Dawn Mission, pre-launch disscusion |
Dawn Mission, pre-launch disscusion |
Jul 21 2007, 08:55 PM
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#331
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
You know, I've heard about this equation since reading Heinlein, but for some reason have never seen it. Would you be so kind as to post it, UG? Tired of being ignorant, here.... Try this: Tsiolkovsky rocket equation. Notice the delta V is proportional to exhaust velocity with fixed mass ratios, there is no time variable in the equation. -------------------- |
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Jul 22 2007, 01:30 AM
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#332
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Merciless Robot Group: Admin Posts: 8783 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
Simple yet powerful; thanks, UG!
-------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Jul 22 2007, 08:13 AM
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#333
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Member Group: Members Posts: 362 Joined: 12-June 05 From: Kiama, Australia Member No.: 409 |
QUOTE And it has been discussed in detail how much more energy it takes to ionize hydrogen than xenon The choice of xenon over hydrogen has very little to do with the ionisation energy, 15.4 eV vs 12eV is inconsequential when compared to the energy needed to accelerate the ions. It also needs to be remembered that if one looks at ionisation energies/ mole and lets assume they are roughly equal for H2 and Xe. Then the ionisation energy/unit of mass is 65X as high for H2 as Xe because of their different atomic weight. The ionisation energy / gram if far more important in calculating efficient use of power than ionisation energies/ mole QUOTE what can be done feasibly with our current technology. The limit of current technology has as much to to with the power source as anything else, that was the point I was trying to make. All the early work with ion thrust was done with caesium and mercury before they switched to xenon. So what have they all got in common? High atomic mass and the ability to be handled as fluids at a convenient temperature. They were after maximum thrust per kW. Cesium and mercury were abandoned because of electrode erosion. Maybe hydogen has a similar electrode erosion problem as well. QUOTE (impulse)...It reins supreme in terms of efficiency If that were our primary consideration then we should design a thruster that runs on nuclear energy and uses a high efficiency light source to expel photons at the speed of light. Our impulse goes up to 3x 10*7. Maybe that will be possible one day but to get any meaningful kind of thrust we are going to need an awful lot of power. QUOTE delta V is proportional to exhaust velocity with fixed mass ratios, there is no time variable in the equation Time constraints are however a practical consideration as I have outline above, no scientist wants to expire before his experiment in complete |
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Jul 24 2007, 05:27 PM
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#334
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
-------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Jul 24 2007, 05:34 PM
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#335
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
Time constraints are however a practical consideration as I have outline above, no scientist wants to expire before his experiment in complete Agreed, but this has nothing to do with what the rocket equation says and yet you were replying to my remark on it. Yes, thrust is important in certain regimes, but in other regimes the actual efficiency of the engine is more important. That's the gist of what the other Doug is trying to say I think. -------------------- |
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Jul 24 2007, 05:58 PM
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#336
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Member Group: Members Posts: 540 Joined: 25-October 05 From: California Member No.: 535 |
The unfortunate sight of seeing Dawn back in its processing facility is made up for by the photos of Phoenix now in place at pad 17-A. -------------------- 2011 JPL Tweetup photos: http://www.rich-parno.com/aa_jpltweetup.html
http://human-spaceflight.blogspot.com |
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Jul 25 2007, 07:55 AM
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#337
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Member Group: Members Posts: 362 Joined: 12-June 05 From: Kiama, Australia Member No.: 409 |
Agreed, but this has nothing to do with what the rocket equation says and yet you were replying to my remark on it. Yes, thrust is important in certain regimes, but in other regimes the actual efficiency of the engine is more important. That's the gist of what the other Doug is trying to say I think. I did wrap the reply to your post with that to Doug's and I suppose it should have been kept separate. I cant resist a comment on your last post and that it to say that "efficiency" is not the same as "impulse". I would venture to say that the hydrogen Shuttle main engines may in fact be more efficient than ion thrusters. Efficiency in engineering terms is the fraction of the input energy converted to the desired output energy. For the Shuttle main engines this is approx 50%. I cant find any accurate figures for ion thrusters but it would surprise me if it were much higher than this. Again the barrier we come to is the limitations on the power sources used today. |
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Jul 25 2007, 08:09 AM
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#338
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Member Group: Members Posts: 610 Joined: 23-February 07 From: Occasionally in Columbia, MD Member No.: 1764 |
I cant resist a comment on your last post and that it to say that "efficiency" is not the same as "impulse". I would venture to say that the hydrogen Shuttle main engines may in fact be more efficient than ion thrusters. Efficiency in engineering terms is the fraction of the input energy converted to the desired output energy. For the Shuttle main engines this is approx 50%. I cant find any accurate figures for ion thrusters but it would surprise me if it were higher than this. Again the barrier we come to is the limitations on the power sources used today. Ion thrusters have a couple of efficiency parameters. The electrical efficiency (beam power, or half mdot v squared, divided by power in) is often 90% or more. You waste a little bit in ionizing the stuff in the first place. There is also a propellant utilization efficiency - again typically over 90% - some propellant leaks out un-ionized, and some must be expended in a neutralizer to balance the charge flux. Of course, there is the question of propulsive efficiency - ion thrusters are great at putting power efficiently into the kinetic energy of the beam, which is not the same as adding kinetic energy to the vehicle. That efficiency is maximized when the vehicle speed approaches the exhaust velocity.... |
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Jul 25 2007, 08:55 AM
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#339
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Member Group: Members Posts: 362 Joined: 12-June 05 From: Kiama, Australia Member No.: 409 |
Found some information.
Thrust efficiency appears to range from about 40% to 75% depending on Impulse with about 60-65% at Isp 2500 http://www.engin.umich.edu/dept/aero/space...IAA-96-2973.pdf |
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Aug 4 2007, 11:11 AM
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#340
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Member Group: Members Posts: 540 Joined: 25-October 05 From: California Member No.: 535 |
Now that Phoenix is safely on its way, September 26 is being eyed as the new launch date for Dawn...
-------------------- 2011 JPL Tweetup photos: http://www.rich-parno.com/aa_jpltweetup.html
http://human-spaceflight.blogspot.com |
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Aug 6 2007, 02:56 PM
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#341
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Interplanetary Dumpster Diver Group: Admin Posts: 4404 Joined: 17-February 04 From: Powell, TN Member No.: 33 |
Slightly OT, but I have reprocessed Hubble's 1994 Vesta FOC image (well, a series of images, but only in one band and of one face). While there are certainly equally good (if not better) images, most of them are in the infrared or visible spectrum, while this is in the near UV. And I don't care what the silly scientists say, if you look at this one closely, it is obvious that Vesta is really a giant sculpture of a smiling pig.
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Aug 6 2007, 03:32 PM
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#342
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Senior Member Group: Moderator Posts: 2785 Joined: 10-November 06 From: Pasadena, CA Member No.: 1345 |
-------------------- Some higher resolution images available at my photostream: http://www.flickr.com/photos/31678681@N07/
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Aug 8 2007, 12:53 AM
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#343
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Solar System Cartographer Group: Members Posts: 10154 Joined: 5-April 05 From: Canada Member No.: 227 |
Nice, Ted! I haven't seen much done with FOC. Also I forgot to tell you how great the Miranda dark side pics were.
Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Aug 9 2007, 07:16 PM
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#344
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Interplanetary Dumpster Diver Group: Admin Posts: 4404 Joined: 17-February 04 From: Powell, TN Member No.: 33 |
Thanks. I also made this composite of the "Big-3"
Ceres and Vesta are HST FOC images, Pallas is from WFPC/2 (It is a super-resolution view, but the images are not from the PC chip). -------------------- |
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Aug 10 2007, 04:53 AM
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#345
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Solar System Cartographer Group: Members Posts: 10154 Joined: 5-April 05 From: Canada Member No.: 227 |
Nice one of Ceres. I'll have to compare it with the color set that appeared recently.
Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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