Shellcalc©

This paper is adapated from the book “Firework Displays:Explosive Entertainment” by Dr Tom Smith (BPA Secretary) – details of the book and ordering information are available at http://www.fd-ee.com

Shellcalc © has been adopted by the BPA and many other organisations around the World as the de facto means of predicting shell and comet trajectories.  It does not address “normal” fallout but it can be used with other calculations to predict “normal” fallout from aerial fireworks.

Shellcalc^© is a tool developed by John Harradine(1) of Queensland, Australia following investigation into an incident at Bray Park and modified by the author of this book (2) which produces numerical and visual representations of shells and comets in flight for use in planning “safety” distances.

Shellcalc© Standard and Shellcalc© Pro are available at www.shellcalc.co.uk and www.facebook.com/shellcalc

The program, written in Microsoft® Excel for convenience, has been extensively modified and allows inputs as shown in the table below:

Table – Shellcalc^© input parameters

Parameter	Comments
Type of effect   Shell or comet	If “Shell” is selected a burst diameter is calculated and a possible fallout trace is produced. If “Comet” is selected then the projectile is assumed to be consumed during flight
Units	Metric or Imperial
Calibre	Diameter of projectile
Mortar angle	Direction fired from vertical
Muzzle velocity	Entered if known, otherwise standard values used
Fuse delay	Entered if known, otherwise standard values used
Shell mass	Entered if known, otherwise standard values used
Tumbling/mortar drift	Allows a factor to be introduced to simulate barrelling and tumbling effects.  If this is not present a shell fired vertically would land exactly where fired if no wind parameters are entered
Wind speed	At ground level
Relative wind direction	Relative to the firing angle
Elevation of launch site	To allow for changes in atmospheric density etc
Height of launch	Height of launch point above the ground – introduced to allow for firing from buildings and to permit debris to extend below launch height
Terrain category	To accommodate open, wooded or built up areas.

Output from the calculations is produced graphically and in a tabulated form.  A selection of typical outputs is given below.  Note that the depiction of the shell burst diameter may be distorted depending on the axes that the program selects to display the results – the alternative would be a highly extended “y” axis in the majority of cases.

Figure  –  4” (100mm) shell  –  Fired 5 degrees from vertical with low wind

Shellcalc^© can also be used to model the trajectories of comets and this is particularly useful for the risk assessment of firing single shot items from buildings.  In this case Shellcalc^© allows modelling of below horizontal firing and allows modelling of “blow back” onto the building in extreme conditions.

Figure – Comet fired from a building

Taken together the calculated data from “normal” debris distances (which are relatively low hazard but high frequency) and shell failures (which are high hazard but low frequency) allow the display designer to design a display which presents an acceptably low risk, whilst also provides objective criteria for curtailment or cancellation of the display.

We usually work on a normal maximum wind speed of Force 4 – this allows a display to be designed to accommodate the vast majority of conditions that would normally be encountered for displays.  If the wind is stronger than this then it is likely that other safety considerations will need to be considered anyway.

Figure  –  6” (150mm) shell with deliberately extended fuse time (i.e. shell fuse failure) – note the position and extent of the predicted burst

From all the data we have amassed and calculated, and with a knowledge of the material chosen for the display we usually then prepare an objective “contingency” table for the display which details what curtailments will need to be made as a function of wind direction and strength.  Direction is important because it may be that from certain directions an additional risk is posed (i.e. towards the audience or other hazards) but if the wind is blowing in the opposite direction then a greater wind strength can be tolerated before curtailment or cancellation is necessary.