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IGNITER.XLS
MS EXCEL 2000 Spreadsheet
Written by R.Nakka Oct..1999, Release 1.00
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Parachute addition by D. Pennington, Mar. 2002
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Latest Release 1.1
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Introduction
For free distribution
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This spreadsheet may be used to estimate rocket motor chamber pressure, or parachute chamber
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pressure that results from the combustion of a pyrotechnic igniter charge.
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The igniter charge is typically Black Powder (BP), although this spreadsheet may be
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used for any pyrotechnic mixture, as long as the charge mass density and impetus* is known.
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Values for impetus for a few mixtures are given in Table 1.
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For BP, the mass density may be determined using the BP worksheet. Either mass ratios or
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volumetric ratios of potassium nitrate (KN), sulphur (S) or charcoal (Ch) may be used as input.
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Rather than inputting igniter charge mass, the igniter volume is required, from which the mass
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is calculated based upon the density. This would seem to be a more useful approach, as
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the volume occupied by an igniter is often a limiting design parameter.
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The method is based on the NASA report SP-8051 Solid Rocket Motor Igniters,and
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makes the reasonable assumption that the products of combustion behave as an ideal gas,
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with the ideal gas equation (PV=mRT) being applicable. Combustion is assumed to be
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complete and adiabatic, with the assumed combustion time being brief, such that no
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pressure loss (e.g. through the nozzle throat) occurs.
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Table 1
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MaterialGrade
Impetus (ft-lbf/lbm)
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Black Powder
Igniter (note 1)
100 000Note 1
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Black Powder
Commercial FFFF
85 000Note 2
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Black Powder
User prepared
50 000 - 70 000
Note 3
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KN-Sucrose powder
User prepared
110 000Note 4
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* Impetus is represented by the term RT in the ideal gas equation, where R is the
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gas constant, and T is the combustion temperature.
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Impetus of a propellant or pyrotechnic mixture represents available energy per unit mass.
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This may be determined for any pyrotechnic mixture by use of a chemical equilibrium
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program such as one of the PEP programs (PROPEP, GUIPEP, CET, etc.)
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Notes:
1. This value is taken from NASA SP-8051, Section 2.1.4. (seems high).
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2. Calculated, based on KN/Ch/S ratio of 74/15.6/10.4
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using GUIPEP.
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3. Estimated
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4. Calculated, KN/Sucrose ratio of 65/35, using GUIPEP. This mixture is
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energetic, but requires generous heat input to initiate. An exploding
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bridgewire (EBW) initiator may be more suitable than a resistive element.
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