June 15, 2004 notes
Perfect test flight
The new actuator covers worked out great. We made these by hand, but we also finally
got around to getting a sheet metal roll / brake / shear for the shop to make
this type of thing easier in the future.
There was intermittent rain around Dallas today, but we
decided to head out to our test site and hope for the best anyway. We taped over all the exposed holes in the
vehicle, but it turned out that we only caught a few drops on the way over, and
the test site was fine. Since we had a
test out there only two and a half weeks ago, we already had some of the gear ready,
and we didnt forget anything this time.
We had the vehicle loaded up and ready to go within 30 minutes of
We pressurized the tank to 300 psi, which is a little high
for the threaded end closures on these tanks, and we could hear a small
pressure leak as the O-ring unloaded a bit.
The engine warmed up predictably, adding further evidence
for the benefit of a compressed hot catalyst pack (although we did lose some
thrust after compressing it). Something
that we noticed this time was that there was still some cloudiness in the
exhaust while I brought it up to temperature, but after it had sat for a couple
seconds before I started it back up again for the launch it was completely
clear. It is likely that there is some
path that was channeling enough propellant to self cool at steady state, but
after letting it heat soak a bit from the surrounding catalyst, it was
uniformly at operating temperature. We
can probably use this effect to shorten our warm-ups by warming for a few
seconds, then pausing for a couple seconds.
I changed the liftoff procedure slightly, opening the valve
to the warmup level and holding it there while engaging the boost command. Previously, I would warm the engine, then
let it shut completely down for an inertial reset, then just engage boost mode,
which pushes the throttle open as fast as it can. This lets more propellant flow into the engine than it would get
if there was already chamber pressure, resulting in a brief period of higher
than normal thrust and stress. This was
quite noticeable in the test on Saturday, which had a momentary kick of nearly
one G. Boosting from warmup made the
flow completely predictable.
The flight parameters were set for 1.8 seconds of boost, -4
m/s^2 minimum acceleration (slightly more than negative one half G) during the stabilization
phase, 3 m/s^2 acceleration in the landing phase, 1 m/s target touchdown
velocity, and a 3 m uncertainty margin for the GPS altitude. I increased the minimum acceleration during
stabilization because of concerns about throttling the ball valve at small open
fractions and low chamber pressures.
This wastes more propellant during the flight, but this vehicle can carry
so much more propellant than we can use without our burn time waiver that it
doesnt really matter.
The flight was perfect.
It went 131 feet high, and landed less than one foot from the launch
Analyzing the telemetry told us the following:
This engine didnt run as well at full throttle with the
increased pressure, giving an acceleration during the wide-open-throttle that
cycled from 15 to 20 m/s^2. We really
need to build a brand new engine to replace this one that has been cut open and
modified a half dozen times.
The acceleration prediction that I added to smooth out the hunt-for-acceleration
modes helped the stabilize mode, but not as much as it smoothed out the
hovering in the test on Saturday. You
can clearly hear the pulsations in the flight video. This is understandable, because the flow curve is changing faster
at the lower ball valve opening. I
should be able to either increase the acceleration prediction, or slow down the
ball valve movement at smaller openings.
I also realized that I can develop this on the test stand, because
hunt-for-chamber-pressure will be effectively the same thing as
hunt-for-acceleration on a vehicle.
We knew our chamber pressure signal was messed up, so we
didnt get that in the data logs this time.
When we got back to the shop we investigated, and found that the porous pressure
snubber in front of the transducer was blocked up. We had this happen once before on a test stand transducer, so we
are going to change up from 1 micron to 7 micron filter size. A little bit of peroxide probably starts
enough surface corrosion on the 303 SS to clog it up.
The auto-land worked perfectly. I had tried several algorithms on the simulator before settling
on this one, and it behaved exactly the same in reality, which is always a
We were planning on doing more tests, but the burn time on
the first one was 14 seconds, so we really didnt have much room under the 15
second burn time limit. I could have
trimmed the stabilization acceleration and GPS uncertainty, but risking the
vehicle to go another 50 feet higher didnt seem worthwhile, and we called it a
(Neil, Phil, Tommy, John Carmack, Russ, John Carr, Matt) Joseph was sick today,
and James teaches class on Tuesday nights, so they missed it
We have about 25 hops on this set of jet vanes now, and they
have taken on an interesting coloration pattern:
We probably wont fly this vehicle again until we build a completely
fresh engine and develop the low throttle hunting algorithm a bit more, but we
are submitting some changes to our burn time waiver request to allow us to do
initial flights with the small vehicle before flying the big one. It can easily do flights three times as
long, which may show up some problems before we hit them with the big vehicle. If the big engine isnt burned from the
leaking valve problem, we should have the big vehicle hovering under the lift
this Saturday, so we may be out next Tuesday doing a boosted hop with it.
Speaking of next week
I think Space Ship One has good odds of success in the
single-person-to-100km flight. I only
see two real issues they may hit: The
extended burn above the atmosphere may run into some control issues as the
nozzle ablates, which will be hard to correct with only cold gas attitude
jets. This would be a fairly benign
failure, with the pilot just shutting off the main engine if he cant hold the
trajectory. The dangerous part of the
test will be the reentry with a significantly bigger drop than the previous
test. At this point, I hope Burt has
everything work out and he is able to make the X-Prize flights soon, because
our prospects are pretty dim for getting everything working perfectly in the
big vehicle in five months and having permission to fly it. I certainly dont want the insurance company
to keep the prize money. If Space Ship
One crashes, we will probably throw ourselves at an attempt, but it will be a
long shot. No, I dont think any of the
other teams are close.
Since the Frontier Files Online diaries dont seem like they
are going to get working again in the near future, I have started a public
Q&A thread on the X-Prize message boards.
It is generally better to ask a question on a public forum, rather than
emailing me, because many people have similar questions and curiosities.