Solar Powered Space Yacht - revisited.

[Solman]

I have conceived a new space yacht using laboratory level thin film solar panels, the ELF-375 engine design, and allowing 100 tonnes for the balance of the spacecraft. I parameterized mast height and settled on 1500 meters. This configuration masses almost 500 tonnes fully fueled, giving a time to accelerate to the moon (8 km/s delta V) of 35.3 hours at high thrust.
 
Thin film solar http://www.nature.com/ncomms/journal/v3/n4/full/ncomms1772.html
 
Note that using 4.2 % efficiency and 4 grams per square meter for this thin film actually gives the power to mass ratio of 14.29 kW/kg instead of 10 kW/kg as given in the reference. With two 1500 meter masts and the resulting four 1500 meter booms, this thin film solar masses 18 tonnes and produces 257.3 megawatts.
 
ELF-375 engine http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA513936
 
The ELF-375 engine gives the highest electric engine thrust to weight ratio that I could find. The design point is 200kW giving high thrust of 95 mN/kW,  (Isp ~ 1500 seconds), and high Isp of 5000 seconds at 35 mN/kW thrust. The engines are still quite massive at 64 tonnes for thrusters and 115 tonnes for the PPUs. The run to the moon, 35.3 hours at high thrust, used 196 tonnes of Xenon propellant and ended with tanks empty. The same run at high Isp took 80 hours, a propellant load of 51.7 tonnes and again ended with tanks empty.
 
Mast height http://yachtpals.com/largest-sailboat-4155
 
Note that the Mirabella V is 765 tonnes, 75.22 m long, with a 14.8 m beam and an 88.5 m mast.  Look at the photos to see a really impressive luxury sailing yacht. The solar panels on my space yacht are configured somewhat like the sails on the Mirabella V, except that my space yacht has the second mast where the keel is on the Mirabella V and of course the deck is domed over air tight. Like the Mirabella V, my space yacht is for party cruises, but around the moon instead of the Caribbean.
 
Comments:
 
Point 1 - The engines are far too massive. What can be expected by way of mass reduction, (thruster, PPU), if the engine design point was 20 or 50 megawatts instead of 200 kW?
 
Point 2 - Xenon propellant from Earth is a show stopper. Lifting 200 tonnes from Earth to LEO for each cruise is not going to happen.

 In reading the referenced thin film solar cell paper I note that the reference is to solar cells and not to solar panels which the paper notes will require different design:
 "For small, laboratory-scale devices the silver grid is unnecessary and detrimental to device performance, but it will doubtlessly be required for large-area devices."
 I suspect this is a reference to the problem of collecting current from a large area. Am I correct in that the mass of the conductors required to collect and deliver this current to the engine system was not included in your mass estimate?
 Resistojets using hydrogen could take advantage of the Oberth effect if the high specific power of the lab device is not too much reduced in a large scale device. A far smaller panel but more propellant mass for that alternative but if there is ISRU you might choose that for your yacht.

Steve 

[aero]

I have conceived a new space yacht using laboratory level thin film solar

snip ...

Comments:
 
Point 1 - The engines are far too massive. What can be expected by way of mass reduction, (thruster, PPU), if the engine design point was 20 or 50 megawatts instead of 200 kW?
 
Point 2 - Xenon propellant from Earth is a show stopper. Lifting 200 tonnes from Earth to LEO for each cruise is not going to happen.

 In reading the referenced thin film solar cell paper I note that the reference is to solar cells and not to solar panels which the paper notes will require different design:
 "For small, laboratory-scale devices the silver grid is unnecessary and detrimental to device performance, but it will doubtlessly be required for large-area devices."
 I suspect this is a reference to the problem of collecting current from a large area. Am I correct in that the mass of the conductors required to collect and deliver this current to the engine system was not included in your mass estimate?
 Resistojets using hydrogen could take advantage of the Oberth effect if the high specific power of the lab device is not too much reduced in a large scale device. A far smaller panel but more propellant mass for that alternative but if there is ISRU you might choose that for your yacht.

Steve 

Yes, the reference is to cells. I used the word "panels" which they are not. I was searching for a word to describe a trangular shaped solar energy collection device that could be suspended by two edges between a mast and a beam. There doesn't seem to be one that won't cause a lot of confusion with the reader.

No, the mass of the current carrying conductor was not estimated and I suspect it will be significant. I don't know how to estimate it though. How would one go about developing that estimate?

Being able to benefit from ISRU would be great if the mass penalties of engines and propellant are not to great. I don't know about relying on the Oberth effect as I may wish to go places where there there is no deep gravity well to make use of for the return trip. I haven't thought it through.

[Solman]

aero - your response got me thinking about a solar sail idea from long ago which I think was called a "Heliogyro". It had several ribbon shaped reflectors that rotated around an axis to keep themselves flat. Something similar might work for the thin film PV and might save mass over trusses. I posted once about a SEV just trailing the ribbon behind it as it accelerated. This might work pretty well in interplanetary trajectories since you're usually not thrusting at too steep an angle to the sun but not too well near planets. You would have to accelerate fast enough to keep the ribbon from fluttering and I'm not really sure what to do with it when you're not accelerating. Deliberately rippling the ribbon might increase the area exposed to the Sun.
 Whether trusses or something else I sure would like to do some yachting.

Steve