Boosted Orbital Tether - a fishing line in space!

The Boosted Orbital Tether is a less extreme version of the space elevator. A rocket is launched on a vertical trajectory to an altitude of several hundred kilometres. An orbiting mass drags a line and hook to catch the rocket and spends momentum to bring the rocket into orbit.

Let's look at the concept in more detail.

Concept

There are three steps into orbit.

The first step is a conventional rocket boosting the payload. The trajectory is nearly vertical. 

The second step involves a 'fishing line' trailing behind the orbital platform. The payload hooks onto this line and uses brakes to match the platform's velocity.

The third step is the longest: the platform returns to its original orbit. High-efficiency engines are used. Nuclear or solar powered electric engines are possible, as is propellant sourced from extraterrestrial operations. Payloads returning from higher energy trajectories can be slowed down by the platform in reverse.

Variations

A rotating tether.

A rotating tether can sync with the payload's motion and catch it with zero relative velocity. There are no losses to friction and heating as with the braking method, but it is riskier due to more moving parts and the requirement that the payload can position itself very accurately. 

Electromagnetic 'fishing lines' can use electric currents to accelerate a payload more gracefully and with fewer losses than by braking. They can also handle higher velocities or accelerations. 

A propellantless tether system from Tethers Unlimited

The Boosted Orbital Tether can be reversed, with the platform being the boosted object and the payload staying in a fixed orbit. By catching the moving platform, the payload can be pulled out of a low orbit into a higher orbit. If the platform is in a heliocentric orbit, it can pull a payload onto an interplanetary trajectory.

Advantages

The Boosted Orbital Tether enjoys most of the advantages of an orbital tether. It allows for very cheap access to space. The orbital platform is a kinetic sink: it stores energy through its orbital position. This energy is nearly perfectly transferred to a smaller mass, our payload, without any propellant expended. 

The BOT is much simpler, cheaper and very much smaller than a space elevator.

It can store additional kinetic by temporarily varying its trajectory in preparation for a catch. On-board engines can continue to accelerate the platform, creating a lop-sided elliptical trajectory. Each time it passes by the lowest altitude point, the 'periapsis', it can catch a rocket. The momentum it loses drops its highest point, the 'apoapsis'. This way, it can continue to use its engines to maximize the mass it can accelerate per catch. This is especially helpful for low thrust engines that are mostly restricted by how much deltaV they can put up over time. 

In the heliocentric variant, the engines keep pushing until escape velocity is reached and exceeded. Further corrections are needed to set up a rendezvous, either with the payload or another platform. This allows for very heavy payloads to be pulled into orbit, or other payloads to reach higher orbits. 

A conventional rotating tether must match the velocity of the tip of its tether to its payload to perform a rendezvous, making it much less flexible and limiting the maximum kinetic energy it can deal with. 

The deltaV requirements on the rocket booster are quite low. This leads to a low mass-ratio. Modern rockets with high deltaV requirements are built like balloons inflated with fuel. Lower mass ratios mean simpler, cheaper and more robust rockets built closer to jet planes than balloons. This also leads to smaller engines, shorter burn times and further cost savings. 

The orbital fishing line (provisional name) is highly scalable. It can be constructed using modern technology, put into orbit tomorrow and drastically reduces cost to orbit. It is also flexible, with the same platform being able to pull the same payload to multiple possible orbits depending on how much time is allowed to raise its apoapsis, to pull heavier payloads by adding bulk mass, or sacrifice itself into an unrecoverable orbit to perform the above-mentioned tasks while under-capacity.

Unlike a geostationary tether, the 'BOT' platform is free to orbit at any altitude, inclination or position.

In case of a failure to reach orbit, a vertical trajectory is safer than a near-orbital velocity horizontal trajectory. Lower mass ratios means that more mass can be dedicated to escape and safety systems.

Another major advantage is development cost. Extreme mass ratios means extreme design requirements. 

Costs

The frame the platform is built around is lightweight. It consists of efficient engines, a power source, and the fishing line. Additional mass can be anything suited to the orbits it takes. 

The simplest to set up would be a solar-electric platform, using solar panels for power and argon for propellant. The majority of the mass is rocket fuel. 

Orbital fuel depot

More advanced versions get their rocket fuel from ISRU on the Moon or captured asteroids. Very large platforms can be used as manned space stations. Even the ISS can be used as the basis for a boosted orbital tether. It is quite possible that this entire design is unsustainable without an orbital refuelling system in place.

Overall, the Boosted Orbital Tether system can be set up for the cost of a regular satellite. The rocket booster can be conventional rockets with second and third stages removed. Payloads only require a very lightweight set of brakes to perform captures. It is assumed that the BOT platforms set up and sustain the orbital presence that makes them worthwhile in the first place. 

This design has some disadvantages. It requires a conventional rocket booster to used, unlike SkyHook or a space elevator, that require zero propellant. The BOT needs constant refuelling, and the propellants it needs are not always the cheapest, such as argon or cadmium.  

The payload itself has to be more complex. It requires stronger, more precise reaction control systems and a bulkier construction than most vacuum designs, however these are offset by the great savings in mass ratios. 

There is also the risk of a collision or failure of the braking tether. At the velocities involved, these would be fatal. Redundant braking lines and 'guide' rails can attenuate this risk. 

Performance numbers

Higher deltaV capacity allows for higher altitudes, where orbital velocity is lower and acceleration forces are lower. Lower mass platforms lose more momentum during a catch and take longer to regain their orbit. Robotic craft are likely able to endure higher accelerations than human payloads.

For reference, we will look at a 10 ton robotic payload with 30G acceleration resistance, and a 10 ton human payload with 5G maximum resistance.

Minimum altitude platform:

At a mass ratio of 5, an average Isp of 300 and a dry mass (engines, fuel tanks, possible recovery equipment) of 10 tons, 4734m/s of deltaV is possible. This rocket flies straight up. 

Gravity and drag losses are lower than if it followed a curving trajectory, and it reaches much lower speeds while in the atmosphere, so we can round off the deltaV to 4500m/s after losses.

This rocket reaches an altitude of 1032km. Orbital velocity at that altitude is 7334m/s. Translated into kinetic energy, this is 268.9GJ that the orbital platform must provide to the payload. 

We assume that kinetic energy after the rendezvous is mostly conserved, minus braking losses. Also, we do not want the platform to drop its periapsis to lower than 150km, where atmospheric drag becomes noticeable. 

The difference between a 1032km apoapsis and 150km periapsis is 235m/s. This means that the orbital platform must mass at least 9738 tons. 

Zylon!

A robotic payload is caught over 25 seconds with a 91.9km braking tether that must withstand 2.94 MN of force. A 10x margin is 29.4MN in tensile strength. This can be achieved by Kevlar wires of 11mm diameter, massing 1078 tons. A Zylon wire will mass 720 tons.

A human payload is caught over 150 seconds using a 548km braking tether. It must withstand 490.5kN of force, or 4.9MN with a 10x safety margin. This can be achieved by Kevlar wires of 4mm diameter, massing 1071 tons. A Zylon wire would mass 715 tons.   

Using 8kN engines with 100km/s exhaust velocity, consuming 400MW produced by 200 tons of solar panels at 50% efficiency, the 2334 ton platform will return to its original orbit in 39 hours. It only needs to dedicate 20% of its mass to perform 39 captures. Even higher exhaust velocities only lead to more captures per propellant mass. 

Elliptic orbit platform:

Types of orbit

Like a jet craft, altitude can be traded for speed. In this case, the Boosted Orbital Tether platform increases its apoapsis to millions of kilometers, and swings by its periapsis at very high velocities. This allows a small platform to capture a very large payload and place it into orbit.

By raising its apoapsis to 10000km, at the cost of 1530m/s deltaV, it can catch and pull a 10 ton payload into 150km orbit using 42.3 times less mass, or alternatively, pull 423 tons into orbit using the same mass, as the minimum orbit platform.

Exponential gains are possible with increasing altitude. At 1 million km, 51.3 tons of platform are all that are required per 10 tons of payload. 

Multiple-step low acceleration network:

This design uses a network of Boosted Orbital Tethers to progressively catch and pull a payload into orbit. 

Each BOT platform orbits so that their periapsides (plural) form a stepped line. The payload is pulled to faster velocities at each step, reaching orbital velocity at the right-most step. 

This allows each individual platform to be smaller and use shorter, more manageable tethers. It is best suited to rotating tethers that can easily control the release angle.

Boosted Orbital Tether using 113 Amalthea:

From Seveneves.

113 Amalthea gained recognition as the asteroid captured around Earth in the Stephen Nealson novel Seveneves. It masses 100 trillion tons.

Using Amalthea at a 1032km orbit, a total of 100000 tons can be pulled into orbit.