|
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Air Ship To Space?
In article ,
Allen Meece wrote: they [JPA] appear to need a positively miraculous L/D ratio to make this thing work as described, and there's no hint of how they could possibly do that. Correct about the no hint. They just say the "ion engine for the orbiter will be tested in the next five months." The doubts about the aerodynamics are actually independent of the exact engine type -- it's a generic problem that isn't sensitive to propulsion details. ...Let's get behind JPA and wish them success. So what if they can't or won't right now tell us how to make an ATO, Airship To Orbit. The concern is that they may not *know* how to make one: they may be kidding themselves about basic feasibility. Indeed, it rather looks that way. I wish them luck, and I remain open to the possibility that they've found some clever loophole... but right now I wouldn't invest money in it. It will be an affordable and wonderful thing to slowly accelerate to orbit. Space will become closer to all of us when this concept flies. Correction: "It *would* be" and "*if* this concept flies". -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#12
|
|||
|
|||
Air Ship To Space?
Jim Davis writes:
Henry Spencer wrote: The doubts about the aerodynamics are actually independent of the exact engine type -- it's a generic problem that isn't sensitive to propulsion details. But even if one ignores aerodynamics (ie, assume infinite L/D) the scheme doesn't add up. Take the following statements from their handout: "The third part of the architecture is an airship/dynamic vehicle that flies directly to orbit. In order to utilize the few molecules of gas at extreme altitudes, this craft is big. The initial test vehicle is 6,000 feet (over a mile) long. The airship uses buoyancy to climb to 200,000 feet. From there it uses electric propulsion to slowly accelerate. As it accelerate it dynamically climbs. Over several days it reaches orbital velocity." "Once in orbit, the airship is a spacecraft. With its solar/electric propulsion, it can now proceed to any destination in the solar system." "The ion engine 120,000 foot flight test for the orbital airship will be flown in the next five months." The airship in orbit has a specific energy of 32,000,000 J/kg. Taking several days to mean 4 days or 345,600 seconds that means the power source has to supply 92.6 W per kg of airship in orbit. Now using data from Larsen and Pranke an ISS 890 kg photovoltaic blanket produces 28000 W for a specific power 31.5 W/kg. So even if the airship were nothing but photovoltaic cells converting electricity to kinetic energy at 100% efficiency it couldn't possibly achieve orbit in several days even ignoring atmospheric drag. Surely the folks at JP Aerospace can do this simple calculation. So what is going on here? Among other things, the ISS 890 kg photovoltaic blanket is not the latest and greatest word in photovoltaic cells. For that matter, I'm pretty sure its mostly not solar cells at all, but support structure and wiring harness and protective coating and whatnot. There are credible solar power system designs in advanced development with specific power levels of ~200 W/kg. If such a system could be tightly integrated with the airship envelope, those numbers would actually add up pretty well, with your 92.6 kW/kg for the overall vehicle a not entirely unreasonable figure. What's going on here is speculation at this point, but the numbers start to fall apart not far beyond the solar power calculation you did. Assuming basic competence on JP Aerospace's part and thus looking for a minimum-number-of-discrete-miscalculations explanation, the simplest hypothesis I can find is that they A: assumed that COTS ion thrusters can be hooked directly to solar arrays, neglecting the requisite power processing units that are the heaviest part of the system, and B: used a specular rather than diffuse reflection model for rarefied gas aerodynamics, leading to the false conclusion that an arbitrarily skinny and low-alpha airfol can have an arbitrarily high lift to drag ratio. Those are actually two fairly common educated-amateur level mistakes in electric propulsion and rarefied gas dynamics, respectively, and if you grant those two mistakes I can almost make the numbers work for the rest of the system. Unfortunately, while direct-drive ion or plasma thrusters may be a theoretical possibility, specular reflection simply does not describe the way gas molecules behave in the relevant environment, period. -- *John Schilling * "Anything worth doing, * *Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" * *Chief Scientist & General Partner * -13th Rule of Acquisition * *White Elephant Research, LLC * "There is no substitute * * for success" * *661-718-0955 or 661-275-6795 * -58th Rule of Acquisition * |
#13
|
|||
|
|||
Air Ship To Space?
John Schilling wrote:
Among other things, the ISS 890 kg photovoltaic blanket is not the latest and greatest word in photovoltaic cells. For that matter, I'm pretty sure its mostly not solar cells at all, but support structure and wiring harness and protective coating and whatnot. I was using only the figure for the photovoltaic blanket. Larsen and Pranke break down the solar array as follows: Photovoltaic blanket 890 kg Mast 330 kg Gimbal 540 kg Electrical equiptment 610 kg Thermal control 730 kg Misc. integration 610 kg Total 3710 kg There are credible solar power system designs in advanced development with specific power levels of ~200 W/kg. If such a system could be tightly integrated with the airship envelope, Very, very tightly integrated. The solar cells are not only going to convert sunlight to electricity but be load carrying structures as well. those numbers would actually add up pretty well, with your 92.6 kW/kg for the overall vehicle a not entirely unreasonable figure. No, it does not make sense even then. Grant them the 200 W/kg power level. Assume the airship is made up of nothing but these cells when it gets to orbit. This means it the it has to convert 46.3% of the electric power to useful work to acheive orbit in 4 days. An ion thruster operating at an Isp of 5000 s over a delta V of 8000 m/s only converts a maximum of 15% of the electric power to useful work with about 15% of the initial mass as propellant. You can improve the situation by lowering Isp but then your propellant loads increase. At 1250 s you can theoretically get the required 46.3% but now 48% of the initial mass is propellant with the rest of the airship made up of nothing but solar cells. It just doesn't add up even ignoring the impossibly high L/D ratios required. Those are actually two fairly common educated-amateur level mistakes in electric propulsion and rarefied gas dynamics, respectively, and if you grant those two mistakes I can almost make the numbers work for the rest of the system. I would be interested in seeing what you can come up with those stipulations. Jim Davis |
#14
|
|||
|
|||
Air Ship To Space?
Jim At 1250 s you can theoretically get the required 46.3% but now
Jim 48% of the initial mass is propellant with the rest of the Jim airship made up of nothing but solar cells. Not that I think the concept flies, but: This airship wants to keep a particular airfoil shape as it rises. Perhaps the fuel for the ion thrusters is the buoyant gas. If half the initial mass is the buoyant gas, and they dump nearly all of that as the atmospheric pressure drops off, then the change in size of the airbag is diminished, which might be a good thing if you are trying to keep the shape constant. You lose buoyancy as you dump gas, of course, and as far as I understand you need a *lot* of velocity before your lateral velocity generates much centripetal acceleration on its own, so this doesn't balance out. As far as I know, COTS ion thrusters use heavy elements like cesium rather than light elements. Presumably lighter ions dropping across the same electric field will get higher velocities, so I would imagine that using hydrogen or helium as a fuel would involve higher Isp and lower thrust. Lower thrust seems very bad for something trying to punch out of the atmosphere, as you point out. Maybe they use space mirrors to boost the available sunlight! A gigantic airship might be a pretty good target for a LEO mirror, and a 10x increase in sunlight density, especially if you selectively target the more efficiently converted wavelengths, will markedly improve the power-to-weight ratio of the solar cells. But you'd need a lot of these mirrors to keep the power level up as they zoomed by overhead. Mirrors won't do squat for the power-to-weight of the ion thrusters or power handling equipment though. Maybe it helps a little that the gas in the bag is heated to high temperatures, but maybe that just makes the bag material problem even more ridiculous than it already is. |
#15
|
|||
|
|||
A more fundamental problem with this approach, is that the craft is
explicitly supposed to be *reusable*. If they exhaust any great fraction of their lift gas on the way *up*, how do they get *back*? They're going to need that H2 to keep from falling out of the sky as they complete their re-entry. Maybe not. The orbiter airship is a mile long and it only needed the gas to float while it slowly accelerated to orbital speed. Coming back in, it is not supposed to make a ground landing, [too big and fragile] but is going to make a stall landing at the high altitude base from which it departed. This might be possible with a little gas pressure and some aerodynamic flying, rather than floating down to base after all the speed has been scrubbed. ?? ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
National Space Policy: NSDD-42 (issued on July 4th, 1982) | Stuf4 | Space Shuttle | 150 | July 28th 04 07:30 AM |
European high technology for the International Space Station | Jacques van Oene | Space Station | 0 | May 10th 04 02:40 PM |
Unofficial Space Shuttle Launch Guide | Steven S. Pietrobon | Space Shuttle | 0 | April 2nd 04 12:01 AM |
Clueless pundits (was High-flight rate Medium vs. New Heavy lift launchers) | Rand Simberg | Space Science Misc | 18 | February 14th 04 03:28 AM |
International Space Station Science - One of NASA's rising stars | Jacques van Oene | Space Station | 0 | December 27th 03 01:32 PM |