In this post, we will describe two improvements to the Boosted Orbital Tether design. One allows for propellant-less acceleration of a payload into orbit, the other vastly reduces the braking intensity required.
A spacecraft at the top of of its parabolic trajectory latches onto an orbital tether and applies its brakes. The friction slows it down relative to the tether, which is travelling at the orbital velocity of that altitude. When it comes to a complete stop relative to the tether, it has effectively extracted kinetic energy from the platform the tether is attached to.
|A diagram of the the rendezvous and braking at a simple Boosted Orbital Tether platform.|
This kinetic energy gained by the spacecraft is lost from the the platform. This translates into a lower orbital velocity. At a 1000km altitude, a 1000 ton platform catching a 10 ton spacecraft loses 1% of its kinetic energy, 36.8m/s velocity and about 150km altitude on the opposite side of its orbit. It is restored by propulsion, which consumes propellant. If there is not enough propellant, its orbit will drop with each spacecraft caught.
|Diagram of an orbital tether attached to a flywheel.|
Flywheels can store kinetic energy. Their energy can be transferred to the spacecraft without changing the platform's velocity or altitude. The tether is wound around free-spinning flywheels instead of being attached to the platform.
They work by pulling the tether against the motion of the incoming spacecraft. This increases the relative velocity between the spacecraft and the tether. We will now work out an example of such a system.
Let us consider seven flywheels of 5m diameter and 5m width, massing 100 tons each. They spin at 3437RPM and are shaped like drums. Surface velocity is 900m/s. A36 Steel is sufficiently strong for this purpose, as hoop stress is 200MPa.
|Flywheel kinetic energy storage system from Ontario.|
Each flywheel contains 40.5GJ. Together, they contain 283.5GJ. A 10 ton spaceship accelerated from 0 to 7350m/s (orbital velocity at 1000km) requires 270GJ. All of the required energy to catch a spacecraft and pull it into orbit can be contained in the flywheels.
Instead of encountering the tether at a relative velocity of 7350m/s, the spacecraft has to brake from 8450m/s, which means braking takes 15% longer.
No propellant is required to return the Boosted Orbital Tether platform to its original orbit. Spinning the flywheels back up to speed can be done using electric motors very efficiently. Using magnetic bearings, friction losses can be minimal.
By storing kinetic energy in flywheels, a platform can bring heavier payloads into orbit than it normally would have been able to.
More advanced materials such as carbon or beryllium flywheels allow for greater rotational velocities and great mass savings. Increasing the rotation speed lowers the mass required by a squared amount.
Flywheels can explode. They prevent a station from turning effectively due to angular momentum.
Pulley train tether
When a spacecraft latches onto a tether and applies its brakes, it is coming to a stop relative to the orbiting platform. From this frame of reference, it is dissipating kinetic energy as heat.
A 10 ton spacecraft braking at a rate of 3G must dissipate about 1.8GW of heat from its brakes. It would be very difficult to make physical brake pads survive that kind of heating. It is also hard to actually apply enough braking force on a thin wire without destroying it.
There is a solution to greatly reduce the amount of braking required. It works by reducing the relative velocity between the spacecraft and the tether, and distributing the braking across several nodes.
The concept consists of multiple pulleys in series, like a train.
On one end of the pulley train is the spacecraft to be captured. On the other is the platform. In between, the tether is composed of multiple segments. Each segment is wound around two wheels of a pulley, in a gun tackle configuration. The free end of the wire is attached to the fixed wheel of the next pulley.
Brakes are applied at each wheel.
Attachment points between the pulleys can be fixed or free to move between brake pads for additional resistance.
The effect is that the velocity difference between the spacecraft and the station is divided between the segments. A 7350m/s relative velocity can be reduced to each segments' wheel moving away from each other at less than 50m/s if 74 segments are used. Even lower velocities can be achieved by increasing the mechanical advantage in each pulley.
|A diagram of the pulley train configuration|
Each segments' wheels first accelerate away from each other then slow back down to 0m/s relative velocity under the braking force. It is quite possible that braking during the acceleration phase means that 50m/s is only the theoretical peak velocity, with actual top speed being lower.
The initial segment of the tether can be shot ahead of the spacecraft to rendezvous at very low relative velocity, like a lasso thrown to wrangle a cow. It only masses a few dozen kilograms, so this only requires a little bit of energy. It allows the spacecraft to need practically no brakes, heat sinks or additional radiators.
Distributing the braking force allows for more effective, lower temperature designs brakes with better friction coefficients. Electromagnetic braking is also possible, if the pulley wheels are designed like reverse electric engines.
By moving the braking from the spacecraft to the pulley wheels, the spacecraft can be lighter and have lower reaction control requirements.
Each segment's tether is quite short. This makes it easier to handle and less vulnerable to shear stress from sideways movement.
There are also backups integrated into the design. If one set of pulley brakes and stop rotating, the braking force is distributed across the remaining brakes. If one set of pulley wheels seize up and stop rotating, the other segments simply lengthen at a faster rate to compensate.
The system is more complex. There are more moving parts, creating more points of failure.
If too many brakes fail initially, a cascade failure of the remaining brakes can be the result.