Let me recap what I remember:
- The tension in the hoop is proportional to the rotational speed squared times the linear density of the hoop (mass/length).
- The velocity of a wave in the hoop (shaking left/right or in/out) is proportional to the square root of the tension, and turns out to be exactly the same as the rotational speed.
- This means that waves travelling in the anti-spinward direction stand still in space, and any little thing pulling on the hoop (like a tide) will pull it further and further from a circle.

I'd have to re-do some DiffEq to confirm this, but it feels right.

with future technology, beanstalks!

i also have trouble following the math, however.

we really need the ssto (and solar power satellites). but what i really want to see is orion...

quote:

---

Originally posted by jhota:
i would think the cheapest way into space (with current tech, anyhow), would be an ssto.

---

quote:

---

with future technology, beanstalks!

i also have trouble following the math, however.

---

quote:

---

we really need the ssto (and solar power satellites). but what i really want to see is orion...

---

i always hated that class.

government control of space is the problem, i think. i mean, they can't even count. how can we expect rational exploitation of space?

as for orion: hell, no i don't want to launch it from earth! but it sure would make the solar system a lot smaller place. make mining asteroids a lot more reasonable possibility: fly out an orion pusher "tugboat," pick up a nice rock, and cruise home. mine it and manufacture stuff right there in orbit. rare earths and minerals that are valuable in the manufacture of high tech products are (theoretically) more available in the belt, right? and, higher quality manufacturing of many of these is easier in microgravity, right?

and as for the solar power satellite(s): how much, really, would it cost to build and launch a few? less than a few stealth bobmers and their development i bet. with greater payoffs.

------------------
Hoc futui quam lude militorum....

quote:

---

Originally posted by jhota:
yeah, sure, calculus.

i always hated that class.

---

Orion as a tugboat is a pretty good idea, but it might not scale as well as mass-drivers or solar sails.  It would be a great way to send out a comet-diversion mission to Save The Earth From Armageddon, though.

quote:

---

and as for the solar power satellite(s): how much, really, would it cost to build and launch a few? less than a few stealth bobmers and their development i bet. with greater payoffs.

---

I never really got a chance to develop a good hate for that class. Only the professor. I went up to him after class one day towards the very beginning of the semester to ask for help. He said, "Oh, this stuff is self-explanatory."

Since he wasn't interested in teaching, I wasn't interested in attending.

government control of space is the problem, i think. i mean, they can't even count. how can we expect rational exploitation of space?

Commercial control would be better somehow? I doubt it. The Government at least can make a good claim to be doing this research in the public interest. Everyone benefits from public research. I'm not saying capitalism won't work for space research. But right now the barrier to entry is so high that only a few commercial entities have the capability to scale it. That reduces the potential competition, which lessens the drive to compete on price.

Commercial companies are also more beholden to their stock owners than the general public. This also limits the potential areas of research. A private company would probably be more interested in research that will help it get more out of its communication satellites (which can make money now) than in , say, exploring Mars. The payoff on Mars exploration in the form of a colony (what company wouldn't want a piece of that contract?) is too far away for now. So it lands square in NASA's hands to do the not-immediately-profitable research which that future colony might be built on.

The government shouldn't completely control space, but it does serve a very important purpose!

(Aside: DAMN TO HELL the person who decided to put the "ALT" key next to the space bar where you hit it accidentally all the time and have to hit "ESCAPE" to get back out of the lousy menus, and DOUBLE-DAMN the idiot who made "ESCAPE" irretrievably blank the text-entry window in Exploder!  And before these people are damned, they should be tortured to death v-e-r-y slowly.)

According to http://www.space.edu/projects/book/chapter19.html, the SSME's are used to bring the ET up to full speed along with the Orbiter.  This means that there is typically no OMS burn until the first apogee, and my calculations assuming that a burn is required to set the apogee are too pessimistic!  Yay!

If the external tank is discarded at an altitude of 63 miles (101 km) and the apogee of the initial orbit is the same 160 nautical miles (295 km), the velocity at perigee is 7718.90 m/sec.  The apogee velocity is 7494.26 m/sec compared to the circular-orbit velocity of 7550.21, so 55.95 m/sec has to be made up.  This is about 27% of numbers based on older assumptions.

Continuing to use the remaining-fuel figures in http://www.orbit6.com/et/carroll.htm, we have to give a 56 m/sec boost to a tank with an initial mass of 80,000 pounds (66,000 lb dry weight, 9000 lb remnant fuel, 5000 pounds payload).  Doing this with an engine with an impulse of 350 seconds requires 1294 pounds of fuel (call it 1300).  This would leave ~7700 pounds of remnant fuel.  This looks downright easy.

After the orbit is taken care of, the remaining hydrogen has to be converted to a storable form or it will leak or have to be vented as it boils.  An idea that just struck me is to burn it with oxygen and use it to heat the hot side of the Stirling motor, then vent the steam to the radiator.  The system could even be designed to use steam/water as the heat-transfer fluid for the radiator system; it would be launched empty and filled on-orbit.  This would allow the Stirling motor to be started even before the solar mirror could be opened and without the overhead of a second power system.  Helium in the gas mix (which I'm certain is used to pressurize the tanks) would be problematic, as it would render the heat pipe non-functional if allowed to accumulate and require a gas/liquid separator somewhere in the system to vent it.  I'm sure this is doable, but it's one more complication.  I'll have to calculate the available energy from burning that fuel, and the impact on the orbit, sometime later; I don't have time to do it right now.

Once you're in LEO, there are two more phases involved in getting something down onto Luna.  They are:

1. Trans-lunar injection and achieving lunar-orbit, and
2. Landing.

quote:

---

Originally posted by Petethelate:
If there's a practical way to do it, I'd try using the H2/O2 with a burner directly on the hot end of the stirling

---

quote:

---

and swipe some He for the heat transfer fluid in the engine.

---

quote:

---

The heat transfer capabilities of helium are ideal for a stirling cycle.

---

quote:

---

Originally posted by Tau Zero:
Otherwise known as a "combined-cycle turbine", if you leave off the afterburner. But it doesn't sound like something you'd find in large quantities on the used freighter market; even the US Navy is only just moving to gas turbines for their smaller vessels. Where are you going to find a bunch of these cheap enough to do a shoestring startup? It sounds a lot better for something in a later phase, where you've got plenty of money and lots of stuff in orbit so that your equipment is busy all the time and repaying your investment faster.

---

Still, ships are big, and you wouldn't be packing them full of cargo. Quite likely you could buy used, then gradually retro-fit the turbines in place of the present burners, as money becomes available.

quote:

---

Saintonge:
I don't have time (again) to do numbers, but here's another idea: I remember something from a book "Power Supplies for Space vehicles." The author noted the advantages of generating power, then using the waste heat from the turbine to heat reaction mass.

Tau:
Which leaves the reaction mass colder, and therefore exitting the nozzle slower.� If reaction mass is the limiting factor (as it appears to be, at least as far as the hydrogen portion is concerned), getting the most energy into it as possible appears to be the optimal strategy; tapping off energy to run other machinery just decreases your performance.

---

Hey, weren't you the person telling me that in space, energy is cheap, it's waste heat disposal that's expensive? Generate the power, dump heat into reaction mass, and add more heat to it. When it's hot, use the electric power to give it extra velocity.

Do I have to think of everything?

quote:

---

Tau Zero:
I did mention the necessity for deploying a 90-meter square receiving array, didn't I?� I can think of possible ways to do this on-orbit (such as inflating a crossed pair of tubes of graphite fiber and uncured thermoset resin, burning a fuel mix inside the tubes to generate the heat for curing, then unrolling and stretching a wire mesh antenna across the framework) but it's not terribly simple and maybe not even very light.� If you've got some scheme for doing this, I'd love to hear it.� It would also be great if you mentioned the limits on your intended applications; inverse square law dictates that the available power would fall by a factor of 4 every time the distance doubles, so the efficiency drops rather rapidly by the time the perigee gets up to 1000 miles altitude.

---

I'm not sure which post you're referring to, but in one of the last Galaxy magazines, there was an article on transmitted power for space ships. As I recall, with a proper transmitting antenna, (it has to be large), you can get good focus on the receiving antenna at quite long distance.

Surely we intend to re-use these hypothetical receivers, don't we? So, we might need a manned mission at the start, for set up, but after that no great problem as long as the weight is under control.

See Kingsbury's novel The Moon Goddess and the Son for some more on lightweight receivers.

------------------
Saintonge

"She just left me. *sniff* She didn't even care enough to cut me head off or set me on fire. *sniff*"

quote:

---

Originally posted by Tau Zero:

I was thinking about having the tubes go directly through the heat-storage medium (whatever you use to keep the Stirling running when you're in shadow).

---

There was an article once in Popular Science on Stirlings and heat storage for them. IIRC, the substance with the greatest heat storage per kilo was a NaF salt with some additions. It melted around 1000K, with a very high heat of crystalization.

This is where my ideas about low rejection temperature power generation and dumping the waste heat into the reaction mass might work out. Rather than cart radiators into orbit, we use the hydrogen we already have. The heat rejection temperature can be down around the critical temperature of Hydrogen if we want, or much higher if desired.

I don't what the optimum trade off between exhaust velocity and power generation is, but I suspect it's rather low, allowing us to keep maximum amount of hydrogen and generate the maximum power for the tether. Or it might be best to use different temperatures at different times. Just what assumptions are we using?

quote:

---

Originally posted by Saintonge:
Surely we intend to re-use these hypothetical receivers, don't we?

---

quote:

---

Hey, weren't you the person telling me that in space, energy is cheap, it's waste heat disposal that's expensive?

---

Seriously, one of the links above points to an inflatable mirror which is something like 5 meters across and weighs, if memory serves, 8 pounds.  When mass is money, that's a lot cheaper than the radiator required to handle its heat.

Would you care to write and then post an analysis of your concept above?  I'm wondering what kind of thermal efficiency you were expecting from the heat engine, and what your exhaust velocity would be vs. just heating the gas directly.  Hydrogen-heater rockets using solar heat have been studied, but that was for missions without the constraints of trying to piggyback on a spent Shuttle ET and going from there.

quote:

---

Do I have to think of everything?

---

quote:

---

Rather than cart radiators into orbit, we use the hydrogen we already have.

---

quote:

---

Just what assumptions are we using?

---