Author Topic: Railgun Loop for space travel  (Read 1211 times)

Offline whazp

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Railgun Loop for space travel
« on: 01/11/2019 01:50 am »
Hi All,

Railgun's can accelerate objects pretty fast. All the ideas i have seen are using rail guns have been along the lines of using a single really big & powerful rail gun to accelerate an object in space.

What about using a rail gun loop that could essentially work similar to a particle accelerator. This could be a rail gun that is actually a large ring tube placed in space (low friction). An object in placed within the rail loop tube and magnetic fields are used to accelerate the object slowly over time around the loop until the desired speed is achieved. It could complete loops as many times as needed to reach massive speeds. Once the desired speed is achieved the loop would 'open' at a point allowing the object to hurtle off into space.
I think you would have to use magnetic fields to keep the object from touching the sides of the tubes, but i think this method would allow a huge amount of energy to accelerate the object close to earth rather than the object having to have a fuel load and accelerate on its journey.

Hopefully that made some sense. Apologies if this idea has already been thought of.

Offline meberbs

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Re: Railgun Loop for space travel
« Reply #1 on: 01/11/2019 03:43 am »
The idea for railguns and variations of them for space launch from Earth is not practical for many reasons.

There are other places this has been covered on this site as recently said in another thread:
...
Also, if you want to consider launches that use some variation of "space gun" from the Earth surface, the long list of reasons why that is just not a good idea has been documented in multiple threads:
https://forum.nasaspaceflight.com/index.php?topic=45961.0
https://forum.nasaspaceflight.com/index.php?topic=42706.0
https://forum.nasaspaceflight.com/index.php?topic=45295.0

Note that the last of the links was started by a proponent of a non-physical propellantless propulsion drive, and some of their claims in that thread demonstrate that they don't understand basic physics.

For your specific variation, the problems are:
-no matter how gentle you accelerate linearly, if you get going fast, continuing to move in a circle applies high acceleration of v^2/R, where R is the radius of the loop. Either the loop is even bigger than the linear equivalent, or the acceleration is still very high.
-The problem with these concepts usually is not that you can't accelerate to high enough speed with a railgun, but that you simply can't move anywhere near fast enough in atmosphere to be worthwhile, without melting your craft.
-It sounds like you may be trying to talk about doing it from once you are already in space, but this has 2 big problems: 1. getting to space in the first place is the actually hard part, and 2. you have to cancel out the equal and opposite reaction of your space station somehow, which still takes fuel.

Offline spaceman100

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Re: Railgun Loop for space travel
« Reply #2 on: 01/12/2019 07:03 am »
Hi All,

Railgun's can accelerate objects pretty fast. All the ideas i have seen are using rail guns have been along the lines of using a single really big & powerful rail gun to accelerate an object in space.

What about using a rail gun loop that could essentially work similar to a particle accelerator. This could be a rail gun that is actually a large ring tube placed in space (low friction). An object in placed within the rail loop tube and magnetic fields are used to accelerate the object slowly over time around the loop until the desired speed is achieved. It could complete loops as many times as needed to reach massive speeds. Once the desired speed is achieved the loop would 'open' at a point allowing the object to hurtle off into space.
I think you would have to use magnetic fields to keep the object from touching the sides of the tubes, but i think this method would allow a huge amount of energy to accelerate the object close to earth rather than the object having to have a fuel load and accelerate on its journey.

Hopefully that made some sense. Apologies if this idea has already been thought of.

NASAs Maglifter study points out that only 600 mph launch speed would yield 80% more payload to LEO.

Huge speed would incinerate the craft on take off.

Enclosed system would be free in energy as operated by wind...and reaches 600 mph launch speed with very simple system.

Offline tyrred

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Re: Railgun Loop for space travel
« Reply #3 on: 01/12/2019 07:22 am »
It's a mistake of imagination to take a single study and extrapolate it to the realm of a better solution to an already solved problem.

Offline spaceman100

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Re: Railgun Loop for space travel
« Reply #4 on: 01/12/2019 09:34 am »
It's a mistake of imagination to take a single study and extrapolate it to the realm of a better solution to an already solved problem.

Which problem has been solved ?

Offline KelvinZero

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Re: Railgun Loop for space travel
« Reply #5 on: 01/12/2019 09:48 am »
What about using a rail gun loop that could essentially work similar to a particle accelerator. This could be a rail gun that is actually a large ring tube placed in space (low friction).
Hi, you might be interested in the https://en.wikipedia.org/wiki/Orbital_ring

It was also mentioned a couple of times in the recent "Commuting to Saturn" thread.

...
Issac Arthur's video describing Orbital Rings
https://youtube.com/watch?v=LMbI6sk-62E

A leo orbital ring could also do something like a BFS earth departure.. If the ring can support 3g acceleration (giving you 4g due to one being cancelled by earth) that gives you around 16km/s .. which I think is equivalent to around 5km/s when you subtract 11km/s earth escape velocity?

Edit:
Also this idea from a few years back: the Satellite Launch Ring:
http://www.launchpnt.com/portfolio/aerospace/satellite-launch-ring

Another application of this idea would be a magnetic rail around the equator of the moon.
Not only could this easily handle the fairly modest lunar orbital velocity requirements (less even than lunar escape velocity) but it could also link a series of settlements around the equator, both for commuting and to share power from solar panels on the day side to the night side. I don't think there is any big technical difficulty. You just need a lot of people living there for some reason to justify it.
« Last Edit: 01/12/2019 08:59 pm by KelvinZero »

Offline spaceman100

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Re: Railgun Loop for space travel
« Reply #6 on: 01/12/2019 10:27 am »
What about using a rail gun loop that could essentially work similar to a particle accelerator. This could be a rail gun that is actually a large ring tube placed in space (low friction).
Hi, you might be interested in the https://en.wikipedia.org/wiki/Orbital_ring

It was also mentioned a couple of times in the recent "Commuting to Saturn" thread.

...
Issac Arthur's video describing Orbital Rings
https://youtube.com/watch?v=LMbI6sk-62E

A leo orbital ring could also do something like a BFS earth departure.. If the ring can support 3g acceleration (giving you 4g due to one being cancelled by earth) that gives you around 16km/s .. which I think is equivalent to around 5km/s when you subtract 11km/s earth escape velocity?

How do they get that system into space in the first place ?
« Last Edit: 01/12/2019 01:41 pm by spaceman100 »

Offline KelvinZero

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Re: Railgun Loop for space travel
« Reply #7 on: 01/12/2019 12:23 pm »
How do get that system into space in the first place ?
Usually such megascale engineering ideas assume there is already a healthy space industry to justify wanting to build such a large transportation system. If you have that then of course you use that industry with it's unlimited sunlight for smelting and materials that do not need to be pulled out of earth's gravity well. If a space industry still is not practical even for this, there is probably no justification to build a superhighway there.

There is also some ability for an orbital ring to bootstrap itself: from a thin ring to a thick ring, or even build the thing on the ground in an evacuated tube around the equator and sort of 'charge it up'. I don't really like that evacuated tube version of the ring though. I much prefer the less extravagant version with discrete components and no single point of failure.
(edited to remove whitespace.. I keep posting with a bunch of blank trailing lines for some reason)
« Last Edit: 01/12/2019 08:52 pm by KelvinZero »

Offline joema

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Re: Railgun Loop for space travel
« Reply #8 on: 01/12/2019 02:02 pm »
Quote from: spaceman100
...NASAs Maglifter study points out that only 600 mph launch speed would yield 80% more payload to LEO...

That does not reflect what the  AlAA 94-2726 study said. It said a VERY PRELIMINARY assessment showed IF you built an electromagnetic catapult up the side of a 10,000 ft mountain, THEN accelerated an ELV to 600 mph (268 m/s) it might increase LEO payload from 2000 lbs to 3800 lbs. HOWEVER -- another study they did for small TSTO vehicles showed ZERO benefit.

The first assessment was something roughly the scale of a Minotaur-C (73,000 kg mass). If you accelerate the entire vehicle to 268 m/s, the kinetic energy imparted is 2621 MJ. By contrast the kinetic energy of the LEO payload is 403,915 MJ. During powered ascent, the total imparted kinetic energy to the changing mass of the vehicle stack will be vastly greater than this. I don't know the equation for that, but let's say it's double, or 800,000 MJ (comments/suggestions welcome).

That means the total kinetic energy imparted to the vehicle by a 268 m/s catapult would be about 0.3% of the total required. The reason is that KE is based on velocity *squared*. Small changes in velocity do not impart much KE.

A "small sat" launcher like the Minotaur-C does OK with a stationary launch. It would require a titanic investment to build (and maintain) an electromagnetic catapult up the side of a 10,000 mountain, which would then only imparts a slight energy boost and only be feasible for certain vehicles which could be adapted. The benefit was slight enough that one case in that study's showed ZERO advantage.

If such a thing was built, and if it worked, it's likely that during the lengthy construction and test phase, continuing progress in conventionally-launched reusable vehicles would make it obsolete. If the next generation SpaceX or Blue Origin vehicles can achieve $300 per kg to LEO, nobody would care about using a hugely expensive maglev system that can only launch certain ruggedized vehicles of a certain mass range in one direction.

Offline spaceman100

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Re: Railgun Loop for space travel
« Reply #9 on: 01/12/2019 02:26 pm »
Quote from: spaceman100
...NASAs Maglifter study points out that only 600 mph launch speed would yield 80% more payload to LEO...

That does not reflect what the  AlAA 94-2726 study said. It said a VERY PRELIMINARY assessment showed IF you built an electromagnetic catapult up the side of a 10,000 ft mountain, THEN accelerated an ELV to 600 mph (268 m/s) it might increase LEO payload from 2000 lbs to 3800 lbs. HOWEVER -- another study they did for small TSTO vehicles showed ZERO benefit.

The first assessment was something roughly the scale of a Minotaur-C (73,000 kg mass). If you accelerate the entire vehicle to 268 m/s, the kinetic energy imparted is 2621 MJ. By contrast the kinetic energy of the LEO payload is 403,915 MJ. During powered ascent, the total imparted kinetic energy to the changing mass of the vehicle stack will be vastly greater than this. I don't know the equation for that, but let's say it's double, or 800,000 MJ (comments/suggestions welcome).

That means the total kinetic energy imparted to the vehicle by a 268 m/s catapult would be about 0.3% of the total required. The reason is that KE is based on velocity *squared*. Small changes in velocity do not impart much KE.

A "small sat" launcher like the Minotaur-C does OK with a stationary launch. It would require a titanic investment to build (and maintain) an electromagnetic catapult up the side of a 10,000 mountain, which would then only imparts a slight energy boost and only be feasible for certain vehicles which could be adapted. The benefit was slight enough that one case in that study's showed ZERO advantage.

If such a thing was built, and if it worked, it's likely that during the lengthy construction and test phase, continuing progress in conventionally-launched reusable vehicles would make it obsolete. If the next generation SpaceX or Blue Origin vehicles can achieve $300 per kg to LEO, nobody would care about using a hugely expensive maglev system that can only launch certain ruggedized vehicles of a certain mass range in one direction.

If you know this for sure, perhaps you correct it then ? In wiki and elsewhere.

Offline spaceman100

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Re: Railgun Loop for space travel
« Reply #10 on: 01/12/2019 03:00 pm »
Here are the main points of the MagLifter study.
« Last Edit: 01/12/2019 03:01 pm by spaceman100 »

Online edzieba

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Re: Railgun Loop for space travel
« Reply #11 on: 01/12/2019 05:28 pm »
That's not a correction of the study, it is a summary of it. It would be a good idea to read and understand the entire study, not just skim the conclusion.

Offline Coastal Ron

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Re: Railgun Loop for space travel
« Reply #12 on: 01/12/2019 05:42 pm »
Here are the main points of the MagLifter study.

People LOVE to propose technical problems, but rarely focus on the NEED for such systems. In other words, who would pay for it.

Because unless you have a market that can support the cost of developing and operating such a system, AND survive the competition that comes with it, then it's the equivalent of arguing over how many angels can dance on the head of a pin;)

So if you really want to validate whether such a system is worth pursuing, I would suggest looking at the market for its capabilities and the competition that it would face.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline joema

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Re: Railgun Loop for space travel
« Reply #13 on: 01/12/2019 05:55 pm »
Here are the main points of the MagLifter study.

Those are NOT the main points. The main points are:

- It was predominately an assisted-SSTO study. The SSTO scenario was the reference case, and they spent most of their time studying that. Of course an SSTO could theoretically benefit; they can barely get a useful payload to orbit at all.

- For a two-stage vehicle, the study admits "the overall economics...seem intuitively prohibitive."

- The reference study was NOT a maglev system running up the side of a 10,000-foot mountain, rather a large-diameter tunnel bored INSIDE a 10,000-foot mountain. The study admitted that "...a highly robust structural support system to altitude is needed." Note this was not a little tunnel for a little launcher. The reference case was a 1.2 million pound vehicle.

- The benefits were less for a multi-stage ELV, and in one case ZERO benefit

- There were no listed studies or modeling of the structural mass penalty of laying a million-pound fully-loaded liquid-fueled launcher on its side. While you can put a solid-fueled booster sideways, you can't safely do that for a large liquid fueled vehicle stuffed full of propellant -- without extensive modeling and likely structural reinforcement which costs mass.

Offline KelvinZero

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Re: Railgun Loop for space travel
« Reply #14 on: 01/12/2019 09:04 pm »
We seem to have scared of whazp.

I think any launch system that does not have a loop in it is off topic for this thread, even if no one wants to discuss the OP any further.

Offline joema

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Re: Railgun Loop for space travel
« Reply #15 on: 01/13/2019 12:48 am »
We seem to have scared of whazp.

I think any launch system that does not have a loop in it is off topic for this thread, even if no one wants to discuss the OP any further.

The OP was referring solely to a hollow tube (presumably circular) IN SPACE which would magnetically levitate and accelerate a payload or vehicle in a circle until a high velocity is reached, then release it in the desired direction. Maybe he was thinking about destinations like the moon or Mars.

As already pointed out, the centrifugal force for any reasonable size structure would be very high. Therefore the loop structure would have to be extremely robust. Also the loop would have to use powerful retro-thrust to compensate for the full impulse of the released mass, similar to a floating astronaut who fires a canon.

If the "railgun loop" was 1 kilometer in diameter and you only needed sufficient delta v to get from LEO to the moon (about 3.1 km/sec), this would produce 2,000 g centrifugal acceleration: http://www.calctool.org/CALC/phys/newtonian/centrifugal

Offline KelvinZero

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Re: Railgun Loop for space travel
« Reply #16 on: 01/13/2019 07:17 am »
The OP was referring solely to a hollow tube (presumably circular) IN SPACE which would magnetically levitate and accelerate a payload or vehicle in a circle until a high velocity is reached, then release it in the desired direction. Maybe he was thinking about destinations like the moon or Mars.

As already pointed out, the centrifugal force for any reasonable size structure would be very high. Therefore the loop structure would have to be extremely robust. Also the loop would have to use powerful retro-thrust to compensate for the full impulse of the released mass, similar to a floating astronaut who fires a canon.

If the "railgun loop" was 1 kilometer in diameter and you only needed sufficient delta v to get from LEO to the moon (about 3.1 km/sec), this would produce 2,000 g centrifugal acceleration: http://www.calctool.org/CALC/phys/newtonian/centrifugal
You can't really define reasonable size without a timeframe. I think it is totally reasonable to discuss Dyson swarms for example and that dwarfs these projects about a billion-fold I guess. I think maglev tracks around the lunar equator are reasonable, and even some versions of orbital rings are reasonable.. given some future level of space industry that we can only hypothesise.

In some cases thousands of g acceleration and tiny payloads could be fine, for example some sort of "beamed propellant propulsion"

Usually, rather than 'retro-thrust', momentum is maintained by either both accelerating and decelerating payloads, or it could be in orbit around some planetary body and launch in both prograde and retrograde directions, waiting to be in the correct point in the orbit if necessary. It is probably much more massive than individual payloads so it has time to even things out with different missions.

Also some links above:
....

Offline alexterrell

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Re: Railgun Loop for space travel
« Reply #17 on: 01/13/2019 09:12 am »
Here are the main points of the MagLifter study.

People LOVE to propose technical problems, but rarely focus on the NEED for such systems. In other words, who would pay for it.

Because unless you have a market that can support the cost of developing and operating such a system, AND survive the competition that comes with it, then it's the equivalent of arguing over how many angels can dance on the head of a pin;)

So if you really want to validate whether such a system is worth pursuing, I would suggest looking at the market for its capabilities and the competition that it would face.
If NASA can spend $20 billion on a launch system for which there is no need and no market,.... the concept of putting a 5 km long tunnel in a mountain, equipping it with a maglev system, and designing a rocket for it starts to make sense.

However, this concept competes with air launched systems - Mach 0.9 at 12,000m. The benefit here being that the air launch stage is reusable.

But now that SpaceX have reusable 1st stages, there seems little point in developing either new (large*) air-launched systems, or high-fixed cost maglevs, or overly priced expendable HLVs.

(*I think there is still an opportunity for small launches from large carrier aircraft. I still think it's a shame T-Space never made progress)

Offline alexterrell

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Re: Railgun Loop for space travel
« Reply #18 on: 01/13/2019 09:16 am »
Hi All,

Railgun's can accelerate objects pretty fast. All the ideas i have seen are using rail guns have been along the lines of using a single really big & powerful rail gun to accelerate an object in space.

What about using a rail gun loop that could essentially work similar to a particle accelerator. This could be a rail gun that is actually a large ring tube placed in space (low friction). An object in placed within the rail loop tube and magnetic fields are used to accelerate the object slowly over time around the loop until the desired speed is achieved. It could complete loops as many times as needed to reach massive speeds. Once the desired speed is achieved the loop would 'open' at a point allowing the object to hurtle off into space.
I think you would have to use magnetic fields to keep the object from touching the sides of the tubes, but i think this method would allow a huge amount of energy to accelerate the object close to earth rather than the object having to have a fuel load and accelerate on its journey.

Hopefully that made some sense. Apologies if this idea has already been thought of.

There are two main issues with rail guns for launching payloads:
1. The very high accelerations required to get any useful mass, and the fact that payloads can't take much acceleration (maybe 10g for humans, 100g for scientific payloads, 1000g for designed electronics, 10000g for military shells?)
2. The power required, needing vast banks of capacitors etc etc.

Your proposal might get round the second issue, by allowing a slow speed up. But the centripetal acceleration is still large - indeed you get more acceleration per unit of length of the accelerator track, than with a linear accelerator.

Offline joema

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Re: Railgun Loop for space travel
« Reply #19 on: 01/13/2019 10:36 am »
...
You can't really define reasonable size without a timeframe. I think it is totally reasonable to discuss Dyson swarms for example and that dwarfs these projects about a billion-fold I guess. I think maglev tracks around the lunar equator are reasonable, and even some versions of orbital rings are reasonable.. given some future level of space industry that we can only hypothesise. ...In some cases thousands of g acceleration and tiny payloads could be fine, for example some sort of "beamed propellant propulsion"

The OP clearly meant a circular maglev tube IN EARTH ORBIT (not on earth or the moon) which accelerated an object "on its journey". He likely meant a functional macro-size object like a space probe, not an inert mass nor a micro-size payload.

He was probably not thinking of a far future civilization wielding god-like technology. If so why would they use a merry-go-round in space for accelerating payloads? He was probably thinking about near-term technology and better ways to produce delta v for injecting orbital payloads toward deep space.

The size can vary (see above calculator) but to achieve meaningful delta V for a useful destination, either the diameter or the g-force (or both) would be very high. It is not clear whether it's even feasible with current material science. However if we assume the orbiting maglev tube is 1 km in diameter and cable-stayed (like Von Braun's early space stations), and the cables are 100 cm in diameter (7,853 cm^2 cross section) and constructed of Zylon (1.6x stronger than Kevlar): https://en.wikipedia.org/wiki/Zylon

It might be theoretically possible from a materials standpoint. For a 1 km diameter orbital maglev tube, a medium-size satellite (1,000 kg) accelerated to trans-lunar injection (3.1 km/sec) would be at 2,000 g and produce a tension force of 1.97E07 Newtons, or about 4.3 million pounds force. If this was counter-balanced by an inert mass 180 deg. opposite, the total force would be 8.6 million pounds force. That force if applied on a 100 cm diameter Zylon cable would produce 2.4 Gigapascals of tensile stress, which is below Zylon's limit of 5.8 GPa.

It would be a gigantic engineering project to produce in essence a swinging bucket on a rope which would be released when pointed in the desired direction. We have better ways to accomplish that from LEO -- they're called rockets.

Quote from: KelvinZero
...Usually, rather than 'retro-thrust', momentum is maintained by either both accelerating and decelerating payloads...

If you mean "catching" an incoming payload to balance the outgoing mass, how could a km-size structure intercept an incoming object at the moment it launches an outgoing payload? More importantly,  what are the chances those two events and trajectories would coincide with the required launch window?

Quote from: KelvinZero
... it could be in orbit around some planetary body and launch in both prograde and retrograde directions, waiting to be in the correct point in the orbit if necessary.

Yes that could theoretically work. Launch the payload in one direction and a compensating dumb mass in the opposite direction. That would be needed anyway to counter-balance the force on the structure.

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