ArcJet Update by Stacey Mills, W4SM, AO-40 command team ============= Dear Folks, I'd like to try to answer a few questions and clear up some misconceptions made in comments on the Amsat-BB about the ArcJet outgassing. 1. Was the old orbit stable? ------------------------- Yes it was. I did the orbital integration and it WAS stable, at least in the sense of not suffering re-entry or melt down. However, as we all know, perigee was very close. In fact in some simulations, it was as low as 160 km. There was considerable drag at perigee, which continuously changed the mean motion and probably contributed to the perturbation in ALON (the mystery effect). The keps on the S/C had to be frequently updated. The rapidity of the perigee flyby made magnetorquing very tricky and required a lot of time to determine attitude. To improve these things we wanted to raise perigee height. That doesn't mean the old orbit wasn't stable, but it certainly wasn't optimal, and it was close enough at perigee in June to be frankly scary. 2. Was the command software controlling the arcjet faulty? ------------------------------------------------------- No, it most certainly was not. I wrote it, the rest of the command team checked it. It functioned perfectly in all the simulations. There is not a shred of telemetry evidence to suggest a software error. 3. Are we out of ammonia? ---------------------- As far as we can tell, yes. 4. Where did it go? ---------------- We don't have all the answers, and hence we have not yet posted a full report. It appears that the TMFC (thermal mass flow controller) failed, passing far more gas than it should have. There was NO INDICATION of this in the telemetry. Spin rate changes and telemetered flow rate, etc., all appeared in a safe range. The software properly set the TMFC to 50% flow rate. This should have been ~25-30 mg/s of flow. Even fully open, the TMFC should not have exhausted more than 100 mg/s, which should have left ~60% of the ammonia unused, even after 58 hours of run time. The telemetry suggests that the gas ran out after approximately 1575 minutes (26.25 hrs.). If the tanks were full, that's 560 mg/s, far in excess of what the TMFC should have allowed. Again, we're still looking at this for better answers. As has been noted, we really only needed about half of the fuel to get to the orbit we're in, so it is quite possible that there was also a slow leak in the ammonia tanks such that we started with only approximately half of the fuel. If this were the case, then the flow would have been ~280 mg/s, still almost three times nominal maximum flow rate. Looked at another way, the new and old orbits indicate an apogee velocity change of 40 m/s. Accelerating a 500 kg mass through 1575 minutes indicates a force (thrust) of 0.213 N. That's over twice as high as would have been achieved if the motor were used in a proper powered ArcJet burn. Unlike the helium tank, the ammonia tanks have a liquid/gas interface in equilibrium, so the pressure in the tanks is not proportional to the amount of liquid they contain. Pressure is a function only of temperature as long as liquid ammonia is present. Thus, there was no way to tell how much "fuel" we had. If we did have a slow leak it is very fortunate we did not wait any longer to use the remaining fuel. 5. Why wasn't the orbit checked after the first test outgassing? ------------------------------------------------------------- Because of changing solar angle, the mystery effect and precession moving ALON, as well as the rapidly decreasing perigee height in late June due to solar/lunar forces (which might make attitude even harder to maintain), it was important to complete the maneuver as soon as possible while conditions were right and then begin the move back to 0/0. A nominal single test burn should have produced so little change in perigee height as to be hard to detect against background "noise". In addition, waiting for good keps following the test could have required more than a week (current case in point). There was nothing in the test telemetry to suggest a problem, so we proceeded with the full maneuver. From a pragmatic point of view it was always clear that any radical departure from expected would make it highly unlikely that we would be able to maintain a stable, low-volume gas flow as needed for a powered ArcJet burn, so loss of extra ammonia would likely have little effect on the powered ArcJet use. 6. Do the command stations know what they're doing? ------------------------------------------------ We like to think so. There is ABSOLUTELY NO EVIDENCE that the ammonia problem was due to a command error. I cannot state that any clearer. Please bear in mind that we all take this task extremely seriously, and we spend a HUGE amount of time on it. Just getting AO-40 into the 270/0 position for the outgassing took several weeks of effort and calculation. When there is a problem, the amount of time required goes up exponentially. The first requirement is to maintain the integrity of the spacecraft, then to gather critical information while it is still available, then to analyze the information, and then to report the findings. I freely admit that my first thought when there's a problem is NOT to post a message to the amsat-bb. This is not because we want a "cover-up," although I have been personally accused of this at least once in quite objectionable terms. The reality is that we are busy trying to figure out if we still have a safe spacecraft, and then determine what's going on. When we are sure that we have the best possible information, we fully report our findings. 7. What's going to break next? --------------------------- The short answer is that we don't know. I sincerely hope that nothing else malfunctions for a long, long time, but this is after all, rocket science. Nothing is guaranteed. However, there has been the suggestion that the command stations are rushing to test things without considering failure scenarios. Nothing could be further from the truth. Weeks of discussion were devoted to the ArcJet and possible failure modes. These discussions involved the designers and builders of the ArcJet, the management team, etc. Out of these discussions came the decision to "cold outgas" before trying hot outgassing, and to orient to raise perigee, since a faulty or stuck valve in this orientation would not destroy the orbit. Consider that if we had been oriented differently we could be at -350 km rather than +850 km perigee height!! The momentum wheels have not yet been tested. Rest assured that when they are, it will be after lengthy discussions regarding failure possibilities, recovery modes, etc. I do not regard this as a particular risky test, unlike the ArcJet, but it will still be approached very very carefully. Finally, the decision to deploy the solar panels will be much more involved. This is a "no return" event and will in all likelihood be a considerable time in the future after prolonged demonstration that the momentum wheels are fully functional and that 3-axis control software is up to the task. If there are ANY problems in this regard, we will stay in spin mode!!! Our primary goal at the moment is to get AO-40 back into an orientation where the transponders can be operational and RUDAK testing can be completed. 8. And finally ----------- I hope this clears up at least a few points. I know this is a frustrating issue for the users and supporters, but believe me, it's also very frustrating for the command stations. A lot of software written and tested for the "hot" ArcJet, will now never be used. And my frustration is totally insignificant next to that of the wonderful folks who put years of effort into building the ArcJet, and now will not see it function. I must admit that my disappointment is compounded by some of the comments that appear on the amsat-bb questioning the abilities and motives of the command stations. I try not to take these personally, and I realize that you would all like answers. So would we. We do have a very stable orbit and we are moving back towards 0/0 to make AO-40 available for your use. All in all, it could be a lot worse, a whole lot worse. I'd like to thank all of you who have posted supportive comments on the amsat-bb. I'd even like to thank those who have questioned our abilities. At least your comments show that you're interested. We can withstand your critical scrutiny! ...if we can't we need a new hobby. Additions by Peter Guelzow, DB2OS, AMSAT-DL President ========= Dear All, I should add a few points to Stacey's excellent summary. > 4. Where did it go? > ---------------- > We don't have all the answers, and hence we have not yet posted a full > report. There are a few things to remember... We've got the ATOS (Arcjet Thruster on Oscar Satellite) for free and we designed/build the power supply for it in return. We haven't yet posted a full report, because those who build and designed the ATOS should have a chance to review all the fact we could provide and probably make an official conclusion out of it. It's their business, not ours.. They are currently looking in every possible direction, doing a failure tree analysis and it would be unfair to post something based on speculation, which is than published everywhere.. We only know that everything was done on our side to make it a success. We were lucky to get the perigee raised enough, so that we don't have to worry about the low perigee. Yes, we were told that it is safe, but who knows... now we can feel *much* better. We were also told that AO-13's orbit would be safe before we burned all the fuel.. What did the users missed? Nothing... There would not have been enough ammonia for a significant change in inclination. Perhaps we could have made 10-20° in inclination change, but than we would periodically have bad coverage due to the precessing orbit, with the apogee being over the wrong hemisphere (from either point of view) for some time... Instead we could have reduced the apogee height making it a 16h orbit, but this also makes not such a big deal at the low inclination and the pathloss also does not really get much better.. so what? Indeed, it is very disappointing that we could not demonstrate the Arcjet burning "hot". On the other hand, ATOS has never flown before and it is highly experimental and rocket science. At least we learned that this type of TFMC (mass flow controller) is not flight worthy.. Something which could not have simulated on ground.. To be honest, we were not even sure if the Arcjet motor was still there.. You all heard the speculations that the bottom plate was blown away.. Obviously at least the motor was still there at the right position and mounting angle... ;) > 7. What's going to break next? > --------------------------- No risk, no fun? If everything would be so easy, than there would be no role for AMSAT in space anymore... There is no other way in trying.. but be assured that everything and every possible failure mode was discussed in detail, before any decision is made. We don't use crystal balls. In fact, we were told that the failure mode of the TFMC is, that it gets shut and not open wide... We will have enough time to check-out the momentum wheels. And to be honest again, I would be surprised if they all work OK after the possible destruction in the inner core of the spacecraft. But we have no other choice, we must try! Also risk is predictable. If everything goes well, we will proof test the 3-axis control for a *long* time before we will deploy the solar panels (if et all). > 8. And finally > ----------- > We can withstand your critical scrutiny! ...if > we can't we need a new hobby. Otherwise I wouldn't be here anymore, since I spent most of my life for it since AO-10.