any limits on mechanical seals?

In article . com,
wrote:

Joe Strout wrote:
Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, ........

An interesting problem but one I would try to avoid entirely by having
no stationary parts at all on the colony, at least not pressurized
ones.

An interesting idea but not practical, in my opinion; there are far too
many uses for a stationary portion. In addition, the problems presented
by trying to dock (especially more than one ship) on a rotating station
far surpass the problems presented by large rotating seals.

The most common supposed need for stationary parts is to allow
docking spaceships but a case could be made for keeping approaching
ships a healthy distance from the main body of the colony (remember the
re-supply ship that collided with Mir?).

A poor case, I think. Can you imagine a town where all vehicles have to
park 1 km outside the city limits, and passengers and cargo transferred
in and out of town by some other means? Yes, it's possible, but it's
highly inefficient and expensive compared to letting the ships dock
directly.

I would have tethered platforms swinging around the colony, 100m or
more away. The ships would drop landing gear and brake to a stop.

Hah! Thank you for a humorous mental image.

Best,
- Joe

Joe Strout wrote:
In article . com,
wrote:

Joe Strout wrote:
Suppose you have a large space station (OK, let's say a colony) with a
rotating portion and a stationary portion, ........

An interesting problem but one I would try to avoid entirely by having
no stationary parts at all on the colony, at least not pressurized
ones.

An interesting idea but not practical, in my opinion; there are far too
many uses for a stationary portion.

Would you list some?

In addition, the problems presented
by trying to dock (especially more than one ship) on a rotating station
far surpass the problems presented by large rotating seals.

I disagree. A 1km radius colony with 100 meter tethers would have
almost 7km of potential landing pad. Say it's a continous
strip/belt/runway. A ship would slowly approach the runway, 'land' at
a relative speed of about 115 m/sec or 259 miles/hr, decelerate at 1
gee, and use about 10% of the runway to do so. Four ships could land
simultaneously with no problem.

The most common supposed need for stationary parts is to allow
docking spaceships but a case could be made for keeping approaching
ships a healthy distance from the main body of the colony (remember the
re-supply ship that collided with Mir?).

A poor case, I think. Can you imagine a town where all vehicles have to
park 1 km outside the city limits, and passengers and cargo transferred
in and out of town by some other means? Yes, it's possible, but it's
highly inefficient and expensive compared to letting the ships dock
directly.

It's only 100m from the inside of the colony, not 1 km. I'm trying to
imagine a town-analogy where vehicles carrying tons of explosive fuel
approach at varying speeds to a town-wall/membrane that, if breached,
could kill thousands quickly. Can't seem to do it but that does
describe the colony situation. Also, colony visitors could probably be
in their hotel room or cargo at it's final shelfspace much faster than
similar visitors/cargo arriving at an airport on Earth.

I would have tethered platforms swinging around the colony, 100m or
more away. The ships would drop landing gear and brake to a stop.

Hah! Thank you for a humorous mental image.

You're welcome but I'm perfectly serious.

In article .com,
wrote:

An interesting idea but not practical, in my opinion; there are far too
many uses for a stationary portion.

Would you list some?

Well, docking is the obvious one. Then there's observation; it's hard
to keep your telescope trained on something when you're whizzing around
at 2 RPM. Also, workers need to travel frequently between the habitat
(where they live) and the big external manufacturing or power facilities
(where they work) -- this could most efficiently be done via transport
tubes, but such tubes need a stationary place to attach.

In addition, the problems presented
by trying to dock (especially more than one ship) on a rotating station
far surpass the problems presented by large rotating seals.

I disagree. A 1km radius colony with 100 meter tethers would have
almost 7km of potential landing pad.

Of *unusable* landing pad. The very idea is ludicrous. Your landing
platforms are whizzing by at over 400 km/hr -- and in circular (rather
than straight!) paths! I'm all for thinking outside the box, but let's
not open our minds so far that our brains fall out.

Say it's a continous
strip/belt/runway. A ship would slowly approach the runway

From what direction can you slowly approach such a runway? Any
direction I imagine involves passing through (a) the runway, (b) the
habitat, or (c) the runway supports that are moving at 400 kph. (A) and
(b) violate the laws of physics, and as for (c), you'd quickly end up
adding to the space-debris problem (see other thread).

If you can solve *that* problem, I think you may have something mildly
interesting -- but only "mildly" since it would mean only very
specialized spacecraft can go to/from the habitat. I prefer to allow
pretty much any ordinary spacecraft to visit. But that's probably moot,
since I don't think this problem is solvable anyway.

A poor case, I think. Can you imagine a town where all vehicles have to
park 1 km outside the city limits, and passengers and cargo transferred
in and out of town by some other means? Yes, it's possible, but it's
highly inefficient and expensive compared to letting the ships dock
directly.

It's only 100m from the inside of the colony, not 1 km.

With your impossible landing-strip scenario, yes. I was addressing the
more reasonable proposition of ships matching orbits with the colony a
short but safe (e.g. 1 km) distance away, and then transferring
passengers and cargos using some sort of ferry, such that only one ferry
would dock at the time. More reasonable, but still horribly inefficient.

I'm trying to imagine a town-analogy where vehicles carrying tons of
explosive fuel approach at varying speeds to a town-wall/membrane that,
if breached, could kill thousands quickly. Can't seem to do it but that
does describe the colony situation.

I don't think it does. There would surely be emergency doors at key
places -- including between the dock and the main habitat -- that would
automatically shut in the event of a drop in pressure. And with
structures this large, even blowing a fairly largish hole in the hull
does not result in immediate depressurization -- it could take weeks or
months, depending on the relative sizes of course. That allows plenty
of time for those in the compromised portion to make it into emergency
chambers, or don emergency suits.

Such an accident would probably result in *some* casualties -- those
close to the explosion, just as on Earth -- but I think not as many as
you're imagining.

Best,
- Joe

Joe Strout wrote:
In article .com,
wrote:

An interesting idea but not practical, in my opinion; there are far too
many uses for a stationary portion.

Would you list some?

Well, docking is the obvious one. Then there's observation; it's hard
to keep your telescope trained on something when you're whizzing around
at 2 RPM.

First of all, I will assume 1 RPM; most large colony designs assume
that's the most comfortable reasonable spin rate, though there's no
real data, I think. Back in the 1940s, Arthur C. Clarke and other
members of then-new British Interplanetary Society invented a
periscope-thingy with rotating mirrors for the purpose of making
outside observations. It's mentioned in one of his books, sorry, can't
recall which. But by the time we build O'Neill colonies, I expect even
amateur telescopes (used in space) will consist of CCD devices on
unmanned platforms or Luna (low vibration), with digital data/control
signals sent to and from the user.

Also, workers need to travel frequently between the habitat
(where they live) and the big external manufacturing or power facilities
(where they work) -- this could most efficiently be done via transport
tubes, but such tubes need a stationary place to attach.

Why would the workspace need to be stationary? Because it makes use of
zero gravity? I'm not sure what manufacturing processes you're
thinking of that require zero-gee, maybe ore smelting or assembling
Solar Power Satellites? If so, those would also be done in vacuum so a
rotating seal between two pressurized spaces, one rotating and one not,
would not be necessary. Imagine two large spheres; one the colony and
the other the workspace. The first is pressurized and rotating and the
second is neither. They are joined by a non-sealed, very large (100m
diameter) rotating bearing/track. The colony side has a number of
airlocks, spread over a 20m diameter area and opening into the vacuum
of the workspace. From the point of view of the workspace these doors
are moving by slowly (2.4 mph), about as fast as an escalator. Not
hard to enter or exit. But I think you should give serious
consideration to keeping high-energy, high-temperature, zero-gee
workspaces disconnected and well separated from living spaces. Let
people suit up and commute in a small space-trolley.

In addition, the problems presented
by trying to dock (especially more than one ship) on a rotating station
far surpass the problems presented by large rotating seals.

I disagree. A 1km radius colony with 100 meter tethers would have
almost 7km of potential landing pad.

Of *unusable* landing pad. The very idea is ludicrous. Your landing
platforms are whizzing by at over 400 km/hr -- and in circular (rather
than straight!) paths! I'm all for thinking outside the box, but let's
not open our minds so far that our brains fall out.

Say it's a continous
strip/belt/runway. A ship would slowly approach the runway

From what direction can you slowly approach such a runway? Any
direction I imagine involves passing through (a) the runway, (b) the
habitat, or (c) the runway supports that are moving at 400 kph. (A) and
(b) violate the laws of physics, and as for (c), you'd quickly end up
adding to the space-debris problem (see other thread).

Imagine harder. Here's some bad ASCII art:
__ The 'ball' in the center is the colony, the vertical
lines are the tethers
! holding the runway, the small horizontal lines are
crossections
----!--- of the runway, the # is the ship moving slowly to
land on the runway.
/ \ You are in the plane of the runway, looking at it
edge on.
\ / So it is physically possible, though I will grant you
that landing
-------- requires landing gear and could be .... interesting.
! #
--!--



If you can solve *that* problem, I think you may have something mildly
interesting -- but only "mildly" since it would mean only very
specialized spacecraft can go to/from the habitat. I prefer to allow
pretty much any ordinary spacecraft to visit. But that's probably moot,
since I don't think this problem is solvable anyway.

Not so fast. Have a long (500m), 20m diameter pressurized tube
extending straight out from one pole of the colony and studded with
airlocks/docks. A ship would approach the tube and connect to a dock
that's moving by at only 2.4 miles/hr. The ship would then be hanging
from the side of the tube and feeling 0.01 gee. It's not unreasonable
to move cargo/people 'up' to the door in 0.01 gee. There could be a
continuous loop conveyor running down the center of the tube, carrying
people into the colony in minutes.

In article .com,
wrote:

From what direction can you slowly approach such a runway?

Imagine harder. Here's some bad ASCII art:...

OK, so you're supposing the runway is only supported in the middle;
essentially a cantilever structure. I'll agree this is physically
possible, but I think it might be very hard to make a decent-sized
runway that could hold itself up under such conditions under more than 1
G acceleration. as you say, landing would be... interesting.

Not so fast. Have a long (500m), 20m diameter pressurized tube
extending straight out from one pole of the colony and studded with
airlocks/docks. A ship would approach the tube and connect to a dock
that's moving by at only 2.4 miles/hr. The ship would then be hanging
from the side of the tube and feeling 0.01 gee. It's not unreasonable
to move cargo/people 'up' to the door in 0.01 gee.

But how *exactly* do you dock with something moving by at 2.4 mph? All
dockings I've seen start by matching velocity with the dock, and then
closing the distance very slowly. Seems like suddenly connecting a
spacecraft to such a rotating tube is going to impart quite a lurch to
one or both. Assuming a largish spacecraft, I'd expect the result would
be rupturing of the tube, which would then start thrashing about,
spraying people in one direction and propelling the habitat in the
other. (Now *there's* a vivid mental image, eh?)

But you certainly get points for creativity (and I mean that sincerely).

Best,
- Joe

On Thu, 13 Jul 2006 14:30:14 -0600, Joe Strout wrote:

In article .com,
wrote:

Not so fast. Have a long (500m), 20m diameter pressurized tube
extending straight out from one pole of the colony and studded with
airlocks/docks. A ship would approach the tube and connect to a dock
that's moving by at only 2.4 miles/hr. The ship would then be hanging
from the side of the tube and feeling 0.01 gee. It's not unreasonable
to move cargo/people 'up' to the door in 0.01 gee.

But how *exactly* do you dock with something moving by at 2.4 mph? All
dockings I've seen start by matching velocity with the dock, and then
closing the distance very slowly. Seems like suddenly connecting a
spacecraft to such a rotating tube is going to impart quite a lurch to
one or both.

This strikes me as being downright easy, and gentle, compared with e.g.
mating a B-52 and a KC-10 at Mach 0.8 or so. Yet the latter is done
many times a day, without incident.

Doesn't mean there won't be plenty of engineers with job security on
account of such a requirement, but it does mean that there shouldn't be
much doubt about the end result.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *

In article ,
John Schilling wrote:

On Thu, 13 Jul 2006 14:30:14 -0600, Joe Strout wrote:

In article .com,
wrote:

Not so fast. Have a long (500m), 20m diameter pressurized tube
extending straight out from one pole of the colony and studded with
airlocks/docks. A ship would approach the tube and connect to a dock
that's moving by at only 2.4 miles/hr. The ship would then be hanging
from the side of the tube and feeling 0.01 gee. It's not unreasonable
to move cargo/people 'up' to the door in 0.01 gee.

But how *exactly* do you dock with something moving by at 2.4 mph? All
dockings I've seen start by matching velocity with the dock, and then
closing the distance very slowly. Seems like suddenly connecting a
spacecraft to such a rotating tube is going to impart quite a lurch to
one or both.

This strikes me as being downright easy, and gentle, compared with e.g.
mating a B-52 and a KC-10 at Mach 0.8 or so. Yet the latter is done
many times a day, without incident.

Um, wait, are we talking about the same thing? I'm quite sure that a
B-52 and a KC-10 don't mate at a *relative* velocity of Mach 0.8. And
that's what we're talking about here -- a spaceship attempting to
connect to a dock that is moving, *relative to the ship* (as it must be,
since the dock is moving in a tight circular path that would be darn
near impossible for any ship to match).

Best,
- Joe

On Fri, 14 Jul 2006 16:56:19 -0600, Joe Strout wrote:

In article ,
John Schilling wrote:

On Thu, 13 Jul 2006 14:30:14 -0600, Joe Strout wrote:

In article .com,
wrote:

Not so fast. Have a long (500m), 20m diameter pressurized tube
extending straight out from one pole of the colony and studded with
airlocks/docks. A ship would approach the tube and connect to a dock
that's moving by at only 2.4 miles/hr. The ship would then be hanging
from the side of the tube and feeling 0.01 gee. It's not unreasonable
to move cargo/people 'up' to the door in 0.01 gee.

But how *exactly* do you dock with something moving by at 2.4 mph? All
dockings I've seen start by matching velocity with the dock, and then
closing the distance very slowly. Seems like suddenly connecting a
spacecraft to such a rotating tube is going to impart quite a lurch to
one or both.

This strikes me as being downright easy, and gentle, compared with e.g.
mating a B-52 and a KC-10 at Mach 0.8 or so. Yet the latter is done
many times a day, without incident.

Um, wait, are we talking about the same thing? I'm quite sure that a
B-52 and a KC-10 don't mate at a *relative* velocity of Mach 0.8. And
that's what we're talking about here -- a spaceship attempting to
connect to a dock that is moving, *relative to the ship* (as it must be,
since the dock is moving in a tight circular path that would be darn
near impossible for any ship to match).


So, there's no middle ground between exactly matching the motion of the
docking port, and just standing "still" in some unspecified reference
frame so as to collide with the docking port as it swings by?

If nothing else, it seems obvious that the docking spacecraft would
approach tangent to the docking port, at the aforementioned 2.4 mph,
so as to have zero velocity relative to the docking port at the estimated
time of docking.

Also seems reasonable that the ship would as it approaches set up a
free rotation at 1 rpm and a radial acceleration of 0.01 gee and, well,
it looks like that's all it takes to perform the "darn near impossible"
task of matching the path of the docking port.

If the controls were properly laid out, I'm pretty sure I could fly that
maneuver entirely manually.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

In article ,
John Schilling wrote:

Also seems reasonable that the ship would as it approaches set up a
free rotation at 1 rpm and a radial acceleration of 0.01 gee and, well,
it looks like that's all it takes to perform the "darn near impossible"
task of matching the path of the docking port.

If the controls were properly laid out, I'm pretty sure I could fly that
maneuver entirely manually.

I admire your confidence, but I'd like to see that myself. From a safe
distance.

Best,
- Joe