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Rocket Equations

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Rocket Thrust Equation

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The Rocket Thrust Equation is given by

F=m˙ve+(pep0)Ae,F = \dot{m} v_e + (p_e - p_0) A_e,
where m˙\dot{m} is the mass flow rate, vev_e is the exhaust velocity, pep_e is the exhaust pressure, p0p_0 is the ambient pressure, and AeA_e is the exhaust area. It is used to determine the thrust produced by a rocket engine.

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Specific Impulse Equation

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The Specific Impulse Equation is given by

Isp=Fm˙g0,I_{sp} = \frac{F}{\dot{m} g_0},
where IspI_{sp} is the specific impulse, FF is the thrust, m˙\dot{m} is the mass flow rate, and g0g_0 is the standard gravity. It's a measure of the efficiency of rocket and jet engines.

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Burnout Velocity Equation

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The Burnout Velocity Equation is determined by combining the Tsiolkovsky rocket equation with the initial mass, propellant mass, and final mass:

vb=veln(mimimp),v_b = v_e \ln\left(\frac{m_i}{m_i-m_p}\right),
where vbv_b is the burnout velocity, vev_e is the exhaust velocity, mim_i is the initial mass, and mpm_p is the propellant mass. It is used to calculate the velocity a rocket reaches after it has consumed all its propellant.

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Delta-V Budget

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Delta-V Budget is estimated via the summation

Δvtotal=Δvi,\Delta v_{total} = \sum{\Delta v_i},
where Δvtotal\Delta v_{total} is the total change in velocity needed for a mission, and Δvi\Delta v_i are the individual velocity changes for each segment of the trajectory. It is critical for mission planning and fuel requirements.

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Breguet Range Equation for Rockets

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Similar to its use in aircraft, the Breguet Range Equation for Rockets can be expressed as

R=veg0ln(m0mf)LD,R = \frac{v_e}{g_0} \ln\left(\frac{m_0}{m_f}\right) \cdot \frac{L}{D},
where RR is the range, vev_e is the exhaust velocity, g0g_0 is standard gravity, LL is the lift, DD is the drag, and m0m_0 and mfm_f are the initial and final mass. It is useful for estimating the range a rocket can achieve given certain parameters.

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Ideal Rocket Equation

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The Ideal Rocket Equation is identical to the Tsiolkovsky Rocket Equation and is given by

Δv=Ispg0ln(m0mf),\Delta v = I_{sp} g_0 \ln\left(\frac{m_0}{m_f}\right),
where IspI_{sp} is the specific impulse, g0g_0 is standard gravity. It's used to calculate the change in velocity in terms of specific impulse.

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Continuous Thrust Trajectory Equation

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For continuous thrust trajectories, the spacecraft's velocity change is given by

v(t)=v0+Fm(tt0),v(t) = v_0 + \frac{F}{m}(t-t_0),
where v(t)v(t) is the velocity at time tt, v0v_0 is the initial velocity, FF is the constant thrust, mm is the spacecraft mass, and t0t_0 is the initial time. This equation can be used for estimating flight trajectories under constant thrust acceleration.

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Tsiolkovsky Rocket Equation

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The Tsiolkovsky rocket equation, given by

Δv=veln(m0mf),\Delta v = v_e \ln\left(\frac{m_0}{m_f}\right),
describes the maximum change in velocity a rocket can achieve without external forces. It relates the mass of propellant, exhaust velocity, and the initial and final mass of the rocket.

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Thrust-to-Weight Ratio

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The Thrust-to-Weight Ratio is calculated by

TWR=Fmg,TWR = \frac{F}{m g},
where TWRTWR is the thrust-to-weight ratio, FF is the thrust, mm is the mass of the object, and gg is the acceleration due to gravity. It indicates whether a rocket can overcome gravitational forces to lift off.

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Mass Ratio Equation

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The Mass Ratio Equation is given by

R=m0mf,R = \frac{m_0}{m_f},
where RR is the mass ratio, m0m_0 is the initial mass, and mfm_f is the final mass of the rocket. It's used to evaluate the proportion of mass that is propellant versus payload and structure.

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Spacecraft Maneuvering Equation

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The Spacecraft Maneuvering Equation, for a small maneuver, is given by

Δv=aΔt,\Delta v = a \cdot \Delta t,
where Δv\Delta v is the change in velocity, aa is the acceleration provided by the spacecraft's engines, and Δt\Delta t is the duration of the burn. This is a simplified version used for quick calculations of velocity change due to short thrusts.

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Gravity Drag Equation

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The Gravity Drag Equation is given by

Δvgd=g0tburn,\Delta v_{gd} = g_0 t_{burn},
where Δvgd\Delta v_{gd} is the change in velocity due to gravity drag, g0g_0 is standard gravity, and tburnt_{burn} is the burn time. It accounts for the velocity loss in a rocket due to gravity.

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Payload Fraction Equation

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The Payload Fraction Equation is given by

λ=mpayloadm0,\lambda = \frac{m_{payload}}{m_0},
where λ\lambda is the payload fraction, mpayloadm_{payload} is the payload mass, and m0m_0 is the initial total mass. This metric helps in the design and efficiency analysis of the rocket.

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Exhaust Velocity Equation

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The Exhaust Velocity Equation, derived from thermodynamic principles, is

ve=2kTcM,v_e = \sqrt{\frac{2\cdot k\cdot T_c}{M}},
where vev_e is the exhaust velocity, kk is the specific heat ratio, TcT_c is the combustion temperature, and MM is the molar mass of the exhaust gases. It is used to determine the velocity of gas exiting a rocket nozzle.

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Oberth Effect Formula

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The Oberth Effect Formula can be quantified by the relation

ΔE=Δvv,\Delta E = \Delta v \cdot v,
where ΔE\Delta E is the change in kinetic energy, Δv\Delta v is the rocket's change in velocity, and vv is the velocity prior to the burn. Exploits the fact that burning propellant at higher velocities produces more useful energy.

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