Phobos First!

[SteveMick]

In reading this thread I feel compelled to point out that a solar concentrator can have very low mass. The 40ft. dia. antenna flown in '92(as I recall) by the Shuttle weighed only 12 lbs. If used as a solar concentrator it would focus about 1.3KW/m^2 * .9 * 130.6 m^2 = 153KW or 100KW thermal at Phobos. With concentrator PV at 40% efficiency, the electrical output would be about 40KW. The PV cells have a mass of about 1KW/kg, so the total mass of this system will be a little more than 5.5kg(concentrator) + 40kg(PV) = 45.5kg. Therefore the specific power that should be used is closer to .8KW/kg not the much lower specific power of DS-1. I argued with the designers of that probe that large low mass concentrators should be used at the time.<br /> Common sense says that the difference in insolation between Earth and Mars can easily be made up for by simply carrying a low mass mirror. In addition, Sandia is now developing a concentrator based system to use extreme temp. to break down water directly(no electricity needed) so the higher thermal power and much lower mass of the concentrator will result in far lower masses for H2 and O2 production at Phobos.<br /> In addition this allows solar thermal rockets powered by the concentrator(at 800 sec.+Isp) and the concentrators can double as antennas for high baud comm. STR's can use almost anything as propellent although obviously lower atomic weight is desirable.<br /><br />Steve <br />Solar Thermal/Electric Propulsion<br />First STEP

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>In addition this allows solar thermal rockets powered by the concentrator(at 800 sec.+Isp) and the concentrators can double as antennas for high baud comm. STR's can use almost anything as propellent although obviously lower atomic weight is desirable.<p><hr /></p></p></blockquote>That sounds good, but can a concentrator still be as lightweight if it has to stand up to acceleration? At low acceleration, ion drives should have a clear ISP advantage.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

<I argued with the designers of [DS-1] that large low mass concentrators should be used... ><br /><br />Interesting. You should have argued with the designers of the DAWN asteroid mission instead. The DAWN probe uses the same SCARLET solar panel technology as DS-1, but scaled up to 10 kilowatt maximum power to compensate for the drop in sunlight beyond Mars. Solar concentrators might have been a lot cheaper and lighter than super-sized solar panels.<br /><br /><In addition this allows solar thermal rockets powered by the concentrator(at 800 sec.+Isp) and the concentrators can double as antennas for high baud comm. /><br /><br />Ah, a fan of STP! <br /><br />I myself was quite enamored of the concept and made several posts regarding STP in some earlier threads discussing Mars missions. Sadly I am not so enamored with STP anymore. <br /><br />It's been demonstrated to me that STP has to use low-acceleration flight trajectories similar to those that electric-propulsion must use and that that kind of trajectory penalizes effective ISP by about 50%. Since the maximum practical ISP of STP using liquid hydrogen propellant is only about 900 seconds, when you cut that in half you are no better off than using a conventional hydrogen/oxygen chemical rocket.

 

[SteveMick]

Great question. The balance between low mass and ability to handle acceleration is critical I'd say but the highest is likely to be .1 g operating in thermal only mode. This is not much of a problem but there is a situation that does involve 1 g or so. This occurs if STEP is used to raise the craft from LEO to escape using a series of thrusts at perigee to create a highly elliptical orbit. On the last pass oxygen can be added to the H2 to increase thrust to acheive a Mars bound trajectory. Since the solar electric mode can be used thereafter, the delta vee can be lessened, but to get max. advantage from thrusting when going fast at perigee you still need a gee or so. <br /> To deal with this I came up with an idea I called inflation insituform with degradable elements. The concentrator is first inflated to shape and while inflated has extra strength from gas pressure inside. After a period of time, the structural elements to remain harden upon exposure to UV. At the same time elements that merely contained gas or are otherwise superfluous, are degaded by UV and fall away or vaporize. The remaining structure can therefore be extremely complex and to take advantage of this I coined a term "fractal architecture" to describe what in its simplest form would be trusses made of trusses down to the microscopic level. In addition this makes possible a "light net" in which the molecular spacing in the thin film of the concentrator is less than visible light's wavelength range. This can reduce overall mass of both concentrator(by a factor of ten) and support structure.<br /> I wish there was a Centinnial Challenge for designing the lowest mass practical concentrator because I'm sure others could do even better.<br /> <br />Steve<br />Solar Thermal/Electric Propulsion<br />First STEP

 

[SteveMick]

This is simply not true! As I said in my previous post a series of perigee thrusts with a final chemical enhanced thrust overcomes these objections. Also a mix of electric and thermal can be used so that there is a further advantage.<br /> There are only two ways to go - solar or nuclear fission at present and on cost, development difficulty and time, specific power, etc. solar kills. The question then is how to use it. If you decide to use a concentrator mirror, then a host of synergies are created since it also can serve as a solar furnace, concentrator PV powerplant, antenna for radar and comm and wireless poer transmission(to the Martian surface for instance) and probably a dozen other things I haven't thought of.<br /> BTW the perigee thrusts have a rather big kinematic advantage especially on the last passes at just under escape velocity not a disadvantage. Also work by Joel Sercel in '86 resulted in 1100 sec. designs for thermal alone using hydrogen.<br />Steve<br />Solar Thermal/Electric Propulsion<br />First STEP