Tsiolkovsky rocket equation

The Tsiolkovsky rocket equation, classical rocket equation, or ideal rocket equation is a mathematical equation that describes the motion of vehicles that follow the basic principle of a rocket: a device that can apply acceleration to itself using thrust by expelling part of its mass with high velocity can thereby move due to the conservation of momentum.
where:
History.
The equation is named after Russian scientist Konstantin Tsiolkovsky () who independently derived it and published it in his 1903 work.
The equation had been derived earlier by the British mathematician William Moore in 1810, and later published in a separate book in 1813. The minister William Leitch, who was a capable scientist, also independently derived the fundamentals of rocketry in 1861.
Robert Goddard in America independently developed the equation in 1912 when he began his research to improve rocket engines for possible space flight. Hermann Oberth in Europe independently derived the equation about 1920 as he studied the feasibility of space travel.
While the derivation of the rocket equation is a straightforward calculus exercise, Tsiolkovsky is honored as being the first to apply it to the question of whether rockets could achieve speeds necessary for space travel.
Derivation.
Most popular derivation.
Consider the following system:
In the following derivation, "the rocket" is taken to mean "the rocket and all of its unburned propellant".
Newton's second law of motion relates external forces (formula_9) to the change in linear momentum of the whole system (including rocket and exhaust) as follows:
where formula_11 is the momentum of the rocket at time formula_12:
and formula_14 is the momentum of the rocket and exhausted mass at time formula_15:
and where, with respect to the observer:
The velocity of the exhaust formula_17 in the observer frame is related to the velocity of the exhaust in the rocket frame formula_18 by (since exhaust velocity is in the negative direction)
Solving yields:
and, using formula_21, since ejecting a positive formula_22 results in a decrease in mass,
If there are no external forces then formula_24 (conservation of linear momentum) and
Assuming formula_26 is constant, this may be integrated as follows:
This then yields
or equivalently
formula_30      or      
formula_31
where formula_3 is the initial total mass including propellant, formula_4 the final mass, and formula_18 the velocity of the rocket exhaust with respect to the rocket (the specific impulse, or, if measured in time, that multiplied by gravity-on-Earth acceleration).
The value formula_35 is the total working mass of propellant expended.
formula_36 (delta v) is the integration over time of the magnitude of the acceleration produced by using the rocket engine (what would be the actual acceleration if external forces were absent). In free space, for the case of acceleration in the direction of the velocity, this is the increase of the speed. In the case of an acceleration in opposite direction (deceleration) it is the decrease of the speed. Of course gravity and drag also accelerate the vehicle, and they can add or subtract to the change in velocity experienced by the vehicle. Hence delta-v may not always be the actual change in speed or velocity of the vehicle.
Other derivations.
Impulse-based.
The equation can also be derived from the basic integral of acceleration in the form of force (thrust) over mass.
By representing the delta-v equation as the following:
where T is thrust, formula_3 is the initial (wet) mass and formula_22 is the initial mass minus the final (dry) mass,
and realising that the integral of a resultant force over time is total impulse, assuming thrust is the only force involved,
The integral is found to be:
Realising that impulse over the change in mass is equivalent to force over propellant mass flow rate (p), which is itself equivalent to exhaust velocity,
the integral can be equated to
Acceleration-based.
Imagine a rocket at rest in space with no forces exerted on it (Newton's First Law of Motion). From the moment its engine is started (clock set to 0) the rocket expels gas mass at a "constant mass flow rate R" (kg/s) and at "exhaust velocity relative to the rocket ve" (m/s). This creates a constant force "F" propelling the rocket that is equal to "R × ve". The rocket is subject to a constant force, but its total mass is decreasing steadily because it is expelling gas. According to Newton's Second Law of Motion, its acceleration at any time "t" is its propelling force "F" divided by its current mass "m":
Now, the mass of fuel the rocket initially has on board is equal to "m0 – mf". For the constant mass flow rate "R" it will therefore take a time "T = (m0 – mf)/R" to burn all this fuel. Integrating both sides of the equation with respect to time from "0" to "T" (and noting that "R = dm/dt" allows a substitution on the right), we obtain
Limit of finite mass "pellet" expulsion.
The rocket equation can also be derived as the limiting case of the speed change for a rocket that expels its fuel in the form of formula_46 pellets consecutively, as formula_47, with an effective exhaust speed formula_48 such that the mechanical energy gained per unit fuel mass is given by formula_49.
Let formula_50 be the initial fuel mass fraction on board and formula_51
the initial fueled-up mass of the rocket. Divide the total mass of fuel formula_52 into formula_46 discrete pellets each of mass formula_54. From momentum conservation when ejecting the formula_55'th pellet, the overall speed change can be shown to be the sum
Notice that for large formula_46 the last term in the denominator formula_58 and can be neglected to give
As formula_62 this Riemann sum becomes the definite integral
Special relativity.
If special relativity is taken into account, the following equation can be derived for a relativistic rocket, with formula_65 again standing for the rocket's final velocity (after expelling all its reaction mass and being reduced to a rest mass of formula_66) in the inertial frame of reference where the rocket started at rest (with the rest mass including fuel being formula_3 initially), and formula_68 standing for the speed of light in a vacuum:
Writing formula_70 as formula_71 allows this equation to be rearranged as
Then, using the identity formula_73 (here "exp" denotes the exponential function; "see also" Natural logarithm as well as the "power" identity at Logarithmic identities) and the identity formula_74 ("see" Hyperbolic function), this is equivalent to
Terms of the equation.
Delta-"v".
Delta-"v" (literally "change in velocity"), symbolised as Δ"v" and pronounced "delta-vee", as used in spacecraft flight dynamics, is a measure of the impulse that is needed to perform a maneuver such as launching from, or landing on a planet or moon, or an in-space orbital maneuver. It is a scalar that has the units of speed. As used in this context, it is "not" the same as the physical change in velocity of the vehicle.
Delta-"v" is produced by reaction engines, such as rocket engines and is proportional to the thrust per unit mass, and burn time, and is used to determine the mass of propellant required for the given manoeuvre through the rocket equation.
For multiple manoeuvres, delta-"v" sums linearly.
For interplanetary missions delta-"v" is often plotted on a porkchop plot which displays the required mission delta-"v" as a function of launch date.
Mass fraction.
In aerospace engineering, the propellant mass fraction is the portion of a vehicle's mass which does not reach the destination, usually used as a measure of the vehicle's performance. In other words, the propellant mass fraction is the ratio between the propellant mass and the initial mass of the vehicle. In a spacecraft, the destination is usually an orbit, while for aircraft it is their landing location. A higher mass fraction represents less weight in a design. Another related measure is the payload fraction, which is the fraction of initial weight that is payload.
Effective exhaust velocity.
The effective exhaust velocity is often specified as a specific impulse and they are related to each other by:
where
Applicability.
The rocket equation captures the essentials of rocket flight physics in a single short equation. It also holds true for rocket-like reaction vehicles whenever the effective exhaust velocity is constant, and can be summed or integrated when the effective exhaust velocity varies. The rocket equation only accounts for the reaction force from the rocket engine; it does not include other forces that may act on a rocket, such as aerodynamic or gravitational forces. As such, when using it to calculate the propellant requirement for launch from (or powered descent to) a planet with an atmosphere, the effects of these forces must be included in the delta-V requirement (see Examples below). In what has been called "the tyranny of the rocket equation", there is a limit to the amount of payload that the rocket can carry, as higher amounts of propellant increment the overall weight, and thus also increase the fuel consumption. The equation does not apply to non-rocket systems such as aerobraking, gun launches, space elevators, launch loops, tether propulsion or light sails.
The rocket equation can be applied to orbital maneuvers in order to determine how much propellant is needed to change to a particular new orbit, or to find the new orbit as the result of a particular propellant burn. When applying to orbital maneuvers, one assumes an impulsive maneuver, in which the propellant is discharged and delta-v applied instantaneously. This assumption is relatively accurate for short-duration burns such as for mid-course corrections and orbital insertion maneuvers. As the burn duration increases, the result is less accurate due to the effect of gravity on the vehicle over the duration of the maneuver. For low-thrust, long duration propulsion, such as electric propulsion, more complicated analysis based on the propagation of the spacecraft's state vector and the integration of thrust are used to predict orbital motion.
Examples.
Assume an exhaust velocity of and a formula_65 of (Earth to LEO, including formula_65 to overcome gravity and aerodynamic drag).
Stages.
In the case of sequentially thrusting rocket stages, the equation applies for each stage, where for each stage the initial mass in the equation is the total mass of the rocket after discarding the previous stage, and the final mass in the equation is the total mass of the rocket just before discarding the stage concerned. For each stage the specific impulse may be different.
For example, if 80% of the mass of a rocket is the fuel of the first stage, and 10% is the dry mass of the first stage, and 10% is the remaining rocket, then
With three similar, subsequently smaller stages with the same formula_18 for each stage, we have
and the payload is 10% × 10% × 10% = 0.1% of the initial mass.
A comparable SSTO rocket, also with a 0.1% payload, could have a mass of 11.1% for fuel tanks and engines, and 88.8% for fuel. This would give
If the motor of a new stage is ignited before the previous stage has been discarded and the simultaneously working motors have a different specific impulse (as is often the case with solid rocket boosters and a liquid-fuel stage), the situation is more complicated.
Common misconceptions.
When viewed as a variable-mass system, a rocket cannot be directly analyzed with Newton's second law of motion because the law is valid for constant-mass systems only. It can cause confusion that the Tsiolkovsky rocket equation looks similar to the relativistic force equation formula_91. Using this formula with formula_92 as the varying mass of the rocket seems to derive the Tsiolkovsky rocket equation, but this derivation is not correct. Notice that the effective exhaust velocity formula_18 does not even appear in this formula.