It used to be that nearly all U.S. planetary spacecraft had their remote sensing instruments mounted on a scan platform. For the past fifteen years or so, all instruments have been body-mounted. I've been wondering if this is a permanent change in planetary spacecraft design. Scan platforms have the advantage of a faster slew rate than moving an entire spacecraft, so more targets can be acquired in a given amount of time. Scan platforms also mean no attitude control gas is used (except to stabilize the spacecraft), although this advantage is nullified if reaction control wheels are used instead. An additional advantage is that using a scan platform means all its instruments can be used at once, whereas body-mounting can mean the spacecraft blocks the view of some instruments when others are able to see the target.

Body-mounting instruments is advantageous only in that it saves money in the overall design of the spacecraft. I don't know of any other advantage. The last spacecraft that would have used a scan platform was Cassini, but the project switched to body-mounting in a cost-cutting descope. Only JIMO would have had a scan platform (or two) because the spacecraft was so monstrous there was no practical way to slew it quickly to change targets.

So, will we ever see a planetary spacecraft with a scan platform again? Is there some engineering reason why scan platforms shouldn't be used again? Or is it all just to save money, sacrificing some science observations to have an affordable spacecraft?

I recall Mariner 10 having a scan platform, and Messenger doesn't. It seems to me thermal protection would be greatly simplified for body mounting, but that gets back to cost again.

Having Cassini protected by the high gain/radar dish for ring plane crossings is fine, but not having the camera aligned with the radar beam axis has been a disappointment too.

So many tradeoffs for cost/performance/ capabilities, tough to second guess decisions regarding what gets flown.

Beg/Borrow/Steal a copy of 'The Titans of Saturn' . It's a management / leadership book - but it talks about the story of the Cassini scan platform at length and is quite insightful. Essentially - given a choice of Cassini without a scan platform, or no Cassini at all - which would you pick?

One detail - having a platform doesn't mean zero prop useage - the very process of turning a scan platform would, I would imagine, impart a moment on the vehicle to which it is attached. Not big - but not zero.

Doug

How many more flyby spacecraft do we expect to see? For an orbiter in a roughly-circular orbit, the instruments are all more or less nadir-pointed all the time anyway. Pointing the spacecraft is done with momentum wheels and costs little additional fuel.

Scan platforms have always had poor pointing accuracy relative to spacecraft, they're heavy and mechanically complex, and they complicate the cable design between the instruments and the spacecraft a lot. Most needed pointing/scanning can be done more effectively within an instrument.

Hmm. Good point; hadn't thought of that. NH might well be the last of the breed, and it doesn't have a scan platform. Still, there may be a few more dedicated flyby missions (a main belt asteroid tour, or perhaps Jupiter's Trojans?), but probably not targeted towards major bodies in the Solar System.

That's the bit of the MER design that keeps Squyres most worried I think.

Doug

I don't blame him. Cabling between a moving object & the main vehicle frame has to be installed just right in the first place, and it's also going to suffer gradual wear & tear from the motion. By way of illustration, lots of military aircraft require that landing gear wire harnesses must be periodically replaced, regardless of physical condition at the inspection interval.

Slip rings are another method of coupling the two, but those also don't last forever & add to mechanical complexity (to say nothing of weight). Perhaps we need to find a way to go wireless in UMSF!

The way I figure this is there wouldn't be any momentum imparted (not permanent anyway). Rotating the scan platform would tend to rotate the entire body of the spacecraft in the opposite direction slightly, but once the platform rotation stopped so would the spacecraft. The end result is the scan platform is rotated X degrees w/respect to the spacecraft, but slightly less than X degrees w/respect to an outside frame of reference. This ought to be easy to compensate for by additional rotation. Only problem is if you're required to maintain precise Earth point on the main antenna and even a slight misalignment hurts. If the rest of the spacecraft bus is massive, this reactionary movement should be very small.

Lacking a scan platform on the other hand usually implies very long slew times - say half an hour for 180 degrees, not very favorable for an orbiter during a busy period such as periapsis passage. Once you're slewed, however, the pointing can be rock solid.

An additional reason for body-mounted instruments on all recent spacecraft: Much bigger, more reliable and faster on-board data storage than on older spacecraft, i.e. solid state recorders instead of tape recorders. You no longer need to downlink most of the data in realtime so the antenna doesn't need to stay Earthpointed almost constantly.

You're ignoring frictional losses in the bearings. This is why momentum wheels still require propellent usage to unload them occasionally.

A few thoughts come to mind. I think if you're designing a spacecraft, body-fixed instruments are better: cheaper, and more reliable.

Every scan platform is custom. Reaction wheels are off the shelf. Propulsion, to dump momentum, is largely off the shelf. Both pretty reliable. If you don't need to observe and transmit at the same time, removing an additional moving part that is custom built is better. Simplifies the thermal design: a big box is easier to spec than a group of individual heated boxes outside the inside. Doing the attitude maintenance code is easier if stuff isn't moving, especially high mass stuff. It also means you won't be trying to maintain a very tight instrument fix on a target while at the same time trying to maintain a very tight fix on Earth. That requires more fuel, for momentum dumps or active thruster control. If the platform movement fails, you're using a spacecraft with a now fixed platform that wasn't designed for it. As someone rightly said, more on-board storage and use of higher downlink frequencies, and many years of development in better antennas (on-board and ground), means you can store a lot of stuff reliably and get it down from further at higher rates. Simpler spacecraft, simpler to build, less errors likely, more likely to meet those launch dates.

I just come at it from a gut level, what I've seen work and seen fail. Stuff that moves is bad, unless you have no choice. All the moving stuff can fail. Its the stuff that starts going bad near the end of mission, causing operational problems and workarounds. And if it does fail, there are no back-ups for that kind of thing. And cost is not such a small thing. Getting down the weight, getting it on a cheap enough launcher. Getting it approved within your budget. Very important. No bucks, no Buck Rogers.

So, I hope those are some ideas to think about. Just empirical, but some of the stuff in the mix.

I must admit I'm having a hard time understanding why friction in scan platform bearings would impart momentum on the rest of the spacecraft. It's still a closed system and any friction would only transfer momentum from one part to another, no? This is different to rotating reaction wheels - friction unloads their momentum onto the spacecraft because they're rotating and in doing so the wheels lose momentum. The total momentum is conserved. Scan platforms are kept pointed at something, they only have slight rotational momentum when they're slewing and that's what I was talking about before. Am I getting something totally wrong here?

I thought reaction wheels need unloading only because environmental torques (effects from outside the spacecraft as a system) build up over time - gravity gradients, solar light pressure, aerodynamic friction, magnetic fields etc., not because of their own friction.

Hmm. I think you're right; I stand corrected.

Pointing precision of a scan platform is still really poor relative to spacecraft precision. If we're talking about rates, that's even more true.

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'm still amazed it worked.

-the other Doug

Most of the time.

That spun/despun interface caused something like 20 safe modes - 6 in the second half of 1993 alone- and having not occured for nearly 5 years, it occured three times inside of 20 hours in July '98 and three times in 10 hours in August '99.

It was a million miles from perfect.

Doug

"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!

Moving parts is bad even when you have no choice.

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.

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)

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.

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.

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.

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

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?

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.

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.