a single 4mm O-ring.
Casing – aluminium pipe, 54x2mm. 6060 T6
Forward closure – aluminium, grade 6082, with a single 4mm o-ring and a M6 thread machined in the top for an eyebolt attachment.
Liner – EDPM foil 1mm thickness, length: 323mm. 1,5 wrap, no tape used to hold it together.
Inhibitor – thick white sketching paper / cardboard +/- 0,5mm thickness, length: 90mm. 2 wraps + 30mm overlap. Glued together with white wood glue.

Design

Grain type – BATES
Grain length – 65mm
Inhibitor length – 70mm
Grain outer diameter – Ø 44,0 – 44,5mm
Inhibitor outer diameter – Ø 46,0mm
Inter segment spacing – 5,0mm (inhibitor minus grain length)
Core diameter – Ø 20,5mm. Same as the outer diameter of the POM /Delrin coring rod
Theoretical weight KNDX at 98% density – 147gr (actual 149gr will be added because 1-2gr extra to make up for some foreseen losses)

Inhibitor making

Making the inhibitor I use two templates made out of 1,5mm thick aluminium sheet metal. One of 360x90mm and one with 300x90mm. The larger one is used to cut out the inhibitor from the cardboard.
Pull the cut paper a couple of times over a sharp square edge, for example I use a 20x20x2mm aluminium angle profile on my desk. After this the cardboard will have a curl but not all the way to the ends.
Use the smaller template to cut the inhibitor to its final length removing the uncurled sections at both ends.
The inner side of the inhibitor which is in direct contact with the KNDX + 10mm overlap is not coated with white glue. Use a marker to draw a line were to apply glue and where not. Use a short haired brush and apply a thin layer of white glue on the designated section.
Roll the inhibitor over a pipe with an OD of 44mm and carefully align the edges. You will only get one shot at this. If the alignment is off by 2+mm just throw it away and start over.
To finish the inhibitor: put the casting casing (aluminium pipe 50x46mm – length 90mm) over the inhibitor, remove the core pipe care and allow to dry.

Casting process

Here under you can find the drawings of the casting stand. I use a spring to get the optimum density, good inhibitor bonding and as-cast dimensions of the grain without the need to machine them to the final dimensions.

Please click here for the CAD drawings of the casting stand: 54mm mold spring loaded

Weigh the inhibitor and write it down on the inhibitor.
Insert the pre-made inhibiter into the casting casing. Add the base and secure it with good quality 25 – 30mm wide paint tape. Two longitudinal tapes + one around the perimeter of the casing. Also tape the top side of the inhibitor to the casing so the inhibitor cannot move upward. INSERT PICTURES
Put the top closure in the freezer.
Tighten the lower nut and washer on the coring rod and coat the Delring coring rod + the surface of the base exposed to the KNDX with a thin layer of WD40.
Put this assembly (base taped to casting tube & the coring rod) into an oven set at 80-100°C.
For 2 grains, weigh out the right amount of KN & DX in a bowl: 234gr KN + 126gr DX = 360gr KDX and mix / blend thoroughly. For 1 grain a batch of 200gr KNDX will do.
Heat on an electrical hot plate (setting 2) approximately 100gr in a small pan. Add 100gr at a time until fully melted. Also swirl the batch a bit to the side where the pouring will take place. When adding the last 100gr of KNDX set the hot plate to “minimum”. Keep the KNDX molten for 5 min so the air enclosed in the KNDX slurry can escape thus improving the density to start with.
Place the pre-heated casting mould on a digital scale (1gr accuracy is sufficient) and set the scale to zero.
Pour the melted KNDX slurry in the casting mould until you have added 149gr (147 ideal + 2gr for small loss during casting) of KNDX propellant.
Get the top closure out of the freezer and push down the propellant along the coring rod. It might be necessary to perform this operation during step 8 to create more space for pouring in the propellant because of the low density of the molten KNDX.
Slide the spring, pusher and washer on the coring rod and tighten the nut. Tighten the nut until it reaches the coring rod and turn back ½ a turn. You can hear the air escape in small bursting bubbles as you tighten the top nut and compress the spring.
After an hour you can remove the grain from the mould
- undo the top nut and remove the washer, pusher and spring
- undo the lower nut, the washer will probably stick to the base
- remove the tape holding the base attached to the casting tube as well as the small pieces at the top of the inhibitor
- clamp the base in a vice and carefully remove the casting tube + grain with small and light wiggles
- secure the top of the coring rod in a vice and pull off the grain by turning it in small steps from left to right
- slide out the grain if not done already at step “e” or push the grain out of the casting tube. I use a bench top drilling machine for this INSERT PICTURES
- with a sharp knife, cut away the excess inhibitor at the top carefully not denting the segment spacing at the bottom or scratching the top press
- secure the by know exposed top closure in a vice and wiggle it again to remove it from the grain
- use a sharp stainless steel scissors to trim the edges off the top of the grain
- weigh the grain on the digital scale and write with a permanent marker: date, grain number of that day, weight, dimensions and calculated density on the outside.

Liner

Cut the EDPM pont foil to the correct dimensions. I use template made out of 1,5mm thick aluminium sheet metal. Dimensions: 323 x 235mm – note for myself – check if correct. Liner is approximately 1,5 times inner diameter of casing
Lay out / stack the 4 grains on the liner and use an empty inhibitor to fill up the free space at the top. This will prevent the grains from moving when static testing the motor nozzle up. Add a line with a marker and cut with a pair of scissors to length. Optionally the inhibitor can be filled with KNDX + red iron oxide to make a make grain.
Now cut 5 stripes of aluminium tape approximately 10x40mm. Wrap the liner around the grains and insert the assembly in the casing.
Align on end of the liner with the casing and add a strip on the outside of the longitudinal edge of the overlap, flip it over the edge of the liner to the inside and use your nail to make a sharp edge at the overlap (long discription of a very simple action).
Now pull out the liner including the grains bit by bit and add an additional 2 strips every +/- 80mm. Not longitudinal but square to the edge.
Slide back the liner including the grains and repeat step 5 at the other side of the casing. Make sure no crease are present and the liner slips in and out of the casing without force.
Now tape the aluminium tape over the full length of the overlap and cut to length. Again, use your nail to make a sharp edge at the overlap.

Motor assembly

Grains

Grains could be taped together with a single wrap of aluminium foil tape to prevent movement.

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Static test # 1 – DECA rocket motor, 4 grains KNDX, December 30th 2008

First test of the 54mm range on a load cell

Excel file – 54mm, 4 BATES grains, KNDX 6535 static test 1 – 30.12.2008 corrected

Video – 54mm, 4 BATES grains, KNDX 6535 static test 1 – 30.12.2008

Thrust curve

Peak thrust – 522N
Average thrust – 361N
Burn time – 1,7s
Total thrust – 621Ns
Propellant weight – 565,0gr
ISP, delivered – 112,2s

Notes:

Pyrogen – mix of 2:1, BP meal powder : flare
Pyrogen coating – 4 gr + NC laquer for grains. +/- 1gr coating per grain.
Igniter – 5gr pyrogen mix as per above, x-mas bulb + McD straw & 500mm igniter wire.
Triangular shape suggests that the end surfaces of the BATES grains were not burning. Although KNDX is not really hard to ignite and all the burning surfaces were painted with a pyrogen, it is believed that the igniter went off like a fire cracker rather than burning for 0,5-1,0 seconde. Also the inter segment spacing of 5mm might be on the short side.