(Henry Cate, Jr) wrote in message . com...
I've been playing with Len Cormier's Bear Cub. As a tether
enthusiast, I'm interested in the impact of using a tether with the
Bear Cub.

A hanging tether can give as much as 800 m/s sub orbital grapple or
trapeze target. The payload of the Bear Cub grows from 455 kg to
about 1200 kg for such a tether.

I use the rocket equation to get a table of the net velocity available
to this configuration with increased payloads. This gave me the
following table.

Payload Ideal DV $/kg
455 9.472 549
620 9.233 403
900 8.865 278
1000 8.744 250
1200 8.516 208

Len estimated a per flight cost of about $250,000. I couldn't resist
adding the cost per kilogram to this table.

I'm a bit tentative about the conversion from "Ideal DV" to orbital
altitude. If I use the Buzz Aldrin 15% loss to gravity, air
resistance, etc. Then 9.474 km/s should produce an effective 8.23
km/s. If I spend 8.07 to get into a transfer orbit, and 0.16 to
circularize the orbit, that looks like 550 km altitude. Does that
work?

Henry

For sanity; these are the figures I used in my spreadsheet.

Len gave an expected ideal DV for each of the pieces; the Tupelov
carrier, the booster stage, and the orbiter. He quoted the ISP for
the RL-10. The booster stage uses Kerosene, Len does not provide an
ISP for the booster stage. I infer an exhaust of about 3200 m/s from
the mass and DV.

Payload Ideal DV ISP m/s Empty Fuel GLOW
455 5.553 4422 Orbiter 1945 6025 8425
3.249 3200 Booster 2340 19000 29765
0.670 Tupelov95 94400
Total 9.472


This last table is essentially correct. The Isp
assumed for the booster is 3195 m/s. However, I
calculate that reduction of the orbiter delta v
requirement by 800 m/s to 4753 m/s would reduce
the required mass ratio to 2.93, which would give
a payload of about 930 kg, rather than 1200 kg--
which is still a very nice improvement. Of course,
the tether isn't free, so this would also add to
total costs. The $250,000 per flight recurring cost
is a goal, not an estimate--although I think it is
a reasonable goal.

The gross orbiter mass should remain the same,
in order to maintain initial thrust-to-weight
ratio. Higher payload means higher burnout and
reentry mass; so this would have an impact on
TPS, reducing the payload somewhat.

The Tu-95 would launch the same mass. The booster
would also remain about the same (although I might
opt for a different recovery concept). And the
total delta v requirement would be the same for
the same basic ISS orbit. BTW most of the delta v
losses are early in the game; this--along with the
near-elimination of the altitude compensation
requirement--are the main benfits of subsonic launch
at altitude.

Our Space Van 2008 envisages subsonic launch at
extreme altitude, by taking some cues from our Condor-X
X PRIZE concept. This can be equivalent to supersonic
launch at low supersonic speeds. It also results in
a relatively gentle ride to altitude with the orbiter
never encountering more than about 7500 Pa dynamic
pressure. Constraints on orbiter size and shape are
also greatly relieved. The altitude compensation
problem is completely eliminated for the booster and
orbiter.

IMO, there are many ways of getting to orbit that are
far superior to what we have been doing. I unabashedly
pursue concept designs, since I feel that system concept
design is by far the most important "technology"--especially
when one is dealing with very limited resources. We shall
focus, when we get serious money to pursue a particular
concept.

Best regards,
Len (Cormier)
PanAero, Inc.
(change x to len)
http://www.tour2space.com