Hybrids 2024

[cfb_rolley]

I have started to work on a 38mm prefilled motor I'd call "AlphaTech" it will have an AlphaHybrids style injector (piston held back by BP) and Aerotech style casing/closures.

I'll enjoy following this. Building my own pre-filled hybrid is pretty high up on my list of things that I'd like to do after I get the hang of a commercial hybrid. Pryo valves are attractive but I wouldn't mind exploring a burst-disc setup similar to a micro hybrid too, even if it's just for a "benchtop" hybrid.

 

[kramer714]

so, I have been watching this thread. From a disclosure standpoint, I have built / designed pressure vessels for commercial, military and aircraft applications.

Not trying to discourage anyone, but there are some things I haven't heard that have me concerned. I would like to get some comments from the folks on here that work in these industries professionally. Not to dump on the 'hobby only' guys, but there are some very good best practices from the commercial world that should flow down to the hobby guys. The big distinction I am making is prefilled vs pad filled. Pad filled has some additional safety built in, no way to maintain pressure, no one next to the tank when it is filled, only limited time filled etc. Once you prefill, you have longer times, no vent, and close proximity.

Things that I haven't heard discussed and am concerned about are (in no particular order);

• Discussions on over pressure relief valve - every pressure vessel I have been involved with that are filled and stored have an over pressure relief system in them, These are small, pretty light weight, and very reliable. Not only to the vent, but usually there is some kind of 'spreader' to keep the valve from becoming a nozzle. Look at any CO2 tank valve there is ALWAYS an over pressure valve. the thing on the left side of the picture.
• Discussion on material allowances. Before you say something like 'I'm not building a certified DOT tank...' or something like that, these tanks are not just designed and certified, but they are filled carefully and have safeties in them (see above). I do get concerned with some of the let’s try it and see what happens approach when we are talking about something that is filled and moved in close proximity to people. The allowances are how much strain the tank sees for a prescribed number of cycles. Remember one high pressure is equal to many lower pressures for fatigue (understand Miners rule)
• Discussions on ullage - there always needs to be a 'gas head' above the liquid. The exact percentage will change with temperature and pressure. Understanding the gas tables for nitrous is very helpful, even just look at Luxfur for C02 tanks look at the fluid volume vs the allowable weight, big difference. Over fill and the pressure can go VERY high with a small temperature excursion. This is factored into the over pressure relief valve, and the material allowances.
• Discussions on temperature - people have been carrying mixed phase gas/liquid in tanks for a long time, the issue is if you design the tank yourself, have you factored in these temperature extremes? With a prefilled tank, it can easily reach over 150F sitting in the sun. Nice day, rocket and tank loaded, on the pad, in the sun, misfires, now you need to handle a MUCH hotter / higher pressure tank to take it off the pad.
• Discussions on testing. EVERY pressure vessel i worked on was hydro tested to a proof pressure, every design was tested to the working pressure plus a margin of safety, (almost) every design was tested to failure to validate the design. Hydro testing doesn’t just see if the vessel can hold the pressure, it also sees if the vessel has yielded permanently. I had a college team that was working on a rocket, they were making a filament wound pressure vessel (I have done many of these), when I asked about testing I was told.. 'we don’t have the time and money to make two pressure vessels, if it fails the test we can’t launch.... " Yeah, it took me a about 20 minutes to unpack that for them and to say failing a test and not launching may be the best case scenario for them.
• Discussion on 'stuck valve / opened valve'. For new valve designs, and trapped liquid, the chance of a valve failure is not zero, so,
  • How do you get the liquid out, safely, if the valve is stuck closed after filling?
  • How have you tested the valve to the max pressure the tank can be at, not just operating pressure? Some valves get 'stuck' closed at higher pressures.
  • Drop Test - is there ANY way the valve can open on a filed tank from dropping it? You will drop it, it is gonna happen....
  • Valve releases before mechanical engagement. Ever get the 'hiss' on an airline before the coupler grabs. No imagine that with 1,000 psi liquid. High pressure disconnects all are designed so the poppet is closed before the mechanical feature disconnects.

As Phil Esterhaus used to say, 'Let's be careful out there.."



Just my thoughts...

 

[kramer714]

An few examples

https://www.amazon.com/Pure-Energy-Tank-Rupture-Disk/dp/B000AP4240

https://www.summitracing.com/parts/...o13PZdjMEdm0dfNTHsrdMJ6_SEaiZtUa7uRQXy35i8duY

https://www.lethalperformance.com/n...g5WqO1t8-nUcoIMg1qDKJtR_owhIQjQAQhGaqA2yRE7yo

These are just to give you an idea, I am not trying to say these would work, should be used, or are safe to use for your tank. Plus the tank needs to be tested beyond the pressure value of the burst disk.

You dont fix bad brakes on a car by buying a better seatbelt.....

 

[kramer714]

Tank valve triggers.

Been a little bit but I did experiment using sprinkler glass ampoules as the trigger. They only respond to heat and temperature, not just temperature. They are rated for failure load (load not pressure), with how many pounds keeps them from crushing. when they do fail from heat, they really disintegrate. Impressive to see. The plan was to use them as the trigger for a piloted valve.

A ring of solid will get them to burst.

https://www.geisslerglass.com/en/products/sprinkler-glass/

 

[hybridfreak]

Thank you for actually raising these topics and hopefully getting a discussion going!

For me it didn't really occur to bring it up, probably because I am concerned with engineering safety topics (different but nevertheless) ever day at work as well. The German word is "betriebsblind" - blinded by routine.

Since I am going down the prefilled path I might as well share some of my thoughts:

- don't make yourself what you can buy and you know will work. In my case paintball equipment rated for use with CO2 (similar pressure levels and also liquid/gas). The tanks are tested, the valves have burst disks installed and I clean them thoroughly from any combustibles and replace all sealing with nitrous safe material

- think through proven designs and then copy: again, paintball CO2 stuff, the way the valve is opened and so on. There are a lot of parts you can just buy, inspect, try and then copy or simply use. Other example: ullage, check how much mass of nitrous gas suppliers will put in their bottles and go with that ratio. Usually 0,75kg/l, I aim for a little less and use a scale when filling tanks.

- be afraid, double check everything, wear PSA..

And lots more!

Prefilled motors intrigue me because of the simplicity and repeatability. The GSE you need is smaller and not sitting next to the pad (sun..), no solenoids, no electronics. And you can get the same amount of nitrous every time which takes out one variable and let's you know how efficient your motor is

 

[kramer714]

Thank you for actually raising these topics and hopefully getting a discussion going!

For me it didn't really occur to bring it up, probably because I am concerned with engineering safety topics (different but nevertheless) ever day at work as well. The German word is "betriebsblind" - blinded by routine.

Since I am going down the prefilled path I might as well share some of my thoughts:

- don't make yourself what you can buy and you know will work. In my case paintball equipment rated for use with CO2 (similar pressure levels and also liquid/gas). The tanks are tested, the valves have burst disks installed and I clean them thoroughly from any combustibles and replace all sealing with nitrous safe material

- think through proven designs and then copy: again, paintball CO2 stuff, the way the valve is opened and so on. There are a lot of parts you can just buy, inspect, try and then copy or simply use. Other example: ullage, check how much mass of nitrous gas suppliers will put in their bottles and go with that ratio. Usually 0,75kg/l, I aim for a little less and use a scale when filling tanks.

- be afraid, double check everything, wear PSA..

And lots more!

Prefilled motors intrigue me because of the simplicity and repeatability. The GSE you need is smaller and not sitting next to the pad (sun..), no solenoids, no electronics. And you can get the same amount of nitrous every time which takes out one variable and let's you know how efficient your motor is

All good comments.

Couple thoughts,
As far as reproducibility, yeah even with prefilled as long as it is self pressurizing, the flow rates will vary. Had a guy once ask me 'is my hybrid a long burn?' My answer was yes.... in July.

One question I have is what flow rates can you get with a paint ball tank, and how much do they change from one brand to the next.

Once you get a system you like and works well, plus standardize all of your fittings, and go wireless, it isnt that hard to setup and use the GSE. The biggest issue, and you mentioned it, is making sure you have a full fill.

And yes, be smart with PSA

 

[hybridfreak]

All good comments.

Couple thoughts,
As far as reproducibility, yeah even with prefilled as long as it is self pressurizing, the flow rates will vary. Had a guy once ask me 'is my hybrid a long burn?' My answer was yes.... in July.

One question I have is what flow rates can you get with a paint ball tank, and how much do they change from one brand to the next.

Once you get a system you like and works well, plus standardize all of your fittings, and go wireless, it isnt that hard to setup and use the GSE. The biggest issue, and you mentioned it, is making sure you have a full fill.

And yes, be smart with PSA

Flow rate is the main issue with those paintball pin valves. It's low and varies from one manufacturer to the next so the only option really is to modify the valves so when it's opened the cross section is big enough for your application. Again, look what others are doing Aerotech did this and Patrick (who's on here as well and provided me with some great tips!) has documented how he replicated that on his website: https://pcarroll2525.com/

Those valves will max out at ~190-200g/s at 25-30bar chamber pressure (6,5-7oz/s at 400psi). More cross section and you'll have to machine new sealing surfaces and that's what I don't feel comfortable doing right now. Need more time on the lathe to get good at turning stuff again

Even if you always get a full fill with your easy-to-use GSE the amount of nitrous will vary quite a bit depending on ambient temperature, vent size and thus chilling of the nitrous during fill and so on. That's all fine if you just want to fly hybrids, but my inner engineer wants more data

 

[cfb_rolley]

On Kramer’s post, I need to remember that forums are public and it’s not just rocketeers that can view them - safety considerations do need to be talked about and I shouldn’t just assume that everyone knows it has to be part of the build - don’t want to accidentally lead people to build rocket motors without all the knowledge they need.

For the pre-filled that I’d like to build, DOT tanks with burst discs are the only thing I’ll use, and they’d have to be hydrostatic tested after a number of cycles etc. like any other certified pressure vessel.

I originally considered a monotube motor that could be pre-filled but the thought process I went through was that as much as I’m very confident that I could design and build something safe no problem at all, I would never feel 100% comfortable holding a pressurised container that I made myself, because a properly handled DOT vessel (in my mind) is as safe as a pressure vessel can get, therefore nothing I can design and build would be better than that anyway.

sure, I could mitigate the handling concern and design a post-fill motor so that handling isn’t an issue, but those motors are already available off the shelf from Contrail - making something that can only be flown privately or at research launches (not sure we even have any clubs that would allow an uncertified hybrid here) already exists as a tried and true product and still commercially available is a bit counterproductive.

Anyway, this thread is filled with all the stuff I love, keep it coming haha.

 

[P_Carroll2525]

With the price of AP going up because of the military buying all they can, maybe this will cause a resurgence of hybrids with more choices available.

 

[cfb_rolley]

With the price of AP going up because of the military buying all they can, maybe this will cause a resurgence of hybrids with more choices available.

Yep, hybrids are definitely gaining popularity in Australia with AP propellants being ridiculously expensive, if it’s even obtainable.

As an example of what’s going on for us in Australia: I used to get everything through Australian Rocketry They were supposed to be doing a big motor order and revamping back in October but went radio silent. Won't answer emails, phone just plays a repeating message saying they’re going through and exciting evolution and will be on hiatus until October 2023, but Blake has been on Facebook saying they’ve sent out emails to people wanting to order, but then multiple people stated they haven’t got it. At one point he said he’d potentially be able to help me out with some small propellant grains to use in hybrids as pre-heater grains but now I can’t even find out if I order any of the little stock they have on the website will even be fulfilled.

The other option is Berg’s hobbies, who keep giving me vague answers and want me to contact back next month, or endeavour aerospace, a new startup, but they’re only doing CTI stuff with a 9 month delay and they don’t send HPR reloads to QLD. They do send some G reloads, but they’re $75 each plus postage of like $40, and I don’t have any CTI hardware anyway.

So, for me, hybrid is my *only* option for anything above a black powder F motor. Victorian and NSW club flyers do seem to have an o.k stock of AP motors but many have turned to hybrid due to cost anyway. Initial setup for me is still expensive but by the time I’ve flown 5-6 times, it would already work out cheaper than continuing to fly AP.

 

[RainierWolfcastle]

sure, I could mitigate the handling concern and design a post-fill motor so that handling isn’t an issue, but those motors are already available off the shelf from Contrail - making something that can only be flown privately or at research launches (not sure we even have any clubs that would allow an uncertified hybrid here) already exists as a tried and true product and still commercially available is a bit counterproductive.

Anyway, this thread is filled with all the stuff I love, keep it coming haha.

Curious if you've reached out to any of the clubs/prefects to check? You definitely wouldn't be the first to fly a research hybrid in Australia so I can't imagine none of them allowing you to if you're certified L2 and can demonstrate you've got a suitable design and flying experience.

Yep, hybrids are definitely gaining popularity in Australia with AP propellants being ridiculously expensive, if it’s even obtainable.

As an example of what’s going on for us in Australia: I used to get everything through Australian Rocketry They were supposed to be doing a big motor order and revamping back in October but went radio silent. Won't answer emails, phone just plays a repeating message saying they’re going through and exciting evolution and will be on hiatus until October 2023, but Blake has been on Facebook saying they’ve sent out emails to people wanting to order, but then multiple people stated they haven’t got it. At one point he said he’d potentially be able to help me out with some small propellant grains to use in hybrids as pre-heater grains but now I can’t even find out if I order any of the little stock they have on the website will even be fulfilled.

The other option is Berg’s hobbies, who keep giving me vague answers and want me to contact back next month, or endeavour aerospace, a new startup, but they’re only doing CTI stuff with a 9 month delay and they don’t send HPR reloads to QLD. They do send some G reloads, but they’re $75 each plus postage of like $40, and I don’t have any CTI hardware anyway.

So, for me, hybrid is my *only* option for anything above a black powder F motor. Victorian and NSW club flyers do seem to have an o.k stock of AP motors but many have turned to hybrid due to cost anyway. Initial setup for me is still expensive but by the time I’ve flown 5-6 times, it would already work out cheaper than continuing to fly AP.

Are you a member of QRS? I'm pretty sure they were asked if they wanted anything in recent group buys.

 

[cfb_rolley]

Curious if you've reached out to any of the clubs/prefects to check? You definitely wouldn't be the first to fly a research hybrid in Australia so I can't imagine none of them allowing you to if you're certified L2 and can demonstrate you've got a suitable design and flying experience.

I haven't checked with the other clubs yet but I didn't get a clear answer from QRS last time I asked them, there's no active QRS members that fly hybrids to my knowledge so I was told I'd probably be the one steering that ship myself. That was some time ago though. A research hybrid is still a long way way off for me anyway though, it's just on my list of long term projects and I'll slowly work on it but I'll definitely wrap my head around commercial hybrids first.

Are you a member of QRS? I'm pretty sure they were asked if they wanted anything in recent group buys.

I think that might be the same group buy that I was referring to, unless it was arranged through someone else. If it is the same group buy, then for a number of people that tried to order, they never received confirmation and were told to try re-sending the order from a different email address, they did that, and then were met with radio silence. Others got an order through, but never received an invoice despite being told invoices had gone out. When they said they definitely hadn't received it despite checking junk mail, radio silence. Then some people were told to email aus rocketry support.... They got radio silence.... last I heard anything about it was 4 weeks ago. Maybe some members got their orders or a different group buy has been organised though.

Actually joining QRS could be an issue too. From my understanding the QRS site was also being hosted by aus rocketry, but it's been offline since last year meaning you can't join, unless you attend a physical launch I guess, which up until recently you couldn't find details about either.... Launch info was on the website or aus rockety forums (also hosted by aus rocketry offline). Myself and a few others did offer to clean up the forum but again... radio silence)....

QRS do now have a forum called rocketrychat, and looks like launch updates are being posted on there but I can't find how to join that forum and ask. At least I might be able to get launch info and I can head out there when the next one doesn't clash with other stuff and try to join then, but I shot them a message through the contact form as well, so we'll see what happens.

 

[RainierWolfcastle]

I haven't checked with the other clubs yet but I didn't get a clear answer from QRS last time I asked them, there's no active QRS members that fly hybrids to my knowledge so I was told I'd probably be the one steering that ship myself. That was some time ago though. A research hybrid is still a long way way off for me anyway though, it's just on my list of long term projects and I'll slowly work on it but I'll definitely wrap my head around commercial hybrids first.


I think that might be the same group buy that I was referring to, unless it was arranged through someone else. If it is the same group buy, then for a number of people that tried to order, they never received confirmation and were told to try re-sending the order from a different email address, they did that, and then were met with radio silence. Others got an order through, but never received an invoice despite being told invoices had gone out. When they said they definitely hadn't received it despite checking junk mail, radio silence. Then some people were told to email aus rocketry support.... They got radio silence.... last I heard anything about it was 4 weeks ago. Maybe some members got their orders or a different group buy has been organised though.

Actually joining QRS could be an issue too. From my understanding the QRS site was also being hosted by aus rocketry, but it's been offline since last year meaning you can't join, unless you attend a physical launch I guess, which up until recently you couldn't find details about either.... Launch info was on the website or aus rockety forums (also hosted by aus rocketry offline). Myself and a few others did offer to clean up the forum but again... radio silence)....

QRS do now have a forum called rocketrychat, and looks like launch updates are being posted on there but I can't find how to join that forum and ask. At least I might be able to get launch info and I can head out there when the next one doesn't clash with other stuff and try to join then, but I shot them a message through the contact form as well, so we'll see what happens.

Prefect contact details are on the TRA website, I'd start there because as far as I'm aware all the Australian prefects are contactable lately, at least for the active flying clubs.

I'm not saying this is definitely the issue as I wasn't part of the conversation, but I can understand why there would be reluctance for anyone to engage with supplying motors to someone if they aren't a known club member, as really you're pretty limited in what you can fly and generally the best transport method for motors is via a key contact at the local club.

I can say with certainty active QRS members have had two opportunities to get in on recent orders, including an in person contact at a launch for anyone having issues with email, and as far as I know all but one person who submitted their request has had their order placed and is in progress. I take comments online with a grain of salt, there are always a few people out there who don't like to help themselves and then blame others when things don't get done for them.

Short answer: anyone serious about rocketry should join their local prefecture, for many reasons.


Back on topic, did I see some talk of printed ABS grains...?

 

[hybridfreak]

Back on topic, did I see some talk of printed ABS grains...?

View attachment 630241View attachment 630242

Oh yeah! Nice!
Can you share more details? Looks like quite a short 54mm?
I have a 16" casing I want to make some hardware for

Looks like internal venting and pre- and post combustion chamber?

 

[OzHybrid]

I haven't checked with the other clubs yet but I didn't get a clear answer from QRS last time I asked them, there's no active QRS members that fly hybrids to my knowledge so I was told I'd probably be the one steering that ship myself. That was some time ago though. A research hybrid is still a long way way off for me anyway though, it's just on my list of long term projects and I'll slowly work on it but I'll definitely wrap my head around commercial hybrids first.


I think that might be the same group buy that I was referring to, unless it was arranged through someone else. If it is the same group buy, then for a number of people that tried to order, they never received confirmation and were told to try re-sending the order from a different email address, they did that, and then were met with radio silence. Others got an order through, but never received an invoice despite being told invoices had gone out. When they said they definitely hadn't received it despite checking junk mail, radio silence. Then some people were told to email aus rocketry support.... They got radio silence.... last I heard anything about it was 4 weeks ago. Maybe some members got their orders or a different group buy has been organised though.

Actually joining QRS could be an issue too. From my understanding the QRS site was also being hosted by aus rocketry, but it's been offline since last year meaning you can't join, unless you attend a physical launch I guess, which up until recently you couldn't find details about either.... Launch info was on the website or aus rockety forums (also hosted by aus rocketry offline). Myself and a few others did offer to clean up the forum but again... radio silence)....

QRS do now have a forum called rocketrychat, and looks like launch updates are being posted on there but I can't find how to join that forum and ask. At least I might be able to get launch info and I can head out there when the next one doesn't clash with other stuff and try to join then, but I shot them a message through the contact form as well, so we'll see what happens.

Canberra Rocketry Group launch Hybrids as does NSW Rocketry. I'm a member of NSWRA but have not launched a hybrid for a while, but am getting back into it. @Voyager1 can probably assist if you want to launch with CRG. They've got a great site at Ardlethan. If you want to launch with NSWRA you can PM me or contact NSWRA through the website nswrocketry.org.au Hybrids can only be launched with NSWRA at the NSWRA HPR site at Mullaley which is closer to you.
Where are you in Qld?

 

[cfb_rolley]

I can understand why there would be reluctance for anyone to engage with supplying motors to someone if they aren't a known club member

Yeah I get that, particularly with such limited supply in Australia. That said all I was trying to do with contacting aus rocketry was see if they were still operating so I could order some more d15 RMS reloads without wondering whether they’ll turn up or not, and although I always take online commentary with a grain of salt, the fact that there were quite a number of people all telling me the same thing that I was experiencing seems odd. I don’t know what the story is, all of my interactions with Blake or others that have worked with aus rocketry have been great in the past, it’s more just.. another difficulty of getting in to rocketry in Australia if you don’t really know anyone.

…but yeah, 3D printed ABS grains?? Hell yes, tell us more!!

 

[drewnickel]

Prefect contact details are on the TRA website, I'd start there because as far as I'm aware all the Australian prefects are contactable lately, at least for the active flying clubs.

I'm not saying this is definitely the issue as I wasn't part of the conversation, but I can understand why there would be reluctance for anyone to engage with supplying motors to someone if they aren't a known club member, as really you're pretty limited in what you can fly and generally the best transport method for motors is via a key contact at the local club.

I can say with certainty active QRS members have had two opportunities to get in on recent orders, including an in person contact at a launch for anyone having issues with email, and as far as I know all but one person who submitted their request has had their order placed and is in progress. I take comments online with a grain of salt, there are always a few people out there who don't like to help themselves and then blame others when things don't get done for them.

Short answer: anyone serious about rocketry should join their local prefecture, for many reasons.


Back on topic, did I see some talk of printed ABS grains...?

View attachment 630241View attachment 630242

Did you ever manage to get some thrust data from this motor? I remember you mentioning you didn't for the particular test shown, but curious to see how it lined up with HRAP predictions. For the tests I've done at least it's usually within a couple percent, but things like two-phase injector flow complicate predictions.

 

[drewnickel]

so, I have been watching this thread. From a disclosure standpoint, I have built / designed pressure vessels for commercial, military and aircraft applications.

Not trying to discourage anyone, but there are some things I haven't heard that have me concerned. I would like to get some comments from the folks on here that work in these industries professionally. Not to dump on the 'hobby only' guys, but there are some very good best practices from the commercial world that should flow down to the hobby guys. The big distinction I am making is prefilled vs pad filled. Pad filled has some additional safety built in, no way to maintain pressure, no one next to the tank when it is filled, only limited time filled etc. Once you prefill, you have longer times, no vent, and close proximity.

Things that I haven't heard discussed and am concerned about are (in no particular order);

• Discussions on over pressure relief valve - every pressure vessel I have been involved with that are filled and stored have an over pressure relief system in them, These are small, pretty light weight, and very reliable. Not only to the vent, but usually there is some kind of 'spreader' to keep the valve from becoming a nozzle. Look at any CO2 tank valve there is ALWAYS an over pressure valve. the thing on the left side of the picture.
• Discussion on material allowances. Before you say something like 'I'm not building a certified DOT tank...' or something like that, these tanks are not just designed and certified, but they are filled carefully and have safeties in them (see above). I do get concerned with some of the let’s try it and see what happens approach when we are talking about something that is filled and moved in close proximity to people. The allowances are how much strain the tank sees for a prescribed number of cycles. Remember one high pressure is equal to many lower pressures for fatigue (understand Miners rule)
• Discussions on ullage - there always needs to be a 'gas head' above the liquid. The exact percentage will change with temperature and pressure. Understanding the gas tables for nitrous is very helpful, even just look at Luxfur for C02 tanks look at the fluid volume vs the allowable weight, big difference. Over fill and the pressure can go VERY high with a small temperature excursion. This is factored into the over pressure relief valve, and the material allowances.
• Discussions on temperature - people have been carrying mixed phase gas/liquid in tanks for a long time, the issue is if you design the tank yourself, have you factored in these temperature extremes? With a prefilled tank, it can easily reach over 150F sitting in the sun. Nice day, rocket and tank loaded, on the pad, in the sun, misfires, now you need to handle a MUCH hotter / higher pressure tank to take it off the pad.
• Discussions on testing. EVERY pressure vessel i worked on was hydro tested to a proof pressure, every design was tested to the working pressure plus a margin of safety, (almost) every design was tested to failure to validate the design. Hydro testing doesn’t just see if the vessel can hold the pressure, it also sees if the vessel has yielded permanently. I had a college team that was working on a rocket, they were making a filament wound pressure vessel (I have done many of these), when I asked about testing I was told.. 'we don’t have the time and money to make two pressure vessels, if it fails the test we can’t launch.... " Yeah, it took me a about 20 minutes to unpack that for them and to say failing a test and not launching may be the best case scenario for them.
• Discussion on 'stuck valve / opened valve'. For new valve designs, and trapped liquid, the chance of a valve failure is not zero, so,
  • How do you get the liquid out, safely, if the valve is stuck closed after filling?
  • How have you tested the valve to the max pressure the tank can be at, not just operating pressure? Some valves get 'stuck' closed at higher pressures.
  • Drop Test - is there ANY way the valve can open on a filed tank from dropping it? You will drop it, it is gonna happen....
  • Valve releases before mechanical engagement. Ever get the 'hiss' on an airline before the coupler grabs. No imagine that with 1,000 psi liquid. High pressure disconnects all are designed so the poppet is closed before the mechanical feature disconnects.

As Phil Esterhaus used to say, 'Let's be careful out there.."



Just my thoughts...


View attachment 630132

At a minimum I see the following being necessary for anyone getting into hybrids:

For design/test work:
1. The flight tank should be designed to no less than 2x MEOP and hydro tested to no less than 1.5x, and it should not be pressurized with people inside the minimum-personnel distance for that impulse class.
2. Injector flow rates should be characterized with at least water flow testing, and possibly with a proper cold flow test to characterize two-phase flow.
3. The motor should be static fired before flying (not always possible with the larger projects like Alex's R10,000, in which case knowledge of how similar but smaller motors perform should be used to judge performance).

For common-sense and safety:
1. All GSE needs some way to purge the tank, preferably a normally open solenoid where if power to the GSE is lost the rocket will automatically safe itself.
2. A manual ball valve on a long rope should be used as an emergency backup for aborts, probably referencing NFPA's "Minimum Diameter of Cleared Area" for a given impulse class for how long that rope should be.
3. Oxygen cleaning and use of compatible materials is important, but less so when you are 2,000 feet away from the rocket and remote filling. If you are using pre-filled tanks oxygen cleaning becomes a life-threatening necessity (by the same merit, all GSE that will be pressurized with you near it should have relief valves and be oxygen cleaned).
4. For remote-filled motors, a way to check if the tank is pressurized and how much oxidizer is loaded is extremely useful and arguably should be a requirement.

There are two easy ways to mitigate risk with motors:
1. A physical barrier, such as those used at FAR. This is more appropriate for static firing and not everyone is going to have a bunker at their disposal.
2. Distance. A catastrophic failure becomes more of a case of "well that sucks" the farther away from the failure you are able to be. This note is why I strongly advocate for remote-filled motors vs pre-filled. You light a pre-heater on a remote-filled motor while standing next to it and you could get burned, but the odds of serious injury are slim to none. You have a BLEVE or decomposition or pyro-valve ignition with a pre-filled motor and it's a completely different story.

 

[hybridfreak]

Making progress on the 38mm prefilled motor
Using this casing (equivalent to a 38/360) gives me 130mm (5.1") of grain length, using a thin liner and a ~ 20mm (3/4") core leaves about 60ccm of fuel volume. That means with 250g (9oz) of nitrous and fuel densities ranging from 1 - 1,4 g/ccm the 9oz tank can take full use of the grain size and a 12oz tank might be too large even.

Does anyone have regression rate data (or a and n) numbers for printed ABS? I have found some a and n values on wikipedia and put them into HDAS but the regression rate I get with those is much higher than e.g. HTPB, which ABS is commonly compared to.

Another option I am looking into is sorbitol/wax/Al, I found some fine (and I mean reeaally fine, less than 5 micron) aluminium and some numbers (see the video description here TU Delft fuel characterization tests ) that I can work with. Its high density might just make it work with the 12oz tank for me

 

[drewnickel]

Making progress on the 38mm prefilled motor
Using this casing (equivalent to a 38/360) gives me 130mm (5.1") of grain length, using a thin liner and a ~ 20mm (3/4") core leaves about 60ccm of fuel volume. That means with 250g (9oz) of nitrous and fuel densities ranging from 1 - 1,4 g/ccm the 9oz tank can take full use of the grain size and a 12oz tank might be too large even.

Does anyone have regression rate data (or a and n) numbers for printed ABS? I have found some a and n values on wikipedia and put them into HDAS but the regression rate I get with those is much higher than e.g. HTPB, which ABS is commonly compared to.

Another option I am looking into is sorbitol/wax/Al, I found some fine (and I mean reeaally fine, less than 5 micron) aluminium and some numbers (see the video description here TU Delft fuel characterization tests ) that I can work with. Its high density might just make it work with the 12oz tank for me

I will note that TU Delft's sorbitol blend is extremely hard to light. I've gotten it to burn with oxygen but not nitrous. Part of what helps them is they have a nitrous purge to get it started before starting full flow.

 

[hybridfreak]

I will note that TU Delft's sorbitol blend is extremely hard to light. I've gotten it to burn with oxygen but not nitrous. Part of what helps them is they have a nitrous purge to get it started before starting full flow.

Good to know, thank you! I think I can make that work because I'm not using a fill tube, which means my preheater can look like a capped off bates grain meaning it can burn for a bit before burning through to the pyrovalve.

 

[drewnickel]

As for ABS regression rates, the regression rates are almost identical to HTPB. I've always just used the same regression coefficient/exponent for getting an idea of regression rates, and it's more than close enough to get a motor on a stand and get actual data. Given ABS has a higher density and lower required OF ratio than HTPB at basically the same regression rate, you can just copy the dimensions of an HTPB grain and directly swap it for ABS in most cases. Note the pink for N2O/ABS and gray for N2O/HTPB basically overlap.

 

[P_Carroll2525]

Good to know, thank you! I think I can make that work because I'm not using a fill tube, which means my preheater can look like a capped off bates grain meaning it can burn for a bit before burning through to the pyrovalve.

I used a sorbitol grain for Balls based a Dutch research paper. 70% sorb, 10% synthetic paraffin, 10% Kn, 8% Al powder and 2% red iron oxide. Lit jist fine with the aerotech pyrovalve set up.

 

[Voyager1]

I used a sorbitol grain for Balls based a Dutch research paper. 70% sorb, 10% synthetic paraffin, 10% Kn, 8% Al powder and 2% red iron oxide. Lit jist fine with the aerotech pyrovalve set up.

Don’t these specific details belong in the Research forum?

 

[cfb_rolley]

Don’t these specific details belong in the Research forum?

Good question. Posting a formula that wouldn't burn as a propellant on it's own could be argued as no different to just posting the formula of ABS plastic. But, Kn is an oxidiser... so it'd be considered a "doped" grain, right? I don't know which side of the line doped grains sit. What defines whether a grain is a "propellant" or just an inert tube of hybrid fuel? Is it a "propellant" because it has an oxidiser present in it? Or is it the amount that changes the definition? Probably a good question for the forum mods since this thread is probably the main thread for everything hybrid related so it may come up, and one of the big reasons for experimental threads being restricted is for complying with US legality.

I'd also be interested to know the legal definition too, but I imagine that may vary wildly. e.g - Australia, you can't make your own solid propellants without a licence to manufacture explosives/pyrotechnics. But I've never found a clear definition on what classifies something as "explosive" for the purposes of the legislation. I've never been able to get a clear answer on whether a doped grain would be considered as such, so never looked too deep in to anything other than 3D-printed fuel grains to err on the side of caution.

 

[Voyager1]

Good question. Posting a formula that wouldn't burn as a propellant on it's own could be argued as no different to just posting the formula of ABS plastic. But, Kn is an oxidiser... so it'd be considered a "doped" grain, right? I don't know which side of the line doped grains sit. What defines whether a grain is a "propellant" or just an inert tube of hybrid fuel? Is it a "propellant" because it has an oxidiser present in it? Or is it the amount that changes the definition? Probably a good question for the forum mods since this thread is probably the main thread for everything hybrid related so it may come up, and one of the big reasons for experimental threads being restricted is for complying with US legality.

I'd also be interested to know the legal definition too, but I imagine that may vary wildly. e.g - Australia, you can't make your own solid propellants without a licence to manufacture explosives/pyrotechnics. But I've never found a clear definition on what classifies something as "explosive" for the purposes of the legislation. I've never been able to get a clear answer on whether a doped grain would be considered as such, so never looked too deep in to anything other than 3D-printed fuel grains to err on the side of caution.

Yes, the Kn was what I was concerned about here. It depends on whether it refers to the propellant/nozzle area ratio (Kn), or Potassium Nitrate (KNO3) quantity?

 

[RainierWolfcastle]

Oh yeah! Nice!
Can you share more details? Looks like quite a short 54mm?
I have a 16" casing I want to make some hardware for

Looks like internal venting and pre- and post combustion chamber?

Not sure how much I can say as us foreigners aren't allowed in the research section, but hopefully some very general info won't get me arrested by the ITAR police:

Close, actually a 60mm as that's an easy to source size locally. I designed it to use varying lengths of tube up to about 1500mm long to get everything from J to L impulse. Short case is for testing ignition and injectors without spending a chunk on N2O fills, plus can have some fun flights with lower ceiling areas. Couple of fittings, a bit of nylon tube and a printed grain makes reloads cheap. Down the road I'd also like to try and develop it a bit further with a shorter grain and see what impulse can be squeezed out of it.

Yes to internal venting, pre and post combustion chamber sections. The latter is roughly based on the Rattworks bigger motors and seems to work well. I try to get the ignitor to burn through the vent line first to start decomposing the gaseous N2O before the fill line breaks. From videos it seems to work well as the motor goes very quick from the fill line bursting to a full size flame, but I look forward to getting test stand data later on to look into this further. This photo is the K size motor frame by frame (30fps) as it burst fill tube and lights.

Did you ever manage to get some thrust data from this motor? I remember you mentioning you didn't for the particular test shown, but curious to see how it lined up with HRAP predictions. For the tests I've done at least it's usually within a couple percent, but things like two-phase injector flow complicate predictions.

Still working on a test stand, actually have the DAQ, load cells and pressure sensors for it. Missing is the time to make it into a physical object I can test a motor with. Plugging the HRAP engine file into Openrocket ended up with a sim pretty close to the measured performance of a flight of the next size up motor (850mm long K), hoping to do some more this flying season to get more than 1 data point through.

 

[cfb_rolley]

Damn, less than a week to ship from the US to Australia, and that’s including a weekend! I don’t feel so bad about the cost of shipping stuff to Australia now!

 

[Voyager1]

Damn, less than a week to ship from the US to Australia, and that’s including a weekend! I don’t feel so bad about the cost of shipping stuff to Australia now!

View attachment 631815

Yes, I’ve generally had prompt service from Contrail. In fact, I’ve had quite fast deliveries from several US-based companies lately.

 

[cfb_rolley]

Yes, I’ve generally had prompt service from Contrail. In fact, I’ve had quite fast deliveries from several US-based companies lately.

It’s bloody good. Tom must have sent it out same day too. I also work for a race team here and we have to get all our stuff from the US and usually it’s a few weeks before anything arrives, minimum. Now that you mention it though, everything I’ve had come from the US via UPS has indeed been quick, might have to recommend that we switch to them haha.