Air Command Water Rockets - How to make your water rocket fly straighter

Help! My rocket doesn't fly straight

For best performance a rocket should fly straight without spinning, however, this rarely happens in real life. There are a number of rocket design factors and environmental conditions that influence a rocket's trajectory.

This article describes what practical steps you can take to help your rocket fly straighter by observing what it does in flight. This article isn't intended to teach the detailed principles of aerodynamics although stability is a key concept in rocket flight.

In order to fly properly a rocket MUST be stable. Technically, a rocket must have it's center of pressure (Cp) behind the center of gravity (Cg) in order to fly without tumbling. A simple way to think about it is if you consider the design of a dart from a darts game. It has a heavy nose (the barrel and tip) and a light tail with the 'fins' (flight). A rocket needs to be designed in a similar way to be stable.

For better explanations and more details on rocket stability here are some good references:

For even more detailed explanations, don't hesitate to use your favourite search engine.

Common Flight Patterns

Here are some common rocket flight patterns, their likely causes and suggestions on how to correct them. Of course a typical rocket may be influenced by a number of factors simultaneously. After fixing one, you may need to fix another.

Flight path	Probable Cause	Potential Remedy
Rocket tumbles end over end soon after launch	Rocket is unstable	Make the rocket stable by doing one or more of the following: Add weight to the nose [1] Increase the fin size Lengthen rocket Move the fins further back Attach fins properly. The fins should be as rigid as possible. They should not flop around. Make the fins from a lighter material

Rocket flies straight but spins around its axis	Misaligned fins, one or more fins are not aligned with the rocket axis	Use fin alignment jig to ensure proper orientation.
	Warped fins	Use materials that don't warp when exposed to moisture or heat.
	Unevenly sized or irregularly shaped fins	Make fins using a template to ensure same size and shape of all fins
	Fins unevenly spaced around the rocket body	Accurately measure the spacing of your fins around the rocket. Use a fin alignment jig.
	Asymmetric protrusions from rocket body such as camera housings or components of recovery systems such as air flaps.	Try to make any protrusions symmetrical. A protrusion may cause a rocket to both spin and fly in an arc.

Rocket flies in an arc	Cross wind	Fly on less windy days or wait for calmer conditions between wind gusts. Rocket will 'weathercock' into the wind if this is the cause.
	Axially unbalanced	Balance internal components in the rocket. The recovery and payload components should be spread around the axis of the rocket to keep it balanced. [2]
	Misaligned nozzle/ obstruction in nozzle	Ensure the nozzle is aligned with the rocket axis. [3]
	Uneven airflow over rocket due to protrusions such as camera housings, air flaps etc.	Make protrusions from the rocket symmetrical and evenly spaced around the rocket.
	Rocket body is bent	Practice good construction techniques to prevent building a bent rocket. The rocket may also bend due to pressure.

Rocket flies in an arc but when water runs out flies straight	Rocket is marginally stable	Improve rocket stability. Rocket is marginally stable during the water phase.
	Nozzle too small	Increase the nozzle size if possible.
	Too much water	Reduce the amount of water in the rocket.
	Not enough pressure	If it is safe to do so, increase the pressure.

Rocket performs an 'S' or fishtail in flight	Rocket is marginally stable	Rocket may be marginally stable until water runs out. Try to improve stability.
Rocket performs an 'S' or fishtail in flight	Water slosh due to the rocket not pointed vertically on the launch pad	Water may be sloshing in the rocket as its launched causing a sideways motion, and then swings back when the internal wave reflects from the other side of the rocket. This can be caused when the launcher is not levelled properly and the water is sitting against the side of the rocket.

Rocket flies straight normally but veers off course at launch	No or too short guide rail	Increase the length of the guide rail(s). This will allow the rocket to reach stable flight sooner while pointing in the right direction.
	Cross wind	Wait for calmer conditions.
	Unstable launcher	Secure launcher to the ground. The launcher may move when the launch string is pulled inducing a pitching motion on the rocket as it releases.
	Low takeoff acceleration potentially due to:
	Too much water	Reduce the amount of water in the rocket.
	Not enough pressure	Increase the pressure but stay within safe limits.
	Nozzle too small	Increase size of nozzle if possible.
	Rocket is too heavy	Reduce weight of rocket

Rocket flies straight for first part of flight but tips over soon after	Loss of power due to blow through effect	This flight path is more obscure but has been observed in rockets that use narrow Robinson couplings and large nozzles. In this instance some of the air escapes leaving some water in the rocket. Try to match the nozzle size to the size of the Robinson couplings. Use a baffle over the Robinson coupling to prevent blow through.
	Broken fin(s)	A fin may come off during launch or become loose resulting in loss of stability.

Practical Tips

[1] When adding weight to the nosecone, try to use heavy yet soft materials in case the recovery system fails, or the rocket explodes on the pad. Plasticine or modelling clay are good options for this. Increase weight in 20 gram increments so that you add just enough to be stable. Adding too much weight will reduce altitude.
[2] To check if a rocket is axially balanced, stand it up only on its nozzle. If the rocket consistently tips over to one side, then it is not axially balanced. Try to move things like batteries, nose weights, cameras, parachutes around until it is balanced.
[3] To check nozzle alignment, insert tight fitting dowel in the nozzle and see if is aligned with the rocket's centerline.
Don't leave your rockets resting on their fins. They are likely to warp over time and cause the rocket to spin on the next flight.
Sometimes it is hard to see what a rocket is doing in flight. You can videotape the flight and then step through frame by frame to get a better idea.
Adding more than 4 fins to the rocket will not increase stability significantly.
When using restricted nozzles, make the rocket more stable since the rocket will spend more time in flight with a heavier tail.
Try to design your rocket so that take off acceleration is 3G or higher in order to reach stable flight sooner.

If you would like your rocket to fly higher, here are some suggestions for flying higher.