The Rocket Launcher

NX CAD Model PlanI first saw paper rockets launched at a church-sponsored campout in the Tonto National Forest in Arizona. Steven Lees had brought his launcher and got all of the kids making their own paper rockets out of construction paper, clear plastic shipping tape, and a few paper plates. I was truly impressed. I love this kind of stuff, and once I understood how the system worked, I couldn't wait to get back to Home Depot and start buying PVC.

I did a bit of preliminary research on the web and whipped up a quick 3D model (using NX) for sizing and layout purposes. Then, after a quick trip to Home Depot and Auto Zone, I was ready to start!




The Video Clips

Here are a couple of short video clips showing the launcher in action and fun reactions from kids in the neighborhood.

Rocket Launcher Video Clip #1

Rocket Launcher Video Clip #2


Required Materials and Tools (costs on 10/30/06)

Description Image Quantity Store Cost Cost per
Launcher
Tubeless Tire Valve Tire Valve 1
(sold in two-packs)
Auto Zone $2.99 $1.50
2" PVC Pipe 2 Inch PVC Pipe 5 feet
(sold in 10-foot lengths)
Home Depot $7.08 $3.54
2" PVC Cap 2 Inch PVC Cap 1 Home Depot $1.16 $1.16
2" PVC Elbow (Female Solvent Both Ends) 2 Inch PVC Elbow 2 Home Depot $1.86 $3.72
2" PVC Coupling (Female Solvent Both Ends) 2 Inch PVC Coupling 1 Home Depot $0.91 $0.91
2" to 3/4" Flush Bushing Flush Bushing 1 Home Depot $1.35 $1.35
3/4" Male Thread to 1/2" Female Solvent Straight Coupling 3/4 to 1/2 Reducer 1 Home Depot $0.64 $0.64
1/2" PVC Pipe 1/2 Inch PVC Pipe 5 feet
(sold in 10-foot lengths)
Home Depot $1.74 $0.87
1/2" Ball Valve Half Inch Ball Valve 1 Home Depot $2.65 $2.65
1/2" Elbow
(Female Thread, Female Solvent)
Half Inch PVC Elbow 1 1 Home Depot $0.31 $0.31
1/2" Elbow
(Male Thread, Female Solvent)
Half Inch PVC Elbow 2 1 Home Depot $0.63 $0.63
PVC Solvent (Christy's Red Hot Blue Glue) PVC Glue 1 (You won't use much...) Home Depot $4.98 $0.25
Total Cost of All Materials       $28.16  
Estimated Cost
per Launcher
        $17.53

Construction Method

Step Description Images
(Click to Enlarge)
 

Mounting the Inflation Valve

Getting the valve to work is a rather critical part of this project, and because the valve must be installed from the *inside* of the pressure chamber, you *must* do this step first.
 
1 Start by making a hole in the 2" cap to receive the valve stem. The Tubeless Tire Valve has a relatively soft rubber body that is designed to be pulled tight through a 0.453" diameter mounting hole.

[NOTE: I'll admit that I completely eyeballed the hole (and to boot, I eyeballed it using my Dremel because I didn't happen to have a 0.453" drill bit lying around my garage. I took it slowly and checked it against the stem frequently, and in the end it worked great.]
step01
2 Once the hole is perfect, insert the valve stem from the inside of the cap and pull it outward until it snaps tightly into place as shown. The key to success here will be getting the hole very round (easier with the right drill bit, of course) and just the right size to engage the stem snap tightly, thus allowing the shoulder of the valve stem assembly to engage with the PVC all the way around and maintain pressure with a tight seal. step02
 

Reservoir Construction

The bulk of the pressure chamber is constructed using the 2" PVC, making a U-shaped chamber for stability when sitting on the ground. The 2" Cap (with the protruding valve stem) will be one end of this chamber, and the Ball Valve assembly will screw into the other end. The three 2" diameter sections should lay flat when assembled, so take care to keep the reservoir assembly pressed flat against something like a garage floor while gluing the segments together.
 
3 Attach the cap to a 24" piece of 2" pipe. Again, make sure the Tubeless Tire Valve Stem is already installed -- it's going to be kinda impossible to install it later. step03
4 Next, attach the first big elbow... step04
5 ...and then a 12" long piece... step05
6 ...and another elbow, an 18" piece, and the straight 2" coupling. step06
7 Then cap this main portion of the reservoir body using the Flush Bushing. step07

step08
 

Valve Area Assembly

In order to keep everything aligned the way you want it (my preference was: handle of the Ball Valve pointing up, with the launch tube pivoting on a horizontal axis) assemble the remaining 1/2" diameter segments in the following order:
 
8 First, glue one three-inch segment of 1/2" pipe into the solvent end of the Straight Coupling. Then (adding a little bit of silicone tape to maintain a tight seal) screw it into the Flush Bushing.

Note that because this joint is a critical part of the pressure chamber, it must be airtight. The straight coupling has flats for a wrench -- you'll definitely want to use them to tighten this joint.

Assembling just this first segment will allow you to glue the Ball Valve onto this segment in the desired orientation -- if the Ball Valve were already glued to this segment before you screw it in, then it would be very difficult to get the orientation correct while establishing a tight seal with the Flush Bushing.
step09

step10
9 Next, assemble the Ball Valve, another 3" segment of 1/2" pipe, and the first small elbow (female solvent on one end, female thread on the other) as shown.

Again, *my* preference was to have the Ball Valve handle pointing UPWARD and the launch tube pivoting around an axis parallel to the ground (a pure "elevation" control), so establishing the correct angle between these two components was rather important.
step11
10 Next, glue this small subassembly to the chamber assembly. Again, be careful to maintain the desired orientation during this step. step12

step13

step14
11 Once the Ball Valve has been attached, you should now have an airtight pressure chamber. This is a good time to test your chamber by closing the valve and pumping it up a bit.

Personally, I use a small portable battery-powered air compressor with an integral pressure gauge. Many manual bicycle pumps similarly have a pressure gauge as well.

Test the chamber by running the pressure up to about 20 psi. With the pump power off, listen for any hissing noise indicating leaking air, and watch the pressure gauge needle to see if any pressure drop is evident. If you're not hearing any leakage and the pressure holds steady, then you're in good shape!

(Remember to open the valve and relieve the pressure after this testing.)
step15
 

Launch Tube Assembly

The final step is the launch tube -- the segment over which your rocket will slide when you launch it.
 
12 First, cut the launch tube to the desired length -- I made mine 24" long so that it could stow flat inside the U-shape of the pressure chamber.

Then, glue the second elbow (female solvent on one end and male thread on the other end) onto one end of the launch tube.

You may want to slightly taper the other end of the launch tube to make it slightly easier to slide the rockets onto the tube before launching.
step16
13 Finally, screw this launch tube assembly into the pressure chamber assembly. It should be loose enough that it can be easily pivoted but tight enough to withstand the blast of compressed air. The "air-tightness" of this joint is less critical as it is not *maintaining* pressure -- just guiding the blast of compressed air for a fraction of a second. step17

step18
 

Making a Rocket

A simple rocket can be constructed using a sheet of construction paper, sheet of cardstock (or a stiff paper plate), and some clear packing tape, as described here.
 
14 Roll the sheet of construction paper around a segment of 1/2" PVC pipe to get the size right. After rolling it fairly tight, relax the spiral just a bit so that the tube can slide smoothly over the outside of the launch tube. Then, use the packing tape to secure the tube together.

[NOTE: I cut a couple of 15" segments of 1/2" PVC pipe to use as "forms" while rolling rocket tubes. These two 15" segments were part of the 5 feet of 1/2" pipe needed for this project.]
 
15 Make a nose cone by forming a half-circle of the cardstock (or stiff paper plate) into a conical shape. Trim it up to fit the size of your rocket, and *SECURELY* tape it on to one end of your tube.

Note that when you open the valve and blast your rocket with compressed air, this nose cone will take the brunt of the force, so make it quite strong and attach it securely. Don't skimp on the tape. :-) The extra mass in the nose will help the flight characteristics a bit as well.

It goes without saying that the landing can be pretty brutal on the nose cone, too. This picture shows a rather flimsy nose (one thickness of stiff paper plate material) after about three landings in the street. A newer design with about three thicknesses of the same heavy paper plate material barely dented when it hit the same street repeatedly. Sturdy nose cones are good.
step20
16 Finally, cut some tail fins from your cardstock (or paper plate) and tape them onto the other end of the tube. You can experiment with different shapes and orientations to achieve different flying effects. step21
 

Launching the Rockets

Prepare to have a *lot* of fun. Children and adults alike are drawn to this little unit, and if everything goes right, you're going to see a lot of smiling and giggling and clapping and general excitement. Just be safe!
 
17 Place the launcher *OUTSIDE* on the ground (or on a sturdy table) and aim the launch tube in a safe direction. Remember to compensate for any breeze that might be blowing -- the rockets are lightweight and can really carry in a stiff wind. :-)

Be careful about launching rockets into or toward streets. While the rockets will not do significant damage to a car, children have a tendency to chase the rockets while looking up at them in the sky, and moving cars can do a lot of damage to a child who is running into the street without watching for traffic.

Slide a rocket down over the launch tube. It should slide smoothly so that it can release easily.
step22
18 Close the Ball Valve, attach your pump, and pressurize the launcher. For reference, my initial launches at about 25 psi [1.7 bar] were sending the rocket about 80 feet [25 meters] into the air (total flight time of about 4.7 seconds -- just like a really good football punt). One subsequent (almost vertical) flight at 35 psi had a total flight time of over seven seconds -- indicating (neglecting air friction) a peak height of close to 200 feet. It gets hard to see these little rockets at that altitude!

Make sure people are out of the line of fire. A tough nose cone at short range under high pressure can seriously injure someone. (Fortunately, I have no pictures to demonstrate possible side-effects.)
 
19 After the obligatory countdown ("Five... four... three... two... one... Blastoff!") quickly open the Ball Valve. Then, watch the rocket soar and listen to the awestruck crowd!

NOTE: The rockets hit the ground with a fair amount of force. This one landed in the grass and embedded itself about five inches into the turf. Don't let kids try to catch these things -- they're sharp and tough and moving awfully fast.
step23


Random Thoughts and Comments

Premanufactured Materials

Completely coincidentally, my oldest daughter brought home a *real* model rocket from a school-sponsored "Mad Science" class the day after I purchased all of the above materials (and the day before she and I assembled the launcher.) I noticed a few things that were interesting:

Pressure Considerations

I wonder a little about the maximum safe operating pressure of this unit. All of the PVC components were rated for at least 150psi (and some for as much as 600psi), but in the case of pressure vessels, averages don't matter at all. I think the "weakest link" in this pressure vessel is the initial one -- the Tubeless Tire Valve Stem through which the launcher gets pressurized.

The packaging for the valve stem does not specify a maximum pressure rating, but seeing as how the average operating pressure of normal car tires (for which people might realistically be buying replacement stems at Auto Zone) is between about 25 and 40 psi, I'm thinking that *my* safe operating range for this unit might be limited to that same range. I would expect that a very reasonable safety factor has been built into the valve stems to accommodate higher transient (shock) pressure loads due to things like potholes and rocks. You don't often (ever?) hear about a car hitting a pothole and having a valve stem pop loose from a wheel. At much higher pressures, though, I could certainly envision the rubber valve stem being forced outward through my imperfect mounting hole.

If anyone has any better specs on the pressure limits of a Tubeless Tire Valve Stem, I'd love to know about them. In the meantime, 20-30 psi shoots rockets *very* well.

When I build the next generation launcher I may look for a couple of additional components...

Reference Links

The above design is based on a blending of a design originally at Steve Spangler's site and my good friend Steven Lees' version.


©2018 Taylor Anderson

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