Shuttle Launch updates for space buffs (1 Viewer)

[Jonathan_E]

Anyone interested in an alternative method of getting into space should go read this article here



http://www.wired.com/wired/archive/1...ic=&topic_set=


It seems kind of far fetched, but the technology is there and it is feasable.

Addition:
Here is the projects website:
http://www.isr.us/SEhome.asp?m=1

 

[BrianW]

Ah, yes, the "Angular Momentum Is Free" approach...

 

[Andrew Testa]

Jack,

Thanks. A small clarification: While losing the Hab module certainly eliminated crew comfort and living convenience, it still would have been possible to have a larger crew aboard. Not seven, but larger. As the additional modules get added and more space becomes available the crew could have "roughed it" like they do in the orbiter. Sleep and eat wherever you're out of the way. Far from having a dedicated sleeping, eating, and washing space, but still better than having to endure trying to live in the service module as they are now. What mandated a crew of three was the cancellation of the Crew Return Vehicle by killing the X-38 prototype. Since the only return vehicle they have now is a Soyuz, which can only hold three, that limits the crew size. There was some talk about trying to have two Soyuz on station at all times, but the Russians laughed at having to double the taxi flights and throwing away two Soyuz every six months. They'd do if we payed the extortion money, but NASA is actually not permitted by law to buy anything from Russia, due to a law restricting trade with Russia while it does arms business with Iran. Tangled web, eh?

Chris,

I'm glad I could be of help. It is indeed painful to experience. It results in either cynical bitterness or acceptance that this is the way a huge government bureaucracy works, and do the best you can. I flirted with the former, but decided to stick with the latter.

Andy

 

[Jonathan_E]

Brian -

I am quite aware of the problem with angular momentum considering that I am currently in school to become an engineer.

If you go to the projects web page, there is a faq section that addresses this. They don't give any calculations or anything, but considering that this project has received funding from NASA and the guy leading it up is the Director of Research at the Institute for Scientific Research (ISR), I would assume that they have considered the problem with angular momentum.

Jonathan

 

[Jack Briggs]

You know, Andrew, I remember well the X-38 debacle and the whole CRV issue being the cause, but I charged ahead with the Hab Module. I think, for the first time ever (in my life perhaps?), something I said about the space effort required clarification! At least it came from somebody within NASA! *blushes, considers slashing wrists*

 

[BrianW]

I'm sorry, Jonathan. I didn't mean to use your post as a segue for a thread crap. I'm usually all over a thread like this like a duck on a June bug. (And I mean that in a good way.) Suffice to say that I've been to their site, read their FAQ, and come away thinking that they haven't adequately addressed many of the problems associated with a tethered orbit. But I'm just a crackpot with a physics degree. Andrew, the true rocket scientist, was able to think of far more issues than I was able to come up with. (Truth be told, Andrew put me to shame. :b) There may be a way to make it work, but I don't think their approach is it. But I'm definitely willing - eager, even - to be proven wrong.

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I think in a sense we've become spoiled. We're so used to new discoveries and new technologies going from tentative lab experiments to profitable, commercial ventures within mere decades, if not sooner. Because manned space flight, after about four decades, hasn't reached the point of accommodating tourist resorts on the Moon, we can't help but wonder where we went wrong in developing and nurturing this technology. What, after all, can be so difficult about space flight technology that makes it defy Moore's Law so profoundly?

Well, Moore's Law doesn't apply to everything, as much as we'd like it to. Even worse, we live in an age in which Murphy's Law has been all but forgotten in the technology sector. Sure, I want to be able to take my kid to Lunar Disney, and I'm disappointed that I'm not able to do that. But given how amazingly difficult manned space flight is, I'm very proud of the progress we've made, and I commend everyone who has played a part in this pioneering effort. It is the most difficult effort ever undertaken by humanity, and it will take more time to commercialize than any of us are likely to live. Though we won't live long enough to see vacation resorts on the Moon, we should take considerable solace and joy in the fact that ours will be the generation of pioneering giants on whose shoulders will stand the lunar explorers and colonists surely to come.

 

[Jack Briggs]

And they will.

At the time of the thirtieth anniversary of the Apollo 11 landing, I was moved to tears (not kidding) when lunar veterans expressed regret that we have not returned yet to the Moon and then suggested that their legacy mandates a return there by future generations. These were our greatest-ever. Our task is to live up to their challenge. Or, rather, to pave the way for such.

 

[Andrew Testa]

Jonathan,

As BrianW said, we've discussed this, but I'd be happy to discuss some of the issues with you. As you continue with your engineering degree, remember that the following:

...but considering that this project has received funding from NASA and the guy leading it up is the Director of Research at the Institute for Scientific Research (ISR), I would assume that they have considered the problem with angular momentum.

is an appeal to authority, one of the logical fallacies. Someone's job, degree, or implied importance has nothing to do with the worth of their opinions. NASA fund many things in small doses hoping that something will pan out. And I've never heard of the ISR: for all I know it's a DBA he bought for $5 at the local courthouse. Also, never trust a PhD in any field other than the one he did his dissertation in. Outside of that, they usually think they know far more than they do, but since the have PhD after their names, nobody questions them. ALWAYS question them. All that matters is the strength of a persons work, and it's up to you to determine that strength by carefully picking apart any claims. That's the heart of science.

Now as for the space elevator, I can assure you that these people have absolutely no clue about the angular momentum consequences of their design. It is doomed to failure. We began discussing it in a thread on Burt Rutan's Spaceship One. My first post on the topic is around #20, I think. I went over in detail several fallacies in the design. Chief among them is that they view the completed structure as a solid building, allowing the angular momentum gained from lifting a payload to be taken from the rotational momentum of the Earth. This is impossible, since it's not a rigid structure. It is instead a satellite, even when anchored, so the angular momentum trade is between raising a payload, and dropping the orbit of the elevator's CG. Continually raising mass will cause the ribbon to deorbit unless they reboost the CG during a lift. So they have to spend a significant portion of their lift capability on bringing fuel up to the center for reboost. So they still have a rocket's drawback of having to bring their fuel along with them, and having the fuel to payload ratio increase as payload mass increases.

While they claim to have a lot of people with high engineering qualifications, they apparently don't have a physicist on staff (When I posted on their forum, they suggested I take some physics classes and learn something about actually working in space. Since that's what I've been doing for the last 20 years, it kind of sealed my opinion of their operation).

You're welcome to peruse the thread and let me know if you have any comments.

Andy

 

[Jonathan_E]

Andrew -

Thanks for your clarifications Andrew, I didn't even know that other post existed.

I now bow down to the NASA expert, besides what do I know? I'm a junior in college that took a year of physics because it's required to be an electrical engineer, I don't know much when it comes to orbital mechanics.

Thanks,
Jonathan

 

[Andrew Testa]

I now bow down to the NASA expert

Now, stop that, you'll give me a head bigger than William Shatner's (have you seen the size of the melon on that guy recently? Sheesh!). Remember appeals to authority. I'm an expert in space robotics, not orbital mechanics. You should double check my work. I have had enough exposure to orbital mechanics to believe I know what I'm talking about, but there are plenty of people within a stone's throw of me right now who may take issue with some of my statements.

I do, however, believe that I am completely right.

Andy the humble

 

[chris_everett]

Andy,

Please destroy my theory

If we developed a sufficiently strong material (impossible, but work with me) and were able to anchor it well enough (again, impossible) and then set up the ribbon so it's CG was somewhat beyond a geosynchronous orbit, so that the ribbon stretched out when it was not ferrying cargo, and had it's CG pulled down to a geosynchronous level when it was, would that do any good at all? (I think I'm missing something obvious here, but I can't seem to think of it)

Alternative #2
What if the "nanotube lattice" or whatever, could be made to have some of the properties of a ridged structure, i.e. not compressing or bending, or with enough "memory" to return to it's original shape if it did bend? I seem to remember some groups working on this kind of tech for other applications...

And in a question actually related to your specialty, how do you overcome the difficulties with temperature swings in space and their effects on motors?

Thanks for taking your time to answer all these questions from us "wanna-be's" I really appreciate your insights here.

 

[Andrew Testa]

set up the ribbon so it's CG was somewhat beyond a geosynchronous orbit, so that the ribbon stretched out when it was not ferrying cargo, and had it's CG pulled down to a geosynchronous level when it was

first, a few notes about orbital mechanics.

1) When moving through a gravitational well, energy must be conserved. If you are in low orbit, your potential energy is low so your kinetic energy is high. lower orbits have faster velocities. Move to a high orbit, and potential energy increases, so kinetic energy decreases. High orbits have slower velocities.

2) Each orbital altitude has a unique velocity required to maintain that orbit, and when no outside force acts on the orbiting object other than the central body gravity, the object is in free-fall.

3) If an orbiting body becomes constrained to move outside of its desired orbital velocity, it will no longer be in free-fall, and will experience more of the Earths gravity the farther it gets from where its CG wants to be.
Example: the elevator is partially deployed, say 1000 miles in both directions. While the CG orbits just fine at GSO, the lower and upper platforms are moving slower and faster, respectively, than would a free flying object at the same altitude. objects in the lower platform experience real gravity, but reduced, toward the Earth. Objects in the upper platform experience the Earth's gravity pulling them down as well, but even more reduced since they're 2000 miles farther from the Earth than the lower platform. The gradient in gravity potential between the platforms creates the tension (tidal forces) to keep the ribbon rigid.

Let's examine the steps required to move the central platform. First, in order to anchor, the orbital platform sends ribbons both toward and away from Earth. To anchor it must be in Geostationary orbit (lets call it GSO), so the central platform stays directly above the anchor point. Now, after anchoring, lets move the central platform up the ribbon beyond it's GSO orbit instantaneously. Angular momentum is conserved, I0W0 = I1W1, and since we increased the CG's distance from Earth, I1 > I0, therefore W1 < W0. So to conserve angular momentum, the rotational velocity decreases. Since we're tethered it can't just fall back in the current orbit. It will be constrained by the tether to a constant distance from the tether point. So it drops in an arc behind the tether point.

Now lets slowly move the platform up ribbon. Since the CG is in free-fall and won't leave free-fall except under external forces, the cable drops to keep the combined CG at GSO. But as cable drops it piles up on the ground at the tether point, so that mass effectively leaves the satellite. The CG shifts outward toward the platform, which wants to maintain it's orbit, dropping more cable. So instead of he platform moving up the cable, the platform winds the cable onto the ground. Once it goes beyond the point where there's enough outward cable to balance to the CG, the CG will move lower, and since lower orbits want faster velocities, it doesn't have the velocity to remain in orbit and reenters.

Now, lets put some jets on the central platform to add the energy to raise it so we don't drop the ribbon, and move the CG farther out from GSO. Then shut off the jets. Now lets examine the consequences of this.
(Extra credit question: where on the platform are the jets located, which direction do they fire, to raise the orbit of the platform?*)

The CG is in an orbit that has a lower velocity than GSO does. We've added potential energy and kinetic energy to keep the platform at a higher orbit and constant angular velocity. Since we have excess kinetic energy for this orbit, the CG wants to move to a higher orbit to balance its new higher energy, but it's constrained by the anchor point. Tension in the ribbon increases keeping the CG in an orbit it doesn't want to be in. It has kinetic energy that it desperately wants to dump into potential energy but can't. This increases tension in the ribbon beyond what the tidal forces produce. Also, the system changes from a balanced tether system, with two end weights equidistant from an orbiting central body, to a rotating inverse double pendulum anchored to the Earth with a central mass beyond GSO and a distance end mass. I don't want to even try the math of a rotating inverse double pendulum in a central gravity field. It certainly is NOT equivalent to simply swinging a ball on a string, where the only force is centripetal, which is the model the designers think applies. MY guess though is that it wouldn't be stable. Oscillations would develop not only between the anchor and the central platform, but between the platform and the outer counterweight. If the rotational velocity oscillates, the tension oscillates, and multiple energy exchanges will probably tear apart the cable or deorbit the system.

I think that covers question 1. Gimme a break and I'll try 2 and 3.

BrianW, any comments? anything I forgot?

Andy

 

[chris_everett]

The CG is in an orbit that has a lower velocity than GSO does. We've added potential energy and kinetic energy to keep the platform at a higher orbit and constant angular velocity. Since we have excess kinetic energy for this orbit, the CG wants to move to a higher orbit to balance its new higher energy, but it's constrained by the anchor point. Tension in the ribbon increases keeping the CG in an orbit it doesn't want to be in. It has kinetic energy that it desperately wants to dump into potential energy but can't. This increases tension in the ribbon beyond what the tidal forces produce. Also, the system changes from a balanced tether system, with two end weights equidistant from an orbiting central body, to a rotating inverse double pendulum anchored to the Earth with a central mass beyond GSO and a distance end mass. I don't want to even try the math of a rotating inverse double pendulum in a central gravity field. It certainly is NOT equivalent to simply swinging a ball on a string, where the only force is centripetal, which is the model the designers think applies. MY guess though is that it wouldn't be stable. Oscillations would develop not only between the anchor and the central platform, but between the platform and the outer counterweight. If the rotational velocity oscillates, the tension oscillates, and multiple energy exchanges will probably tear apart the cable or deorbit the system.

That's what I figured, hence my preface of an impossibly strong cable and anchor point, but sending cargo up the cable, in this scenario, in and of itself, would not cause the cable to deorbit, right? Or would the cable, in an attempt to dump energy, bend and deorbit, a process made even worse by the addition of more mass to the system? Or is that what you just said???

 

[BrianW]

Also, the system changes from a balanced tether system, with two end weights equidistant from an orbiting central body, to a rotating inverse double pendulum anchored to the Earth with a central mass beyond GSO and a distance end mass.

Man, I HATE when that happens! I couldn’t begin to add anything except to say that if the central platform’s position is maintained by tension in the ribbon to the anchor point, then it would be best to do away with the portion of the ribbon that extends beyond the central platform altogether. Besides making the math really difficult, the extended ribbon risks the danger of de-orbiting as a result of the constrained platform’s oscillations flinging it around. And when the upper half of the ribbon de-orbits, the platform will certainly follow, despite the tension in the bottom half of the rribbon.

As for question 2: What if the "nanotube lattice" or whatever, could be made to have some of the properties of a ridged structure, i.e. not compressing or bending, or with enough "memory" to return to it's original shape if it did bend? I seem to remember some groups working on this kind of tech for other applications...

Is this to suggest that the ribbon actually be rigid enough so that the central platform is like a weight attached to the Earth with a memory spring? Like attaching a golf ball to a basket ball with a stiff screen-door spring? If so, and ignoring the advances in materials science that would be necessary for this to happen, then the tethering spring will have its own oscillations to contend with. If such a rigid structure has any resonances that can’t be eliminated, then any time a resonant oscillation of the rigid structure is constructively combined with the orbitally-induced movements of the platform, then the rigid structure will serve only to bring the platform down more quickly and more violently. If the idea is to provide a tethering structure that naturally provides a force in the opposite direction of the platform’s orbital oscillations, then a spring is an intuitive choice, but probably not the best choice. (Indeed, attempting to counter the orbital movements of the platform with any structural force may not be a good idea in any case.) A rigid tether also introduces the prospect of having to deal with longitudinal waves and forces being transmitted up and down the tether. (A non-rigid ribbon already transmits transverse waves and forces, which the Space Elevator people say doesn’t matter). The introduction of longitudinal forces in a rigid tether, constructively combined with the platform's tendency to convert potential/kinetic energy, could conceivably turn the platform into a paddle ball. I’m not saying that having a rigid tether is incapable of solving more problems than it introduces and that these destructive forces can't be diminished or eliminated, but it introduces a sufficiently huge number of problems that I wouldn’t even begin to know how to calculate or simulate the effects along with everything else that’s going on. Someone smarter than I will have to figure it out.

As for question 3: how do you overcome the difficulties with temperature swings in space and their effects on motors?

I think they use Vaseline.

[Edit: Chris, you got your post in while I was composing mine. Sorry if it looks like I'm ignoring you. ]

 

[Lee L]

Heh, heh, my friends and people at work think I am a geek.

 

[Andrew Testa]

Lee, I'm far from the worst around here. But then again, if you don't get excited arguing over orbital dynamics, you probably wouldn't have applied for a job here in the first place!

Everyone gets a gold star on the bonus question. One caveat is that the jets would need to fire continuously to maintain the same angular velocity. A two-burn system would raise the orbit, but it would also slow the forward velocity. So we fire continuously to keep the velocity up and provide some upward force. Actually, we'd probably provide TOO MUCH upward force if we're creeping along the cable, since it would take a lot of added energy to keep the forward velocity the same, and the platform's going to want to jump into a very eccentric orbit. We'd have to ride the brakes. This would probably put us into the double pendulum situation even sooner.


Chris, you are correct that both of the first two cases cause a deorbit. After thinking more about the complexity of the third case, I'm still not sure of the end result. The tension in the cable is redirecting a LOT of energy from the orbital mechanics, and it has to be dissipated somehow. IF we assume that the Inertial properties are fixed, since our altitude is fixed by the cable, then changes in kinetic energy are proprtional to 0.5W^2, so if the excess energy is dissipated through heat and vibrational excitation, then as the kinetic energy drops, then W must increase. So even if the ribbon is in tension, with the CG beyond GSO, if the CG slows down at all because of energy dissipation (which entropy demands) then the angular velocity increases, pulling the CG forward, and down since we're tethered. The counter-weight suddenly finds its tether point accelerating forward, pulling it to a lower potential energy, but higher kinetic energy. Since the proprotions are unknown, the angular velocity of the counter-weight ABOUT THE CENTRAL PLATFORM will either increase or decrease. Hence our oscillations begin.

Eliminating the outer ribbon would quicken the end, since it's a major factor in the location of the CG. It also cuts in half the altitude at which payloads could be released, substantially reducing the initial energy imparted (if it were stable).

But let's assume that it IS stable, and the CG merrily swings around past GSO. Let's send up a payload. It's mass is small compared to the elevator, but is added to the ribbon mass on it's way up. As it moves up the ribbon the inertial moment I increases as the mass moves farther out. Conserve angular momentum: if I increases, W decreases, so the angular velocity lowers and the ribbon starts to lag behind. The kinetic energy of the system will drop, since the lowered W is squared vs. the Inertial. so the angular velocity slows and the kinetic energy drops, reducing tension in the cable. If the payload is large, this will happen quickly and deorbit the elevator.

If the payload is small, let's assume it makes it all the way to the top with only a small decrease in W. It can't take too long though, because W is a rate, and even a small rate translates into a large distance over time. So it's at the top, the CG has slowed and is lagging behind the GSO anchor. The payload is released and flys up and away from the cable. But, conserve energy. The payload absorbed a lot of potential and kinetic energy on it's trip up, far more than what was added by the motors driving the payload. (this transfer is responsible for the reduced energy that causes the lag). Since total energy combined = total energy separate, the elevator loses a lot of kinetic energy when the payload is released. The inertia I suddenly decreases to it's original value, but kinetic energy is reduced, so W suddenly decreases below the geostationary velocity.

Lower W and lower KE with each payload eventually lead to two possible results: loss of enough energy to deorbit the system, or a reboost is required to add back the lost energy.

Think about that carefully: It took a set amount of energy to get that payload to that height and velocity; energy that is taken from the elevator. To stay up, that energy must be added back into the elevator with jets at the CG. That same amount of energy would have been expended to get the payload there with a conventional rocket. I don't see the advantage even if it did work.

Andy

 

[Andrew Testa]

Question 2, How about a rigid cable?

The only way the elevator system would work is if the cable was completely rigid and supported it's own weight, just like a mega-huge skyscraper. In that case it would not be an orbiting body, although orbital mechanics would induce stresses into the structure. In an impossibly rigid structure that reached beyond GSO, you could indeed crawl a payload up the structure and release it with the increased energy gained from the climb. Since the structure is impossibly rigid, the energy gained by the payload is pulled from the structure, which pulls from it's base. So in this case the payload does indeed rob angular momentum from the Earth, just as you do every time you climb a set of stairs.

(Hey everybody, lets all ride every elevator in the world from ground to roof at the same time and slow down the Earth's rotation! More hours in the day! As long as we don't ever come back down, that is.)

Question 3 is the easiest of all. We use blankets, heaters, and the Earth. You wouldn't think so, but LEO is a pretty warm place. The Earth radiates a lot of heat, so objects in LEO are always being heated by the radiation. We never get the 4 degrees above absolute zero of deep space. The motor temperatures are supposed to range between 0F to +150F. We have alarms to let us know if they go beyond those limits. These are maintained by electric heaters and blankets. Everything is wrapped in a pretty elaborate blanket system so there is a minimum of bare metal exposed to space. The orbiter generally orbits in a way that slowly rolls components into or out of hot or cold spots, so nothing bakes or freezes. Every system on the orbiter or its payloads has temperature limits, so a lot of the choreography involves keeping everything's temperature in the proper range.

Andy

 

[chris_everett]

I realize that I am ignoring the materials questions here, and I am doing so intentionally, and believe me, I see that the actual developers are completely off base. I remember when the concept was first proposed and the developers indicated that they had NASA support, I assumed (mistake) that they had the basic problems worked out, and didn't think about it in any detail.

[Edit: Chris, you got your post in while I was composing mine. Sorry if it looks like I'm ignoring you. ]

Thanks for your input Brian! Not feeling ignored at all! Just feeling like I need to take a dust-buster to the parts of my brain that store all of my physics knowledge. As you guys are answering my questions I'm starting to remember stuff, but it's been way too long since I've used any of this knowledge.

 

[chris_everett]

Apparently, it's now it's my turn to edit a post while other people are posting...

 

[chris_everett]

Okay,

We use blankets, heaters, and the Earth. You wouldn't think so, but LEO is a pretty warm place. The Earth radiates a lot of heat, so objects in LEO are always being heated by the radiation. We never get the 4 degrees above absolute zero of deep space. The motor temperatures are supposed to range between 0F to +150F. We have alarms to let us know if they go beyond those limits. These are maintained by electric heaters and blankets. Everything is wrapped in a pretty elaborate blanket system so there is a minimum of bare metal exposed to space. The orbiter generally orbits in a way that slowly rolls components into or out of hot or cold spots, so nothing bakes or freezes. Every system on the orbiter or its payloads has temperature limits, so a lot of the choreography involves keeping everything's temperature in the proper range

Wow! That's a much smaller temperature range than I expected!
Thanks again for the answers Andy and Brian, I appreciate it. And now that my brain is throughly fried, I think I'll go over to the Maxim Hot 100 thread for awhile
I'll post again if I think of any other questions!