Talk:Nuclear photonic rocket

Too bad Sanger wasn't around for the invention of lasers. See my discussion of the relativistic rocket problem. http://en.wikipedia.org/wiki/Talk:Relativistic_rocket

(Onerock 23:40, 26 Aug 2004 (UTC) At short range, the force of the laser beam could be multiplied by bouncing the light back and forth repeatedly between two retro-reflectors. This would provide a good take-off boost until the reflections start missing the reflectors. If a laser gun and one reflector are on the moon and the other reflector is on a projectile a short distance away, it might be possible to impart a significant amount of momentum to the projectile.

A solar light sail can be propelled by the pressure of ultra high energy laser or maser beams. A solar light sail can use natural sun light to achieve a velocity of 300 to 3000 km/sec by flying within .10 A U (930,000 miles ) of the sun. The laser or maser beams would be based on earth , or the inner solar system . If the laser or maser beams have enough power they can accelerate a space craft to near the velocity of light.

Unfortunately, light is made up of waves and obeys the law of diffraction. To focus light beams over interplanetary distances requires apertures of hundreds of kilometers in diameter. DonPMitchell (talk) 00:27, 5 July 2008 (UTC)Reply

I've just substantially rewritten this article, and I'm left with a number of questions we should try to address:

• Why is a hotter temperature better? Certainly the blackbody radiation flux density goes up as something like the 4th power of temperature, but is this important? 10W of infrared photons carries the same momentum as 10W of ultraviolet, so making things hotter seems to just make things harder to deal with. (probable answer (not clear from article): Higher temperatures produce more photons because the greater difference in temp between the emitter and the background means more photons travelling outwards - radiative cooling goes up as the 4th power.) (121.222.165.152 (talk) 09:52, 12 August 2008 (UTC)Another Answer - hotter means the radiating area is less for the same power level. A radiator 10 times hotter needs to be 1/10,000th of the size. The bulk of vehicle mass is probably tied up in the radiator.)Reply

• Why convert to electricity then back to thermal energy? If we convert nuclear heat to electricity surely a more efficient emitter of photons (such as a gas discharge light) would make sense. Otherwise, why not just have the core radiating directly as exhaust? (Answer: This is a nuclear light bulb - NLB)

• Are lasers better still? A laser does not require collimation to the same extent and could perhaps be directly pumped from the reactor. (Answered; however the physic faq states that a direct antimatter graser (gamma ray laser) would be the best choice, which is basically my intuition. We should distinguish between theoretically uncertain, theoretically possible, and existing technologies carefully in an article like this)

• Where is the line between photonic rockets and Fission-fragment rocket? (Partially answered: What is the advantage of hanging onto spent fuel? If spent fuel is worthless why not use it as exhaust?)

• Are non-nuclear photonic rocket plausible? (Answered: yes, but not practically)

• Considering the huge energy requirements, where does a photonic rocket make more sense than say an ion drive or non-photonic nuclear drive?

njh 05:21, 31 December 2005 (UTC)Reply

Ok, thanks for the corrections, there are still issues, I believe:

• Why is cooling and aux power a show stopper for a NLB rocket? If I have a NLB generating 10GW of light out the back I can always mount a few solar panels and illuminate them through a tiny hole in the mirror. Cooling is best done in space with radiation, which is what we're doing! So I don't buy those arguments.

• Saying 'For this reason pure nuclear light bulb type photon engines are not used.' sounds a bit like 'We don't use warp drives because we don't have transparent aluminum', we currently don't use any serious nuclear propulsion. I've removed this sentence.

Thanks Tmayes1965 for answering some of my questions! njh 23:02, 3 January 2006 (UTC)Reply

• It doesn't make sense to me to say that a NPR has an Isp = c, just because the exhaust is photons. While technically correct, it's misleading, since you have many times that amount of nuclear fuel you need to carry. It seems to me it's better viewed as Ns / kg, where the mass measured is the nuclear fuel carried, not the photons being expelled. On a similar note, it would actually be *more* efficient to expel the used fuel, for two reasons. First, it reduces the final mass to expel used fuel along the way, thus improving mass ratio and delta-V. Second, if the energy is imparted into the additional mass and so the nuclear fuel is expelled as exhaust mass, then the Isp goes up -- the same energy spread over 4 times the mass produces half the velocity but twice the momentum, and therefore twice the impulse. Evand 21:11, 19 March 2006 (UTC)Reply

The article says that a 100 ton space ship using 3 Mw of laser power for propulsion would achieve speeds of over 500 km/s over 20 years. However, when I tried to calculate the speed that the vehicle would reach, the result I came up with was that even 100% efficient lasers would only achieve a pitiful 126 m/s. I may have made a mistake, but I think that the article is wrong.

The setup mentiond in the article seems like a pretty foolish way to build a rocket. Why use millions of very small lasers instead of a few big ones? And why use lasers at all when nuclear light bulbs can produce a much higher specific power and ion engines can convert electric power into thrust much more efficiently?--Todd Kloos 06:21, 15 August 2006 (UTC)Reply

I get the same result. Accordingly, I have deleted the section, which really had very little of value after that. Evand 04:15, 17 August 2006 (UTC)Reply

I had been just about to fix that error. The information was based upon a old article I had on my computer, and I only recently tried to check the maths myself. I posted it because it is a way such rockets can be built, and 'laser rockets' are often cited in science fiction. Even the most optomistic fission-photonic design I could come up with can only achieve 50kmps. Given the number of articles on nuclear propulsion, would an article highlighting the fundimental limits of such technology be useful? The Rocket page contains much information in this reguard. Even if you assume advanced technology or new physics technology (Nuclear magic rocket?) the very limits of the fuel itself stop you from building the kinds of engines you get in space opera. Upshot of the article being if you make you engine too powerful you'll irradiate your crew, melt your engine or make it a mile across, and could use your fuel up very quickly. Given the lack of direct information about nuclear fuels, I'm not sure this point is made sufficiently clear. ANTIcarrot 12:43, 18 August 2006 (UTC)Reply

The accidental radioisotope rocket produced by New Horizons follows the same principle, though of course on a far smaller scale.