[X] My Assistant

[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?

[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

[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.

[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.

[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.