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.
 
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