Delta-v budget

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Delta-v in feet per second, and fuel requirements for a typical Apollo Lunar Landing mission. ApolloEnergyRequirementsMSC1966.png
Delta-v in feet per second, and fuel requirements for a typical Apollo Lunar Landing mission.

In astrodynamics and aerospace, a delta-v budget is an estimate of the total change in velocity (delta-v) required for a space mission. It is calculated as the sum of the delta-v required to perform each propulsive maneuver needed during the mission. As input to the Tsiolkovsky rocket equation, it determines how much propellant is required for a vehicle of given empty mass and propulsion system.

Contents

Delta-v is a scalar quantity dependent only on the desired trajectory and not on the mass of the space vehicle. For example, although more fuel is needed to transfer a heavier communication satellite from low Earth orbit to geosynchronous orbit than for a lighter one, the delta-v required is the same. Delta-v is also additive, as contrasted to rocket burn time, the latter having greater effect later in the mission when more fuel has been used up.

Tables of the delta-v required to move between different space regime are useful in the conceptual planning of space missions. In the absence of an atmosphere, the delta-v is typically the same for changes in orbit in either direction; in particular, gaining and losing speed cost an equal effort. An atmosphere can be used to slow a spacecraft by aerobraking.

A typical delta-v budget might enumerate various classes of maneuvers, delta-v per maneuver, and number of each maneuver required over the life of the mission, then simply sum the total delta-v, much like a typical financial budget. Because the delta-v needed to achieve the mission usually varies with the relative position of the gravitating bodies, launch windows are often calculated from porkchop plots that show delta-v plotted against the launch time.

General principles

The Tsiolkovsky rocket equation shows that the delta-v of a rocket (stage) is proportional to the logarithm of the fuelled-to-empty mass ratio of the vehicle, and to the specific impulse of the rocket engine. A key goal in designing space-mission trajectories is to minimize the required delta-v to reduce the size and expense of the rocket that would be needed to successfully deliver any particular payload to its destination.

The simplest delta-v budget can be calculated with Hohmann transfer, which moves from one circular orbit to another coplanar circular orbit via an elliptical transfer orbit. In some cases a bi-elliptic transfer can give a lower delta-v.

Hohmann transfer orbit, labelled 2, from an orbit (1) to a higher orbit (3). This is a very commonly used maneuver between orbits. Hohmann transfer orbit.svg
Hohmann transfer orbit, labelled 2, from an orbit (1) to a higher orbit (3). This is a very commonly used maneuver between orbits.

A more complex transfer occurs when the orbits are not coplanar. In that case there is an additional delta-v necessary to change the plane of the orbit. The velocity of the vehicle needs substantial burns at the intersection of the two orbital planes and the delta-v is usually extremely high. However, these plane changes can be almost free in some cases if the gravity and mass of a planetary body are used to perform the deflection[ citation needed ]. In other cases, boosting up to a relatively high altitude apoapsis gives low speed before performing the plane change, thus requiring lower total delta-v.

The slingshot effect can be used to give a boost of speed/energy; if a vehicle goes past a planetary or lunar body, it is possible to pick up (or lose) some of that body's orbital velocity relative to the sun or another planet.

Another effect is the Oberth effect—this can be used to greatly decrease the delta-v needed, because using propellant at low potential energy/high speed multiplies the effect of a burn. Thus for example the delta-v for a Hohmann transfer from Earth's orbital radius to Mars's orbital radius (to overcome the sun's gravity) is many kilometres per second, but the incremental burn from low Earth orbit (LEO) over and above the burn to overcome Earth's gravity is far less if the burn is done close to Earth than if the burn to reach a Mars transfer orbit is performed at Earth's orbit, but far away from Earth.

A less used effect is low energy transfers. These are highly nonlinear effects that work by orbital resonances and by choosing trajectories close to Lagrange points. They can be very slow, but use very little delta-v.

Because delta-v depends on the position and motion of celestial bodies, particularly when using the slingshot effect and Oberth effect, the delta-v budget changes with launch time. These can be plotted on a porkchop plot.

Course corrections usually also require some propellant budget. Propulsion systems never provide precisely the right propulsion in precisely the right direction at all times, and navigation also introduces some uncertainty. Some propellant needs to be reserved to correct variations from the optimum trajectory.

Budget

Delta-v map of selected bodies in the solar system, assuming burns are at periapsis, and gravity assist and inclination changes are ignored (full size) Solar system delta v map.svg
Delta-v map of selected bodies in the solar system, assuming burns are at periapsis, and gravity assist and inclination changes are ignored (full size)

Launch/landing

The delta-v requirements for sub-orbital spaceflight are much lower than for orbital spaceflight. For the Ansari X Prize altitude of 100 km, Space Ship One required a delta-v of roughly 1.4 km/s. To reach the initial low Earth orbit of the International Space Station of 300 km (now 400 km), the delta-v is over six times higher, about 9.4 km/s. Because of the exponential nature of the rocket equation the orbital rocket needs to be considerably bigger.

Earth–Moon space—high thrust

Delta-v needed to move inside the Earth–Moon system (speeds lower than escape velocity) are given in km/s. This table assumes that the Oberth effect is being used—this is possible with high thrust chemical propulsion but not with current (as of 2018) electrical propulsion.

The return to LEO figures assume that a heat shield and aerobraking/aerocapture are used to reduce the speed by up to 3.2 km/s. The heat shield increases the mass, possibly by 15%. Where a heat shield is not used the higher "from LEO" Delta-v figure applies. The extra propellant needed to replace the aerobraking is likely to be heavier than a heat shield. LEO-Ken refers to a low Earth orbit with an inclination to the equator of 28 degrees, corresponding to a launch from Kennedy Space Center. LEO-Eq is an equatorial orbit.[ citation needed ]

qLEO-KenLEO-EqGEOEML-1EML-2 [1] EML-4/5LLOMoonC3=0
Earth 9.3–10
Low Earth orbit (LEO-Ken)4.244.333.773.433.974.045.933.22
Low Earth orbit (LEO-Eq)4.243.903.773.433.994.045.933.22
Geostationary orbit (GEO)2.061.631.381.471.712.053.921.30
Lagrangian point 1 (EML-1)0.770.771.380.140.330.642.520.14
Lagrangian point 2 (EML-2)0.330.331.470.140.340.642.520.14
Lagrangian point 4/5 (EML-4/5)0.840.981.710.330.340.982.580.43
Low lunar orbit (LLO)0.900.902.050.640.650.981.871.40
Moon surface 2.742.743.922.522.532.581.872.80
Earth escape velocity (C3=0)001.300.140.140.431.402.80

Earth–Moon space—low thrust

Current electric ion thrusters produce a very low thrust (milli-newtons, yielding a small fraction of a g), so the Oberth effect cannot normally be used. This results in the journey requiring a higher delta-v and frequently a large increase in time compared to a high thrust chemical rocket. Nonetheless, the high specific impulse of electrical thrusters may significantly reduce the cost of the flight. For missions in the Earth–Moon system, an increase in journey time from days to months could be unacceptable for human space flight, but differences in flight time for interplanetary flights are less significant and could be favorable.

The table below presents delta-v's in km/s, normally accurate to 2 significant figures and will be the same in both directions, unless aerobraking is used as described in the high thrust section above. [2]

FromToDelta-v (km/s)
Low Earth orbit (LEO)Earth–Moon Lagrangian 1 (EML-1)7.0
Low Earth orbit (LEO)Geostationary Earth orbit (GEO)6.0
Low Earth orbit (LEO)Low Lunar orbit (LLO)8.0
Low Earth orbit (LEO)Sun–Earth Lagrangian 1 (SEL-1)7.4
Low Earth orbit (LEO)Sun–Earth Lagrangian 2 (SEL-2)7.4
Earth–Moon Lagrangian 1 (EML-1)Low Lunar orbit (LLO)0.60–0.80
Earth–Moon Lagrangian 1 (EML-1)Geostationary Earth orbit (GEO)1.4–1.75
Earth–Moon Lagrangian 1 (EML-1)Sun-Earth Lagrangian 2 (SEL-2)0.30–0.40

[2]

Earth Lunar Gateway—high thrust

The Lunar Gateway space station is planned to be deployed in a highly elliptical seven-day near-rectilinear halo orbit (NRHO) around the Moon. Spacecraft launched from Earth would perform a powered flyby of the Moon followed by a NRHO orbit insertion burn to dock with the Gateway as it approaches the apoapsis point of its orbit. [3]

FromToDelta-v (km/s)
Low Earth orbit (LEO)Trans-Lunar Injection (TLI)3.20
Trans-Lunar Injection (TLI)Low (polar) lunar orbit (LLO)0.90
Trans-Lunar Injection (TLI)Lunar Gateway0.43
Lunar GatewayLow (polar) lunar orbit0.73
Low (polar) lunar orbitLunar Gateway0.73
Lunar GatewayEarth Interface (EI)0.41

[3]

Interplanetary

The spacecraft is assumed to be using chemical propulsion and the Oberth effect.

FromToDelta-v (km/s)
LEO Mars transfer orbit4.3 [4] ("typical", not minimal)
Earth escape velocity (C3=0) Mars transfer orbit 0.6 [5]
Mars transfer orbit Mars capture orbit 0.9 [5]
Mars capture orbit Deimos transfer orbit 0.2 [5]
Deimos transfer orbit Deimos surface0.7 [5]
Deimos transfer orbit Phobos transfer orbit 0.3 [5]
Phobos transfer orbit Phobos surface0.5 [5]
Mars capture orbit Low Mars orbit1.4 [5]
Low Mars orbit Mars surface4.1 [5]
Earth–Moon Lagrange point 2 Mars transfer orbit<1.0 [4]
Mars transfer orbitLow Mars orbit2.7 [4] (not minimal)
Earth escape velocity (C3=0)Closest NEO [6] 0.8–2.0

According to Marsden and Ross, "The energy levels of the Sun–Earth L1 and L2 points differ from those of the Earth–Moon system by only 50 m/s (as measured by maneuver velocity)." [7]

We may apply the formula

(where μ = GM is the standard gravitational parameter of the sun, see Hohmann transfer orbit) to calculate the Δv in km/s needed to arrive at various destinations from Earth (assuming circular orbits for the planets, and using perihelion distance for Pluto). In this table, the column labeled "Δv to enter Hohmann orbit from Earth's orbit" gives the change from Earth's velocity to the velocity needed to get on a Hohmann ellipse whose other end will be at the desired distance from the sun. The column labeled "v exiting LEO" gives the velocity needed (in a non-rotating frame of reference centred on Earth) when 300 km above Earth's surface. This is obtained by adding to the specific kinetic energy the square of the speed (7.73 km/s) of this low Earth orbit (that is, the depth of Earth's gravity well at this LEO). The column "Δv from LEO" is simply the previous speed minus 7.73 km/s. The transit time is calculated as years.

Note that the values in the table only give the Δv needed to get to the orbital distance of the planet. The speed relative to the planet will still be considerable, and in order to go into orbit around the planet either aerocapture is needed using the planet's atmosphere, or more Δv is needed.

DestinationOrbital radius
(AU)
Δv to enter Hohmann orbit
from Earth's orbit
Δv
exiting LEO
Δv
from LEO
Transit time
Sun 029.831.724.02.1 months
Mercury 0.397.513.35.53.5 months
Venus 0.722.511.23.54.8 months
Mars 1.522.911.33.68.5 months
Jupiter 5.28.814.06.32.7 years
Saturn 9.5410.315.07.36.0 years
Uranus 19.1911.315.78.016.0 years
Neptune 30.0711.716.08.230.6 years
Pluto 29.66 (perih.)11.616.08.245.5 years
1 light-year 63,24112.316.58.82.8 million years

The New Horizons space probe to Pluto achieved a near-Earth speed of over 16 km/s which was enough to escape from the sun. (It also got a boost from a fly-by of Jupiter.)

To get to the sun, it is actually not necessary to use a Δv of 24 km/s. One can use 8.8 km/s to go very far away from the sun, then use a negligible Δv to bring the angular momentum to zero, and then fall into the sun. This can be considered a sequence of two Hohmann transfers, one up and one down. Also, the table does not give the values that would apply when using the moon for a gravity assist. There are also possibilities of using one planet, like Venus which is the easiest to get to, to assist getting to other planets or the sun. The Galileo spacecraft used Venus once and Earth twice in order to reach Jupiter. The Ulysses solar probe used Jupiter to attain polar orbit around the sun.

Delta-vs between Earth, Moon and Mars

Delta-Vs for inner Solar System.svg

Delta-v needed for various orbital manoeuvers using conventional rockets. [5] [8]

Abbreviations key

Near-Earth objects

Near-Earth objects are asteroids whose orbits can bring them within about 0.3 astronomical units of the Earth. There are thousands of such objects that are easier to reach than the Moon or Mars. Their one-way delta-v budgets from LEO range upwards from 3.8 km/s (12,000 ft/s), which is less than 2/3 of the delta-v needed to reach the Moon's surface. [10] But NEOs with low delta-v budgets have long synodic periods, and the intervals between times of closest approach to the Earth (and thus most efficient missions) can be decades long. [11] [12]

The delta-v required to return from Near-Earth objects is usually quite small, sometimes as low as 60 m/s (200 ft/s), with aerocapture using Earth's atmosphere. [10] However, heat shields are required for this, which add mass and constrain spacecraft geometry. The orbital phasing can be problematic; once rendezvous has been achieved, low delta-v return windows can be fairly far apart (more than a year, often many years), depending on the body.

In general, bodies that are much further away or closer to the sun than Earth, have more frequent windows for travel, but usually require larger delta-vs.

See also

Notes

  1. The sum of LEO to GTO and GTO to GEO should equal LEO to GEO. The precise figures depend on what low Earth orbit is used. According to Geostationary transfer orbit, the speed of a GTO at perigee can be just 9.8 km/s. This corresponds to an LEO at about 700 km altitude, where its speed would be 7.5 km/s, giving a delta-v of 2.3 km/s. Starting from a lower LEO would require more delta-v to get to GTO, but then the total for LEO to GEO would have to be higher.
  2. Earth's speed in its orbit around the sun is, on average, 29.78 km/s, equivalent to a specific kinetic energy of 443 km2/s2. One must add to this the potential energy depth of LEO, about 61 km2/s2, to give a kinetic energy close to Earth of 504 km2/s2, corresponding to a speed of 31.8 km/s. Since the LEO speed is 7.8 km/s, the delta-v is only 24 km/s. It would be possible to reach the sun with less delta-v using gravity assists. See Parker Solar Probe. It is also possible to take the long route of going far away from the sun (Δv 8.8 km/s) and then using a very small Δv to cancel the angular momentum and fall into the sun.

Related Research Articles

Interplanetary spaceflight Crewed or uncrewed travel between stars or planets

Interplanetary spaceflight or interplanetary travel is the crewed or uncrewed travel between stars and planets, usually within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System. Uncrewed space probes have flown to all the observed planets in the Solar System as well as to dwarf planets Pluto and Ceres, and several asteroids. Orbiters and landers return more information than fly-by missions. Crewed flights have landed on the Moon and have been planned, from time to time, for Mars and Venus. While many scientists appreciate the knowledge value that uncrewed flights provide, the value of crewed missions is more controversial. Science fiction writers propose a number of benefits, including the mining of asteroids, access to solar power, and room for colonization in the event of an Earth catastrophe.

Spacecraft propulsion Method used to accelerate spacecraft

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry.

Trans-lunar injection Propulsive maneuver used to arrive at the Moon

A trans-lunar injection (TLI) is a propulsive maneuver used to set a spacecraft on a trajectory that will cause it to arrive at the Moon.

Gravity assist Space navigation technique

In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.

Hohmann transfer orbit Elliptical orbit used to transfer between two orbits of different altitudes, in the same plane

In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits of different radii around a central body in the same plane that is sometimes tangential to both. The Hohmann transfer often uses the lowest possible amount of propellant in traveling between these orbits, but bi-elliptic transfers can use less in some cases.

Geostationary transfer orbit Hohmann transfer orbit used to reach geosynchronous or geostationary orbit

A geosynchronous transfer orbit or geostationary transfer orbit (GTO) is a type of geocentric orbit. Satellites that are destined for geosynchronous (GSO) or geostationary orbit (GEO) are (almost) always put into a GTO as an intermediate step for reaching their final orbit.

Orbital mechanics Field of classical mechanics concerned with the motion of spacecraft

Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and law of universal gravitation. Orbital mechanics is a core discipline within space-mission design and control.

Delta-v, symbolized as v and pronounced delta-vee, as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass 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.

Sub-orbital spaceflight Spaceflight that does not complete an orbit around the earth

A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it will not complete one orbital revolution or reach escape velocity.

A geocentric orbit or Earth orbit involves any object orbiting Earth, such as the Moon or artificial satellites. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. More than 16,291 objects previously launched have undergone orbital decay and entered Earth's atmosphere.

Orbital spaceflight Spaceflight where spacecraft orbits an astronomical body

An orbital spaceflight is a spaceflight in which a spacecraft is placed on a trajectory where it could remain in space for at least one orbit. To do this around the Earth, it must be on a free trajectory which has an altitude at perigee around 80 kilometers (50 mi); this is the boundary of space as defined by NASA, the US Air Force and the FAA. To remain in orbit at this altitude requires an orbital speed of ~7.8 km/s. Orbital speed is slower for higher orbits, but attaining them requires greater delta-v. The Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself.

In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft. For spacecraft far from Earth an orbital maneuver is called a deep-space maneuver (DSM).

In astrodynamics and rocketry, gravity loss is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field. In other words, it is the cost of having to hold the rocket up in a gravity field.

Hiten 1990 Japanese lunar probe

The Hiten spacecraft, given the English name Celestial Maiden and known before launch as MUSES-A, part of the MUSES Program, was built by the Institute of Space and Astronautical Science of Japan and launched on January 24, 1990. It was Japan's first lunar probe, the first robotic lunar probe since the Soviet Union's Luna 24 in 1976, and the first lunar probe launched by a country other than the Soviet Union or the United States. The spacecraft was named after flying heavenly beings in Buddhism.

This is an alphabetical list of articles pertaining specifically to aerospace engineering. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.

Bi-elliptic transfer Maneuver that moves a spacecraft from one orbit to another

In astronautics and aerospace engineering, the bi-elliptic transfer is an orbital maneuver that moves a spacecraft from one orbit to another and may, in certain situations, require less delta-v than a Hohmann transfer maneuver.

Low-energy transfer

A low-energy transfer, or low-energy trajectory, is a route in space that allows spacecraft to change orbits using very little fuel. These routes work in the Earth–Moon system and also in other systems, such as between the moons of Jupiter. The drawback of such trajectories is that they take longer to complete than higher-energy (more-fuel) transfers, such as Hohmann transfer orbits.

A reaction engine is an engine or motor that produces thrust by expelling reaction mass, in accordance with Newton's third law of motion. This law of motion is commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force."

Oberth effect Maneuver in which a spacecraft falls into a gravitational well, and then accelerates when its fall reaches maximum speed

In astronautics, a powered flyby, or Oberth maneuver, is a maneuver in which a spacecraft falls into a gravitational well and then uses its engines to further accelerate as it is falling, thereby achieving additional speed. The resulting maneuver is a more efficient way to gain kinetic energy than applying the same impulse outside of a gravitational well. The gain in efficiency is explained by the Oberth effect, wherein the use of a reaction engine at higher speeds generates a greater change in mechanical energy than its use at lower speeds. In practical terms, this means that the most energy-efficient method for a spacecraft to burn its fuel is at the lowest possible orbital periapsis, when its orbital velocity is greatest. In some cases, it is even worth spending fuel on slowing the spacecraft into a gravity well to take advantage of the efficiencies of the Oberth effect. The maneuver and effect are named after the person who first described them in 1927, Hermann Oberth, an Austro-Hungarian-born German physicist and a founder of modern rocketry.

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