Author Topic: Space cannons & mass drivers, launching into specific orbital planes?  (Read 4719 times)

Offline KelvinZero

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Fortunately, there's no such thing as wind, so nothing could possibly go wrong with a 50 km tall string of airships piled on top of one another.
Again with the strange strawmen. There are many ways you could design this, and we haven't even discussed how wide. I think the wind is potentially a huge resource, as I mentioned.

Since 50km is the limit of what we can do, aiming for specifically that height is probably extreme. Whatever.

Online ChrisWilson68

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Fortunately, there's no such thing as wind, so nothing could possibly go wrong with a 50 km tall string of airships piled on top of one another.
Again with the strange strawmen. There are many ways you could design this, and we haven't even discussed how wide. I think the wind is potentially a huge resource, as I mentioned.

Since 50km is the limit of what we can do, aiming for specifically that height is probably extreme. Whatever.

Strawman?  Are you claiming you weren't talking about a 50km lighter-than-air tower?  Because you were replying to a post about a 50km tower, you talked about liking the idea of lighter-than-air skyscrapers, and you mentioned that the record for balloons is 50km.  Any reasonable reader would conclude you were talking about a 50km lighter-than-air tower.

Offline Nilof

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A pair of parallel wires carrying currents I1 and I2 at distance d from each other repel each other with a force per unit length of μo I1 I2 / 2*pi*d

In SI units, this becomes F/L = (2*10^-7 N / A^2) I1 I2 / d

A YBCO superconducting wire can achieve current densities on the order of a megaAmpere per cm/2, so fairly compact MegaAmpere wires are doable. If the tunnel to be elevated carries 10 MA, and the ground wire carries 50 MA, at a distance of 50 km, and we can lift 2 tonnes per meter. No need for lighter-than-air structures.

You can scale up the lifted weight straightforwardly by increasing the currents. Because the force scales quadratically with the current but only a linearly with the distance, it's actually fairly easy to scale up.
« Last Edit: 04/20/2017 10:29 AM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline KelvinZero

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Fortunately, there's no such thing as wind, so nothing could possibly go wrong with a 50 km tall string of airships piled on top of one another.
Again with the strange strawmen. There are many ways you could design this, and we haven't even discussed how wide. I think the wind is potentially a huge resource, as I mentioned.

Since 50km is the limit of what we can do, aiming for specifically that height is probably extreme. Whatever.

Strawman?  Are you claiming you weren't talking about a 50km lighter-than-air tower?  Because you were replying to a post about a 50km tower, you talked about liking the idea of lighter-than-air skyscrapers, and you mentioned that the record for balloons is 50km.  Any reasonable reader would conclude you were talking about a 50km lighter-than-air tower.
No I was just clarifying that what I said implied 50km was the upper limit, rather than what you would sensibly aim at.

Your strawman was to introduce a specific presumably very narrow design, but not a tether. Something simultaneously prone to twisting and vulnerable to it. Do you actually have an argument that a lighter than air structure cannot be 50km high due to wind?

You would have to argue from basic principles that cellular inflatable structures do not scale up. Perhaps something to do with the well known area/volume problem of scaling an ant up to the size of an elephant. Something along those lines.

You can't just make up silly implementations and then mock that. That is a straw-man.

Offline Robotbeat

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You can make a heavier than air structure higher than 50km. 50km isn't the upper limit.
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Online ChrisWilson68

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Fortunately, there's no such thing as wind, so nothing could possibly go wrong with a 50 km tall string of airships piled on top of one another.
Again with the strange strawmen. There are many ways you could design this, and we haven't even discussed how wide. I think the wind is potentially a huge resource, as I mentioned.

Since 50km is the limit of what we can do, aiming for specifically that height is probably extreme. Whatever.

Strawman?  Are you claiming you weren't talking about a 50km lighter-than-air tower?  Because you were replying to a post about a 50km tower, you talked about liking the idea of lighter-than-air skyscrapers, and you mentioned that the record for balloons is 50km.  Any reasonable reader would conclude you were talking about a 50km lighter-than-air tower.
No I was just clarifying that what I said implied 50km was the upper limit, rather than what you would sensibly aim at.

Your strawman was to introduce a specific presumably very narrow design, but not a tether. Something simultaneously prone to twisting and vulnerable to it. Do you actually have an argument that a lighter than air structure cannot be 50km high due to wind?

You would have to argue from basic principles that cellular inflatable structures do not scale up. Perhaps something to do with the well known area/volume problem of scaling an ant up to the size of an elephant. Something along those lines.

You can't just make up silly implementations and then mock that. That is a straw-man.

The irony here is that while I didn't make up a strawman to argue against, you just did to argue against me.

You said "skyscraper", "lighter than air", and "50km".  I said "50 km tall string of airships piled on top of one another", which is simply adding some humor to the characterization of a 50km tall lighter-than-air skyscraper.  Obviously it doesn't mean you're literally going to take ordinary airships and put them one on top of another.  It just means that any sort of 50km tall skyscraper is going to be roughly similar in some important ways to airships piled one on top of another.  The idea that I was limiting what you said to a particular design is just silly.

And I never said that it can't be done because of wind.  I was just pointing out that wind is going to be a big, big problem for a lighter-than-air tower that tall.

Offline Hanelyp

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... If the tunnel to be elevated carries 10 MA, and the ground wire carries 50 MA, ...
... You have a logistic problem of 40MA return current for the ground wire that has to take a long path around to not significantly detract from the lift magnetic field.  I'm thinking it'll be easier to work if elevated and ground wires carry the same current.

Offline Dao Angkan

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 You could use a rocket assisted projectile. Next generation XM1113 has three times the thrust of it's predecessor. It also uses "electrically-responsive energetics";

Quote
"If you can control the burn rate and energy output of a propellant with electric voltage, this opens a whole new capability," said David Thompson, a chemical engineer and member of the research team.

"Right now, we're considering it [electrically-responsive energetics] for rocket propellants found in extended range artillery rounds."

Hyper Velocity Projectile designed for use with various systems including the US Navy's railgun prototypes has high-acceleration tolerant electronic components. A next-gen rocket assisted high velocity projectile could probably already launch from the Moon and maybe even Mars via the Navy railgun prototypes. A more powerful railgun would likely be required for launching from Earth.
« Last Edit: 05/08/2017 12:46 PM by Dao Angkan »

Online Lars-J

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All the talk about mass drivers and space cannons seem to omit an important point... They can't launch you into an orbit. Unless you give it escape velocity, whatever you launch will come down. Each launched payload needs to have its own propulsion to circularize the orbit which rather complicates the idea of a cheap & dumb way to shoot cargo into space.

Offline QuantumG

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You could catch it with a tether, etc, but putting enough propulsion in the payload to circularise the orbit isn't a show stopper.
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Offline Asteroza

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The general implication though is if you have a large accelerator infrastructure project (which by design usually needs to be both large and high launch rate), it implies there would be a matching orbital infrastructure to receive projectiles/payloads. If you want to avoid propulsion systems on the payload (turning it into a potentially uncooperative passive object), you need an orbital momentum exchange to make up for the lack of circularization propulsion (for those cases where a burn is otherwise necessary). These are typically mass catchers like a giant cone, or the reduced set which is a tether with active grapple, or the minimum set which is an OTV, but they all feature some other form of propulsion to make up the momentum loss.

The logistics problem here is that if you have a fixed launch azimuth, launch/catch scheduling and placement of the catcher is easier but still not trivial (the trivial case is equatorial launch to 0 azimuth, which avoids catcher orbit precession timing issues post-catch). Variable launch azimuth doesn't buy you much, since there will usually be only a few popular orbits for payloads (LEO parking, GTO, moon bound parking, LTO, ecliptic, and various SSO or megaconstellation deployment parking orbits, maybe Molniya)

Offline Nilof

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Assuming the apoapsis is 200 km and the periapsis is close to ground level, a BOE calculation for circularization delta-v gives me 60 m/s.

A kick motor could do this with almost no mass penalty, but if you want to shoot dumb payloads, you could catch it with a small orbital tug acting as a reverse bomber. You'd likely want some tugs anyway to handle logistics in LEO.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Dao Angkan

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With propulsion on the payload you're also still able to retrieve the sabot. Maybe the sabot could have a mechanism to push the payload into orbit, and in turn, the sabot to the surface.
« Last Edit: 05/11/2017 10:32 AM by Dao Angkan »

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