Author Topic: Two Stage Tether to Orbit Launch System  (Read 23802 times)

Offline johncarpinelli

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Re: Two Stage Tether to Orbit Launch System
« Reply #20 on: 02/27/2013 11:06 pm »
Good idea about using chemical rockets for the upper stage. The atmospheric lower stage is the first part of the project. Once it's in operation, you should have significantly cheaper launch with chemical rockets. You could use that capability to build up the orbital stage.

Latest updates:
-Submitted the design in a proposal to NIAC NNH13ZUA001N this month.
-Working on a prototype wing tether constructed with sheets of polyethylene, balsa wood and packing tape.

All suggestions and feedback are appreciated. Building a stable wing tether is harder than it sounds. There are some similarities with kite-building, but I've not seen a kite design that meets our specific requirement.

Offline RanulfC

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Re: Two Stage Tether to Orbit Launch System
« Reply #21 on: 02/28/2013 04:14 pm »
Good idea about using chemical rockets for the upper stage. The atmospheric lower stage is the first part of the project. Once it's in operation, you should have significantly cheaper launch with chemical rockets. You could use that capability to build up the orbital stage.
Might find some helpful stuff here:
http://forum.nasaspaceflight.com/index.php?topic=25095.0;all

(Jet Powered First Stage thread)

Once the lower-stage is up and running you could launch ramjet/rocket vehicles from the upper-tether.
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Latest updates:
-Submitted the design in a proposal to NIAC NNH13ZUA001N this month.
-Working on a prototype wing tether constructed with sheets of polyethylene, balsa wood and packing tape.

All suggestions and feedback are appreciated. Building a stable wing tether is harder than it sounds. There are some similarities with kite-building, but I've not seen a kite design that meets our specific requirement.
Hmmm, hear of the "Princton Sail-Wing" at all?
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0275307
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790015726_1979015726.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690009905_1969009905.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670021814_1967021814.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740008656_1974008656.pdf

The idea is that a solid "leading-edge" shape wrapped with cloth or other flexable material and around a tension stiffened wire as a "trailing-edge" takes on various required "shapes" as in aerodynamically needed at various points from standing still to what-ever the design speed is.

Not sure how you'd make the "leading" edge flexible yet able to support the "wing" but you might look into an inflatable tubular structure.

What kind of "kite" technology were you looking at by the way?

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Joel

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Re: Two Stage Tether to Orbit Launch System
« Reply #22 on: 05/25/2013 10:34 am »
Makani Power has demonstrated tethered flight with their 8m 58kg prototype flying wing. Their drone flies in a vertical circle to capture wind power, whereas our design needs a horizontal circle.


Google has now gone from being an investor to acquiring Makani. It might lead to significantly accelerated research.
http://news.cnet.com/8301-1023_3-57585834-93/google-x-acquires-kite-power-startup-makani/

All suggestions and feedback are appreciated. Building a stable wing tether is harder than it sounds. There are some similarities with kite-building, but I've not seen a kite design that meets our specific requirement.

Note that you don't need to have any electric propellers. An alternative approach is to use the "inverse pumping mode" to keep the airfoil afloat using a linear generator on the ground. If you are interested in the trajectory optimization and control of tethered airfoils, my research group has open-source software for this:
http://homes.esat.kuleuven.be/~highwind/

Offline Joel

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Re: Two Stage Tether to Orbit Launch System
« Reply #23 on: 05/25/2013 11:50 am »
Some other points:

1. Using solar power is a complete non-starter. Wind power at these altitudes is extremely abundant and very consistent.

2. Flying these giant circles that you are proposing will suffer from too much tether drag. Consider having the cable split up into two airfoils higher up (a.k.a. "dancing kites"). This will limit the amount of tether exposed to fast crosswind.

3. What you are proposing has a lot of similarity with airborne wind energy (AWE), and research in this field goes back 30 years. You really should try to leverage that.

4. AWE is feasible up to very high altitudes since the weight of the cable is small compared to the amount of lift generated. Joby Energy (www.jobyenergy.com), whose airborne wind energy department has now merged with Makani, wanted to put tethered airfoils in the jet stream (~10 km). You can probably go significantly higher than that.

5. You need to put ground station off-shore and have restricted airspace. These stuff are potentially very dangerous so you don't want to put them anywhere near populated areas.

Offline johncarpinelli

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Re: Two Stage Tether to Orbit Launch System
« Reply #24 on: 05/26/2013 06:48 am »
That is good news about Makani being acquired. Google has much greater resources and will hopefully scale up the technology. I would love to see them deploy the system commercially.

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Note that you don't need to have any electric propellers. An alternative approach is to use the "inverse pumping mode" to keep the airfoil afloat using a linear generator on the ground. If you are interested in the trajectory optimization and control of tethered airfoils, my research group has open-source software for this:
http://homes.esat.kuleuven.be/~highwind/

I don't understand how this would help the electric takeoff proposal. We need thrust at high altitude to tow the tether. I don't think it could be provided from the ground. Can you elaborate?

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1. Using solar power is a complete non-starter. Wind power at these altitudes is extremely abundant and very consistent.

The solar panels are deployed on the ground and electricity is transmitted along the tether to the tow aircraft. Electricity gives you flexibility in combining wind, solar, hydro and other energy sources. This would be useful for remote locations (e.g. Australian outback) to provide clean air transport. A major goal of the technology is to move aviation away from fossil fuels.

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3. What you are proposing has a lot of similarity with airborne wind energy (AWE), and research in this field goes back 30 years. You really should try to leverage that.

I have not seen much research on tethered electric tow aircraft for point-to-point transportation. Nate Saint was using circular flights in the 1950's to exchange packages with the ground. I do not know of any commercial use of tethered electric aviation for transport.

CyPhy Works is a new startup using tethered drones that looks promising.

http://cyphyworks.com/robots/

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5. You need to put ground station off-shore and have restricted airspace. These stuff are potentially very dangerous so you don't want to put them anywhere near populated areas.

I disagree. The tether will be a light-weight inflatable wing that flies above conventional airspace (e.g. at 30 km). If the system fails, the tether will slow as it descends into the thicker atmosphere at low altitudes. It will flutter harmlessly to the ground and should cause minimal damage.

Offline Joel

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Re: Two Stage Tether to Orbit Launch System
« Reply #25 on: 05/26/2013 09:46 am »
I don't understand how this would help the electric takeoff proposal. We need thrust at high altitude to tow the tether. I don't think it could be provided from the ground. Can you elaborate?
Maybe I didn't understand the idea fully. So the cable that goes from the electric aircraft to the ground is not able to support any tention, only transmit electricity? An alternative is to have a cable that can support tention, but not necessarily electricity and then "pump" the aircraft to higher altitudes. It's the inverse of the pumping cycle for AWE http://homes.esat.kuleuven.be/~highwind/?p=36.

The solar panels are deployed on the ground and electricity is transmitted along the tether to the tow aircraft. Electricity gives you flexibility in combining wind, solar, hydro and other energy sources. This would be useful for remote locations (e.g. Australian outback) to provide clean air transport. A major goal of the technology is to move aviation away from fossil fuels.

Or, you can use a cable that can transmit tension, flying it like a kite, in which case you hardly need any power at all.

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3. What you are proposing has a lot of similarity with airborne wind energy (AWE), and research in this field goes back 30 years. You really should try to leverage that.

I have not seen much research on tethered electric tow aircraft for point-to-point transportation. Nate Saint was using circular flights in the 1950's to exchange packages with the ground. I do not know of any commercial use of tethered electric aviation for transport.

The physics has a lot of similarity. Tethered flight is a tricky thing.

CyPhy Works is a new startup using tethered drones that looks promising.

http://cyphyworks.com/robots/

From what I can see, that is just a quadcopter with a power cable. I'd rather have a look at Makani or the other companies in AWE, such as http://www.ampyxpower.com/


The tether will be a light-weight inflatable wing that flies above conventional airspace (e.g. at 30 km).

Rigid airfoils are superior to inflatable wings when it comes to supporting forces.

Offline johncarpinelli

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Re: Two Stage Tether to Orbit Launch System
« Reply #26 on: 05/26/2013 06:54 pm »
The tether from hub to tow aircraft would support both tension and electricity. I understand how you could pump the aircraft higher by increasing tension on the tether. To keep the aircraft circling the hub, you would need to have a tow vehicle on the ground moving in a circle.

One option would be to use a car or truck to drive in a circle on a flat plain like a salt lake. We discussed that concept earlier in the thread.

Another option would be to use a boat to tow the tether in a circle. This is similar to para-gliders being towed by boats at tourist beaches.

Towing the tether from a ground vehicle would be a cheap way to test tether designs, but it would complicate the process of attaching a payload to the tether. The payload would have to be on the ground vehicle prior to startup of the system.

http://www.ampyxpower.com/ looks very promising. I hope they succeed.

Offline cordwainer

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Re: Two Stage Tether to Orbit Launch System
« Reply #27 on: 05/29/2013 08:21 pm »
Doesn't seem feasible without the use of a high-speed aerostat or rigid airfoil. A Lofstrom Loop seems more likely given current tech, even if it might be more expensive.

Offline Joel

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Re: Two Stage Tether to Orbit Launch System
« Reply #28 on: 05/29/2013 10:06 pm »
Doesn't seem feasible without the use of a high-speed aerostat or rigid airfoil. A Lofstrom Loop seems more likely given current tech, even if it might be more expensive.

Makani was working on soft kites for several years, before they realised that rigid airfoils is the only reasonable way forward... But of course, if they crash, they can easily kill someone on the ground...

Offline RanulfC

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Re: Two Stage Tether to Orbit Launch System
« Reply #29 on: 05/30/2013 07:05 pm »
Doesn't seem feasible without the use of a high-speed aerostat or rigid airfoil. A Lofstrom Loop seems more likely given current tech, even if it might be more expensive.

Makani was working on soft kites for several years, before they realised that rigid airfoils is the only reasonable way forward... But of course, if they crash, they can easily kill someone on the ground...
"Soft" airfoils have been extensivly tested before. One example I pointed out before was the "Princton Sailwing" design:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0275307

(Note: I had originally included several NTRS links on studies done by NASA on the Sailwing. However it now seems that none of them are working and that the data has been removed from NTRS at this time. They are included here again JUST in case they should begin working again as some point:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790015726_1979015726.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690009905_1969009905.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670021814_1967021814.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740008656_1974008656.pdf
End Note)

Several other inflatable or soft airfoils have been suggested in past posts also. My suspicion at this point is the "ribbon" would be some sort of hybrid with the power supply cables and supports making up the main "hard" sections with inflated or air supported sections of very light weight materials in between making up the majority of the surface.

Some other points:
1. Using solar power is a complete non-starter. Wind power at these altitudes is extremely abundant and very consistent.
The concept is a tether-and-tow-aircraft/motors flying a powered circle and 'towing' an extended tether/mass that is a larger circle whose tip speed enters the hypersonic speed range. You can't really "extract" wind energy under those conditions.

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2. Flying these giant circles that you are proposing will suffer from too much tether drag. Consider having the cable split up into two airfoils higher up (a.k.a. "dancing kites"). This will limit the amount of tether exposed to fast crosswind.

3. What you are proposing has a lot of similarity with airborne wind energy (AWE), and research in this field goes back 30 years. You really should try to leverage that.

4. AWE is feasible up to very high altitudes since the weight of the cable is small compared to the amount of lift generated. Joby Energy (www.jobyenergy.com), whose airborne wind energy department has now merged with Makani, wanted to put tethered airfoils in the jet stream (~10 km). You can probably go significantly higher than that.
Not AWE compatible due to the required powered nature of the concept. AWE requires tapping a "drag" factor of an airfoil or airfoils interaction with upper atmospheric winds. This concept uses the "lift" of an aerodynamic tether to offset the mass of the tether and allow a "payload" to advance up the tether while gaining energy from the rotational motion of the system. Since this motion has to be provided by the system this requires a powered segment to move the system in a circular motion.

While some AWE work is relevent this is a very different system and concept from most AWE systems and there is very limited crossover due to those system requirement differences.

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5. You need to put ground station off-shore and have restricted airspace. These stuff are potentially very dangerous so you don't want to put them anywhere near populated areas.
I've suggested that option for very different reasons but in general since the system is "powered" at all times and the tethers are going to have to be aerodynamic as well as stong and light there are many more options for safely lowering, landing, or crashing the system if needed.

For example the powered tow-plane will have an emergency on-board power source in case of a power failure or break in the power tether which will allow it to continue flying as required long enough to retract and store the upper tether and reel-in and land with the lower tether segment if needed.

Maybe I didn't understand the idea fully. So the cable that goes from the electric aircraft to the ground is not able to support any tention, only transmit electricity? An alternative is to have a cable that can support tention, but not necessarily electricity and then "pump" the aircraft to higher altitudes. It's the inverse of the pumping cycle for AWE http://homes.esat.kuleuven.be/~highwind/?p=36.
The concept uses a tether in tension (IIRC from the cited paper someting like several hundred TONS of tension in fact) AND transmitting electricty to a powered "tow" vehicle (aircraft) that is then used to "drag" another aerodynamic tether in an even larger circle to build up and store potential energy to be transfered to a payload moved along the tether by a transfer vehicle.

At some pre-determine point along the upper atmospheric tether the payload is released in a subsonic, supersonic, or hypersonic glide/trajectory having used no on-board power.

Since the system requires input power at a steady rate and must not only support that propulsion system but a "towed" upper atmosphere tether simple "drag" aerodynamic flight is not sufficent to power the system and active input is needed.

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Or, you can use a cable that can transmit tension, flying it like a kite, in which case you hardly need any power at all.

Not compatabile with the system being discussed which requires an ACTIVE propulsion source to provide steady thrust to power the system dynamics. Kites would not work.

The physics has a lot of similarity. Tethered flight is a tricky thing.
In general yes, in specifics not so much as the MAJORITY of the work (especially AWE) is based on totally different flight physics as well as propulsion and system management. Since this system is powered in a manner similar to the cited system and avoids as much as possible "interaction" with upper atmospheric winds some tether dynamics can be applied to lower power/energy portions of the system, but that's about all.

On the other hand considering the requirements for meeting the aerodynamic, weight and strength goals of this system, AWE sytems will greatly benifit from research and testing of this system.

From what I can see, that is just a quadcopter with a power cable. I'd rather have a look at Makani or the other companies in AWE, such as http://www.ampyxpower.com/
Actually the "cables" are extremly light weight designs for the power they carry which I believe was the point :)

The PARC quad-copter isn't really applicable to this system either I'd say. It wouldn't work well for the forward speed and towing needs. Something more like the EASY ring-wing drone would be more applicable I think in that once airborne and on-station the entire thrust of the vehicle can then be devoted to towing duty unlike the quad-copter design.
http://cyphyworks.com/robots/ease/

Rigid airfoils are superior to inflatable wings when it comes to supporting forces.
Normally but they have a mass penalty as well as an inflexability that preclude their consideration for this purpose.

Doesn't seem feasible without the use of a high-speed aerostat or rigid airfoil. A Lofstrom Loop seems more likely given current tech, even if it might be more expensive.
Is there such a thing as a "high-speed-aerostat"? LTA flight is pretty limited as to speed obtainable without serious power and/or constant transition shapes.

There are a couple of other concepts out there using very-high-speed cable systems as well as magnetically accellerated/controlled enclosed "pellet" systems for launch assist/launch purposes. Scaling them down to "demonstration" or "research" sizes is a bit of an issue though :)

As we've discussed previously here the inital testing and vetting of the concept can be done at a fairly low "garage/hobby" level really. So it should be pretty cost effective to test and refine.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline cordwainer

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Re: Two Stage Tether to Orbit Launch System
« Reply #30 on: 06/02/2013 05:14 am »
An improvement to this is to have planes deploy the tether but have the tethers towing taken over by a ground-based propulsion system, like an electric train or linear motor. Make the circular circuit the tether travels along slight wider in radius and you shouldn't have any problem overcoming  air resistance. Problems would arise with either model though if you had to overcome adverse weather like cyclones. A ground based towing/rigging system would be easier to maintain and might offer advantages when you have to take the tether down in a hurry.

Offline johncarpinelli

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Re: Two Stage Tether to Orbit Launch System
« Reply #31 on: 06/03/2013 05:44 am »
I like the idea of ground based towing for research purposes. I suspect it would take more air-space than a tow aircraft based system. The aircraft could provide extra lift to make the initial ascent of the tether steeper. 

A rigid airfoil would be tricky to deploy from the ground.  How would you unreel a 100km rigid wing?  I guess it would have to be segmented. 

Offline cordwainer

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Re: Two Stage Tether to Orbit Launch System
« Reply #32 on: 06/03/2013 06:27 am »
I agree a semi-rigid foil might be better, maybe you could deploy it using light-than-air gas bags that would inflate during deployment and deflate upon being statically cast. Manned aircraft could be used to help lift the tether during deployment, gas bags would be inflated to create rigidity. The aircraft would initially tow the tether to obtain aerodynamic lift, gas bags would be deflated and reeled in at this point. Once enough aerodynamic lift on the tether was created a ground towing unit would takeover. Don't need UAV's or electric planes, and the maintenance costs are lowered overall.

Offline RanulfC

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Re: Two Stage Tether to Orbit Launch System
« Reply #33 on: 06/03/2013 04:51 pm »
For testing purposes I've been playing with some ideas for low-cost test rigs. Then I got to thinking of what specifically (and how to document) the things you want to test at each step and what each step would entail?

I'm playing with some nomenclature (naming) as well, mostly due to trying to explain the concept and having people get stuck on the fact there is what I'm calling a "Lower-Atmospheric-Tether" (LAT)portion (ground-to-10km) and an "Upper-Atmospheric-Tether" (UAT) portion (10km-400km) as the first "stage" of the system.

"Ground" testing of the LAT I suspect are going to have to be "powered/towed" to get a proper understanding of the dynamics while the UAT portion can be done with a central powered hub or a "vehicle" on a fixed track.

Currently I'm looking to "test" extension/retraction work for the UAT using a electric motor driven "cage" similar the illustration in figure 3 (imagine all the dotted lines are solid bars) from the paper cited earlier. Mounted on the cage at a spot to simulate the "tow-plane" would be a simple fishing reel with an attached electric motor to control the speed of the weighted line as it deploys or retracts.

I know the idea is to use a "conventional" aircraft for the tow-plane but my read on how the concept works says that there are going to be some rather heavy loads in several planes that a standard fuselage won't be able to handle. How much "lift" capacity is the tow-plane going to need overall? (How much wing-area I guess is what I'm asking)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline johncarpinelli

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Re: Two Stage Tether to Orbit Launch System
« Reply #34 on: 06/04/2013 08:46 am »
Hi Randy,

I like your prototype concept.  Try the spreadsheet below for calculating the lift of a 10km system.  I can upload the Excel file if this link doesn't work for you. 

https://spreadsheets.google.com/pub?key=0ArN3BBRS37YcdHVkTGN0dl81NWFFSDdWcDBaTDd0MlE&hl=en&single=true&gid=0&output=html

I think the concept of a wing tether (the UAT) is yet to be demonstrated.  I would love to see some video of a test flight. 

John

Offline Shevek23

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Re: Two Stage Tether to Orbit Launch System
« Reply #35 on: 06/10/2013 03:33 pm »
Hi, all!

I've been reading this thread and thinking about it for some days now.

What I'm interested in is achieving mass to orbit at a per ton cost drastically lower than rocket-based systems. When you have a mass in low Earth orbit it is as Heinlein said, not quite accurately, "Halfway to anywhere." (It's certainly halfway to escaping the Earth-Moon system and thus more than halfway to anywhere in that system; to get anywhere much beyond that in the Solar System we need more of course). There's quite a few promising proposals for getting around above LEO and beyond once we are in orbit, including tether-based ones. It seems to me the trick is, to bring the cost of getting stuff into orbit down to something reasonable.

There are other ways of looking at it; one could argue that we already have "economical" access to space in that there is a market for certain space services--comsats mostly, other utility satellites like GPS (a kind of comsat, sort of) and various Earth surveillance platforms--weather satellites, Earth resource satellites, and the historical granddaddy of them all (going back to the Corona project of the 1950s) "spy" satellites. All of these are routinely launched, at a steep price per ton (or kilogram, as we miniaturize them more and more effectively) but one the operators find affordable; voila the free market at work!

In my humble opinion, many of us, and indeed many people on Earth, would like to see lots more activity in space, particularly human space exploration, and if we wait for a market-based "pull" we will wait in vain, because the markets will not emerge in the sense of sensible investors believing in them enough to risk money investments until after a certain amount of fooling around in space for the hell of it demonstrates both access to space at an affordable cost and applications that aren't needed now and are therefore invisible to the market, but will be appreciated after the fact. Just as comsats are a vital part of the world's routine operations today, but obviously weren't in 1960 when they only existed as a modest proposal by Arthur Clarke.

Prior to reading this thread, I knew about proposals of space elevators, which in their original form required materials technology far beyond anything currently available though there seems to be more confidence nowadays that such awesomely strong materials might be attainable someday. And variations on the theme, such as a single rotating tether to orbit, like the second-stage one in this scheme except of course being faster and thus capable of single-stage to orbit--at the cost of course of again requiring materials we don't actually have yet and possibly may never. Or the notion of a band of orbiting material (faster than orbital speed actually so it is under tension) girding the Earth from which small tether-elevators can be deployed downward; I gather it is even supposed to be possible, by anchoring those various lifting links, to so torque the faster-than-orbiting band that it is held over a geosynchronous path despite orbiting at a much lower than GEO altitude, and that such a grand scheme can be attained using existing materials. It would of course be a tremendous capital investment!

But this latter is what I'd be looking for, hoping to get the investment down to a level where someone might realistically fund it.

And also the idea of "launch loops," or rather dynamic structures in general, which was first introduced to me under the name "Loftstrom Loop." Ever since I've first looked into it, the Lofstrom Loop, or a suitable variation thereof, seemed to me like the most promising approach. I don't know if I can assume everyone here is familiar with how a Launch Loop is supposed to work. I tried doing a search of this site under the names "Loftstrom" and "Launch Loop" and got very few hits; the most substantial thread being one where someone came in and asserted there were dozens or hundreds of threads "wasting time" discussing the concept to no good end and he wished this one would use the Search function and shut up. That ended any discussion there and I have yet to find any of these alleged cluttering threads. So perhaps those have been moved to where I can't find them or deleted, or perhaps they never really existed. Anyway this thread is not one of those, so I'll assume the basic idea is widely known and just refer to the idea for comparison to this proposal.

In fact since I have yet to see any discussion of the idea that goes beyond reprinting what Wikipedia says about it here (or naysaying that in no way brings in substantial criticisms, which are addressed in the off-site references but not by the naysayers, who simply shout it down ::) )here's that link for convenience.

For everyone's further convenience, I found more on the subject of the basic "Aerovator," in the form of a discussion of a single-stage-to-orbit version (that would unfortunately also require materials stronger than we currently have) here. Interestingly this write-up is not published as an actual Wikipedia article, it appears to be a work in progress or not approved for posting or some such (I don't know much about teh Wiki's internal workings and policies :-[). But as it stands it does shed more light on this new-to-me (anyway) Aerovator concept. Just bear in mind this version is not currently attainable, which is a shame as doing without the orbital tether strikes me as quite a nice goal to work toward--for one thing it would allow launches to any orbital inclination attainable from any launch latitude.

Now first of all--by the attached paper, which focuses almost entirely on the "Aerovator" first stage component, the sorts of masses per launch recommended, about 5 tonnes or less, compare to Loftstrom's smaller but less cost-effective minimal proposal pretty directly. The estimated capital cost of the two-rotor system is claimed to be about 1/10 the $10 billion Loftstrom quotes for building the smaller Loop system he suggests, whereas operating costs per payload (presumably operating at full capacity to amortize fixed capital costs) are of the same low level Loftstrom projects for his bigger system--again assuming full usage. So if we can believe these respective estimates, the per-tonne costs and minimal capital requirements strongly favor the two-tether system.

The biggest price we pay by going for the two-tether versus single Launch Loop is, launches to orbit are only possible when the two tethers link up, whereas LL systems can keep sending up more packets as soon as the track ahead is sufficiently clear of the previous one. Thus the potential throughput rate of the 2-tether system is much less than that of the LL--however, according to some critics of the latter, that's actually a good thing, since their minds boggle at the notion of the hundreds of thousands of tonnes the LL, even the smaller one, could launch per year (and should, to bring per launch costs down). The much more restricted launch rates allowed by the two tethers synching up every couple of hours or so ought to be a lot less frightening to them! ;D (Still, if we can do 12 launches a day, and each is 5 tonnes, that's the equivalent of 180 or so Saturn V launches to LEO each year! We'd outdo the whole Apollo program in about 3 months and the whole STS program in about 2 years. Cool, if you ask me--contemptibly presumptuous apparently according to some people. Zeus will zap us any second for Hubris.)

The attached paper does not address a number of issues that bother me though:

Synchronization seems a bit worrisome. To pass a package from the first stage Aerovator to the upper-stage orbiting Rotovator requires that both tethers intersect at the same time and place. I can see that with enough "grapple" they can afford to miss by meters, maybe tens, perhaps even hundreds of meters, but the precision required is still pretty tight. Whereas both tethers will have irregular forces and torques operating on them that will tend to throw them out of whack and need to be compensated for continually. I can see how this might be done but the proposal needs to address the issues.

Whereas a Launch Loop, being a single-stage system, does not have this problem; assuming it works at all, each payload is launched direct to orbit. (They then have to be assembled to a space station or space ship project in parking orbit to be sure, but essentially the same is true of stuff the orbiting Rotovator puts into orbit as well).

The "real estate" footprint of an Aerovator seems lesser, as a single ground station that takes up no more than a 10 KM diameter circle of reserved airspace at typical aviation altitudes pretty much covers it; the high-altitude, high-speed tether is whirling at altitudes between those aircraft can typically reach and those at which satellites can orbit. Loftstrom's "Loop" (and other, rival variations thereof) require two ground stations which take up considerable surface area (for a power plant, turnaround loop and so on) and supplementary tether anchors along its 2000 km length--the working stretch of linear accelerator however is suspended in those same intermediate, clear altitudes.

Much is made in the paper of the auxiliary uses of the Aerovator aside from space launch. That's just as well as far as it goes because the proposal to orbit stuff does seem practically restricted to the Equator to me, based on the numbers tossed out. Specifically, the upper limit of speed that the proposed existing material can sustain is 3000 m/sec, whereas the orbital velocity given--presumably that of the core of the Rotovator tether system--is around 7600; obviously both tethers are stretched just a bit even with a full 500 m/sec coming out of the total due to the Earth's rotation; moving the Aerovator hub to higher latitudes means even more stretching of the limits, admittedly not a lot until we get to really high latitudes but we're on the wrong side of the limits given already. Obviously we can hope for improvements in the state of the art of material strength and need only modest ones to make this idea work even for a polar orbit, but with the numbers given it seems to me we'd better restrict orbital launches to equatorial ones.

Also there's the question of the ground track of the Rotovator. If it is dead-on in an equatorial orbit, that strikes me as the simplest situation; then the thing will indeed pass over equatorial launch sites every orbit. Otherwise we are in some situation where generally speaking each orbit passes over new territory and it might be days or even months before the Rotovator station passes over a given higher-latitude ground station. It is possible to aim for an exact orbit that is synchronized to pass over a given track or cycle of tracks, to be sure. But that's yet another synchronization constraint to meet, when the Rotovator by its very nature of operation is subject to torques and drags that alter both its orbit and its rotation speed--these effects must be compensated for. Indeed the article acknowledges that the Rotovator must be continually boosted.

Again it seems to me the system is most workable for achieving orbit from the Equator to a purely equatorial orbit.

And vice versa, with a Rotovator constantly sweeping the band up to 1000 km above the Equator with its tether arms, it seems we've pretty well pre-empted LEO equatorial orbits, and had better give some careful thought to traffic control to allow satellites orbiting anywhere in LEO safe passage past them.

A Launch Loop also works best launching stuff into LEO along the Equator, but there's no reason they can't be made just a bit longer and more power-demanding and launch to other inclinations, from other latitudes.

----
The more I think of it, the more exciting and innovative a single-stage to orbit Aerovator would be. It seems to combine the best aspects of a Launch Loop with flexibility in the matter of orbital inclinations and a smaller ground footprint, and eliminate the orbital clutter that an orbiting tether system represents. Again it's just too bad we need an order of magnitude improvement in strength of materials to get it!  :'(

In the meantime, y'all have alerted me to the most competitive alternative to Launch Loops I've seen yet in terms of cost-effective mass transit to orbit. Indeed if it works as advertised, even with the clumsy 2-rotor requirement, it would seem to cost 1/10 as much as a Launch Loop to achieve at least 1/10 the capacity; given that a major objection to proposing development of an LL is the steep capital cost to provide a few orders of magnitude more mass to orbit (at net sticker price not much more than current space budgets to be sure) than we currently "need," doing less for less is actually attractive.

I'm pretty dubious about most of the "auxiliary" uses for the Aerovators that you seem to think will be its real bread and butter. The notion of starting with towed advertising banners strikes me as downright obnoxious; a thing like that might get the whole technology banned. :o Anyway I'd be somewhat sympathetic with the grumblers!

Similarly it's pretty hard to believe that it would make economic sense to haul up cargo planes and then release them for subsonic cruise. Certainly takeoff costs a lot in terms of fuel consumption rates and also in design constraints--the planes have to take stresses and the engines generate thrusts not needed in most of the flight. But while it would be technically possible to land the tether-launched planes by having them hook on to a tether at the destination, I don't think you'd seriously propose that, would you? The issue is one of risk, both to the airplane (if it misses making hook-on at all) and to the whole tether system--if it misses the other way, failing to grapple and instead colliding with the tether, it might take the whole thing down. I gather that poses little risk to bystanders but it wrecks the tether and strands anyone else who might be queued up to hook on in their turn. So that's out, I should think.

So instead the airplanes have to land in the normal way. This doesn't take as much thrust or structural strain as takeoff, perhaps, but just about as much, and if anything I gather the accident rate is higher for landings than takeoffs. So you can't design the plane to be any lighter, and you save only half the extra fuel cost that TO/L represents.

Enabling hypersonic transport aircraft might have some promise, I guess. I'm a skeptic about the wisdom of skip-gliders. In particular it seems to me it behooves them to use thrust to make up for lost energy with each skip rather than allow the plane to be gradually slowed into a continued dive. But these skips will be frequent and to provide enough lift to reverse the descent into an ascent, each will dip low enough to make for serious sonic booms, I'd think. So the same sorts of path restrictions that currently block possible SSTs from taking direct routes, the same ban on flights over land and requirements of offshore routes, still apply. Even if it's true you spare some people along the ground track the noise, that doesn't help the ones you do blast.

But if it can be done so as to address the issues, certainly a suborbital plane flung along at 3000 m/sec aka Mach 10 will beat even REL's proposed Mach 5.2 LAPCAT SST.

However I doubt very much even a relatively small, DC-3 sized passenger load HS/SB craft, with or without rocket supplemental thrust, could be made that masses under 5 tonnes! ::) Nope, you'd need to upgrade the Aerovator to lift something like 10 times the masses you've been talking about to launch speeds and altitudes.

Which brings us to another approach to simplifying the system to eliminate the orbiting Rotorvator, while we wait for stronger materials allowing the Aerovator to put things directly into orbit...

Use the Aerovator to boost a rocket nearly halfway to orbital speeds, and let the rocket engines finish the job instead of a second rotor.

This obviously would prevent the system from being nearly as cheap to orbit as the two-rotor system. But it does enable the flexibility of a single, horizontal Aerovator able to launch to any azimuth at any time. Launching from the Equator we need to gain an additional 4500 m/sec--with hydrogen/oxygen rockets that means a mass ratio around 2.5-3, or with Ker-LOX around 4 or so. Either way it's a lot more favorable than a ground launch or even a subsonic air launch; the tether tip launch speed is twice as great as Skylon's SABRE engines hope to achieve in airbreathing mode.

If you can get development funding with the various subsonic applications, even God Help Us advertising banners :o that's fine, but I think what we have here is a promising approach to space launch, perhaps low suborbital/high supersonic flight, and it should be developed with that goal in mind.

And ideally without the second orbiting rotor which tends to complicate things considerably.

Offline ChrisWilson68

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Re: Two Stage Tether to Orbit Launch System
« Reply #36 on: 06/18/2013 11:32 am »
The tow aircraft is supposed to fly at 8-10 km altitude but the end of the tether is supposed to be at 100 km altitude.  What keeps it up?

Offline Skeptical1

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Re: Two Stage Tether to Orbit Launch System
« Reply #37 on: 06/18/2013 02:54 pm »
Aerodynamic lift like a kite or a paraglider. The wing can fly higher than the tow vehicle.  Consider a paraglider towed by a boat for an example. 

http://www.towmeup.com/

Offline Shevek23

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Re: Two Stage Tether to Orbit Launch System
« Reply #38 on: 06/20/2013 04:08 am »
Later edit: I goofed. :-[

Much of my original post is correct as far as it goes, but see corrections below in
Teletype font...

Centrifugal "force" also contributes. On an infinite-flat-plane model it wouldn't but the Earth is spherical, so a perfectly straight tether whirling around in vacuum would be angled up relative to a radius from the center to any point. Toward the middle this is a tiny factor; toward the ends the outward pull is nearing maximum while the angle is significant.

All true as far as it goes. Unfortunately, I later figured out that an element of the tether must be going at orbital speed to lift itself; if the tether extended beyond the radius where the speed is orbital, this outer part could lift the inner part by tension--so what follows below is only true if we use such an extra-speed tether. Of course such extensions will demand a much stronger and heavier tether than the minimum necessary to achieve orbital speed! :o

So we could have a version of this on the Moon for instance. It might be necessary to have a tower to hold up the middle but the outer ends will be being held down by the tether's tension, and the momentum of the outer part will at least help hold the inner part up.

True if we make it big enough, with outer part greatly exceeding Lunar low orbit speed of 1600+ meter/sec. For the Moon it might be doable with existing materials. But you'd need a lot of it.

This is a much bigger factor in the full-size, direct to orbit version; with the tethers split into two, one rotating horizontal to the planet surface, one vertical, the former is under much less tension and the tether is much shorter. Still a half-speed intermeshing horizontal Earth based tether that achieved 3000 m/sec at the tip and pulled 3 G's there would be some 300 kilometers in radius; that's not insignificant compared to Earth's radius.

But inadequate to lift itself, by far! Even a full-orbital speed radius tether (not possible with existing materials) would need the vast majority of its weight to be lifted aerodynamically; one that only reaches a tip speed of 3/8 orbital velocity would be centrifugally lifted only to a very tiny extent.

For the small version the centrifugal lift won't be great and mostly it would have to be aerodynamic lift. Boy, I'm glad I said that! I didn't then realize just how little the inertial lift would be.For full orbital speed on one tether though it would mostly be centrifugal lift I think,I thought, wrongly and the radius is more like 2000 km--almost a third the Earth's radius, or about 20 degrees of Earth's circumference.

Yes indeed! So the tension of a hypothetical tether pulling straight toward the axis of rotation would be countered by an inertial force angled up, relative to the vertical radius at that point, by 20 degrees, and this, applied to an acceleration of 3 G's, would indeed lift the weight. Of objects exactly at this tip! Objects (such as segments of the tether) farther in would have a lift that goes as the cube of the fraction of the radius needed to reach actual orbital speed. And unless we extended the tether well beyond a length that corresponds to full orbital velocity, since lift would be aerodynamic the tether would not straighten out into a straight line but would curve with the Earth's curve, following isobars in the atmosphere, so the angle of the tether's tension to the local vertical would be closer to 90 degrees and thus the lift that much less--it would be reduced at all points.
« Last Edit: 06/20/2013 11:54 pm by Shevek23 »

Offline ChrisWilson68

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Re: Two Stage Tether to Orbit Launch System
« Reply #39 on: 06/23/2013 08:55 am »
Aerodynamic lift like a kite or a paraglider. The wing can fly higher than the tow vehicle.  Consider a paraglider towed by a boat for an example. 

http://www.towmeup.com/

That's what I was afraid of.  Afraid of because aerodynamic lift always comes with drag.  Here you're talking about hundreds of kilometers of wing, essentially.  How could a single tow plane drive hundreds of kilometers of wing, most of it going much faster than the plane itself (because the plane is making a circle with a much smaller radius than that of the tip of the tether)?  It seems like you'd need an engine every few hundred feet, at least, along the tether to drive it.

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