
Mass Driver - pmoriarty
https://en.wikipedia.org/wiki/Mass_driver
======
trhway
[https://en.wikipedia.org/wiki/Project_HARP](https://en.wikipedia.org/wiki/Project_HARP)

"On November 18, 1966 the Yuma gun fired a 400 lb (180 kg) Martlet 2
projectile at 7,000 ft/s (2,100 m/s)[4] sending it briefly into space and
setting an altitude record of 180 km (590,000 ft; 110 mi)"

If my math napkin correct the projectile lost less than 1.5 Mach of the speed
when it reached 30km height, ie. where air is just 2% density of the sea
level.

[https://en.wikipedia.org/wiki/Project_Babylon](https://en.wikipedia.org/wiki/Project_Babylon)

"The second supergun, "Big Babylon", of which a pair were planned (one to be
mounted horizontally, at least for test purposes), was much larger. The barrel
was to be 156 metres (512 feet) long, with a bore of 1 metre (3.3 feet).[1]
Originally intended to be suspended by cables from a steel framework, it would
have been over 100 metres (300 feet) high at the tip. The complete device
weighed about 2,100 tonnes (the barrel alone weighed 1,655 tons). It was a
space gun intended to shoot projectiles into orbit, a theme of Bull's work
since Project HARP."

what a pity :

"The project began in 1988; it was halted in 1990 after Bull was assassinated,
and parts of the superguns were seized in transit around Europe. The
components that remained in Iraq were destroyed by the United Nations after
the 1991 Persian Gulf War. "

one can dream - put such a gun on plateau at 4km altitude, half sea level
pressure, somewhere in Bolivia or Peru (it is even close to equator) or may be
Tibet, and shoot the fuel, food, water, etc. into the orbit to build up the
Moon, Mars, etc, missions. If only we could find a few tens of millions
dollars :). I'm waiting for Musk to hopefully get to it as he has a business
case and money.

------
vortico
Reading this made me wonder a few physics questions.

\- If you shoot a gun on the moon, will it return to your location from the
behind, no matter what angle you shoot it?

\- The escape velocity of the moon is 2.38km/s. Is this the velocity required
for a bullet to leave orbit no matter what angle you shoot it? Or is the
required velocity higher at smaller angles?

~~~
gnode
> If you shoot a gun on the moon, will it return to your location from the
> behind, no matter what angle you shoot it?

In most cases no. Although the orbit of the bullet is cyclical (below escape
velocity), unless you shoot perfectly level with the surface, its orbit will
intersect the moon. Obviously if you shoot slightly down, it'll hit the
ground. If you shoot slightly up, it'll hit the ground behind you on its
return.

> Is this the velocity required for a bullet to leave orbit no matter what
> angle you shoot it?

Yes. Although if it hits the moon first, it'll slow down and not escape
(unless you were to shoot through the moon).

~~~
vortico
\- Ah, I see. Does there always exist a velocity `v(theta)` that will make the
bullet hit you in the back after one orbit? A slightly different question, but
if you shoot a bullet perpendicular to the moon's surface at any speed (as
long as it won't hit the ground), it will always hit you in the back after one
orbit, right?

\- I suppose if you think about it in terms of kinetic energy, this makes
sense.

~~~
gnode
I realize that I made an assumption in my first answer, that the moon is not
rotating. A rotation may cause you to move out of the orbital path of the
bullet before it gets back to you. If you shot from the poles though, you
would remain in the bullet's orbit.

That said, in ideal conditions, even with rotation, the bullet will hit you
eventually, but not for a long time on average, given the scale of the moon.
In reality it's orbit would probably destabilise due to collisions with dust
and perturbations from other gravitational bodies (the Earth, Sun, etc. create
an n-body situation which slowly changes the orbit over time).

> Does there always exist a velocity `v(theta)` that will make the bullet hit
> you in the back after one orbit?

The velocity must be great enough to stay above the surface, and not high
enough to escape. Additionally, you must shoot horizontally, like I mentioned
before, and not rotate out of the orbit (e.g. by being at the poles, or at the
rotational equator, shooting along the equator).

------
ceejayoz
This simple concept leads to some really wacky concepts.

[https://en.wikipedia.org/wiki/Launch_loop](https://en.wikipedia.org/wiki/Launch_loop)

[https://en.wikipedia.org/wiki/Space_fountain](https://en.wikipedia.org/wiki/Space_fountain)

~~~
quakeguy
To extend your post, this man[1] did some research way back in the fifties
about magneto-implosive generators and applications.

[https://en.wikipedia.org/wiki/Andrei_Sakharov#Magneto-
implos...](https://en.wikipedia.org/wiki/Andrei_Sakharov#Magneto-
implosive_generators)

------
thinkingkong
I think the Mars series has a bunch of references to this, too. Basically if
we ever had to go to war with the moon or mars colonies, it would just be
kinetic weapons.

~~~
Razengan
If we come upon the aftermath of such a weapon that launched meteors at a
planet, would there be any signs that it wasn't a natural event?

~~~
jerf
On Earth it would absolutely show up. Because Earth is tectonically active,
and has an active atmosphere that erodes things in relatively predictable
ways, along with a number of other processes that can date things, we'd be
able to tell that there's a whole bunch of impacts that seem to have taken
place at suspiciously close to the same time, even millions of years after the
fact. If there's more than a couple of them, it may even be the case that it
becomes clear that they were specifically targeted; e.g., 2/3rds of the planet
is water, if we found 8 impacts on land or coasts at the same time and could
show that there were 0 in the middle of the water, that would be very strong
evidence for intelligent action, for which "military" is the most likely
conclusion.

In other environments, it may be much more difficult, but similar effects on
more detailed scales may be in play.

It's possible the space war was won with One Big Rock, but for a variety of
reasons, I'd expect space war is more likely to occur with a variety of
smaller rocks. Primarily, One Big Rock is harder to steer at militarily useful
speeds, and you have to be pretty committed to wiping out not just your enemy,
but the entire ecosystem. A lot of much smaller rocks would be more militarily
useful in all sorts of ways, speed of deployment and redundancy being among
the most important. If there is a war between a space-faring civilization and
the groundpounders, the space-farers win if they just degrade the ground
civilization to the point that they are no longer space-faring and unable to
interfere. Spending the considerably greater resources to massively overkill
past that goal is possible, but would not be the most likely outcome in my
opinion because of the outsize expenditures and much longer time to return on
said expenditure vs. smaller, more nimble attacks.

~~~
maxerickson
That would be quite the thing if the dinosaurs won the war with Mars.

~~~
riffraff
Or maybe mutual annihilation? The dinosaurs where more ruthless and ended up
wiping out the whole biome while martians limited themselves and ended up only
killing the dinosaurs but letting Life survive.

Thank you, interplanetary brothers.

------
theothermkn
> "In contrast to cargo-only chemical space-gun concepts, a mass driver could
> be any length, affordable, and with relatively smooth acceleration
> throughout, optionally even lengthy enough to reach target velocity without
> excessive g forces for passengers."

Wikipedia can generally be pretty good, but the above text from the subsection
_On Earth_ shows what can go wrong when deluded space enthusiasts go nuts on
an article; it's pure wishful thinking. No major construction project of "any
length" is "affordable," especially not one of the sophistication of a mass
driver. At about 3g, about 30 m/s^2, it would take you about 270 seconds to
get to 8 km/second, during which time you are traveling at an average speed of
4 km/second. This is a structure, with attendant control system and
safeguards, about 1080 km, or 670 mi, in length. This is not affordable. By
anyone.

What _might_ be affordable (sticking my wet finger in the breeze) is getting 1
or 2 km/s at 5 or 6 g, or higher, and then doing the rest with a rocket,
provided that you're firing continuously to amortize your initial cost.
Skimming a couple km/s off the high end of the rocket equation brings your
mass ratio way down, especially considering the reduced gravitational losses.
(I'm assuming, in a fit of witless optimism, that we're launching from a high
enough altitude that the increased aerodynamic losses are mitigated to the
point that gravitational loss savings dominate, and that this adds no cost.)

Mass drivers are a fun idea because they _seem_ to get around the rocket
equation, but this affordability nonsense is just over the top.

~~~
loeg
The major problem with mass drivers in Earth atmosphere is that anything like
escape velocity combined with atmosphere introduces a _lot_ of G forces and a
_ton_ of heat. (For intuition, imagine reentry, except even faster at lower
altitudes.)

~~~
evgen
Run it in a tunnel (cut and cover to reduce costs) with a reduced atmospheric
pressure that goes up the side of an equatorial mountain. Mt. Kenya is the
best prospect from what I remember the last time I looked at this. It is just
south of the equator so you get max angular momentum from the spin of the
earth, it is about 5km above sea level so you are at half the atmospheric
pressure as sea level, and it is relatively close to a downrange ocean in case
the launch does not go as planned.

~~~
loeg
The tunnel can't just be cut and cover, because you can't take sharp turns of
any kind and there's no straight-line gradient up a mountain. Yes, a
termination at a high equatorial peak helps, but there's still a lot of air
there that you're immediately buffeted by as soon as you leave the tunnel.

------
Symmetry
The blog for the game _Children of a Dead Earth_ had some excellent posts on
railguns as weapons.

[https://childrenofadeadearth.wordpress.com/tag/railguns/](https://childrenofadeadearth.wordpress.com/tag/railguns/)

~~~
jacquesm
That's an incredible level of detail in that explanation.

------
canthonytucci
[https://en.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress](https://en.wikipedia.org/wiki/The_Moon_Is_a_Harsh_Mistress)

Mike, the protagonist of this book, runs the catapult on the moon, among other
duties.

------
RocketSyntax
Hmm. Precious cargo would require gradual acceleration.

~~~
savethefuture
What defines precious cargo? Satellites and astronauts seem to do alright
going into space attached to a giant rocket, but I will admit I'm not well
versed on the forces created from a electromagnetic launcher.

~~~
savethefuture
Asks a real question about whats precious cargo, downvoted.

~~~
RocketSyntax
fragile =)

------
amelius
Can't they use an accelerator ring such as used in a particle collider, to get
the objects at the desired speed before launching them?

------
kerkeslager
Is there any research a hybrid approach? That is, start with a gravity driver
and then continue accelerating with rockets?

~~~
andbberger
You would need a rocket to use a mass driver at all, you need something to
circularize the orbit. Otherwise, it's not very useful.

------
henearkr
Wouldn't that be a solution to the never-ending objections saying that Jupiter
cannot be used as a resources for various gases because that gravity sink
would use too much fuel to leave?

As the spacecraft launcher can be station-powered the whole time of the
acceleration, there is no more any worry about fuel mass, isn't it...

------
Animats
Is there a bot that posts random Wikipedia articles to HN, or what?

~~~
dang
I don't think so. wikipedia.org used to have a mild downweight on it, the same
one HN has on most major media websites. A few months ago a reader emailed and
asked us to try removing that. There seemed to be enough randomly interesting
Wikipedia submissions that we agreed to try it. Randomly interesting is sort
of the highest good on HN, so on the whole I think it's working. We tend to
moderate so there's no more than one on the front page at a time.

------
m4r35n357
Babylon 5

