Someone asked the question:  "If we could, in principle, throw something
into lunar orbit from Mare Crisium, why couldn't we throw something into
Earth orbit from the Rockies?"  (Or words to that effect.)

So here's the item to talk about different ways to get into space.

32 responses total.

Slinging something off the Moon is a fairly simple engineering
task.  The speed to orbit the Moon is only about a mile per second,
and the vacuum is good enough that you can pretty much ignore
drag.  Lack of drag means that the payloads can be about any
convenient size or shape.  Small payloads require little energy
and can be slung at a high, continuous rate.

Slinging something off of Earth is, ah, more difficult.  First,
orbital velocity is about 5 miles a second, so the energy requirement
is 25 times as high.  To get through the atmosphere, you need a
minimum size.  It needs a heat shield or else it's going to suffer.

I've seen proposals for catapults to launch bulk items into orbit.
The canisters would be about the size and shape of a telephone pole
and carry a foot or so of graphite on the forward end.   IT would be
launched at about 6.5 miles per second and lose about 1.5 miles per
second on the way up.

Engineering such a beast would be difficult.  The payloads are big
and infrequent instead of small and frequent.  The energy and power
demands are much bigger, power storage is a problem, sonic booms,
the catapult has to be protected from the plasma produced when
the payload slams into air, you've got environmental issues...
Not to mention that the demand for the bulk commodities which are
the only things which can take such a rough ride (thousands of
gravities) isn't there yet.

So yes, in principle it can be done.  But the engineering, economic
and political problems mean it probably won't be any time soon.

too bad dr. bull is dead.

Thanks, russ..

I had been hoping that some new technologies had evolved.. *sigh*

I presume, then, that it would be less economically feasible than current
launchs? How about switching to strict H2&O engines?

Oh, catapult launches would be wonderfully cheap if you were lifting
millions of tons a year.  For a few hundred tons a year, it would
make as much sense as building a conveyor belt to move a few bundles
of newspapers once a week.

Hm...got me thinking...suppose you were sitting on the moon, feeling
bored, and partook of a few lunar brewskies.  With nothing better to
do, you decide to shoot the earth.  With a regular .22 caliber rifle,
do you think your bullet would be able to hit the earth's atmosphere?

Wouldn't make it.  Lunar orbital velocity is about a mile per
second.  That's 5300 ft/sec to 2 significant figures.  If memory
serves, 2000 ft/sec is pretty fast for a rifle bullet.  So, your
.22 bullet wouldn't even make it into lunar orbit, much less back
to Terra.

You could make your neighbors cower, though.  That .22 bullet
would have a range of many, many miles in vacuum.

Hmmm...maybe if you first place a few small dense masses in lunar orbit (black
holes would be dramatic, but lumps of neutronium ought to suffice), and you
were a real sharp shooter, you might be able to use the slingshot effect to
staircase you pellet out of the moon's gravity well.

Ummm... What about firin to the East? Wouldn't the RPM rate add some
delta-V?

Sum, but not nearly enough.  The moon only rotates once a month after all.
It's not a fast spinner.

Wouldn't the lower gravity be a much bigger factor in how far a bullet goes
than the rare atmosphere? (ie: 1/2 gt^2)

in the case of the moon, i'd think not.  (without actually doing the math)
the gravity is only one sixth, but the drag effect of air resistance is
infinitessimal above three inches

If you can do a gravity-whip off a very dense mass in lunar orbit, you
could take a hand-thrown rock and send it to Earth.  From the POV of
the mass, the projectile is on a parabolic or hyperbolic trajectory.
Assume a parabola, for simplicity.  At the limit, a parabola reverses
its direction.  Since no energy is lost, the speed  of exit is the same
as the speed of entry (okay, OOCQ me).  The projectile comes in at
orbital speed (~1 mile/second) and leaves at the same relative speed,
going in the opposite direction, still moving at 1 mile/sec but in
the direction of the mass's orbital motion.

Change this to the POV of the projectile-thrower.  THe projectile goes
up, at some small speed.  It makes a close pass by a mass orbitting at
about 1 mile/second.   It leaves going at 2 miles/second, more than enough
to escape Luna, cancel out all of LUna's orbital speed, and fall strraight
down to Earth.

And now for another topic transplanted from item #79:

Mining the asteroid belt.

It's a subject which has often appeared in science fiction stories
and comes up regularly in proposals from space enthusiasts and
companies in the industry.  Yet nobody seems to *do* anything
about it.

Probably because it's not quite as easy as it looks, and when
you're trying to get money for a venture, uncertainty's a killer.

In item 79.21, pfv mused:
>hmm...
>
>       Drives and automation... A good computer system, hardened.. An Ion
>       Drive, even a Fission Drive.. Lightsails... Long lags between
>       "deliveries".. 

All sorts of issues with this one.  Good computer systems are hard
to come by.  The technology advances, but the same sub-micron chips
which make them cheap also make them more vulnerable to radiation
effects.  Ion drives:  You may be able to get 10,000 lb-sec of thrust
per pound of propellant, but if you're trying to push a million tons
of asteroid, a ton of propellant only gets you about 10 cm/sec of
delta-V.  Ion drives are starting to make appearances on geosync
communications satellites, but NASA has never flown one on a deep space
mission.  (Something about "never used one for propulsion before, so
why should we be the first?" aka chicken/egg.)  Lightsails:  Nobody's
ever flown one, and they'd produce a few Newtons of force per square
kilometer of area.  Maximum.  You won't move anything very fast.
Getting one built, launched and deployed without tangling is going to
be quite a learning process, you can bet.

>       Taking along some H-bombs and simple-minded Guidance Systems: kick
>       the ore toward the Moon/Earth Orbit and let the GS finetune the
>       flight.. Flying "Catchers Mitts" (the design is old L5 related).

The H-bomb idea would be great for giving small pushes to bodies
which threaten to strike Earth, and deflect them just enough to
miss us.  Trying to use them for anything else would likely run
afoul of one or another test ban treaty (and you don't even want
to ask about the paperwork and background check for purchase).
The "catcher's mitt" concept doesn't apply to asteroid capture;
it is a rather specialized idea for capturing relatively slow-moving
slugs of moon dirt at a site over the far side of the Moon, not
whole asteroids winging in at escape-velocity-plus.  It was designed
out of Kevlar, which stops bullets, not meteors.

>       "The Man Who Corrupted Earth" covers a lot of the socio-economic
>       horrors, too.. What happens when someone parks an asteroid
>       in orbit containing more nickel-iron than Man has ever even mined?

What happens?  Well, the world price of nickel takes a dive.  I suppose
that this brings cities-full of cars running on nickel-hydrogen
batteries a bit closer to plausibility, but you never know.

So let's see.  Light sails big enough to do the job almost
certainly couldn't be made and launched, not as a first attempt.
Ion drives would require many tons of propellant, too costly
to launch.  Nuclear pulse, not politically feasible.  What's left?

The concept being pushed in the late 70's/early 80's by the Space
Studies Institute was to take propellant from the asteroid itself.
Using a page from the moon-mining handbook, they proposed building
a catapult and just slinging pieces of asteroid.  Action/reaction;
for every slug of rock slung one way, the rest of the asteroid
gathers a bit of speed the opposite way.  You might use up half or
so of your rock, but you get there.  Unfortunately, you can't
launch anything that big in one piece, so it would have to be
built on-site.  Building a catapult of substantial size takes a
crew, and launching a crew and the supplies to sustain them would
cost a lot of money and take a lot of engineering.  Which brings
it back to money.

The real problems are political and financial.  The political
problems with landing the fruits of your labor might be considerable;
if you think that nuclear power plants had problems in the courts,
I think it would be a church picnic compared to the brouhaha over
bringing down space stuff.  Everyone with an interest to be damaged,
e.g. nickel mine owners, would be paying lawyers to get you shut
down.  And in that sort of climate it's unlikely you'd ever get the
venture capital to get off the ground.  The Christic Institute
wasn't able to prohibit NASA from flying Galileo only 100 miles
from Earth, but there wasn't any money at stake.  Now think about
fat fees for lawyers and campaign money for politicos....

pfv asked about technologies for getting to orbit cheaply.  He
specifically asked about H2/O2 rockets.

Of the chemically stable, non-toxic chemical rocket fuels, hydrogen
and oxygen have the highest performance.  The specific impulse Isp
(a figure of merit for rocket motors) is around 460.  (It occurs
to me that the Science conference could use a glossary item.)

The problem:  hydrogen tanks are HUGE.  The Shuttle External Tank
is divided into two sections, and the hydrogen section has more
than twice the volume of the oxygen section.  This is despite
the Shuttle burning about 6 pounds of oxygen for every pound of
hydrogen.  Liquid hydrogen has a density of about 0.07 (water is
1.0), and the big tanks required to hold large volumes add weight
and drag.  On a reusable vehicle, these tanks require thermal
protection during re-entry, adding even more weight.  This cuts
the performance still further.  This doesn't make it impossible
to use (hardly!) but it does show that the best propellant doesn't
always make the best vehicle.

One of the most interesting concepts for getting to orbit cheaply
uses propellants you could buy at the gas station and drug store.
Called Black Horse, it would burn jet fuel (kerosene) and hydrogen
peroxide (H2O2), which are both liquid at room temperature.  The
H2O2 would not be the garden-variety disinfectant nor even the
70% industrial oxidizer, but 98% pure.

Black Horse would look like a fighter plane and have two seats.
It would take off with a full load of fuel, but only enough oxidizer
to get up to an aerial tanker plane.  After filling its peroxide
tanks from a KC-135, it would push the rest of the way to orbit.
Starting from 30,000 feet and 400+ MPH, it would have a big
advantage over vehicles starting from sea level and zero.  Being
able to fight gravity with lift from a wing instead of rocket thrust
helps a lot too.  Taking off mostly empty makes the landing gear
and other parasitic weight quite a bit smaller, and every pound
removed from the vehicle is a potential pound of payload.

Want to know more?  See http://www.im.lcs.mit.edu/bh/

Could you repeat that please???  *rotfl*

#14 is cool but I think a mother of a slingshot would work better. Get
Dennis the Menace and Bart Simpson together and anything would be possible.

Won't happen.   Washington will shut the venture down as soon as
it looks like an arms race in rubber-band guns is developing, and
they'll establish treaties to restrict world trade in latex to
prevent hostile governments and punks from getting too much.

Iraq might still be able to build one, if it hasn't been specifically
banned by the UN agreement.  But they'd probably only be interested in
slinging things at Israel.  It could double as a handy way to dispose
of traitorous guards and family members in the royal palace.  "What's
that?  You want to defect to the West?  I'll defect you to the West!"

the sick thing is .... #18 really happened.
  
the NucTestBanTrty however .. .could be modified to allow vessels
exceeding the thrust of escape-velocity restrictions ... so that
nothing could/would "fall" back to earth. ????????????
  
<<is this linked from science.cf? >>
  
there is a large puddle of rocket-scientist brains working on the
engineering and physics for stuff liek this ... it will take salesmanship
adn 'will' to make it happen in our life time.
  
and i hope it happens

I assume you're referring to Iraq's "supergun," which really happened.
Well, mostly happened, until they were forced to dismantle it.  They
haven't yet embarked on a "superslingshot," but that's probably just
because they haven't thought of it yet.

("science" still points to the melvin conference, so I assume that
 the conf does not exist yet.)

Agora 87 <--> Science 3

I'm pretty sure I read once that some test ban treaty explicitly prohibited
the detonation of nulear devices in space.  Maybe that has been ammended to
allow detonation for thrust?

That Black Horse idea sounds like a death trap.  The German military used a
similar set of fuels for the ME-163 Kommet rocket fighter.  The one of the
two fuel components was caustic enough that a spoonfull would make a nice
tunnel through your thigh if you sat still enough.  The other burst into
violent flames on contact with almost all organic substances such as clothing
and flesh.  Together they produced a wonderfully powerful explosion.  If any
of you know of a Kommet pilot who lived through his 'Kommet tour', and doesn't
look just like Freddy Krueger, I'd like to know his name.  So on that note,
*you* fly the blackhorse, I'll watch.  

Oh, and I forget one or both of those fuels also had a nitroglycerin reaction
of exploding at the slightest bump.  Have fun...  >:}

Nope...

        the _Orion_ Project was predicated on setting off nukes behind the
tail, and (after already proving the design with tnt and a model) the
thing was canned because it required nukes..

        We got the tech, but we also have Governments that don't WANT
folks to exploit resources out from under their controls..

i read a book by david brin entitled "earth".. science fiction of course
a microscopic black hole falls into the earth.. in order to "battle the 
beast", instruments that fire some type of gravtational beam are used. these
beams affect the black hole, producing a slingshot effect...entire chunks
of the earth's crust were flung into space. farfetched, sure, but i found
it to be an interesting theory.

Re #25:  I don't think "Earth" is the book you mean.

Re #23:  According to the data I've received, H2O2 is *more* stable
the purer it is.  Oddly enough, its decomposition is catalyzed by water.
Yes, it was used in the Komet, and yes, the Komet was a deathtrap, but
that's like comparing a GeeBee racer to a 747 because they both burn
petroleum.

Getting back to the subject of electromagnetic catapults
(slingshots), I'm sure some people are interested in just how
rough the ride would be on one.

The answer is, it depends.  It depends on the speed required
and the length of the catapult.  Assuming that the acceleration
is constant over the entire length and a standing start, the
speed at the end is given by the equation:

                v = sqrt(2  * a * d) 

If the catapult is on the Moon, v is roughly 1 mile per second
to get to orbit.  Assume d is 1 mile (1610 meters) just for a
number that's easy to grasp.  Converting to metric:

        1610 m/sec = sqrt( 2 * a * 1610 meters )

Solving for a:

        (1610)^2 m^2/sec^2 = 2 * a * 1610 meters

        a = 805 m/sec^2 = ~82 G's.

From Earth things are a bit more difficult.  Anything launched
from low down has to plow through the remaining atmosphere before
getting to space.  Aside from issues of heat shielding, this
causes some loss of speed.

Assuming a desired orbit at 5 miles/second and 1 mile/second loss
of speed on the way up, the speed at the end of the catapult must
be 6 miles/second.  If the catapult is 10 miles long, we can 
once again solve the acceleration equation for "a":

        9660 m/sec = sqrt( 2 * a * 16100 meters )

        a = (9660)^2 m^2/sec^2 / (2 * 16100 meters)
        a = 2898 m/sec^2 = ~295 G's

This is okay for bulk materials, but delicate equipment may not
survive.  Don't ask to buy a ticket.

yup, Earth was the book.. David Brin the author.. jusst finished it
actually.... pretty good, IMHO

Robert Heinlein's "The Moon is a Harsh Mistress" had one of these thingies,
and talked a bit about the physics.  The main character ended up using it to
get to Earth from the Moon.

In TMIAHM, Heinlein also describes the catapult "rings" as being
spaced very far apart near the end.  To have constant acceleration
you need constant force, and roughly constant ring spacing.  Making
the whole machine much longer than necessary would just increase
the requirement for travel by construction crews, power cabling and
other materials.  I'm amazed that Heinlein made that mistake.

Maybe they wanted the "kicks" equally spaced in time, not in distance, which
would imply that the rings get further apart as you go along.  Constant
acceleration is not necessarily the best way to do it, but it's the easiest
to calculate.

The effective "range" of each ring would be about the same
regardless of how fast the load was moving.  This implies that
each "kick" would last less and less time as the load moved faster.

It occurs to me that you might want such a thing to even out the
power demand, but if energy is stored you shouldn't need to.  A
curious design feature indeed.

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