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.
This write-up of the core burner test is condensed as it is very similar to the test performed with the hybrid finocyl / core burner test. On Friday 21st of February the KILO core burner motor was static tested on a 66/12 reload. This was motor was designed for maximum volumetric loading, lowest core mass flux thus reducing erosive burning at start-up and followed by an expected progressive thrust curve.
Upgraded igniter consisted of a 4gr CuO/Fe03/Mg thermite 10mm heat shrink tube igniter inserted in a 20mm heat shrink tubing with mix of 6gr thermite and 8gr of plain APCP shavings sealed with some toilet paper rolled up into a ball.
Chamber pressure and thrust was again independently measured. After days of rain, the weather was dry and partial sunny at 9°C. After a 5sec count down the remote controlled ignition was pushed, the igniter popped the thermite charge and after 6 seconds the motor came up to full pressure. Thrust profile showed an expected and acceptable erosive spike at start up followed by a progressive burn and 3-4s tail off. Basically the thrust curve is all over the place but a good sustainer motor. The motor reached a total impulse of 16.780Ns at a Isp of 215s. This motor delivered 6.300Ns or approximately 60% more total impulse than the previous KILO 5GXL sustainer motor which flew in the GIGA III.
For now,this test concludes the development for the GIGA IV sustainer motor which is now flight ready.
On Sunday 26th of January the KILO hybrid finocyl core burner motor was static tested on a 66/12 reload. This 66/12 propellant is a modified version of the Slow White propellant where the 4%Zn was replaced with 4% AP resulting in 66% AP and 12% Al at 78% solids. The result is a very docile propellant even in the Kn 600+ range with good pour-ability, density and improved Isp. Furthermore it remains slow burning though a bit faster than Slow White.
Similar to the KILO 4GXXL, this motor uses a Black Cat Rocketry 3” phenolic airframe tube as liner (longer than standard PML phenolic tubing) into which the 66/12 propellant was directly cast to form a monolithic grain. The rather unconventional core geometry consisted out of a finocyl in the top half and a core burner at the bottom half (nozzle) of the grain.
The idea behind this was to test a more direct thrust shut off and reduce tail off time for potentially using a pourable HTPB liner, spin casted directly into a casing. This will prevent buying expensive custom phenolic liners, splicing liners for greater length etc. For such a HTPB liner to successfully protect the casing against the hot burning flames, it requires the motor to directly shut off without extended burn time due to tail off and possible burning away the HTPB liner. For this to happen to goal was to have the top half of the grain burn away in the same time as the lower half. The lower half experiences erosive burning and burns away faster than the top in a conventional core geometry causing tail off. Hence the finocyl was placed on top with the core burner in the bottom of the grain maximizing web thickness near the nozzle.
A 4gr CuO/Fe03/Mg thermite 10mm heat shrink tube igniter was used to get the motor started in combination with a separate, paper wrapped booster charge consisting of 8gr plain APCP shavings with another 8gr of CuO/Fe03/Mg thermite.
Chamber pressure and thrust was again independently measured. Weather was dry but cloudy at 3-4°C. After a 5sec count down the remote controlled ignition was pushed and two distinct ‘pops’ were heard, indicating the charges ignited after each other instead of one uniform ignition charge. Thrust profile showed an expected erosive spike at start up with an simulated progressive burn and an unexpected 3s tail off. The motor reached a total impulse of 16.500Ns with a Isp of 217s. This was approximately 2000Ns more than the previously tested KILO 4GXXL motor in the same casing. The motor was easily disassembled with the silicone grease now also applied to the inside of the casing and not only to the outside of the liner. Post inspection:.....
This particular thrust curve shows that, with this core geometry, it will likely not be used as-is for the sustainer reload on GIGA IV. However with some modifications (like shortening the finocyl and lengthening the core burner - to reduce the erosive spike at the beginning) it might be a possible to get a flyable thrust curve. However a 100% core burner minimizes the erosive spike for this motor configuration while keeping the peak chamber pressure at the end of the burn within acceptable (casing) limits. This time accepting both erosive burning and thrust tail off for this slow burning propellant.
Some photos of the activities and casting of the hybrid finocyl core burner.
Notes:
Cylindrical foam core length: for simplicity 580mm / same length as the standard foam plate length. 580 is also approximately 1/2 of the liner length (but slightly less).
Finocyl foam core will be trimmed to length in casting jig and also represents approx half of the liner length.
On Friday the 13th, (an omen of bad luck?) JVDB & LD tested the KILO 4GXXL motor. This motor has an extended casing to hold more propellant when compared to the KILO 5GXL. The 4GXXL uses a Black Cat Rocketry 3" phenolic tubing for liner and casting tubes. With its four XXL bates grains, the motor geometry is designed to have an even more simulated, progressive burn profile to limit core mass flux erosive burning at start up. However due to the erosive nature of the Slow White propellant a relatively neutral burn is expected.
A 4gr CuO/Fe03/Mg thermite 10mm heat shrink tube igniter was used to get the motor started in combination with a booster charge created at the top bates grain consisting of 8gr plain APCP shaving with another 8gr of CuO/Fe03/Mg thermite.
Chamber pressure and thrust was independently measured. Despite typical Dutch cold and wet weather 4°C, setup op the test stand went smooth. After a 5sec count down the remote controlled ignition button was pushed and the motor roared to life with a 7-8 second burn. Thrust profile was relative neutral. A respectable total impulse of 14.500Ns was achieved for a 86mm OD motor. The motor was easily disassembled with the silicone grease now also applied to the inside of the casing and not only to the outside of the liner. Post inspection showed a cracked liner over the entire lenght - we have seen this happen before and quite typical. Unfortunately the nozzle cracked as well and required to be replaced for a new one. A potential cause for the nozzle cracking could be a bit of play between graphite nozzle and the nozzle retainer which cause the nozzle to slam against the retainer upon pressurization or simple the increased thermal expansion of the graphite because of the bigger reload. The nozzle had over 5 previous firings. Thrust curve shows this is a suitable candidate for the GIGA IV sustainer motor.
Below is a quick representation of the head-end-ignition setup as used on the GIGA III rocket. There are many different configurations which work, including a more integrated, pyrotechnic, flame thrower type solution. However for the sake of robustness and proven designs I remained close to the modified NERO style M10x1 head end igniter body and the LD style heat shrink tubing thermite igniter.
On February 3rd we had a nice day of testing with blue skies, little wind and 6° Celsius. Two motors were to be tested both with the goal of having high thrust and short burns so they could be used as a booster for the forthcoming 2 stage rocket.
The first motor made by LD was a stretched monolithic 12 point star grain. Thanks to a different supplier a new, 1200mm long, phenolic liner is now available with the same dimensions as the PML 3" phenolic body tube which fits perfectly inside our 80mm ID motors. This motor contained a different formulation than the Slow White propellant. Now containing 65% AP, 10% Al with RIO as burn rate modifier and some additives like BDO and TETA for cross linking and bonding agent. The motor showed a clean burn with little erosive burning, high thrust and short burn. By the looks of this we now have a superior booster for the forthcoming 2 stage launch.
Second motor was a full length PML phenolic liner, 6 fin true finocyl of which the construction photos and notes can be found here. This core shape consisted out a thin 6 fins around a central core AKA finocyl . Since erosive burning at start up was expected the motor was designed to burn progressively. Core Mass Flux (CMF) simmed at 1.7 lb/sec-in². I erroneously installed a 1000psi pressure transducer whereas normally I use the 2500psi version. So the pressure transducer maxed out at start up. However the load cell picked up the thrust spike at 7000N. Obviously this motor is not flight worthy but nonetheless a nice data set was obtained. For future finocyls it is not recommended to use thin long fins but shorter and wider fins as typically seen in large monolithic finocyls such as the Qu8k rocket.
I also tried a new ignition mix of 3gr BP, 6gr CuO/RIO/Mg thermite with 12gr of fine APCP shavings which resulted in a pressure spike of 55bar but no ignition. Clearly it is easier to light this APCP heavy on Al with a longer burning, pressureless ignition mix. The traditional thermite only igniter in some heat shrink, kindly provided by LD, together with some APCP shavings sprinkled into the motor saved the day and reliably lit the motor.
On Friday 19h October we tested a smaller KILO 3G motor with a simulated total impulse of approximately 5.000Ns. The famous Alumaflame propellant formula was modified by removing the AP to make it pourable but maintaining the same CuO/Al percentages. This 3G motor was designed to be a full L or baby M1650 with a 3s burn suitable for launching rockets on ASK 't Harde to about 2-2,5km. However it proved the burn rate was considerably slower than expected resulting in a (disappointing) 6,7s. Also the signature blue "Alumaflame" flame was washed out to a pale yellow / white flame with purple edges likely a result of the HTPB rich propellant. The datalogger, which required some TLC / maintenance, performed as expected and recorded the chamber pressure at 200Hz.
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.
Core design:
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.
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).
KILO 5GXL
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.
Below is a quick representation of the head-end-ignition setup as used on the GIGA III rocket. There are many different configurations which work, including a more integrated, pyrotechnic, flame thrower type solution. However for the sake of robustness and proven designs I remained close to the modified NERO style M10x1 head end igniter body and the LD style heat shrink tubing thermite igniter.
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
" order_by="filename" order_direction="ASC" returns="included" maximum_entity_count="500"]"slow white" formula courtesy of RG.
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.
" order_by="filename" order_direction="ASC" returns="included" maximum_entity_count="500"] "slow white" formula courtesy of RG.
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.
