last updated: 5th September 2017 - Day 190 - Polaron G3 and Drone Mods

Safety First

Search

Site Index

Tutorials

Articles

Rocket Gallery

Labs

Where To Buy

10 Challenges

Links

Blog

Glossary

Contact Us

About


Construction - Basic

Body

Ring Fins

Flat Fins

Nozzle

Nosecone

Construction - Advanced

Robinson Coupling

Splicing Bottles #1

Splicing Bottles AS#5

Reinforcing Bottles

Side Deploy #1

Side Deploy #2

Mk3 Staging Mechanism

Multi-stage Parachutes

Fairings

Construction - Launchers

Gardena Launcher

Clark Cable-tie

Medium Launcher

Cluster Launcher

Launch Abort Valve

Quick Launcher

How It Works

Drop Away Boosters

Katz Stager Mk2.

Katz Stager Mk3.

DetMech

Dark Shadow Deployment

Articles

Recovery Guide

Parachutes

How Much Water?

Flying Higher

Flying Straight

Building a Launcher

Using Scuba Tanks

Nozzles

Video Taping Tips

MD-80 clone

Making Panoramas

Procedures

Burst Testing

Filling

Launching

Recovery

Flight Computer

Servo Timer II

V1.6

V1.5

V1.4

V1.3, V1.3.1, V1.3.2

V1.2

Deploy Timer 1.1

Project Builds

The Shadow

Shadow II

Inverter

Polaron G2

Dark Shadow

L1ght Shadow

Flight Log Updates

#190 - Polaron G3

#189 - Casual Flights

#188 - Skittles Part #2

#187 - Skittles Part #1

#186 - Level 1 HPR

#185 - Liquids in Zero-G

#184 - More Axion G6

#183 - Axion G6

#182 - Casual Flights

#181 - Acoustic Apogee 2

#180 - Light Shadow

#179 - Stratologger

#178 - Acoustic Apogee 1

#177 - Reefing Chutes

#176 - 10 Years

#175 - NSWRA Events

#174 - Mullaley Launch

#173 - Oobleck Rocket

#172 - Coming Soon

#171 - Measuring Altitude

#170 - How Much Water?

#169 - Windy

#168 - Casual Flights 2

#167 - Casual Flights

#166 - Dark Shadow II

#165 - Liquid Density 2

#164 - Liquid Density 1

#163 - Channel 7 News

#162 - Axion and Polaron

#161 - Fog and Boom

#1 to #160 (Updates)

 

Flying Straight

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.
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.

Back To Top
 



Copyright © 2006-2017 Air Command Water Rockets

Total page hits since 1 Aug 2006:

George Katz - Google Plus