Another Space Elevator Design

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paperburn1
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Re: Another Space Elevator Design

Postby paperburn1 » Tue Aug 18, 2015 6:48 pm

The first rule of fight club is you do not talk about fight club.
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Skipjack
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Re: Another Space Elevator Design

Postby Skipjack » Tue Aug 18, 2015 8:32 pm

Yeah, the whole aircraft launch concept has its own problems, as Stratolaunch is learning right now. Like with the space elevator, the size of the rocket is limited by the plane. And then your rocket has the problem that it has loads towards the sides for a long time. Most current rocket designs are meant to have the loads vertically (as if the rocket is standing upright). So you need a custom rocket design that can take the loads and wont be too heavy or you lose even more payload and you have not gained anything from launching from a plane. They looked into solids (like Pegasus was), but have since abandoned this again. Solids just are not cost competitive (and Pegasus is among the most expensive launchers of all).

JoeP
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Re: Another Space Elevator Design

Postby JoeP » Tue Aug 18, 2015 9:58 pm

We just have too much gravity.

Tom Ligon
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Re: Another Space Elevator Design

Postby Tom Ligon » Wed Aug 19, 2015 1:47 am

williatw wrote:An Earth based mass-driver that goes up Kilimanjaro mountain is probably more feasible; topped off with laser boosting:

http://everything2.com/title/The+Millennial+Project

Loved that book, still have and look at it once in awhile; to bad it hasn't happened yet.


A few years back I was asked about an idea to use a launch cannon (electromagnetic mass driver or fuel-air combustion gun) in which the area ahead of the projectile was mostly evacuated. The thought was to run it up a tall mountain (call it 20,000 ft) and open a door on the end and shoot it out at orbital velocity or slightly higher. I had issues with that. Aside from some shock wave issues inside the almost (but not quite) evacuated launch tube, I ran the air density at that altitude and did some 1/2 ro vee squared calculations and did not like the dynamic pressure I was getting. If you were on it you would feel like you had hit cotton bales, and I'd expect the thing would need a heat shield that would go incandescent almost immediately.

Then there's the hypersonic shock wave from a projectile at 20,000 ft and orbital velocity. The neighbors might not like being deafened and having their windows blown out.

The idea might have some merit for launching a scramjet to, say, Mach 5, and going the rest of the way up on that way.

This "elevator" design might have some utility as support columns for such a gun. They're looking at altitudes that might make the air density almost low enough to get away with a gun launch.

The trouble with patenting this now is that the time horizon for making anything work is beyond the life of the patent. If the idea is any good, there ought to be other applications, much less ambitious but beyond the capability of conventional structures. Towers several miles high, that can be inflated as needed, for instance. Shape shifting structures.

I personally like scramjet launch from high altitude jet aircraft as a practical way of getting around heavy first stages, and maximizing payload fraction to orbit.

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Re: Another Space Elevator Design

Postby Skipjack » Wed Aug 19, 2015 2:31 am

Tom Ligon wrote:I personally like scramjet launch from high altitude jet aircraft as a practical way of getting around heavy first stages, and maximizing payload fraction to orbit.

The problem I see with hypersonic air breathing first stages is that they have to be in the atmosphere fir the air breathing to work. That means more structural stresses on an already more complicated body and friction losses and you have to deal with heating of the hull somehow (and it is different from reentry heating as it affects other parts of your spacecraft). With a normal VTOL rocket, you usually try to get out of he atmosphere as quickly as possible.

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Re: Another Space Elevator Design

Postby Tom Ligon » Wed Aug 19, 2015 3:47 am

At the moment, rockets are all we have. And they work pretty well.

But long term, to get launch costs down we need to reduce the amount of fuel and/or oxidizer carried from launch. The scramjet eliminates oxidizer for most of the climb.

To make it work to orbit you need to have one that can work over a wide speed range, designed to climb as speed increases so that it does not raise dynamic pressure and drag as the speed increases. Ro must drop as v increases. If you work out the swept area of air, the amount of oxygen entering the combustion zone per second actually doesn't drop of much as you climb thru thinner air, because you're sweeping that area so fast. The principle is, if there is enough air to make drag, there's enough to run an engine.

At some point you can't maintain combustion and have to use some on-board oxidizer, but it should be possible to burn to something like 60-70 miles and almost to orbital velocity.

It has been years since I've modeled the swept air problem ... I used a TRS-80 Model III. The simulation took as long to run as it would have taken to get the ship into orbit.

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Re: Another Space Elevator Design

Postby Skipjack » Wed Aug 19, 2015 8:22 am

Tom Ligon wrote:At the moment, rockets are all we have. And they work pretty well.
But long term, to get launch costs down we need to reduce the amount of fuel and/or oxidizer carried from launch. The scramjet eliminates oxidizer for most of the climb.

Meh, fuel is cheap. The total cost of fuel for an F9 launch is according to Elon Musk about 200,000 USD.
What you need is that the total space launch vehicle cost and the cost for its life time launches (which is of course only one for an expendable) plus the cost for maintenance divided by the number of life time launches multiplied by the amount of payload per launch is less than for competing systems. If the super scram jet does not safe any money, because it requires lots of maintenance or drives up the cost for the vehicle so much that it does not safe any money, then it serves no purpose. The problem with the Space Shuttle e.g. was that while it was partially reusable, it cost so much to service between flights that it did not safe any money over expendable systems. Scramjets are nice, but they drive up system complexity and system weight a lot. The latter reduces payload. Or you do a two stage system with a scram jet first stage. But the the question is what the advantage of a two stage system with two different kinds of engines (and presumably also airframes) is over a two stage system with just one type of engine. The F9 e.g. saves a lot of money because of the huge amount of commonality between the two stages.

AcesHigh
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Re: Another Space Elevator Design

Postby AcesHigh » Wed Aug 19, 2015 12:01 pm

Tom Ligon, I am sure that you already heard about the Skylon Spaceplane, right?

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Re: Another Space Elevator Design

Postby Tom Ligon » Wed Aug 19, 2015 2:39 pm

AcesHigh wrote:Tom Ligon, I am sure that you already heard about the Skylon Spaceplane, right?


One of several over the years.

Back in the last millennium, I wrote a story for Analog called "Amateurs", based on an actual card-carrying crazy rocket scientist named Jim Hill. This is before Astronaut Farmer, before Space Ship One. The X Prize was annonuced after I had submitted the story. Jim was working on a SSTO spaceplane in his barn, and I fictionalized how it might work out, with a fact article to pair with the story.

To make it realistic, I had the fictional version of him (Jake Knoll) fail. And Jim was OK with that ... he figured that was the most likely outcome. He died of a heart attack before he could attempt a flight.

In the fact article, my main thrust was that small aerospace companies have an ability NASA and the big outfits don't. They can fail without causing a national crisis. Looking at the budget Jim was hoping to get for his project, failure would have about the same impact as a gas station going out of business. That's true of a lot of the small shops building kit aircraft, as well, and that is the sort of scale Jim was working at.

I was critical of some straw men bureaucrats in the story, but it turns out that the FAA was creating the Commercial Space Transportation group at that time. This was the brainchild of one of my favorite Republicans, Dole. Not Bob, Elizabeth. Should have been our first woman president. The FAA CST, it turns out, was created to provide a route for the Jim Hills, Burt Rutans, and their like to actually pull this off, and I wound up with a fan club in that group.

You can have a hundred failures. You only need one success. Elon Musk has pulled it off, using the straightforward rocket approach. No quibbles ... it works, and he has made smart choices. But this will not be the last stage of spacecraft evolution. Point to any single project, such a Skylon, and the smart money will be they will fail. You would be right 99% of the time.

Those predictions were made for SpaceX, too. Oh, well, can't win them all.

Space elevators and strange servo feedback inflatable towers will have vastly more failures than successes, too. There are hundreds of ideas to explore. We only need the best few to succeed.

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Re: Another Space Elevator Design

Postby Skipjack » Wed Aug 19, 2015 7:48 pm

I like Skylon a lot, been following Alan Bonds efforts since the HOTOL days. The problem is that they need A LOT of money to develop Skylon and then they have to somehow make that money back. So a single Skylon costs a lot of money to build. This means that many Skylons have to fly many times before the whole concept becomes economic compared to a much cheaper Falcon 9.
The question is whether there is a big enough market for this business case to close. Many people with expensive degrees will tell you that the market is not (sufficiently) elastic for the case for expensive RLVs like this to close. I think it may just be elastic enough. At least for the reusable Falcon 9, I am pretty sure it is. We will see who is right.

D Tibbets
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Re: Another Space Elevator Design

Postby D Tibbets » Wed Aug 19, 2015 8:17 pm

krenshala wrote:Orbital velocity in a geosynchronous (Earth) orbit is 3.07 km/s.


My messy calculation was... messy.
Assuming a 50,000 km orbital height and circumference of 2 pi r gives ~ 314,000 km. This is per day for geostationary requirements so 314,000 km/ 24 hr= ~13,000 km/hr or ~3.6 km /s. At least this time I am close, considering that the altitude was a guesstimate.

My point that any velocity gained by a tower of 12 km in height was insignificant stands.

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Skipjack
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Re: Another Space Elevator Design

Postby Skipjack » Wed Aug 19, 2015 8:24 pm

D Tibbets wrote:My point that any velocity gained by a tower of 12 km in height was insignificant stands.

Absolutely

Tom Ligon
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Re: Another Space Elevator Design

Postby Tom Ligon » Wed Aug 19, 2015 8:33 pm

The market demand problem drives the whole space elevator problem, too. A true space elevator would be a massive undertaking, requiring huge investment, disruption not only of the launch industry but of orbits ... the things would stretch out above the equator, potentially in the way of virtually every orbit except for well-coordinated sun synchronous orbits, to beyond geosynch. To give a return on investment, they would need to hum like a major international airport plus a major shipping port.

But I'm enough of a starry-eyed dreamer to think we will reach that level of traffic.

On the way there, we'll reach a level where scramjets, or something like them, provide an economic break. That economic break will, itself, open up launch markets. True enough, rocket fuel is cheap and gets us there. However, the payload fractions to orbit are low. Most of what is launched is fuel and oxidizer. And ask Musk just how fond he is of oxidizer right now ... a tank of it just set him back quite a bit.

Not only does a scramjet not need to carry much oxidizer, 80% of what it is breathing in is nitrogen and a pinch of argon. That's reaction mass, something few rockets carry in excess of their fuel.

Net result, more payload per launch. The economics of this are compelling because only payload pays.

Regarding the complexity argument, I would counter that the oxidizer carried by liquid fueled rockets adds considerable complexity and hazard. The pumping problem alone is non-trivial. Scramjets only need the fuel system. And their engines are simple, some versions are even external combustion.

I didn't have much faith in the idea when we were running engine tests with a 2 second engine life. But tests up in the range of minutes have me hoping. If we can hit 15 minutes, I think the concept is viable.

The trouble for the people dreaming this up is, by the time we do have that level of launch demand, and the materials technology catches up with the dream, all present patents will have expired.

Skipjack
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Re: Another Space Elevator Design

Postby Skipjack » Wed Aug 19, 2015 9:30 pm

Tom Ligon wrote:On the way there, we'll reach a level where scramjets, or something like them, provide an economic break. That economic break will, itself, open up launch markets. True enough, rocket fuel is cheap and gets us there. However, the payload fractions to orbit are low. Most of what is launched is fuel and oxidizer. And ask Musk just how fond he is of oxidizer right now ... a tank of it just set him back quite a bit.

Well, the failure had nothing to do with the tank itself but rather a single bolt holding a strut holding a helium tank for pressurization. You need that in all the tanks anyway.

Tom Ligon wrote:Regarding the complexity argument, I would counter that the oxidizer carried by liquid fueled rockets adds considerable complexity and hazard. The pumping problem alone is non-trivial. Scramjets only need the fuel system. And their engines are simple, some versions are even external combustion.

Scramjets need fuel pumps too, but I agree that they have less issue with pumping. But the scramjets can only get you part of the way to orbit.
From all I know about scramjets, they only have a very limited speed range at which they can operate.
First, you need to get the launch vehicle to the speeds that are high enough for scramjets to work. What do you use for that? Right now it is rocket engines. This is particularly troublesome since the way to Mach 6 is where rockets use most of their propellant. The F9 first stage essentially does just that.
Then you have to get it all the way from the maximum scramjet speed to orbital speed. What do you use for that? Rocket engines. So you really don't get around the problem of having rocket engines and their fuel systems. You add the weight, complexity and heat issues of scramjets on top of that and of course you need a more complicated and heavier air frame.
I am not saying that it wont work and that it wont be better than what we have now. But to me the big question is whether the gains are big enough to justify the huge development and research effort.
Fuel is cheap, oxidizer is cheap, tanks are cheap. If you want more payload, you can always build a bigger rocket. SpaceX is planning on doing just that with the BFR which will be powered by their new Raptor Methane/LOX engines. That will be very interesting.

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Re: Another Space Elevator Design

Postby Tom Ligon » Thu Aug 20, 2015 4:12 am

There's no intrinsic reason why we can't learn to run scramjets up to Mach 17. Present models work only in a tight speed range, but that's because we're still pretty stupid in this technology. And if we can morph to optimal shape for a wide range of speeds, giving them common ramjet technology may be part of the flight envelope. You can launch a ramjet from a turbojet carrier at transonic speeds.

It may very well be necessary to use "morphing" aircraft technology to give them the broad speed range. This sounds pretty fantastic, but the Skunk Works seems to be treating this as a real avenue for future progress.

We may be talking about nanotechnology self-assembling ceramic Legos to pull it off. Dang, though, that would be a whole disruptive technology all by itself. Like maybe self-assembling, shape-shifting stratospheric launch towers.


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