After the CATO of the KILO 12G KNSB “lite” motor shortly after take-off, a switch was made to Ammonium perchlorate Composite Propellant (APCP). Keeping the same design philosophy and re-building from the recovered GIGA I hardware, GIGA II will again be a true minimum diameter “sustainer ready” rocket.
Note: * – to be updated according to as-built values
Read more: Flight 1 – Flights of Experimental Rockets (LRE) / Drawsko Pomorskie, April 2018.
KISS design – Form Follows Material, the non-optimised rocket geometry follows from matching, readily available materials.
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.
Some pictures of KILO 5GXL motor on a Slow white APCP reload.
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.
Nose cone avionics
The nose cone avionics of the GIGA II have been re-built after the CATO. Keeping the original basic design with some minor modifications. One of the failure modes evident was that the nose cone tip with central M6 threaded rod was missing and ripped itself, including the nut, through an aluminium U-profile taking along the G10 board holding the electronics with it. Mitigating action is an added washer between the profile and the nut. However it is expected that is a similar CATO happens the same failure can happen again.
Improvements:
The downward looking Mobius mini camera was re-located from an outside camera shroud to be housed inside of the nose cone looking out. The design of the nose cone electronics is dictated by the rather large 4000mAh 2S LiPo and keeping it centered in the nose cone to reduce the likelihood of coning as much as possible. Re-using the original nose cone - with the hole for the pull pin switches already drilled and prevent drilling any extra holes - proved quite the engineering challenge. So, although the arrangement of parts and electronics can be more compact, it is a result of existing rocket parts. In the Mobius GUI the “auto record external power” was toggled to start the recording. The camera was connected via a pull pin switch and step down converter set to 5V to an angled USB mini cable.
Nose cone avionics consisting of:
Electronics bay
Similar to the nose cone avionics the GIGA II electronics bay has been re-built from the ground up after the CATO. Almost all components were replace with new replacements such as: RDAS Tiny, battery holder, pull pin switches, U-profiles and G10 board. The basic design was kept the same but the CATO of GIGA I showed two failure modes which are mitigated by design improvements:
Recovery notes such as actual flight desent speed will be mentioned below.
Mass empty: Lift off 14,7kg – 5,27kg APCP propellant = 9,4kg mass emtpy.
GIGA recovery hardware consisting of:
