Coordinator: Excuse me, at this time I'd like to inform all participants that today's call is being recorded. If you have any objections you may disconnect at this time.
And I'd like to turn today's call over to Miss Trina Ray. Thank you. You may begin.
Trina Ray: Thank you. Well I'd like to welcome everybody to the CHARM telecon for this month. We've got a fantastic topic today. It's one that you all suggested when I took ideas a few CHARM telecons ago. I wrote down a whole bunch of suggestions and one of them was a spacecraft tutorial.
And today we're joined by Julie Webster, who is the Cassini Spacecraft Operations Manager. And she'll have a wonderful talk for us on the spacecraft.
Just a few reminders about the telecon. If you don't have mute capability on your phone, that's okay. Just press star six at any time to mute and unmute. If you have any trouble at all press star 0 and the operator will come on and help you.
And Julie is welcome to take questions during the actual talk. And with that, I will go ahead and turn it over to Julie.
Julie Webster: Okay, this is, of course, my favorite subject. I've been working on Cassini since 1995. I started back in the assembly, test and launch operations phase. So I hope today - I've got a few unique pictures for you that were taken during that time to just kind of give you a different perspective.
You know, when the scientists put out their beautiful pictures and things like that and the engineers look at all the wiring and electronics and the assembly that had to be put together to take that picture.
So I tried to arrange it with picture/words, picture/words and I'm going to try to use the words to talk to the picture before. If this works out. In hindsight, I should have probably done it the other way. But this, this will go.
Man: Julie, I'm sorry to interrupt but you have a bad connection.
Trina Ray: I was going to ask you...are you on a…
Julie Webster: Okay.
Trina Ray: ...are you on a hand - you know, on a phone or can you...
Julie Webster: I was.
Trina Ray: ...can you pick up the handset?
Julie Webster: I just picked up the handset. Is that better?
Trina Ray: Oh, much better.
Man: It’s pretty much the same.
Julie Webster: Oh, good. Okay.
It's pretty much the same?
Man: It's still bad.
Julie Webster: Oh.
Man: It's much better here.
Trina Ray: It's much better here too. You know what Julie, hold on a second. Let me go ask the operator if she can see any problem. Let's just hold on for a sec.
Julie Webster: Okay.
Man: I don't think it's the telecon. (See what happens when) you have engineering problems?
Julie Webster: Especially on telecommunications. It stops everything.
Man: Especially once you have - if you're in a wonderful situation. It certainly got better. I mean, it's perfect here. I guess maybe it's not so good up where the other - I think if you're on a speakerphone sometimes it's not so good. I don't know.
Julie Webster: Yeah. She told me to pick up the handset but apparently, it's not much better.
Man: I think it's better but it's not like, you know, it's not perfect. There's still some kind of crackling that you hear.
Man: Is that what the other person is experiencing? The crackling?
Man: I was getting some crackling too; it seemed like when she went to the handset it got better.
Man: Yeah, I agree with both statements.
Julie Webster: Okay, well let's hope Trina solves the crackling or handset problem.
Man: See if she can bring some cookies too.
Trina Ray: Okay, the operator is going to come take a look at it.
Man: It's pretty marvelous that this could even happen.
Trina Ray: It's amazing.
Well, the entire Cassini mission is basically driven by telecon. We have so many telecons because our scientists are spread out all over the...
Trina Ray: ...all over the world. All of the engineers are co-located at JPL and - basically in one building. But even, I mean, I routinely from my office will call into a telecon that is just, you know, down the hallway.
Julie Webster: Do you want to try again?
Trina Ray: Okay. Do you want - are you working with the operator Julie?
Julie Webster: No. Was I supposed to?
Trina Ray: She was going to take a look at your line.
Julie Webster: Okay.
Trina Ray: And see if, whether she though you should call back in or whether she thought...
Julie Webster: I can certainly do that. That's easy.
Trina Ray: Yeah. I wonder what she would recommend. I guess we can hold on a sec.
Julie Webster: Okay.
Man: It's interesting because the local lady in Southern California, I guess or wherever, her voice is much clearer than Julie's. So there is a difference for some reason. Of…
Trina Ray: That's because I'm calling from home. I am sure it's just the whole atmosphere. Calling from home you get a better connection.
Man: And maybe that's why I'm hearing you better too.
Trina Ray: I have to say Julie calls in from her office to telecons that are right down the hallway too. I know. I've seen it happen.
(Jane): Trina, this is (Jane). A lot of people when they mute on and off, also, you know, if they go back and forth a lot, that creates a little blip. And actually, when we're writing the transcripts, that makes it almost impossible sometimes. Because the Operator - the transcript - all they hear is the blips all the time.
So people should try to also just keep the mute on when they're listening to the talk and then ask questions at the end or whatever.
Man: Okay, so we'll save our questions for the end?
Trina Ray: No, I don't think so. We can ask...
(Jane): Well, if you do, just be sure that you mute, you know, not with frequency.
Man: All right.
(Jane): Who is this asking all these questions?
(Ron Ignelsi): Oh, (Ron) - I didn't ask all of them but I asked a lot of them - (Ron Ignelsi).
(Jane): And are you a Solar System Ambassador?
(Ron Ignelsi): Yes.
(Jane): Great. Thanks.
Trina Ray: So (Jane), were you hearing Julie okay before or...
Julie Webster: I was hearing Julie just fine. I think we should just go ahead and...
Coordinator: Excuse me, this is the coordinator. I have placed Ms. Webster back to conference. And her line is much better at this time.
Trina Ray: Oh, excellent. Do we need to start the recording over?
Coordinator: I can...
Trina Ray: Or is it still going?
Coordinator: It's still going right now.
Trina Ray: Would it be okay if we sort of stopped it and restarted it so that it's a nice clean recording? Is that possible?
Coordinator: Yes, we can go ahead, I can go ahead and we can disconnect that. And go ahead and re-do it after the call is ended. And we'll just go ahead and record it now. And then we can go ahead and fix that for you.
Trina Ray: Oh, okay.
Trina Ray: That's fine also.
Coordinator: Okay, all right and then you can go ahead and...
Trina Ray: We'll just let Julie go ahead. And then once you start over, we can figure that...
Julie Webster: Okay. Do I need to start all over or can you just cut out all the conversation about bad telecon?
Trina Ray: I guess we should just start from scratch.
Julie Webster: Okay.
Again, I've been on Cassini since 1995. So I started back in the assembly, test and launch operations phase. And what I was trying to show with the pictures today, I hope I got some unique pictures, was how engineers look that the spacecraft in terms of the buildup of it instead of just the pretty pictures.
So anyway, on page 3, we are the largest outer planetary spacecraft ever built. I believe - and I have never verified this and I've given this in many, many talks - that Phobos, the Russian spacecraft that went to Mars, that had problems in the late '80s was actually a larger spacecraft.
At launch we weighed 507 - almost 5,600 kilograms. And of that, 3100 kilograms was just pure propellant. Today we are down to just 422 of that 3132 left, after three major maneuvers and dozens of smaller maneuvers.
We do have 58 microprocessors on board, which is pretty amazing. Every instrument has its own microprocessor. Several of the attitude control systems have microprocessors. And of course, there's microprocessors that run the computer and the … attitude flight computer.
The 12 science instruments - and the only thing I want to put out here on the picture is the optical remote sensing, the things that you get most of the times on these telecons are the pictures - and the four cameras are collocated on what they call the remote sensing pallet, in the picture. So the cameras are all facing collinear so you get, you can get pictures from all four of the cameras for one spacecraft pointing.
The fields and particles instrument are mostly collinear - well, over on the fields and particles pallet. But they are also spread out around the spacecraft. So they are a little bit different there.
The electronics in the spacecraft is mostly in - there is a 12 sided bay that's the upper equipment module where the louvers are shown. And I'll have a better picture of that later. JPL has almost always built their spacecraft in a bay configuration. I think we started out with an 8 bay for Voyager. And some of the Mariners, I think were like 6 six bays.
We were up to 12 bays, so I think we have the most bays in terms of a kind of circular configuration that JPL has ever flown.
Man: A bay is a side?
Julie Webster: It's a side. You know, like an octagon, like an 8 sided would be like a stop sign?
Julie Webster: And so we are a 12 sided. A dodecahedron, is that correct? Or is that 20?
Julie Webster: I never can remember.
Man: You're right. But are we looking at the picture on page...
Julie Webster: Page 2. Yes.
Man: Okay, thank you.
Julie Webster: I'll go into quite a bit of details in a second on the next grouping.
There's three of the radioisotope thermoelectric generators. And those are shown in the cartoon around the bottom. And they have shields on the top of them to stop some of the heat from going back in in a different way from what we wanted, on the spacecraft.
So I'm going to go on to page 4.And...
(Ken Kramer): Julie, could I ask you, ask you a question about...
Julie Webster: Sure.
(Ken Kramer): ...a propellant, please? It's (Ken Kramer) from Princeton, New Jersey.
About the propellant, have you used - or how much have you used in the past years since the Saturn orbit insertion?
Julie Webster: Let me see, I've got that detail written right over here.
In the past year, we've used a fair amount because we did what we call Periapsis Raise Maneuver. And I'll show that in a later slide. We were about two-thirds down in propellant before that Periapsis Raise Maneuver. And so I'll go through a little bit of detail on that.
If you want some additional details, I can also email you those kind of things.
(Ken Kramer): I will ask you that then. Just one last question, about the instuments - basically everything is functioning normally on the spacecraft, right? There are no problems at this time?
Julie Webster: There are no problems on this. It's been the most amazing spacecraft I ever worked on.
Okay, page 4 is the picture of the radioisotope thermoelectric generator. Page 5 is the details.
We started out with three, what we call RTGs. We had about 875 watts of power at launch. If you think about it, that's less than half of your hairdryer. Of course, that's a little misleading because your hairdryer is on a 110 volt system and we are on a 30 volt system. But it still is an amazing amount of - not very much power to run this whole spacecraft.
The RTGs themselves, we get heat from the radioactive decay. And that heat is turned into power. And of course, the more they decay, the less heat they produce. So we lose about one watt a month. So we're down to, today, about 721 watts. So we've lost 150 watts since the launch. And we'll continue to lose that. It's about 9 watts a year starting around 2007.
The excess power - we still have, out of all of that, out of that small amount of power - we still have excess power. And that's distributed out to a (shunt) radiator. And I'll point that out in one of the later pictures. I think that solar thermal vac has a good picture of the (shunt) radiator. And that is just radiated out to space.
One of the major upgrades on Cassini, as opposed to like a Galileo or a Magellan or the Voyagers, or some of the earlier Earth spacecraft, is that we have - the power is distributed to the electronics and the instruments through 192 solid state power switches. And the solid state power switches are like little circuit breakers.
This, for me, this was a major step forward in the engineering because relays - we used to use relays. So you know, if you had a critical thing, you would put four relays on it. And you would have to close two different relays to get the power through the electronics. And relays, we were always worried about relays sticking, or relays frying together, or other things about faulty relays.
So these power switches have been a major upgrade for us. They - the critical loads, you know, we tried to make the spacecraft idiot proof, even for us. So all the critical loads have redundant switches an automatic turn on circuitry. So if the spacecraft is on, they're on- and a lot of things that we've never turned off since we first powered the spacecraft at launch.
There is special fault protection involved. We've worried about these switches occasionally, if they're off or on, they tend to just trip off, not for an over current situation but because of a faulty design. And we've uploaded special fault protection for these cases.
The other thing that is unique about the power system is we don't have enough power to keep all the instruments on all the time. We keep 10 of the 12 at any given time but not all 12. The radar and radio science guys, tend to be high power draws so we go into certain operational modes to be able to carry.
So we say if you want radar on, these instruments have to be in sleep, these instruments have to be off, et cetera, et cetera.
Let's see, I don't think there's anything else. I'm on to page - with the next page is a picture of the 1750A processor the microprocessor that is actually the main computer chip on the spacecraft. So the engineering flight computer, the command and data system, we have two identical flight computers.
Of course, when you send an asset, a spacecraft like this out to Saturn, we do all lot of redundancy. Almost everything that is a critical item has a redundant.
So we have a redundant computer, we have a redundant bus that distributes the computer out to the different electronics and the science subsystems. It's, I think it's always kind of amazing that we only take 512 - these computers are only 512K of RAM. There isn't - you don't need all lot of processing power to run a spacecraft.
We upload background sequences about every 3 to 6 weeks. So we put everything that is needed for the computer to fly for the next 3 to 6 weeks. And then sometimes we still need real-time commands so those are up-linked during downlink passes that we'll talk about in a little while.
Let's see, we do process all the telemetry for the spacecraft engineering subsystems, and the instruments, the instruments - when we're not in sight of a downlink station - we store the information, and the pictures and the science data, on two solid state recorders.
And each solid state recorder is the 2.3 gigabits; 2.1 is available for telemetry. On any given day, we only downlink probably a little bit less than 2 gigabits a day.
And going on to the - the next is the propulsion system. And maybe I can answer some more prop questions on this one. We have two completely different, two completely isolated systems.
One is a mono propellant system which is hydrazine. And hydrazine is put through a heated catalyst bed to rapidly expand and get us this small thrust.
These are one Newton thrusters. We have 4 thrusters per cluster. And the cluster, if you look in the photograph, those are the lower items that look like little squares that are out on the black supporting struts. And there's one in each corner. We have - we only use 8 at a time. And there are 2 downward facing and 2 outward facing in the Z direction.
And in between the down and Z, between the downward facing and the outward facing in one direction, we can thrust the spacecraft in any direction that we need to go.
These small thrusters, are used in what we call a blowdown mode. That means that they're pressurized to a certain thing using helium as a pressurant at launch. And since launch, they've been - of course, every time they thrust, they lower the pressure just a little bit.
We - and because there's less gas in there, we started out with these at one Newton thrusters and we're down now to about .65 Newtons today. And the reason that I'm going into a little bit of detail on this, is a week from next Monday, on April 10, we're actually going to fire - oh, I just drew a blank here.
We're actually going to fire a pyro device. And if you look at the pictures, there is a small ball - and I have a helium recharged tank for mono prop - there is a small ball on the lower right part of the picture. We're actually going to fire a pyro that will open up that small ball of highly pressurized helium. And it will go back in to the large mono prop ball. And it will pressurize these thrusters back up to one Newton.
That gives us more attitude control authority for going very close to the Titan passes. So that we have more thrust capability to overcome Titan (torques).
Let's see, what else can we talk about that? Oh, we have a complete backup system. All of those thrusters have a redundant thruster with them. We have never gone to the backup redundant system in flight. The spacecraft has been remarkably performing well.
The other system that is a larger thrust is the bi-propellant system. It's nitrogen tetroxide and (monomethyl) hydrazine. And it - we have main engines. The picture is not a very good picture for the main engines - I'm hoping that I'll have a better picture later on - because of the support cages are in the way. But they are kind of the blue-gray nozzles down at the bottom of the picture, if you look carefully.
These are 445 Newtons engines, or just large thrusters. And they're used to - there's only one engine that we've ever used. We've only used engine A. But because you go out to Saturn, after 7 years, you don't want an engine failure to stop you from going into orbit, we carried a backup engine. And we still have that backup engine available to us should anything happen with the main engine.
The bi-prop also is mostly used in a blowdown mode in terms of that it was pressurized, except for real long burns. For long burns, for large burns like a Saturn Orbit Insertion or the Periapse Raise Maneuver, or the Deep Space Maneuver that are very, very long, over several minutes, you know, 5 up to maybe 90 minutes burn, those are all done in a regulated mode. And we have a regulator that provides a constant pressure to the engines so that we get the same thrust throughout the maneuver.
Let's see, the engines, almost - it wasn't really an afterthought, but late in the design game, they came back with some worry about these two engines kind of hanging out in the breeze during the asteroid crossing, was our original concern. And so we put on an articulated baby buggy, baby buggy cover that is a, it has metal stays, and thermal blanket material, several layers of thermal blanket material. And we actually articulate, or you know, baby buggy this over the main engines during any dust hazards.
We kept the engine covered almost all the time in cruise, and only opened it when we wanted to do a trajectory correction maneuver. In tour, we tended to do just the opposite. We leave it open except for dust hazard crossings where we get near the E-ring, I believe. And when we go in to the closer rings where we're worried about dust hazards, we'll close the cover at that time.
Man: Julie, you would begin to, obviously, have a problem if the dust cover never opened. Do you take special precautions to make sure it will open under (all) circumstances?
Julie Webster: Absolutely. Engineers are nothing if not cautious. There's two motors, there's two driving motors for this. You can use either motor A or B, or you can use both if there's an issue with driving the material.
And in the worst case conditions, since it's easier to avoid dust hazards than it is to not fire a trajectory, you can also blow this off. We can blow this cover off with a pyro fire should we need to. And we always keep that option in the back of our mind.
Man: Thanks, (that was) my question, if you...
Julie Webster: And...
Man: ...to not chance things.
Julie Webster: And actually, in '88 - not '88, gosh, 10 years too early - in 1998/1999 we actually had quite a change in where the cover opened to because of the stiffness of the material after it's been out in flight for a while. And we watch that very closely. Since then it's kind of settled into the same. It opens up about 2/3 of the way. It doesn't get quite all the way up, like it used to, anymore. And that's still good enough to fire the main engines. But we watch that every single time when we close and open the cover.
All right. Let's go on. Now the workhorse subsystem, of course every subsystem thinks they are the most important subsystem, but attitude control is the essential thing for science. We have to be able to point the spacecraft reliably, we have to be able to point it accurately, and we have to point it very stable, should we need to.
That attitude control system, the picture is kind of showing that the attitude control system is distributed all over the spacecraft. At the top, and I don't think I wrote that in the words, but there's two holes in the high gain antenna. And those are where the sun sensors go.
So the very first thing the spacecraft does, if it doesn't know where it's at, - the flight software on board tells it to go find the sun. So it will turn the spacecraft in a slow motion around until it gets the sun in the field of view of the sun sensors going through the high gain.
And then it tries to find stars. So the actual attitude determination is done by what we called stellar reference units. And what these are, are basically very, very fancy cameras with CCDs, charge coupled devices. Where they have, they are comparing against, all the time, against an onboard star catalog.
We have about a 3500 star catalog. So it takes a picture. It picks out the four or five brightest stars in the field of view at any given time and it tries to match them up to its star catalog to make sure that it knows where it's at.
If it can't find more than three stars, it starts a search program that it will continue to try and move around until it can find those four or five brightest stars at any given time.
Now, the stellar reference unit, still, it can be spoofed on occasion if there's bright bodies, like Saturn is a bright body, Titan is a bright body, even the reflection of the rings is considered a bright body. So it can be spoofed into seeing brighter bodies than the stars that we want it to see.
So during that time, we may tell it not to look for stars for a few minutes, to a few hours. And we hold the attitude control by gyroscopes or inertial reference units. And we have two, although we only used one, hemispheric resonator gyros. And that's how we hold the attitude if we can't pick it up with the SRU.