Death of the scan platform? Options

[monitorlizard]

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?

[tasp]

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.

[djellison]

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

[mcaplinger]

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?

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.

[nprev]

How many more flyby spacecraft do we expect to see?

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.

[djellison]

the cable design between the instruments and the spacecraft

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

Doug

[nprev]

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!

[ugordan]

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.

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.

[Bjorn Jonsson]

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.

[mcaplinger]

The way I figure this is there wouldn't be any momentum imparted (not permanent anyway)..

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

[cndwrld]

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.

[ugordan]

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

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.

[mcaplinger]

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.

[dvandorn]

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

[djellison]

I'm still amazed it worked.

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