January 22 and 26, 2002 meeting notes
After coming down tilted last Saturday, we decided to
shorten the legs to make the stronger, and it seems to have helped out.
The one thing that we noticed after the hop on Saturday was
that one of the engines wasnt tilted very much, so it didnt have as much roll
control as it should have had. I played
around with parameters on the simulator, and I was able to get it to do the
same kind of roll-and-tip behavior that we saw if I offset the CG and actually
put two of the engines canted the wrong way.
On Tuesday I double checked, and while the didnt have a whole lot of
cant, the engines were all pointing the correct directions (we have arrows on
the frame, but this was a brand new base, so I was afraid I might have drawn
them wrong). We increased the cant on
the engines, and made sure that they were uniform all around.
We did two more hops, and it still showed the same
roll-and-tip behavior, although in comparing the telemetry with Saturday, the
roll was now only going to five degrees a second instead of ten degrees a
second in the first second after liftoff, so the increased cant was fighting
The roll-and-tip is due to the way the attitude control uses
the same engines for roll control as for tipping control. The roll control correction is prioritized
at 25% of the tipping control, so it initially lifts off with the engines
keeping it level even though it is starting to roll. After a half second or so, the roll has gotten to the point that
it looks four times as bad as the tip, so the engines are prioritized to
fighting the roll, which means that one tip axis begins to loose its control
authority. If the CG is offset at all,
this will cause it to always start to tip towards the CG if it is off the
ground. I have tested some changes to
the flight control software that will cause it to never allow it to tip (if it
can help it) much, even if it starts spinning like a top, which is probably the
desired behavior, but I didnt put the code on the flight computer, because we
wanted to change one thing at a time.
It looked like we could get the roll under control if we
canted the engines a huge amount, like 40 degrees or so, but Im not sure the
flight control software would like that.
We decided that the problem was due to the banged up nozzle on the main
engine. When we had the splat crash,
the motor exit cone hit the ground hard enough to deform it a fair amount. Russ touched it back up on the lathe, but
there is a distinct bell shape to it now that isnt completely symmetric. The CG is about four inches rearward from
the center when we dont have any pilot ballast on it, so if the nozzle was
giving any thrust vector to the pilots right, it would cause a continuous
positive roll thrust.
We shimmed the engine away from that direction for the tests
on Saturday, and it looks like we were correct, because the roll is now
The tether attach points inside the attitude engine are
structurally a lot stronger, but tugs on them are causing a lot more problems
for the vehicle than when we had them on the outside of the legs. The next time we fly, we will probably
connect the two tether points on the vehicle together with chain a few feet
under the main engine, and just use a single chain from the lander to the
ground. The tether is mostly to prevent
a fly-away-and-land-on-someones-car scenario, not really to try and keep it
The engines arent getting full decomposition. Even though there isnt a lot of cloudiness,
there are a lot of peroxide fumes when it is operating, and the total
performance isnt as good as it should be.
We will be making new engines when we have the silver / stainless
catalyst packs worked out.
The laser altimeter should be here within a week or so, so
we will probably do a set of manually controlled flights to test the interfacing
on that before trying to use it for active guidance.
High Temp Engines
Russ chucked up the TZM bar and faced it off to get a little
feel for machining it. It isnt very
pleasant, with a narrow range of speeds that cut well.
NASA SP-8124 has a lot of relevant information for our next
It recommends columbium alloys over TZM, even though TZM has
a higher melting point, better strength at temperature, less thermal expansion,
and a higher thermal conductivity. The primary reason is that after
thermal cycling, TZM has a ductile to brittle transition at 60 degrees, which
means that it can be brittle at room temperature. Solenoid thrusters can
easily have significant overpressures on initial startup, but if we use a
throttled valve, or a sequence of warmup pulses, we should be able to guarantee
that we have warmed the chamber above 60 before opening up the throttle.
TZM has an ultimate tensile strength of 14,000 psi at 3000
F, and 120,000 psi at 70 F.
They recommend silicide coatings over aluminide coatings.
Slurry application followed by vacuum furnace curing is recommended over flame
spraying. I have two quote requests in
for various coating work on our test chambers.
They recommend using clamped flanges instead of bolted
flanges for high temperature chambers. As
well as reducing stress risers, that also has the benefit of removing some
machining steps from the expensive, hard to work materials. There is probably some normal industrial
flange clamp that we can use for testing, but I havent found anything appropriate
There was an interesting concept discussed in the context of
heat sink engines: "interegen cooling", a term which, amazingly,
turns up zero hits on all the search engines I tried.
The basic idea is that instead of spraying the fuel into the
center of the chamber, it is sprayed directly onto a (thick) chamber wall.
The heat of vaporization is then taken directly out of the wall, instead of out
of the chamber gasses, then it functions like normal film cooling.
They talk about it allowing some heat sink engines to run at
a steady state with lower pressures, but it seems like it should also be of
some benefit to thick walled radiatively cooled engines.
A possible modification of that idea would be to submerge
the nozzle throat inside the main chamber, and have the fuel sprayed directly
at the back sides of the nozzle wall. There would be a danger of puddling
fuel in the submerged space, but it would direct lots of cooling at the area
with high convective thermal transfer loads, while the rest of the chamber just
used radiation to cool the lower loads.
In thinking about it more, while I have been looking around
for low temperature / low performance biprop combinations, it might not be all
that hard to make a radiation cooled peroxide biprop with normal fuels and
moderate pressures. Almost all of the radiation cooled thrusters are nitrogen
tetroxide / hydrazine based, which burns quite a bit hotter than peroxide
combinations. They are low pressure, and have to use a fair amount of
film cooling, but they also tend to be very small, and radiation cooling
benefits with scale up.
In discussing low temperature biprop options, Jeff Greason of
XCOR mentioned that there are a lot of alcohols that are water soluble, and
they probably aren't all miscable with peroxide (and hence capable of making
Furfuryl alcohol and Propargyl alcohol have been noted
as "potentially hypergolic" with peroxide, which sounds rather
non-miscable :-) I have just ordered lab quantities of them from http://www.citychemical.com , so we are
going to find out. Propargyl alcohol is
supposed to have somewhat superior performance to kerosene in both Isp and
density, but it is a suspected carcinogen, so we probably wouldnt use it in
production, but we can at least get a definitive answer on the hypergolic
We will probably try firing ethane first in our heat sink test
motor, because the gas will have quicker combustion and wont have a danger of
We had been planning on making just nozzles out of the TZM
for hybrid testing, under the assumption that we would run the grain all the
way down to the nozzle, intentionally sacrificing some combustion quality for
simplicity and a lot of film cooling.
Research has shown that the combustion efficiency would probably be a
LOT worse than we want. A 2 diameter
hybrid test motor with a 1 core needed a 4 post-grain combustion area to
achieve good C* efficiency, which would have been 16 at the core diameter. If we need to have chamber space for the
hybrid testing, we might as well set it up so we can do both biprop and hybrid testing
on the same chamber / nozzle setup.
This does mean that we are only going to get two test articles out of
our bar of TZM, so if the coatings dont work well and we melt them, it could
take an extra month (and another $1000) to get more TZM.
The big rotor blades are ordered, and should be here in
three to four weeks.
This line of linear actuators looks fast and powerful enough
for our gimbal control needs, at about $1700 each:
We are considering possibly using an ejectable fin can to
keep stability on the ascent and descent.