Tuesday, June 24, 2014

HPR L1 - part one - Building the Rocket

The next few blog entries concern my attempt for NAR Level One High Power Certification.

the motivation

At the end of July, I plan to attend NARAM 56 in Pueblo with my family. The initial motivation for this was to watch some of the really big HPR stuff, in particular the ULA launches. But of course I have to participate to some degree. In talking about it with my wife, she actually was the one who convinced me to go for my certification, reasoning that as long as we're taking several days to go down there, I might as well make the most of the event. How could I argue?

the rocket

For my L1 build, I selected the Madcow Torrent, for several reasons:
  • I like the kit designer. I've had one other Madcow kit, and really liked it.
  • The kit is not a giant stretch in terms of construction techniques, since it is all cardboard and wood.
  • I like the vendor. The Torrent is available (only) from Apogee. Their website is really excellent in terms of education and information, including a whole series of videos on construction of the Torrent. They have great customer service, and are always responsive with questions. Plus I like to buy from a local (Colorado) company, and it doesn't hurt that I was in the Springs so I didn't have to pay shipping.
  • I wanted something with a good size payload so I can experiment with my own custom-built avionics.
  • I can grow quite a bit with this kit. If all goes well, after I get my L1, I can further pursue L2 with this same rocket. Also, I can get experience with dual deployment easily since it comes with an ebay.

The one thing I don't like about the rocket is that, due to its weight, I can't fly it at the local CRASH launches. So for future flights, I'm going to have to attend either NAR national events, or else join up with NCR. Not a big deal though, since the 38mm H+ motors for this aren't cheap - I won't be flying it all that often. 

the build

I don't see the need to post any pictures of the build, because it's basically been "by the book" - or video in this case, following along with the Apogee tutorials. There are a couple things I will note:

  • Glue or epoxy? I struggled with this decision quite a bit. In general, I'd rather not use epoxy unless I need to. And reading around, the results are interesting. Everyone says that for wood and paper, wood glue with give you just as strong a bond as epoxy. But it's rare to see anyone say that they've done an HPR build with wood glue, with the very notable exception of the Apogee videos. I even went so far as to email Mike Stoop at Madcow to ask his opinion. His answer was interesting:  
me: I'm following the Super DX3 instructions from Madcow, which say to use epoxy.
On Apogee's website, they have a series of construction videos in which they use wood glue for all the wood/cardboard joints.
Reading various forums, the general consensus seems to be that for wood/cardboard, wood glue is as strong as epoxy, and lighter. Yet a lot of people still exclusively use epoxy on their HP builds.
I was just wondering what your take is on this. 
Mike Stoop: I would tend to agree on the wood glue - it is much easier to use.  However, we have not built any of our kits with wood glue, we only use epoxy.  Sorry can’t be more help on this. 
In the end, I decided to go with wood glue, though I still lose a little sleep over the decision, particularly since it really wouldn't have hurt to just have used the darn epoxy. Will it hold up? time will tell. But it's only my first HPR build - probably won't be my last.
  • Shock cord/mount. Not a big modification, but I did not tie my shock cord to the eye bolt on the forward ring. On this 4" diameter rocket, it is easy enough for me to reach in there and attach it with a quick link. I like the flexibility this offers. Also, I purchased a nomex sleeve for the tubular nylon shock cord.
  • Internal fillets. There are a couple of nicely documented builds of this model on the Rocketry Forum. Following the lead of a couple of these, I put some good internal fillets on my fins and motor mount. First I glued only the forward centering ring onto the motor tube. I then glued this inside the airframe, using the UNglued aft ring to position it. Removing the aft ring, I was able to put good heavy fillets on both sides of the forward ring. Next I mounted the fins, and again, with the aft ring not in place, I was able to put good internal fillets on the fins where they connected to the airframe and motor tube. Then finally I glued in the aft ring.
  • Motor retention. I bought an Aeropack retainer to use with this rocket, which would've been great if I hadn't messed up and left not quite enough of the motor tube exposed to attach it fully. So instead I went with a Madcow retainer.
  • Finally, I added parachute swivel.

I've considered getting a larger parachute than the 36" that comes with the kit. Some reviews indicate that it may be a bit undersized, and my simulations with OpenRocket bear this out, showing an approximate descent rate of 28 fps for the fully built rocket with ebay sled. I've decided to stick with the stock chute for the initial flight and see what the actual descent rate is from my Jolly Logic 2 altimeter. Again, hopefully I won't regret this decision, as a damaged rocket will result in a failed certification attempt.

Throughout the build, I've weighed all components and re-weighed them as they'd been assembled, updating a detailed OpenRocket design. I'll do the final real-world CG measurements when I've go the paint on, but for now it looks like I've got good stability (1.5 to 1.6 cal) for H and I motors.

One other thing I'll comment on. Being a little old-fashioned, I like books. And you can't argue that Harry Stine's Handbook of Model Rocketry is anything but a great book to have. I also like the comprehensiveness and level of detail in Tim Van Milligan's Model Rocket Design and Construction. So I decided to buy Modern High Power Rocketry - 2nd ed. from NARTS. I won't mince words here - it was mostly a waste of money. While the book is a good overview of the topic, it doesn't really go into great depth on anything. The information it provides is readily available from vendor/organization websites, casual web browsing, or even the two books formerly mentioned. Many of the diagrams look like they were done on a Commodore 64. Further, it is extremely repetitive. Each chapter not only rehashes much information from other related chapters, but there are numerous sidebars that add nothing other than repeating information from the main text. The chapters on Level 3 are interesting and fun to read, but most likely won't ever be of practical use to me. There is exactly one new and worthwhile piece of information I got from this book: substitute closed (forged) eyebolts for the open eyebolts generally supplied with kits. They don't weigh much more and are much stronger. Alas, I read this after I'd already mounted my open bolts with epoxy, but I don't imagine it will be a problem on the four pound Torrent. But definitely something to keep in mind for future HPR builds, especially if I ever get to something big and really heavy.

the motor

For my certification attempt, I'll be using a Cesaroni H120 in a Pro38 casing. I've never used a reloadable motor before, so the simplicity of the Cesaroni system (compared to Aerotech) was appealing. Also, I like having the ability to put spacers in the casing, so the Pro38 case can be used with various sizes of reloads. I purchased 3-grain case with two spacers, but plan to purchase 6-grain case as well in the future for L2 certification. This will allow me to use a huge range of Cesaroni 38mm reloads.

The clincher, however, was the Cesaroni certification special, which gives you the casing for free. Can't beat that.

the payload

On the certification flight, I plan to have my trusty little Jolly Logic 2 on board, as well as a custom-built data logger. I've got all sorts of plans and ideas for avionics, but for this initial attempt, I plan to build something to log GPS, altimeter, and accelerometer data. Details on this in my next post…