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PostPosted: Sep 22, 2014 2:08 pm 
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Despite decades of promises from enthusiasts, putting stuff into orbit, especially large orbits, is still very expensive. Geosynchronous orbit currently costing $15000-20000/kg.

SpaceX and such are promising lower costs near $5000/kg but it seems it won't be cheap with chemical rockets and complicated engines. Rockets are expensive and have very unfavorable self weight to payload ratios.

Clearly we need a new paradigm. Some pretty crazy ideas are out there such as ground based laser heated hydrogen propulsion.

But what if we really concentrate to the most favorable (lowest) self weight to payload ratio, without exotic (and in my mind dangerous and crazy) ideas like space elvators?

Can we make a linear accellerator, like a gauss launcher? We'd need is a really tall vertical tower. How tall would this have to be while keeping accelleration to a level where robots and probes can still survive? This seems like an efficient method of propulsion.


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PostPosted: Sep 22, 2014 3:44 pm 
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Use a momentum-exchange, electrodynamic-reboost (MXER) tether system to propel satellites from LEO to GTO. This will cut initial mass in LEO in half. It's fully reusable and uses solar power to restore orbital energy to the system after each use.


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PostPosted: Sep 22, 2014 6:26 pm 
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That's awesome Kirk, thanks.

Cyril, what would you say is the maximum design G-force for a probe launch? We could do some napkin calculations to see how tall the tower would have to be.


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PostPosted: Sep 22, 2014 9:37 pm 
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Well I am not really sure space elevators would be particularily dangerous if you have the materials necessary to make one..... (debris can be handled in the accident scenario - even if you resort to burying cutting charges in the cable structure).

Best thing we got in terms of making launchers cheap is probably the OTRAG concept.
Just pump out tens of thousands of 'thrust launchers' on a huge car style production line with the minimum number of moving parts (and definitely no turbopumps).


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PostPosted: Sep 23, 2014 2:57 am 
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E Ireland wrote:
Well I am not really sure space elevators would be particularily dangerous if you have the materials necessary to make one..... (debris can be handled in the accident scenario - even if you resort to burying cutting charges in the cable structure).


My main concern is the cable breaking off. If it breaks midway (due to say comet strike) then the lower half will plummet to earth and its kinetic energy will be a sort of "whip from hell" weapon of mass destruction. Imagine a cable moving at high speed through the entire atmosphere of the earth, potentially killing people on the other side of the world! Not in my backyard becomes not on my planet.


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PostPosted: Sep 23, 2014 3:08 am 
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Joshua Maurice wrote:
That's awesome Kirk, thanks.

Cyril, what would you say is the maximum design G-force for a probe launch? We could do some napkin calculations to see how tall the tower would have to be.


I think 10-20 G is no problem, more can be done but it gets awkward/costly with precision robotics "astronauts".


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PostPosted: Sep 23, 2014 3:35 am 
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Here's what I'm thinking of.

We make a long horizontal but inclined linear accellerator track, like EM powered (like a maglev). We use a location that has a tall mountain with gradual slope, to build the track on to cut down on tall structure costs. At the end the track curves up to launch the cargo.

Would it make a big difference in track length if we use 100 G as max.


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PostPosted: Sep 23, 2014 5:10 am 
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Cyril R wrote:
Here's what I'm thinking of.

We make a long horizontal but inclined linear accellerator track, like EM powered (like a maglev). We use a location that has a tall mountain with gradual slope, to build the track on to cut down on tall structure costs. At the end the track curves up to launch the cargo.

Would it make a big difference in track length if we use 100 G as max.


I quite like that idea, too bad it would only be useful for cargo launches.

An idea did hit me though of how something like this could be adapted for human launch.

Making the G load survivable can only be achieved by reducing the exit velocity and stretching out the acceleration period as long as it is economical to do so. Of course what would happen is now you would not have enough energy and cannot maintain exit velocity .

But if you don't confine yourself to the thinking of having only the accelerator as the sole propulsion you open up the option for on-board propulsion in the launch vehicle to make up the shortfall. In short merging old world tech with new.

For cargo launches, the vehicle is a dead load container. For human launches you design a vehicle that has something like a pulse rocket engine (fuel efficiency, reducing fuel load). The purpose of the engine would be to maintain the launch vehicle at exit velocity, thus the ground accelerator would be used to provide the initial propulsion to exit velocity, meaning that the energy/fuel requirements for the launch would effectively moved to an external source, reducing the dead load and per kg cost of a launch. I haven't run any numbers on napkins, but since most of the fuel load on existing rockets is entirely for achieving exit velocity, it wouldn't be a stretch to think that such a system would cut on-board fuel by more than half, three quarters even.

In fact, you could even make a recoverable detachable launch section a la what traditional rocket stages do, further streamlining the orbiter vehicle. And to make the materials needed to construct the launch vehicle more flexible you could use a sabot like system for the railgun for the load carrier on the initial EM Rail run.

The perfect analogy for this would be carrier catapults and the jet aircraft they sling off the deck, only on a MUCH grander scale.

Thoughts?

Ben


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PostPosted: Sep 23, 2014 6:31 am 
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Cyril R wrote:
Here's what I'm thinking of.

We make a long horizontal but inclined linear accellerator track, like EM powered (like a maglev). We use a location that has a tall mountain with gradual slope, to build the track on to cut down on tall structure costs. At the end the track curves up to launch the cargo.

Would it make a big difference in track length if we use 100 G as max.



that was my idea too,
but you dont have to launch straight up at 90 degrees
however, there are not many suitable locations
the most obvious place in the tibet plateau, not only you have the himalayas near by for the final section of the maglev, and a very large plateau, because you will need a few hundreds of kilometers for the mostly flat track, but even more importantly all this teritory is located in china, which translates into cheap work force, cheap land , no environmentalist harasement ,no political endless debate which in the west can kill almost any project and no cost overruns due to 1 guy working and 10 others signing for the work done by the first.

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PostPosted: Sep 23, 2014 6:55 am 
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Cyril R wrote:
My main concern is the cable breaking off. If it breaks midway (due to say comet strike) then the lower half will plummet to earth and its kinetic energy will be a sort of "whip from hell" weapon of mass destruction. Imagine a cable moving at high speed through the entire atmosphere of the earth, potentially killing people on the other side of the world! Not in my backyard becomes not on my planet.

AIUI the lower portion of the cable close to the launch site will not cause much damage as it will simply drop to the ground - this is likely to be the greatest concentration of infrastructure along the cable route for at least a few thousand kilometres.
(If the cable is sited correctly by an ocean on the equator then the cable will drop into open sea - can't remember which coast it is though).

You can place charges that will cause the cable to break up and turn into a series of independent objects - as well as one guillotuine charge that will cut the cable at the altitude where the component above it will always remain in an (admittedly low) earth orbit as the section below it drops away.

I actually write science fiction as a hobby and my 'fissionpunk' future setting has low yield nuclear devices built into the cable structure to obliterate large fractions of it in this scenario (but then the cables in that scenario have grown to become enormous structures far larger than anything we would need in anything but the very long term).


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PostPosted: Sep 23, 2014 8:23 am 
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PebbleMaster wrote:
Cyril R wrote:
Here's what I'm thinking of.

We make a long horizontal but inclined linear accellerator track, like EM powered (like a maglev). We use a location that has a tall mountain with gradual slope, to build the track on to cut down on tall structure costs. At the end the track curves up to launch the cargo.

Would it make a big difference in track length if we use 100 G as max.



that was my idea too,
but you dont have to launch straight up at 90 degrees
however, there are not many suitable locations
the most obvious place in the tibet plateau, not only you have the himalayas near by for the final section of the maglev, and a very large plateau, because you will need a few hundreds of kilometers for the mostly flat track, but even more importantly all this teritory is located in china, which translates into cheap work force, cheap land , no environmentalist harasement ,no political endless debate which in the west can kill almost any project and no cost overruns due to 1 guy working and 10 others signing for the work done by the first.


Tibet plateau and Himilayas, that's a good place for this scheme.

If we don't go straight up (90 degrees) then we have to go through a lot more atmosphere. Seems bad. OTOH a shallower angle would allow lower G (or shorter bend for the same G). What's the optimal angle?


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PostPosted: Sep 23, 2014 8:45 am 
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Going straight up does not get you to orbit though.


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PostPosted: Sep 23, 2014 8:49 am 
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My guestimation would be between 70 and 80 degrees

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PostPosted: Sep 23, 2014 10:08 am 
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Guys, shooting something straight up, or even at an angle, will not get it into orbit. You primarily need lateral (horizontal) velocity, but if you try to apply that in the atmosphere your item will just burn up. That's why rockets first go up (to get above the atmosphere) but then bend over (to go fast). A true ascent trajectory (which I spent a number of years at NASA and the Army modeling) is a combination of both objectives, optimized into a path that is the best for a given set of initial and later conditions. Each one is dependent on the thrust of engines, both initially and during ascent, staging conditions if any, and of course the overall mass of the rocket and how it changes as propellant is consumed. If it sounds complicated that's because it is. NASA probably has less than a dozen people who know how to do it. I took great satisfaction in being one of them.

Even if you do all this, you still need a "circularization burn" applied at about 180 degrees in your initial orbit from the site where you launched, in order to stay in orbit. This isn't a big deal for most payloads that carry onboard propulsion systems, but if you wish to launch some "dumb" payload like water or bulk supplies this ends up being a non-trivial problem. It's not much delta-V to circularize an orbit but it's more than zero, and if you don't do it, and don't it at the right place, then all your work was for naught because the payload will follow an orbit that intersects the original launch site and will burn up in the atmosphere about 80-90 minutes after you launch it.

There's a good reason we use rockets to get things into space instead of things like big EM guns or whatever speculative device you wish to imagine.


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PostPosted: Sep 23, 2014 11:25 am 
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Kirk Sorensen wrote:
Guys, shooting something straight up, or even at an angle, will not get it into orbit. You primarily need lateral (horizontal) velocity, but if you try to apply that in the atmosphere your item will just burn up. That's why rockets first go up (to get above the atmosphere) but then bend over (to go fast). A true ascent trajectory (which I spent a number of years at NASA and the Army modeling) is a combination of both objectives, optimized into a path that is the best for a given set of initial and later conditions. Each one is dependent on the thrust of engines, both initially and during ascent, staging conditions if any, and of course the overall mass of the rocket and how it changes as propellant is consumed. If it sounds complicated that's because it is. NASA probably has less than a dozen people who know how to do it. I took great satisfaction in being one of them.


So, could you tell us if the calculus changes if you have a low energy and variable cost way to launch and you have a relatively low frontal area (thin) cargo? With rockets you're lumping all that heavy fuel, engine and shell around, fighting the thick early portions of the atmosphere with such a system by inclining the orbit too much would be foolish.

What if we shoot the cargo capsule at the right angle to get the required velocity (for geosynchronous orbit) and orbit altitude all in one go (with minor adjustment rockets on the cargo)?

Quote:
There's a good reason we use rockets to get things into space instead of things like big EM guns or whatever speculative device you wish to imagine.


Well, in that line of reasoning, one might also say there's a good reason we use LWRs rather than MSRs today. But I don't think the reason is entirely in the physics and engineering, and I'm pretty sure that you of all people will agree (haven written much on the subject yourself). The politics, military reasons (guided rockets, good weapons), and early space program where cost was not as important as getting it done, play a major role in the evolution of our space technology. Today we have different needs and wants. Plus advances in electronics and computers mean levitation technology is well advanced today.


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