Hybrids 2023
- Thread starter DRAGON64
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https://whipit.com/products/whip-it-elite-cream-chargersCan you send me some, they wont post to Australia!!!
cfb_rolley
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Damn it! So close to being able to fly at the club
I emailed bestwhip about whether their 16g chargers were stocked somewhere in Australia too but never got a response. So likely if we wanted to fly minis, we would have to buy the growlerwerks ones which are super expensive and who knows how reliable the supply is.
I have a very small handful of some old 12g canisters you can have if you're interested. Can't remember what the casing material was, vague memory recalls them being quite light though.
TP
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Yes please Troy
cfb_rolley
Well-Known Member
I gotta know more about this.
Happy to share whatever it is you were curious about. It's basically just a pyroless substitute for a hose cutter, although no hose is actually cut. Think standard push in hose fitting that's pneumatically actuated (on command) by utilising the pneumatic pressure within the hose.
TP
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Is there a drawing of it available? V interested....
Uploaded just for you
Note: for this particular device, a small hole needs to be drilled into the hose at a nominal distance from the end (27mm IIRC) to allow the device access to the gas (pressure) for actuation.
TP
cfb_rolley
Well-Known Member
This is really cool, but honest to god I swear I'm missing something here - I asked about servo actuated ball valves earlier in the thread and Contrail stated that valves need to fail closed:Uploaded just for you
...That makes perfect sense for the fill valve. At power loss a solenoid has the unique advantage of closing automatically to cease filling, and that's not as easily done for other valve types. In the case of the dump valve at communication or power loss though, then a solenoid, pyro-based line cutter, servo actuated ball valve and your non-pyro mechanism would ALL fail closed and cannot be triggered without power... So the only possible way the motor can empty without anyone approaching the motor is vent at the top of the motor, which might take ages since it'll be venting gaseous NO2, not liquid NO2 while you let it run down - which then brings us straight back to this bit:
...Forgive me for being a bit confused but.... I'm confused. If your dump valve fails the motor won't dump no matter what type of valve it is, and approaching the motor is not allowed, but allowing the motor to dump through the vent is not an approved procedure either. Does anyone have a direct link to the actual NAR and Tripoli rules specifically about hybrid motors?
Apologies, for the confusion, the device I linked was only designed for filling disconnect, not a valve.
Nevertheless, the standard setup for remote filling N2O hybrids is to use 2 solenoid valves - a fill and a dump. But are fail closed (as per usual for solenoid valves).
I achieved a similar config with a geared motor driving a ball valve with a DPDT toggle switch & DPDT relay that would act as a limit switch and H-Bridge driver for both directions ie. if I didn't have my finger on the fill button, the valve would auto shut, much like a solenoid or spring return actuated valve most common in industry. An ultra cap as part of the power rail provided pretty much effectively a dead man switch.
Problem with that was the toggle switch would eventually wear out and fail after, say, a decade of service which was a PITA when it happened.
TP
Nevertheless, the standard setup for remote filling N2O hybrids is to use 2 solenoid valves - a fill and a dump. But are fail closed (as per usual for solenoid valves).
I achieved a similar config with a geared motor driving a ball valve with a DPDT toggle switch & DPDT relay that would act as a limit switch and H-Bridge driver for both directions ie. if I didn't have my finger on the fill button, the valve would auto shut, much like a solenoid or spring return actuated valve most common in industry. An ultra cap as part of the power rail provided pretty much effectively a dead man switch.
Problem with that was the toggle switch would eventually wear out and fail after, say, a decade of service which was a PITA when it happened.
TP
cfb_rolley
Well-Known Member
Nah that’s ok. I’m familiar with the standard full setup of two solenoids, but it sounds like the requirement to fail closed is really only relevant for the fill solenoid.
Am of the correct understanding that to reduce initial costs, there’s actually nothing stopping anyone from using a single fill solenoid with a servo actuated ball valve for the dump? If that’s the case, then you could have a nice and simple single line setup by using your disconnect device inline between the fill solenoid and the motor (nitrous bottle -> solenoid -> device -> motor). Because when activated, the hose be disconnected, allowing the motor to dump liquid NO2 out the bottom, as required.
Assuming your device is cheaper than a solenoid that’d be a pretty damn good for us aussies subject to the godawful exchange rate at the moment haha!
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100%
I see what you're saying now. It's not what the device was designed for, but yes it could be used for that application. It's probably a bit overkill to be used as a dump valve when you can just mount an electrically driven ball valve to your fill manifold to dump. Like cheap Chinese 1/4" reduced bore ball valve, 1000:1 ratio N20 geared motor and coupler. Seriously, I reckon I could knock up a CHAD dump with those components for less than 20 bucks, probably <$10.
Like this:
https://www.propulsionlabs.com.au/V2_Recovery/IMG_1816.JPG
Or, as you suggested, a servo is probably easier to implement.
TP
A-ron
Well-Known Member
With all this discussion of valves and venting, I'd like to throw this idea out here.
In the interest of developing a more efficient 29mm hybrid motor, I wanted to find some way to automatically close the vent when the flight tank is full. The design I've come up with is a compact float valve that screws into the forward tank bulkhead, see attached image.
When the tank is nearly full, an extremely lightweight aluminum float slides up a slotted push pin until the flow path through the slot is blocked. At this point, the float continues sliding upward and compresses the spring until stopped by the boss just below the threads. When the tank is depressurized, the spring will push the float back down and re-open the vent.
To use this vent valve without compromising safety, the GSE has to incorporate a fail-open dump valve of some kind. My current design has a pneumatic dump valve that was inspired by the Half Cat valve:
https://www.halfcatrocketry.com/half-cat-valves
The idea is to have the dump valve connected to a long air hose that runs all the way back to the control station, where it can be safely depressurized even in the event of a total electrical failure.
In the interest of developing a more efficient 29mm hybrid motor, I wanted to find some way to automatically close the vent when the flight tank is full. The design I've come up with is a compact float valve that screws into the forward tank bulkhead, see attached image.
When the tank is nearly full, an extremely lightweight aluminum float slides up a slotted push pin until the flow path through the slot is blocked. At this point, the float continues sliding upward and compresses the spring until stopped by the boss just below the threads. When the tank is depressurized, the spring will push the float back down and re-open the vent.
To use this vent valve without compromising safety, the GSE has to incorporate a fail-open dump valve of some kind. My current design has a pneumatic dump valve that was inspired by the Half Cat valve:
https://www.halfcatrocketry.com/half-cat-valves
The idea is to have the dump valve connected to a long air hose that runs all the way back to the control station, where it can be safely depressurized even in the event of a total electrical failure.
Attachments
Couple things: I don't like the idea of a really long hose. In 25 years of testing and flying hybrids I've never experienced (or even heard of) a total electrical failure with GSE. I've experienced lots of GSE failures and seen lots from others, but never a total electrical failure.
One of the many virtues of a UC system is that the nylon hose not only acts as your "on" valve, not only acts as your auto disconnecting fill umbilical; it also acts as a pressure relieving device in the event of over-pressurization ie. it acts as a "burst disc". That's (of course) provided you're using the standard compressed air line "flexible" grade nylon fill hose that will reliably burst @1100 psi.
So, given that your tank should be good for at least 1.5x that, approaching the filled rocket in the extremely (unheard of) rare occurrence of a complete electrical failure of the GSE isn't crazy.
But let's not get ahead of ourselves here: your proposition of the float valve, whilst sounding like a simple enough project for the general hobbyist doesn't even come close to approaching serious engineering challenges hidden in the details. Let's delve into some of those:
Let's assume your vent port is static ie. not regulating like a PRV or bang-bang active modulation - it's just a static port. Therefor, your tank vapour pressure and temperature will be subject to environmental conditions eg. ambient temperature outside. Of course, that will also be significantly determined by the vent orifice area throughput, but being a 29mm motor, your vent throughput will be small hence a hot day could push up your vapour pressure to something approaching 1000psi. That could push your N2O density down to something approaching 0.5g/cc at the extreme end. That's 50% of water @STP!
So you need to build a float that has an overall density less that 0.5g/cc using Al-alloy materials that have a specific gravity of 2.7g/cc. Okay, so just make it rather "empty" you say. Well, problem is, this float needs to withstand buckling stresses from 1000ish psi pressure. But it gets worse: the density differential also needs to be enough to overcome the compressive force of the spring AND provide a high pressure seal, so we're most likely getting into the <0.4g/cc targeted ball park.
As you can see, this isn't something that can be whipped up in a spare evening with ad hoc "looks close enough" improvisation. Significant thought and cleverness is required to sort through these engineering challenges, likely coupled with precision machine shop work to even get close.
TP
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A-ron
Well-Known Member
@rocket_troy thanks for your input!
I personally don't like the idea of a long hose either, but so far it's the best solution I've been able to think of for a remotely operated non-electric dump valve that fails open. A pneumatic dump valve also has the advantage of acting as a safety valve if the deltaP between N2O and air pressure exceeds a certain value.
The float valve idea is something that I've been pondering for a long time and you correctly identified the challenges in getting such a proposition to work as intended. Due to the cooler-on-average weather in Canada, I've been assuming a N2O supply pressure of about 750 psi in my design process. At that pressure, the float's density can be as high as ~0.77g/cc.
Do note that the float does not engage the spring until it has blocked the vent flow path through the pin slot. At this point, the tight radial clearance between the float and the pin becomes the choke point for the vent flow, which should cause the pressure within the float to drop significantly. The resulting boost in deltaP is what allows the float to overcome the force of the spring and reach the end of its travel.
A perfect seal between the float and the vent pin is actually not one of the design goals. It would be satisfactory to simply minimize the vent outflow to a rate below that of a static port. The main goal is to minimize the loss of N2O during motor operation.
I personally don't like the idea of a long hose either, but so far it's the best solution I've been able to think of for a remotely operated non-electric dump valve that fails open. A pneumatic dump valve also has the advantage of acting as a safety valve if the deltaP between N2O and air pressure exceeds a certain value.
The float valve idea is something that I've been pondering for a long time and you correctly identified the challenges in getting such a proposition to work as intended. Due to the cooler-on-average weather in Canada, I've been assuming a N2O supply pressure of about 750 psi in my design process. At that pressure, the float's density can be as high as ~0.77g/cc.
Do note that the float does not engage the spring until it has blocked the vent flow path through the pin slot. At this point, the tight radial clearance between the float and the pin becomes the choke point for the vent flow, which should cause the pressure within the float to drop significantly. The resulting boost in deltaP is what allows the float to overcome the force of the spring and reach the end of its travel.
A perfect seal between the float and the vent pin is actually not one of the design goals. It would be satisfactory to simply minimize the vent outflow to a rate below that of a static port. The main goal is to minimize the loss of N2O during motor operation.
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cfb_rolley
Well-Known Member
Just on this bit here, is there a specific brand or type of push-to-connect fitting that's used? I can get PTCs here from lots of places but most don't list the pressure rating. Or do they all handle higher pressure than what the hose can take anyway? You do make a good point on the burst pressure of the hose though, pretty impossible for the motor to fail if it's burst pressure is higher than the hose, and even then it'd be 2x the normal operating pressure of the fill system (the nitrous bottle I'm going to be using also has a burst disc, too).
The max pressure those non-rated push in fittings can take is all over the place. For small diameters, it can often be higher than the tube's bursting pressure. For large diameters, it'll often be less.
For the fill tank side of things (non consumable) it's probably worth investing in a proper rated push in fitting like a Camozzi for the assurance it just won't fail on you. If you're using them in the CC, then... ehhhh... don't go to the bargain barrel... but perhaps use something off the shelf from a reputable hose supply store.
TP
I always followed the rocketry of Jeff Jakob out in California who built incredible all-aluminum hybrids. He had a web site at one time.
cfb_rolley
Well-Known Member
Going to throw out a few hypothetical questions in to the mix here.
Oxidiser to fuel ratio - one of the complications of hybrids is shifting OF ratio during the burn. Hypothetically, is there some way to read or calculate at least the overall OF ratio during the burn? For example, some kind of sensor near the nozzle end? If so, then two things come to mind - first, optimising the fuel grain geometry based on the OF ratio data from previous burns. You could then use this data to help design a grain that has a large starting surface area that actually reduces as it regressed to try to keep the OF close to optimal (eg - a star port where the points burn away and the port becomes cylindrical). Or, think about how modern fuel injection on cars works… could be possible (hypothetically of course) to control oxidiser flow rate or even add additional liquid fuel based on the OF readings to get closer to optimum efficiency throughout the whole burn?
Secondly, doped grains and type of wax. I know some university groups have used paraffin wax and additives like aluminium or sorbitol with good results. This could be something to explore for amateurs that want to try making grains, because casting wax is something well within the realm of possibility and opens up the possibility of additives. How much does the type of wax matter though? Soy wax has a higher melting point, which is a bit better in terms of sitting in the sun on a hot day, for example, but I assume would be less energetic. But is there a big difference? And also, at what point does it become “making your own propellant”? In Australia you can’t make your own propellant but inert fuel grains for hybrids are fine. So I assume aluminium powder or sorbitol in wax is fine, but adding solid oxidisers? Is that a propellant the moment you put any amount in or is it only a propellant if there’s enough oxidiser in it to be ignited and burn on it’s own?
Finally, internal structure for wax fuel grains. This is actually something I’ll test with a micro hybrid eventually but with the ability to create things like a porous ABS mesh with 3D printing, I reckon I’ll have a go at printing a gyroid infill grain and then casting wax in to it to take up the voids. What’s the hypothesis on how well this will work?
Oxidiser to fuel ratio - one of the complications of hybrids is shifting OF ratio during the burn. Hypothetically, is there some way to read or calculate at least the overall OF ratio during the burn? For example, some kind of sensor near the nozzle end? If so, then two things come to mind - first, optimising the fuel grain geometry based on the OF ratio data from previous burns. You could then use this data to help design a grain that has a large starting surface area that actually reduces as it regressed to try to keep the OF close to optimal (eg - a star port where the points burn away and the port becomes cylindrical). Or, think about how modern fuel injection on cars works… could be possible (hypothetically of course) to control oxidiser flow rate or even add additional liquid fuel based on the OF readings to get closer to optimum efficiency throughout the whole burn?
Secondly, doped grains and type of wax. I know some university groups have used paraffin wax and additives like aluminium or sorbitol with good results. This could be something to explore for amateurs that want to try making grains, because casting wax is something well within the realm of possibility and opens up the possibility of additives. How much does the type of wax matter though? Soy wax has a higher melting point, which is a bit better in terms of sitting in the sun on a hot day, for example, but I assume would be less energetic. But is there a big difference? And also, at what point does it become “making your own propellant”? In Australia you can’t make your own propellant but inert fuel grains for hybrids are fine. So I assume aluminium powder or sorbitol in wax is fine, but adding solid oxidisers? Is that a propellant the moment you put any amount in or is it only a propellant if there’s enough oxidiser in it to be ignited and burn on it’s own?
Finally, internal structure for wax fuel grains. This is actually something I’ll test with a micro hybrid eventually but with the ability to create things like a porous ABS mesh with 3D printing, I reckon I’ll have a go at printing a gyroid infill grain and then casting wax in to it to take up the voids. What’s the hypothesis on how well this will work?
The N2O will also be boiling. That makes it less dense where the float is.
O:F - not difficult to calculate but difficult to measure precisely. You'd probably want to utilise spectrum analysis. However, unlike GOX/LOX, an N2O hybrid will generally gather 45% of its energy release from the decomposition of the N2O molecules ie. before there's any O+F reaction. This allows for generous tolerances for O:F ratios ie. the specific impulse is pretty similar between 5:1 to 10:1 as opposed to a propellant combination utilising straight oxygen which will peak at stoichiometric (around 2.6 IIRC) but taper off quite steeply from that point.
As for waxes, I'm yet to see a comparison. Generally, paraffin is the popular choice as it's generally a pure hydrocarbon like kero, only with a larger molecule. If you're after pure performance, you probably don't want wax as it generally doesn't offer the most efficient combustion (in terms of chamber residence) without additional chambers, geometries and potentially modifiers.
Saying that, it's a delight to work with compared to something like HTPB ie. you can melt, mix and pour without worrying about moisture, without worrying about wastage (can just remelt leftovers for the next) and without worrying about toxic curatives.
Without an oxidizer mixed in, I'm sure you'd be pretty safe from the regulators.
As for utilising wax for filling porosity: sure, if you can get the wax to migrate through, mind you, a bit of porosity might be beneficial for regression rates (if that's important to you).
TP
I find Hybrids fascinating. 
A-ron
Well-Known Member
Thank you so much for that insight! I never thought to consider the effect that the fizz would have on buoyancy. Time to go back to the drawing board as they say.
ContrailRockets
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Flight tanks are to be designed to 3000 psi and tested to 2000 psi for Contrail Rockets or any other commercial hybrid. That being said ex standards are lower. In any event approaching a filled motor is against the safety code.
As for closing the vent after filling Contrail has done that as well as stage separation of two stage. Beware over pressurizing your motor if filled and the vent is closed. We developed an external expansion chamber that allows you to have a fill and on pad hold. My L3 sat on the pad for five minutes filled with the vent which came out the combustion chamber closed. Just a few of the innovations.
Again, it's not "against the safety code" because it's not *in* the safety code and for the reasons I explained.
TP
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The O/F ratio can be improved more by better injectors than anything else. There were a series of RattWorks tests on the K240 I think with different fuels and multi outlet injectors. The results were interesting and would seem to show that multi point injectors work better and smoothed the engine thrust data. With cheap small CNC machines now readily available, it would be easy to make those injectors. The tests could be found on the Aerocon site wierdly. Tests below.
RattWorks link http://rattworks.net/docs/rattworks_k240-injector_tests.pdf
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A-ron
Well-Known Member
Could you please provide more info on staging hybrids? This is something that I'd like to try someday.
Assuming that the fill valve is a solenoid, wouldn't it be possible to use the GSE tank as an expansion chamber? My thinking is that an unpowered fill solenoid would function as relief valve in the event that the flight tank pressure exceeds the ground tank pressure.
ContrailRockets
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What size and brand hybrid motors are you flying. I can not do smaller that 38mm and both motors need to be internally vented back through the combustion chamber. So we fill the booster and then the sustainer via a single fill line and vent line. I have a non BP line cutter that cuts both the sustainer and vent lines at the same time. Where do you fly?
PM me if you want.
Injector tests added to my OP.
