None of the following parameters are included in the novice version.

With the exception of Atmospheric Pressure, each of the variables has shortcuts to commonly used values. For example, Gamma of Gas in Rocket can be typed in as any value but by pressing a letter corresponding to the Gas required (a for Air ... e for CO₂) the value will be inserted into the appropriate variable field.

The Gamma and Density of Gas in Rocket are linked in that, if you make the letter you press into an uppercase, it will insert the values for both Gamma and Density. So, by pressing E, the Gamma and Density for CO₂ will be inserted into the appropriate fields.

The valid letters are displayed in the help panel on the upper right of the model.

	Gamma of Gas in Rocket This is a measure of the compressibility of the gas. The number itself (Gamma) is derived from the ratio of the specific heat capacities of the compressed gas at constant pressure and constant volume thus . . .   Gamma = Cp   Cv Therefore, Gamma can be measured in the laboratory using simple equipment that gives real-life numbers rather than fiddle-factors. You can look up the values of Gamma in any useful physical constants book but just in case you haven't got one to hand and you are thinking about trying out some other gasses in the rocket, here are some Gammas of common gasses - remember to be careful with gasses and familiarise yourself with any appropriate safety data sheets or other information.	Gamma of gas in rocket Gas Gamma air 1.402 Ar 1.667 He 1.66 C2H4 1.26 CO2 1.304

	Density of Gas in Rocket (kg/m3) Each gas has a different density - proportional to the molecular weight of the gas or gas mixture in question. The density has an effect on the amount of push it can give the rocket when it is expelled at the end of the water thrust phase. On the right are the densities of the same set of gasses as above.	Density of gas in rocket Gas Density (kg/m3) air 1.293 Ar 1.784 He 0.179 C2H4 1.260 CO2 1.977

	Density of Liquid in Rocket (kg/m3) You are not confined to using water in the rocket although it would make most sense to limit yourself to water. The density of the liquid in the rocket affects the push on the rocket during this phase of thrust. The densities of some common liquids are listed on the right.	Density of liquid in rocket Liquid Density (kg/m3) Water 998 Sea Water 1025 Hg 13546 Olive oil 920 CCl4 1632 Br 3100

	Acceleration due to Gravity (m/s2) Gravity (g) affects the rocket in that it provides a fairly constant force that tries to bring the rocket back to earth (or whichever planetary body you decided to launch it from). For most purposes, g is 9.81m/s2 but if you want to be pedantic, standard earth gravity is 9.80665m/s2 and, for those who want to be even more pedantic there is a formula that works for heights above sea-level that are reasonable on the Earth's surface (acceleration due to gravity is inversely proportional to the square of the distance from the centre but with Mt. Everest being only about 5 miles tall and roughly 4,000 miles from the Earth's centre, it is proportional for the purposes of this equation) . . .   g /ms-2   =   9.80616 - 0.025928cos2L + 0.000069cos²2L - 0.000003h         where:     h   =   height above sea-level in metres; and,         L   =   latitude. If you wish to experiment with launches off different planets/moons and so on, here are the figures for this solar system . . .	Some places to launch Body Acceleration (m/s2) Sun 274 Moon 1.62 Mercury 3.76 Venus 8.77 Earth 9.81 Mars 3.80 Jupiter 24.9 Saturn 10.4 Uranus 10.4 Neptune 13.8 Pluto 4

	Atmospheric Pressure (mBar) This affects the pressure along the nozzle and the amount of drag on the rocket (see Density of Air below). Standard air pressure is 101325 Nm-2 or 1013.25 mBar. I have used mBar as the air pressure unit in the model because it is a value that is available to us all in weather reports The air pressure is less at greater altitudes and the air pressures that you see on weather maps are all converted to atmospheric pressure at sea level. If your launch site is on raised ground, you can take this into account with the following table which uses standard atmospheric pressure at sea level. Taking into account the fact that there are plenty of out of control factors in any launch, a rough estimate can be made by taking the pressure from the weather report and multiplying it by the factor for your altitude in the table on the right to give you the pressure at the launch. eg, weather report says 998 mBar, you are at 240 metres and the factor for 250 metres is 0.9707 therefore . . . 998 x 0.9707 = 969 mBar (or, in practical terms, 970 mBar) Note that a millibar is 1/1,000th of a Bar and 1 Bar is 100,000 Nm-2	Pressure against Height Geometric Height /m Pressure /mBar Factor 0 1013.25 1.0 250 983.58 0.9707 500 954.61 0.9421 750 926.35 0.9142 1,000 898.76 0.9768 1,500 845.60 0.8345 2,000 798.01 0.7876 2,500 749.92 0.7401 3,000 701.21 0.6920

Density of Air at STP (kg/m3) This is the density of the gas on the outside of the rocket at Standard Temperature and Pressure. This figure affects the amount of drag experienced by the rocket - the higher the density, the more force is required to move the air out of the way during flight. If you are launching in a low pressure atmosphere such as on the Moon, use the density of the gas (the solar wind - mainly hydrogen) at STP (see the table on the right) and put the lower Atmospheric Pressure in, in the previous data field. The model will give the rocket the correct drag for the reduced pressure.

Some common atmospheres Atmosphere Density (kg/m3) air 1.293 NH3 0.771 He 0.179 H2 0.090 CH4 0.717 SO2 2.927 CO2 1.977

Copyright ©2000 Paul Grosse. All Rights Reserved.