The Aestus rocket is intended to be another one of my "stepping stone" rockets. That is, it will be my first hybrid rocket and not designed for any particular purpose other than to give me experience in hybrid engine rocket flight. At over 14' long and 8" in diameter, it certainly isn't a small rocket. I wanted to keep my altitude down, yet I wanted to fly a good sized engine and I do despise "big dumb rockets". So the Aestus will be a compromise, designed to fly on a "N" to "O" class impulse engine to altitudes from 10,000 to 15,000'. Peak speed should remain sub sonic on the "N" load, and pushing mach on an "O" load.
This rocket will be of a new and rather unusual design. Notice the four centering rings on the graphic above, while not shown in the graphic, I'll use stringers to connect the four centering rings, making the entire engine and tank assembly an internal skeleton and removable. The outer body tube will also provide support as an exoskeleton. The body tubes will start as 8" cardboard tubes, with a liberal fiberglass composite covering. The fins will need to be surface mounted, since I've used surface mount fins on my last two rockets, I have confidence I can adequately lay up the fiberglass to give me the required strength. Once the engine assembly is inserted into the exoskeleton, the two will be screwed together from the outside, which should make for a very strong air frame.
Here's a shot that should give you a better idea of the internal structure. The centering rings are 1/2" plywood, the stringers are 3/4" square clear poplar.
The fin cores were cut from 1/4" plywood. Two layers of 6 ounce glass cloth were applied to each side, then the fins were sandwiched between Duralar plastic sheets, the glass counter top and a piece of 1/2 inch acrylic. After this picture was taken more weight was applied on top the acrylic to keep pressure on the lamination as it cured.. This first two layers of glass are primarily to flatten the plywood, but also it starts the process of strengthening the fins.
Here are the 4 fins after the epoxy-glass cloth lay up. I used a utility knife to trim the excess from the edges.
The fin leading and trailing edges were then given a wedge shape in my 12" disc sander. I used 80 grit paper in a palm sander to smooth the edges and rough up the flat surfaces for good epoxy adhesion when more layers of glass are applied in the tip to tip glassing of the fins. In the back you can see the 8" tube which will form the body of the rocket. The tube where the fins will attach has already been given 4 layers of fiberglass.
To begin attaching the fins, I laid out the body tube and fins pattern on piece of plywood with old fashion drafting tools. The factory end cut on the tube wasn't perfect, to get the tube perpendicular to the plywood I had to shim up one side with some thin cardboard (blue square in the picture). I also tacked the tube to the plywood with four spots of hot melt glue between where the fins would be mounted.
This has been my standard method of aligning fins. A square is used on each side of the fin and aligned to the marks on the plywood. The fins are then tacked in place with quick set epoxy. If the tube is perpendicular to the plywood, and the squares are square, the fins should be in perfect alignment.
After all four fins were tacked in place. I added a strip of glass cloth in each corner to solidify the joint. Once the epoxy cured, these fins are quite strong already as the rocket can be handled by the fins alone.
On a side project, the nose cone has been plotted and laid out on paper, I'll be using a von Karman profile. I measure out stations at 2" intervals and measured the diameter at each station.
Here I've sanded the fin joints and I'm ready to start tip to tip glassing. In the background you can see blocks of pink Styrofoam. These 2" thick blocks will be cut to approximate size as determined by my station measurements on the nose cone profile.
I cut the foam disks to rough size, then drilled a hole in the center for a 5/8" threaded rod. I started with a plywood disk at one end, then started stacking the disks on the rod. I used 3M Spray adhesive in center of each disk to glue them in place as they were stacked. Care must be taken to keep the glue from the outside edges as this makes it impossible to turn the surface smooth if there is glue in the joint. The threaded rod was center drilled to allow one end to held in my lathe dead center.
Here's the foam nose cone core after the initial rough shaping. I used a cheese grater style tool for rough removal of the foam, then I moved to some very course 40 grit sand paper. I finished up with 150 grit sand paper for a pretty smooth surface on the high density foam. One thing I think I'd do next time is to pour some epoxy down the rod as I stacked the disks. The threaded rod can enlarge the center holes in the foam if too much force is applied during the turning process. I think some epoxy in the center holes would firm that up and prevent enlargement.
At this point I have two layers of glass laid around the nose cone in strips, and one layer running in lengthwise
strips. The nose cone tip was a cone formed from light cardboard. I added about two layers extra at the tip to
reinforce it. The tube at the bottom will be the coupler section that goes into the body tube, at the moment it's
unattached and the nose cone is just sitting on the coupler section. This isn't turning out to be the best looking
nose cone I've ever done... But I think it will do the job. This being my first full attempt at this type of nose
cone it's a learning experience, I'm sure future builds will improve.
Here's the first tip to tip glassing on the fins. I used three layers of 6 ounce cloth for the lay up. Once I've done all four sets and the epoxy has fully hardened I'll decide if I need more layers. Since this is designed as a subsonic rocket, I suspect the fins will be strong enough with the first two strengthening layers, the fillet layer and the 3 tip to tip layers. That's 10 full layers of cloth on each fin with 8 layers supporting the fin roots.
As you can see here the fin tip to tip glassing is done. The nose cone now has the coupler section fiberglassed to it as well.
I couldn't quite get the whole rocket in the view even backing into my hall.
I'm having serious doubts I approached this project from the right direction. I had debated using a large pipe as a mandrel and just doing a pure fiberglass body tube. But I thought it would be cheaper and easier to use these 4' long tube sections. The problem I'm going to have is nasty bumps, or rings, where the tubes meet. The tube ends aren't perfectly square, so I inserted my engine support into the tubes to hold them together, then used quick set epoxy to butt glue the joints until the first couple of layers of glass were on. This left ridges around the tube. Then of course the length of this body tube precludes applying the layers of fiberglass in one long shot. So there are numerous ridges where layers of glass overlap as well. Then there's the issue of keeping the body tube straight over such a long distance, I think it's pretty good, well I hope it is! I've used 16 square yards of cloth already, and I'm no where near being done. I ordered another gallon of epoxy too...
I guess what it all boils down to is this is going to be one nasty looking rocket. I'm going to be tempted to prime it in flat gray and leave it that way. I'm going to have to make it fairly strong. New simulations I've run indicate I'll have to run the engine into the "O" class to break 10,000', and that puts the peak speed into the transonic region. I'm too far along to change directions now, so I'll keep moving forward with this body design. Although how many layers of glass I end up with is anybody's guess at this point.
I've been working on the plumbing for the engine in the rocket. This configuration will be almost identical to the static test configuration. I decided to run the nylon fill line down the inside of the rocket and out the lower bulkhead. That way there is one less penetration hole in the side of the rocket. I've ordered the material for the flight tank as well, I decided to go with a 5" x .125" wall x 5' long tank. This should give me enough tankage to run the engine in the low to medium "O" impulse range. I was somewhat concerned the PIRM2 release key could hang up inside the tube when released. A partially opened valve at launch could be disastrous. So rather than use the release key, I'll just loop the stranded steel cable over the retaining piston in the PIRM2.
The four thrust tabs are 1/8" x 1" aluminum angle and bolted to the stringers with 1/4" flush mounted bolts.
To retain the engine after motor burn out, I used 3 more aluminum angle tabs. Not only will they retain the engine, but give me three more points to transfer thrust to the frame.
Most of the body tube has four layers of glass cloth on it now. I decided to try a strength test of the body. With the support brackets just ahead of the fins, and about 2' behind the top of the tube, I put all my 230 pounds of weight in the middle of the tube. Not only did it hold, but it didn't flex either. While it seems very strong now, I picked up another 12 yards of glass fabric today. I'll put at least two more full layers on the body tube, and I think I'll give the fins another 2 layers tip to tip. Since I've decided to fly this rocket with an "O" class motor, I'd rather be safe than sorry.
Here are some early weight measurements and calculations:
Engine with support structure, no fuel or tank includes flight valve and hardware: 21.4 pounds
Body with nose cone and electronics module, no electronics or recovery hardware: 24.4 pounds
Fuel Grain: 9.8 pounds
Flight Tank Tube: 11.3 pounds
Flight Tank Bulkheads: 3.8 pounds
Flight Tank Hardware: 1.0 pound
Recovery System Estimate: 8 pounds
More Body Tube Glassing: 2.0 pounds
N2O Load: 25 pounds
Nose Payload: 2.0 pounds
Estimated Gross Lift Off Weight: 108.7 pounds
Ever wondered how to get something lined up along the center line of a body tube?
The above picture shows one way to get holes lined up along a body tube. Of course, you can always lay a piece of angle iron on the body, that does a reasonable job of giving you a straight line. But I think this method works a little better. You need to have your tube leveled first, well, it could be out of level and that wouldn't hurt, but you don't want the tube in pitch and yaw at the same time. So it's best just to make sure your tube is level from the start. You also don't want any bumps or humps in the tube, it must be a good surface for the centering tool legs to rest on. Just level the body tube, then place the centering tool on the tube with a level on the upright. Now move the tool until the bubble indicates it's level. Then mark your point on the tube using the bottom of the measure. Make sure rocket does not move, and repeat the process along the length of the body tube. The more marks you make the better. Then you can draw a line through the marks to have a line parallel to the body tube. This is good for launch lugs, or in my case here, I needed to drill holes to attach the engine frame stringers to the body tube.
To retain the engine assembly in the rocket body, I installed 3 T-nuts in each stringer.
Here you can see the 1/4" bolts through the air frame and into the T-nuts on the stringers. Assuming 800 pounds of thrust, that's only about 67 pounds of force on each bolt/nut. Even then, I think I'll add four more bolts through body tube into the lower centering ring. That would bring the load on each bolt down to 50 pounds.
I did add two more layers tip to tip on the fins. That brings me to 5 layers on each surface tip to tip, one fillet layer and two initial full layers of glass on the fins. The body is probably plenty strong now, but I'll add two more full layers to be safe, giving me a minimum of 6 layers on the body tube. I wanted to get the holes drilled through the body tube now for attachment to the engine retainer, that way if I screwed up and made an extra hole or two I could get them patched up and covered with the final layers of glass. Which, as it turns out I did make a couple of extra holes. It turns out my engine retention package has a slight twist in the stringers amounting to about 1/4". Not that that will hurt anything, but it took me two miss-drilled holes to figure out where the alignment problem was.
Here is the basic electronics bay.
This cylinder slides into the upper body tube and is retained by 26 screws from the outside. 13 into each bulkhead on the bay. The two ends of the bay are fiberglass covered 1/2" plywood. I know 26 screws sounds like overkill, and it is, but all of the recovery force will be absorbed by this module, so it needs a good grip on the body tube. Three 5/16" aluminum threaded rods keep both ends secured to each other.
Here I've cut a hatch in the side of the electronics bay.
I debated this one for a while, but in the end I decided to use a hatch on the side of the rocket to access the electronics. The reason I did it was to make launch day easier. In the past, I've found it difficult to flip switches and push buttons through small holes in the body tube. Even after practicing numerous times in the shop, it still proved problematic. I suppose launch day jitters and a long check list added to the rush to arm electronics, and actually slowed me down. Not to mention having to do this while climbing up a tower... Another reason I decided to go with the hatch is for the camera. I can make multiple hatches and use either a down looking mirror, or use a side looking window. The corresponding hatch in the body tube will be slightly larger, and I'll bolt the hatch on through the electronics module wall.
You can see the two power switches in the upper left hand side of the hatch opening. I'm planning on running my Missile Works altimeter with a timer as back up on apogee. I'll run Mike Bennett's flight computer as well, since it's a recording unit and I always like flight data!
Here is the electronics bay pretty much done. I made a new PIRM2, and installed a couple of PVC tubes to hold the apogee deployment charges.
For the first deployment test I strapped the rocket to the last 12' of tower left in my yard. I installed 2) #4 nylon shear screws to keep the nose cone in place until the charge goes off.
Here is the moment the 12 grams of black powder go off.
Click Here for a short video of the deployment test.
It was a very crisp ejection of the nose cone, perhaps a bit too crisp as the nose cone extended the 40' of tubular nylon and pulled the bulkhead off the nose cone. Well, there were only a couple of screws holding the bulkhead on at the moment, and they were too short anyway. I'll test it again with a drogue chute and some wadding in the tube and see how it looks then. I was really hoping I'd break the tip off the nose cone so I'd have to replace it. But no, it sticks in the ground and doesn't even scratch the tip of the nose cone.
My Parachute arrived today. This is a military surplus, new old stock 15' diameter chute with drogue and deployment bag.
Here's the chute out in a breeze.
I'll have to admit, I'm really impressed with this parachute. It cost $29, there's no way I could have bought the material and made it for that kind of money. It's also very light for its size. I don't think I'll be able to use the drogue, I'll need something a little bigger for this project. But since this chute has a line attached to the top, I think I'll use my drogue to pull this chute from the top, as it was intended. I'll still have to cut the deployment bag off the chute. I need to attach the deployment bag to the rocket, so it stays in the body tube. The deployment bag is canvas, and a little small to fit all the shroud lines into. So I think I'll make a double bag, you can see it in the picture above, the green thing at the chute apex is a sleeve off an Air Force pilot jump suit. Since it's Nomex, it will be more resistant to flames than the canvas, and being a little bigger I can get all the shroud lines inside as well.
The nose cone step and a shear bolt hole.
When I made the nose cone, I had to lay up layers of fiberglass fabric over the coupler section that goes into the body tube and the outside of the cone itself, to attach the two parts and make them one solid unit. That left an imperfect mating line between the body tube and the nose cone. To smooth up the mating line, I painted two heavy coats of PVA parting compound to the body tube end, both inside, outside and on the tube edge. Then I drilled and tapped 2 holes for #4 nylon shear screws, I installed the shear screws to hold the nose in place. Then I mixed up a batch of epoxy and thickened it with talcum powder until it was paste like, then the epoxy paste was applied like body putting to the joint. Once cured, I rough sanded the joint down to the outside tube diameter. I removed the screws and gave the nose cone a sharp whack with the palm of my hand and the two broke free. That PVA really works, and is very handy for odd jobs like this. You can see how well the joint mates up now, making an almost flawless transition from nose cone to body tube. There's still a lot of work to do on the surface of the body tube and the nose. But now I have a strong, clean joint.
Here's my homage to Slim Pickens in Dr. Strangelove. No wait, that is Slim...
Ok, here's the homage!
Notice I've started priming and spot coating the fin section. I don't think I'll get carried away and shoot for a beautiful finish on the entire rocket. Maybe this winter if it survives the flight this Fall. I have work I need to get done outside before the snow flies. When I started this rocket, I was looking for a quick and easy rocket to flight test my engine. This rocket hasn't been quick, easy or cheap. I must have used close to 30 yards of fiberglass fabric and almost 2 gallons of epoxy, but it is quite strong!
I've got the gray primer coat on. I had considered just flying it with a primer coat, but I decided to give it a color coat. Anyone want guess what color I'm going to paint it?
The oxidizer tank and bulkhead materials finally arrived today. Notice anything wrong? Probably not unless you've followed my comments closely. The oxidizer tank tube is only 36" and I ordered a 60" tube. So much for having that done this week. At least the materials for the bulkheads were the right size.
Notice the body tube has been given a top coat of paint. I'm ashamed to show a picture of the entire rocket as it looks so bad. I wasn't going to spend much time making this a pretty rocket, but I don't know if I can live with this paint job. I should have gone with white paint, white doesn't show imperfections like other solid colors will. This paint was an industrial type equipment paint shot with a pressure fed paint gun. I had an odd problem of bubbles appearing on the surface, most noticeably on the fins, when these air bubbles burst, they left little fish eyes on the surface. Of course the rest of the fiberglass surfaces don't look great either, because I didn't fill the glass weave. I'll have to see how fast this paint cures, if it cures to the point I can sand it, I'll probably sand, prime and repaint it white. If not, I may have to just live with it.
I'm using 1" thick 6061 aluminum for the oxidizer bulkheads. In this picture I've turned the disk down to 4.74" and now I'm cutting the second of two o-ring glands. Notice the threaded rod holding the disk in place. My lathe jaws don't allow me to get close enough to turn the face and cut the glands, so I center drilled the disk with a 5/8" drill, then ran the threaded rod through the bulkhead and into the lathe chuck. Since I need to tap each bulkhead anyway, this procedure works fine. Of course chatter is always a problem when using a cut off tool, and the threaded rod doesn't help since it's a weaker mounting point. I take light cuts and use cutting fluid, and I still get some chatter...
Here I'm starting the threading process by chucking up the 3/8" tap in my drill chuck. This really helps to get the tap started straight and true. Once I'm in a few turns I unchuck the tap and use a wrench on the tap.
Here is the completed bulkhead. I labeled the bulkhead inside and outside, and also upper, indicating this goes at the forward end. While this bulkhead will be nearly identical to the lower bulkhead, it will have a nipple extending into the tank to give my ~10% ullage. I cut the threads deeper on the high pressure outside, and have fewer threads on the ullage dip tube nipple on the inside.
Here's the lower bulkhead. Not that it was really needed, but to improve N2O flow I cut a 20 degree cone transitioning to a 60 degree cone into the port. The outside is again threaded for a 3/8" pipe fitting.
I nice sunny day to help cure the paint.
I decided to run the oxidizer tank vent inside the rocket body, down the engine frame and out the bottom of the rocket. I had some 1/8" nylon tube and fittings so I decided to make use of them. The only problem was, I needed to reduce the line size down to a suitable vent opening. So I turned a .25" diameter aluminum rod down to fit inside the pipe to tube adapter fitting, the outside end I tapered slightly so I could press fit the plug into the brass fitting.
My first static test using a vent used a .065" diameter hole, which worked fine but seemed a little too big. The next test used a .032" vent hole, which worked fine too, but now it slowed the fill time. So I drilled the plug out to .044", a seemingly good compromise.
My new oxidizer tank tube arrived today, in short order I had the holes drilled for the bulkhead retaining pins. In the picture above, I'm cleaning up the first few inches of inside wall. I do this for two reasons, first, to give the o-rings a good surface to seal on, second is to clean up any rough edges around the drill holes. I use an automotive cylinder hone and light weight oil (WD40).
Here is the finished surface. Any dark areas would indicated an imperfection, but you can see here it's nice and shiny.
Once all the machine work was done. I cleaned everything as best I could, assuming this would be used as a pure oxygen system for added safety. Everything was cleaned with original Dawn dishwashing soap and nylon brushes, then rinsed well in hot water and allowed to air dry in a clean room. The o-rings are Viton and the grease is Krytox, while these are much more expensive than standard o-rings and silicone grease, it is the safest way to go.
To the left is the forward bulkhead, it has a 6" aluminum dip tube installed to give me 6" of ullage, or free gas space above the liquid N2O. 6" is more than I really needed, but using a 6" dip tube should give me a N2O load of about 25 pounds, and I really didn't want to go over 25 pounds on this flight. Notice how nice and clean the inside of the cylinder is. I used white cloth to make sure there was no residue detectable on any inside surfaces.
Here the forward bulkhead is installed. The port coming out will hook up to the vent tube, the little brass fitting you see on the end is the one I installed the .044" port reducer in. Both ends of the tank are covered in plastic to keep out contamination.
Now I need to do a couple of tests on the tank. I need to do a low pressure leak test, I could just use air, but air from a shop air compressor isn't clean enough. So I think I'll rob an almost empty bottle of extra pure helium or nitrogen from the laser shop, and run some pressure in the tank from that source. I also need to do a full pressure test on the tank. The calculated burst pressure is 2,250 psi. I doubt my N2O pressure will go over 650 psi this Fall on the flight, so I'll likely try to test it at about 1,000 to 1,250 psi.
I just couldn't stand the black paint job on the rocket. So I waited a week or so and sanded as much of the black off as needed. I found a high solids primer at an auto parts store and shot it with a coat of primer, then went over the entire rocket again with spot putty wherever needed. After power sanding I hand sanded one last time, then shot another coat of primer.
I was really debating colors. I bought some white paint, and intended to paint it white with some blue trim. I picked up some masking tape that was supposed to be useable on fresh paint, it's purple in color. While it doesn't have much holding power, it's just enough and worked great. Here you can see the nice clean line it left between the blue top coat and the primer. I just couldn't bring myself to paint the main rocket body white either, I searched through my paint supply, debating what would look good with the blue.
I finally decided the blue didn't look too bad, maybe if I broke it up a little with a couple of color bands it would help. I shot a coat of clear over the whole rocket after the color had dried. It's still most certainly not a beautiful rocket, but a vast improvement over the black paint job! I'll paint the nose cone gray I think, to offset the color at the top of the rocket as well.
Here is a picture of the tank upper bulkhead with the vent line running down the side.
Here is the base of the engine assembly, notice the N2O vent line exits under the rocket and opposite the fill line.
The plumbing from one angle.
Here's the plumbing from another angle. Notice the oxidizer tank is now secured to the frame with 3) 1.5" x 1/8" brackets.
Here is the assembled rocket.
I needed to do more accurate stability calculations, so I assembled the rocket in nearly flight ready configuration. The only thing I didn't have was a fuel grain to install. But I didn't want a full fuel grain, I wanted the weight of a spent fuel grain. What I'm up to here, is simulating the balance of the rocket after engine burn out. For a hybrid that's usually the moment the rocket is least stable, which is opposite the condition of a solid fueled rocket, as it becomes more stable as the aft mass of propellant is burned. Since my hybrid has all the oxidizer ahead of the center of gravity, the rocket will become less stable as the propellant is burned.
So with the rocket fully prepped with no oxidizer weight and 7.6 pounds of weight in the combustion chamber, I balanced the rocket on a piece of 3" PVC pipe to get the center of gravity. The CG was 117.875" behind the nose cone tip. Using AeroLab, the Barrowman center of pressure was calculated at 135.955" from the nose tip. So I have an 18.08" margin of stability at engine burn out, or 2.178 caliber's of stability minimum.
I also took the opportunity to weigh the rocket with no oxidizer or fuel grain. The rocket dry weight came in at 72.8 pounds. Assuming 25 pounds of N2O and 6.4 pounds of fuel, the rocket comes in at 104.2 pounds. I'll have just a little more nose weight for the GPS tracking system, so the final lift off weight should come in near 107 pounds.
I haven't heard from the FAA in a couple of weeks and was getting nervous. So I had to call and bug them again today. Since we're still over a month from the launch date, they didn't think it was such a big rush and I was assured all was in order.
Here's the Aestus on the launch rail. I guess I'm lucky I have a building with a 25' ceiling to do this in.
I haven't been idle, I had loaned out my launch rail trailer last Spring, now that I have it back I've been getting the rail back on the trailer and doing some other maintenance work on the trailer and launch rail. I bought a new Mig welder last week, I wanted to make some outrigger supports for the trailer rather than using guy wire supports. You can see one of the outrigger supports at the bottom of the picture. I also needed the rocket on the rail to set up the tube cutter on the N2O fill line.
Here you can see the tube cutter attached to the rocket support standoff bracket. When the tube is cut, the spring swings the tube cutter away from the nozzle to keep it away from the brunt of the exhaust plume.
Click Here for a short video of the tube cutter in action.
The good news here is that I have plenty of room for electronics in the nose cone. I needed room for the GPS and my MaxStream transmitter. Since I had all this room, I decided to leave the transmitter in the case I had installed it in. I used a steel bar heated with a torch to melt out a rough opening in the foam core (pink foam) of the nose. I wrapped the transmitter enclosure in plastic wrap and wax paper and installed it in the rough opening. Then shot some expanding foam around the enclosure, after it cured I removed the enclosure and had an opening that fit the box like a glove. I epoxied two pieces of plywood to the bottom of the foam to give me something to retain the transmitter from the bottom.
Here's the transmitter installed in the opening.
I used two layers of neoprene rubber to add a little soft compression between the retaining board and the transmitter.
Here you can see the Garmin eTrex H mounted to the bulkhead, the battery pack for the transmitter is installed adjacent to the GPS. Turning the transmitter on is a simple matter of connecting the power lead clips.
Here is the other electronics compartment, the altimeter bay. Again plenty of room for more devices if I should need to add anything. I'll be using a fairly simple setup of two altimeters, Mike Bennett's RMCS and a Missile Works RRC2X.
I had to move the power switches twice, notice the holes in the left side of the board. I couldn't get the switches away from the 5/16" threaded aluminum rod on that side, so I moved the swtiches to the next board over. The small two cell battery pack is for the camera. The camera is retained using the tripod threads and a bolt through the bottom, to keep the camera from pivoting I glued two small wood blocks to the base.
