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Rockets launched from high-altitude
balloons, or cannons lofted by such
balloons, have both been proposed as a
means of making it easier to get things
up
to orbital altitude and/or velocity.
I believe I have hit upon an alternative
which benefits from being highly
impractical.
The
balloon itself can afford to carry a
fairly high payload (since gas lift is
cheap).
Suspended beneath the balloon are two
satellites, on either end of a 100m-wide
arm which pivoted at its centre, so that it
can rotate in the horizontal plane (so, the
whole thing looks like an upside-down
helicopter).
A modest electric motor (perhaps solar
powered) starts to turn the rotor.
Gradually, arm spins faster and faster, up
to a not-unreasonable 23 revolutions per
second.
At this point, the speed of the rotor tips
is
about 7km/s, good enough for low-earth
orbit. A signal from the ground fires
explosive charges on the satellites,
releasing them and sending them
hurtling
off in opposite directions - two launches
for the price of one.
Some minor problems. (1) Just before
release, the satellites are experiencing a
centrifugal force of about 845,000G.
This
will need some serious engineering,
although I note that ultracentrifuges have
operated up to 1 million G. Note that the
centrifugal forces (for a given release
velocity) become smaller as the rotor
diameter increases. If the rotor were
1km
across, the G-forces are only 84,000G to
attain a 7km/s release speed. A 10km
rotor (perhaps in the form of tethers
rather than rigid arms) takes you down to
8,400G. To get down to a human-
survivable 8.4G you'd need a rotor only
1000km in diameter - easy!! (2) This
strategy sends the two
satellites outward horizontally, with no
vertical motion. It would be preferable to
angle them upwards, but this means that
one has to be released downwards; so,
you would probably want one of the
satellites to be a sacrificial
counterweight. (3) Half an orbit later, the
two satellites will smash into eachother
at a closing speed of 14km/s. (No, not
really; they'd miss by at least the
diameter of the rotor.)
Centripetal Space Junk Drive
Centripetal_20Space_20Junk_20Drive
Shares some of those minor problems [lurch, Feb 24 2008]
[link]
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If you tried to spin a 100m wide arm with two satellites attached to it by motor, you would just end up spinning your enormous balloon around. It's a simple comparison of rotational moments of inertia. |
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We need some mass here somewhere..... |
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The chance of the two satellites hitting each other is so small that I wouldn't even entertain the notion. Quite imaginative. |
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The tail would wag the dog. |
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//Impractically possible// [marked-for-tagline] |
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// It's a simple comparison of rotational
moments of inertia.// |
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Ah yes. Errr, glad you spotted that one.
So, we have two counter-rotating rotors,
and we launch 4 satellites. |
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Another minor problem: Since these arms are rotating at orbital velocity in the atmosphere, wouldn't they be cooking at around 5,000 degrees? |
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One could publish a line of cookbooks
that exploit this effect. |
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//Impractically possible// [marked-for-tagline] |
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They'll be at about 50km up, the limit of
balloons. Air density here is about
1/500th that at ground level and, since air
resistance is proportional to air density,
the tips will experience the same
resistance (and heating) as if they were
travelling at about 14 metres per second
(31mph) at sea level. |
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It seems to me that if this was going to work then launching a large missile from a high altitude jet flying at supersonic speeds would work. Please explain what I am missing. |
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Well, yes. Basically, our ultracentrifugal
launcher is a means of getting something
up to orbital speeds (>7km/s) without that
thing having to carry its own propulsion
system. So, if you can get a high altitude
jet up to 150,000 ft, and then get it up to
about mach 21, it will achieve the same
result. |
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Do balloons get you high enough to achieve orbit? They would need to expand with the change in pressure and even so, with a payload, we still top out in the upper atmosphere. What if we launch the payload from the high altitude (reusable) jet? |
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//Do balloons get you high enough to
achieve orbit?// No, but they can get you
up to 150,000ft or so, at which point air
resistance is much less. |
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//the tips will experience the same resistance (and heating) as if they were travelling at about 14 metres per second (31mph) at sea level.//
Oddly enough, meteors burn up at that altitude (ie, 150k feet). Good try, though. |
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//Oddly enough// Well, no. Meteors
typically are travelling at >>7km/sec. |
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[Maxwell], I'm tempted to take this off to another idea, but: Make the rotor arms even more like a helicopter's blades, and fly the thing up even higher. |
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You'll want to build up the RPM in flat pitch, do a "jump takeoff" for maximum altitude, and wind the RPM back up while coasting upward in even thinner air. |
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//It would be preferable to angle them upwards// No, 'cause if you do, the one that went up will be coming down again on the other side of the planet. |
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I wasn't sure if you could get much
"helicopter lift". I don't know enough
about helicoptering, but if the atmosphere
is 1/500 as dense as at ground level, I
presume you'd have 1/500th the lift, all
else being equal. On the other hand, we
have a high rotor speed, so maybe.. |
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Maybe. I think it should work. I've a toy around somewhere that is a helicopter "rocket". |
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I also considered this prior to the "blow it all up with C4" idea, but I thought that max rotor tip velocity was independant of radius (is that right?). This max tip velocity is about 1500m/s using some exotic materials. I imagined a rotor being gradually accelerated until it exploded, hurling off the mini-satellites. Anyway, the explosions thought led to C4 & etc. |
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//since air resistance is proportional to air density, the tips will experience the same resistance (and heating) as if they were travelling at about 14 metres per second (31mph) at sea level..//
I'm assuming that this is a joke idea, and this nonsensical defense of it is also a joke. Otherwise it's very bad science. |
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OK, miscalculation; the equivalent
speed would be 313 metres per second,
or 700mph. This generates heat, but
not a ludicrous amount of heat. |
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Air resistance (drag, to be precise) is
defined as being proportional to the
density of the fluid and to the square of
the velocity. If I'm wrong, as frequently
happens, I'd rather someone
explained my mistake than simply
calling
it nonsense. |
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I hope this is a joke, otherwise it is terrible science. What would provide the energy to turn the arms? It would take the same about of energy to accelerate the satellites to escape velocity regardless of how you do it: in fact, more, because you are now accelerating the rotor arms. You may save a bit by launching from a higher altitude, but again, that is lost by the rotor arms. |
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Furthermore, the savings in fuel (ignoring the rotor arms loss) is completely negated by the added cost of such a massive piece of equipment and the cost of lift gas. |
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Agamemnon, it is intended tongue in
cheek (the phrases "impractically
possible" and "highly impractical" were,
sort of like, clues there), but your
annotation shows that
you have embarrassed yourself by
missing the purpose of this. bless. |
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The need is to get sufficient velocity in
a projectile. If you do this by using a
rocket, then the rocket has to carry all
of its fuel, meaning that the rocket
wastes most of its energy in
accelerating the fuel itself that it needs
for the rest of the flight. (A typical SRB
is 90% propellant, and puts only a few
percent of its mass into orbit.) |
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So, you want to get your projectile up to
speed by providing an _external_ input
of energy. |
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One way to do this is by means of a
railgun or other cannon-type device,
but that is inelegant. My solution
achieves the same end, in a more
elegant way. |
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The power, as stated, comes from solar
panels on the balloon. If you prefer,
you can put any other power source on
the balloon - bouyancy lift is dirt cheap
($37 per cubic metre of helium, if you
like, which is actually cheaper than a
cubic metre of good topsoil.) |
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So, please think at least once before
annotating - it saves a great deal of
embarrassment all round. |
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You could also get power from ramjet engines on the rotors, if you want to lift the fuel for them. Somewhere along the length of the blades will be a spot moving at the right speed through the air for a ramjet to work. That would also let you get back to a single rotor, perhaps. |
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True. I'd originally thought of ion drives
on the satellites themselves, but their
impulse would be way too low. Ramjets
make more sense if there's enough air for
them, which I guess there would be. |
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Of course, if we could speed up the Earth's
rotation by a factor of 16, the problem
would be solved. |
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//you would probably want one of the satellites to be a sacrificial counterweight// Well, it's going to end up a satellite, so you may as well put some functionality in it. |
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//A signal from the ground fires explosive charges on the satellites// Overly complicated and could damage the satellites - suggest whistle attachment at end of arms, tone pitched so that at correct speed it excites the balloon pilot's terrier into biting the large red toggle switch that operates a servo-release. |
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Also if you articulate the arms at the pivot point, some interesting effects might be possible (think ice skaters twirling) Much easier to maintain balance in the whole structure by accelerating to say 600 revolutions per second with the arms folded and pointing straight down below the balloon with the two payloads close together - then release the connection between the two halves of the arm... |
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//Well, it's going to end up a satellite,
so you may as well put some
functionality in it. // No, because it
(the sacrificial one) will be shooting
downwards rather than upwards, in the
situation I described. |
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//excites the balloon pilot's terrier into
biting the large red toggle switch// If
anyone manages to build a terrier that'll
be happy with those centrifugal forces, I
really don't want to meet it. |
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Two things about the sacrificial mass. If you do fire it downward, the other one, the intended satellite, will be going up, yes, but will be coming back down on the other side of the planet. As was understood above, the two masses must meet somewhere, theoretically, on the other side of the world. Unless the satellite has some means of circularizing its orbit, it is going to "try" to pass through its launch point again, on the same path, which means it should be coming UP out of the air. |
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Second, if both masses are thrown perfectly horizontally, the one thrown eastward will be going faster than the one thrown westward against the Earth's rotation. The factors I babbled about above will still apply, but differently, I think. |
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[MaxBun], another suggested major modification: Make the axis of rotation horizontal and north-south. Support the axis with balloons on each side. Make the rotor with two arms. On one arm, put a ramjet. On the other arm, put the satellite. Run the ramjet every time it dips down into thicker air. When the satellite is at the top, going east, have the terrier release it into orbit, and also release the ramjet to keep things balanced. Then release gas from the balloons, descend to the ground, retrieve the ramjet from its parachute, and reload for another trip. |
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[Bacon] What you say is true, alas. It
occurred to me the other day that, for
the purposes of trajectories, you can
imagine the spherical earth as being
"unwrapped" into a flat sheet. Then, a
circular orbit becomes a straight
horizontal line above the plane.
Conversely, throwing something
upwards will cause it to travel in a
parabola (on the "gravitationally
unwrapped" earth), which will
correspond to a distorted parabola (eg,
a cardioid or something else weird) in
the real world. |
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So, you're quite right. If we start with
the "flat earth" model, we've got to
throw something upwards but then give
it a purely horizontal (flat) velocity
(which corresponds to a circular orbit). |
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And to think they thought that thinking
the earth was flat was silly. |
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