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Hey Cameron - Kudos for reaching out and beginning a conversation on the forum rather than just blind googling. Please reach out to us at Brazoswood - We can discuss some of the details of the big questions you have. We don't have it all figured out (far from it) but we can help you with some resources for the safety, math, and testing you will need to be comfortable with to make your process more likely to succeed. Shoot me a PM if you need contact details, I do not post much on this forum.

As you are likely picking up from the post, safety is the biggest concern when we build rockets, and it starts way before launch day. Safety begins with design, especially with hybrids. There are a thousand things that must go right for a hybrid to fly and recover, and any one of those going wrong can at best lead to a poor launch, and at worst lead to someone getting hurt or killed.

-

To be clear to the other posters, this is not quite as Wild West as what you may infer. Students launch in a controlled environment at WSMR, and are completely evacuated to a bunker before filling ever begins. The pressure vessels must be proof tested and a long series of flight checks happen before the rockets ever hit the rail. There is a range of experience at this senior level program, and students are encouraged to go out look for solutions rather than copy and paste a model design. Most of the students have built and flown J/K COTS hybrids before getting into this large SRAD stuff. And yes, P/Q class motors have successfully flown several times by high school students.

That said, I completely agree with the concerns, especially mixing fuel. In the workshop, it is easy to see when a tool is being used poorly, but often chemical safety is overlooked as it is not always as apparent when something is not being done correctly.

And to get on my teacher podium for a minute: The dismissive posts are not helpful in situations like this. While the intention may be to prevent an accident, it often rather has an opposite effect as student will seek answers elsewhere, or not at all. Dialogue and communication towards the correct resources and mentors would be much more helpful.
 
cmcleod said:
Dialogue and communication towards the correct resources and mentors would be much more helpful.
Click to expand...
It took how many posts to figure out this wasn't a troll???? Dialog is a two way street............
 
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im not going to repeat many similar posts i have made, but, it is ready aim then fire,

These are the steps the I would recommend a school team follow going into hybrids. Plus assuming you have a mentor that has actually flown a hybrid....

  1. By and fly an off the shelf hybrid rocket
    1. Understand what hybrid GSE looks like and build, buy, borrow a GSE
    2. purchase an off the shelf hybrid rocket motor
    3. Predict its performance AHEAD of time
    4. Borrow or design a test stand, understand how to use it
      1. do an intentional cold flow just to check out how the test stand works
    5. 'launch' it with the reloads that came with, on a test stand a few times
      1. Does the prediction match the test? if so great, if not figure out way and repeat
  2. Launch it in a rocket (have you done rockets with electronics?
    1. If you havent done electronic before. USE A SOLID MOTOR and launch the rocket using simple electronics and use the motor eject as a backup.
    2. Predict the performance of the rocket with the off the shelf hybrid motor
    3. Launch it
      1. record tank pressure, temperature
    4. Did the rocket perform like you thought it would with the hybrid? if yes continue, if not understand why
  3. Design your own reload - USING THE OFF THE SHELF MOTOR, NOZZLE, and INJECTOR
    1. Make sure you understand all the 'goesintos'
      1. average thrust
      2. peak thrust
      3. pressure drop
      4. operating pressure
      5. injector Cd
      6. Port Diameter
      7. Nozzle Diameter
      8. fuel Isp
      9. OF Ratio
    2. Make a minimum of (7) reloads
      1. cut up the best and worst looking ones, to look for defects. Do you have a good DOCUMENTED manufacturing process
      2. 'launch' three on the test stand
      3. analyze the data
      4. CAREFULLY inspect the nozzle and reload AFTER test stand launch,, is the weight what you thought it would be after launching? Does it look like the grain collapsed? is the nozzle eroded, how much form before and after your test.
    3. Launch it, launch the home made reload in the off the shelf motor case
      1. record tank pressure, temperature
      2. Did the rocket perform like you thought it would with the hybrid? if yes continue, if not understand why
      3. did the grain and nozzle perform like the test launches?
  4. Now think about making your own motor case / flight tank / injector / nozzle / cup holder / etc...
Seems like a bunch of 'extra work' but it really isnt. If you do these steps first, you will save time and money overall. You can do step 1 within a few weeks, way faster than trying to build a full motor from the beginning. This also gets you familiar with loading, unloading, launching etc for hybrids using the off the shelf grains.
Just making the reload in step 2 can be done in a few weeks.

Ready, aim then fire.....
 
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one last thing... Understand an o-ring, really, I'm not kidding...

If you don't have an actual copy of the Parker O-RIng Handbook or at least spent some time understanding how to use and how to design for orings, please dont go any further.... the parker o-ring handbook is the best way to learn this.

https://www.parker.com/content/dam/Parker-com/Literature/O-Ring-Division-Literature/ORD-5700.pdf

I have a hardcopy that is at leas 25 years old... And yeah, everything you need to know about o-rings and hybrids is at least that old. YouTube is not your friend here.

Had a college team ask me about a design they were doing, when I asked what he groove geometry for their gland seal was, the answer was.. 'squeezed the crap out of it' followed by... 'What is a gland seal?'

Mike (kids these days...) K
 
kramer714 said:
one last thing... Understand an o-ring, really, I'm not kidding...

If you don't have an actual copy of the Parker O-RIng Handbook or at least spent some time understanding how to use and how to design for orings, please dont go any further.... the parker o-ring handbook is the best way to learn this.

https://www.parker.com/content/dam/Parker-com/Literature/O-Ring-Division-Literature/ORD-5700.pdf

I have a hardcopy that is at leas 25 years old... And yeah, everything you need to know about o-rings and hybrids is at least that old. YouTube is not your friend here.

Had a college team ask me about a design they were doing, when I asked what he groove geometry for their gland seal was, the answer was.. 'squeezed the crap out of it' followed by... 'What is a gland seal?'

Mike (kids these days...) K
Click to expand...
I second that... with one clear important caveat: don't trust ANY (including Parker) of the o-ring compatibility tables for Nitrous. They are all clearly wrong.

TP
 
rocket_troy said:
I second that... with one clear important caveat: don't trust ANY (including Parker) of the o-ring compatibility tables for Nitrous. They are all clearly wrong.

TP
Click to expand...
We found that urethane O-rings worked in the AeroTech Hybrid, at least for the O-ring on the bottle valve.
 
AeroTech said:
We found that urethane O-rings worked in the AeroTech Hybrid, at least for the O-ring on the bottle valve.
Click to expand...
What other materials would you recommend for the high pressure (tank side) for pad fill hybrids? I'm using BunaN for most. On the chamber side I use Silicone if i want a clean surface when i am done, Buna for the rest.
 
kramer714 said:
What other materials would you recommend for the high pressure (tank side) for pad fill hybrids? I'm using BunaN for most. On the chamber side I use Silicone if i want a clean surface when i am done, Buna for the rest.
Click to expand...
For static (non moving) o-rings, BUNA/Nitrile is okay, but for dynamic you definitely don't want to be using BUNA or any material (silicone is another) that N2O will very happily permeate into creating not just significant swelling if one side of the o-ring is unpressurized, but also a potential explosive. That's the problem with the o-ring compatibility charts - these particular issues aren't really measured.

TP
 
Haven't really considered that, for the hybrids I do they are all fill on the pad and single use o-rings, i do have experience with dynamic seals, they do behave differently.
Other question is grease, I typically use Krytox, but Conrail instructions call out mobile 1 synthetic grease.
 
I don't recall offhand which O-rings are in my flight tank at the moment.

https://www.marcorubber.com/o-ring-chemical-compatibility-chart.htm#chemId=1483

Check other sources as well. This was just at the top of the list of a quick google search.

Keep in mind you also want your o-ring to be able to take some temperature excursions. For instance when the nitrous goes from the pressure line to the unpressurized tank, it starts out somewhere around -130F IIRC before repressurization.

https://www.nes-ips.com/o-ring-for-low-temperatures/ but look for additional sources of info.

https://www.rocketseals.com/low-temperature-o-rings-and-seals/ This one caught my eye just because of the name!

When I designed THRP-1 I didn't pay enough attention to thermal + pressure shock in the flight tank. I use chilled nitrous but not low enough to cause any issues with any O-ring materials in that regard. But flash-freezing an O-ring while pressurizing could possibly lead to cracking - and that would be bad.

However I have had my tank pressurized / depressurized a few times, and aged a few years, between disassembly. The O-rings were still perfect. I just don't recall offhand what ones I used. I might have re-assembled it with the same O-rings. I don't recall that either. Sorry! And it's too late for me to want to dig through my records.

Your grease for the O-rings definitely absolutely - I cannot stress this enough - MUST be compatible with nitrous oxide. There is only one manufacturer choice here, that I know of (and I'm probably wrong about that). It is expensive (that I'm not wrong about). You don't need to use it around the nozzle end but IMHO you definitely do need to use it on the flight tank etc. Krytox has gotten expensive. There are multiple versions. I'm not sure we can get our hands on the aerospace grades. I'm using 206 because it was what I could get when I needed it last time around. Others who have researched it better might be able to give you a better recommendation.

But in any event, you definitely do not want to be using incompatible grease. Nitrous Oxide is a good solvent, which is not one of the properties we're normally thinking about. It will dissolve some hydrocarbons and organics just fine, including some types of grease. Then N2O + hydrocarbon contamination = explosive. It won't take much to make it rather more sensitive then you want to be messing with.

So consider the grease has to be safe with pressurized oxygen (that's part of what you get if the nitrous starts to decompose for some reason), safe with Nitrous which is an organic solvent, safe over a wide temperature range, and safe with your O-ring or seal material of choice.

Gerald

PS - IIRC, the Mobile 1 was for the outside of the fuel grain. Not for the O-rings that get exposed to nitrous oxide.
 
Pure teflon would be a solid. The question is what is the carrier? Will it sensitize N2O?

Krytox uses a fluorinated hydrocarbon if I recall correctly... Fluorine doesn't let go of things easily! Teflon is fluorinated, but without info on the carrier I wouldn't use it. First thing to check for IMHO is it rated as safe for oxygen service. If not, it is a non-starter.

(Generic comments, not necessarily specifically about that one product)

Gerald
 
G_T said:
Pure teflon would be a solid. The question is what is the carrier? Will it sensitize N2O?

Krytox uses a fluorinated hydrocarbon if I recall correctly... Fluorine doesn't let go of things easily! Teflon is fluorinated, but without info on the carrier I wouldn't use it. First thing to check for IMHO is it rated as safe for oxygen service. If not, it is a non-starter.

(Generic comments, not necessarily specifically about that one product)

Gerald
Click to expand...
​
Its PFPE not PTFE. Krytox is a PFPE. perfluoropolyether​
Here another one, 28g enough for around 4000 flights.​
​
Search PFPE and you will find many economical solutions.​
 
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I read too quickly. I saw the PTFE in the first listing, missed the PFPE. Regardless I don't trust drawer grease to not have other things in it that could make nitrous more easily go boom.

Anyway the second listing I like a whole lot better. It mentions the appropriate compatibility, and price is better than Krytox.
 
G_T said:
I read too quickly. I saw the PTFE in the first listing, missed the PFPE. Regardless I don't trust drawer grease to not have other things in it that could make nitrous more easily go boom.

Anyway the second listing I like a whole lot better. It mentions the appropriate compatibility, and price is better than Krytox.
Click to expand...
I was (an still am) a bit skeptical about the hazard of N2O on the minute amount of grease that is needed on a static o-ring in N2O service for exposures that last less than 3 minutes. That doesn't mean I stop using the Krytox i have left.

In my earlier hybrid days I did experiments trying to get N2O from near depleted tanks impinging on wheel bearing grease (Mobil 1 grease) to ignite or do any reaction at all. I couldn't. In many attempts.
 
I figure it is a bit of a square/cube problem. For large motors, the nitrous mass vs grease mass ratio is quite high. For tiny motors, the ratio is not as high.

The impinging temp would have been, what, somewhere around -160F? That would seriously decrease solvent action, by orders of magnitude, and greatly increase required ignition power. Cold flow nitrous almost makes a good fire extinguisher. Until it doesn't.

Unrelated, I learned a few years back (Jerry and I doing propellant tests) that grease on the face of solid propellant is one heck of an inhibitor! There was this washer of propellant left behind at the end of the burn...
 
kramer714 said:
What other materials would you recommend for the high pressure (tank side) for pad fill hybrids? I'm using BunaN for most. On the chamber side I use Silicone if i want a clean surface when i am done, Buna for the rest.
Click to expand...
I would only recommend the urethane at this point.
 
Would like to see something that says "excellent, no swelling" sometime! It would become a candidate for a composite flight tank liner coating.

With the swelling that does occur, I wonder if we should resize our O-ring grooves in the axial direction to allow for a bit of expansion without pinching? Admittedly, with my machining in my current motor I probably left enough room anyway.

Good summary link John.
 
jderimig said:
Found this.

https://www.canyoncomponents.com/chem/nitrous-oxide
Click to expand...
Yet another useless o-ring compatibility chart that has no correlation to test results or link to any tests.

The people who have actually done tests - surprise surprise - all sing from the same hymn sheet:

XCOR Aerospace (Doug Jones):
"No, I only used orings that I could order easily from the usual
suspects. FVMQ and VMQ both sound like flavors of silicone and probably
have similar N2O saturation levels. I tested silicone, FEP-coated
silicone, viton, EPDM, and kalrez.

Only EPDM and Kalrez didn`t get bent. The FEP encapsulated silicone
apparently had a lot of permeation through the coating and when
pressure was released the coating inflated until it burst. Only viton
and kalrez didn`t burn. Kalrez was the only material that passed both tests."

Kuwait Space Rocket:
https://www.instagram.com/kuwaitspacerocket/p/CYBk_ODKub4/?img_index=1

Scaled Composites report (post multiple fatality N2O mishap):
"Experience has shown that some materials such as silicone (which is generally considered
N2O compatible) absorb N2O. The N2O saturated silicone exhibits impact sensitivity,
whereas the raw material does not."

From my testing and from all the feedback from other testing I've heard/read in the past, I would tend to trust this table:
https://www.ualberta.ca/chemistry/media-library/safety/gasmaterialcompatability.pdf

Although, if Gary said urethane works, I'll go along with that - I've never tested urethane.

TP
 
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jderimig said:
Even though your trusted table says its incompatible (polyurethane) with N2O even though you have no data but hearsay. ;)
Click to expand...
With N2O, I've learned to place more trust with actual experience than what's listed in tables. If it was a dynamic application, I'd definitely want to run some testing of my own to verify.

I should caveat (with all this) that common o-ring elastomer materials like BUNA/Nitrile do come in various durometers. Typically 70, but 80 or 90 are not uncommon and the permeability of N2O into a given material seems to generally be inversely proportional to the rigidity of the material. So... it is possible (although unlikely) that for some materials, the absorption tests might fail for a softer duro but pass for a harder.

TP
 
kramer714 said:
Haven't really considered that, for the hybrids I do they are all fill on the pad and single use o-rings, i do have experience with dynamic seals, they do behave differently.
Other question is grease, I typically use Krytox, but Conrail instructions call out mobile 1 synthetic grease.
Click to expand...
For me personally, I only use LOX 8 or particular grades of Krytox for anything exposed to N2O upstream of the injectors. Mind you, I do run all my HPR hybrids well outside standard practice guidelines, so I went to those extremes perhaps as precautionary overkill. Not sure, I would trust any greases that don't have a significant fluoropolymer fraction for anything upstream of the injectors. Couldn't care less for anything downstream.

TP
 
rocket_troy said:
With N2O, I've learned to place more trust with actual experience than what's listed in tables. If it was a dynamic application, I'd definitely want to run some testing of my own to verify.

I should caveat (with all this) that common o-ring elastomer materials like BUNA/Nitrile do come in various durometers. Typically 70, but 80 or 90 are not uncommon and the permeability of N2O into a given material seems to generally be inversely proportional to the rigidity of the material. So... it is possible ( although unlikely) that for some materials, the absorption tests might fail for a softer duro but pass for a harder.

TP
Click to expand...
True! experience beats tables on the internet. I have been flying hybrids since 2004 starting with Rattworks. By now I have easily put 200lbs of N20 through my motors. O-rings were all BunaN70. No issues or hint of issues. The o-rings on the cold bulkhead could easily be reused 3-4 times if I wanted.

The handwringing of oring selection on an amateur hobby rocket motor where the oring exposure to N2O is on the order of minutes is fun, but mostly unwarranted in my honest opinion.

Now the use of grease, that deserves some attention, but there is so much availability of PFPE greases around for scuba and other uses that are chemically the same as Krytox there is no reason not to use it. You'll likely not use close to all of it on your hybrid motors but you can use the extra to lube your kitchen drawer slides.
 
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