|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Alternative to Rockets
George Kinley wrote:
Are there any way for rockets to fly in space , other then throwing mass out in one direction and moving in other If they don't work that way, then we don't call them rockets. One other possibility is a solar sail that uses the pressure of sunlight, but such a sail would have very low thrust. |
#12
|
|||
|
|||
Alternative to Rockets
In sci.physics Gordon D. Pusch wrote:
2.) Anything that does _not_ "throw mass out the back" (or more precisely, _momentum_) in order to accelerate would violate Newton's 3rd Law of Motion (AKA, the conservation of Momentum). In 300 years, _NO ONE_ has observed a replicatable violation of Conservation of Momentum. -- Gordon D. Pusch Ummm, how about "catching" momentum, i.e. a sail. Yeah, I know, it is still conserved. Homework problem: Given: A. A light sail. B. A light sail that is also a "solar cell" and uses the electricity to power an ion rocket. Assume equal mass for A and B (at the start), that everything is 100% efficient and your speed is nowhere near relativistic. At the start, do you get more "go" from B or are they the same? Why? -- Jim Pennino Remove -spam-sux to reply. |
#13
|
|||
|
|||
Alternative to Rockets
"Mike Miller" wrote in message om... "George Kinley" wrote in message ... 2) Mass drivers/railguns/coilgun launchers. Unfortunately, the big electromagnetic launchers that can fling a spaceship into orbit without turning passengers to goo is really, really long, like 600-700 miles for a 3G launch. As was pointed out in an earlier post to this web site, well designed and packed "cargo" can withstand hundreds to thousands of gees acceleration. A 1000G electromagnetic launcher would only be about 3 miles long. Also I seem to recall talk of a "supergun". The astronauts could go by a small cheap rocket - I think 5 astronauts plus short-term life support mass about one ton. |
#14
|
|||
|
|||
Alternative to Rockets
"George Kinley" wrote in message ...
Are there any way for rockets to fly in space , other then throwing mass out in one direction and moving in other Only if you can find a way around Newton's third law of motion. |
#15
|
|||
|
|||
Alternative to Rockets
"George Kinley" wrote in message ...
Are there any way for rockets to fly in space , other then throwing mass out in one direction and moving in other No. All the laws of physics forbid a reactionless system, ie there is *always* some reaction mass. But there are reaction based propulsion that give the same kinda results. Solar sails are my choice and we don't need nano anything to make useful spacecraft. Greg |
#16
|
|||
|
|||
Alternative to Rockets
On Mon, 08 Mar 2004 16:30:28 GMT, "George Kinley"
wrote: Are there any way for rockets to fly in space , other then throwing mass out in one direction and moving in other This isn't exactly in response to the question, but what is the current opinion on laser rockets? The idea is fairly simple, but I get the impression that currently it is not a very strong candidate. The latest news I found (and that was from November 2000) is that they've lifted a 51g craft to an altitude of 71 meters using a 10kW laser. "The 51 g, 12 cm diameter Lightcraft is propelled skyward when the laser beam hits a parabolic condensing reflector on its underside. This ablates a thin plastic coating, sending the craft upwards." (http://optics.org/articles/news/6/11/9/1) This thing is developed by a company called Lightcraft Technologies, Inc. (http://www.lightcrafttechnologies.com/news.html) as a method of launching micro-satellites. I wonder how they are currently doing - the "Latest Developements" section of their website is last updated in Dec 2000. The website describes the technology thus: "The back side of the craft is a large, highly polished parabolic mirror that is designed to capture the laser beam projected at it from the ground. The mirror focuses the beam, rapidly heating the air to 5 TIMES the temperature of the sun, creating a blast wave out the back that pushes the vehicle upward. As the beam is rapidly pulsed, the vehicle is continuously propelled forward, on its way to orbit." This seems to imply that no propellant is used as such, but the news article mentions a platic coating. If it does need to carry some kind of reaction mass, some easily produced and non-polluting substance would be necessary (ice, if it was practical, would probably be superb in terms of cost and ease of production). Laser rockets have a kind of simplicity that appeals to me, but I lack the knowledge of rocket or laser technology to form any concrete opinion about their practicality. -- Olli Wilkman |
#17
|
|||
|
|||
Alternative to Rockets
|
#18
|
|||
|
|||
Alternative to Rockets
"George Kinley" wrote in message ...
Are there any way for rockets to fly in space , other then throwing mass out in one direction and moving in other No. What's wrong with throwing mass? Check it out; for non relativistic speeds; F = mdot * Ve where mdot = kg/sec = mass flow rate Ve = m/sec = exhaust velocity (Isp*g0 = Ve) F = N = force (in Newtons, N/g0 = kg, g0=9.82 m/s/s) and the speed you can achieve is; Vf = Ve * LN(1/(1-u)) where Vf = m/sec = final velocity Ve = m/sec = exhaust velocity (Isp*g0 = Ve) LN(...) = natural logarithm function u = propellant fraction the propellant fraction is a number between zero and 1. Its the amount of propellant contained in the vehicle divided by the full up vehicle weight. Vf = a * t Vf = m/sec = final velocity a = m/sec/sec = vehicle acceleration t = sec = time D = 1/2 * a * t^2 D = m = distance a = m/sec/sec = vehicle acceleration t = sec = time E = 1/2 * m * Ve^2 E = joules = total energy in the propellant mass m = kg = mass of propellant Ve = m/sec = exhaust velocity P = 1/2 * mdot * Ve^2 P = watts = power of rocket engine mdot = kg/sec = mass flow rate Ve = m/sec = exhaust velocity Now here are some interesting things to know; ENERGY - this gives you Ve CHEMICAL ROCKET: Hydrogen/Oxygen - 15.7 MJ/kg -- 5.6 km/sec FISSION ROCKET: Pu -- Ac - 900 million MJ/kg -- 42,000 km/sec FUSION ROCKET: 4H2 -- 2He - 9 billion MJ/kg -- 134,000 km/sec ANTIMATTER ROCKET: Matter -- energy - 90 billion MJ/kg -- 300,000 km/s LASER LIGHT SAIL: Reflect energy - limitless - 300,000 km/sec Now, to achieve various missions requires that we achieve various speeds. Here are some important ones; This gives you Vf Minimums Earth Orbit: 7 km/sec Earth Escape: 11 km/sec Lunar Mission: 22 km/sec Mars Mission: 30 km/sec Solar System: 50 km/sec Cool Interplanetary 1/10th gee constant - inner solar system - 150 km/sec 1 gee constant - inner solar system - 500 km/sec 1/10th gee constant - outer solar system - 1,000 km/sec 1 gee constant - outer solar system - 3,000 km/sec Interstellar Minimum - 15,000 km/sec - alpha centauri in 100 years Reasonable - 90,000 km/sec - alpha centauri in 18 years Cool - 180,000 km/sec - alpha centauri in 10 years NOTE: 1 gee for one year equals the speed of light (300,000 km/sec) So, using these formulae and figures we can see some interesting things How big is your gas tank; Vf Chemic Fission Fusion Photon Laser Ve 5.6 42000 134000 300000 infinity MINIMUMS Earth Orbit 7 0.71 0.00 0.00 0.00 0.00 Earth Escape 11 0.86 0.00 0.00 0.00 0.00 Lunar Mission 22 0.98 0.00 0.00 0.00 0.00 Mars Mission 30 1.00 0.00 0.00 0.00 0.00 Solar System 50 1.00 0.00 0.00 0.00 0.00 Cool Interplanetary 1/10th gee inner 150 1.00 0.00 0.00 0.00 0.00 1/10th gee outer 500 1.00 0.01 0.00 0.00 0.00 1 gee inner 1000 1.00 0.02 0.01 0.00 0.00 1 gee outer 3000 1.00 0.07 0.02 0.01 0.00 Interstellar Flyby Minimum 1/20th c 15000 1.00 0.30 0.11 0.05 0.00 Reasonable 1/3 c 90000 1.00 0.88 0.49 0.26 0.00 Cool 2/3 c 180000 1.00 0.99 0.74 0.45 0.00 Interstellar One Way Minimum 1/20th c 30000 1.00 0.51 0.20 0.10 0.00 Reasonable 1/3 c 180000 1.00 0.99 0.74 0.45 0.00 Cool 2/3 c 360000 1.00 1.00 0.93 0.70 0.00 Interstellar Round Trip Minimum 1/20th c 60000 1.00 0.76 0.36 0.18 0.00 Reasonable 1/3 c 360000 1.00 1.00 0.93 0.70 0.00 Cool 2/3 c 720000 1.00 1.00 1.00 0.91 0.00 When the propellant fraction given in the table above exceeds 0.95, then the rocket is generally impossible since you need 0.05 structural fraction to build it. When the propellant fraction in the table above falls below 0.05 then the propellant fraction approaches that of a typical automobile. Another interesting table is how powerful is your engine? VEHICLE SIZE Auto Airline Supertanker Type Ve W/kg HP/lb 1000kg 3E+5kg 500E+6 Chemical 5.6 2800 1.71 2.8E+6 840E+6 1.4E+12 Fission 42000 21E+6 12800 21E+9 6.3E+12 1.05E+16 Fusion 134000 67E+6 40840 67E+9 2.01E+13 3.35E+16 Photon 300000 150E+6 91433 1.5E+11 4.5E+13 7.5E+16 So, the interesting thing is that a chemical rocket needs 2.8 MWatts to move an automobile sized rocket around but to apply the same forces with a fusion rocket requires over 30,000 times as much energy! To compare these usage rates (in Watts here - there are 745.7 watts per horsepower if you wish to convert them) to typical rates here are a few interesting numbers; US Household - 833 watts 200 hp engine - 149kW - 149e+3 Total US utlity net - 800GW - 8e+11 Total humanity - 4TW - 4e+12 So, a photon rocket adapted for use in a home automobile hovering in place would consume as much energy as about 1 billion people! Such a rocket would have to be pretty damn efficient not to be dangerous. One way to achieve that - since we're dreaming and not building it right now - is to use something that transfers momentum and nothing else - something like neutrinos for exhaust, or maybe a graviton rocket - which are both versions of a photon rocket since both gravitons and neutrinos are massless. Industrial humanity with all of its fuel fired technology when compared to a truly space faring species would seem as a tribe huddled around a rather small campfire in the wilderness. Our fighting over oil and such would be akin to people huddled around that fire arguing over this or that ember in the campfire. Laser sustained rockets and laser light sails are an interesting innovation as are microrockets. Micro nuclear rockets might also be interesting. Or solar pumped lasers in space (the sun is a fusion reactor) driving laser rockets and laser sails around would also be interesting. http://clifton.mech.northwestern.edu...crorockets.gif http://www.grc.nasa.gov/WWW/RT1998/i...schneider2.jpg Basically, you have modified inkjet printheads so that they blow rocket propellants through shaped holes rather than ink onto paper. With this technology you can create utterly reliable, utterly safe, quiet, highly controllable propulsive skins for vehicles. With a laser powered, or nuclear fusion powered, microrocket skinned spaceship you could hop in a vehicle and fly to mars in a few days, while feeling 1 gee acceleration all the way. You might meet up and dock with larger hotel like vehicles to while away your time as you traverse the expanse between worlds. In short, with the right rocket technology, interplanetary space is as accessible to humanity as ocean travel by ship makes the continents available. You can cross the solar system in a week at 1 gee. That's not too shabby. It takes a lot of power to sustain this lifestyle. But, there is lots of power in the sun, and there are lots of nuclear fuels available in the gas giants. So, what are we waiting for? Is the squabbling over dwindling resources here really stopping us from the greatest adventure, and greatest advance of all time? Yes, yes it really is. And that's a damn shame - and a failure of vision of those who believe they lead us. Phht! |
#19
|
|||
|
|||
Alternative to Rockets
One interesting alternative might be what could be called a "recursive
trebuchet": essentially, a highly damped and well-controlled whip. On the surface of a planet, anchor a catapult with a very short moment (swing), perhaps less than one degree. On top of that, put a smaller catapult, and another on that, and so on. If kept static, such a structure could not exceed 10-12 km in height because of limitations of structural strength, but if it were kept in continuous motion, the sum of centrifugal forces balanced against the pull of gravity might allow something of considerable length and enormous flexibility and strength. If it could be made long enough to allow its tip to describe an arc of, say, 1000 km, acceleration of the tip to orbital velocity might be kept down to the vicinity of three gravities. This would be a very large structure, and its mass would probably allow the launching of enormous projectiles. Think of capsules the size of cruise ships being plucked from the ocean and flung into space; that will give you a notion of the scale. Jim McCauley |
#20
|
|||
|
|||
Alternative to Rockets
"George Kinley" wrote in message
... Are there any way for rockets to fly in space , other then throwing mass out in one direction and moving in other Throwing planets often helps a lot. Mark Folsom |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Alternative to Rockets | George Kinley | Science | 53 | March 31st 04 02:45 AM |
Pressure fed versus pump fed rockets | Larry Gales | Technology | 16 | November 19th 03 11:18 PM |
Rockets not carrying fuel. | Robert Clark | Technology | 3 | August 7th 03 01:22 PM |
"Why I won't invest in rockets for space tourism ... yet" | RAILROAD SPIKE | Space Station | 0 | July 30th 03 12:06 AM |
Rockets | George Kinley | Technology | 37 | July 17th 03 01:18 AM |