As noted here, the New Centurion came to a bad end at the August SPARC launch. I've just gotten around to getting repair parts lined up, and decided to post my thoughts here.
I still have the 1x18 and 2x18 engine mounts in good condition. I've ordered and received replacement fins and a length of BT-60 to use for the lower tube; the upper tube is in good shape. I've also assembled a Semroc baffle for this rocket; I'll epoxy it into the tube below a section of coupler. This time I'll mark the tube well so the engine mounts screw in symmetrically (so I don't have to orient them just one way to mount them).
I think I'll paint the lower section fire red this time. Pictures coming when I have it done.
I've simulated this rocket with OpenRocket. According to OpenRocket, there are just no good single 18mm engine configurations for the New Centurion; the best is a single C6-5, which has the rocket deploying somewhat late while falling at 28 fps. The alternative, a C6-3, has the rocket ascending at a bit over 36 fps at deployment... much worse situation.
In 2x18 mode, OpenRocket says the rocket deploys while still going up at 25 fps on 2 ea. C6-5 engines, or coming down at 26 fps on 2 ea. C6-7 engines. Neither is all that good. On A8-3 engines, deployment happens while descending at 14 fps. On B6-4 engines, it's ascending at 9 fps at deployment. Both very good choices.
Perhaps I should forget about the convertible design and just glue in that 2x18 engine mount. I don't plan to build a replacement 1x24 at this point anyway.
Here are the OpenRocket files:
I note major discrepancies between the OpenRocket and the older RockSim simulations. RockSim said I'd have nearly-optimal Dv values for all the 18mm engine choices I simmed, even 1xA8-3 (but with an altitude of just 22' I wasn't planning to try it). OpenRocket says I have few good choices for C6-x engines in either 1x18 or 2x18 configuration. Curiously, there are other differences as well... OpenRocket gives an apogee on 2xC6-7 engines of 972', while RockSim predicted just 815' for 2xC6-5 engines. (2xC6-5 engines in OpenRocket results in an early deployment and an apogee of 965'.)
Possibly I need to start over and build new simulations from scratch rather than using the converted RockSim files.
I submitted an article to EMRR about this rocket build; in the process, I discovered some bits left out below. Rather than try to fill in the blanks, so to speak, I'm copying the text here that I submitted to EMRR.
I started by assembling the three engine mounts. I used the stock engine mount parts along with a coupler for each. I had to sand them down to make them fit inside the body tube. The BT-60 couplers from Uncle Mike's Rocket Shack are really tight in the hobby-shop body tube.
I next glued the nose cone into the upper sustainer tube (after gluing in a screw eye and affixing the shock cord, of course) and assembled the bypass-tube baffle (two short BT-20 tubes offset from each other, then off-centering rings, coupler, and Kevlar® thread installed).
After marking the lower sustainer tube for fin and launch lug attachment, I mixed up a little slow-setting epoxy and used it to glue in the BT-60 coupler segment which serves as a "thrust ring" for the interchangeable engine mounts. At this point, I didn't know how I would secure the engine mounts. Save that, I knew I was not going to use an engine hook (as was done in the old Centuri RX-16 rocket). I used epoxy to avoid the "freeze" effect of yellow glue. I was able to insert an engine mount slowly, pushing the ring into place without fear that the mount would suddenly become non-interchangeable. I used the remaining epoxy to glue in the baffle, leaving half of it sticking out of course.
Next I attached the fins, in the standard fashion, using yellow glue. Like all the Semroc fin sets I've worked with, they were excellently cut. This set of fins also proved to be the hardest balsa I've ever handled which is another big plus for a heavy rocket.
When all was good and solid, I followed up with fillets, also yellow glued. Launch lugs were attached in the same way. I decided with this rocket to try brushing thinned Elmer's Wood Filler on the fins; some complain that it adds too much weight. I didn't think it would make much difference for this big bird.
Now came the quandary, I had the rocket done, except for the pesky business of securing the engine mounts. The rocket stood forlorn on my shelf for more than five months before I figured out how to go about it.
I started by measuring two locations, just aft of the lower sustainer thrust ring and evenly spaced between two fins (and clear of the launch lug line). I inserted the 1x18 engine mount and using a cordless drill and a 1/16" drill bit, I carefully made holes through the sustainer and engine mount coupler at the two marked points. I removed that engine mount, inserted the 2x18 mount (with the engine tubes 90 degrees from the holes) and drilled holes in this mount also. I finished by doing the same to the 1x24 engine mount.
Next, I mixed up some more epoxy. Using a syringe from the local farm store (without a needle), I injected some of the epoxy into the holes of each engine mount; then I placed each mount upright, forward end down so that the epoxy would puddle on the forward centering ring. I put a piece of cellophane tape over each hole temporarily to prevent leaking.
When the epoxy was good and hard I inserted a self-tapping screw into each hole to open them up. I use two self-tapping screws, scavenged from computer cases to hold the engine mount in place in flight.
With all this work finally finished, I primed, sanded and eventually painted the rocket. And, almost as an afterthought, I installed a handmade mylar chute.
Flight: When I first conceived of this project, I had a trial version of Rocksim. This is the sort of rocket Rocksim is made for (whereas several other rockets of mine are very hard to simulate with it). The simulations told me to use:
I've flown it on 1x B6-4, 2x B6-4, and 1x C6-5 and it has turned in an excellent flight each time.
I had the opportunity to launch this rocket on an E9-4 for the first time, at the SPARC launch on September 27th. The flight was high and straight, and deployment was perfect. I cut about a 3" spill hole in the parachute (made from a Mylar balloon) to speed descent, resulting in the rocket landing beautifully within the field. However, when I went to remove the engine mount (using the tool shown in the breakdown picture), I tore it up instead. Evidently, the relatively long piece of engine tube sticking up from the engine mount was not tough enough to survive the force needed to pull it out, even though I had done it several times during the build process. I'll make a new 24mm mount this winter, and this time I'll wrap the upper end of the engine tube in two or three layers of 110# paper to reinforce it.
Here's a breakdown of the rocket. Note the bypass tube baffle... no wadding for this rocket. I built the "off centering rings" by hand using layers of posterboard. I did discover that it's almost impossible to extract the engine mounts by hand, so I created the tool shown in the breakdown to pull them out. It's just a piece of #9 fence wire bent into shape... if you've ever handled that sort of wire you'll know it's plenty strong for the purpose.
The engine mounts were inserted into the rocket and drilled from the outside, so that the holes line up the same on all of them. I then removed them, and used a disposeable syringe (without needle) to force epoxy into the holes, standing them on end to ensure the epoxy would pool on the upper centering ring. After the epoxy was totally solid, I redrilled the holes. The screws that hold the engine mounts in place are ordinary self-tapping screws made to hold the plastic pieces in place on a computer case.
The parachute was made from a mylar balloon; it's tougher material than the thin mylar emergency blanket material I use for smaller rockets.
I've completed assembly of the three engine mounts for this rocket. I had to sand them down to make them fit inside the body tube... the BT-60 couplers from Uncle Mike's Rocket Shack are really tight in the hobby-shop body tube.
I have also glued the nose cone into the upper sustainer tube (after gluing in a screw eye and affixing some shock cord, of course) and assembled the bypass-tube baffle. I think I'll glue the engine mount thrust ring and baffle into the lower sustainer tube next.
I am building a rocket which I am calling the "New Centurion." It's inspired by the classic Centuri Centurion, which had an innovative (for the time) ejection baffle system. My New Centurion will have an offset-tube baffle, will break at the midline (just above the baffle), and will feature three interchangeable engine mounts (1x18mm, 2x18mm, 1x24mm D/E).
I created RockSim files for this design last summer. It was a trial install, and I didn't buy it, so I must admit the files may not be entirely up to date. However, if you are interested, here they are:
According to those simulations, here are the "recommended" engines for this design:
|A8-3||22'||0.28 fps||A8-3||88'||0.09 fps||D12-3||783'||6.57 fps|
|B4-2||77'||0.08 fps||B4-4||275'||1.00 fps||E9-4||1357'||10.91 fps|
|B6-4||83'||2.30 fps||B6-4||286'||3.32 fps|
|C6-3||283'||3.10 fps||C6-5||815'||6.90 fps|
Now, I think it should be obvious I'm not actually going to fly this bird on a single A8-3; with only 22' of altitude, I have no room for mistakes. Likewise, I'm not wasting B4-2 or B4-4 engines on it (though I may fly it once on a pair of A8-3's just to say I did it). A pair of C6-5 engines gives a pretty respectable flight for a big bird, and I suspect that will be a common rig for me for my small regular launch field. Likewise, the D12-3 looks like a good engine for a crowd-pleasing flight.
I'm hoping to make it to St. Louis at least once this year to launch with AKPilot and his club, and if I do I plan to buy a pack of E9-4's for it. This may well be just the rocket the E9-4 is designed for.