Solar-pumped laser power transmission, a way to dramatically decrease launch costs?
in [Physics]
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From: Marvin the Martian on 8 Mar 2010 13:08 On Sun, 07 Mar 2010 12:50:27 -0500, J. Clarke wrote: > On 3/7/2010 11:53 AM, Marvin the Martian wrote: >> NASA has been looking into laser transfer of energy for propulsion >> since at least 1976. >> >> http://hdl.handle.net/2060/19760014427 > > Yep, and it doesn't address the main driver in launch costs. > > All these schemes aimed at saving gas are worthless until gas is the > major cost driver. Gas is the major cost driver? This isn't like driving a car, it has to do with the rocket equation, specific impulse of the fuel, and the percentage of the mass that can be driven to a given delta V. Specific impulse being the change in impulse for a given weight of fuel, and it turns out to be the exhaust velocity of the gases out of the rocket. The thrust, T = v * dm/dt, where v = g * I_sp and dm/dt is the rate at which mass leaves the rocket engine. However, the energy of dm at velocity v is dE = dm * v^2 /2 or power P = 1/2 v^2 dm/dt. That energy has to be in the fuel itself for conventional rockets. This limits the percentage of the mass of the rocket. Chemical fueled rockets top out at about an Isp =<450. (Note the ratio T/P = 2/v) IF you can get large amounts of power to the rocket from an external source, then: 1) You are not limited to reactive masses with a high I_sp. Any mass will do. 2) You can achieve a much higher exhaust velocity out of the rocket, which increases the payload to total mass of the rocket ratio. So, this laser idea has some merit, and that's why NASA has been looking at it for some time. It isn't about 'saving gas'. It is about physics.
From: J. Clarke on 8 Mar 2010 17:50 On 3/8/2010 1:08 PM, Marvin the Martian wrote: > On Sun, 07 Mar 2010 12:50:27 -0500, J. Clarke wrote: > >> On 3/7/2010 11:53 AM, Marvin the Martian wrote: > >>> NASA has been looking into laser transfer of energy for propulsion >>> since at least 1976. >>> >>> http://hdl.handle.net/2060/19760014427 >> >> Yep, and it doesn't address the main driver in launch costs. >> >> All these schemes aimed at saving gas are worthless until gas is the >> major cost driver. > > Gas is the major cost driver? This isn't like driving a car, it has to do > with the rocket equation, specific impulse of the fuel, and the > percentage of the mass that can be driven to a given delta V. > > Specific impulse being the change in impulse for a given weight of fuel, > and it turns out to be the exhaust velocity of the gases out of the > rocket. > > The thrust, T = v * dm/dt, where v = g * I_sp and dm/dt is the rate at > which mass leaves the rocket engine. However, the energy of dm at > velocity v is dE = dm * v^2 /2 or power P = 1/2 v^2 dm/dt. That energy > has to be in the fuel itself for conventional rockets. This limits the > percentage of the mass of the rocket. Chemical fueled rockets top out at > about an Isp =<450. > > (Note the ratio T/P = 2/v) > > IF you can get large amounts of power to the rocket from an external > source, then: > 1) You are not limited to reactive masses with a high I_sp. Any mass will > do. > 2) You can achieve a much higher exhaust velocity out of the rocket, > which increases the payload to total mass of the rocket ratio. > > So, this laser idea has some merit, and that's why NASA has been looking > at it for some time. > > It isn't about 'saving gas'. It is about physics. Seven paragraphs, not one of which addresses its actual effect on launch costs. Nice try at a straw man.
From: Marvin the Martian on 8 Mar 2010 22:01 On Mon, 08 Mar 2010 15:46:00 -0600, OM wrote: > On Mon, 08 Mar 2010 11:43:08 -0600, Marvin the Martian > <marvin(a)ontomars.org> wrote: > >>McCall is a first class idiot. >> >>The only way you don't win is if you don't killfile that jackass. > > ...Fred's the longest-standing resident in my killfile, having been > thrown in there *before* .history came into being. With the exception of > a few months where I'd given him a second chance at the urgings of one > other .history regular - a second chance he blew right out of the > starting gate as I predicted - that's a period spanning almost 20 years, > from the days when his lover's feud with Eric Chumpko derailed .policy > into the morass it's been ever since. I'm sorry for the suffering that McCall has caused you. > Then again, I will point out that Mookie's got the same rep with a lot > of other people, and he's been sentenced to Killfile Hell for almost as > long... > OM MR. Mook does real stuff. That calls for some respect. His ideas are often different, but in and of itself, that isn't bad. It isn't like he's saying Quantum Mechanics is bad because it disagrees with classical mechanics, or the moon is hollow, or some other baseless idiot thing. This laser idea is not even new and the physics behind it is the rocket equation. The payoff of getting something like this is so great that it is worth speculation.
From: Brad Guth on 9 Mar 2010 15:56 On Mar 8, 8:29 am, William Mook <mokmedi...(a)gmail.com> wrote: > On Mar 8, 10:46 am, Fred J. McCall <fjmcc...(a)gmail.com> wrote: > > > > > William Mook <mokmedi...(a)gmail.com> wrote: > > > :On Mar 7, 8:16 pm, Fred J. McCall <fjmcc...(a)gmail.com> wrote: > > :> I think you get some idea of how 'good' Mookie's numbers are by the > > :> simple fact that he claims that SSTO is 'NOT POSSIBLE' with > > :> conventional propellant types. There's lots of data out there from > > :> people who are actually knowledgeable in the field (unlike Mookie) > > :> that says otherwise. > > :> > > :> For example:http://www.dunnspace.com/alternate_ssto_propellants.htm > > :> > > :> Or perhaps:http://www.reactionengines.co.uk/downloads/JBIS_v56_108-117.pdf > > :> > > :> Mookie thinks that an ability to do arithmetic is sufficient to > > :> 'prove' things. It's not. > > :> > > : > > :Fred, is nuts. He's saying that a rocket with a 4.0 km/sec exhaust > > :speed and 13.4% structural fraction can be made into an SSTO rocket. > > :It can't. He'd know that if he knew how to do the calculations, which > > :he obviously doesn't. > > : > > > In other words, Mookie is rigging the numbers to come out the way he > > wants them to, just like he does with his 500 launches required Mars > > missions. > > > : > > :GIven these exhaust velocities and structural fractions - SSTO is not > > :possible. These values are based on actual data, they are soundly > > :based. This is supported by the fact that there are no systems flying > > :today that are SSTO. The SSTO programs supported by NASA over the > > :past 25 years are regarded generally as failures. > > : > > > I see. Mookie knows better than the experts, writing in the cites I > > gave. > > > : > > :Also, flight systems derived from SSTO research may not have the > > :reliability and safety required for piloted operation. One expert > > :said, we are trying to achieve the structural fraction of eggs (5% to > > :7%) - we shouldn't be surprised if the results are as fragile as > > :eggs. > > : > > > And which 'expert' would that be? Is his name "Mook"? > > > : > > :The destruction of a leading edge on the space shuttle made of > > :reduced carbon composites by a piece of foam flying at subsonic speeds > > :killed the SSTO program for NASA leadership because the reduced > > :structural fractions were going to be achieved by extensive use of RCC > > :throughout the vehicle. > > : > > > Absolute bullshit. > > > : > > :SSTO is not possible with today's performance with cyrogens. None > > :exist. According to Fred, that makes them fictional. > > : > > > Actually, you might want to go do some looking at this. The old Atlas > > was damned near an SSTO. Much less fictional than your imaginary > > maunderings. > > > : > > :Even so, the engineering argument is compelling. Shoud we spend a > > :significant amount of money to slightly improve these performances and > > :achieve SSTO? > > : > > :I think not. Because total cost isn't radically reduced. That's > > :because vehicle size is not reduced, and performance remains largely > > :the same. So, we are better off investing in laser rockets, which is > > :my point. > > : > > > Mookie thinks a totally untried technology will be CHEAPER to develop > > than merely enhancing what we have. No doubt he started running 9.5 > > second 100 meter dashes before he learned to walk. > > > : > > :Let's look at SSTO program as approached by NASA for the past 25 years > > :and see what might be possible if we assume someslight improvements in > > :structural fraction and performance. > > : > > :The argument is sound. Slight advances in the art, might make SSTO > > :possible. Reduced structural fractions to say 7%, and increased > > :exhaust velocities as achieved with flourine oxygen engines > > : > > > Once again we see Mookie jiggering the numbers. One of the cites I > > gave him specifically ruled out things like fluorine engines and had > > no problems getting to an SSTO. > > > : > > :- to 4.5 > > :km/sec from lift off through ascent - and we have a single stage > > :capability. The numbers given these slightly improved numbers are; > > : > > :Ve=4.5 km/sec > > : u = 7% > > : > > : Propellant Fraction 0.87055 > > : Structure Fraction: 0.07000 > > : Payload Fraction: 0.05945 > > : > > :Which is interesting. Obviously if structure were to rise to over > > :12.945% we would have no useful payload and SSTO is not possible. > > :Since I assumed 13.4% structural fraction in my cryogen case, and 4.0 > > :km/sec no useful payload is possible with those assumptions - which is > > :all that meant - and which Fred is trying to lie to us and say I meant > > :something more than that. > > : > > > So you jiggered the numbers to make your pet fiction look better. > > Anyone surprised? > > > Now he's going to go into a dance that makes it look like it's all > > about exhaust velocity. It's not. SSTO is probably actually easier > > with lower exhaust velocity fuels because you can lose all the mass > > margin required for handling cryo fluids in large quantities. A > > non-cryo fuel can have tank masses as low as 1% of the fuel weight. > > LH2 usually requires something up around 10%. LOX with any of a > > number of non-cryo fuels will double your payload to LEO. > > > : > > :For this SSTO system - which is only possible if exhaust speeds are > > :4.5 km/sec throughout the flight cycle, and only possible if > > :structural fractions of 7% are attained, a vehicle that lofts a 60 > > :tonne payload to LEO would mass 1,009.2 tonnes and mass 70.7 tonnes > > :empty. > > : > > :A two stage system with the same performance is smaller overall. > > :714.4 tonnes take off weight, 50.0 tonne structural weight - with far > > :more chances of success if performance or structural fraction > > :capabilities slip slightly. > > : > > : > > :So, with a program dedicated to improve structural fraction from 13.4% > > :to 7% and to improve engine performance from 4.0 km/sec to 4.5 km/sec > > :we have the following for a 60 tonne payload capability; > > : > > : 4.0 km/sec - 13.4% > > : > > : TOW structure > > : 4 stage 763.04 102.46 > > : 3 stage 1,665.45 223.65 > > : 2 stage 9,895.24 1,328.79 > > : 1 stage NOT POSSIBLE > > : > > : 4.5 km/sec - 7.0% > > : > > : 2 stage 714.4 50.0 > > : 1 stage 1,009.2 70.6 > > : > > :Total launch infrastructure remains about the same as a three or four > > :stage cryogen rocket of the older performance type, and total vehicle > > :cost is cut in half. The big issue is reusability. Can a vehicle > > :built with a very small structural fraction by flown again and again > > :and again reliably? If its easy to break, or even easy to dent, its > > :not going to have the robustness, or low cost that we're seeking. > > : > > :Even so, assuming we do our jobs right, it might be possible to attain > > :an SSTO capability, with no reduction in overall costs from a four > > :stage capability using the older technical approaches. Apply the > > :improved capabilities to a two stage system and we might cut costs by > > :30% or more. > > : > > :Applied to our Mars mission example from the previous post we can see > > :that total numbers are not radically changed by this approach. Even > > :if we cut costs in half, we still only get a footprint mission on Mars > > :for $100 billion in a decade- after spending a decade and another $50 > > :billion developing the technology. Far better to pay the russians to > > :adapt their launcher technology to the task and get the whole > > :footprint on Mars done in four years and $50 billion . > > : > > :So, this discussion of SSTO emphasizes my original point. For $30 > > :billion and a few years research, we can radically transform space > > :launch and our relationship to the cosmos while solving our energy > > :problems and raw material problems. > > : > > > Utter hogwash. Just look at what Mookie claims can be developed for > > that $30 billion and just what a crock it is becomes obvious to > > everyone but Mookie. He thinks it takes almost twice as much money to > > make small improvements in what we already do than it does to come up > > with a combined laser launcher and 1g lightsail combination. > > > The man is obviously batty as a belfry. > > > -- > > "Ordinarily he is insane. But he has lucid moments when he is > > only stupid." > > -- Heinrich Heine > > Interesting that thorughout all of Fred's objections he doesn't > provide any numbers of his own, or any real rationale for his > commentary. That speaks volumes. Fred knows nothing of the rocket > equation or how to use it to estimate system performance or put it > together in any meaningful way. All of Fred's ZNR friends (those of ARPA/DARPA and NASA) are no longer available, so he has almost nothing to work with. ~ BG
From: Brad Guth on 9 Mar 2010 16:06
On Mar 8, 10:08 am, Marvin the Martian <mar...(a)ontomars.org> wrote: > On Sun, 07 Mar 2010 12:50:27 -0500, J. Clarke wrote: > > On 3/7/2010 11:53 AM, Marvin the Martian wrote: > >> NASA has been looking into laser transfer of energy for propulsion > >> since at least 1976. > > >>http://hdl.handle.net/2060/19760014427 > > > Yep, and it doesn't address the main driver in launch costs. > > > All these schemes aimed at saving gas are worthless until gas is the > > major cost driver. > > Gas is the major cost driver? This isn't like driving a car, it has to do > with the rocket equation, specific impulse of the fuel, and the > percentage of the mass that can be driven to a given delta V. > > Specific impulse being the change in impulse for a given weight of fuel, > and it turns out to be the exhaust velocity of the gases out of the > rocket. > > The thrust, T = v * dm/dt, where v = g * I_sp and dm/dt is the rate at > which mass leaves the rocket engine. However, the energy of dm at > velocity v is dE = dm * v^2 /2 or power P = 1/2 v^2 dm/dt. That energy > has to be in the fuel itself for conventional rockets. This limits the > percentage of the mass of the rocket. Chemical fueled rockets top out at > about an Isp =<450. > > (Note the ratio T/P = 2/v) > > IF you can get large amounts of power to the rocket from an external > source, then: > 1) You are not limited to reactive masses with a high I_sp. Any mass will > do. > 2) You can achieve a much higher exhaust velocity out of the rocket, > which increases the payload to total mass of the rocket ratio. > > So, this laser idea has some merit, and that's why NASA has been looking > at it for some time. > > It isn't about 'saving gas'. It is about physics. Correct, and it's not even about cost when the return on investment is equal or greater. Mook isn't even the fly-by-rocket or laser-sail inventor, just the utilizer. ~ BG |