"

BI, you said you wanted to talk

At the bottom of the lead section you have written:

""Its promoter speculated in 1994 that the skyhook concept could be cost competitive with what is realistically thought to be achievable using a space elevator, but the skyhook is not competitive with other rotating tether concepts. In addition, the rotating skyhook is in fact deemed ""not engineeringly feasible using presently available materials"".""

Regarding:  In addition, the rotating skyhook is in fact deemed ""not engineeringly feasible using presently available materials""

That statement appears to come from Ref [3] on page 10.  The full quote is

""If the mass of the tether alone started to exceed 200 times the mass of the payload,
then that was an indication the particular scenario being considered was not engineeringly
feasible using presently available materials, although the application might become feasible in
the near future as better materials become available with higher tensile strengths at higher
operational temperatures.""

It then goes on to say

""As we shall see, presently available commercial materials will suffice to make the HASTOL
tethers needed. The primary message we want to leave with the Reader is:
“We don't need magic materials like ‘Buckminster-Fuller-carbon-nanotubes’ to make
the space tether facility for a HASTOL system. Existing materials will do.”""

So it would appear that you misread your reference.  The tether mass of 200 times mass of the payload mass is the upper limit to the problem.  Systems where the tether mass is less than 200 times the payload mass can be built with ""presently available commercial materials.""

To further reinforce this the Conclusion to the report states

""The fundamental conclusion of the Phase I HASTOL study effort is that the concept is
technically feasible. We have evaluated a number of alternate system configurations that will
allow hypersonic air-breathing vehicle technologies to be combined with orbiting, spinning space
tether technologies to provide a method of moving payloads from the surface of the Earth into
Earth orbit. For more than one HASTOL architecture concept, we have developed a design
solution using existing, or near-term technologies. We expect that a number of the other
HASTOL architecture concepts will prove similarly technically feasible when subjected to
detailed design studies. The systems are completely reusable and have the potential of drastically
reducing the cost of Earth-to-orbit space access.""

Regarding: ""but the skyhook is not competitive with other rotating tether concepts."" and 

""A 2000 Boeing report on the possibillity of combining skyhooks with hypersonic planes concluded that ""In general, the non-spinning tether HyperSkyhook concept does not look competitive with the spinning tether concepts."".""

The full quote is:

""HyperSkyhook

In 1995 Zubrin proposed the “Hypersonic Skyhook” as a solution to the mismatch between
the attainable atmospheric speeds of a hypersonic airplane and the orbital speeds of space tethers.
Since the orbital speed of the space tether decreases with increasing altitu