On Sat, 26 Jun 2004 09:03:02 -0700, Tim Ward wrote:


"Ron Wanttaja" wrote in message
...
Space Elevators are obvious, but how does a Space Trim Tab work? :-)

Ron Wanttaja

I dunno, but this seems as good a time as any to bring up a stupid,
complicated idea of mine for access to space.

First, you should be familiar with the Kelly Aerospace idea of towing the
spaceship to altitude. If not, Google for "Eclipse project", NASA, and
perhaps F106.

They towed an idling F106 behind a C141 as a proof of concept project.

Second, you should be aware of the "payout winches" used to ground launch
hang gliders. These just pay the line out at a constant tension, rather
than reeling them in at a high rate of speed, as in sailplane launches.

So here's the scheme:
You build a tow plane about the size of a 747. The payout winch is mounted
such that it "pays out" from the CG of the airplane, on top. You have
somewhere around 100,000 lbs of Vectran tow rope (several tens of
kilometers) on the payout device. This is within the cargo capability of a
747, though you may want to throw on a couple of extra engines because of
the additional drag.

The spacecraft has a CG hook on the bottom.

You take off, and climb as high as you can, while paying out the tow line.
The spacecraft pilot basically controls the pay out. Pitch up, and a little
more line pays out. Pitch down, and it stops.

If the spacecraft can maintain a 45 degree angle behind the towplane, it
will be 70% of the towrope's length higher than the towplane.

At some point, the true airspeed of the tow plane will not provide enough
airspeed for the spacecraft to continue to climb. So the towplane starts to
turn, and the spacecraft maneuvers to the outside of the turn. Now it's
just like playing "crack the whip". The air-breathing booster is down in
the (relatively speaking) thick atmosphere at 50,000 feet, while the
spacecraft is above most of the atmosphere at say, 100,000 feet.

That's when the spacecraft releases and fires its rockets.
Because the atmosphere is so much thinner, and the spacecraft is going
faster than it would be at lower altitudes, the increase in peak altitude
achievable should be much higher than just the 50,000 ft altitude difference
between the tow plane and the spacecraft.

After the spacecraft releases, the towplane also releases the towline, and
it descends under a parachute, separately.

There, I feel better.

I'm fascinated by this idea, but I'm still not sure if I've got my head
round it.

One thing that worries me, is that stubby little planes like the
space-shuttle or spaceship 1 need to be flying very fast in order to stay
up at high altitudes. For example, the U2 cruised at this speed near
100,000 feet (unladen). If the 747 was to tow a spaceplane up to this
altitude in straight and level flight, I expect it would need to look like
an U2 and might have problems during re-entry.

Imagine the 747 was flying in 1km (radius) circles; if the spaceship flew
concentric 4km circles the ratio of its speed to that of the 747 would be
4:1. At it began to approach this speed maybe it could ascend above the
747? If the 747 could reduce the turning circle down to 0.5km radius,
you'd be up to 8 times its velocity - well on the way to orbit. I think
this is a fair simplification of your 'crack the whip' - but this is what
I'm not sure of.

3KM of 1cm wide costlium wire would have 30M^2 of frontal area which
doesn't sound unmanageable, but perhaps it would cause more drag than
that suggests. But presumably it would get quite toasty...

If our 747 is flying 0.5km radius circles at 500kph, acceleration on it
will be V^2/r = (500000/3600)^2/500 = 38.5 M/S - about 4 G's

If our spaceship is flying 4km radius circles at 8*500kph it will be
pulling (8*500000/3600)^2/4000 = about 30 G's.

Hmm...

To keep that down to 7G's at 4000kph I get 20km of cable required
80KM for 8000kph

Escape velocity is about 40,000kph

If people are seriously considering cables strong enough for
space-elevators, maybe there could be something in this. If you can
tolerate higher G-forces then you can use much more feasible lengths of
cable - perhaps it would be a practical way of launching freight into
orbit, even if it wasn't suitable for people.

Maybe you could fix one end of the cable to a mountain-top, and send
electrical power through the cable to a prop-unit mounted part of the way
along it.

AC