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Thursday, 26 January 2017

The Orbital Runway - two more improvements

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.

Flywheel braking


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.

Advantages:

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.

Disadvantages:

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.

Friction heating.
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.
Pulley types.
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.   

Advantages:

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. 

Disadvantages:

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. 


18 comments:

  1. How about this idea: you use the energy from the pulled tether line to (re)set the mechanism for a "space ballista" earthbound launch. The space ballista will be some kind of "spring-loaded" stored energy system (pressurised gas piston, spring-leaf, straight coil spring, wound coil spring, etc). The space ballista will then launch a payload for (re)entry, which (if properly matched for mass) should return the "catcher" to its originalorbital configuration. Alternatively, it should be possible to use the tether as a pull to operate a space-trebuchet or space-sling/bola system to launch a previously prepared load at the moment of capture for the outbound load.
    Also, I thnk that either system should work with a "yo-yo" tether draw system. The tether is initially in a retracted state. To reduce the strain on the tether line, the line will be wound around a "flywheel" configured pretty much exactly like the hub of a y-yo. After the tether has reached maximum length, the spinning hub will now have sufficient energy to retract the line. Again, this should work in conjunction with the arthbound lunch system.

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    1. The yo-yo design is incredible! OF COURSE! The flywheels start motionless. They are pulled into motion by the spacecraft, like a ripcord, then they reel the spacecraft and the tether back in!

      It also prolongs the available braking time by x2, so you can dissipate the energy the flywheels gain even more slowly!!

      We can also combine the concept with a set of flywheels that pull another tether in the opposite direction. Instead of a spring-loaded ballista, it is flywheels.

      The spacecraft pulls out tether A, thereby putting the flywheels into motion. Tether B is attached to the flywheels in the opposite direction. A payload is attached to tether B, and flies out with 270GJ of energy to drop straight down.

      The more I think of this, the more I realize that the same set of flywheels can provide a boost in ANY direction. Just set the tether to pull through a guide rail from a specific angle relative to the flywheels.

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    2. Yes, it can provide boost in any direction. However, I was more concerned about maintaining the original orbital conditions without having to expend propellant.

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    3. As long as a flywheel is being used, the platform won't lose any kinetic energy, so won't need propellant.

      I wonder how we could go about implementing the full pulley train + flywheel kinetic sink + yoyo launcher configuration. The yoyo will need a pulley train in reverse to allow for multiple kms/s ejection speeds though.

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    4. The flywheel does not quite work that way. You are not taking into account the energy/momentum of retracting the cable, which has a rather significant mass, even if you cut loose the payload.

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    5. Thanks. I went through different sources and asked around, and the BOT seems to have fatal flaws. I'll explain them in a future post, but here they are for now:

      -Flywheel momentum is angular, which means there is no clever way to convert it into linear momentum without requiring propellant.

      -Braking a payload up to orbital speeds is doable, but actually making the rendezvous between a 7km/s+ tip of the tether and the payload will be very difficult, if not impossible.

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    6. The rendezvous would be difficult, yes... but NOT impossible. It WOULD, however, require a manoeuverable tether head and/or a manoeuverable booster. A carrier landing approach to the rendezvous would probably be most useful. The booster would follow commands to align its path with a "landing strip" that supports multiple tether heads, probably stretched out in a trapeze arrangement. The payload would then only need to contact and hold one of several tethers. The manoeuvering payload would need to assure an appropriate glide slope, relative to the rendezvous platform. There would be many guides assisting this task, similar to the "ball" on a carrier.

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    7. When I looked at it, all I saw was that some element of the tether is going to touch some element of the spacecraft going at 7.3km/s+ of relative velocity. Even 'arrestor cables' would be vaporized by an impact at those velocities.

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    8. Okay, I see what you mean. OTOH, that is not exactly what I would call a rendezvous problem (which I interpreted as being the problem of arriving at the same point at the same time, which was another issue).
      So... the actual problem appears to be finding a material that can survive a 750g acceleration. You are correct that it WOULD be toast if the caple were held taught. However, if the cable were to move with the "impact" point, then the energy would be converted into kinetic rather than thermal. Ideally, both contacts should be highly deformable. Even better, they would have a like electrical charge that would cause the tether to start moving before actual contact. I have also been thinking that the tether head should be designed more like a net than a wire.
      So, the strands of the net should be very strong, highly elastic (stretchable), charged elements; while the contact head of the payload should be thick, highly compressable, and charged foam-like material. So long as there is sufficient give, you should not have the problem with vapourisation.

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    9. Moving the impact point implies accelerating one component of the space station, whether a tether or a cage ect., to at or near orbital velocity. This would require a lot of energy even for a few kg.

      Once braking starts, the spaceship will be holding on to the component mentioned above. This means it must withstand the full braking force, and so has to be as strong as the main cable. This implies a minimum mass per length... and so on. It becomes difficult to model and calculate, and even harder to find an appropriate solution.

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    10. I'm not certain you quite understood what I meant: the tether end would "give" with the impact of the payload. Think of it in terms of the kevlar in a "bulletproof" vest. If you had a stiff sheet of kevlar, even at twice (or even ten times) the thickness and mass of an actual vest, the bullet would pass through the vest as if it were soft butter. Instead, the kevlar "gives" a little with the bullet, allowing the impact energy to be transmitted through the length of the fibres. The wearer feels as if s/he were being hit by a sledge hammer, but the bullet does not penetrate.
      Again, though, even better would be if the tether head "net" begins to move along the vector before actual contact, as would happen if both parts had a like charge. No need to actually generate much energy for this, just a little static electrical charge, which will be pushed by the static electrical charge of the approaching payload. Granted, this won't have much of an effect, in all likelihood, but even a little extra "give" helps.

      Yes, of course the components will have to withstand the breaking force. I don't think I was saying otherwise. I WAS, however, supporting you argument that the breaking force is better distributed throughout the cable, which avoids the entire load being focused on a single shear point.

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  2. How does this design not violate conservation of momentum? As near as I can tell, it does. All you have to do to show this is to take the momentum of the spacecraft + momentum of the tether satellite. If no propellant is being consumed and there is no interaction with the planet and momentum is changed (which it is), then momentum is not being conserved.

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    1. The 'propellant' being consumed is the rotation of the flywheel. It slows down and eventually stops at the same time as the payload reaches maximum extension of the tether.

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  3. While this is interesting, I am not sure that you want a "Rube Goldberg" device in orbit. All these moving parts interacting with spacecraft travelling at speeds measured in *kilometres per second* is trouble just waiting to happen. A bit of extra friction, a thread of carbon nanotube coming unravelled at the wrong moment, magnetic bearings in the pulley or flywheel system failing....the disaster movie almost writes itself.

    Following the simplicity and add lightness school, I'd go with a rotating Moronic Tether, where the structure is essentially the flywheel as well. No moving parts, no fuss, no muss (unless your traffic is unbalanced, in which case a multi thousand ton tether hits the ground with incredible results) and minimal maintenance.

    True, the tether should have some moving parts (perhaps the ends have a few degrees of freedom to match orbits with the spacecraft, much like a tanker extends the boom to fuel up a bomber), and possible a small reactor or microwave target at the spin axis to receive energy to allow for electrodynamic flight inside the magnetosphere, but the overall system is much simpler, and I would guess that a mass comparison might actually come out fairly close as well.

    So there are systems which can do the same thing as the orbital runway with pulleys and moving tether parts, but use simple brute force to achieve the same ends.

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  4. Here's a very long and active thread going on about my orbital tethers. Some critical points are the sheer difficulty of docking the spacecraft to the tether before braking even begins.

    http://bbs.stardestroyer.net/viewtopic.php?f=5&t=166030

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    1. Yes... it IS very long. I noticed one criticism about the cable still moving with the same velocity in respect to the pulley. If this has not been mentioned yet, I would point out that there is one system of pulleys attached to the ship, but there is a SECOND system that is "free-floating", and intended to move with the tether head. Thus, between the two systems working together, the relative motion between cables and pulleys is NOT the same.

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    2. I would also point out that even with a single system, the actual force on the cable would be distributed between all the points of contact. Instead of tension building up on a single midpoint on the cable, it will be distributed among all of the midpoints between pulleys. This means less tension (of course), but also less friction at any single point.

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    3. I intend to make a follow-up article based on what was discussed on that forum thread.

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