Section 2.9 - External Interaction Methods

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External interactions are transport methods which create a force via fields or physical interaction with objects outside the vehicle itself.

Magnetic Interactions[edit]

72 Ionospheric Current Engine[edit]

Alternate Names: Electrodynamic Engine, Alfven Engine

Type: Electrical via Magnetic Field

Description: A current-carrying wire in a planetary magnetic field will experience an IxB force (current cross product with magnetic field, which is perpendicular to both). This is the same type of force created in electric motors. Currents which only flow in one direction cannot be sustained, electric charges would accumulate at one end. Therefore for this method the current loop is closed through an Ionosphere, the portion of a body's atmosphere which is ionized and can carry a current. The wire accelerates in one direction (pulling a vehicle along), and the ionosphere accelerates in the other direction. The ionosphere motion eventually dissipates within itself. When current is driven through the wire by a power supply, this functions as a motor, generating positive thrust. When current is allowed to flow in the other direction unopposed, it functions as a generator, producing power which can be used on the vehicle, and creating drag. Both power and drag can be useful in come circumstances, but any power generated will be at the expense of orbital kinetic energy.

A current loop thruster produces more thrust/watt than an ion or plasma engine. No propellant is consumed directly, although some material is consumed by a Plasma Contactor to produce a plasma that enables good electrical contact with the ionosphere. Effectively this gives an equivalent exhaust velocity of 250 km/s. Advantages are this is a relatively simple device, and relatively high thrust for an electric thruster. Limitations of this method are the planet must have both a substantial magnetic field and sufficient ion density to operate effectively, and the direction of thrust is governed by the local magnetic field direction. Planetary magnetic fields are often offset and tilted with respect to their poles, and in any case only point in one direction at any given point in orbit. The thrust direction is restricted to a circle perpendicular to the field direction. Earth orbit up to around 1000 km, and some areas around Jupiter are good candidates, other places less so. Uses include orbit maintenance for low orbit vehicles and Skyhooks, and apogee raising for reaching higher orbits. This method may be combined with other electric thrusters and share the same power source, changing which thrust method is used as appropriate for altitude or thrust direction.

The motion of a long wire in a magnetic field will generate a static voltage even when not used for propulsion. Such voltage build-ups happen to all spacecraft, a topic known as Spacecraft Charging, but are much larger with a long conducting wire. Therefore proper insulation and equipment grounding is needed to avoid damage. Even without the plasma contactor running, spark discharges to the surrounding ionosphere can happen.

Status: An experiment called the Tethered Satellite System was flown on the Space Shuttle in the 1980's.



  • Belcher, J. W. "The Jupiter-Io Connection: an Alfven Engine in Space", Science vol 238 no 4824 pp 170-6, 9 Oct 1987.

Gravity Interactions[edit]

73 Gravity Assist[edit]

Alternate Names: Planetary Flyby, Celestial Billiards

Type: Kinetic Energy by Gravity Field

Description: Momentum exchange between a planetary body or large satellite and a vehicle allows changing the vehicle's direction and velocity in other reference frames. When considering just the vehicle and large body, a hyperbolic orbit has the same velocity on approach and departure, but gravity forces change the direction. When the large body is in turn in orbit around another larger planet or star, the change in vehicle direction can increase or decrease it's velocity relative to the larger velocity. By conservation of momentum, such gravity assist flybys affect the object you fly past, but typically the vehicle is so much smaller than the body it is flying past, so the change in the body's orbit is too small to measure.

The amount of velocity change is theoretically limited to twice the escape velocity from the body, by changing the vehicle direction 180 degrees. Thus larger bodies can produce larger velocity changes. As a practical matter gravity assist usually results in significantly less than this because the vehicle arrives with excess velocity and the desired final direction limits the flyby parameters. Excess velocity reduces the time the gravity of the body can act. More typical values are 0.5 to 1.0 times escape velocity measured from 1.5 body radii from it's center. The velocity changes are still large enough that they are extensively used in planetary exploration missions, often using multiple flybys of different planets, or even the same planet multiple times. The fuel cost of lining up for a gravity assist in these cases is much lower than doing the equivalent velocity change directly. The disadvantage is longer mission times consumed by the gravity assists compared to a direct transfer orbit, and needing to select mission dates according to when the planets are in the right positions. Lunar gravity assist to escape from or return to Earth orbit is particularly useful, as the Moon's orbit is short enough to give frequent opportunities.

Status: Used extensively in planetary missions, often multiple times for a single spacecraft.



74 Gravity Counterweight[edit]

Alternate Names: Dumb-Waiter System

Type: Potential Energy by Gravity Field

Description: In this method mass falling down a gravity well can be an energy source to power payloads going up the gravity well. Most typically this would be via a space elevator, using the falling mass to directly lift a cargo via cable, or to generate power to lift the cargo electrically. This is most efficient when cargo is going in both directions and are the same mass. In that case cargo delivery only consumes the inefficiency of the motors, which can be just a few percent. Another variation is to use braking energy of cargo above synchronous altitude, which sees a centrifugal force higher than gravity, to power lifting cargo from the body surface to synchronous altitude. The energy for this comes at the expense of rotation of the body by slowing it down. Braking will induce sideways forces on the elevator.

Status: Counterweights are commonly used in elevators on Earth. Space applications have not been tried.



Aerodynamic Interactions[edit]

Interaction with the atmosphere of large bodies which have one, such as the Earth, can produce significant forces. These forces are distinct from propulsion using an atmosphere which was discussed under Air-Breathing Engines, which are principally thrust forces.

75 Aerodynamic Forces[edit]

Alternate Names: Aerobrake, Airfoils

Type: Kinetic Energy by Aerodynamic Forces

Description: This method uses aerodynamic interactions with an atmosphere to provide forces perpendicular to forward motion (lift), intentionally oppose forward motion (drag) or change orbit parameters (using lift, with some drag). Buoyancy, or lift by displacement without forward motion, is covered by method 3 Aerostat. There is a wide range of conditions and applications for aerodynamic forces and it is a well developed field of engineering.

The range of velocities is from well below the local speed of sound, or Subsonic, to many times the speed of sound, or Hypersonic, even beyond escape velocities. All bodies which are not symmetric or pointing directly parallel with the velocity direction will generate some lift, and all moving bodies generate drag. The ratio of lift to drag, known as L/D Ratio ranges from less than 1.0 for blunt re-entry bodies to 25 or more for good subsonic Airfoil shapes designed to maximize lift. Structures whose main purpose is to generate lift are called Wings. All lifting bodies induce some drag by virtue of the lift force not being entirely perpendicular, as well as body drag from other parts of the vehicle besides the wings.

Lift forces can be used to gain altitude as part of a climb to orbit. Drag forces would be minimized as they counteract thrust to reach orbital velocity. On return from orbit, purposely designed devices to slow down include parachutes, heat shields, and High Q Aerobrakes. While already in orbit, pure drag generating Low Q aerobraking can be done without special devices as long as the drag and heating are within what the structure can withstand. Orbital speed lift devices can change orbit direction, at the cost of some drag loss.

Status: Powered aircraft have operated for over 100 years. Jet aircraft have been used as the carriers for rockets. They use a combination of 25 Fanjet for forward thrust and wings for lift.


  • 75a Parachute - Relatively low Mach number drag device to come to a complete stop or low enough velocity for a terminal landing device.
  • 75b High Q Aerobrake - Q is dynamic pressure, the pressure caused by motion through an atmosphere. High Q aerobrakes generate a lot of drag at high Mach numbers. They are often designed as inflatable devices, called Ballutes (from balloon and parachute), but extendable flaps or panels would also fall into this category.
  • 75c Heat Shields - They are drag devices integrated into a vehicle structure, but primarily designed to dissipate the extreme heat of re-entry.
  • 75d Low Q Aerobrake - These are often re-purposed existing parts of a vehicle which are used for braking. By keeping the braking drag low, and using multiple orbits, often the braking can be done without special components on the spacecraft. Obviously that will take longer.


Mechanical Interactions[edit]

This category involves direct physical contact with a natural or human-made body in space.

76 Rheobrake[edit]

Alternate Names: Lithobrake, Crashportation

Type: Lower Kinetic Energy by Friction

Description: This methods uses mechanical friction against a planetary surface to slow down. For example, imagine a rail made of cast basalt on the lunar surface. It is laid level to the ground, and is shaped like the concrete highway dividers. A vehicle wanting to land is in a low grazing orbit. It aligns with the rail, just above it, then extends some clamps over the rail. By applying clamping pressure, the vehicle can brake from lunar orbit to a stop. Obviously the brake will be dissipating a lot of heat, and will therefore have to be made of high temperature material such as graphite. Another approach is to have a 'runway' which is a smoothed area on the lunar surface. The arriving vehicle slows down to below orbital speed, then gravity pulls it down to the runway, and friction with skids on the bottom of the vehicle slows it down.

In order to not melt the brake, it should maximize surface area and possibly have cooling systems like heat pipes. The rail will not see such concentrated heating, so cooling is not as much of a challenge. The main advantage is not requiring fuel to land on a body. The main disadvantage is the large size and mechanical accuracy required for the landing rail or runway. When the velocity is greater than the speed of sound in the brake materials, irregularities in the landing surface can create shock waves in the device. This method is easier to implement on smaller bodies where the amount of orbital kinetic energy to dissipate is less.

Status: Mechanical brakes are used in many vehicles on Earth. High performance ones are used in passenger aircraft. Use for space transport is theoretical at this point.


  • 76a Rock Cloud - This involves creating an artificial 'atmosphere' of particles to slow down against. A cloud of lunar dust could be raised by electrostatic forces and an arriving vehicle slows by impact of the dust particles, or deflection by charged surfaces on the vehicle.