The main challenge is building the combustion chamber. It will be made out of an aluminum cylinder, the dimensions of which are to be determined. I do not yet know the size of the cylinder because of one little complication... phenolics. Phenolics are the insulating component that separates the fuel grain from the metal combustion chamber wall. Without these, the heat and pressure overcome the limits of the metal walls, causing the combustion chamber to be compromised. With any decent scale, the energy and pressure created as the fuel grain burns will be too much for the aluminum or steel chamber alone, causing an explosion. By casting a fuel grain (the chunk of solid fuel to be placed in the rocket) inside pre-made phenolics (these are too impractical to manufacture for my project), the rocket will operate properly and be reusable; without the phenolics, the assembly more resembles a one-use pipe bomb. So for now, my combustion chamber size relies on the size of the phenolics I can most easily source.
While I can't say for certain the diameter of my combustion chamber, I can speak of a couple certainties. It will be made of aluminum (whether I modify tubing or decide to machine one using a lathe), and will host a graphite fuel nozzle as well as an opening for an injector and ignitor.
Regarding the ignition, I have two options: one chemical, one electric. The chemical one is a repurposing of an estes engine, i.e. the little single stage, cardboard wrapped rocket motors that are used in the small model rockets. This one may prove more practical as it can also be used to initiate the flow of oxidizer into the combustion chamber. My other option is to apply a 14v current (from a lantern battery) to a 12v ignition relay, the relay will provide a continuous spark as opposed to the standard ~1/2 second spark intervals. This will allow for the energy necessary to perpetuate the oxidizer-fuel reactions to be applied throughout the reaction.
The whole assembly will be mounted on a mobile cart, allowing easy transport between fabrication facilities and test facilities. The design is sketched below:
If you can't see, the cart includes a tank full of the oxidizer, N2O in this case. That is connected to a 1/4in line that runs to a control valve, and then to the injector inside the combustion chamber, which is mounted horizontally. A pressure transducer will relay values to a data acquisition tool.
So for now, I'm finding and making parts to this rocket engine... 29mm, 38mm, 54mm diameter for the combustion chamber, I do not know yet. Once I have those phenolics, I will promptly tell you all.
What I am really looking forward to is casting the fuel grains! That is when I can get into the real chemical engineering behind this project but sadly this is also reliant on having phenolics.... Here is my tentative list of fuels and additives (my oxidizers are limited to Nitrous Oxide and liquid Oxygen)
HTPB+Magnesium
HTPB+Aluminum
HTPB+UFA (ultra fine aluminum)
Paraffin+Mg
Paraffin+Al
Paraffin+UFA
Asphalt
Paper-based fuel
I might also be able to test some aluminum borohydride Al(BH4)3 but that depends on time and availability. This stuff is very very reactive with water, so it has to be prepared and used with lots of care, lest it all react with atmospheric water and no longer be a feasible fuel. Could possibly use powdered teflon or viton, too.I might have time to test another fuel/additive combination, so if you have any you want to see, let me know! I'm interested in what the readers might like to see. (Solid fuels only.... and that's a firm no to any suggestions for using hydrazine. I don't have a deathwish.) Remember, with enough electrons and energy applied, a hybrid rocket can make nearly anything burn (even salami)!
Wish me well in my search for phenolics. Updates and technical drawings soon to come. And thanks for viewing again!
Sounds like phenolics might be important.
ReplyDeleteAlso, I submit bacon as a possible fuel.
Good work on the update