Tuesday, April 30, 2013

My Take on a Mid-Power Launch Pad

Having recently completed my first mid-power build (a Madcow Mozzie) I realized that I probably needed to upgrade my ground support equipment with a beefier pad. The two most readily available options looked like these:

Estes Porta-Pad E  While this is quite inexpensive and does come with a 1/4" rod, several reviews I read described it being crushed under the force of an F or G engine, so that ruled it out for me. (And also put to rest once and for all any thoughts of trying to use my standard Estes LPR pad!)

Aerotech Mantis  Reviews on this seem to be mixed. Some people say it is great, some people report issues. Plus it's huge and doesn't even break down well for storage. For $100+, I'd want something that was absolutely ideal. (Some people seem to like this because the rocket sits so high off the ground, making it easier to set up. Really? You can't get on your knees every now and then? Do you train flying monkeys to chase down your rockets?)

There are other (harder to find and equally expensive) options out there, but the general consensus on discussion boards seems to be to build your own out of PVC. Inexpensive, accessible, reliable, and - of course - fun.

I found plans/descriptions of a few things that came close to what I wanted, but they all have, to my mind, some shortcomings (see the appendix below). Ultimately I used these ideas and came up with my own design.

My criteria:
  • Low cost.
  • Easily obtainable parts.
  • Easy to build.
  • Compact for storage/transportation.
  • Adjustable launch rod angle.
  • Easy way to change launch rod diameters.

Parts List
1" PVC, 10' (*)

1.25" PVC, 5'

1" PVC side outlet
(Be sure all joints are slip-fit. Not everyone has this - Lowe's does)
1" PVC end cap (2)

1.25" PVC end cap

1" PVC couplers (2)

1.5" 1/4-20 thumb screws (2)

1/4-20 wing nuts (2)

2.5" 3/8-24 hex bolt

drill chuck
(Salvaged from an old cordless drill, but probably obtainable cheap at a thrift store)
stainless steel bowl or pot lid for blast plate
(from thrift store)

Total: $19.02 (Plus launch rod(s) - I went for 4 foot,  1/4" diameter stainless from a local supply house - less than $10)
Low cost - check!
Easily obtainable parts - check!

(*) Aside from strength, the choice of 1" PVC for the legs was deliberate. Since one leg is made adjustable with a telescoping tube arrangement, I needed two PVC sizes, one sliding neatly inside the other the other. 3/4" PVC does not fit into 1" PVC.

Tools needed: drill, drill bits, saw, PVC cement or other suitable glue. Optional but nice: drill press, mitre box

figure 1
  1. Cut the 1" PVC into three lengths. I did 16", 16", and 14". (Longer if you so desire)
  2. Cut off a 14" section of 1.25" PVC.
  3. Glue one 1" PVC end cap onto one (only one!) of the 16" PVC legs.
  4. Slide the 14" 1" PVC section into the 14" 1.25" section, with an inch or so sticking out. Drill 10 1/4" holes straight through both pieces at one inch intervals. A drill press would be ideal here because the idea is that you want the holes to line up no matter how far you slide in/out the 1" section, giving you an adjustable length leg that can be secured in place with the thumb screws and wing nuts. But even with my lousy drilling skills I was able to get this to work after widening the holes a bit. (See figure 3 below to see how this looks on the assembled pad.)
  5. Glue the 1.25" PVC end cap on to one end of 1.25" PVC section.
  6. Drill a 3/8" hole straight through the top of the PVC side outlet, all the way through to the bottom. Slide the 3/8-24 bolt through from the bottom and screw on the chuck (see figure 1)
  7. Drill a 1/4" hole in the center of your blast plate.

Easy to build - check! The hardest part for me was getting the chuck off the old drill.

Putting your launch pad to good use
figure 2
For storage/transport, Slide one holey leg all the way into the other and put in the thumbscrews/wing nuts to hold them together. Add the other two 16" legs onto this with the two 1" PVC couplers, storing your launch rods inside. This protects your rods during transport, and it all stores easily in the rafters of a garage or the corner of a closet. (OK, I know, the side-outlet-chuck, one PVC end cap, and the blast plate float around loose. Not perfect. I'm sure I'll forget to bring them to a launch one day.)
Compact for storage/transportation - check!

figure 3
To set up, separate the legs, remove the couplers, insert the legs into the tripod top, add the extra end cap on the third leg, put the launch rod in the chuck, and slide on your blast plate. Adjust the angle of your pad by removing the thumb screws and changing the length of the telescoping leg.
Adjustable launch rod angle - check!
Easy way to change launch rod diameters - check!

I just tested mine out last weekend, launching B through F engines with varying rod diameters, and it worked great.

I admit that the adjustable leg doesn't allow for a huge range of angular adjustment - maybe 6 degrees is all. But rarely if ever do I launch at a higher angle, and I especially wouldn't do it with a mid-power rocket.

One modification that I've made since this is to widen the blast plate hole to 3/8" and put in a 2" threaded wiring post from an old lamp. This will prevent the blast plate from tipping and wobbling so much at launch. I'm also considering creating an easy way to stake in the ends of the tripod legs to hold the pad solidly down.

Appendix - Inspiration

Four Legged Launch Pad by John DeMar
Judging from the number of references I've seen to these instructions, this seems like a very popular design. What I don't like about it however is the lack of angle adjustment, the less compact head assembly, and the fact that four legs could be inherently unstable on uneven ground.

This is an interesting idea for angular adjustment, but it's complex and bulky.

I think these two references show a great idea to allow for angular adjustment, and I think a drill chuck is ideal for holding/changing the rod. However, as noted in the discussion thread, a typical drill chuck has 3/8-24 mounting threads. Good luck finding a suitable eye bolt. I called all over town with no luck. I found them online for around $6 - but with $14 shipping. Ha! Still, if I ever feel like I really need this, it could easily be added to my rig as an upgrade.

NOTE:    These  instructions  are  presented  without  warranty  of  any  kind.  The  author  is  not   responsible  for  any  liability  or  loss  related  to  the  construction  and  operation  of  the  launch  pad. 

Wednesday, April 24, 2013

Rear Ejection

The latest build I'm working on is Pemberton's Little Bucky Jones, and I think it is going to be a slick looking model. Last week I was silently lamenting the fact that it will have a seam where the nose cone detaches, and I thought about the idea of making it, or really any other model, a rear-ejection system. It wouldn't be too difficult: seal on the nose cone and construct an engine mount that is blocked in the front by some sort of engine block, but can slide out the back and drag the parachute with it when the ejection charge fires. Should work.

But if it is so obvious, why isn't it done? The best reason I could think of concerns what would happen if your parachute failed to open for any reason. Since the node cone would still be on, you'd have an aerodynamically stable rocket in the sky, making a very ballistic return to earth. 

Ironically, I was just talking about this very scenario to my father-in-law on Saturday as I prepped my US Rockets Dual 18mm Rear Eject  (also linking to Apogee since they have a much nicer presentation and pics of the rocket) for a flight with two C6-5 engines. The rocket took off, flew high, arced downward, and as soon as the ejection charges fired it was clear things were not good. Two separate pieces were visible, and the big one was headed downward fast.

We recovered the rocket with the nose cone and crumpled front of the airframe buried six inches in the soft ground. At that point it broke off, and the rest of the rocket was intact, except of course for the missing parachute, which had detached as a result of a broken shock cord.

(I should point out that on a previous flight the Kevlar shock cord that came with the kit actually burned all the way through. I replaced it with a cloth-elastic cord mounted in the end of the tube with a paper mount, Estes-style.)

So from what I can tell, one, or possibly two things went wrong:

First, the delay was too long. From what I observed, the rocket had fully arced over and was starting to head down with some speed when the ejection charge blew. I'm sure this put a lot of stress on the chute as it popped out, and it likely led to the snapped cord.

Second, I had set up a camera to get a movie of the take off from the pad. Watching this frame-by-frame clarified what seemed like a not-quite-right launch. Despite the parallel hookup of the two engines, the video clearly shows that they ignited at different times, maybe 1/15th of a second apart. Not a big difference, but I wonder if this led to the first ejection charge pushing out the wadding and chute, but the second charge burning the shock cord on its way out and weakening it. Though I didn't see much evidence of this on what we recovered (we never did find the chute and the end plug to which it was attached).

  1. Verify the proper ejection delay with flight simulation software whenever possible.
  2. I'm done with the idea of rear-ejection. I've had my share of chute failures in other models, but they've always had enough drag after popping the nose that they are recovered with little or no damage.

And here's a semi-related epilogue:
About fifteen to twenty minutes after that launch, I was at the launch base preparing another rocket while my father-in-law was out still looking for the parachute. A police officer drove up and approached, saying that he saw the launch from the road and came to check things out. I told him that although this was my first time launching there (at the Northwest Open Space in Northglenn, CO) I had checked the Northglenn fire code (which is basically the IFC) and verified that model rocketry was permitted. He responded that "it may be allowed by fire code but prohibited by municipal code," and added that there was currently no fire ban in effect - a big issue at the end of last summer around here.

The municipal code issue is something I'd never thought of. But he said that in a very shrugging-it-off sort of matter, then just asked a few general questions about my rockets (how high, do I ever use payloads like an altimeter) and casually looked over my gear. His last question was whether I planned to launch any more, so I figured I was in the clear. He checked my ID and wrote down the info, then left with a not-too-stern "be safe." Another police car rolled up as he was leaving, and my father-in-law overheard him say to the other officer "just someone launching model rockets." 

I've since looked up Northglenn municipal code and can't find anything about rockets. I've really never heard of low power model rockets being illegal to launch anywhere. They're certainly sold all over the metro area.

I still wonder if what he saw from the road was a scary ballistic return - though he never mentioned that.

Update - 06-May-2013
I attended the CRASH launch yesterday (my first time - it was pretty great actually) and talked to one guy briefly as he was preparing to launch his Estes Gemini DC, sharing my experiences. Guess what happened next? Lawn dart.

Monday, April 8, 2013

Then and Now

Here's where I'm coming from:

As a kid, I launched rockets, but never to any degree of sophistication. My neighborhood friends and I all did. We built Estes models, shot them with spray paint, launched them, and had a lot of fun. Did we meticulously seal the balsa parts? No. Did we join the NAR? No. Did we even use a launcher or even a launch pad? Well, no - we had speaker wire with a 9V and paper clips, and a piece of music wire stuck in the ground. And no, we never bought flame-proof wadding either. I probably reached farther than anyone in my group when I checked out Harry Stine's book from the library. But I certainly didn't read it cover to cover or follow all his advice.

I don't exactly remember when it stopped. Certainly by the end of high school. I think I may have launched one or two rockets as a young adult.

Jump forward 20+ years, to the summer of 2012. I'm a father of a two year old girl and a five year old boy who is intensely curious and physical science-minded. And friends of his/ours announce plans for a rocket party. My son, Jack, and I get psyched and in advance of the party I pull an old Estes starter set out of the garage - it's got the launcher, the pad, and a rocket - all we need. So we do the build together and go out for a launch, and we're hooked. The day of the rocket party arrives and it is insanely fun.

So now here I am as an adult. I don't exactly have lots of spare time, but I do spend almost every minute outside of my job with my kids, and I need a hobby. Jack is still interested in rockets, and we work on some together, but my interest can't remain at the pace of a five year old. And I'm older, more thorough. And I have more money. Plus, this rocketry business - it's a whole different deal now. Sure, the Estes-level stuff is still at the local hobby store, almost identical to 1978. But now there is the Internet, with loads of information, loads of online vendors for parts and models. And microelectronics - cameras, altimeters! So, so much to explore.

I won't say I've become obsessed, but sometimes it feels like it. I've built about a dozen models since last summer, and have a half dozen kits in my closet waiting to be built. Plus an impressive stock of pieces and parts, with ideas for at least three or four custom models. With every new build, I'm exploring new techniques - adhesives, finishes. I've got my first composite motors that I'm itching to use. I just joined the NAR, and am hoping to go to my first club launch this spring. 

 So this is my blog, to document and reflect on what I do. I may not have anything original to say, but I will try to distill what I learn.