Hybrids 2020

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Not Quite Nominal

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Disclaimer: Static tested over 2000 hybrids, from H to O sized.

Definitely, you can have impinging injectors. One great thing about nitrous oxide is that it atomizes very, very easily because it flashes from liquid to gas in the chamber. I never saw that like-like impinging injectors were substantially better than the the two I described, other than making machining more difficult.
Would a like-impinging pintle be easier to machine than a liquid-gas pintle?

While we're at it, do you use unlike impinging injectors in your tribrids?
 

AlphaHybrids

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I use a pintle for the tribrid. Fuel through the center and oxidizer along the outside. The ones I've tested with alcohol and nitrous the ISP is 205-220. In O-P size motors I have seen numbers from other groups in the same range. They typically try to run 4-6 O:F ratio.

Like impinging pintle would be easier, but I've found that gas/nitrous was just as easy because I use the setup from the tribrid. With like impinging I found the pattern has been wider than I like and with gas/liquid it produces a narrower cone.
 

Rocket501

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In the research community, one of the more favored fuels is 50p with swirl injection. It is produced by mixing (by weight) 50% liquid paraffin wax with 50% of an HTPB mixture consisting of (92% R-45M and 8% IPDI) all mixed at 80 degrees Celsius. You don't have nearly as much of a problem with grain melting and still get higher regression rates than HTPB alone. It is often run fuel-rich to help prevent excessive nozzle erosion. I've been planning on building a hybrid rocket motor using this fuel for awhile now.

Regression rates are usually just under 2mm/s and the isp is usually somewhere in the 210-220 range for a well-designed engine. Most of these engines were operated with gaseous oxygen, so i'll have to look for one of the N2O variants papers.
 
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AlphaHybrids

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Thank you for this information - do you know how heavily this fuel and injector combination is favored? I've done *a lot* of HTPB and parrafin, but have never mixed them, does the HTPB cure? How do you control the cooling and size of the wax particles? Are the two fluids miscible?

GOX is a completely different characteristic than N2O, apples and oranges.
 

Rocket501

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I've never tried the combination myself, most of my knowledge is second-hand from research papers and a friend at college. From what I've heard, the HTPB does cure, but you have to use a high-temperature curative to get it to work. I don't think it has to be IPDI, but if you use something else, you'll have to adjust the ratio. I think that medium-temp microcrystalline wax works the best (you want it to melt at just under 80c). From what I've heard, it works best to mix the HTPB/curative separately and add it to the hot melted wax while mixing. I think it is at least somewhat miscible, but the temperature is rather important to control.

As for how heavily favored it is, I'd say right now it is pretty obscure but picking up steam. It's an easy way to reduce the problem of wax grain slumping if you intent to relight the motor and allow the fuel grain to maintain integrity during hot days. These are more practical problems that occur when you actually want to use the motors than something most researchers have to deal with, so I think this sounds promising for us amateurs.

Also, many of the experiments that use "paraffin" are actually using a highly refined type of microcrystalline wax that has greater mechanical properties and higher melting points than the ordinary stuff. Centrifugally cast Sasol 0907 is one of the favored types. Standard paraffin waxes melt at between 50-70c. Sasol 0907 melts at between 90-100c and is mechanically rather harder. It also usually works best if cast under pressure or centrifugally, otherwise the high thermal contraction of wax as it cools can cause internal cracking and other troublesome issues that compromise the grain.

Other interesting options involve the addition of aluminium to HTPB or wax in order to reduce the optimum O/F ratio. I've generally not found major ISP gains from doing so, but reducing this ratio can lead to major overall impulse gains as well as also increasing the fuel grain density.
 
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AlphaHybrids

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Interesting - I've seen it, but didn't know that it was heavily favored.

If you want the same performance, with much less work, mix powdered wax (choose your preferred type) with HTPB. The type of wax didn't have a noticable impact on impulse - it was just there as a hydrocarbon. I typically use wax that has particles about the size of table salt or sugar. Very easy to cure with HTPB, ability to add metal additives. Instead of having a homogeneous mixture, you get a mixture where the wax melts easily and leaves *a lot* of surface area in the HTPB that helps with combustion.

All of this comes from first hand test stand experience with a 5" hybrid motor, 20 pounds nitrous oxide, 7 pounds fuel, 30,000Ns nominal total impulse. This was with 5% aluminum (5-20 micron particle size) in the fuel matrix. There was an approximately 10% (varied from 8% to 14%) gain in impulse from the addition of the aluminum. The injector was a 12 orifice showerhead. There were a total of 35 tests of this configuration.

Edward
 

Rocket501

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What were the dimensions of the fuel grain? What sort of regression rates were you getting?
 
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AlphaHybrids

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The fuel grain was 4.5" OD x 30" long. Average of 4mm/s. Some tests were up to 7mm/s. They had a long beautiful yellow flame with white exhaust (aluminum oxide). They used a 6 fin finocyl grain. Pre/post combustion chambers.

Edward
 

Rocket501

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Nice! However, those regression rates are a bit higher than I desire. Ideally, I'm looking for about 1.2-1.5 mm/s, which I believe can be done with either 50p or HPTB/AL.
 

AlphaHybrids

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Regression rates can easily be adjusted by the HTPB/Powdered wax ratios. When the initial testing was done in 4-inch hardware they were able tailor that to the 5" motor. As I said, much easier than trying to heat wax, pour in, keep headed, centrifugal cast, etc. I've done a lot of centrifugal casting with wax mixtures. Produce really nice grains, but a lot of work and energy go into it. I found a 50% mixture of wax and 50% mixture EVA (hot glue sticks) produced really great results.

Edward
 

Rocket501

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I'll keep that in mind. What sort of ISP were you getting from the wax and EVA mixture?

Also, has anyone ever worked with liquid oxygen as an oxidizer? If so, what injector did you use and what isp did you see?
 

AlphaHybrids

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I have seen from 190-210 with wax and EVA. Some people think that there is this magic fuel combination for maximum ISP. There isn't. It is a system parameter that depends highly on the specific system. Oxidizer pressure and density, chamber pressure, injector, grain configuration, mdot Ox, everything is a component. Just adding a magic widget isn't going to suddenly boost performance of the system.

One thing about the hybrids that I test is that I use a pre-filled tank. The amount of oxidizer is known, as well as the pressure and density at the start of the test. This lets me get very repeatable results and calculate ISP and mass flow with a high degree of confidence. With a U/C or continuously vent to fill hybrid it is harder to know those initial conditions unless your test stand has additional load cells for filling and firing.

Liquid oxygen is beyond most amateurs, even Universities due to the cryogenic nature and extra hardware required (pressure tank or small pump, plus valving) for a hybrid motor. Nitrous is nice because it is self pressurizing and allows a relatively high feed pressure. I'd suggest reading Sutton for liquids, as well as searching the NASA technical papers.

Edward
 

Rocket501

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Yeah, I didn't think it would be better, I just was wondering if there were any major performance penalties. I've already worked on a small kerolox engine at work and for my university rocket team. Also, liquid oxygen isn't that bad. For ground testing, you can simply use blowdown pressurization and a large free volume of ullage space.
 

Louis Wu

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If one wants to use n2o cartridges in a caldera-style motor, where does one find piercing pins? I’ve checked air gun, cross man, brewers sites, etc but not sure what to get.
 

Louis Wu

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Thanks, Steve. Looking into those now. Im trying to design/build a cluster of N2O canisters (to feed a larger engine) instead of a long fillable tank And supporting ground equipment.
 

Steve Shannon

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Thanks, Steve. Looking into those now. Im trying to design/build a cluster of N2O canisters (to feed a larger engine) instead of a long fillable tank And supporting ground equipment.
Sounds like all of your impulse will be used up lifting the many tanks and other hardware. It’s also much more expensive buying whipped cream charger refills rather than filling a large nitrous tank.
Finally, if those whipped cream charger tanks are steel (I think they are) then they’re not allowed at a Tripoli Research Launch. That last may not be important to you.
 

Louis Wu

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....
One thing about the hybrids that I test is that I use a pre-filled tank. The amount of oxidizer is known, as well as the pressure and density at the start of the test. This lets me get very repeatable results and calculate ISP and mass flow with a high degree of confidence. With a U/C or continuously vent to fill hybrid it is harder to know those initial conditions unless your test stand has additional load cells for filling and firing.

Edward
Hello Edward,
what pre-filled tank do/did you use?
thanks
 

Louis Wu

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Sounds like all of your impulse will be used up lifting the many tanks and other hardware. It’s also much more expensive buying whipped cream charger refills rather than filling a large nitrous tank.
Finally, if those whipped cream charger tanks are steel (I think they are) then they’re not allowed at a Tripoli Research Launch. That last may not be important to you.
Thanks for your input, Steve. I do intend to become a tripoli member, as soon as I get my butt in gear and start building a rocket (for certification), so it is important.
 

dhbarr

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Thanks for your input, Steve. I do intend to become a tripoli member, as soon as I get my butt in gear and start building a rocket (for certification), so it is important.
Sodastream CO2 bottles and their generic equivalents are DOT 3AL. Been meaning to write that down in one of these threads, thanks for the reminder.
 

Stefan2k4

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Finally, if those whipped cream charger tanks are steel (I think they are) then they’re not allowed at a Tripoli Research Launch. That last may not be important to you.
Does that mean the original micro and mini-hybrids are not legal as well? Does the fact they are contained inside an outer aluminum engine casing matter?

I emailed Aerocon Systems about their S’Creamer motors. Anyone has experience with them? Are they’re still around?
I don't know about the S'Creamer, but Lee Dexter still makes and sells the Mini and Micro Hybrids. In his design the piercer and orifice are one piece that fits inside the injector body and burst disk holder.
 
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Steve Shannon

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Does that mean the original micro and mini-hybrids are not legal as well? Does the fact they are contained inside an outer aluminum engine casing matter?



I don't know about the S'Creamer, but Lee Dexter still makes and sells the Mini and Micro Hybrids. In his design the piercer and orifice are one piece that fits inside the injector body and burst disk holder.
If the charger nitrous container is steel (which I suspect but don’t know), then they are not allowed to be flown at Tripoli launches. It doesn’t matter if the outside motor case is aluminum.
 

dhbarr

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DOT 3AL, meaning aluminum? Equivalents include N2O bottles?
DOT-3AL1800: “DOT-3AL” is the US Department of Transportation’s specification for seamless
aluminum cylinders with a minimum service pressure greater than 150 psi. “1800” is the service pressure rating of the cylinder in psi.

IIRC sodastream and all the others are OEMed by Catalina Cylinders to maximum no-inspection-required dimensions, 2ft * 2in. There'll be other markings on the neck for tare weight, etc -- speed shop probably won't fill 'em with NOS for you, but they should be great for aerotech or hypertek style 54mm work.
 

Louis Wu

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I see. A sodastream bottle is compliant with Tripoli 'metal in rockets' rules. Filling it yourself means getting your own N2O tank. Earlier I was told this was cheaper than buying individual N2O 'whip it' cartridges. I'm looking at NOS tanks and they are expensive. At almost $300 for a ten pound tank, how many flights can I get out of the sodastream bottle? Indications are about twenty. Another $35 for the sodastream bottle, and still need the hardware. Does it make sense?
 

dhbarr

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I see. A sodastream bottle is compliant with Tripoli 'metal in rockets' rules. Filling it yourself means getting your own N2O tank. Earlier I was told this was cheaper than buying individual N2O 'whip it' cartridges. I'm looking at NOS tanks and they are expensive. At almost $300 for a ten pound tank, how many flights can I get out of the sodastream bottle? Indications are about twenty. Another $35 for the sodastream bottle, and still need the hardware. Does it make sense?
In theory as many flights as you like as you shouldn't be exposing it directly to heat or stress -- please don't mistake my intent, there is almost zero chance developing a hybrid from spare parts saves anyone time, money, or frustration.
 

Louis Wu

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In theory as many flights
I meant how many fillings (flights) before I have to refill the big tank. Twenty or more, I guess.

there is almost zero chance developing a hybrid from spare parts saves anyone time, money, or frustration.
the thrill of victory, the agony of defeat. Still looking to buy one but no one answers the call...
 

DRAGON64

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I am currently focusing on one motor design based off the 98mm "bolted" motor from Contrail. The motor is 60", with a 5300cc nitrous capacity. Such a large motor will require a suitable rocket kit that I can modify to accept the motor w/ additional room for dual recovery. I have purchased the Madcow 5.5" DX3 for the vehicle. I should have a quote for new GSE this coming weekend to start tying everything together.

Although I considered design and building a 98mm pre-fill design, which would help to keep GSE costs down, but time and available materials seem to be a bit of a draw back.
 
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AlphaHybrids

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For pre-filled tanks I used the 9000 series from Catalina Cylinders. I had them oxygen cleaned. The 300cc cylinder could be filled to 204 grams. The 440cc cylinder could be filled to 300 grams. The 9000 series has three options that are 2" outside diameter. They also have a 150cc option (102 grams). If I was filling the 440 cc cylinder I would budget 1 pound per fill based on GSE losses. Also, in a 20 pound tank it will be very hard to get the last 3-5 pounds out of the tank, it tends to expand and turn to gas. You have to chill it down cold to get it to stay a liquid. I used K bottles with 76 pounds of nitrous and filled from those.

Edward
 
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