The KILO series represents motors based on a 90x5mm or 86x3mm aluminium casing sharing the 80mm ID. The idea was to use readily available 3” PML phenolic airframe and coupler tubing as liner and casting tubes which fits nicely in 80mm ID aluminium casing.
KILO monolithic finocyl
On September 20th, LD and JVDB static tested a joint monolithic motor project. JVDB provided the design, foam cores & motor hardware. LD developed the grain casting process, assembled the motor and provided the test stand. This motor was our first try at a monolithic grain motor with a complex foam core. We believe case bonded, monolithic grains are a requirement in case we wish to scale up our APCP motors. Hence we started with smaller 10-11kNs proof of concept motor cast in a PML 3" phenolic body tube mainly to investigate startup.
The core design was gratefully provided by AV and duly copied by us. AV had tested the design with great results in an approx 20kNs motor. The shape can be considered a finocyl or a finned core. Below regression graphs, as copied from this great page about solid fuel regression, describes the difference between finned and slotted core regression. In the finned core design the propellant itself protect the casing from the hot gasses. We did experience quite some erosive burning at the beginning. This could be due to the Slow White propellant and / or the grain design. See below thrust curve picture with comments.
KILO monolithic moonburner
On 20.09.2018 we tested 3 motors of which the monolithic moonburner was my own and a monolithic finned core / star grain was a joint venture with LD. The moonburner burn was succesful and the thrust curve spot on according to Bursim simulations. At the end of the burn there seems some off-center thrust. Upon post firing inspection it was noticed that the liner, even at its thickest point with 3 overlapping PML phenolic tube parts, did suffer a burn through although the casing seems to look untouched at the inside. Furthermore the 2mm thick HTPB inhibitor coating at the top of the grain resembled charred paper. Because of the liner burn through and off-center thrust I doubt this will make it as flight motor.
To cope with the extended exposure to hot gases due to the moonburner grain configuration I decided to apply additional phenolic protection on the inside of the liner. Rather than adding a full length casting tube I decided to cut the casting tube in two pieces and applied an additional layer of phenolic near the core of the moonburner and no extra liner protection on the opposite site.
- The liner (3" PML phenolic airframe tube) and casting tube (3" PML phenolic coupler tube) were sanded from the inside with a honing device and 60grit sand paper taped to the stones. This worked surprisingly well.
- The casting tube was indexed into 6 and marked on the outside for alignment.
- The casting tube was cut to 897mm length so it is recessed in the liner approx. 9mm at both side corresponding with the nozzle and forward closure step(s).
- This tube was then longitudinally cut in 2 pieces with table saw. A 240° and 120° piece or a 2/3 and 1/3 strips.
- Both pieces were coated with a layer of HTPB on the outside. A 60gr batch of HTPB proved to be sufficient.
- First the coated 240° piece was inserted into the liner at an angle (to prevent the HTPB to be scraped of) and secondly the 120° piece was inserted.
- The casting base was wrapped in food wrap and inserted into the liner to make sure the casting tube inserts were recessed at the correct distance.
- Finally a 70mm OD aluminium rod was inserted to apply the necessary pressure and left to cure for 24 hours. Due to the casting base in place and a large overhang of the bar the pressure was not optimal at the casting base end. Next time make sure there is even pressure (no casting base in place, use a distance piece to position the inserts and keep the aluminium rod centered).
Motor design considerations:
The idea was to use 3” PML phenolic airframe and coupler tubing as liner and casting tubes which fits nicely in 80mm ID aluminium tubing. To maximize propellant loading a full length of 3” PML phenolic airframe tube was selected for the liner requiring no further trimming. The motor was designed in BurnSim with a progressive burn. This is to use a high L/D ratio while keeping mass flux at a minimum and resulted in 5 progressive burning BATES grains hence the “5GXL” name. Peak mass flux simulated to be 1,09gr/s-mm² @ 34,5bar (1.547 lb/s-in² @ 500psi) which is between the non-erosive and max erosive limits at that pressure. However, since no bonding agent such as Tepanol was used, I expected erosive burning to happen at slightly lower limits than the literature suggests. The idea was that a bit of erosive burning at the beginning would level out the progressive burn and give it an extra kick out of the rail when flown in single stage configuration. With a little bit of luck and a little bit of planning I got the exactly what I wanted.
On Saturday 04.02.2018 we organized a small static test party. I wanted to test the KILO 5GXL motor measuring both thrust as well as chamber pressure. My current set up only allows for chamber pressure and to determine the Isp I needed actual thrust readings for which I needed the static test stand from LD. Weather was overcast, approx. 1°C and lightly snowing every now and then.
Setting up and testing went quick with interfaces tested and prepared at home. After a quick discussion it was decided to test the KILO 5GXL first and secondly the smaller 2G motor by LD. Both motors used the same propellant and similar motor hardware. After a quick 5 sec count down the 2+2gr CuO/FeO3/Mg thermite igniter / booster charge popped and a small flame started licking from the nozzle. After a 3 second delay the motor quickly roared to life and continued to burn for 8s. Post examination showed the motor hardware had held fine. After a brief study of the data it showed there was a subtle erosive spike followed by a flat to regressive burn.
Motor disassembly showed the liner + grease at cemented itself to the casing. Guess I will have to use a more heat-resistant grease next time. Forward closure, nozzle retainer and nozzle had held up fine and after a quick sanding in the lathe they can be used for the flight motor. The nozzle did have nasty slag which is difficult to remove. In the forthcoming days I will try to dissolve the aluminium slag in caustic soda as per LD’s instructions.
Characterization tests of the Slow White pourable propellant with unimodal AP 200um propellant at Kn 424-643. This is a 60mm OD / 50mm ID test motor with 2G neutral burning bates grains. As the name already implies the propellant is indeed slow burning and needs a high Kn to get the most out of the Isp in this short motor.
A modified straw igniter consisting of heat-shrink tubing with 0,5gr CuOMg thermite was used to ignite the motor according to the rule of thumb 1gr thermite / 1000Ns. The CuOMg reaction is short and the delay in ignition is to be investigated. For a similar bimodal AP test, MnO2Mg thermite is considered
Mixing of a small 700gr batch of "slow white" APCP pourable propellant for characterization tests. Actually this bimodal formula calls for ¼ part op 90µm AP but I have found this difficult to obtain. Hence the all 200µm AP. The AP I have contains 25% by weight of particles 0-150µm so I hope this comes close to the original formula calling for a 3:1 course to fine ratio. I might try some day to get some 90µm AP by ball milling the 200µm stuff.
The propellant was casting into two, 2 grain casting tubes each being placed onto a vibrating table while casting.
Awaiting some final ingredients I started to characterize the old-stock HTPB. I expect the HTPB I have is similar to R45HTLO so an EW of 1190 is used in the curative calculations. Small 50gr batches (measured to a hundreds of a gram) were made to evaluate the different Cure Indexes 0.9 - 1.3. The curative used is from a fresh can of PS120b - MDI with an EW of 133 (%NCO 31.5).
Attached is a graph of the results. The Shore A hardness was determined with a Bareiss durometer and showed consistent results over several measurements on the same sample. The Cure Index is an approximation since I don’t know the exact EW of the HTPB. Compared to the 24h and 72h / 1 week results, the HTPB significantly hardens over time.