Mro On Approach, TCM-3 not required |
Mro On Approach, TCM-3 not required |
Feb 17 2006, 04:05 AM
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#31
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Senior Member Group: Members Posts: 2542 Joined: 13-September 05 Member No.: 497 |
Hmmm... using the "ideal" kernel gives periapsis on 15MAR2006 at 06:24UTC, 399km, at 67S 28E, although it's on the night side so the picture is dark. Pretty big difference there. The software I'm using is a C program that I wrote to use the CSPICE library--so there could be a bug or three there. The same program works pretty well with Cassini, but at least in that case I have actual images to compare against for testing. Choice of kernels seems to be a big factor. That result sounds pretty close to ours, so I'd say your code is working well. Those kernels just appeared on the NAIF website and I don't know what sort of burn performance differences they represent; it would surprise me if plausible burn variations would change the orbit timing so much. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Feb 17 2006, 12:18 PM
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#32
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Senior Member Group: Members Posts: 2488 Joined: 17-April 05 From: Glasgow, Scotland, UK Member No.: 239 |
That result sounds pretty close to ours, so I'd say your code is working well. Those kernels just appeared on the NAIF website and I don't know what sort of burn performance differences they represent; it would surprise me if plausible burn variations would change the orbit timing so much. Guys, can you *please* work out whoever is right, you're beginning to (gulp) worry me! MOI has already eaten several pretty toys... Bob Shaw -------------------- Remember: Time Flies like the wind - but Fruit Flies like bananas!
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Feb 17 2006, 12:47 PM
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#33
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Founder Group: Chairman Posts: 14434 Joined: 8-February 04 Member No.: 1 |
Thing is - only a tiny tiny change in the altitiude at MOI, and the duration of MI will produce quite a large change at aphelion (it's a very eliptical orbit) and so a tiny fraction of a change to either of those numbers, will put quite a different bit of Mars under the spacecraft 5 days later
Doug |
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Guest_AlexBlackwell_* |
Feb 17 2006, 06:13 PM
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#34
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Feb 17 2006, 06:28 PM
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#35
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
You think Mike might be using metric figures, while jmknapp is using English units? Hmm. That sounds vaguely familiar. I tend to prefer pixels per fortnight. Be that as it may, any non-hyperbolic insertion orbit that doesn't intersect the surface is a good one. -------------------- |
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Guest_AlexBlackwell_* |
Feb 17 2006, 06:34 PM
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#36
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Feb 17 2006, 06:36 PM
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#37
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
That might be why the MRO mission designers baselined for aerobraking instead of lithobraking I was going to crack that joke but I thought "Naw -- it's too old and tired." I guess no joke is too old and tired for an academic --Emily -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Guest_AlexBlackwell_* |
Feb 17 2006, 06:50 PM
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#38
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I was going to crack that joke but I thought "Naw -- it's too old and tired." I guess no joke is too old and tired for an academic --Emily Yeah, the joke was pretty stale. In fact, as Mike has pointed out over the years, the public has gotten an incomplete if not distorted view of the MCO/MPL/DS2 losses, mainly that the root causes, especially of the MCO loss, can't be simply described as "Oh, they screwed up because they didn't know the difference between metric and English units." For balance, I would also recommend the following: Euler, Edward A., Steven D. Jolly, and H.H. Curtis; The Failures of the Mars Climate Orbiter and Mars Polar Lander: A Perspective From the People Involved; AAS 01-074, 24th Annual AAS Guidance and Control Conference, Breckenridge, CO, January 31-February 4, 2001. A few years ago, Steve Jolly (of LMAO) also sent me some PowerPoint slides, prepared, I believe, for a subsequent conference. These offered some nice perspective, as well as being pretty instructive. * EDIT - Before "[f]or balance" above, I should have inserted "In addition to the 'official' mishap investigation reports, which can downloaded, among other places, here, as well as the popular press coverage (of varying degrees of accuracy),..." This post has been edited by AlexBlackwell: Feb 17 2006, 07:56 PM |
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Feb 17 2006, 06:58 PM
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#39
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Solar System Cartographer Group: Members Posts: 10229 Joined: 5-April 05 From: Canada Member No.: 227 |
Emily: "I guess no joke is too old and tired for an academic"
And no academic is too old and tired for a good joke. 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 PDF: 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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Feb 17 2006, 07:50 PM
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#40
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Senior Member Group: Members Posts: 2542 Joined: 13-September 05 Member No.: 497 |
You think Mike might be using metric figures, while jmknapp is using English units? Very funny, Alex. If it makes anyone feel better, I don't have anything to do with planning MOI. jmknapp has performed a valuable public service by highlighting that those three kernels produce significant orbital timing changes; I hadn't appreciated that the MOI performance could induce that large a change, but now we're prepared. We weren't given any context about what those kernels might mean -- they just showed up on the NAIF website. They may be for training purposes, or they may be physically realistic. -------------------- Disclaimer: This post is based on public information only. Any opinions are my own.
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Feb 17 2006, 08:06 PM
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#41
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Senior Member Group: Members Posts: 1281 Joined: 18-December 04 From: San Diego, CA Member No.: 124 |
Emily: "I guess no joke is too old and tired for an academic" And no academic is too old and tired for a good joke. Phil And I was going to crack that joke..... It's getting to the point where I log on to UMSF and just nod in assent to almost every post -------------------- Lyford Rome
"Zis is not nuts, zis is super-nuts!" Mathematician Richard Courant on viewing an Orion test |
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Guest_AlexBlackwell_* |
Feb 17 2006, 08:06 PM
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#42
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Very funny, Alex. If it makes anyone feel better, I don't have anything to do with planning MOI. I couldn't resist the tweak, Mike, especially since I know the way the MCO/MPL/DS2 losses were reported has grated on you. It's not the same, though, as needling Bruce. Now that is fun jmknapp has performed a valuable public service by highlighting that those three kernels produce significant orbital timing changes; I hadn't appreciated that the MOI performance could induce that large a change, but now we're prepared. We weren't given any context about what those kernels might mean -- they just showed up on the NAIF website. They may be for training purposes, or they may be physically realistic. Is this similar to the slight differences between MSSS-generated and JPL-designed targeting boxes that were evident early in MOC campaign? If I remember correctly, sometimes your orbit predicts disagreed with JPL's. |
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Feb 17 2006, 08:09 PM
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#43
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Founder Group: Chairman Posts: 14434 Joined: 8-February 04 Member No.: 1 |
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Feb 17 2006, 09:39 PM
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#44
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Senior Member Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 |
jmknapp has performed a valuable public service by highlighting that those three kernels produce significant orbital timing changes; I hadn't appreciated that the MOI performance could induce that large a change, but now we're prepared. We weren't given any context about what those kernels might mean -- they just showed up on the NAIF website. They may be for training purposes, or they may be physically realistic. FWIW, here's the difference between the three kernels in terms of altitude. Looks like they are very different. Not sure of the context as you say--both high perf and low perf have shorter periods than ideal. -------------------- |
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Feb 17 2006, 09:59 PM
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#45
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Founder Group: Chairman Posts: 14434 Joined: 8-February 04 Member No.: 1 |
Being VERY unscientific, approx speed at periareion using Orbiter with these apareion's....
10000km - 4330 m/sec 18000km - 4460 m/sec 28000km - 4590 m/sec 38000km - 4650 m/sec 45000km - 4670 m/sec So to vary between 28000 and 45000 is only a 80ish m/sec difference, in an MOI burn of I believe roughly 1000 m/sec over 25 minutes - so +/- 40m/sec is about a 4% error, or 1 minute of the burn 200 x 400km orbit is approx 3520 - 3320 m/sec ish -so aerobraking is giving us another 1000 m/sec of delta V. All figures very VERY roughly done in orbiter. Doug |
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