Questions about "The High Frontier"

On Oct 24, 12:26 am, Hop David wrote:

Asteroid mined metal would come from a metallic asteroid which is
unlikely to have volatiles.

That's a reasonable objection. At best, you can overcome it by
assuming (he waves his hands here) the construction of spheres
designed for reentry with different materials coming from different
asteroids -- which I admit would add hugely to the cost and time in
what would already be a very high cost, long-haul endeavor.

There's a navigation error: it flies past Earth at or somewhat below
its aerocapture threshold. Well, then, maybe it makes it all the way
down. BUT, if it has a very low mass/volume ration, maybe it just
falls to a certain altitude in the atmosphere and ... floats.

It depends on duration of transit through the atmosphere. If its passage
lasts long enough for heat to be conducted through the metal shell, the
volatiles will turn to gas and provide supporting pressure. But if the
volatiles don't vaporize fast enough, I'd expect the metal ball to
crumple. Is it possible to have a metal shell strong enough to keep its
shape while enclosing vacuum but light enough to float? I don't know.

I think Buckminster Fuller worked out long ago that big hollow spheres
open at the bottom -- if made big enough -- could float in the
atmosphere by virtue of only solar heating of the shell. Which
inspires a possible workaround to your objection: allow some of
Earth's atmosphere into the sphere as it descends. Smallish holes in
the shell might be all to the good, allowing in air that the sphere
has heated by reentry. This hot air would help keep the sphere
buoyant, and with the right valve design, keep it buoyant. Or --
forget the complexity of valves, just make the sphere's shell thicker
and heavier on one end, and locate the atmosphere inlets there. At
the right mass/volume ratio, the thing stays afloat just from solar
heating, over the long term.

Note that I keep saying "sphere". I have no idea whether this is the
optimal geometry. Most likely something spherOIDAL, though.

Given your conditions above, the object has a low flight path angle so
it might have a long enough trip for heat to conduct through the shell.

I'm not sure internal volatiles are required anyway. The problematic
propellant tanks mentioned in this RFP

http://www.openfos.com/supply_histor...CEPT_STUDY.htm

didn't have much propellant in them when they landed, largely intact,
on the ground, after which they were still largely intact. They are
called "debris" in that RFP only because they were no longer useful as
tanks, and were still coming down fast enough to have damaged
something or hurt somebody, though the chances were very slight. So
there doesn't seem to be any question that some metal propellant tanks
can make it down alive. But the ones they speak of were aluminum and/
or titanium, which have high melting temperatures and high strength
compared to more abundant metals out there, so those are some
variables you'd have to look at, not just radius, shell thickness, and
details of geometry.

... I agree
density should be considered when establishing payload ceilings. Hollow
payloads may well be more amenable to aerobraking.

I think the main value would be "aerobrake AS (much or most of the)
payload", or, to put it another way, "payload doubling as aerobrake".
You'd lose far less of your aerobrake, and what you'd have left of it
would have value -- as materials. Maybe even as prefab structures,
for some applications. A big enough spheroidal shell could be spun
for artifical gravity, for example. A thick enough shell gives you
debris-strike protection and some radiation shielding.

-michael turner

Michael Turner wrote:
That's a reasonable objection. At best, you can overcome it by
assuming (he waves his hands here) the construction of spheres
designed for reentry with different materials coming from different
asteroids -- which I admit would add hugely to the cost and time in
what would already be a very high cost, long-haul endeavor.


I want to see someone explain where the energy to move an asteroid down
into a orbit that gets near Earth comes from.
That's going to take a lot of Estes "D" engines. :-)
We're still trying to figure out a reasonable way to move one on a
collision course with Earth fifty thousand miles to one side given
several months to a year or so notice.

Pat

Pat

There's a navigation error: it flies past Earth at or somewhat below
its aerocapture threshold. Well, then, maybe it makes it all the way
down. BUT, if it has a very low mass/volume ration, maybe it just
falls to a certain altitude in the atmosphere and ... floats.

It depends on duration of transit through the atmosphere. If its passage
lasts long enough for heat to be conducted through the metal shell, the
volatiles will turn to gas and provide supporting pressure. But if the
volatiles don't vaporize fast enough, I'd expect the metal ball to
crumple. Is it possible to have a metal shell strong enough to keep its
shape while enclosing vacuum but light enough to float? I don't know.


I think Buckminster Fuller worked out long ago that big hollow spheres
open at the bottom -- if made big enough -- could float in the
atmosphere by virtue of only solar heating of the shell. Which
inspires a possible workaround to your objection: allow some of
Earth's atmosphere into the sphere as it descends. Smallish holes in
the shell might be all to the good, allowing in air that the sphere
has heated by reentry. This hot air would help keep the sphere
buoyant, and with the right valve design, keep it buoyant. Or --
forget the complexity of valves, just make the sphere's shell thicker
and heavier on one end, and locate the atmosphere inlets there. At
the right mass/volume ratio, the thing stays afloat just from solar
heating, over the long term.

Note that I keep saying "sphere". I have no idea whether this is the
optimal geometry. Most likely something spherOIDAL, though.


Given your conditions above, the object has a low flight path angle so
it might have a long enough trip for heat to conduct through the shell.


I'm not sure internal volatiles are required anyway. The problematic
propellant tanks mentioned in this RFP

http://www.openfos.com/supply_histor...CEPT_STUDY.htm

didn't have much propellant in them when they landed, largely intact,
on the ground, after which they were still largely intact. They are
called "debris" in that RFP only because they were no longer useful as
tanks, and were still coming down fast enough to have damaged
something or hurt somebody, though the chances were very slight. So
there doesn't seem to be any question that some metal propellant tanks
can make it down alive. But the ones they speak of were aluminum and/
or titanium, which have high melting temperatures and high strength
compared to more abundant metals out there, so those are some
variables you'd have to look at, not just radius, shell thickness, and
details of geometry.


... I agree
density should be considered when establishing payload ceilings. Hollow
payloads may well be more amenable to aerobraking.


I think the main value would be "aerobrake AS (much or most of the)
payload", or, to put it another way, "payload doubling as aerobrake".
You'd lose far less of your aerobrake, and what you'd have left of it
would have value -- as materials. Maybe even as prefab structures,
for some applications. A big enough spheroidal shell could be spun
for artifical gravity, for example. A thick enough shell gives you
debris-strike protection and some radiation shielding.

-michael turner

Re-entry systems are a real problem to design. Both Apollo and the
Shuttle's re-entry systems were incorrectly modeled (which has
implications for Orion - narrow entry corridor). These were designed
on a spherical section, because that was easy to model. The
"spotlight" shape of the Soyuz and Dragon capsules is reportedly a
better design than Apollo's cone.

A spherical design with no centre of mass offset would tumble, but
that might be nice as it would allow part of the heatshield to cool.

Sphere-cones look like your ICBM re-entry vehicles, ie like a
shuttlecock. The advantage of these is aerodynamic stability.The
maximum half-angle you can have on a cone is about 60 degrees in Earth
atmosphere (ie with the point of the cone downwards). Mars landers can
have flatter cones of about 70 degrees half-angle. Using less than
these angles results in aerodynamic instability. The aft end can be
flat, conical or domed.

Radiatively cooled designs were used on early Mercury capsules and
some ballistic missile designs. For re-entry purposes (assuming
descent from orbital velocity and not meteoric re-entry), I think that
a sufficient thickness of metal can serve as both heat sink and
radiator. There are some high-tech designs like hafnium diboride but I
think that's getting a little too fancy for what is basically a metal
ingot.

On Oct 24, 1:06 pm, Pat Flannery wrote:
Michael Turner wrote:
That's a reasonable objection. At best, you can overcome it by
assuming (he waves his hands here) the construction of spheres
designed for reentry with different materials coming from different
asteroids -- which I admit would add hugely to the cost and time in
what would already be a very high cost, long-haul endeavor.

I want to see someone explain where the energy to move an asteroid down
into a orbit that gets near Earth comes from.
That's going to take a lot of Estes "D" engines. :-)
We're still trying to figure out a reasonable way to move one on a
collision course with Earth fifty thousand miles to one side given
several months to a year or so notice.

Pat

Hm, not quite a fair comparison, in this context. For several
reasons:

(1) For purposes of avoiding catastrophic collision events, the
natural focus is on deflection propulsion for NEA asteroids in certain
size ranges. What I'm talking about, if it could work, would probably
be fine for metallic asteroids much smaller than the kind people are
worried about.

(2) I'm talking about moving refined materials of certain types
(mainly metallic), not entire asteroids. If you could carve or sweep
ores off a large asteroid, you might send the refined materials to
Earth in a series of shipments, the sum total of which might never be
more than single-digit percentages of the asteroid's mass. And so
much the better if the way those shipments are propelled from the
asteroid surface tend to deflect a potential Earth-crossing asteroid
from a collision path. (BTW, I'm aware of time-value-of-money
arguments about why it's better to just make asteroid mining a single,
two-way trip, because minimal-delta-V return-trajectory launch windows
will be infrequent.)

(3) The "several months to a year" real-time constraint for preventing
a collision doesn't apply to resource extraction, which is not an
emergency, just (we would hope) an opportunity. Or rather, time
constraints don't apply quite as much -- time *does* matter if you're
trying to deflect a colossal state of bankruptcy. The longer it takes
to get there, process ores into good stuff, and get good stuff back,
the less promising your Net Present Value calculations will seem.
From economic analyses of asteroid mining I've seen, time-to-profit is
a much bigger factor than most space enthusiasts realize, hardly
insignificant compared to initial capital investment.

(4) As for "where the energy comes from" in any case, I don't see why
solar photovoltaic and/or nuclear wouldn't work. So there are at
least two ways to get energy, and there a number of ways (some
admittedly very speculative at this point) to economize on energy.
For example, gravitational assist and aerobraking from other planets
(Mars/Venus) is a delta-V source, if your finances can survive the
long detours. I rather like the minimagnetosphere solar sailing and
plasma beaming propulsion concepts as well, though they are a very
long way from proven. Perhaps more practically, for smaller payloads,
tethers anchored to the asteroid being mined might provide a valuable
initial boost. And I think we're still in a very steep part of the S-
curve for electric (ion) propulsion technology, assuming we've even
hit the first inflection point yet. There's probably no single silver
bullet; practical transportation of materials from mined asteroids
back to Earth orbit would probably require a mix of techniques. That
might seem messy, I suppose, but any package that gets delivered to
your door will have gotten to you by way of several different
transportation modes (truck-train-plane-train-truck) using several
different fuels and engine types.

-michael turner

On Oct 24, 2:41 pm, Troy wrote:
Re-entry systems are a real problem to design. Both Apollo and the
Shuttle's re-entry systems were incorrectly modeled (which has
implications for Orion - narrow entry corridor). These were designed
on a spherical section, because that was easy to model. The
"spotlight" shape of the Soyuz and Dragon capsules is reportedly a
better design than Apollo's cone.

A spherical design with no centre of mass offset would tumble, but
that might be nice as it would allow part of the heatshield to cool.

Sphere-cones look like your ICBM re-entry vehicles, ie like a
shuttlecock. The advantage of these is aerodynamic stability.The
maximum half-angle you can have on a cone is about 60 degrees in Earth
atmosphere (ie with the point of the cone downwards). Mars landers can
have flatter cones of about 70 degrees half-angle. Using less than
these angles results in aerodynamic instability. The aft end can be
flat, conical or domed.

Radiatively cooled designs were used on early Mercury capsules and
some ballistic missile designs. For re-entry purposes (assuming
descent from orbital velocity and not meteoric re-entry), I think that
a sufficient thickness of metal can serve as both heat sink and
radiator. There are some high-tech designs like hafnium diboride but I
think that's getting a little too fancy for what is basically a metal
ingot.

I admit to having no idea whether tumbling down would be better than
presenting one face to the atmosphere, for the aerobraking/aerocapture
modes I'm thinking of.

When asked about demisable propellant tank concepts, I suggested a
slightly oblate spheroid (or spherical shell with gradually higher
shell thickness toward its "equator") with finely scored grooves
radiating spirally out from the two points at the flattest/thinnest
parts. But that was with demisability in mind: making a hollow,
oblate metal spheroid orient itself and "pinwheel" very fast while
facing in one direction would tend to cause disintegration high in the
atmosphere, if it was hot enough and spinning fast enough, and maybe
it would come apart high enough that the resulting pieces would
evaporate/incinerate further on in the reentry path.[*] I also
suggested providing for a small aperature at the lower point of the
pinwheeling axis. This would, in my suggested concept, admit enough
superheated air during reentry to apply a lot of heat load from the
interior as well, perhaps even more so if those gases were powerfully
centrifuged to the sides, thus heating up even more under the added
pressure, as well as applying added internal pressure to the sides.

However, if there were compelling reasons why a *slightly* oblate
spheroid were better for surviving aerobraking/aerocapture, some
variations on the concept might work for helping to prevent demise,
rather than to accelerate it. Just place and angle the spiral grooves
(or whatever passive control surfaces) so that the spheroid never
spins very fast. And -- in the aerocapture scenario -- don't open
that lower central aperture (which, in an object of this size, you
could create with explosives if necessary) until you really need to do
that in order to prevent a collapse that might otherwise be caused by
mounting air pressure and softening of the shell from reentry heating.

As for the shell being a good enough heat sink, I worry about that.
This is why I mention coating the object with something highly IR-
reflective -- maybe gold or silver, taken along to the asteroid, or
perhaps mined from it, and possibly applied by very slow sputtering
processes, in very slow orbits around the shell during the long
transit to Earth. With the shell radiating into cosmic background and
reflecting 98% of incoming IR, its equilibrium temperature would
probably be cryogenic long before it got to the upper atmosphere of a
planet, and would thus sink more heat. Maybe this coating step would
only be important if studies revealed that an adequate thickness for
heat sinking would leave the sphere too heavy to float in the
atmosphere after aerocapture. Would you lose all of the IR shield in
the first aerobraking pass? Probably, especially if you designed for
tumbling. If didn't design for tumbling, could you hold on to half of
the IR shield for a subsequent pass, and get some (more temporary)
cooling value from it, using the other side of the (presumably oblate)
spheroid? Who knows?

Any such detailed questions should wait on getting a first-order
analysis done, though. Like I say, I don't really know what I'm
talking about here. I find the concept fascinating (obviously) but I
don't have the modeling chops to really investigate. Oblate
spheroids? Eggs? Domed cones? Who knows? Maybe it's a nonstarter
with any geometry, and almost any substance, beyond a certain size
range. All I really know is that satellite propellant tanks come
back from space to haunt us, almost unharmed, and I find that very
thought-provoking.

-michael turner
[*] Because I'm a space nut amateur who likes to think about stuff way
above his $0/year pay grade, I also considered whether you could use
some such aerobrake geometry to harvest rotational kinetic energy
during aerobrake passes, to be converted to electricity for electrical
propulsion, or perhaps transferred to a tether. My insanity is
clearly boundless.

Pat Flannery wrote:


Michael Turner wrote:

That's a reasonable objection. At best, you can overcome it by
assuming (he waves his hands here) the construction of spheres
designed for reentry with different materials coming from different
asteroids -- which I admit would add hugely to the cost and time in
what would already be a very high cost, long-haul endeavor.



I want to see someone explain where the energy to move an asteroid down
into a orbit that gets near Earth comes from.

We're talking about payloads sent from an asteroid, not the entire
asteroid. The former is much more plausible than the latter.

Hop

John Schilling wrote:


But, better still to explain why you think the relative energy differences
are so important in the first place.

One of Henry Spencer's mantras is "fuel is cheap". I should know better
than to disagree with Henry, every one of my efforts to get a t-shirt
have been (slightly embarrassing) learning experiences.

You and Henry are more knowledgeable than I but nevertheless I have a
hard time swallowing this. For these reasons:

1) Delta vee budgets several times exhaust velocity make single-stage,
reusable vehicles very difficult. Multi-stage vehicles generally include
expendable engines and fuel tanks. Reliance on expensive expendables for
every mission will keep missions extremely expensive.

2) Fuel at LEO and beyond isn't cheap. Let's say we had Goff's orbital
fuel depots supplied by Herbert's big dumb boosters from earth, Kuck
Mosquitoes, and oxygen from lunar ilmenite. Even with this very
optimistic scenario, fuel at different staging points on earth's gravity
well _still_ wouldn't be cheap.

Are either 1) or 2) erroneous notions?

Hop

"Jim Davis" wrote in message
. 96.26...

Mike, you're a good guy and everything, but the above is a textbook
example of thinking with your heart instead of your head. In any
other context except space (you yourself bring up oil rigs) you
would quickly recognize the absurdities. But since this is space
we're talking about...well, things are different in space, right?

This big difference is that a person on an oil rig can pop back and forth
between it and land much more quickly and inexpensively than a person
working in HEO can pop back and forth to Earth.

Yeah, I'm aware that I'm arguing events will go in the direction of my
fondest dreams. So caution is called for. But I think part of my point was
that a company might have a perfectly selfish reason for wanting to provide
their workers with decent places to live (namely, a desire to not have to
train and educate new recruits every two years because nobody wants to stay
with the company any longer than that).

--


Regards,
Mike Combs
----------------------------------------------------------------------
By all that you hold dear on this good Earth
I bid you stand, Men of the West!
Aragorn

Hop David wrote:

We're talking about payloads sent from an asteroid, not the entire
asteroid. The former is much more plausible than the latter.

Still a significant amount of energy required per kilogram of material
ending up in HEO.
At some point it makes more sense just to shoot it up from Earth, or at
least from the Moon.
The other problem is mining the asteroids per-say... solar flux at that
distance is a lot lower than here on Earth, so a major source of free
energy is not as effective.
The other thing is that with there low gravity fields getting around on
them is going to be more like EVAing than driving or walking.
If you are going to try and colonize them, then it makes more sense just
to leave the material right in the orbit it's in, and build your
colonies out in the asteroid belt itself, rather than using all the
energy to move it inwards toward Earth.
The Moon is a lot closer at hand for mining, you can walk and drive
around on it, it has a lot more sunlight falling on it, and you have a
reasonable transit time to and from Earth.
One of the big problems of this scenario though is that it presumes that
humanity will continue growing in numbers like yeast colonies and
eventually run out of room on Earth to live.
There's no particular reason for that to be the case, as Spartan Condoms
are cheaper than Space Colonies.
Dropped to about 1/2 - 3/4 its present size by natural mortality and
lowered birth rates over the next century if desired, there's no reason
that the human population of Earth...particularly given new sources of
renewable energy and recycling of materials... can't be quite happy and
stable for the next few millenia.
By that time our technology may allow us to colonize other Earth-like
planets throughout the galaxy by shoving people, living quarters, and
cargo through a wormhole directly onto their surface.
Alternately, if some of the parallel dimension theories are right, we
may be able to colonize other Earths where Homo Sapiens never evolved
and sell all the dinosaurs to McDonald's Restaurants, or, in a different
timeline, turn the bipedal apes into servants, with hilarious results
somewhere down the line when they learn to speak, and begin to plot
against us. :-)

Pat

On Oct 23, 5:58 pm, Jim Davis wrote:
Mike Combs wrote:
On the other hand, one might note that with the SMF with the
spartan living conditions one sees quick turnover in skilled
workers. People sign-on for 2 year tours (or whatever), fill up
their bank accounts, and then come back to Earth to spend their
money, because what are they going to spend it on living in an
aluminum can in HEO? Then you might note that since you've got
mining facilities on the moon and/or a NEA, a means of
transporting ore to HEO, ore refineries and parts fabrication
facilities in that same orbit, most of what you need to build a
Bernal Sphere or Stanford Torus is already in place (and perhaps
already paid for by SPS profits). So it might be worth a bit of
investment for your workers to be able to live under natural
sunlight surrounded by greenery, and able to do ordinary things
like fish in a pond or walk in a park. Perhaps highly-trained
and skilled workers might be more apt to spend their entire
careers with you if they can look at their apartment or house in
Bernal Alpha as "home" rather than some place on Earth.
Families might be more apt to form in such a place than in a
place which more resembled an off-shore oil rig.

Mike, you're a good guy and everything, but the above is a textbook
example of thinking with your heart instead of your head. In any
other context except space (you yourself bring up oil rigs) you
would quickly recognize the absurdities. But since this is space
we're talking about...well, things are different in space, right?

Jim Davis- Hide quoted text -

- Show quoted text -

The only way any of that would make sense is if the cost of returning
workers to Earth, and the related turnover, was less than the cost of
constructing a habitat. Slot in selected assumptions about relative
cost and you can reach an answer. The answer is almost surely going
to be 'no'.