[Ground-station] pop quiz!
Zach Leffke
zleffke at vt.edu
Mon Jun 11 23:27:11 PDT 2018
All,
OK, I'm going to give this a go! Spoiler alert and lots of whitespace
before my attempts at answers (please be gentle if I'm in left field! hi
hi).....................................
------------------------------------- POTENTIAL SPOILER
ALERT--------------------------------------------------
-------------------------------------STOP READING IF YOU DON'T WANT TO
SEE MY ANSWER
ATTEMPTS-------------------------------------------------------
-----------------------OK, YOU'VE BEEN
WARNED.....TWICE---------------------------------------------------------------------
------------------BEGIN ANSWER---------------------
So first off, this is a Motor Firing set of questions....which lead me
to the LIU set of schematics (marked page 129b, PDF page number 158).
The first question plus hints leads towards a hunt for a hardware timing
error. The golden nugget I believe is the indication of a 1.68 factor
(longer burn time than expected).....which I'll get to in a second.
Bit of my logic/attempt at understanding the schematic:
I think I narrowed things down to the 4015 chip (U4, dual 4 bit serial
to parallel shift registers, configured to be an 8 bit shift register)
and the pair of 4029 chips (U2, U3) near the top center of the page
(with the 'burn counter' lines coming out). Also relevant is the 4029
chip (no 'U' label) below U2/U3 that I think is the 'kick' to start the
firing sequence by setting the 'JAM' signals (aka preset enable) on
U2/U3 to initialize the countdown values in U2/U3 that are loaded into
U4. The 'kick' signal out of the 4029 is tied to Q4, so 8 cycles of the
undecimated clock passes before this bit is set (and stopped since its
also tied to the inhibit), allowing the serial to parallel registers to
be clocked in (from the pn sequence decoders) before the 'kick' that
takes their values and maps them to the U2/U3 chips. The 4040 Chip
(U11) looks like its being used for clock decimation. The 4022 chip
(U5) looks like it is the valve sequencer that controls the order and
repetition rate of the opening and closing of the helium, UDMH, and N2O4
valves, ultimately controlling the rate at which fuel and oxidizer flows
to the motor.
If I'm reading the schematic correctly, it looks like U2/U3 comprise an
8 bit timer with 256 (0 - 255) possible countdown times. U2 is the low
nibble, U3 is the high nibble. Every time U2 reaches the all zeros
state, the U2 Carry Out toggles Low for one clock period, which allows
U3 to decrement by one count. U2 is fed by the 'B' side of U4 and U3 is
fed by the 'A' side of U4. Via a sequence of NAND gates, it looks like
the initial 'kick' signal triggers a Monostable multivibrator (aka a
'one shot') on the U21 555 Timer that is the 'START' of the burn. When
the countdown finishes, it looks like another monostable multivibrator
555 (U9) fires the 'STOP' command. This is where my understanding
begins to break down, because it could be that the 555s are astable
multivibrator configurations (maybe some kind of charge pump for the
ignition coils?, or maybe the ignition is being fired in sync with the
valve sequencing?) that are being toggled on and off rather than actual
one shots, and I'm not positive about how the valves are actually
'actuated' by the control circuitry. I am sure that I am missing lots
of details, but I think I have a pretty good understanding of what the
'regions' of the schematic are responsible for.
To the point......
Answer to Question 1:
I think there is a transposition of the high nibble and the low nibble
when transferred from the 4015 to the respective 4029s for the
countdown. The bits are 'mirrored' about their 4 bit lanes when
transferred. More specifically, the 4015 clocks in the serial data on
the 'B' side first, and then those bits are transferred to the 'A' side
as four more bits are moved into the 'B' side. I think the fundamental
problem is that what becomes the least significant bit of each nibble in
the 4015 becomes the most significant bit in the respective nibbles of
the overall countdown timer. Lets say that the intended 8 bit sequence
that is clocked into the 4015 is the following: 0010 1111, where the
0010 is the High Nibble, side A of the 4015 and 'jammed' into U3 on
'kick' and 1111 is the Low Nibble, side B of the 4015, and 'jammed' into
U2 on 'kick'. When the 'kick' signal is applied, the 0010 of the 'A'
side of the 4015 then becomes 0100 in U3. A similar process happens on
the Low Nibble, but since it is 1111, the transposed result is also 1111.
Why did I pick that bit sequence?...This is where the 1.68 factor comes
in. I'm not sure of the actual clock period, so lets assume for the
moment that the clock frequency is 1 Hz, or a rate of 1 clock period per
second. For an intended countdown of 0010 1111, this would result in a
47 second burn. With the transposition of the high nibble, the actual
countdown timer is then 0100 1111, which results in a 79 second burn.
79/47 = 1.680851064. Thus the motor burns longer than expected by a
factor of 1.68. I figured out the bit sequence using a giant excel
spreadsheet and listing out all 256 combinations of 'expected' sequences
and the resulting 'transposed' sequences, with the associated burn times
for each, and thus the ratio of burn times and only one bit sequence
shows a factor of 1.68 of Actual burn time over expected burn time
(programmers reading this.....please don't throw any rotten tomatoes,
copy and paste is quick in excel...hi hi).
Answer to Question 2:
IF a maximal length burn was tested on the ground, the transposition in
the 'jamming' from the 4015 to the 4029s would not have been detected.
1111 1111 with each of the nibbles transposed is 1111 1111. Still
works! The most OCD test (and expensive if actual propellant was used
during the test) would be to test all 256 potential burn periods (one of
them would be '0' and thus the cheapest!). Measuring the burn time of
each burn and comparing against the expected burn times for the sequence
loaded would have revealed the problem.
Answer to Question 3:
No idea really. Zero experience with how these kinds of valves work.
I'm guessing if the problem is 'electrical' in nature it maybe has
something to do with the timing between the 4040 clock decimation chip
('pyro speed' indicators here, without remote control of the selector,
suggesting the developers were testing various rates, and then settled
on a jumper for the fastest rate) and the 4022 valve sequencer that is
'self inhibiting' (maybe another step of clock decimation?). It looks
like the Helium control valve is the first valve in the sequence at one
of the lower bit values (bit '2') relative to the higher bit values. So
maybe the helium valve is getting 'toggled' more frequently, meaning
less 'time open' and thus reduced helium pressure in the
propellant/oxidizer tanks (like maybe it was getting 'stuck' and the
toggle rate was too rapid to allow sufficient helium to make it into the
tanks?). Maybe this was less of a problem earlier before the big burn
because the propellant/oxidizer tanks had sufficient pressure on their
own when they were more full (requiring less helium to pressurize them
sufficiently)? Last shot in the dark.........The 4022 is tied back into
the pn sequence decoder reset lines.....so maybe some kind of reset
occuring relatively quicker than expected even when the right pn
sequences are sent. maybe the 4022 hits the reset condition, which
clears the pn sequences from the registers, requiring some number of
clock periods (8?) before they are 'reloaded' with the correct pn
sequence, reenabling the process.....so sort of a 'stop and go'
situation with the valve sequencing.....again, maybe not a big deal when
the tanks were full and had their own pressure, but more of a concern
later when more helium is needed to pressurize fully?
That's it.......again, I'm sure I screwed up a lot of the details, but
that's my best guesses for the three questions. Fun game! I look
forward to partial credit!
-Zach, KJ4QLP
Research Associate
Aerospace Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305
On 6/11/2018 9:31 AM, Douglas Quagliana wrote:
> Friends,
>
> Even if you know the history, you might not have seen the actual
> blueprints, and you still have to find it in the blueprints.
>
> Regards,
> Douglas
>
> On Jun 11, 2018, at 6:59 AM, Michelle Thompson via Ground-Station
> <ground-station at lists.openresearch.institute
> <mailto:ground-station at lists.openresearch.institute>> wrote:
>
>> Good question - post with a generous spoiler alert style header. Cc
>> Phil for a faster reply.
>>
>> On Mon, Jun 11, 2018, 04:52 Zach Leffke via Ground-Station
>> <ground-station at lists.openresearch.institute
>> <mailto:ground-station at lists.openresearch.institute>> wrote:
>>
>> Do you want answers (or attempts at answers) on the list or
>> direct to you?
>>
>>
>> -Zach, KJ4QLP
>>
>> Research Associate
>> Aerospace Systems Lab
>> Ted & Karyn Hume Center for National Security & Technology
>> Virginia Polytechnic Institute & State University
>> Work Phone: 540-231-4174
>> Cell Phone: 540-808-6305
>>
>> On 6/8/2018 10:36 PM, Michelle Thompson via Ground-Station wrote:
>>> Phil Karn KA9Q has been working on something fun for us. If you
>>> enjoy this quiz, please feel free to share it and let him know!
>>>
>>> Given the documents that you can find at:
>>> https://github.com/phase4space/AO-10-blueprints
>>>
>>> Quiz for fans of the AMSAT Phase IIIB/AO-10 blueprints
>>> Phil Karn, KA9Q
>>>
>>> Question 1: AO-10 carried a 400-newton kick (rocket) motor that
>>> used hypergolic propellants. It was fired only once, but the
>>> burn duration was 1.68 times longer than planned. Why? (Note:
>>> the burn did *not* end because of propellant depletion.)
>>>
>>> The cause can be discerned from the prints; i.e., it was a
>>> hardware problem, not a software bug or operational mistake.
>>>
>>> Optional hint 1: The burn was timed in hardware.
>>>
>>> Question 2: The ground test of the hardware consisted of
>>> successfully executing a maximal length burn. What tests could
>>> have revealed it?
>>>
>>> Question 3: Although the first burn did not deplete the
>>> propellants, it was not possible to fire the engine again
>>> because of insufficient helium pressure. Why? (Hint: the cause
>>> was not insufficient helium loading before launch.)
>>>
>>>
>>> _______________________________________________
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>>> Ground-Station at lists.openresearch.institute
>>> <mailto:Ground-Station at lists.openresearch.institute>
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>>
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