Active Structures

Active structures rely on constant power input, in addition to the material and mechanical properties of their construction materials (active support). This is in contrast to passive structures, which solely rely on the aforementioned properties (passive support). An example of an active structure is the force of a jet of water holding up a tethered lid of a trashcan in the air, versus the passive structure of a concrete pillar.

Nearly everything, from skyscrapers to houses are passive structures. Low-power active structures are in use now, for things like roof support.

The advantage of active structures is that they can be much more massive than passive structures , enabling structures many kilometers tall without requiring significant tapering. Some proposals for non-rocket launch infrastructure rely on active support, with the advantage of the option for being built by modern, existing materials.

Most known designs of active structures rely on the force of a stream of mass to support them, using an accelerator to drive the mass stream.

Active Support Principles

As gravityis what pulls down objects, active support must counteract gravity. Since it is the acceleration that causes objects to be pulled down, it follows that active support should accelerate in the opposite direction; the acceleration must be equal to gravity to support the structure.

The gravitational acceleration of the planet is given by:

$A_{g}=G(M_{p}/r_{p}^{2})$

• Where $A_{g}$ is the gravitational acceleration.
• Where $G$ is the universal gravitational constant, defined to be 6.6743e11 m³/kg/s².
• Where $M_{p}$ is the mass of the planet.
• Where $r_{p}$ is the radius of the planet.

On Earth, $A_{g}$ equals 9.80665 m/s², a constant known as $g$.

Mass Streams

Mass streams generally use particle accelerators or similar technology to create the streams. They use a deflector, usually magnetic, to receive the force from the stream and redirect it back towards the ground to create a loop.

The force required to accelerate the active structure is given by Newton’s second law of motion.

$F=MA$

• Where $F$ is the force exerted on the deflector.
• Where $M$ is the mass of the structure.
• Where $A$ is the antigravity acceleration.

This also applies to the mass stream, the cumulative force of the stream must be equal to this force. If the total mass of the mass stream is lower than the mass of the structure, the acceleration for the stream must be higher.

The acceleration required for each particle or pellet of the stream is calculated with:

$A_{s}=F/N_{m}/M_{m}$

• Where $A_{s}$ is the acceleration of the particle.
• Where $N_{m}$ is the amount of particles or pellets in the stream. The amount for particles is given by:

$M_{s}/N_{s}$

• Where $N_{s}$ is the molar mass of the stream material.

• Where $M_{s}$ is the mass of the stream.

• Where $M_{m}$ is the mass of the particle or the pellet.

Active Structures

Existing

• Small-scale demonstration of incomplete mass stream technology using water.

• The air-supported fabric roofs of the Tokyo Dome, Japan, and the Silverdome, USA use (and for the latter, used) constant fan pressure to keep the roofs aloft.

Proposed

• The Lofstrom Launch Loop is a thin 2000+ km long and 80 km tall active structure, and uses its own mass stream to help launch payloads to orbit. It uses attractive magnetic levitation for the mass stream. The mass stream is a solid continuous iron rotor. The loop suffers from some unaddressed instability concerns.

• The Space Cable is a similar concept to the launch loop. It differs from the launch loop in that it uses magnetically interacting bolts instead of a continuous rotor, is smaller in length, and has addressed instability concerns.

• The Bolonkin Kinetic Space Tower / Anti-Gravitator, using cables and rollers instead of using rotors or bolts or pellets to support various things. It has completely unaddressed instability concerns.

• The Space Fountain / Space Tower, which aims to make a space elevator using active support. Its technology can also extend to shorter buildings.

• The Orbital Ring, which uses a mass stream travelling faster than the orbital velocity to support a ring above a planet, as the stream keeps it from falling through momentum, and is tethered to the earth for stability.

• The Pneumatic Freestanding Tower, which uses pressurized gas to support large structures such as a space tower. It utilizes compressors to provide pressurized gas and alleviate leaks. The main concerns are buckling due to the height of the tower, though it has mechanisms in place to prevent this.

Control Systems

Control Systems
Active structures can suffer from stability issues as mentioned before, such as for example in the launch loop unstable attractive magnetic levitation of the mass-stream in the launch-loop requiring active control of the deflector magnet. The unpredictable winds in the atmosphere are also a concern. Control systems are also needed in even just skyscrapers, with devices like tuned mass dampers to deal with vibration.

Safety Engineering

Active structures are subject to the problem of how to ensure that they don't fail, or a bit worse, only fail gracefully, when something in their active systems breaks down. This is not a question of if; entropy breaks everything. All electrical and mechanical systems have a mean time between failure. If an active structure is only supported by a single active support "string", the failure of that "string" will cause a catastrophic failure.

By adding redundancy to our structure, we can ensure it can tolerate the failure of some of its components, and possible "fail gracefully" with a time period allowing for evacuation and response measures to be taken. Accepting an increase in mass, we split the support power required between some $N$ "strings" operating in parallel. Each string only operates at a partial power, with an oversize factor added on top. If one or a few of the strings in the parallel system fail, the other strings are ramped to full power, generating sufficient support power to ensure the active structure remains standing despite the failure of some of its "strings". We can also use this to shut down strings intentionally for inspections, maintenance, overhauls, or other work, overall allowing us also to keep the active structure alive over time by incrementally replacing and upgrading its parts.

Usually safety redundancies have at least three systems operating in parallel. You may consider having a larger number of systems.

Note: in terms of safety engineering, no degree of redundancy reduces the chance of a total, catastrophic failure to zero. There is some chance that even a very redundant system may experience the failure of all its critical components at once. But this is given for any system, and you could consider pushing the safety factor of an active structure to the same point (or beyond) any passive structure.

Additional References

Derivation of the Gravitational Acceleration Equation

1. The gravitational force equation is $F=G(m_{1}m_{1}/r^{2})$, where $F$ is the force, $G$ is the universal gravitational constant, $m_{1}$ is the mass of the first object, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle m_2} is the mass of the second object and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle r} is the distance between their centers of mass.
2. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle r} becomes the radius of the planet from the frame of reference of a planet.
3. The equation Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle F = MA} , which gives the force needed to accelerate an object is rearranged to give acceleration, thus Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A=F/M}
4. Since Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A =F/M} , where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A} is the acceleration, divide by the mass of the second object and therefore cancel out its inclusion in the expression, giving you the gravitational acceleration equation.

Reference for the Derivation of the Gravitational Acceleration Equation

Derivation of the Particle/Pellet Acceleration Equation

1. If only the total force is known, we must operate on it with something in order to get the force per particle/pellet. We divide the total force by the amount of particles/pellets in the stream.
2. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A = F/m} gives the acceleration needed for the particle to exert that particular amount of force on the deflector.

Credit

To Tshhmon for writing the article

• To SOPHONT SIMP and pMXoTJFu for sweeping the article.
• To AdAstraGames for contributing useful information and sweeping the article.