Author Topic: Space cannons & mass drivers, launching into specific orbital planes?  (Read 3996 times)

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 »

Offline 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.


Offline 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.

Offline 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.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline 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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Offline 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.

Offline 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.

Offline 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.

Offline 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.

Offline 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.

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