Water bottle rocket

I was challenged with building a rocket over a weekend for a challenge at work. No time for testing or planning or, well any actual real science, but ok. 60 hours of elapsed time went into designing, printing, testing, redesigning, reprinting, retesting… etc.

In the limited low-pressure testing we did, everything was fine… in the medium-pressure testing nearly everything went wrong, so when we turned up on the day, I was just hoping the rocket left the launcher… In the first round, objective met… just. A couple of tweaks, and round 2… we won the furthest launched rocket of the entire day award (not actually a good thing for what we were attempting). The third round and we almost made it to the 70m zone everyone was aiming at.

This set of prints is just for the different fins and nose cones I played with. Nozzles and launchers to come later.

The results

Round 1 was the first test round. The internet says 120psi, we thought we'd creep up on it, so started lower

• 600ml water
• No nozzle
• Straight fins (so65-h100)
• ping-pong ball in the nose cone
• Fill pressure: 80psi,
• Launch angle: 45 degree(ish)
• Distance: about 20m
• Outcome: Very unstable flight, cracked nose cone.

Round 2 was the furthest we went (although we were aiming for 70m)

• 600ml water
• No nozzle
• Straight fins (so65-h100)
• Tennis ball in the nose cone
• Fill pressure: 80psi,
• Launch angle: 45 degree(ish)
• Distance: over 120m… further if the trees hadn't been there. 
• Outcome: Very stable flight, broken nose cone.

Round 3 was the egg challenge

• 600ml water
• No nozzle
• Straight fins (so64-h100)
• Egg and padding in the nose cone
• Fill pressure: 60psi,
• Launch angle: 45 degree(ish)
• Distance: about 80m
• Outcome: Very stable flight, broken nose cone…. RIP eggy.

Bottles

I have been using 2-litre Schweppes bottles in the UK, Namely 2-litre Coke-a-cola, coke zero, lemonade, and Pepsi/Max bottles. The diameter of these bottles is about 97mm when resting, and 99mm or so, when pressurised, The threads are a 28mm standard for pressurised bottles in the EU apparently.

Fins

The bigger the fins and the further back they are, the more aerodynamically stable your rocket is. The more they stick out as you look down on them, the more drag they produce, the more drag, the stronger the forces at take-off… 30x the force of gravity can be a painful experience… ohhhh so much to fiddle with.

Curved fins will produce a rotation which will increase stability but significantly increase drag.

Print the fins in vase mode with no base infill and a 0.4mm nozzle. Slight over extrusion is probably good.

Fins can be scaled to the size of your bottle, but they should slide up and down the bottle when the bottle is not pressurised. test pieces are supplied for you to test.

We were building a specific rocket to be tested only 3 times… but I still produced fins for a few other alternatives.

If you can print in PETG then great, I could not, they kept warping the whole time. PLA was superb though.

Fin sticky-outy-ness

• so50 - these fins only stick out 50mm from the side of the bottle. They print on a Prusa mini
• so65 - these provide the best performance but need a bigger print bed - the Mk3

Fin height

• h100 - perfect for the 2-litre bottle we were testing.
• h175 - if you're using a bigger water bottle (like 2 stuck together) this maxes out the mini
• h200 - again with the bigger rocket, but will max out the mk3.

Rotation angle

• RA00 - straight fins. The best way to go
• RA30 - a slight twist, a gentle rotation
• RA70 - a steep rotation, will probably end up shredded by any launch

For all of the challenge outcomes, we used the V1-SO65-H100-RA00 model.

Further refinement

After the competition, I refined the fins further including a symmetric airfoil design (mostly for printing ease), joining the body back at a narrow point (again for printing ease) and controlling the how much cut-off in the first layer (this is a 2 perimeter sticky out bit. it will lift if given a chance).

• AS00 - The sine wave starts at zero degrees (in the body)
• AS70 - The sine wave starts at 70 degrees (giving some print profile)
• AE180 - The sine wave ends at 180 degrees after going trough the 90 degree peak.

I also dabbled with some 2-fin designs that are theoretically stable enough to fly.

https://www.rocketreviews.com/descon-two-fins-no-kidding-daniel-kirk.html

Best fins?

For all of the challenge outcomes, we used the V1-SO65-H100-RA00 model. The V4 version prints a little easier and flies really well also. Which ones are the best? well, that's experimentation for you :)

Nose cones

So what ballast are you going to be using? I tried golf balls, ping-pong balls (with adjustable ballast) and tennis balls. This will all depend on what you are attempting to achieve. Furthest anything and you are looking to reduce mass and increase launch pressure to the absolute max. That is not what I was going for. Stability and retrievability were the objectives on a regular-length football pitch.

The long nose cone will allow for a golf ball to be placed further forward. This means less mass, but good stability. The outcome will most likely be a ruptured nose cone - which will save the fins :)

The other 2 cones allow for a golf ball or tennis ball to be placed on the end of the bottle. This means your nose cone will probably survive the landing. The downside is either more mass (with the tennis ball), or less stability (with the golf ball). All fun factors to play with.

Print the nose cone in vase mode with no base infill and a 0.4mm nozzle. Slight over-extrusion in PETG is probably good.

The nose cones can be scaled to the size of your bottle, but they should be a ‘comfort fit' when the bottle is not pressurised.

Wanna know a little bit more science? I gotcha!

Water rockets are a perfect example of Newton's third law of motion, basically, every action has an equal and opposite reaction. As water mass is ejected out of the nozzle of a water rocket, the opposite force propels the rocket in the opposite direction.

Slightly more sciencey science concerns the centre of gravity and the centre of pressure. 

The centre of gravity is the point where all forces on a rigid body will appear to react. Find this by balancing the rocket lengthwise on your finger. For a stable rocket, this should be close to your rocket's nose, or at least past the midpoint. You move this point forward by adding mass to the nose or move it back by removing mass to the nose (or adding mass to the tail)

The centre of pressure is the point where aerodynamic forces appear to apply. When your rocket is in motion, the centre of pressure will generally always follow behind the centre of gravity, and your rocket will be stable in that configuration. While finding this point is tricky, if you attach a piece of string around your rocket at its centre of gravity and spin it above your head. If the tail follows the nose, you're probably good. The bigger the fins, or the further back they are, the further back your centre of pressure is… However, you are moving the centre of gravity backward as well.

These 2 points should generally be within 2 rocket body diameters of each other, but if they are further apart, your rocket can be “Overstable”. This basically just means that all the huge forces at takeoff and the early flight will have a bigger influence on newton and his third law, which might be less desirable. The close together they are, the less stable your rocket is on the return to earth phase so you can try and make that landing a little softer.