Fisher Space Systems LLC

Viper Rocket Trike

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Updated May 2022
04-30-2022: This month, I did a static engine test of the flight system. The thrust ramped up from ~15 N to around 20 N over a 7 sec burn time. The longer than usual burn time is attributed to ignition occurring in ~0.3 sec and thus leaving more oxidizer to fuel the engine. This was due to a small amount of HTPE leaking into the fuel grain during initialization of the receiver, a preheating event.

 The center of mass (CM) is below the cockpit area and ~ 2 cm off the thrust vector. The off axis thrust will cause the Viper to rotate after leaving the guide rail. Due to the low thrust at ignition, the liftoff velocity will not be enough to correct the rotation of the MkI Viper using the elevators and the vertical stabilizer. I will rearrange the masses inside the fuselage and bring the CM closer to the line of thrust. Also, I believe I can increase the thrust by increasing the initial nozzle throat diameter from 5 to 6 mm. Read all about it in the April EOM report.

03-31-2022: This month, I mounted a static engine test stand, a load cell to measure thrust, and a test article with a blast shield around the oxidizer tank to my rocket engine test stand. To run a static engine test, I need to pressurize the oxidizer tank to 140 psig and turn off the fill valve. Adding a blast shield around the oxidizer tank makes it safer and is part of my KISSES principle of "Keeping it Safe". Over the next month or two I'll be testing fuel grain lengths of 11.5, 12.5, and 13.5 cm to narrow done the ideal fuel grain lengths for best overall performance. Read more in the March EOM report.

02-28-2022: This month, in an attempt to increase the thrust of the class I engine, I increased the O/F ratio by shortening the fuel grain from 15 cm to 12.5 cm. This reduced the contact surface area of the fuel grain and also, reduced the mass of the rocket engine by about 20 gm. Ignition occurred around 0.9 sec with a characteristic velocity (c*) of 1,521 m/sec and a c* efficiency of over 100%. I observed a net positive thrust of greater than 19.1 N at ignition .

                  Also this month, I printed a 15 cm PLA/Al fuel core using off the shelf aluminized PLA with an aluminum content of ~ 13%. I infused the PLA/Al fuel core with KMnO4 and assembled a rocket engine based on the PLA/Al/KMnO4 fuel core. All other parameters were the same. Ignition occurred in ~ 1.3 sec and burn time was ~ 5.8 sec. The c* was 1,477 m/s with a c* efficiency of 96%. This too looks promising. If ignition time is reduced by running the engine oxidizer rich as noted above, it may be worth pursuing. Also, using a finer grain of Al with the PLA may improve performance. Read more in the February EOM report.

01-30-2022: This month I worked on the forward and aft struts for the cockpit, the paraglider box, the battery pack, and the placement of the RC receiver and the servos. I inserted the oxidizer tank and fuel grain (pictured below) into the carbon fabric fuselage. Everything fits and is ready for static testing. The mass is ~1.3 kg including the oxidizer and fuel. Read more in the January EOM report.

12-31-2021: I began this month by making a wooden template to hand layup the carbon fabric for the Mk I Viper. I found that by laying Saran Wrap on the wooden template it was easier to remove the carbon fabric panel from the template. I cut three wooden panels, made some PLA mold brackets to hold everything together, and inserted some supports and a 1" CPVC pipe. I wrapped some Saran Wrap around the mold and hand laid up the carbon fabric in two layers. The mold and carbon fabric fuselage is shown on the left.

                  Next, I worked on the nose cone and forward strut. Originally, I tried to make the nose cone out of PLA. But, it look terrible and the mass was greater than 50 gm. I decided to go with a styrofoam nose cone. I cut a styrofoam block into a rough shape of a nose cone and then sanded down the edges until I reached a tapered shape resembling a nose cone. I glued some hinges on it and attached it to the forward strut. The forward strut is made of PLA and is secured to the fuselage by three small screws. When inserted into the fuselage, the propellant tank rest against the forward strut, pushing on the forward strut during take off. The forward strut and nose cone are shown on the left.

                  Finally, after Christmas, I worked on the aft strut. The aft strut centers the rocket engine and supports the fin assembly. I made three fins using the thinnest wall setting on the 3D printer. I glued some K'Nex pieces to the fins to hold everything together. The aft strut and fin assemble are shown on the left.

                  I made some plumbing modifications to the propellant tank, solenoid valve, check valve, injector, and rocket engine assembly. Before the modifications, the subsystem mass was ~ 930 gm. After the mods, the subsystem mass, shown on the left, is now ~ 575 gm, a savings of 355 gm.

                 The forward and aft cockpit struts are next. The forward cockpit strut supports the fuselage as well as a micro servo motor to open the cover and deploy the paraglider . The aft cockpit strut also supports the fuselage and three micro servo motors for the fins. The cockpit itself houses the battery, receiver, and paraglider. The total flight system mass is an estimated 1,292 gm, well below the 1.5 kg class I requirement. The mass allotment is summarized in the table below. Read more in the December EOM report.
"Simplicity is the ultimate sophistication."
- Leonardo da Vinci
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Copyright (c) 2021 Jerry F Fisher
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Subsystem Mass (gm)
Prop Tank, valves, rocket engine with fuel  
 Forward strut and nose cone  42 
 Cockpit forward strut (includes servo motor)   40 (est.)
Cockpit (includes batt, receiver, & paraglider)  130 (est.) 
Cockpit aft strut (includes three servo motors)   70 (est.) 
 Aft strut and fins 70 
HTPE Oxidizer  75 
 Carbon Fabric Fuselage   290 
Total System       1,292 (est.)
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