[Ground-station] pop quiz! (SPOILERS!)

[Phil Karn]

Hi Zach,

You're pretty close.

You are correct that the LIU consists of an 8-bit count-down timer
(U2/U3) fed by a 8-bit serial-to-parallel converter (U4), but  I think
the schematic is correct as drawn. There was a PC layout error that
reversed the four bits in the high nybble of the commanded burn interval
between U4 and U3. The low nybble connections from U4 to U2 are correct.

That is, U4 pins 5, 4, 3 and 10 should go to U3 pins 4, 12, 13, and 3 as
shown in the schematic. Instead they went in to U3 reverse order, i.e.,
to pins 3, 13, 12 and 4.

So the commanded bit sequence

76543210

got loaded as

45673210

i.e., 0x2f became 0x4f. I calculated 1.68 myself from those two values;
good work on re-deriving them!

> 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.

The motor has separate 'start' and 'stop' commands. The fuel and
oxidizer valves are pneumatically actuated by pressurized helium that is
in turn controlled by a single latching-type solenoid. A 200 ms wide
negative pulse on the coil opens it, and it remains open until a
positive pulse on the coil closes it. Q10 and Q11 form a DC-DC converter
to produce the negative voltage to open the helium valve.

Latching type solenoids are often used in aerospace relays to save
power, but I can't imagine that would be much of a consideration here. I
presume it was done to avoid a mechanical spring that could fail.

I think the same or very similar engine was flown on AO-40, and you may
remember its failure due to a red vent cap that should have been removed
before flight. If I recall correctly, the capped port vented the helium
control manifold to vacuum when the control solenoid was closed. When
the solenoid closed, the helium pressure was not properly relieved and
at least one of the propellant valves stayed open.

> 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.

There's no need to test electronics with actual propellant, or even to
try all 256 possible durations. Just do a walking-bit test: command the
eight values 0x80, 0x40, 0x20...0x01 and see that they all work
correctly. Also, the burn timing counter was telemetered where it could
also have been checked in software.

Personally, I would have put most of this in software. The only hardware
you really need, aside from power supplies to pulse the solenoid, would
be a safe-arm circuit to keep an insane computer from firing the motor
unintentionally. I'd use just one PN sequence detector. Send it the arm
sequence and it would turn on the solenoid power supplies just long
enough for the software to conduct a maximal-length burn.

Of course all of this is obvious in hindsight.

The helium pyro valves were single-shot. They were fired before the
first burn and remained open thereafter. The helium expanded
adiabatically from the high pressure cylinder, which meant it cooled
quite a bit. Since the first burn was unexpectedly long, the thinking is
that the fittings on the helium tank got so cold that differential
contraction allowed them to leak and lose pressure. So there wasn't
enough to perform another burn.

73, Phil