Water Rocket

Out on the west coast to visit her maternal grandparents for a bit, Alice and I had to put out some extra effort to get in at least one rocket launch this month. With help from Uncle Brian and Grandpop we built a water rocket and launcher, got in a couple more-or-less successful flights, and learned a lot to put toward second generation designs.

Looks like a rocket!

Design

The launch mechanism we settled on is made from garden hose quick releases, a “Gardena style” water rocket launcher. AirCommand, RaketFued, and US Water Rockets each have useful writeups on building these. The positive side of the quick release is attached to the rocket, doing double duty as both catch and nozzle. You fill the rocket about 1/3 full of water, plug it into the negative side of the quick release, and then pump air through that to pressurize the rocket. Pulling a string attached to the quick release collar frees the catch and lets the rocket launch. Our design incorporates two quick releases to provide an abort string as well, in case we need to dump pressure without launching the rocket. This style launcher also needs a check valve or water trap so water can’t flow back to the pump or compressor. You could use a regular garden hose to run the air supply, but we used an actual air hose because it was smaller diameter—less to fill if using a hand pump—and actually cheaper.

THE PLAN.

Uncle Brian cutting a garden hose quick release to fit into the throat of the bottle while Alice draws a sign for the family July 4 party.

Grinding down that nozzle to perfection.

Lining up a drill hole in the launcher base. (Alice took this picture)

Alice rightfully supervising my use of power tools in building the launch tower. To be clear, because she was hassling me about it, in the previous picture I’m just lining things up; I put on safety glasses before actually drilling.

Between the PET material and the flexibility, it’s surprisingly difficult to glue things to soda bottles. Epoxies don’t usually work well. We used liquid nails here, which is somewhat of a mess to work with and tends to produce a less-than-beautiful result, but was functional enough. Fins were made out of coroplast, the corrugated plastic which lawn signs are made out of. A substantial lump of kids’ modeling clay went under the tennis ball nosecone to bring the center of gravity forward, hopefully ahead of the center of pressure—which I did not compute or measure; YOLO!

The assembled rocket.

Nozzle assembly.

Nosecone with substantial ballast weight.

Alice handled the most important part of the rocket construction herself: Decoration and finishing. The fins were colored with permanent markers. We used narrow colored duct tape on the body in order to have adhesion to the PET bottle and keep the whole rocket waterproof. Compared to our usual micro-rockets that come in around 3–5g, this rocket is quite the beast, very hefty at something like two orders of magnitude more mass than that, so I figured “Tape away, kid!”

Alice’s very exciting color scheme.

See-through panel and fill line to watch how much water we’re putting in.

The assembled launcher and rocket.

Ready for go-time!

First Flight

The debut launch was more entertaining than successful. But at least our little crowd of family spectators had also waved over the neighbors and their kids so everybody could watch the first trial of the completely untested system…

It turned out there was a leak in the air supply connection to the tower, so the rocket didn’t fully pressurize. Probably in part because of that it then also didn’t cleanly release on the first tug of the launch cord either so it lost even more pressure. However, it did still have enough energy on the second pull to at least demonstrate reasonable basic soundness of the system with a short hop.

 

Second Flight

After some field modifications and tightening various joints in the launcher stack, on the second attempt we got a solid release and good flight… arcing directly toward a tree. After an extensive all-hands search and consulting some providential video evidence which we did not at first realize we had captured, the rocket was located buried deep into the very top section of the tree.

 

 

Fortunately our crack team of retrieval experts managed to knock it down with a pole and then scuttle inside the heavy ground cover to grab the rocket, fully intact.

You can go 3000 miles away and you can completely change your engine type, but you’re still gonna need a big ole pole in yer rocket kit…

Another rocket retrieved by our rocket retrieval pro!

Final Flight

On the third and final flight we increased the pressure just a bit more, targeting 100psi. Unfortunately the nozzle on the rocket broke right as we were about to start recording video, releasing the rocket unexpectedly. From there though it had an exceptional flight, nearly straight up to some good height and then straight back down… nose first, at high speed, whistling loudly like a V2 while the launch team and spectators all scurried for cover. Fortunately it didn’t hit anyone, and, obviously even more importantly, neither did it hit the solar panels on the house…

It did though lawn dart into the grass hard enough to displace a noticeable crater and break off the nosecone. So back to the workshop everything went for repairs and improvements after a reasonably successful set of first flights.

Thoughts

I chose this quick release style launcher because at the moment, away from my own workshop, I didn’t want to build something like a Clark cable tie release. That was probably the right call for a first launcher. This style is pretty much plug & play. Toying with the quick releases when they originally came I was worried they required too much force to release, but they wore in rapidly and developed a smooth enough action.

However, with how far away you want to get from the launcher, enough force is involved, and with enough leverage and potential cut points involved, that the pull strings must be chosen with some care. The twine I chose broke in several places. That’s not necessarily the worst thing ever, better that than force through some blockage. But they’ll be the first thing upgraded when we get to revisions.

With more time and access to my more hobby-oriented tools and collections of bits & materials I would probably do the fins differently. Gluing to the bottle is problematic and messy enough that I would consider shrouding or bracketing all around it to create a cleaner mount for the fins. Another alternative is 3D printing a bracket or fin can that screws onto the bottle. Some investigation of other adhesive options is also in order. I was pleasantly surprised though at how straight the rocket flew. Between the ad hoc, arbitrary design of the fins, the largely unknown mass distribution and consequent unknown relationship between the centers of gravity and pressure, and not constructing guide rods for the launcher, I was expecting it to be a lot more squirrelly.

Segueing from the mass distribution though, even without the nose weight, a rocket like this is deceptively heavy and could go surprisingly high. Include the nose weight and it’s a kinetic bomb. That’d be one thing in a wide open field, but with any kind of structure, vehicles, etc. even remotely nearby, some serious thoughts about a parachute deployment mechanism are in order.

All in all though this design worked out well for a first shot. There was a bit of frustration to the launches as we got the kinks worked out and various bits broke. But we couldn’t actually have expected much better for our initial water rocket attempt.

Onward and upward!

Our rocket and launcher construction schematic.

R2

Recently I finished R2, a scratchbuilt somewhat over-engineered BT60 sized 18mm model, and got in its first two flights at the April 2021 PARA launch. It came out pretty much exactly how I had envisioned, and wound up with a very satisfying second flight after a nearly catastrophic inaugural flight.

R2 finished (one side).

R2 finished (one side).

Construction

R2 began with modeling in OpenRocket, inspired by and based on a BT60 tube and nosecone from a ~26 year old Estes Designer’s box. The design has a payload compartment just for fun, through-the-wall papered fins that interlock with the motor mount rings, redundant Kevlar threads running from the motor mount to the shock cord, conformal angled launch lugs, and a couple other unnecessary touches. I just wanted to build a big, substantive feeling, low power rocket, and experiment with some construction techniques for strength and redundancy. I wasn’t too concerned about weight or overkill features for a rocket intended to fly on C motors at most. Slow, heavy-feeling flights would be just fine (provided stability was good & flights safe).

Modeling R2 in OpenRocket.

Modeling R2 in OpenRocket.

Other than the tubes, nosecone, and parachute, R2 was built up with a mix of 3D printed and CNC cut parts. The boattail was printed on a Wanhao i3+ in ABS for heat resistance, and the launch lugs the same for convenience. The tube coupler is in PLA printed on an Ender 3. Since the coupler isn’t exposed to significant direct heat I’m not too worried about the PLA, but in hindsight I wish I’d done it in ABS as well just to have no doubts about it sitting in a car trunk on a hot day. The fins, centering rings, and bulkheads were cut from 1/16″ balsa using a Cricut Maker. Strength was added by papering the fins with adhesive paper and coating the edges with CA. Rings and bulkheads were beefed up with a PVA glue coating.

Drawing the fin pattern in OpenSCAD.

Drawing the fin pattern in OpenSCAD.

3D printing the boattail.

3D printing the boattail.

All R2's pieces ready for assembly.

All R2’s pieces ready for assembly.

Marking the tube for the fin slots.

Marking the tube for the fin slots.

Gluing together a combo coupler and payload bay bulkhead.

Gluing together a combo coupler and payload bay bulkhead.

R2 fully assembled.

R2 fully assembled.

Finishing

Painting was done with Montana spray paints over a Rustoleum primer sanded down. The Montana paints go on kind of thick & heavy, they’re made for graffiti art, but because of that lend themselves to consistent, smooth, very opaque feeling finishes. The big “R2” lettering was done using vinyl stencils and spray paint while the other details are custom vinyl stickers. The stickers were printed on an inkjet and then they & the stencils cut out with the Cricut. The “SPACECRAFT” icon is a stock art logo I bought as tongue-in-cheek representation of a generic NASA/ESA/whatever. The US, Canadian, and Korean flags were chosen just because they look cool, it doesn’t reflect any actual mission partnership or such.

Really my only disappointments with the rocket came in finishing. One of the stickers on the nosecone smudged a bit in application. I also didn’t think enough about the striping on the payload bay, so there are 10 stripes instead of 8 and they don’t line up exactly with the quartered paint scheme of the body. But overall I think it came out well and looks real cool in the background on video calls.

R2 finished (other side).

R2 finished (other side).

R2 finished (nosecone detail).

R2 finished (nosecone detail).

First Flight—Disaster!

There isn’t anywhere immediately near home to fly anything larger than maybe a 1/2A, so the first R2 launch had to wait until the next PARA meet. I got a good slow motion video of the launch itself, there was a successful parachute ejection, but then disaster: It was a windy day and the rocket drifted into trees immediately behind us.

R2 on the launchpad for its debut.

R2 on the launchpad for its debut.

Ignition!

Ignition! Photo by Mike S.

Up in a tree!

Up in a tree!

Fortunately the club has a telescoping pole with which we were just barely able to reach the rocket. It took some finagling, but eventually we were able to get hands on the rocket. The shock cord stretched a ridiculous amount so it took some effort even beyond that point, but we were eventually able to rip the rocket down.

Getting out the rescue pole.

Getting out the rescue pole.

Hooking the line.

Hooking the line.

Pulling R2 off the tree.

Pulling R2 off the tree.

Second Flight—Success!

Initial damage assessment indicated the parachute was ripped in several places, one shroud line was still in the tree, and a bulkhead was gone. All in all R2 was in reasonable shape but I assumed I wouldn’t be able to fly it again that day. I kept looking at it as the day went on though and really wanted to get in another flight. So between helping Alice launch rockets and other goings-ons, I got to work. The parachute got more or less taped up, lines replaced and retied, the remaining bulkhead reinforced, and so on. It all came together at literally the end of the meet as people started talking about packing up the launch pads. With the very last launch of the day, R2 pulled off a picturesque second flight, up to 365 feet and almost back to my hands.

Staged for second launch.

Staged for second launch.

Wrap

R2 came out great. It looks exactly as I wished, it was a fun build, and it had a perfect second flight with just enough drama beforehand to accentuate the success. I don’t know how much more I’ll fly it and risk R2 getting captured for good by those hungry trees! But it flew well, looks real nice in my office, and was a good little project.