Building, building, and almost there!

So, "what happened over spring break?", you may ask.

Fabricating.  The rocket system is finally coming together!  First, I cut the stainless steel base plates that are mounted to the testing cart.  Little did I know, when cutting steel with a chopsaw (I had only ever cut wood and tile with one before), the friction between the saw blade and steel produces enough thermal energy to melt the steel.  Forget to pull the saw back and you sling bright orange molten metal around a workshop full of people/chemicals.  I later drilled holes in the steel plates with the drill press, and those were then mounted on the test cart (pictured below).

The mount for the rocket assembly

   The bent vertical component is standard aluminum.  After this, I finally got the prefabbed rocket cases I need! That was the highlight of the week. However, the bulkhead and injector were inadequate.  So as you probably can guess,I made it work.  With grinders, blowtorches, solder, copper tubing, more drills, and epoxy, the injector's orifice shrunk from 1/8in to 1/16in... I am happy with the result.

Pre-modification Case

Modified bulkhead (top view)

  I also used the chopsaw to cut the beautiful phenolic tube into ten small baby tubes... Apparently their true color is yellow!  Last post I made remarks about the phenolics' red-brown color; the red-brown arises as a result of exposure to ultraviolet light.  The same phenomenon occurred with the old Space Shuttle booster rockets: UV turned the colors over time.

About to cut the Phenolics

Ten 4.5in Phenolic tubes, almost ready for casting

My nitrous valve also came in.  What that leaves left to do is fabricate the graphite nozzle (the lathe at the shop is too big!  Gotta find a smaller lathe somewhere.), finish up my control box (built with automobile accessory switches and a three-dollar Walmart "trinket box" that I'll likely spray paint black to make it look more professional).  The tubing is all left, but won't be too hard.  And last, but certainly not least, is to cast the fuel grains.  Finally I jump into the chemical engineering. Aluminum, magnesium, polyvinyl chlorate, more aluminum, and a vast array of hydrocarbons lay ahead.  I'm so excited.

Oh, one last thing... to measure pressure vs. time during the testing, I'll need to fit a pressure transducer into the bulkhead of the rocket.  That's likely prove horrifically inconvenient.  But we'll see...

Tomorrow, if I am lucky, I will cast my first HTPB (that's hydroxyl-terminated polybutadiene, for those of you who forgot) fuel grain.  Also, the warning on the casing told me the pyrotechnic igniters that I made (matches, BKNO3, wire) were not to be used.  But rules are meant to be broken.  Sorry, printed label, I do not take my orders from you.

  Jury-rigged pyro igniter    

"Ignite by Electrical Means Only"... eh, no thanks.

If all goes well, I could be firing next week. Fire, smoke, nitrous, and technical analysis lay ahead!

Updates

Thursday was rather productive. I got the phenolic tubing! Its a 29mm diameter tube about a meter long, with web length (the thickness of the tubing) about 2mm.  I have two of these tubes, enough to make approximately 16-20 rocket fuel grains. 

So as I stated earlier, the purpose of the phenolic tubing is to insulate the aluminum combustion chamber during reaction.  This also has a secondary purpose: it serves as the mold in which I will cast the fuel grains.  This task I will accomplish in the coming days, but only after I have the prefabbed aluminum cases shipped in.  I need to cut the tubing to proper length: too long will waste precious material, too short and the aluminum will be unshielded.

As for right now, I'm ordering a Nitrous oxide valve.  It's the exact same type that is used in automotive racing: you know, the Fast and Furious flames-shooting-out-the-exhaust-pipe deal.  Same piece.  NOS valves are great for high flow, high pressure, and great responsiveness.  The downside is that they burn out fast; anything past 60 seconds is most likely going to fry them.  That isn't a problem for me. The burn time of my rockets will not approach that mark because of the thin web length of the fuel grain. 

This week's agenda is to create fuel grains and start assembly of the entire platform.  Having done the mathematics with my advisor, I know the dimensions of all pieces in this puzzle.  I will then create the pyrotechnic ignition system (the basic idea is to ignite a small grain of fuel with an electric current, then flood it with oxidizer and start the rocket).

More updates/drawings/pics soon to come!

March: Fabrication Begins!

It's been a while since I last posted, but today begins my March fabrication phase for the rocket engine.  I will be busy finding/creating the appropriate parts to start construction of the whole assembly. 

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:

The cart assembly is left, the combustion chamber is sketched to the right.  I will make more precise technical drawings and upload those too. 

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!