Manned Spaceflight Gets a Kick in the Pants (1 Viewer)

[Andrew Testa]

As a NASA contractor I've been following the X-Prize candidate's progress. Rutan's entry is probably the best shot. However, it's a one trick pony, designed specifically to meet the X-Prize criteria. There is a HUGE technology gap between this craft and anything that could reach orbit and survive reentry.

I believe that Rutan will use the momentum from winning the X-Prize to gather significant venture capital and work to a more advanced vehicle. The goal would probably be to ferry tourists for a short suborbital hop. I don't see him trying to compete in the launch services market. It's oversaturated as it is and there's little money to be made given the exorbitant insurance required and the small payloads he could deliver. Orbital Science's Pegasus already covers the delivery method Rutan is exploring (small rocket dropped from a carrier plane).

And just to stir the pot on the Space Elevator: physically impossible. Reason being that there is no place on Earth to anchor it that isn't moving relative to the Elevator's CG at the 'Geostationary' orbit. Being a nonuniform spheroid and subject to tidal forces, the Earth wobbles, and significantly so. Gyroscopic Precession and nutation keep it wobbling on its own, and tidal forces continually move the land as well as the oceans. Also, due to the gravitational influence of the moon and sun, the orbit of the elevator would constantly drift. The Elevator would wiggle like spaghetti if there was any material that could withstand the incredible tension loads.

Cool idea for a book though.

Andy

 

[Jack Briggs]

Quick, somebody tell Arthur C. Clarke!

Nice post, Andrew.

 

[Max Leung]

A space elevator would probably work well on Mars...no oceans and moon to create unwanted wobble.

I like Gregory Benford's tragic idea for a space elevator...it gets destroyed by an alien invasion force out to exterminate the human race (Tides of Light series, I think) just before it was complete...

 

[Andrew Testa]

Still won't work on Mars: there's still gyroscopic precession and nutation that cause it to wobble. Everything wobbles, no getting around it. But if you like the elevator concept, you might be interested in tethered vehicle delivery systems.

The single largest obstacle to leaving Earth is that you have to bring your fuel with you. The higher you want to go, the more fuel you need and the larger your rocket has to be, so mass goes up requiring MORE fuel to lift, etc. Low cost access to space will require a way to get there without having to lug your fuel along.

Fortunately for us, physics doesn't care WHERE you get energy, as long as it's there when it's needed. In a tether system, orbital mechanics transfers kinetic energy between two vehicles in place of on board fuel. Here's the scenario:

Launch into low earth orbit a dual vehicle connected by a LONG (several mile) reeled tether. Your only fuel is that required to get into low earth orbit. Now, extend the tether. Orbital mechanics and tidal forces will keep the tether pointing at the Earth. The center of mass of the tethered system will travel at the proper orbital velocity. But, the lower vehicle is now travelling SLOWER than is required to orbit on it's own at it's lower altitude. The higher vehicle is traveling FASTER than is required to orbit on its own at its higher altitude.

Kinetic energy has been transfered from the lower to the higher vehicle with no fuel involved. Now: release the tether connections to each vehicle. The lower vehicle, having lost kinetic energy and traveling too slow to orbit, will deorbit and reenter with no need for costly deorbit burn. The higher vehicle, having gained the kinetic energy from the lower vehicle and traveling faster than its orbital velocity, will enter an eliptical transfer orbit that takes it to a greater altitude, with no burn required. When it reached the peak of it's eliptical orbit a small circularizing burn will keep it at the higher orbit.

You've now delivered a vehicle to a high orbit with only the fuel required to attain low orbit. How much altitude gain you can achieve depends on the relative masses of the vehicles and the length of the tether. Your physics dollars at work!

Andy

 

[RobertR]

How much altitude gain you can achieve depends on the relative masses of the vehicles and the length of the tether.

When you say "relative", do you mean that the lower orbit vehicle would be more massive? Is there any reason why the tether couldn't be HUNDREDS or even THOUSANDS of miles long?

 

[Joe Szott]

Andrew,

Thanks for the insight, that is really fascinating. All the fun physics in college dealt with getting out into space and ~=light travel. Glad to see it is still alive and kicking

 

[David-S]

When you say "relative", do you mean that the lower orbit vehicle would be more massive? Is there any reason why the tether couldn't be HUNDREDS or even THOUSANDS of miles long?

Materials constraints??

That's a LOT of energy, and i don't think we'd have anything that could remotely support such a long length without having problems supporting it's own mass (friend did a similar problem to this for a senior design project, and found that even running thin electrical wire down the length of such a tether would make it impractibaly heavy...) IIRC the only material that theoretically came close was beryllium, and, well, a 100 mile cord of beryllium just ain't gonna happen...

 

[BrianW]

If carbon nanotubes will work for a space elevator, then they'll easily tow a vehicle through the atmosphere.

 

[Jack Briggs]

I'm not used to playing a mere supporting role in a space-related thread. Go ahead and have your intelligent physics discussion. Jack go home now.

 

[Andrew Testa]

"Relative" does refer to the difference in vehicle masses. The larger the lower vehicle is relative to the higher vehicle, the more energy can be transfered and the greater altitude that can be reached. However there is a point of diminishing returns: if the lower vehicle mass is significantly greater, than you've expended a large amount of fuel for a relatively small payload, similar to the present launch vehicle fuel problem we'd like to improve.

Length of the tether is determined by how much energy you need to transfer, but you quickly run past the limits of known material strengths. The tension in such a tether is incredible. In 1996 STS-75 flew a tether experiment with a 14 mile tether. Due to a small kink in the tether it snapped. The tethered satellite disappeared quickly, separating from the orbiter at a rate of 450 miles every orbit. Here's a quote on the construction of the tether:

Manufactured for Martin Marietta Astronautics by the Cortland Cable Company of New York, the tether has a center of Nomex® wrapped with copper wire, which acts as the electrical conductor. The layer of wire is insulated with Teflon®, which is then covered with braided Kevlar® 29 to give strength to the tether. The outer jacket of the tether is braided Nomex®, which protects the tether against abrasion and the corrosive effects of atomic oxygen

details can be read Here.

It's fairly easy to calculate the tension involved, since it's the same as tidal forces. Developing a material that can be made miles long, very thin, with incredible tensile strength, and flexible are all pretty hard. But physics on paper is always a lot easier than physics in space.

Andy

 

[David-S]

If carbon nanotubes will work for a space elevator, then they'll easily tow a vehicle through the atmosphere

All i'll say is I'd be VERY impressed if they could get that nanotube/epoxy composite to work (most epoxies need high curing temperatures for several hours, and there are severe problems in making nano-scale composites at high enough volume fractions to do anything worthwhile...)

The best composite I've seen w/ C nanotubes so far had something like a 1-2% strength increase...

I'm currently working on mixing nano-particles, and it's tough to get them over 30-40% volume percent, because the surface area just gets so large for the volume...

It could happen, I just happen to believe the time scale will be a bit longer than that...

 

[RobertR]

Okay, so the materials problems are similar to those for the space elevator (enormous stress on the material), but the problem of not having a fixed "anchor" on the ground is not an issue, correct?

 

[Andrew Testa]

Well, the materials problems aren't even close, really. We're talking tethers on the order of tens of miles. A space elevator would be 42,800 miles long, since its center of mass has to be at geostationary orbit, at 21,400 miles. But you're correct that the tethers don't have to worry about that pesky wobbly anchor.

Jack,

Please stay and play! It's great that your such a supporter of the space program, I'd love your input.

Andy.

 

[BrianW]

Well, I didn't say it wasn't a BIG "if." So far, we've been able to produce carbon nanotubes in just above microscopic lengths, so the prospect of producing them in lengths of tens of miles, or even tens of thousands of miles, is still just a pipe dream for now. Although there have been two-foot lengths of nanotube ribbons produced, they are not made from two-foot long nanotubes. IMO, using epoxy (or anything else) to bind shorter nanotubes into a longer structure will not provide the tensile strength we need for the applications we're discussing. Too bad, because I'd really like to see something like this in my lifetime.

 

[Eric_L]

Andrew;

I hope you don't mind but I copied (er, sampled) part of your post and placed it on the space elevator message board. Here:

http://forums.highliftsystems.com/cg...ct=ST;f=47;t=8

I am hopeing to get their answer to this. Forgive me if I presume that maybe they've already thought of this. Not being an engineer myself all I can do is read and ask.

 

[Eric_L]

Deposit is refundable until after a successful transitioning flight has occurred. Thereafter deposits are refundable only if Final Delivery Price exceeds List Price (as adjusted for CPI-W) by 5%, OR Standard Equipment List has been shortened OR Guaranteed Performance Specifications are not met, OR FAA Certification Date of the M400 Skycar occurs after December 31, 2005 or a Purchase Agreement is executed prior to FAA certification.

The car was successfully tested in 8/02 and a prototype was recently listed on EBAY. (1/03)

 

[Andrew Testa]

Eric,

I don't mind, so long as you didn't include my email address. I didn't want to register to read the forum, but I read the summary material on the home page, and I think there are more problems than I originally thought.

There seems to be a missing paradigm in their concept. The ribbon is a satellite, with one end dangleing into the atmosphere and being attached to some structure. This does not suddenly make the ribbon a very tall building held up by centripetal acceleration. It remains a satellite and must behave according to orbital mechanics.

Some issues I have:

The ribbon is designed originally to handle some 990 kg. If this is the maximum load that can be put on the ribbon above the attachment load, then they are perillously close to the breaking point. The force exerted on the attachment point by the ribbon is going to easily be in the multiple tons. Remember that the ribbon is a satellite. Once it gets attached to the floating anchor, the anchor becomes a part of the satellite and is essentially suspended from the ribbon, rather than the ribbon being anchored to the Earth. The anchor platform will move with the ribbon, not the other way around.

Since their attachment point is a floating platform, it will rise and fall with the tides. While this is not a very large distance, it will cause the ribbon to slacken and then increase tension with the tides. The ribbon will be in a constant state of varying tension. That quickly leads to material failure.

The mass of the ribbon is incredible, also. With a 2 square mm cross section and 100,000 km length, the ribbon will mass in the 100s of millions of tons, even assuming a density of the material near 1. A floating platform isn't going to be any kind of anchor for a satellite of that mass.

Also, their method of growing the ribbon is flawed. The concept for the original ribon is correct: launch a satellite to geostationary orbit, then extend your ribbon in both directions. However, all subsequent fibers being attached must follow the same pattern. The concept of sending a climber up from the anchor with the additional fiber will destroy the ribbon. Imagine, the original ribbon has mass X for a single ribbon. The climber bringing the second ribbon up has to bring the entire mass in a spool, laying it out as it goes (the weight would be impossible to lift from a spool on the anchor). Since the original ribbon will only carry 990 kg, that's the limit for the spool of fiber. No 100,000 Km fiber ribbon is going to weigh less than 1000 Kg.

So suddenly the mass of the satellite Increases, with the majority of the mass at the lower end. As the climber ascends the center of mass of the satellite changes radically, and the ribbon will want to orbit at its center of mass. With the center of mass (CM) below the Geostationary position for a long time during the climb, the CM is moving slower than the required velocity to maintain orbit. The CM orbit will begin to decay, and the whole structure will rapidly de-orbit.

Okay, that's enough for just off the top of my head and reading the summary. Anybody have any other dreams I can destroy, or dare contradict me?

Andy "Physics Reaper" Testa

 

[Max Leung]

You'd figure someone would have run a computer simulation of a space elevator (construction and in operation) by now!

 

[Eric_L]

Dang I wish I could borrow your brain for a weekend and read it. Well, I'd probably need MUCH longer than a weekend, maybe a METRIC weekend.

I'll go check see if there is any reasonable reply to our (your) post on the other board and share it here. With your permission I'll also share your insights again over there.

I am intrigued by the whole idea.

My cursory and limited insights agree with much of what you said, however....

Most structures are under some form of varying load. Consider bridges and skyscrapers. (people inside and winds) They seem to handle it well.

As far as the mass, well, in a geosynchronous orbit, I'd presume the heavies load would be borne not at the base, but 50% of the way up. That is where centrifical force exceeds gravity, so half the mass is pulling toward Earth and half is pulling away. I'ts probably still a pretty dang big number.

Tides vary in extremity in different parts of the world, some places by yards, others by inches.

whole structure will rapidly de-orbit

Now I KNOW you work for NASA. Talk about understated.

 

[BrianW]

As far as the mass, well, in a geosynchronous orbit, I'd presume the heavies load would be borne not at the base, but 50% of the way up. That is where centrifical force exceeds gravity, so half the mass is pulling toward Earth and half is pulling away.

If by "50% of the way up," you mean half way to the geostationary platform, then not quite. The "halfway point" you described occurs right at the geostationary obit position. So everything below the geostationary platform is being pulled down toward Earth. That's why the ribbon needs to extend in both directions from the geostationary position.

Andy's concerns are valid and reflect a much more thorough look at the problem than I've had the inclination to consider. I won't say that it can't be done, but I do think that the people involved with these ventures are presenting a simplistic analysis of the forces and variables involved.