Talk:Mars Climate Orbiter

The metric flaw

The loss of the MCO has become a case study frequently cited in a software engineering context, so I thought it worthwhile to update the article to correct some common misunderstandings. I've cited my references. My impression from the presentations I've heard is that it was a single *constant* which was computed incorrectly by an engineer (who forgot to include the necessary conversion factor) -- which, for the case studies, means that compiler technology to automatically do unit checking or inference likely wouldn't have helped here. It was just a bad "magic number" that was input. However, the references I can find refer to an "equation" not just a bad constant, so that is the language I have used in my update. C. Scott Ananian 23:34, 17 November 2005 (UTC)[reply]

More details (since this was reverted yesterday, presumably before my comments here were read): See page 7 of the cited AAS paper. There is a JPL-internal memo (JPL D-18441, November 1999) that presumably has the precise details, but I don't have access to it. C. Scott Ananian 15:43, 18 November 2005 (UTC)[reply]

In your last edit summary, you said "revert. This is a direct quote, from the single reference I added. (pg 7) See discussion page."

You do know how to indicate a direct quote, don't you? You know, those little squiggly things called quotation marks—which I just used to enclose your edit summary. See also Wikipedia:Manual of Style#Quotation marks.
Of course, not only do I not see any of those quotation marks in this section of the article, I also do not see any direct quote from the cited page. At least nothing longer than a word or two.

The biggest problem in your edits is the statement "The conversion factor from pound-seconds to newton-seconds was buried in the original equation and not immediately identifiable, and so it was not included in the updated equation."

This gives the misleading impression that a conversion factor was there in the MCO software, but just didn't get used. It wasn't, however. All there was in the MCO software was a formula which would give results consistent with its input. It probably would have worked equally well for gravitational fps units an both input and output, or for SI units as both input and output.
The conversion factor was only built into the formula used in different software, the software used on the Mars Global Surveyor, which was used as a pattern for the MCO software. It wasn't ever in the MCO software.

This article never was written as if it were discussing some "constant"; rather, it discussed the output of the calculations using the formula. A variable, whose value depends on the input. GIGO, etc. Gene Nygaard 18:10, 18 November 2005 (UTC)[reply]

Your response seems needlessly personal. Here is the relevant paragraph from the reference I have been citing (Euler, E.E., Jolly, S.D., and Curtis, H.H. "The Failures of the Mars Climate Orbiter and Mars Polar Lander: A Perspective from the People Involved." Proceedings of Guidance and Control 2001, American Astronautical Society, paper AAS 01-074, 2001, page 7):

"The equations used to make the I-bit calculation are vendor supplied and in English units (in the case of this vendor). Although starting from MGS-heritage software, the coded MGS thruster equation had to be changed because of the different size RCS thruster that MCO employed (same vendor). As luck would have it, the 4.45 conversion factor, although correctly included in the MGS equation by the previous development team, was not immediately identifiable by inspection (being buried in the equation) or commented in the code in an obvious way that the MCO team recognized it. Thus, although the SIS required SI units, the new thruster equation was inserted in place of the MGS equation - without the conversion factor."

I do not believe that the "impression" I gave is misleading: according to this primary source, the conversion factor was in the original MGS equation, but not in the replacement equation. I don't really think a direct quotation of this paragraph is necessary in the article, but I'll be happy to include it if you feel this is appropriate. In fact, misunderstanding of the root flaw here is very wide-spread (google will turn up lots of sloppy quotes of the investigation board's simplified explanation), and my edits were specifically made to ensure that Wikipedia did not repeat the error. C. Scott Ananian 16:36, 21 November 2005 (UTC)[reply]

What error? There was no conversion factor in the MCO software. It really is that simple. Your source says so too. Don't be throwing out some alphabet soup about soem MGS software which wasn't used on the MCO. The Mars Global Surveyor made it to Mars and accomplished its mission five years ago, and is now on an extended mission, using that software which did include the conversion factor. Gene Nygaard 00:46, 22 November 2005 (UTC)[reply]

I think we agree, then. As you say, there is no conversion factor in the MCO software. As you also say, there was a conversion factor in the MGS software. This particular piece of MCO and MGS software ("SM_FORCES") is identical on both craft except for the equation for the RCS thrusters (the MSOP readiness review, page 65 shows that SM_FORCES is a minor modification of MGS software). According to Euler et al, page 7, the conversion factor was "buried in the equation" and thus not "immediately identifiable", and it was not "commented in the code in an obvious way". The sentence from the article we seem to be discussing is:

The conversion factor from pound-seconds to newton-seconds was buried in the original equation and not immediately identifiable, and so it was not included in the updated equation.

It seems that we agree that, as this sentence states, the conversion factor "was not included in the updated equation". C. Scott Ananian 22:44, 28 November 2005 (UTC)[reply]

But it is still badly worded, confusing and misleading, and wrong. When I have mor time I'll try to figure it out. The "velocity change" stuff appears to me to be misplaced and part of the confusion; that's just another exacerbating factor, isn't it? Not a cause of the problem, but something that might have detected the existence of the problem--is that right? If it is right or not, the transitions and distinctions need to be made clearer. Gene Nygaard 01:19, 29 November 2005 (UTC)[reply]

English Imperial Units

Hello Gene,

I sort of thought my change would provoke a response from someone!

There are a few good reasons why it should be left as "Imperial" and that "English" should be dispensed with.

If you click on English units you will be redirected to Imperial units. This in itself indicates that the preferred term is Imperial.
While English Units may be a common term in the US, it is actually uncommon internationally. Here in Australia it is referred to as Imperial.
The term also denotes that England still uses such units of measurement. This is partly true but the nation is pretty much Metrified.

I have reverted the article to say Imperial, but I will leave it up to you to make the final change back to English if you so desire, and if you do not think my reasoning is sound. One Salient Oversight 23:10, 17 Feb 2005 (UTC)

Here are my comments

The current Imperial units page is a mess--sometimes distinguishing those used in the British empire--after the United States opted out--and sometimes including the United States.
In none of the cases where there is a difference between British English and U.S. English or either of them and Australian English or Canadian English or whatever is the title of a Wikipedia article to be taken as an indication of any "preferred term".
There is another U.S. customary units article.
What we need is one English units article that is more comprehensive, with the existing ones more limited.
Like I pointed out in my edit, nobody uses Roman units any more, but they are still Roman units. In other words, these designations are based on history and derivation--not on current usage.
Unlike the United Kingdom, it is legal to sell draft beer by the liter in the United States. Note also that we use the real he-man sized liters, which only take 3.8 to make a gallon, rather than those dinky little litres the British as well as you in Oz use, which take 4.5 to make a gallon. ;-)
Most of the units, such as the avoirdupois pound of mass invented in 1303, are actually pre-imperial. (Or post-imperial, in reference to the Roman empire from which these units derived.) However, even in common usage outside the United States, "imperial" is most often used only for those units which remained in use in the British empire after the Weights and Measures Act of 1924 (in other words, not including Queen Anne's wine gallon and the Winchester bushel, for example).
Pound-force seconds, OTOH, are probably best classified as post-imperial. There were never used in the days of the British empire. (Pounds force were never well defined units before the 20th century, and these rocketry units of impulse only came along well into the century.)
This article is about usage in the United States--and in the United States, "imperial" is used only to distinguish those volume units introduced in 1924 (the best the Brits could come up with for a decimal system, even 30 years after the metric system, was to take a unit that had been the volume of 8 pounds of whatever and make it the volume of 10 pounds of water). Gene Nygaard 04:06, 18 Feb 2005 (UTC)

Well I'm from the UK and I didn't understand what english units were until reading this discussion page, also at the top of the article it cites imperial measurements and later on its then referred to as english. Can someone make their mind up once and for all please. 84.68.230.185 21:06, 9 January 2007 (UTC)[reply]

The article was using both imperial and english units, in the interests of consistency i've changed the english reference to be imperial. Looking at the wikipedia article on "english units" refers to it as being a mostly american term. Seeing as both terms are valid as they contain the "pound" unit of measurement (which was the one incorrectly converted) and given that wikipedia is a global entity, it makes sense to me that imperial units should be used as it is known both inside and outside of the USA. I'm canadian and I'd never heard the term "english unit" until reading this article. Shad0w 23:18, 9 January 2007 (UTC)[reply]

Would have been?

The second paragraph of the lead section says "The spacecraft would have been destroyed by atmospheric stresses and friction at this low altitude." It would have been destroyed? Don't we mean it was destroyed? --Angr (t·c) 15:40, 20 December 2005 (UTC)[reply]

Fixed.

Shouldn't it be more along the lines of "The spacecraft would most likely have been destroyed by atmospheric stresses and friction at this low altitude." as it is an unobserved event. It is possible that the spacecraft bounced off the atmosphere and was not "destroyed" but I guess that is really nitpicking and that the craft was rendered useless by the fault.