Death of the scan platform? Options

[edstrick]

"I'm still amazed it worked."
I had real concens of it lasting the primary mission. It lasted to the end of the mission. I'm still amazed.
The only reason it worked was some ??? 100 million $$$ ??? spent on that part and related parts of the spacraft design.
(I think that's the rough figure.. it may have been more!

[tty]

Stuff that moves is bad, unless you have no choice.

Moving parts is bad even when you have no choice.

[mchan]

I always thought that the ultimate in scan platforms was Galileo, which, IIRC, was a spinning spacecraft with a central bus that was entirely despun. All of the control cabling, instrumentation data flow and power flow between the spinning portion and the despun bus had to connect along continually moving surfaces.

I wish I can remember more about this interface. All I can recall is something about a "roll ring" which went thru some re-design for reliability.

[rlorenz]

One could argue that a lot of the FY1992 savings in deleting Cassini's scan platforms have been eaten up
and more by the increased operations complexity - but FY2000-2008 $$$ were not on people's mind then.

Fundamental issue is different things pointing in different places. This occurs at least more systematically
on orbiters than on flyby s/c where every encounter is a custom job (and Cassini, though it orbits Saturn,
is really just a 100-flyby s/c..) and so the thermal design at least may be simplified. You will note though
that many Mars orbiters have gimballed antennas - that are turning for at least part of the orbit, every orbit,
for years - so mechanisms are still with us. Magellan-style point-and-shoot orbiters work ok, but a lot
depends on how much time you have (e.g. if you are short on radiation lifetime, orbital stability or whatever,
it may be necessary to be acquiring and transmitting data simultaneously)

[mchan]

Unfortunately, the up-front costs are more immediately visible. You need the dollars to spend now to get the program started. If the program doesn't start, then there is no opportunity to save costs later.

I have read there was a proposal for a single axis scan platform vs. the usual 2-axis platform and fixed mounting. The spacecraft would roll to provide one axis of rotation while the platform can rotate to track a target on fast flybys. As I recall, a single axis platform would have allowed much more imaging during times when Cassini had to be in a fixed orientation. e.g., dish forward during ring plane crossings. I don't recall seeing info on the cost tradeoffs between the three options of full 2-axis scan, 1-axis scan, and fixed mounting.

[monitorlizard]

I've learned a lot about scan platforms versus body-mounted instruments from all the responses in this thread, and I really appreciate everyone's input. But all of this has generated a new question for me: If scan platfroms are heavier, more complex, and have inferior pointing compared to body-mounting, why did the early planetary spacecraft (Mariners 4,6,7,9, etc.) use scan platforms? I'm especially thinking of Mariner 4, where a succesful first Mars flyby before the Soviets was a national priority. It seems that the simplest possible science system (body-mounting) would have been the first choice in the spacecraft design.

[tedstryk]

At the time, the risk of turning the spacecraft around the flyby was too risky in those days. Mariner 4 was designed at the time when the Rangers were failing one after the other. Not to mention the fact that the other instruments were transmitting in real time, so there would have been gaps while the spacecraft was turned to take pictures. Not to mention the risk if the signal wasn't recovered right away. Better a stuck scan platform than a lost spacecraft, especially when one is trying for a "first" as we were with Mariner 4.

[elakdawalla]

I wish I can remember more about this interface. All I can recall is something about a "roll ring" which went thru some re-design for reliability.

Here's what I wrote down while researching Galileo's mission: "Beginning in March 1991, Galileo suffered safing events, in which it canceled its ongoing command sequence and radioed Earth for help, due to spurious reset signals that were generated by short circuits in its spin bearing assembly slip ring. Galileo had two sections, one of them spinning (for the fields and particles instruments) and one of them not spinning (for the optical remote sensing instruments), and the slip ring allowed electronic signals and power to be passed from the rotating to the non-rotating sections of the spacecraft. The short circuits caused Galileo's computer to think that the spacecraft bus on the despun section had suffered a reset."

--Emily

[hendric]

Just googling:

http://www.ruag.com/ruag/juice?pageID=166718
http://www2.jpl.nasa.gov/galileo/messenger...s/SpinBear.html

Galileo had 48 slip rings, and 23 "rotary transformers". Even for Galileo's time that seems like serious overkill. I suppose it was too hard programmatically to take all the data, packetize it onto one bus, and send it over the rings that way? I know I've seen a picture of the Galileo Spin Bearing Assembly somewhere (probably here!).

Re: going wireless

Probably not that bad an idea! If you can send your command/data wirelessly, you can get away with a much simpler interconnect, say a 2 contact slip ring (no more complicated than a motor or generator). Just put a battery/capacitor on the downstream side of the power bus to even out any fluctuations, and you're good to go.

Were there any other aspects of the spun/despun dual nature of Galileo that caused problems? If there was a simpler/more reliable way of doing it, would it be useful today?

[mcaplinger]

Between the Galileo experience and the total failure of the JPL SEASAT mission due to slip ring arcing, it's unlikely that slip rings will find much favor in future applications.

It would have been simpler to split Galileo into two spacecraft, a spinning particles and fields mission and a three-axis imaging mission, and indeed this was discussed after Challenger.

Juno of course is a spinner, but imaging is not the main focus of that mission. Imaging has to be done in spite of the spinning.

[cndwrld]

Just to add a a note, these spun/despun spacecraft have been done for a very long time, and can be incredibly reliable. Before it was sucked into Boeing, Hughes Space and Communications developed a lot of spun/despun spacecraft whose reliability was one reason that they were built until pretty recently. Boeing would probably still sell you one today. They were used with simple commercial payloads, all the way up to very complex payloads for agencies which cannot be named. The only real reason they have gone out of favor is that the power requirements today are so much larger than they used to be, and you can't scale up a spinner very easily to get a lot more power. Because of that, it was never cost effective to redesign the electronics and propulsion systems to bring them into the modern age.
See http://en.wikipedia.org/wiki/Boeing_Satellite_Systems.

There's nothing inherently wrong with using a spun/despun section. You just have to build it right.