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Re: Changing Cycles 'till it Hertz
Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>
> Bottom line is that, as Faraday said, "names are one thing and science
> is another". I doubt that any competent engineer anywhere in the world
> was ever confused by the expression of units in either the english or
> metric systems. The changes were silly and didn't help the engineering
> profession at all! There are alleged to have been some colossal butches
> made by confusing metric and english units, as for example in the loss
> of one of the Mars probes, but I don't believe that. Careless
> arithmetic, perhaps.........
I feel compelled to defend our (JPL) honor here....
JPL has used metric units (km, Newtons, etc.) for navigation of spacecraft
for decades (probably as long as the lab has been here, but certainly since
the 70's). The same is not true of mechanical dimensions (although,
currently, most (if not all) released drawings are metric), and, more to the
point, it is not necessarily true of various and sundry vendors and
suppliers to JPL (in this particular case, Lockheed-Martin).
The Nav guys get data from the vendor, which according to the
specification/contract/whathaveyou is in metric, in this case, thrust in
Newtons (impulse in Newton seconds), which is fairly standard in the rocket
propelling business (for instance, if you buy "Estes" rocket engines,
they're marked in metric units).
Turns out though, that nobody on either side actually checked the data to
make sure it was really being supplied in Newtons, rather than pounds (which
is what it was). On the JPL side, I'm sure it never occurred to them that
anyone would do anything else (the last navigators to use non-metric units
probably having retired, died, or moved into senior management years ago).
I'm not sure on the LMA side, but probably, nobody really looked at it
carefully, it being just one set of numbers out of the pallet loads
(literally.. I get documentation delivered to me on pallets), and everyone
just assuming that everybody is doing the "right thing".
Then, when the actual navigation was being done, the numbers didn't quite
match up (i.e. the expected doppler shift and range weren't quite what they
were expected to be), but, then, there are lots of things that can cause
this, and it was "in the right general ballpark".. Complicating this is
that the designers and specifiers of the propulsion weren't the ones doing
Digression: how spacecraft get navigated.. all you can directly measure is
the range (to an accuracy of about a meter) and the doppler shift (the
velocity of the s/c relative to earth)... there's no GPS between here and
Mars... You can also get a bit of information (sometimes) on the
orientation of the spacecraft (if it has a star tracker or sun sensor). So,
you basically rely on using Newton's laws (a = F/m... deltaV = a * time,
etc..) and comparing what you told the s/c to do (turn on a rocket with a
given amount of thrust for a given amount of time when pointing a particular
direction) with what you observe (which is only range and speed away from
earth). For a given burn, there are multiple possible things that would
give the same observable data. However, after some time, you can look again,
and the laws of gravitation and celestial mechanics will have made the s/c
go somewhere where it shouldn't, and hopefully the range and doppler are
different, so you know somethings wrong (then, they call up the tiger teams,
have lots of meetings, wring their hands, have review boards, etc.)..
All of this takes time, and the deviations are tiny and the "signal" you are
looking for (the expected range/doppler) is noisy. Small errors can get
masked, especially if you don't expect things to be wrong. Normally, the
nav goes just as planned, and they hit the mark right on... an accuracy of
100 meters after travelling all the way to Mars (hundreds of million km)
wouldn't be unrealistic.... after doing this dozens of times, if the
measurement was a bit off, you'd assume it was measurement error, not a nav
So, getting thrust data in pounds instead of Newtons meant that the delta V
was off by a factor of 4.5... I don't know how much total delta V was in
the various nav maneuvers, but it was probably a few meters per second,
total (most of the trip is set up when leaving Earth orbit, and the
corrections are tiny "trim" burns), out of a typical velocity of say, 20,000
km/sec (so, you're making a change of a part per million...). A small
difference doesn't get noticed until it's too late...