Difference between revisions of "Rocket equation"

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(Relativistic Rocket Equation)
 
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The rocket equation for speeds that become relativistic (~0.1c) is:  
 
The rocket equation for speeds that become relativistic (~0.1c) is:  
  
     <math>\Delta</math>v \over c = ''tanh'' ( {v<sub>e</sub> \over c} ''ln'' R)
+
     <math>\frac{\Delta v}{c} = \tanh(\frac{v_e \ln(m_i/m_f)}{c\sqrt{1-\frac{v_e^2}{c^2}}})</math>
  
 
''tanh'' is the hyperbolic tangent function.
 
''tanh'' is the hyperbolic tangent function.
 
 
 
  
 
==External Links==
 
==External Links==
  
 
*[http://en.wikipedia.org/wiki/Rocket_equation Wikipedia Article: Tsiolkovsky Rocket Equation]
 
*[http://en.wikipedia.org/wiki/Rocket_equation Wikipedia Article: Tsiolkovsky Rocket Equation]

Latest revision as of 14:04, 9 April 2010

The rocket equation is one solution to the momentum method of calculating a rocket's delta v. The equation is:

vf - vi = ve lnR

where:

<math>\Delta</math>v = vf - vi
vf = Final Velocity of Rocket
vi = Initial Velocity of Rocket
ve = Exhaust Velocity of the rocket
R = Mass Ratio of Rocket

lnR is the natural logarithm of R.

Mathematics

The rocket equation is derived from the conservation of momentum.

    <math>\Sigma p_{i} = \Sigma p_{f} = Constant </math>

If mass is exhausted from a rocket engine at some mass flow rate m'e and some exhaust velocity ve, then that exhaust steam carries momentum, manifested as thrust:

    <math>{dp \over dt} = m'_{e} v_{e} = v_{e} {dm \over dt} = m {dv \over dt}</math>

Using the algebraic representation of derivatives, this differential equation can be reduced to:

    <math>dv = v_{e} {dm \over m}</math>

If the exhaust velocity is constant, then the integral of this is the rocket equation.

    <math>\Delta v = \int dv = v_{e} \int {dm \over m} = v_{e} ln (R)</math>

Conservation of momentum can be applied to more propulsion situations than the simple case presented above. For example, under the influence of constant gravity:

    <math>{dp \over dt} = m'_{e} v_{e} + g cos( \theta )</math>

Using the substitutions listed above, the integral of this relationship becomes:

    <math>\Delta v = v_{e} ln (R) + g t cos( /theta )</math>

If the exhaust consists of two separate gas streams whose mass flow rates keep some constant ratio, r, to each other:

    <math>r = {m'_{1} \over m'_{2}}</math>
    <math>v_{avg} = { v_{1} m'_{1} + v_{2} m'_{2} \over m'_{1} + m'_{2}}</math>

Thus:

    <math>v_{avg} = { r v_{1} + v_{2} \over r + 1 }</math>

and vavg can be substituted into the rocket equation rather than ve.

Example

Relativistic Rocket Equation

The rocket equation for speeds that become relativistic (~0.1c) is:

    <math>\frac{\Delta v}{c} = \tanh(\frac{v_e \ln(m_i/m_f)}{c\sqrt{1-\frac{v_e^2}{c^2}}})</math>

tanh is the hyperbolic tangent function.

External Links