Counterweights and Payload Energy

The original Launch Loop consisted of a rotor, a track, and stabilization cables. The largest perturbation is from a vehicle/payload passage, which dumps vertical momentum into the rotor. Another large perturbation is from wind gusts, which dumps unpredictable perturbations into the track. Track and rotor perturbations must be synchronized, or the track and ribbon will separate or crash catastrophically. The total force must be adjusted with stabilization cables, or the rotor and track will deflect away from the position of the stations and the incline ramps.

Given the large speed-of-sound propagation times through the stabilization cables, actuator forces launched from the surface will take about 30 seconds to propagate from the ground. While these forces can eventually restore equilibrium, that is a long time to respond to unpredicted perturbations, such as impacts into the sheath, wind gusts, anomalous vehicle position, etc. On the original design, this required that the spacing controllers on the track be agile and capable of large force changes.

The new, improved Launch Loop has an additional component added below the track; solenoid-started, induction-powered counterweight masses. The mass per meter of the track is reduced by the mass of the counterweights, which travel up and down a hundred meters or so of a wire-wound portion of the stabilization cables. The counterweights are magnetized, and current is applied to the wires to move the counter up and down from a latched, neutral position.

In the old design, a 3 kg/m rotor moving at 14000 m/s supported 7 kg/m of track and cable. With the new design, about 1kg/m of actuator is added, and the track and cable weight are reduced accordingly.

Accelerating above orbital velocity

The maximum deflection of the rotor and track combination occurs when a large payload is about to leave the loop at above escape velocity. It is following a curve, faster than orbital velocity, and the rotor and track are deflected downwards to provide the force to hold it down (and after release, stay out of the way of the vehicle magnet). For the numbers above, if the vehicle is moving at 12000 m/s, it is passing the stationary track at that speed, while the rotor passes it at 2000 m/s. The upward centrifugal force on the vehicle is about 60 kiloNewtons ( about 1.2 gees upwards! ), and all that force is passed to the rotor, instantaneously deflecting it upwards with a velocity of 10 m/s. The track should follow, quickly, or the nominal spacing of 1 cm between the track and the rotor will increase beyond the ability of the spacing control electromagnets to correct.

So the track must also deflect upwards, behind the vehicle, with a delta V of 10 m/s (assume it changes from 5 m/s down, to 10 m/s up ). The force needed to do this for a 7 kilogram track structure (including counterweights and cables) is 840 kiloNewtons.

With infinitely strong control magnets, this sudden momentum change would be instantaneously pulled out of the rotor, applied as an impulse force directly under the vehicle magnet. In this infinitely-strongly coupled system, the deflection would be substantially reduced, with most of the momentum change ending up in the track - 56 kN to the track, 4kN to the rotor, resulting in only 0.67 m/s upwards deflection. However, the normal track-to-rotor force is 70 Newtons/meter , so to drive 56 kN into the track in 10 meters would require 800x the ordinary spacing control force. If the magnetic field is confined to a region of 4 square centimeters area, it would require a field of 0.6 Tesla and an energy change of 560 joules in a fraction of 1 millisecond; a vast power level.

The passage of the vehicle magnets, outboard magnets, or conductive surfaces on the vehicle wings, might be able to generate this pulse energy, then subtract it back out when the vehicle passes. The power electronics on the track might only be responsible for modulating part of the power. A 5000kg vehicle moving at 12000 m/s and accelerating at 30 m/s2 is pulling 1.8 GW out of the rotor, and dumping 300MW into heat, so perhaps a megawatt or so of the power can be temporarily diverted by windings in the rotor into pulling up the track.

However, the traveling perturbation will be interrupted if for some reason the vehicle releases prematurely. During all this movement up and down, the stabilization cables are pulling down an extra 60000 Newtons in the region of the vehicle. It may take 30 seconds to tell the ground actuators to stop pulling, and for the relaxed strain to travel back up the cable. In the meantime, we are dumping unwanted radial momentum into the track, perhaps causing the rotor to approach east station too low.

This is where the counterweights become handy. First, they can move up and down a long distance as a payload passes, adding and subtracting vertical momentum while strain waves propagate up from the ground. This gives us an extra degree of freedom in the control system, and makes overall control much easier. We are still using the traveling rotor to redistribute momentum between portions of the track, but we are not dependent on it to keep the track absolutely stable. If we have plus or minus 100 meters of vertical play in the counterweights, and the ratio to track mass is 5 to 1, we can move the track up and down by 20 meters just using the counterweights. If the track velocity change during a vehicle passage is 0.67 meters per second, we can accomodate that velocity change for 30 seconds while waiting for cable tensions to change, without using controller power and a big magnetic field on the rotor, along with downstream rotor perturbations, to do so.

Accelerating below orbital velocity

The vertical forces of a payload passage below orbital velocity are smaller, maybe half of the forces at escape velocity, peaking at about 0.6 times circular orbital velocity, or around 4800 meters per second. However, the track now needs to be pushed downwards, rather than pulled upwards, and this can't be done with a DC magnetic field from the rotor. At 4800 meters per second, and the other parameters above, an ideal system instantaneously changes track velocity downwards by 0.35 m/s, while changing rotor velocity downwards by 0.93 m/s. This generates a lifting force of 32 kiloNewtons, joining the centrifugal force to support the vehicle against gravity. A 10 meter vehicle magnet passes overhead in 2 milliseconds; if we tried to accelerate the track 0.35 m/s in that time, we would need to pull it down at 175 m/s2, or nearly 9 gees. Perhaps there is some clever way to do this with the vehicle wings, but it is easier to do with counterweights.

Note that the track does not need to do all the acceleration underneath the vehicle; 0.35 meters per second in 10 milliseconds will only displace the track from the ideal position by 3.5 millimeters. However, we are still expecting the control electronics to generate a lot of energy. It is easier to actively pull the track downwards by pushing counterweights upwards, perhaps moving them upwards at 1.75 meters per second until additional stabilization cable forces arrive from the ground.

Programming Forces with Vehicle Magnet Rail Outriggers

It may be possible to avoid a lot of control complexity by hanging DC magnets and conductive plates off the side of the central vehicle magnet rail, which pass over matching coil and magnet outriggers on the track. Vehicles with different masses, exit velocities, and acceleration profiles could be attached to different magnet rails. While the electronics can still adjust or even override this programming, we can save a lot of high power electronics switching expense by using "dumb" magnets and conductors to do most of the work. For more detailed resume help information about how to use "dumb" magnets you can see in the custom paper.

The payload capsule or sled, provided by the customer and shipped up from the ground, should not have much control over the launch process. Control belongs to the distributed track controllers on the loop itself, along with a small amount of global control. This can prevent damage to the loop by balky payload operation. Of course, the payload should be able to report conditions and request launch aborts, for the loop control system to respond to in the safest manner possible.


Counterweights Acting Against Wind Gusts

An unexpected wind gust, acting against the outer sheath of the incline section, should never push it into the inner sheath, or push the inner sheath into the rotor. Further, the rotor should always arrive at the station or the incline ramps properly aligned, within a few millimeters of nominal position. This is not easy to do when the wind forces are unpredictable.


Counterweights (last edited 2010-10-02 22:33:43 by HSI-KBW-091-089-106-131)