Date: Mon, 20 Oct 2008 09:50:39 -0500
From: John Carmack <***>
Subject: [AR] I think we are ready.
To: arocket

We have burned well over 10,000 pounds of propellant in the week and a
half since we finally got our tether testing waivers sorted out. We
were very fortunate to only have a couple days of rain, or things might
not have been possible, and I would have been very, VERY bitter at AST
over the tether ruling. As it is, we crammed most of the testing we
wanted to do into one quarter the time we wanted to do it in. We did
lots of full duration burns for both vehicles (Matt has a great video of
another 180 second Pixel flight with the sun literally going down behind
it), a ground liftoff and touchdown, and a bunch of trim adjustments on
Pixel. The only thing we haven't done this year is a full, untethered,
pad-to-pad free flight, although both vehicles have done several of them
in past years.

There are still a few things that aren't perfect:

Pixel has never started a significant translation with a full propellant
load. Several flights have been made at XPC '06 and in Oklahoma, but
they were all with half propellant load for 90 second flights. We could
choose to burn off the first 90 seconds of propellant over the starting
pad before doing the traverse, but that would have us in full slosh
wiggle during the motions, and I want as much time as possible to dial
in the landing on the lunar surface so we don't land on a rock or in a
crater.

Positional control gets marginal on Pixel near the end of a three minute
burn, which is another reason I would prefer to be sitting right over my
landing spot, rather than moving towards it in the last part of the flight.

Balance and trim issues are still a concern with Pixel, we had one 180
second flight that ended with the residual propellants unbalanced enough
that the engine started sucking some helium. We can control this with
manual trim if we stay on top of it.

The flight computer box we have on Pixel isn't quite as good as the one
on the module. The main battery drains faster, and the gyros drift a
bit more. If we had more time, we would have tested swapping the boxes
around to make sure they are as interchangeable as we think they are.
As it is, we are going to stick with what we have tested unless forced
to switch.

We had one communication upset post-flight with Pixel. It seems very
coincidental that the video receiving media stack was unplugged just
about at the time of the upset, so we are not going to do that in the
future.

We had some odd problems with initial fuel valve movement on both the
Mod and Pixel that caused some no-lights. We replaced an actuator and
it seems to have gone away, but we don't understand the root cause. I
wouldn't be stunned if we experience a no-light on one of our attempts,
but hopefully just recycling and trying again will fix it.

The lunar surface may yet trip us up for level 2. In testing trim
issues for pixel, relatively minor level changes can make drastic
differences in propellant balance, and cause significant problems.

There are always worries about transporting everything long distances
over the road.

I think we have a decent chance of taking both prizes on our first try,
but we have a backup engine for each vehicle and lots of spare parts if
we need to fix anything.

...

John Carmack