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November 1, 2013 

Weight saving tips for improving water rocket performance

Reducing the weight of your rocket is a great way to make it go higher. If you're competing in a competition, or just trying to set a new personal best, weight reduction is a guaranteed way to get more altitude, since the same launch pressure will require less work to lift the rocket, resulting in more speed and a higher flight. This article will focus on how we reduced the weight of our rocket payload, as well as a few other tips we have discovered so that we can fly higher.

When we talk about the payload, we are referring to the portion of the rocket that sits on top and contains the electronics and deploy system. This part of our rockets houses a few components:

  1. Altimeter used to log the altitude of the flight.
  2. Camera which records video of the flight.
  3. Recovery system to insure a safe return to the ground.
  4. Flight Controller to coordinate the electronics.
  5. Batteries to power the systems.

Over the years, we have gone through several iterations of our water rocket payload designs. We have found a few easy ways to make the payload lighter. The following sections detail ways in which we reduced the weight of our electronics payload to a minimum, and also share some bits of history from our past designs and world record flights.

Weight Reduction Tip #1: Size Matters

The most obvious way you can imagine to cur weight would be to fabricate everything as small as possible. We have achieved this by shrinking all of the individual parts to the smallest size we can obtain, and packing them all into as small of a compartment as we are able to. Shortening the length of the compartment itself is good for a few grams of weight, since the body tube has a lot of mass. Shown below is a fiberglass body tube we used for the smallest payload we have flown. The tube could be a bit lighter if it were made from carbon fiber, but the carbon fiber blocks radio waves, and we use radio communications for our flight controller, so we had to make use of fiberglass for the main tube. The top is capped with a carbon fiber dome which protects the electronics in the event that the parachute fails and the rocket crash lands. The coupling holding the payload to the rocket is simply threaded PVC pipe, eliminating any need for unnecessary and illegal metal parts from the design.

Weight Reduction Tip #2: Throw dead weight overboard

A basic step to reducing weight we always employ is the strip unnecessary weight from the payload itself. The altimeter we use is a good example of how we do this. The altimeter has a number of heavy components which we can remove without interfering with the operation of the board. The first thing to go is the huge capacitor on the board which would be used for a pyrotechnic deploy on a regular rocket. Since pyrotechnics are not legal in water rocket competitions, we remove the capacitor that would be used to fire the deploy charge. The altimeter we use also has a bank of DIP switches which can be used to program the parameters in the field. The same settings can be made over a USB cable from a PC, so the switch block is redundant and can be removed as well. Another big weigh savings can be made by removing the screw terminal blocks from the altimeter and soldering the wiring harness

Weight Reduction Tip #3: Only take along as much battery as you need

Another way which we were able to save weight was to reduce the size of the batteries to the minimum needed. Since our rocket only needs to fly for about 2 minutes, we only need a power source good for 2 minutes, plus some margin of error. We have several systems that all run on batteries, but we opted to power everything from a pair of LiPo batteries connected in series. the series configuration was needed because our altimeter would not function on a single battery and needed the voltage of both batteries in series to function. We chose batteries which were small enough that the pair would fit inside the PVC coupler, utilizing the otherwise empty space and saving volume inside the payload compartment.

Weight Reduction Tip #4: Find the lightest camera you can possibly use

Trimming battery capacity to only 3-4 minutes presents a different problem. When competing for a world record, the rocket has to be pressurized for several minutes to prove it can safely hold the launch pressure and is not being flown with a sudden burst of air in the launcher. It is unsafe and against competition rules to approach a pressurized rocket, so there is no way we would approach the rocket to press buttons or flip switches. It would be against the rules, and frankly it would be insanely risky.

Luckily, our past designs had to account for a limited duration of a different sort. 10 years ago, the only tiny video cameras available recorded their movies to internal memory, and these were limited to clips of 2-3 minutes in length, so we had already solved the problem of turning the camera on and initiating the video mode with our prior designs, so we had some experience which we could apply to this new lightweight payload. By connecting wires to the camera electronics and bringing them to an external connector that plugs into our custom flight controller, we can operate the camera with the control software instead of poking the switches. Since we already had a hole in the payload compartment for the air vent needed for out altimeter, we positioned the lens of the camera to view through the same hole,

Weight Reduction Tip #5: Learn from past designs and improve them

In the past we had built different remote control transceiver circuits into the flight controller, which allowed us to remotely operate the camera, and also get some status information from the rocket. Our first designs used IrDA links for the ground communications, but we found that bright sunlight reflections could interfere with the system, so we began using small 433MHz transceivers for the job. Our early designs used an optical switch to detect if the rocket had launched. This way, we could still get some video if the remote link was not working. We also felt that if the launcher ever failed and let the rocket launch, it would also trigger the camera and we would still get video. The launch detect switch was eventually eliminated because we figured out how to read the data output of the altimeter and sense the launch without any additional hardware.

Weight Reduction Tip #6: Don't be afraid to try new things

For this new lightweight design, we decided to use a brand new product development tool from Texas Instruments called the EZ430-RF2500. This little piece of kit is an evaluation board for a microprocessor radio communications system for the MSP340 line of microcontrollers. The kit consists of two very tiny communications boards and a USB dongle that could load software into them. Being that these devices were based on a microcontroller that could accept user programs, we moved our entire rocket software suite into the tiny board, and used that to replace several discrete boards for the communications and the flight controller. All flight control could now be done with the EZ430-RF2500 board.

Weight Reduction Tip #7: Move functions to the ground if possible

On the ground based side, we built a battery operated control which we call the "Magic Wand". This simple control contained one of the EZ430-RF2500 boards, some LED indicators, a LiPo battery, and some switches. The components were small enough to fit inside a piece of 1/2" PVC pipe, with some fittings added to dress it up. The LEDs were glued into some holes drilled into the pipe, and the switches were placed into the exposed ends of the pipe such that they were recessed into the pipe where they would be easy to operate, yet immune from accidental actuation.

Weight Reduction Tip #8: Reduce, Reuse, Recycle

The 433MHz Receiver and antenna were leftovers from our previous launches, but they were perfectly good for this new design. The only change was the addition of a camera mount to the boom of the 433MHz directional antenna, so that one person could track the rocket with the camera and be guaranteed to have the antenna pointed at the rocket as the same time. The 433MHz receiver module has a data output which we could connect to a laptop using a USB connection, and an audio output which we amplified and used to drive a small speaker. We invented a method in which we used a number of audio tones to signify events detected by the flight controller, meaning we could tell without taking our eyes off the rocket during flight if it had detected apogee and deployed the parachute, or if it was in trouble.

The parachute compartment for this payload bay is merely a piece of T-12 Fluorescent Light Tube covering. This is a very light polycarbonate tube we have used for parachute compartments for many years. We prefer this tube because it is transparent and we can see that the parachute lines are not binding on the side of the tube or in the deploy mechanism itself. An opaque tube would mean it would be harder to check for these problems before launch. Notice that we use electrical tape to join and seal the sections together to prevent them from separating in flight and in our case to keep the electronics from getting wet if we land in a body of water.

We put together a short video which shows how we set up and launch our high pressure water rocket using this system. Many people have asked what it takes to launch a rocket like ours, so we thought it would be fun to put together a video showing how we do it. Enjoy the video!


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