Hybrid Rocket Engine 5

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Preface: Since the start of the HR-5 hybrid engine project, the engine has been static tested 5 times in varying configurations, using different fuels and injector flow rates. The engine has also been successfully flight tested in 2 flights, one to over 15,000' and another to over 23,000'. While I may have a use for the HR-5 engine again in the future, for now I'm moving on to a similar design in a 6" engine. Below are links to the individual HR-5 test pages.

Static Test 139: HR5 Hybrid First Test

Static Test 140: HR5 Hybrid Second Test

Static Test 141: HR5 Hybrid Third Test

Static Test 143: HR5 Hybrid with full ground support

Static Test 144: HR5 Hybrid Test 5

Launch Test 148: Aestus "O" Class Hybrid 15,000'+

Launch Test 149: Defiance Hybrid "O" Class 23,000'+

After the first successful test of the #4 hybrid engine, I was stuck trying to decide if I should do more tests of the 2.5" engine, or move on to a larger engine. Ultimately I want to design some rather large hybrids, not sure how well a small hybrid scales up, I decided to go larger right away. I settled on a 5" engine casing, simply because I had some hardware from the Defiance rocket I could use on this project. Most notably were the nose cone mold for a 5" minimum diameter rocket, the fin can and the electronics module. I also have a large supply of heavy walled cardboard tube, which should make for easier fuel grain casting.

I did decide to go with slightly thicker wall casing, I'll use .1875" for the engine casing, and revert back to .125" tube for the upper body tube, and perhaps even the oxidizer tank. The .1875" tube has a burst pressure of about 3330 psi, while the .125" tube has a burst pressure of about 2250 psi. With N2O having a critical pressure less than 1,000 psi, I should be able to get by with the thinner tube, it just depends on what level of safety I want. And to a lesser degree the optimum weight of the rocket. The impetus behind the thicker casing wall is that I want more thermal mass in the engine. I'm looking at longer burn times, and it seems the FAA may drop the 15 second burn time limit in the near future. That fits in nicely with a hybrid burn profile, high thrust off the pad when the N2O pressure is high, then tapering thrust as the N2O cools and oxidizer pressure drops.

Here's a look at the injector design. The plate on the left is the forward bulkhead and inlet for the N2O. The N2O will come in from the down side (as it lays in the photo). The injector plate to the right mates to the bulkhead with the cone that's up now, into the concave portion of the bulkhead plate. When mated the two cones are .125" apart, creating a channel for the oxidizer flow. I'm not sure if this cone shaped flow path is needed, probably not. But, I thought it may serve several purposes. First it should keep the oxidizer flowing a little better in the direction it needs to flow; Secondly, it reduces what would be a rather large void above the injector plate which could lead to some turbulence from oxidizer flashing from liquid to gas; Third, it may help to reduce the possibility of flash back into the oxidizer feed line and tank.

Here you can see the two plates mated together. The six outer holes are threaded for 1/4" -28 bolts in the bulkhead. The eight inner injector holes are .125" diameter.

Here's a rough drawing showing the side view.

I'm going to need a new test stand for this motor. The last hybrid engine I tested in the vertical position, and blew a nice hole in the ground and to mention about lifted the trailer off its wheels. So I decided to use another one of my old boat trailers, and set up a test stand in the horizontal position.

As you can see here, I've got a good start on the new test stand. The engine will attach to the channel iron tray on top, the tray pivots on 3/4" bolts through 3/8" arms and thrust terminates at the load cell to the far right. Everything is constructed very heavy, the load cell is a 2,000 pound unit and I'm pretty sure everything will handle at least that much thrust. The big problem is keeping the trailer in place. I'll have to decide on a test location and construct mounting points to tie the trailer in place.

This is a still capture from a cold flow test I did on the injector. I used 3 pounds of N2O in my 20 pound static test tank. The liquid flow didn't last long.

Here is a moment later as the liquid N2O is depleted. I did this test because I wanted to get a good visual idea of how this injector plate would flow. I was hoping for even, symmetrical flow. It looks like I got what I wanted, as I can't see anything that looks unexpected.

Click Here for a short video of two camera angles of the cold flow test.

June 23, 2007:

I've been doing what I can on the engine, but it turned out my graphite order was misplaced by my supplier. After 11 days I called them, they found it and it's now on the way, but it set back my test date slightly. I've been contemplating what fuel to use. I picked up a 100 pound "keg" of asphalt from a roofing company a couple of months ago, and I've been playing around with the asphalt. Well, playing perhaps isn't the correct term to use, as nothing is fun about asphalt at all! It really is a mess to use, and stinks like crazy too. But it is inexpensive, fairly easy to get, has some desirable properties for use in a hybrid and should give good performance.

My first test with asphalt was to cast a 3" test grain. I used a cardboard tube with an aluminum foil covered wood dowel to form the port. I used one of my Presto Multi Cookers to melt the asphalt. I originally set it to about 325 F., then increased the heat to 375 F. I'm using a standard type III asphalt from Owens Corning. If you notice on the data sheet, the usual temperature for application in roofing is 410 to 430 degrees F. with a flash point of over 500 F. I found it easy to pour at the 375 degree setting. The 3" fuel grain worked out fine. I did notice there is a fair amount of shrinkage at the top of grain, but the rest of the grain looked great. I let the grain cool to room temperature, then chilled it at 38 degrees for an hour before removing the core. I was pleased to find core removal easy, and the aluminum foil peeled easily off the inside port surface after core rod removal.

For my next casting attempt, I wanted to try a full 5" diameter grain, about 9" long. I use an old wood chisel and hammer to break chunks off the asphalt block. It works pretty well if the asphalt is 75 degrees or cooler. But if it's too warm, the asphalt won't break off, but remains sticky and somewhat taffy like. While pouring the 5" grain, I managed to make a mess and pour a large glob of asphalt down the side of the 5" tube. The asphalt has an absolute tenacious grip on the cardboard, I scraped off what I could while it was hot, but still had a lot of asphalt stuck to tube.

This time I tried freezing the asphalt grain in my freezer. When I tried to remove the coring rod and base, I had to use a lot of force because of the spilled asphalt. After the base and core were out, I noticed several cracks running the length of the grain. I would guess the cracks were caused by the cold asphalt and force needed to remove the core and base. In hindsight, the freezer probably wasn't a good idea.

As a secondary test, I wanted to see if I could cut the asphalt grain. Not really sure what to do, I tried cutting the grain in my power miter saw. As I expected, it was a big mess and didn't work very well. Considering you have to cut through heavy cardboard first, then the asphalt, I'm not really sure I see a good way of doing it.

For my first attempt a useable fuel grain, I decided to try to pour the grain as one full length grain. I used foil tape to tape the 9" cardboard tube sections together, then made a wide funnel to channel the melted asphalt into the tube. The mandrel was a 36" aluminum motor casing covered with two layers of aluminum foil. The casting base was turned from a solid slug of cast aluminum and the top centering ring from turned Plexiglas.

This is after my first full fuel grain pour attempt.

I had to use 2 cook pots to get the 1.5 gallons of asphalt required. With the melting pots rather full, I had to use the max setting of 400 degrees to get the asphalt liquid, even then, it took some 2 hours. The funnel worked well, but I still managed to get a spill down the side when an unmelted clump of asphalt plugged the spout. After allowing it to cool for an hour and a half, I added more asphalt to bring the top level back up to compensate for the shrinkage.

After several more hours of cooling. I removed the base and core mandrel, both came out rather easily. Next I wanted to make sure the fuel grain would actually fit in the casing, as it is a rather snug fit even without the foil tape on the tube joints. I used some talcum powder to slick things up a bit, and the fuel grain slid in with only modest pressure. The aluminum foil left on the core surface however, didn't want to come off. I suppose I may need to cool the grain to facilitate that. Or, I may just leave it in place. This is the very thin foil and since it burns/melts under a match, my guess is it will burn out almost instantly at ignition.

Here's a chart showing N2O density vs. Temperature

At 97.5 degrees F. and 1051 psi N2O reaches it's critical temperature and pressure, called the critical point, where N2O exits as neither a gas nor a liquid, but somewhere in between. The bottle pressure of this super critical fluid will now be determined by both temperature and density.

I made this chart to show the Isp change between different ratios of N2O to asphalt at a chamber pressure of 350 psi.

Here's similar chart using the same range, but R45M as the fuel. Again, Pc of 350 psi.

Lastly, a look at the two fuels side by side.

I found a source that has asphalt with a listed density of .04 pounds per cubic inch. I measured my full length fuel grain at a density of .0365 pounds per cubic inch. I also tested a chunk of asphalt in water, it just sank after shaking off any small air bubbles from the surface. With water density at .0361 pounds per cubic inch, my measured density of .0365 seems pretty close. I'm sure different types of asphalt may well have slightly different densities, that may account from my slight variation from the reported density.

Here is the completed nozzle. Notice the step to allow the fuel grain casting tube to drop into, also the 3 o-ring glands.

This is the pyro starter or pre-heater grain. It uses 60 grams of APCP cast into a 3" diameter cardboard tube, the outer ring was filled with asphalt.

Here the fuel grain and starter grain have been installed in the engine. I applied high temp RTV to the top surfaces of the starter grain to help keep some of the heat off the injector plate by inhibiting the upper surface of the APCP starter grain.

The engine has been static tested for the first time, see the details here.

I did a little research on HDPE, it's the most common recycled plastic (it's the #2 recycle symbol). It's usually processed at 350 to 525 degrees F. So I collected up a pile of empty HDPE containers, cut them into small strips and tried melting them in an oven between 375 and 425 degrees. I had some success, but some of the plastics melted, while others just softened. If I tried heating more than 425 degrees for any amount of time, the plastic started to brown from overheating. It did seem possible to compress the heated plastic into a solid form once heated, but to get any thickness with good density was difficult. Extruded HDPE is processed using around 15,000 psi, that would be hard for me to do... I considered making small sheets of my own plastic, then cutting the sheets into small squares, then casting these squares in polyester or epoxy resin as a binder. But that was going to be a lot of work and take a lot of time.

Another option I have is to buy HDPE in sheet form, cut circles out of the sheet, then cut out a port hole and stack the disks to form a grain. This would give me a 100% HDPE fuel grain. But again, cutting all those disks out to the correct size would be a lot of work.

I decided to see if I could get my hands on some HDPE pellets that a plastic manufacturer uses. I have small plastics company about 13 miles from me, so I went over to see if I could talk the owner into selling some pellets. The owner of the plastics company was gracious enough to put up with me, and sold me a bag of pellets. He also told me he thought I'd have better luck melting these virgin pellets in a mold. They also sell 1/2" HDPE sheet, since I didn't have a trailer with me, I'd have to hold off on that.

I tired melting some pellets in a small Pyrex dish in the oven. While it did work better than the recycled plastic. I was still going to have problems with removing trapped air. So I was back to plan A, adding the pellets to polyester resin. I sealed the base of my casting stand with melted wax, and covered the core mandrel with Duralar (Mylar without the metal coating). I mixed up 1.6 pounds of resin, then added about 2" of HDPE pellets to the empty casting tube. The resin was poured in, then I worked the pellets and resin with a small wood dowel to get it all mixed. I continued this until the first casting tube was an inch short of full, then allowed the resin to set up.

To keep the polyester resin from overheating during the cure. I only used about 1/3 the amount called for in the instructions. Even then, the casting tube did get warm to the touch and cured solid in about 1.5 hours. It was a simple matter of adding another casting tube, and repeating the process until all three tubes were filled with the resin/pellet mix.

This is the new fuel grain cast from HDPE pellets and polyester resin.

Looks odd in some way doesn't it? Fish eggs maybe? I really don't know how well this will work. My guess is the polyester resin will burn more rapidly than the HDPE pellets. Which could be a good thing, if the pellets create a somewhat turbulent surface, it may improve the mixing of gases and give me better performance. Of course, it's possible too the pellets may let loose as the polyester resin burns, and I may have created a giant pea shooter!

Fuel Grain:

Length: 27"

Weight w/Tube: 11.60 pounds

Tube Weight: 1.09 pounds

Fuel Weight: 10.51 pounds

HDPE: 6 pounds

Polyester Resin: 4.51 pounds

Ratio HDPE to PR: 57.1/42.9

The engine and new fuel grain was static tested in HR5ST2. While the engine performed well, I did suffer a burn through in the casing just below the injector plate. After the first test, I had noticed no heating at all in that area, and honestly didn't think it was going to be a problem area. Much to my dismay, I was informed (after the fact) by other hybrid builders this is very common. So it looks like I'll need to spend a little more time working on thermal protection in that area.

In preparation of my next static test, I ordered some 5" tube for a new casing. I cut the original case down 4" to remove the burned through area, I certainly could use it that way, but I wanted consistent engine parameters to judge performance.

Here is a disaster. I really worked to get a perfect fuel grain. But when I went to remove the grain from the mandrel, it just wouldn't break loose. I used aluminum foil to cover the mandrel, I suspect I poked a hole in the foil while poking and mixing the HDPE pellets and polyester resin. Notice how I even mushroomed the end of the aluminum tube mandrel, not to mention cracking the grain.

For my next attempt I wrapped the mandrel in Duralar. I coated the base of the casting stand with melted wax to keep it from sticking and to seal the base and prevent leaking of the polyester resin. I had to purchase more polyester resin, as well a HDPE pellets. I picked up 50 pounds this time so I wouldn't have to run after more plastic in the near future.

Here's the second level of the fuel grain after casting. It's sort of like building a skyscraper, just work your way up!

After the previous grain casting fiasco, I no longer had a mandrel long enough. So after the second pour cured, I pushed the mandrel up from the bottom to give me enough length to cast the final section.

Here is the completed grain. You can see a thermal protection "cap" on top of the fuel grain. The white powder is talc which is used to slick things up for insertion into the engine casing.

In this look down the core, you can see the thermal cap. The cap is PVC with a layer of EPDM rubber on the outside. Prior to installing it I'll add some high temp RTV as an added thermal barrier. This grain is just about as perfect as I could get it. The core is nicely centered and the outside of grain is just snug in the casing but slides in easily.

A few thoughts on hybrid engines and what I'm trying to do here:

I've been reading what materials I have, and searching the web for data. And I've found some interesting information. There are formulas and software for calculating a hybrid burn. You can calculate regression rates and determine r, a and n values from empirical test data. But the problem is that every hybrid is different, injector design plays a huge roll in regression rates in a hybrid. I've seen data where changing the angle of the injectors changes the regression rate by 150%. Another source I found indicated a 650% increase in regression rate from using a swirl injection design. Of course your oxidizer pressure also plays a big roll, and that changes with ambient weather and fill procedures. A hybrid also suffers from the inevitable oxidizer pressure drop from cooling during a burn, which also changes the regression rate and oxidizer to fuel ratio.

So what does this all mean to a hybrid builder? I really don't think there is any way to accurately predict with a high degree of certainty how any given engine will perform, other than by testing it. There are just too many variables with a hybrid. I think it boils down to getting the engine designed with some basic guidelines: Port to throat ratio, injector area to throat area ratio and fuel surface to oxidizer mass flow ratio. Using those parameters as a starting point, then using test data to fine tune the engine seems the best approach to me.

What I'm looking to do with this engine is gather data on a good sized hybrid, for future even larger hybrid engines. For now I've been testing this engine in the "N" class, once I'm comfortable with tests in this range, I'll move to larger nitrous loads and test it at an "O" total impulse level. When moving to the "O" level, I plan to open the injectors up a little more. For an "O" class hybrid flight to be practical, I think I'll need at least 800 pounds of initial thrust. Depending on how the next few static tests go, I may reconfigure and fly the Defiance rocket this Fall as an "N" class hybrid. Or perhaps build a new rocket and fly a "big dumb" "O" class hybrid. The reason I say "big dumb" is because I really don't want a real high altitude flight, I'm not looking at setting any altitude records just yet, but I'd like to get some hybrid flights in for the learning experience.

August 7, 2207:

After one static test of an asphalt fuel grain and two tests of polyester/HDPE fuel, I've come to the conclusion I need something in between the two in terms of fuel burned. With a pure asphalt fuel grain at 27", the mixture was too fuel rich, with polyester/HDPE it was too fuel lean. A shorter engine case with asphalt would work no doubt, but I like the longer case for better fuel-oxidizer mixing. For my next test I've decided to use the original casing that was cut down 4" after the burn through in the second test. I'll cut the asphalt fuel down to about 17" rather than 27" and use a short polyester/HDPE fuel grain with a slightly larger port as a post combustion chamber.

This graphic shows the intended fuel grain design.

One other change I'm making to this fuel grain is to add an epoxy/graphite/fiberglass cap to the fuel grain to try to eliminate burning of the thermal liner at the top of the engine. This cap will go all the way to the top of chamber and touch the forward bulkhead. The cap is enlarged to allow the injector plate to descend into the cap. I cast this cap from 100 grams of epoxy, 63 grams of graphite powder and 6 grams of fiberglas strands 1 to 2" long.

Parts:

Self-Aligning Brass Compression Tube Fitting Adapter for 1/4" Tube OD X 1/4" NPTF Male Pipe McMaster-Carr #5220K65

Nylon Vacuum Tubing .150" ID, 1/4" OD, .05" Wall Thk, Semi-Clear White McMaster-Carr #5173K43

PTFE Braided Hose W/Type 304 SS Braid SS Swivel Fem X Fem Fitting, 48" L, 3/16" ID, 3000 PSI McMaser-Carr #4552K438