People seem to be diddling over a 10% improvement in the cost or flexibility of SSTOs, of flyback boosters or of big Dumb rockets over the shuttle. Bah. Rockets have been optimized over the last 30 years, and that's 30 years of evidence that rockets fail to support any greater strides in manned spaceflight than what has already been achieved.

A skyhook would open doors IMO where rocket technology is just plain stupid. My skyhook design would launch 45 tonnes of payload into space every 66 minutes. That is roughly a 9000-fold improvement over the shuttle. The inherent rotational freqency of a skyhook would be hard to keep up with. But even if we produced a payload to send up on every 100th cycle, the skyhook would still represent a 90-fold improvement over the current shuttle. Rockets alone could not hope to compete with a skyhook. Period.

Here is a re-cap of my post to the Space Frontier Foundation wrt The Cube Skyhook Infrastructure :

With a sufficiently tapered geometry, nearly any conventional high strength material would suffice for the tether, including carbon fiber, zylon, dyneema, and several other polymer-based materials. This is definitely NOT an unvaried tether geometry.

My Skyhook design also uses a Boeing 747, capable of lofting 120 tonnes of payload to 45,000 ft altitude. Cruise speed is about Mach 0.7. The Boeing 747 carries only 45 tonnes, which leaves plenty of carrying capacity for a command & control center, radar systems, supercomputers, telepresence systems, et cetera, inside the fuselage. It also leaves the 747 flying light, able to achieve better climb & dive rates during aerial chase maneuvers at high altitude.

The tether extremity is outfitted with a transonic drone modelled after the 1960's era Skunk Works D-21 Drone (top secret classified Project Senior Bowl) and with a mass of 5 tonnes. It's engine was a Marquardt ramjet with a cruise speed of Mach 4.5 at full power, orginally used in cruise missiles. A modern version of this drone should have a significantly more versatile powerplant, with a translating inlet spike for a wider flight envelope. As well it should have external tanks for liquid oxygen & isopropyl alcohol, able to be swapped out or refuelled quickly in transit, used to power both the airbreathing powerplant and various rocket thrusters for on-orbit attitude control. Also the wings should house 2 or 3 high power rocket thrusters to dampen tether contraction after the payload is released.

The tethered drone re-enters the atmosphere at high supersonic speed, about Mach 5, but must use the ramjet for powered descent. The thrust from the ramjet balances the aerodynamic drag on the tether during re-entry and keeps it straight. Due to the counter-rotating motion of the tether w.r.t. the orbiting Hub, the Drone's airspeed will continually slow down in spite of being under power. In fact, its airspeed would reduce to nil without a powerplant, and needs something to match speed with the Boeing 747 at the bottom of its cycle. Note that the airborne rendezvous occurs at subsonic speed, at a reasonable altitude of 35,000 ft. The drone uses its inherent ability to slow down, to approach the 747 backwards and land on it's piggybacked cargo container, thus avoiding the turbulant wingtip vortices of the 747. The cargo container, with wedge-like supersonic airflow fairings, has a top surface that is designed like an aircraft carrier flight deck. Capture is possible even with significant difference in relative airspeeds, and is very fast, allowing for a short rendezvous window. Once the drone is locked in place on top of the cargo container it must wait for sufficient tension to build in the tether before releasing it from the Boeing 747. I calculate that during this time the tether will stretch as much as 30 kilometers (beyond its unloaded 3950 km length). Upon release from the 747 it will ascend upwards with 1-gee of acceleration, passing various subsonic and supersonic thresholds on its way out of the atmosphere. The cargo container itself will have aerodynamic control surfaces at its tail end, and will house small rocket engines used to circularize its orbit upon reaching apogee and for attitude control in orbit. The cargo container will also house fuel to re-supply the Drone. This will be accomplished in-transit, before the cargo is released by the Drone.

Another arrangement of the airborne rendezvous has the Drone itself being quickly swapped out with a new one, so that they can undergo periodic maintenance at ground-based facilities.

The orbiting Hub is a massive counterweight, about 14,000 tonnes, used for momentum exchange with the 45-tonne cargo. To maintain orbit altitude it has powerful electrodynamic stationkeeping engines, able to put out 4.5 gigawatts of work to repel the earth's magnetosphere. The stationkeeping engines are beam-powered, either microwave beam or electron laser, from ground-based equatorial nuclear powerplants, each with a triple-set of 50-meter diameter directional transmitting dishes. These nuclear powerplants would be located around the equator, matching the ideal orbit for a Skyhook, due to the absence of disruptive precession that would be caused by the oblate spherical shape of the earth on inclined orbits. Microwave power can be transmitted with roughly 60% efficiency, and taking other efficiencies into account, each nuclear station would probably need to generate approximately 13.5 gigawatts of electrical power (a very large commercial generating station).

The tether itself has a varied geometry: the furthest 200 km extremity would be a simple rope-like structure to cleanly cut through the atmosphere, with thermal cladding. The farthest 200 meters would likely be simple steel wire with a ceramic impregnation to protect from rocket plasma from the Drone, fired during Payload release. Most of the length of the tether would be a 2 - 3 meter diameter hollow cylinder, segmented into 400-500 km lengths, connected by load-bearing nodes. A wide diameter ensures that a micrometeorite impact will not sever the tether completely, and any payload in transit will make it safely to its intended orbit. The segments of the tether allow shorter sections of tether to be repaired by a robotic climber, without replacing the entire tether or retracting it and disrupting operations.

At the root of the tether is THE CUBE:

The Cube is the most powerful machine component of the skyhook. Without it adequate feedback control over tether harmonics, orbital trajectory, and re-entry location could not be accomplished. But most importantly, The Cube allows a smaller mass to be used as a counterweight, only 14,000 tonnes instead of 57,000 tonnes. Therefore The Cube drastically and astonishingly reduces both the time needed to construct a tether, and the cost to launch components into orbit. Furthermore, The Cube protects the skyhook from self-destructing in the event of a power failure, keeping it from intersecting the Earth's atmosphere which would drag it down to its demise. What is the Cube? Wait and see. :-)

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The evolution of the species requires more than just a few labs making advances. There has to be a diffusion of knowledge to the people. Everyone has to DO THE THING that is supposedly so great an improvement over the old.

Is a 9000-fold increase in launch capacity an improvement? I say it is. Still others will wonder where it is all going.... I say once millions of people are in space they can once again decide for themselves whether it is worthwhile. The point is, at least they will have a choice.

Today, we are grumbling about the space program because something has changed. We expect more. Something in our minds is telling us to take bigger steps. We have outgrown the old.