Un- conventional ways to LEO

cjchandler · 2006-10-20 17:10:54

I don't see any problems, a very short number cruching sesion reveals that yes, it is probably fesible, however, 7 GPa tensile strengh is I think at the moment still unatainable. Would a tapered cable help? Don't know enough math (yet) to figure that one out but I'd guess yes. The only thing I'd change would be the ribbon shape generating the lift, that seem way to optimistic since it lacks any control surfaces. waverider kites spaced out along it with radio controlled flaps would probably be a better option.

C M Edwards · 2006-10-25 07:48:59

If you have to crunch numbers to figure out if it will work or not, chances are it won't.

The curmudgeon is right! All hail the power of the curmudgeon!

Aerovator will not work as advertised.

It is relatively simple to derive that there is some minimum tip velocity at which the fully deployed system is stable. From there, one can then use that formula to find that there is some minimum altitude at which the tip's kinetic energy plus its potential energy is greater than the kinetic energy of a circular orbit. This is the "working aerovator" regime of parameters, and cjchandler is right: it really exists. There is an entire range of deployed configurations that are stable and can suspend a vehicle on its way to orbit. Further, the drive situation for the fully deployed aerovator is more favorable than the "catherine wheel" plan shown in the cited Wikipedia article. The only inflection in the curve of the tether occurs at its pivot. (The picture in the article, with two inflections on each arm of a working aerovator, is physically impossible even if the aerovator can generate lift.) So, it won't spiral, and the tension should be sufficient to turn it using a central motor. The spin rate is only a few dozen rpm when fully deployed, so the fully deployed tether won't exceed the speed of sound until it's well clear of the lower atmosphere.

The trouble is that it can't be deployed.

The tip speed is smaller for shorter tethers. However, it's impossible to reel it in to the Earth's surface because the minimum tip speed doesn't decrease fast enough as the tether is reeled in to keep the tip speed below Mach 6 in the lower troposphere. I can't see it surviving that at <10 km elevation. Also, the additional drag of supersonic travel without the compensating tension of the fully deployed tether will cause the tether to start winding around its driver once the reduced line tension falls below the aerodynamic drag (which it will at some elevation less than 10km without a HUGE end weight to keep up the tension - and it could still disintegrate at > Mach 6 anyway).

If there's no way to reel it in, there's no way to real it out, either. Using the catherine wheel configuration won't eliminate the problem - it will just change the winding points.

Now, it is theoretically possible to lower the aerovator into place. Fully deployed, it's lower reaches never exceed the speed of sound due to the reduced angular velocity requirements. However, any rocketry capable of doing this already exceeds the capabilities of the aerovator it's hauling. (By its nature, the aerovator can only lift a fraction of its total mass into orbit.) And any linear space tether capable of doing the job is already performing more efficiently. Balloon deployment can't provide the necessary torque for a central driver and the fuel requirements for a catherine wheel driver are prohibitive at best without a steady source.

An aerovator is only superior to other launch technologies if it can be deployed from the surface. It's useless for Earth.

However, not everywhere in the solar system is Earth. Hmm... I wonder where we could put it without those pesky tropospheric air densities?

GCNRevenger · 2006-10-25 20:18:28

The curmudgeon is right! All hail the power of the curmudgeon!

I love how grandiose, clever, and especially counterintuitive ideas are brought down by little details.

C M Edwards · 2006-10-26 08:00:51

I love how grandiose, clever, and especially counterintuitive ideas are brought down by little details.

Yeah, I get a little thrill from it, too. It's the physics, not the phenomena.

Interestingly, a spectra strap tether aerovator-style sling could - just barely - function on the moon. It would probably never deliver much payload, though. And it would need to be solid spectra tape - woven wouldn't do.

publiusr · 2006-10-27 12:37:55

Howabout we just forget the Rube Goldberg nonsense--and...I don't know...buckle down and build a big simple rocket?

Nah--that will never work.

C M Edwards · 2006-10-27 13:27:25

Where's the fun in that? :?

publiusr · 2006-10-27 13:28:24

Its about capability and throw-weight.

That is fun too.

cjchandler · 2006-10-28 14:03:05

Hum, yes I didn't even think of the spiraling from drag, bit of an oversight. :oops: However, if we add one more "Rube Goldberg" step, pephaps it could work? The problem seems to be that the drag would be far too high for the centripidal force to counter and prevent spiraling correct? So what if electric cables were placed in the cable and it had some kind of electrostatic propulsion system ever meter or so, that could counter the drag? Since the power could come from the ground, it would not have to be terribly efficent.

Why not a rocket? I think rockets are dandy for what we do now (puttering) but to bring a good number of people into space, I'm not sure they can ever be made cheap enough. Pephaps not safe enough either, though that's debatable.

C M Edwards · 2006-10-30 08:33:59

Hum, yes I didn't even think of the spiraling from drag, bit of an oversight. :oops:

Well, technically, it's not spiralling. When a winch reels in a 1 ton jeep, the line doesn't spiral even if you stand on it. What it does do is wind around the drum at the point where the torque exerted by the drum exceeds the moment exerted by the jeep. Aerovator would do something similar.

You would not want to go overboard with connecting things to the tether while its in the tropophere. Putting anything on the line is bad in terms of drag and terrible for stability. I'd be hesitant to even add an elevator car, but everything a trade-off I guess.

An ultra-high altitude catherine wheel would need to start working fast enough to keep the tether from sagging and work long enough to allow a central motor to take over when it ran out of fuel. And it will run out - fuel could never be lifted at a high enough rate to keep the catherine wheel operating indefinitely.

It is possible to raise a tethered aerostat to 25 km carrying about 100 T or so of payload (a rough approximation taken from previous research of my own - the mass limit is determined by what it takes to get the aerostat through the tropopause, not the maximum possible balloon size), but that's (probably) not enough fuel to spin up an aerovator using conventional rocketry. The tether can only be reeled out so fast without binding, and the catherine wheel engines would need to run for at least an hour or two to get it to a safe extension.

Still, there is that island of stability at full deployment.

Hmm...

If you can come up with a means to get it through the troposphere and keep it there, it could be a physical possibility. Then all you'd need to do is pull half a million square kilometers of controlled airspace out of your hat, and you're in business.

noosfractal · 2006-10-30 13:21:17

If you can come up with a means to get it through the troposphere and keep it there, it could be a physical possibility.

Balloons?

Then all you'd need to do is pull half a million square kilometers of controlled airspace out of your hat, and you're in business.

Closer to ten thousand - only a fraction of the ribbon is under 21 kms altitude.

noosfractal · 2007-04-29 16:04:28

If you can come up with a means to get it through the troposphere and keep it there, it could be a physical possibility.

Balloons?

Sorry about the balloons thing, I didn't read your posts thoroughly enough.

So, you've got this good steady state, but there seem to be issues during deployment. If I understand you right, during deployment, the ribbon will have insufficient lift & tension at reasonable speeds. Can we augment lift & tension during deployment with some sort of launch vehicle - perhaps emulating the steady state (or some reasonably stable minimum)?

Suppose our launch vehicle is a 747. It takes off from the hub trailing the ribbon and spirals up and out until it is circling the hub at with a radius of, say, 20km, at a height of 12km, at a speed of 0.8 Mach (standard cruise). The article has a ribbon weight of 240kg/km, so at this point the payload is < 10 tons, which is not a problem for the 747. Also, the mass of the 747 (< 300 tons) is approximately the mass of the ribbon to be deployed.

Now the ribbon tip is above the troposphere without having gone supersonic and the ribbon has approximately the tension & lift of the stable steady state. So far the hub has been passive, but now the hub can add power to maintain the rotational velocity while letting out the ribbon – with the 747 transitioning from launch vehicle to tensioning mass (ideally shedding mass as the ribbon lengthens).

Probably not convincing, but how about plausible? Is there a deployment strategy in there somewhere?

noosfractal · 2007-05-03 23:55:04

I did the simplest possible finite element analysis on the aerovator, and I think it is deployable as described in the wikipedia article – self lifting from the hub – if the hub can vary it’s rate of rotation. I’ll describe what I did so that you can poke holes in it.

Each segment of the ribbon is at some angle with respect to the ground in the radial direction (the climb angle). Lift is generated perpendicular to the lifting surface, so it will have a horizontal component (back towards the hub) and a vertical component that lifts the ribbon. The horizontal component is balanced by the centrifugal force of the rotating ribbon mass, the vertical component is balanced by the weight of the ribbon.

The ribbon is light weight, and early on the air is dense, so the climb angle can be steep and still support the ribbon weight, even though the airspeed is also low. The centrifugal force generated by a ribbon segment is proportional to it’s distance from the hub, while most of the opposing force generated by lift occurs early on generated by the steep climb angles.

When the above forces are balanced (I’ll get to drag next), the ribbon profile looks like this ...

http://www.geocities.com/noosfractal/aerovator.pdf

This is for a tip speed of 8 km/s ( = 13 minutes per rotation ). I used a specific mass of 240g per linear meter (because the wikipedia article had the total mass at 240 tons) and a ribbon width of 100mm (less width = less drag and a 1000 km wing just doesn’t need that much lift). For the coefficient of lift I used 0.5 (thin plate with an angle of attack of 5 degrees). I used this simple model of the atmosphere.

The above profile isn’t practical because the forces are exactly balanced and unstable in the direction back towards the hub, but a stabilizing tensioning mass at the tip can be quite modest. On the other hand, even larger tensioning masses don’t seriously deform the profile because induced small changes in climb angle generate large counterforces.

The profile isn’t optimized to reduce drag. I used conservative coefficients of drag (0.04 for subsonic and 0.20 for supersonic, transitioning at 270 m/s) and the total drag generated was less than 50% of the radial tension – enough to cause the tip to trail, but not enough to prevent the hub from accelerating the tip. I haven’t properly modeled drag – it should peak at Mach 1 and then drop again – and the coefficients are probably too high, so I didn’t look into reducing drag, but it is definitely possible to operate at higher altitudes than shown in the profile, and, as a reference, a tensioning mass of 60 tons at the tip produces a force of the same magnitude as the total calculated drag.

I used the same model to look at deployment scenarios. Because most of the counterbalancing centrifugal force is generated after the 500 km mark, you need a tensioning mass for deployment. At 1/13th rpm you need a peak tensioning mass of 450 tons (at altitude 16 kms, tip speed 45 m/s). Not completely out of the question if the tensioning mass is a self lifting launch vehicle, but problematic. However, if the hub can operate at up to 5 rpm, the required peak tensioning mass drops to 30 tons (at altitude 15 kms, tip speed 180 m/s) which is imaginable as a more or less unpowered wing designed to survive the entire deployment.

Once you pass the peak tensioning requirement, the hub can gradually slow to the target velocity. Note that after this point, there is no real rush to deploy since the situation is quite stable. Also, the ribbon tip and tensioning mass are well clear of the troposphere before going supersonic.

I haven't looked much at payload launch dynamics, but the profile above provides a peak vertical acceleration of about 0.1g from about the 50 km mark, so the payload would have to provide for its own altitude gain. After the 5 km mark, the radial acceleration rises linearly to 6g as advertised.

noosfractal · 2007-05-05 02:44:34

I had a look to see if stresses were reasonable during steady state and deployment. The stress/density ratio graph for the scenario from the previous post looks like this ...

http://www.geocities.com/noosfractal/st … seline.pdf

i.e., it peaks at 0.41 M Pa m^3 / kg. This is well within the capability of modern carbon fiber composites such as those sold by Toray ...

http://torayusa.com/cfa/product.html

Their baseline T300 product has a tensile strength of 3.5 GPa at a density of 1760 kg/m^3 = 2.0 M Pa m^3 / kg. Their high end T1000G product rates 3.5 M Pa m^3 / kg.

This density would give a ribbon thickness ~ 1 mm to maintain the 240g/m of the wikipedia article.

As a bit of an aside (since it doesn’t seem to be necessary and may be undesirable since it lowers ribbon tension), while playing around with passive control ideas, I found that if you can get some control over lift (and thus the climb angle), then a slightly different ribbon profile can cause the stress to peak at about half the baseline ...

http://www.geocities.com/noosfractal/stress_control.pdf

The 0.2 M Pa m^3 / kg figure would allow you to use high end Titanium alloys, but I'll stick with carbon fiber composites.

The T300 product wouldn’t make it through the 5 rpm deployment scenario, and the T1000G product goes right to the limit. Halving the ribbon width for the first 50 kms deployed keeps the stresses under the T300 level.

Keeping the lift relatively constant is a challenge. It doesn’t have to be rock steady, but it does have to be kept within bounds to keep drag under control. There are also problematic failure modes like the ribbon twisting into a helix. As well as a wing, the ribbon also has to be a launch rail, so payload transients have to be taken into account.

The are some clever ideas out there for passive regulation (the space elevator needs something like this as well for the section of the ribbon in the atmosphere): “kite tails”, shaping the airfoil so that it generates counter-twist vortices and various anti-stall mechanisms. A distributed version of Gurney microtabs might be an active control option.

===

Edit: oops, the stress curves were upside down. They need to be zero at the free end unless there is a tensioning mass. I've updated the graphs.

noosfractal · 2007-05-06 01:40:30

I updated the drag model so that I could look at deflection due to drag - still not anything amazing mind you - the coefficient of drag jumps from .04 to .2 at Mach 1, stays there until Mach 2 and then drops to 0.125. The actual equations for calculating this stuff are completely out of control, but a L/D ratio of 4 looks doable at Mach numbers 2 through 25 from waverider discussions. In the baseline, the ribbon hits Mach 1 around 48 kms altitude and Mach 2 around 59 kms altitude.

The ribbon does spiral a little bit - up to 50 km from nominal without a tensioning mass. With a 1 ton tensioning mass at the tip, the deflection at 1000 kms is under 3 kms ...

http://www.geocities.com/noosfractal/pr … _above.pdf

The 1 ton tensioning mass raises the max stress/density ratio to 0.47 M Pa m^3 / kg ...

http://www.geocities.com/noosfractal/st … weight.pdf

noosfractal · 2007-05-06 21:50:46

When a payload is accelerated by the ribbon, it exerts a force in the same direction as drag, deflecting the ribbon (I assume the payload is aerodynamic and provides it's own lift, but I ignore drag on the payload in the following). Presumably that deflection needs to be limited to some extent, setting a limit on the payload. The deflection is vaguely proportional to the ratio of the force exerted by the payload to the tension in the ribbon (high tension, less deflection). It's hard to guess what the limit ratio might be - almost certainly < 1, very probably < 0.5, may be as low as 0.01 - I chose 0.1 for the following.

To accelerate to 8 km/s over 1000 km ribbon with

max stress (M Pa m^3 / kg) --- requiring tensioning mass (tons) --- max payload is (tons)

1.7 --- 25 --- 1.0
3.5 --- 50 --- 2.5
10 --- 150 --- 7.5
45 --- 700 --- 35
100 --- 1600 --- 78

3.5 M Pa m^3 / kg is the max stress that the high end Toray product can handle. 1.7 is that with a x2 safety factor. 100 is the figure assumed by the NIAC space elevator report. 45 is the figure quoted by liftport as the minimum economically feasible for a space elevator (and is roughly the figure implied by the LANL SuperThread press release). 10 is a predictable figure for near term CNT fiber composites.

If the limit ratio turns out to be 0.01, you'll have to divide the payload figures by 10, if 0.5 is doable, you can multiply them by 5. If you double the length of the ribbon to 2000 km (and half the rotation rate) you can double the payloads, but you also have to double the tensioning mass figures.

publiusr · 2007-06-23 12:39:29

An asteroid bola dragging a tether tail. This tail snags an object traveling at speed on the ocean surface, and gets "wrapped" into orbit to prevent a sudden jerk.

I have been wondering about cutting asteroids up with cables as in the Kursk, and lowering one half of such a bola to the surface to do asteroid mining on the ground...the other segments chutes/ballutes down, then floats and the tether goes to a tug and is dragged to shore--ballute still inflated.

A molten asteroid can have a bomb inside to blow it into a bubble. The sphere can be filled with fluid and used for SPS in orbit around the sun--or to discharge Jupiter's field for power...

Terraformer · 2007-12-01 09:55:20

A tethered airship aroun 100 km up? Can't find any info on air density up there though. People and craft could be lifted up there on an elevator car riding the cable. Once up there a spacecraft takes off like a plane then fires its rockets and boosts up to orbit.

Maybe something similar to a solar sail? A ground based laser would shine onto the bottom of the craft accelerating it slowly to orbital velocity. One problem: the wind would be blowing the craft off the beam.

Plasma launches?

JoshNH4H · 2007-12-01 16:14:19

The Karman Line is an internationally designated altitude commonly used to define outer space. According to definitions by the Fdration Aronautique Internationale (FAI), the Karman or Krmn line lies at a height of 100 km (about 62 miles) above Earth's surface (ie. in technical terms 100 km above mean sea level).
Around this altitude the Earth's atmosphere becomes negligible for aeronautic purposes, and there is an abrupt increase in atmospheric temperature and interaction with solar radiation.
It was named after Theodore von Krmn (May 11, 1881 - May 6, 1963), an engineer and physicist who was active primarily in the fields of aeronautics during the seminal era in the 1940s and 1950s. He is personally responsible for many key advances in aerodynamics, notably his work on supersonic and hypersonic airflow characterization

I've told you over and over. YOU CANNOT USE A BALLOON TO GET ANYWHERE NEAR ORBIT.

JoshNH4H · 2007-12-01 16:20:15

As you go further and further from the surface of the earth, the atmosphere gradually fades until it’s difficult to notice any air around you. No physical boundary clearly separates space from the atmosphere – even thousands of miles away from the planet, a few air molecules zoom around. As a result, it’s difficult to put a precise total on the number of travelers who have tasted space.
The line distinguishing spaceflight from ordinary flight evolved from the efforts of Theodore Von Karman, a Hungarian-American physicist and engineer, in the 1950’s. In ordinary flight, an aircraft relies on the atmosphere to create lift. Without air, the wings of an airplane and the helium or hot air of a balloon are useless.
Von Karman knew that as the earth’s atmosphere became thinner and thinner, an airplane would have to fly faster and faster to generate the same lift, because there is less air for the aircraft to push against. At some critical height, the plane would need to travel so fast to generate lift that its motion would be more like a satellite, which orbits the earth in perpetual free-fall — without additional thrust or lift — due to the earth’s gravity.

More proof. And I would prefer if you didn't say that the article says helium, so Hydrogen would be fine, because that doesn't really make sense, don't you think?

Terraformer · 2007-12-02 06:30:59

Heated Hydrogen would be fine. and it says around that line, which means about 10 km below that line you can use a blimp using heated hydrogen. A suborbital craft only needs to boost 10 km up after launch to qualify as a spacecraft.

JoshNH4H · 2007-12-02 15:00:43

I have some actual info for you about atmospheric pressure:

fraction of 1 atm average altitude
(m) (ft)
1 0 0
1/2 5,486 18,000
1/3 8,376 27,480
1/10 16,132 52,926
1/100 30,901 101,381
1/1000 48,467 159,013
1/10000 69,464 227,899
1/100000 96,282 283,076
Or you can see it more clearly at [url] http://en.wikipedia.org/wiki/Atmospheri … _variation [/url]

so 4 km below the kamaran line, the air pressure/density averages 1/100,000th of that at sea level. This is from wikipedia.

Hydrogen and helium are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference is inconsequential. (Both provide about 1 kilogram of lift per cubic meter of gas at room temperature and sea level pressure.) Helium is preferred because it is not combustible.

So, at 96 km, you will need ( assuming that the material you use to enclose it has no mass ) ~100,000 square meters of ambient-temperature helium/hydrogen for every kilogram, or a box of about 47 m x 47 m x 47 m to lift one kilogram. If you can find something airtight, that weighs 1 kg for every ~ 13,500 m2, Kudos to you, but you still have no lifting power. Of course that is a cube, but I believe that these figures show the infeasibility of using a balloon to go anywhere near space.[/url][/list]

JoshNH4H · 2007-12-02 16:27:26

And if you say that we'll use monatomic Hydrogen (H) instead of Diatomic Hydrogen (H2), then I will tell you that (in case you didn't know) At temperatures higher than abs. 0, hydrogen only comes diatomically.

JoshNH4H · 2007-12-13 16:27:12

Sorry terraformer, it had to be said. Now, my favorite is suborbital craft to smaller space elevator. It would orbit the earth in about a 4 hour orbit, and begin at ~ 2-3 km above the karman line. (102-103 km above earth's surface)

This cable would be no more than a couple hundred of km, and steel, or at worst carbon fiber, would suffice (note- household carbon fiber, not CNT's or super advanced materials.) Minimal fuel would be needed, it would travel at its lowest point, ~ Mach 6 or 6.5.

Of course, cnt's would lighten the load.

This means away w/ making thousands of km of cnt's. Away with small weight quotas (smaller weight would of course be better, but bigger isn't deadly.) Away w/ power isssues (See flywheels at http://www.liftport.com/component/optio … opic,793.0, they look good for this kind of thing) And radiation damage ( think that even the top of the cable will be below the radiation belts.

What do you think?

Terraformer · 2007-12-14 16:30:41

Uneconomical. You would have to keep transporting fuel up to keep the craft going. At hypersonic speeds the air resistence would increase and drag the craft down. The ISS loses speed due to air resistence meaning it has to have its orbit boosted every five years. Anything other than wikipedia? I don't like it any more because they put out a deletion notice for my wiki on Terraforming Mercury saying I had 'incorrect physics' and directed me to, wait for it, a page on wikipedia.

Sorry jumpboy11j, it had to be said.

JoshNH4H · 2007-12-14 20:10:04

Actually, on the matter of it being slowed down, I agree with you 100%. I envision, I could be wrong, anyone correct me if I am, but it could be constantly accellerated, possibly by nasa's m2p2 concept. However, the atmosphere at any part of the elevator being so thin, and the cable a cable, wind resistance should be minimal. It could be built slightly topheavy to account for that. Failing those, bringing just a little fuel on every supply ship, or 1 supply ship in 25 or so in fuel, will still drastically improve over chemical rockets. It's not a craft, really, anyway, more of a floating ribbon.

Sorry jumpboy11j, it had to be said
Sorry terraformer, it had to be said

very funny-- Actually, I'm just kidding, a decent joke.

And the newmars wiki gladly accepts new ideas. Put an article there (www.newmars.com/wiki) and list it as a wikipedia source.

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#26 2006-10-20 17:10:54

Re: Un- conventional ways to LEO

#27 2006-10-25 07:48:59

Re: Un- conventional ways to LEO

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Re: Un- conventional ways to LEO

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#36 2007-04-29 16:04:28

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#38 2007-05-05 02:44:34

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#40 2007-05-06 21:50:46

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#42 2007-12-01 09:55:20

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