I found plans/descriptions of a few things that came close to what I wanted, but they all have, to my mind, some shortcomings. Ultimately I used these ideas and came up with my own design.
- Low cost.
- Easily obtainable parts.
- Easy to build.
- Compact for storage/transportation.
- Adjustable launch rod angle.
- Easy way to change launch rod diameters.
Step 1: Parts List
1" PVC, 10' (*) - $3.38
1.25" PVC, 5' - $3.73
1" PVC side outlet - $2.05 (Be sure all joints are slip-fit. Not everyone has this - Lowe's does)
1" PVC end cap (2) - $1.98
1.25" PVC end cap - $0.86
1" PVC couplers (2) - $0.92
1.5" 1/4-20 thumb screws (2) - $2.54
1/4-20 wing nuts (2) - $1.18
2.5" 3/8-24 hex bolt - $1.21
drill chuck - $0.00 (Salvaged from an old cordless drill, but probably obtainable cheap at a thrift store)
stainless steel bowl or pot lid for blast plate - $0.99 (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.
Step 2: Construction
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. I later added a 2" threaded wiring post from an old lamp. This prevents the blast plate from tipping and wobbling so much at launch.
Easy to build - check! The hardest part for me was getting the chuck off the old drill.
Step 3: Putting Your Launch Pad to Good Use
Compact for storage/transportation - check!
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!
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.
I've been using mine for a year now, launching A through F engines with varying rod diameters, and it works great.
Step 4: Appendix
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.