[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

*To*: starship-design@lists.uoregon.edu (Starship list)*Subject*: Re: Re: The fuelsail is stupid (was starship-design: Hull Materials)*From*: kuo@bit.csc.lsu.edu (Isaac Kuo)*Date*: Mon, 17 Nov 1997 06:52:29 -0600 (CST)*In-Reply-To*: <971116235811_-1607580981@mrin52.mail.aol.com> from "KellySt@aol.com" at Nov 16, 97 11:58:12 pm*Reply-To*: kuo@bit.csc.lsu.edu (Isaac Kuo)*Sender*: owner-starship-design@lists.uoregon.edu

KellySt@aol.com wrote: >>In a message dated 11/15/97 5:31:48 AM, kuo@bit.csc.lsu.edu wrote: >>>>So there you have it. If the fuel/sail concept is even possible, >>>>there is no good reason to do it. Even if the scoop I mentionned >>>>weighed 10 _times_ as much as the payload, you'll probably save >>>>in the cost of the laser more than you'll save in the cost of the >>>>lithium. >>In this design I assume that the ship itself has to maneuver in order >>to catch the fuel packages. I assume this requires 50% more fuel >>just to catch the fuel packages! And I assume the scoop used weighs >>as much as the rest of the payload! >>This increases the fuel requirements by a factor of 6. >>However, considering the laser's cost is reduced by a factor of 100, >>it's easily worth it. Consider that the laser will weigh many >>magnitudes more than the fueled starship in the fuel/sail design. >Two assumptions I'm not comfortable with. One: given the major delta V >requirements for manuvers to intercept fuel ships, assuming they are close >enough together that the mainship could intercept them (were talking about >fractions of light speed and potentially light minutes of lateral drift. So what? The amount of fuel expense I assume would be enough to handle light _days_ of lateral maneuvering. Remember that this ship is using an entire 1/3 of its fuel capacity in order to make the 200 intercepts. To a rough degree, this allows 4/400 light years of maneuvering (assuming the design target system is Alpha Centauri, 4 light years away). I am assuming what I consider to be absolutely obviously worse than would ever be the case. >Two: the power savings of the laser array might be minimal. Wrong. The power required is proportional to the mass of the sailships. >intercept with the followon fuel 'ships' kinda forces you to use far higher >boost rates so you get up to speed closer to sol, and fusion motors with >higher thrust to boost faster. Why are higher boost rates required? Because of diffraction limits, you need to boost with the laser near sol to begin with. Besides, even if we assume the boost rate needs to be 10 _times_ as much, the new design is _still_ only about 10% of the cost of the original. The fusion motors already need to be relatively high thrust in order to complete the deceleration run in a reasonable amount of time. I doubt any increased thrust requirement is going to increase the payload mass by more than 100% more (the assumed increased payload mass including the scoop). You have to show that these factors some how add up to making it cost at least 50 times as much before the original fuel/sail design becomes competitive. >Also since the fuel and ship have to meet at >nearly the same speed and position (at least within 1% or less) the fuel will >need to be launched at the same time the ship boosts out. In this design I do assume the fuel and ship need to meet at around the same velocity. I also assume the intercept is accomplished entirely using the ship's maneuvering (which is ridiculous, of course). That is why I assume the ship blows such outrageous amounts of fuel making those intercepts. However, the fuel does _not_ need to be launched at exactly the same time as the ship. If we assume Starwisp like acceleration runs with the laser, 200 shots will take about 400 weeks. Since the laser isn't used to power the ship, these can start before and during the ship's acceleration run. (The ship catches up with the shots fired before/during the acceleration run; the shots fired afterward catch up with the ship.) If half of the shots are shot before and half afterward, they'll only be about 200 weeks off. I would continue with back-of-the-envelope calculations to show you that these fuel shots could be intercepted for far less than the amount of fuel I describe, but now that I think about it it isn't worth it, because you probably will simply dismiss it without even conjuring up a single rough number estimate to back your claims up. Kelly, if you want to talk numbers, then this argument can go further. It's all about overall cost. I have made a bold claim--that the modified design is about 1% the cost of the original fuel/sail design. I've given at least rough calculations to show why. If you want to dismiss my argument, do it with NUMBERS. >Since your talking about the same total weight, 'sailing' out at >about the same time as fuel/sail (but with far greater delta-V >needs and dry weight for the ship) No, the total weight in the modified design is 6 times as much as the weight before. Pay attention. As I recall from the discussion of the acceleration/deceleration track ramjet design, you show difficulty paying attention. >But fuel/sail doesn't need to boost out >at the same rate since it doesn't need to do its bosting (and the fuel >intercepts) close to the sol system. Actually it does require acceleration close to sol because of diffraction limits. How close? You tell me how big you want the huge honking laser's lens to be, and what frequency it's using. >All in all, I'ld expect fuel sail to use less maser power to boost out. Back it up with some numbers. I don't ask for precise ones, just rough guestimates. >>>The expence of the launching maser platforms >>>is considerable, but the power levels and fusion motor needed are less, >>>acceleration can take place over longer times, and you don't need any >precise >>>intercepts. >>The power level of the maser platform is _much_ higher than anything >> else in the entire system. What in the world in the entire starship >> system has comparable power levels? >I was reffering to the power needs of the maser launcher platforms, and those >of the fusion boost motor. Okay. But if you really did _any_ numbers on this design, you'd see that the power levels of the masers were _much_ higher than that of the fusion motor. Really, this should be quite obvious. If the on board fusion reactor produced about the same power level as the masers, then it could be used directly to power a laser in order to make the deceleration run (and/or the acceleration run). But that's completely ludicrous. >> Assuming the new design requires a scoop which weighs as much as the >> entire payload, the fusion motor only needs to be twice as powerful. >The higher acceleration rates needed by the main ship would probably require >the motor be scaled up more than that. How much more? >>>D-D or D-T need a tank (one likely to outweigh the unfueled starship), will >>>boil off into space over the years, and are very rare and expensive. > Lithium >>>is extreamly common and cheap (well under a dollar a pound assuming you >>>refine medical grade Lithium to Lithum-6), can be chemically bonded with >>>hydrogen to carry it, and can be used as a structural metal. >> However the simple fact is that if you can't achieve Lithium-hydrogen >> fusion, you can't use it. Period. D-D or D-T fusion will be acheived >> much sooner, probably. >Possibly, but the development time can be accelerated considferably given a >R&D budget. Given the needs of this project would require a huge budget, the >funds to develop a fusion system to our needs would be trivial. That's not the way science works. You don't just funnel in money, and get results proportional to your money. No matter how much you spend, R&D requires experiments which take a certain amount of time to accomplish. And even then there's no guarantee of success. The fact is that we could build a vehicle powered primarily by D-D reactions _today_ (a crude Orion rocket). Within 10 or 20 years, we could even do a pretty good job of it (a refined MagOrion). By the time we can even consider a manned interstellar mission, it should be easy technology. The fact is that there is no telling how long it would take to power something with lithium fusion, no matter how much money is spent researching it. How long do you think it will take, assuming we spend 100 billion dollars a year on fusion research specifically aimed at acheiving lithium fusion? Rough estimate? Ten years? 100 years? 500 years? >Oh I agree by the way that the cost of the maser stations to boost the ship >out would be the dominent cost factor of a maser sail systems, but don't >think you variation would reduce that. Back it up with some numbers. Just rough guestimates. -- _____ Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo __|_)o(_|__ /___________\ "Mari-san... Yokatta... \=\)-----(/=/ ...Yokatta go-buji de..." - Karigari Hiroshi

**References**:**Re: Re: The fuelsail is stupid (was starship-design: Hull Materials)***From:*KellySt@aol.com

- Prev by Date:
**Re: Re: The fuelsail is stupid (was starship-design: Hull Materials)** - Next by Date:
**starship-design: RE: Moore's Law & Vinge's Singularity** - Prev by thread:
**Re: Re: The fuelsail is stupid (was starship-design: Hull Materials)** - Next by thread:
**Re: Re: Re: The fuelsail is stupid (was starship-design: Hull Materials)** - Index(es):