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.
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.
"Black Cat Rocketry" liners are dimensional equivalent to PML phenolic tubes except that they come in 1225mm lengths whereas a PML tube is 916-912mm. This gives the possibility of an additional 310mm propellant without the need of splicing a liner. This BCR liner was already used in the BOOSTAR-ONE motor as a liner / casting tube for a monolithic star grain. As usual a heavy wall thickness 90x5mm static test casing with M5 bolts and a 86x3mm flight casing with M4 retaining bolts were machined. New casing length is 1360mm. Fin holders were gratefully machined by JWS.
One of the design changes for the GIGA IV is a common bulkhead to join the previously central avionics bay of GIGA I-III (in control of recovery) with the updated nose cone electronics (camera and GPS tracker). With this modification there is no longer space for a separate main parachute compartment as can be expected in a normal HPR dual deployment set up. Instead a single 2ft kevlar Rocketman parachute is deployed at apogee. Considering landing at sea or in a forest, the resulting higher decent rate is therefore unlikely to damage the rocket.
Since optionally landing at sea was used as a starting point, a lot of consideration has gone into the design and production of this concept to ensure both the avionics bay as well as the nose cone electronics are air / water tight.
The camera lens is glued / sealed in place through the bulkhead wall. For a next revision the lens band of the bulkhead is to be a bit longer than the current 13mm since only a thin edge remains due to the 12mm hole.
The GIGA III was built completely from aluminium but heavy like a tank. That is probably the reason why it was so successful recovering from non-flight-critical failures. In order to shave some weight for the from the total rocket mass of the GIGA IV it was decided to replace the aluminium tubes with fiberglass (FG) tubes. In the end, to produce accurate custom fiberglass parts, it proved to be a lot of work including fabricating of many custom jigs. Mass density savings FG overs aluminium: 1,68 vs 2,7 gr/cm³. However with the new single parachute design (effectively shortening the airframe) the weight saving of the FG airframe and av-bay coupler tube for GIGA IV was 886gr or a 61% reduction when compared to the GIGA III. Aluminium GIGA III 1440gr. Shortened FG GIGA IV 554gr = weight saving of 886gr.
Notes airframe: 300mm section.
280gr/m2 FG gives a layer thickness of 0,25-0,30mm.
ID of 80mm + 12*0,25= 3mm => 12 wraps minimum proved to be sufficient - as-built OD: 87mm.
(π*0,086)=0,27m per wrap. 12*0,27=3,24m => 3 piece of FG, each 1,08m long.
0,4 (width)*1,08 (length of FG cloth)*3 (pieces of FG cloth)*280 (FG mass/m2)*0,85 (FG/epoxy ratio) = 308gr epoxy. Based on FG / Epoxy = 1: 0,85.
3 cups of each 70g epoxy / 35gr harder = 315gr epoxy.
OD mandel with polyester paper is 79,9mm.
Notes coupler to house av-bay: 132mm section.
280gr/m2 FG gives a layer thickness of 0,25-0,30mm.
ID of 76mm + 8x0,25= 2mm => 8 wraps minimum proved to be sufficient - as-built OD: 80,5mm.
(π*0,08)=0,2m per wrap. 8*0,25=2m => 2 piece of FG, each 1m long.
0,3 (width)*1 (length of FG cloth)*2 (pieces of FG cloth)*280 (FG mass/m2)*0,85 (FG/epoxy ratio) = 142gr epoxy. Based on FG / Epoxy = 1: 0,85.
2 cups of each 50g epoxy / 25gr harder = 150gr epoxy.
OD mandel with polyester paper is 76,3mm.
The same avionics as flown in GIGA II and GIGA III.
*** Check to add some bungee cord (snap-back) or velcro (no snap-back but bulky) to absorb some of the energy / load and prevent to nose cone from popping off ***.
Single stage recovery by means of a 2ft kevlar Rocketman chute and 25mm wide 10m kevlar strap deployed at apogee. Kevlar strap folded in 30cm zig zags and subsequent bundle zigzag folded in 3 with 1m (up to pre-sewn loop) left loose. This compact bundle is than inserted into motor coupler.
Rocket – Booster built by LD, aluminium airframe, minimum diameter, experimental booster.
Diameter – 130mm.
Length – 2600mm*.
Weight fully loaded – 42.0kg*.
Maximum velocity: 600m/s*.
Booster Motor – experimental LD 12 point star core (130mm).
Propellant: 65/10 AP/Al with RIO (75,5% solids).
Total impulse: approx 41.500Ns*.
Isp: approx 215s*.
Burn time: 7s*.
Propellant mass: 20.0kg*.
Equivalent to a P5300*.
Flight control – Stratologger*.
Onboard camera – 2 pcs.
Tracking – Talky GPS, 1 Watt by LD; 433mHz AM emergency beacon.
Note: * – to be updated according to as-built values