Adam has given the best answer so far in terms of fundamentals: sure this technology or that technology has not yet been funded, etc.. but WHY is it so hard to create a light-driven rocket instead of a gas or plasma-driven rocket? Adam explains it is because the ratio of POWER to FORCE is much larger for photons than for regular rocket exhaust. WHY is this the case?
Let's dig a little further into the fundamentals of momentum and energy of a particle.
Momentum for a gas molecule = m v (mass times velocity)
Energy for a molecule ~ 1/2 m v^2 (1/2 mass times velocity squared)
Momentum for a photon = h f / c (Plank's constant) x (frequency) / (light speed)
Energy for a photon = h f (Plank) x (frequency)
Remember that rocket thrust = FORCE = dp/dt = (momentum change per time)
While POWER = dE/dt (work or energy used per time).
Re-arranging the above terms for molecules, we can see that the ratio of POWER / THRUST for a gas molecule (rocket fuel exhaust gas) is velocity / 2. Actually there is an additional factor of 3 or 5 in there because in a hot gas there are 3 or 5 or more degrees of freedom due to thermodynamics; but that factor of 3 hardly matters for the argument that follows.
For a PHOTON, the ratio of POWER / THRUST is "c" the speed of light. At temperatures that rocket materials can withstand, rocket exhaust molecules move at most several thousand meters per second, while the speed of light is 300 million meters per second.
To conclude, the fundamental physics difference between a photon moving 300 million meters per second, and any gas atom --such as hydrogen, the lightest and fastest atom, moving 3000 meters per second at about 3000 Kelvin temperature-- means that to get the same thrust from a light beam you need roughly 100,000 times as much POWER. And there is your fundamental problem.
If I go to the government and say, I have two kinds of rockets that will accelerate a human being at "1 G-force acceleration". One of them requires 1,000 megawatts of rocket power, and the other rocket requires 100,000,000 megawatts of rocket power to achieve the same thing... which project do you think is going to get NASA funding? OF course-- the gas or plasma engine that is 100,000 times easier to build using existing technology.
Good luck with that "light-drive"-- I think it is far, far in the distant future for humanity... unless Adam's idea of "multiple light bounces" can be achieved in a practical geometry. Try to figure out a way to bounce a gigawatt light-beam between a planet and a rocket 100,000 times without significant dissipation. If you can do that, you can reach the stars... more easily than we can now.
Craft that use laser or solar radiation pressure? Or something else?
Missions that rely on radiation pressure, both natural and artificial, have been studied relentlessly in the past. The cost tends to be an inhibitor, and I was in contact with Colin McInnes of Glasgow for a time on this topic.
thank You very much for Your interest in the problem. This is the technologies of 1970s and they are not developed due to many things. I recently sent an E-mail to Propfessor Dr. Leik Myrabo from NASA who is looking for some money to complete the project. Also, Prof. Manfred Laxy and his Laxy Corp is the sole possessor of the so-called LTF and TF technologies. Prof. Manfred Laxy has stated that NASA has taken many things from the LTF technologies, se the attached file.
Indeed, 'lightcraft's use laser . Could You share with me what You did and the research and achievements of Professor Colin McInnes of Glasgow. I see that already many researchers deal with lightcraft technologies from Australia, Canada, USA, and UK. The technoloties are proven and work well, but there is no money to create a lightcraft. It is possible to create a lightcraft which can orbit Moon or Mars serving as a 5-star Hotel for 10,000 persons!!!!
The link to "howstuffworks" above, is actually not a "light sail", i.e. it does not use light as the pusher, rather it uses the light to create a plasma (in the atmosphere), which "explodes" behind the craft to propel it forward. Thus this type of craft could not be used beyond the atmosphere (unlike the Project Orion type spacecraft http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) ), as the light intensity on the craft in space would not provide the same "thrust". Light sail craft are much more useful for long-haul missions due to the very low thrust, but not human traffic. At Earth, in space, the thrust is a maximum of around 9 micro Newtons/ m^2. Which is pretty small. For comparison a Saturn V rocket produced 35 Mega Newtons of thrust to go to the moon in 3 days. Now of course less thrust just means more time is needed for the same acceleration, but to put massive human cargo on a solar sail will require very, very large sails to make durations bearable, which is where the problem becomes much more difficult (for now) than either chemical or even nuclear propulsion, which is why solar sails or light sails, driven by lasers, are not being funded. Now talk to me when laser diodes are a few cents /kW and then everything will be driven by lasers!
The work is public domain - I was informally chatting to Colin about it - a comparison between chemical propulsion and solar-sail methods for the Rosetta mission.
The usual caveats applied (which Kirk has ably listed), namely;
a) Vanishingly small thrusts (F is roughly = (1+reflectivity)*I/c, for intensity I), and thus either microscopic payloads or lengthy transfer durations.
We've all read Bob Forward's works on this topic, I'm sure.
b) Poor technology maturity - and the money simply isn't there to build a 1km^2 sail and learn the hard way how to deploy and control it.
So it goes.
Lobby your law makers.
Note, a number of space agencies have toyed with solar sails (JAXA, NASA Ames+Marshall) in orbit, but purely as technology demonstrators.
Kirk,
I'm with you on the utility of Orion. I've read Dyson's historical overview a few times and remain impressed as to its practicality. One merely (ha!) needs to rewrite a few pesky laws and get ready to stake some claims in the asteroid belt.
if I rightly understood, lightcraft engines relate to green technologies because no pollution and the work medium such as air will be not polluted outside the engine! Please, read it carefully! It is well-known that a vacuum has no air, but lightcraft can fly to stars in a vacuum. Air is a work material inside the engine!!!
James, thanks. In the "howstuffworks" the NASA group of Professor Dr. Leik Myrabo has demonstrated that a lightcraft can start from a planet. This actually demonstrates that a lightcraft can start from a planet and fly in space, but not to use light engines only for trajectory corrections. Also, Professor Manfred Laxy has stated that he possesses the authentic lightcraft technologies such as the LTF technologies which can move both cars and spacecrafts. Probably, these tech are different from that Professor Dr. Leik Myrabo developed. Toyota has a great interest.
Beamed power has been touted for many years as an energy source for propulsion systems. I'll date myself - I remember reading of Dr Myrabo's first work with beamed power back in the early 80s or late 70s and I remember the tail end of Dr Glaser's work on SPSS in, ooh, the late 70s - I wasn't old enough to join the L5 society.
Without doubt, beamed power is certainly possible as a propulsion system. Once one has collecting areas of the order of km^2 or so then respectable powers can be delivered to craft close to hand (1.2 lambda over d, that's in radians, multiply by 1AU... mumble) but probably not useful beyond a light second or so. Beam steering isn't that hard - but building a km^2 of collecting surface was, and will be, the challenge.
Till we have cheap access to orbit then I don't see orbital microwave stations. Chicken, meet egg.
For what it's worth, I spent a couple of years as a Fortran monkey working on SSTO craft, that's surely where effort should be focussed - without reliable access to LEO/MEO these power station-based schemes will remain paper studies. I reckon.
James, thanks. I have found something about the SSTO craft. They also use electromagnetic waves. Note that my research also deals with the electromagnetic waves coupled with the elastic waves for smart materials and devices. Also, I voted up all the answers because I respect hard work on a computor. I am familia with mankey working because I have a Diploma (MSc) for programming, this is a Russian red Diploma with excellence. Also, I prefer a MonkeyWare laptop with a 13GHz i9 processor than an Alienware laptop with a 3GHz i7 processor. Have a nice day!
There are two problems with pure light propulsion. A single newton of thrust requires 300 MW of light-power, if the light is used just once. That's the first problem.
Second problem: Bouncing the light between very reflective mirrors allows the thrust to be multiplied, increasing the efficiency of power to thrust conversion. If the light can be bounced 3000 times, then the power per newton decreases to 100 kW - but this needs to be dissipated by the optics. Plus a 100 kW beam isn't very healthy for anyone who crosses it.
thank You for Your answer. I always thought that there is some ecological problem in the light technology. However, I think that nobody will walk inside a light engine!
Adam has given the best answer so far in terms of fundamentals: sure this technology or that technology has not yet been funded, etc.. but WHY is it so hard to create a light-driven rocket instead of a gas or plasma-driven rocket? Adam explains it is because the ratio of POWER to FORCE is much larger for photons than for regular rocket exhaust. WHY is this the case?
Let's dig a little further into the fundamentals of momentum and energy of a particle.
Momentum for a gas molecule = m v (mass times velocity)
Energy for a molecule ~ 1/2 m v^2 (1/2 mass times velocity squared)
Momentum for a photon = h f / c (Plank's constant) x (frequency) / (light speed)
Energy for a photon = h f (Plank) x (frequency)
Remember that rocket thrust = FORCE = dp/dt = (momentum change per time)
While POWER = dE/dt (work or energy used per time).
Re-arranging the above terms for molecules, we can see that the ratio of POWER / THRUST for a gas molecule (rocket fuel exhaust gas) is velocity / 2. Actually there is an additional factor of 3 or 5 in there because in a hot gas there are 3 or 5 or more degrees of freedom due to thermodynamics; but that factor of 3 hardly matters for the argument that follows.
For a PHOTON, the ratio of POWER / THRUST is "c" the speed of light. At temperatures that rocket materials can withstand, rocket exhaust molecules move at most several thousand meters per second, while the speed of light is 300 million meters per second.
To conclude, the fundamental physics difference between a photon moving 300 million meters per second, and any gas atom --such as hydrogen, the lightest and fastest atom, moving 3000 meters per second at about 3000 Kelvin temperature-- means that to get the same thrust from a light beam you need roughly 100,000 times as much POWER. And there is your fundamental problem.
If I go to the government and say, I have two kinds of rockets that will accelerate a human being at "1 G-force acceleration". One of them requires 1,000 megawatts of rocket power, and the other rocket requires 100,000,000 megawatts of rocket power to achieve the same thing... which project do you think is going to get NASA funding? OF course-- the gas or plasma engine that is 100,000 times easier to build using existing technology.
Good luck with that "light-drive"-- I think it is far, far in the distant future for humanity... unless Adam's idea of "multiple light bounces" can be achieved in a practical geometry. Try to figure out a way to bounce a gigawatt light-beam between a planet and a rocket 100,000 times without significant dissipation. If you can do that, you can reach the stars... more easily than we can now.
Oh yeah, I just had to mention: Science fiction writer Larry Niven in his collection of short stories "Tales of Known Space" (1975) pointed out that, if you could create a gigawatt-light-beam with enough momentum transfer to accelerate rockets carrying human beings across space... you would have a hellishly powerful beam-weapon that, properly focused, could slice through any material at extremely long (planetary) ranges. So be careful what kind of rocket engine you wish for... it might turn out to be a "dual use" technology. ;-)
Indeed, many researchers state that almost all state governments have their interests in military utilization in the first queue concerning many new old technologies which are wellknown and are going to be forgotten due to no interest. This can be the answer on the question why light engines are not developed for non-military use.
My interest is to travel among stars or among the planets of the Solar system.
It is all about kinetic energy imparted to the remaining rocket from loosing fuel. The most optimal kinetic energy is imparted if the fuel leaving the rocket completely converts to photons thus imparting the highest momentum. The big problem with this mechanism is that as the matter completely converts to photons, it is very hard to control the direction of the generated energy. In fact it might need very clever mirroring/lensing to direct the leaving photon away from the desired path of the rocket.