March 5, 2007 notes:
We made several flight tests this month with Pixel. All of the things that I wanted to improve with
the software are now done.
The roll control has been relaxed so tank slosh doesnt
cause the thrusters to fire nearly continuously, depleting the ullage pressure.
I have added discontinuity tracking to the raw GPS
position, since the integrated velocity value that I was previously using did
have a drift of six meters on the real flights we did at the X-Prize Cup. The drift was always small when we were hanging
at a hover, but when we did the up-50-meters-over-100-meters-down-50-meters
flight the drift was a lot higher. I
still use the integrated position as the input to the flight control because it
is much smoother than the raw GPS position, but I allow the integrated position
to slowly drift towards the corrected absolute position. Using differential GPS for the raw position would
be even better, but I would prefer not to add another box, antenna, and cable
to my ground station equipment.
Eventually, when we outfit a dedicated van for our operations we will
permanently mount everything and likely include DGPS.
Speaking of GPS, I am seriously considering buying a
multi-antenna attitude sensing GPS system.
This product is still over $13k, but it is much cheaper than previous
The flight control system now has a good parameter file /
command line override setup, rather than having some hard coded assumptions
about the lunar lander challenge flight profiles. We will just walk the electronics box from
the liftoff pad to the landing pad to get our exact coordinates, rather than
relying on a pad survey.
One of the test flights this month was done in absolutely
crazy wind conditions. We measured steady winds of 30 mph, with gusts
significantly higher. The fire station had seen gusts as high as 55 knots
(63 mph) earlier in the day. Flights were being canceled out of DFW
airport due to the wind. Pixel didn't
care a bit. Almost two minutes of hovering there, completely unperturbed. As Neil put it, Pixel laughs at 40 mph
We planned to try for a 180 second flight, but we managed to
put the engine back together with the o-ring pinched out of its groove, and the
resulting fuel leak into the chamber messed up the mixture ratio badly and
eroded the graphite chamber a fair amount.
We should have a new chamber ready to go this week. We also made a new injector that has more
elements, because the current one was requiring nearly 100% throttle to stay
aloft with the heavy propellant load during the blow down flight. I have also added a guard lip that will
hopefully protect the chamber o-ring. We
still have the metal helicoflex seals coming, but if
we can continue to use elastomer o-rings, life will
be better. I dont expect to make 180
seconds on the first try, I figure we will have to make some adjustments to the
propellant load and/or starting pressures and element hole
sizes to guarantee simultaneous propellant depletion, and we may need to strip
some weight out of the vehicle.
It is interesting to note that we just dont do short flight
tests anymore, every flight is at least a 90 second propellant load. That might be different if we could still do
test flights behind our shop, but we have been testing at the Greyson County Airport, which is a little over an hours
drive from our shop, and the packing and unpacking add a couple hours more. We are paying $6.50 / gallon for our 90%
ethanol, and $570 for a six-pack of helium.
I dont know our lox costs off hand, but it is the least expensive of
the three consumables. Counting crane truck
rental and facility fees, it winds up being about $3000 to do a pair of flight
tests. Full up 100 km space shots will
not be much different, but operating out of New Mexico will increase our costs. We are probably going to try using E85 to
save a few hundred dollars on fuel costs, but the gasoline content makes it
more of an environmental issue for launch permits / licenses.
We have FAA-AST experimental permit #003 in hand now for
doing lunar lander challenge flight profiles at the
Oklahoma Spaceport. There are still some
issues to be resolved regarding some cross waivers between the FAA and Oklahoma
(which came as a surprise to all involved!), and I still need to write the big
check to start our insurance coverage, but we should be good to go soon. We probably wont do any operations there in
March, but we will likely do an official level 2 qualification flight in April.
We are going to have to make another permit application for
the 2007 X-Prize Cup flights. The fact
that we will have three separate permits for the same flight profile is pretty
silly a permit should allow just a single population exclusion zone to be
specified for a given flight profile. If
we ever want to be able to fly, say, vertical dragsters at multiple airshows, some arrangement like that will be
necessary. FAA may need to issue a
waiver instead of a permit/license for something like that, because there are
issues about the show site being a spaceport.
Neil is going to start working on the next experimental permit
to allow us to do flights to 4000 in Oklahoma,
and we are probably going to apply for a full launch license at Spaceport America in New Mexico for 100km flights.
Almost all of the pieces have been machined for our batch of
modules, and everything fits.
I am really happy with how we are able to fit everything
conveniently together at the base of the vehicle, and we will be able to easily
change to different tanks if desired. We
will be able to fly the individual modules with the gimbaled engine, but if the
four module fixed engine / differentially throttled configuration works out, we
will make a different modular design for the next rev that has the engine
practically butted up against the tank, with no gimbals, flex lines, or
connectors. We will probably save 30
pounds per module, a foot of height, and a fair amount of cost.
We have added slosh baffles back to the modular vehicles. We had some in the original VDR, but we
removed them on the quads. We are pretty
sure some of the oscillations we have seen on quad flights are slosh related,
so we decided to go back to them again.
The baffles are 0.032 thick 5052 aluminum, 12 ID, and 30 / 35
OD. They were laser cut at Wooten Metals
in Dallas. To give some expansion capability and make
welding easier, each baffle is attached to the tank by 16 tabs of 1/8 thick
aluminum, slightly bent up to allow them to flex. The slosh baffles leave an open ring at the
edge to allow propellant to drain.
My current plan-of-record (always subject to change) for
upper stages is to use extremely low tank and chamber pressures. Since our boosters will be so wide, we arent
at all diameter constrained, and I think that using self-pressurized propellants
(VaPak) will allow efficient combustion even with
quite low pressure drops across the injectors.
Some lightweight and inexpensive engine fabrication techniques may also
become viable at very low chamber pressures.
We made a test tank to get some experience with this level of
The 27.5" ID sphere was spun from 0.050" 5052
aluminum and weighed 10 pounds dry, and 396 pounds wet, for a water mass ratio
of 39.6. It burst at only 115 psi, right in the
middle of the weld. I was expecting something around
150 psi if we achieved equal weld quality with
our thicker, better alloy tanks.
James found welding the ultra-thin hemispheres challenging, and the failure was
almost certainly in an area where the spheres had separated some due to puckering
from the tack welds. He thinks it could be improved with a better clamp
band design and practice. The fact that the hemispheres were brushed
instead of delivered as-spun didnt help, but wasnt the main factor. Other alternatives would be a girth ring that
both hemispheres are welded to, or a backing plate behind the girth weld.
AMS's recommended spinning limits for hemispheres are:
<=27.5" diameter = 0.050" thick (possibly somewhat thinner, this
is on the hand-spinning machine)
<=42" diameter = 0.102" thick due to the hydraulic spinning
machine tearing thinner material
<=65" diameter = 0.375" thick due to thinner material not being
available in the necessary sizes
If we do pursue thin-wall welding, going with 0.102" thick at 42"
diameter is probably a better direction. The mass ratio would be a little
lower than the 0.050" at 27.5" diameter, but the extra capacity at
158 gallons would work out better for an upper stage module. If we use a
good aluminum and get the welding process down, we are probably still looking
at a burst pressure of around 250 psi, which is
higher than really necessary for the ultra-low-pressure vapak
upper stage work, meaning that we would prefer to have lighter tanks if we
could fabricate them.
These minimum-gauge ratios make life a bit awkward for one of our other
possible technical directions -- high strength maraging
steels would need to be around those thickness limits just to have the same
weight as our aluminum tanks. They would have a burst pressure around
1500 psi, allowing higher pressure engines that are
more efficient and higher thrust to weight, but they wouldn't help the raw mass
ratio very much, and we would probably wind up losing some after we convert the
various things welded onto the tanks to maraging
steel from aluminum.
These results encouraged me to put more emphasis on following up on the chem-etch costs. It would allow us to thin the
already formed hemispheres down as needed, while also leaving a thicker weld
land to make fabrication easier and allow weld qualities somewhat less than
100% to still give the same tank burst pressures. The tradeoff in land
thickness is that both the initial spinning costs will be determined by the
land thickness. If we stay with 5xxx series aluminum, starting with
1/4" aluminum and taking 1/16" off outside the weld land would save
us some weight without hurting our pressure rating much. If we go to
2219, we may be able to go all the way down to 1/8" thickness on most of
the sphere, but we might want to start with 5/16" initial thickness
because of the lower as-welded strength.
A good article on chemical milling: http://www.jobshop.com/techinfo/papers/chemmillpaper.shtml
I got a quote for chem-etch work
from Ducommun Aerospace: http://www.ducommunaero.com/chem_mill.html It was quite reasonable for 36 hemispheres:
about $700 in tooling, $1100 extra for each first-article that changes process
parameters, and $450 for each hemisphere, with a minimum lot of five
hemispheres. The total turnaround for
the first batch would be around two months, so I decided not to pursue it for
our current run of modules, but I probably will for the next batch. Im sure we can pull 30+ pounds out of each
module with no loss of burst pressure with our current alloy.
See some of you later this month at Space Access 07!