25'107 Intro' to Engineering Session 17: Unit Conversion II

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Title: 25'107 Intro' to Engineering Session 17: Unit Conversion II

1
25.107 Intro. to EngineeringSession 17 Unit
Conversion II

2
Module ThreeEngineering Analysis

3
Agenda

4
Case StudyMars Polar Lander
5
Previous Space Failures

Propulsion systems have leaked exploded.
Power systems have short-circuited.
Observation instruments have failed to work or
have been pointed in wrong directions.
But until 1999 no CFIT had occurred.
CFIT is a Controlled Flight Into Terrain
It occurs when human error flies a perfectly good
airplane right into the ground

6
During the CFIT

Trajectory errors were accumulating.
Systems were assumed to be good.
30-year-old trajectory code was not allowed to be
run, seen, or verified outside JPL.
Mid-course corrections resulted in erratic
behavior.
No fix found given limited time during final
approach.
The spacecraft was almost certainly destroyed
when its hydrazine propellant tank heated to the
point of self-ignition of the remaining fuel.
"There was enough explosive force there to level
a city block, so the spacecraft probably was
blown apart into shards of scrap metal that soon
burned up in the Martian atmosphere.
Whether the debris burned up, fell to the
surface, or grazed the atmosphere fast enough to
have passed out into space on the other side of
Mars is not known.
For now, it is truly "lost in space."

7
After the CFIT
8
The Source of ErrorUnit Conversion!!!

The Mars Climate Orbiter performed momentum dump
operations to slow the aircraft at its
destination. According to NASA, the trajectory
problem began at this point.
The unit that JPL wanted was the Newton (the
force that accelerates a 1 kilogram mass at a
rate of 1 meter per second per second).
Engineers at Lockheed Martin Astronautics Co.,
Denver, Colo., had used pounds force, and these
figures would have been too large by a factor of
4.45.
In engineering terms, these two values are on the
same order of magnitude.
There was no large mismatch in the scale of
calculations made with the one or the other.
These two units were close enough in magnitude
that the unintentional substitution of one for
the other apparently rang no warning bells.

9
The Failure Mode?Adaptive Computer Control

According to a JPL spokesman, every maneuver
intended to dump momentum added a velocity error
of about 0.001 meter per second, on a probe that
was traveling at a rate of tens of kilometers per
second.
These deflections themselves were not the
problem, but their incorrect modeling was, when
the computer was told the spacecraft had received
a force of four or five times as great as it
really had.
"Every momentum dump will introduce an additional
bogus force into the navigation software.
Ultimately the real-time model of the spacecraft
was warped by this application of a non-existent
force."
At the 10 November press conference, chief
investigator Arthur Stephenson explained "Had we
done end-to-end testing, we believe this error
would have been caught." But the rushed and
inadequate preparations left no time to do it
right.