Hypervelocity Landing Track

Executive Summary

Hypervelocity is generally defined as speeds over one kilometer per second. Hypervelocities must be dealt with routinely in space travel. Rockets are the most mature technology for this, but the rocket equation means these become exponentially expensive when major velocity shifts (Δv, pronounced "delta-vee") are substantial. Leaving the Earth's gravity well is one such case, leaving space inaccessible to the common human being.

Hypervelocity Landing Tracks may reduce the cost of space travel substantially, by allowing momentum to be stored in relatively large inertial bodies, either planetary surfaces or artificial satellites in controlled orbit.

Comparison to other NRSL Systems

There are many approaches to non-rocket space launch. Most of these have been documented in Space Transport and Engineering Methods, and have dedicated Wikipedia articles as well. Few such ideas are new, although some are obscure.

Familiar examples include rotating tethers (aka rotovators), mass drivers, and coilguns. A less familiar, but relevant example is the crashportation, also known as rheobraking or lithobraking.

Conceptually an HLT functions similar to the manner in which an aircraft uses a landing strip to slow itself down. In that case, the wheels of the landing gear are rotated to allow coupling with the asphalt via static friction, then friction against the brake pads is used to slow the wheels and remove momentum from the aircraft.

In practice, this concept is somewhat similar to the rotovator or hypersonic tether in the sense that momentum is borrowed from an existing body to propel a craft to orbital velocity (or other hypervelocity). The primary difference is that the HLT does not act as an "elevator" in the sense of altering the altitude of the craft, just as a linear track for "stopping", relative to the inertial body to which the track is attached.

This type of structure can also be compared to an orbital ring, which also utilizes a hypervelocity mass stream which the payload is magnetically coupled to as a mechanism to get it to orbit. Launch loop, a type of Partial Orbital Ring System (PORS) works in a similar fashion, but within a structure which is anchored to the ground at its ends and via tethers. The main difference is that HLT as described here lacks a structure or tether constantly attaching it to Earth. It can be thought of as a Sectional Orbital Ring System (SORS), since extending the length by about 400 times would allow connecting both ends, resulting in a fully circular orbital ring system (ORS) capable of supporting a tether to the ground (what Paul Birch called a Jacob's Ladder).

Example Systems

HLT For Reaching Low Earth Orbit

We will describe a HLT for reaching low Earth orbit (HLT-LEO). The assumptions are as follows:

A significant mass of ballast is in orbit already. A 1000:1 ratio of ballast to payload is assumed to be conservative, e.g. 1 ton per kilogram.
Inertial energy is added slowly to this mass by a propellant-efficient mechanism such as an ion rocket powered by solar energy.
A high-tensile cable is anchored to and trailed behind the inertial mass at approximately the same altitude and velocity.
Inertial mass as well as tail end of track are actively synchronized, at equal speeds and distance from the Earth.
A payload may be inexpensively lifted to match altitude but not velocity with the station, and maintained inexpensively at that altitude for a matter of 130 seconds or less.
Magnetic coupling with the track is possible, such that the payload can experience continuous acceleration along its length.
Energy generated by magnetic or mechanical coupling through induction or friction is dispersed or stored safely.

A human-friendly version of HLT-LEO would be 500 kilometers in length, and an acceleration rate of 60 m/s² (~6 g) would be maintained for 130 seconds to reach 7.8 km/s. To obtain tension in newtons, one can multiply 60 times the number of kilograms. A 1 kg payload would exert 60 newtons of force on the track (equivalent to suspending 6 kilograms under Earth gravity). This is well within the capabilities of a square millimeter cross section of A36 steel (which can hold 400 newtons).

Such a wire would mass around 3.9 tons, given a volume of 0.5 m³ and a density of 7.8 g/ml. If loads of 6 kg are decelerated along its length at a time cost of 130 seconds each, the number of such loads that can be put up in a day are 664 (~4 tons), suggesting that the cable could more than double its strength in 24 hours, if momentum can be replenished quickly enough. Moreover, carbon composites have much higher tensile strength to weight ratios than steel, which implies that doubling time can be even more rapid in that case.

A realistic 'starter kit' version of HLT-LEO would probably be much shorter and use much higher accelerations, as it would only be used to boost its own components and propellants, which can be designed to not be sensitive to high acceleration. Going shorter does not give advantages for tensile strength of materials in this context, but may make for a more manageable / less complex design. A major concern with long structures would be resonance, although this could be damped out with appropriate design. Also, long structures are more stable when oriented vertically, so constant small corrections are needed to keep it horizontal. Longer versions of HLT-LEO may conceivably consist of multiple physically disconnected tracks kept in precisely aligned orbits using ion thrusters. In fact, a fully non-tensile version of the concept could use thousands of small weights with ion thrusters ('smart ballastites') to keep them in synchronized orbits.

HLT For Reaching High Velocity Objects

A similar system can be considered for matching speeds with asteroids, or comparable high-velocity objects (HLT-HVO). In this case, an anchoring system is needed. For example, the asteroid may be wrapped in a thin sheet of fabric or netting. The track would need to be fastened to the pole of the asteroid, or a bearing that "moves" with respect to the main mass to remain stationary, so as to avoid high tension from centrifugal force. The far end would be kept extended against the asteroid's gravity with a low-energy ion drive that does not impart significant momentum.

In this case, we do not worry about replenishing the stolen momentum, as asteroids come with plenty of natural momentum and are not orbiting the Earth so there is no issue with descent into the atmosphere from the momentum loss. In many cases, the velocity differential is low enough that a shorter cable designed for 1-2km/sec is adequate, and craft may use low-intensity ion drives to come closer to the target velocity anyway. The primary advantage is lower transit times and a broader range of suitable targets and time windows (especially in the case of human activity, which relies on short enough trips for astronauts to survive in good health).

In this context, it is worth paying particular attention to tracks manufactured using asteroid materials, as opposed to additional landed materials. Although an HLT-LEO system could lead to much less expensive missions in general, materials launched from Earth always come at a cost whereas many asteroids are formed of an iron alloy that naturally has high tensile strength. Delivery of equipment suitable to process this into track structure may be cheaper than delivery of additional track materials.

HLT To The Lunar Surface And LLO

Landing on the Moon with rockets has the complication of kicking up dust, and the rocket equation makes it considerably more expensive than achieving a low lunar orbit (LLO). Thus, a HLT system based on the lunar surface (HLT-LS) can give substantial advantages to lunar industries. This consists of a track about 34 km long, anchored between two towers of modest height. The craft in LLO uses a small amount of fuel to approach, couples to the track, and decelerates at 6 g for about 30 seconds. Using the same thickness as for the HLT-LEO system described above, the mass (using our steel reference example) would be ~0.27 tons. Its theoretical doubling time at 30 seconds per landing with 6 kg payloads is a little over 10 minutes. So the cost of delivering materials could approach the cost of putting them in LLO very closely.

An alternate design for HLT-LS (originally concieved as a variant on crashportation) would be similar in shape to a concrete road barrier, also 34 km long, produced from sintered regolith, and covered with metal using vacuum deposition. The metal coating permits a craft with magnetic coils to induce magnetic friction (i.e. induction) on the track.

Setting up an HLT-LLO system for getting materials off of the Moon and into LLO is also relatively simple, as it would work along the same lines as HLT-LEO. The required length of the track with the same 60 m/s² acceleration rate is 34 km just as for the surface based track. Payloads can be lofted to the height of the LLO track by relatively simple mechanical means.

Optimizations

Hypervelocity Carriage

The mechanism of coupling craft with track is the most speculative aspect. The high relative speed may be insurmountable for realistic grappling mechanisms, particularly for HLT-LEO. One way around it might be to surround a section of the track with a length of material which is designed to ride along it smoothly at high speeds, either mechanically or magnetically. This is attached to the track ahead of time, and accelerated rapidly just prior to the craft reaching the track altitude. The craft grasps the carriage (which is moving at the same speed as itself), after which it begins to acquire momentum due to the braking action of the carriage against the track.

Electromagnetic Induction Braking

Mechanical friction is likely to induce wear and tear on the track (which would need to be repaired), and necessitates that it be smooth. So the most likely form of this concept would be using electromagnetic induction to convert relative velocity to electricity. The launch loop uses a similar concept.

Ablative Laser Thrusters

Thrust for stationkeeping in the HLT-LEO context needs to be well above what chemical rockets will handle in terms of propellant efficiency, as the cost to deliver any mass by this method is around 7.8 kNs/kg. There's no limit in principle as to how massive the equipment can be relative to thrust, but extremely massive systems would slow the bootstrapping process. One system that has promise for the required range of thrust (250 kN for a 1 million ton per year system) is ablative laser propulsion. The power can be generated on site using solar energy, and the laser equipment can be part of the station, so there is no need for advanced targeting systems to deliver the laser power remotely. The system could also serve a double purpose in preventing Kessler Syndrome by eliminating space debris.

Electrodynamic Thrusters

Electrodynamic tethers (vertically hanging conductors) can boost the orbital velocity of a satellite if a current is induced in the opposite direction of that generated by the Earth's magnetic field. EDTs have also been suggested as a method to deorbit materials from higher orbits (by inductive conversion of kinetic energy to electric current), which has further implications for inexpensive fuel/propellant delivery using materials from higher orbits.

Crashportation of Propellant and Ballast

As the station grows in scale, a steel or aluminum impact area could be created which would be used to catch fast-moving chunks of orbital debris. These would approach from higher orbits, picking up velocity as they move towards the Earth in a manner analogous to a hydroelectric power source. The impacts would create a large amount of heat, and the rest of the station would need to be cushioned against abrupt velocity changes. The materials could later be used as propellant for a solar-driven plasma rocket to create even more velocity, or could be piled together to serve as passive ballast.

Economic Analysis

The most important near term application for this concept is HLT-LEO. The primary cost for each kilogram lifted to the altitude of LEO is that of a sounding rocket payload, which we can estimate at around $250/kg (needs verified). The cost of replenishing momentum for the inertial mass must also be accounted for. There needs to be, at a minimum, a propulsion system with exhaust velocities greater than orbital velocity. Existing ion thrusters do reach this, but with very low impulse levels. A better option is ablative laser propulsion.

Analysis assumes that $250/kg (or approximately 1/10th of typical costs to orbit) is reasonable for "lofting" the loads to sufficient height (160km-400km) to reach the track. If the cost is closer to orbital cost, the profitability of this method is much lower, and bootstrapping in an economical fashion requiring short term repayments at each stage would require much longer time frames.

The primary advantage of HLT is that there is no need to start with a very large system. A single track costing $20 million to launch into orbit using traditional methods may be used as a basis to bootstrap a much larger track by successive doubling of capability over time. Even given an extremely conservative estimate of materials strength, single-day doubling times are plausible, which suggests that other economic and practical factors coming into play would not be insurmountable in the range of weeks to months. Twenty doublings would increase the power of the track to approximately one million times its original parameters, and could be accomplished in the $1-2 billion range if costs are $250/kg.