Hybrids For 2015

A bit of a delay... I used calipers to transfer a dimension. The calipers shifted slightly. The result is the coupler no longer being a snug fit. That wouldn't be an issue if there were a tube to go outside the motor but there is not. So, no slop allowed.

Time for some more metal. At least it was only the coupler I messed up.

Gerald

Edward..throtterable? YES! My aspiration, an rc, ME 163 Komet..hybrid powered. Gosh...just fly more with laughing gas.........

G_T said:

A bit of a delay... I used calipers to transfer a dimension. The calipers shifted slightly. The result is the coupler no longer being a snug fit. That wouldn't be an issue if there were a tube to go outside the motor but there is not. So, no slop allowed.

Time for some more metal. At least it was only the coupler I messed up.

Gerald

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Live and learn the easy way is always better then the resulting alternative... G_T do you have any "engineering napkin" drawings of your design showing the basics of what you are building?

Here is one of three projects I am working on for use with hybrids; this is my Rocketry Warehouse Fire Flyer (2.6" thin wall fiberglass) and the largest motor I could possibly fit in it. The motor is a SkyRipper Systems 54mm by 36" long K-motor.



This rocket will also handle the Alpha Hybrids 54mm I250...

I do, but I'm not posting anyting too detailed on the design until it is proven to work - or not.

I was looking for an interference fit in machining, taking passes of about a thousandth on the lathe. Unfortunately the calipers had shifted more than a few thousandths... When I went to check the fit, as I was getting "close" it was already loose. Calipers can be annoying for measurements. But I don't have micrometers big enough for the job. Next time I'll just machine to fit, checking each pass. It will be slower but more certain.

In a nutshell, the coupler is a tube with variable wall thickness. Doubled O-rings at each end, single O-rings at the tube joint (to aid in vibration dampening if free play develops for thermal reasons, not as a pressure seal). Bolt circles on the low pressure side of the doubled O-rings, into thicker parts of the tube. An internal removable bulkhead for the injector assembly with its own O-rings and bolts used as pins. Liner slip fits outside the coupler in the chamber side, with O-ring to help prevent hot oxidizer from passing through to the aluminum combustion chamber tube at the head end. I'm not concerned about the nozzle side of the liner as the hot gasses will no longer be oxidizing.

The upper bulkhead is a puck with doubled O-rings, a bolt circle (probably doubled), and a threaded hole for the valve used for pressure regulation and venting. I'll also likely have the usual threaded hole for an eyebolt. In the minimum diameter configuration I plan to break it in the middle by blowing the motor off the bottom of the upper electronics and recovery section. That section will be shear pinned to the motor case.

Also for that configuration the spiral retaining ring will be removed from the outside of the motor case and the fin can will be clamped on. One conformal rail guide will go on the fin can, and one on the rear of the electronics and recovery section. The extra drag of the guides will also help keep the altitude down. I can always add a few thousand feet later by discarding them and using a small tower. But I'll want flight data first.

That should let you picture what I'm doing pretty well, except for the exact dimensions.

Gerald

So you are making the tank from 76mm tubing, and coupling it together with a 76mm combustion chamber, and employing a floating injector? Your idea of using bolts as pins to retain the bulkheads is a favorite design for me. Contrail does this on their 76mm hardware. Snap rings in the larger hardware can get quite hazardous for the end user.

Conformal launch lugs causing drag?

The injector is not floating, but bolt pinned inside the coupler. So, I can test radically different injector assemblies without making a new motor or even a new coupler. The first injector assembly I intend to use has three injectors in a swirl pattern.

I've used snap rings for 114mm motors and consider them fine for normal usage. I have had issues with them as small as 76mm for high pressure EX motors though. In real life they fail somewhere around 1500psi best I can tell, if there is any gap between the bulkhead or nozzle washer and the case wall. Most commercial nozzle washers have a gap, which is what causes the nozzle to blow out of the case before the upper bulkhead blows out (for low G flights; high G gets additional loading to encourage nozzle side failure due to mass of liner and fuel grains).

With good snap ring pliers the chance of launching the ring is low (Knipex in various sizes - highly recommend!).

I do like bolts though. This will be the first motor where I've done that. I did have some Contrail hardware at one point. Wish I had a supply of the bolts they use! Do you have a source for them?

Yes, conformal launch lugs have drag. Anything sticking in the airstream has drag. Some shapes are better than others. That channel down the center of commercial ones for instance is bad for drag, particularly transonic and supersonic, as it should trap shock waves. Think of it as a small form of a multi-tube fin with open slots. It just happens to be better than the big round buttons. I'll probably machine my own guides. That's a very easy job if I've got the scrap metal around here somewhere.

Gerald

Ah, when said drag, I thought you meant drag on the rail itsels, with the fin can being a slightly larger diameter than the airframe... For some reason I had ACME fin can in mind when you described your vehicle. Then yes, ultimately, buttons guides and lugs have drag.

I've switched from bolts to dowel pins. No stress concentrations from threads and you don't have to tap umpteen times. You just need to use a dividing head so that everything lines up, or make a template that you can use on all the parts to drill them. I've tested many injectors and so far haven't found anything that works much better than a showerhead. I've done swirls, impinging, annular, aerospike type. Simple is the best. I've found that the fuel geometry matters far more than the injector geometry.

I just got done doing the initial test on the motor with the throttle. It worked, but I'm a perfectionist and would like to see some issues resolved before putting it in the air. Had good start up, throttle to a set pressure range and movement of the valve to keep it there, then good shutdown when the tank pressure reached 100 PSI and closed by the time it was at 50 PSI.

Edward

Edward,

What is the specific goal in making a hybrid motor with a throttle (other than because you can)? I'm assuming this is a long burn design, so I was wondering at what point in a burn would you initiate the use of a throttle? I remember early on, that the hybrid concept was sold with the idea that they could be turned/switched off in case of an "event." Unlike a solid motor that had to play out the event etc. Here, EAC is firing a large hybrid motor that they were able to start & stop and start again (second video on page)... Are you looking to something similar with your throttle design?

The purpose is so that I can avoid some of the downward sloping profile that all hybrids have that are self pressurized. As the tank pressure drops, so does the chamber pressure and thrust. With this you start with a second of full throttle to get it off the pad, then set the valve so the chamber pressure is a bit lower than the ending chamber pressure if the motor was not throttled. For example, this motor without the throttle had an average liquid nitrous chamber pressure of 425 psi. This is average, so the start was much higher and it ended lower. This test was set to have a chamber pressure target of 400 PSI. This made the liquid phase last longer and become more of a square shape. Once the liquid runs out you will have the tail off no matter what. There are a couple of other reasons I wanted to go with the actively controlled valve. The first is that it eliminates a mechanical setup to initiate ignition. With a U/C or pyrovalve you are relying on a sequence to happen the same every time. Sometimes that does not and you have an off nominal flight. The U/C tube might burn through in a different spot or the pyrovalve might not open all the way. This helps eliminate some of the guesswork. The preheater is lit, burns for around 5 seconds, then the nitrous flow is barely opened for 1/8 of a second, then it is opened all the way and the flight start. The other reason is to isolate the flight tank from the combustion chamber at the end of the burn. I have seen quite a few times soot and other particles get sucked back through the injector to tank. With this it turns the tank off with pressure still inside so that no particles can go back through the injector.

The system is more complicated and has more moving parts, but I like the more definite and repeatable aspect of the motor. I was already using the valve as the shut of between the tank and forward closure, so the parts addition is the servo and control electronics. It also allows you more control when using different fuels. A crazy idea that a friend and I were throwing around was to cast a very thick HTPB liner in the motor, and then on the inside of that cast a typical solid motor, probably a fin-o-cyl. You light the solid motor to get you going and then as pressure in that motor drops you turn on the nitrous and burn the HTPB liner as a hybrid. I think that idea is a long time off - you would have to insulate the injector face from the soild burn and make sure you release the nitrous timed so it decomposes and doesn't quench the motor.

Edward

Makes sense... took me a couple reads to grasp it, but I believe I understand what you are trying to accomplish. Thank you for the detailed replay. As for the solid/hybrid combo, sounds counter intuitive, yet a cool concept. Keep us posted, we would all love to see the results of your labor etc.

The solid hybrid combo is just an idea on how to get more impulse out of a motor. The solid would light and take you off of the pad and probably to a thousand or so feet before you would open the valve and turn on the hybrid. It also could provide you with a much higher initial thrust than a hybrid can to get going and get stable. BUT, the devil is in the details on how you sense when you are going to open the valve. Too soon and the pressure could be too high and have a flash back event. To late and there might not be enough heat left to start the hybrid. Solids also tend to shut down rather quickly, going from operating pressure to zero very fast- it could be faster than the electronics I'm use can sense and react. A few thoughts around this are to use a thicker and slower top grain that burns out much later than the rest of the motor, essentially a smoke grain. That way there is something still burning and then you can light your hybrid. This definitely won't happen for a while, I want to fly the throttled motor first, actually, I want it to test out on the stand first, then fly it.

Edward

Last October i flew a Contrail K2655 54mm motor and got a nice loud flight to about 3000 feet.

Today I repeated that flight, same rocket same motor, but iced the N20 tank so that gauge pressure read about 700 psi. Last years flight was closer to 800 psi.

Today's flight went to 4400 feet with a farting sound very reminiscent of Hyperteks. Last year's flight was a screaming roar. Would a 100 psi be expected to make this much difference? I am thinking of repeating this flight yet again tomorrow at 800 to test.

PS. My gauge may not be accurate,actual pressure might be lower but difference is probably right.

The loss of 100psi will reduce the ISP and thrust. HOWEVER, N2O has a density which is strongly sensitive to temperature. Lower temperature yields much higher density. Higher density means more oxidizer in the tank for the same volume tank. That's why I'm using a valve to control tank pressure on my EX project. Two reasons actually - more oxidizer in same volume, and much greater consistency of motor performance.

https://www.thefintels.com/aer/n2odensity.jpg

Gerald

I will have data when I get from the launch. Today I flew the same combination at around 800 psi indicated. 3300ft versus 4400 yesterday and totally different sound, roar instead of long fart. I will be able to generate thrust curves from the altimeter data.

Hybrid flatulence is very high oscillations between chamber and tank pressure. Your delta P between the two was closer at 700 psi than 800 psi. The increased density gave you more oxidizer and a higher flight.

Edward

Here's the data. This is thrust curve data backed out from acceleration readings.

Contrail K265 at "800psi" fill. Calculated motor designation K264, 1610N-s. Very close to certification parameters.


Contrail K265 at "700psi" fill. Note higher thrust level (average) and about 1 second longer liquid phase. Calculated motor: K303 2083N-s!

John - you've highlighted on of the hard points when certifying hybrids, how do you know how much nitrous is in the motor. As you saw, fill at lower temperatures and you have more impulse. This is one of the tips that hybrid fliers going after altitude records use. Chill the fill tank, then fill the motor, when it vents, keep filling, and filling and filling and filling to chill down the nitrous in the motor. This is the main reason I prefer tank/prefill hybrids. I know exactly how much nitrous I'm filling the tank with each time. With the throttle hybrid I even know the pressure in the tank due to the sensors. It is more repeatable than U/C, in my opinion.

Edward

I've started machining the new coupler for my EX hybrid, round 2. It's not going to be a precision machined part. It's a one-off, just for this one motor. I wouldn't do it quite the same way again anyway. I think I've figured a lighter easier method for the same size motor, and a far easier method for larger ones. I'm planning on going larger than M next time anyway, up to a 5" diameter tank, so...

It always amazes me how big a pile of chips one creates from removing only a few cubic inches of metal. The machine and boring bar are not rigid enough so it takes lots of little passes. I have hours of machining to go on this one part.

Gerald

Going to try a composite grain along the lines of a college thesis paper i found.

Student used a 3D printed form (ABS) and paraffin. Going to give it a try using a Contrail hardware setup. Still need to do some calculations if the stock injector size and nozzle combinations will work for a start.

here a picture of the grain. The wax (IGI 1250) will go in between those spoke segments. Comments and suggestions are welcome.

IGI 1250 has a melting point of 145° I would consider looking for a higher melting point wax, like a hurricane wax or similar. Looking forward to more updates on this research!

DRAGON64 said:

IGI 1250 has a melting point of 145° I would consider looking for a higher melting point wax, like a hurricane wax or similar. Looking forward to more updates on this research!

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Initially I am just repeating the research that the University did and they used IGI 1250. One consideration is that the melting temp is lower than the TG of the ABS. FYI, Paper is here.

jderimig said:

Initially I am just repeating the research that the University did and they used IGI 1250. One consideration is that the melting temp is lower than the TG of the ABS. FYI, Paper is here.

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A good read so far, thanks for the link.

John - I worked with a college group competing in ESRA this past year and they built a very nice hybrid. They did 3-inch subscale tests of many grains/fuels. They did do one of something very similar to the wax/wagon wheel grain. They found that they could get better performance from plain HTPB with 50% powdered wax in a finocyl shape. They tested many fuels, HTPB, PVC, PP, PE, ABS as well as many powders in HTPB. I think in total they did over 50 subscale test with nitrous oxide. They kept the injector and nozzle the same, the grain had a pre and post combustion chamber. For the finocyl they used foam and melted it out with acetone after. I kept trying to convince them to do HTPB, Teflon and Magnesium.

Edward

I have looked up powdered waxes in the past, like shuffleboard or dance floor wax etc. Have also come across a Carnauba powdered wax, are these the type of waxes you are referring too?

AlphaHybrids said:

John - I worked with a college group competing in ESRA this past year and they built a very nice hybrid. They did 3-inch subscale tests of many grains/fuels. They did do one of something very similar to the wax/wagon wheel grain. They found that they could get better performance from plain HTPB with 50% powdered wax in a finocyl shape. They tested many fuels, HTPB, PVC, PP, PE, ABS as well as many powders in HTPB. I think in total they did over 50 subscale test with nitrous oxide. They kept the injector and nozzle the same, the grain had a pre and post combustion chamber. For the finocyl they used foam and melted it out with acetone after. I kept trying to convince them to do HTPB, Teflon and Magnesium.

Edward

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Cool. I am willing to trade off performance for ease of fabrication. The 3D printer works when I sleep or do other things, like interact with the family.

They did do have some sliver ejection through the nozzle with wagon wheels - something to think of. Ease of fabrication is important. I also know they found that some of the different ABS grades tended to strip off and go through the nozzle. On the early test they had a bunch of 1/2" long strings on the ground.

The wax they used was granular, about the consistency of sand. I don't know what specification it is, but came in a 40 gallon paper drum. Their first attempt was setting up a booth with hot wax in a HVLP sprayer and was charged like a powder coat setup. They sprayed the wax at a charged sheet and it cooled in the air and stuck. The particle size was tiny and it was labor intensive. The 40 gallon drum just appeared on their dock Larger particles also tested better than smaller ones.

My preferred easy mix is 50% HTPB and 25% Wax and 25% Polyethylene Powder.

Edward

I was thinking of just heating up the ABS wagon wheel past the melting point of the wax and pouring the melted wax in between the spokes, maybe pull a vacuum to debubble. let cool and top off for shrinkage.

I am thinking of making these in 3 inch long stackable grains that interlock.

What style injector are you going to use? Single orifice or multi-orifice. If you are doing multi I would say skip the wax and have an injector down each wagon wheel.

Edward

AlphaHybrids said:

What style injector are you going to use? Single orifice or multi-orifice. If you are doing multi I would say skip the wax and have an injector down each wagon wheel.

Edward

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Good idea but I am just going to use stock Contrail or Skyripper 54 hardware with the single compression fitting injector.