Earth to LEO - discuss

Tom Kalbfus · 2006-09-23 09:21:35

You wait for FTL and your going to wait a long time if not forever. FTL depends on the Universe being just so, I don't think you can count on that.

GCNRevenger · 2006-09-23 09:41:40

Again, if you are wanting mass Lunar launch, the solution is a Lunar space elevator; ride it all the way from the surface up to L1, and then a gentle push to wherever you want to go in the solar system. I think this is the only sane option for mass transport from the Moon, and perhaps down to the surface as well. Why not an elevator? Even Kevlar or Spectra fibers are strong enough, no need for advanced nanocomposites.

The reason for this is simple chemistry, that you need two liquid chemicals one with a high and one with a low oxidation state for use as a practical RLV fuel. On the Moon you only have one of the two readily available, liquid oxygen, but not the other. Since only some of the fuel for Lunar launch is available on the Moon, importing industrial quantities of the other component isn't happening. No massive multikilometer railgun or gigawatt laser required; send up a heavy lifter with the cable spool to L1 and drop a controlled deorbiting vehicle to the surface with the cable end.

I also want to remind you that launching up to L1 from the Moon requires about as much fuel just to launch back to Earth or to other planets directly; going up to L1 and getting a push from there doesn't make much sense. The notion that you could somehow time and direct a stream of objects from L1 tward Earth with sufficent accuracy to pass through a particular flight corridor for an Earth-launched suborbiter is nonsense. Because they aren't fired simultainiously, they will never experience precisely the same gravitational pull.

You'd have to give these things a push from L1 to Earth first of all, second the projectiles must be launched before the suborbiter by days nessesitating a super-tight launch schedule, and finally you just couldn't prevent them from spreading out away from the meter-accurate trajectory you need to hit.

And even if you do, the suborbiter can't survive such a collision in the first place, even with leftover fragments of such a projectile following impact with the "particle cusion" the suborbiter drops, and a laser too? Come on, too many things have to happen at precisely the right instant, its impossible. Again you also have the same problems of laser launch, that the beam will spread and not be focused enough to hit anything really small without a huge mirror, and the suborbiter will go over the horizon before the laser could aid acceleration to orbital velocity. Even if you did hit them with the laser, they would explode like a shock wave, not a nice gentle, constant wall of gas.

And just one misfire, just one targeting error and kaboom, the suborbiter is dead. That one time the projectile or its fragments slip past all the suborbiters' hair-thin defenses, the "little ship that shouldn't" would be shredded to bits.

The worst problem though is that the suborbiter will need to carry at least as much "cusion stuff" or a blast shield (including shock absorbers) as it would rocket fuel for the acent all the way to orbit!

A few side notes...

-GPS only works in low Earth orbit, and then only over long time scales. The projectiles will not have either characteristic.
-Radar ranges for small onboard antennas is usually only 100-200km or so, and the reaction time required will be too short for the suborbiter to make much correction. A few seconds is probably all you have!
-The allignment required between the Lunar source of projectiles and the suborbiters' path will ensure you only get launch opportunities infrequently, dooming the idea.

GCNRevenger · 2006-09-23 09:47:26

You can't use the pellet gun idea for interstellar or even short transit interplanetary mission for the simple fact you can't accelerate any faster than the maximum speed of the railgun.

As you move further away from the gun, the accuracy out in the void becomes more and more difficult too. And the damage inflicted by the projectile impacts will again be difficult to protect from. Shock waves don't readily dissapate as the run through the ships' structure either, which is why Orion needed a massive shock absorber.

cjchandler · 2006-09-23 10:02:55

I agree, it is not really practical, however, I think the concept of intersteller travel with pellets was that the pellets are shot in a strait line one after another very quickly, another aspect we haven't discussed, and the starship catches up with each pellets and then with an onbord powersorce and probably some kind of magnet pushes the pellet backward, thus gaining momentum. So it's different from the earth to LEO version. Sort of line a liner electric motor, with pellets instead of a track.

GCNRevenger · 2006-09-23 10:21:39

No way, the acceleration from "pushing off" from slower moving projectiles is to large extent controlled by the mass of the projectile, and since they weigh much less then the ship you'll hardly get any push. When you really start getting into percents-of-C, the projectiles will be going by so fast that the magnets or whatever won't have time to exert much pull on them before they sail past. And if you think accurately guiding projectiles from the Moon to Earth is a problem, try delivering them with meter-scale accuracy to, say, Alpha Centauri. Finally, the sheer number of projectiles is absurd, you'd have to carve up half the Moon to get enough.

noosfractal · 2006-09-23 12:55:52

I have no personal intrest in traveling at reletevistic velocities, holding out for FTL , so I leave you on your own to figure out the magsail. One thing that does intrest me about them is that once I read an article somewhere that claimed besides their thurst they provided excellent radiation shielding. Wish I remembered where that was...

May be hereish ...

http://www.newmars.com/forums/viewtopic.php?p=84792

Tom's right about the magsail being the solution to slowing down at the destination star - very nice properties like faster you're going the better it works ...

http://www.niac.usra.edu/files/library/ … Zubrin.pdf

Another interstellar sail idea: the sail is a relatively smallish square of uranium. It tows a box of antimatter which is released at the sail. The collision causes a smallish nuclear reaction, ablating the sail and providing thrust. Has an Isp of a million seconds ...

http://www.niac.usra.edu/files/library/ … 40Howe.pdf

Tom Kalbfus · 2006-09-23 14:58:05

I also want to remind you that launching up to L1 from the Moon requires about as much fuel just to launch back to Earth or to other planets directly; going up to L1 and getting a push from there doesn't make much sense. The notion that you could somehow time and direct a stream of objects from L1 tward Earth with sufficent accuracy to pass through a particular flight corridor for an Earth-launched suborbiter is nonsense. Because they aren't fired simultainiously, they will never experience precisely the same gravitational pull.
You'd have to give these things a push from L1 to Earth first of all, second the projectiles must be launched before the suborbiter by days nessesitating a super-tight launch schedule, and finally you just couldn't prevent them from spreading out away from the meter-accurate trajectory you need to hit.

Just one point I want to make here. L1 remains in a fixed position relative to the Moon more or less. You can collect all the pellets at L1 in a way that O'Neill envisioned, then you can string the pellets together and let them all fall to earth together play it out and then seperate right before the encounter with the suborbiter then they all come in a stream quite close together. You could make it so the bulk of each pellet is oxygen gas processed from lunar materials all containe in easily vaporized hollow spheres. A puff of oxygen hits the impact shield and pushes on the suborbiter. I imagine the oxygen at such high velocities would tend to erode the shield, but that is replaceable.

GCNRevenger · 2006-09-23 17:35:26

No, again you add another hyper-critical timing event (projetile package separation) that you still can't aim with meter-scale accuracy. Regular mechanics are quite good with kilometer-scale accuracy, but slight variations in the Earth's gravitation field (its NOT constant), variations in upper atmospheric density (even the ISS experiences a little drag), and the fact that L1 is not completly stationary all adds up to an impossible shot.

And even then, when and not if one of the projectiles or its shrapnel fails to explode the impact at trans-Lunar velocity will shatter the suborbiter without an absrudly heavy shield. You fail to really comprihend the destructive power of even a small chunk of metal traveling at Mach 20+ I think.

And oxygen? If you store it as a pressurized gas, its density will be close low, so each projectile simply won't carry much momentum with it, requiring an unreasonable number of direct hits to reach orbital velocity. The momentum it does though won't be a gentle nebulous puff, an explosion is nothing more than a wall of supersonic gas, and the impact will be jarring... jarring enough perhaps to wreck the suborbiter or push it off course.

Tom Kalbfus · 2006-09-23 20:06:43

Didn't the Apollo capsules hit a similar wall of gas when they hit the Earth's atmosphere when returning from the Moon, that was in fact a greater velocity than the would be puffs of oxygen falling from L1, as that velocity was above the Escape velocity. if those Apollo capsules missed the Earths atmosphere, their velocity was such as to completely escape from Earth's gravity, yet those Apollo heat shields were sufficient to break in teh Earth's atmosphere and slow down from above the Earth's escape velocity to a gentle parachute-assisted splash down in the Pacific Ocean. You seem to be a pessimist in all this, you mention an engineering problem and then you wave your hands and say its impossible to over come. If all it is is a question of accuracy, then accuracy can be approved. The gas can be ejected from each capsule one way propelling the solid parts of the pellets in the other direction so the solid shells all miss the spacecraft and the space craft is only hit by expanding clouds of oxygen gas. We can do this first with unmanned cargos and when the safety is sufficiently proven, we can transport passengers by this method. And by this method we can lift additional supplies to the moon, including computer chips and pellet guidance systems to be attached to the pellets. So long as people wave their hands and say it cannot be done, we'll never find out if it can be. One big plus though, you don't need carbon nanotubes, so we don't depend on finding out how to build cables of sufficient strength.

I don't get the point of Lunar Space elevator, there is nothing on the moon and no one living there anyway. The problem is getting off Earth, and if we lowered the Lunar cable all the way to the Earth, we'd still encounter the strength problems we'd encounter in lowering a geosychronious cable to the Earth's surface, except that the cable would have to be much longer and stronger and it botom end would be moving in relation to the Earth surface. The cable would make a sonic boom as it dangled in Earth's atmosphere, as that is the equivalent of an airplane that circles the Earth at the equator about once per 24 hours.

GCNRevenger · 2006-09-23 22:21:45

No Tom, the difference is that Apollo's entry was comparitively gradual, but yours is the exact opposit, that it will be a relatively much denser, much more concentrated shockwave that hits.

Accuracy can be improved? Yes and no, there comes a point where achieveing a sufficent level of accuracy in something is simply not practical, or at least not affordable. Firing a missile from a metastable Lunar orbit that drifts around by the kilometers across a 150,000mi gulf through the interplay of three gravity fields (Earth, Moon, Sun), Earth's gravity varying slightly with orientation, and through the upper layers of the atmosphere... To hit a target only meters wide on an essentially unmaneuvering vehicle with perfectly consistant precision every time. Even being off aim a little bit would knock the suborbiter off the required flight path and would either have to abort or perhaps even be destroyed.

you must be completly out of your mind

The system will never be safe because it is not practical to ever acheive this level of consistancy and accuracy. All it takes is one failure of one projectile, nobody would ever use your ships. The risk will never be low enough for it to ever be reliable. Its like trying to make an airplane fly by shooting cannon shells at it from the other side of the ocean! Its just a bad idea.

"So long as people wave their hands and say it cannot be done, we'll never find out if it can be"

This is anti-rational, that there are lots of things we know that can't reasonably be done that we haven't tried, we know this because we can estimate how difficult they are; one of the principle ways of doing this is to count the number of things that have to go right for it to work, and how hard they are. For each "hard" thing you add, it decreases the reasonable-ness of sucess geometrically. Yours is one of them, too many difficult things all have to work exactly right, so we know that it can't be done.

Tom Kalbfus · 2006-09-24 09:57:56

How can you be sure that you know what is possible and what isn't? What if those pellets were solar sails of the same mass, and since they are so thin they could vaporize easily or perhaps they don't need to be vaporized? I think as they are, they could hit the spaceship flat on without concentrating any force on any particular location. Solar sails can manuever in space, simply by becoming selectively reflective and non reflective, they can altern their angle of attack on the sunlight and through this maneuving the sunlight and their maneuverings would push them on the proper course to hit the ship. And it just so happens that another starship idea involves laser sails and the sails themselves being used as reaction mass against a starship. One must be open to new ideas, to reject them out of hand because you think you know all their is to know about them risks rejecting a perfectly valid idea. I think some things you can't really know whether they will work or not, which is why you try them out, just like many people did up to the period the Wright Brothers flew their first airplane. Remember Aristotle, who thought he could deduce the entire Universe through reason alone? I think he missed the mark widely. I don't think chemical rockets which carry all their own reaction mass and energy supply will be the ultimate answer for getting of the Earth's surface. The Lunar Elevator doesn't really solve this problem, it instead solves a problem of getting people off the lunar surface, a problem that doesn't really exist. A terrestrial space elevator would solve this problem provided we can find strong enough material, but I think having a plan B in case this doesn't work out would also be a good idea.

cjchandler · 2006-09-24 10:25:34

Have a spat if you like but leave Aristotle out of it. Without his philosophy western civilization would be very different. While logic cannot explain the universe, it is necessary for everyday life and the scientific method, and therefore going into space , which depends on scientific research, needs logic and Aristotle.

Tom Kalbfus · 2006-09-24 23:25:19

Logic depends on the initial assumptions made. A person with the tiniest scrap of information can deduce a whole lot, which is what Plato and Aristotle did. The problem was, that tiny scrap of information was insufficient for making the logical deductions they did. One can assume from 19th century information that the Earth is the only thing in the Universe that does not move, because the speed of light is the same in any direction you measure it in. Since other objects are moving, it can be assumed that they would measure different speeds of light depending on which direction their measuring it in.

If you start with the wrong assumption, your logic can be flawless, but you end up with the wrong conclusion or as they say, Garbage in Garbage out.

Plato and Aristotle were not very big on experimentation, their idea was to withdraw from the world and remove all distractions of the material world so they can think purely logical thoughts in their world of ideas, this idea really didn't advance the Greeks too far I'm afraid. they did great in Math, since that did not require too much experimental evidence, but in the World at large, they came up with bizarre theories of a world made up of four elements called air, earth, fire and water, and that everything was made up of atoms of those four elements. This led many people to try to rearrange these atoms so they could make gold out of lead, because "surely these learned scholars from ancient times must be right, just look at thos giant marble monuments they built, anyone who could build that must be right, and thus we must stand on their shoulders and discover a way to turn lead into gold. - Alchemy."

idiom · 2006-09-25 00:23:10

Well Lead is Protons and Neutrons... sort of and Gold is also sort of Protons and Neutrons. So the logic was okay they just missed the possibility of a heirarchy or two (or three) being in between.

GCNRevenger · 2006-09-25 08:33:17

Oh come on Tom, you sound like that lunatic gaetano, just a bit more eloquent; its the same argument though, "oh but we don't KNOW it will/won't work until we try it."

Nonsense! That is a rejection of reason, and is counter to the very core principles of engineering. When lots of delicate, precise things have to happen at precisely the same place and precisely the same time, the onus lies on the inventor to prove why such a scheme that defies such huge probabilistic odds would ever work. "Oh but you don't know I won't win the lottery - three times" isn't a sane argument!

Solar sails? Solar sails are hard to build, and unless they are extremely thin then their net acceleration is essentially zero, so they would be essentially unmaneuverable. This mechanism to change the reflectivity and the angle of the sail also sounds too expensive to make in bulk. If they are extremely thin, they they don't carry much mass, and you'd need too many of them to impart momentum over suborbital flight time scales. Besides, they have to be huge by area to have any appreciable thrust to even maneuver, and if you have vast numbers of them in tight formation orbit, they would block eachothers' light. Solar sails also don't have 6-degrees-of-freedom maneuverability required either.

And even if you did get it to work, they would still carry the same momentum as a projectile of similar mass, and still impart the same jarring shock to the vehicle which nessesitates the heavy shock absorber mechanism. And the difficulty of getting it to hit exactly normal to the blast shield is much harder than just getting a projectile to hit, especially since the sail doesn't have the ability to maneuver. And even then, the aluminum sail would probably actually stick to the vehicle due to impact heating, increasing its mass and decreasing the acceleration, plus endangering the aerodynamics for reentry.

Oh! And don't forget the atmospheres' effect on the solar sails too, given their huge area and small mass the effects will be signifigant. Since the center of the sail probably is more dense than the outside, it would probably cause the sail to collapse with the differences in deceleration.

This is an even worse idea

One must be open to new ideas, to reject them out of hand

No. Even cursory analysis of this idea proves that its totally crazy, there is no need nor any sort of obligation to be "open" to obviously terrible ideas. There are a virtually unlimited number of awful ideas, which far far out number good ones.

cjchandler · 2006-09-25 09:27:13

I fail to see how aristotle's few innitial assumtions that were wrong, makes our innitial assumptions about the accuracy and presision of pellets, laser pulse power and accuracy, fluxuations in the earth gravity field, the destructive shock waves in a vaporised pellet and so on wrong. Another problem with a solar sail is that since they are so thin and dellecate, unless your suborbiter is the same size as the sail you will only get a little bit of momentum exchange, most of the sial will rip off and go past the suborbiter.

Tom Kalbfus · 2006-09-25 10:06:37

Gravity does most of the accelerating of the solar sails. They are not so thin that they can accelerate significantly. What the light pressure does is hold them on course as they fall toward Earth. These aren't the most efficient of solar sails that I'm talking about, they are only efficient enough to make tiny course correction to compensate for fluxuations in the gravity field and the spaces between the solar sails are timed to converge just before impacting with the spaceship, so that many thousands upon thousands of them impact with the spaceship in a short period of time. The sails converge much like an accordion. They manuever into position, their rotations and everything is taken into account so that they are right on target and hit flat on and normal. In the final moments before impact it doesn't matter if some are shadowing others, their courses have already been predetermined at much closer range than L1 by their maneuverings and they maneuver themselves to converge together prior to impact, so that the density of material hitting the space ship is appreciable and provides it with substantial acceleration. Much of the aluminun will evaporate upon impact as they are hitting it at a very high velocity, Perhaps some of it will stick, but also some of the shield will ablate and much more of the aluminum will spread outward as a high temperature plasma of aluminum gas. I think this all has to be computer modeled to get any more educated answers about the feasiblity of this.

I don't know GCNRevenger, you sound similar to one of those "man will never fly" people from the 19th century. Such a person would have said things like: "If you examine a bird's bones you will find that their bones are hollow, now a bird's muscular strength to weight ratio is much higher than a humans, so therefore, no matter how many feathers you can glue to a man's arms and no matter how hard he flaps them, he still won't be able to fly. I tell you this endeavor to build heavier than air flying contraptions is doomed to failure, and we should concentrate our efforts into building faster and more powerful steam locomotives. The steam engine is the wave of the future and its here to stay."

idiom · 2006-09-25 10:32:43

Wouldn't it be easier to make the pellets explode rather than trying to vaporise them? Sort of like an external Orion (big Orion) system.

Also, won't this fill LEO with a huge pile of debris?

Tom Kalbfus · 2006-09-25 12:02:32

Its probably possible to make Solar sails explode into some sort of gas. have two reactive parts that are separated suddenly come together for a chemical reaction that instantly vaporizes the sail. the sail is already spread out, so it may not be necessary to vaporize it to distribute its impact. Vaporizing them might distribute the gases so that its one continuous incomming stream of gas rather than bang bang bang off multiple solar sails hitting the spaceship. Doesn't matter too much, mass is mass, whether its a gas or a solid. Perhaps the gas may be allowed to expand a little more before impact initially and then later on when the ship gets up to speed the gas expands less so prior to impart to impart greater acceleration.

I'd hate for space travel to remain a spectator sport for the rest of the new century as the pessimists seem to have it.

cjchandler · 2006-11-02 19:22:50

I had an idea for rocket stages, and would like to know if anyone's thought about this before. If you built a large tube out of plastic or some mildly flamable material and fuilled it with a mono propellent like hydrogen peroxide that would give off extra oxygen, then could a sort of rocket engine plunger rig be built for the botom that would covver the bottom and slide up the tube, burrning the tank walls for propellent as it goes? Or maybe for a better isp use bipropellents with a similar boiling point (oxygen and methane?) seperated by a thin flexible ballon? This seems good to be true, so is there some major problem I'm missing here?

RobS · 2006-11-02 19:53:01

Sounds like it'd blow up to me! You're burning the walls that are containing the fuel; what if they burn too fast somewhere and burn through? How would you slide the "bottom" of the rocket upward against burning walls against the high pressure of the gasses? Slide it up too slowly and the bottom of the rocket falls off; my my, a mess!

-- RobS

cIclops · 2006-11-03 00:35:32

Sounds like it'd blow up to me! You're burning the walls that are containing the fuel; what if they burn too fast somewhere and burn through? How would you slide the "bottom" of the rocket upward against burning walls against the high pressure of the gasses? Slide it up too slowly and the bottom of the rocket falls off; my my, a mess!
-- RobS

Yes it's a good idea to use the inert mass of the rocket structure as fuel but it's hard to see a safe and workable engineering solution. The U-235 solar sail concept uses a similar idea, the sail structure is consumed as fuel, unfortunately it requires antimatter as the "oxidizer" which also has massive engineering problems.

Rob, your Mars story was a good read, thanks for putting it online! Thinking of a sequel perhaps with Kim Robinson as a coauthor?

C M Edwards · 2006-11-03 15:21:31

You're burning the walls that are containing the fuel; what if they burn too fast somewhere and burn through?

I imagine the same thing that would happen if you were launching a conventional solid fueled rocket with a metal combustion chamber. It would be the doom of any solid fueled rocket.

If the propellant were not the typical cast elastomer but something exceptionally strong, like high density polyethylene, it could hold the chamber walls in shape. That would allow the chamber walls to be very thin - just strong enough to keep the motor from exploding as the propellant burned away. This is desirable not to enable the survival of an expendable stage that is discarded regardless, but to allow combustion of every last bit of fuel.

The idea of a sliding nozzle would have to be abandoned, though, because accomplishing it without leakage would require reinforcing the chamber wall, adding more weight.

But I can see how using fuel to reinforce the combustion chamber could allow a higher mass fraction.

cjchandler · 2006-11-03 19:55:05

I guess I really didn't explain it properly, my idea was that a large plastic pipe opened on each end could be a fuel tank. The engine assembly, probably an aerospike engine would have a gasket and large ring so that it could just fit in the bottom of the pipe to be a plug, then at the top, the payload would cover the top of the pipe. The interior cavity created could be filled with oxidizer and fuel, and maybe pressurized to avoid turbo pumps, though that wouldn't be absolutely necessary. Then the bottom engine assembly would burn the propellant and as the tank emptied, could climb up the walls of the pipe, maybe by spinning and having the inside of the pipe threaded, of some other way. This would expose some of the pipe behind the engine, where it could be burnt sort of like a hybrid rocket, or since the engine will probably be an areospike just left there to burn off and reduce weight as it goes. If there was a problem with too much of it burning quickly, the outside could be covered with some fire retardant that the engine assembly scraps off as it climbs, but I don't think that will be a problem since it can only burn where there is oxidizer, that is at the end of the pipe.

dicktice · 2006-11-06 10:42:07

How about (getting back to simple-minded basics) just launching your supply rocket vertically through the atmosphere, retaining the horizontal component due to Earth's rotation from wherever you launch from ... vectored and timed to be where the Moon will be when it coasts there ... go into orbit and then land tail-first where the supplies and the used rocketship parts can be salvaged of junked by commercial interest on the Moon? Might be used as a one-way transportation system for emergency personnel. Or (one might as admit the possibility) as prisoner ships for convicts sentenced to "Transportation" at Lunar mining camps as alternative to death sentences on Earth. Let's kick that around for a bit: feasible? profitable? socially acceptable?

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