Moment of Inertia

I’m starting on an extra little side project. I’m going to build a Mach-breaker rocket using the Cesaroni 29mm H410 Vmax motor. Here’s a picture of the basic layout.

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I imagine the fist question I’m going to get is, “Why did you choose tube fins? Tube fins are high drag. Don’t you need low drag for a Mach-breaker?”

Well…it depends.

The Vmax motor is a real beast. Approximately 100 lbs of thrust for four tenths of a second in a 29mm three grain reload. I ran a simulation. At Mach one, the rocket experiences 90 Newtons of drag. If drag were the only force on the rocket, it would be decelerating at 30 G’s. However, at that point in the simulation the motor is putting out 490 Newtons of thrust. The net result is that the rocket is accelerating at 135 G’s.

If I were to put in a heroic effort and reduce drag by 50%, the acceleration would increase to 150 G’s. I could get that same increase with only a 10% reduction in rocket mass. The moral is if you only care about max velocity and your drag is a small fraction of your thrust, then you get more bang for the buck minimizing mass than drag. The absolute magnitude of the drag doesn’t matter, only the ratio of drag to thrust.

So the real challenge with this rocket is structural. Withstand over a hundred G’s with minimum mass. Bundles of tubes have high strength-to-weight and stiffness-to-weight so tube fins are a good choice for minimizing mass.

The high drag of the tube fins also has a secondary advantage. I’m only going for maximum speed, not altitude, and I don’t want to lose this small rocket so I’d like to keep the apogee as low as possible. Once the motor burns out, the high drag of the tube fins helps accomplish that.

I’m going to call the rocket Moment of Inertia because all of the acceleration happens in a moment and is dominated by inertia. Drag and gravity have very little to say about it.

Rocket Weight

All of the major components are in for the first configuration of the 6″ rocket.  So I weighed them all together.  They came in at 40.8 lbs.  There’s little odds and ends that will still be needed so it will probably wind up closer to 45 lbs when it’s all ready.  And then the motor will be about 5 lbs.

That’s good because the Pro54 6GXL IMAX motor has an initial thrust of about 300 lbs so it will have a 6:1 liftoff T/W, which is good enough for a passively stable rocket.

Past 6″ Rocket Posts

Mostly what I’m going to be blogging about here is my 6″ rocket project.  There are some past posts from the beginning of the project on my other blog, which is going away soon, so I thought I would copy those here so that people could read about the project from the beginning.

Fin Mount

I got the fin mount done for one of the fins on the rocket. I wanted to design the fin mount to restrict the inside diameter as little as possible. I’m going to need as much thrust from the peroxide engine as I can get so I want to maximize the diameter that I can make the catalyst pack. However, there aren’t any off-the-shelf 6″ diameter external fin cans, and getting one made was pretty expensive.

The design I settled on was to sandwich the body tube between two pieces of flat bar stock and fill the fillet between the flat bar and curved tube with epoxy. That gives a flat mounting surface onto which I can bolt a fin.

fin_mount1 fin_mount2 fin_mount3 fin_mount4

Non-Floating Nut Plates

I missed the update last week, but I have been making some progress. I decided to try epoxying nuts in place instead of getting more floating nut plates. The method I use is to put a bolt into the nut to keep the threads from getting epoxy on them, then apply epoxy to the nut, push the nut into place, and then unscrew the bolt.

It actually works with a few caveats. The bolt gets epoxy on it’s threads. I thought maybe I could clean them up, but not very well. I’ve been throwing out the bolts and using new ones. There’s also a failure rate of nuts that get their threads epoxied anyway or don’t stay put in the right location. That rate has been going down as I get more practice. If I check them before the epoxy completely cures I can remove the bad ones and just do it again with a new nut.

My conclusion about non-floating epoxied nuts is that it’s an acceptable construction technique for amateur rockets as long as you can live with the fact that a certain percentage of the ones you do will have to be re-done, and of the ones that are re-done the same percentage will have to be re-re-done.  If you don’t like that you can pay $3.10 each for aviation grade floating nutplates.

glue_nut_after glue_nut_before

Motor Mount Progress

I’ve made more progress putting together the removable solid motor mount. It’s actually coming together well, and I like the design.

motor_mount_1 motor_mount_2 motor_mount_3

Batteries and Body Tubes

Happy New Year! There was no update last week because of the holidays. This week I have gotten some things done. I received the batteries that I ordered. They are three cylindrical LiPo cells. Each one is about the size of a double A battery, only a little longer. I decided to get three separate cylindrical cells instead of a 3S pack. From the specs it weighs less although the difference is a few grams, which might be eaten up by the extra connectors. I also read that cylindrical cells have slightly higher performance than the rectangular “prism” shaped packs because of shorter internal distances. I can also charge each cell individually so I don’t have to worry about balance charging, although the charger I bought does have the capability to balance charge because all of the chargers do nowadays. Basically, it probably doesn’t really matter and I just got the three separate cells because I liked the idea.

The nominal voltage of a LiPo cell is 3.7 volts. Wired in series that makes 11.1 volts, although fully charged they are almost 12. I figure they will be within the tolerance of most stuff that expects nominally 12 volts. They are 1.2 amp-hour batteries with a 15C discharge rating so in theory they should be able to supply 1.2 * 15 = 18 amps. I tried them on the automotive nitrous solenoid that’s going to be the biggest load. It pulled 7.5 amps and actuated just fine. They are quite capable for their weight and cost. Below is a picture of the three cells with a quarter for scale and a wiring harness I made from JST connectors to connect them in series.

three_batteries

The BeagleBone Black that I’m going to be using as a flight computer takes 5 volts. Luckily, included in the parts I inherited is an AnyVolt, a circuit board that takes any DC input voltage and provides a regulated DC output voltage. So the AnyVolt will provide 5 volts and handle battery voltage dips from things like the 7.5 amp nitrous solenoid deployment event.

I got a basic hello world setup running with the batteries powering the flight computer through the AnyVolt. I also set it up to monitor the battery voltage. The BeagleBone is limited in some ways. One of those ways is that its analog inputs can only read 0-1.8 volts. So I created a voltage divider resistor network to bring the batteries’ ~12 volts down below 1.8. Here’s a picture of the setup. I’ve got a long way to go with packaging, but it’s a flight computer running plugs out monitoring a voltage. I’d say that qualifies as hello world.

flight_computer_hello_world

The last thing I got done was fitting all of the body tubes and couplers together. Not all of the fasteners are installed, but this gives an idea of the scale of the thing. This picture is with the nitrogen and peroxide tank sections included. The initial solid engine configuration will be shorter. It’s almost 14 feet long. For a 6″ diameter that’s a fineness ratio of 28. That’s higher than conventional wisdom recommends, but I think that conventional wisdom was formed at a larger scale. HPR rockets should be able to get away with higher fineness ratios than orbital boosters just like an ant can carry many more times its body weight than a human can. The Mean Machine Estes model has a fineness ratio of 48.

rocket_body_tubes

Received Parts

I haven’t gotten much done on the rocket this week, but I did receive a bunch of parts in the mail. This picture is a mix of the new parts with ones I already had.

rocket_parts

The next thing I want to do is a dry fit of the overall arrangement of the rocket. Especially with two configurations I want to make sure that everything fits without interference, and all of the holes line up. I’m also going to label everything as to what part it is and how it is oriented. This is most important with the multiple couplers that could easily be mixed up. Not all of the floating nutplates had been glued on to the inherited couplers so I started by finishing that up.

glueing_nutplates

Floating Nutplates

The parts of the rocket that I inherited use these interesting fasteners called floating nutplates. There’s a plate that glues to a surface and it holds a captive nut. The nut is not hard attached to the plate. It’s held in a cage with a little play for alignment, hence floating.

nutplate1

In the inherited hardware there were some extras of these floating nutplates. Before installation they have a rubber thingy that keeps the nut threads from being fouled with glue when they are installed.

nutplate2 nutplate3

These fasteners are just awesome. They are light and convenient. They are just exactly what you’d want for bolting together thin walled pieces with one blind side such as when you attach rocket body tubes and couplers.

There’s only one problem with them. They are used on certified aircraft. In fact, they seem to be used almost exclusively on certified aircraft. Anything used on a certified aircraft is specified to the Nth degree and winds up being expensive. I’m going to need a few more to finish my new body sections. I went looking to buy some and the cheapest I could find was $3.10. That’s $3.10 apiece for what’s essentially a nut crimped in a little piece of sheet metal. Most places I found them used the three most expensive words in the English language, “Call for quote.”

It would be really nice if there were some non-aviation-grade floating nutplates, but I couldn’t find any. So I don’t know if I’m going to buy the nutplates, or if I’ll just see if I can reach in and hold the nuts during assembly, or maybe I’ll see if I can glue down non-floating regular nuts without getting glue on the threads.

More Motor Mount

If I want to use a K2045 motor for my rocket the peak thrust is 2231 N (227.7 kgf). The motor mount will have to handle that with a 2x safety factor so I want to build the motor mount to hold 455.4 kgf of static load, almost half a ton! This brings up two problems: how to design it to handle that, and how to test it?

The RRS guys machined some beautiful brackets to hold the regulator and parachute attachment hardpoint.

rrs_brackets

But me, I took a trip to the local hardware store.

speedup_bracket

Four of the angle brackets still need their tips chopped off, but you get the idea. The angles are bolted to the centering ring and will be bolted to the body tube. The wooden centering ring will be in compression between the metal motor retaining ring and the tips of the metal angles. Other than that no wood or cardboard part will carry any load. The forward centering ring will be floating, epoxied to the motor mount tube, but not attached to the body tube. It will perform a centering function only.

For testing the strength of the mount I’m probably going to have to find someone nearby with a hydraulic press.

CO2 Parachute Ejection System

I’ve put together the system I’m going to use to eject the parachute. It’s a CO2 cartridge holder connected to a solenoid valve. The solenoid is an automotive nitrous oxide solenoid. It’s a suitable valve to use because CO2 and nitrous have very similar vapor pressure curves and critical points and CO2 has fewer material compatibility restrictions. I also put a burst disk from a paintball tank in the system.

co2_ejection

The released CO2 will blow off the nose cone and eject the drogue chute. The drogue chute will pull out the main chute. I’m planning eventually to have something hold the line to the main chute and deploy with a pin puller at a lower altitude, but initial flights will be fairly low so for now I’m just going to have the drogue pull out the main at apogee.

It’s heavier than some possible alternatives, but it has two advantages:

  1. It’s completely pyro-free.  Some CO2 systems use a pyro valve to release the CO2.
  2. It’s made of parts I already had on hand.

Now all I have to do is get a battery that can source enough amps to open the solenoid valve and a switch that can be controlled by the onboard computer. I’ve been able to get the valve to open under pressure with a 4 amp power supply. Based on the measured resistance of the coil, for a 12 volt automotive electrical system that it was designed for it should draw just under 10 amps, which probably includes some margin.

Here’s a list of possible deployment triggers I’m thinking of including in the system:

  1. GPS apogee detection
  2. Barometric apogee detection
  3. Inertial apogee detection
  4. Timer
  5. Ground remote button

Motor Mount

The parts for the solid motor mount arrived. I ordered them from Apogee. I ordered some other stuff from Hawk Mountain because they have the biggest parachutes I could find. That hasn’t come in yet so I don’t have everything to show the whole rocket layed out.

motor_mount_parts

Please excuse the low quality cell phone photo. It’s interesting to think that this motor mount is essentially the same as the ones for little Estes rockets. A smaller tube that goes inside the larger body tube with centering rings, and something to hold the motor, in this case a screw on cap instead of a hook. I’m going to JB weld the parts of the motor mount together, but I’m not going to glue it inside the body tube. Instead I’ll have several brackets that attach with screws through the body tube so the whole motor mount can be removed.

The bottom row of parts is the case for a Cesaroni Pro54 motor. I got the 6-grain-XL case so I could make sure the mount would hold the longest motor I would ever put in it. And I got a couple of spacers so I can use some smaller loads as well. I like the Cesaroni motors the best. The reloads come as a self-contained sealed plastic case with the nozzle and delay grain integrated. That slides into an aluminum tube for strength, but it doesn’t have to seal against the aluminum tube so you don’t have to mess with o-rings and grease.

I will probably use the L1030-p reload. A plugged motor would be nice because I’m not using the deployment charge for recovery. I want a lot of thrust because even the short configuration of my rocket will be relatively heavy for a 54mm motor. To maintain a 5:1 thrust/weight ratio the 1030 will be able to lift 21kg. If I need more than that I can go all the way to the 4-grain K2045. If I do that I will need to get a shorter case because there is a limit of two spacers. Over 200 kgf of thrust for 0.7 seconds. Whew, that motor must be something to see.

Recovery System Testing

The first system that I am going to work on is the recovery system. This is a system that seems to suffer a high fraction of the failures of HPR rockets. I want to have a testbed where I can test the recovery system before risking the peroxide engine components. So I’m going to fly a modified version of the rocket first.

rrs_recovery_test_rocket

I will use the nose cone, recovery section, and fin can that will be part of the final rocket, but I will leave out the tankage and plumbing. I will design the fin can to take a removable commercial solid motor mount as an alternative to the peroxide engine. So ironically, my first step in building a liquid propellant rocket is to build a solid propellant rocket.

Another benefit of this configuration is that it doesn’t use any inherited components. It will be all my construction and use a commercial motor so I can use it for my Tripoli level 2 certification flight. I never actually got level 2 certified, which is required to fly experimental engines, which the peroxide engine will be. All of my work with the lunar lander challenge and my other engine testing was never done under Tripoli research rules. I was just a private citizen following FAR part 101. But I’d like to fly this one at Tripoli launches, and getting certification is an overall good thing to have.

And level 2 won’t be the end of it. Level 2 allows up to 5120 Ns impulse, which for peroxide monoprop will be around 5-6 kg of propellant depending on what Isp I get. That will be enough to do initial flights up to parachute deployment altitude, but I will need to get level 3 certified to fly it with a full tank. This is going to be a long-term incremental development project.

Peroxide Rocket High Level Design

For the high level design I’ve decided to split the rocket into four sections. These sections will be body segments that can be separated for transportation and storage, and they will also contain separate systems of the rocket.

rrs_full_rocket

Starting from the front of the rocket, the nose cone will contain a payload bay. A 6″ nose cone has a decent amount of space. I don’t have any plans as of yet about what I will put in the payload bay. The plan for this section is just to buy an off the shelf nose cone.

The next section will house avionics and a parachute recovery system. The avionics will have to do a little more than just pop the parachute so I’m planning to use a Beagle Bone computer that I got when our local Radio Shack had its going out of business sale. Parachute deployment will be with a CO2 cartridge and solenoid valve. I’m planning on this configuration being completely pyro-free. It will wind up heavier than a pyro system would be, but it’s a neat capability.

The next section will be the inherited tankage and plumbing components. No real modifications needed to this section. Just cleaning, passivating, and double checking.

The final section will be the engine and guidance. The T/W will be fairly low so I’m planning on having jet vanes to prevent a gravity turn in addition to fins for passive stability. With the heavy engine in the back the C.G. will be fairly far back and I’m not confident in my ability to make a control system for an unstable rocket with jet vanes alone so I’m going to have fins too. However, after I fly it in a stable configuration I may try it with smaller and smaller fins to see if I can get it to fly straight in a neutrally stable configuration.

I’ve got some rough mass budget numbers, except for the tankage section where I have a measured weight. With some conservatism in my estimates it comes out fairly heavy for an HPR: 64 kg dry mass, about 20 kg propellant, and a couple kilograms nitrogen or a few hundred grams if I pressurize with helium. I’m targeting at least 200 kgf thrust from the engine, but that’s barely above 1 G liftoff acceleration and an 8-10 second burn so I will definitely need the jet vanes to keep the pointy end up.

RRS Peroxide Rocket

I’m setting out on a new project that should hopefully produce regular updates for a while. I have received a partially complete peroxide rocket from Bill Clabaugh. It was to be a copy of a rocket that Bill and other Reaction Research Society members flew that was itself an updated version of a rocket that RRS members Lee Rosenthal and Mike Evans flew in 1951. Bill is now focussing on solids instead of liquids and he offered to give the peroxide hardware to someone who could use it.

The hardware that I am inheriting is a pressurant tank, a peroxide tank, a regulator, valves, plumbing, and structure holding all of those parts together. It’s probably about halfway to being an HPR rocket. Here’s a picture of it in its shipping crate.

rrs_rocket

My plans for the hardware are starting to gel and I think it’s going to turn into a really cool project. I’ll hold off talking about the details of the design until next post.

 

Two Mustangs

My project to modify my mustang rocket has gotten more complicated.  In my original attempt to modify the rocket I broke some things.  It was really hard modifying things inside the tube with everything already glued together.  So I bought another Mustang kit to complete the project.  But then I realized that with some of the parts from the new kit that I wasn’t going to need, I could rework the original rocket as well.  So I’m winding up making two rockets.

0705160910

The bottom one is an original size mustang, but with the engine mount extended to fit up to 6 grain motors.  That’s now all done except for painting, and it’s the one I will use for my level one certification flight.

The top two tubes fit together to make a stretched version that can hold up to 27″ long motors and will be dual-deployment.  For that one, the engine-mount/fin can section is done, but I still have to make the e-bay in the recovery section.  I won’t be able to use that one for my level two certification because there aren’t any certified J motors in 29mm size, but after I get my level two certification with another rocket, I know someone who is working on an experimental hybrid motor who thinks he can fit J impulse into a 29mm x 27″ long motor.

I haven’t checked these new designs for stability yet, but if necessary I will add modelling clay to the hollow plastic nose cones.

Stretched Mustang

For my L1 certification flight I have a Mustang model that I am modding to a stretched version that can take longer motors.  The Cesaroni Pro-29 3-grain case (pictured) has some H size reloads.0615161337

The Mustang model is from AeroTech.  It uses a 1.9″ body tube, which doesn’t appear to be a common size.  In fact, AeroTech models are the only ones I’ve found that use 1.9″.  I found places where I could order the body tube and coupler, but the problem was getting the centering rings.  The only off-the-shelf way that I could find to order the centering rings was as part of a combined motor mount/Fin-Lok™ Kit.  I already have fins and I wanted more than the two centering rings that come in a kit so I would have had to order multiple kits.

In the end, I found a place called Balsa Machining Service in Pahrump, Nevada that had the 1.9″ body tube and coupler and was willing to cut some centering rings for me on their laser cutter.  I was really happy with their service.  I got 100 centering rings cut so if anyone needs a centering ring for a 29mm motor mount tube in a 1.9″ body tube I can give you some.