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Re: ArcJet Update

This is the type of response that we have yearned for in the past. Great
post, Stacey.

----- Original Message -----
From: Stacey E. Mills <w4sm@cstone.net>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, July 09, 2001 6:17 PM
Subject: [amsat-bb] ArcJet Update

> 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
> perigee was very close.  In fact in some simulations, it was as low as
> 160 km.  There was considerable drag at perigee, which continuously
> 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
> 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
> suggests that the gas ran out after approximately 1575 minutes (26.25
> 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
> 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
> 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
> 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
> 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
> were right and then begin the move back to 0/0.  A nominal single test
> 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
> 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,
> 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
> 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
> 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
> 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
> stations.  I try not to take these personally, and I realize that you
> 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
> that you're interested.  We can withstand your critical scrutiny!   ...if
> we can't we need a new hobby.
> --
>   ________________________________________________________________________
>   Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
>     Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
>   ________________________________________________________________________
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