While recovering from New Year's celebrations, I have reread James
Frysinger's USMA 16948. I am glad that he now agrees with me that the Mars
Climate orbiter disaster due to NASA issuing commands to the MCO in newtons
that the Lockheed-Martin-built MCO interpreted as pounds-force, and was not
due to a confusion between metres and feet. If it had been a confusion
between metres and feet the MCO would have flown past Mars and out of the
solar system.
>"Joseph B. Reid" wrote:
>>
>> James Frysinger wrote in USMA 16936:
>....
>> >A simplified analogy would be this. Assume NASA asked L-M how much to
>> >change in velocity of the MCO was needed. L-M provides the number 10
>> >[feet per second] and NASA interprets that as 10 [meters per second]
>> >because the contract said that meters per second were the specified
>> >units. Note that L-M merely says "10" and NASA hears "10", but both are
>> >thinking in terms of different units. The result would be that the
>> >orbiter would be given orders to change velocity by about 3.3 times as
>> >much as it should have. L-M was culpable because the contract specified
>> >the use of (certain) metric units for the provided data and they failed
>> >to comply.
>>
>> I can't agree with Jim's explanation. When the Mars Climate Orbiter was
>> launched its main thruster rockets burned out in a few seconds. From then
>> on the speed of the orbiter was determined. The snafu was with
>> instructions to the side-acting jets whose function was to steer the
>> orbiter. The orders from NASA to the orbiter were to control its
>> direction, not its speed. NASA ordered newtons to steer the orbiter into
>> an orbit around Mars The orbiter, built by Lockheed Martin, interpreted
>> the orders as pounds-force, and the orbiter crashed into the back side of
>> Mars.
>
> I agree with your more precise wording, Joe, except for the units. I
>had cross-line-of-sight velocity in mind but did not state that. And of
>course, the platform used jets (hydrazine?) instead of propulsion
>rockets to effect that change in transverse velocity. What neither of us
>mentioned is why NASA needed L-M to provide the data. Velocity in the
>line of sight was fairly well set by the intial launch towards a Mars
>rendezvous and was fairly easily measured with decent precision by
>doppler analysis of the signals coming from the MCO. This allowed NASA
>to easily integrate velocities to determine how far the MCO was along
>its intended track ("downtrack"). However, the direction in space to the
>MCO could not be measured with sufficient precision to determine how far
>left/right/up/down the MCO was with respect to its intended track.
>Worse, stabilization firings of those jets (to maintain orientation)
>were off-center from the cneter of mass, imparting a torque to the
>orbiter. These torques were absorbed by a flywheel, essentially, and
>periodically that accumulated angular momentum in the flywheel would
>have to be shed by means of a special sequence of simultaneous flywheel
>decelerations and countertorque applications via steering jets. These
>caused track errors across the intended path, which L-M was in charge of
>calculating. It was this countertorque data that they passed to NASA in
>the wrong units. They were supposed to pass it in newton meters but
>passed it in pound (force) feet instead. In short, the bad data were
>torque values, not force values. With that data, NASA applied an
>unfortunately erroneous steering correction to the MCO.
>
>Jim
>
>--
>Metric Methods(SM) "Don't be late to metricate!"
>James R. Frysinger, CAMS http://www.metricmethods.com/
>10 Captiva Row e-mail: [EMAIL PROTECTED]
>Charleston, SC 29407 phone/FAX: 843.225.6789
Joseph B.Reid
17 Glebe Road West
Toronto M5P 1C8 TEL. 416-486-6071
