Author Topic: Space cannons & mass drivers, launching into specific orbital planes?  (Read 4692 times)

Online Nilof

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Mass drivers are possibly the most practical option for non-rocket space launch. The Startram generation 1 design promises launch costs on the order of 40-50$ per kg ammortizing the construction cost over ten years, assuming ten 35 ton launches per day (which is admittedly very high compared to present launch rates).

The upfront construction cost is listed as ~20 billion in 2010 dollars, which is very high but less than the inflation-adjusted development budget of the space shuttle, and within a factor of two of Skylon's proposed budget. Max g-load would be 30 gee, which isn't usable for humans, but is quite flexible for cargo when compared to space cannons.

One thing that got me wondering however, is what inclinations you can launch into. I'm assuming the mass driver is at ~35 degrees latitude, directed straight eastward, and can launch into any altitude between 200 and 800 km. Clearly, the orbital plane you launch into depends on the time of the day, and launching ten times a day would mean you launch into ten different orbital planes. This is a significant downside if you want to launch a lot of payloads to a fixed given orbit.

You can use nodal precession to match orbital planes with the same inclination. However, I'm getting a ballpark figure of ~1 year for the inclination of a body in a 200 km orbit to "catch up" to a body in an 800 km orbit if they were launched 12 hours apart (please correct me if I am wrong). This is acceptable for cargo, but it's still a significant limitation and requires some delta-v in for a 200 km to 800 km altitude Hohmann transfer.

What options exist for changing the inclinations of a large number of payloads from one fixed orbit to another fixed orbit? Could momentum exchange tethers do this?

Another option would be to focus on an equatorial mass driver and use equatorial orbits for everything. I recall that equatorial LEO was a particularly benign environment for a space station, in terms of radiation?
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 Hanelyp

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Presuming you don't use delta-V for plane changes, you could potentially launch into any inclination passing over the launch site by orienting the mass driver for that orbit.  But most mass driver concepts would need to be built in a specific orientation, and would not be easily moved.

Offline DarkenedOne

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots. 

Offline Hanelyp

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.

Offline Rei

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots.

HARP did a pretty good job demonstrating that when properly encapsulated, sensitive payloads can survive far higher G-loads than 30g.

I do think there's more interesting use for mass drivers on asteroids and moons that lack atmospheres (made with telescoping, collapsibe, or rollable trusses, of course; there's no realistic way you'd make such long structures out of components that can't pack down).  I particularly like the concept for asteroid mining. As with one of the NIAC Phase 2 concepts, if you sinter regolith, you can end up with an optimal-shaped aeroshell for aerobraking - the material to be recovered arriving as its own aerobraking and entry vehicle. But beyond that, it has elemental iron, meaning it can be accelerated by a coilgun or quenchgun (the latter being generally superior for the role) - a small propulsive stage would only be needed for guidance.  dV requirements can still be significant, however, which means a very long quenchgun. I'd imagine that gravitational assists (Mars, Jupiter, etc, depending on the asteroid's orbit) might help get lower dV returns to Earth, and I'd think that trojans would offer some especially cheap return trajectories (gravitational assist with the parent and/or its moons, aka, you only need to escape L4/L5).

At least in microgravity, aiming should be much more achievable than on Earth.  Very slow, but you don't need much of a slew rate.

On bodies where you have a significant atmosphere, however, you either have to deal with huge atmospheric resistance, or build an a very massive vacuum tube - not so much of a problem for its own mass, but of the mass of the megastructure that has to support such a huge thing.   You can hold up a vacuum tube without preposterous support mass and/or materials requirements if you go with kinetic suspension, like a Lofstrom loop, but that comes with its own technological challenges.  You could float it with balloons, but the needed size would get huge, fast with increasing altitude.  I guess if something like that "Martian evacuated balloon" NIAC concept works out, a tube might be able to float itself, and only need stabilization cables. But that's really pushing the bounds of materials engineering, and the tube would have to constantly change in design, or exponentially expand, with increasing altitude.
« Last Edit: 04/12/2017 08:40 AM by Rei »

Offline as58

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To be honest, even the quoted $20 billion cost estimate sounds optimistic.

Offline Robotbeat

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.
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Offline CameronD

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots.

If you think that's all they're good for... then if you do happen to build one, (a) please don't tell your military about it and (b) please don't aim it in the direction of North Korea! (..or the Middle East, Russia or anywhere else on Earth, pretty much) :o


« Last Edit: 04/13/2017 01:25 AM by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Rei

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).

Offline Rei

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots.

If you think that's all they're good for... then if you do happen to build one, (a) please don't tell your military about it and (b) please don't aim it in the direction of North Korea! (..or the Middle East, Russia or anywhere else on Earth, pretty much) :o

Not exactly. Countries have built giant guns over the years, but as a general rule they've proven impractical as weapons - slow firing rates, highly vulnerable to attack, etc.  Gerald Bull, for example, wasn't assassinated over the Babylon supergun, which the Mossad didn't really care about, but for his work on improving the accuracy of Iraq's scuds (a project he didn't have any particular interest in, but was part of the agreement for getting his gun project built)

Offline Robotbeat

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).
Space elevator doesn't require unobtainium, it's just that with current materials (which require an AREA taper ratio of on the order of a million, so a diameter ratio of 1000), the space elevator would be a meter in diameter in the middle but for only a small payload (100kg), so would never pay for its own mass because a space elevator is really slow.

And actually the taper ratio improves a lot as you increase specific strength, so it's possible we'll reduce the taper ratio to 10,000, but even then, the really long travel time kills you.

Rockets FTW. People underestimate how efficient rockets are. ITS, for example, would be about 10% efficient at converting chemical energy of methane into potential and kinetic energy of reusable payload in orbit (15% if we're talking expendable). Considering it runs on methane, which is the cheapest source of energy on Earth right now at 1/kWh, you could beat the energy cost of a laser-powered space elevator or any sort of electrically powered launch method that has to pay 10/kWh for electricity. (Of course there are costs to properly condition that chemical energy so it is cryogenic, etc, but that's true for converting electricity into laser/rail gun power, too.)

ITS is supposed to get the cost of propellant in LEO down to just $9/kg. Nine. Dollars.

And ITS isn't the end-all, be-all, either. It's possible to improve efficiency by increasing chamber pressure, modifying mixture ratio so you run either oxygen-rich or stoichiometric, improving mass fraction, using hydrogen on the upper stage to better match the Isp to the flight speed, perhaps using a lower speed launch rail to accelerate the first little bit where it's not possible to lower the Isp far enough to match the flight speed, launch from high altitude, etc.

But for a first shot at a truly large, full, rapidly reusable RLV, ITS (and possibly whatever Bezos has planned for New Armstrong) would already demolish the $50/kg benchmark that these alt-launch concepts are struggling to meet.
« Last Edit: 04/13/2017 04:04 PM by Robotbeat »
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Offline DarkenedOne

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.

Yes that is always true, but you have to remain within a reason length.  In order to accelerate a mass up to 5 km per second with  max G force of 50g you would need a barrel 25 km long. 

Offline Rei

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).
Space elevator doesn't require unobtainium, it's just that with current materials (which require an AREA taper ratio of on the order of a million, so a diameter ratio of 1000), the space elevator would be a meter in diameter in the middle but for only a small payload (100kg), so would never pay for its own mass because a space elevator is really slow.

And actually the taper ratio improves a lot as you increase specific strength, so it's possible we'll reduce the taper ratio to 10,000, but even then, the really long travel time kills you.

Most of the proposals I've seen for space elevators that try to come up with plausible cost figures have a density around that of graphite and a tensile strength of 100-120 GPa.  But individual carbon nanotubes (let alone CNT bundles, let alone bulk fibers) usually measure in at 60 GPa or less.  It's not entirely clear that it's even physically possible to make a material with that sort of strength to weight ratio, let alone achievable.  And space elevators are awash in problems beyond that (not just travel time). Oscillations (they're undamped), micrometeoroids, atomic oxygen, lightning (both tropospheric, and sprites, and the potential to build up a charge on its own), etc, etc.  The worst is power transmission - space elevators are horribly inefficient because of the combination of how distant the receiver is with how small it is and how light it must be.  And the throughput is horrible too, because of the combination of travel time with how few climbers the elevator can support at once.

A launch loop avoids all of the problems of space elevators. I see no reason at all favour the latter over the former. But as you say...

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Rockets FTW.

At least in the foreseeable future, that's the bet that the good money is on  ;)  Technically, a launch loop should beat rockets, with over 50% efficiency, efficiency not dropping off with increased dV beyond that needed for LEO, virtually no wear-and-tear (versus the great stresses of rocket launch, particularly on the engines), and crazy-high throughput. But right now it's nothing but a concept, while rockets are pushing hard in the direction of increasingly affordable reusability.  :)

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ITS is supposed to get the cost of propellant in LEO down to just $9/kg. Nine. Dollars.

I will say that while I *very* much hope they can, I'm not holding my breath  ;)

Quote
And ITS isn't the end-all, be-all, either. It's possible to improve efficiency by increasing chamber pressure, modifying mixture ratio so you run either oxygen-rich or stoichiometric, improving mass fraction, using hydrogen on the upper stage to better match the Isp to the flight speed, perhaps using a lower speed launch rail to accelerate the first little bit where it's not possible to lower the Isp far enough to match the flight speed, launch from high altitude, etc.

And propellants aren't done advancing, either.  Let's not forget that ITS is intended to be the first large scale methalox rocket in history - I mean, they're looking to advance propellants even in that case.

The big technology jump will be if ITS can succeed with composites. We've been stuck with aluminum because it works with LOX and, historically, composites don't. But there's been a lot of work trying to change the latter. The strength to weight ratios on composites are far higher than aluminum, and there's significant potential for further advancement; aluminum's strength to weight ratio is not going to advance much with time.

Offline Robotbeat

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I like composites, but aluminum lithium alloy is pretty great. Aluminum 737 has lighter dry weight per passenger than composite 787, for instance. There are actually some manufacturing advantages to composites, which is why you have fiberglass boats and wind turbines instead of just aluminum. Composites are easier to make compound curves, for instance. That may actually be as much of a reason as strength to weight.

Anyway, I don't think that's the big enabling technology. They could build an aluminum ITS if they wanted to. Raptor is more important. EDIT: trying to keep on topic: New Armstrong could be nearly as cheap even if it's just aluminum and the lower performing BE-4. The REAL trick why rockets can beat alt-launch concepts is because of rapid reuse. Reusing the booster 1000 times and the upper stage hundreds of times with zero refurb on the booster and virtually none on the upper stage. Rapid reuse and the ability to use LOx and LNG, both ridiculously cheap, is the biggest threat to alt-launch.
« Last Edit: 04/13/2017 06:22 PM by Robotbeat »
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Offline DarkenedOne

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).
Space elevator doesn't require unobtainium, it's just that with current materials (which require an AREA taper ratio of on the order of a million, so a diameter ratio of 1000), the space elevator would be a meter in diameter in the middle but for only a small payload (100kg), so would never pay for its own mass because a space elevator is really slow.

And actually the taper ratio improves a lot as you increase specific strength, so it's possible we'll reduce the taper ratio to 10,000, but even then, the really long travel time kills you.

Rockets FTW. People underestimate how efficient rockets are. ITS, for example, would be about 10% efficient at converting chemical energy of methane into potential and kinetic energy of reusable payload in orbit (15% if we're talking expendable). Considering it runs on methane, which is the cheapest source of energy on Earth right now at 1/kWh, you could beat the energy cost of a laser-powered space elevator or any sort of electrically powered launch method that has to pay 10/kWh for electricity. (Of course there are costs to properly condition that chemical energy so it is cryogenic, etc, but that's true for converting electricity into laser/rail gun power, too.)

ITS is supposed to get the cost of propellant in LEO down to just $9/kg. Nine. Dollars.

And ITS isn't the end-all, be-all, either. It's possible to improve efficiency by increasing chamber pressure, modifying mixture ratio so you run either oxygen-rich or stoichiometric, improving mass fraction, using hydrogen on the upper stage to better match the Isp to the flight speed, perhaps using a lower speed launch rail to accelerate the first little bit where it's not possible to lower the Isp far enough to match the flight speed, launch from high altitude, etc.

But for a first shot at a truly large, full, rapidly reusable RLV, ITS (and possibly whatever Bezos has planned for New Armstrong) would already demolish the $50/kg benchmark that these alt-launch concepts are struggling to meet.

You can deduce that space guns and space elevators will likely beat RLV if you reason by first principles.  Suppose you had the perfect RLV.  For the sake of the scenario lets define the perfect RLV as one that is infinitely reusable, requires no maintenance, and is completely automated so there are no labor costs.  The cost of using such a RLV would be in fuel alone.  Elon Musk said that the fuel costs for the Falcon 9 are about $200,000 per launch.  Sure you can use a cheaper fuel like methane, but you still have to deal with the rocket equation.  At the end of the day you will still consume fuel at a 10:1 fuel to payload ratio if you are really lucky. 

The perfect space elevator would only require an amount of energy roughly less than the orbital energy of GSO.  The perfect space gun would only require the orbital energy minus the drag the projectile initially experiences.  In order to put it in perspective the orbital energy for LEO is about 33 MJ/kg, which is less than what is in a kg of gasoline.

Fact of the matter is that if it comes down to energy space elevators and space guns will always win over RLV.
« Last Edit: 04/13/2017 08:40 PM by DarkenedOne »

Offline Robotbeat

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That is due to a misunderstanding of the significance of the rocket equation. If you adjust the Isp of the rocket to match your flight velocity, you can get PERFECT efficiency. Now, as chemical rockets cannot get more than 5km/s (vs 7.5km/s orbital speed) there's a limit to the efficiency, but you're still able to get maybe 75% of the way to perfect efficiency just with chemical rockets.

Guns have to deal with going at full speed in the dense atmosphere and still needing circularization. Additionally, since it isn't expanding into vacuum, there is inherent loss there as well.

With space elevator, you have loss due to conversion of electrical energy to laser energy then conversion back to electrical energy. That alone gives you maybe 25% efficiency.

And a big problem with gun and elevator cost is the dry mass that the payload carrier brings with. That is pure loss. Suppose your elevator climber is equal in mass to the payload it carries. That's ANOTHER 50% loss, so you're ALREADY back to the same efficiency of ITS, PLUS your source of energy (electricity) costs an order of magnitude more than ITS's methane. And actually dry mass in orbit, this is one thing that rockets (especially multistage rockets) can excel at. The F9 upper stage has a dry mass of about 4.5 tons, and supposedly can put 22 tons payload in orbit. That's an incredibly good ratio. ITS is supposed to do similar while also being fully reusable.

Again, we get hung up on the exponential part of the rocket equation and tend to ignore everything else. Rockets do dang good. Gun launch cannot compete even with finely divisible & g-tolerant payloads.

(This may be different for the Moon, by the way.)
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Offline Robotbeat

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Let me repeat: the rocket equation does not preclude perfect efficiency.
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Offline Robotbeat

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A gun is also not perfectly efficient. Maybe 30% efficiency is typical. Good rocket engines are better, with some oxygen-rich Soviet engines approaching 50% in vacuum (and you can get 80% or even 99% efficient with high enough pressure and big enough area ratio). And that's ON TOP of the inefficiency induced by the dry mass of your sabot, payload carrier (with heatshield), and crazy friction.
« Last Edit: 04/13/2017 09:10 PM by Robotbeat »
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Online KelvinZero

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.
Yes that is always true, but you have to remain within a reason length.  In order to accelerate a mass up to 5 km per second with  max G force of 50g you would need a barrel 25 km long.
There is an possible workaround to this that can be exploited in some cases. It might be hard to apply to direct launch from earth but could be used in other cases.

Rather than a huge rail gun used only momentarily, you could have a much smaller rail gun that fires a stream of momentum carrying elements. Then you would need a similar rail on your vehicle to catch this stream and send it back to the first rail gun in a loop. It would sort of be equivalent to having a temporary magnetic rail that stretches over say a million kilometers.

If you do not attempt to recover these elements then the catcher on the vehicle can be lighter than the launcher. In the extreme case with smaller elements this becomes a variation on beamed powered propulsion.
https://en.wikipedia.org/wiki/Beam-powered_propulsion#Direct_impulse

Online ChrisWilson68

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.
Yes that is always true, but you have to remain within a reason length.  In order to accelerate a mass up to 5 km per second with  max G force of 50g you would need a barrel 25 km long.
There is an possible workaround to this that can be exploited in some cases. It might be hard to apply to direct launch from earth but could be used in other cases.

Rather than a huge rail gun used only momentarily, you could have a much smaller rail gun that fires a stream of momentum carrying elements. Then you would need a similar rail on your vehicle to catch this stream and send it back to the first rail gun in a loop. It would sort of be equivalent to having a temporary magnetic rail that stretches over say a million kilometers.

If you do not attempt to recover these elements then the catcher on the vehicle can be lighter than the launcher. In the extreme case with smaller elements this becomes a variation on beamed powered propulsion.
https://en.wikipedia.org/wiki/Beam-powered_propulsion#Direct_impulse

Good luck aiming the slugs to hit the rail on a vehicle a million kilometers away.

Offline Paul451

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Good luck aiming the slugs to hit the rail on a vehicle a million kilometers away.

"Railgun" is the wrong device. It's would need to be a variant on a maglev launcher, with the "net" at each end being a long magnetic funnel. to steer the projectile into the braking/accelerating tube(s).

[And if you really don't care about delicacy, you can always use the Orion model. Big impact plate. "Kinetic Sail". Bang bang bang.]

[edit:typo]
« Last Edit: 04/13/2017 11:15 PM by Paul451 »

Online ChrisWilson68

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Good luck aiming the slugs to hit the rail on a vehicle a million kilometers away.

"Railgun" is the wrong device. It's would been to be a variant on a maglev launcher, with the "net" at each end being a long magnetic funnel. to steer the projectile into the braking/accelerating tube(s).

[And if you really don't care about delicacy, you can always use the Orion model. Big impact plate. "Kinetic Sail". Bang bang bang.]

OK, but the bigger you make your magnetic net, the more expensive it is.  It also introduces the possibility of drag as it interacts with ions the ship is moving past.


Online KelvinZero

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Good luck aiming the slugs to hit the rail on a vehicle a million kilometers away.
Im sure you could think of alternatives yourself. I think what you are actually doing is continuing the topic of recent posts, pointing out the huge gulf of technical hurdles before ideas like this could supplant reusable rockets. I totally agree there.

I was really just making the academic point that there is a potential loophole to escape the obvious problem of scaling magnetic schemes up to HSF scales and down to HSF-survivable accelerations.

Online Nilof

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I generally agree with the sentiment that reusable Rockets are very hard to beat for HSF space launch. The quadratic dependence of acceleration distance as a function of delta-v really optimizes for a launch concept with no track.

Space elevators are not something I'd even consider viable because of the absolutely tiny payload throughput they can support in relation to their ludicrous size, and are hard to make viable even if you magically got the tether in place for free. Rotovators are a lot more viable but they can easily become very large if you limit centrifugal acceleration while increasing tip velocities to orbital speeds.

Afaik, the only options that could compete with reusable rockets from an energy efficiency viewpoint and which don't have some inherent limit to launch rates are either electromagnetic acceleration along a track, or momentum exchange with a stream of mass that you fire at a vehicle.

Startram is an example of the former, but the long track really handicaps it. You can only launch in one direction from a given launch site. The second hasn't been explored much. I think the optimal solution for that route would be a dynamic version of the space fountain, where the top isn't gradually lifted up by the pellet stream, but accelerated to escape velocity. This would be my favourite pet idea since it can have near-optimal energy efficiency all the way up to escape velocity and doesn't require a track.

Reusable rockets like the ITS are really difficult to beat though, especially when you consider that it is an incremental step rather than a completely new technology.
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 Rei

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I like composites, but aluminum lithium alloy is pretty great. Aluminum 737 has lighter dry weight per passenger than composite 787, for instance.

That's of course a very meaningless comparison to make, just picking two random objects, of which only a fraction of their masses are made of the material in question (composites are only a fraction of the total) and ignoring everything else that makes them different (for example, the fact that the 787 is pressurized to a higher pressure, which corresponds linearly with stress on the skins) - and a bunch of other features that add mass. And even still - reguardless how how people choose to configure seats (why would that be a reasonable measure, exactly?), the 787 has a higher ratio of maximum takeoff weight to empty weight:

https://en.wikipedia.org/wiki/Boeing_737_Next_Generation
https://en.wikipedia.org/wiki/Boeing_787_Dreamliner

You don't compare materials by picking random structures that happen to include them as part of their mass, and comparing those structures on one randomly selected property. You compare their material properties.  And from a material properties perspective, there's no comparison; carbon fibre has a far superior strength to weight ratio.

Quote
There are actually some manufacturing advantages to composites, which is why you have fiberglass boats and wind turbines instead of just aluminum. Composites are easier to make compound curves, for instance.

I really can't wait to see the manufacturing process for large composite rockets. Because it's basically going to be done the same way as for COPVs (since they're big cylinders). Have you ever seen that?



I recommend putting it on fast and skipping through, as it takes a while.  But picture a skyscraper-sized system doing that  :)

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The REAL trick why rockets can beat alt-launch concepts is because of rapid reuse. Reusing the booster 1000 times and the upper stage hundreds of times with zero refurb on the booster and virtually none on the upper stage. Rapid reuse and the ability to use LOx and LNG, both ridiculously cheap, is the biggest threat to alt-launch.

If it can be pulled off   :)

Offline Robotbeat

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Agreed with all that. I'm pro-composites, I just think it's possible to do extremely well with aluminum lithium (don't need to sandbag the design as much with metal... The flip side of that is: Composites that need to be certified for aerospace might not be THAT much lighter for the same purpose, but will likely be stronger with a lot of hidden margin).

SpaceX's prototype ITS composite tank was not filament wound, by the way.
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Offline Paul451

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I think the optimal solution for that route would be a dynamic version of the space fountain, where the top isn't gradually lifted up by the pellet stream, but accelerated to escape velocity. This would be my favourite pet idea since it can have near-optimal energy efficiency all the way up to escape velocity and doesn't require a track.

Confused here. In order to accelerate the ship to orbital velocity, the pellets need to be have more than orbital velocity when they reach the ship, and so haven't you just invented a space-cannon for the pellets?

Composites are easier [than metal] to make compound curves, for instance.

Que?

Offline Robotbeat

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You can do it with metal, but you need a big press or need to rivet smaller pieces together or stamp them with a hammer, etc. It's more difficult to achieve complicated curves with metal parts. That's why when you see an aluminum fairing, it's a biconic shape. Just a simple, single curve (cone) for each segment. For a composite fairing, you always see a more complexly curved (double curved) shape, because it's as easy to do that as the simpler shape, so you might as well do the more complex one.
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Offline Paul451

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You can do it with metal, but you need a big press or need to rivet smaller pieces together or stamp them with a hammer, etc. It's more difficult to achieve complicated curves with metal parts.

To do complex shapes in metal requires a mould and a press. (Or an oven and a cast. Or a block and a routing machine.) To do complex shapes in composites requires a mould and usually a vacuum-oven. There's no savings in equipment.

But for any given shape, composites are generally much more time consuming and labour intensive, which is why they are always more expensive. If you've ever shaped metal, or made composites (I've only done a bit of fibreglassing), you'll know that metal is much much easier and quicker to work with. The only advantage of composites is the amazing strength/weight ratios.

[A game changer would be if you could form the target fibres chemically from a raw liquid, after being moulded. Then you could just injection mould, which is cheaper and easier than either metal working or composite lay-up. Carbon, basalt and glass are all heat-extruded into fibre, and seem unlikely to ever allow chemical creation, but CNTs might be possible. (There are crude versions of bulk-CNT reinforced materials, even CNT-laced resin for use with carbon-fibre cloth, but the CNTs are simply mixed with the resin and hence mechanically tangled rather than chemically forming an interconnected web.)]



[edit:

That's too off-topic for this thread... so... ummm, Nuclear Verne Gun.

Bit like a light-gas gun, but powered by a 150kT nuclear warhead. Tunnel a couple of miles into a geological feature, usual example given is a salt-dome, excavate out a chamber, fill the chamber with water. At the centre you place the 150kT warhead, wrapped in a boron blanket. A 1000+ tonne payload sits part-way up the tunnel, underneath is a compressible fluid like oil. Set off the nuke, energy is absorbed by the water, steam/plasma/etc push the oil which pushes the payload. Payload is accelerated to escape velocity. Suits any nation with nuclear weapons, poor environmental policies, and a separate conventional manned space program, so Russia and China. Or North Korea as a demonstration alone.]
« Last Edit: 04/16/2017 05:00 AM by Paul451 »

Offline Robotbeat

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To do simple shapes in composites requires a mold and an oven. That's my point.
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Online Nilof

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I think the optimal solution for that route would be a dynamic version of the space fountain, where the top isn't gradually lifted up by the pellet stream, but accelerated to escape velocity. This would be my favourite pet idea since it can have near-optimal energy efficiency all the way up to escape velocity and doesn't require a track.

Confused here. In order to accelerate the ship to orbital velocity, the pellets need to be have more than orbital velocity when they reach the ship, and so haven't you just invented a space-cannon for the pellets?


Yes. But the pellets don't have to be accelerated at HSF-tolerable accelerations. The acceleration on the pellets can be several orders of magnitude higher than the acceleration felt by the payload. So you eliminate the need for a long solid track, and you can build it anywhere on Earth.
« Last Edit: 04/16/2017 11:53 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 stefan r

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If you think that's all they're good for... then if you do happen to build one, (a) please don't tell your military about it and (b) please don't aim it in the direction of North Korea! (..or the Middle East, Russia or anywhere else on Earth, pretty much) :o

If it is built in the continental USA it is likely going to be aimed at Australia.   :P

No need to worry too much.  Rockets can be fired at any target so simultaneous aiming over Korea, Tehran, and Moscow would be challenging.  The mass driver gets 10 shots per day.  Rockets can all launch at once.  The mass driver is less dangerous than an equivalent rocket launch capability.

Offline Hop_David

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Mass drivers are possibly the most practical option for non-rocket space launch.

Something moving 7.7 km/s at sea level would have a dynamic pressure of 37,000 kilo pascals. This speed at the top of Chimborazo would 17,000 kilopascals. A severe hurricane is about 3 kilo pascals. Max Q for ascent of many space craft is around 35 kilopascals.

And what is flight path angle at launch? If straight up, it'd get out of the thick atmosphere pretty quick. But with no horizontal velocity, it'd come straight back down.

If launched at near horizontal flight path angle, it'd take 20 minutes or so to ascend above the thick atmosphere. Given a .04 drag coefficient, radius of 1.8 meters and a 500,000 kg rocket, that'd be 20 to 30 minutes of 1.5 g deceleration. Nine minutes at that drag would completely kill a 7.7 km/s velocity.

Offline Robotbeat

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Build a 50km tall tower. Way easier than a space elevator! :)
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Online ChrisWilson68

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Good luck aiming the slugs to hit the rail on a vehicle a million kilometers away.
Im sure you could think of alternatives yourself. I think what you are actually doing is continuing the topic of recent posts, pointing out the huge gulf of technical hurdles before ideas like this could supplant reusable rockets. I totally agree there.

I was really just making the academic point that there is a potential loophole to escape the obvious problem of scaling magnetic schemes up to HSF scales and down to HSF-survivable accelerations.

Sure, I'm not faulting you for bringing up ideas, but if we're going to bring up possible solutions, I think it's also worth pointing out weaknesses of those possible solutions.

I'm not convinced that it's just a matter of overcoming technical hurdles before these ideas are viable.  It may well be that even if we have 10 million years of development rockets will still be a better choice than shooting slugs from a rail gun at a vehicle.

I'm not saying it couldn't be made to work, just that making it work will require enough additional complexity that it will always be cheaper and more reliable to use a simple reusable rocket.

Of course, entirely other things might be superior to chemical rockets in the long run, it's just that I wouldn't bet on it being mass drivers throwing slugs over enormous distances at vehicles.

Online Nilof

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Mass drivers are possibly the most practical option for non-rocket space launch.

Something moving 7.7 km/s at sea level would have a dynamic pressure of 37,000 kilo pascals. This speed at the top of Chimborazo would 17,000 kilopascals. A severe hurricane is about 3 kilo pascals. Max Q for ascent of many space craft is around 35 kilopascals.

And what is flight path angle at launch? If straight up, it'd get out of the thick atmosphere pretty quick. But with no horizontal velocity, it'd come straight back down.

If launched at near horizontal flight path angle, it'd take 20 minutes or so to ascend above the thick atmosphere. Given a .04 drag coefficient, radius of 1.8 meters and a 500,000 kg rocket, that'd be 20 to 30 minutes of 1.5 g deceleration. Nine minutes at that drag would completely kill a 7.7 km/s velocity.

You need to elevate the last part of the tube if you want to do HSF. Startram does it magnetically with a JxB force. It's unproven, but the power required for this kind of active support is surprisingly small.

For unmanned stuff, extremely high velocities through lower parts of the atmosphere have been reasonably well studied for the purpose of ICBM warheads, and anti-ICBM weapons like Sprint.
« Last Edit: 04/19/2017 04:42 PM 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 Hop_David

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Build a 50km tall tower. Way easier than a space elevator! :)

And way easier than warp drives as well. But still not plausible.

Online KelvinZero

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Of course, entirely other things might be superior to chemical rockets in the long run, it's just that I wouldn't bet on it being mass drivers throwing slugs over enormous distances at vehicles.
Yep, It is well beyond the window I would bet on either. Im still awed by the F9R first stage. I think lots of people miss it's potential.

Online KelvinZero

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Build a 50km tall tower. Way easier than a space elevator! :)
And way easier than warp drives as well. But still not plausible.
I mentioned earlier in the thread the idea of having lighter than air skyscrapers. I think the record for high altitude balloons is about 50km.

Having a tall structure like that could also give you a lot of control over your altitude also, by shuttling some pressurised compartments up and down like elevators. Near the top they would transform into dead weight.

Access to all these different heights could also allow you to tack against different streams and perhaps also generate power.

Online ChrisWilson68

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Build a 50km tall tower. Way easier than a space elevator! :)
And way easier than warp drives as well. But still not plausible.
I mentioned earlier in the thread the idea of having lighter than air skyscrapers. I think the record for high altitude balloons is about 50km.

Having a tall structure like that could also give you a lot of control over your altitude also, by shuttling some pressurised compartments up and down like elevators. Near the top they would transform into dead weight.

Access to all these different heights could also allow you to tack against different streams and perhaps also generate power.

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.

Online 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.

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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.

Online 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 »

Offline 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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Online 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)

Online 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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