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Launch Services: The Science of SIRTF
 
Question and Answer Board

Pierre from Omaha
How long does it take to assemble a Delta II on the pad and how many people are involved in the process?
Well Pierre, to assemble a Delta II on the pad it takes approximately six weeks or 45 days. The Delta II Heavy, which this is the second time we've flown this, takes a little bit longer than that. But we were able to shave eight days off of the optimum Delta II schedule even with a more difficult launch vehicle to put up. This was a challenge that the Opt Agency gave us after the two MER missions and trying to get SIRTF up this quarter. So we got there right on the mark and we're dealing with things such as weather in the Indian Ocean. So answering your question, 37 days is the record up to this point for the Delta II Heavy.

Host: Could you share with us briefly some examples of what you did to shave that time?

We worked weekends; that's how we got there. Usually we take Saturdays and Sundays off and this time we worked Saturdays. We also learned a lot from doing the first heavy vehicle and were able to shave some time off of that and get some efficiency.
Rich O. from Elk Grove, Illinois
If I remember correctly the Delta II heavy was first used on the MER-B launch a couple months ago. What did you learn from that launch that you have improved on this launch vehicle? How has going to a heavy launch vehicle improved the launch process?
Rich O, what we've learned is just process improvement. There's really nothing that we can go from one vehicle to the next in this close proximity to improve the vehicle in this short of time. So what we have done for the efficiencies have been in the processing time. It's not the first time we're going through it anymore, and that's where we're getting the most efficiency. And then the other part of the question you say, how is going to the heavy launch vehicle improved the launch process? It really hasn't, it's actually a little more difficult. What the heavy does is it widens the Delta II fleet and allows us to get into bigger payloads or hotter trajectories or getting out to the distant planets more readily with more weight; so it expands our fleet.
Peter from Basel, Switzerland
How Heavy is the Telescope?
The telescope is 851 kilograms, Peter, and that is the total spacecraft weight. So I wasn't able to give you the actual telescope weight, but the whole complete package is 851 kilograms.

Host: Is this typically about the weight of a payload for that size of a vehicle?

Another very good question. Sandstorms are not uncommon on Mars and so that has always been a concern for all our landed Mars missions. And this particular mission is not an exception. One of the reasons we worked extremely hard to prepare these rovers as quickly as we could was to get them launched in this launch opportunity. And we only have a limited number of opportunities to launch to Mars because where Mars is relative to Earth and they only line up roughly about every two years. If we miss this opportunity the next opportunity will be a year to two years later. And the good thing with respect to sand storms in regards to this launch opportunity is that it is just beyond the season when we tend to see sandstorms on Mars in the areas we're going to. So while it's not impossible that we're going to land in a dust storm, it's probably somewhat unlikely. If we were to get a dust storm or a sand storm on Mars, it depends on the severity, the dust effects what we call the optical opacity which then effects our ability to get power on our solar panels from the sun and if a dust storm is not too severe we could continue on with our mission in a degraded fashion and do just a little less science, a little less driving and that sort of thing. If the sandstorm is severe we may need to go into a hibernate type of mode. And we do have contingency plans in the event that that needs to be done. And hibernate, the rover is pretty smart little fellow and they have the ability to basically go to sleep and when they go to sleep they use almost no power. It's just extremely small amounts of power on the order of just a couple of watts. But it also means that the rover can't do much. It can't communicate with Earth. It can't do science and it can't drive so when it goes into hibernate mode we'll periodically wake it up to talk to it. And when we believe its acceptable and safe to continue on with the mission we will then reactivate the daily cycles and mission scenarios that we had planned originally.
Lenora from Lakeland, Florida
Does the weight of the cargo determine the window you use to carry the payload into space? If not just how do you determine the window?
Usually what we try to do is hit a point in space, an imaginary point in space, where the telescope is going to be able to do its best science. And weight has a little bit to do with it, but it's really the vehicle performance and getting there. Some vehicles have the added flexibility of having variable azimuths, in other words they're able to change the direction in which they are flying almost instantaneously throughout a set time period. The Delta II does not have a variable azimuth; you can pick one or two at most for every attempt. So I don’t know if I answered the question completely, but those are some of the pieces and it's not just the weight variable.
Andy from Grangeville
What do you feel is the most difficult part of vehicle processing? Where could the most issues occur?
The most issues can occur at L-4 and beyond. That's when the launch vehicle becomes its most active. We load the RP1 kerosene onboard the first stage. We load the cryogenic locks onboard. We also start pressuring the second stage. So things become very active in that time period. We’re also dealing during that time with the guidance system, the range assets, and the weather. So it all becomes very exciting in those last four hours with all those things interchanging and having to work correctly to get us off on time.
Jerry from Shrewsbury
Do you anticipate any issues with the cork this time as you had with the MER-B mission?
Absolutely not, we already went through our locks loading a couple of weeks ago and we had no deterioration of the cork bond. So the SIRTF booster is pristine and ready to go.

Host: For those viewers who are not familiar with the cork issues; can you just give us a real brief summary?

Sure, what we had is, we've got this cork around the center body of the Delta II rocket and the original booster that was slated for SIRTF which we flew on MER-B, had been sitting out in the elements for a while. And some water, rain induced and also thermally induced when we load the locks onboard had gotten behind this cork and expanded some of it and actually caused a disbond of the cork to the booster. And so we went through a lot of iterations trying to get some more cork bonded to the MER-B booster and suffered some setbacks in the way that the bond was setting for us. We were finally able to get that off, and in early July we were able to launch MER-B. This one is not going to suffer from that because we've added some leak holes to the area to make sure that any water that's behind there will drain properly and the adhesive that's on there is the original adhesive that should hold very well for this application.
Juan Rodriguez from Gijon, Spain
Do you have to take special safety precautions to prevent problems with the SIRTF liquid helium tank during this launch?
Juan, there's safety precautions that are taken while we're loading the helium onboard the spacecraft, and that is to protect the personnel and obviously the spacecraft from suffering any damage. And helium in itself is not very dangerous, it's not explosive, but in the cryogenic condition it can currently cause some burning if folks are exposed to it or if it increases rapidly it can asphyxiate some folks if they're in a confined space. So all that is taken before the launch countdown ever takes place. So we disconnect at L-12 hours and at that point the helium system is really locked up and there are no other special safety precautions to take other than the ones we would use around a fully loaded vehicle.
Juan Rodriguez from Gijon, Spain
I have read that you had to reschedule the launch of SIRTF because you are using a tracking and instrumentation ship in the Indian Ocean to support launch. Why don´t you use tracking and data relay satellites?
That's a great question Juan, and I'll tell in the back of our minds we have been developing a transmitter to use on the Delta's and the smaller Pegasus launch vehicle. Unfortunately developing it and implementing it takes quite some time. What we run into is SIRTF will be approximately 90 nautical miles in altitude during this second stage phase, which we were going to be able to recover data from this ship in the Indian Ocean. So it takes very low power to get the signal down from the spacecraft, or from the second stage down to the ground. It’s only 90 miles so we're able to catch it very easily with a telemetry ship, or ground base type systems, or even an airplane if we had one available. But if you go to a tracking and data relay satellite you've got to remember that these satellites are up at geosynchronous orbit, so you need high power and you need to be pointing your antenna in the opposite direction out into space in the geosynchronous. So when you go to high power you need higher weight and so you have to make your satellite smaller or your intended science takes a hit in mass and you have to take up some additional batteries and you have to be able to reject heat, which means you need to have some radiators or heat syncs. So you take quite a bit of hit if we were to have these types of transmitters, but we're headed that way. We're trying to make them smaller and miniaturize some of the components and hopefully we'll get there in a couple of years.