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Alternative to Rockets
Henry Spencer wrote:
The idea is not ridiculous, but advanced propulsion in general is very poorly funded. The laser rocket suffers from needing a very large laser, which is a big up-front capital expense. Yes. The rule of thumb is that a laser to launch 1kg payload to orbit needs 1 MW. Another rule of thumb is that currently lasers cost order $1/watt and are getting rapidly cheaper. However a laser rocket might currently make more sense as a second stage- the power of the laser is proportional to the thrust required and the thrust required goes down with burn time, and after staging; this means that less than the 1MW rule of thumb is needed. In addition, fuel for laser rockets is very lightweight (pun not intended, often hydrogen is proposed), and so the first stage would benefit from not having to carry so much mass. For near-term applications, it is not obvious that the money needed for that wouldn't be better spent on improved application of conventional rocket technology. Possibly- but it's not completely clear. In particular laser launch usually deals with smaller orbital payload sizes than conventional rocketry sensibly can address. Whether there is a market for small payloads isn't all that clear however. |
#53
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Alternative to Rockets
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#54
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Alternative to Rockets
In article , (Greg) writes:
wrote in message The only extra gain you can have beyond the numbers I provided is by getting the sail tilted around the closest approach point, to get a force component along the trajectory. This only yields an advantage along a short fraction of the trajectory in the case described. The overall result doesn't chage appreciably. You're welcome to try and calculate. my point is that your *proof* was flawed. Your derived bound is not indeed a bound. It is close enough to a bound to not make any significant difference. Example: Tether pair of solar sails with accurate independant tilt control (also rapid), there speeds of rotation can simply continue to incress untill the teather breaks... You can play all the games you want, it doesn't matter. The scenario I described is one in which you already played *all the possible games*, within the system, using if needed multiple orbits to get yourself to the optimal *final pass*. And the optimal final pass is one where the excentricity of your orbit is as close to 1 as possible (i.e. sum of kinetic and gravitational energy close to 0) and your final perihelion is as close as survivable. And at this point you unfold your sail and go out. Remeber, that's the finite pass. So, what more you can do at this point? Well, the scenario I calculated for is one where you let the sail point straight away from the Sun. The optimal scenario (energy transfer wise) is one where the sail points halfway between the "straight away" direction and the "tangent to the orbit" direction. The difference between the two is negligible except in the immediate vicinity of the Sun. And, the contribution of this difference can be estimated as well. In the limit where the force on the sail is just barely larger than gravity, you can, by maintaining optimal orientation, improve on the final energy by a tad less than 30% and on the finite velocity by about 15%. But, as I said, that's for a poor sail. The better the sail (in the sense of area/mass ratio) the smaller the improvement since the better the sail the faster do the tangential and straight away directions approach each other. For a realisticall good sail you won't gain more than few extra percent above the previous estimate. And there are no more gains available. Using the reverse angular momentum manover speeds of up to 50 AU per year are possable with moddest tech addvance in solar sails. 50 AU per year is about the number I got (and no, we're talking serious, not modest advance). 210 km/s is 44 AU per year. And at this rate it'll take you a very long time to get anywhere. Sure you need to wait a long time to get to the nearest star. But it *is* doable. This kind of space exploration will need missions lasting many 10's of years and even lasting gererations. Can you calculate? No, it is not "10s of years" or "generations". We're talking below 0.001c. At this rate it'll take upwards of 4000 years to get to the nearest stars. That's comparable to the time since the construction of the pyramids till today. Even the assumption that the civilization which launched the probe will still be around when the probe gets somewhere is quite optimistic (based on prior data). Mati Meron | "When you argue with a fool, | chances are he is doing just the same" |
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