Hybrid Rocket Build Thread

[diyaerospace]

The length of the grain has nothing directly to do with burn time

I understand that grain length does not determine burn time. What I am getting at is that the amount of propellant allowable in a short grain is less than the amount of propellant allowable in a long grain if their diameter and port size is fixed.

Somewhere in there is also a decision on how much pressure drop you choose across the injectors.

I will have the tank pressure 30 percent greater than the combustion chamber pressure. (in this case 153 psi difference)

However IMHO it is well worth understanding what is going on, at least at a solid conceptual level even devoid of any math, before entering upon simulations. Then you could even design a motor somewhat in the ballpark with back of the envilope computations using simple math. The simulations would then be for verification and fine tuning.

I agree with this. I have successfully built and tested a "micro hybrid" design using 16 gram n2o whip cream cartridges and 3d printed PLA as a fuel. I have done the first few designs of this larger hybrid with mathematic equations in a notebook (following calculation tutorials mentioned in original post). I feel familiar with the concepts behind a hybrid rocket engine and am continuing to post on this thread to receive feedback on my design.

All the feedback so far has been greatly appreciated,
Walter

 

[diyaerospace]

Here is the updated forward closure design.

The first and smallest "layer" has a phenolic insulator adhered to it. This section will be bonded to the inside of the liner with RTV or ET5422 epoxy . This part will be tapped to fit the injector.

The second layer slides into the casing and will be bonded in place with RTV or ET5422 epoxy. https://www.permabond.com/resource-...-structural-epoxy-ideal-for-aluminum-bonding/

The last "layer" has glands for two viton O rings and will slide into the Oxidizer tank which is made from aluminum.
https://www.onlinemetals.com/buy/al...aluminum-round-tube-6061-t6-extruded/pid/4369

These will be attached with radial bolts. (I won't drill through the aluminum to prevent nitrous from leaking in between the bolt and the hole.)

 

[OzHybrid]

What I'm stating here is just an opinion, and not meant as a criticism.

In particular, I applaud and want to encourage you on your attempt to make your own hybrid motor of your own design.

The problem with having simulators available is they provide a shortcut to getting to a design or an evaluation of a design, without necessarily understanding the underlying interrelationships in the item being simulated. If the item is sufficiently compilcated as to be essentially not understandable, then that's the best we can do.

However IMHO it is well worth understanding what is going on, at least at a solid conceptual level even devoid of any math, before entering upon simulations. Then you could even design a motor somewhat in the ballpark with back of the envilope computations using simple math. The simulations would then be for verification and fine tuning.

Otherwise the process can easily become an exercise in GIGO.

Gerald

I put some garbage in one time when I had 2 different regression rates for PVC in 2 different hybrid programs. I wanted a 28 sec burn time and went with the better one for the result I wanted. Bad idea, as the hole in the casing showed me. And expensive.

 

[G_T]

If working with N2O for a hybrid, this attached pic is worth studying.

If working with a prefilled cylinder, then mass is fixed, but pressure, temperature, and density are not. The percentage that is liquid vs the percentage that is gas ullage is going to vary.

If working with a cylinder filled at the pad, then nothing is fixed.

Or if you try something like the game I'm playing on mine, everything is relatively fixed but very hard to light.

Pick your poison. Without instrumentation those are the options...

Chances are -

The temperature, pressure, and density are not fixed.

The pressure drop across the injector may be different each time it is fired.

The thrust produced may be bit different each time it is fired.

The total impulse may be a fair bit different each time it is fired.

The supply bottle pressure will likely be a fair bit different each time.

A motor should be designed to operate over a range of conditions (meaning temperature in this case). A simulation will run over one set of conditions, whatever you specified.

Gerald

 

[G_T]

http://www.aspirespace.org.uk/downloads/The physics of nitrous oxide.pdf
Also for anyone contemplating working with N2O hybrids, it is worth reading one paper at least on N2O safety.

Gerald

 

[rocket_troy]

Yep, given a 4-4.5 mm regression rate and an injector flow rate of 257 g/s. (I am still trying to figure out the effects of adding a CNVA to the flow rates of Contrail's injectors. I can find the flow rate of their orafice injectors by looking at motor certification data. The question of adding a CNVA seems to come down to changed Cd(0.57) and injector area.)

I used the equations in the hybrid rocket video tutorial (found in original post) and NASA CEA to calculate the motor dimensions.

Side note: I am looking for a better fuel than paraffin to burn as the wax is relatively brittle and has a high regression rate meaning a higher ox flow is required. The length of the grain will have to be pretty short to to maintain the ox to fuel ratio which limits the burn time of the motor.

I am thinking of ABS because it allows me to skip the casting process.

Thanks,
Walter

Just a few notes to add:
As suggested before, if wax is too brittle then adding either hot melt glue or even PE will improve its mechanical properties. Both additives seem to provide a bit of an eutectic effect when mixed with paraffin.
Also, because something like (typically) 45% of the energy released from an N2O hybrid comes from the decomposition of the N2O, the tolerance of O:F ratios is quite generous ie. there's not much difference in specific impulse from 5:1 to 10:1. Typically, density impulse will favour lower O:F ratios, but that depends on other factors like how much you're chilling your N2O and how packed up your fuel is with heavier compounds. Although, high O:F ratios will generally make your nozzle more susceptible to oxidation which might be a limiting factor.
Saying that, you'll probably find improved performance from ABS over paraffin via combustion efficiency and additive manufacture of hybrid fuels does seem the way of the future.

TP

 

[diyaerospace]

Hello,

I used this spreadsheet tool to get some rough dimensions of my hybrid rocket. These will only be used as a starting point and with each static test I will make adjustments to improve preformance.

The spreadsheet itself does not have ABS as a fuel. After some reading it seems ABS has somewhat similar properties to HTPB if not a slightly lower regression rate. Because of this HTPB was chosen as a fuel in the spreadsheet.

Combustion parameters were generated with NASA CEA.

The grain will be 3d printed with 0.6 mm layer height in two sections as my printer has limited print height. They will both be epoxied into the liner.

Thanks for everyone's help,
Walter

 

[cfb_rolley]

I have successfully built and tested a "micro hybrid" design using 16 gram n2o whip cream cartridges and 3d printed PLA as a fuel.

Sorry if this is a bit off topic for the thread, but how did you go with PLA as fuel? I’m using ABS in my 8g micro hybrid with really good results, but avoided testing with PLA due to worries that the really low melting point may cause the remnants of the grain to melt itself in to the components and prevent disassembly. I’ve had no such issues with ABS though, and from the few flights I’ve had using different port geometry have all been pretty high performance for a micro-hybrid.

 

[diyaerospace]

Sorry if this is a bit off topic for the thread, but how did you go with PLA as fuel? I’m using ABS in my 8g micro hybrid with really good results, but avoided testing with PLA due to worries that the really low melting point may cause the remnants of the grain to melt itself in to the components and prevent disassembly. I’ve had no such issues with ABS though, and from the few flights I’ve had using different port geometry have all been pretty high performance for a micro-hybrid.

I am using ABS as a fuel

 

[kramer714]

hmmm, like many on here I do applaud your build. Having said that, it looks like you have a number of different things that you will be trying all at once, From my experience, that never really ends well.

A suggestion,

A) buy an off the shelf hybrid (new or used), model the stock grain and nozzle, launch it a few times with the off the shelf reload and compare the actual results with your model, does it correlate?

B) replace the off the shelf reload with one of yours, using the stock case, and fire it on a test stand (3 times). Does it correlate with your predictions? Does it actually work? (no grain collapse or nozzle plugging, clean, steady burn, etc.

C) New Case design,

• understand how bolted joints really work
• understand bearing and yield about a pin
• Determine the actual tolerances in the joint based on what you can make, repeatably
• Understand how missing pin design works
• then....
• create a test plan and a way of hydro testing your design to a pressure that accounts for margins for temperature, and materials allowables

D) test fire your new desgin and reload on a test stand.

 

[diyaerospace]

hmmm, like many on here I do applaud your build. Having said that, it looks like you have a number of different things that you will be trying all at once, From my experience, that never really ends well.

A suggestion,

A) buy an off the shelf hybrid (new or used), model the stock grain and nozzle, launch it a few times with the off the shelf reload and compare the actual results with your model, does it correlate?

B) replace the off the shelf reload with one of yours, using the stock case, and fire it on a test stand (3 times). Does it correlate with your predictions? Does it actually work? (no grain collapse or nozzle plugging, clean, steady burn, etc.

C) New Case design,

• understand how bolted joints really work
• understand bearing and yield about a pin
• Determine the actual tolerances in the joint based on what you can make, repeatably
• Understand how missing pin design works
• then....
• create a test plan and a way of hydro testing your design to a pressure that accounts for margins for temperature, and materials allowables

D) test fire your new desgin and reload on a test stand.

That makes sense.

Right now I will try to get my hands on some Contrail 38mm hardware to get more experience with hybrids.

I will continue to post updates here but right now my time to work on this project is limited.


Thanks for reading,
Walter